The effect of abdominal wall morphology on ultrasonic pulse distortion. Part I. Measurements

Abstract

The relative importance of the fat and muscle layers of the human abdominal wall in producing ultrasonic wavefront distortion was assessed by means of direct measurements. Specimens employed included six whole abdominal wall specimens and twelve partial specimens obtained by dividing each whole specimen into a fat and a muscle layer. In the measurement technique employed, a hemispheric transducer transmitted a 3.75-MHz ultrasonic pulse through a tissue section. The received wavefront was measured by a linear array translated in the elevation direction to synthesize a two-dimensional aperture. Insertion loss was also measured at various locations on each specimen. Differences in arrival time and energy level between the measured waveforms and computed references that account for geometric delay and spreading were calculated. After correction for the effects of geometry, the received waveforms were synthetically focused. The characteristics of the distortion produced by each specimen and the quality of the resulting focus were analyzed and compared. The measurements show that muscle produces greater arrival time distortion than fat while fat produces greater energy level distortion than muscle, but that the distortion produced by the entire abdominal wall is not equivalent to a simple combination of distortion effects produced by the layers. The results also indicate that both fat and muscle layers contribute significantly to the distortion of ultrasonic beams by the abdominal wall. However, the spatial characteristics of the distortion produced by fat and muscle layers differ substantially. Distortion produced by muscle layers, as well as focal images aberrated by muscle layers, show considerable anisotropy associated with muscle fiber orientation. Distortion produced by fat layers shows smaller-scale, granular structure associated with scattering from the septa surrounding individual fat lobules. Thick layers of fat may be expected to cause poor image quality due to both scattering and bulk absorption effects, while thick muscle layers may be expected to cause focus aberration due to large arrival time fluctuations. Correction of aberrated focuses using time-shift compensation shows more complete correction for muscle sections than for fat sections, so that correction methods based on phase screen models may be more appropriate for muscle layers than for fat layers.

Automatic Measurement of Human Subcutaneous Fat with Ultrasound

“…Pomaroli et al [70] performed a histologic analysis and showed that fatty tissues with more connective tissues appeared to be more echogenic in B-mode than fatty tissues with fewer connective tissues. Moreover, Hinkelman et al [66] also mentioned that thick layers of fat may cause poor B-mode image quality at the abdominal wall because they distort ultrasound beams due to their scattering and absorption effects.…”

“…As suggested by Hinkelman at el. [66], the subcutaneous fat has greater energy level and waveform distortion than muscle when they investigated the effect of abdominal wall morphology on ultrasonic pulse distortion. Therefore, we imply that the high variation in the tissue structure of subcutaneous fat causes more fluctuations in the received spectrum, and this results in a greater change in the spectrum variance.…”

Distributed aberrators for emulation of ultrasonic pulse distortion by abdominal wall

“…Data acquisition was performed with an apparatus used in previously reported aberration measurements on tissue specimens. 2,13 Ultrasonic pulses with 3.75 MHz center frequency and 2.2 MHz -6-dB bandwidth were emitted from a hemispheric source positioned 185 mm below a 128-element linear array with the same center frequency and bandwidth. The lateral array pitch was 0.72 mm, and the elements were masked to produce an effective height of 1.44 mm in the out-of-plane direction.…”

“…Aberration is particularly severe when imaging through the abdominal wall where sound speed variations and strong scattering can occur in both muscle layers and subcutaneous fat. [1][2][3] Arrival time, energy level, and shape changes in propagating waveforms are often encountered, resulting in a decrease in the peak amplitude at the focus, a broader focus, and an increase in the peripheral energy. This focus degradation is a major barrier to achieving diffraction-limited resolution with large aperture transducers.…”

Modeling and Analysis of Ultrasonic Echoes Reflected from a Surface under Two Layers

“…In this case, the parameter set consists of the two depths of the planar interfaces, given by D 1 and D 2 . The scalar error that is minimized is given by E(p) = n (y n − y(n; p)) 2 where y n and y(n; p) are the data signal and the synthesized signal respectively.…”