Transcranial focused ultrasound systems (Exablate Neuro, SonoCloud)

Insightec's Exablate Neuro places the patient's shaved head, fixed in a stereotactic frame, inside a hemispherical transducer of over a thousand elements filled with cooled degassed water, inside an MRI scanner. CT of the skull lets the software correct each element's phase for the bone it passes through, so the beams converge on a target a few millimetres across. At high intensity the system ablates tissue, the basis of its approval for essential tremor in 2016; at low intensity with intravenous microbubbles it makes the vessel walls of the blood-brain barrier temporarily permeable, the mode being trialled in glioblastoma with temozolomide or chemotherapy, in brain metastases with antibodies, and for releasing tumour DNA into the blood as a liquid biopsy. Sunnybrook in Toronto performed the first such openings in brain tumour patients and continues the trials. Carthera's SonoCloud takes a different route: a small unfocused ultrasound implant fixed in the skull window left after tumour surgery, activated at each chemotherapy visit to open the barrier across a wide region in a few minutes without MRI. NaviFUS in Taiwan uses a neuronavigated headset.

Both approaches are still investigational for cancer. The helmet demands MRI time, head shaving and a frame, and reaches only a limited volume per session; the implant covers more brain but needs surgery to place and cannot be steered. Whether opening the barrier improves survival is the question the current trials are asking.