# Assessment of intercostal sonication capabilities of a dedicated transducer by MR-ARFI/thermometry

Fanny Dabrin, Mathieu Guyot, Valéry Ozenne, Stéphane Chemouny, Pierre

Bour, Bruno Quesson

► To cite this version:

Fanny Dabrin, Mathieu Guyot, Valéry Ozenne, Stéphane Chemouny, Pierre Bour, et al. Assessment of intercostal sonication capabilities of a dedicated transducer by MR-ARFI/thermometry. 13th interventional MRI symposium 2022, Leipzig, Germany, Jan 2022, Leipzig (DE), Germany. [hal-04684818](https://hal.science/hal-04684818v1)

HAL Id: hal-04684818

https://hal.science/hal-04684818v1

Submitted on 3 Sep 2024

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# Assessment of intercostal sonication capabilities of a dedicated transducer by MR-ARFI/thermometry

**Author List:** Fanny Dabrin¹, Mathieu Guyot², Valéry Ozenne², Stéphane Chemouny¹, Pierre Bour¹, Bruno Quesson². ¹Certis Therapeutics, Pessac, France; ²Univ. Bordeaux, CNRS, CRMSB, UMR 5536, IHU Liryc, F-33000 Bordeaux, France,

## Purpose:

Intercostal HIFU ablation using spherical transducers remains challenging due to absorption of the ultrasound by the ribs (risks of inducing unwanted heating) and phase aberrations/reflection resulting in significant acoustic losses at the focal spot (Quesson, B. et al. Medical Physics 37,2533-2540(2010)). MR-ARFI/thermometry is used to assess sonication capabilities of a dedicated HIFU transducer for different rib spacing.

## Material and Methods:

Two ribs from a pig were positioned over a piece of muscle on a mounting enabling mechanical spacing adjustment (Figure 1). Tested values were 0.8, 1 and 1.5 cm. A 32-element transducer (13 cm focal length, with asymmetric apertures of 2 and 12 cm) was powered by a 1.6 kW generator operating at 1 MHz. A modified echo-planar imaging sequence integrated a bipolar motion encoding gradient (MEG with 20 mT/m amplitude and 3 ms duration) to image the displacement and temperature rise induced by a single ultrasound pulse [Bour, P. et al. Magn Reson Med 78, 1911-1921(2017)]. Sequence parameters: 5 slices, TE/TR/FA = 25ms/2000ms/90°, FOV= 186x192 mm², resolution 1.5x1.5x3 mm³, 4 averages (8s update time per stack of slices). Slices were centered on the HIFU focus in a plane perpendicular or parallel to ultrasound propagation direction. A HIFU pulse of 5 ms duration (1.25% duty cycle) was synchronized with the MEG. Experiments were repeated for different acoustic powers: 68, 170 and 308 W<sub>acoustic</sub>. Displacement and temperature images were reconstructed using the method proposed in [Bour, P. et al. Magn Reson Med 78, 1911-1921(2017)] for each condition of rib spacing and acoustic power.

<figure><img src="image_1.png" /><figcaption>**Figure 1:** experimental setup showing transducer position relative to the two ribs. Yellow rectangle indicates position of the central slice of the MR-ARFI/thermometry sequence.</figcaption></figure>

## Results:

<figure><img src="image_2.png" /><figcaption>(B) Displacement µm</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>(C) Temperature °C</figcaption></figure>

<figure><img src="image_4.png" /></figure>

**Figure 2:** (A) Evolution of maximal displacement for different intercostal spaces. Maximal displacement decreases when intercostal space is 0.8cm. (B) Representative displacement map for 1 cm rib spacing during the shot (C) Representative temperature map for 1 cm rib spacing during the shot.

Table 1 shows that maximal displacement is similar without ribs and for rib spacing down to 1 cm. Reducing spacing to 8 mm resulted in significant reduction (> 50%) of maximal displacement.

<table><caption>**Table 1:** Summary of the displacement and the temperature values</caption><thead><tr><th>Intercostal size in cm</th><th>Acoustic power W</th><th>Maximal displacement µm</th><th>Maximal temperature °C</th></tr></thead><tbody><tr><td rowspan="3">No ribs</td><td>68</td><td>16</td><td>1.0</td></tr><tr><td>170</td><td>31</td><td>2.4</td></tr><tr><td>308</td><td>96</td><td>0.4</td></tr><tr><td rowspan="3">1.80</td><td>67</td><td>14</td><td>1.4</td></tr><tr><td>168</td><td>30</td><td>2.3</td></tr><tr><td>307</td><td>77</td><td>4.3</td></tr><tr><td rowspan="3">1.00</td><td>70</td><td>15</td><td>1.4</td></tr><tr><td>175</td><td>35</td><td>2.1</td></tr><tr><td>312</td><td>85</td><td>6.4</td></tr><tr><td rowspan="3">0.80</td><td>67</td><td>11</td><td>0.6</td></tr><tr><td>169</td><td>20</td><td>1.2</td></tr><tr><td>307</td><td>39</td><td>3.0</td></tr></tbody></table>

## Conclusion:

Intercostal sonication is feasible with the proposed HIFU device without significant acoustic energy loss for rib spacing above 1 cm. Further studies are required to investigate the possibility to perform non-invasive intercostal ablation using higher sonication duty cycles.