Quantification of Transcranial Magnetic Stimulation and Low-intensity Focused Ultrasound Energy Field Focality in the...

Transcranial magnetic stimulation (TMS) and low-intensity focused ultrasound (LIFU) are neuromodulation techniques that use different types of energy and have disparate spatial profiles. The study aimed to quantify and compare both modalities' focality on a common cortical target.

Magnetic resonance imaging-derived individualized head models were used to simulate TMS-induced electric fields and LIFU acoustic fields at 250 kHz and 500 kHz for a common standardized target located in the left dorsolateral prefrontal cortex in 20 subjects. Energy distribution was quantified using three relative thresholds (30%/50%/70%) and two metrics: % Engaged (percentage of a region's volume energized) and % Deposited (percentage of total energized tissue lying within a region). Regional analyses focused on the middle frontal gyrus (_MFGyr) and middle frontal sulcus (_MFSulc).

TMS produced notably larger activation volumes than did both LIFU frequencies. TMS engaged 59% of _MFGyr and 29% of _MFSulc, whereas LIFU-250 kHz engaged 3% and 8%, and LIFU-500 kHz engaged <1% of each. TMS mainly engaged _MFGyr, whereas LIFU dispersed its energy more evenly between gyri and sulci. LIFU also indicated greater focality but exhibited significantly higher intersubject variability, particularly at 500 kHz. Reducing the fundamental frequency to 250 kHz increased coverage while maintaining focality.

Our modeling provides a quantification and contrast to the concept of focality between TMS and LIFU. It shows a trade-off between focality and sensitivity to anatomical variability: the greater the focality, the more sensitivity to anatomical variability, thus potentially requiring more sophisticated targeting methods and apparatus, and vice versa.

The Clinicaltrials.gov registration number for the study is NCT02800226.