Approach Considerations
The value of SPECT in the diagnosis of CAD was confirmed in a meta-analysis, which demonstrated that it is highly effective in assessing myocardial perfusion, with a quoted sensitivity of 86%, a specificity of 74%, and a normalcy rate of 89%. [16]
The negative predictive value of the test has been shown to be as high as 98%, offering excellent prognostic value. A normal technetium-201 or technetium-99m scan is generally associated with low risk of future cardiac events (< 1%/yr). A report of 2946 patients with a normal scan showed a low risk for cardiac death or myocardial infarction (< 0.5%/yr and < 0.3%/yr, respectively) at 1.8-year follow-up. [48]
High-risk features on SPECT that predict an increased risk of cardiac events include extensive ischemia involving more than 20% of the left ventricle, defects in more than one coronary vascular supply region, reverse ischemia in multiple segments, transient or persistent left ventricular cavity dilatation, left ventricular ejection fraction less than 45%, [48] and increased lung uptake of thallium or sestamibi. [49, 50, 51] These factors predict an annual mortality of more than 3%. For patients with normal, mildly abnormal, moderately abnormal, or severely abnormal perfusion defects, the annual rate of cardiac death in one study was 0.5%, 2.7%, 2.9%, and 4.2%, respectively. [48]
Standard (not-gated) radionuclide scanning of the heart includes data from throughout the cardiac cycle in a single image, without discrimination of systole and diastole. Using a technique called ECG gating, sequential data can be obtained with timing to the cardiac cycle, thus providing images specific to phases of systole and diastole. [41] With this technique, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility. This, in turn, allows calculations of left ventricular ejection fraction, stroke volume, and cardiac output.
The laboratory workup needed before a patient undergoes SPECT is namely related to contraindications, including testing for cardiac enzymes to rule out acute coronary syndrome.
Dynamic Stress
Exercise testing must be undertaken by an appropriately trained healthcare professional. If a physician is not performing the test, a physician experienced in cardiovascular stress should be available for assistance in case of an emergency. [52] Rapid access to personnel trained in advanced life support should be available.
Dynamic exercise can be undertaken using a treadmill or a bicycle ergometer.
Regardless of the exercise protocol, an intravenous line should be secured and flushed to ensure patency before starting the test.
The patient’s heart rate, blood pressure, and ECG should be monitored at rest and throughout the test. In a standard rest/stress protocol, a patient is imaged twice: once at rest and then after the patient is stressed by exercise or pharmacologic stimulation. [53] Monitoring with a 12-lead ECG is required for the detection of ST-segment and T-wave changes and for the diagnosis of arrhythmias.
Pharmacologic Stress
For administration of adenosine, an intravenous line is required and a 3-way connector should be used to allow tracer injection without interruption of the adenosine infusion. The adenosine is infused at 0.14 mg/kg/min IV for 6 minutes (total cumulative dose of 0.84 mg/kg). Heart rate, blood pressure, and ECG should be measured and recorded at baseline and every 2 minutes during the infusion. The radiopharmaceutical agent (ie, thallium-201) is injected after 3-4 minutes of the adenosine infusion. Thallium-201 is physically compatible with adenosine and may be injected directly in the adenosine infusion set.
Intravenous dipyridamole is infused at a rate of 0.142 mg/kg/min IV for 4 minutes (not to exceed a cumulative dose of 0.57 mg/kg). Vital signs and ECG are measured and recorded at baseline and every 2 minutes. The radiopharmaceutical should be injected 4 minutes after completion of the infusion.
Dobutamine infusion is commonly used when dynamic exercise is not feasible and contraindications to vasodilator stress are recognized. It is given as an intravenous infusion at incremental doses. Vital signs and ECG are recorded. The radiopharmaceutical should be injected when more than 85% of the age and sex adjusted maximum predicted heart rate is reached. The dobutamine infusion should be continued for 1 minute after the injection of thallium-201 or 1-2 minutes after the injection of technetium-99m-labeled tracers and is then stopped.
