Protocol
for the QA of Computed Radiography Systems
Commissioning
and Annual QA Tests
This document describes a series of tests to assess CR plate and reader
performance. The tests are intended to detect artefacts and monitor image
quality and sensitivity. The tests are split into the following categories,
- commissioning tests
- annual QA tests.
All the tests described should be performed on all available reader
systems.
KCARE have data from performing tests on all manufacturers CR systems
and are available for advice and information.
1 Commissioning Tests
List of equipment
·
Tape measure
·
Adhesive tape
·
1.0 mm Copper filtration (>10 x 10 cm)
·
1.5 mm Copper filtration
(>10 x 10 cm)- for Agfa only
·
0.5 mm Copper and 1mm Aluminium filtration (>10 x
10 cm) –for Kodak only
·
TO20 threshold contrast test object
·
Resolution test object (e.g. Huttner 18)
·
M1 geometry test object or lead ruler
·
Contact mesh
·
Ionisation chamber
·
Small lead or Copper block (~5 x 5 cm)
·
Steel ruler
In all tests described the unique plate identification code should be
recorded.
These tests should be performed using an x-ray unit that has recently
passed QC tests. In particular, the accuracy of the kVp selected for detector
dose indicator consistency and calibration should be tested.
It may be possible to undertake some of these tests on a review
workstation depending on the processing tools available (which will be
dependent on the manufacturer). However certain tests require the use of the
higher quality reporting workstation.
1.1 Dark Noise
Purpose: To assess the level of noise inherent in the
system
a) Erase an image plate and without making an exposure
read it using the following parameters
b) Examine the
images visually for uniformity and record the detector dose indicator value for
Agfa (SAL – at centre of plate) and Kodak (Exposure Index).
c) Record a mean
pixel value using region of interest analysis (for systems not offering ROI
analysis see appendix for details of how to measure a mean pixel value).
d) If possible
either archive or print the image for future reference.
Tolerance: For Agfa and Fuji
systems a uniform artefact free image should be expected. Kodak systems add a collector profile to the
image to compensate for non uniform collection efficiency across the place.
This results in series of bands appearing across the image. Agfa systems should
have an SAL < 100. For Fuji
the pixel value should be <280. For Kodak the EI value should be <80 for
GP plates and <380 for GP plates. For Konica a pixel value >3975 should
be expected.
1.2 Dosimetry
Purpose: To measure
receptor doses required for later tests 1.3,1.5, 1.6, 1.7 and 1.11
a)
Position an ion chamber at ~1.2 m from the focus
(see figure 1) and at least 30 cm above the table (record the actual
distances). Collimate to the ion chamber.
 |
b)
Expose the chamber at 70kVp with 1.0mm of copper in
filtration at the tube head. The mAs should, by ‘trial and error’, be set such
that the inverse square law corrected dose to the table level is approximately
10mGy
c)
Record the measured dose and repeat twice.
d)
Under the same beam conditions determine the mAs
required to deliver a receptor entrance air kerma of 1mGy, 4mGy 12mGy and 50mGy
e)
If a Fuji
system is being tested determine the mAs to deliver a receptor entrance air
kerma of 10mGy at 80kVp with
no filtration in the beam.
If a Kodak system
is being tested determine the mAs to deliver a receptor entrance air kerma of
10mGy at 80kVp with
1mm Al and 0.5mm Cu filtration at the tube head.
1.3 Linearity and System
transfer properties
Purpose: To establish the
relationship between receptor dose and pixel value so that this relationship
can be corrected for in tests 1.4 and 1.7. Also to establish that the indicated
exposure (calculated from the detector dose indicator) responds linearly to
increases in dose.
a) Place a 24 x 30 cm cassette on the table at
~1.50m (as described for test 1.2). Set the field to just cover the cassette.
Mark the corners of the cassette on the table with transpore, so that the
cassette can be easily repositioned.
b) Expose a plate at 70kVp with 1.0mm copper at
the tube head to deliver a dose of order 1mGy as measured in test 1.2.
c) After a minimal fixed time delay (e.g.1 to 5
mins), read the plate as described below.
d) Record the detector dose indicator value.
e) Record
a pixel value from the centre of the image.
§
For Agfa systems the SAL values obtained from ROI
analysis on the review workstation should be used.
§
For Fuji,
Konica and Kodak systems the images should be transferred to reporting
workstations to use ROI analysis tools if available.
f)
Repeat for doses of order 4mGy, 12mGy and 50mGy.
g) Plot a graph of pixel value versus receptor
dose using a graph plotting package (e.g. Microsoft excel). Obtain the equation
of the trend-line for this graph (i.e. the pixel value as a function of
receptor dose). This equation is the system transfer properties (STP) equation
and is used for making corrections in tests 1.4 and 1.7. An equation of the
form
dose = f (pixel value)
where
f is some arbitrary function is required.
Tolerance: For all images
the ratio, k, of indicated exposure to exposure should not differ by greater
than ±10% from the mean
k value. The trend-line plotted in excel should have an R2 fit value
>0.95. There is no tolerance for the STP equation. However the pixel value
to dose relationship should be a simple relationship (e.g. log, linear or
square root). For systems evaluated by KCARE the following has been found.
Manufacturer
|
STP Relationship
|
Agfa
|
Square root *
|
Fuji
|
Logarithmic
|
Kodak
|
Logarithmic
|
Konica
|
Logarithmic
|
* For the Agfa system there is a square root relationship between SAL
values and dose. The relationship between raw data pixel values and dose
however was logarithmic for systems evaluated by KCARE
1.4 Erasure cycle efficiency
Purpose: To test that minimal residual signal
(ghosting) remains on a plate after readout and erasure.
a) Position a plate
on the table at ~1.5 m. Set a 10 cm x 10 cm field and position a piece of
attenuating material (e.g. Copper or lead) at the centre of the CR plate.
Expose at 80kVp, 25mAs with no filtration.
b) Read the plate
(the readout parameters are not important).
c) Re-expose the
plate with a 9 cm x 9 cm field centred on the same point on the plate with no
attenuating material in place, using 80kVp, 0.5mAs and no filtration.
d) Read the plate
using the following parameters.
 |
e)
Set a very narrow window and adjust the level. Visually inspect the image for any remnant of the previous image (look for both the attenuating material and the position of the collimators). If a remnant is visible, use region of interest analysis to quantify the difference in pixel value between the ghosted and unghosted areas.
Set a very narrow window and adjust the level. Visually inspect the image for any remnant of the previous image (look for both the attenuating material and the position of the collimators). If a remnant is visible, use region of interest analysis to quantify the difference in pixel value between the ghosted and unghosted areas.
¨ For Agfa systems
the SAL values obtained from ROI analysis on the review workstation should be
used.
¨ For Fuji, Konica and Kodak
systems the images should be transferred to reporting workstations to use ROI
analysis tools if available.
The ROI values should be used to calculate
indicated receptor doses using the STP equation established in test 1.3.
Tolerance: If no evidence of ghosting is found from
visual inspection of the images then the test is passed and there is no need to
perform ROI analysis. There should be <1% (remedial) difference between the
STP corrected pixel values in the ghosted region and the surrounding areas. A
suspension level of <5% is set.
1.5 Detector dose indicator
calibration
Purpose: To assess the accuracy of the plate exposure
values calculated using exposure indicators.
a) Position
a 24´30 plate on the
table as described for test 1.3
 |
c) Read the plate out as described below
d) Record the detector dose indicator, and
calculate the indicated exposure using the equations given below.
e) Repeat twice and take a mean value of the indicated exposures.
Tolerance:
The
indicated exposure should agree with the measured exposure within 20%.
1.6 Detector dose indicator
consistency
Purpose: To assess the variation of sensitivity
between plates, and set a baseline for monitoring system sensitivity for future
QA testing
a)
Place a 24 x 30 cm CR cassette on the couch and set
up as described for test 1.2/1.3 (see figure 1) and with 1.0mm Cu filtration.
b)
Expose the plate at 70kVp to give a known dose of
~10 mGy. The dose to
the plate calculated from inverse square law corrected ion chamber measurements
should be recorded (see test 1.2)
c)
Read the plate as described for test 1.5.
d) Record the detector dose indicator, and calculate
the indicated exposure using equations 1-3. Repeat twice for the same plate.
e)
Calculate the indicated exposure using equations
given in test 1.5
f)
Repeat this test for all plates for acceptance
testing(making only one exposure to each plate). It is helpful at this point to
identify a plate that has a detector dose indicator in the middle of the range
for future QA.
Tolerance: The variation in
the calculated indicated exposures should not differ by greater than 20%
between plates. The measurements repeated on the same plate should be used to
lay down a baseline for future QA tests. Al images should be inspected for
gross artefacts
1.7 Uniformity
Purpose: To assess the uniformity of the recorded
signal from a uniformly exposed plate. A non-uniform response could affect
clinical image quality.
a) Expose a plate as
described for test 1.6 but with half the mAs.
b) Rotate the plate
through 180o about the vertical axis and re-expose using the same
parameters (this should largely cancel out the non uniformities due to the
anode heal affect).
c) Read the plate as
described for test 1.3.
d) Visually inspect
all images obtained in test 1.3, 1.5 and 1.6 for uniformity and artefacts.
Likely artefacts include dust on the plate or readout optics, and scratches on
plates.
e) The uniformity of
the image obtained in 1.7b should be assessed using region of interest analysis
(ROI ) if available, to measure the mean pixel values in positions a-e, as
indicated in figure 2 below (i.e.at the centre of the image, and at the centre
of each of the four quadrants of the image). The size of ROI should be of order
10000 pixels.
