A Comprehensive Review of Low-Intensity Focused Ultrasound Parameters and Applications in Neurologic and Psychiatric Disorders

Abstract

Objectives:

Low-intensity focused ultrasound (LIFU) is gaining increased interest as a potential therapeutic modality for a range of neuropsychiatric diseases. Current neuromodulation modalities often require a choice between high spatial fidelity or invasiveness. LIFU is unique in this regard because it provides high spatial acuity of both superficial and deep neural structures while remaining noninvasive. This new form of noninvasive brain stimulation may provide exciting potential treatment options for a variety of neuropsychiatric disorders involving aberrant neurocircuitry within deep brain structures, including pain and substance use disorders. Furthermore, LIFU is compatible with noninvasive neuroimaging techniques, such as functional magnetic resonance imaging and electroencephalography, making it a useful tool for more precise clinical neuroscience research to further understand the central nervous system.

Materials and Methods:

In this study, we provide a review of the most recent LIFU literature covering three key domains: 1) the history of focused ultrasound technology, comparing it with other forms of neuromodulation, 2) the parameters and most up-to-date proposed mechanisms of LIFU, and finally, 3) a consolidation of the current literature to date surrounding the clinical research that has used LIFU for the modification or amelioration of several neuropsychiatric conditions.

Results:

The impact of LIFU including poststroke motor changes, pain, mood disorders, disorders of consciousness, dementia, and substance abuse is discussed.

Conclusions:

Although still in its infancy, LIFU is a promising tool that has the potential to change the way we approach and treat neuropsychiatric disorders. In this quickly evolving field, this review serves as a snapshot of the current understanding of LIFU in neuropsychiatric research.

INTRODUCTION

Until recently, the only available treatments for neuropsychiatric disorders, ranging from depression and anxiety to chronic pain, have generally been the administration of psychopharmacologic or behavioral interventions. However, there is an increasing desire to advance treatments to involve a more neuroanatomically specific, circuit-based approach to treatment. For this reason, there has been an overwhelming growth in the area of neuromodulation techniques over the past 25 years as a means of directly modifying the activity of neural substrates, with much success.^1–3 Although brain stimulation has been shown to improve clinical outcomes and has gained several United States Food and Drug Administration approvals, there remain several drawbacks to each modality. The two commonly implanted forms of neuromodulation include vagus nerve stimulation (VNS) and deep brain stimulation (DBS).^1,4,5 Technology such as DBS can reach deep brain structures with a high spatial resolution but requires electrode placement surgically in the brain and is accompanied by serious neurosurgical risk, along with the semipermanence of an implant. VNS involves implanting electrodes around the vagus nerve in the neck, and although more commonly used than DBS, its effects are diffuse and nonspecific and still pose significant risks related to surgery.^4,5 Noninvasive alternatives, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), allow excitation or inhibition of neural activity through the skull and can improve outcomes of conditions such as major depressive disorder (MDD), obsessive compulsive disorder, and substance use disorders (SUD).^2,3,6–8 However, owing to technologic limitations, TMS and tDCS effects are limited to cortical structures and do not afford strong spatial fidelity, stimulating regions as large as centimeters.^5,9 In contrast to all commonly clinically available brain stimulation techniques, transcranial low-intensity focused ultrasound (LIFU) offers the ability to perform both deep and focal stimulation on a region or circuit-specific level, without the need for invasive procedures.¹⁰ This opens the potential for exploration of therapeutic interventions otherwise inaccessible and previously not feasible.

The use of low-intensity ultrasound as a neuromodulatory tool started in the early 20th century, with some of the first documentation of sound waves modifying cellular activity in the brain in 1929.¹¹ In the 1950s, ultrasound was shown to reversibly suppress both neuronal firing rate and visual-evoked potentials in the lateral geniculate nucleus of the cat cortex.^12,13 This seminal work did not initially spur a meaningful change in the utilization of LIFU as a noninvasive tool for modulating neural tissue; instead, it went largely unnoticed. It was not until the 21st century, with additional preclinical evidence suggesting LIFU can modulate voltage-gated channels in the hippocampus in vitro and produce motor responses in awake mice when focused on the motor cortex, that interest in the application of LIFU for neuromodulation resumed.^9,14,15

Parameter Considerations and Mechanisms of LIFU

Parameter Considerations

LIFU uses ultrasonic sound waves (above the limit of human perception, >20 kHz) often pulsed in a sinusoidal waveform to propagate through bone and tissue. There are five main stimulus parameters for ultrasound: fundamental frequency (FF), peak intensity, sonication duration (SD), pulse repetition frequency (PRF), and duty cycle (DC).^4,9 Additional parameters include pulse width (PW), pulse repetition period (PRP), interstimulus interval, and mechanical index (MI)¹⁶ (Fig. 1 presents the schematic of stimulus parameters). The frequency can dramatically alter the outcome of the stimulation; high-frequency stimulation has shorter wave-lengths and therefore allows higher spatial resolution (resolution is inversely related to wavelength). Higher frequency, however, also is associated with increased acoustic attenuation through the skull. For the human skull, when skull thickness is a concern, the range used is often 250 to 650 kHz.^17,18

Figure 1.

Schematic of primary Transcranial focused ultrasound (tFUS) stimulation parameters. [Color figure can be viewed at www.neuromodulationjournal.org]

The intensity is often characterized by the spatial peak temporal average (I_SPTA; the average intensity across entirety of sonication) and the spatial peak pulse average (I_SPPA; the average intensity of a single pulse). The intensity of the ultrasound stimulus is equal to the ultrasound pressure squared, and with a higher intensity come larger effects on the tissue. However, evidence suggests that the effects of the tissues at different intensities may be governed by different mechanisms. For example, it is hypothesized that neuronal changes seen with low-intensity stimulation are due more to mechanical changes at the level of the cellular membrane, whereas changes at high intensity (used as an irreversible, ablative tool) are induced by a thermal mechanism.^4,9,19 Intensity may therefore be a primary factor for the nature of the effects of the ultrasound. In a study of electrocorticographic (ECoG) signals from the cat cortex, sonication with high-intensity ultrasound generally produced cortical inhibitory effects whereas lower intensities tended to cause excitatory changes to ECoG rhythms.²⁰ Although the ultrasound was delivered at much higher intensities than in human LIFU, this finding suggests the biological effect of ultrasonic mechanical forces on neural tissue may be intensity dependent. Alongside I_SPPA and I_SPTA, MI is another commonly used measure of acoustic output aimed at predicting potential biological effects of ultrasound exposure. MI, the maximum value of the peak negative pressure divided by the square root of the acoustic center frequency, is used to determine the likelihood of biomechanical changes (often cavitation) that can occur with sonication.²¹ All three of these variables can be used to identify potential physiologic or adverse effects of a particular LIFU protocol.