Image Acquisition
The image obtained by the gamma cameras is a 2-dimensional (2D) view of a 3-dimensional (3D) distribution of radionuclide. SPECT imaging is performed by using a gamma camera to acquire multiple 2D images (called projections) from different angles. A computer then yields a 3D image by using by using a tomographic reconstruction algorithm for the multiple projections. This 3D dataset can then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other topographic techniques, such as MRI, CT, or positron emission tomography (PET).
Reconstruction images typically have resolutions of 64x64 or 128x128 pixels, with the pixel size ranging from 3 to 6 mm. The number of projections required is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images have lower resolution, have more noise than planar images, and are susceptible to artifacts.
To acquire SPECT images, the patient lies still in a supine position, and the gamma camera is rotated around the patient. The patient must not move during the scan time. Movement can cause significant degradation of the reconstructed images, [54] although movement compensation reconstruction techniques can help. A highly uneven distribution of the radiopharmaceutical also has the potential of causing artifacts. An intense area of activity (eg, the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity. The traditional filtered back projection reconstruction algorithm has this limitation. However, alternative techniques like the interactive reconstruction algorithm is less sensitive to artifacts and can also correct for attenuation and depth dependent blurring.
Several projections are obtained at defined points during the rotation (on average, every 3-6°). In most cases, a full 360° is used to obtain an optimal reconstruction. Typically, it takes 15-20 seconds to obtain each projection, for a total scan time of 15-20 minutes.
Multiheaded gamma cameras are able to acquire projections in a shorter amount of time. A dual-headed camera (each gamma camera spaced 180° apart) allows 2 projections to be acquired simultaneously, with each head requiring a 180° rotation. A triple-head camera requires only a 120° rotation. With multiheaded SPECT systems, imaging can often be completed in less than 10 minutes.
Because the radionuclide slowly redistributes, performing both sets of images on the same day is not possible. Therefore, a second attendance is sometimes required 1-7 days later. However, with a TI-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as 2 hours post stress. If stress imaging is normal, rest imaging is unnecessary. For this reason, stress imaging is obtained first.
Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. and can therefore decrease specificity of SPECT imaging. [28] Approximate correction is possible, based on relative position of the activity. However, optimal correction is obtained with measured attenuation values. Modern SPECT equipment has an integrated x-ray CT scanner. Because x-ray CT images are an attenuation map of tissues, this data can be incorporated into the SPECT reconstruction to correct for the attenuation.
CT attenuation correction (CTAC) images are considered nondiagnostic; however, clinically significant incidental findings requiring follow-up have been reported. [55, 56]
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Device summary. To acquire single-photon emission computed tomography (SPECT) images, the patient lies still in a supine position, and the gamma camera is rotated around the patient. Courtesy of Patrick J. Lynch, Yale University.
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Single-photon emission computed tomography (SPECT) images compared with illustrations of the heart from similar views. The SPECT images (bottom row) show the distribution of the tracer and therefore the relative blood flow to the different regions of the myocardium. The top rows are illustrations of the heart to compare with the SPECT images from similar views. Notice there are more enhancements in the area that corresponds to the left ventricle because of its greater thickness. Courtesy of Patrick J. Lynch, Yale University.
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Planar imaging. The gamma camera detects the gamma rays emitted from the tracer, and the image of where the tracer is found in the organs is transmitted to the computer. In planar technique, the computer provides a 2-dimensional myocardial perfusion image. Courtesy of Patrick J. Lynch, Yale University.
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SPECT imaging performed after stress. The upper row shows short-axis slices after pharmacologic stress; the lower row shows the same slices when the body is at rest in conventional cardiac SPECT with Tc-99m tetrofosmin as a radiotracer. Arrows indicate a small perfusion defect on the backside of the heart (visible only on the stress images), showing ischemia in this region of the heart wall. Courtesy of Philipp A Kaufmann, MD, and Oliver Gämperli, MD, University Hospital Zurich.
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Positioning. The patient should be supine with both arms above the head. Courtesy of Philips Healthcare.
