MRI provides a wealth of structural and biochemical information about tissue (Hilal SK, Mohr JP, 1992. Dunbabin DW, Sandercock PAG, 1991. Caplan LR. Stroke: a Clinical Approach. Boston, Butterworth-Heinemann, 1993, pp 99-150]. The technique is based on the interaction between radio frequency waves and the nuclei of different atoms in the body in the presence of a strong magnetic field. Hydrogen (proton) is the most common nuclear magnetic resonance-observable nucleus within the human body. Clinically, water protons and fat protons are the most extensively imaged nuclei.
In the presence of the powerful magnetic field, the protons are susceptible to excitation by selective radio frequency pulses. The energy from these pulses is absorbed and then released until the tissue being scanned has completely reemitted the energy absorbed and has undergone complete relaxation. The energy released from the excited tissue occurs over a short period of time according to two relaxation constants known as T1 and T2. In T1-weighted images, cerebrospinal fluid has a low-density relative to brain, while the fat has a high-signal intensity. In T2-weighted images, cerebrospinal fluid has an increased signal relative to brain, and fat has almost no signal.
Conventional MRI shows tomographic sections of the brain in multiple plans of proton distribution modified by T1 and T2 relaxation times as well by a third type of image with a balanced T1-and T2-weighting.
Ischemia alters water content in the brain cells, changing their response to a magnetic field. Infarction prolongs the T1 and T2 relaxation films. Intracerebral hemorrhage (ICH) has different appearances that are complex and depend on the duration of time since the bleed, the strength of the magnetic field, and the settings used to obtain the images [Bruno A. Geriatrics. 1993; 48;26].