Computed Radiography: Acceptance Testing and Quality Control

Transcription

1 Computed Radiography: Acceptance Testing and Quality Control J. Anthony Seibert, Ph.D. University of California Davis Department of Radiology Sacramento, California Introduction CR is the primary means to capture D images in a PACS environment Acceptance testing validates performance Quality control verifies optimal operation Considerations: Knowledge of CR attributes and operation Understanding the tests Determining appropriate results Presentation Outline Overview of CR How does it work? What are the issues? Acceptance test procedures What tests? Why? How? Quality control When? What? How often? Computed Radiography (CR)...is the generic term applied to an imaging system comprised of: Photostimulable Storage Phosphor to acquire the x-ray x projection image CR Reader to extract the electronic latent image Digital electronics to convert the signals to digital form. X-ray Exposure Patient CR Image Acquisition unexposed 5. Computed Radiograph. Acquisition Transmitted x-raysx through patient Digital Pixel Matrix. Display Digital to Analog Conversion X-ray system exposed. Image Reader Phosphor plate 3. Image Scaling. Image Record Charge collection device Analog to Digital Conversion X-ray converter x-rays electrons Digital processing 3. Archiving

2 CR Reader CR - QC Workstation DICOM Film laser printer PACS Soft-copy review CR Networking PACS and DICOM Digital Imaging COmmunications in Medicine Provides standard for modality interfaces, storage/retrieval, and print Modality Worklist Input (from RIS via HL-7) Technologist QC Workstation Image manipulation and processing Processed / Unprocessed images DICOM image output PSL 3. ev CR: How does it work? Photostimulated Luminescence phonon f 6 5d f 7 t Eu F/F + Eu 3+ / Eu + Incident x-rays t tunneling e - e t recombination Laser stimulation. ev Conduction band 8.3 ev Energy Band BaFBr Valence band PSLC complexes (F centers) are created in numbers proportional to incident x-ray x intensity Relative intensity Stimulation and Emission Spectra Stimulation 7.75 Diode 68 nm 6 BaFBr: Eu + Optical Barrier 5.5 Emission 3 λ (nm) Energy (ev) 3 Exposed Imaging Plate Photostimulated Luminescence Incident Laser Beam Light Scattering Light guide Photostimulated Luminescence Laser Light Spread "Effective" readout diameter PMT PSL Signal Protective Layer Phosphor Layer Base Support Laser Source Reference detector Polygonal Mirror Laser beam: Scan direction CR: Latent Image Readout f-θ lens Plate translation: Sub-scan direction Cylindrical mirror Light channeling guide PMT Output Signal ADC x= 79 y= To 333 image processor z= 5

3 Phosphor Plate Cycle Scan Direction Laser beam deflection Sub-scan Direction Plate translation Typical CR resolution: 35 x 3 cm --.5 lp/mm ( µm) x 3 cm lp/mm (5 µm) 8 x cm lp/mm ( µm) Screen/film resolution: 7- lp/mm (8 µm - 5 µm) reuse x-ray exposure laser beam scan light erasure PSP Base support plate exposure: create latent image plate readout: extract latent image plate erasure: remove residuals Computed Radiography Exposure Latitude: Dynamic Range Acquisition, Display and Archive are separate functions Variable speed detector to speed Wide dynamic range. to mr Image processing is a crucial requirement Signal output Film : CR : Log relative exposure Raw image Processing the Image Inherent subject contrast displayed Contrast inverted (to screen-film) PSL signal amplitude log amplified Image pre-processing processing Find pertinent image information (histogram analysis) Scale data to an appropriate range Contrast enhancement Anatomy specific grayscale manipulation Spatial frequency enhancement 3

4 Finding the Image Location Histogram analysis Image recognition phase Collimation (Agfa) EDR, automatic mode (Fuji) Segmentation (Kodak) Finding collimation borders and edges Frequency distribution of pixel values within a defined area in the image Shape is anatomy specific Sets minimum and maximum useful pixel values Histogram: frequency distribution of pixel values in an image Histogram Distribution Frequency Value 3 Frequency Value Frequency Histogram Distribution Collimated area Anatomy Useful signal Direct x-ray area Pixel value The shape is dependent on radiographic study, positioning and technique Frequency of Digital Number.mR Histogram Distribution S.mR S K mr S Latitude (L) mr 3 Q 5 Q mr Sensitivity (S) Digital value

5 Relative PSL Data conversion Exposure into digital number - Histogram - Exposure input min max Raw Digital Output Grayscale transformation Input to output digital number, number, Output digital number 8 6 6, Input digital number. Find the. Scale to 3. Create film signal range look-alike Frequency 8 6 Histogram: pediatric image to 9368 to Digital value Useful image range for anatomy Relative PSL Data conversion for overexposure - Exposure into digital number Reduce overall gain Pre-processed raw image Scaled and inverted: unprocessed image Exposure input overexposure - min 3 max 5 3 Raw Digital Output (scaled and log amplified) Screen-Film Computed Radiography Underexposed Overexposed Underexposed Overexposed 5

