Detection of high-intensity focused ultrasound liver lesions using dynamic elastometry

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 (less than or equal to 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%.