Magnetic resonance imaging (MRI) is a noninvasive procedure that does not use ionizing radiation (x-rays). An MR image measures the ability of hydrogen nuclei to absorb radio frequency energy. All soft tissues can be visualized with MRI, however, dense bone and air are difficult to image with this procedure. Magnetic resonance relies on a very powerful magnetic field. (Range of 0.06-2.0 tesla. One tesla equals 10,000 gauss. The earth's magnetic field equals 0.5 gauss.) The high magnetic fields can produce a very strong pull on metallic objects and attention needs to be paid to this fact or flying projectiles may result.
The MR image, like a CAT scan image, is dependent on computer processing of the signals. Diagnoses utilizing MRI are based on the fact that abnormal tissue displaces normal structures and results in contrast-enhanced areas which can be visualized. MRI is extremely sensitive at showing areas of hemorrhage and timing the duration of the hemorrhage. The hemorrhage appears different depending on the location of the bleeding and on the strength of the magnet. As the red blood cells break down, the MR image changes. Biopsies guided by the MRI can be performed when indicated.
General anesthesia is necessary, since the MRI generates considerable noise and requires that the patient remain still for 10-60 minutes. Due to expense and space requirements, veterinarians who want to use MRI technology usually have the procedure performed at a nearby human hospital.
MRI is not indicated in acutely traumatized patients, since life support systems can not be safely used in the magnetic field. Artifacts in images are generally caused by motion or deformations in the magnetic field from implanted metallic objects. These could include pacemakers (which will malfunction in a magnetic field), microchips and other steel objects (BB pellets). Some types of implants may absorb sufficient energy to cause discomfort from local heating. Hemoclips may actually be pulled away from the tissue by the magnetic field. Personnel working with a patient receiving MRI must be aware of the magnetic field and potential problems it may cause with items such as pacemakers they may have themselves.