Accuracy and calibration of integrated radiation output indicatorsin diagnostic radiology: A report of the AAPM Imaging PhysicsCommittee Task Group 190

Pei-Jan P. Lina)

Virginia Commonwealth University Medical Center, Richmond, Virginia 23298

Beth A. SchuelerMayo Clinic, Rochester, Minnesota 55905

Stephen BalterColumbia University Medical Center, New York, New York 10032

Keith J. StraussChildren’s Hospital Medical Center, Cincinnati, Ohio 45229

Kevin A. WunderleCleveland Clinic Foundation, Cleveland, Ohio 44195

M. Terry LaFranceBaystate Health Systems, Inc., Springfield, Massachusetts 01199

Don-Soo KimChildren’s Hospital Boston, Boston, Massachusetts 02115

Richard H. BehrmanBoston University Medical Center, Boston, Massachusetts 02118

S. Jeff ShepardUniversity of Texas MD Anderson Cancer Center, Houston, Texas 77096

Ishtiaq H. BerchaChildren’s Hospital Colorado, Aurora, Colorado 80045

(Received 29 April 2015; revised 4 September 2015; accepted for publication 19 October 2015;published 6 November 2015)

Due to the proliferation of disciplines employing fluoroscopy as their primary imaging tool and theprolonged extensive use of fluoroscopy in interventional and cardiovascular angiography procedures,“dose-area-product” (DAP) meters were installed to monitor and record the radiation dose deliveredto patients. In some cases, the radiation dose or the output value is calculated, rather than measured,using the pertinent radiological parameters and geometrical information. The AAPM Task Group190 (TG-190) was established to evaluate the accuracy of the DAP meter in 2008. Since then, theterm “DAP-meter” has been revised to air kerma-area product (KAP) meter. The charge of TG 190(Accuracy and Calibration of Integrated Radiation Output Indicators in Diagnostic Radiology) hasalso been realigned to investigate the “Accuracy and Calibration of Integrated Radiation OutputIndicators” which is reflected in the title of the task group, to include situations where the KAPmay be acquired with or without the presence of a physical “meter.” To accomplish this goal,validation test protocols were developed to compare the displayed radiation output value to anexternal measurement. These test protocols were applied to a number of clinical systems to collectinformation on the accuracy of dose display values in the field. C 2015 American Association ofPhysicists in Medicine. [http://dx.doi.org/10.1118/1.4934831]

Key words: fluoroscopy, dose-area-product, kerma-area-product, calibration of KAP meters, patientexposure

TABLE OF CONTENTS

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 BACKGROUND INFORMATION . . . . . . . . . . . . . . . 2

2.A KAP, the units, and the geometry . . . . . . . . . . 2

2.B Regulatory requirement . . . . . . . . . . . . . . . . . . . 3

2.C Digital imaging and communications inmedicine (DICOM) radiation dosestructured report (RDSR) . . . . . . . . . . . . . . . . . 3

2.D Integrated radiation output indicators . . . . . . . 4

3 INTEGRATED RADIATION OUTPUT SYSTEMVALIDATION METHOD . . . . . . . . . . . . . . . . . . . . . . . 4

6815 Med. Phys. 42 (12), December 2015 0094-2405/2015/42(12)/6815/15/$30.00 © 2015 Am. Assoc. Phys. Med. 6815

6816 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6816

3.A Protocol overview . . . . . . . . . . . . . . . . . . . . . . . . 43.A.1 An external radiation dosimeter . . . . . 43.A.2 Attenuator . . . . . . . . . . . . . . . . . . . . . . . . 53.A.3 Field-size measurement plate . . . . . . . 53.A.4 Radiation detector/FSMP stand . . . . . 5

3.B Interventional fluoroscopy system protocol(vertical geometry) . . . . . . . . . . . . . . . . . . . . . . . 53.B.1 Determination of focal spot location. 63.B.2 Determination of isocenter location . 63.B.3 Measurement setup for Ka,r and

radiation field size . . . . . . . . . . . . . . . . . 63.B.4 Measurement process . . . . . . . . . . . . . . 73.B.5 Calculation process . . . . . . . . . . . . . . . . 8

3.C Interventional fluoroscopy system protocol(horizontal geometry) . . . . . . . . . . . . . . . . . . . . 83.C.1 Determination of focal spot location. 83.C.2 Measurement setup . . . . . . . . . . . . . . . . 83.C.3 Measurement process . . . . . . . . . . . . . . 83.C.4 Calculation process . . . . . . . . . . . . . . . . 8

3.D Undertable fluoroscopy system protocol . . . . 93.D.1 Measurement setup . . . . . . . . . . . . . . . . 93.D.2 Measurement process . . . . . . . . . . . . . . 93.D.3 Calculation process . . . . . . . . . . . . . . . . 9

3.E Overtable fluoroscopy system protocol . . . . . 93.E.1 Measurement setup . . . . . . . . . . . . . . . . 93.E.2 Measurement process . . . . . . . . . . . . . . 93.E.3 Calculation process . . . . . . . . . . . . . . . . 9

3.F Multipurpose fluoroscopy system protocol . . 93.G Mobile C-arm system protocol . . . . . . . . . . . . 9

3.G.1 Measurement setup . . . . . . . . . . . . . . . . 103.G.2 Measurement process . . . . . . . . . . . . . . 103.G.3 Calculation process . . . . . . . . . . . . . . . . 10

3.H Mini-C-arm protocol . . . . . . . . . . . . . . . . . . . . . 103.H.1 Measurement setup . . . . . . . . . . . . . . . . 103.H.2 Measurement process . . . . . . . . . . . . . . 103.H.3 Calculation process . . . . . . . . . . . . . . . . 10

3.I Radiographic systems protocol . . . . . . . . . . . . 113.I.1 Measurement setup . . . . . . . . . . . . . . . . 113.I.2 Measurement process . . . . . . . . . . . . . . 113.I.3 Calculation process . . . . . . . . . . . . . . . . 11

4 DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.A Sources of uncertainty . . . . . . . . . . . . . . . . . . . . 12

4.A.1 External dosimeter readinguncertainty . . . . . . . . . . . . . . . . . . . . . . . 12

4.A.2 External detector locationuncertainty . . . . . . . . . . . . . . . . . . . . . . . 12

4.A.3 Displayed dose value accuracy. . . . . . 124.A.4 X-ray field-size measurement

uncertainty . . . . . . . . . . . . . . . . . . . . . . . 124.B KAP meter performance variation with

beam quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . 12A APPENDIX A: THE FIELD-SIZE

MEASUREMENT PLATE AND STAND. . . . . . . . . 13B APPENDIX B: THE HORIZONTAL

GEOMETRICAL ARRANGEMENT . . . . . . . . . . . . 14NOMENCLATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. INTRODUCTION

Over the past two decades, several publications have addressedconcerns on the dramatic increase of radiation dose patientsreceive from fluoroscopic examinations has increased dramat-ically.1–3 This has been due in part to the proliferation of med-ical disciplines that are new to the application of fluoroscopy,in employing this imaging tool in their patient care.1,2 On theother hand, the development of complex surgical proceduresresulted in prolonged extensive use of fluoroscopy in inter-ventional and cardiovascular angiography procedures whichcontributed substantial increase in radiation exposures to pa-tients.3–5 To better understand and control this increase indose, manufacturers began providing real-time displays ofthe radiation dose that a patient was receiving. These real-time displays work by either performing calculations using thepertinent generator parameters and geometrical information orby measuring the x-ray beam directly with what is known as adose-area-product (DAP) meter.

