LETTER TO THE EDITOR
Reduced global myocardial thallium uptake—an important marker of severe coronary artery disease: Reply
Timothy F. Christian, MDa
a Mayo Clinic, Cardiovascular Diseases & Internal Medicine, Rochester, Minnesota, USA
We appreciate the interest by Keeble et al. in our study concerning T1-201 scintigraphic abnormalities in patients with severe coronary artery disease (CAD) (1). Certainly inadequate exercise is a potential cause for limited perfusion abnormalities in patients with severe disease, because the development of blood flow heterogeneity is restricted. We doubt that the method proposed by Keeble et al. in estimating global thallium uptake would alter our findings (2,3). The ratio of left ventricular thallium activity to injected thallium-201 counts in the syringe will offer little new data, because the injected dose for all patients was the same (4 mCi). More importantly, the impact of individual body configuration on gamma camera photon detection for thallium-201 is large. We have previously shown (4) that detected thallium activity varies threefold to fourfold in normal myocardium even in patients of similar weight (and therefore cannot be confidently estimated). It is this factor (among others) that precludes estimates of absolute myocardial blood flow with conventional radionuclide techniques. The article by Keeble et al. showed a two-fold variability in normal volunteers for their method of estimation of global thallium-201 uptake (2). Perhaps the method proposed was helpful in improving CAD detection because planar imaging (which has lower sensitivity compared with single photon emission computed tomography (SPECT) for any CAD) was used (3). Balanced ischemia (apparently normal scans due to symmetric reduction in flow reserve in all three coronary territories) is a concept that was derived when planar imaging was common. We have previously shown that true "balanced ischemia" occurs in <5% of all patients with triple-vessel CAD using SPECT imaging (5).
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References
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1. Kwok JMF, Christian TF, Miller TD, Hodge DO, Gibbons RJ. Identification of severe coronary artery disease in patients with a single abnormal coronary territory on exercise thallium-201 imaging. J Am Coll Cardiol. 2000;35:335–344[Abstract/Free Full Text]
2. Tweddel AC, Martin W, McGhie I, Hutton I. Improved detection of coronary artery disease by estimated myocardial thallium uptake. Eur J Nucl Med. 1989;15:336–340[Medline]
3. Martin W, Tweddel AC, Hutton I. Balanced triple-vessel disease: enhanced detection by estimated myocardial thallium uptake. Nucl Med Commun. 1992;13:149–153[Medline]
4. O’Connor MK, Bothun E, Gibbons RJ. Influence of patient height and weight and type of stress on myocardial count density during SPECT imaging with thallium-201 and technetium 99m-sestamibi. J Nucl Cardiol. 1998;5:304–312[Medline]
5. Christian TF, Miller TD, Bailey KR, Gibbons RJ. Noninvasive identification of severe coronary artery disease using exercise tomographic thallium-201 imaging. Am J Cardiol. 1992;70:14–20[CrossRef][Medline]
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