Longitudinal Functional Assessment of Brain Injury Induced by High-Intensity Ultrasound Pulse Sequences.

Exposure of the brain to high-intensity stress waves creates the potential for long-term functional deficits not related to thermal or cavitational damage. Possible sources of such exposure include overpressure from blast explosions or high-intensity focused ultrasound (HIFU). While current ultrasound clinical protocols do not normally produce long-term neurological deficits, the rapid expansion of potential therapeutic applications and ultrasound pulse-train protocols highlights the importance of establishing a safety envelope beyond which therapeutic ultrasound can cause neurological deficits not detectable by standard histological assessment for thermal and cavitational damage. In this study, we assessed the neuroinflammatory response, behavioral effects, and brain micro-electrocorticographic (µECoG) signals in mice following exposure to a train of transcranial pulses above normal clinical parameters. We found that the HIFU exposure induced a mild regional neuroinflammation not localized to the primary focal site, and impaired locomotor and exploratory behavior for up to 1 month post-exposure. In addition, low frequency (δ) and high frequency (β, γ) oscillations recorded by ECoG were altered at acute and chronic time points following HIFU application. ECoG signal changes on the hemisphere ipsilateral to HIFU exposure are of greater magnitude than the contralateral hemisphere, and persist for up to three months. These results are useful for describing the upper limit of transcranial ultrasound protocols, and the neurological sequelae of injury induced by high-intensity stress waves.

Introduction

Purpose Transcranial ultrasound stimulation

Study Objective To determine whether transcranial high-intensity ultrasound pulse trains produce long-term neurological deficits in mice and to quantify those deficits using neuroinflammatory, behavioral, and µECoG measures.

Animal model / Human subject Mouse (Mus musculus), C57BL/6J, 2–12 months old, male

Disease model Healthy

MRI or image guidance method Stereotactic: animals were positioned in a stereotaxic apparatus and the transducer/nosecone placed ~2 mm above the skull/scalp; targeting was guided by surface anatomical landmarks and Bregma-referenced coordinates (e.g., ~1.5 mm lateral to the midline between the eyes and midway between eye and ear, over primary motor cortex).

Targeted brain region(s) Primary Motor Cortex

Target coordinates µECoG/craniotomy over right motor cortex: AP +1.5 to −1.0 mm, ML 0.5 to 3.0 mm (relative to Bregma). HIFU histology exposure: approximately over left primary motor cortex; described as ~1.5 mm lateral to the crossing of the midline between the eyes and the mid-way between eye and ear (no explicit AP or DV given).

Cargo name and characteristics High-intensity focused ultrasound (HIFU) — physical therapeutic modality delivered transcranially using a 1.1 MHz ring-geometry transducer (5 mm nosecone orifice). Dose: train of 40 sinusoidal pulses, each 10 ms on with 500 ms off. Acoustic pressures measured in water: peak negative 13.5 ± 1.5 MPa, peak positive 65 ± 10 MPa. In situ temperature rise ~3–3.5 °C. Applied ~2 mm above scalp/skull to target motor cortex; used for acute focal high-intensity stress-wave exposure in mice.

Route of administration Inhalational (isoflurane) for anesthesia; intraperitoneal injection (sodium pentobarbital, 100 mg/kg, i.p.) for terminal anesthesia/perfusion.

Outcomes and Safety

Summary of Outcomes High-intensity transcranial ultrasound pulse-train exposure produced mild diffuse neuroinflammation, persistent locomotor and exploratory deficits for at least one month, and acute-to-chronic alterations in cortical electrophysiology (increased δ and decreased β/γ activity, especially ipsilateral) lasting weeks to months.

Duration of biological effect 3 months

Safety-related matter High-intensity transcranial ultrasound pulse-trains caused mild diffuse neuroinflammation, impaired locomotor and exploratory behavior lasting at least one month, and acute-to-chronic ECoG abnormalities persisting up to three to six months, indicating potential long-term neurological injury despite absence of focal thermal or cavitational damage.

Brain Region

Ultrasound Parameters

Ultrasound instrument 1.1 MHz ring-geometry ultrasound transducer, H102 model (Sonic Concepts); coupling water-filled nosecone with 5 mm diameter orifice. System components included power amplifier A300 (Electronics & Innovation) and function generator AFG 3102C (Tektronix).

FUS Frequency 1.1 MHz

FUS Intensity Not reported in W/cm2

FUS Pressure Peak negative pressure: 13.5 ± 1.5 MPa (measured in water); Peak positive pressure: 65 ± 10 MPa

FUS Mode pulsed

Pulse duration 10 ms

Duration of a single FUS session Sonication: 40 pulses × (10 ms on + 500 ms off) = 20.4 s; entire procedure (including preparation/anesthesia): 15–30 min

Focal Characteristics Not specified in the paper

Treatment frequency single session

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