Other research suggests it is instead the pulsing protocol that dictates the stimulus outcomes. For example, studies using short pulses with a low DC (defined as the ratio of time the ultrasound is firing [PW] compared with the PRP) were more likely to cause inhibitory effects. In contrast, longer pulses with higher DC may cause more excitation.^4,22,23 Another parameter pertaining to pulsing protocol is the PRF. PRF is defined as the inverse of the PRP and measures the frequency at which a PRP will occur within a cycle. Some preliminary evidence suggests increasing PRF may positively enhance excitatory effects,²⁴ but it did not seem to have a significant effect on suppressive effects.²³

SD may affect the likelihood of generating a behavioral response. In fact, preclinical studies of motor output in both Caenorhabditis elegans and rodents indicated the behavioral effects of sonication increase with duration until peaking at approximately 100 milliseconds.^25,26 It remains unclear whether these findings correlate to humans, given much of the clinical research currently uses durations >100 milliseconds.^18,27,28 Further elucidation into the cellular and clinical impact of these variables is needed to ensure peak efficacy and safety in future clinical studies.

LIFU Target Engagement

As previously discussed, one of the advantages of LIFU is the ability to specifically target both superficial and deep brain structures by combining sonication with various imaging modalities. Several imaging and computational techniques have been used to ensure sonication has the proposed impact on neural tissue in vivo. A recent study by Yaakub et al used magnetic resonance-spectroscopy to assess levels of γ-aminobutyric acid (GABA) and glutamate in the areas of sonication as a replicable measure of target substrate engagement; the authors were able to show durable changes in GABA levels in the target, the posterior cingulate cortex, for up to 50 minutes after stimulation.²⁹ Other studies have successfully used blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI), which reflects the change in local deoxyhemoglobin levels driven by changes in brain blood flow and blood oxygenation, an indirect measure of neuronal activity, to assess the impact of LIFU in regions of sonication.³⁰ Furthermore, researchers can identify differences in target activity across stimulation parameters.³¹ Arterial spin labeling (ASL), a magnetic resonance imaging (MRI) technique using a diffusible tracer, also has been used. ASL, which measures tissue perfusion alone, is a beneficial tool because it reduces variability seen in BOLD and is superior in assessing changes in perfusion across longer periods. A study by Cain et al³⁰ measured the inhibitory effects of LIFU using both BOLD and ASL and was able to detect a decrease activity in the target region using both modalities. However, ASL detects inhibition up to minutes after sonication, as opposed to only seconds in BOLD. Other methods such as electrophysiological recordings through electroencephalography (EEG) scalp leads also have been helpful in showing target engagement, although its use is limited to more superficial regions of the brain.³² Significant research also is under way estimating target engagement through indirect methods such as three-dimensional modeling of acoustic wave propagation. The technical details of these methods are outside the scope of this review but are described elsewhere.²⁹ The technology behind target engagement confirmation of LIFU is rapidly growing and will continue to allow researchers to verify even subcortical LIFU target engagement at ever more precise coordinates, and to refine LIFU stimulation parameters for therapeutic efficacy.

LIFU Proposed Mechanism of Action

Despite a growing body of literature and myriad hypotheses, the exact mechanism of action of LIFU for neuromodulation remains not known. There are currently three primary proposed mechanisms at the cellular level: thermal, mechanical, and cavitation. The low-intensity protocols used for reversible neuromodulation have indicated modulation of neural tissue with temperature increases of as little as <0.01 °C, lower than natural fluctuations seen in the brain.^19,33,34 Additional preclinical evidence using C elegans devoid of thermosensation showed they still responded to focused ultrasound (FUS), whereas those unable to sense mechanical changes did not.^4,25 There also is evidence of the presence of thermo-sensitive ion channels, specifically potassium channels such as TREK 1 and 2, throughout in neural tissue that may be responsible for resting potential changes after sonication,³⁵ which requires further study. However, current evidence suggests the thermal effects may be negligible in LIFU.

An alternative proposed mechanism is based on acoustic cavitation, which occurs when local pressure falls below the vaporization point of the lipophilic component of the cell membrane. This causes bubbles to form and vibrate according to pressure variations and ultimately can change membrane capacitance or permeability.^16,22,36,37 Treatments that use low frequency and high pressure, or the presence of injected gaseous nuclei, have used this mechanism for the purposes of histotripsy or lithotripsy, in addition to opening the blood-brain barrier.^38–41 However, at intensities used in current clinical trials (>100 mW/cm²), the presence of nanobubbles has not been confirmed, making this mechanism unlikely in LIFU.

