Routine Ultrasound Equipment Tests for Quality Assurance. Contents

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1 Routine Ultrasound Equipment Tests for Quality Assurance James A. Zagzebski, Ph.D. Nick Rupert, M.S. Ryan DeWall, M.S. Tomy Varghese, Ph.D. Dept. of Medical Physics University of Wisconsin, Madison, WI, USA Contents Elements of a Routine Test Program What system components are considered most important in routine testing? Concentrate on transducers! Steps to carry out tests on a regular basis Activity of Ultrasound Lab Accreditation Groups Basic QA methods Phantoms and tests 1

2 Routine QA Program Guidelines available from US facility accreditation bodies (ACR; AIUM) Emphasis is given to safety, cleanliness, and very basic performance tests. Issues related to image performance, such as spatial resolution, are not included (except ACR breast). Traditional parameters such as distance measurements are believed by many to be unnecessary. Includedi n some protocols, but only briefly. Routine QA (ACR General US Program) System sensitivity and/or penetration capability 1999: Do for 2 probes (?) Image Uniformity Look for element dropout Photography and other hard copy recording 2010: workstation monitor display Low contrast object detectability Assurance of electrical and mechanical safety and sterility Verify measurement accuracy during program initiation 2

3 American College of Radiology Requirements: General Sonography ACR physics committee developing this standard based it on measurements using a phantom. A phantom was selected (fall,1998) but turned out to be much more costly than originally promised. Physics Committee chose to leave the choice of phantoms to the user ACR staff published the following: Using a phantom will be helpful in responding to questions about low contrast detectability. However, the use of a phantom is optional at this time. Questions relating to characteristics associated with system sensitivity and image uniformity may be answered without the use of a phantom or test object. American Institute of Ultrasound in Medicine: General US QC 2008 Originally called QA in the Clinic Sonographers and clinicians helped draft Outlines what to do Sonographers Physicists/engineers Limited information on methodology Requires a phantom Phantom left to users See 3

4 American Institute of Ultrasound in Medicine: General US QC American Institute of Ultrasound in Medicine: General Sonography (physicist, engineer) Transducer Uniformity Phantom Electronic probe tester Sensitivity, Maximum depth of visualization into a phantom Distance measurement accuracy Target detection and imaging Focal targets such as simulated cysts or low contrast objects Choice of phantoms left to users Image display fidelity Cables OK, air filters clean, mechanical and electrical inspection Minimum frequency Annual for physicists/engineer tests Program includes daily, weekly checks by sonographers 4

5 Water-based gels Currently used materials: Advantages: Speed of sound = 1540 m/s Attenuation proportional to frequency Backscatter Disadvantages: Subject to desiccation Must be kept in containers Requires scanning window Currently used TM materials: Solid, non-water-based materials (urethane) Advantages: Not subject to desiccation No need for scanning window Produce tissue-like backscatter Disadvantages: C= m/s Attenuation not proportional to frequency (~f 1.6 ) Surface easily damaged if not cleaned regularly to remove gels 5

6 Maximum Depth of Visualization Considered by many as a good overall check of the integrity of the system FOV at 18 cm (or set to match the phantom) Output power (MI) at max Transmit focus at deepest settings Gains, TGC for visualization to the maximum distance possible Maximum Depth of Visualization How far can you see the speckle pattern in the material? 6

7 Objective Maximum Depth of Visualization Gibson, Dudley, Griffith, A computerized Quality Control System for B-mode Ultrasound, Ultrasound Med Biol 27:1697 : , 1711, Shi, Al-Sadah, Mackie, Zagzebski, Signal to Noise Ratio Estimates on Ultrasound Depth of Penetration (abstract only), Medical Physics 30: : 11367, Gorny, Tradup, Bernatz, Stekel, and Hangiandreou, Evaluation of an Automated Depth of Penetration Measurement ofr the Purpose of Ultrasonic Scanner Comparison, (abstract only), J. Ultrasound Med 23: S76, MPV Compute mean pixel value vs depth for phantom (signal) and for noise only (noise) Depth where signal/noise = 1.5 =DOP SNR -DOP tracks well Observer-DOP 16 Observers-DOP & SNR-DOP comparision (ATL C5-2) Depth (cm) (Al-Sadah, UW-Madison) Observer's DOP DOP using SNR'= Mechanical Index MI 7

8 Routine QA: Image Uniformity Considered to be the most impor- tant and useful test! Ideally: No evidence of element dropout No vertical shadows No loss of sensitivity near edges Uniformity Turn off spatial compounding, speckle reduction Use 1 tx focus Search for shadows emanating from transducer Common in new and old probes! 8

9 Coupling to a flat surface phantom scanning window with curvilinear transducers Coupling to a flat surface phantom scanning window with curvilinear transducers Solution 1: rock transducer from side to side 9

10 Coupling to a flat surface phantom scanning window with curvilinear transducers Solutside to side Coupling to a flat surface phantom scanning window with curvilinear transducers 10

