Magnetic resonance imaging

Although MRI-compatible life-support equipment is available, it is generally more expensive and may be underused. This, combined with poor patient access, makes MRI logistically more difficult than CT in the critically ill patient. MRI is generally preferred to CT not only because it is more accurate, but also to avoid the high X-ray dose delivered by CT (Royal..Cojleg.e...of..Radi Party 1995); however, considerations of radiation dose are usually far outweighed by other factors in critically ill patients.

The main contraindications to MRI are cardiac pacemakers and other implanted metallic devices such as cochlear implants, heart valves, and intracranial aneurysm clips. Not every device in each of these classes is an absolute contraindication. In practice, however, the necessary MRI compatibility details are often not available in time and, because the potential consequences can themselves be fatal, care is required. A common misconception is that orthopedic metal implants and hemostatic vascular clips in the soft tissues are a contraindication. Most implants do not move in the magnetic fields and, if they do, they are sufficiently embedded in fibrous or osseous tissue not to loosen, although generally a period of 6 weeks after insertion is preferred before exposure to MRI. Heating effects induced by electric currents are negligible.

The advantage of MRI over CT is its far greater sensitivity to soft tissue damage ( SoyleO et...a.l 1994). It has replaced myelography as the test of choice to demonstrate or exclude spinal cord compression; however, it may not demonstrate fractures. Hematomas that are less than 1 or 2 days old, although usually shown, may present a non-specific appearance.

Special techniques using MRI

Many novel magnetic resonance contrast mechanisms such as diffusion, perfusion, and magnetization transfer are available, as are many methods of image analysis.

At present most are too constrained by limited computing power and other design features to be applied in routine clinical practice. Two which have been used are magnetic resonance angiography and, thus far to a more limited extent, diffusion-weighted imaging.

Magnetic resonance angiography can be relied upon to demonstrate carotid stenoses, most carotid and major intracranial branch occlusions, virtually all aneurysms greater than 3 or 4 mm in size, and the patency of dural sinuses and cerebral veins. It is usually not sufficient to display vascular malformations adequately. Magnetic resonance angiography is prone to artefacts, and is sensitive even to slight patient movement and magnetic field distortion.

Diffusion-weighted imaging requires the application of a strong magnetic field gradient, usually in one of three possible directions. Diffusion causes loss of signal, so that areas of restricted diffusion appear brighter. Diffusion-weighted imaging is used to identify ischemic brain damage at an earlier stage than is possible with routine MRI, but early signal changes detected by this technique can be reversible.

Healthy Fat Loss For A Longer Life

Healthy Fat Loss For A Longer Life

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