QUALITY ASSUARANCE FOR COMPUTED...

QUALITY ASSUARANCE FOR

COMPUTED RADIOGRAPHY

Khadijah Ramli Department of Biomedical Imaging

Faculty of MedicineUniversity of Malaya

[email protected]

Medical Imaging & Radiation Therapy Updates

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CONTENTS

• Introduction• CR Workflow• QA Test• Note• Conclusion• Reference


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INTRODUCTION

• Quality assurance (QA) in CR is as important as QA in conventional film‐screen system.

• Purpose:

– As a part of a facility’s radiation safety/quality assurance program.

– To reduce radiation exposure to patients, optimize diagnostic image quality and foster facility involvement in the responsibility for quality assurance (QA).


CR WORKFLOW

CR READER

WORKSTATION

IMAGING

LASER PRINTER

REPORTINGSTATION


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QA TEST

• Inspection Test (Artefacts)

• Brightness, Contrast and Spatial Resolution of Monitor

• Printer Performance

• Erasure Thoroughness

• Dark noise test

• Uniformity: Reproducibility and density uniformity

• Linearity and Response

• Spatial Resolution

• Contrast Detectability

• Wire Mesh Test/Blurring

• Grid Response

• Reject Analysis


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NOTE

• Record plate identification code for all test.

• Use X‐ray equipment that is calibrated. (Accuracy of kVp selected for detector dose indicator consistency and calibration).

• Some test are undertaken on a review workstation depending on the processing tools available.

• Certain test require the use of higher quality reporting workstation.

INSPECTION

OBJECTIVE:

• To assess cassette and image plate (IP).

PROCEDURE:

• Visual examined for external defects, each cassette should then be opened and the IP extracted to inspect for dust or scratches.


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INSPECTION

Cassette Number Cassette ID Cassette

Condition Plate Condition

A1-1a 40647333 Scratched Thorn Layer

A1-2a 40647357 Scratched Dirty

A1-4a 40647289 Scratched Good

A1-1b 40539478 Good Thorn Layer

A1-2b 40539508 Scratched Good


A1-3c 40683454 Good Good

A2-1a 40647241 Scratched Dirty




A2-2b 40539287 Good Good

A2-3b 40539386 Good Good

A2-4b 40539379 Scratched Good


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ARTIFACTS

• Check each image, softcopy and hardcopy, for artifacts.

• May be caused by dust particles, scratches, lines, mechanical friction marks or other factors.

• If found common to a particular Imaging Plate (IP), that plate shall be removed from service until the cause of the artifact iseliminated.

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DIRT ON THE LIGHT GUIDE DIRTY SCREEN


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ARTIFACTS

DIRT ON THE LIGHT GUIDE ELECTRONICS – PMT BOARD


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Cesar et al

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Cesar et al

ARTIFACTS


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IP CLEANING

• Visually inspect and clean all cassettes and imaging plates with methods that are approved by the equipment manufacturer.

• The cleaning frequency is based upon the particular environment that the plate is used in.

• Specific cleaning methods are available from the manufacturer.

• Lint‐free cotton gauze and lens cleaner is recommended by vendor.

IMAGING PLATE CRACKS BACKSCATTER


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ARTIFACTS

INCORRECT ERASURE SETTING SCATTER RADIATION


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Cesar et al

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ARTIFACTS


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ASSESSMENT OF BRIGHTNESS, CONTRAST AND SPATIAL RESOLUTION OF MONITOR

Society of Motion Picture and Television Engineers ‐ SMPTE


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MONITOR BRIGHTNESS & CONTRAST

• 0% and 100% squares each contain smaller squares within them that represent signal level steps of 5% and 95%, respectively.

• Differentiate the inner square from the larger square that contains it.

• Reducing ambient light improves the visibility of the 5% square.


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SPATIAL RESOLUTION

• High contrast bar patterns in the test image ‐ patterns of black and white pairs.

• 6 squares filled with varying widths of alternating black/white horizontal and vertical lines.

• Able to differentiate all the lines, from fat to narrow and both horizontally and vertically


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PRINTER PERFORMANCE

• Film base‐plus‐fog = 100% patch

• Speed index of printer = 40% patch

• Contrast index of printer =10% minus 70% patches

• Proper control of high and low contrast display = visible –0/5% and 95/100% patches


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For softcopy (HIC workstation):

• Measure and record average pixel value using region of interest analysis

For hardcopy (film):

• Print sensitometry strip from the printer and measure Base + Fog OD

• Measure and record average OD

PRINTER PERFORMANCE


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Month: January 2007

Step No 22.01.07 23.01.07 24.01.07 25.01.07 26.01.071 0.2 0.2 0.2 0.2 0.22 0.22 0.22 0.22 0.22 0.223 0.24 0.24 0.24 0.25 0.254 0.27 0.28 0.28 0.28 0.285 0.3 0.31 0.31 0.32 0.316 0.34 0.35 0.35 0.35 0.357 0.39 0.4 0.4 0.41 0.48 0.45 0.45 0.46 0.46 0.459 0.51 0.52 0.52 0.54 0.5210 0.61 0.61 0.62 0.63 0.6112 0.73 0.73 0.75 0.76 0.7413 0.88 0.88 0.88 0.9 0.8914 1.07 1.07 1.07 1.09 1.0715 1.28 1.28 1.29 1.3 1.2816 1.51 1.51 1.52 1.54 1.5217 1.77 1.77 1.8 1.8 1.77

