Ultrasound Physics and Instrumentation MRD535 Quality Assurance PDF

Ultrasound Physics and Instrumentation MRD535 Quality Assurance By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and ins...

Ultrasound Physics and Instrumentation MRD535 Quality Assurance By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Analyse numerical and visual data related to the physics and instrumentation in ultrasonography. (C4) Contents Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2). Discuss the equipment QA Program. Understand the Phantoms- spatial / contrast resolution. Quality Assurance To ensure a consistent and reliability of results. To check for deterioration in performance. Testing been the responsibility of Medical Physics Departments. Sonographers has been recognized and recent publications have included tests to be performed by ultrasound users. Quality Assurance (cont) British Medical Ultrasound Society (BMUS) guidelines includes three levels of Quality Assurance (QA): Level 1. Infection control and scanner damage Level 2. Basic scanner and transducer testing Level 3. Further scanner and transducer testing BMUS guidelines for the regular quality assurance testing of ultrasound scanners by sonographers Level 1 Level 1 procedures have a two-fold purpose: to ensure that the scanner is clean and that infection control risks to patients and staff are minimized. Infection control measures are relevant for every patient so that some of these actions are performed several times each day. checks are performed to detect any damage to the scanner, especially to the transducers and their cables. Checks for scanner damage are performed weekly. Level 1 (cont) Cleaning of cables and transducers. Transducers should be wiped gently with a non- textured paper towel, taking care not to cause wear to the lens. Use of a manufacturer-recommended cleaning agent is acceptable. Avoid use of alcohol, such as alcohol wipes, as these can damage the transducer. Level 1 (cont) Transducer storage. Transducers should be stored in their holder, usually on the side of the ultrasound scanner, when not in use. Avoid leaving the transducer on other surfaces such as the patient couch as they can fall off and be damaged. Care of transducer cables. For transducers not in use, the cables should be correctly stowed so that they do not trail on the ground. For transducers in use, some of the cables may trail on the ground; in these circumstances care should be taken when moving the scanner or chair to avoid running over the cable as this is a potential cause of damage. Level 1 (cont) Cleaning of monitors. Clean dust from the monitor gently with a soft dry cloth. Stains can be removed by using a moist (not wet) soft cloth. A manufacturer-approved screen cleaner is also acceptable. Avoid paper towels as these can scratch the screen. Avoid ammonia-based products as these can damage flat screen LCD monitors. Level 1 Checking of main scanner controls. Check that the most used controls are functioning normally, and that there are no faults such as controls which fail to respond or stick, or respond intermittently. Inspection of transducers. Visually inspect all transducers for signs of wear and damage. Attention needs to be paid to the edge of the lens where it is bonded to the plastic casing. Applying a light pressure to the probe at the junction between the lens and plastic casing may reveal movement suggesting damage to the bonding. incorrect use of cleaning agents can result in the casing becoming brittle, making it prone to stress fractures (hairline cracks). Level 1 Inspection of cables. This should be performed with the scanner switched off. Inspect the probe cables, mains cables, plugs and cables to any attached peripherals for damage. Disconnect the probes (never do this with the probe operating) and check the connectors for physical damage and signs of stress (e.g. twisted or misaligned pins, abrasion or corrosion for surface connections). Level 1 Inspection of the console and main body of the scanner. Brake and wheel function. Air filters. Check the filters (often located towards the bottom of the scanner body) for dust and fluff, which could allow damaging heating to occur. Level 2 To ensure appropriate setting of video monitor controls for consistency of imaging. They also provide a first-line evaluation of scanner performance but without the use of test tools or test objects. These tests are performed daily. Level 3 Provide further evaluation of scanner performance using a very simple test tool. These checks are designed to look for scanner faults and are performed monthly. Air reverberation pattern (sensitivity) 1. Operate the transducer dry in air. 2. Use settings recorded at baseline: alter output to 100%; increase the overall gain to maximum ensure Time Gain Compensation (TGC) sliders are positioned centrally (or as set at baseline); move a single focus to the most superficial setting 3. A reverberation pattern should be seen consisting of a series of lines parallel to the transducer face. Air reverberation pattern (cont) 4. Any local change in the reverberation pattern may indicate dropout. 5. Any local change in the reverberation pattern may indicate dropout. 6. Measure and record the distance from the top of the image to the deepest visible reverberation in the centre of the image. 7. Record the image and note and report any changes to images recorded at baseline. Air reverberation pattern (sensitivity) measurement, from the top of the image to the deepest visible reverberation line Element dropout test 1. Run the smooth edge of a paperclip along the probe. 2. The paperclip will produce strong echoes localised at the point of contact. 3. Any loss of echoes indicates element dropout, which is likely to be clinically significant. Electronic noise assessment 1. Reduce the overall gain to the point where noise has just disappeared from the image. 