Page 315 - Cardiac Nursing

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CHAPTER
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N N N N Nuclear, Magnetic Resonance, and
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C C C Computed Tomography Imaging
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Laurie A. Soine / Peter J. Cawley
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M Many advances have been madde in no invasiive ca diiovasc lular im- th the most common traccerss used in PETT imaging to assess blood
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ag agingg over the lastt 20 years. Fueled byy scientific advances inn the e flow; fluorinne-18-2-fluoro-2-deoxyglucose is used in PET imaging
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fields of computer technology, processing and storing images, to assess myocardial metabolismm.
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ph b tr a nd e ng in ee ri ng si di I Injjectiion and detection of these radiotracers allow for the
hysiics, biio hchemiistryy, and engineering noniinva ive cardiovascullar
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imaging has become a widely available clinical tool. Nuclear car- measurement of relative myocardial blood flow. The radiotracers
diology, magnetic resonance imaging (MRI), computed tomogra- follow blood flow and are extracted from the blood pool by my-
phy (CT), and ultrasound (echocardiography, or echo) are differ- ocytes in proportion to blood flow. The mechanism by which the
ent imaging modalities able to create tomographic images of various radiotracers, used in SPECT and PET imaging, are ex-
cardiovascular structures. A tomogram is an imaging “section” or tracted from the blood stream and taken up by myocytes varies.
“slice” of an object created by an imaging modality. In SPECT imaging, the radioactive decay of tracer is detected out-
side the body as scintillation (flash of light) by a gamma scintilla-
tion camera. The camera functions as a scanning device (large
NUCLEAR CARDIOLOGY Geiger counter) to detect the distribution of radioactivity emit-
ting from the myocardium through the chest wall, allowing ex-
amination of myocardial structure and function. 201 Tl is a potas-
Radionuclides, substances that emit radioactivity, have been used
as a tracer in the body for more than 65 years. Since the develop- sium analog. Because of the dynamic equilibrium of potassium
201
ment of the -ray camera by Anger some 30 years ago and the in- between cells and the blood pool, potassium and therefore Tl,
troduction of radioactive potassium analogs, the use of radionu- distributes in the myocardium in proportion to the blood flow.
201
clides (radiotracers) to study the heart has been the subject of Myocytes with an intact sodium–potassium pump take up 201 Tl.
increasing clinical application. The introduction of new and im- However, the relatively low energy (60 keV) emitted by Tl
proved radioisotopes and imaging techniques in the 1980s led to makes imaging of this tracer suboptimal in large patients. The
widespread clinical application. technetium 99m-labeled radiotracers passively distribute across
sarcolemmal and mitochondrial membranes and remain
intracellularly bound. The relatively higher energy (140 keV)
Radioisotope Pharmaceuticals emitted by this group of tracers allows for improved transmission
through the chest tissue resulting in improved image quality, par-
Radionuclides are atoms in an unstable form. They have a finite
probability of spontaneously converting to a more stable configu- ticularly in large patients.
ration. When they do so, small amounts of energy in the form of PET imaging likewise allows for quantitative and tomographic
rays are emitted. The rate at which atoms in a given sample un- imaging of myocardial perfusion and metabolism without intrin-
dergo this conversion is denoted by the half-life, the time required sically altering these processes. A positron-emitting radiotracer
for one half of the sample to undergo the conversion. Half-lives of travels only a short distance in tissue prior to encountering an
radioactive substances vary from a fraction of a second to a mil- electron. This interaction causes annihilation of both particles,
lennia; the half-life for any given radionuclide is always the same. producing two high-energy photons that depart at an angle of
The characteristics of an ideal radiotracer to assess myocardial 180 degrees from each other. PET imaging systems are designed to
blood flow would include: a half-life long enough to allow for detect the two photons, which travel in opposite directions at es-
convenient imaging, easy combination with biologic substances, sentially the speed of light (511 keV). By measuring the time that
100% myocardial extraction across the entire spectrum of achiev- it takes for each photon to encounter the circumferential ring of
able or inducible coronary blood flow states, instantaneous intra- detectors, localization of the event can be mathematically derived.
cellular binding, low extraction and clearance by organs adjacent
to the heart, and extraction by only viable cells. Unfortunately, to Diagnostic Indications for
date, no commercially available radiotracer of myocardial perfu- Nuclear Imaging
sion meets all of these criteria.
Single-photon emission computed tomography (SPECT) and Table 14-1 summarizes the diagnostic uses of the imaging modal-
positron emission tomography (PET) are the two most common ities discussed in this chapter. Stress myocardial perfusion with
nuclear cardiovascular imaging modalities. The three most com- SPECT and PET imaging is useful clinically for the detection of
mon radiotracers used in SPECT imaging are thallium 201 flow-limiting coronary artery disease and risk stratification for pa-
tients with known coronary heart disease. The clinical aim ofof
technetium
two
Tc
labeled
99m
Tl)
( ( 201 Tl) and the two technetium 99m-labeled tracers:tracers: 99m Tc- tients with kno wn cor onar y hear t disease 1 The clinical aim
the
and
sestamibi (Cardiolite) and 99m Tc-tetrofosmin (Myoview). Nitro- these studies is to evaluate the physiology of coronary blood flow.
gen-13-ammonia, oxygen-15 labeled water, and rubidium-82 are Myocardial perfusion images provide the clinician and patient
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