The AAPM Task Group 190 (TG-190) was established in2008 initially to assess the accuracy of the DAP meter. Sincethen, the term “DAP-meter” has been revised to air kerma-areaproduct (KAP) meter. Furthermore, the charge of TG 190 hasbeen realigned to investigate the “Accuracy and Calibration ofIntegrated Radiation Output Indicators” which is reflected inthe title of the task group (Accuracy and Calibration of Inte-grated Radiation Output Indicators in Diagnostic Radiology),to include situations where the KAP may be acquired with orwithout a physical meter. To accomplish that goal, validationtest protocols for different equipment types were developed tocompare the displayed radiation output value to an externalmeasurement. These test protocols were applied to a numberof clinical systems to collect information on the accuracy ofradiation output in the field.

2. BACKGROUND INFORMATION2.A. KAP, the units, and the geometry

The KAP is the air kerma integrated over the area of theexposure field in the plane perpendicular to the beam axis.The value of KAP is independent of the distance from the x-ray source since the air kerma decreases proportionally to thesquare of the distance from the source while the x-ray beamarea increases proportionally to the square of that distance.

When displayed on radiographic and fluoroscopic systems,the units of KAP used will vary with manufacturer, equipmenttype, and software revision, including (Gy-cm2), (cGy-cm2),(mGy-m2), and (µGy-m2). The mGy displayed by fluoroscopicsystems is the accumulated air kerma at a demarcated refer-ence point (RP) Ka,r . This quantity is defined under low-scatter conditions with all removable attenuators (e.g., tabletopand pad) removed from the beam-path between the x-raysource and the measurement point.

The RP is defined to approximate the “location” of thepatient’s entrance skin surface. This location will differ forvarious fluoroscopic equipment configurations. The RP mayalso be specified by the manufacturer at an alternative

Medical Physics, Vol. 42, No. 12, December 2015

6817 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6817

location which represents the location of the patient’s entranceskin surface. Listed in Table I is a summary of RP locationsdefined by the U.S. Food and Drug Administration (FDA,21CFR1020.32).6

It should be noted that actual patient skin dose will likelybe different from the displayed Ka,r value. Ka,r representsan approximate sum of air kerma delivered to all areas ofthe patient’s skin exposed to fluoroscopic radiation during theprocedure. In most procedures, the x-ray beam is moved todifferent entrance locations, exposing different skin areas, sothat the air kerma to any one anatomical area will be lowerthan the accumulated total. As pointed out by Jones and Pas-ciak, skin dose estimation must also account for backscatter,tabletop and pad attenuation, soft tissue f -factor, and the actualsource to skin distance.7

2.B. Regulatory requirement

Both International Electrotechnical Commission (IEC2000)8 and (IEC 2010)9 and the FDA,6 for equipment man-ufactured after June 2006, require fluoroscopy equipmentto display the cumulative Ka,r and the Ka,r rate during theprocedure at the operator’s working position. IEC also re-quires that an indication of the KAP value be provided. As a

T I. Nominal reference point (RP) location specifications for differentfluoroscope types.a

Fluoroscope type Reference point (RP) location

Vertical orientationfixed C-arm system

15 cm from the isocenter toward thex-ray source along the centerline of x-raybeam

Horizontal orientationfixed C-arm, L-arm, orlateral system

15 cm from the centerline of the x-raytable and in the direction of the x-raysource with the end of the beam-limitingdevice or spacer positioned as closely aspossible to the point of measurement

Undertable x-ray tube 1 cm above the tabletop or cradleOvertable x-ray tube 30 cm above the tabletop with the end of

the beam-limiting device or spacerpositioned as closely as possible to thepoint of measurement

Mobile C-arm 30 cm from the entrance surface of theimage receptor toward the x-ray sourcealong the centerline of x-ray beam

Mini-C-arm Typically located 3–6 cm from the imagereceptor toward the x-ray source alongthe centerline of x-ray beam

Radiographic unit 30 cm above the tabletop toward thex-ray source along the centerline of x-raybeam, with the end of the beam-limitingdevice or spacer positioned as closely aspossible to the point of measurement

Note: The cumulative air kerma represents the value for conditions of free-in-airirradiation at one of the RP locations specified according to the type of fluoro-scope. Alternative locations of the RP may be specified by the manufacturer. Theuser is advised to refer to Instructions for Use or Operator’s Manual to verify theRP location for the system.a21CFR1020.32, Code of Federal Regulations, Title 21, Volume 8, PerformanceStandards for Ionizing Radiation Emitting Products: Fluoroscopic Equipment,April 1, 2013.

result, both Ka,r and KAP are displayed on most fluoroscopicsystems. These requirements and/or regulations also specifythe accuracy requirements for Ka,r and KAP values. Thedisplayed Ka,r value shall not deviate from the actual valueby more than ±35% above 100 mGy (Refs. 6 and 9) and KAPshall not deviate from the actual value by more than ±35%above 2.5 Gy-cm2.7,9

2.C. Digital imaging and communications in medicine(DICOM) radiation dose structured report (RDSR)

More recently, some fluoroscopic systems have begun toprovide a report file that contains a summary of procedure doseinformation for later analysis and recording. The RDSR, asspecified by DICOM 2011 standards,10 contains detailed doseand geometry data for each irradiation event (individual fluo-roscopy or image acquisition sequence) and an accumulateddose summary for the entire procedure. In addition, the RDSRincludes fields for the recording of information related to thecalibration of dose values and calibration factors to account forthe deviation of the displayed dose from the external measure-ment. Table II is a summary of the dose calibration data fieldsspecified.

It should be noted that the calibration factor is not uti-lized by the equipment manufacturer to modify the displayeddose values. Instead, the calibration information is to beutilized by the customer, typically a medical physicist. Thecalibration factor, accounting for tabletop attenuation, back-scatter, and geometry considerations, for example, may beapplied to the dose values that are displayed or recorded in theRDSR to allow for greater accuracy in individual patient doseestimation.

Additional specification of RDSR content has been pro-vided in a publicly available specification from IEC (IEC2007).11 This prestandard defines four levels of conformancewith specified RDSR elements required for each level. For alllevels above level 0 limited conformance (level 1 limited dosemonitoring, level 2 general dose monitoring, level 3 advanceddose monitoring), the dose calibration elements included inTable II must be provided. Furthermore, level 2 conformanceincludes the source to reference point distance and the colli-mated field area per irradiation event.

T II. RDSR dose calibration definitions.

Attribute Definition

Calibration Procedure used to calibratemeasurements or measurement devices

Calibration date Last calibration date for the integrateddose meter or dose calculation

Calibrationfactor

Factor by which a measured or calculatedvalue is multiplied to obtain the estimatedreal-world value

Calibrationuncertainty

Uncertainty of the “actual” value. Valueranges from 0% to 100%

Calibrationresponsible party

Individual or organization responsible forcalibration

Medical Physics, Vol. 42, No. 12, December 2015

6818 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6818

Section 2.D of this report includes detailed methods formeasurement of the calibration factor for various equipmenttypes. To provide a method to enter this information into theRDSR for individual interventional fluoroscopy systems, auser quality control mode has been specified in a NationalElectrical Manufacturers Association standard (NEMA XR27-2012).12 Note that currently the RDSR provides for entryof a single calibration factor. A single calibration factor maynot be sufficient to account for the complexity encounteredin all types of fluoroscopy equipment and clinical geometry.However, the attenuation due to the tabletop for a given systemmay be applied. Therefore, it is possible that the dose displaycorrection factor may be different for Ka,r and KAP and/ordiffer for variations in beam quality and dose rate. Additionalexamination of this issue is included in Sec. 4.

For convenience, a summary of acronyms and abbrevia-tions employed in this report is provided in Nomenclature.

2.D. Integrated radiation output indicators

Several different methods are currently used by manufac-turers of radiographic and fluoroscopic equipment to providea measurement or estimate of Ka,r and KAP. One commonmethod is direct measurement of KAP with a KAP-meter.The KAP-meter is a thin, parallel-plate transmission ionizationchamber that is fixed in the x-ray tube-collimator assembly,typically at the end of the collimator and at all times interceptsthe entire x-ray field.