2. Record the gain value as the noise threshold. 3. Note and report any change from the baseline Electronic noise assessment. The gain has been reduced to the point where noise has disappeared (2DG 90 in the image; at gain of 91 noise becomes visible) Equipment Quality Assurance (QA) Program Uses a tissue-equivalent phantom. Mimic all / most the properties of tissue (velocity, scattering and attenuation). Most of the phantoms are composed of Zerdine®, a solid-elastic, water-based polymer that exhibits echogenic patterns like those encountered in human liver parenchyma. Zerdine is elastic and is not damaged by heavier scanning pressures. Multipurpose Tissue/Cyst Ultrasound Phantom Multi-Purpose, Multi-Tissue Ultrasound Phantom (Model 84-317) Model 040GSE Establishing A Baseline Before performing routine quality assurance measurements, establish: System settings for each measurement Establish and record what system settings should be used for each of the quality assurance tests. These same settings should be used each time the test is performed. Baseline measurements The first set of measurements taken will be the baseline measurements for the combination of system settings and phantom On subsequent scans, refer to the baseline results to determine if the ultrasound system has drifted to an unacceptable level Establishing A Baseline (cont) Allowable deviation from baseline measurements Each facility needs to determine the action level for each test. If the measurement accuracy for your system is 10% for distances up to 2 cm, the scanner may detect 2.0 cm as being any where from 1.8 cm to 2.2 cm and still be functioning properly. Frequency of system assessment Determined by the facility At least manually Testing Procedure Uniformity testing the ability of the machine to display echoes of the same magnitude and depth with equal brightness on the display. This is a good test to ensure all crystals within the transducer are functioning.  Apply coupling gel to the scanning surface. Position the transducer on the scanning surface in a region with a minimum number of targets. Adjust the instrument settings (gain, TGC, output, etc). Record these settings for use on subsequent testing.  Align the probe so that the targets are maximized. Freeze the image. Observe the general appearance of the phantom. Note if all regions at the same depth are displayed with the same intensity across the width of the image. Testing Procedure (cont) Depth of penetration (aka maximum depth of visualization/ sensitivity) Is the greatest distance in a phantom for which echo signals due to the scatterers within the tissue-mimicking background material can be detected on the display The depth of penetration is determined by the frequency of the transducer, the attenuation of the medium being imaged and the system settings.  Apply coupling gel to the scanning surface. Position the transducer to acquire an image of a vertical plane target. (The wires should appear as dots, not lines). Adjust the instrument settings (gain, TGC, output, etc.). Record these settings for use on subsequent testing.  Align the probe so that all the vertical targets are displayed at their maximum intensity level. While actively scanning, look to see where the backscattered echoes within the background material disappear. Be careful not to confuse electronic noise with the background backscattered echoes. Electronic noise will move but back- scattered echoes will remain stationary while maintaining the transducer in a fixed position. Freeze the image. With electronic calipers measure the distance between the scanning surface and the last identifiable echoes due to scattering. Remember to measure the distance to the scattered echoes, not to the last visible wire. Testing Procedure (cont) Beam profile, focal zone and lateral response width The beam profile is the shape of the ultrasound beam. The narrowest region within the beam profile is indicative of the focal point. The best images are obtained while within the focal zone.  Apply coupling gel to the scanning surface or fill the water trough with tap water. Position the transducer in a vertical plane. (The wires should appear as dots, not lines). Adjust the instrument settings (gain, TGC, out put, etc.) Record these settings for use on subsequent testing.  Align the probe so that all the vertical targets are displayed at their maximum intensity level to insure the transducer is imaging a vertical plane. Freeze the image and obtain a hard copy. Some of the targets will appear as short horizontal lines rather than dots on the frozen image. Measure the horizontal length of the targets. These measurements represent the lateral response width of the system at the different depths and setup. The minimum length is indicative of the location of the focal point. Testing Procedure (cont) Vertical distance measurements Is defined as the distance along the axis of the beam. The vertical wire targets are used to assess the accuracy of vertical distance measurements Horizontal distance measurements Is used to determine the accuracy of measurements made perpendicular to the beam axis. This group has 7 wires positioned 20 mm apart at a depth of 9 cm  Apply coupling gel to the scanning surface or fill the water trough with tap water. Position the transducer in a vertical plane. (The wires should appear as dots, not lines). Do not apply excessive pressure when performing vertical distance measurement as this may temporarily compress the target and skew the measurements. Adjust the instrument settings (gain, TGC, out put, etc.) Record these settings for use on subsequent testing.  