Thoracic Imaging 2003 - Society of Thoracic Radiology

TUESDAY
150
Nuclear Cardiology Update
David K. Shelton, M.D.
Professor, University of California Davis, Medical Center
Cardiovascular radiotracers were first utilized in the 1927 by
Blumgart and Weiss to measure cardiovascular transit times.
Then in 1948 the first clinical success for scintigraphic evaluation
of cardiac pump function was accomplished by Prinzmetal
who introduced the “radiocardiograph”. Radioactive potassium
was introduced for myocardial perfusion at rest and under stress
in the 1970’s. This was followed by numerous studies involving
the key potassium analogue, thallium-201 as a myocardial perfusion
agent. The 1980’s brought the development of several
new technetium based radiopharmaceuticals for myocardial perfusion.
There were also new tracers for the evaluation of
myocardial metabolism, myocardial innervation and acute
myocardial necrosis. During this decade, there was also the
development of tomographic imaging techniques utilizing single
photon emission computed tomography (SPECT) and positron
emission tomography (PET). The 1990’s saw the development
of additional technetium based radiotracers, new PET radiotracers
and multiheaded SPECT cameras. The subsequent development
of ECG gated SPECT techniques allowed the evaluation of
cardiac function and wall motion in addition to myocardial perfusion
imaging.
Now at the beginning of the new millenium, most nuclear
cardiology studies are performed for the assessment of myocardial
perfusion imaging utilizing thallium-201, Tc-99m sestamibi
or Tc-99m tetrofosmin in association with ECG gated SPECT.
The utilization of gated blood pool radionuclide ventriculography
(MUGA) and first pass imaging has dramatically decreased.
Infarct imaging with technetium 99m pyrophosphate or
antimyosin antibodies, innervation studies utilizing I-123 MIBG
and fatty acid metabolism utilizing I-123 BMIPP are limited in
numbers and are primarily being utilized at research centers.
RADIOTRACERS:
Thallium-201 remains a highly utilized and highly effective
radiotracer which is a potassium analogue transported across the
cell membrane by the sodium/potassium pump system. It has a
high initial myocardial uptake, proportional to blood flow which
is increased during stress conditions approximately 5 times that
of rest conditions. After its initial myocardial uptake, thallium
begins to wash out and will reach equalibrium with the blood
pool effect. The stress study can demonstrate hypoperfusion of
myocardium distal to a significant coronary stenosis. This can
be followed by 4 hour delayed imaging with thallium redistribution.
Reversible defects indicate ischemic and viable myocardi-
um. Thallium’s long half life of 73 hours can be advantageous
for further delayed imaging at 24 hours to differentiate areas of
critical stenosis with associated hibernating myocardium.
Disadvantages of thallium include its lower energies at 73-81
keV as well as its longer half life which requires a lower prescription
dose at 3-4 mCi. The introduction of technetium
based radiotracers has offered the advantages of the monoenergetic
140 keV higher energy which is associated with fewer
problems due to attenuation, as well as improved imaging properties
for modern SPECT cameras. The shorter half life of 6
hours for technetium also allows for higher doses to be administered
safely in the 10-30 mCi range. Technetium teboroxime
(Cardiotec) is a highly lipophilic cation which has the highest
myocardial extraction rate of all the technetium radiotracers.
However, because of its fast washout rate, its clinical utility for
post stress imaging has been very limited. Technetium sestamibi
(Cardiolite) was approved for clinical use in 1989 and
has had widespread clinical utility. It is a monovalent cation
with hydrophilic properties and is also very lipophilic, facilitating
entry into myocardial cells. Once sestamibi has entered the
myocyte, it remains trapped with very little washout over 4
hours. Technetium tetrofosmin (Myoview) is a monovalent,
highly lipophilic cation. Its uptake is also bloodflow dependent
and like sestamibi its greatest concentration is in the mitochondria.
Also like sestamibi, once in the myocyte there is very little
washout. One advantage of tetrofosmin is that there is rapid
clearance from non-cardiac structures, especially the liver.
Two new technetium based radiotracers are currently undergoing
clinical trials. Tc-furifosmin has properties that are similar
to Tc-tetrofosmin. Tc-NOET is a neutral lipophilic myocardial
perfusion agent with a very high extraction fraction over a
wide range of flow. It appears that Tc-NOET has many kinetic
and imaging properties similar to thallium-201 but with the
advantage of high photon flux and higher energy. Tc-NOET
does have significant washout and redistribution over time due
to the absence of intracellular binding and to the high circulating
blood levels of this radiotracer.
Table 1 provides a listing of most of the commonly utilized
cardiac radiotracers and those radiotracers being utilized in
research. There is strong interest in the neuroreceptor imaging
agents, the fatty acid metabolism agents and the ability to image
myocardial hypoxia directly, as well as myocytes undergoing
apoptosis. Research interest is also strong in imaging developing
arterial plaque and vulnerable plaque.