6 Data conversion for wide latitude Exposure into digital number Change gradient (auto mode) Screen-Film 8 kvp, 8 mas 8 kvp, 6 mas CR 8 kvp, 8 mas Relative PSL - Exposure input - 3 low kvp (wide range) min max 5 3 Raw Digital Output (scaled and log amplified) speed screen - film L=, wide latitude Contrast Enhancement Look-up up-table transformation Optimize image contrast via non-linear transformation curves Unprocessed images display linear subject contrast Gradation processing (Fuji) Tone scaling (Kodak) MUSICA (Agfa) Output digital number, 8 6 M L E A GT Gradient Type Fuji System Example LUTs 6 8, Input digital number Spatial Frequency Processing Edge Enhancement Solid: Edge Difference: Original Enhanced: original MTF response Dash: Difference Original low pass filtered - + filtered Original Response Sum Original Blurred Raw Unprocessed Contrast Enhanced low low low high high Spatial frequency Difference Edge enhanced 6

7 Fuji CR Parameter Settings Anatomy Anatomical region GA LUT shape parameters GT GC GS Frequency enhancement parameters General chest (LAT). B R. General chest (PA).6 D R. Port Chest GRID.8 F T. Port Chest NO GRID. D R.5 Peds chest NICU/PICU. D R.5 Finger.9 O T.5 Wrist.8 O T.5 Forearm.8 O T.5 Plaster cast (arm).8 O T.5 Elbow*.8 O T. Upper Ribs*.8 O R. Pelvis*.9 O T. Pelvis portable.9 O.6.. T.5 Tib/Fib.9 N F.5 Foot.8 O T.5 Foot*. N T.5 Os Calcis.8 O F. Foot cast.8 O F.5 C-spine. F P.5 T-spine.8 F T. Swimmers. J T.5 Lumbar spine. N T. Breast specimen.5 D P. RN RT RE Acceptance Test / QC considerations Image acquisition Electro-Mechanical readout Image processing PACS / RIS interfaces Image handling Recommended Acceptance Tests Recommended Acceptance Tests Physical Inspection Inventory Inventory PACS Interfaces Imaging Plate Uniformity and Dark Noise Signal Response: Linearity and Slope Signal Response: Exposure calibration and beam quality Laser Beam Function High Contrast Resolution Noise / Low-Contrast Response Distortion Erasure Thoroughness Artifact Analysis: Hardware/Software Positioning and collimation robustness Imaging Plate Throughput Acceptance test tools required Exposure meter/dosimeter Spatial resolution phantom Low contrast phantom Vendor QC phantom (periodic tests) SMPTE test pattern Anthropomorphic phantom Documentation log / spreadsheet / instructions CR: Spatial Resolution Phosphor plate sizes: impact on resolution 35x3 (x7). mm pixels x3 (x) 8x (8x). mm pixels. mm pixels 7

8 High Contrast (Spatial) Resolution MTF Curves 35 x 3 cm 8 x cm 35 x 3 cm MTF Sampled MTF: Standard CR K x K matrix 35 x 3 cm Screen-film 6 8 Spatial Frequency (lp/mm) Pre-sampled MTF Scan Subscan Hi res CR Standard CR X-ray Absorption Efficiency Low Contrast Response: Leeds TO-6 Photon absorption fraction BaFBr, 5 mg/cm² BaFBr, mg/cm² 6 8 Energy (kev) Gd O S, mg/cm 3.5 mr 7 kvp.5 mr Uniformity mas mas

