M

M



Magnetic Resonance Imaging Scan



SYNONYMS


MRI, MR



OVERVIEW AND GOALS



• Magnetic resonance imaging (MRI) is a method of cross-sectional imaging that does not involve ionizing radiation. When compared to radiography, MRI allows improved evaluation of areas of complex anatomy by avoiding superimposition of multiple structures. The contrast resolution of MRI is vastly superior to that of radiography and computed tomography (CT).

• Magnetic resonance imaging can be used for assessing any area of the body. In veterinary medicine, common uses include assessment of the brain and spinal column. Musculoskeletal imaging (scapulohumeral joint, stifle etc) is also increasing in popularity in small animal practice.


INDICATIONS


Disease of the central nervous system; best method available for imaging of the brain



CONTRAINDICATIONS



• Inability to tolerate general anesthesia

• Presence of a pacemaker: in a minority of patients, the MRI magnetic field may alter the pacemaker program or cause the pacemaker to oscillate in the patient. In human medicine, MRI suites bear warnings forbidding entry to any person with a pacemaker, so it is unlikely that an MRI technician will allow a veterinary patient with a pacemaker to be scanned.

• Presence of ferrous (iron-containing) metal including implants and intestinal foreign bodies:
○ Movement of the metal when in the magnetic field, causing soft-tissue trauma

• Presence of large metallic implants (nonferrous) in the area of interest:
○ Heating of the metallic implants → tissue damage

○ Imaging artifact severely limits the value of the study.

GENERAL MRI SAFETY CONSIDERATIONS:


• Injury to the patient and/or damage to the MRI magnet can occur if careful attention is not paid to basic MR safety as regards the presence of metallic objects within the MRI suite.

• The MR magnet is the core of the MRI unit, and the magnet is always “on,” 24 hours a day. Therefore, it is never safe to have loose metallic objects in the vicinity of the magnet—that is, anywhere in the same room as the MRI unit (MRI suite).

• Since the chemical composition of metal objects (ferrous or nonferrous) may not be known, all metallic objects should be regarded as potential safety hazards and excluded from the MRI suite.

• All equipment used in the area of the MRI magnet must be constructed of plastic and/or nonferrous metal or must be kept a safe distance from the magnet and secured. Recorded instances where this guideline was not followed have produced injury and death in patients and hospital personnel through blunt trauma caused by large objects which transform into projectiles owing to their strong attraction to the magnet.

• The safe distance from the magnet will depend on the field strength of the magnet, the shielding of the magnet and the amount of ferrous metal in the object. A distance of 36 inches (3 meters) is generally sufficient for a magnet of 1.0 tesla or less field strength.

• Because of the safety hazards associated with MRI imaging, it is best to limit the number of 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 intravenous 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 only be used if the patient is intubated in an area isolated from the MRI magnet.

• Gas anesthesia:
○ Anesthesia machine:
▪ Unit specified as being MRI safe (nonferrous metals and plastic used in construction)

▪ Generic anesthesia unit with long breathing circuit; minimum distance to magnet ∼3 feet (1 meter)

• Constant rate infusion:
○ Infusion pump:
▪ Unit specified as being MRI safe (nonferrous metals and plastic used in construction)

▪ Generic infusion pump with extension tubing; minimum distance to magnet ∼3 feet (1 meter)

Monitoring equipment:


• Pulse oximeter:
○ The patient attachment and cables must be specially constructed of nonferrous metal and/or plastic:
▪ Fiberoptic technology is employed; an example is the Nonin 7500 FO unit.

• Blood pressure/CO2 measurement:
○ The patient attachment and cables must be specially constructed of nonferrous metal and/or plastic:
▪ Surgivet monitors

• Electrocardiogram (ECG):
○ ECG electrodes for MRI use are available:
▪ Because of radiofrequency interference, usable ECG tracings cannot be acquired during image acquisition.

• Emergency kit:
○ As with any procedure performed under general anesthesia, the equipment and drugs necessary for emergency cardiopulmonary cerebral resuscitation (CPCR) should be immediately available.

○ Great care should be exercised to ensure that metallic objects are not introduced near the magnet during CPCR. In the case of emergency, the patient will need to be removed from the MRI suite and resuscitative efforts performed at a safe distance from the magnet.

