RADIATION SAFETY ACT 1975

DIAGNOSTIC X-RAY EQUIPMENT

COMPLIANCE TESTING

WORKBOOK 6

COMPUTED TOMOGRAPHY EQUIPMENT

2006

Radiological Council
Locked Bag 2006 P O
NEDLANDS WA 6009

Telephone: (08) 9346 2260

Facsimile: (08) 9381 1423
email: radiation.health@health.wa.gov.au
www.radiologicalcouncil.wa.gov.au/index.html

© Radiological Council of Western Australia

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DIAGNOSTIC X-RAY EQUIPMENT
COMPLIANCE TESTING

WORKBOOK 6
COMPUTED TOMOGRAPHY EQUIPMENT
2006

Department of Health of Western Australia

ISBN 0-9775570-6-5
 PREFACE
This workbook is one of a series prepared by a working group of the Radiological Council1 , the
statutory body established under the Western Australian Radiation Safety Act 1975.

The workbook describes standard test protocols approved by the Radiological Council for routine
compliance testing of computed tomography (CT) x-ray equipment. Variations to the recommended
test methods may be used provided they are approved by the Radiological Council. Documentation on
alternative test methods will need to be provided.

Persons who perform these tests in Western Australia must hold a licence under the Radiation Safety
Act for the purpose, or be acting under the direction and immediate personal supervision2 of a
licensed compliance tester.

Most of the tests assess compliance with the regulations to the Western Australian Radiation Safety
Act and with other Council requirements that may apply to the class of equipment under test. Where
practicable, tests are traceable to relevant Australian and International Electrotechnical Commission
(IEC) standards.

Directed essentially at radiation safety parameters, this workbook does not include all tests that might
be considered as part of a comprehensive quality assurance (QA) program. However, should the
workbook be used as a basis for a QA program, it is recommended that additional radiographic,
sensitometric and image quality tests be performed.

The Radiological Council is grateful to the personnel of Radiation Health Branch and the other
members of the compliance testing working group who contributed their time and effort to the
production of the workbooks for the compliance testing program. It is particularly grateful for the
substantial contributions of Mr Peter Henson in redrafting these workbooks. The working group
which undertook the program revision has comprised Richard Fox (Chairman); Janette Atkinson (Sir
Charles Gairdner Hospital); John Burrage & David Causer (Royal Perth Hospital); Bill Cooke
(Community Dental Services); Janelle D’Souza & Tim Finney (Coordinators, Radiation Health
Branch); Bruce Dwyer (Philips Medical Systems); Luke Ferris (MedDent); Peter Henson (Radiation
Health Branch); Gary Johnson; Brett McLoughlin & John Pereira (Health Technology Consultancy);
Max Ross (Biomedical Engineering); Roger Sandercock (Toshiba (Australia) Pty Ltd); Tony
Shackleton (Sir Charles Gairdner Hospital); Jonathon Thwaites (Medical & Scientific); Ann Watson
(ADAW Pty Ltd); Graham Wood (Siemens Pty Ltd).

Further comments during editing by Barry Cobb (Radiological Council), Hazel Upton (Secretary,
Radiological Council) and Leif Dahlskog (Radiation Health Branch) are also gratefully acknowledged.
1
The Radiological Council is the statutory regulatory authority responsible for administration of the State’s
Radiation Safety Act. For use in other jurisdictions the relevant regulatory authority would need to be
consulted where the Radiological Council is referred to in these workbooks. Organisations which wish to
reproduce all or part of the workbook for use in their own testing programs should contact the Radiological
Council for permission to do so.
2
“Immediate personal supervision” requires the licensee to be physically present and to directly observe
persons working under their direction and supervision.
 CONTENTS

1. INTRODUCTION 7
1.1 SCOPE AND APPLICATION 7

1.2 TEST EQUIPMENT 8

1.3 DEFINITIONS 10

1.4 PRECAUTIONS 12

2. GENERATOR, X-RAY TUBE AND SCANNER 14

2.1 CHECK LIST 14

2.2 TUBE VOLTAGE ACCURACY 15

2.3 RADIATION OUTPUT AND OUTPUT LINEARITY 17

2.4 HALF VALUE LAYER 19

2.5 CT DOSE INDEX (CTDI) 21

2.6 TUBE HOUSING LEAKAGE 24

3. IMAGE QUALITY 27
3.1 INTRODUCTION 27

3.2 MEAN CT NUMBER AND UNIFORMITY OF IMAGE 28

3.3 LINEARITY OF RESPONSE 30

3.4 HIGH CONTRAST RESOLUTION 32

3.5 SLICE THICKNESS 34

3.6 TABLE INDEXING AND REPRODUCIBILITY 36

3.7 ALIGNMENT LIGHT AND IMAGE SLICE CONGRUENCE 38

RELATED PUBLICATIONS 39

APPENDIX 1 - ASSESSMENT OF MEASUREMENT ERRORS 41

APPENDIX 2 - OUTPUT LINEARITY COEFFICIENT 45

APPENDIX 3 - TEST REPORT FORM 49

Workbook 6 Computed Tomographic Equipment

1. INTRODUCTION
1.1 SCOPE AND APPLICATION
This workbook describes protocols for the compliance testing of computed
tomographic (CT) x-ray equipment.

Directed essentially at radiation safety parameters, this workbook does not

include all tests that might be considered part of a comprehensive quality
assurance (QA) program. Examination of structural radiation protection,
personal protective clothing, condition of ancillary equipment etc, is also
excluded, being undertaken in a routine inspectio n program conducted by the
Radiological Council.

It is expected that the CT equipment will be subject to a regular service

contract which includes calibrations and image quality checks. It is also
expected that daily quality assurance tests will be performed that include CT
number and noise measurements on a suitable test phantom.

Whilst other test methods may be available for spiral CT scanners, it is

assumed in this book that the relevant tests will be conducted with the couch
stationary.

Persons intending to use this workbook should seek advice from the
Radiological Council on any necessary approvals required to perform the tests.

The frequency of testing for each class of x-ray equipment is prescribed by the
Radiological Council in the Program Requirements.

Following service or maintenance, parameters which have the potential to

affect the performance of the equipment may require additional testing to
ensure compliance has not been compromised.

 Radiological Council of Western Australia [3rd edition] 7

Computed Tomographic Equipment Workbook 6

1.2 TEST EQUIPMENT

Test equipment 1 that may be required includes —

Ø Aluminium filters (type 1100 > 99% purity)

Ø CT test phantom

There are a number of CT test phantoms in use, both commercial and in-
house, some of which may not fully satisfy the requirements of this
Workbook. The CT phantom(s) required to properly perform the tests in
this Workbook must contain the following inserts —

Noise: a plain, water filled section. Fresh distilled water should be

used. The use of anti-fungal agents is not recommended but
if used their concentration must be kept to a minimum and
the agent must not contain any high atomic number
elements.

