Exploring potential mechanisms responsible for observed changes of ultrasonic backscattered energy with temperature variations

Abstract

Purpose: Previous studies have provided the observation that the ultrasonic backscattered energy from a tissue region will change due to a change of temperature. The mechanism responsible for the changes in backscattered energy (CBE) with temperature has been hypothesized to be from the changes in scattering properties of local aqueous and lipid scatterers. An alternative mechanism is hypothesized here to be capable of producing similar CBE curves, i.e., changes in speckle resulting from changes in summation of scattered wavelets. Methods: Both simulations and experiments were conducted with a 5.5 MHz, 128-element linear array and synthetic and physical phantoms containing randomly spaced scatterers. The speckle pattern resulting from summation of scattered wavelets was changed in simulations and experiments by directly increasing the background sound speed from 1520 to 1540 m/s, and changing the temperature from 37• C to 48• C, respectively. Shifts in the backscattered signal were compensated using 2D cross-correlation techniques. Results: Excellent agreement between simulations and experiments was observed, with each pixel in the CBE images on average undergoing either a monotonic increase (up to 3.2 dB) or a monotonic decrease (down to −1.9 dB) with increasing sound speed or temperature. Similar CBE curves were also produced by shifting the image plane in the elevational and axial directions even after correcting for apparent motion. Conclusions: CBE curves were produced by changing the sound speed or temperature in tissue mimicking phantoms or by shifting the image plane in the elevational and axial directions and the production of these CBE curves did not require the presence of lipid and aqueous scatterers.

Thermometry and ablation monitoring with ultrasound

“…In three mouse tumors heated from 37.5 to 43°C with HIFU, after motion compensation, the means of image pixels with positive CBE monotonically increased by a mean of 3.5 dB (3.1 to 3.6 dB); the negative CBE monotonically decreased by −3.4 dB (−2.0 to −4.5 dB). Li et al reported similar findings for small transducer displacements in both elevation and axial directions (140). Breaking from the motion- and echo shifted-compensation CBE paradigm though, Tsui and colleagues proposed in 2012 that CBE imaging without echo shifting may have improved sensitivity to increased temperature detection (141).…”

Analysis of tissue changes, measurement system effects, and motion artifacts in echo decorrelation imaging

“…Other ultrasound approaches to ablation monitoring have been based on tissue echogenicity changes, including conventional B-mode imaging 6,12 as well as quantitative tracking of echo energy changes (e.g., integrated backscatter) [29][30][31][32] and statistical parameters of the backscattered ultrasound echoes. 33,34 However, the local echogenicity of tissue is dependent on heat-induced changes in tissue attenuation, sound speed, and bubble activity, which do not consistently correspond with local tissue coagulation.…”

“…1,34,35 Echo decorrelation imaging, which tracks stochastic changes in echo waveforms over short time scales (e.g., 10-50 ms), 1,2 may potentially be a more robust predictor of ablation. Other ablation monitoring approaches have also tracked stochastic changes in echo signal amplitude or energy [32][33][34] but over longer time scales (e.g., comparison of speckle statistics between frames temporally separated by 20 s), 33 so that these approaches may not be suitable for in vivo monitoring in the presence of tissue motion. Experimental studies of echo decorrelation imaging have indicated that, in comparisons using the same ex vivo and in vivo echo data, local tissue ablation was predicted with significantly greater accuracy using echo decorrelation than using integrated backscatter.…”

Temperature Monitoring during Focused Ultrasound Treatment by Means of the Homodyned K Distribution

“…Temperature monitoring by means of intensity was reported to be effective in several works [6][7][8]. In our case, intensity was the best individual parameter.…”

“…To monitor temperature variations, several US-based methods were proposed. First of all, the change in signal's backscattered energy (CBE) was reported as a good temperate indicator in several studies [6][7][8]. CBE is based on the fact that heating modifies acoustic impedance of the tissue and subsequently the reflectivity [9].…”