Density

PRINTER PERFORMANCE


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LASER PRINTER CONSISTENCY

Date 22.01.07 23.01.07 24.01.07 25.01.07 26.01.07Speed 1.2 1.2 1.2 1.2 1.2

Base fog 0.2 0.2 0.2 0.2 0.2Contrast 0.88 0.88 0.88 0.9 0.89

INADEQUATE ERASURE THOROUGHNESS

• This image was spoiled when the erasure thoroughness was inadequate, the previous image was not eliminate.



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ERASURE THOROUGHNESS ASSESSMENTPROCEDURE

• Position cassette on Pb apron

• Set a 10 X 10 cm field and position a piece of attenuating material (at the centre of IP.

• Read IP using

Imaging process ‐ Test / Sensitivity / Semi‐EDR parameters

IP

attenuating material Cu/Pb

Pb apron

FFD = 180 cmExposure = 80 kVp/25 mAs without Filteration Field Size = 10 X 10cm


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• Re‐expose IP with a 9 X 9 cm field centred on the same point on the IP with no attenuating material in place.

• Change the exposure mAs to 0.5.

• Read the IP with the same parameters.

• Visually inspect the image for any remnant of the previous image

IP

Pb apron

FFD = 180 cmExposure = 80 kVp/0.5 mAs without Filteration Field Size = 9 X 9 cm



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80kVp, 25mAs, 10cm X 10cm. 80kVp, 25mAs, 9cm X 9cm.

Absence of ghost image of the previous image from the first exposure in the re-exposed image. Ghosting’ image was the result of inadequate erasure.


DARK NOISE TEST

OBJECTIVE:

• To ensure the level of noise inherent in the system.

• To ensure removal of all residual signal from background radiation or other sources.

PROCEDURE:

• Use the primary erasure to erase an IP without making an exposure, read it using the following parameters:

Fuji: Readout mode – ‘Fixed’

S = 1000

L = 1

• Examine the images visually for uniformity


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For softcopy (HIC workstation):

• Measure and record average pixel value using region of interest analysis

For hardcopy (film):

• Print sensitometry strip from the printer and measure Base + Fog OD

• Measure and record average OD


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DARK NOISE TEST

RESULT:

• Each IP should demonstrate a clear, uniform and artefact‐free image.

ANALYSIS:

• Compare the measured OD and Base + Fog OD.

Softcopy Cirteria:

• Average pixel value (PV) < 280

Hardcopy Criteria:

• OD measured randomly over the image area should be within 0.05 OD of base + fog values determined from the sensitometry strip film.


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DARK NOISE TEST

UNIFORMITY

OBJECTIVE:

• To assess the uniformity of the recorded signal from an exposed plate.

• A non‐uniform response could affect clinical image quality.

PROCEDURE:

• Position cassette on the lead apron.

• Expose with no filteration at 80 kVp, FFD = 150 cm, mAs to deliver ½ .10mR.

• Rotate the plate through 1800 about the vertical axis and re‐expose using the same parameters (cancel out the non uniformities due to the anode heal effect).


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• Read IP using the following parameters:

Fuji: Readout mode – ‘Fixed’

S = 200

L =2

• Print the image onto laser films and measure the OD as indicated in the image beside.

• Visually inspect image obtained for uniformity and artefacts.

• Measure OD and PV at the centre and each quadrant of each film and record.

1

3

4

2

5

UNIFORMITY


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ANALYSIS

• The images should not have obvious artefacts.

• If measuring uniformity from film the maximum variation in OD should be less than 10%.

• Using ROI analysis, values should be within a range of 10% of each other.

UNIFORMITY


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1 2 3 4 5

14 X 7 (A4) 40647319 1.53 1.66 1.6 1.62 1.68 1.62 0.013

14 X 7 (B6) 40647340 1.48 1.64 1.6 1.56 1.64 1.58 0.025

14 X 7 (B6) 40647449 1.39 1.67 1.58 1.42 1.7 1.55 0.08

14 X 17 (B4) 41322246 1.46 1.67 1.62 1.53 1.68 1.59 0.093

14 X 17 (B4) 40647272 1.39 1.64 1.61 1.45 1.67 1.55 0.062

14 X 17 (B4) 41287637 1.51 1.68 1.62 1.46 1.71 1.6 0.104

14 X 17 Control 43403608 1.53 1.62 1.62 1.56 1.66 1.6 0.011

Average Denisty

Standard Deviation

Cassette Size Cassette ID

Optical Density

UNIFORMITY

Linearity:

• To establish that the indicated exposure (calculated from the detector dose indicator) responds linearly to increases in dose.


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LINEARITY

Curve of ln(S) and ln(E)

0

1

2

3

4

5

6

7

8

-3 -2 -1 0 1 2 3

ln(E)

ln(S

)

SPATIAL RESOLUTION

Objective:

• To test the high contrast limit of the systems ability to resolve details.