Linearity: a section, either of water or other low atomic number

material e.g. nylon, containing inserts of air, water (if the
matrix is not water), teflon and at least two other inserts
from the following:- low density polyethylene (LDPE),
polyethylene, polystyrene, nylon, lexan, Perspex, delrin.

Slice width: a single or multiple ramp system of known angle(s) to the

slice plane and the means to accurately align the section
correctly with respect to the slice plane.

High contrast resolution: a section containing a series of holes of

varying diameters, line-pair insert(s), or a bead, wire or
edge and the means to accurately align the section correctly
with respect to the slice plane.

Ø Digital kV meter, voltage divider with measuring display and oscilloscope,

non- invasive beam analyser with probe suitable for the CT scanner beam
or other approved method of CT scanner kV determination.
Ø Perspex2 body phantom for CTDI measurement
Ø Small volume (< 0.6 cm3 ) ion chamber with a sensitive region ≤ 6 mm in
length. The reduction in sensitive volume to less than 6 mm in length may
be achieved through the use of a metal (lead filled) occluding cap. The
occluded ion chamber must be calibrated for the electrometer used and the
energy of the x-ray beam to be measured.

1
All radiation measuring instruments used as part of a test must be of a suitable type and calibrated by an
organisation recognised by the Radiological Council at not more than two yearly intervals.
2
Perspex - polymethylmethacrylate (PMMA) density 1.19 g.cm-3

8  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

Ø 10 or 14 cm pencil ion chamber and electrometer

Ø Retort stands
Ø Short length of wire or solder
Ø Spirit level
Ø Tape measure
Ø Test report form (see Appendix 3)

Invasive measurements using appropriately calibrated equipment and test

methods are also acceptable. (See Program Requirements, section 1.8 for
accuracy specifications).

Non-invasive beam analysers may be useful for some measurements.

 Radiological Council of Western Australia [3rd edition] 9

Computed Tomographic Equipment Workbook 6

1.3 DEFINITIONS

Tube Voltage

For the purpose of this workbook the term kVp (kilovoltage peak) has been
adopted although for true constant potential units the term kV should be used.
Tube voltage measurements will normally be specified as kVp average, being
the average of the voltage peaks measured in the sampling period.

See Appendix 1 for a discussion of kVp measurement errors.

Radiation Units

Exposure (C kg-1 )

A measure of the ionisation produced in air by x and γ radiations. The

quantity 'exposure' has limited use in relation to the biological effect of
radiation.

Absorbed Dose (Gy)

The energy absorbed per unit mass in a nominated medium. Because the
amount of energy deposited from a beam of radiation will depend on the
medium irradiated, the nature of that medium should also be stated. The
gray (Gy), the unit of absorbed dose, is equal to the deposition of energy of
1 joule in a mass of 1 kg of the nominated medium.

Equivalent Dose (Sv)

This quantity takes into consideration the type of radiation being measured.
The unit of equivalent dose is the sievert (Sv).

Equivalent Dose ( Sv ) = ∑ ( Absorbed Dose × WR )

where WR is the Radiation Weighting Factor and Absorbed Dose refers to the
average dose over a tissue or organ.

WR relates to the biological effects that result from exposure to different types
of radiation. For x-rays, WR = 1 and therefore an absorbed dose of 1 gray (Gy)
of x-rays gives an equivalent dose of 1 sievert (Sv).

Effective Dose (Sv)

Effective dose is perhaps the most meaningful quantity to be used in radiation

protection because it relates the equivalent dose to certain tissues or organs to
a whole body dose of radiation.

10  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

Effective Dose = ∑ (Equivalent Dose ×WΤ )

where WT is the Tissue Weighting Factor.

SI and non-SI Units

The following table compares SI and non-SI units.

Quantity SI unit Old unit Conversion

Exposure C kg-1 roentgen (R) 1 C kg-1 = 3876 R
Absorbed Dose gray (Gy) rad (rad) 1 Gy = 100 rad
Equivalent Dose sievert (Sv) rem (rem) 1 Sv = 100 rem
Effective Dose sievert (Sv) rem (rem) 1 Sv = 100 rem

For this workbook, all radiation output measurements are to be recorded as

absorbed dose in air. The unit for absorbed dose is the gray (Gy).

Commonly used sub- multiples are —

milligray (mGy) = 10-3 Gy

microgray (µGy) = 10-6 Gy

For radiation detectors using non-SI units, an exposure of 1 R = 8.73 mGy in

air (1 mR = 8.73 µGy in air) for x-rays.

Optical Density of X-ray film

Optical density (OD) refers to the gross optical density (blackening) on x-ray
(or photographic) film.

OD = log 10 ( I 0 I t )

where I0 is the intensity of light without the film and It is the intensity of light
transmitted through the film. For example, a density of 1 means that only
10% of the light has been transmitted through the film; a density of 2 means
1% transmission, etc.

Abbreviations

The following abbreviations may be used in this workbook:-

SID source (focal spot) to image (image receptor, film) distance,

SSD source (focal spot) to skin distance,
SDD source (focal spot) to (radiation) detector distance.

 Radiological Council of Western Australia [3rd edition] 11

Computed Tomographic Equipment Workbook 6

1.4 PRECAUTIONS

Permissions

Before performing any of the tests in this workbook, ensure that approvals to
operate the x-ray equipment have been obtained from —

Ø the Radiological Council, which requires a licence to be held.

Ø the equipment registrant or an authorised delegated person, for example,

the Radiation Safety Officer.

Equipment Ratings and Tube Voltage Range

Particular care is required to ensure that neither the instantaneous nor

continuous ratings of the x-ray tube are exceeded. The manufacturer's
specifications need to be strictly followed in this regard.

Tube warm-up is under computer control and should present no problem for
the tester. However, should an alarm, error code (or message), equipment shut
down or an abnormal waveform occur, the tester should:-

Ø abort the compliance test at the kVp concerned (unless technically

qualified to conduct further tests)
Ø notify the responsible person that an error has occurred.

Calibration of scanner

Before any scanner image parameter tests are performed at least an air, and
preferably an air/water, calibration should be performed on the scanner to
correct for any drift which may have occurred in CT numbers. The scanner
may need to be re-calibrated should the test sequence be interrupted for any
significant time.

Beam compensating filters (“bowtie” etc)

All image parameter tests and dose measurements must be conducted in the
routine operating mode with any beam compensation filter in position.

Use of non-invasive x-ray beam analysing equipment

It is important to position the detector according to the manufacturer's

recommended orientation and distance from the x-ray tube focal spot. Ensure
that the x-ray beam covers the area of the detector recommended by the
manufacturer.

Errors in measurement may result if these steps are not followed correctly.

