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A test phantom for estimating changes in the effective frequency of an ultrasonic scanner
Thaddeus Wilson1, James Zagzebski, Yadong Li
1Department of Radiology, University of Tennessee, Memphis 38163, USA.
Objective:
Ultrasonic frequency is an important performance feature of B-mode scanners. It is particularly relevant when comparing instruments from different manufacturers and reporting clinical results. We investigated a test phantom to independently measure an effective imaging frequency, including effects of depth-dependent attenuation and frequency filtering during echo reception.
Methods:
The approach capitalizes on variations of the frequency dependence of backscatter with scatterer size. A tissue-mimicking phantom containing 48-microm-diameter scatterers was constructed. Embedded at depths of 1, 3, 7, and 9 cm were sets of cylindrical inclusions, each containing tissue-mimicking material with a different scatterer size and number density. Computer simulations helped establish scatterer parameters for the cylinder bodies that resulted in image contrast versus the background that varied with frequency, with each cylinder transitioning from negative to positive contrast at a different frequency. Acoustic properties of the phantom were verified by a laboratory apparatus. Initial tests of the effectiveness of the phantom were done by imaging with several scanners using various frequency and imaging settings on transducers.
Results:
Images were obtained with 2 clinical scanners in which modest changes in the image acquisition parameters were adjusted. Image contrast between test cylinders and background corresponded to operating frequency with a multihertz transducer. Changes in observable contrast consistent with a shift in operating frequency were not always accompanied by visual indicators that such changes in the scanning protocol had occurred.
Conclusions:
The test phantom performs as predicted by computer simulations and theoretical calculations of backscatter versus frequency. Contrast on images of the test phantom produced by clinical systems correlates with scanner frequency settings, showing feasibility. Relative shifts in effective frequency and operating bandwidth can be assessed from these contrast differences between settings with this test phantom.