Lesion formation and visualization using dual-mode ultrasound phased arrays

A 1 MHz spherical-section 64-element linear piezo-composite phased array and a supporting driving and data acquisition system were recently tested for use as a dual-mode HIFU applicator system. In therapeutic mode, the array was shown to be capable of producing focal intensities in excess of 3500 W/cm² at its geometric center (100 mm radius of curvature). Imaging tests of quality assurance tissue-mimicking phantoms as well as computer simulations confirmed that the array has an oval-shaped imaging field of view (IxFOV) with nearly 50 dB dynamic range extending from 70 to 120 mm in the axial direction and ± 20 mm in the transverse direction. This is larger than its therapeutic operating field (ThxOF), which is defined as the region around the geometric focus where the loss in intensity gain is within 1 dB from the intensity gain at the geometric center. We have tested the array imaging capability in the visualization of the formation of discrete and volumetric lesions in freshly excised porcine liver samples. Experimental results clearly show that the enhancement of the tissue echogenecity at the lesion location is more pronounced in the harmonic images when compared to images at the fundamental. Results also clearly show that the harmonic imaging mode produces more accurate mapping of the lesion size and shape as determined by histologic evaluation.