Thallium is a radioactive agent which is
taken up my the myocardium (heart muscle) in proportion to blood flow.
Sestamibi (Cardiolite) is another agent used in nuclear cardiology. With
thallium the radioactive material is injected during peak exercise and
images are obtained within 5 minutes of completion of exercise. These
images represent the blood flow to the heart muscle during stress or
exercise. Images are then obtained 3 to 4 hours later, representing the
blood flow at rest. With sestamibi, the radioactive material is injected
at rest, the patient is asked to walk for a few minutes and then rest
scans are obtained. The stress test is then performed and sestamibi is
again injected at peak exercise, with stress scans performed several
minutes later.
Normal
Scan
The
top line are scans at rest, the second line are scans after stress
testing. The yellow color represents good blood flow to the heart
muscle.
Normal
Anatomy
Short
Axis
anterior wall
septum 
lateral
wall
inferior wall
Horizontal
Long Axis
apex
septum 
lateral wall
Vertical
Long Axis
anterior wall
apex
inferior wall
Myocardial Ischemia (Reversible Defect)
In this scan, there is a
large anterior wall defect seen during stress (second line). There
is no yellow on the front wall (anterior wall) of the heart.
The rest images show
redistribution and filling in of the anterior wall defect
(first line).
Therefore, when the heart
is under stress or the demand for blood flow is increased, there is no
flow to the anterior wall. At rest, the demand decreases and there is
blood flow to this wall.
Myocardial Infarction (Fixed Defect)
In these scans, there is no blood flow to the inferior wall during
stress and at rest. This represents a fixed defect or myocardial
infarction. Since the heart muscle is necrotic, it doesn't require blood
flow.