More recent research has hypothesized the primary mechanism causing neural excitability changes in LIFU is due to the acoustic radiation force (ARF) on mechanosensitive ion channels in the brain.^42–44 The ARF is the transfer of momentum from the sound wave field to the cell membrane and unlike the pulsatile sound waves, exerts a constant pressure on the membrane during sonication. This constant pressure may alter the cellular membrane to allow conformation change of ion channels.³⁵ Another possibility supposes the pressure may allow the dispersion of lipid rafts that allow increased production of mechanosensory ligands on the cell membrane by permitting substrate-enzyme interaction.⁴⁵ Certain mechanosensitive channels that are responsive to ultrasound have been identified in cultured brain cells; however, those necessary for neuromodulation continue to be debated.^46–49

In terms of the impact of LIFU on neural function, its effects can be discussed as either “online” or “offline.” Although online effects are observed during or immediately aftersonication, offline effects can be assessed minutes to even days after the last session. The durable offline impact of sonication may reflect lasting neuroplastic changes beyond the online effects of the ultrasound waves themselves. As discussed above, LIFU can induce conformation changes in mechanosensitive ion channels, including calcium and sodium channels. This allows the depolarization and subsequent influx of calcium into N-methyl-D-aspartic (NMDA) receptors, critical for long-term potentiation (LTP) and depression, a critical cellular step in learning and memory. In a model of aging using senescent mice, Blackmore et al showed that ultrasound sonication of the hippocampus restored LTP through NMDA receptors and reversed spatial learning deficits.⁵⁰ Although this study used ultrasound combined with microbubbles, another study in rats without the use of microbubbles, and an FF similar to that used in human studies, found an increase in the excitatory postsynaptic potentials in the hippocampus over 30 minutes after sonication.⁵¹

There remain several outstanding questions concerning the impact of LIFU on neural tissue, one being the difference in the mechanism of sonication between homogenous white matter and more cellularly heterogenous gray matter areas. Most of the in vivo neuromodulation refers to the sonication of gray matter; however, there are several in vitro studies that have explored the impact of LIFU on white matter axonal transmission. Seminal work performed by Tagaki et al showed LIFU of the sciatic nerve of a frog caused an increase in action potentials at lower intensities and a deconstructive property on action potentials at higher intensities.⁵² Studies also have shown sonication of giant axonal fibers of earthworms at higher intensities (600 mW/cm I_SPTA) temporarily decrease action potential magnitude, but lower intensity (200 mW/cm I_SPTA) did not affect the magnitude of action potentials.⁵³ Recent research using murine sciatic nerve fibers worked to explain these findings and found that action potentials were not directly evoked by LIFU but instead increased nerve conduction velocities in a reversible fashion that is independent of any local temperature change.⁵⁴ Exploration of the neurophysiological mechanism of changes in neuronal signaling has discovered that mammalian nerve fibers possess, among others, mechanosensitive K+ channels, such as TWIK-related arachidonic acid-activated K+ channel, that are found in high concentrations in the nodes of Ranvier, which, when mechanically stimulated, contribute to potassium leakage in the nerve fiber. This hyperpolarization allows an increase in sodium channel availability for action potential propagation.⁵⁵ Additional work related to temperature-sensitive K+ channels showed no impact on their activity after LIFU in the nodes of Ranvier, again suggesting sonication-induced neural activity change depends on mechanical stretch and not local temperature change.⁵⁶ These mechanosensors, not voltage-gated or temperature sensitive K+ channels, appear to play a primary role in action potential repolarization in the nodes of Ranvier.⁵⁷

Additional evidence has emerged related to the impact ultrasound has on nonneuronal cells in the brain. For example, Newman et al⁵⁸ examined the sensitivity of murine astrocytes, pericytes, and endothelial cells to LIFU using an in vitro model. They concluded that all nonneuronal cell lines examined were sensitive to LIFU stimulation through mechanosensitive ion channels.⁵⁸ Taking these results one step further, Oh et al⁵⁹ observed that ultrasound can modulate mechanosensitive channels on astrocyte membranes to cause an influx of calcium and a subsequent release of glutamate that act on NMDA receptors on adjacent hippocampal neurons. Overall, it remains unclear whether the primary mechanism of action varies in white compared with gray matter. It is possible that sonication in white matter tracts more specifically affects mechanosensitive K+ channels, whereas gray matter, on the basis of its cellular homogeneity, is affected through multiple pathways, including lipid rafts⁴⁵ and mechanosensitive calcium channels on neurons⁴⁶ and on nonneuronal cells.^58,59

Another outstanding question in the field is related to the impact of sonication parameters on the directionality of the neural response. As discussed above, studies have shown both excitatory and inhibitory effects of LIFU, and there is currently much debate as to the underlying mechanism of the polarity of sonication. It has been proposed in the modified neuronal intramembrane cavitation excitation (NICE) model that the DC may predict the polarity of the response independent of other parameters,²² given numerous laboratories have concluded that higher DC produces excitatory effects and is an independent predictor of the polarity of sonication response.⁶⁰ This hypothesis has been under contention because equivalent numbers of studies indicate low DC, believed to be within the “suppressive” range, did not produce suppressive effects. Furthermore, there also were experiments showing suppressive effects at higher DC (reviewed in greater detail in¹⁶). Taken together, these empirical findings do not corroborate the modified NICE model’s predictions for effect polarity.

An alternative electrophysiological-mechanical coupling theory in the neuronal membrane considers both mechanosensitive ion channels and the cell types being stimulated as the driver for sonication polarity.⁶¹ For example, Yaakub et al²⁹ observed a significant change in GABA concentration after theta burst FUS in the posterior cingulate cortex but not the anterior cingulate cortex. The authors intimated that sonication differences may be explained by tissue composition differences in ion channels between the regions.

Questions remain whether LIFU can induce sustained ion channel and ultimately dynamically change neuronal activity, and this requires continued scrutiny. Regardless, given LIFU has revealed a strong safety profile, research is rapidly moving beyond basic mechanistic studies to clinical trials to assess its impact on symptom improvement in numerous neural disorders. In the next sections, we discuss the current literature evaluating the effect of low-intensity ultrasound neuromodulation in multiple neuropsychiatric conditions.

Neurologic and Psychiatric Applications

Motor Function and Rehabilitation

The most targeted region to date in human research has been the primary motor cortex (M1) (Table 1). Targeted sonication of the M1 was shown to reduce reaction times in a simple stimulus-response task in healthy participants.⁶² Furthermore, LIFU significantly potentiated movement-related M1 cortical potential in participants practicing a voluntary foot tapping task compared with the sham ultrasound condition.²⁴ Another study used a visuomotor task, in which participants were asked to quickly and accurately move a cursor to a target, to evaluate changes in voluntary motor behavior. Participants in the active stimulation group had significantly shorter reaction time to the presentation stimulus than did the sham group; however, accuracy between groups did not differ.²³ A separate experiment also recorded a decrease in movement time in a similar visuomotor task in healthy participants who received theta burst ultrasound of the M1 vs sham controls.⁶³ Finally, a 15-minute LIFU protocol focused on the left M1 in 24 healthy participants significantly potentiated motor evoked potentials during sonication. In addition, participants had a significant decrease in stop-signal task time (ie, improved motor inhibitory control) compared with presonication, a change that was not seen in the sham control group.⁶⁴

Table 1.