9 Radiation Dose for CR Variable Speed Detector Optimal dose (UC Davis) Adult chest image: S=-3 Neonates/pediatrics: S=-6 Extremities: S= 75- Lower detection efficiency, luminance and readout noise Anti-scatter grids needed Sensitivity number, S Estimate of the incident exposure on the IP Comparable to screen-film speed Amplification required to map median value of histogram to 5 ( to 3 grayscale) Dependent on histogram shape and examination selected Date: 7//98 Location: UCDMC, ACC, 3 Physicist: Anthony Seibert, Identification: CR unit 3 Medical Ph.D. System UC Davis Medical Center CR Reader and Screens Signal Response: Calibration and Beam Quality Note: Use mas values to provide an approximate exposure of of mr to to the IP. Menu = TEST Exposure Conditions x7 SubMenu = Ave. Focal spot Time delay SID (cm) SMD (cm) IP Type: ST IP SN: L =,, EDR = semi. mm ~ min 3 kvp Dependency kvp Filtration mas mr-meter mr-ip S S (mr) OD NA 6 Al/.5 Cu NA 8 Al/.5 Cu NA 5 Al/.5 Cu NA Maximum Difference:.33.7 NA Filtration Dependency kvp Filtration mas mr-m eter mr-ip S S (mr) OD NA 8 none NA 8 Al/.5 Cu NA 8 Al/.5Cu NA Maximum Difference: NA Response S (mr) kvp Response S (mr).. none Al/.5 Cu Cu Al/.5Cu Filtration Guidelines for QC based on Exposure Sensitivity number > <. mr mr mr mr mr mr <5 >. mr Indication Underexposed: repeat Underexposed: QC exception Underexposed: QC review Acceptable range Overexposed: QC review Overexposed: QC exception Overexposed: repeat Sensitivity number, S Imaging plate (HR vs. ST) differences Examination specific histogram shapes S number varies with examination type EDR mode effects Automatic (determines S and S values on histogram) Semi-automatic (average value within ROI) Fixed (system acts like screen-film detector) X-ray beam spectrum effects S number varies with beam hardness (calibration required) What S value is appropriate? Determined by examination UCDMC targets Adult exams (CXR, abdomen, etc) 5 3 Extremities (ST plates) 75 5 Pediatrics 3 6 CR s variable speed should be used to advantage Anatomical information can be lost with too high or too low exposure 9

10 Adult portable chest calculated exposures #exams Low First half, 99, 57 exams % 53.9% 7.8% 3 Incident Exposure System speed (S #) Q Q <5 High Target exposure range Adult portable chest calculated exposures #exams Low Second half, 99, 66 exams 5 3.% 73.5% 3.% 3 System speed (S #) Incident Exposure Q3 Q <5 High Target exposure range Number of examinations April - 7, 996 Adult Portable Chest > > > > > Sensitivity number Exposure Creep Grid technique without a grid Wrong exam 5% Motion 6% Reprinting 9% Underexposure % Other % Repeated Examinations with CR Overexposure % Positioning 6% Total # repeats = 3 from Willis, RSNA 996 Variable Speed Detector Radiation Dose for CR Optimal dose for typical adult chest image is X higher than speed screen/film Lower absorption efficiency Quantum and electronic noise Readout inefficiencies of latent image Anti-scatter grids necessary for most procedures AEC adjustment procedures A -speed equivalent exposure is desirable Empirically determine AEC setting(s) with simple uniform phantoms UCDMC technique: use fixed mode (S=) and sensitivity test menu; adjust AEC response according to changes in film optical density Verify settings with semi-auto mode Verify patient exposure S number; recheck often

11 Date: 7//98 Location: UCDMC, ACC, 3 Medical Physicist: Anthony Seibert, Ph.D. System Identification: CR unit 3 UC Davis Medical Center CR Reader and Screens Inspection Results Summary Acceptable. Physical Inspection - Inventory Yes. Imaging Plate Uniformity and Dark Noise Yes 3. Signal Response: Linearity and Slope Yes. Signal Response: Calibration and Beam Quality Yes 5. Laser Beam Function Yes 6. High-Contrast Resolution Yes 7. Noise/Low-Contrast Response Yes 8. Distortion Yes 9. Erasure Thoroughness Yes*. Anti-Aliasing Yes. Positioning and Collimation Errors Yes. Throughput Yes Problem areas Film-based performance measurements Digital image analysis tools and evaluation methods not readily available Low contrast resolution measurements Lack of a standardized QC phantom Comments: Quality Control Three levels of system performance quality control. Routine: Technologist level - no radiation measurements. Full inspection: Physicist level - radiation measurements and non-invasive adjustments 3. System adjustment: Vendor service level - hardware and software maintenance Daily (technologist) Periodic Quality Control Inspect CR system and status. Interfaces: PACS broker, ID terminal, QC workstation Erase image receptors (if status unknown). 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 out-of of-tolerance issues. Clean screens with recommended agents.

12 Periodic Quality Control Annually (Physicist) Perform linearity / sensitivity / uniformity tests Inspect / evaluate image quality Re-establish establish baseline values (Acceptance Tests) What is needed? Computer friendly phantoms Objective quantitative analysis methods System performance tracking and database logs Exposure monitoring tools and database tracking Review retakes, exposures, service records. Fuji Agfa Line pair phantoms (contrast transfer tests) Lumisys Diagonal bar (laser jitter test) line/cm grid (visual aliasing) Home built Fiducial Markers (distance accuracy) Lead attenuator (dynamic range) Notches (geometric accuracy tests) Resolution Bar Pattern (qualitative) Single exposure, qualitative and quantitative 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 Additional Information / Help AAPM Task Group # document: jaseibert@ucdavis ucdavis.edu Dr. Ehsan Samei spreadsheets deckard.mc.duke..mc.duke.edu/~samei/downloads 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