Computed Radiography: Acceptance Testing and Quality ... - AAPM

Computed Radiography: Acceptance Testing and Quality Control

Introduction CR is the primary means to capture 2D images in a PACS environment • Acceptance testing validates performance • Quality control verifies optimal operation

J. Anthony Seibert, Ph.D. University of California Davis Department of Radiology Sacramento, California

Considerations: • Knowledge of CR attributes and operation • Understanding the tests • Determining appropriate results

Presentation Outline

Computed Radiography (CR) ...is the generic term applied to an imaging system comprised of:

Overview of CR How does it work? What are the issues?

Photostimulable Storage Phosphor

to acquire the xx-ray projection image

Acceptance test procedures What tests? Why? Quality control When ?

CR Reader

How?

to extract the electronic latent image

Digital electronics

to convert the signals to digital form

What?

How often?

CR Image Acquisition 1. X-ray Exposure Patient

5.

unexposed

2.

Image Reader X-ray system

2. Display

3.

Image Scaling

Computed Radiograph

1. Acquisition

Digital to Analog Conversion

Transmitted xx-rays through patient Digital processing

4.

Image Record

Analog to Digital Conversion

exposed

Phosphor plate

Digital Pixel Matrix

Charge collection device

X-ray converter x-rays → electrons

3. Archiving

Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.

1

CR Networking

Film laser printer

CR Reader

• PACS and DICOM – Digital Imaging COmmunications in Medicine – Provides standard for modality interfaces, storage/retrieval, and print

• Modality Worklist Input (from RIS via HLHL-7) DICOM

• Technologist QC Workstation – Image manipulation and processing – Processed / Unprocessed images

CR - QC Workstation

• DICOM image output

PACS SoftSoft-copy review

Stimulation and Emission Spectra

CR: How does it work? Photostimulated Luminescence t

recombination

4f 6 5d

F/F+

PSL 3.0 eV

t

e-

Laser stimulation

Energy Band BaFBr 8.3 eV

2.0 eV

Eu

Emission

Optical Barrier

Conduction band tunneling

Relative intensity

t

phonon

BaFBr: Eu2+

Stimulation

1.0

0.5 Diode 680 nm

4f 7

Eu

3+

/ Eu 2+

Incident x-rays

e

Valence band PSLC complexes (F centers) are created in numbers proportional to incident xx-ray intensity

0.0

800

1.5

PSL Signal

PMT

Exposed Imaging Plate

Light Scattering

Photostimulated Luminescence

Protective Layer

2

f-θ lens

Laser Source Polygonal Mirror

Laser beam: Scan direction

Phosphor Layer

Laser Light Spread

"Effective" readout diameter

1.75

Reference detector

Light guide

600

500 2.5

400

300

3

4

λ (nm)

Energy (eV)

CR: Latent Image Readout

Photostimulated Luminescence Incident Laser Beam

700

Base Support

Cylindrical mirror Light channeling guide

Output Signal PMT

ADC ADC

x= 1279 To1333 image y= processor z= 500

Plate translation: SubSub-scan direction


2

Phosphor Plate Cycle PSP

SubSub-scan Direction

Typical CR resolution:

Base support

x-ray exposure

Plate translation reuse

35 x 43 cm -- 2.5 lp/mm (200 µm) 24 x 30 cm -- 3.3 lp/mm (150 µm) 18 x 24 cm -- 5.0 lp/mm (100 µm)

plate exposure: create latent image laser beam scan plate readout: extract latent image light erasure

Screen/film resolution:

Laser beam deflection

plate erasure: remove residuals

7-10 lp/mm (80 µm - 25 µm)

Computed Radiography • Acquisition, Display and Archive are separate functions • Variable speed detector – 20 to 2000 speed

• Wide dynamic range – 0.01 to 100 mR

Exposure Latitude: Dynamic Range Film

Signal output

Scan Direction

CR

100:1 10000:1

• Image processing is a crucial requirement

Log relative exposure

Raw image

Processing the Image • Image prepre-processing

• Inherent subject contrast displayed

– Find pertinent image information (histogram analysis) – Scale data to an appropriate range

• Contrast inverted (to screenscreen-film)

