Page 159 - Canine Lameness

10.6 ­pclear Medicine 131

Intravenous contrast media are frequently used with MR and CT imaging to improve contrast reso-
lution and to evaluate the blood flow (i.e. perfusion and vascularity) of tissues. This can help differ-
entiate different pathologic processes (e.g. neoplastic versus degenerative). The type of intravenous
contrast agents differs between MR and CT. Specifically, MR utilizes gadolinium based while CT
utilizes iodinated contrast media.
Standard imaging sequences obtained in MR include T1, T2, and proton density (PD)‐
weighted sequences, which demonstrate the molecular differences in various tissues and can
detect abnormalities due to differences in tissue appearance in the sequences. MR terminology
uses the term “intensity” to describe tissue characteristics and appearance on various sequences,
whereby tissues that are bright are described as hyperintense; dark tissues as hypointense; and tis-
sues of a similar intensity as isointense. Fluids are typically hypointense on T1‐weighted images,
hyperintense on T2‐weighted images, and of intermediate signal intensity on PD‐weighted images.
An additional technique often used in MR imaging is called “fat saturation.” This technique makes
fat appear hypointense on T1‐, T2‐, and PD‐weighted images and can highlight inflammation and
edema in tissues.

10.6 Nuclear Medicine

In the past, nuclear medicine imaging in small animals was limited to an imaging modality
described as bone scan or bone scintigraphy. This technology uses a gamma camera that captures
gamma radiation emitted by specific radiopharmaceuticals (typically 99m Technetium methylene
diphosphonate) after intravenous administration, thereby highlighting areas of increased uptake.
It has been found to be helpful in dogs with obscure lameness by several authors (Schwarz et al.
2004; Samoy et al. 2008). Scintigraphy localizes areas of increased radiopharmaceutical uptake due
to inflammation or neoplasia; however, it does not provide a specific diagnosis. Therefore, further
structural imaging of the identified areas is always needed to establish a diagnosis.
Currently, the newest imaging modality for evaluation of the musculoskeletal system is PET
imaging combined with a conventional CT, termed PET/CT (Figure 10.6). PET is a form of nuclear
medicine that uses radiopharmaceuticals that are positron emitters. Positrons are positively
+
charged particles, also called beta + particles, or β . These particles travel a short distance (1–2 mm)
before colliding with a negatively charged electron. When the two collide, two annihilation gamma
photons are created and travel 180° from each other. The special detectors in a PET scanner detect
and register these coincident photons – photons that are emitted 180° from each other and arrive
within a few nanoseconds of each other. Fluorine‐18‐fluorodeoxyglucose (FDG), an analog of glu-
cose, is widely used for PET imaging in human medicine and veterinary oncology. FDG is charac-
terized by uptake and retention by hypermetabolic cells, hence it is frequently used for diagnosis
of cancer and metastatic disease; however, it can also be used to evaluate muscle activity. The most
common way to quantify PET tracer accumulation is by standardized uptake values (SUVs), and
the tissue activity concentration is normalized by the fraction of the injected dose/unit weight
(Kinehan and Fletcher 2010).
When this technology was first developed, PET/CT acquired a PET scan and CT scan on separate
machines at different times. However, advances in machine technology now provide the ability to
acquire images using a dual PET/CT scanner as part of one imaging exam. The CT images offer
excellent anatomic depiction of normal and abnormal structures, while PET with FDG identifies
areas of high glucose metabolism. Once obtained, CT and PET images are compared, side by side
and fused, to determine if areas of noticeably high metabolic activity are normal or abnormal on