Page 44 - Feline diagnostic imaging

38 4 Nuclear Imaging
newer imaging modalities have replaced many of the early
applications of scintigraphy, including brain scans, liver/
spleen scans, perfusion and ventilation lung scans, as well
as cardiac scans to determine ejection fraction and diag-
nose left‐to‐right shunts.
The procedures most often used in feline patients today
are thyroid scintigraphy, renal scintigraphy (GFR), and por-
tosystemic shunt (PSS) scintigraphy. Lymphoscintigraphy
and skeletal scintigraphy also remain clinically applicable.
Specific procedural details for these procedures will not be
provided here. For those interested in performing the proce-
dures or in more background information, the most com-
prehensive reference is the Textbook of Veterinary Nuclear
Medicine [7] published by the American College of
Veterinary Radiology (ACVR) and available for purchase on
the ACVR website at www.acvr.org. New developments in
nuclear imaging continue to be reported in veterinary scien-
tific publications.

4.2 Thyroid Scintigraphy

Feline hyperthyroidism has been diagnosed with increased
frequency in recent years. This may indicate a true increase
in frequency of the disease but inclusion of thyroid hor-
Figure 4.1 A gamma camera set up for veterinary imaging. The mone assay in laboratory wellness profiles for cats has also
rectangular scintillation crystal is behind the collimator on the contributed to increased detection. Laboratory results from
face of the detector. Cats are small enough to be imaged while annual physical examinations often provide early diagno-
lying directly on the detector face. Dogs may be imaged on a sis of hyperthyroidism before evidence of clinical signs,
radiolucent tabletop positioned over the detector. The detector
can be rotated to any angle so that patients may be imaged allowing early and perhaps more effective treatment [8].
from above, below or horizontally. This particular detector is Hyperthyroidism has been treated with iodine‐restricted
131
mounted on a ceiling-suspended gantry so that it can be raised, diets, antithyroid medications or I injection. Radioiodine
lowered, and moved around the room to facilitate imaging of is generally considered the most effective treatment
standing equine patients.
because of its high rate of control with a single dose [9].
Dietary management may not completely control the con-
In the 1960s, it was discovered that technetium‐99m dition and antithyroid medications require daily or twice‐
( 99m Tc) mimicked the physiologic distribution of 131 I, had daily administration for the life of the patient.
ideal properties of short half‐life (six hours) and pure gamma Radioiodine for hyperthyroid cats is typically adminis-
emission (140 kv) that were excellent for scintigraphic imag- tered by subcutaneous injection at doses ranging from
ing, and could be made readily available in commercially 2 mCi (millicuries) to 6 mCi. The dose is determined by a
produced generators. It also turned out that technetium was number of factors including clinical signs, palpation of
easy to combine with other compounds to produce radiop- thyroid enlargement, thyroxine (T4), free thyroxine (fT4),
harmaceuticals for imaging other organs and tissues. Many and thyroid‐stimulating hormone (TSH) levels, renal sta-
other radionuclides have been applied in nuclear imaging tus and results of 99m Tc scintigraphy, including the number,
but 99m Tc remains the most commonly used. size and site(s) of thyroid uptake as well as quantitative
Nuclear imaging came into practical use in veterinary determination [10] of thyroid/salivary ratios, thyroid/back-
medicine in the late 1970s. A large number of clinically ground ratios, and percent dose uptake. Evidence that
applicable scintigraphic procedures have been developed hyperthyroidism increases glomerular filtration rate (GFR)
and used over the years [4–7]. Those procedures have been and can mask renal dysfunction and also that hypothyroid-
well documented in veterinary publications. Their use pre- ism adversely affects renal function in older cats has
dated the development of ultrasonography, computed increased concern for overtreatment as well as undertreat-
tomography, and magnetic resonance imaging. Those ment of the disease [11].