In order to estimate Ka,r , the KAP is divided by the irradi-ated field area at the RP location. The exposure area is deter-mined by system indicators of the collimator blade position.Using this method, KAP accuracy will depend on the accuracyof the KAP-meter itself and Ka,r accuracy will depend on boththe KAP-meter accuracy and the field-size measurement.

Instead of direct KAP measurement, some models deter-mine Ka,r computationally based on x-ray tube output forgiven technique factors and added filtration. KAP can then beestimated by multiplying Ka,r by the x-ray beam area. For thismethod, Ka,r accuracy will depend on the accuracy of x-rayoutput values that are used and KAP accuracy will depend onboth the Ka,r computation and the field-size measurement.

3. INTEGRATED RADIATION OUTPUT SYSTEMVALIDATION METHOD3.A. Protocol overview

The following sections describe integrated radiation outputindicator validation protocols for various types of fluoroscopicand radiographic equipment configurations.

3.B. Interventional fluoroscopy, vertical x-ray beam.3.C. Interventional fluoroscopy, horizontal x-ray beam.3.D. Undertable x-ray fluoroscopy.3.E. Overtable x-ray fluoroscopy.3.F. Multipurpose fluoroscopy (C-arm with integrated ta-

ble).3.G. Mobile C-arms.

3.H. Minimobile C-arms.3.I. Radiographic systems.

For each equipment configuration, irradiation data arecollected from both an external dosimeter and the system’sintegrated radiation output display. The measurements fromthe external dosimeter are compared to the system’s internaldisplay. This process yields a correction factor C where C(x)=measured external value/system’s displayed value, where xis either Ka,r or KAP.

A system with C < 1.0 will display a value that is greaterthan the external value, and a system with C > 1.0 will displaya value that is smaller than the external value.

The following materials are required to perform validationmeasurements.

3.A.1. An external radiation dosimeter

The dosimeter should be able to make accurate measure-ments over the appropriate air kerma and beam quality range.It should have a traceable current calibration based on relevantbeam qualities. The measured external air kerma value shouldinclude necessary adjustments for the calibration factor of thedosimeter and appropriate temperature-pressure corrections.

A note on the requirements of this section: Typically, thedosimeter is calibrated with standard radiation quality RQR 5[70 kilovolt peak (kVp), homogeneity coefficient 0.71, nom-inal first HVL 2.58 mm Al] and/or RQR 8 (100 kVp, homo-geneity coefficient 0.68, nominal first HVL 3.97 mm Al) asspecified in Table 4 of IEC 61267 ed. 2.0. The energy responseover the 50–150 kV should be better than ±2.5% of the RQRcalibration points.

It is possible to use an external KAP-meter to performvalidation measurements. This device will allow for directvalidation of displayed KAP values and calculated Ka,r valueswith measurement of the field area. Also available are dualchamber KAP-meters which incorporate a small detector toallow for simultaneous KAP and Ka,r measurement. For eithertype of meter, a current calibration is needed for accuratevalidation measurements.

It is important to note that when performing validationmeasurements, external KAP-meters and radiation dosimeterswithout incorporated lead backing should be placed so thatthe sensitive area of the dosimeter is not in a region wherethe measurement may be impacted by backscatter from theimage receptor. A spacing of at least 10 cm away from theimage receptor is recommended, for an irradiated field size of50–100 cm2.

The dosimeter will be operated in “integrate” mode toallow for accumulation of sufficient air kerma (a minimumof 50 mGy is suggested) for improved radiation output accu-racy. In integrate mode, multiple acquisitions or fluoroscopyirradiation events can be combined to achieve the desired airkerma total. Use of an “automatic reset” mode is allowable;in this mode, the integrated dose is reset at the start of eachexposure. Use of a dose rate mode for fluoroscopy measure-ments is not recommended due to variations in the radiationoutput delivered over time. This is particularly important in

Medical Physics, Vol. 42, No. 12, December 2015

6819 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6819

interventional systems in acquisition (cine) modes, which canhave substantial variations by design.

3.A.2. Attenuator

An attenuator consisting of copper sheets of approximately1 mm totaling at least 8 mm is required. Copper is selectedas the attenuator to reduce weight and excessive x-ray scatterproduction. The copper sheets should be large enough (at least30×30 cm) to cover the collimated exposure field and placedas close as possible to the image receptor. The copper sheets donot need to be high purity. Commonly available copper sheets,type C101 with 99.99% purity, are acceptable. The purpose ofthe copper is only to drive the fluoroscope within the targetkVp range. Individual copper sheets approximately 1 mm(or, alternatively 1/32 in.) thick are recommended to allowfor adjustment of the total attenuator thickness as needed.This thickness of attenuator will typically drive a fluoroscopicsystem under automatic dose rate control to moderately highkVp and Ka,r rates. Use of high kVp minimizes deviations inKAP-meter accuracy that are common at low kVp and use ofhigh Ka,r rates allows for more rapid accumulated air kermameasurements.13

3.A.3. Field-size measurement plate

The plate should contain radio-opaque ruled demarcationsin orthogonal directions to allow for measurement of the expo-sure field area. A sample field-size measurement plate (FSMP)is depicted in Fig. 1.

On the FSMP, the cutout is 7×7 cm in size which servestwo purposes. It is a blank space to accommodate a 30 cm3 flatpancake-type ionization chamber typically 5 cm in diameteror smaller. The physical size of the square (7× 7 cm) pro-

F. 1. Field-size measurement plate (FSMP).

F. 2. Enlarged view of FSMP.

vides a 49 cm2 area for KAP measurement. (To be exact, a7.071 cm2 would provide a 50 cm2 area.) The 10, 15, and 20 cmsquare boxes and the radio-opaque ruled demarcations workas landmarks aiding setup and measurements of the radiationfield size. Small lead numbers may be embedded to assist inidentifying the field size.

One enlarged section of the radio-opaque ruler on the FSMPis shown in Fig. 2. Note that the linewidth of radio-opaque rulerand the square boxes is 1 mm.

3.A.4. Radiation detector/FSMP stand

If using the interventional fluoroscopy system protocol(vertical geometry) method described below, a stand to holdthe radiation dosimeter and field-size measurement plate isuseful. The stand may be as simple as using the FSMP itself, asdescribed later in Sec. 3.C.3, securely affixed and extended outfrom the tabletop for the field-size measurement. The FSMPmay also be employed as a supporting device to hold theradiation detector. Alternatively, a more elaborate stand canbe fabricated to hold the FSMP and the detector in a moreconvenient configuration. One such example of FSMP standis shown in Appendix A.

3.B. Interventional fluoroscopy system protocol(vertical geometry)

This section of the report is specific to isocentric fluoro-scopes with a fixed focal spot to isocenter distance [sourceto axis distance (SAD)]. The default RP location is along thecentral ray of the x-ray beam at a distance of 15 cm from theisocenter toward the x-ray tube. Note that any particular systemmight have a different RP location. This and other importantgeometric dimensions are available in the Instructions for Useor Operator’s Manual that the manufacturer is required tosupply with each system.

This method makes use of the FSMP at the plane of radi-ation measurements for the positioning of the dosimeter aswell as the placement and thickness adjustment of the coppersheets. In addition, the physical sizes of the copper sheets needto be large enough to encompass the entire area of the x-raybeam at the location of the tabletop.

Medical Physics, Vol. 42, No. 12, December 2015

6820 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6820

The measurement procedure includes a method to deter-mine the exact location of the focal spot within the x-ray tubehousing and a method to verify the location of the isocenterof the C-arm gantry. Once this information is obtained for asystem, it need not be repeated for subsequent measurements.

3.B.1. Determination of focal spot location

In some cases, the focal spot location may not be markedon the x-ray tube housing or a precise determination of thefocal spot location is desired. To find the location, two testobjects will be employed; a 1× 1 cm and a 2× 2 cm coppersheets (1 mm thick). The 1×1 cm copper sheet can be placedon the tabletop or a stand where the FSMP is located whilethe 2×2 cm copper sheet is attached to the front face of theimage receptor. The exact location of the focal spot can thenbe determined as follows.

