Chapter 10: Special Topics - Human Health Campus · PDF fileTo familiarize the student with...

IAEAInternational Atomic Energy Agency

Slide set of 71 slides based on the chapter authored by

D. McLean and J. Shepherd

of the IAEA publication (ISBN 978-92-0-131010-1):

Diagnostic Radiology Physics:

A Handbook for Teachers and Students

Objective:

To familiarize the student with Dental radiography, Mobile

Radiography and fluoroscopy, Dual-Energy X-Ray absorptiometry,

Conventional tomography and tomosynthesis.

Chapter 10: Special Topics

Slide set prepared

by S. Edyvean

IAEA

CHAPTER 10. SPECIAL TOPICS IN RADIOGRAPHY

10.1. Introduction

10.2. Dental radiography

10.3. Mobile Radiography and fluoroscopy

10.4. Dual-Energy X-Ray absorptiometry

10.5. Conventional tomography and tomosynthesis

Bibliography

Diagnostic Radiology Physics: a Handbook for Teachers and Students – chapter 10, 2

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� Previous chapters covered 2- dimensional imaging

� Later chapters cover cross-sectional imaging (CT,MR,

ultrasound)

� This chapter presents a number of special X ray imaging

modalities and their associated techniques - forming a

transition between projection and cross sectional imaging

� Special X-ray imaging techniques

• Dental radiography

• Mobile Radiography and fluoroscopy

• Dual-Energy X-Ray absorptiometry

• Conventional tomography and tomosynthesis

10.1. INTRODUCTION


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� The tooth can be imaged in three ways

• Intra oral examination with the x-ray tube external and a bitewing

film placed inside the mouth

• Extra oral examination where both the X-ray tube and detector is

external to the patient to form an OPG

• A conebeam CT image

10.2. DENTAL RADIOGRAPHY

10.2.1. Introduction


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� An intra oral examination with bite-wing

• Is the most common examination, and is a low cost technique

• Places very small demands on X ray generation since the tooth is

a low attenuation static object

• The image receptor is placed inside the mouth, and irradiated

externally.



X-ray tube

bite-wing film

teeth


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� Orthopantomograph (OPG),

• Two dimensional images when radiographs of the entire set of

teeth are required

• The image receptor and the X ray source are external to the

patient

• The X ray beam is transmitted through the head - demanding

significant X ray generation power and complex motion control for

the X ray tube and image receptor

• Image receptors are film or digital detectors




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� Cone-beam dental CT

• For three dimensional information




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� Intra oral radiography

� The intra oral X ray tube is a small robust device with a

stationary target operating with a tube current of only a few

mA

Dental X ray tube with a stationary anode.


10.2.2. Technology


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� Intra oral radiography (continued)

� The generator is typically very simple often with fixed tube

voltage and tube current allowing output changes only by

variations in exposure time.

� Major concerns with this device are for the stability of the

tube head and the collimation of the beam.


10.2.2. Technology


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� International standards require that the focus to the patient

surface distance (FSD) be 200 mm.

� This is assured with the use of a collimating attachment

that also restricts the beam to the region of the mouth

being radiographed.


10.2.2. Technology

X-ray

tube

bite-wing film

teeth200

mm


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� The X ray equipment requires periodic QC checking

� The process of film processing requires more diligent

attention.

� The unscreened film is removed from the light tight

moisture protective wrapping and is processed either

manually or with varying degrees of automation.

� Hand processing is probably most common and ideally

requires control of temperature and processing time.


10.2.2. Technology


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� For higher volume clinics this can be automated with film

mounted on hangers that progress through the

development, stop bath, fixation and rinse processes.

� Typically these devices have timing and temperature

control but do not control chemical activity through

replenishment.

� This is achieved in fully automatic processors, however

these are typically restricted to major dental hospitals.


10.2.2. Technology


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� The uncertainties in film processing are best controlled

through sensitometry.

� Light sensitometers are rare in dentistry due to the small

film format,

• However adequate results can be achieved by using a simple

radiograph of a 3 step ‘wedge’

• This can be easily manufactured by folding the lead foil found in

the film wrap or purchased commercially

� Increasingly digital detectors are replacing film.


