Histologic safety of transcranial focused ultrasound neuromodulation and magnetic resonance acoustic radiation force imaging in rhesus macaques and sheep

Abstract

Background Neuromodulation by transcranial focused ultrasound (FUS) offers the potential to non-invasively treat specific brain regions, with treatment location verified by magnetic resonance acoustic radiation force imaging (MR-ARFI). Objective To investigate the safety of these methods prior to widespread clinical use, we report histologic findings in two large animal models following FUS neuromodulation and MR-ARFI. Methods Two rhesus macaques and thirteen Dorset sheep were studied. FUS neuromodulation was targeted to the primary visual cortex in rhesus macaques and to subcortical locations, verified by MR-ARFI, in eleven sheep. Both rhesus macaques and five sheep received a single FUS session, whereas six sheep received repeated sessions three to six days apart. The remaining two control sheep did not receive ultrasound but otherwise underwent the same anesthetic and MRI procedures as the eleven experimental sheep. Hematoxylin and eosin-stained sections of brain tissue (harvested zero to eleven days following FUS) were evaluated for tissue damage at FUS and control locations as well as tissue within the path of the FUS beam. TUNEL staining was used to evaluate for the presence of apoptosis in sheep receiving high dose FUS. Results No FUS-related pre-mortem histologic findings were observed in the rhesus macaques or in any of the examined sheep. Extravascular red blood cells (RBCs) were present within the meninges of all sheep, regardless of treatment group. Similarly, small aggregates of perivascular RBCs were rarely noted in non-target regions of neural parenchyma of FUS-treated (8/11) and untreated (2/2) sheep. However, no concurrent histologic abnormalities were observed, consistent with RBC extravasation occurring as post-mortem artifact following brain extraction. Sheep within the high dose FUS group were TUNEL-negative at the targeted site of FUS. Conclusions The absence of FUS-related histologic findings suggests that the neuromodulation and MR-ARFI protocols evaluated do not cause tissue damage.

Abstract via europepmc.

Flags from extraction

• n_subjects — Combined total across two species (2 rhesus macaques + 13 Dorset sheep, of which 11 received FUS and 2 were untreated controls); see provenance.notes for the breakdown.
• n_sessions_per_subject — Sessions varied by subgroup: macaques and 5 sheep received a single session, 6 sheep received repeated sessions 3-6 days apart, and the high-dose group received additional prolonged sessions (up to 70 consecutive MR-ARFI repetitions at one location); not reducible to a single figure.
• randomised — Paper does not state whether animals were randomly allocated to dose/location or control groups.
• sham_type — The two control sheep received no ultrasound at all (true untreated controls matched for anaesthesia/MRI) rather than a sham stimulus mimicking active treatment; classified as sham_type 'none' for lack of a closer fit in the controlled vocabulary.
• auditory_control — All animals were anaesthetised during FUS; the paper does not address auditory confounds (not relevant/discussed for this histology-focused study).
• exposures[0].timing.pulse_repetition_frequency_hz — PRF (1 Hz) is derived from the stated repetition period ('occurring every 1 s'); not stated as a Hz value directly in the text.
• exposures[0].timing.duty_cycle_pct — Stated 50% duty cycle for a 300 ms burst repeated every 1 s implies ~30% duty if computed at the burst-repetition level (300 ms x 1 Hz); the discrepancy suggests the stated 50% describes an unspecified higher-frequency pulsing within each 300 ms burst window rather than the burst-to-repetition duty cycle. Recorded as stated; a 'burst rule' ambiguity.
• exposures[0].in_situ.pressure_kpa — Only in-situ ISPTA (not in-situ peak pressure) is given for the macaque experiment; recorded as not_reported.
• exposures[1].timing.pulse_repetition_frequency_hz — PRF values (2 Hz for MR-ARFI's 500 ms window, 1 Hz for neuromodulation's 1 s repetition) are derived from stated repetition periods, not given as Hz values directly.
• exposures[1].timing — Sheep transducer (550 kHz) was used for two protocols with very different timing (MR-ARFI: 16 ms bursts within a 500 ms window; neuromodulation: 200-300 ms bursts every 1 s, CW or 50% duty) combined here as list values within one exposure per the target x frequency exposure rule; see protocol_description for which values belong to which protocol.
• exposures[1].in_situ.reported_as — In-situ pressure/intensity given as ranges across sonications/animals (not single values or explicit means); classified as mean_or_range_across_subjects.