LIFU Trials in Motor Function.

Author, year	Sonication target	LIFU parameters	Outcome measures	Findings	Adverse events
Fomenko et al,23 2020	M1	Basic parameters: FF: 0.001 MHz PRF: 0.001 MHz PW: 0.002 ms DC: 30% SD: 0.5 s ISI: 5 s ISPTA: 0.69 W/cm2 ISPPA: 2.32 W/cm2 MI: 0.19 Total duration: 10–45 sonications Sham controlled: yes Varied parameters: FF: 0.5 MHz PRF: 0.0002 MHz, 0.0005 MHz, 0.001 MHz PW: 0.002 ms DC: 10%, 30%, 50% SD: 0.1 s, 0.2 s, 0.3 s, 0.4 s, 0.5 s ISI: 5 s ISPTA: 0.23 W/cm2, 0.69 W/cm2, 1.16 W/cm2 ISPPA: 2.32 W/cm2 MI: 0.19 Total duration: 60–90 sonications Sham controlled: yes	Effects of varied acoustic parameters for cortical motor suppression Visuomotor behavioral task	Longer SD more efficacious None for suppression Lower DC more efficacious for suppression No effect of PRF on suppression Increased short-interval intracortical inhibition Decreased reaction time	None
Yu et al,24 2021	Primary leg motor cortical region	FF: 0.5 MHz PRF: 0.0003 MHz, 0.003 MHz PW: 0.2 ms DC: 6%, 60% SD: 0.5 s ISI: 2.5–4.5 s ISPTA: 0.703 W/cm2 ISPPA: 1.17 W/cm2 MI: not reported Total duration: ~120 sonications Sham controlled: yes	Movement-related cortical excitability	Excitatory modulation Enhanced voluntary movement-related cortical activity PRF positively amplifies modulatory effects	None
Legon et al,62 2018	M1	FF: 0.5 MHz PRF: 0.001 MHz PW: 0.36 ms DC: 36% SD: 0.5 s ISI: 10 s, 3–6 s ISPTA: 6.16 W/cm2* ISPPA: 17.12 W/cm2* MI: 0.9 Total duration: 10, 100 sonications Sham controlled: yes	Cortical excitability Response reaction time	Amplitude inhibition of single-pulse MEPs Significant attenuation of intracortical facilitation No effect on intracortical inhibition Significant reduction in reaction time	None
Zeng et al,63 2022	Left motor cortex/left occipital cortex	tb-LIFU FF: 0.5 MHz PRF: 0.000005 MHz PW: 20 ms DC: 10% SD: 80 s ISI: N/A ISPTA: 0.23 W/cm2* ISPPA: 2.26 W/cm2* MI: not reported Total duration: 80 s r-LIFU FF: 0.5 MHz PRF: 0.001 MHz PW: 0.32 ms DC: 32% SD: 0.5 s ISI: 1.1 s ISPTA: 0.72 W/cm2* ISPPA: 2.26 W/cm2* MI: not reported Total duration: 80 s Sham controlled: yes	Effects of varied LIFU delivery methods on motor cortical activity Motor cortical activity with LIFU of the orbital cortex Visuomotor behavioral task	tb-LIFU produced consistent increase in motor cortical excitability for 30 min r-LIFU produced no significant change tb-LIFU decreased short-interval intracortical inhibition tb-LIFU increased intracortical facilitation LIFU to the occipital cortex did not change motor cortical excitability tb-LIFU shortened movement time in a visuomotor task	None
Zhang et al,64 2021	M1	FF: 0.5 MHz PRF: 0.0001 MHz PW: 0.5 ms DC: 5% SD: 0.5 s ISI: 8 s ISPTA: 0.142 W/cm2 ISPPA: 2.846 W/cm2 MI: 0.420 Total duration: 15 min Sham controlled: yes	Motor cortex activity Stop-signal task Temperature change	Altered long-lasting cortex plasticity for ≥30 min Did not cause changes past 7 d Improved behavioral inhibition Minimal temperature change that is unlikely to induce brain injury	None

ISI, interstimulus interval; MEPs, motor evoked potentials; r-LIFU, regularly patterned LIFU; tb-LIFU, theta burst patterned LIFU.

Total duration is reported in total time of LIFU intervention or the number of sonications delivered in one session.

^*Intensity reported is not the intracranial estimation. The reported values represent either the intensity measured in freeform water or are a parameter output of the LIFU system. Intracranial estimations are derived from these values.

Somatosensation and Chronic Pain

The continued need for nonopioid alternatives for treating chronic pain has sparked a growing interest in LIFU for the purposes of somatosensory modulation. There has been a substantial amount of LIFU research targeting the somatosensory cortex to evaluate its impact on sensory and discrimination changes⁶⁵ (Table 2). For example, LIFU targeted at the somatosensory cortex (S1) showed significant modulation of S1 somatosensory evoked potentials, and behavioral analysis revealed that LIFU targeted to S1 enhanced performance on both two-point discrimination using pins and frequency discrimination using air puffs, compared with sham controls.²⁸ In another study, participants who received LIFU had an increased percentage of correct responses in a vibration frequency discrimination test compared with shams.⁶⁶ Moreover, sonication of the S1 generated sonication-specific evoked potentials (SEP) on EEG, while also eliciting transient tactile sensations on the corresponding contralateral hand and fingers without direct tactile stimulation, with a high level of anatomical specificity.⁶⁷ Similarly, a separate study stimulated either S1, S2, or both using multiple ultrasound transducers and was able to elicit various tactile sensations in the contralateral hand in the absence of any external sensory stimuli.⁶⁸

Table 2.

LIFU Trials in Sensory Perception.

