©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das
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MAGNETIC RESONANCE IMAGING - V.J.T.I.
Magnetic Resonance Imaging (MRI) is a new diagnostic imaging technique that produces cross-sectional images. It utilises interactions between a magnetic field, radio waves and hydrogen atoms. Hydrogen nuclei in the body are a principal constituent of all tissues. They are normally oriented randomly but are affected by the external field introduced by the MRI magnet and undergo a net alignment parallel to the field. When subjected to brief periods of RF energy, the H+ nuclei in the tissue takes up the RF energy and alter their orientation. The absorbed RF energy is subsequently released by the nuclei, which function as RF transmitters as they return to their previous parallel alignment over characteristic relaxation times T1,T2 and T2*.This emitted energy is measured using an external RF antenna(coil).Repetitive systematic variations in the strength of a second magnetic field(generated by gradient coil inside the magnet) during the time the nuclei are stimulated by the external RF signal makes it possible to selectively excite individual areas of the body. Recording and subsequent processing of this spatially localized data result in the clinical MRI scan. The N x N numerical values that make up the image are referred to as pixels(picture elements).These values are generated from the MRI signals acquired during scanning in a computational process known as Fourier reconstruction.Any MRI study directly show received amplitude as a function of position. For a given pixel, amplitude depends on relaxation times T1,T2 and T2* as well as proton density of the substance being measured. Taking advantage of the fact that different kinds of tissue(such as bone, cerebrospinal fluid, fat etc.) have different T1 and T2,contrasts in tissue images can be obtained using 'T1 weighted' or "T2 weighted' sequences at the same time suppressing T2* effects as the latter is due to magnetic inhomogeneities and leads to error. Accordingly the main types of pulse sequences that can be applied to a tissue being examined are saturation recovery, spin-echo and inversion recovery. The techniques mainly differ in the duration and combination of RF pulses applied. MRI is being increasingly used in a variety of diagnostic applications especially of the soft tissues in the brain and spinal cord. Its multi-planar scanning capability makes it superior to the CT scan.
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©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das |