Caractérisation de la vascularisation de la moelle épinière par un algorithme d'ondes planes pour l'imagerie ultrasonore Doppler

Abstract

For orthopedic surgeons, one of the main issues is to evaluate spinal cord condition during surgery. Its vascular condition is a representative parameter of its functional abilities, as well as the initial circumstances of injury. Ultrasound imaging is particularly adapted for an intraoperative use. However the algorithms contained in the current clinical devices do not have enough resolution to describe the blood vessels with very small diameters. This study aims to adapt a Doppler ultrasound plane wave imaging sequence to the spinal cord. The conventional focused wave and plane wave imaging algorithms were adjusted based on an equivalent sampling frequency of the Doppler signal, in order to reach a similar quantifiable flow velocity interval. For the same acquisition time, the amount of Doppler data is about 13 times higher with the plane wave sequence. Firstly, the quantification of parameters associated with the vascularization was performed using medullar vessel phantoms, and the results obtained from the two imaging sequences were compared. More specifically, for plane wave Doppler imaging, the experiments carried out during this research project showed an improvement of the signal-to-noise ratio that can reach 18 dB, as well as a more precise estimation of the velocities and diameters associated to the flow. For three different phantom configurations, the mean difference in the flow estimates compared to the theoretical flows was 25%, while the values were almost twice the theoretical flow in the images reconstructed from the focused waves algorithm. Secondly, ultrasound imaging combined with the administration of a contrast agent applied on a porcine spinal cord in vivo confirmed the previous results. Synchronization of wave emission with the cardiac cycle prevented variations associated with this parameter. Moreover, the addition of a delay between two emission sequences led to a slower destruction of the contrast agent on the one hand, and on the other hand, allowed it to fill the intra-medullary vessels again. A larger number of blood vessels were detected after the application of the plane wave Doppler imaging sequence. More specifically, spinal veins and arteries were visualized, as well as some intra-medullary vessels. Furthermore, the addition of an injury using a dropped weight enabled evaluation of the extent of the spinal cord damage. All of the results obtained during this research project show that the plane wave ultrasound Doppler algorithm leads to a significant improvement for the characterization of artificial blood vessels representative of the spinal cord vascularization, and more precisely measures the associated parameters. These conclusions could be more accurately detailed using higher emission frequencies or with the application of a non-linear image reconstruction algorithm following the contrast product administration. The results suggest that a plane wave Doppler ultrasound imaging sequence could be used to characterize and quantify the spinal cord blood flow. In the long term, a use in the operating room could provide additional information to the surgeons in order to evaluate the spinal cord’s vascular condition.

Date	15 Mar 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor) & Jean Marc Mac-Thiong (Co-supervisor)