Experimental Techniques in Brain Imaging

Signal Source: MRI utilizes the $^1H$ nucleus (water protons), as the body is approximately $2/3$ water.
Proton Properties: * Spin: Protons rotate about an axis. * Nuclear magnetic moment: Protons align with an external magnetic field (polarization).
Signal Generation: * Protons align parallel (low energy) or anti-parallel (high energy) to a magnetic field. * A Radio frequency (RF) pulse flips the magnetization direction (excitation). * Protons relax and realign, creating a voltage change detected by a receiver coil. * Contrast is generated because relaxation rates differ across tissues (e.g., fat vs. water).

T1 relaxation: Longitudinal relaxation involving the return of Magnetisation ( $M$ ) in the z plane.
T2 relaxation: Transverse relaxation involving the loss of $M$ in the xy plane.
Brain Tissue Values at $1.5 ext{ Tesla}$ : * White matter: T1 = $600\,ms$ ; T2 = $80\,ms$ . * Grey matter: T1 = $950\,ms$ ; T2 = $100\,ms$ . * CSF: T1 = $4500\,ms$ ; T2 = $2200\,ms$ .

System Components: 1. Strong Magnet: Polarizes the sample. 2. Radio frequency coils: Handles excitation and detection. 3. Magnetic gradients: Provides spatial encoding for imaging.
Safety Considerations: * Magnetic Field Strength: Clinical units ( $1.5$ or $3\,Tesla$ ); preclinical units up to $11\,T$ . * Projectile Effect: Objects can accelerate into the magnet at speeds up to $70\,mph$ . * Metal Implants: Pacemakers, aneurysm clips, or shrapnel can cause localized heating or burns.

T1 Weighted Imaging: CSF appears hypointense (dark); used for anatomical detail and assessing Blood-Brain Barrier (BBB) integrity with Gadolinium contrast.
T2 Weighted Imaging: CSF appears hyperintense (bright); used to detect pathology such as tumors, hemorrhage, and vasogenic oedema.
Diffusion Weighted Imaging (DWI): Assesses acute stroke damage (cytotoxic oedema) and white matter damage (DTI).
Perfusion Weighted Imaging (PWI): Assesses cerebral blood flow ( $CBF$ ) using contrast agents or Arterial Spin Labelling.
Functional MRI (BOLD): Blood Oxygen Level Dependent imaging measures the hemodynamic response to neural activity.
Cell Tracking: Uses Superparamagnetic (Iron oxide) or Paramagnetic (Manganese/Gadolinium) agents to track transplants.

Acute Phase: Energy failure leads to $Na^+/K^+$ pump failure, causing cytotoxic oedema (cell swelling). This appears as a hyperintense signal on DWI within minutes.
Vasogenic Oedema: Occurs $3-4\,hours$ post-stroke due to BBB breakdown; visible as hyperintensity on T2 images.
Ischaemic Penumbra: Defined as the "mismatch" between the larger PWI hypoperfusion volume and the smaller DWI lesion. This tissue is at risk but potentially salvageable by thrombolysis (e.g., $rtPA$ ).
Quantification: Infarct volume is calculated by summing hyperintense areas on slices multiplied by slice thickness.