Arterial Vulnerable Plaque Characterization Using Ultrasound-Induced Thermal Strain Imaging (TSI)

Abstract

Thermal strain imaging (TSI) is demonstrated in two model systems mimicking two potential clinical applications. First, a custom ultrasound (US) microscope produced high-resolution TSI images of an excised porcine coronary artery. Samples were placed in a temperature-controlled water chamber and scanned transversely and longitudinally. Phase-sensitive, correlation-based speckle tracking was applied to map the spatial distribution of temporal strain across the sample. TSI differentiated fatty tissue from water-based arterial wall and muscle with high contrast and a spatial resolution of 60 microm for a 50-MHz transducer. Both transverse and longitudinal TSI images compared well with B-scans of arterial wall structures, including intima, media, adventitia, and overlying fatty tissue. A second model system was used to test the hypothesis that US can produce the heating pattern required for TSI of internal structures. A 2-D phased array with independent drive electronics was combined with a conventional US scanner (iU22, Philips, Bothell, WA) for these studies. This 513-element array, originally designed for the US therapy, acted as the US heat source. To quantify the temporal strain induced by this system, TSI was performed on a homogeneous rubber phantom. TSI temperature estimates were within 3% error for a 3.2 degrees C temperature rise produced within 2 s using a specially designed beamformer and pulse sequencer. The system was then used to produce TSI scanning of an excised kidney containing an intact piece of fat below the collecting system. These images were validated using an magnetic resonance imaging (MRI) pulse sequence designed for lipid quantification. TSI scans matched well MRI scans and histology both anatomically and quantitatively. Finally, to test the potential of US-induced TSI for a significant clinical problem, images were obtained on an excised canine aorta with fatty tissue inside the lumen. Both longitudinal and transversal TSI agreed well with anatomy. These in vitro results demonstrate the potential of high-resolution US-induced TSI with a small temperature change (<1 degrees C) for plaque characterization.

Thermal strain imaging: a review

“…Also, an US heating/ imaging scheme using a single US probe connected to a commercial US scanner will provide significant system practicality, and it does not require a specially designed electromagnetically shielded room. In summary, both electromagnetic [20][21][22][23][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] and US heating [24][25][26][27][28][29][30][31] have been used to demonstrate the feasibility of TSI. Future clinical applications will primarily use US heating for TSI unless electromagnetic heating is already integrated into the system for therapeutic purposes.…”

“…As an extension of this work, a 513 element two-dimensional therapeutic phased array (Besançon, FR, USA) was combined with a conventional US scanner (iU22, Philips, WA, USA) [28]. The array was used as the US heat source [29].…”

“…As an initial test of the ability of TSI to remotely detect lipid-bearing tissue at depth using conventional US equipment for both heating and imaging, a set of experiments was performed on the right kidney of a normal 3 kg New Zealand white rabbit [28]. Figure 4a represents the B-scan of the kidney.…”

“…noninvasively), a canine aorta was prepared with fatty tissue inside the lumen [28]. Figure 5b,e depicts TSI maps overlaid on the B-scan.…”

An adaptive displacement estimation algorithm for improved reconstruction of thermal strain

“…For temperature changes in this range, thermally induced mechanical expansion can be ignored and the shift between the reference and post-heating images can be considered to be solely the result of the temperature dependence of the speed of sound [4]. The derivative of this apparent displacement (“thermal strain”) can be used to differentiate between water and lipid-based tissues [8]. TSI-based detection of lipids has a number of potential medical applications including the identification of lipid pools in atherosclerotic plaques to assess plaque vulnerability [4], [9].…”