Key points
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The first fundamental assumption of myocardial perfusion SPECT imaging is that the radiotracer is distributed in the myocardium directly proportional to the blood flow at the time of injection.
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The second fundamental assumption of myocardial perfusion SPECT imaging is that the count value in each myocardial pixel (voxel) is directly proportional to the radiotracer concentration in the myocardium that corresponds to that pixel.
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The relative differences in count values between myocardial pixels are represented in the images as a change in either brightness (in black-and-white images) or color (in color images). This is done through the use of a translation formula (translation table) that converts the number of counts to brightness or color in the image. The usual representation is the higher the number of counts the brighter the pixel.
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Following the logic in the preceding key points, the brighter the pixel, the higher the radiotracer concentration, and the higher the regional blood flow. SPECT imaging of normal subjects should then generate a uniform (homogeneous) brightness or color in the myocardial pixels.
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It would also be reasonable to assume that if the brightness of a myocardial segment is half the brightness of another segment, the first segment receives half the blood flow of the second segment.
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Some fundamental assumptions apply to the assessment of LV regional and global function from the ECG-gated tomographic slices that represent different time intervals in the cardiac cycle. The most fundamental assumption is that our eyes and the computer techniques can detect and track the LV borders throughout the cardiac cycle as a change in intensity (or color).
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In practice, our ability to detect and track the endocardial and epicardial borders throughout the cardiac cycle is limited by radiation scatter and by the spatial and contrast resolutions inherent in the imaging systems. Because of this limitation we rely on the partial volume effect concept to detect changes in myocardial thickness (i.e., changes in myocardial thickness are directly proportional to changes in brightness [or color]). The various software tools used to measure LVEF, wall motion, and wall thickening apply this concept to varying degrees.
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In practice, wall motion is assessed by tracking the apparent endocardial borders from the black-and-white images, whereas wall thickening is assessed by changes in color using color images. The normal LVEF by visual or quantitative analysis is ≥ 50% with some variation between software packages and specific protocols.
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Although these key points form the fundamental assumptions as to how a normal myocardial perfusion SPECT study will appear to the observer (and how it should be interpreted), these are theories that can vary significantly in everyday clinical practice because of differences in radiotracer, imaging equipment, imaging protocols, reconstruction algorithm and filters, the patient’s body habitus and gender, stressors, artifacts from patient motion, display monitor, the physician’s vision, and many other issues.
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Many of these variations or “exceptions” are illustrated in this book, particularly in this chapter, the chapter on image interpretation, and the chapter on image artifacts. The ability to recognize the normal variants and artifacts is what separates the expert interpreter from the novice.
Background
Recognition of the normal patterns of myocardial count distribution, wall motion, and wall thickening are imperative to properly interpret ECG-gated myocardial perfusion SPECT studies. The normal perfusion patterns vary depending on the specific protocols used such as differences in radiopharmaceuticals, imaging equipment, count density, reconstruction algorithm and filters, stressors, artifacts such as patient motion, patient’s size and gender, display monitor, and others. Similarly, when quantitative software tools are used to assist with the interpretation, the reader should be aware of the quantitative criteria used to call a specific parameter abnormal. The more aware the reader is of the scientific principles used to generate the images and the expected normal variations, the more likely it is that the correct diagnosis will be reached. In this chapter, we will emphasize what normal myocardial perfusion SPECT studies look like.
Case 1-1
Normal Tc-99m Perfusion Study (Nonobese Man) ( Figure 1-1 )
An 84-year-old, 163-pound, 6-foot 1-inch man with hypertension, aortic insufficiency, and heart failure presented with 2-week history of atypical chest pain. Tc-99m tetrofosmin perfusion SPECT was performed using a 1-day rest/stress (12 mCi/39 mCi) protocol. The patient underwent a standard modified Bruce treadmill protocol. The resting ECG was normal. The patient exercised for 4 minutes 28 seconds, reached 88% of maximum predicted heart rate, and stopped because of fatigue. There were no ECG changes during exercise. SPECT images were acquired using a 90-degree–angled dual-head camera and a 180-degree imaging arc from the 45-degree RAO projection to the 45-degree LPO projection. Rest and poststress ECG gating were performed using eight frames per cardiac cycle.
Comments
This is an example of a male patient with a normal perfusion study of excellent image quality. Note that the stress planar projections are of higher quality than the rest planar projections. The higher radiotracer dose injected at stress generates more counts per pixel. Despite the difference in stress and rest count density, the tomographic stress and rest images are both of high quality due to appropriate reconstruction and filtering. They exhibit high spatial and contrast resolution as depicted by the well-defined endocardial and epicardial myocardial borders and a well-defined LV chamber. RV activity is seen in the SA and HLA slices. Note the fairly uniform count distribution throughout the LV myocardium. A mild count reduction in the inferior wall (particularly in the basal inferior segment) is consistent with diaphragmatic attenuation. The gated images show normal segmental and global LV wall motion and normal LV ejection fraction. The EDV and ESV are mildly increased, which further enhances the LV myocardium/LV cavity contrast. For this quantitative program, EDV < 171 mL and ESV < 70 mL are within normal limits.
Case 1-2
Normal Tc-99m Perfusion Study (Nonobese Woman) ( Figure 1-2 )
A 52-year-old, 117-pound, 5-foot 1-inch woman with hypertension and family history of CAD had recurrent atypical chest pain. Tc-99m tetrofosmin myocardial perfusion SPECT was performed using a 1-day rest/stress (12 mCi/35 mCi) protocol. The patient underwent standard adenosine stress testing. The resting ECG was normal. The patient experienced no chest pain and there were no ECG changes during adenosine infusion. SPECT images were acquired using a 90-degree–angled dual-head camera and a 180-degree imaging arc from the 45-degree RAO projection to the 45-degree LPO projection. Rest and poststress ECG gating were performed using eight frames per cardiac cycle.
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