Evaluating Myocardial Perfusion SPECT: The Normal Study

Key points

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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).

  • 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.

  • 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.

  • 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.

  • 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.

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.

Figure 1-1
Normal Tc-99m perfusion study (nonobese male). A, Top right , black-and-white panels are the stress and rest planar projection images demonstrating excellent image quality. Color images are the corresponding tomographic slices display with the resting results interleaved between the stress slices. Top four rows display the LV SA from apex (top left) to base (bottom right) . Next two rows display the stress and rest VLA slices from the septum to the lateral wall and the last two rows show the stress and rest HLA slices from the inferior to the anterior wall. Note the high image quality and fairly homogeneous tracer uptake throughout the LV. B, Polar maps representation and quantification of this patient’s LV tracer uptake. The three panels in the top row correspond to the stress, rest and reversibility (normalized rest-stress) LV distributions. The brighter colors represent higher counts (perfusion) and the darker colors, fewer counts as depicted by the translation table on the rightmost column. Note the relative homogeneity of the stress and rest polar maps. The middle row is the defect extent polar maps display where areas that are abnormal in comparison to the normal database are highlighted in black. Note the absence of blackout regions, indicating a normal study by quantitative analysis. The bottom row is the defect severity polar maps display where each pixel (voxel) is color coded to the number of standard deviations below the mean normal distribution with the scale shown by the translation table. C, Stress and rest LV myocardial perfusion polar maps with superimposed 17-segment coordinate system using the 0-to-4 score for each segment (0 = normal, 1 = mildly reduced, 2 = moderately reduced, 3 = severely reduced, 4 = absent uptake). Note that for this patient the sum of all 17 segment scores at stress (SSS) and at rest (SRS) is zero. The lower the segmental scoring the more likely the perfusion study is normal. Non-AC images are usually considered abnormal if the summed score is > 4. D, Mean normal LV stress and rest Tc-99m myocardial perfusion distribution in males generated from a population of 30 male patients with < 5% probability of CAD. Note the relatively homogeneous perfusion distribution but the somewhat reduced uptake in the inferior wall as compared to the distal septum and lateral walls. Although less discernable there is also a count reduction in the anterior wall at the 11 o’clock position seen typically when using 180-degree acquisition. The cause of this normal variant is the change in resolution of depth during a 180-degree acquisition orbit and is not seen with 360-degree acquisition. The four rows of color images show end-dyastolic (E) and end-systolic (F) poststress (top row) and rest (bottom row), VLA, HLA, and SA LV tomographic display of this patient’s ECG-gated images. Note the normal uniform regional inward motion of the endocardial and epicardial borders from end-diastole to end-systole. Also note the normal uniform change in myocardial color from diastole to systole due to normal LV thickening. The color polar maps represent the quantification of the poststress (top) and rest (bottom) regional thickening. Note the labeled thickening scale to the right of the maps. The poststress (top left) and rest (bottom left) panels show the patient’s averaged LV volume curves per cardiac cycle, LVEF, EDV, ESV, SV and LV mass. Note that both the rest and poststress LVEFs are 59%, above the 50% normal threshold for this quantitative program. These panels are accompanied by dynamic displays (Video 1-1).

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.

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.

Figure 1-2
Normal Tc-99m perfusion study (nonobese female). A, Top right , black-and-white panels are the planar projection images demonstrating excellent image quality. Note the prominent extra-cardiac activity. Color images are the corresponding tomographic slices display with the resting results interleaved between the stress slices. Top four rows display the LV SA from apex (top left) to base (bottom right) . Next two rows display the stress and rest VLA slices from the septum to the lateral wall and the last two rows show the stress and rest HLA slices from the inferior to the anterior wall. Note the high image quality and fairly homogeneous tracer uptake throughout the LV. B, Polar maps representation and quantification of this patient’s LV tracer uptake using the same format as in Figure 1-1, B . Note the relative homogeneity of the stress and rest polar maps. The middle row shows the defect extent polar maps where areas that are abnormal in comparison to the normal database are highlighted in black. Note the absence of blackout regions, indicating a normal study by quantitative analysis. The bottom row is the defect severity polar maps display. C, Stress and rest LV myocardial perfusion polar maps with superimposed 17-segment coordinate system using the 0-to-4 score for each segment. Note that for this patient the sum of all 17-segment scores at stress (SSS) and rest (SRS) is zero. D, Mean normal LV stress and rest Tc-99m myocardial perfusion distribution in females generated from a population of 30 female patients with < 5% probability of CAD. Note the relatively homogeneous perfusion distribution. Compared to the normal distributions in males (see Figure 1-1, D ) there is less diaphragmatic attenuation of the inferior wall. Although less discernable, there is also a count reduction in the anterior wall at the 11 o’clock position often seen in normal studies when using 180-degree acquisition. The four rows of color images show end-dyastolic (E) and end-systolic (F) stress (top row) and rest (bottom row) VLA, HLA, and SA LV tomographic display of this patient’s ECG-gated images. Note the normal uniform regional inward motion of the myocardial borders from end-diastole to end-systole. Also note the significant uniform change in myocardial color from diastole to systole due to normal LV thickening. The color polar maps in E represent the quantification of the poststress (top) and rest (bottom) regional thickening. Note the labeled thickening scale to the right of the maps. The poststress (top left) and rest (bottom left) panels show the patient’s averaged LV volume curves per cardiac cycle, LVEF, EDV, ESV, SV and LV mass. Note that both the rest and poststress LVEFs are above the 50% normal threshold for this quantitative program. These panels are accompanied by dynamic displays (Video 1-2).

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Jan 27, 2019 | Posted by in CARDIOLOGY | Comments Off on Evaluating Myocardial Perfusion SPECT: The Normal Study

Full access? Get Clinical Tree

Get Clinical Tree app for offline access