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Providence, RI
Dr. Afshar is a Clinical Instructor of Medicine, Brown University. Dr. Tilkemeier is Associate Professor of Medicine, Brown University, and Director of Nuclear Cardiology, Miriam Hospital.
Correspondence to: Peter Tilkemeier, MD, The Miriam Hospital, 164 Summit Ave, Providence, RI 02906
There are many methods for evaluating left systolic ejection fraction and regional wall motion. Gated single photon emission CT (SPECT) imaging is the most frequently used method used in nuclear cardiology for estimating ejection fraction. The majority of nuclear cardiology laboratories in the United States perform gated SPECT imaging routinely in all patients who are referred for myocardial perfusion imaging, with an estimate of > 90% of all images acquired with gated SPECT. It takes minimal additional effort to obtain gated images without the need for additional hardware or an increase in image acquisition times while providing important additional clinical information.
Gating has become a valuable tool in nuclear cardiology because of its ability to yield more accurate diagnostic and prognostic information through increasing specificity. This is particularly useful in the setting of differentiating attenuation from infarction. Many authors have shown this benefit when assessing "fixed" perfusion defects (possible infarction) that have preserved wall motion. In this issue of CHEST (see page 778), Lapeyre et al also have demonstrated this benefit in a healthy population that had decreased inferior wall counts in men compared to women, which is suggestive of diaphragmatic attenuation.
Quantification of Gated SPECT Imaging
Most gated SPECT quantification methods result in absolute volumes. Currently, there are several automated and semi-automated algorithms that are available for the quantitative measurement of myocardial left ventricular (LV) function using gated SPECT scan images that require minimal operator interface.
Some of these methods use epicardial/endocardial borders, and others use count, volume, and geometric fitting. Finally, a partial volume effect has been used to estimate the wall thickening and LV ejection fraction. The method described by Lapeyre et al has excellent reproducibility with minimal interobserver/intraobserver variability. Important variations in septal contraction patterns have been validated in this and other studies. Lapeyre et al also emphasized the importance of a fixed-center method compared to a variable-center method for the assessment of regional wall motion and function, particularly among patients who have experienced infarctions.
Using gated imaging for the assessment of normal myocardial contraction, Sharir et al1 reported a progressive decline in wall thickening going from an apical to a basal region. They also showed that there is variability in circumferential LV wall thickening and heterogeneity in endocardial wall motion in the normal myocardium during contraction. By assigning specific cutoff parameters for thickening and contraction in each myocardial region, the sensitivity and specificity for the detection of regional myocardium dysfunction was much improved in their study.
In the study by Lapeyre et al, the wide range in ejection fraction values for individual sections of normal myocardial slices was speculated to be due to a number of parameters, including the following: patient motion during the acquisition time; intrinsic properties of the LV; variability in the myocardial edges after the enlargement of images and image processing; patient hemodynamics and medication effects; and, finally, subdiaphragmatic activity that might affect edge detection, mostly in the inferior wall. This variability is important when utilizing this technique in patients with impaired regional motion following infarction as well as when comparing the gated SPECT scan results to those from other modalities such as echocardiography (Lapeyre et al) or MRI and CT scan images.
Gated SPECT Images in Postinfarct Patients
The value of obtaining myocardial perfusion images in patients after they have experienced a myocardial infarction has been well established. Also, there has been many studies showing the close relationship between LV ejection fraction and event-free survival in patients after myocardial infarction. ECG gating of myocardial perfusion images provides the additional ability to assess the severity of the infarction using wall motion and wall thickening, while also providing information regarding the remaining ischemic territories and global as well as regional LV systolic function. The combination of all these data can augment the clinical value of perfusion imaging, and can result in more precise diagnostic and prognostic information without the added expense of additional imaging studies and with no additional time or radiation exposure to the patient. The simultaneity of a single resting gated SPECT scan image in a postinfarct patient can give information regarding both infarct size, as well as regional and global systolic function, as demonstrated in the study by Lapeyre et al.
The addition of data on regional ejection fraction in patients following thrombolytic therapy for infarction can provide valuable clinical information for the prediction of future events in this population of patients. Regional function is another clinical marker, in addition to perfusion defect size, that can determine the success of reperfusion therapy and can contribute to the long-term prognostic data in these patients.
The quantitative assessment of regional contraction and wall thickening by gated SPECT imaging can assist in the follow-up of patients with myocardial dysfunction and contributes to the ongoing evaluation of medical or invasive therapy in these patients.
Limitations of SPECT Imaging
Despite the tremendous clinical advantages of gated SPECT imaging, there are limitations to the technique that must be considered when acquiring and interpreting the images. Most errors in gated SPECT can images can be attributed to the following:
Conclusion
Gated SPECT imaging has developed to provide useful functional information. It has evolved into a valuable clinical adjunct to SPECT imaging. It not only improves the test specificity but also adds incremental clinical value to the study. Lapeyre et al have demonstrated the additional benefit of SPECT imaging in evaluating infract size and regional function in patients following thrombolytic therapy.
The gated perfusion images are easy to obtain, reliable, and highly reproducible without incurring significant additional cost or patient risk. They provide information about the global function and regional contraction simultaneously with perfusion imaging with minimal limitations, and increase the ability to provide accurate diagnostic imaging in healthy and ischemic patients, as well as in patients who have experienced myocardial infarction.
References
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