• Contrast enhancement

• PSL signal amplitude log amplified

• Spatial frequency enhancement

– Anatomy specific grayscale manipulation


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Histogram analysis

Finding the Image Location

• Frequency distribution of pixel values within a defined area in the image

• Image recognition phase – Collimation (Agfa) – EDR, automatic mode (Fuji)

• Shape is anatomy specific

– Segmentation (Kodak)

• Sets minimum and maximum “useful” pixel values

• Finding collimation borders and edges

Histogram:

Frequency

1

4

4

4

1

4

2

2

2

4

4

2

3

2

4

4

2

2

2

4

1

4

4

4

1

Frequency

Histogram Distribution

frequency distribution of pixel values in an image

16 14 12 10 8 6 4 2 0

0

1

2

Value

Pixel value Useful signal The shape is dependent on radiographic study, positioning and technique

1023 Frequency of Digital Number

Direct x-ray area

Frequency

Anatomy

5

Histogram Distribution

Histogram Distribution Collimated area

3 4 Value

Q2

Digital value

511

S1 0.01mR 0.1mR

SK 1mR

Q1 0

S2 10mR

100mR

Latitude (L) 20000

2000

200

20 2 Sensitivity (S)


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Data conversion

Input to output digital number 1,000

102 101 100 10-1

511 1023 10-1 100 101 102 103 0 Raw Digital Output Exposure input min

600 400 200 0

600 400 200

200 600 1,000 Input digital number

0 0

max

1. Find the signal

2. Scale to

3. Create film

range

to 8323

to 9368 800

800

Frequency

Output digital number

Relative PSL

Exposure into digital number

Histogram

Histogram: pediatric image

Grayscale transformation

200

400

600

800

1000

Digital value

Useful image range for anatomy

looklook-alike

Data conversion for overexposure Exposure into digital number

Relative PSL

Reduce overall gain

102 101 100 10-1

Exposure input

PrePre-processed “raw” image

Scaled and inverted: “unprocessed” image

10-1 100 101 102

0

511

1023

Raw Digital Output

overexposure

Screen-Film

Underexposed

103

min

max

(scaled and log amplified)

Computed Radiography

Overexposed

Underexposed

Overexposed


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Data conversion for wide latitude

ScreenScreen-Film 80 kVp, 18 mAs

Exposure into digital number

CR

80 kVp, 64 mAs

80 kVp, 18 mAs

Change gradient (auto mode)

Relative PSL

102 101 100 10-1

Exposure input

10-1 100 101 102 103

low kVp (wide range)

0

511

1023

Raw Digital Output (scaled and log amplified) min

400 speed screen - film

max

L=4, wide latitude

LookLook-upup-table transformation

Contrast Enhancement

1,000

Output digital number

• Optimize image contrast via nonnon-linear transformation curves • Unprocessed images display linear “subject contrast” – “Gradation processing” (Fuji) – “Tone scaling” (Kodak) – “MUSICA” (Agfa)

M

E

L

A

800 600

GT

Gradient Type

Fuji System

Example LUTs

400 200 0

0

200

400

600

800 1,000

Input digital number

Spatial Frequency Processing “Edge Enhancement”

Response

Solid: Original originalMTF response Edge Enhanced: Difference: Dash: low pass filtered Difference Original Original - + filtered

Raw

Unprocessed

Contrast Enhanced

Sum

low low low

Original

Blurred

high high high

Spatial frequency

Difference

Edge enhanced


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Fuji CR Parameter Settings LUT shape parameters

Anatomy Anatomical region General chest (LAT) General chest (PA) Port Chest GRID Port Chest NO GRID Peds chest NICU/PICU Finger Wrist Forearm Plaster cast (arm) Elbow* Upper Ribs* Pelvis* Pelvis portable Tib/Fib Tib/Fib Foot Foot* Os Calcis Foot cast C-spine T-spine Swimmers Lumbar spine Breast specimen

GA 1.0 0.6 0.8 1.0 1.1 0.9 0.8 0.8 0.8 0.8 0.8 0.9 0.9 0.9 0.8 1.2 0.8 0.8 1.1 0.8 1.2 1.0 2.5

GT B D F D D O O O O O O O O N O N O O F F J N D

GC 1.6 1.6 1.8 1.6 1.6 0.6 0.6 0.6 0.6 0.6 1.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 1.8 0.9 0.9 0.6

