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How Does MRgFUS Work? — The Technology of Focused Ultrasound Neurosurgery, Explained at KIMS Hyderabad
1,024 converging ultrasound beams, real-time MRI thermometry, electronic beam steering, transcranial phase correction, and a 55–60°C thermal lesion in the Vim thalamus — the complete technology explanation at KIMS Neuroscience, Secunderabad, Hyderabad.
Focusing Energy Through Solid Bone — With Sub-Millimetre Precision
The idea of focusing energy through a solid object to heat a precise internal target without touching the object's surface is not unique to medicine — it is the same principle as focusing sunlight through a magnifying glass to heat a point beneath the glass without touching it. What makes MRgFUS extraordinary is the precision with which this focusing is achieved in three dimensions inside the human brain — to within 1–2 millimetres — and the real-time MRI guidance that monitors the temperature at every point during the treatment, making the procedure safe for a structure as critical as the living brain.
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The KIMS team walks every patient through the technology in detail at the pre-procedure consultation, using the actual treatment planning images.
040-44885000Emergency line: 040-463243651,024 Converging Beams · 55–60°C Target · Real-Time MRI Thermometry
1,024 focused ultrasound beams converge from outside the skull to a single point — the Vim nucleus of the thalamus. Each individual beam is too weak to damage tissue on its own. At the convergence point, the combined energy of all 1,024 beams heats the target tissue to 55–60°C for 10–15 seconds, creating a precisely defined thermal lesion. Real-time MRI thermometry monitors the temperature at the target and at all surrounding structures during heating, stopping the treatment instantly if temperatures deviate from the plan. The skull, scalp, and all brain tissue outside the target are not damaged.
- 1,024 converging beams — the energy geometry
- The Insightec Exablate Neuro system has 1,024 piezoelectric transducers arranged across a hemispherical helmet. Each beam is clinically harmless as it passes through the skull and brain. Only at the convergence point — where all 1,024 beams focus simultaneously — does the accumulated acoustic energy reach therapeutic intensity, heating the target to 55–60°C and creating a precise thermal lesion. This is the same converging-beam principle as Gamma Knife radiosurgery (192 converging gamma rays) — but using mechanical sound energy instead of radiation, and heating rather than ionising the tissue.
- MRI thermometry — the real-time safety system
- Proton resonance frequency (PRF) shift thermometry is built into the MRI scanner used during the procedure. By measuring the PRF shift across the entire brain volume in real time, the MRI generates a temperature map updated every 3–5 seconds. This confirms the target is heating as planned and that adjacent structures stay below safe thermal thresholds. Heating can be stopped within seconds if the map shows anything unexpected — the fundamental safety mechanism that makes MRgFUS safe for the living human brain.
- Transcranial phase correction — the breakthrough
- Ultrasound does not pass through bone without distortion: each of the 1,024 beams is refracted, reflected, and absorbed to a different degree by the skull. Without compensation these distortions would scatter the beams and prevent convergence. Transcranial phase correction uses the pre-procedure CT scan to model precisely how each beam will be distorted, then applies individual time delays (phase shifts) to each transducer so all 1,024 beams arrive at the target simultaneously, in phase. This algorithm was the fundamental breakthrough that made transcranial focused ultrasound clinically viable.
- Electronic beam steering — fractional-millimetre adjustment
- The convergence point can be moved electronically in three dimensions by introducing phase shifts between transducers — without physically moving the helmet or the patient. During the procedure, the targeting physician can shift the focal point by fractions of a millimetre in real time in response to patient feedback or thermometry data, iteratively refining the target before committing to therapeutic energy levels.
- Three-stage sonication — the safety-first escalation protocol
- Stage 1 (Alignment): low-energy sonications confirm that the focal point is correctly positioned on the target; thermometry confirms location. Stage 2 (Testing Phase / Verification): sub-therapeutic sonications at 45–48°C — the patient reports any sensations, and patient feedback and thermometry confirm the target is correct. Stage 3 (therapeutic): sonications at 55–60°C for 10–15 seconds create the permanent thermal lesion. Post-treatment MRI immediately confirms lesion location and verifies adjacent structures are unaffected. This is why MRgFUS can be performed safely in an awake patient.
- Skull cooling — preventing thermal injury through bone
- As ultrasound passes through the skull, some energy is absorbed by bone and converted to heat. Without countermeasures this would cause scalp and skull thermal injury during sonication. The Exablate Neuro system uses a continuous flow of cold degassed water (15–20°C) between the transducer array and the scalp, which cools the skull and also serves as the acoustic coupling medium. Degassed water is required because air bubbles would scatter the ultrasound beams.
The MRgFUS Technology Stack
The six engineering and physics concepts that make MRgFUS possible — each explained in detail in the sections below.
1,024 Transducer Array
Insightec Exablate Neuro hemisphere with 1,024 piezoelectric transducers — each individually harmless, converging at the focal point to deliver therapeutic energy. Same converging-beam principle as Gamma Knife — but sound, not radiation.
Electronic Beam Steering
Time delays (phase shifts) between transducers move the convergence point in 3D without moving the helmet or the patient. Enables fractional-millimetre real-time target adjustment during the procedure.
