Functional Neurosurgery · Frontier & Investigational
Focused Ultrasound: Emerging Indications and LIFU
Beyond Ablation — Neuromodulation, Blood-Brain-Barrier Opening, and the Non-Thermal Frontier
The same transducer that burns a thalamic lesion can, at a fraction of the power, open the blood-brain barrier or nudge a circuit without destroying it. This is where focused ultrasound stops being a scalpel and starts being a platform — promising, fast-moving, and almost entirely investigational. Read it as a map of what is coming, with the evidence tier stated honestly for each.
Orientation
The approved uses of cranial focused ultrasound — thalamotomy for tremor, pallidal and pallidothalamic ablation for Parkinson disease — all rely on high-intensity energy to coagulate tissue. But intensity is a dial. Turn it down, and ultrasound stops destroying and starts doing other things: transiently opening the blood-brain barrier, modulating neuronal excitability, releasing drugs from carriers, or mechanically fractionating tissue. These low-intensity (LIFU) and non-thermal applications are the reason the field increasingly describes focused ultrasound as a platform rather than a single operation.
Almost everything on this page is investigational. The purpose here is to separate what is mechanistically plausible and in trials from what is approved, so that the promise is communicated without overselling. Each application is tagged by where it actually sits: early human trials, single-centre case series, or preclinical concept. The honest framing matters most precisely where enthusiasm is highest.
One Transducer, Many Intensities
1.HIFU versus LIFU — the governing distinction
The single most useful concept on this page is the difference between high- and low-intensity focused ultrasound. High-intensity focused ultrasound (HIFU) deposits enough acoustic energy to raise tissue above ~55°C and create a permanent thermal lesion — the mechanism of the approved ablations. Low-intensity focused ultrasound (LIFU) uses orders-of-magnitude lower energy that produces little or no heating; its effects are mechanical and reversible, mediated by acoustic radiation force and, when intravenous microbubbles are added, by their oscillation (stable cavitation) against vessel walls and membranes. The same hemispheric array, and often the same MR guidance, can do either — the difference is the dose, not the device.
| Property | HIFU (high-intensity) | LIFU (low-intensity) |
|---|---|---|
| Primary effect | Thermal coagulation | Mechanical / non-thermal, reversible |
| Tissue result | Permanent lesion | No destruction (transient effect) |
| Microbubbles | Avoided (cavitation is a hazard) | Often required (mediate the effect) |
| Representative use | Thalamotomy, pallidotomy | BBB opening, neuromodulation |
| Regulatory status | Approved (tremor, PD) | Investigational |
Blood-Brain-Barrier Opening
2.Opening the barrier on purpose
The blood-brain barrier protects the brain but also excludes most large-molecule drugs, antibodies, and chemotherapies. LIFU with intravenous microbubbles can transiently and reversibly open the barrier in a precisely targeted volume: the oscillating microbubbles loosen endothelial tight junctions for a window of hours, after which the barrier reseals. The first-in-human Alzheimer's trial (Lipsman et al., 2018) opened the barrier in five patients safely, reversibly, and repeatedly with no serious adverse events — a proof of concept that has since been extended to amyotrophic lateral sclerosis, Parkinson disease, glioma, and brain metastases.
The most tangible therapeutic rationale is in neuro-oncology. The Sunnybrook group (Mainprize et al., 2019) opened the barrier around recurrent high-grade glioma the day before resection and demonstrated higher chemotherapy (liposomal doxorubicin) concentrations in sonicated tumour than in unsonicated tissue, with the barrier resealing within about a day. The strategy under active study is to use BBB opening to enhance delivery of temozolomide, antibodies, immunotherapy, and gene therapy — and, separately, to use sonication-induced clearance effects as a possible disease-modifying approach in Alzheimer's. All of this remains investigational, with efficacy (as opposed to feasibility and safety) still unproven.
Neuromodulation and Other Ablative Frontiers
3.Reversible neuromodulation with LIFU
At low intensity and without microbubbles, focused ultrasound can transiently raise or lower the excitability of a targeted region — the effect depending on pulse parameters (duty cycle, intensity, duration). Its appeal is unique among neuromodulation tools: it is non-invasive, yet unlike transcranial magnetic or electrical stimulation it can reach deep targets (amygdala, thalamus, basal ganglia) with millimetric focus. Early human work is exploring LIFU neuromodulation in depression, chronic pain, disorders of consciousness, and epilepsy, and as a mapping tool to test a target reversibly before a permanent procedure. This is among the most exciting and least mature areas — predominantly small, early-phase studies.
4.Ablative frontiers: psychiatric, pain, and epilepsy targets
HIFU ablation is also being extended beyond movement disorders, reusing the incisionless-lesion advantage:
- Refractory OCD and depression — bilateral MRgFUS anterior capsulotomy reproduces the classic stereotactic capsulotomy lesion without an incision, with response rates in early series broadly comparable to other capsulotomy techniques (Jung 2015; Davidson 2020). Covered alongside DBS and radiosurgical capsulotomy on the psychiatric-neurosurgery pages.
