Page 376 - Adams and Stashak's Lameness in Horses, 7th Edition

342 Chapter 3

NUCLEAR MEDICINE/SCINTIGRAPHY

VetBooks.ir Kurt SelBerg, elizaBeth acutt, and alejandro

ValdéS‐Martínez

Nuclear medicine techniques image the blood flow to conducted using radiolabeled polyphosphonates, which
bone as well as the function or the physiological activity have a high affinity for the Ca‐hydroxyapatite molecules
of bone with minimal anatomic detail compared with in bone. Images made at 2–4 hours’ postinjection are a
ultrasound, computerized tomography (CT), and mag­ representation of the uptake pattern in the bones. A very
netic resonance imaging (MRI) produce images that predictable uptake pattern is seen in normal animals,
reveal anatomic detail. Nuclear medicine imaging is one and abnormal radiotracer is seen with increased or
tool that may augment but does not replace the basic decreased blood flow or increased osteoblastic activity.
lameness examination. 10,58,61,65,74,80,81,95,99 This chapter Either 99m Tc‐oxidronate (HDP) or 99m Tc‐methylene
discusses the principles of, techniques of, indications for, diphosphanate (MDP) is administered intravenously at
and interpretations of nuclear medicine imaging in the a dose of about 0.35‐mCi/kg or 12.95‐MBq/kg body
evaluation of the musculoskeletal system of horses. weight. 99m Tc‐HDP has the advantage of faster soft tis­
sue clearance, thus allowing image acquisition to begin
sooner after injection. A second advantage of 99m Tc‐
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PRINCIPLES OF NUCLEAR MEDICINE HDP is improved visualization of bones surrounded by
large amounts of muscle, e.g. spine, pelvis, and hips.
Nuclear medicine imaging, also known as scintigra­ An average 450‐kg horse receives about 160 mCi
phy, is based on the functional distribution of a radi­ (5.92 GBq) of the radiolabel. The dose rate can be
otracer also known as radiopharmaceutical in the body. adjusted for age, i.e. increased by about 10% in older
The radiotracer is made of a radionuclide, most com­ patients and decreased by about 10% in juveniles
monly Technetium‐99m ( 99m Tc), and is labeled to a phar­ because of the difference in metabolic activity of bone
maceutical, which determines the target tissue of the tissue. Approximately 50% of the injected radiolabel is
radiopharmaceutical in the body. Technetium‐99m excreted in the urine, which results in the effective T
1/2
decays by emitting a 140‐kEv γ‐ray. A γ‐ray is identical being shorter than the natural T . 70
1/2
to an X‐ray, except that it originates from the nucleus of A nuclear medicine evaluation of the musculoskeletal
an unstable atom ( 99m Tc in this case) as the atom strives system may consist of three phases. Phase 1, known as
toward a more stable state. Nuclear medicine imaging the blood flow or vascular phase, represents the radiotracer
can also be described as an emission imaging technique in the blood vessels before diffusion into the extracellu­
because the image is made by γ‐rays that are emitted by lar fluid. It lasts for 1 or 2 minutes after injection. The
the 99m Tc inside the horse. Radiography is considered a body region to be evaluated must be positioned in front
transmission imaging technique because the X‐rays that of the γ camera at the time of injection, and dynamic
produce the image are transmitted through the patient. rapid frame acquisition is made as the radiolabel per­
The pharmaceutical part of the radiopharmaceutical fuses the vasculature. Multiple images are acquired over
determines the distribution of the tracer radionuclide in the first few minutes while the radiolabel is within the
the body. There are various molecules or cells that can vascular system, before diffusion into the extravascular
be labeled. Red blood cells can be labeled for the space occurs. The vascular phase is used to compare the
evaluation of the circulating blood compartment, most blood flow, especially to the distal limbs, e.g. in cases of
commonly to study cardiac function. Intravenous degloving injuries, but it can also be used to document
administration of 99m Tc‐pertechnetate ( 99m Tc0 ) or 99m Tc‐ perfusion deficits in different anatomical regions.
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labeled red blood cells ( 99m TcRBCs) is scintigraphic tech­ Phase 2, known as the pool or soft tissue phase, rep­
niques looking at the perfusion (blood flow) of soft resents the radiopharmaceutical distribution in the
tissue structures such as the joints of the distal limbs. extracellular fluid and is visualized from 3 to approxi­
White blood cells can be selectively labeled with 99m Tc‐ mately 10 minutes postinjection. This phase is used to
hexamethylpropyleneamine oxime (HMPAO) to look evaluate blood flow to soft tissues. An increased signal is
for areas of active inflammation/infection. 9,51,63 99m Tc‐ observed with hyperemia due to edema, inflammation,

labeled biotin ( 99m TcEB1) has also been used to detect etc. Increased radioactivity during the pool or soft tissue
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soft tissue inflammation in horses. Renal function can phase is best used in the distal limbs and has been asso­
be studied using 99m Tc‐diethylenetriamine pentaacetate ciated with navicular bone disease, inflamed joints, ten­
(DTPA), and quantitative hepatobiliary studies can be dinopathy, and desmopathy.
performed with 99m Tc‐disofenin. Functional lung ventila­ Early intense bone uptake of the radiopharmaceutical
tion studies have been described using aerosolized 99m Tc‐ ( 99m Tc‐HDP or 99m Tc‐MDP) can sometimes be seen as
DTPA and lung perfusion with 99m Tc macroaggregates soon as 5 minutes after injection, especially in cases of
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of albumin (MAA). Although each of these techniques intense delayed phase bone uptake, e.g. fractures or
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uses 99m Tc, the distribution of the radiotracer varies, infectious processes. This can sometimes make the eval­
based on the biokinetics of the pharmaceutical or cell to uation of soft tissues challenging at best, if not impossi­
which the 99m Tc has been labeled. ble. A scintigraphic technique for looking at soft tissues
The most common use of nuclear scintigraphic exams only, without any possibility of bone uptake, is to use
in the equine is orthopedic related. Bone scans are 99m Tc‐O (pertechnetate, unlabeled to a pharmaceutical)
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