Frequency enhancement parameters

GS -0.2 -0.5 -0.05 -0.15 -0.2 0.3 0.2 0.3 0.4 0.4 0.0 0.2 0.2 0.25 0.3 -0.05 0.4 0.5 0.5 -0.05 0.3 0.4 0.35

RN 4.0 4.0 4.0 4.0 3.0 5.0 5.0 5.0 5.0 7.0 5.0 6.0 4.0 5.0 5.0 7.0 5.0 5.0 5.0 4.0 5.0 5.0 9.0

RT R R T R R T T T T T R T T F T T F F P T T T P

Recommended Acceptance Tests • Physical Inspection– Inspection–Inventory– Inventory–PACS Interfaces

RE 0.2 0.2 0.2 0.5 0.5 0.5 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 0.5 1.0 0.5 0.5 0.2 0.5 1.0 1.0

Acceptance Test / QC considerations • Image acquisition • ElectroElectro-Mechanical readout • Image processing • PACS / RIS interfaces • Image handling

Recommended Acceptance Tests • Noise / LowLow-Contrast Response

• Imaging Plate Uniformity and Dark Noise

• Distortion

• Signal Response: Linearity and Slope

• Erasure Thoroughness

• Signal Response: Exposure calibration and beam quality

• Artifact Analysis: Hardware/Software

• Laser Beam Function

• Positioning and collimation robustness

• High Contrast Resolution

• Imaging Plate Throughput

Acceptance test tools required

CR: Spatial Resolution • Phosphor plate sizes: impact on resolution

• Exposure meter/dosimeter • Spatial resolution phantom • Low contrast phantom • Vendor QC phantom (periodic tests) • SMPTE test pattern • Anthropomorphic phantom • Documentation log / spreadsheet / instructions

35x43 (14x17)

24x30 (10x12)

18x24 (8x10)

0.2 mm pixels

0.14 mm pixels

0.1 mm pixels


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High Contrast (Spatial) Resolution 18 x 24 cm

MTF Curves

35 x 43 cm 1

MTF

0.8 PrePre-sampled MTF

ScreenScreen-film

0.6

Scan Subscan

0.4

Sampled MTF: Standard CR 0.2 2K x 2K matrix 35 x 43 cm

0

Photon absorption fraction

0

2 4 6 8 10 Spatial Frequency (lp/mm)

Low Contrast Response: Leeds TOTO-16

X-ray Absorption Efficiency 1

Hi res CR Standard CR

BaFBr, 100 mg/cm²

0.8

Gd2O2S, 120 mg/cm2

0.6 0.4 0.2 BaFBr, 50 mg/cm² 0

0

20

40

60

80

100

120

140

Energy (keV)

3.5 mR

70 kVp

0.5 mR

Uniformity 498

508

537

490

497

10 mAs

480

513

505

544

487

20 mAs


8

Radiation Dose for CR

Sensitivity number, S

• Variable Speed Detector

• Estimate of the incident exposure on the IP

• Optimal dose (UC Davis) – Adult chest image: – Neonates/pediatrics: – Extremities:

• Comparable to screenscreen-film “speed”

S=200S=200-300 S=400S=400-600 S= 7575-100

• Amplification required to map median value of histogram to 511 (0 to 1023 grayscale)

– Lower detection efficiency, luminance and readout noise

• Dependent on histogram shape and examination selected

• AntiAnti-scatter grids needed

Date: 7/10/98 Date: 7/10/98 Medical Medical Physicist: Physicist: Anthony Anthony Seibert, Seibert, Ph.D. Ph.D.