Skull Cooling
Continuous flow of cold (15–20°C) degassed water between the transducer array and the scalp — cools the skull during sonication and serves as the acoustic coupling medium.
MRI Thermometry
Proton resonance frequency (PRF) shift gives a real-time temperature map of the entire brain, updated every 3–5 seconds. Confirms target heating, monitors all adjacent structures, stops the sonication instantly if anything goes wrong.
Three-Stage Sonication
Low-energy verification sonications at 45–48°C → patient feedback and thermometry confirmation → therapeutic sonications at 55–60°C for 10–15 seconds → post-treatment MRI verification.
Transcranial Phase Correction
Sophisticated algorithms model how each of the 1,024 beams is distorted by the skull bone (from the CT scan) and apply per-beam phase corrections so they all arrive at the target in phase — the breakthrough that made transcranial MRgFUS possible.
The Exablate Neuro System — What the Technology Consists Of
The transducer array — 1,024 sources of focused energy
The Insightec Exablate Neuro system (the MRgFUS device used at KIMS) consists of a hemispherical transducer array containing 1,024 individual piezoelectric ultrasound transducers arranged across the inner surface of a helmet-like device that fits around the patient's shaved head. Each transducer generates a focused ultrasound beam directed toward the centre of the hemisphere — the planned focal point (isocenter). By itself, each transducer delivers ultrasound at an intensity that is clinically harmless to the skull and brain tissue it traverses — only at the focal point, where all 1,024 beams converge, does the accumulated intensity reach therapeutic levels. This is the same principle as Gamma Knife radiosurgery (192 converging radiation beams) — but using mechanical sound waves instead of electromagnetic radiation, and heating rather than ionising the target tissue.
Electronic beam steering — how the target is moved without moving the device
The Exablate Neuro system does not need to physically move to change where the beams converge. The target position can be shifted electronically: by introducing small, precisely calculated time delays between the firing of different transducers (phase shifts), the convergence point of the 1,024 beams can be moved in three dimensions without changing the position of the helmet or the patient's head. This allows the targeting physician to make fine adjustments to the focal point in real time — moving it by fractions of a millimetre in response to patient feedback or thermometry findings — without touching the patient. This electronic steering capability is essential for the step-by-step targeting process used in MRgFUS.
Skull cooling — preventing scalp and skull heating
As ultrasound passes through the skull bone, some energy is absorbed by the bone and converted to heat. Without countermeasures, this bone heating would cause scalp and skull burns during the sonication. The Exablate Neuro helmet addresses this with a continuous flow of cold degassed water (at approximately 20–15°C) between the transducer array and the patient's scalp. This water coupling also serves as the acoustic medium for the ultrasound transmission (ultrasound does not travel through air). The cooling water prevents skull heating while allowing the focused ultrasound energy to reach the brain target.
MRI Thermometry — The Real-Time Safety System
What MRI thermometry is and why it is essential
MRI thermometry is the technique that makes MRgFUS safe. In conventional surgical procedures, the surgeon sees the tissue directly and can judge whether an area is being damaged. In MRgFUS, the target is deep inside the skull and invisible. MRI thermometry provides a real-time temperature map of the entire brain — updated every few seconds during the sonication — that shows the temperature at the focal point AND at every other point in the brain simultaneously. This allows the team to confirm that: (1) The target is reaching the planned therapeutic temperature (55–60°C). (2) The surrounding brain, skull, and scalp are NOT being heated beyond safe limits. (3) If any unexpected heating appears in an unintended location, the sonication can be stopped within seconds — before tissue damage occurs.
How MRI measures temperature in real time
MRI thermometry exploits the physics of water proton resonance frequency (PRF) shift: the resonant frequency of water protons in tissue changes predictably with temperature — approximately −0.01 ppm/°C. By measuring this frequency shift on rapidly acquired phase-sensitive MRI sequences during sonication, the MRI scanner can calculate the temperature change at every voxel (3D pixel) in the imaging volume, updated every 3–5 seconds. The temperature map is displayed in real time on the treatment workstation as a colour-coded overlay on the anatomical MRI: cool areas in blue, heated areas transitioning through green, yellow, and red as temperature rises. The team watches this thermometry display continuously during every sonication — it is the primary safety monitor of the entire procedure.
Treatment windows — sub-therapeutic, therapeutic, and verification
MRgFUS treatment proceeds in a series of sonications (pulses of ultrasound) at progressively increasing energy levels. This staged approach is a critical safety feature: Early low-energy sonications: delivered at sub-therapeutic temperatures (45–48°C, insufficient to create a permanent lesion). Purpose: to verify that the thermal focus is precisely at the planned target, that the thermometry map matches the expected heating pattern, and that the patient is responding as expected · The patient is asked for feedback during and after each sonication: “Does the tremor look better? Do you feel any tingling or numbness?” · If the low-energy sonications are on-target and well-tolerated, energy is incrementally increased. Therapeutic sonications: delivered at 55–60°C for 10–15 seconds, creating the permanent thermal lesion. Post-treatment verification: final MRI (T1 + T2 + DWI) to confirm the lesion location and rule out any unintended heating effects before discharge.