(1) All clinically removable attenuators except the x-raytable (i.e., removable without tools) shall be removedfrom the path between the x-ray tube assembly and themeasuring detector before acquiring data.

(2) Place the 1×1 cm copper sheet on the FSMP (which issecurely affixed on and extended from the tabletop) andattach the 2×2 cm copper sheet on the front cover of theimage receptor housing assembly as shown in Fig. 3.Both copper sheets should be aligned and placed in thecenter of the imaging field as shown on the top right ofFig. 3.

(3) During fluoroscopy with the x-ray beam vertical, eitherchange the elevation of the tabletop or the elevationof the image receptor until the size of the two coppersheet test objects in the fluoroscopic image is identical.This will place the small 1×1 cm copper sheet exactlymidway between the focal spot and the 2×2 cm coppersheet. The images of copper sheets appear as shown

F. 3. Geometry of utilizing two copper sheets for localization of focal spot,under vertical geometry.

on bottom right of Fig. 3. When both sheets appearthe same in size, by triangulation, “A” is equal to “B”(A= B).

(4) Without moving the vertical position of the examina-tion table or the detector, carefully measure the distancebetween the two copper sheets. Make sure that the tapemeasure is vertical during this measurement; it may benecessary to move the table horizontally to ensure this.Then, measure the distance from the 1×1 cm coppersheet test object to the x-ray tube. This will providethe location of the focal spot. This location can bemarked permanently on the x-ray tube housing surfacefor future reference.

(5) Using the determined focal spot location, measure thedistance from the focal spot to the exit point of the x-raytube housing assembly (SHD).

3.B.2. Determination of isocenter location

(1) All clinically removable attenuators (i.e., removablewithout tools) shall be removed from the path betweenthe x-ray tube assembly and the measuring detectorbefore acquiring data. This can be accomplished eitherby retracting the examination table or using a hori-zontal x-ray beam.

(2) Set the system to the medium field-of-view (FOV) withthe collimator opened to its fullest extent. The mediumFOV, typically, is a 23 cm image intensifier input fieldsize or a 25×25 cm flat panel image receptor.

(3) Set the gantry to the maximum source to image receptordistance (SID) that allows free rotation of the gantry.

(4) Tape a small lead marker to the surface of the tabletop.Or, use the FSMP in place of the tabletop as continua-tion of Sec. 3.B.1.

(5) Set the x-ray beam to a vertical orientation: The smallradio-opaque marker is placed approximately at isocen-ter by moving the tabletop horizontally.

(6) Set the x-ray beam to a horizontal orientation: Centerthe radio-opaque marker by adjusting tabletop height.

(7) Repeat steps (5) and (6) until the marker does not moveacross the field as the gantry is rotated. The marker isnow at the isocenter.

(8) Orient the x-ray beam vertically with the tube over thetable.

(9) Without moving the vertical elevation of the x-ray ta-ble, carefully measure the vertical distance from thefocal spot marking on the x-ray tube housing to the leadmarker taped to the tabletop. Note that moving the tablehorizontally may be included during this measurementto ensure that the tape measure is vertically oriented.Record this distance as the focal spot to isocenter dis-tance, SAD.

3.B.3. Measurement setup for Ka,r and radiationfield size

(1) All clinically removable attenuators (i.e., removablewithout tools) shall be removed from the path between

Medical Physics, Vol. 42, No. 12, December 2015

6821 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6821

the x-ray tube assembly and the measuring detectorbefore acquiring data.

(2) The examination tabletop should be moved to itsmaximum height. To the extent practicable, thereshould be no objects, including the x-ray table, in thex-ray beam that can scatter x-rays from within 10 cmof the sensitive volume of the measuring detector.

(3) Set the x-ray beam to a vertical orientation.(4) Set the gantry to the maximum SID.(5) Set the system to a medium FOV with open collima-

tion.(6) Affix the FSMP on the tabletop as shown in Fig. 4. The

field-size measurement pattern is extended out fromthe tabletop and into air. Place the external dosimeterin the cutout of the FSMP and centered within FSMP.

(7) Using a tape measure, carefully measure and recordthe distance from the focal spot to the center of theexternal dosimeter (source to detector distance, SDD).

(8) Set the system to a FOV of approximately 17 cm(diagonal) at the image receptor and maximum SID.Note: A legacy of circular image intensifier specifi-cation. A 17 cm diagonal flat panel image receptor isequivalent to 6–7 in. round FOV of an image intensi-fier.

(9) Set the collimators using the 10×10 cm2 marker on theFSMP. All edges of the irradiated field should be seenon the monitor almost superimposed on the squarebox and well within the full FOV border. (If the pen-umbra makes the location of the edge uncertain, placethe corresponding edge of the 10×10 cm2 marker onthe FSMP to the middle of the penumbra.)

(10) Place an attenuator (approximately a total of 8 mmCu) on the tabletop so it completely intercepts the x-ray beam.

(11) Select the medium dose rate modes for fluoroscopy. Ifthe system only has two dose rate mode choices, thenselect the higher.

F. 4. Measurement setup for Ka,r and radiation field size.

(12) Do a test irradiation. Adjust the attenuator thicknessif this is necessary to bring the displayed tube voltageinto the 90–100 kVp range and record the attenuatorthickness required.

Note on the selection of tube voltage in the range of90–100 kVp.

(i) It is preferred to have one measurement point forverification of accuracy and calibration of integratedoutput indicators.

(ii) The tube potential for various fluoroscopic examina-tions, typically, falls in the 90–100 kVp range.

(iii) The TG 190 members compared a half dozen radiationdetectors typically employed in the field and foundbetter agreement amongst different types of detectorsat 90–100 kVp (<5%) than at 70–90 kVp (∼15%).

3.B.4. Measurement process

The following procedure should be done both with the fluo-roscopic and the acquisition mode, using the amount of filtra-tion appropriate for each to bring the displayed tube voltageinto the 90–100 kVp range.

(1) Collect data using the external dosimeter in the inte-grate mode.

(2) Precision is increased if each measurement is appro-priately replicated (three is suggested). The coeffi-cient of variation in the value of C calculated from therepeated measurements (see Sec. 3.B.5) should be lessthan 0.01.

(3) Record the initial displayed system values of Ka,r andKAP and the external dosimeter reading before eachirradiation. If using the external dosimeter’s “auto-dose” mode, you need not record the external reading.

(4) Each irradiation should be long enough so that theresolution of the digital displays does not significantlylimit the accuracy of the measurements. For example,for a system that displays Ka,r in units of mGy in awhole number (rounded to the nearest integer value),the external dosimeter should show approximately50 mGy after each irradiation.

(5) Record the final displayed system values of Ka,r andKAP and the external dosimeter reading after eachirradiation. Record the associated generator factors(kVp, mA, ms, beam filter) if available.

(6) Calculate C(Ka,r) using the process described inEq. (3) in Sec. 3.B.5.

(7) Remove the external dosimeter from the beam withoutdisturbing the collimator setting.

(8) Place the FSMP at isocenter by rotating the gantry be-tween the vertical and the horizontal imaging projec-tions to confirm the correct placement of the FSMP atthe isocenter.

(9) Perform a fluorographic irradiation (nonsubtracted) toimage the FSMP, or a fluoroscopic irradiation withlast image hold.

Medical Physics, Vol. 42, No. 12, December 2015

6822 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6822

(10) Determine the field size at isocenter by observing theimage of the FSMP on the system monitor. If the sys-tem is equipped with “black shutters” or “automaticmasking” these should be turned off before makingthis measurement. Record the two dimensions of therectangular field as AL and AW.

3.B.5. Calculation process

(1) Calculate the measured KAP,

KAP= [measured Kamas]∗ AW ∗ AL, (1)

where Kamas is the accumulated air kerma measuredwith the external dosimeter at isocenter. If using inte-grate rather than autodose mode, this is the valuerecorded after an irradiation event minus the valuerecorded before the event.

