MRI is rapidly becoming an indispensable tool in the accurate diagnosis of veterinary diseases. It is an enormous resource that allows for fast non-invasive three-dimensional visualization of internal pathology.
1) The Basics – How Does MRI Work?
A dog’s body is 70 percent water. As you know, water has an oxygen atom with 2 electrons and two hydrogen atoms each with a proton which are spinning randomly. When a dog is placed in the center of the MRI magnet, these protons become aligned in the same direction. A radiofrequency (RF) pulse is introduced which misaligns the direction of these protons from the magnetic field. When the RF pulse is turned off the protons “flip back” into their original alignment in the magnetic field. As these protons “flip back” they release energy which is detected by the MRI sensors and the MRI computer converts this released energy into an image. The protons in each tissue type release different levels of energy thereby creating the various shades of “color” which delineate the anatomic structures.
The strength of the MRI magnet is measured in Teslas (T). The majority of MRI scanners are either 1.5T or 3T in strength. The more powerful the magnet, the faster and more detailed the image.
2) When to choose MRI as a Diagnostic Tool
MRI is not the diagnostic tool for every pathologic state. X-rays and CT scans are best for evaluating lung parenchyma and bones. MRI is the diagnostic tool of choice for the evaluation of soft tissue in disease states like but not limited to:
- Herniated discs
- Brain tumors, abscesses and inflammation
- Strokes
- Muscles and tendons
- Spinal cord tumors
- Intra-abdominal disease
3) MRI Safety
The magnet in an MRI scanner is a powerful attractant for ferrous metal. The magnet of a 3T MRI is 60,000 times stronger than the earth’s magnetic field. Any object containing ferrous metal can literally become a flying missile for potential injury to a technician or the patient. Consequently, all objects within a calibrated distance from the scanner, called the Gauss Line, must be without ferrous metal or MRI safe. Consequently, all patient monitors, anesthesia machines, IV poles and patient support equipment must be MRI safe.
4) Anesthesia During MRI
The majority of MRI scans take from 1 ½ to 2 hours during which the patient cannot move. Consequently, every veterinary patient requires general anesthesia. This requires that the anesthetic machine, patient monitors, IV poles and any other equipment in the room be MRI safe.
Anesthesia during MRI presents its unique set of challenges. The patient is sequestered inside the circle of the magnet for 1 ½ to 2 hours and virtually inaccessible. Dislodgement of an endotracheal tube may not be detected until vital signs change on the monitor because there are no immediate eyes on the patient. Patient monitors can also become dislodged as the patient moves through the scanner.
General anesthesia causes vasodilation of peripheral blood vessels and consequently a shunting of warm core blood to the periphery. The patient literally becomes a heat radiator. Veterinary patients’ very large surface area to mass ratio coupled with the shunting of warm core blood to the periphery makes hypothermia a perpetual problem in MRI. After 1 1/2 to 2 hours in an MRI scanner it is not uncommon for patients to be 2 to 3 degrees Fahrenheit below normal body temperature. Since active warming devices, like warm air blowers and electrical warmers, contain ferrous metals, they cannot be used in MRI to combat hypothermia.
The physiologic effects of hypothermia can be profound. Hypothermia inhibits the metabolism of anesthetic drugs by the liver and kidneys potentially prolonging post anesthesia recovery by 1 ½ to 2 hours. Without going into the physiologic mechanisms of each complication, the deleterious effects of hypothermia can include but not limited to:
- Cardiac arrhythmias
- Decreased platelet aggregation
- Decreased blood oxygen
- Postoperative protein breakdown
- Impaired kidney function
- Decreased drug metabolism
- Impaired liver function
- Decreased cardiac output
- Peripheral vasoconstriction
- Surgical site infections
- Poor wound healing
- Altered mental status
- Increased recovery time
- Death
This non-exhaustive list of potential physiologic complications of hypothermia was provided to impress on you the seriousness of patient hypothermia in MRI. Allowing a patient to spiral into hypothermia is no longer an acceptable mode of patient care.
5) The MRI Safe – Conrad Thermal Blankets
The MRI Safe – ConRad Thermal Blankets are currently the only passive patient warming device that is MRI safe and effective in maintaining normothermia. Their MRI safety have been proven by two safety studies and their effectiveness in maintaining normothermia by two clinical studies. The two proprietary internal component layers act as a barrier to the three mechanisms of heat loss, conduction, convection and radiation. The patient is simply wrapped circumferentially with the MRI Safe – ConRad Thermal Blanket and placed in the scanner.
<< See the ConRad Thermal Blanket Instructions >>
MRI in veterinary medicine continues to improve disease diagnosis and patient care. Understanding how MRI works, patient safety and the clinical challenges is imperative in providing the standard of care that all veterinary patients deserve.