Align the probe so that all the vertical targets are displayed at their maximum intensity level- (vertical distance). Align the probe so that all the horizontal targets are displayed at their maximum intensity level- (horizontal distance). Freeze the image and obtain a hard copy. Using electronic calipers measure the distances between two wires at various depths– (vertical distance)/ measure the distances between two wires along the horizontal plane (horizontal distance). Testing Procedure (cont) Axial resolution testing Axial resolution is defined as the ability of an ultrasound system to resolve objects in close proximity along the axis of the beam. it determines how close can two objects be along the axis of the beam and still be detected as two distinct objects. Axial resolution is proportional to the length of the system’s transmitted ultrasonic pulse or pulse length. Testing Procedure (cont) Lateral resolution testing Lateral resolution is defined as the ability of an ultrasound system to resolve objects in close proximity perpendicular the axis of the beam. Lateral resolution will improve with a narrowing of the beam width.  Apply coupling gel to the scanning surface or fill the water trough with tap water. Position the transducer in a vertical plane. (The wires should appear as dots, not lines). Adjust the instrument settings (gain, TGC, out put, etc.) Record these settings for use on subsequent testing.  Align the probe so that all the targets are displayed at their maximum intensity level and adjust the focus to match the depth of the targets assessed. Freeze and save the image Determine the last pair of axial wire targets to be distinguished as two separate entities and record the value Determine the last pair of lateral wire targets to be distinguished as two separate entities and record the value Testing Procedure (cont) Elevational testing A measurement of elevational resolution, or slice thickness. Elevational resolution can be assessed by rotating the transducer 45° with respect to the vertical wire targets.  Apply coupling gel to the scanning surface or fill the water trough with tap water. Position the transducer in a vertical plane. (The wires should appear as dots, not lines). Adjust the instrument settings (gain, TGC, out put, etc.) Record these settings for use on subsequent testing.  Orient the transducer to obtain an image with the vertical target wires in the center. Be sure to adjust the tilt of the transducer so that the wires are lined up in a vertical column. Rotate the transducer 45°, being careful to keep the tilt of the transducer so that the wires are still lined up in a vertical column. Adjust the focus to match the depth of the pin with the smallest width. Freeze the image and measure the length of key wire segments with the electronic calibers  The most important wire to include in this set is the smallest wire segment, the depth of which defines the elevational focus of the transducer. Elevational resolution measurement. The fourth wire measured, at a depth of 10 cm, indicates the location of the elevational focus. Testing Procedure (cont) Low-contrast target detectability (Contrast resolution) Ability to detect and display the size, shape, and depth of dark, or anechoic, target structures. This test indicates the smallest diameter target a system can resolve at a given depth.  Apply coupling gel to the scanning surface. Position the transducer above the cyst of interest and perpendicular to the wires. Adjust the instrument settings (gain, TGC, output, etc.). Record these settings for use on subsequent testing. Align the probe so that the target is maximized. Freeze the image and obtain a hard copy.  When performing this test, anechoic targets should appear circular with sharp, clearly defined edges, and be free of any observable shading (echo fill- in). When imaging anechoic targets, bright echoes may be observed at the top and bottom of anechoic targets Targets appear to shrink with depth Testing Procedure (cont) Grayscale contrast sensitivity To create greyscale images in B-mode ultrasound, the ultrasound scanner converts the amplitude of echoes received by the transducer to brightness values on the display. The range from a just noticeable echo at the lowest grey level to the maximum echo brightness at the highest is referred to as the displayed dynamic range or greyscale contrast sensitivity.  Apply coupling gel to the scanning surface or fill the water trough with tap water. Position the transducer above the greyscale targets (appear as a circular region). Adjust the instrument settings (gain, TGC, output, etc.) Record these settings for use on subsequent testing. Not clearly visible Observe the general appearance of each target. Note if you are able to see each of the target. Targets should have a clear (not distorted/ blurry), progress in brightness from low to high based on the contrast levels. Testing Procedure (cont) Dead zone assessment The near field group is used to assess the distance from the front face of the transducer to the closest identifiable echo. This region, where no useful information is obtained, is commonly referred to as the “dead-zone”, “ring-down distance”, or "near field resolution." The dead zone occurs because the ultrasound system cannot send and receive data simultaneously. It is instrument dependent and is diminished as frequency is increased.  Apply coupling gel to the scanning surface. Position the transducer above the near field resolution target and perpendicular to the wires. (The wires should appear as dots, not lines). Adjust the instrument settings (gain, TGC, output, etc.) to maximize resolution in the near field. Record these settings for use on subsequent testing.  With the electronic calipers, measure the distance from the transducer face to the closest resolvable wire target. Record the distance. Note that the actual dead zone is smaller than this distance. If the first target to be resolved is at 4 mm, then the dead zone distance is “something less than 4 mm”. Grayscale Thank you