For Agfa systems
the SAL values obtained from ROI analysis on the review workstation should be
used.
For Fuji,
Konica and Kodak systems the images should be transferred
to reporting workstations to use ROI analysis
tools if available
 |
f) The five values obtained from ROI analysis should be used to calculate five indicated receptor dose values using the STP equation obtained in test 1.3
Tolerance: The images should
not have obvious artefacts. The ratio of the standard deviation of the 5 STP
corrected ROI values to their mean (the coefficient of variation) should be
less than 10%.
1.8 Scaling errors
Purpose: To assess the accuracy of software distance
indicators and check for distortion.
a) Position the M1
test object directly on the centre of a CR cassette at > 150 FDD.
b) Expose at 50-60
kVp with no filtration and 10mAs.
N.B. A lead ruler
could be used in place of the M1 test object. If so 2 exposures should be made
with the ruler placed in first the scan direction then the subscan
direction.
c) Read out plate
using processing as for test 1.3.
d) Using the
distance measuring software tools measure the dimensions (x and y) of five
central squares in both fast and slow scan directions. Calculate the aspect
ratio x/y. For Agfa systems the review workstation software can be used. For Fuji, Kodak and Konica
systems the images should be transferred to the reporting workstation to use
distance measuring software tools if available. If images are reported from
film then they should be printed at full size. Distances can then be measured
with a ruler.
 |
f)
Along the edge of the plate measure the horizontal
(x1) and vertical (y1) sizes of two squares as indicated in figure 4. Calculate
the aspect ratio x1/y1.
g) If possible
download the image as a DICOM file. Open the image using a DICOM viewer such as
Santeviewer. Hold the curser over a corner of a square in the grid. Record the
position within the image (i.e. the x and y coordinates). Move the curser to
the corner of the square of the grid 10cm from the first corner in the x
direction. Record the coordinates again. Calculate the pixel pitch,
p(mm)=100/n, where n =number of pixels covering 10cm of the grid. Repeat for
the y direction. This test is only necessary on commissioning. Compare the pixel
pitch to that stated by the manufacturer. The difference should be no greater
than the estimated measurement error.
 |
Tolerance: The measured distances x and y should agree within 3% of the actual distances at the centre of the plate and 5% at the edge. All calculated aspect ratios should be within 1.00 ± 0.03 at the centre of the plate, or 1.00 ± 0.05 at the edge.
1.9 Blurring
Purpose: To test for any localised distortion or
blurring of the image.
a) With the contact
mesh in placed on the cassette at >150cm FDD, expose at 50-60 kVp, fine
focus, with no filtration and 10mAs.Read the plate as described for test 1.3.
b) Visually inspect
the image for distortions. If distortion occurs clean the plate and repeat.
c) Repeat for at
least two other plates.
d) Repeat with a
fine mesh if available.
Tolerance: No blurring
should be present. If blurring is present on all plates this suggests the
reader is at fault, whilst imperfections in individual plates may also lead to
blurring. If blurring remains on a region of a plate after cleaning it should
not be used clinically.
1.10 Limiting Spatial
Resolution
Purpose: To
test the high contrast limit of the systems ability to resolve details.
a) Place a general purpose cassette on the couch with
the Huttner test object positioned at its centre aligned at 45o to
its edges.
NB A Huttner test
object with line spacings up to 8 lp.mm-1 may be required.
b)
Set 50-60 kVp, fine focus, and expose the cassette
using 10mAs.
c)
Readout the plate using the following parameters
d)
Adjust the window level and magnification to
optimise the resolution. Score the number of resolvable groups of lines from
the screen. Look up the corresponding resolution. The image should be scored at
a magnification of order x 5. If this facility is not available on the review
workstation then images should be transferred to the reporting workstation for
scoring.
e)
Repeat the measurement twice with the resolution
test object placed at a slight angle to the first the lateral or then the
longitudinal axis.
f)
Repeat this process for all available image pixel
pitches (nb different plate sizes will often default to being scanned at
different pixel pitches).
g) If possible
either archive or print the image for future reference.
Tolerance: These
measurements should be used to set a baseline for future QA tests. Print or
save the images for future reference, if possible. Comparable images are
available for most systems through KCARE.
N.B. The limiting
resolution should be expected to approach the Nyquist limit. At 45o
the Nyquist frequency is defined by Ö2/2p where p is
the pixel pitch. The measured limiting resolution may be limited by display
when scoring from a review workstation, particularly if no zoom or limited zoom
facilities are available.
