Detection of High-Intensity Focused Ultrasound Liver Lesions Using Dynamic Elastometry

Abstract

A novel ultrasound technique was developed for detecting the distribution of stiffness in biological tissue. The method, which we call 'dynamic elastometry,' involves applying a low-frequency vibration (< or = 5 Hz) to the tissue and measuring the resulting velocity pattern within the sample using Doppler spectral analysis. Based upon the velocity differences, an elastically stiff region can be differentiated from surrounding soft tissue. Dynamic elastometry was used to both detect and quantify lesions produced by high-intensity focused ultrasound (HIFU) in porcine livers. Measurements of the lesion position and length agreed well with independent geometric measurements. The mean and standard deviation of the differences between the two types of measurement were -0.01 cm and 0.10 cm for lesion position, and -0.05 cm and 0.12 cm for lesion length, respectively. The relative stiffness between lesions and normal liver tissue was estimated by the velocity gradient ratio. Results were compared with the Young's modulus ratios between lesion and normal liver tissue obtained from mechanical measurement. The dynamic elastometric estimates had a strong linear correlation with the mechanical measurements (r = 0.93) but were smaller than the latter by 26%.

High-intensity focused ultrasound: past, present, and future in neurosurgery

“…Ultrasound produces frictional heat by causing vibration of molecules in tissue; a temperature of > 56°C maintained for 2 seconds or more leads to coagulative necrosis. 13,14,75 The thermal damage leads to unplanned cell death. The targeted cells retain their outline, their proteins coagulate, and their metabolic activity halts.…”

Therapeutic ultrasound: Surgery and drug delivery

“…image contrast [28,29]. Diagnostic ultrasound is principally used in B-mode for lesion guidance and targeting [30], in Doppler-mode for hemostasis applications [31], and in the new elastography and vibro-acoustography modalities for lesion monitoring [32][33][34]. B-mode scans exploit the transient minimal bubble activity that is present before the onset of a thermal lesion and indicates the area where the energy is delivered as hyperechoic in the image.…”

Changes in backscatter of liver tissue due to thermal coagulation induced by focused ultrasound

“…5 However, the change due to thermal coagulation as well as the change due to temperature rise without coagulation is not understood. Techniques for estimating the elasticity of tissue, known such as elastography, 6 acoustic radiation force impulse (ARFI), 7 and shear wave imaging, 8 are under investigation for monitoring HIFU treatment, based on the fact that tissue tends to become elastically stiffer due to coagulation.…”