- Neuropathic pain — central lateral thalamotomy by MRgFUS (Jeanmonod and colleagues) has been used for chronic neuropathic pain in small series, with mixed and incompletely durable results — investigational.
- Epilepsy — both ablation of an epileptogenic focus and LIFU neuromodulation of seizure networks (for example anterior thalamic or hypothalamic-hamartoma targets) are under early investigation; no MRgFUS epilepsy indication is approved.
5.The preclinical horizon
Further out are approaches still largely in the laboratory: histotripsy (purely mechanical, non-thermal tissue fractionation by controlled cavitation, avoiding the heat-and-skull-efficiency constraints of thermal ablation), sonodynamic therapy (ultrasound-activated sonosensitizers for glioma), focused-ultrasound-enabled liquid biopsy (sonication to release tumour biomarkers into blood), and ultrasound-triggered drug-carrier release. These are concept-stage to early-preclinical and are noted here only to map the trajectory, not to imply clinical readiness.
| Application | Modality | Maturity |
|---|---|---|
| BBB opening for drug/antibody delivery | LIFU + microbubbles | Early human trials (glioma, AD, ALS, PD) |
| Reversible neuromodulation | LIFU (no microbubbles) | Early human / mapping studies |
| Anterior capsulotomy (OCD, MDD) | HIFU ablation | Early human series |
| Central lateral thalamotomy (pain) | HIFU ablation | Small series; mixed durability |
| Epilepsy ablation / neuromodulation | HIFU or LIFU | Early investigation |
| Histotripsy / sonodynamic / liquid biopsy | Non-thermal / adjunctive | Preclinical to concept |
Key points
- Intensity is the dial: HIFU coagulates tissue (the approved ablations); LIFU, at far lower energy, produces reversible, mechanical effects — the basis of the entire emerging field.
- LIFU with intravenous microbubbles transiently and reversibly opens the blood-brain barrier; first shown safely in Alzheimer's (Lipsman 2018) and used to raise intratumoural chemotherapy levels in glioblastoma (Mainprize 2019). Efficacy remains unproven.
- LIFU neuromodulation is uniquely non-invasive yet deep-reaching; under early study for depression, pain, epilepsy, and as a reversible target-mapping tool.
- HIFU ablation is extending to incisionless anterior capsulotomy (OCD/MDD), central lateral thalamotomy (pain), and epilepsy targets — all investigational.
- Histotripsy, sonodynamic therapy, and FUS-enabled liquid biopsy are preclinical-to-concept frontiers worth tracking, not offering.
- The discipline of this topic is tier-labeling: approved use is limited to movement disorders; everything here is investigational, and saying so is part of saying it accurately.
Selected References
- Lipsman N, Meng Y, Bethune AJ, et al. Blood-brain barrier opening in Alzheimer's disease using MR-guided focused ultrasound. Nat Commun. 2018;9(1):2336. PubMed First-in-human reversible BBB opening.
- Mainprize T, Lipsman N, Huang Y, et al. Blood-brain barrier opening in primary brain tumors with non-invasive MR-guided focused ultrasound: a clinical safety and feasibility study. Sci Rep. 2019;9(1):321. PubMed Enhanced intratumoural drug delivery in glioblastoma.
- Jung HH, Kim SJ, Roh D, et al. Bilateral thermal capsulotomy with MR-guided focused ultrasound for patients with treatment-refractory obsessive-compulsive disorder. Mol Psychiatry. 2015;20(10):1205–1211. PubMed Incisionless capsulotomy for OCD.
- Davidson B, Hamani C, Rabin JS, et al. Magnetic resonance-guided focused ultrasound capsulotomy for refractory obsessive-compulsive disorder and major depressive disorder. Mol Psychiatry / Neurosurgery series. PubMed search OCD and depression outcomes.
- Jeanmonod D, Werner B, Morel A, et al. Transcranial magnetic resonance imaging-guided focused ultrasound: noninvasive central lateral thalamotomy for chronic neuropathic pain. Neurosurg Focus. 2012;32(1):E1. PubMed MRgFUS thalamotomy for pain.
- Meng Y, Hynynen K, Lipsman N. Applications of focused ultrasound in the brain: from thermoablation to drug delivery. Nat Rev Neurol. 2021;17(1):7–22. PubMed Platform review across intensities.
Educational synthesis for neurosurgery trainees; not a treatment directive. All applications described here are investigational; maturity tiers are stated as of 2026 and will change. Landmark feasibility studies verified against primary publications during review; where an exact citation was uncertain, a PubMed search link is provided rather than a fabricated identifier.