Principles of magnetic resonance imaging and evaluation of different modalities

Abstract

Nuclear Magnetic Resonance Imaging is an imaging modality whichproduces cross-sectional transaxial, coronal and sagittaltomographic images to those of X-Ray computed tomography (CT),and also, it is non-ionizing, non-invasive and without knownrisk.Certain atomic nuclei that have an odd number of protons orneutrons possess a characteristic known as 'spin' and they behavelike small bar magnets and tend to align with external magneticfield. Actually they precess about the axis of the externalmagnetic field at a frequency that depends on the strength of thefield. The object in the external field is said to be temporarilymagnetized.If the magnetized object is then exposed to a short burst ofRF energy at exactly the same frequency as that of precessingnucle1., the nuclei start precessing and emit a coherent signal.This RF signal, FID (Free Induction Decay), is detectable bythe nearby RF coil and contains information on the object bymeans of having two relaxation parameters; Tl and T2.By applying different RF pulse sequences, it is possible·toobtain different contrast and resolution levels.In th.i.s worJ{, basic issues related to NMR Imaging phenomenon;such as, principles of NMR, relaxation processes, relaxationtimes, special pulse sequences and techniques, gradient fields,phase encoding, frequency encoding, instrumentation, safety,economics, and design considerations of a magnet are discussedin a comprehensive manner, and several conclusions are made.