Carotid atherosclerotic plaque characterisation by measurement of ultrasound sound speed in vitro at high frequency, 20 MHz

Abstract

The first part of the study was to characterise the acoustic properties of an IEC agar-based tissue mimicking material (TMM) at ultrasound frequencies centred around 20 MHz. The TMM acoustic properties measured were the amplitude attenuation coefficient (dB cm-1MHz-1), the sound speed (ms-1) and the backscattered power spectral density characteristics of spectral slope (dB MHz-1), y-axis intercept (dB) and reflected power (dB). The acoustic properties were measured over a temperature range of 22 - 37oC. Both the attenuation coefficient and sound speed, both group and phase, showed good agreement with the expected values of 0.5 dB cm-1 MHz-1 and 1540 ms-1 respectively with average values of 0.49 dB cm-1MHz-1 (st.dev. ± 0.03) and 1541.9 ms-1 (st.dev. ± 8.5). Overall, this non-commercial agar-based TMM was shown to perform as expected at the higher frequency range of 17-23 MHz and was seen to retain its acoustic properties of attenuation and speed of sound over a three year period. For the second part of the study, composite sound speed was measured in carotid plaque embedded in TMM. The IEC TMM was adapted to a clear agar gel. The contour maps from the attenuation plots were used to match the composite sound speed data to the photographic mask of plaque outline and thus the histological data. By solution of sets of simultaneous equations using a matrix inversion, the individual speed values for five plaque components were derived; TMM, elastin, fibrous/collagen, calcification and lipid. The results for derived sound speed in the adapted TMM were consistently close to the expected value of soft tissue, 1540 ms-1. The fibrous tissue showed a mean value of 1584 ms-1 at body temperature, 37oC. The derived sound speeds for elastic and lipid exhibited large inter-quartile ranges. The calcification had a significantly higher sound speed than the other plaque components at 1760 - 2000 ms-1.