Magnetic Resonance Imaging
Musculoskeletal
Basic Information 
Overview and Goal(S)
• Magnetic resonance imaging (MRI) produces images based on the magnetic properties of the hydrogen protons within the body.
• MRI provides exquisite detail of the soft tissues. Osseous structures are also well visualized, particularly bone marrow and subchondral bone. However, due to the minimal resonance produced by normal cortical bone, periarticular proliferation and small osseous bodies can be easily overlooked compared with computed tomography (CT).
• MRI differs from CT in that multiple scan planes are acquired rather than being reconstructed from a single axial plane. This leads to much longer scanning times.
• The patient is placed within a relatively strong magnetic field (see “Equipment” below), aligning the hydrogen protons of the body.
• A pulse sequence is applied to the anatomic area of interest, and the response of the protons to that pulse sequence produces an image. Within the pulse sequence, a radiofrequency (RF) pulse is introduced into the patient from a transmit coil, energizing the tissue. As the tissue reverts to its original state, RF energy is emitted from the patient and recorded by a receiving coil.
• The differences in the magnetic properties of tissues are demonstrated by using multiple types of pulse sequence. By altering these parameters, the contrast of various tissues is altered, allowing differentiation of anatomic structures.
• Spin-echo sequences use multiple introductions of RF signal that include a 180-degree rephasing pulse. This rephasing pulse corrects for magnetic susceptibility.
• The most commonly used scanning sequences are:
T1 spin-echo: T1 recovery is a measure of the exchange of energy with the surrounding environment. T1 sequences have high signal-to-noise ratio and are more subject to small changes in the local magnetic field, making them ideal for contrast studies.
Fat saturation sequences are similar in end result to STIR sequences (decreased signal from fat). These sequences can have various names depending on the manufacturer.
Indications
• Generally reserved for patients with a localized source of lameness.
• Other imaging modalities have failed to identify a lesion believed to cause lameness or the severity or duration evident clinically.
• Lameness localized to areas that are not amenable to accurate ultrasound evaluation of the soft tissues, such as the foot or proximal suspensory ligaments.
Equipment, Anesthesia
• It is imperative that patient positioning be accurately entered into the initial patient setup to avoid mislocalization of lesions. Additional markers, commonly vitamin E capsules (used because of the bright fat signal produced), can be used to delineate right and left sides.
• Sand bags or foam wedges are commonly used to stabilize the limb and minimize respiratory motion. This is especially important with the nondependent limb because it is more prone to motion.
Low field (<1 T): The majority of low-field magnets used in equine imaging are permanent magnets that do not require cooling.
High field (≥1 T): These units require supercooling with helium. As the field strength of a magnet increases, the signal to noise ratio increases (in general) and better quality images are obtained (Table 1).• Coils: The coils used with MRI function like an antenna, receiving or transmitting (or both) RF signals. Multiple coils are in use during image acquisition performing different tasks, and multiple coil types are available to receive the signal from the patient that produces the image information.
Gradient coils: This coil system coordinates the anatomy being scanned into a matrix by producing a gradient of magnetic strengths along each axis. As the strength of the gradient increases, the spatial resolution increases (measured in mT/m) (Figure 1).
Surface coils: Receive coils that detect signal from the patient. These coils have good signal to noise ratio, with the signal degrading as distance from the coil increases.
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