12  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

Radiation Protection

Persons operating x-ray equipment must protect themselves from unnecessary

radiation exposure and must not expose themselves to the direct x-ray beam.

Unless exempted, CT equipment must be operated from behind a fixed

protective barrier. When this cannot be used, other protective measures must
be adopted including maximising distance (> 2 m); wearing lead protective
clothing (≥ 0.25 mm Pb equivalence); and/or the use of other structural
shielding such as brick walls.

Compliance testers may be required to wear an approved personal monitoring

device to record their occupational radiation exposure.

Recording Compliance Test Results

All measurements must be recorded using the standard reporting form. A

copy of a test report form is provided in Appendix 3.

Faults found during testing should be included on the test report, even if such
faults are corrected before the completion of testing. Some faults may be
common to a particular model of x -ray equipment and failure to report them
may put other users and patients at risk.

 Radiological Council of Western Australia [3rd edition] 13

Computed Tomographic Equipment Workbook 6

2. GENERATOR, X-RAY TUBE AND SCANNER

2.1 CHECK LIST

Record on the test report form the following —

A X-ray Control

Ø manufacturer
Ø model
Ø serial number
Ø generator type (three phase, medium frequency, etc.)
Ø generator ratings (maximum kVp, mA or mAs)
Ø exposure warning devices

B X-ray Tube Housing

Ø manufacturer
Ø model
Ø serial number
Ø inherent filtration (mm Al at a nominal kVp)
Ø added filtration
Ø filtration non-removable (without tools)
Ø marking of focal spot position

C X-ray Tube Insert

Ø manufacturer
Ø model
Ø serial number
Ø maximum rated kV
Ø focal spot sizes

D CT Scanner
Ø scanner movements (rotate/rotate, rotate only, spiral, other)
Ø reconstruction matrix sizes available
Ø type of detectors, solid or gas (name of material if available)

Assessment and Evaluation

Most of this information should be displayed by either permanent markings or

labels on either the x -ray control or the x -ray tube housing. If the data are not
easily accessible, they may be available in the equipment manuals or from the
manufacturer or manufacturer’s agent.

Unless specifically exempted, the exposure switch must be arranged so that it

cannot be operated outside the shielded area.

14  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

2.2 TUBE VOLTAGE ACCURACY

Purpose of Test

To determine the accuracy of the tube voltage.

Equipment Required

Ø Digital kV meter, non- invasive beam analyser, voltage divider with

measuring display and oscilloscope or other approved method
Ø Tape measure

Method

1. Ensure that any necessary x-ray tube warm up procedure has been
followed (section 1.4).

2. Position the detector according to the relevant requirements noted in

section 1.4.

3. Obtain a radiation waveform at the most commonly used kVp, typically

120 kVp, to demonstrate any unusual characteristics which may affect
subsequent measurements.

4. Obtain a kVp accuracy measurement at each kVp available to the

operator, i.e. excluding values which may be available only in service
mode.

Obtain radiation output measurements simultaneously with the kVp

measurements at each kVp step.

Assessment and Evaluation

Care must be taken to ensure that the tube voltage obtained is the average of
the peak voltage throughout the sampling period (kVp average). Some
instruments provide different high voltage parameters such as kVp maximum,
kV effective, etc.

The tube voltage must satisfy the following accuracy requirements within
100 ms of initiating the exposure.

Inclusive of measuring instrument errors, the tube voltage fails compliance if

the measurement obtained differs from the set or nominal tube voltage by
> ± 6.0% for voltages less than or equal to 100 kVp or > ± 6.0 kVp for
voltages greater than 100 kVp. (See Appendix 1 for further information).

 Radiological Council of Western Australia [3rd edition] 15

Computed Tomographic Equipment Workbook 6

The radiation waveform is used to determine that the measurement has not
been affected by any unusual waveform characteristics. These may include
voltage spikes, inconsistent output, rising radiation waveform, etc.

The radiation output over a range of tube voltages may be used to compare
outputs from different tubes and generators within a department. For fixed
mA and time or fixed mAs exposures, a graph of the logarithm of the kVp
versus the logarithm of the output should be a straight line between, typically,
50 kVp and 100 kVp, with a correlation coefficient of r2 ≥ 0.99. The gradients
of such graphs have been found to vary between 2.0 and 2.8. A significant
deviation of the measured points from linearity on such a plot may indicate a
generator problem.

16  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

2.3 RADIATION OUTPUT AND OUTPUT LINEARITY

Purposes of Test

To measure the free in air radiation dose at the centre of the CT gantry and to
measure the output linearity coefficient of the scanner.

Equipment Required

Ø Small volume ion chamber with an occluding cap so that the sensitive
region of the ion chamber is less than 6 mm in length1
Ø Suitably calibrated electrometer for the small volume ion chamber
Ø Retort stand and clamps
Ø Tape measure

Method
1. Using the retort stand, locate and clamp the ion chamber with the
occluding cap at the centre of the gantry (free in air).

2. Confirm with a tape measure and the positioning lights that the chamber
is centred within the gantry opening and that the centre of the chamber is
within the slice plane.

3. Set the ion chamber electrometer to integrate mode.

4. Move the scanner table 6 mm “out”.

5. Using an 8 mm or 10 mm slice width, expose the ion chamber to

120 kVp, 400 mAs (or the closest practicable value). Record the
electrometer reading.

6. Repeat step 5 with table increments of 2 mm so that the range from 6 mm

“out” to 6 mm “in” is covered. Record the maximum electrometer
reading.

7. Convert the maximum electrometer reading to absorbed dose in air using

the ion chamber calibration data. For the method of correction of partial
length exposures, see section 2.5. Express the result as absorbed dose
per 100 mAs.

8. Using the same kVp, scan time and slice thickness, repeat the slice dose
readings with other mA values to cover the available range. Convert the
results to absorbed dose as in step 7.
1
The unoccluded length of the chamber should not be at the end but preferably in the central region of the
chamber. This is to avoid significant inaccuracy being introduced through uncertainty in the end wall thickness
and end field distortion region when correcting the recorded result by the active length/unoccluded length ratio.

 Radiological Council of Western Australia [3rd edition] 17

Computed Tomographic Equipment Workbook 6

Assessment and Evaluation

The dose in mGy.100 mAs-1 at a specified kVp and slice width can be used for
patient dose calculations and for comparisons between scanners.

As a check on the validity of the results, radiation outputs in air at the centre
of the gantry at 120 kVp are typically in the region of 20 – 40 mGy.100 mAs-1 .

The radiation output in mGy.100 mAs-1 should be constant with mA over the
available tube current range or mAs over the available range of settings.

In the case of a series of calculated values of radiation output, the linearity

coefficient is given by —

-
linearity coefficient = X max X min
X max + X min

where Xmax and Xmin are respectively the maximum and minimum output
values. The numerical value of the linearity coefficient must be ≤ 0.1, but the
parameter range over which it is measured depends on the type of x-ray
generator. See Appendix 2 for further information.