Author, year	Sonication target	LIFU parameters	Outcome measures	Findings	Adverse events
Badran et al,27 2022	Right anterior thalamus	FF: 0.65 MHz PRF: 0.00001 MHz PW: 5 ms DC: 5% SD: 30 s ISI: 30 s ISPTA: 0.719 W/cm2* ISPPA: not reported MI: not reported Total duration: 20 min Sham controlled: yes	QST	Attenuation of the sensory component to pain perception Significant increase in thermal pain thresholds Blunted sensitization to thermal stimuli	None
Legon et al,28 2014	SI	FF: 0.5 MHz PRF: 0.001 MHz PW: 0.36 ms DC: 36% SD: 0.5 s ISI: 6 s ISPTA: not reported ISPPA: 5.895 W/cm2 MI: 1.13 Total duration: 120 sonications Sham controlled: yes	Sensory-evoked brain activity Sensory discrimination task	Significant attenuation of the SEP amplitudes Significant modulation of spectral content sensory-evoked brain oscillations Above changes not present when LIFU shifted 1 cm anterior or 1 cm posterior Enhanced performance on sensory discrimination tasks	None
Liu et al,66 2021	SI	FF: 0.5 MHz PRF: 0.0003 MHz PW: 0.2 ms DC: 6% SD: 0.5 s ISI: 2 s ISPTA: 0.06713 W/cm2 ISPPA: 1.10 W/cm2 MI: not reported Total duration: 56 sonications Sham controlled: yes	Somatosensory cortical activity Sensory discrimination task	Excitatory neuromodulatory effects at S1 Higher source profile amplitude at S1 Enhanced performance on sensory discrimination task	None
Lee et al,67 2015	SI	FF: 0.25 MHz PRF: 0.0005 MHz PW: 1 ms DC: 50% SD: 0.3 s ISI: 2.7 s ISPTA: 0.35 W/cm2 ISPPA: maximum 2.5 W/cm2 MI: 0.62 Total duration: 200 sonications Sham controlled: yes	LIFU evoked sensations Somatosensory cortical activity Safety	Elicited explicit tactile sensations Sensations occurred in the hand area contralateral to the sonicated hemisphere Stimulatory effects were transient and reversible Cortical evoked potentials akin to classical SEP generated by median nerve stimulation No acute neurologic or neuroradiologic abnormalities after stimulation	None
Lee et al,68 2016	SI SII	FF: 0.21 MHz PRF: 0.0005 MHz PW: 1 ms DC: 50% SD: 0.5 s ISI: 6.5 s ISPTA: 3.5–4.4 W/cm2 ISPPA: 7.0–8.8 W/cm2 MI: not reported Total duration: 20 sonications Sham controlled: yes	LIFU evoked sensations across different target regions	Simultaneous stimulation of the SI/SII in the same hemisphere and elicited various tactile sensations Stimulation of the SII area induced perception of tactile sensations Multiple FUS transducers can be used for simultaneous stimulation of different targets	None
Legon et al,69 2018	Ventro-posterior lateral nucleus of the thalamus	FF: 0.5 MHz PRF: 0.001 MHz PW: 0.36 ms DC: 36% SD: 0.5s ISI: 4 s ISPTA: not reported ISPPA: 7.03 W/cm2 MI: 0.56 Total duration: 300 sonications Sham controlled: yes	Activity of unilateral sensory thalamus and cortical somatosensory SI area Acoustic modeling Sensory discrimination task	Inhibited the SEP amplitude at unilateral sensory thalamu No effect of LIFU on somatosensory cortex SI area Frequency inhibition in mostly beta and gamma power Skull produces some LIFU beam distortion Significantly worse than chance on a discrimination task	None
Hameroff et al,70 2013	Fronto-temporal cortex	Unfocused transcranial ultrasound FF: 8 MHz PRF: N/A PW: N/A DC: N/A SD: 15 s ISI: N/A ISPTA: 0.152 W/cm2 ISPPA: N/A MI: 0.7 Total duration: 15 s Sham controlled: yes	VAMS NRS Vital signs	Significant improvement in global affect at 10 min Significant improvement in global affect at 40 min Slight improvement in pain ratings (NRS) 40 min No changes in any vital signs 40 min after exposure	None
Legon et al,71 2023	AI PI	FF: 0.5 MHz PRF: 0.001 MHz PW: 0.36 ms DC: 36% SD: 1 s ISI: 12–20 s ISPTA: not reported ISPPA: 3.5 W/cm2* MI: not reported Total duration: 40 sonications Sham controlled: yes	CHEP Autonomic measures Pain ratings	LIFU of PI affected earlier EEG amplitudes ~300 ms LIFU of AI affected EEG amplitudes ~500 ms LIFU to both AI and PI reduced the amplitude at 824–903 ms LIFU to both AI and PI reduced perceived pain ratings LIFU of the AI affected HRV	None

AI, anterior insula; CHEP, contact head evoked potentials; HRV, heart rate variability; ISI, interstimulus interval (s); NRS, numerical rating scale; PI, posterior insula; QST, quantitative sensory thresholding; SI, primary hand somatosensory cortex; SII, secondary hand somatosensory cortex; SEP, sensory evoked potentials; VAMS, Visual Analog Mood Scale.

Total duration is reported in total time of LIFU intervention or the number of sonications delivered in one session.

^*Intensity reported is not the intracranial estimation. The reported values represent either the intensity measured in freeform water or a parameter output of the LIFU system. Both values can be used to estimate intracranial intensity.

