Page 2280 - Cote clinical veterinary advisor dogs and cats 4th

Magnetic Resonance Imaging Scan 1133

GENERAL MRI SAFETY CONSIDERATIONS and/or plastic. Fiberoptic technology is insufficiency. This syndrome has not
• Injury to the patient and/or damage to the employed; an example is the Nonin 7500 been reported in veterinary patients, but
VetBooks.ir is not paid to basic safety regarding the • Blood pressure/CO 2 measurements ing these agents to patients with renal
care should be taken when administer-
FO unit.
MRI magnet can occur if careful attention
presence of metallic objects within the MRI
○ The patient attachment and cables must
dysfunction. Gadodiamide has a higher
safety ratio than Gd-DTPA.
suite.
• The magnet is the core of the MRI unit, and be specially constructed of nonferrous • Intravenous fluids
metal and/or plastic (e.g., SurgiVet
the magnet is always on, 24 hours/day. It is monitors) • Blankets/heated fluid pads or bags
never safe to have loose metallic objects in • Electrocardiogram (ECG)
the vicinity of the magnet—that is, anywhere ○ ECG electrodes for MRI use are available. Anticipated Time
in the same room as the MRI unit (MRI Because of radiofrequency interference, The time required to perform the study depends
suite). usable ECG tracings cannot be acquired on many factors:
• Because the chemical composition of metal during image acquisition. • Field strength of the magnet: the higher the
objects (ferrous or nonferrous) may not • Emergency kit field strength of the magnet, the more quickly
be known, all metallic objects should be ○ As with any procedure performed under the study can be performed.
regarded as potential safety hazards and general anesthesia, the equipment and • Configuration/size of the radiofrequency Procedures and Techniques
excluded from the MRI suite. drugs necessary for emergency cardio- coils
• All equipment used in the area of the MRI pulmonary cerebral resuscitation (CPCR) • Size of the area to be scanned
magnet must be constructed of plastic and/ should be immediately available. • Number and type of imaging sequences
or nonferrous metal or must be kept a safe ○ Great care should be exercised to ensure performed: with MRI, images are acquired in
distance from the magnet and secured. that metallic objects are not introduced multiple planes and using multiple different
Recorded instances of this guideline not near the magnet during CPCR. In case imaging sequences; the more acquisitions
being followed have produced injury and of emergency, the patient will need to performed, the longer the study
death in patients and hospital personnel be removed from the MRI suite and Approximate times (based on use of a 1.0-tesla
through blunt trauma caused by large objects resuscitative efforts performed at a safe magnet):
that transform into projectiles because of distance from the magnet. • Brain: 1-1.5 hours
their strong attraction to the magnet. Intravenous contrast agents: • Spinal column
• The safe distance from the magnet will • Paramagnetic agents ○ Cervical: 1-1.5 hours
depend on the field strength of the magnet, ○ Gadolinium diethylenetriamine penta- ○ T3-L3: 1.5-2 hours
the shielding of the magnet, and the amount acetic acid (Gd-DTPA): Magnevist ○ Lumbar: 1-1.5 hours
of ferrous metal in the object. A distance of ○ Gadodiamide: Omniscan Because of long scan times, it is important
36 inches (≈1 meter) is generally sufficient for ■ Nephrogenic systemic fibrosis has been to determine neuroanatomic localization of
a magnet of 1.0 tesla or less field strength. reported as a sequela to the injection spinal lesions so that only the affected area
• Because of the safety hazards associated of intravenous paramagnetic contrast of the spinal column is scanned, particularly
with MRI, it is best to limit the number of agents in human patients with renal in large dogs.
personnel in the MRI suite to the minimum
necessary to perform the study and monitor
the patient.
Equipment, Anesthesia
General anesthesia is required. MRI studies
are prolonged and generally performed using
gas anesthesia. Constant-rate infusion of R L R L
intravenous (IV) anesthetic agents can be used,
but gas anesthesia is preferred because of the
duration of the procedure.
• Endotracheal tube, not wire reinforced
○ A laryngoscope can be used only if the
patient is intubated in an area isolated
from the MRI magnet.
• Anesthesia machine for gas anesthesia
○ Unit specified as being MRI safe
(nonferrous metals and plastic used in
construction)
○ Generic anesthesia unit with long breath-
ing circuit; minimum distance to magnet
≈3 feet (1 meter) A B
• IV pump for constant-rate infusion
○ Unit specified as being MRI safe MAGNETIC RESONANCE IMAGING SCAN T1-weighted magnetic resonance images from a normal
(nonferrous metals and plastic used in (A) and abnormal (B) canine brain. Images are from rostral to midcerebrum. In MRI, the imaging characteristics
construction) of tissues are determined by the imaging sequence used. To define the tissue characteristics of a lesion, multiple
○ Generic infusion pump with extension different imaging sequences are performed, and the appearance of the lesion in these images is compared. The
tubing; minimum distance to magnet term intensity is used for describing the appearance of tissues in scans. In the normal patient (A), there is a subtle
≈3 feet (1 meter) difference in intensity between gray matter and white matter of the brain, and intensity of muscle is different
Monitoring equipment: from that of brain. This illustrates the level of soft-tissue contrast achieved with MRI. In the abnormal patient
(B), note the large, round hypointense lesion with a slightly hyperintense rim in the right ventral cerebrum;
• Pulse oximeter the lesion causes left deviation of the falx cerebri. Differential diagnoses for mass lesions in the brain include
○ The patient attachment and cables must be neoplasia, granuloma formation, and abscess formation. In this case, the appearance of the lesion in multiple
specially constructed of nonferrous metal imaging sequences was consistent with a diagnosis of neoplasia with a central necrotic component.

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