18  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

2.4 HALF VALUE LAYER

Purpose of Test

To assess the x-ray beam quality (half value layer, HVL).

Equipment Required

Ø Aluminium filters (type 1100 > 99% purity)

Ø Retort stand and clamps
Ø Small volume ion chamber with an occluding cap so that the sensitive
region of the ion chamber is less than 6 mm in length
Ø Suitably calibrated electrometer for the small volume ion chamber
Ø Masking tape
Ø Tape measure

Method

1. Repeat steps 1 to 3 in section 2.3.

2. Select the scout 1 mode and 120 kVp tube voltage.

3. Confirm that movement of the table will not entangle the cable from the
ion chamber to the electrometer.

4. Perform a plain scan of the ion chamber from 6 mm ‘out’ to 6 mm ‘in’

and record the electrometer reading.

5. Add aluminium filtration between the x-ray tube and the ion chamber by
positioning the filters at the gantry aperture ensuring that the x-ray beam
is intercepted.

6. Repeat the scan of the ion chamber and record the reading.

7. Continue adding aluminium filters until the electrometer reading is

approximately 30% of the unattenuated reading.

Assessment and Evaluation

The Half Value Layer (HVL) is that thickness of a nominated material

required to reduce the x -ray beam output to one half of its unattenuated value.
The HVL can be determined by plotting the thickness of added filter against
the dose in air (or % transmission) using the integrated readings from the
series of scout scan measurements, or approximated from the following
formula:

1
also known as a scanogram, topogram or scan projection radiograph.

 Radiological Council of Western Australia [3rd edition] 19

Computed Tomographic Equipment Workbook 6

t b ln (2 D a D 0 ) − t a ln (2 D b D 0 )
HVL =
ln (D a D b )

where HVL is in the same units as t a , tb .

Dose reading without any filtration D0
For the two readings that bracket D0 /2:-
Dose reading that is greater than D0 /2 Da
Thickness of aluminium used for this reading ta
Dose reading that is less than D0 /2 Db
Thickness of aluminium used for this reading tb

The HVL measured will increase with an increase in beam filtration, tube
voltage and generator output pulse frequency.

While the HVL could be measured at any nominated kVp, 120 kVp is
recommended for consistency.

The minimum HVL requirements are –

Measured kVp W A regulations SAA Recommendation1

mm Al mm Al
90 2.5 2.7
100 2.7 3.0
110 3.0 3.4
120 3.2 3.8
130 3.5 4.2
140 3.8 4.6
150 4.1 extrapolate

CT scanner HVLs are typically in the range of 6 – 8 mm Al at 120 kVp.

1
AS/NZS 3200.2.44:2000 Medical electrical equipment
Part 2.44: Particular requirements for safety – X-ray equipment for computed tomography.

20  Radiological Council of Western Australia [3rd edition]

Workbook 6 Computed Tomographic Equipment

2.5 CT DOSE INDEX (CTDI)

Purpose of Test

To measure the CTDI from single CT slices at various locations within a

Perspex CT phantom.

Equipment Required

Ø Pencil type ion chamber and electrometer

Ø Perspex body phantom, typically 320 mm in diameter with 3 x 50 mm
thick sections of Perspex, each with holes of a diameter suitable for the
pencil ion chamber (Figure 1)

B
E
CT phantom C
A

Ion chamber

Electrometer

Figure 1 - CTDI measurement

Method

1. Locate the complete Perspex body phantom in the centre of the gantry.
Ensure that the centre hole of the phantom is congruent with the centre of
the gantry.

2. Position the phantom so that the slice plane passes through the centre of
the phantom.

3. Place the pencil ion chamber in the central hole of the body phantom,
shown as A in Figure 1.

4. Check that the slice plane will pass through the centre of the ion
chamber.

 Radiological Council of Western Australia [3rd edition] 21

Computed Tomographic Equipment Workbook 6

5. Set the electrometer to integrate with a suitable scale.

6. Using an 8 mm or 10 mm slice width, expose the ion chamber to

120 kVp, 400 mAs (or the closest practicable value). Record the
electrometer reading.

7. Multiply the electrometer reading by the active chamber length and

divide by the slice thickness (in the same unit of length). If necessary,
convert CTDI exposure data to absorbed dose in air. Calculate the
CTDI .100 mAs –1 by multiplying the result by 100 and dividing by the
mAs per slice.

Note: The software associated with some available detectors will

automatically perform these calculations.

8. Repeat the same reading. Calculate the mean value.

9. Repeat at the 4 locations near the edge and within the body phantom
which are shown as B, C, D and E in Figure 1.

10. Expose the ion chamber at the centre of the gantry without the Perspex
phantom in place using a slice thickness of 8 to 10 mm. This will be
referred to as the free-in-air CTDI for the nominated slice thickness.

Assessment and Evaluation

The CTDI of a single slice is the line integral of the total dose measured over
the length of the pencil ionisation chamber and can be expressed by the
equation —
DL
CTDI =
t
where D is the dose reading given by the electrometer, L is the active length of
the pencil chamber and t is the nominal slice thickness

The CTDI may be compared with the values quot ed in the manufacturer’s
literature. For comparative purposes the full test conditions must be stated
i.e., exposure factors used, the position of the chamber within the phantom and
the slice thickness.

For the purpose of this workbook, the “standard” CTDI is expressed as the
dose per 100 mAs with the chamber located in the centre of the phantom.

Examples of published CTDI100 values, in mGy.100 mAs-1 (for a 100 mm length

ion chamber), obtained for head and body scans and measured at the centre of
the phantoms (160 mm and 320 mm diameter Perspex respectively), are given
in Table 1.

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Workbook 6 Computed Tomographic Equipment

Slice width CTDI CTDI

CT Scanner kVp
(mm) head Body
GE HiSpeed CT/i 120 10 11.4 3.2
FX/I, LX/i 120 10 17.9 5.0
HiSpeed ZX/i, NX/i 120 10 14.8 4.4
QX/i, LightSpeed and
120 10 21.0 6.2
LightSpeed Plus
LightSpeed Ultra 120 10 22.5 7.0
LightSpeed 16 120 10 21.1 7.2
ProSpeed 120 10 20.7 5.7
Philips AV, LX SR7000 120 10 13.6 4.3
SR5000 120 10 11.8 3.6
Aura 120 10 21.8 7.2
Philips/Marconi Mx8000 120 10 14.7 4.7
Picker PQ series 120 10 14.0 4.8
Ultra Z 120 10 14.3 4.9
Shimadzu SCT 120 10 12.0 4.0
Siemens Somatom Plus 4 120 10 13.6 4.4
Balance, Emotion 130 10 22.5 7.3
Emotion Duo 130 10 20.7 6.2
Sensation 4 120 10 19.0 5.1
Sensation 16 120 10 15.4 4.2
Toshiba Xpress HS 120 10 12.9 4.7
Aquilion Multi/4 120 8 21.5 7.1
Aquilion 16 120 8 (4 x 2) 24.5 8.0
Asteion Multi 120 8 25.6 7.5
Asteion Dual 120 10 26.0 8.1

Table 1 - Examples of CTDI values1

The free in air CTDI may be used in patient dose assessments, such as the CT
dose software package from the UK National Radiological Protection Board
NRPB SR250.

