Michael McNitt-Gray, PhD, DABR, FAAPMProfessor, Department of Radiology

Director, Biomedical Physics Graduate ProgramDavid Geffen School of Medicine at UCLA

mmcnittgray@mednet.ucla.edu

KEY CT PARAMETERS - WHAT ARE THEY CALLED AND WHAT DO THEY

MEAN?

DISCLOSURES

• Institutional research agreement, Siemens AG

• Recipient research support Siemens AG• Consultant, Flaherty Sensabaugh

Bonasso PLLC• Consultant, Fulbright and Jaworski, LLC

PURPOSE• Introduce some of the important tech.

parameters in CT scanning that affect both radiation dose and image quality

• Describe the terms used by the major manufacturers

• Discuss similarities and differences between them.

IMPORTANT REFERENCE• AAPM Website (www.aapm.org)• CT Protocols Link• Lexicon Tab• Excel document• http://www.aapm.org/pubs/CTProtocols/documents/

CTTerminologyLexicon.pdf

AAPM LEXICON• from Working Group on nomenclature and CT

protocols

TECHNICAL PARAMETERS• CT localizer radiograph• kV• mA, mAs, effective mAs (aka mAs/slice)• Pitch• Tube Current Modulation (TCM) Systems

• One form of Automatic Exposure Control (AEC) Systems

TECH. PARAMETERS: CT LOCALIZER RADIOGRAPH

• The scanned projection radiograph, often acquired by the CT system to allow the user to prescribe the start and end locations of the scan range

• Used for Planning CT Scan Start and End Locations• ALSO – All Automatic Exposure Control systems use

this to plan adjustments based on patient size/attenuation

TECHNICAL PARAMETERS•

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CT LOCALIZER RADIOGRAPH

Generic Termzs

GE Philips

Siemens

Toshiba Hitachi Neusoft

Neuroligica

CT localizer Radiogragh

Scout

Surview

Topogram

Scanogram

Scanogram

Surview

Scout

CT LOCALIZER RADIOGRAPH• Importance of centering

Each manufacturer has a different name for the projectional image that is used for planning a CT exam, including Scout, Surview, Topogram, and Scanogram, but the generic name is actually the:

1. Planning View 2. CT localizer

radiograph 3. CT survey projection4. Localizer Scan 5. Monitoring Scan

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Each manufacturer has a different name for the projectional image that is used for planning a CT exam, including Scout, Surview, Topogram, and Scanogram, but the generic name is actually the:

1. Planning View 2. CT localizer radiograph 3. CT survey projection4. Localizer Scan 5. Monitoring Scan

Answer: 2, CT localizer radiographRef: AAPM CT Lexicon version 1.3 04/20/2012

TECH. PARAMETERS: KV• Tube potential• The electric potential applied across an x-ray

tube to accelerate electrons towards a target material, expressed in units of kilovolts (kV)

• Often reduced in peds/smaller patients• kV selection methods part of AEC• NOTE: In CT, all scans are constant kV; There is

no kV modulation or varying of kV within the scan

TUBE POTENTIAL

Generic Termzs

GE Philips

Siemens

Toshiba Hitachi Neusoft

Neuroligica

Tube potential

kV kVp kV kV kVp kV kV

TECH. PARAMETERS: KV• Contrast in image

• Lower kV can give more contrast, especially with iodinated contrast agents (exploit k-edge)

• Tube output (mR/mAs)• Lower kV yields lower tube output –> noise

increase• So, reducing kV often involves increasing

mAs to offset noise increase

TECH. PARAMETERS: KV• Dose

• CTDIvol (kV)2.5

• So, reducing kV from 120 to 80• (80/120) 2.5 = .36 (64% reduction)• IF mAs is held constant

TECH. PARAMETERS: KV• Dose

• CTDIvol (kV)2.5

• So, reducing kV from 120 to 80• (80/120) 2.5 = .36 (64% reduction)• IF mAs is held constant

TECH. PARAMETERS: TUBE CURRENT, ETC. • Tube current (in mA)• Tube Current time product (in mAs)• Effective Tube Current Time Product

• Effective mAs• mAs/Slice• = mAs/pitch

TUBE CURRENT, ETC. Generic Terms

GE Philips Siemens

Toshiba Hitachi

Neusoft Neuroligica

mA mA

mA mA mA

mAs mAs (axial)

mAs(axial)

mAs mAs mAs

Eff. mAs = mAs/pitch

mAs/slice (helical)

Eff. mAs(helical)

Eff. mAs(helical)

mAs/slice

mAs

Manufacturers use different terms for the tube current, tube current time product or the effective tube current time product. The definition of the effective tube current time product is:

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2. The product of tube current and exposure time per rotation, expressed in units of milliampere • seconds (mAs).

