Focusing of High-Intensity Ultrasound Through the Rib Cage Using a Therapeutic Random Phased Array

Abstract

A method for focusing high intensity ultrasound through a rib cage that aims to minimize heating of the ribs whilst maintaining high intensities at the focus (or foci) is proposed and tested theoretically and experimentally. Two approaches, one based on geometric acoustics and the other accounting for diffraction effects associated with propagation through the rib cage, are investigated theoretically for idealized source conditions. It is shown that for an idealized radiator the diffraction approach provides a 23% gain in peak intensity and results in significantly less power losses on the ribs (1% versus 7.5% of the irradiated power) compared with the geometric one. A 2D 1-MHz phased array with 254 randomly distributed elements, tissue mimicking phantoms, and samples of porcine rib cages are used in experiments; the geometric approach is used to configure how the array is driven. Intensity distributions are measured in the plane of the ribs and in the focal plane using an infra-red camera. Theoretical and experimental results show that it is possible to provide adequate focusing through the ribs without overheating them for a single focus and several foci, including steering at ± 10–15 mm off and ± 20 mm along the array axis. Focus splitting due to the periodic spatial structure of ribs is demonstrated both in simulations and experiments; the parameters of splitting are quantified. The ability to produce thermal lesions with a split focal pattern in ex vivo porcine tissue placed beyond the rib phantom is also demonstrated. The results suggest that the method is potentially useful for clinical applications of HIFU for which the rib cage lies between the transducer(s) and the targeted tissue.

A novel sound-blocking structure based on the muffler principle for rib-sparing transcostal high-intensity focused ultrasound treatment

“…In the case of transcostal HIFU, the rib cage not only accumulates heat, resulting in patient discomfort, but also acts as an aberrator to the focusing beam [12][13][14]. Bobkova et al [15] compared rib-sparing HIFU approaches to 'geometric' and 'diffraction' approaches. The earlier 'geometric' approaches switch off the transducers that are 'geometrically' obstructed by the ribs [16][17][18][19].…”

“…Aubry et al proposed time-reversal focusing to control the firing of ultrasound pulses so that they build up at the tumour site and cause ablation [20,21]. Bobkova et al took into account the diffraction effects and used a phase conjugation method to improve focusing and provide better protection of the ribs [15]. Simulation of transcostal HIFU involves the solution of the RayleighSommerfeld integral [17].…”

Infrared mapping of ultrasound fields generated by medical transducers: Feasibility of determining absolute intensity levels

“…There has been considerable interest in using the infrared (IR) technique to investigate the acoustical fields of HIFU transducers [3][4][5] which seems to have increased in recent years. [6][7][8][9][10][11][12] The IR method has specific advantages in comparison with these other methods. Hydrophone field mapping in three dimensions is very slow, with even a single planar raster scan taking typically several hours.…”

“…In this study, the assumption was that the measured temperature rise at the surface of the absorber is proportional to the free field intensity. Other studies have extended the method to account for the effect of heat diffusion by introducing correction factors in the conversion of temperature images to intensity, 8,9,11,12 and measuring the temperature distribution on the surface of a thick (40 mm) less absorbing tissue mimicking gel (0.6 dB cm À1 MHz…”

Evaluation of intensity based beam-shaping method with Rib-phantom HIFU sonications

“…With phased-array transducers it is convenient to reduce the exposure by shaping the ultrasound beam through switching off part of the elements. The decision which elements are turned off can be made for example by using geometrical shadowing [1] or by backward-propagating fields from the target location towards the transducer [2]. However, neither of these methods estimate the temperature increase of the bone and may thus end up being too conservative.…”