The Physics of Ultrasound in the Brain
Why ultrasound can penetrate the skull — and when it cannot
Bone is not a perfect barrier to sound. Ultrasound waves are sound waves at frequencies above the range of human hearing (the Exablate Neuro operates at approximately 650 kHz). At this frequency, ultrasound does penetrate bone — but with significant reflection, scattering, and absorption. The key innovation that made transcranial focused ultrasound possible (in the 2000s) was combining MRI-based skull CT reconstruction with sophisticated phase correction algorithms: the computer calculates the skull bone geometry from the CT scan, models how each individual beam from each of the 1,024 transducers will be distorted and delayed by the skull, and applies phase corrections to compensate for these distortions — ensuring that all 1,024 beams arrive at the target simultaneously and in phase, despite passing through an irregular skull bone. Without this phase correction, the beams would arrive out of phase and would not converge to a tight focal point.
Thermal vs mechanical effects — what actually destroys the target tissue
The therapeutic effect of MRgFUS in thalamotomy is thermal: the focused energy heats the target tissue to 55–60°C, denaturing proteins and causing coagulative necrosis of the neurons in the focal zone. This is the same type of tissue destruction as thermal ablation used in cardiac procedures (radiofrequency ablation) and liver tumour ablation — but delivered non-invasively through the skull. The resulting lesion is a small zone of coagulative necrosis approximately 3–5 mm in diameter surrounded by a rim of reversible oedema that resolves over weeks. At sub-therapeutic temperatures (<55°C), the ultrasound energy causes temporary, reversible suppression of neuronal activity — which is what the patient experiences as partial tremor improvement during the early low-energy sonications, before the therapeutic lesion is created.
What the Technology Achieves
MRgFUS at KIMS uses the Insightec Exablate Neuro platform — a system engineered to deliver therapeutic ultrasound through the intact skull with sub-millimetre precision. The following points summarise the most clinically significant technical facts every patient should understand.
Exablate Neuro uses 1,024 piezoelectric ultrasound transducers arranged in a hemispherical helmet — each individually harmless, converging at a single focal point
Operating frequency approximately 650 kHz — above the range of human hearing, capable of penetrating the skull with phase correction
Therapeutic target temperature: 55–60°C for 10–15 seconds — produces a 3–5 mm zone of coagulative necrosis surrounded by reversible oedema
MRI thermometry uses water proton resonance frequency (PRF) shift of approximately −0.01 ppm/°C — temperature map updated every 3–5 seconds
Electronic beam steering by phase-shifting transducer firing moves the focal point in 3D without moving the helmet or the patient's head
Continuous cold degassed water (15–20°C) circulates between the transducer array and the scalp — prevents skull heating and provides acoustic coupling
Sub-therapeutic sonications (45–48°C) cause temporary reversible neural suppression — patients often see tremor improve before the permanent lesion is created
MRgFUS uses no ionising radiation — fundamentally different from Gamma Knife thalamotomy (the radiation-based alternative for the same Vim target)
How the Technology Works at a Glance
| Fact | Details |
|---|---|
| The physics | 1,024 piezoelectric transducers converge ultrasound at the Vim nucleus · Each beam individually harmless · Therapeutic only at the convergence point · Same principle as Gamma Knife but acoustic, not radiation |
| Real-time safety | MRI thermometry via PRF shift · Whole-brain temperature map updated every 3–5 seconds · Heating stopped instantly if anything deviates from plan · The fundamental safety mechanism |
| Transcranial phase correction | Per-beam phase shifts derived from pre-procedure CT of skull · Compensates for individual beam distortion by bone · All 1,024 beams arrive at target in phase · The core breakthrough that made MRgFUS viable |
| Electronic beam steering | Phase delays move convergence point in 3D without moving device · Fractional-millimetre real-time adjustment during procedure · Enables iterative target refinement before therapeutic sonications |
| Three-stage sonication | Stage 1: Alignment (low-energy, focal point positioning) · Stage 2: Testing Phase / Verification (45–48°C, sub-therapeutic) — patient feedback + thermometry confirmation · Stage 3: 55–60°C for 10–15 seconds (therapeutic lesion) · Post-treatment MRI verification |
| Skull cooling | 15–20°C cold degassed water between transducer array and scalp · Prevents skull thermal injury during sonication · Also the acoustic coupling medium · Degassed to eliminate air-bubble scattering |
| Device | Insightec Exablate Neuro · FDA-registered · Same device as NEJM clinical trials · First in Telangana and AP at KIMS |
Related Treatments
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Frequently Asked Questions
Understanding the Technology is the Foundation for Trusting It — MRgFUS at KIMS Hyderabad.
Understanding how MRgFUS works is the foundation for trusting it. At KIMS Hyderabad, the team walks every patient through the technology in detail during the pre-procedure consultation — using the actual treatment planning images to show where the target is, what the thermometry looks like, and what the lesion will appear on post-procedure MRI. Call 040-44885000.
International enquiries: info@kimsneurosciences.com