Nuclear medicine (also known as "un-clear" medicine) is a valuable and rarely used imaging tool. Unlike radiographs, where a beam of external radiation is directed at a patient, in nuclear medicine a radioactive substance is injected into the patient. The patient then acts as the "source" of radiation. Radiation that is emitted from the patient is collected and an image created. Because of the scatter and overlap of photons, the spatial resolution is much much poorer than radiographs.

The advantage of nuclear medicine lies in our ability to tag radioisotopes so that they are concentrated by specific cells. This allows us to easily image either a whole body or a large region of the body and find concentrations of the radioisotopes. This leads us to areas that we can do follow up imaging on, if required, for further evaluation.

By far the most common radioisotope used for traditional nuclear medicine is technetium 99m (Tc99m), which is then labeled with various compounds to allow for imaging specific disease. Tc99m has the advantage of being easily transported in a stable form, short half-life, low energy and possessing chemical properties that make it valuable for imaging things like the thyroid gland.

Some of the more common nuclear scintigraphy studies are discussed below, as well as their importance to cancer imaging.

Bone Scintigraphy

 Uses Tc99m-MDP or Tc99m-HDP

 Both forms very sensitive to areas of active bone turnover; not specific (degenerative changes, bony metastasis, osteomyelitis)

 Is not picked up by purely lytic lesions (i.e., multiple myeloma or other round cell tumors)

 Used primarily for cancer staging to look for bony metastasis (reportedly seen in ~5% of animals, we think is actually higher)

 Can also be used to differentiate between bony or soft tissue causes of lameness, and to evaluate bony healing

Thyroid Scintigraphy

 Can use just Tc99m; this is a "halide" meaning it acts like chloride or iodine (these are in the same column in the periodic table)

 Can also use I-123; better target to background ratio but more expensive and longer half life

 Both radioisotopes concentrated in the thyroid gland via the Na-I symporter; eliminated via feces, urine and glandular secretions

 Can help us determine if dealing with unilateral or bilateral hyperthyroidism, ectopic thyroid tissue or even thyroid carcinoma, and thyroid carcinoma metastatic disease

 If an active thyroid tumor (not common in dogs) can then use high doses of I-131 to treat (i.e., theranostics, the combination of therapy and diagnostics to create precise medicine)

Glomerular Filtration Rate

 Use Tc99m-DTPA, Tc99m-MAG3, or I131-OIH; all of these substances are secreted in the urine without any filtering

 GFR is best way to determine renal function non-invasively (inulin excretion is gold standard, but requires serial blood and urine sampling)

 Can quantify individual kidney function

 Can detect renal dysfunction prior to a rise in blood parameters (BUN and creatinine)

 Important to assess function of contralateral kidney if planning a nephrectomy

Portal Scintigraphy

 Uses Tc99m-mebrofenin or Tc99mO4-

 Administered per rectum or transplenically

 In a normal dog will be collected in portal vein and go to liver first, then heart

 In a dog with a congenital portosystemic shunt, or multiple acquired portosystemic shunts, radioisotope is seen in the heart either before or at the same time as the liver

In conclusion, nuclear medicine can be a valuable tool in the right hands to assist with a specific diagnosis. We are limited to a certain extent by poor resolution, so it usually has to be combined with some other form of imaging.