Thoracic Imaging 2003 - Society of Thoracic Radiology

TUESDAY
154
Imaging protocols:
Utilizing the physical and physiologic properties of thallium
and technetium radiotracers allows the physician to establish
several different MPI SPECT protocols. For a one day protocol,
thallium can be injected at peak stress followed by imaging
within 15 minutes. Due to its washout, blood pool, and redistribution
effects, the redistribution imaging can be accomplished at
3-4 hours. This amount of time may not be enough for critical
stenosis and thus further delayed imaging at 12-24 hours can be
accomplished to detect viable myocardium, even in those
patients with 100% stenosis. Alternatively a 1-1.5 mCi thallium
booster dose can be given prior to the 4 hour redistribution
imaging, thus eliminating the necessity for most of the more
delayed imaging.
An alternative one-day dual isotope study can be accomplished
with a resting thallium scan (3-3.5 mCi) followed
almost immediately by a stress technetium radiotracer scan.
This protocol matches the best properties of thallium and technetium
and allows for the shortest delay time between rest and
stress imaging. It also allows 24 hour delayed redistribution
images in those cases requiring viability assessment. The main
drawback to this protocol is that it does involve 2 different
radiotracers with different imaging characteristics.
For technetium sestamibi and technetium tetrofosmin, one
can utilize a two-day protocol or a one-day protocol. The twoday
protocol would usually involve stress testing with the technetium
radiotracer injection, and imaging which is usually
accomplished at 30 minutes for tetrofosmin and 60 minutes for
sestamibi. The patient returns a second day if necessary for the
same 25 mCi technetium dose and rest scan imaging.
For a one-day protocol with sestamibi or tetrofosmin, a 10
mCi rest injection, followed by SPECT imaging is usually
accomplished. This is followed by a 3-5 hour delay before
stress testing and radiotracer infusion with 25-30 mCi of technetium
agent. The lower resting dose, lower myocardial blood
flow and decay time delay allow for the most accurate 1 day
protocol with the least influence of the rest or stress imaging on
each other.
Viability:
The detection of viable myocardium is of extreme importance
in prognostic assessment and in choosing the appropriate
therapy for a given patient. “Hibernating myocardium” usually
refers to myocardium with high grade stenosis with resulting
chronic hypoperfusion, altered metabolism from free fatty acid
metabolism to glucose metabolism and hypokinesia or akinesia
of that myocardial segment. “Stunned myocardium” usually
refers to myocardium which has experienced prolonged
ischemia/recovery with resulting hypokinesia or akinesia.
Prolonged, stunned, myocardium can occur after myocardial
infarction with remaining viable myocardium. In the post injury
setting, so called “reverse redistribution” can indicate higher
uptake of thallium post stress with worsening of the perfusion
abnormality on delayed imaging. This usually occurs in those
patients with viable myocardium which has experienced prolonged
ischemia and damage to the sodium potassium pump and
thus an inability to maintain the myocardial thallium concentration
levels over the redistribution time period. In the setting of
recent injury, reverse redistribution can be a good prognostic
sign demonstrating reestablishment of blood flow and viable
myocardium.
With sestamibi or tetrofosmin, visible wall motion and more
importantly, visible wall thickening and brightening indicate
viability. Because there is no significant redistribution however,critical
stenosis with resulting hibernating myocardium, may
demonstrate fixed defects during rest and stress. This may
make detection of viable myocardium difficult for these agents.
Administration of nitroglycerin prior to the infusion of the technetium
radiotracer has been shown to improve the resting
uptake, and thus improve detection of viable myocardium. The
standard thallium protocol to detect viable myocardium is best
accomplished with a 3.5 mCi resting injection followed by 12-
24 hour redistribution imaging. The resting SPECT images can
be ECG gated, although the delayed ECG gated imaging is
more difficult to obtain. PET imaging with perfusion radiotracers,
such as 13 N-ammonia or Rubidium-82 can be followed by
F-18 fluorodeoxyglucose. The perfusion tracers will demonstrate
hypoperfusion secondary to stenosis where as the FDG
images will demonstrate normal or increased FDG uptake in
those hypoperfused segments. The combined use of a PET flow
tracer and FDG metabolic tracer is considered the gold standard
for viability assessment. The amount of viable myocardium
detected is predictive for functional improvement after revascularization
and is also coupled with reduced peri-operative risk
and improved long term outcome.
Acute Ischemic Syndrome:
Sestamibi and tetrofosmin given as a resting injection may
show regionally reduced perfusion which is a sensitive marker
of jeopardized myocardium. Recent studies on chest pain
patients in the E.R. have demonstrated the efficacy of MPI
SPECT imaging to detect those patients with acute ischemic
syndromes. If the patient’s MPI SPECT imaging is normal it
has been shown that the patient may be safely discharged for
further evaluation of their chest pain on an outpatient basis. If
the MPI SPECT imaging demonstrates hypokinesia or reduced
perfusion, the patient is admitted for observation to rule out
myocardial infarction. This form of triage on E.R. chest pain
patients has been shown to be highly efficacious as well as cost
effective.
Pre-Operative Risk Assessment:
MPI SPECT imaging has been utilized effectively to predict
those patients at high risk for cardiac events during major noncardiac
surgery. These patients are typically diabetics, patients
with prior CABG, or known CAD who are being evaluated for
non-cardiac vascular surgery. A positive MPI SPECT scan indicates
significant underlying CAD and high risk, whereas a negative
scan is predictive of low risk. In general, a negative
myocardial perfusion SPECT scan is associated with less than
1% risk for cardiac event or cardiac death per year.
CONCLUSION:
Nuclear imaging techniques for the heart provide a highly
efficacious and cost effective technique for detecting CAD, evaluating
the significance of known CAD, detecting viable
myocardium and triaging patients with suspected ischemic cardiomyopathy
for medical therapy, reperfusion techniques,
CABG or cardiac transplantation. Further evaluation of metabolic
agents for fatty acid metabolism, direct ischemic labeling,
apoptosis and neurocardiac imaging should bring promising
results. Direct imaging of developing atheromatous arterial
plague is also an exciting development.