10.2.2. Technology


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� Digital image capture can be achieved from an intensifying

screen that is linked to a CCD camera through a tapered

fibre optic coupling.

� The electronic signal can be transferred to an acquisition

computer either through a direct cable or through ‘blue

tooth’ radio frequency transmission.


10.2.2. Technology


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� OPG (Orthopantomograph)

� An OPG image is created by complex equipment where

the X ray tube and image receptor assembly move in a

horizontal plane around the head of the patient.

OPG image of the teeth


10.2.2. Technology


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� OPG (Orthopantomograph) (continued)

• A narrow beam of radiation is formed by the tube collimation

• the image receptor moves within the assembly behind a lead

aperture


10.2.2. Technology

The basic movements

of the OPG unit

around the mandible

are illustrated


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� OPG (Orthopantomograph) (continued)

� The device uses the principle of tomography and more

importantly the principle of panoramic photography.

� This process can be illustrated through consideration of

the panoramic camera used in photography.

� Here an acquisition aperture is used to expose an image

plate that is moved behind the aperture slit to capture the

image of a ‘panorama’ while the camera simultaneously

slowly rotates to scan a scene.


10.2.2. Technology


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� Cone-beam CT

� CT imaging has been used for some time in dentistry,

including the use of custom designed units for dental

applications.

� Their use has become more widespread recently with the

advent of cone beam technology


10.2.2. Technology


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� Cone-beam CT (continued)

� There are many cone beam CT (CBCT) models available

using a variety of acquisition schemes

� They have in common a flat panel detector for acquisition,

typically using either DR technology or an intensifying

screen with a CCD camera (see chapter 7).


10.2.2. Technology


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� Cone-beam CT (continued)

� Typically a CBCT can acquire a full field of view (FOV) that

covers the whole head

• although acquisitions that are restricted to the mandible with as

little as 10% of full FOV are possible.

� The use of these lower cost CT units opens up new

potentials in some areas of dental diagnosis

� However they have significantly higher dose compared to

OPG procedures


10.2.2. Technology


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� Since dental examinations are amongst the most

numerous, the dosimetry of these procedures is of high

interest.

� Relevant principles and measurement techniques of

dosimetry can be found in

• Chapter 21 of this handbook

• and in the IAEA Technical Report Series No.457


10.2.3. Dental Dosimetry


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� There are large variations in recorded doses between X-

ray facilities.

� A recent study in Europe showed that for an intra oral

bitewing projection

• the average incident air kerma varied from 1 to 2 mGy

• with a corresponding KAP measurement of 20 to 40 mGy cm2.

• The dose in centres that use slower film would be expected to be

significantly higher.

� Data for OPG examinations from Europe showed

• KAP values ranging from 40 to 150 mGy cm2




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� The estimation of a population effective dose is difficult

owing to the complex distribution of critical organs

� There are few radiosensitive organs around the mandible

with some exceptions

• The thyroid, red bone marrow, brain, and salivary glands




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� The thyroid is the main radiosensitive organ around the

mandible

• Well collimated X ray units should not directly irradiate this organ

• but it will probably receive appreciable scattered radiation

� Other radiosensitive organs include

• the red bone marrow of the mandible

• the brain

� The salivary glands also need to be considered as they

are extensively irradiated

• They are now included as a remainder organ in the calculation of

effective dose in accordance with ICRP 103.




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� OPG examinations estimates of effective dose give

• average values of ~ 7 mSv using weighting factors from ICRP 60

• The use of ICRP 103 weighting factors has been variously

estimated to increase effective dose in dentistry by 50% to 400%.

� Since CBCT units operate with a large FOV, their effective

doses are considerably higher than for OPG

• with estimates of dose varying from 60 mSv to 550mSv, for full

FOV

• still considerably lower than conventional head CT with effective

doses of about 2 mSv.




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� Mobile X ray equipment ranges from small dental units to

CT and MRI units carried in a large vehicle.

� However this chapter is restricted to simple radiographic

and fluoroscopy equipment.

� Mobile equipment is needed when the patient cannot be

brought to a fixed installation for a radiographic

examination.

10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY



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� Limitations of mobile equipment relate to

• the availability of a suitable electrical power supply,

• the size and weight of the equipment and the consequent effort

required to move it.

