P1B-11 A 2D-Array for Transcranial Ultrasound Focusing Using Shear-Mode Conversion: A Numerical Study
Notice bibliographique
Résumé
An incident ultrasonic acoustic wave with a frequency around 1 MHz creates longitudinal and shear waves inside the skull. Both waves propagate independently through the skull and create two longitudinal waves after the skull: a purely longitudinal wave and a longitudinal-shear-longitudinal wave. The process of the last wave is identified here as the shear-mode conversion (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ). The interest of developing ultrasound techniques based on S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> resides in the fact that the transmitted wave due to Sc has less phase-shift. The present study shows the feasibility of focusing ultrasound after the skull using a 2D-array (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> TX) conceived to take advantage of the Sc and with dimensions comparable to near-flat skull regions such as the anterior part of the frontal bone. The S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> TX array is composed of two columns of 44 times 7 elements each one separated of 3 cm following its length. The configuration in column is aimed to produce an entering wavefront favorable to the shear-mode conversion. Each element is independently driven at 1 MHz and square-shaped with a length of 1.5 mm. The space between elements is 0.4 mm. The length and width of the device are, respectively, 9.7 and 5.9 cm. The new device was compared to a conventional 2D-array (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> TX) which has the same specifications of the S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> TX device, with the exception of the inter-column space. The degree of focus steering of both devices was compared for depths of 1, 2, 4 and 6 cm from the inner face of the skull (d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fs</sub> ). For each depth, and assuming X and Y perpendicular to the sound propagation axis Z, four focal spots were tested at (x,y)=(0, 0), (2 cm, 0), (0, 2 cm) and (2 cm, 2 cm). A prismatic volume of 10.7 times 6.5 times 0.9 cm, located 1 cm from the device, was used to simulate the skull bone. The required phase of elements was calculated using a back-propagation (BP) method. Tests were done with the corrected phase calculated with only shear-mode BP (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> BP), only longitudinal BP (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> BP) and sum of both back-propagated waves (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> BP+L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> BP). The resulting acoustic field for each focal point considered the forward-propagation due to the longitudinal and shear-mode transmissions. S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> TX with S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> BP produced the most focused fields for targets found close to the face of the skull (d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fs</sub> = 1 cm) and steered following the X direction. For deeper targets (d/s= 2 cm, 4 cm, 6 cm), the correction S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> BP+L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> BP was more effective to focus ultrasound. For the same targets, the correction S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> BP produced focal zones located closer to the device than expected. Calculation of surface area at -3 dB on XZ and YZ planes indicated that the S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> TX device created more focused fields for most of tested cases, excepting a few cases involving steering following the Y direction. In conclusion, Focusing using shear mode conversion is feasible using a simple device that is compatible with the geometry of the frontal human bone. Using the proposed device, this focusing is limited to tissues located about 1 cm from the inner face of the skull.
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| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,002 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,001 | 0,001 |
| Méta-épidémiologie (sens large) | 0,002 | 0,001 |
| Bibliométrie | 0,001 | 0,002 |
| Études des sciences et des technologies | 0,001 | 0,000 |
| Communication savante | 0,001 | 0,001 |
| Science ouverte | 0,001 | 0,000 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
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