11 Coupling to a flat surface phantom scanning window with curvilinear transducers Solution 2: Use a liquid TM material Coupling to a flat surface with curvilinear transducers 1 Sensitivity 0.8 Volts p-p Elements Electronic Probe test Solution 2: Use a liquid TM material (pseudo liquid TM slurry; Madsen et al.) 11

12 Coupling to a flat surface phantom scanning window with curvilinear transducers Solution 3: Use a phantom having concave windows (Goodsitt et al) Visualizing 1-2 element dropouts Use persistence; translate transducer. 12

13 Visualizing 1-2 element dropouts Use persistence; translate transducer. Visualizing 1-2 element dropouts Record Image loops while translating the transducer; Compress a 100 frame loop into 1 image (averages speckle) 1 image Ultrasonix SonixTouch Enables masking off individual elements Elements 95 and 96 masked 13

14 Visualizing 1-2 element dropouts Record Image loops while translating the transducer; Compress a 100 frame loop into 1 image (averages speckle) Average image Ultrasonix SonixTouch Enables masking off individual elements Elements 95 and 96 masked Visualizing 1-2 element dropouts Use persistence; translate transducer Use Record Image loops while translating the transducer; compress a 100 frame loop into 1 image (averages speckle) Phased array transducer performance Need electronic probe tester Transducer test modes on the ultrasound instrument are possible; manufacturers need to be encouraged to provide these Future 2-D probes, probes that do part of the beam forming within the probe housing 14

15 Gray Scale Photography, Workstation Fidelity Important for monitor on machine to be set up properly to view all echo levels available and entire gray bar pattern. Set up during acceptance testing Take steps to avoid the meat hook effect (mark or inscribe contrast and brightness controls) Workstation monitors and/or hardcopy devices should be matched to the machine monitor. AAPM, July, 2010 Check hardcopy, workstation displays. Are all gray bar transitions visible? 15

16 SMPTE Test Pattern Available on most scanners 0% to 100% gray bar pattern Squares for detecting geometric distortion Are all gray transitions visible? Distance Measurement Accuracy: Vertical Actual 2.5 Measure 2.46 error 1.5% Acceptable 16

17 Distance Measurement Accuracy: Horizontal Actual 1.0 Measure 1.01 error <1% Acceptable Routine QA (ACR General US Program) Distance Measurement Accuracy tests as needed Necessary? ( Scanner is a transducer tied to a computer. ) May be important for specific uses Images reregistered from 3-D data sets Workstation measurements Radiation seed implants 17

18 ACR Recommended QC for Breast US Maximum Depth of visualization Vertical and horiz measurement accuracy Semiannually Semiannually Service eng./physicist Service eng./physicist Hardcopy Recording Semiannually Service eng./physicist Uniformity Semiannually Service eng./physicist Elec-mechanical condition Semiannually Service eng./physicist Anechoic void perception Semiannually Service eng./physicist Ring down Semiannually Service eng./physicist Lateral Resolution Semiannually Service eng./physicist QC Checklist Semiannually Service eng./physicist Adherence to infection control procedures After each biopsy Sonographer Clean transducers After each biopsy Sonographer Vertical and horiz measurement accuracy Quarterly Sonographer Grey-scale photography Quarterly Sonographer Dead Zone? No suitable targets for dead zone testing, even in phantoms that advertise deadzone tragets! 18

19 Anechoic Sphere Imaging, a better choice Anechoic Sphere Imaging 19

20 Image of a phantom is useful for qualitative comparisons! Conventional Spatial Compounding Images obtained during routine Breast QC testing, 3/2010 Image of a phantom is useful! Conventional Spatial Compounding Images obtained 1 month later, after a software change; 20

21 Routine QA: Cleanliness, safety Transducer Inspection Delaminations Frayed cables Proper cleaning Console Air filters Viewing monitor, keyboard clean Lights, indicators Wheels, wheel locks Proper cleaning Summary Setting up, maintaining an equipment QA program is straight forward The ACR listed procedures are a useful, basic QA program Directed by physicist or lab personnel Integrated effort including lab and technical staff Requires a Phantom Closely correlates with AIUM list of factors to test Transducer uniformity is a frequent fault in today s scanning machines Computational methods can be developed for objective tests 21

22 References Goodsitt et al, Real-time B-mode ultrasound quality control test procedures, Medical Physics 1998; 25: J Zagzebski, US quality assurance with phantoms. In Categorical Course in Diagnostic Radiology Physics: CT and US Cross-Sectional Imaging, Edited by L. Goldman and B. Fowlkes, 2000, Oak Brook, IL: Radiological Society of North America, pp J Zagzebski and J Kofler, Ultrasound Equipment Quality Assurance, in Quality Management in the Imaging Sciences, ed. By J Papp, 2002, St. Louis, Mosby, pp QA Manual for Gray Scale Ultrasound Scanners, 1995, American Institute of Ultrasound in Medicine, Laurel, MA. D Groth et al, Cathode ray tube quality control and acceptance program: initial results for clinical PACS displays, Radiographics 2001, 21: