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Nuclear Imaging
William Brawner

Department of Clinical Sciences, College of Veterinary Medicine, Auburn, AL, USA

4.1 Nuclear Imaging nuclear imaging systems that allow tomographic imaging.
SPECT is a system with detector heads that rotate around
Nuclear medicine is a broad term encompassing all diagnos- the patient yielding cross‐sectional images that allow
tic and therapeutic medical procedures that use radioactive assessment of depth and elimination of superimposition.
materials. Nuclear imaging includes procedures in which PET is accomplished by taking advantage of the properties
radiopharmaceuticals are administered and subsequent of positron‐emitting radiopharmaceuticals. When a posi-
images are made to determine their distribution in the tron is emitted, it immediately interacts with an electron in
patient. This ability to trace the distribution of chemical a process called annihilation in which the particles disap-
agents in the body provides images based primarily on physi- pear and equal and oppositely directed gamma rays are
ology and quantitative organ function. While not offering the emitted. The gamma rays are detected by an array of scin-
morphologic detail of medical imaging systems based pri- tillation crystals around the patient and converted into a
marily on anatomy, scintigraphy provides important infor- tomographic image. PET/CT is the latest development in
mation that those systems cannot. In nuclear imaging nuclear imaging, allowing fusion of simultaneous PET and
devices, gamma rays emitted from the patient are absorbed CT images. These are true physiologic/anatomic images
by a scintillation crystal which then emits flashes of light that combining the physiologic patterns of radionuclide distri-
are detected by photomultiplier tubes and sent as electrical bution with the anatomic detail of CT.
signals to a computer that forms an image of the distribution The high cost of nuclear imaging systems limits their use
and intensity of the radiopharmaceutical in the patient. in general veterinary practice but conventional gamma
Because the primary detector is a scintillation crystal, the cameras are available in most veterinary teaching hospitals
process is known as scintigraphy or scintigraphic imaging. and many specialty referral hospitals. Some of those insti-
The earliest device used for nuclear imaging had a small tutions offer SPECT and/or PET imaging and a few now
crystal and single photomultiplier tube on an arm that have PET/CT units available for clinical patients as well as
scanned back and forth across the patient and was known research [1–3].
as a rectilinear scanner. That is why the images are still The first radionuclide to gain extensive use in nuclear
often referred to as scans. Current systems for nuclear medicine in the 1950s and 1960s was iodine‐131 ( 131 I)
imaging are called gamma cameras. They have large crys- which is selectively accumulated by thyroid glands. It was
tals and multiple photomultiplier tubes that allow a large initially used in treatment of thyroid disease and to meas-
field of view and dynamic imaging so that the distribution ure thyroid uptake by simple radiation counts. As scinti-
131
of radiopharmaceuticals can be tracked over time in a graphic imaging developed, I was used to acquire thyroid
patient’s body following injection. Gamma cameras are the scans. Unfortunately, its high gamma ray energy was not
most commonly used systems in veterinary nuclear imag- ideal for scintillation detection. Its beta particle emission
ing (Figure 4.1). Single photon emission computed tomog- and long half‐life were good for treatment of thyroid
raphy (SPECT), positron emission tomography (PET) and tumors but delivered an unacceptably high patient radia-
PET/computed tomography (CT) are more advanced tion dose for diagnostic procedures.

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.