Procedure:

• Place a cassette with the Huttner test object (line spacing up to 8 lp/mm) positioned at its centre aligned at 450 to its edges.

• Set 50 kVp expose the cassette to 10 mAs.


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SPATIAL RESOLUTION

• Readout the plate using:

– Fuji: Readout mode – ‘Fix’ , L = 2 and S = 200

• 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.

Criteria:

• For the 450 angled test object the resolved line pairs per mm should be > 1.2/2p where the p is the pixel dimension in mm.

• In the scan and subscan directions the limiting resolution should be > 0.85/2p.


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CONTRAST DETAIL DETECTABILITY

Objective:

• To monitor image quality by assessing the visibility of low contrast detail.

Procedure:

• With the tube, plate and copper in the same position as for sensitivity test, place the TO20 test object on the plate.

• Collimate to the size of the test object.

• Set 75 kVp and an mAs to deliver 4μGy.• Read the plate using: • Fuji: Readout mode ‐ ‘Semi‐auto’ with

test/sensitivity GA = 1Medical Imaging & Radiation Therapy

Updates36

CONTRAST DETAIL DETECTABILITY

• Ascertain whether clinical images are most commonly viewed soft or hardcopy.

• If 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 image on a masked light box, and score each detail size using fixed distance viewing (< 1 m).

• If images are viewed on softcopy, score them on a reporting workstation optimising window width and level settings for each detail size.

Criteria:

• The results of this test are used to set a baseline for future QA tests.


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WIRE MESH TEST/BLURRING

Objective:

• To test any localised distortion or blurring of the image.

Procedure:

• With the contact mesh in place expose and read a plate as in Scaling Errors.

• Visual inspect the image for distortions.

• If distortion occurs clean the plate and repeat.


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Criteria:

• The resulting image should distortion free and sharp over the whole FOV.

• 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.

WIRE MESH TEST/BLURRING


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ALIASING / GRID RESPONSE

Objective:

• To test for the presence of Moire pattern artefacts caused by grids.

Procedure:

• 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.

• Expose at 70 kVp using the AEC with 1.0mm of copper in the beam, and the grid in place.


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ALIASING / GRID RESPONSE / MOIRE PATTERN

• Read the plate using the following parameters:

• Maging process: Test / Contrast / Semi‐EDR (test 1.4)

• Visually inspect the image for Moire line pattern artefacts.

Criteria:

• No Moire patterns should be visible.

• If Moire patterns are visible with a particular grid, it should not be used with CR plates.

Cesar et alMedical Imaging & Radiation Therapy

Updates41


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GRID RESPONSE

GRiID CUT OFF180 cm, 80 kVp + AEC

UNIFORM DENSITY 100cm, 80 kVp + AEC


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• Review the image retake rate and determine the causes of unacceptable PSP images.

• Review the QC exposure indicator database.

• Determine the cause of under /overexposures, implement corrective action and generate a quarterly report that can be reviewed by the Radiation Safety Committee or RSO.

• The incident exposures on the IPs should be tracked to analyze any trends and in particular “dose or CR creep” as there is an optimal level of exposure for each type of exam that is performed.

REJECT ANALYSIS


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“EXPOSURE CREEP”

• High mAs produces CR images that are extremely crisp and unlikely to be rejected by a radiologist.

• Violation of the ALARA principle

S = 50S = 200S = 1000Receptor Exposure

Pixe

l Val

ues

4096

2048

0

High Noise Optimum Very Good

High Exposure

Optimum Exposure

Low Exposure

Effects of Exposure on Image Quality

GRID CUT‐OFF SELECTION OF EXPOSURE


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REJECT IMAGES

WRONG SETTING SELECTION OF EXPOSURE


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REJECT IMAGES

DOUBLE EXPOSUREEXPOSURE TO THE BACK OF

CASSETTE


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Cesar et al

REJECT IMAGES


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CONCLUSIONS

• Errors can still arise in the practice of CR, and QA processes must be in place to detect and correct those errors when they occur.

• Radiographers need to be trained to use the entire computed radiography system.


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REFERENCES

• Smith R, August 2002, Quality control for Computed Radiography, Imaging Economics articles:2002‐08_15.

• Cowen A R,Clarke O F,Coleman N J,Craven D M, McArdle S and Hay G A, 1992. Leed X‐ray Test Object : Instruction Manual.

• W A Jennifer, 2006, High Contrast (Spatial) Resolution Testing for Computed Radiography,

• Lu Z F, Nickloff E L and Terilli, 1999. Monthly monitoring program on DryVeiw laser imager : One year experience on five Imagion units. Med. Phys. 26(9).

• Willis C.E,etl, 1995, Objective Measures of Quality Assurance in a Computed Radiography‐Based Radiology Department.

• KCARE,2004, Protocol for the QA of Computed Radiography System Commissioning and Annual QA Test.

• WWW.healthline.com/galecontent/computed radiography.

• Http://en.wikipedia.org/wiki/computed ‐radiography.

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