Thursday, July 12, 2007

Nuclear Stress Test

Patients that feel chest pain or any sort of discomfort that might be related to the heart are usually told to get a nuclear stress test done. There are a few different ways of performing a nuclear test, but in the end, they all asses the same thing - the distribution of blood in the heart. The test is not always necessary since there are apparently other ways of assessing a problem in the heart. For example, if a patient complains about chest pain, a doctor can take an EKG, notice a drop in the ST segments and then deduce that the patient probably had a heart attack. One could also underscore this claim with a blood test - looking for increased levels of troponin. The list goes on….

Fortunately, a nuclear stress test offers the clinician a quick way of assessing common problems in the heart. As a result, most patients with heart related problems will most likely have one done. The idea is simple; inject the patient with a radioactive isotope, thalium201, which gets concentrated in the tissues of the heart receiving blood supply. Then use a gamma scintillator camera to read the signal from different angles of the body. In the end, the doctor receives a low spatial resolution image that depicts portions of the heart that receive less blood as dark spots while others that receive a lot of blood as bright spots (figure).

Unfortunately, the signal detected by the gamma scintillator camera is not calibrated to an absolute scale. In fact, since the signal is relative, the doctor cannot say much if he observes dark spots in regions of the heart where one would intuitively expect bright spots. Evidently, one can claim that the dark spots are due to below normal levels of perfusion while someone else can claim that the dark spots are normal since the bright spots are due to above normal levels of perfusion. Partially for this reason, after obtaining images of the heart at rest, the patient is told to exercise, so that the blood supply can be maximized at all portions of the heart. For patient’s that can’t exercise, the alternative is to be injected with a vasodilator (dipyridamole (Persantine), adenosine (Adenoscan), or dobutimine). Shortly following, the patient is injected with another isotope, Technetium -99m pyrophosphate or Mibi, in the case of a dual isotope scan. Each isotope peaks at a different energy level (in keV) so the two are easily filtered and thereby differentiated. In terms of differentiating between the two sets of images (one at rest and the other under stress), one must rely on a little nuclear physics. The radiation emitted by mibi is usually stronger and scatters less; thus, the spatial resolution of the set of images of the heart under stress is usually better. Furthermore, mibi is filtered by the body differently form thalium. So another way of differentiating between the two sets of images is to see where they are concentrated in the human body; typically either the kidneys or the gall bladder.

Anyway, the images of the heart at rest and under stress make the claims about the distribution of blood in the heart more plausible. They can help the doctor identify coronary artery disease or other potential anomalies in the heart. Yet, this imaging modality can use some improvement. I used to wonder why it was not possible to integrate the resolution of CT with the perfusion identifying capabilities of a NST. Fortunately, I recently discovered that there has been some work underway on this and one might see a device/software that does just that in the near future.

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