MRI Principles Applications

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     1  MRI: Principles & Applications Carrie L. Lotenero, D.O. EM Weekly Conference 7/14/05
     2  Major Advantages: #1 DOES NOT use ionizing radiation & no short/long-term effects demonstrated (preferred over CT in peds & childbearing females) #2 Produces variable thickness, 2-D slices in any orientation (optimizes visualization) #3 Provides better contrast resolution & tissue discrimination (replacing invasive methods)
     3  Physical Basis Nuclear: nuclei of H+ in water & fat behave like small spinning bar magnets Place in Magnetic field? align to form net polarization nearly parallel to external magnetic field Apply radio frequency energy? nuclei absorb energy, change alignment, return to equilibrium positions Voltage generated= nuclear magnetic resonance signal
     4  Tissue Appearance
     5  SAFETY NO definite long-term harmful effects Known potential safety concerns due to large static magnetic field: Internal cardiac pacemakers Steel cerebral aneurysm clips (ferromagnetic) Small steel slivers embedded in eye Life-support equipment with magnetic steel Cochlear implants ***ALL ABOVE CANNOT BE SCANNED WITH MRI
     6  SAFETY Malfunction: ICDs, neurostimulators, bone growth stimulators (prosthetic heart valves) Superficial burns (uninsulated wire leads) Obese (> 300#): low-field magnets NEED sedation: infants, younger peds, agitated adults (claustrophobia) Precautions: magnetic plastic cards, watches, hearing aids, ferromagnetic steel objects (LEAVE OUTSIDE) Loud noise (long-term hearing loss)
     7  APPLICATIONS IV contrast agents? less toxicity and reactions (caution in asthma, early pregnancy) Echo planar & diffusion imaging: possible to dx cytotoxic cerebral edema almost immediately after acute ischemic event (i.e., detect early strokes) MRI may completely replace CT head (EXCEPT acute ICH, skull fx, calcified brain lesions) More accurate than CT or plain films in detecting acute fxs; sensitivity to marrow & trabecular changes (although MRI NOT indicated for most acute fx) MR cholangiography Cardiac MRI (limited in ED)
     8  APPLICATIONS (cont’d) MRI Soft tissue: rotator cuff tear, knee, small joints, spinal cord, brain Posttraumatic avascular necrosis (hips) Infection (bone, ST) Mets in bone (after +bone scan) Stress/occult fx Carpal tunnel CT Bone: head, spine (optimally visualizes fracture fragment relationships & bone detail) Abdominal injuries
     9  MRI in Emergent Setting ***Procedure of Choice: #1 Spinal cord compression #2 radiographically occult femoral intertrochanteric and neck fx Unique ability to form images in axial, coronal, sagittal planes Superb contrast resolution Higher sensitivity and specificity than radionuclide bone scan, tomography, CT in detection of OCCULT fxs
     10  MRI in Emergent Setting Potential : Aortic Dissection superior to contrast-enhanced CT and Transesophageal US in delineating aortic intimal flap Drawback: few good candidates (unstable hemodynamically and agitated; require life support and sedation)
     11  MRI in Emergent Setting Potential: Pediatric Fxs When suspect significant injury to unossified cartilage around open growth plates (not seen on plain films)
     12  MRI in the Emergent Setting Low/very low magnetic field systems Easier to accommodate life-support equipment Eval of subacute ICH & brain edema Drawback: less signal at low field (decreased diagnostic quality)
     13  MRI in the Emergent Setting MR angiography Noninvasive May eventually be method of choice in emergent eval of suspected SAH or in leaking aortic aneurysms