Front Matter

Chapter 20 Imaging in Canine Sports Medicine 505

than general anesthesia. It is also much less
expensive in most cases than the gold standard
modality for soft tissue evaluation, MRI.

Magnetic resonance imaging

Magnetic resonance imaging (MRI) is superior
to any other imaging modality for providing
exquisite 3‐D anatomic detail, especially of soft
tissue structures (Figure 20.4). Unlike other
modalities, MRI relies on physical and chemical
interactions within the tissue to obtain an
image. These interactions are primarily exploi-
tation of the physical properties of atomic
nuclei throughout the body, in particular the
hydrogen atom (Berry, 2002; Gavin & Bagley,
2009). The magnetic resonance (MR) unit pro-
duces a signal from changes in hydrogen atoms
induced by the application of the magnetic field
and radiofrequency pulses. These signals, using
Fourier transformation, generate a 3‐D image
through computer processing. The images are
produced in high resolution gray‐scale with
increased signal showing as white and decreased
signal as black. A higher proportion of hydro-
gen ions are contained within water and fat,
which constitute a large percentage of tissues
throughout the body. Most of the signal
acquired is from these components.

When an MRI study is performed, multiple

image sequences are obtained. There are

standard sequences such as T1‐weighted, T2‐

weighted, T2*‐weighted, and STIR, described

below. Many more are available and new

sequences are being developed. The combina-

tions of sequences selected will depend on the

type of MR unit being used (high field vs. low

field), the area being studied, and the type of

suspected lesion. Some sequences may be more

valuable in highlighting a specific lesion or

process. T1‐weighted, T2‐weighted, and STIR

are the most common MR sequences (Gavin &

Bagley, 2009). In T1‐weighted images, fat has

an increased or hyperintense signal and water a

relatively decreased or hypointense signal.

Anatomic detail is superior in T1‐weighted

images due to less tissue contrast. In T2‐

weighted images water has an increased signal

and fat a more hypointense signal. T2‐weighted

images are best for visualizing abnormalities,

but have less anatomic definition. In most inju-

ries, increased fluid signal on MRI is a hall-

mark of tissue damage. This increased signal

can stem from any abnormal fluid signal within

the tissue such as inflammation, edema, or

hemorrhage. STIR and other fat‐suppressed

sequences suppress the fat signal within tis-

sues and are very good for confirming or iden-

tifying pathology in bone characterized by

increased signal, as the marrow cavity contains

a significant amount of fat. Sclerosis within

bone is seen as decreased signal in non‐fat‐

suppressed sequence images.

Gadolinium‐based contrast agents can also

be used to enhance MR sequences. They are

commonly used in human and veterinary med-

icine in angiography, and when assessing neo-

plasia or cerebrospinal disease for improved

visualization and characterization. Indirect

and direct arthrography are considered gold

standards for MR evaluation of the joint in

human medicine, and have been reported in the

hip, knee, ankle, wrist, and shoulder (Applegate

et al., 1993; Waldt et al., 2007; Cerezal et al., 2008;

Jung et  al., 2009; Pozzi et  al., 2009; Van Dyck

et  al., 2009; Rakhra, 2011). Indirect arthrogra-

phy is performed by intravenous injection of a

contrast agent prior to obtaining the desired

MRI sequence, whereas direct arthrography is

via injection of the contrast agent into the joint.

Differences in specificity and sensitivity for

Figure 20.4 Sagittal T1‐weighted image of the shoulder
joint. T1‐weighted images provide excellent anatomic
detail. Source: Image courtesy of Dr. Pat Gavin.