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A theoretical expression for the variance of scatterer size estimates is derived for a modified least squares size estimator used in conjunction with a reference phantom method for backscatter coefficient measurement. A Gaussian spatial autocorrelation function is assumed. Simulations and phantom experiments were performed to verify the results for backscatter and size variances. The dependence of size estimate errors upon free experimental parameters is explored. Implications of the findings for the optimization of scatterer size estimation are discussed. The utility of scatterer size parametric imaging is examined through the signal to noise ratio comparison with standard ultrasonic B-mode imaging.

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.

A finite amplitude insert-substitution method has been used to determine the ultrasonic nonlinearity parameter B/A of nine versions of water-based, macroscopically uniform ultrasonically tissue-mimicking (TM) nonfat and fat materials. In this method, the amplitude of the second harmonic following transmission through degassed distilled water with known B/A (B/A = 5.2) and the amplitude of the second harmonic following transmission through the unknown sample are measured. The ratio of these amplitudes allows calculation of the B/A of the sample. Measured B/A values of the nonfat materials range from 5.6 to 6.6. These values compare favorably with published values for nonfat soft tissues. In contrast, the measured B/A values for two tissue-mimicking fat materials are 9.8 and 11.1; these two values represent the low and high end of B/A for most fresh fatty tissues. For comparison, B/A was measured for two commonly available uniform materials, corn oil and ethylene glycol, and the results are in good agreement with published values.

The autocorrelation function pertaining to spatial distributions of ultrasonic scatterers in soft tissue is believed to contain useful information related to tissue morphology. A simple processing method applied to radio-frequency echo signals estimates this function for a sample having isotropic scattering conditions. It utilizes backscattered echo signals from the sample and echo signals from a reference object having defined scattering properties. The ratio of the echo signal power spectrum from the sample to the echo signal power spectrum from the reference object is obtained, and corrected for attenuation differences between the two media. This yields a "form factor" for the sample, whose inverse Fourier transform is the autocorrelation function. The method was tested using tissue-mimicking samples for which spatial autocorrelation functions could be modeled from the dimensions of embedded scatterers. The shapes of the measured autocorrelation functions were in reasonable agreement with those estimated, although measured functions overestimated the function at small lag distances. Scatterer diameters estimated from the zeros of the autocorrelation function agreed to within 6% of expected values when the measurement system bandwidth satisfied minimal criteria.

Tissue-mimicking phantoms and an automated system have been developed for testing the resolution performance of ultrasound scanners by determining detectability of low to higher contrast spherical lesions over the entire depth of field. Axial, lateral and elevational resolutions are accounted for simultaneously and equally. Tissue-mimicking spherical simulated lesions are either 3 or 4 mm in diameter and have one of four different intrinsic material contrasts. For each diameter and contrast, there is a set of 109 lesions in a regular array with coplanar centers extending from 0.5-15.5 cm in depth. With the scan slice superimposed on the spheres, the image is frame-grabbed for automated analysis. A diameter-dependent lesion signal-to-noise ratio is computed for each pixel position in the image, excluding a 5-mm boundary. Two universal thresholds, resulting from maximization of agreement between the automated system and human observers, give rise to a depth range, or "resolution zone", over which detection exists for each type lesion.

A new method is tested to compensate for attenuation losses through the intervening layers in quantitative ultrasound imaging. The method subtracts the echo signal power spectrum acquired from a uniform region beneath the overlying layers from the signal power spectrum obtained from a reference phantom using the same instrumentation system settings. Changes in spectral components with frequency are then used to estimate the attenuation of the overlying layers. Several phantoms were used to test the method, among which was a phantom having three windows, one with no overlying layers and the other two with fat and muscle mimicking layers of different degrees of irregularity. Attenuation losses through the windows were compensated for using the technique, producing backscatter estimator images of a simulated tumor inside the phantom. After applying the method, consistent results for the backscatter estimator of the tumor, as well as the backscatter coefficient of the background material, were obtained from the various windows.