The free-in-air radiation output at the centre of the gantry, as measured in

section 2.3, and the free-in-air CTDI should be in close agreement. If they are
not, this may indicate errors in the electrometer / ion chamber calibration or
measurement technique.

1
ImPACT National CT Scanner Dose Survey, 2000-2004. http://www.impactscan.org/dosesurvey.htm

 Radiological Council of Western Australia [3rd edition] 23

Computed Tomographic Equipment Workbook 6

2.6 TUBE HOUSING LEAKAGE

Purpose of Test

To determine any areas of radiation leakage through the x-ray tube housing
and collimation, and to measure this leakage. Alternative methods are
described:-

Ø The preferred general method (A) describes the use of x-ray film or other
suitable image receptors to identify any areas of untoward leakage
radiation so that those discrete areas can be measured using an ion
chamber rate meter or an integrating dosemeter as may be appropriate to
the x-ray equipment under test.

Ø The alternative general method (B) should be applied if method A cannot

be used. It describes the procedure for undertaking a number of
dosemeter measurements around the x-ray tube assembly in accessible
directions. This must be undertaken with care to prevent heat damage to
the x-ray tube. The time required for this method also may be
significantly longer than for method A.

Note: Once this test has been performed, subsequent testing should only
be necessary if there has been service or maintenance requiring
disassembly of the x -ray tube housing.

Caution: This test will require the assistance of licensed service personnel to
gain access to the x -ray tube housing. The x -ray tube must also be
stationary during the test.

Warning: Before using high voltage for this test, consult section 1.4,
Equipment Rating and Tube Voltage Range.

Equipment Required

Ø Ionisation chamber with detector area not exceeding 100 cm2

Ø Loaded 35 cm x 43 cm cassette or envelope-wrapped x-ray film
Ø Tape measure
Ø Masking tape
Ø Lead section 3 mm thick with an area large enough to fully intercept the
x-ray beam at the x-ray tube’s primary collimator

Methods

A – Image receptor method

1. Set the scanner exposure parameters for a scout scan that provides the
highest tube voltage and the lowest tube current. Record these values.

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2. Open the covers of the scanner gantry and fix the 3 mm lead section
across the collimator of the x-ray tube so that the primary x-ray beam
will be fully intercepted.

3. Support or fix the cassette or envelope-wrapped film in accessible areas

on or close to the tube housing and, with the tube stationary, make an
exposure so as to achieve an optical density of approximately 1.0 on the
developed film.

4. Develop the film and visually check for any areas of significant 1
radiation leakage.

Note: Depending on the gantry design, it may only be practical to

check one side of the tube housing.

5. Measure any areas of significant leakage using either a dose rate or an

integrated dose method. If measuring dose rate, an exposure time greater
than the response time of the ion chamber is required.

B - Dosemeter method

1. Set the scanner exposure parameters for a scout scan that provides the
highest tube voltage and the lowest tube current. Record these values.

2. Open the covers of the scanner gantry and fix the 3 mm lead section
across the collimator of the x-ray tube so that the primary x-ray beam
will be fully intercepted.

3. Set the ionisation chamber, in integrate mode, at 100 cm from the tube
focal spot in an accessible direction. With the tube stationary, make an
exposure to give a suitable detector reading. Repeat the exposure with
the dosemeter in other accessible directions around the tube housing.

Caution: This procedure may require a number of exposures. Observe

the manufacturer’s duty cycle and ensure that neither the
instantaneous nor continuous rating of the x -ray tube is
exceeded.

4. Select the maximum value of dose obtained and use it to calculate the
leakage radiation value.

1
"Significant" is difficult to define and depends to some extent on judgement and experience. Any areas of
increased film density could be considered for measurement.

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Computed Tomographic Equipment Workbook 6

Assessment and Evaluation

The maximum permitted leakage radiation is 1 mGy in 1 h at 1 m with the

x-ray tube operating at the maximum rated voltage and the maximum rated
continuous current. The manufacturer’s tube loading specifications will need
to be consulted so that measurements can be corrected to meet the one-hour
continuous rating requirement (generally restricted by the maximum cooling
rate of the tube housing).

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3. IMAGE QUALITY
3.1 INTRODUCTION
Mean CT number, uniformity and noise are important measures of the
imaging performance of a CT scanner. The owner should ensure that these
parameters are tested at regular intervals not exceeding one month.

A water filled phantom is used to perform these measurements.

A region of interest (ROI) is used to determine the CT number statistics. The

mean of the central ROI is the mean CT number for the phantom. Uniformity
of the image is determined by comparing the mean for a ROI at two-thirds of
the radius from the centre of the phantom with the mean of the central ROI.

To facilitate positioning and to secure the phantom, a suitable supporting

cradle should be available. It should be constructed of a uniform and
relatively radio-translucent material.

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3.2 MEAN CT NUMBER AND UNIFORMITY OF IMAGE

Purpose of test

To measure the mean CT number in water and the uniformity of CT numbers

and noise in the image

Equipment Required

Ø CT water filled phantom

Method

1. Position and secure the CT water phantom in the centre of the gantry.

2. Scan the phantom through the central region using exposure factors of
120 kVp, 300 mAs, 8 mm slice width for both head and body sized
phantoms. If these values of kVp, mAs and slice width are not available,
use the nearest practicable values. Record these values.

A standard convolution filter for head CT must be used. i.e. no

smoothing or edge enhancing filter is to be used.

3. Select regions of interest (ROIs) at the centre of the phantom image and
at least four other sites approximately two-thirds of the phantom’s radius
towards the phantom’s edge. The ROIs should be circles of 20 mm
diameter.

For each ROI record —

♦ mean CT numbers for water at the central and outer ROI positions;

♦ the standard deviations (noise) of the mean CT numbers at these

positions

4. Print a hard copy of the image(s), recording the scan factors and other
data.

Assessment and Evaluation

The mean CT number of water in the central ROI should not deviate from zero
by more than ± 4 CT numbers.

The uniformity of the CT numbers in the image should be such that the mean
CT number of the central ROI is within ± 2 CT numbers of the mean CT
number of each outer ROI.