The thalamus also is a brain region heavily studied for its role in sensory perception and pain. In one such experiment elucidating the role of unilateral LIFU sonication of the unilateral sensory thalamus, researchers showed alteration in the amplitude of the SEP as compared with sham, in addition to an alteration in participants’ performance on a discrimination task during LIFU stimulation.⁶⁹ In a double-blind sham-controlled pilot study in 19 healthy subjects, volunteers received MRI-guided LIFU (or sham) directed at the anterior nucleus of the thalamus,²⁷ a region known to regulate pain perception.⁷² Two 10-minute sessions of sonication were sufficient to produce antinociceptive effects, as measured by an attenuated sensitivity to thermal pain stimulus before versus after sonication.²⁷ To date, only one study has explored the effects of ultrasound in patients with chronic pain. In a double-blind crossover pilot study, subjects with chronic pain received diagnostic ultrasound (dUS) to the posterior frontal cortex contralateral to their identified maximal pain site, and subjective mood and numerical pain ratings were assessed 10 and 40 minutes after sonication.⁷⁰ Mood significantly improved in active versus sham at 10 and 40 minutes, and although the pain rating score decreased in the active group, it was not significantly different from that in the sham group (p = 0.07).⁷⁰

In a unique experiment, Legon et al assessed the impact of neuromodulation of the insular cortex on perceived pain perception.⁷¹ They also evaluated the impact of LIFU on the anterior insula (AI) compared with the posterior insula (PI), two anatomically distinct subregions. Although the study is in preprint, the authors found a significant decrease in perceived pain to a noxious stimulus with neuromodulation of both the AI and PI. However, only sonication of the AI caused an alteration in heart rate variability, which, the authors argue, may be critical in both pain and other conditions with concomitant autonomic dysregulation.⁷¹

Mood and Anxiety Disorders

Another area of interest for using LIFU is the treatment of mood and anxiety disorders (Table 3). Some initial research has focused on the impact ultrasound has on emotional and behavioral consequences in healthy participants. For example, a large double-blind clinical trial in >150 volunteers assessed the impact of sonication of the right prefrontal cortex (PFC) on a virtual T-maze task, a behavioral output established to measure motivational conflict by assessing approach versus withdrawal behavior. After sonication of the PFC, a region implicated in mood and emotional regulation, participants showed significantly potentiated approach and decreased withdrawal behaviors.⁷³

Table 3.

LIFU Trials in Psychiatric Disorders.

Author/y	Sonication target	LIFU parameters	Outcome measures	Findings	Adverse events
Ziebell et al,73 2023	Right prefrontal cortex	FF: 0.5 MHz PRF: 0.00004 MHz PW: 0.125 ms DC: 0.5% SD: 120 s ISI: N/A ISPTA: 0.199 W/cm2* ISPPA: 40 W/cm2* MI: 1.54* Total duration: 120 s Sham controlled: yes	Approach versus with drawal behavior with MFT during T-maze task VAMS	Greater approach and fewer withdrawal behaviors MFT decrease was present across all T-maze event types LIFU-induced MFT differences consistently explained greater approach and less withdrawal No significant subjective mood changes	None
Sanguinetti et al,74 2020	rIFG	Experiment 1 FF: 0.5 MHz PRF: 0.00004 MHz PW: 0.065 ms DC: 0.26% SD: 30 s ISI: N/A ISPTA: 0.130 W/cm2* ISPPA: 54 W/cm2* MI: 1.79* Total duration: 30 s Sham controlled: yes Experiment 2 FF: 0.5 MHz PRF: 0.00004 MHz PW: 0.125 ms DC: 0.50% SD: 120 s ISI: N/A ISPTA: 0.272 W/cm2* ISPPA: 54 W/cm2* MI: 1.79* Total duration: 120 s Sham controlled: no	Experiment 1 VAMS Experiment 2 FC with rsfMRI	Significant increase in mood 20 and 30 min after LIFU No significant difference in global vigor Increase in connectivity between the rIFG and rMFG Decrease in connectivity between rIFG with left prefrontal and limbic areas Regions within the DMN showed a general decrease in FC	None
Reznik et al,75 2020	Right prefrontal cortex	FF: 0.5 MHz PRF: 0.00004 MHz PW: 0.065 ms DC: 0.26% SD: 30 s ISI: N/A ISPTA: 0.071 W/cm2* ISPPA: 14 W/cm2* MI: 0.92* Total duration: 30 s Sham controlled: yes	Depressive symptoms measured with neuropsychologic battery VAMS	No change in depression severity Decrease in trait rumination No change in anxiety severity Increased happiness over the days of the study	None
Oh et al,76 2023	dlPFC	FF: 0.25 MHz PRF: 0.0005 MHz PW: 1 ms DC: 50% SD: 0.3 s ISI: 6 s ISPTA: 0.3 W/cm2 ISPPA: 0.6 W/cm2 MI: 0.27 Total duration: 20 min Sham controlled: Yes	Depressive symptoms measured with MADRS Additional neuro psychologic battery FC evaluated with rsfMRI	Significant treatment effect. Reduction of suicidal ideation and depressive symptoms. More pronounced reduction of MADRS scores More pronounced reduction of anxiety symptoms Significant network level FC changes in the right subgenual anterior cingulate cortex.	None
Mahdavi et al,77 2023		FF: 0.65 MHz PRF: 0.00001 MHz PW: 5 ms DC: 5% SD: 30 s ISI: 30 s ISPTA: 0.719.73 W/cm2* ISPPA: 14.39 W/cm2* MI: 0.75* Total duration: 10 min Sham controlled: no	Anxiety severity measured with HAM-A and BAI Participant perceived change in clinical status	60% showed a meaningful reduction (>30%) of HAM-A score at completion. 32% achieved remission of GAD symptoms according to a completion HAM-A score <14 64% reported a PGI-I score of >2, indicating significant perceived benefit	None
Peng et al,78 2024	Left NAc	FF: 0.65 MHz PRF: 0.00001 MHz PW: 5 ms DC: 5% SD: 30 s ISI: 30 s ISPTA: 0.719.73 W/cm2* ISPPA: not reported MI: not reported Total duration: 20 min Sham controlled: yes	fMRI FC Safety, tolerability, and feasibility	Inhibition in bilateral NAc Increased FC between NAc and medial PFC LIFU was safe and well tolerated	None
Mahoney et al,79 2023	Bilateral NAc	Enhanced dose FF: 0.22 MHz PRF: 0.00000033 MHz PW: 100 ms DC: 3.3% SD: 5 s ISI: 10 s ISPTA: not reported ISPPA: not reported MI: not reported Total duration: 20 min Lower dose FF: 0.22 MHz PRF: 0.00000033 MHz PW: 100 ms DC: 3.3% SD: 5 s ISI: 10 s ISPTA: not reported ISPPA: not reported MI: not reported Total duration: 20 min Sham controlled: yes	Safety, tolerability, and feasibility Cue-induced substance craving, assessed through VAS	Both LIFU doses were safe and well tolerated Brain MRIs did not indicate edema, hemorrhage, or notable changes in brain structure Attenuated craving primary substance of abuse in participants receiving enhanced dose Reductions in cue-induced craving for several substances persisted 90d after treatment.	None

BAI, Beck Anxiety Inventory; dlPFC, left dorsolateral prefrontal cortex; DMN, default mode network; FC, functional connectivity; HAM-A, Hamilton Anxiety Inventory; ISI, interstimulus interval; rMFG, right middle frontal gyrus; MADRS, Montgomery-Asberg Depression Rating Scale; PGI-I, Patient Global Impression–Improvement; rsfMRI, resting state functional magnetic resonance imaging; VAMS, Visual Analog Mood Scale; VAS, visual analog scale.

