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MRI
Magnetic resonance imaging
MRI uses strong magnetic fields to align spinning atomic nuclei (usually
hydrogen protons) within body tissues, then uses a radio signal to disturb the
axis of rotation of these nuclei and observes the radio frequency signal
generated as the nuclei return to their baseline states. The radio signals are
collected by small antennae, called coils, placed near the area of interest. An
advantage of MRI is its ability to produce images in axial, coronal, sagittal
and multiple oblique planes with equal ease. MRI scans give the best soft tissue
contrast of all the imaging modalities. With advances in scanning speed and
spatial resolution, and improvements in computer 3D algorithms and hardware, MRI
has become an essential tool in musculoskeltal radiology and neuroradiology.
One disadvantage is that the patient has to hold still for long periods of time
in a noisy, cramped space while the imaging is performed. Claustrophobia severe
enough to terminate the MRI exam is reported in up to 5% of patients. Recent
improvements in magnet design including stronger magnetic fields (3 Tesla),
shortening exam times, wider, shorter magnet bores and more open magnet designs,
have brought some relief for claustrophobic patients. However, in magnets of
equal field strength there is often a trade-off between image quality and open
design. MRI has great benefit in imaging the brain, spine, and musculoskeletal
system. The modality is currently contraindicated for patients with pacemakers,
cochlear implants, some indwelling medication pumps, certain types of cerebral
aneurysm clips, metal fragments in the eyes and some metallic hardware due to
the powerful magnetic fields and strong fluctuating radio signals the body is
exposed to. Areas of potential advancement include functional imaging,
cardiovascular MRI, as well as MR image guided therapy.
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