Location: Location: System System Identification: Identification:

UCDMC, UCDMC, ACC, ACC, 33 CR CR unit unit 33

Guidelines for QC based on Exposure

UC UC Davis Davis Medical Medical Center Center CR CR Reader Reader and and Screens Screens

Signal Signal Response: Response: Calibration Calibration and and Beam Beam Quality Quality

Sensitivity number

Note: Note: Use Use mAs mAs values values to to provide provide an an approximate approximate exposure exposure of of 11 mR mR to to the the IP. IP. Menu Menu == TEST TEST IP ST14x17 14x17 IP Type: Type: ST IP IP SN: SN:

Exposure Exposure Conditions Conditions

SubMenu SubMenu==Ave Ave2.0 2.0

Focal Focalspot spot

Time Timedelay delay

SID SID(cm) (cm)

SMD SMD(cm) (cm)

LL == 2, 2, EDR EDR == semi semi

1.2 1.2 mm mm

~2 ~2 min min

140 140

130 130

kVp kVp Dependency Dependency kVp kVp

Filtration Filtration

m mAs As

m mR-m R-meter eter

60 60 80 80 115 115

11 Al/0.5 Al/0.5 Cu Cu 11 Al/0.5 Al/0.5 Cu Cu 11 Al/0.5 Al/0.5 Cu Cu

15.00 15.00 4.5 4.5 1.13 1.13

1.06 1.06 1.06 1.06 1.14 1.14

Filtration Filtration

m mAs As

m mR-m R-meter eter

80 80 80 80 80 80

none none 11 Al/0.5 Al/0.5 Cu Cu 1Al/2.5Cu 1Al/2.5Cu

0.50 0.50 4.50 4.50 60.00 60.00

0.96 0.96 1.06 1.06 0.99 0.99

1000 • 600 – 1000

SS SS (1mR) (1mR) 0.91 121.00 110.59 0.91 121.00 110.59 0.91 108.00 98.71 0.91 108.00 98.71 0.98 115.00 113.04 0.98 115.00 113.04 Maximum Difference: 14.33 Maximum Difference: 14.33 m mR-IP R-IP

Filtration Filtration Dependency Dependency kVp kVp

Indication

SS(1mR) (1mR) 70 70

90 90 kVp kVp

110 110

130 130

20.00 20.00 0.00 0.00 none none

SS(1mR) (1mR) 11 Al/0.5 Al/0.5 Cu Cu Filtrati Filtration on

1Al/2.5Cu 1Al/2.5Cu

Sensitivity number, S • Imaging plate (HR vs. ST) differences • Examination specific histogram shapes – S number varies with examination type

• EDR mode effects – Automatic (determines S1 and S2 values on histogram) – SemiSemi-automatic (average value within ROI) – Fixed (system acts like screenscreen-film detector)

• X-ray beam spectrum effects

What S value is appropriate? • Determined by examination – Adult exams (CXR, abdomen, etc) – Extremities (ST plates) – Pediatrics

UCDMC targets 150 – 300 75 – 150 300 – 600

• CR’s variable speed should be used to advantage • Anatomical information can be lost with too high or too low exposure

– S number varies with beam hardness (calibration required)


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Adult portable chest calculated exposures First half, 1994, 4572 exams 38.3%

600

53.9%

7.8%

Adult portable chest calculated exposures Second half, 1994, 4661 exams

Target exposure range

600

500

3.4%

Q3

300

Q4

Target exposure range

Low

High

Low

Incident Exposure

Other 12%

“Exposure Creep”

180 180 160 160 140 140 120 120 100 100 80 80 60 60 40 40 20 20 0 0

1 100 000 6600 00-6699 99 5500 00554 99 4400 00-4444 99 3355 0-3 74 7 4 3300 00-3322 44 2255 002277 44 2200 00-222 1155 244 00-1177 44 1100 0-1 2244 5500 --77 44

Number of examinations

73.5%

500

400

500

#exams

23.1%

Sensitivity number

Radiation Dose for CR • Variable Speed Detector • Optimal dose for typical adult chest image is 2X higher than 400 speed screen/film – Lower absorption efficiency – Quantum and electronic noise – Readout inefficiencies of latent image

• AntiAnti-scatter grids necessary for most procedures

Wrong exam 5%

High

Repeated Examinations with CR

Motion 6%

Positioning 46%

Reprinting 9% Underexposure 10% Overexposure 12%

Total # repeats = 1043 from Willis, RSNA 1996

AEC adjustment procedures • A 200200-speed equivalent exposure is desirable • Empirically determine AEC setting(s) with simple uniform phantoms • UCDMC technique: use “fixed” mode (S=200) and sensitivity test menu; adjust AEC response according to changes in film optical density • Verify settings with semisemi-auto mode • Verify patient exposure “S” number; recheck often


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Date: 7/10/98 Date: 7/10/98 Medical Medical Physicist: Physicist: Anthony Anthony Seibert, Seibert, Ph.D. Ph.D.

