3D Ultrasound Image Guidance and Therapy Through the Rib Cage with a Therapeutic Random Phased Array

Therapeutic and imaging capabilities of a 1 MHz random phased array are described. Multiple simultaneous foci were generated and steered in simulations as well as experimentally. In imaging mode, the random phased array was integrated with Verasonics system via custom-built connectors to acquire raw RF data. Imaging was implemented using synthetic aperture beamforming to produce 3D images of the region of interest. Ex vivo rib phantoms were imaged both in simulation and experimentally in water bath.

to provide acoustic window (5). The use of phased array to 40 sonicate the liver tumour between the ribs was suggested in 41 late 1990s but could not be tested experimentally (6; 7; 8) . 42 Civale et. al. used a linearly segmented transducer to lower 43 the temperature on the ribs but the remaining elements were 44 not able to produce sufficient energy(9). Liu 2007 deactivated 45 array elements being obstructed by the ribs to minimize heat 46 deposition over the ribs in a numerical study (10). Cochard et. 47 al. used a method based on DORT by analysing backscattered 48 echoes from ex-vivo ribs immersed in water and deactivated 49 the blocked elements in order to minimize heat on the ribs 50 (11). Quesson et. al. described a method to enable intercostal 51 HIFU ablation while sparing ribs and surrounding tissues 52 (12). The method selected elements based on the relative 53 location of focal point and anatomical images of ribs prior 54 to heating. The technique was implemented both ex-vivo and 55 in-vivo but it relies on MRI images to identify the rib cage 56 and turn off the blocked elements. Aubry et al. used time 57 reversal technique to focus through the rib cage and reported a 58 mean temperature increase of only 0.3, however, this technique 59 required an invasive procedure to insert an acoustic source at 60 the focal point. Bobkova et al. quantified the parameters of 61 focus splitting and determined that the rib cage acts as an 62 aberrator (13).

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In this work, we show that random phased array is capable 64 of detecting and imaging obstruction that lie in the therapeutic 65 beam path. We have further shown that sets of multiple foci 66 can be generated and steered in the treatment volume in 67 different regions without any bones obstruction such as uterine 68 fibroid and with bone hindrance such as liver. The suitability 69 of random phased array for imaging the rib cage in simulation 70 and experiments is demonstrated.
where p 0 is the pressure on elements surface, a a is radius of 76 the element, k is the wave number, Z R = ka 2 2 is the Rayleigh For a particular focus configuration, the distribution of 82 phases and velocity for each element is calculated using the 83 pseudoinverse method mentioned (15) and described by Ebbini 84 and Cain 1991 as follows: The intensity distribution was calculated both in the ribs 86 plane and in the focal plane from the pressure distribution as Where p is the pressure amplitude.
Where y i,j is the echo element received by element j when 118 transmitting with element i.

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The random phased array was integrated with Verason-120 ics system via custom-built connectors to acquire RF data. 121 Synthetic Aperture 3D beamforming technique was used to 122 achieve 3D volumetric images of the region of interest. Wire 123 phantom was imaged in simulation and metallic rib phantoms 124 and ex-vivo ovine ribs were imaged both in simulation and 125 experimentally in water bath.

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A wire target array was imaged using the STA approach 129 to determine the spatial resolution as well as the maximum 130 extent of the imaging field of view. Wire targets were placed 131 from -10mm to 10mm in lateral and 100 mm to 130 mm along 132 axial direction. The spacing between each pair of wire is 5mm 133 in lateral and 10mm axially.  The lateral resolution was determined for STA beamforming 138 for 1 emission and 256 emissions as shown below. The 139 sidelobe level is significantly reduced from -15dB down to 140 -30 dB, whereas the main lobe is also narrowed.

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To study the capability of the RPA to image the ribs, two 142 experiments were conducted in which two ovine ribs were 143 placed near the geometric focus in degassed water and a 3D 144 image was obtained. at various heights from the center of the array and geometric 175 ray tracing was performed to determine the elements being 176 blocked by the strips while focusing at the geometric focus 177 of the array. The elements whose normal was crossing any of 178 these strips was deactivated and acoustic field was calculated 179 with the remaining elements. For multiple simultaneous foci, 180 the normals from all elements to each focus was used to 181 find the blocked elements. Intensity distribution in both rib 182 plane and focal planes were calculated. It was observed that 183 the ribs are clearly spared and the energy is distributed only 184 intercostally.

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When the ribs were placed at 40 mm from the array 186 center, only 100 elements were activated and the peak intensity 187 was reduced to 194.2 W/cm 2 . When the rib phantom was 188 positioned at a depth of 100mm from the transducer, the 189 number of active elements was changed to 132 and the peak 190 intensity at the focus plane was slightly increased to 198.8 191 W/cm 2 . The focus splitting phenomena was observed. Two 192 secondary lobes at ±4.5mm were observed off the main lobe 193 with amplitude of 38% of the main lobe. when the rib phantom 194 was positioned at 100mm, the secondary lobes appeared closer 195 to the main lobe at a distance of 1.7mm. The level of secondary 196 lobe increased to 62% of the main lobe.

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A rib topology was obtained in STL format from an adult 198 male cadaver. Ribs 8 to 12 of the left side rib cage were used 199 in the simulation after truncating from the spine. A MATLAB 200 function was used to voxelize the phantom. The rib cage was 201 positioned in such a way that the lowest point of the ribs was 202 39 mm away from the array axially and the highest point was 203 at a distance of 96mm. Rib 9 was straight above the geometric 204 focus.

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For a single focus at the geometric focus, it was observed 206 that the secondary lobes were very low as compared to the 207 previous geometries. The peak level in the secondary peaks 208 was only 6% of the main lobe and the spacing from the main 209 Simulation studies and laboratory tests have shown that 218 random phased array is not only capable of generating and 219 steering single and multiple foci along lateral width and axial 220 depth to ablate the tissue but is also capable of imaging the 221 bone such as ribs. Since integration of an external imaging 222 modality with HIFU is a hindrance in its clinical use, we have 223 shown that such an array is capable of targeting and ablating 224 through the rib cage without overheating the ribs.