(2) Calculate the air kerma Ka,r at the RP by multiplyingthe measured air kerma at isocenter Ka,SAD by a geo-metric factor G1,

G1= [SAD/(SAD−RPD)]2, (2)

where RPD= isocenter to reference point distance

Ka,r = [measured Ka,SAD]∗G1. (3)

For example, for SAD = 750 mm, RPD = 150 mm,G1= [750/(750−150)]2= 1.56.

(3) Calculate the correction factor C separately for eachirradiation event for both Ka,r and KAP. Caution: Thefollowing processes may need further corrections toaccount for the dose units in which a particular fluo-roscope displays KAP.(a) C(KAP) is determined by dividing the measured

KAP value by the KAP value displayed by thesystem.

(b) C(Ka,r) is determined by dividing the measuredKa,r value by the Ka,r value displayed by thesystem.

(4) Average the individual C factors over the measurementrepetitions for Ka,r , and separately for KAP.

3.C. Interventional fluoroscopy system protocol(horizontal geometry)

While this section is similar to Sec. 3.B, an importantdifference is that the dosimetry measurements are completedwith a horizontal as opposed to a vertical x-ray beam geometry.

3.C.1. Determination of focal spot location

Follow the procedure in Sec. 3.B.1.

3.C.2. Measurement setup

(1) All clinically removable attenuators (i.e., removablewithout tools) shall be removed from the path between

the x-ray tube assembly and the measuring detectorbefore acquiring data.

(2) As much as practicable and possible, there should beno objects that can scatter x-rays within 10 cm ofthe sensitive volume of the measuring detector. Spe-cific attention should be directed toward the locationof the tabletop and the supports for the measuringdetector.

(3) Set the system to a medium FOV with open collima-tion. (Typically 6–9 in. or 17–22 cm FOV.)

(4) Set the gantry to the maximum SID that allows freerotation of the gantry.

(5) Set the x-ray beam to a vertical orientation. Theexternal dosimeter is placed approximately at isocen-ter by moving the table horizontally. Note that theradiation detector may be suspended in air or placedon the FSMP. This is described in Sec. 3.B.3, Fig. 4,and the corresponding photographs in Fig. 7 of theAppendix.

(6) Set the x-ray beam to a horizontal orientation. Cen-ter the detector by adjusting table height. See Ap-pendix B for a photograph and description of thesetup.

(7) When the detector appears to rotate in the image butremains fixed at the center of the field of view as thegantry rotates, it is at isocenter. Repeat steps (5) and(6) until the detector does not move across the field asthe gantry is rotated.

(8) Use a tape measure to determine the distance from thefocal spot to the chamber and record this distance asthe SAD.

(9) Rotate the system for a horizontal x-ray beam.(10) Set the system to a FOV of approximately 17 cm (or

22 cm) at the image receptor and maximum SID.(11) Set the collimators using the 10×10 cm2 marker on the

FSMP. All edges of the irradiated field should be seenon the monitor almost superimposed on the squarebox and well within the full FOV border.

(12) Attach an attenuator (∼8 mm Cu) to the face of theimage receptor. The attenuator thickness may need tobe adjusted later to yield 90–100 kVp while takingmeasurements.

(13) Select the medium dose rate modes for fluoroscopy. Ifthe system only has two dose rate mode choices, thenselect the higher.

(14) Do a test irradiation. Adjust the attenuator thicknessif this is necessary to bring the displayed tube voltageinto the 90–100 kVp range and record the attenuatorthickness required.

3.C.3. Measurement process

Follow the procedure described in Sec. 3.B.4.

3.C.4. Calculation process

Follow the procedure described in Sec. 3.B.5.

Medical Physics, Vol. 42, No. 12, December 2015

6823 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6823

3.D. Undertable fluoroscopy system protocol

This section of the report addresses fluoroscopic systemswith x-ray tubes mounted under the procedure table. Theseunits typically have variable SIDs between 75 and 120 cm. Thedefault RP is located 1 cm above the procedure table. (Note thatany particular make or model may have a different RP locationand this should be confirmed in the vendor documentation.)

3.D.1. Measurement setup

(1) Remove any table pads from the path between the x-raytube and the measuring detector before acquiring data.To the extent practicable, there should be no objects thatcan scatter x-rays within 10 cm of the sensitive volumeof the external dosimeter.

(2) Set the system to a FOV of approximately 22 cm andraise the image receptor tower to the maximum height.

(3) Place the FSMP and the external dosimeter 1 cm abovethe tabletop or at the manufacturer specified RP ifdifferent from “1 cm above the tabletop.” If necessary,raise the FSMP with spacers. Set the collimators usingthe 10×10 cm2 marker on the FSMP. All edges of theirradiated field should be seen on the monitor almostsuperimposed on the square box and well within thefull FOV.

(4) Place an attenuator (approximately 5 mm Cu) above theradiation detector in the primary beam. The attenuatorsheets may be attached to the image receptor tower orplaced on a stand to hold the sheets at least 10 cm abovethe radiation detector.

(5) Select the medium dose rate mode for fluoroscopy. Ifthe system only has two dose modes, select the higher.

(6) Do a test irradiation. Adjust the attenuator thicknessif this is necessary to bring the tube voltage into the90–100 kVp range.

3.D.2. Measurement process

Follow the procedure described in Sec. 3.B.4.

3.D.3. Calculation process

Follow the procedure described in Sec. 3.B.5. Note thatsince air kerma is measured at the RP, the geometric factorG1 = 1. If the external dosimeter could not be placed at theRP, use an appropriate value of G1.

3.E. Overtable fluoroscopy system protocol

This section of the report addresses fluoroscopic systemswith x-ray tubes mounted above the procedure table. Theseunits typically have SIDs between 115 and 150 cm whichcan be fixed or variable. The typical RP is located along thecentral ray of the x-ray beam 30 cm above the procedure table.(Note that any particular make or model may have a differentRP location and this should be confirmed in the vendordocumentation.)

3.E.1. Measurement setup

(1) Set the system to a FOV of approximately 22 cm fieldof view at the image receptor. If adjustable, set the SIDto the maximum possible.

(2) Place attenuator (approximately 5 mm Cu) on theprocedure table in the primary beam.

(3) Select the medium dose rate mode for fluoroscopy. Ifthe system only has two dose modes, select the higher.

(4) Place the FSMP and the external dosimeter at the RPand center in the field. Typically, the RP is 30 cm abovethe tabletop. Raise the FSMP with spacers as needed.Set the collimators using the 10×10 cm2 marker on theFSMP. All edges of the irradiated field should be seenon the monitor almost superimposed on the square boxand well within the full FOV border.

(5) Do a test irradiation. Adjust the attenuator thicknessif this is necessary to bring the tube voltage into the90–100 kVp range.

3.E.2. Measurement process

Follow the procedure described in Sec. 3.B.4.

3.E.3. Calculation process

Follow the procedure described in Sec. 3.B.5. Note thatsince air kerma is measured at the RP, the geometric factorG1= 1.

3.F. Multipurpose fluoroscopy system protocol

Multipurpose fluoroscopy systems (sometimes referred toas “universal” or “tilt-C” systems) generally consist of a floor-mounted C-arm stand with an integrated patient table. Thefluoroscopic C-arm is capable of angulation about the table andthe table and C-arm can be tilted together from the stand base.

Either the interventional fluoroscopy system protocol (ver-tical geometry) described in Sec. 3.B or the interventionalfluoroscopy system protocol (horizontal geometry) describedin Sec. 3.C can be used to determine Ka,r and KAP correctionfactors for this configuration. However, some manufacturershave chosen to design their multipurpose fluoroscopy systemsto comply with IEC radiography standards (IEC, 2009)14 andas a result, assume that the patient table is present in the x-raybeam when calibrating Ka,r and KAP displayed values.

