The fundamental working principle of magnetic resonance imaging (MRI) is based on the induction of proton spin and the detection of the proton spin response when radio frequency pulse is emitted. Protons in water and fat inside the human body act as magnetic dipoles (small magnets). These magnets get oriented along an applied magnetic field, causing a net longitudinal magnetization. Radio frequency pulse can influence magnetisation in the transverse direction.
The radio frequency pulse is then stopped, casing the proton spins to relax back to their equilibrium situation via two relaxation processes, namely the T1 and T2 decay. Rotating transverse magnetisation induces magnetic resonance signal in receive coil. By using a pulse sequence, which is series of RF pulses gradients, contrast can be created based upon differences in T1,T2 and proton density between different tissues.
The Bloch equations are three coupled ordinary linear differential equations, that indicate exactly how the magnetization responds to an electromagnetic field of any duration, strength, and frequency. The magnetic resonance image actually represents a mapping of proton density weighted with nuclear relaxation times associated with different molecular environments.
Basically, the strength of the nuclear magnetic resonance signal produced by precessing protons in a tissue depends on T1, T2 of the tissue and the density of protons in the tissue. Motion of the protons is either flow or diffusion kind. The magnetic resonance imaging pulse sequence used in a T1 “weighted” image the pulse sequence is chosen so that T1 has a larger effect than T2. Images can also be made to be T1, T2, proton density or flow/diffusion weighted.
Comparative intensity in the nuclear magnetic resonance signal for different body tissues can be calculated as a function of the relaxation times T1 and T2 for different choices of retardation time and echo time. The relative intensity of a signal for body tissue is the right choice of pulse repetition and echo-time allows one to emphasize the T1 relaxation time characteristics for different body tissues. Short repetition time emphasizes tissues with short relaxation times T1 like fat and blood, short echo time minimizes T2 decay effects. Long repetition times repetition time emphasize tissues with long relaxation times T1 like cerebral tissues.
Sometimes there is a need to administer exogenous contrast usually an intravenous injection of some paramagnetic agent despite the fact that magnetic resonance imaging delivers excellent soft-tissue contrast,. The effect of this agent is to shorten the relaxation time of local spins causing a decrease in signal on T2-weighted images and an increase on T1-weighted images.
Disruptions in the blood-brain barrier can be investigated for brain images both before and after contrast enhancement. The increased vascularity of tumours produces a preferential uptake of contrast agent and the technique can be used to better visualise these from surrounding normal tissue. If magnetic resonance scans are repeatedly acquired following the contrast injection, the dynamic nature of contrast uptake can be examined, which may improve the differentiation of benign and malignant disease.
Magnetic resonance imaging is an advanced imaging techniques which is very costly. To be widely used on a routine basis, the cost needs to fall, with perhaps technological break-through in the future. While current magnetic resonance imaging techniques are touted to be relatively safe, chronic and long-term health effects still needs to be established.