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To make allowance for varying scan factors being used for the test, standard
scan factors of 120 kVp, 300 mAs, 8 mm slice width may be used to assess
values of noise variations between the different ROIs. To convert the noise
value, σm, of the measured set (m) of scan parameters to the estimated noise
value σs for the standard set, use the formula —

kVm mAsm × slice widthm

σ s = σm
120 300 × 8

The noise values of the outer ROI’s should not deviate by more than 2 CT at
the standard factors i.e.-
maximum noise – minimum noise ≤ 2 CT.

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Computed Tomographic Equipment Workbook 6

3.3 LINEARITY OF RESPONSE

Purpose of Test

To assess the linearity of CT numbers with the electron densities of suitable

materials.

Equipment Required

Ø CT phantom linearity section containing samples of air, water, teflon and

at least two other materials from those listed in the table of electron
densities in Table 2

Method

1. Position and secure the CT phantom so that a slice will pass through the
central area of the linearity insert.

2. Using factors of 120 kVp, 300 mAs and 8 mm slice width, or as near
these factors as practicable, scan the phantom through the central region.

3. Select a circular region of interest of 20 mm diameter and record the

mean value of the CT numbers at the centre of all insert samples. If the
phantom is of the type in which the insert samples are embedded in a
uniform nylon matrix, the nylon CT number can also be found from a
ROI at the same distance from the phantom centre as the other insert
samples.

4. Print a hard copy of the image(s) recording the scan factors and other
data.

Assessment and Evaluation

The CT numbers of materials in the image of a CT scanner are expected to

have a linear relationship with their respective linear attenuation coefficients.
For ease of use, the linear attenuation coefficient of the material at the
effective beam energy is replaced in this workbook by the slightly less rigorous
electron density per unit volume. The electron densities to be used in assessing
the data are given in Table 2.

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Material Density Electron density

(g.cm-3 ) (electrons.m-3 x 1026 )
Air 0.00129 4
LDPE 0.925 3180
Polyethylene 0.942 3235
Water 1.000 3345
Polystyrene 1.052 3405
Nylon 1.145 3775
Lexan 1.202 3815
Perspex 1.190 3865
Delrin 1.420 4560
Teflon 2.151 6220

Table 2 - Electron densities of suitable materials for linearity test

Calculate the correlation coefficient of the regression line of CT number

(y axis) on electron density (x axis). This can be done by e.g. using the
statistical function package in Microsoft Excel. The correlation coefficient
should be greater than 0.99.

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3.4 HIGH CONTRAST RESOLUTION

Purpose of test

To assess the high contrast resolution capability of the scanner.

Equipment Required

Ø CT head-sized performance phantom containing a set of high contrast

holes of varying diameters, or a line-pair insert or a bead/wire for
modulation transfer function (MTF) assessment

Method

1. If necessary, re-calibrate the scanner.

2. Set up and align the phantom according to the manufacturer’s

instructions. It is important that the alignment is accurate and that the
critical part of the insert is correctly positioned with respect to the slice
plane.

3. Scan the phantom and examine the image using the values of the
parameters shown in Table 3.

Parameter Recommended value

Phantom diameter ≈200 mm
Scan factors
Scan field diameter (mm) All scanners: ≈250 (head)
Focal spot size minimum
Number of projections maximum
Slice width (mm) 1-3
mAs 700 - max
kVp 125 ± 5
Display parameters
Standard head: no smoothing or
Reconstruction filter
enhancement
Zoom factor 5 - max
Window width ≥ 1000
(line-pair or hole insert)

Table 3 - Scan parameters for high contrast resolution test

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4. If the scanner is of a hybrid type which has both translate-rotate and

rotate-rotate movements, perform the test in both modes.

5. Print hard copies of all images taken, recording the scan factors and other
data. Record the values of any scan factor used which does not appear
on the films.

Assessment and Evaluation

For hole or line-pair test inserts use a wide window to visually assess the
scanner resolution.

If the test insert consists of rows of holes, record the diameter of the holes
from the row in which the smallest holes are completely resolved from each
other.

If a line-pair test object is used, record the highest frequency which can be
completely resolved in line pairs.mm-1 .

In the case of MTF calculations, if the test insert is, for example, a bead,
ensure that the row of pixels from which the bead profile is taken runs through
the centre of the image in either, or both of, the horizontal and vertical
directions. The profile must also run fully across the bead from one side to the
other so that the CT numbers start and end in those of the background
material. Avoid including any noise spikes present in the background as these
will influence the high frequency end of the MTF calculated from the bead
point spread function.

Take the MTF cut -off value for assessing resolution as 0.02 unless the MTF
curve is approaching the spatial frequency axis extremely slowly in this
region, in which case use 0.05.

Acceptable values of high contrast resolution for three different reconstruction

matrix sizes are given in Table 4.

Reconstruction MTF cut-off lp.mm-1 hole diameter

matrix (mm-1 ) (mm)

256 ≥ 0.5 ≥ 0.5 ≤ 1.0

512 ≥ 1.0 ≥ 1.0 ≤ 0.5
1024 ≥ 2.0 ≥ 2.0 ≤ 0.3

Table 4 - High contrast resolution values for various matrix sizes

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3.5 SLICE THICKNESS

Purpose of test

To compare the nominal slice width with the full width half maximum
(FWHM) value of the slice sensitivity profile for axial, helical and multiple
slice scans.

Equipment required

Ø CT phantom section containing a single- or multiple-ramp insert

Method

Axial scanner

1. If necessary, re-calibrate the scanner.

2. Position the 200 mm diameter phantom according to the manufacturer’s

instructions so that the ramp(s) pass through the slice plane at the correct
angle. Particular attention must be paid to ensuring that the phantom is
correctly levelled and aligned so that the ramp angle is as specified (see
Assessment and Evaluation).

3. Select a slice width and scan the phantom using factors of 120 kVp,
300 mAs, or as close as practicable, and the number of projections, scan
field diameter and reconstruction filter normally used for a head scan.

4. Analyse the image of the ramp using the profile software of the scanner
and determine the FWHM value of the profile.

If suitable software is not available, a manual method of finding the

FWHM requires setting a narrow ROI across the ramp image and
identifying the pixel columns on either side which have CT numbers one-
half the maximum value.

The image width can then found by setting the cursor on each column
and using the scanner software to give the distance interval, or by
multiplying the pixel size by the number of pixels between these two
columns. The result must be multiplied by the tangent of the ramp angle
with the scanner plane to give the slice thickness.
e.g. for 45° the tangent = 1; for 26.6° the tangent = 0.5

5. Print a hard copy of the image containing the profile data obtained.

6. Repeat the process for all other slice widths.

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Helical scanner

Perform steps 1 - 6 above as for the axial scan method with zero table
movement.

Multiple slice scanner

1. Perform steps 1 – 3 as above for the axial scanner mode.

2. Analyse the image of the ramp for each of the multiple slice profiles
obtained and determine the FWHM value of at least one profile at the
centre of the beam and one at the edge.