Consultation with the manufacturer is recommended todetermine if a particular model of multipurpose fluoroscopysystem includes the table in the x-ray beam for Ka,r and KAPdisplays. For these systems, the vertical geometry measure-ment setup should be followed with the external dosimeter andFSMP positioned above the tabletop.

3.G. Mobile C-arm system protocol

This section of the report addresses mobile C-arm fluo-roscopic systems. For mobile C-arms, typically the RP islocated 30 cm from the entrance surface of the image receptor

Medical Physics, Vol. 42, No. 12, December 2015

6824 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6824

assembly. Note that any particular model might have a differentRP location and this should be confirmed in the vendordocumentation.

3.G.1. Measurement setup

(1) Remove any material such as tabletop and pad from thepath between the x-ray tube and the measuring detectorbefore acquiring data.

(2) To the extent practicable, there should be no objects thatcan scatter x-rays within 10 cm of the sensitive volumeof the measuring detector.

(3) Set the system to a normal FOV for that system.(4) Set the x-ray beam to either a vertical or horizontal

orientation. For discussion purposes, the description inthe main text will employ a vertical orientation withthe x-ray tube at the top and the image receptor at thebottom.

(5) Place attenuator (approximately 3–5 mm Cu) on theimage receptor.

(6) Select the medium dose rate mode for fluoroscopy. Ifthe system only has two dose modes, select the higher.

(7) Secure and suspend the FSMP and the external dosim-eter, mechanically with appropriate means like a stand,at the RP and center in the field. As mentioned previ-ously, typically the RP is set to 30 cm above the imagereceptor.

(8) Set the collimators using the 10×10 cm2 marker on theFSMP. All edges of the irradiated field should be seenon the monitor almost superimposed on the square boxand well within the full FOV border.

(9) Do a test irradiation. Adjust the attenuator thicknessif this is necessary to bring the tube voltage into the90–100 kVp range.

3.G.2. Measurement process

Follow the procedure described in Sec. 3.B.4, with thefollowing addition.

(1) If the radiation field is hexagonal, octagonal, or circu-lar, determine the diameter of field (D) with the radio-opaque ruler on the FSMP.

3.G.3. Calculation process

Follow the procedure described in Sec. 3.B.5, with thefollowing change.

(1) The measured KAP is

KAP= [measured Ka,r]∗ AW ∗ AL

or

KAP= [measured Ka,r]∗π ∗ (D/2)2. (4)

Note that since air kerma is measured at the RP, the geo-metric factor G1= 1.

3.H. Mini-C-arm protocol

This section of the report is specific to mobile C-arm fluoro-scopes with a fixed SID less than or equal to 45 cm. Typically,the RP for these devices is located 3–6 cm from the entrancesurface of the image receptor assembly. (Note that any partic-ular make or model may have a different RP location and thisshould be confirmed in the vendor documentation.)

3.H.1. Measurement setup

(1) Remove any material such as tabletop and pad from thepath between the x-ray tube and the measuring detectorbefore acquiring data.

(2) To the extent practicable, there should be no objects thatcan scatter x-rays within 10 cm of the sensitive volumeof the measuring detector.

(3) Set the system to a normal FOV for that system. Ifcollimation is adjustable, open the collimation to itsfullest extent.

(4) Set the x-ray beam to either a vertical or horizontalorientation. For discussion purposes, the description inthe main text will employ a vertical orientation withthe x-ray tube at the top and the image receptor at thebottom.

(5) Place attenuator (approximately 2–3 mm Cu) on theface of the image receptor.

(6) Select the normal dose rate mode for fluoroscopy. If thesystem only has two dose modes, select the higher.

(7) Secure and suspend the FSMP and the external dosim-eter, mechanically with appropriate means like a stand,at the RP and center in the field. As mentioned previ-ously, typically the RP is 3–6 cm above the imagereceptor.

(8) Set the collimators using the 7×7 cm2 marker on theFSMP. All edges of the irradiated field should be seenon the monitor almost superimposed on the 49 cm2

square box and well within the full FOV border.(9) Do a test irradiation. Adjust the attenuator thickness

if this is necessary to bring the tube voltage to themaximum value (typically less than 80 kVp).

3.H.2. Measurement process

Follow the procedure described in Sec. 3.B.4, with thefollowing addition.

(1) If the radiation field is hexagonal, octagonal, or circu-lar, determine the diameter of field (D) with the radio-opaque ruler on the FSMP.

3.H.3. Calculation process

Follow the procedure described in Sec. 3.B.5, with thefollowing change.

(1) The measured KAP is

KAP= [measured Ka,r]∗ AW ∗ AL

Medical Physics, Vol. 42, No. 12, December 2015

6825 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6825

or

KAP= [measured Ka,r]∗π ∗ (D/2)2. (5)

Note that since air kerma is measured at the RP, the geo-metric factor G1= 1.

3.I. Radiographic systems protocol

This section of the report is specific to radiographic sys-tems. The RP location of these systems may be based on userdata entry or set to a default location. Review of the vendordocumentation is recommended to determine how the RP isdefined for any individual radiographic system. For measure-ment of the radiographic x-ray field size, any standard radio-graphic collimation test tool measurement plate with radio-opaque fiducial markers can be used.

3.I.1. Measurement setup

(1) As much as practicable and possible, ensure that thereare no objects that could potentially produce scatterwithin 10 cm of the sensitive detector volume.

(2) For a stationary system, position the x-ray tube abovethe tabletop [Fig. 5(A)]. If a radiographic table is notpresent (e.g., in a dedicated chest room), either directthe x-ray tube downward toward a flat horizontal sur-face or position the image receptor on supports for ahorizontal x-ray beam. For a portable or mobile x-raysystem, place the x-ray tube above a flat horizontalsurface such as a tabletop of an examination room[Fig. 5(B)]. Notice the difference in the location of theimage receptor. A 35 × 43 cm (14 × 17 in.) cassettesize image receptor is employed for the test proceduredescribed here.

F. 5. Measurement setup for radiographic systems. (A) is for a radio-graphic x-ray room with a Bucky tray or an integrated DR image receptor.(B) is for a portable radiographic unit using a tabletop for support.

(3) Orient the x-ray tube so the central ray of the beam isnormal (90◦) to the tabletop/flat surface and the imagereceptor. Set the SID to the default setup of the radio-graphic room typically; SID= 100 cm.

(4) Place the FSMP 30 cm over the tabletop or the imagereceptor, using spacer rods to support the FSMP. Centerthe FSMP within the radiation field.

(5) Collimate the x-ray field so that it falls inside the imagereceptor and the FSMP. The collimated beam should beno smaller than 15×15 cm at the plane of the FSMP.It is desirable to use the light field projected on the15×15 cm box drawn on the FSMP. Or, a larger fieldsize should be employed to account for radiographicapplications.

(6) For systems with a Ka,r display, place the externaldosimeter at the RP if different from the setup shownin Fig. 5. Record the SDD. The setup in Fig. 5 allowsfor direct air kerma measurement assuming a typicalpatient size of 30 cm.

3.I.2. Measurement process

(1) Set the x-ray generator to 100 kVp and a minimum of50 mAs.

(2) Precision is increased if each measurement is appro-priately replicated (3× is suggested). The coefficient ofvariation in the repeated measurements should be lessthan 0.01.

(3) Make an exposure and record the measured Ka,r andthe displayed KAP and Ka,r , if applicable.

Note: For screen-film systems, make one additionalexposure at a lower technique to ensure the fiducialmarkers of the field-size measurement device can beread on the developed film. Do not record the Ka,r orKAP for this exposure.

(4) Calculate C(Ka,r) using the process described below.(5) Determine the field size by observing the image of the

FSMP on the recorded image. Record the two dimen-sions of the rectangular field as AL and AW.