1.11 Threshold Contrast
Detail Detectability
Purpose: To monitor image quality by assessing the
visibility of low contrast details.
a) With the tube, plate, and 1.0mm copper filtration in
the same positions as for the sensitivity tests, place the TO20 (or equivalent)
test object on the plate. Collimate down to the size of the test object.
b) Set 70 kVp and an mAs to deliver ~4 mGy. Read the plate using the
following parameters.
c) Ascertain whether clinical images are most commonly
viewed soft or hard copy. If they view hardcopy, adjust the window to optimise
the visibility of the details, ensuring that background noise is perceptible,
and print the image out on the largest film size. View the image on a masked
light box, and score each detail size using fixed distance viewing (<1m). If
images are viewed softcopy, score them on a reporting workstation optimising
window and level settings for each detail size.
d) Calculate an
image quality factor, IQF,
(4)
where
HT(A) = threshold contrast detail
index values calculated from the image,
HTref(A) = threshold
contrast detail index values calculated from a reference image of a system known to be in good
adjustment
D = the dose to the
image plate
Dref = the
dose to the image plate for the reference image
n = the number of
details in the test object.
d) Repeat this test
for two other imaging plates and also for a single plate at exposures of ~1mGy and ~12mGy.
e) If possible
either archive or print the images for future reference.
Tolerance: The results of
this test are used to set a baseline for future QA tests. Results could be
compared to those from other similar systems if available.
1.12 Laser beam function
Purpose: To assess laser beam scanline integrity and
jitter
a) Place a steel
ruler slightly angled to the subscan direction on a large cassette.
b) Expose at
~70 kVp, 150cm FSD and an mAs to deliver an incident exposure of ~50mGy. Read the plate as
described for test 1.3
c) Using the
software magnify the image x10. This will usually require the image to be
viewed from a reporting monitor. Select a narrow window width such that the
image appears largely polarised to black or white. This should allow the edge
to be easily differentiated from the background. Laser beam jitter can be evaluated
by examining the edge of the ruler on the image.
Tolerance: The edge should
be continuous across the full length of the image. Stair step characteristics
should be uniform across the length of the image. Regions of over or undershoot
of the scan lines indicate a timer or laser beam modulation problem.
1.13 Moiré Patterns
Purpose: To test for the presence of Moiré pattern
artefacts caused by grids.
a) Place a CR
cassette in the bucky such that the scan lines are vertical to the gridlines.
The cassette should be 1.5m from the focus, and the collimation should cover
the whole plate.
b) Expose at 70 kVp
using the AEC with 1.0 mm of copper in the beam, and the grid in place.
c) Read the plate as
described for test 1.3.
d) Visually inspect
the image for Moiré line pattern artefacts.
e) Repeat with the
CR cassette positioned in the bucky such that the scan lines are horizontal to
the gridlines.
f)
Repeat for all buckies and grids that may be used
with the CR system, including any grids used in mobile radiography.
Tolerance: No Moiré patterns should be visible. If Moiré patterns are
visible with a particular grid, it should not be used with the CR plates. The
cause of Moire patterns may be the failure of the motion of moving grids or
insufficient grid density.
2 Annual QA tests
The following routine QA tests should be performed approximately
annually
1.2 Dosimetry (only for 4mGy and 10mGy with 70kVp and 1.0mmCu)
1.4 Erasure
cycle efficiency
1.6 Detector
dose indicator consistency/sensitivity (for 1 plate of each size)
1.7 Uniformity
1.8 Scaling errors
1.9 Blurring
1.10 Limiting resolution (45o only)
1.11 TCDD (only 4mGy).
The tests should be performed as described in the previous section.
Table 1 below summarises the relevant remedial levels where these are different
to those described for commissioning
Test
|
Remedial Level
|
detector dose indicator consistency (sensitivity)
|
baseline ± 20% exposure
equivalent
|
limiting resolution
|
baseline ± 20%
|
TCDD (Quality Index)
|
baseline ± 30%
|
It should be noted that TCDD
and limiting resolution are subjective measures. Some effort should therefore
be made to train scorers to score to similar thresholds.
Appendix A – Measuring a
mean pixel value using a Fuji
CR system
Reduce the window width to 1, so that the image has only pixels appear
as either one of just two levels, black or white. Adjust the level until
approximately half the pixels are black and half are white. This level value is
the mean pixel value of the image.
References
[1] Draft Report of Task
Group #10, American Association of Physicists in Medicine, Acceptance Testing
and Quality Control of Photostimulable Storage Phosphor Imaging Systems, August
1998
[2] British Institute of
Radiology, ‘Assurance of the quality in the diagnostic imaging department’,
2001, ISBN 0-905749-48-0
[3] IPEM draft CR QC
protocol
[4] Samei E,
Seibert JA, Willis CE, Flynn MJ, Mah E, Junck KL, ‘Performance evaluation of
computed radiography systems’, 2001, Med.Phys. Vol28 (3) p361-371.
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