Transcranial MRI-Guided Focused Ultrasound: A Review of the Technologic and Neurologic Applications

Abstract

Objective: This article reviews the physical principles of MRI-guided focused ultra-sound and discusses current and potential applications of this exciting technology.

Conclusion: MRI-guided focused ultrasound is a new minimally invasive method of targeted tissue thermal ablation that may be of use to treat central neuropathic pain, essential tremor, Parkinson tremor, and brain tumors. The system has also been used to temporarily disrupt the blood-brain barrier to allow targeted drug delivery to brain tumors.

Keywords: MRI-guided focused ultrasound; brain; brain tumors; movement disorders; neuropathic pain; stroke.

Figure 1

Photograph and schematic of the InSightec Exablate transcranial focused ultrasound system. A membrane holds in the water between the transducer and the patient head. The patient is fixed to the table by the frame, while the transducer can move independently in order to place the target as close as possible to the geometric center of the hemisphere.

Figure 2

Two example beams traveling through water, a simplified bone model with a single elevated velocity, then soft tissue to the target. The increased speed of sound through bone results in a change in wavelength. These two beams pass through different lengths of bone, resulting in differences in phase when the beams hit the target. Such differences in phase mean that the beams do not add constructively and the resulting temperature rise is lower than it could be. Phase aberration correction methods estimate this phase and apply the negative of it to each transducer element.

Figure 3

MR temperature maps in a and d derived from the change in proton resonant frequency seen with temperature. The skull is overlaid on the temperature image in green. A cross hair is placed at the location of the focal spot, with the maximum temperature within a 3×3 region of interest plotted in b), along with the average in the 3×3 pixel region of interest. These curves demonstrate an exponential rise in temperature during the sonication, and the exponential decay in temperature after the sonication is ended.

Figure 4

a) MR-ARFI relies on the acoustic radiation force at the focus displacing the spins. This displacement is encoded into the phase of the spins b) and c) and therefore the phase of the image d). These images were collected in an ex vivo porcine brain as described in Kaye et al.

Figure 5

Treatment planning images. Sagittal MR images acquired before treatment (a) and on the treatment day (b) are co-registered using the CT overlay (in green). The anterior and posterior commissures are marked in red. “No pass” zones in blue mark intrancranial calcifications in the choroid plexus, pineal gland, falx, basal ganglia or vessels on the CT (c), as is air in the sinuses, and air trapped between the membrane and scalp is marked on the treatment day MRI (d). Anatomic measurements for targeting the Vim nucleus of the thalamus provide a starting point for essential tremor treatments (e), with final targeting confirmed using intra-procedural neurological feedback from the awake patient.

Figure 5

Treatment planning images. Sagittal MR images acquired before treatment (a) and on the treatment day (b) are co-registered using the CT overlay (in green). The anterior and posterior commissures are marked in red. “No pass” zones in blue mark intrancranial calcifications in the choroid plexus, pineal gland, falx, basal ganglia or vessels on the CT (c), as is air in the sinuses, and air trapped between the membrane and scalp is marked on the treatment day MRI (d). Anatomic measurements for targeting the Vim nucleus of the thalamus provide a starting point for essential tremor treatments (e), with final targeting confirmed using intra-procedural neurological feedback from the awake patient.

Figure 6

Conventional MR imaging features after MRgFUS lesioning of the left ventralis intermedius nucleus (Vim) of the thalamus in a right-handed patient with essential tremor. The MRgFUS lesion appears on T2-weighted images at 24 hours and initially restricts diffusion. It demonstrates faint enhancement at 1 month, and the cavity collapses by 3 months. Blood product staining is seen immediately after the MRgFUS treatment and persists over time.

Figure 7

A clot was induced in the M1 segment of the right middle cerebral artery by injecting thrombin through an endovascular microcatheter introduced through the femoral artery in a fresh human cadaver, and flow in the arteries was maintained by a saline pumping system. MRgFUS targeted the right M1 clot (arrows) and with a total of 4 sonications, the clot was completely sonothrombolyzed over 10 minutes.