Location: Location: System System Identification: Identification:

UCDMC, UCDMC, ACC, ACC, 33 CR CR unit unit 33

Problem areas

UC Davis Medical Center CR CR Reader Reader and and Screens Screens

Inspection Results Summary

• FilmFilm-based performance measurements Acceptab Acceptable le

1. 1. Physical Physical Inspection Inspection -- Inventory Inventory 2. 2. Imaging Imaging Plate Plate Uniformity Uniformity and and Dark Dark Noise Noise 3. 3. Signal Signal Response: Response: Linearity Linearity and and Slope Slope 4. 4. Signal Signal Response: Response: Calibration Calibration and and Beam Beam Quality Quality 5. 5. Laser Laser Beam Beam Function Function 6. 6. High-Contrast High-Contrast Resolution Resolution 7. 7. Noise/Low-Contrast Noise/Low-Contrast Response Response 8. 8. Distortion Distortion 9. 9. Erasure Erasure Thoroughness Thoroughness 10. 10. Anti-Aliasing Anti-Aliasing 11. 11. Positioning Positioning and and Collimation Collimation Errors Errors 12. 12. Throughput Throughput

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes* Yes* Yes Yes Yes Yes Yes Yes

• Digital image analysis tools and evaluation methods not readily available • Low contrast resolution measurements • Lack of a standardized QC phantom

Comments: Comments:

Quality Control Three levels of system performance quality control 1. Routine: Technologist level - no radiation measurements

2. Full inspection: Physicist level

Periodic Quality Control • Daily (technologist) – Inspect CR system and status. – Interfaces: PACS broker, ID terminal, QC workstation

- radiation measurements and nonnon-invasive adjustments

3. System adjustment: Vendor service level

– Erase image receptors (if status unknown).

- hardware and software maintenance

Periodic Quality Control • Weekly / Biweekly (technologist) – Calibrate review workstation monitors (SMPTE). – Acquire QC phantom test images. Verify performance. – Check filters / vents and clean as necessary.

Periodic Quality Control • Quarterly (Technologist) – Inspect cassettes. Clean with recommended agents. – Review image retake rate and exposure trends. – Update QC log. Review outout-ofof-tolerance issues.

– Clean screens with recommended agents.


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Periodic Quality Control • Annually (Physicist)

What is needed? • Computer friendly phantoms

– Perform linearity / sensitivity / uniformity tests – Inspect / evaluate image quality

• Objective quantitative analysis methods • System performance tracking and database logs

– ReRe-establish baseline values (Acceptance Tests)

• Exposure monitoring tools and database tracking

– Review retakes, exposures, service records.

Fuji

Agfa

Line pair phantoms (contrast transfer tests)

Lumisys

Home built

Diagonal bar (laser jitter test)

40 line/cm grid (visual aliasing)

Fiducial Markers (distance accuracy)

Lead attenuator (dynamic range) Resolution Bar Pattern (qualitative)

Notches (geometric accuracy tests)

Step wedge (signal, signal to noise and linearity response tests)

Open area (scan uniformity test)

Copper step wedge (dynamic range, linearity, SNR)

Edge for Presampled MTF

Single exposure, qualitative and quantitative

Additional Information / Help • AAPM Task Group #10 document: • Email: jaseibert@ jaseibert@ucdavis. ucdavis.edu

• Dr. Ehsan Samei spreadsheets – http://deckard .mc.duke.edu edu/~ /~samei samei/downloads /downloads http://deckard.mc.duke.

• Vendor efforts for QC phantom development and analysis

Summary • CR is the mainstay for direct digital acquisition of projection radiographs • CR acceptance testing and QC are essential for optimal operation • A TEAM

approach is necessary

– Technologists, Radiologists, Physicists, – Clinical Engineering, Information System Group


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