Optoacoustic Imaging of HIFU-induced Thermal Lesions in Tissue

A 3-D optoacoustic imaging system was used to visualize thermal lesions produced in excised tissue specimens and in vivo mice using high intensity focused ultrasound (HIFU). A 7.5 MHz surgical, focused transducer with a radius of curvature of 35 mm and an aperture diameter of 23 mm was used to generate HIFU. A pulsed laser, which could operate at 755 nm and 1064 nm, was used to illuminate excised tissue and mice using a bifurcated fiber bundle resulting in two wide beams of light. Tomographic images were obtained while the specimens were rotated within a sphere outlined by a concave arc-shaped array of 64 piezo-composite transducers. These images were then combined to reconstruct 3-D volume images (voxel resolution 0.5 mm), which were acquired before and after HIFU exposure. Optoacoustic images acquired at 1064 nm provided visualization of HIFU lesions. The lesion in excised tissue was indicated by an increase in the optoacoustic signal; the in vivo lesion was indicated by a decrease in the optoacoustic signal. The location and the extent of the lesions were confirmed upon dissection. The discrepancy between the ex vivo and the in vivo results might be attributed to the different effective thermal deposition in the two cases. These preliminary results demonstrate the potential of optoacoustic imaging to assess and monitor the progress of HIFU therapy.