Quality Control in Ultrasound Imaging

Ultrasound Quality Control: Test Objects and Phantoms

Quality control is essential in imaging departments, especially those using ultrasound, to ensure equipment functions properly.
Testing is performed by service engineers, and some tests can be done by clinical sonographers.
Test phantoms or test objects are used to evaluate system performance.

Traditional Phantoms:
- Fluid-filled with wires at known distances.
- Used to check calibration by measuring distances between wires.
- Used to assess resolution.
Modern Multipurpose Phantoms:
- Hydrogel-based with a membrane for scanning.
- Low attenuation coefficient (similar to water, e.g., 0.7 \, \text{dB/cm/MHz}) to mimic tissue properties.
- Test resolution, calibration, and contrast resolution.
- Contain structures that mimic cysts and echogenic masses to evaluate how well different structures are defined.
Speed of Sound:
- Phantoms mimic soft tissue; the speed of sound within the phantom should match that of soft tissue.
- Data sheets specify the speed of sound for the device (e.g., 1540 \, \text{m/s}).
Tissue Mimicking:
- Phantoms are designed to mimic tissue structures, allowing for realistic system testing.

General Ultrasound Training Phantoms:
- Various phantoms are available for different scenarios, like breast phantoms.
Biopsy Phantoms:
- Contain vessels for practicing needle tip location and insertion.
Pelvic Phantoms:
- Useful for transvaginal ultrasound training, with simulated ovaries and a fluid-filled bladder to learn anatomical relationships.

"Test objects" and "phantoms" can be used interchangeably.
Modern phantoms are multipurpose, testing various aspects of the ultrasound system, including calibration, resolution, and lesion detection.
Dedicated phantoms exist for specific tests, like slice thickness evaluation.

Axial Resolution:
- Phantoms help determine how close two structures can be, above and below each other, and still be resolved as separate entities.
Lateral Resolution:
- Dependent on beam width (left to right on the image).
- Phantoms contain a series of wires to assess how close structures can still be distinguished.
Slice Thickness:
- Refers to the beam thickness in and out of the image plane.
- Specialized phantoms measure slice thickness.

Compare different transducers.
Monitor a transducer's performance over time.
Assess registration: ensuring the location of a structure on the image matches its real-life location.

Correct placement of echoes is crucial.
Poor registration occurs when structures appear distorted on the image compared to their actual arrangement in the test object.

Some test objects have wires near the surface to test for the main bang artifact in the near field.
Different transducers show varying clarity in the near field.

Axial Resolution:
- Evaluated using closely spaced dots. If the dots are too close, they appear as one on the image.
Lateral Resolution:
- Evaluated with closely spaced wires. The ability to distinguish the wires indicates lateral resolution.
- Lateral resolution is influenced by beam width.
- Lateral resolution can vary at different depths or focus settings.

Test objects with known distances between wires are used to verify the accuracy of caliper measurements on the ultrasound system.
Measurements are taken in both vertical and horizontal planes.
A 2% error or less is generally considered acceptable, according to manufacturers.

Test objects with small wires at increasing depths can visualize the beam profile.
The appearance of these wires indicates the beam's focus and width at different depths.
Traditional transmit focus methods may produce an hourglass-shaped beam profile, with the best lateral resolution at the focus.

Phantoms often contain subtle lesions at various depths to test the system's sensitivity in detecting slight differences in echogenicity.
The ability to detect these lesions is assessed over time to monitor the transducer's performance.

The hydrogel within the phantom should have uniform density and acoustic properties.
After adjusting TGC settings, the image should appear uniform with consistent echogenicity and architecture throughout.
Patchiness suggests potential issues with the transducer or system at certain depths.

Test phantoms can help detect dead elements in the transducer.
A clear vertical line on the image indicates a dead element, provided there is not too much compounding or post-processing.

Specialized phantoms contain cones or simulated lesions with varying echogenicity.
Contrast resolution is determined by the smallest difference in echogenicity that is visible.
Harmonic imaging or different transducers can be compared to evaluate contrast resolution.

The ultrasound beam has a three-dimensional aspect including beam width and slice thickness (in and out of the image plane).
Echoes from regions outside the 2D scan plane reduce contrast resolution, so a thinner slice thickness improves contrast resolution.

Specialized phantoms contain a diagonal reflecting surface.
When scanned correctly, the image displays a band whose thickness indicates the relative slice thickness.

Printout image quality depends on the printer or camera (if applicable).
Monitor calibration is crucial for accurate image display.
Post-processing and offline measurement systems (e.g., PACS) must be calibrated correctly.