AAPM Task Group 128: Quality assurance tests for prostate brachytherapy ultrasound systems
Prostate brachytherapy transrectal ultrasound probe. In (a), note dual connectors required for the orthogonal axial and longitudinal arrays. In (b), the white arrow indicates the axial array and the striped arrow indicates the longitudinal array.
Test setup. The transrectal ultrasound probe is shown passing under the needle template. Both are fixed to the stepper, which is connected to a stabilizing arm locked to the treatment table. The CIRS Brachytherapy Phantom is used in this example.
Phantom schematic. This is the CIRS Model 45 phantom, and is used here for illustrative purposes only; no endorsement is implied.
Grayscale determination. (a) Measurement using discrete steps. The arrowheads indicate the maximum range of discrete steps visible. (b) Measurement using a grayscale gradient. The limits of the visible range are indicated by the plus signs and the length between them indicated at the bottom of the image.
Depth of penetration measurement. In this example, the maximum depth, indicated by the (solid arrow) is approximately (dotted arrow). The marker is located at the position where the speckle of the phantom is overcome by the electronic noise of the system.
Resolution using single filament targets. In (a), axial resolution is demonstrated with the axial array, while (b) demonstrates lateral resolution with the axial array. In (c) axial resolution using the longitudinal array is demonstrated, and (d) demonstrates lateral resolution using the longitudinal array. Note the loss of lateral resolution with depth in (b) due to the targets being out of the focal zone of the transducer. Axial resolution is seen to be constant with depth in (a) as axial resolution is primarily dependent upon the frequency of the transducer.
Resolution using paired filament targets. The axial and lateral separation of the nylon fibers are , , , , . A transducer frequency of is used in (a) and in (b). Note the improved axial resolution with the higher frequency in (b).
Phantom containing high contrast fibers for distance verification.
Axial distance measurement accuracy. Ideally, the fibers should be centered in the field of view. If this is not possible, image a column of fibers and note that the horizontal spatial calibration also contributes to the measured vertical distance. When using the electronic calipers for a vertical distance measurement, place the caliper centered on or along the right or left side (be consistent) and centered vertically with respect to the target image.
Measurements of horizontal distance accuracy when several rows of targets are available. For a horizontal distance measurement, the calipers should be placed just above or below the target and centered horizontally with respect to the target. It is critical to be consistent in placement of the caliper with respect to the target image.
Distance accuracy measurements using a cylindrical, sphere, or egg-shaped object. (a) Axial distance measurement; (b) lateral distance measurement.
Using an electronically generated grid to verify distance accuracy throughout the field, of view. Note that fibers near the transducer are rendered farther apart than corresponding grid points. The lateral distance registration at deeper depths is more accurate.
Volume measurement using a target of known volume.
Artifact caused by poor contact between the ultrasound probe and the scan surface. In this case, the source of poor contact is insufficient coupling gel, leaving an air gap between the probe and scan surface.
The displayed depth will not be correct if the scan plane is not perpendicular to the fiber targets. The illustration on the right shows a greater distance from the probe to the target due to the orientation of the probe.
The top illustration shows the circular cross section of the cylindrical target when the scan plane is perpendicular to the axis of the target. The lower illustration shows an elongated image due to the orientation of the probe. Distance measurements recorded from this image would not correctly reflect the performance of the imager.
A ring-down artifact generated by imaging a needle. Note that the “tail” of the artifact always points along a scan line and away from the transducer.
A time sequence showing the formation of a ring-down artifact. Echoes that correspond to the “true” image of the objects are recorded at the times labeled 3 and 5 in the illustration. However, as the pulse continues to “ping pong” between the two interfaces, additional echoes reach the transducer and appear as equally spaced artifacts on the image. Additionally, the pulse continuously loses intensity as it travels; therefore, the artifact appears to taper off with depth.
Sound speed of selected materials (Ref. 10).
Quality control tests: frequencies and action levels.
Percent error associated with vertical (axial) distance accuracy measurements due to material speed of sound.
Percent error for distance measurements with various caliper misplacements.
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