3. Print a hard copy of the image containing the profile data obtained.

4. Repeat the process for all other slice width/profile settings.

Assessment and Evaluation

The slice width is the FWHM value of the slice sensitivity profile. The service
contract for the scanner should involve collimator tests to ensure that only the
minimum volume of tissue necessary is exposed to x -rays in obtaining the
image.

Ramp inserts must be set up at the correct angle to the slice plane and also not
rotated with respect to the horizontal plane. The double opposed ramp
technique allows self cancellation of small angular errors in alignment of the
test insert with respect to the slice plane, but care should always be taken to
achieve accuracy in set -up to within at least 5°. Single-ramp techniques,
however, have no means of correcting for an introduced error in set -up angle.
If, for example, a ramp using a factor of 0.5 is misaligned by ± 3°, errors in
calculated slice width of –12% and +15% respectively will result. For a ramp
of factor 1, the respective errors will be -10% and +11%.

The calculated slice width should be within ± 0.5 mm of the nominal value for
all slice widths. If this criterion cannot be met but the result is within the
manufacturer’s specifications, include a copy of the relevant specification item
together with the completed report form.

For multiple slice scanners, differences in profile FWHM values may indicate
misalignment in the target-collimator-detector system.

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Computed Tomographic Equipment Workbook 6

3.6 TABLE INDEXING AND REPRODUCIBILITY

Purpose of Test

To assess the accuracy of table indexing and reproducibility of positioning.

Equipment Required

Ø CT phantom section containing the slice width ramp insert(s)

Ø Patient-equivalent weight
Ø Ruler and pointer

Method

1. Position the CT phantom so that a slice will pass through the central area
of the slice thickness insert.

2. Scan the phantom through the central region using 120 kVp, 300 mAs, or
as near as practicable, and the minimum slice thickness.

3. From the CT control console move the phantom into the scanner the
width of the slice thickness selected.

4. Repeat the scan using the previous factors.

5. If the software is available, digitally subtract or superimpose the images.

If the software is not available, set the cursor on the corresponding slice
image edges in the separate images and determine the separation from
the pixel column numbers and pixel size.

The table movement is found by multiplying the measured image

separation by the tangent of the ramp angle to the slice plane e.g. for 45°
the tangent = 1; for 26.6° the tangent = 0.5

6. Repeat the table movement four more times and find the average value of
the five table movements.

7. Remove the CT phantom and load the table with the simulated patient.
Fix the ruler to the table support and the marker to the table. Set the
marker to the required zero position and note the position indicator
reading.

8. Move the table, either manually or under computer control, to its extreme
position in one direction and return it to its zero point. Note any
difference in the position indicator reading. Drive the table to its extreme
position in the opposite direction and note any position discrepancy.

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9. Repeat the procedure at least three times and find the mean of the values
of position discrepancy.

Assessment and Evaluation

Accurate table indexing is important to ensure that adjacent CT slices do not

overlap at the nominal full width half maximum thicknesses specified and thus
no unnecessary x-ray exposure of patients occurs. The table movement should
be within 0.5 mm of that selected.

There should also be no gaps between the images if the table is indexing
properly.

Position reproducibility should be achieved to ± 1 mm.

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Computed Tomographic Equipment Workbook 6

3.7 ALIGNMENT LIGHT AND IMAGE SLICE CONGRUENCE

Purpose of Test

To assess the accuracy of the slice positioning light with respect to the x-ray
beam.

Equipment Required

Ø CT phantom
Ø Short lengths of ≤ 1 mm diameter wire or solder
Ø Adhesive tape

Method

1. Position the CT phantom at the centre of the gantry and turn on the
positioning light.

2. Tape three short lengths of wire or solder to the phantom surface as

markers in the line of the light beam, one on top and two at the sides of
the phantom.

3. Scan the phantom with a single slice using the minimum slice width
available.

4. Examine the image and make a hard copy.

Assessment and Evaluation

Accurate light and slice alignment is important to ensure correct positioning,

particularly for example for biopsy set -up.

The alignment is acceptable if the full lengths of the marker segments are
visible with high contrast in the image. If the markers are not seen at all in
the image, the alignment is not acceptable.

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RELATED PUBLICATIONS
1. Radiation Safety Act 1975 (Western Australia), and Amendments
Radiation Safety (General) Regulations 1983 (Western Australia), and Amendments,
Schedule IX

2. Measurement of the Performance Characteristics of Diagnostic X-ray Systems Used

in Medicine. Part III. The Physical Specification of Computed Tomography X-ray
Scanners: Measurements and Use of the Associated Performance Parameters. The
Hospital Physicists' Association.

3. Jones D G and Shrimpton P C. Survey of CT practice in the UK Part 3: Normalised

organ doses calculated using Monte Carlo techniques. NRPB, Oxon, 1991. NRPB -
R250.

4. AAPM 1977 Phantoms for performance evaluation and quality assurance of CT

scanners. American Association of Physics in Medicine Report Number 1

5. Standards Australia publications relevant to diagnostic x-ray equipment, including —

AS/NZS 3200.2.7:1999 Medical electrical equipment.

Part 2.7: Particular requirements for safety – High voltage generators of diagnostic
X-ray generators.

AS/NZS 3200.1.3:1996 Medical electrical equipment.

Part 1.3: General requirements for safety — Collateral Standard. Requirements for
radiation protection in diagnostic X-ray equipment.

AS/NZS 4184.2.6:1995 Evaluation and routine testing in medical imaging

departments.
Part 2.6:Constancy tests – X-ray equipment for computed tomography

AS/NZS 3200.2.44:2000 Medical electrical equipment

Part 2.44: Particular requirements for safety – X-ray equipment for computed
tomography.

6. Seibert J A, Barnes G T and Gould R G. Specification, Acceptance Testing and

Quality Control of Diagnostic X-ray Imaging Equipment. American Association of
Physics in Medicine Monograph 20, 1994. Published by American Institute of
Physics, Woodbury,New York.

7. Poletti J. Radiation Dose and Image Qua lity of CT Scanners: Summary of NRL
Surveys 1980 - 1987. Report NRL 1988/1, 1988. National Radiation Laboratory,
Christchurch, New Zealand

8. 1990 Recommendations of the International Commission on Radiological Protection

ICRP Publication 60

 Radiological Council of Western Australia [3rd edition] 39

Computed Tomographic Equipment Workbook 6

9. Recommendations for limiting exposure to ionizing radiation (1995)

Series No 39
National Health and Medical Research Council

10. Polacin A & Kalender W, Measurement of slice sensitivity profiles in spiral CT

Medical Physics 21, 1994, 133 - 140

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APPENDIX 1

ASSESSMENT OF MEASUREMENT ERRORS

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Measurement Errors

Two general cases of error are recognised; namely the error in the measurement of a quantity
such as dose rate and the combination of the required limit on error in kVp with the error of
the measuring instrument. It has been acknowledged in the workbook that allowance needs to
be made for the error in measuring instruments under defined conditions.