� The equipment design of mobile X ray equipment is varied

and innovative in order to maximise the benefit given the

above constraints.




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� Fixed angiographic X ray machines require a capacity to

draw up to 100 kW with a high current multiphase supply.

� Assuming no loss in the high voltage transformer

• the X ray output power in the secondary circuit will equal that of the

primary power drawn from the electrical supply (cf Chapter 6)

� Therefore a domestic single phase electric supply may

typically be limited to 2.4 kW

� While low power is usually not a limitation for fluoroscopic

application – this is a challenge for some radiography.


10.3.2. Technology


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� One solution is to charge a capacitor which is discharged

across the X ray tube – the ‘capacitor discharge’ mobile.

� However the tube voltage will fall rapidly during the

discharge of the capacitor

• leading to excessive surface kerma for large patient thicknesses.

� It is more advantageous to have an integral battery power

supply which is converted to a medium to high frequency

AC signal (cf chapter 5)

• This leads to substantial reductions in the thickness of the coils

needed in the transformer design.

• There is also the added advantage that it can be used when there

is no electrical power supply available at the examination site.


10.3.2. Technology


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� The variety of possible generator designs leads to the

possibility of many types of radiographic waveforms being

used in the high voltage circuit for X ray generation.

� This leads to varying tube outputs and beam qualities for

the same radiographic settings of tube voltage and tube

current (cf chapter 5)

� Care is therefore needed when determining dosimetric

factors for mobile units.


10.3.2. Technology


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� Image quality and general quality control for mobile X ray

units generally follows that used for fixed units.

� The use of high fluoroscopic image quality can lead to

reduced procedural time, and hence reduced radiation

exposure time.

� An important part of image quality is the setup of viewing

monitors and the ambient conditions used for operation.

� Every effort should be made to view monitors in low

ambient lighting conditions.


10.3.3. Image quality


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� Mobile X ray equipment raises concerns about

occupational and the public radiation exposure, as it is not

operated in a purpose-built shielded environment.

� Assuming all X ray equipment has been checked for tube

leakage, the source of radiation of occupational concern

during the procedure is scatter from the input surface of

the patient.


10.3.4. Radiation protection


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� It is advised that the medical physicist take field

measurements of air kerma levels due to the patient

scattered radiation using a patient phantom for typical

radiographic and fluoroscopic procedures.

� As mobile radiography may take place in environments

where other patients or members of the public may be in

close proximity it is essential that good communication

exists between the medical physicist and the staff at the

location for the radiographic procedure.

� These staff should attend appropriate radiation safety

courses that include information about radiation risk from

mobile radiography.




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� In many cases, such as for mobile chest radiography, the

use of good radiographic practice with basic radiation

protection allows safe usage in most hospital

environments.

� Simple measurements should be made to demonstrate the

safety (or otherwise) of mobile X ray equipment use.




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� The principle of operation involves two images

• from the attenuation of a low and a high X-ray energy beam

� Using special imaging equipment

• comprising of special beam filtering and near-perfect spatial

registration of the two attenuation maps

10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY

Detector(s) within

gantry head

x-ray source

within couch


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� Schematic showing the

components of a DXA system.

� The gantry configuration

shows a pencil beam system

• pinhole source collimator

• and a single detector

� These scan the patient to

acquire the attneuation data


(courtesy of J. Shepherd, UCSF).

Movement of source

and detector to

acquire attenuation

data


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� Other systems with slit may source collimators and

segmented line detectors are called fan-beam systems,

and have the advantage of higher spatial resolution and

shorter scan times



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� The process for determining material composition can be

outlined from consideration of the total attenuation of an

X-ray flux passing through a subject as represented by

the following formula

• where Io is the unattenuated X-ray intensity before it passes

through a N materials with a thicknesses, ti ,

• µi is the total linear attenuation, (µ/ρ)i is the mass attenuation

coefficient of the ith material, and ξi is the ith areal density = ρiti.


∑=

∑=

∑=∑=

====

−

−−−

N

1i i

i

N

1i ii

ii

iN

1i iiN

1i ii

ξρ

µ

O

ρtρ

µ

O

)ρ

ρ(tµ

O

tµ

O eIeIeIeII


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� DXA can only solve for two materials simultaneously.

