The proposed project will investigate the neurobiological mechanisms of accelerated intermittent Theta Burst Stimulation (iTBS) in major depressive disorder (MDD) using an advanced multimodal imaging approach. This single-arm, within-subject study will deliver one week of accelerated iTBS and use pre-/post-treatment PET/MRI to quantify changes in synaptic density, functional connectivity, and microstructural integrity. We will combine [¹⁸F]SynVesT-1 PET with functional, neurochemical and anatomical MRI, such as resting-state fMRI, magnetic resonance spectroscopy (MRS) and neurite orientation dispersion and density imaging (NODDI), to capture treatment-related plasticity. This integrated design will link molecular and network-level mechanisms to clinical improvement, providing an unprecedented mechanistic map of how accelerated iTBS restores brain function in depression.

Major depressive disorder (MDD) is one of the most prevalent and disabling disorders worldwide, affecting approximately one in 20 Canadians at any given time and ranking among the leading causes of lost productivity, poor quality of life, and suicide. Despite major advances in pharmacological and psychotherapeutic interventions, only 40-60% of patients respond to first-line treatments. Theta Burst Stimulation (TBS) represents the next generation of rTMS technology: by delivering patterned bursts of magnetic pulses that mimic intrinsic theta-gamma coupling, TBS is thought to more efficiently engage synaptic plasticity mechanisms that underlie mood regulation. Clinically, iTBS achieves antidepressant efficacy comparable to conventional 10 Hz rTMS in one-tenth of the stimulation time, enabling faster, more accessible treatments. Yet, despite its growing clinical use and regulatory approval in multiple countries, the fundamental mechanisms by which iTBS modulates limbic-cortical networks to alleviate depressive symptoms remain poorly understood. Addressing this knowledge gap is essential to optimizing treatment protocols and advancing precision neuromodulation strategies.

Understanding how iTBS drives recovery thus requires moving beyond traditional symptom-based approaches toward multi-level indices of brain plasticity that capture functional, neurochemical, and microstructural change. Current evidence remains largely descriptive, with limited direct insight into the underlying synaptic or cellular mechanisms of iTBS-induced modulation. Integrating PET with high-resolution MRI techniques provides a unique window on these processes. We now have full capacity for in-house synthesis and imaging with [F18]SynVesT-1. Thus, this tracer quantifies synaptic density in vivo, providing a direct molecular measure of plasticity. The ability to pair [F18]SynVesT-1 PET simultaneously with MRI represents a transformative advance for mechanistic neuromodulation research.

Clinical trials show that accelerated TBS, individually targeted to the DLPFC region most anti-correlated to sgACC, can produce rapid symptom relief within days rather than weeks. However, the neurobiological mechanisms underlying these effects remain unknown. It is hypothesized that repeated stimulation sessions promote cumulative synaptic potentiation and large-scale network reorganization. Elucidating these processes is crucial to optimize dosing parameters, understand inter-individual variability in response, and guide the next generation of biologically informed treatment strategies.

The proposed project will investigate the neurobiological mechanisms of accelerated TBS in MDD using an advanced multimodal imaging approach. In this single-arm, within-subject study, participants will undergo one week of accelerated iTBS treatment while completing pre- and post-treatment positron emission tomography (PET) and magnetic resonance imaging (MRI). PET imaging with the synaptic vesicle tracer [¹⁸F]SynVesT-1 will quantify changes in synaptic density, while MRI sequences such as resting-state functional MRI, magnetic resonance spectroscopy, and neurite orientation dispersion and density imaging (NODDI) will assess functional connectivity and microstructural plasticity. By integrating molecular, functional, and structural measures of brain plasticity, the study will provide new insight into how accelerated iTBS alters brain circuits implicated in depression and how these changes relate to clinical improvement.

Inclusion Criteria

For inclusion in the study, participants must fulfill all the following criteria:

1. 18 to 55 years old.
2. Competent to provide voluntary informed consent.
3. English comprehension and verbal communication (participants must be able to both understand and speak English sufficiently to follow study procedures and be understood by study personnel
4. Referred by their treating physician.
5. Mini-International Neuropsychiatric Interview-confirmed diagnosis of MDD, as a single or recurring episode.
6. Symptoms of MDD have not improved after ≥ 1 adequate antidepressant medication trial in the current depressive episode29.
7. Baseline score of ≥18 on the 17-item Hamilton Rating Scale for Depression (HRSD-17).
8. Maintained a stable treatment regimen for at least four weeks prior to entering the study, defined as being on a stable antidepressant regimen, a stable psychotherapy regimen, both, or neither (i.e., no treatment), with no changes during this period.

Exclusion Criteria

Participants fulfilling any of the following criteria will be excluded from the study:

1. Any comorbid mental health disorders (including, but not limited to lifetime history of psychotic disorders, OCD, and/or bipolar I or II disorder) with the exception of anxiety/panic disorders, posttraumatic stress disorder and ADHD
2. Current or past (< 3 months) substance (including nicotine) or alcohol abuse/dependence, as defined in DSM-5 criteria.
3. Positive urine test for illegal substances, cannabis, or cotinine.
4. Significant unstable medical or neurologic illness confirmed by medical history (e.g. uncontrolled diabetes, or renal dysfunction).
5. Breastfeeding or pregnant (confirmed via urine test).
6. BMI > 30 or BMI < 18.
7. Contraindication for TMS (e.g., personal history of epilepsy or convulsion, metallic head implant, pacemaker).
8. Contraindication for MRI (e.g. metallic implant, claustrophobia).
9. Have received a cumulative radioactivity dose > 15.2mSv during the last 12 months.
10. Have active malignancies (due to high chance of undergoing radiation therapy).
11. Suicide attempt in the past three months and/or active suicidal intent.
12. Failed (non-response) course of ECT or rTMS treatment in the current depressive episode.
13. Benzodiazepine or lithium use. Other psychotropic medications (e.g. ADHD medications) are permitted, if stable in the 4 weeks prior to and during the treatment course)
14. Any other condition that, in the opinion of the investigators, would adversely affect the participant's ability to complete the study.

After participants have completed their participation, we will ask them for informed consent for the storage of their de-identified data on OpenNeuro (an open access database) for the purpose of unspecified research for unspecified future research.

If they do agree to participate, they will be given a unique study code (different from the participant ID used in the previous study) will be assigned to all of the study data instead of personal identifying information (ex. 'John Smith' replaced by the unique study code 'A283422'). This unique study code will be used in the Open Access database to ensure privacy and confidentiality (i.e. de-identified study data cannot be linked to participant in any way).

In addition to an unique study code, their de-identified study data used in the Open Access database will include: age, sex, handedness, education level, medical diagnosis, medication, summary scores from clinical interviews, and MRI scans.

Once data has been included in the OpenNeuro database, data cannot be removed or withdrawn as we will be unable to identify participant information to separate it from the database.

The shared data will be hosted on the OpenNeuro (https://openneuro.org/) platform. The dataset becomes publicly available under the Creative Commons Zero (CC0) Public Domain Dedication which places no restrictions on who can use the data or what can be done with them. The OpenNeuro is supported by the United States National Institutes of Mental Health BRAIN Initiative. The OpenNeuro follows the FAIR principles, which states that in order for shared data to be maximally useful for the scientific community, they need to be Findable, Accessible, Interoperable, and Reusable.