Orthogonally combined motion‐ and diffusion‐sensitized driven equilibrium (OC‐MDSDE) preparation for vessel signal suppression in 3D turbo spin echo imaging of peripheral nerves in the extremities

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Thin‐section, high‐resolution sequences are gaining interest in imaging of the peripheral nerves that are dedicated to optimally increase the conspicuity of nerve tissue signal 1. Although neurographical sequences have made substantial progress through time and many technical aspects contributing to image quality have been addressed 9, a major challenge that persists is the difficulty to address unwanted small‐vessel signal without disrupting the signal of nerves 2, attributed to similar T1 relaxation times between blood and nerves and the long T2 relaxation times of the two tissues. Previous techniques have used a flow‐sensitized preparation in spin echo imaging 16 or low b‐value diffusion weighting in gradient echo imaging 19 to suppress such vessel signal and thus improve the visualization of the peripheral nerves.
Flow‐sensitized preparations, such as the motion‐sensitized driven equilibrium (MSDE) preparation, have been shown to help reduce the signal of fast flowing blood in imaging the vessel wall of large vessels 20 and in angiography 22. However, in order to reduce the signal of slowly flowing blood in small vessels, the first gradient moment (m1) of MSDE needs to be significantly increased, which unavoidably increases the sensitivity to motion and can result in artifacts. In contrast, flow‐compensated diffusion preparation is less sensitive to motion‐induced phase errors. In nerve imaging, such a flow‐compensated diffusion preparation is also suitable for attenuating vessel signal, given the significantly higher diffusion coefficient of blood compared to nerve when the diffusion gradients are applied perpendicular to the nerve axis. By considering the tendency of peripheral nerves and surrounding vessels in the extremities to have a common preferential orientation, as well as the high dependence of motion sensitivity on the choice of gradient direction relative to the anatomy, it is possible to combine flow and diffusion sensitization in orthogonal axes to benefit from both dephasing mechanisms to improve nerve conspicuity.
The extremities have been shown to be a region where vessel signal interferes with the visualization of the nerves 19. Given that large veins and arteries with fast flow in this region tend to run in the same direction as the nerves and smaller vessels with slow flow are more tortuous and run in many directions, the extremities appear as a region where the orthogonal application of motion (flow) and diffusion sensitization can be highly advantageous for vessel suppression. Moreover, given that sagittal and coronal views of long segments of peripheral nerves in the extremities can be useful in the identification of regions with pathology, it is advantageous to combine signal preparation with a 3D readout utlizing isotropic resolution.
The purpose of the present work is to develop an orthogonally combined motion‐ and diffusion‐sensitized driven equilibrium (OC‐MDSDE) preparation sequence that yields improved vessel‐signal suppression and high nerve signal for imaging of peripheral nerves in the extremities. The proposed preparation scheme is combined with a 3D turbo spin echo (TSE) acquisition and its vessel suppression capability is demonstrated in vivo in the knee region.

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