Error in the measurement of a quantity

This is a straightforward situation in which a measured quantity, such as the maximum

fluoroscopic dose rate, will carry the inherent error in the measurement instrument. No
allowance for instrument errors in the measurement of such quantities is incorporated in the
relevant limits required by the workbook. If the measured value marginally exceeds the
required limit, some latitude is possible for the judgment of the qualified expert to be
exercised as to whether the value is accepted as complying or not in view of the instrument
error. However, it needs to be borne in mind that the rigorously applied requirement is that
the limit must not be exceeded.

Combination of errors in assessing kilovoltage accuracy

This is a case in which the required limit is given as an error in a quantity rather than the
quantity itself. In this case the following approach has been taken.

The required kVp accuracy stated in the first edition of the workbook is the lower of ± 5.0%
or ± 5 kVp. This requirement still stands but an allowance for the error introduced into the
measurement by the measuring instrument used has now been incorporated into the
assessment by using the root mean square method of combining independent errors of x and y
to give a total probable measured error, where

combined measurement error = (x 2

+ y2 )
Replacing x with the requirement of 5.0% and stipulating a maximum acceptable measuring
instrument error, y, of 3.0% leads to a combined measured error of 5.8%. As this value is
intermediate between 5.5% and 6.0% it is used as a basis for introducing three assessment
bands for the measured error —-

< ± 5.5%,
≥ ± 5.5% and ≤ ± 6.0%,
> ± 6.0%.

These are applied in the following way —

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Computed Tomographic Equipment Workbook 6

1. Assessment of error at less than or equal to 100 kVp

• the kVp passes if the measured error is less than ± 5.5%

• the kVp is borderline and attention may be required to the unit if the measured
error is equal to or greater than ± 5.5% and less than or equal to ± 6.0%. In this
case the qualified expert must exercise judgment on whether or not the unit meets
compliance requirements (e.g. in considering the number of kVp settings
involved).

• the kVp fails if the measured error is greater than ± 6.0%.

2. Assessment of error at greater than 100 kVp

At tube voltages in excess of 100 kVp, the basic requirement remains at ± 5.0 kVp.
For the same assumed measurement instrument error of 3.0% the three assessment
bands are then taken as

• the kVp passes if the measured error is less than ± 5.5 kVp

• the kVp is borderline and attention may be required to the unit if the measured
error is equal to or greater than ± 5.5 kVp and less than or equal to ± 6.0 kVp. In
this case the qualified expert must exercise judgment on whether or not the unit
meets compliance requirements (e.g. in considering the number of kVp settings
involved).

• the kVp fails if the measured error is greater than ± 6.0 kVp.

Summary

The tube voltage compliance status of the x-ray equipment —

Passes — is borderline a — Fails —

if the difference between the set and measured kVp is —
≥ ± 5.5% and
for =100 kVp set < ± 5.5% ≤ ± 6.0% > ± 6.0%

≥ ± 5.5 kVp and

for >100 kVp set < ± 5.5 kVp ≤ ± 6.0 kVp > ± 6.0 kVp
a
Qualified expert (see Program Requirements, section 3) is to consider compliance in relation to other factors.

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APPENDIX 2

OUTPUT LINEARITY COEFFICIENT

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Linearity coefficient and errors in tube current and exposure time

The radiation output linearity coefficient is given as

X1 − X 2
LC =
X1 + X 2

where X1 and X2 are the radiation outputs per mAs at two different mA or time settings. The
coefficient is a function of either tube current or exposure time in machines in which these
can be selected and one maintained constant, and on the mAs in machines in which they
cannot. The linearity coefficient value is therefore a function of the errors in these quantities.
There are three cases to consider:-

a) For constant mA units, eg dental units, the linearity coefficient depends only on the
timer error, assuming the coefficient of variation of the current reproducibility is small
by comparison. The allowed timer error is subject to recommendation by Standards
Australia. It can be easily shown from the above equation that if the allowed error in
the timer is ± p%, the value of the linearity coefficient is p/100. This simple
relationship does not hold if the error is stated as a fixed parameter value e.g.
± 1 pulse. In the timer range where the requirement is ± 10%, this gives the generally
accepted value of 0.1 for the linearity coefficient.

For this type of machine, if the mA were truly constant, the linearity coefficient test
would be redundant in those cases where timer linearity is shown by direct
measurement of timer accuracy. There are some cases, however, where a direct
comparison with a known time setting may not be possible, eg in object programmed
dental units. The linearity test is necessary in this case and it is required for all others
because it can also give information on the constancy of the tube current across the
timer range.

b) For units in which the mA can be varied and the time held constant for the linearity
test, the coefficient is a function of the mA error. The use of 0.1 for the linearity
coefficient implies acceptance of a ± 10% error in mA, and practical results of tests
imply that errors less than this can be routinely achieved in variable current
radiographic units. The test can also be applied to fluoroscopic units by determining
the ratios of output dose rate to the set mA over a range of tube currents at a given
kVp.

c) The use of 0.1 for the coefficient implies a combined error in the mA and time in mAs
machines of ± 10%. Again, practical results of the linearity coefficient test indicate
that errors of less than ± 10% can be routinely achieved in these units.

Capacitor discharge equipment is excluded from linearity coefficient testing because

the kV and mA do not remain constant during the exposure.

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Variability of the linearity coefficient

In practice, the recommendations for timer accuracy can be used as a model to set the linearity
coefficient for all types of x-ray units. If an accuracy of ± 10% is used in all cases for the
higher range of the parameter, the linearity coefficient will be uniformly 0.1.

However, if a fixed value of the error parameter is recommended, that value becomes a
varying percentage of the parameter setting. This results in the linearity coefficient varying
with the setting. If variable linearity coefficients were tabulated in the relevant working
ranges for each of the foregoing three types of unit, the workbooks, and their application,
would become unnecessarily complicated. The use of the linearity coefficient is therefore
discontinued in these potentially diffic ult, low parameter range and patient dose regions.

Summary

The linearity coefficient test requirements are summarised as:-

X-ray unit Linearity Coefficient Comment

Fixed mA -
Half-wave rectified 0.1 at ≥ 200 ms No test < 200 ms
Full-wave rectified 0.1 at ≥ 100 ms No test < 100 ms
Other generators 0.1 at ≥ 100 ms No test < 100 ms
Variable mA –
tested at constant time in 0.1 At all available mA settings
radiographic mode
Variable mA –
tested as dose rate in 0.1 at ≥ 1 mA No test < 1 mA
fluoroscopic mode
mAs units – At all available mAs
0.1
(ex capacitor discharge) settings
Capacitor discharge units No test No test

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