Overview of Nuclear Brain Imaging Techniques: PET, MRI, and fMRI

In the previous article we introduced EEG, MEG, TMS, and NIRS, noting their limited spatial resolution for deep brain structures. This article shifts focus to three nuclear brain imaging techniques—PET, MRI, and fMRI—that rely on fundamentally different physical principles.

Positron Emission Tomography (PET) : PET uses biologically relevant molecules (e.g., FDG) labeled with short‑lived radioactive isotopes. After injection, the emitted positrons annihilate with electrons, producing photons that are detected by the PET scanner. The distribution of the radiotracer reflects metabolic activity, enabling three‑dimensional functional and structural imaging of the whole brain. PET is highly sensitive and specific, widely used for tumor diagnosis, therapy monitoring, organ‑function studies, and drug development, but it has relatively low spatial resolution and involves ionizing radiation, limiting repeat scans.

Magnetic Resonance Imaging (MRI) : MRI exploits nuclear magnetic resonance of hydrogen nuclei in water and fat. A strong static magnetic field (B0) aligns the nuclear spins; a short RF pulse (B1) perturbs this alignment, and the subsequent relaxation (characterized by T1 and T2 time constants) generates signals that are spatially encoded using magnetic field gradients. The resulting images provide high‑resolution anatomical detail without ionizing radiation. MRI safety requires the absence of metal implants and can be uncomfortable due to loud noise and confined space.

Functional Magnetic Resonance Imaging (fMRI) : fMRI measures the blood‑oxygen‑level‑dependent (BOLD) signal, which arises because deoxygenated hemoglobin is paramagnetic and alters local magnetic fields, shortening T2* relaxation. Neural activation increases cerebral blood flow and oxygenated hemoglobin, reducing the BOLD signal dip. This technique offers millimeter spatial resolution and sub‑second temporal resolution, allowing mapping of task‑evoked activations, resting‑state functional connectivity, and network dynamics. fMRI is non‑invasive, does not require radioactive tracers, and can be combined with diffusion tensor imaging (DTI), MEG, or TMS for multimodal studies.

Overall, PET excels in quantifying metabolic processes, MRI provides unparalleled structural detail, and fMRI bridges the gap by offering high‑resolution functional mapping. Their complementary strengths make them indispensable tools in both basic neuroscience research and clinical diagnostics.