Total duration is reported in total time of LIFU intervention or the number of sonications delivered in one session.

^*Intensity reported is not the intracranial estimation. The reported values either represent the intensity measured in freeform water or are a parameter output of the LIFU system. Intracranial estimations are derived from these values.

One study exploring the impact of LIFU on mood in healthy volunteers targeted the right inferior frontal gyrus (rIFG), a region in the ventrolateral PFC and shown to be a critical node in the regulation of negative moods and emotions,⁸⁰ had a potentiation in self-reported mood.⁷⁴ After sonication, the volunteers also had a statistically significant increase in the functional connectivity between the rIFG and the right middle frontal gyrus, a region that includes the dorsolateral PFC, that also is associated with emotional regulation.^74,81 The authors speculate the connectivity change may have contributed to the participants’ enhanced ability to regulate emotional experience and mood during the experiment.⁷⁴

A subsequent experiment in 24 participants with mild-to-moderate depressive symptoms (as measured by the Beck Depression Inventory) with no concomitant antidepressant medication use received five days of LIFU targeted at the right frontotemporal cortex. Sonication did not have any effect on participants’ self-reported mood scores. There was, however, a significant reduction in trait worry compared with placebo.⁷⁵ In a recent randomized double-blind sham-controlled trial in patients with MDD, LIFU of the left dorsolateral PFC for six sessions over six weeks showed improved Montgomery–Åsberg Depression Rating Scale compared with placebo, both during the two weeks of treatment and at two weeks after treatment.⁷⁶ In addition, patients who received LIFU showed increased resting-state functional connectivity (rsFC) between the subgenual anterior cingulate and the medial PFC, middle frontal gyrus, and orbitofrontal cortex, although no significant correlation was found between change in rsFC and symptom improvement.⁷⁶

Looking at a different patient population, 25 participants with treatment-refractory Generalized Anxiety Disorder (GAD) received weekly 10-minute LIFU targeting the right amygdala for eight weeks.⁷⁷ Researchers found that, compared with baseline scores of anxiety (as measured by the Hamilton Anxiety Inventory and Beck Anxiety Inventory), there was a significant decrease in scores after eight weeks of treatment. Moreover, 16 of 25 patients experienced clinically significant benefits as measured by the Patient Global Impression–Improvement Scale.⁷⁷

Substance Use Disorders

Fewer studies have focused on the impact of LIFU on SUD; however, there are some initial promising results. Owing to the importance of the nucleus accumbens (NAc) as a primary node in reward learning and the growth of goal-directed behaviors in the development of substance abuse, our laboratory performed a single-blind, sham-controlled trial in healthy adults in which changes in the NAc activity, as measured by fMRI, were assessed. Our results indicated that bilateral NAc activity was attenuated with LIFU targeted exclusively at the left NAc, compared with sham controls.⁷⁸ Moreover, functional connectivity between the NAc and medial PFC was increased. These results suggested LIFU can directly modify reward circuitry in healthy subjects.⁷⁸ Expanding on this work, another study used four participants with a history of polysubstance abuse who were receiving intensive outpatient treatment for opioid use disorder and were enrolled to evaluate the effect of LIFU targeted at the bilateral NAc, a region considered to act as the interface of motivation and action and shown to be dysregulated in addictive behaviors.⁸² Participants received 10 minutes of sham followed by 10 minutes of active sonication per hemisphere (20 minutes total), and cue-induced substance craving was assessed immediately after and 90 days after sonication.⁷⁹ Two of the four participants did not see any significant change in their craving scores at a low stimulation dose (60 W). However, in the other participants who received an increased dose (90 W), initial craving scores were attenuated after sonication. Furthermore, for all patients, long-term craving was decreased up to 90 days after sonication for certain drugs of abuse⁷⁹ (Table 3).

Neurodegenerative Dementia

More attention is being paid to LIFU as it relates to dementias (Table 4) based on promising preclinical evidence of reduced τ protein levels and improved cognitive performance in a rodent model of Alzheimer’s disease (AD).⁸⁹ An increasing number of clinical trials have been performed with these outcomes in mind to analyze clinical outcomes in this patient population. In one such pilot study, the metabolic rate change and cognition were assessed after LIFU of the hippocampus in four patients with AD. The study showed a significant improvement in metabolic rate in regions of the hippocampus after sonication but only a minor change in any cognitive test results.⁸³

Table 4.

LIFU Trials in Neurologic Disorders.