3.I.3. Calculation process

(1) Calculate the measured KAP,

KAP= [measured Ka,r]∗ AW ∗ AL. (6)

(2) Calculate the correction factor C separately for eachirradiation event for both Ka,r and KAP. Caution: Thefollowing processes may need further corrections toaccount for the dose units in which a particular fluo-roscope displays KAP.(a) C (KAP) is determined by dividing the measured

KAP value by the KAP value displayed by thesystem.

(b) C(Ka,r) is determined by dividing the measuredKa,r value by the Ka,r value displayed by thesystem.

Medical Physics, Vol. 42, No. 12, December 2015

6826 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6826

(3) Average the individual C factors over the measurementrepetitions for Ka,r , and separately for KAP.

4. DISCUSSION4.A. Sources of uncertainty

There are multiple factors that contribute to inaccuracy inthe correction factor value. These factors include uncertaintyin the external detector reading, the external detector loca-tion relative to the x-ray source, the accuracy of displayeddose values, and accuracy of the x-ray field-size measure-ment. Additional discussion of each source of error is providedbelow.

4.A.1. External dosimeter reading uncertainty

Accurate calibration of the external dosimeter is critical tominimize this source of variation. A mismatch between thecalibration beam spectrum and the clinical beam spectrumcauses an additional source of error for x-ray beams with heavyfiltration (AAPM Report No. 125).15 Depending on the type ofcalibration, the external dosimeter reading should be accurateto within 2% and 6%.

4.A.2. External detector location uncertainty

Inaccuracy in positioning of the external detector at thecorrect distance from the x-ray source (either at isocenter orthe RP) will result in an erroneous air kerma measurement.Utilization of the isocenter localization technique for fixed C-arm systems described in Sec. 3.A will generally reduce thiserror. An error in detector positioning of ±1 cm at a 65 cmsource-chamber distance will result in an air kerma error ofapproximately ±3%.

4.A.3. Displayed dose value accuracy

Error in the displayed dose value can become particularlylarge if Ka,r is displayed in units of mGy in a whole number(rounded to the nearest integer value) and an insufficient totalair kerma is not accumulated during the measurement. Forexample, a 20 mGy displayed Ka,r value may have an error of±0.5 mGy in the initial value and ±0.5 mGy in the final value,resulting in a total error of ±1 mGy or 5%. Accumulating adose of at least 50 mGy will reduce this error to ±2%.

4.A.4. X-ray field-size measurement uncertainty

Uncertainty in the x-ray exposure field size will affect theaccuracy of the measured KAP value only. For a rectangularfield with the exposure field edges clearly visible, an errorof ±1 mm in reading the template rule on each side wouldyield a KAP error of ±3% for a 50 cm2 area. Collimating toa larger FOV decreases this error. When electronic imagingshutters obstruct the exposure edges and cannot be readilyeliminated, increased accuracy can be accomplished by using

film, a CR, or a DR cassette to record the exposure FOV sizeinstead of reading the field size from the monitor. Use of theFSMP will assist in setting up the actual radiation field atthe time of data collection. However, if a software distancemeasurement function is built into the control console, thefield size can be determined with a higher accuracy. If anymagnification/minification exists, the ruler on the FSMP canbe employed to provide the scaling corrections.

4.B. KAP meter performance variationwith beam quality

It is known that KAP meters may have a significant x-raybeam energy dependence which is affected by the materialsand design of the chamber.16 The greatest dependence onbeam filtration occurs at low kVp values (50–80 kVp). Heavierfiltered beams (0.1–0.2 mm Cu) exhibit a higher dependenceas compared to less filtered beams in the range of 10%–15%for 70–90 kVp beams. Since clinical systems may have modesthat include heavier beam filters, it is recommended that aninitial validation of the system includes expanded measure-ments of the calibration for a range of operating modes. If asingle calibration factor is desired (as for entry in the RSDR),the average of these values over the range of clinically usedoperating modes is recommended. Alternatively, if additionalaccuracy in the correction factor is desired, separate correctionfactors for different modes may be determined.

Whether the radiological imaging equipment in questionis a radiographic unit or a fluoroscopic unit, both types ofKAP meters, physical and virtual, employed for dose measure-ments must be properly calibrated to the radiation beam qualityencountered in clinical practices. This is also applicable to theexternal dosimeter to evaluate the accuracy of the KAP meters.

5. SUMMARY AND CONCLUSIONS

In this report, the accuracy of a KAP-meter or more pre-cisely, the integrated radiation output indicators employed indiagnostic radiology has been investigated and the calibrationmethodology described in detail for various types of fluoros-copy system. The TG 190 Report has also been reviewed byvarious x-ray equipment manufacturers so that the calibrationprotocol is also acceptable to the industry. For the patientradiation dose estimation, the measurement protocol yields theradiation dose that can be employed for further modification toinclude corrections due to the geometry, the backscatter, andattenuation of the tabletop and the mattress.

The RP for the interventional angiography fluoroscopy sys-tems has been specified by IEC. The location of RP for otherfluoroscopy systems was left in the hands of equipment manu-facturers. TG 190 Report deals with the measurement andverification of the KAP-meter accuracy at or associated withthe RP independent of manufacturers while permitting medicalphysicists to follow a unified approach in achieving more real-istic radiation dose estimation. For these reasons, this report islikely to be of great interest to standard organizations such asIEC, MITA, and NEMA.

Medical Physics, Vol. 42, No. 12, December 2015

6827 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6827

F. 6. Drawing of the FSMP and the stand.

APPENDIX A: THE FIELD-SIZE MEASUREMENTPLATE AND STAND

It should be pointed out that the “stand” shown here isdesigned for a convenient measurement of KAP and Ka,r ,as well as to perform other tasks related to the evaluationof fluoroscopy systems. While the details of the FSMPare given in the main text, the stand shown here may bereplaced by a simple cardboard box of appropriate size.A sample FSMP with associated stand parts is depicted inFig. 6.

The drawing, including the actual dimensions, is intendedfor illustration purposes only. Note that the “bottom plate ofFSMP” shown on the top left of the drawing is the sameas that described in the main text. All parts are made of

1/2 in. (1.27 cm) thick PMMA plastic with the exception ofthe embedded FSMP gratings section. The FSMP section isembedded in an 8×8 in. (20×20 cm) square and is machinedon a 1/4 in. (0.635 cm) thick PMMA plate. The length of thebottom plate of FSMP needs to be sufficiently long (∼18 in.,∼46 cm) so that the device can be properly secured at the edgeof the examination table.

In addition, the 8 × 8 × 1/4 in. (20 × 20 × 0.635 cm)FSMP can be removed from the bottom plate of FSMP andinterchanged with others designed for different tasks involvedin the testing of fluoroscopic equipment.

The “top plate of FSMP” is designed to hold the cop-per sheets for attenuation. The “spacer rods” shown arefabricated to provide a distance of 30 cm from the bottomto the top of the entire stand. Different sets of spacerrods may be necessary for specific fluoroscopic units beingevaluated.

Depicted in Fig. 7 is a series of photographs showing theexperimental arrangement of the FSMP stand setup with amobile C-arm fluoroscopy unit. (Note, the FSMP stand is ofdifferent design and size than the drawing.) Inset (A) on theleft of the photographs shows the measurement arrangementsimilar to Fig. 4 in the main text. However, as describedpreviously, the FSMP is embedded in a 1/4 in. (0.635 cm) platewhich is interchangeable and may be removed or replaced withother test objects if desired.

Inset (B) in Fig. 7 shows a closeup view of the FSMP withan ionization chamber placed in the middle, aligned to thecenter of the radiation field. Note that the ionization chamberand the copper attenuation sheets shown are for illustrationpurpose only. The copper sheets should be positioned close tothe image receptor in actual measurements.

Under this measurement arrangement, two correctionsmay be necessary for better accuracy;1 the locations of theFSMP and the sensing volume of the ionization chamber aredisplaced by one half of the ionization chamber thickness(0.6 cm), and2 attenuation due to the FSMP itself which ismade of PMMA plastic (1/4 in., 0.635 cm).

