Principles and uses of MRI

As with NMR, the physics underlying MRI is extremely complex and thus falls outside the remit of this textbook, hut the following provides a brief summary of the main principles and uses of this increasingly widespread and powerful imaging technology. 

MRI is particularly useful for anatomical examination, and is often seen on television as the futuristic means by which doctors scan patients to see internal structiues such as the brain or other localized areas. Historically, MRI studies mainly focused on the head and extremities (e.g. hand or leg), as imaging of moving organs, particularly the heart and lungs, was difficult to resolve and thus more challenging experimentally. However, MRI technology has continued to evolve rapidly, and modem MRI equipment does not suffer from the limitations of earlier instmments. 

MRI contrast is dependent on relaxation, which occurs on cessation of the brief exposure time to the RF pulses. The RF pulse adds energy to the nuclei, which is absorbed, and during relaxation the RF pulse energy transferred to the nuclei is dissipated to the surrounding environment (or lattice) and the nuclei relax and 

Other key safety points are given briefly below  

Projectiles many metal objects can become airborne missiles in the presence of a strong magnetic field, being attracted to the centre of the magnet. For example, a simple paperclip or hairpin can reach a velocity of 40 miles per hour in the presence of a 1.5 T magnet. Given this, it is easy to see why there are reported cases of MRI-related injury and death. 

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