� However, three materials may be quantified: bone, lean,

and fat mass, by using three fundamental assumptions,

1. X-ray transmission through the body for the two energy windows

can be accurately described by exponential attenuation

processes

2. Pixels of the human body image can describe two-components

• i.e. either soft tissue and bone mineral, or, when bone is not present, fat and

lean mass. Thus, although DXA can only solve for two compartments within

individual pixels, it can describe a 3-component model for body composition.

3. The soft tissue overlaying the bone in the image has a

composition and X-ray properties

• that can be predicted by the composition and X-ray properties of the tissue

near, but not overlaying, the bone.



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� For example - simplified DXA equations will be derived

for two monochromatic X-ray exposures with different

energies (a high and low energy).

� The full solution would require integration of the

attenuation across the x-ray spectrum for each energy.

� The attenuation equation for each exposure results in the

following two equations:

where the H and L superscripts represent the high and low

energy X-ray beams respectively, and the “s” and “b” subscripts

represent soft tissue and bone.


+

−

=b

L

b

s

L

seII O

L

ξρµ

ξρµ

+

−

=b

H

b

s

H

seII O

H

ξρµ

ξρµ


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� The solution of these equations for the areal density of

bone is given by

• Where RS is commonly referred to as the “ratio value” for soft

tissue measured for tissue surrounding but not containing the

bone.


)(

lnln

aBMDDensityMineralBoneareal

R

I

I

I

IR

S

H

b

L

b

L

O

L

H

O

H

S

b =

−

−

=

ρµ

ρµ

ξ

H

S

L

SSR

=

ρµ

ρµ


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� Principle of DXA is shown

with 4 intensity profiles

� The high energy absorption

profile is multiplied by the soft

tissue R-value, Rs, which

accounts for differences in

high and low energy

absorption of soft tissue.

� Rs is calculated for pixels that

do not contain bone


(drawing courtesy of J. Shepherd, UCSF).


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� In order to use a DXA unit to determine bone mineral

density the DXA unit must be calibrated with a phantom

suitable for a particular examination, for example spine,

and for a particular DXA system type.

� Universal phantoms that can be used between different

types of systems have been developed, however the

calibration of DXA units is an important practical subject

essential for the viability of DXA usage.


Bio-Imaging Technologies, Inc

European spine

Phantom - QRM

Hologic

Examples of standard phantoms available


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� The T-score and the Z-score are parameters used.

� The T-score is the primary diagnostic value used for

osteoporosis.

� The T-score is inversely related to fracture risk.

� By international convention, the T-score is the difference

between the patient’s aBMD and a young reference

aBMD in units of the population standard deviation:

• where SD is the standard deviation of the population of young

adults.


MeanAdultYoung

MeanAdultYoungpatient

SD

aBMDaBMDscoreT

−=−


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� The Z-score is used to diagnose low bone mass in young

adults and children.

� It is the difference between the patient’s aBMD and an

age- and typically ethnicity-matched reference aBMD

and standard deviations:

� The reference values used to calculate T and Z-scores

are derived from normative databases of local

populations.


MeanAdultmatchedEthnicity,Age

MeanAdultmatchedEthnicity,Agepatient

SD

aBMDaBMDscoreZ

−−

−−−=−


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� More information on the standards used to calculate T

and Z-scores can be found in the Postitions of the

International Society for Clinical Densitometry

(www.iscd.org).



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� The usefulness of sectional images, that remove the

image of unwanted overlying tissues, has been well

understood since the early days of X ray imaging.

� The formation of such images is through an analogue

process known as conventional tomography.

10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS


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� Conventional tomography uses the principle of image

blurring to remove overlying structures from a radiological

image while allowing one section of body to remain in

focus.


10.5.1. Principles


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� During image acquisition the X

ray tube is in motion

� The image receptor moves

simultaneously in the opposite

direction

� The projected image in the

focal plane moves in same

direction as the image receptor


10.5.1. Principles

movement of image receptor

movement of

X-ray tube

focal plane

movement

of images

objects in

focal plane


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� The section in focus is the focal

plane

� Regions of the body above and

below the focal plane are

increasingly blurred as their

distance from this plane

increases.


