Synopsis

Transcranial focused ultrasound is an old idea which has become a clinical reality within recent years. MR image guidance and treatment monitoring are key enablers of focused ultrasound technology for use in the brain. The origins, current state and future roadmap of this technology will be discussed. Given the challenges of accessing the brain and the high cost and limitations of some current approaches, this non-invasive technology has the potential to revolutionize the treatment of many brain disorders.

Investigation of the use of focused ultrasound (FUS) in the brain began in the middle of the 20th century, both in the United States and Europe. These early efforts required the removal of bone to create an acoustic window and were further challenged by the absence of practical image guidance. However, with the advent of CT and MR, large phased-array transducers, and the computational resources required for effective penetration of the intact skull by FUS, the technology has achieved clinical adoption. Beginning with the treatment of movement disorders, transcranial FUS is under investigation for an ever expanding list of neurological indications including the treatment of epilepsy, brain tumors, OCD, depression, neuropathic pain and a variety of drug delivery applications.

MR plays a key role in transcranial focused ultrasound including image guided procedure planning, treatment monitoring and follow up. Treatment monitoring using MR thermometry provides real time measurement of temperature increases in targeted tissue, allowing verification of focal heating and treatment completion. Current proton resonance frequency (PRF) based thermometry techniques are currently limited to monitoring 2D slices through the target. Investigation of rapid 3D thermometry techniques is maturing along with the potential of monitoring temperature change in the skull.

While current clinical activity with transcranial FUS is predominantly based on thermal ablation, a wide variety of other bioeffects achievable by the technology is under investigation. These include reversible blood-brain-barrier opening for drug delivery, hyperthermia, neuromodulation and histotripsy.

It is an exciting time for focused ultrasound in the brain. Propelled by a recent FDA approval for the treatment of essential tremor, a vibrant community of researchers and clinicians are engaged in expanding the reach and impact of this technology.

Acknowledgements

References

Odéen et al, "Treatment envelope evaluation in transcranialmagnetic resonance-guided focused ultrasoundutilizing 3D MR thermometry", Journal of Therapeutic Ultrasound 2014,2:19

Foley, et al, "Image-guided focused ultrasound: State of the technology and the challenges that lie ahead", Imaging in medicine 5(4):1190-1203 August 2013

Monteith et al, "Potential intracranial applications of magnetic resonance-guided focused ultrasound surgery ; A review", Journal of Neurosurgery 118(2) November 2012