Image‐guided navigation of single‐element focused ultrasound transducer

Abstract

The spatial specificity and controllability of focused ultrasound (FUS), in addition to its ability to modify the excitability of neural tissue, allows for the selective and reversible neuromodulation of the brain function, with great potential in neurotherapeutics. Intra-operative magnetic resonance imaging (MRI) guidance (in short, MRg) has limitations due to its complicated examination logistics, such as fixation through skull screws to mount the stereotactic frame, simultaneous sonication in the MRI environment, and restrictions in choosing MR-compatible materials. In order to overcome these limitations, an image-guidance system based on optical tracking and pre-operative imaging data is developed, separating the imaging acquisition for guidance and sonication procedure for treatment. Techniques to define the local coordinates of the focal point of sonication are presented. First, mechanical calibration detects the concentric rotational motion of a rigid-body optical tracker, attached to a straight rod mimicking the sonication path, pivoted at the virtual FUS focus. The spatial error presented in the mechanical calibration was compensated further by MRI-based calibration, which estimates the spatial offset between the navigated focal point and the ground-truth location of the sonication focus obtained from a temperature-sensitive MR sequence. MRI-based calibration offered a significant decrease in spatial errors (1.9±0.8 mm; 57% reduction) compared to the mechanical calibration method alone (4.4±0.9 mm). Using the presented method, pulse-mode FUS was applied to the motor area of the rat brain, and successfully stimulated the motor cortex. The presented techniques can be readily adapted for the transcranial application of FUS to intact human brain.

Estimation of the spatial profile of neuromodulation and the temporal latency in motor responses induced by focused ultrasound brain stimulation

“…This result is supported by previous findings in which small regions of cortical areas were selectively stimulated by FUS to specifically elicit only whisker, paw, or tail movements in rats 15,19. The stimulatory effect occurred over roughly the size of the acoustic focus (on the basis of the full-width at half-maximum criterion, shown in Fig.…”

“…The same set of sonication parameters (350 kHz FF, 0.5 ms TBD, 1 kHz PRF, 300 ms SD) as the PET experiment was initially used, although an increased AI (4.5 W/cm 2 I spta ) was used briefly to elicit definite tail movement in the rats 15. After localizing the area that responded to the stimulation, the AIs were changed to determine the minimum AIs (3.5±1.5 W/cm 2 I spta ) inducing the motor responses, while varying the TBDs (0.25, 0.5, 1, 2, 3, and 5 ms) at three different DCs ( n =9, 287±36 g for 30% DC; n =9, 278±23 g for 50% DC; n =5, 294±21 g for 70% DC).…”

Focused Ultrasound-mediated Non-invasive Brain Stimulation: Examination of Sonication Parameters

“…Among the various somatomotor cortical areas that could be stimulated by sonication ( e.g. causing whisker or limb movements), stimulation of the tail somatomotor area was selected since the resulting tail motion can be readily detected via visual inspection and a motion sensor (Piezoelectric Pulse Transducer, MLT1-1010/D, AD Instruments, CO) (29). In some of the previous studies, EMG had been adopted to detect peripheral motor responses induced by FUS stimulation (18, 23).…”

“…The threshold AI that elicited stimulation-related motor responses was determined for different combinations of the sonication parameters. Initially, we used a set of sonication parameters (0.5 ms TBD, 1 kHz PRF, 300 ms SD, 10 W/cm 2 I sppa ), which has been previously shown to stimulate somatomotor areas in rats (29). An ISI of 3 s was used to allow for enough time gaps between the stimulations.…”

Simultaneous acoustic stimulation of human primary and secondary somatosensory cortices using transcranial focused ultrasound

“…For image-guided alignment of the FUS focus to the intended target, the relative location and orientation of the transducers with respect to the helmet (i.e., subject’s head) were tracked in real-time, whereby the coordinates of the focus can be visualized on the individual-specific neuroanatomy (as well as the planned sonication target) via a custom-built image-guidance system as previously described [20, 31]. An optical tracker was attached to the helmet and each of the two FUS transducers for motion tracking.…”