F. 7. Photographs of the FSMP stand.

Medical Physics, Vol. 42, No. 12, December 2015

6828 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6828

F. 8. Fluoroscopy images.

Inset (C) in Fig. 7 shows that the small solid state detectoris held in an interchangeable holder (with a thin transparencyfilm) so that the sensing detector lies in the same plane asthe FSMP. The corrections needed due to the attenuation ofFSMP plastic plate and distance displacement are eliminated.The small amount of scattered radiation from the 10×10 cmradiation field, for example, is further minimized.

Shown in Fig. 8 are fluoroscopy images of the FSMP.Inset (A) is a last-image-hold image corresponding to theexperimental arrangement shown in Fig. 7, inset (B).

In Fig. 8, inset (B) shows the fluoroscopy image withthe ionization chamber removed. The center circular cutoutshould be large enough to accommodate the ionizationchamber employed. As indicated previously, the FSMP isfabricated such that it can be removed and replaced with aholder designed to accommodate the detector.

In Fig. 8, inset (C) is the fluoroscopy image of an ionizationchamber place in a holder so that the center plane of theionization chamber is placed at the same plane where theFSMP is located.

APPENDIX B: THE HORIZONTAL GEOMETRICALARRANGEMENT

Isocentric fluoroscopes can also be evaluated without thestand. Figure 9 illustrates a horizontal-beam measurementsetup. The radiation detector is shown at system isocenter.Its location was confirmed by rotating the gantry 90◦.

Note that the description of each key component isidentified with the annotation in the photograph with thedescription of each item listed in Fig. 9. The SAD may beobtained either from system documentation or measurement.

Field size at isocenter is measured after radiation dataare collected by removing the attenuator (C) and dosimeter(D) and then, sliding the field-size plate (F) such that theplate is perpendicular to the radiation beam (B) and atthe fluoroscope’s isocenter. As shown in Fig. 9, the platecan be slid into position by sliding it along the tight gapbetween blocks (G). Plate position can be confirmed byrotating the gantry 90◦ and verifying that the plate is seen onedge.

F. 9. The horizontal geometry measurement setup. (A) is the x-ray tube assembly. (B) is the central ray of horizontal x-ray beam. (C) is image receptor withcopper attenuation plate. (D) is the radiation detector (note that the sensitive volume is placed at the fluoroscope’s isocenter). (E) is the tabletop (note that thetable height was adjusted to place the horizontal scale of the field-size plate at approximately the same height as the central ray of the beam). (F) is the field-sizeplate with radio-opaque scale (any appropriate plate may be used). For illustrative purposes, the plate is shown outside the x-ray beam in this picture. (G) arealuminum blocks used as the supports to hold the field-size plate (F).

Medical Physics, Vol. 42, No. 12, December 2015

6829 Lin et al.: AAPM TG 190 Report on KAP-meter accuracy 6829

NOMENCLATUREAL, AW Length and width of a rectangular exposure field

of viewC(x) Correction factor equal to the measured external

value divided by the system’s displayed value,where x is either Ka,r or KAP

D Diameter of a circular or octagonal exposure fieldof view

DAP Dose-area-productDICOM Digital imaging and communications in medicineDID Dosimeter to image receptor distance, detector to

image receptor distanceFSMP Field-size measurement plateFDA U.S. Food and Drug Administrationf -factor Factor used to convert air kerma to dose in

tissue.FOV Field of viewFSMP Field-size measurement plateG1 Geometric factor 1= [SAD/(SAD−RPD)]2IEC International Electrotechnical CommissionKa,SAD Air kerma at source to axis distanceKa,SDD Air kerma at source to external dosimeter

distanceKa,r Air kerma at the reference pointKAP Air kerma-area-productKAP-meter A thin, parallel-plate transmission ionization

chamber that is fixed in the x-ray tube housing,typically at the end of the collimator

kVp Kilovolt peakmA MilliamperemGy MilligrayNEMA National Electrical Manufacturers AssociationRDSR Radiation Dose Structured ReportRP Reference pointRPD Isocenter to reference point distanceSAD Source to axis distance (i.e., source-isocenter

distance)SDD Source to external dosimeter distanceSHD Source to the exit point of the x-ray tube housing

assembly distanceSID Source to image receptor distance

a)Author to whom correspondence should be addressed. Electronic mail:Pei-Jan.Lin@vcuhealth.org

1M. Mahesh, “Fluoroscopy: Patient radiation exposure issues,” Radio-Graphics 21, 1033–1045 (2001).

2T. B. Shope, “Radiation-induced skin injuries from fluoroscopy,” Radio-Graphics 16, 1195–1199 (1996).

3NCRP, “Ionizing radiation exposure of the population of the United States,”NCRP Report 160, Bethesda, MD, 2009.

4Interventional Fluoroscopy, Reducing Radiation Risks for Patients and Staff,NIH Publication No. 05-5286, March 2005.

5S. Balter, J. W. Hopewell, D. L. Miller, L. K. Wagner, and M. J. Zelefsky,“Fluoroscopically guided interventional procedures: A review of radiationeffects on patients’ skin and hair,” Radiology 254, 327–341 (2010).

621CFR1020.32, Code of Federal Regulations, Title 21, Volume 8, Perfor-mance Standards for Ionizing Radiation Emitting Products: FluoroscopicEquipment, April 1, 2013.

7A. K. Jones and A. S. Pasciak, “Calculating the peak skin dose resultingfrom fluoroscopically guided interventions. Part I: Methods,” JACMP 12,231–244 (2011).

8International Electrotechnical Commission (IEC2000), Medical ElectricalEquipment–Part 2-43: Particular Requirements for the Safety of X-rayEquipment for Interventional Procedures, IEC 60601-2-43 ed1.0, Interna-tional Electrotechnical Commission, Geneva, 2000.

9International Electrotechnical Commission (IEC2010), Medical ElectricalEquipment–Part 2-43: Particular Requirements for the Basic Safety andEssential Performance of X-ray Equipment for Interventional Procedures,IEC 60601-2-43 ed2.0, International Electrotechnical Commission, Geneva,2010.

10Digital Imaging and Communications in Medicine (IEC2011), Supplement94: Diagnostic X-ray Radiation Dose Reporting (Dose SR), National Elec-trical Manufacturers Association, Rosslyn, Virginia, 2011.

11International Electrotechnical Commission (IEC2007), Radiation DoseDocumentation–Part 1: Equipment for Radiography and Radioscopy, PAS61910–1 ed1.0, International Electrotechnical Commission, Geneva, 2007.

12NEMA XR 27-2012 X-ray Equipment for Interventional Procedures UserQuality Control Mode, National Electrical Manufacturers Association,Rosslyn, Virginia, 2012.

13P. Toroi, T. Komppa, and A. Kosunen, “A tandem calibration method forkerma–area product meters,” Phys. Med. Biol. 53, 4941–4958 (2008).

14International Electrotechnical Commission (IEC2009), Medical ElectricalEquipment–Part 2-54: Particular Requirements for the Basic Safety and Es-sential Performance of X-ray Equipment for Radiography and Radioscopy,IEC 60601-2-54 ed1.0, International Electrotechnical Commission, Geneva,2009.

15P. J. Lin, P. Rauch, S. Balter, A. Fukuda, A. Goode, G. Hartwell, T. Lafrance,E. Nickoloff, J. Shepard, and K. Strauss, “Functionality and operation offluoroscopic automatic brightness control/automatic dose rate control logicin modern cardiovascular and interventional angiography systems,” AAPMReport No. 125, 2012.

16P. Toroi, T. Komppa, A. Kosunen, and M. Tapiovaara, “Effects of radiationquality on the calibration of kerma-area product meters in x-ray beams,”Phys. Med. Biol. 53, 5207–5221 (2008).

Medical Physics, Vol. 42, No. 12, December 2015