10.5.1. Principles

fulcrum or

pivot point

movement of image receptor

movement of

X-ray tube

Only the red triangle remains in the

same position on the image receptor

Illustration of final image

focal

plane


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� Conventional tomography with curved focal plane

� Conventional tomography is easily be extended for use

in dental radiography by acquiring a curved focal lane

� The principle is to use a variable speed for the image

receptor

• The focal plane is known as the focal trough

• The variable speed gives rise to a curved curved focal trough


10.5.1. Principles


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� Conventional tomography with curved focal plane

• The principle is that if the image receptor speed is increased the

focal plane moves upwards in this example (and vice versa)


10.5.1. Principles


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� For the curved focal trough the image

receptor speed changes during motion

� In this example

• The X ray tube moves at constant speed to

the right

• The image receptor accelerates to the left

during motion

• Consequently the focal plane moves away

from the image receptor.

• Note the thickness of the focal trough

changes in accordance to distance from the

image receptor


10.5.1. Principles

focal

plane

Accelerating movement

of image receptor

X-ray tube

moving at

constant

speed

focal

trough


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� Tomosynthesis - a development of conventional

tomography with the use of digital technology to ‘digitally’

change the speed of the image receptor

� In this case one acquisition run might consist of 10

individual X ray images each read and erased in

sequence throughout the one tube movement.

� The images are digitally added to reconstruct different

focal planes in the body. It can be seen that the focal

plane can be altered by advancing or retarding each

image in the series by an increasing amount.


10.5.1. Principles


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� Illustration of Tomosynthesis

• The X ray tube moves at a

constant speed to the right

• The image receptor moves at a

constant speed to the left.

• In this figure 4 samplings of the

image are shown at tube positions

A, B, C and D.

• Tomographic images focused on

planes I, II and III are created by

combining the 4 sampled images

with appropriate offsets


10.5.1. Principles


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� Tomosynthesis is a method for performing high-

resolution limited-angle tomography

� It can be treated as a special case of computed

tomography in which data is acquired over a limited

angular range.

� The computed image can then be obtained using the

various CT reconstruction methods.


10.5.1. Principles


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� Conventional tomography has been almost completely

replaced by computed tomography in the modern

radiology department.

� Areas where it is still used are

• intravenous pyelograms (IVPs) where contrast in the kidney can be

conveniently placed within the focal plane to allow clear

visualisation of the contrast agent. This examination is largely

replaced by CT.

• pantomographic dental radiography

(Orthopantomogram - OPG)


10.5.2. Tomographic applications

OPG

IVP


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� Conventional tomography requires one tube acquisition for

each focal plane image or slice.

• Therefore examinations requiring many slices are inherently high

dose procedures.

� The use of tomosynthesis, on the other hand, requires only

one tube motion to capture enough data to reconstruct

multiple slices within the body.

• Today it is an emerging technology, with its most notable application

so far being in mammography


10.5.2. Tomographic applications


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� CENTRE FOR EVIDENCE-BASED PURCHASING, Digital

cone beam tomography (DCBT) systems. CEP 10048;

NHS PASA March 2010 [online] (2010).

http://www.pasa.nhs.uk/PASAWeb/NHSprocurement/CEP/

CEPproducts.htm.

� HELMROT, E., ALM CARLSSON, G., Measurement of

radiation dose in dental radiology, Radiation Protection

Dosimetry 114 (2005) 168-171.

� INTERNATIONAL ATOMIC ENERGY AGENCY,

Guidelines for the use of DXA in measuring bone density

and soft tissue body composition, Rep. TBA, IAEA, Vienna

(2010).

BIBLIOGRAPHY


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� LANGLAND, O.E., LANGLAIS, R.P., Principles of Dental

Imaging, Williams & Wilkins, Baltimore, MA (1997).

� LUDLOW, J.B., DAVIES-LUDLOW, L.E., BROOKS, S.L.,

HOWERTON, W.B., Dosimetry of 3 CBCT devices for oral

and maxillofacial radiology: CB Mercuray, NewTom 3G

and i-CAT, Dentomaxillofacial Radiology 35 (2006) 219–

226.

� WILLIAMS, J.R., MONTGOMERY, A., Measurement of

dose in panoramic dental radiology, British Journal of

Radiology 73 (2000) 1002-1006.

BIBLIOGRAPHY


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