The use of a segmented transducer for rib sparing in HIFU treatments.

The use of focused ultrasound as a minimally invasive treatment for tumours is rapidly expanding. Target organs include the liver and kidneys. Both single element and phased array transducers may be used in the clinic. The presence of the rib cage presents a problem in high intensity focused ultrasound (HIFU) treatment planning, due to its high attenuation of the HIFU beam resulting in a loss of power at the focus as well as an increase in the risk of damage at the rib and to overlying tissues, including the skin. In this paper, a linearly segmented transducer, in which all active elements are driven in phase, has been investigated. The aim of the study was to investigate how a beam with a clinically useful profile could be achieved by removing the contribution of edge segments from one side of the transducer to the field. We have considered the case in which the HIFU beam approaches the rib cage during a treatment and investigated configurations of the transducer for which up to three segments on the edge are switched off. This problem has been studied initially using a linear acoustic field program to model the segmented transducer's acoustic beam profile. Experimental measurements of the transducer's acoustic field were performed using an automated beam plotting system. Temperature measurements were made on a rib surface for two transducer configurations using a fine wire thermocouple. A thermochromic liquid crystal material was used to assess qualitatively the heating pattern generated by the ultrasound beam. We show the rib sparing potential of the segmented transducer during HIFU treatment by demonstrating a reduction in the prefocal width of the ultrasound beam when edge segments are switched off. This has been predicted with the acoustic field model and demonstrated experimentally by acoustic field measurements and observations of the heating pattern generated by the ultrasound beam. A significant decrease in the temperature rise on a rib was observed in the case for which three edge segments were switched off compared with when all segments were active. We conclude that a segmented transducer extends the potential for treating liver tumours. In the case where the tumour lies behind, but close to the edge of, the ribs, energy loss at the focus and excessive heating in the rib and overlying tissue can be avoided by switching off edge segments.