3. In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch

4. In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time.

5. In helical mode, this is equal to tube current × rotation time.

Manufacturers use different terms for the tube current, tube current time product or the effective tube current time product. The definition of the effective tube current time product is:

1. The number of electrons accelerated across an x-ray tube per unit time, expressed in units of milliampere (mA)

2. The product of tube current and exposure time per rotation, expressed in units of milliampere • seconds (mAs).

3. In helical scan mode, the product of tube current and rotation time (expressed in mAs) ÷ pitch

4. In axial mode, this is equal to tube current × (scan angle ÷ 360) × rotation time.

5. In helical mode, this is equal to tube current × rotation time.

Answer: (3) mAs ÷ pitch; this is also known as mAs/Slice in some systems.  Ref: AAPM CT Lexicon version 1.3 04/20/2012

TECH. PARAMETERS: PITCH• Pitch = Table feed per rotation/nominal

collimation• Pitch = I/NT• Influences:

• Total scan time (e.g. breathold)• Dose (?)• Effective width of reconstructed image thickness

• minor effect in most MDCT)

TECH. PARAMETERS: PITCH• ONLY influences dose if everything else is constant• GE, Toshiba – use mA and Pitch independently

• If Pitch , CTDIvol and patient dose• Philips, Siemens – use effective mAs or mAs/slice

• Eff mAs = mAs/pitch• System AUTOMATICALLY adjusts mAs with changes in

pitch to provide a constant eff mAs• If Pitch then mAs and no net change in CTDIvol

TECH. PARAMETERS: COLLIMATION• Detector Configuration• Nominal Collimation - NxT• N = Number of Detector Channels• T = Width of each Detector Channel• Example: 64 x 0.625mm • N= 64, T=0.625mm, NT = 40mm

DETECTOR CONFIGURATION (DET CONF)

Generic Terms

GE Philips Siemens

Toshiba

Hitachi

Neusoft Neuroligica

DetectorConfig

DetConf

Collimation N x T (mm)

DetConf or Acq

DetConf

DetConf

Collimation N x T (mm)

DetConf

TECH. PARAMETERS: COLLIMATION• Changing Collimation has some influence

on dose• Wider Collimation settings are usually

more efficient

TECH. PARAMETERS: COLLIMATIONCollimation CTDIw

(mGy/100 mAs)

64x.625mm 8.5

32x.625mm 9.0

16x.625mm 10.5

8x.625mm 12.5

4x.625mm 12.4

2x.625mm 15.1

TECH. PARAMETERS: TUBE CURRENT MODULATION

CARE Dose 4DTopogram Evaluation: a.p. and lat.

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Shoulder Region

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Breast Tissue

Long Axis Modulation

CTDIVOL IN CONTEXT OF AEC

• When Tube current modulation is used:• CTDIvol reported is based on the average

mA used throughout the scan

Scan where Tube Current Modulation was usedBlue Curve Represents actual instantaneous mARed Curve Represents avg mA for each imageYellow Curve Represents avg mA over entire scanOverall avg is used for CTDIvol reported in Dose Report

TUBE CURRENT MODULATION• LOTS of Different Names• Siemens: CareDose4D• GE: Smart Scan, Auto mA, Smart mA• Philips: DOM, Z-DOM• Toshiba: SureExposure, SureExposure3D

TUBE CURRENT MODULATION• Siemens: CareDose4D• User sets a “Quality Reference mAs”• System uses online modulation (180 degree lag)• The mAs (or effective mAs, if helical scan) that would be used on

a “standard sized” patient• Quality Reference mAs is NOT the max or min• ACTUAL mAs (eff. mAs) can be larger than this (should be for

large patients)• ACTUAL mAs (eff. mAs) can be less than this (should be for

smaller patients)

TUBE CURRENT MODULATION• GE SmartmA• User sets: Max mA, min mA and Noise Index (NI)• NI is approximately the standard deviation in a 20 cm water

phantom scanned under these conditions• The higher the NI, the lower the mA• The lower the NI, the higher the mA• Scanner output is influenced by recon. image thickness

(Kanal AJR 2007)• Attempts to keep noise constant across patient size/anatomy

While all tube current modulation systems base their calculations from the CT localizer radiograph, the image quality reference parameters vary from system to system. Which of the following will result in an increase in dose for a patient of a given size where the scan is being performed with AEC

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25% 1. Decreasing the Noise Index (NI) on a GE Scanner 2. Decreasing the Quality Reference mAs on a Siemens

Scanner3. Increasing the Standard Deviation on a Toshiba Scanner4. Increasing the Standard Deviation (% ) a Hitachi

Scanner

While all tube current modulation systems base their calculations from the CT localizer radiograph, the image quality reference parameters vary from system to system. Which of the following will result in an increase in dose for a patient of a given size where the scan is being performed with AEC

1. Decreasing the Noise Index (NI) on a GE Scanner 2. Decreasing the Quality Reference mAs on a

Siemens Scanner3. Increasing the Standard Deviation on a Toshiba

Scanner4. Increasing the Noise Index (NI) on a GE Scanner 5. Increasing the Standard Deviation (% ) a Hitachi

Scanner Answer: 1, Decreasing the Noise Index on a GE Scanner Ref: AAPM CT Lexicon version 1.3 04/20/2012 Kanal et al. AJR 2007 Jul;189(1):219-25 and Kanal et al. AJR 2011 Aug;197(2):437-41

SUMMARY• Introduce some of the important tech. parameters

that affect both radiation dose and image quality• CT localizer radiograph, kV, mA/mAs/effective mAs, pitch and

TCM• Describe the terms used by the major manufacturers• Discuss similarities and differences between them.• Important Resources – AAPM CT Protocols Lexicon