Ultrasound Physics and QA (MRD535)

Ultrasound Physics and Instrumentation (MRD535) - Quality Assurance

• The course covers ultrasound physics, instrumentation, accessories, and image recording.
• It analyses numerical and visual data related to ultrasound physics and instrumentation.
• Quality assurance is vital to ensure consistent and reliable results, checking for performance deterioration.

Contents

• Describe the principle, physics, instrumentation, accessories, and image recording in ultrasonography.
• Discuss the equipment Quality Assurance (QA) program.
• Understand spatial and contrast resolution.

Quality Assurance

• Ensures consistent and reliable results.
• Checks for performance deterioration.
• Testing is usually the responsibility of Medical Physics Departments.
• Sonographers are recognized, and recent publications include tests performed by ultrasound users.

Quality Assurance (cont.)

• British Medical Ultrasound Society (BMUS) guidelines define three Quality Assurance (QA) levels:
  • Level 1: Infection control and scanner damage checks (daily)
  • Level 2: Basic scanner and transducer testing (daily)
  • Level 3: Further scanner and transducer testing (monthly)

Level 1

• Procedures aim to ensure a clean scanner and minimize infection control risks to patients and staff.
• Daily checks for damage to the scanner, especially transducers and cables, are performed.
• Weekly checks for scanner damage are performed.
• Cleaning of cables and transducers (use non-textured paper towels, avoid alcohol).
• Storage of transducers in their holder when not in use.
• Avoid leaving transducers on other surfaces.
• Correct stowing of cables for non-use transducers.
• Clean monitors using soft cloths and manufacturer-approved screen cleaner; avoid paper towels and ammonia-based products.
• Check the functionality of main scanner controls.
• Inspect transducers for wear and damage, paying attention to the edge of the lens where it is bonded to the plastic casing and checking the junction for potential damage.

Level 1 (cont.)

• Inspect cables (scanner switched off), checking probe cables, mains cables, plugs, and cables connected to peripherals for damage.
• Disconnect probes (never operating) and check connectors for damage, signs of stress (e.g., twisted or misaligned pins, abrasion, or corrosion).
• Inspect the console and main body of the scanner.
• Check the functionality of brakes and wheels. Inspect air filters for dust and fluff.

Level 2

• Ensures appropriate video monitor settings for consistent imaging.
• Provides first-line evaluation of scanner performance without test tools.
• Performed daily.

Level 3

• Provides further scanner performance evaluation using simple test tools.
• Performed monthly.

Air Reverberation Pattern (Sensitivity)

• Operate transducer dry in air.
• Use baseline settings (output to 100%, increase overall gain to maximum, ensure central TGC sliders, and set focus to the most superficial setting).
• Observe a reverberation pattern of lines parallel to the transducer face.
• Any local changes in the pattern indicate dropout.
• Measure the distance from the image top to the deepest visible reverberation in the image center.

Element Dropout Test

• Run the edge of a paperclip along the probe.
• The paperclip produces strong localised echoes at the contact point.
• Any echo loss indicates element dropout, which may be clinically significant.

Electronic Noise Assessment

• Reduce overall gain until noise disappears from the image.
• Record the gain value as the noise threshold.
• Note any changes from the baseline.

Equipment Quality Assurance (QA) Program

• Uses tissue-equivalent phantoms that mimic tissue properties (velocity, scattering, attenuation).
• Zerdine® phantoms are solid-elastic, water-based polymers with echogenic patterns similar to human liver parenchyma.
• Zerdine® is elastic and not damaged by heavy scanning pressures.

Establishing a Baseline

• Before QA measurements, establish system settings for each measurement.
• Establish and record the system settings used for each test.
• The first set of measurements will serve as the baseline.
• Subsequent scans refer to the baseline to check for system drifts.

Establishing a Baseline (cont.)

• Each facility determines the action level for each test.
• If the system's measurement accuracy is 10% for distances up to 2cm, then the scanner may detect 2cm as anywhere from 1.8 to 2.2 cm while still functioning properly
• The frequency of system assessment is determined by the facility; at least manual assessments are required.

Testing Procedure - Uniformity Testing

• Measures the machine's ability to display echoes of similar magnitude and depth with equal brightness.
• Ensures all crystals within the transducer are functioning as expected.

Testing Procedure (cont.)

• Depth of penetration (maximum depth of visualization/sensitivity)
• Determines the furthest distance in a phantom where echo signals due to scatterers in the tissue mimicking background material are detected on the display.
• Depth of penetration is dependent on the transducer frequency, medium attenuation, and system settings.

Testing Procedure (cont.)

• Beam profile, focal zone, and lateral response width
• The beam profile illustrates the ultrasound beam's shape.
• The narrowest region within the beam profile corresponds to the focal point.
• The best images are typically obtained within the focal zone.

Testing Procedure (cont.)

• Vertical distance measurements
• Determined by measuring the distance along the axis of the beam. Using vertical wire targets to check accuracy.
• Horizontal distance measurements
• The accuracy of measurements perpendicular to the beam axis is determined. The measurements use horizontal wire targets to check accuracy.

Testing Procedure (cont.)

• Axial resolution testing
• The ability of the ultrasound system to distinguish closely spaced objects along the beam axis.
• Axial resolution is proportional to the length of the transmitted ultrasonic pulse.

Testing Procedure (cont.)

• Lateral resolution testing
• The ability of the ultrasound system to distinguish closely spaced objects in a plane perpendicular to the beam axis. Lateral resolution improves with a narrower beam width.

Testing Procedure (cont.)

• Elevational testing
• A measurement of elevational resolution (or slice thickness).
• Assess elevational resolution by rotating the transducer by 45° relative to the vertical wire targets.

Testing Procedure (cont.)

• Low-contrast target detectability (contrast resolution)
• Measures the ability to distinguish small targets with varying degrees of contrast.
• Evaluates a system's ability to detect and display the size, shape, and depth of dark or anechoic (non-reflective) structures.

Testing Procedure (cont.)

• Dead zone assessment
• An assessment of the transducer's near field and the distance from the front face of the transducer to its closest identifiable echo.
• Also known as the "dead zone", "ring-down distance", and "near-field resolution". This region is where the system does not provide the most useful information.

Ultrasound Image Quality Phantom

• Used for testing various aspects of the ultrasound image, such as grayscale, resolution, and depth of field.

Additional Notes

• The course covers various testing procedures and measurements, including specific equipment.
• Specific phantoms, like those made of Zerdine, are part of these procedures.
• Maintaining consistent and accurate baseline measurements are key for accurate analysis.
• Important considerations include the correct use of equipment.