Intravenous contrast agent:


• Paramagnetic agents:
○ Gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA): Magnevist

○ Gadodiamide: Omniscan
▪ Nephrogenic systemic fibrosis has been reported as a sequela to the injection of intravenous paramagnetic contrast agents in human patients with renal insufficiency. This syndrome has not been reported in veterinary patients, but care should be taken when administering these agents to patients with renal dysfunction. Gadodiamide has a higher safety ratio than Gd-DTPA.

• Intravenous fluids

• Blankets/heated fluid pads or bags


ANTICIPATED TIME


The time required to perform the study depends on many factors:


• Field strength of the magnet: the higher the field strength of the magnet, the more quickly the study can be performed.

• Configuration/size of the radiofrequency coils

• Size of the area to be scanned

• Number and type of imaging sequences performed: unlike CT, MRI involves multiple separate image acquisitions. Images are obtained in multiple planes and using multiple different imaging parameters; the more acquisitions performed, the longer the study.

APPROXIMATE TIMES (Based on use of a 1.0-tesla magnet):


• Brain: 1-1.5 hours

• Spinal column:
○ Cervical: 1-1.5 hours

○ T3-L3: 1.5-2 hours

○ Lumbar: 1-1.5 hours

Because of long scan times, it is important to determine neuroanatomic localization of spinal lesions so that only the affected area of the spinal column is scanned, particularly in large dogs.



PREPARATION: IMPORTANT CHECKPOINTS



• Obtain thorough patient history: pacemaker? nonferrous metallic implants/foreign bodies? ferrous metallic implants/foreign bodies?

• Routine laboratory evaluation: CBC, serum biochemistry panel

• +/− Thoracic radiographs

• +/− Abdominal radiographs or ultrasound

• Many patients undergoing MR imaging are older and may have multiple diseases. Routine laboratory evaluation and thoracic and abdominal screening may be indicated in these patients for two reasons: to rule out the presence of concurrent disease that may affect the patient’s ability to tolerate general anesthesia and to identify the presence of any potentially life-shortening disease process unrelated to the primary complaint (e.g., “incidental” splenic mass).

• Make sure all needed equipment is available and functional. This is especially important if the study is to be performed at an outside (e.g. nonveterinary) facility.

• Preparation for general anesthesia:
○ Routine preanesthetic fasting

○ Place IV catheter: when placing the catheter, the anticipated position of the patient within the MRI unit must be considered, and the catheter should be placed to allow easy access to it during the procedure. For example, in patients undergoing MR scan of the head, it is convenient to have the IV catheter placed in the saphenous vein. This is, however, not an absolute requirement, because the forelimb can be extended caudally for ease of access if the catheter is in a cephalic vein.

○ Clipping of hair in areas where monitoring devices are to be applied (dependent on type of monitoring devices used)


image MAGNETIC RESONANCE IMAGING SCAN A-B, T1-weighted magnetic resonance images from a normal (A) and abnormal (B) canine brain. Images are from rostral to midcerebrum. In MRI, the imaging characteristics of tissues are determined by the imaging sequence used. To define the tissue characteristics of a lesion, multiple different imaging sequences are performed, and the appearance of the lesion in these images is compared. The term intensity is used for describing the appearance of tissues in MRI.

In the normal patient, there is a subtle difference in intensity between gray matter and white matter of the brain, and intensity of muscle is different from that of brain. This illustrates the level of soft-tissue contrast achieved with MRI.


In the abnormal patient, note the large, round hypointense lesion with a slightly hyperintense rim within the right ventral cerebrum; the lesion causes left deviation of falx cerebri. Differential diagnoses for mass lesions within the brain include neoplasia, granuloma formation, and abscess formation. In this case, the appearance of the lesion in multiple imaging sequences was consistent with a diagnosis of neoplasia with a central necrotic component.



POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID



• Complications associated with general anesthesia

• Burns induced by placement of cables
○ Objects within the magnet can become hot due to the effect of radiofrequency pulses. Monitoring cables within the magnet should not be coiled and should not be placed on the patient. Wrapping the cables in fabric (towels, blankets, etc.) will provide further protection.

• Injury to the patient and/or damage to the magnet can occur if careful attention is not paid to basic MR safety as regards the presence of metallic objects within range of the magnet. See above.


PROCEDURE



• Anesthetize and intubate the patient.
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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on M

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