On the statistics of ultrasonic spectral parameters.

Several factors affect the accuracy and precision of ultrasonic spectrum analysis, which is used for characterization of normal and diseased tissue in a variety of organs. For example, averaging procedures and the sequence of operations affect the accuracy and precision of spectrum analysis. Averaging procedures and logarithmic conversion (i.e., conversion to dB) introduce a constant bias that affects spectral amplitudes and the values of intercept and midband fit; the bias depends on the sequencing of the log conversion and averaging as well as the number of independent spectra or spectral parameters that are averaged. We derive expressions that permit correction of such biases. Furthermore, we show that standard deviations for slope and midband-fit estimation can be minimized by averaging spectra before dB conversion and before computing spectral parameters by linear regression. Experimental results using phantoms agree remarkably with theoretical predictions for the data window functions studied in this article, Hamming and rectangular.

Radiation-force technique to monitor lesions during ultrasonic therapy.

This report describes a monitoring technique for high-intensity focused ultrasound (US), or HIFU, lesions, including protein-denaturing lesions (PDLs) and those made for noninvasive cardiac therapy and tumor treatment in the eye, liver and other organs. Designed to sense the increased stiffness of a HIFU lesion, this technique uniquely utilizes the radiation force of the therapeutic US beam as an elastographic push to detect relative stiffness changes. Feasibility was demonstrated with computer simulations (treating acoustically induced displacements, concomitant heating, and US displacement-estimation algorithms) and pilot in vitro experimental studies, which agree qualitatively in differentiating HIFU lesions from normal tissue. Detectable motion can be induced by a single 5 ms push with temperatures well below those needed to form a lesion. Conversely, because the characteristic heat diffusion time is much longer than the characteristic relaxation time following a push, properly timed multiple therapy pulses will form lesions while providing precise control during therapy.

Fluid infusions from catheters into elastic tissue: I. Azimuthally symmetric backflow in homogeneous media.

Directly injecting therapeutics into brain tissue has been investigated both experimentally and theoretically. Paul Morrison and others from the National Institutes of Health pointed out the importance of backflow along and outside a catheter inserted into the tissue, once steady state conditions have been reached. Here we investigate and extend their model. We begin with a reformulation of their results and demonstrate an exact solution that exhibits the scaling behavior of the model where the surrounding tissue medium is homogeneous and isotropic. We report on experimental tests of our predictions in agarose gels. We describe the limitations of the assumptions used and the utility of our reformulation. Extensions of the model, including improvements on some of its crude assumptions and generalizations to inhomogeneous media, will be submitted separately.