Author, year	Sonication target	LIFU parameters	Outcome measures	Findings	Adverse events
Jeong et al,83 2021	Right hippocampus	FF: 0.25 MHz PRF: 0.000002 MHz PW: 20 ms DC: 4% SD: 180 s ISI: N/A ISPTA: 0.02–0.12 W/cm2 ISPPA: 0.5–3 W/cm2 MI: 0.27 Total duration: 180 s Sham controlled: No	Cerebral glucose metabolism measured with F-fluoro-2-deoxyglucose positron emission tomography. BBB monitoring using MRI Neuropsychologic assessments	Significant increase in resting cerebral glucose metabolism in superior frontal gyrus. Significant increase in resting cerebral glucose metabolism in middle cingulate gyrus. Significant increase in resting cerebral glucose metabolism in fusiform gyrus. No evidence of BBB opening Mild improvement in memory, executive function, and global cognitive function.	None
Beisteiner et al,84 2020	AD population: Bilateral frontal cortex extending to Broca’s area Bilateral lateral parietal cortex extending to Wernicke’s area Precuneus cortex Healthy population: primary somatosensory	tpFUS AD population: FF: 0.000004 MHz maximum PRF: 0.000005 MHz PW: 0.003 ms DC: N/A SD: nonstandardized ISI: 0.2–0.3 s ISPTA: 0.1 W/cm2 maximum ISPPA: not reported MI: not reported Total duration: 6000 pulsed sonications Sham controlled: no Healthy population: FF: 0.000004 MHz maximum PRF: 0.000001 MHz–0.000005 MHz PW: 0.003 ms DC: N/A SD: 2.5 s, 25 s, 250 s ISI: 6.15 min ISPTA: not reported ISPPA: not reported MI: not reported Total duration: 1110 pulsed sonications Sham controlled: yes	AD population Neuropsychologic assessments fMRI Healthy population SEP at 3 different SD parameters	AD population Significant improvements in the language domains Significant improvement in memory performance Effects lasted 3 mo after treatment Enhanced activation and connectivity of the memory network Healthy population tpFUS shows dose-dependent effects predicated by total energy over time	None
Popescu et al,85 2021	AD population data obtained post hoc from Beisteiner et al84	AD population data obtained post hoc from Beisteiner et al84	Cortical thickness	Reduced cortical atrophy within the DMN Significant correlation between changes in the neuropsychologic improvement and cortical thickness increase in AD-critical brain areas Cortical thickness effects were localized to AD-relevant regions	None
Nicodemus et al,86 2019	Hippocampus Substantia nigra	Transcranial ultrasound FF: 2 MHz PRF: N/A PW: N/A DC: N/A SD: 1 h ISI: N/A ISPTA: 0.52 W/cm2 ISPPA: N/A MI: not reported Total duration: 1 h	Neuropsychologic battery	62.5% improved cognitive scores 87% stable or improved fine motor scores 87.5% had stable or improved gross motor scores	None
Monti et al,87 2016	Thalamus	FF: 0.65 MHz PRF: 0.0001 MHz PW: 0.5 ms DC: 5% SD: 30 s ISI: 30 s ISPTA: 0.72 W/cm2 ISPPA: not reported MI: not reported Total duration: 10 min	Minimally conscious state status measured with CRS-R	Patient presented a CRS-R of 13 on day of stimulation and 17 1 d after stimulation 3 d after stimulation, patient exhibited behaviors consistent with emergence from minimally conscious state 5 d after stimulation, patient attempted to walk	None
Cain et al,88 2021	Thalamus	FF: 0.65 MHz PRF: 0.0001 MHz PW: 0.5 ms DC: 5% SD: 30 s ISI: 30 s ISPTA: 0.72 W/cm2 ISSPA: 14.39 W/cm2 MI: not reported Total duration: 10 min	Minimally conscious state status measured with CRS-R	Clinically significant increases in behavioral responsiveness after exposure to each dose in 2/3 patients Novel behaviors observed before the procedure despite years after onset in 2/3 patients 1/3 patients exhibited no perceived benefit	None

BBB, blood-brain-barrier; CRS-R, Coma Recovery Scale-Revised; DMN, default mode network; ISI, interstimulus interval; tpFUS, transcranial pulse focused ultrasound.

Total duration is reported in total time of LIFU intervention or the number of sonications delivered in one session.

One experiment in 35 patients with AD used a novel LIFU technique known as transcranial pulse stimulation (TPS), which uses single, short ultrasound pulses. Two weeks of TPS treatment directed at the frontoparietal lobe in this population improved language and memory scores that lasted for the three months of follow-up. This change in cognitive functioning was associated with a significantly increased level of functional connectivity between the frontal cortex and hippocampal structures.⁸⁴ In a follow-up to that study, researchers then performed a pre- vs post-TPS analysis of cortical thickness and found a positive correlation between the neuropsychologic improvement after TPS and cortical thickness change in the active group compared with sham.⁸⁵

Finally, in a trial assessing the effects of dUS on neurodegenerative dementia, individuals with a previous diagnosis of either AD or Parkinson’s disease (PD) with a Clinical Dementia rating of 0.5 (mild cognitive impairment) to 2 (moderate dementia) were recruited and received MRI-guided sonication of the hippocampus (AD) or the substantia nigra (PD) once a week for eight weeks. The results of this experiment were unclear; in most of the 22 patients, cognitive functioning was stable throughout the study, with 14 of 22 having ≥one improved cognitive score, whereas seven of 22 had ≥one cognitive score decrease.⁸⁶

Disorders of Consciousness

A unique clinical population in whom behavioral studies have been performed during and after LIFU are those experiencing disorders of consciousness (DOC) after severe brain injuries and coma (Table 4). Many patients do not fully recover after comas or have DOC that may be indefinite.^90,91 In a set of preliminary studies in a total of three patients with chronic DOC, the patients underwent two MRI-guided thalamic LIFU sessions one week apart, and behavioral changes were assessed using the JFK Coma Recovery Scale–Revised (CRS-R). Overall, the researchers saw improvements in the percentage change of CRS-R before versus after sonication initially in seeing patients, with one seeing a decrease in their score. Furthermore, there was no maintained improvement in functional behavioral output in the three- and six-month follow-ups.^87,88

CONCLUSIONS

LIFU is an exciting neuromodulatory tool that is unique in its noninvasive nature, combined with high spatial precision and depth of substrate penetration. There is ever-expanding evidence that points to LIFU becoming a clinically relevant tool for treating myriad neuropsychiatric disorders, from motor deficits after stroke, to anxiety and depression. However, there remain substantial gaps in our fundamental understanding of the primary mechanism, in addition to the ideal parameters for neuronal modulation and the clinical populations that stand to benefit the most from its use. Continued research in the field seeks to gain clarity in these domains even as studies still indicated both safety and efficacy in both healthy and neuropsychiatric populations. Therefore, a more thorough understanding of the physiologic and adverse effects of LIFU will ultimately lead to a better-established role for its use in treating disorders of the brain.