New York University School of Medicine, United States
Introduction:: Brain diseases affect 1 out of 6 people worldwide including a significant portion who suffer from drug resistive conditions1. Non-invasive brain stimulation (NIBS) methods, when effective, play a unique role in the treatment of such patients, as they could prevent invasive procedures. Some of such methods (e.g. TDCS, TACS, TMS) are now widely used in clinical practices, including rehabilitation and treatment of mental disease. Each of these methods has proven several beneficial effects but also come with limitations. Therefore, the search for novel techniques is of great interest for the scientific community. In this report we study the utilization of transcranial radio frequency stimulation (TRFS) as a novel non-invasive brain stimulation technique.
In our previous experiments, we examined whether RF exposure in a non-thermal regime can affect neural activity in-vivo. Using RF-artifact-free 1-photon calcium imaging, we observed that RF exposure, several folds higher than what is permitted by regulatory limits, does not affect the ongoing neuronal activity2. It is, however, well known that RF, when applied with proper parameters (e.g. frequency, power, pulse duration, etc.), can induce temperature increase in the exposed biological tissue such as the brain3. On the other hand, it has been shown that changes in the local temperature affect the ongoing activity of neurons4. Combining these concepts, we use RF energy to increase brain temperature in a controlled manner to affect the ongoing activity of neurons in the nearby tissue, hence introducing a novel neuromodulation technique.
Materials and Methods:: We developed an in-house ‘stub’ antenna design (composed of an open-end coaxial RF cable with an unshielded tip which is matched by a simple PCB including a transmission line and a modifiable parallel capacitor) that can induce RF-induced temperature rises in the brain of head-fixed mice without significantly affecting the body temperature (Fig.1).
We used functional magnetic resonance imaging (fMRI) to evaluate the changes in blood-oxygen-level-dependent (BOLD) signal in response to RF stimulation. Wild-type (C57BL/6 strain; n=14) mice were anesthetized by IP injection of urethane (1.5g/kg dose). They then went through stereotaxic surgery where a 3D-printed headcap (specially designed for RF stimulation using our stub antenna; Fig.2A) was attached to their skulls and an optical temperature probe was implanted in their brain (as illustrated in Fig.2B). They were then head-fixed to a 3D-print small animal conditioning cradle for imaging experiments (Fig.2C-D) that we have formerly designed5. RF stimulation (950MHz; 20W) was applied in an intermittent manner (15 pulses of 15s ON followed by 81s OFF) in a way that brain temperature raised about 2°C during the RF ON periods (Fig.2E-F). Resting-state and TRFS-induced fMRI data were acquired using T2*-weighted single-shot gradient echo–echo planar imaging (GE-EPI) sequence (18,27) with the following parameters: TE 13.7 ms, TR 1500 ms, Field Of View 30x9 mm2, Matrix size 256x256, Number of axial slices 14, and slice thickness 0.8 mm. Imaging data are processed using our pipeline described in a former report5 and TRFS-induce BOLD signal changes are evaluated.
Results, Conclusions, and Discussions:: Fig.1A illustrates the structure of our antenna design and its application on a mouse where it shows that brain temperature can be effectively increased (1.5°C over 15s) without significantly affecting the body temperature (Fig.1B-D).
Fig.2D shows the animal preparation for fMRI experiments under TRFS-paradigm. Fig.2E shows the resulting temperature profile in an example imaging session including resting-state, Sham and TRFS fMRI sequences. Fig.2F shows the resulting temperature change due to the TRFS-paradigm and Fig.2G shows the average temperature change over RF stimulation pulses (15s ON 81s OFF, total of 64 scans). Due to interference between the RF stimulus and the scanner’s RF coil, images during the RF-ON periods (10 out of 64 scans) were noisy and not usable. To evaluate the effects of thermal deposition of the TRFS-paradigm we compared the BOLD signal between the 15 first scans of each stimulation cycle (total of 225; High Temperature (HT) scans) where the brain temperature is most elevated with the last 15 scans (total of 225; Low Temperature (LT) scans) where the brain temperature has lowered (ΔT(avg)=1.41C; Fig.2H-I). Fig.2J illustrates the percentage of change in the BOLD signal (BOLD_HT-BOLD_LT) in various axial planes across the brain (n=14 mice), where both positive and negative changes in the BOLD signal in response to the brain temperature increase are observed.
The dominant negative response is due to changes in the temperature-dependent MRI parameters such as T2*6. Fig.3A-C illustrates changes in the BOLD signal in an area with positive response. The presence of such positive response is associated with changes in neural activity combined with physiological changes (e.g. local blood flow)6. Other comparisons (e.g., TRFS vs. Sham, etc.) will also be presented at the conference.
Although the observed effects do not project pure neural effects, they, nevertheless, demonstrate TRFS-induced changes in brain activity and physiology, giving rise to a novel non-invasive brain stimulation paradigm. We are currently using optical Ca2+-imaging in cortical areas in mouse brain to study the direct neuronal response to TRFS. Next, we will study TRFS application in affecting rodents’ behavior and its translational application for treatment of brain disorders in mice models.
Acknowledgements (Optional): : Authors declare no competing interest. This work was supported by NIH grant #1R01NS113782-01A1 and TL1 postdoctoral fellowship # 2TL1TR001447-06A1 to OY. The imaging experiments were performed at the NYU Langone Health Preclinical Imaging Laboratory and all experiments were conducted in accordance with the Institutional Animal Care and Use Committee (IACUC) of New York University Medical Center.
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