The review and interpretation of myocardial perfusion images are perhaps the key duty of a nuclear cardiologist. It is critical that image interpretation be performed in a systematic fashion so as to maximize the clinical value of the study and to ensure the highest-quality result of the entire procedure, providing optimal clinical information and assisting in clinical decision making. As discussed extensively in Chapter 5, the quality of the study must be reviewed and technical abnormalities be recognized. A comprehensive evaluation of all available imaging data must then be performed so as not to exclude potentially vital information.

A number of guidelines and tools have been recommended for the interpretation of myocardial perfusion studies.^1–5 These policies and guidelines have been developed by experts in the field and should be used as a guide to the successful interpretation of myocardial perfusion imaging (MPI). This chapter will provide suggestions for approaches for interpretation based upon these recommendations. Of note, as single-photon emission computed tomography (SPECT) imaging is performed in the vast majority of patients undergoing radionuclide imaging, this chapter focuses on the tomographic evaluation of perfusion and function with SPECT MPI, although the methods recommended in this chapter are largely applicable to PET imaging.

The sequence of imaging should include (1) review of the raw planar images, (2) analysis of the tomographic slices, (3) interpretation of gated SPECT data, and (4) incorporation of clinical data (Table 12-1).

Image Display

It is highly recommended that myocardial perfusion images be reviewed on a computer monitor as opposed to x-ray film or paper. While other media may provide useful information, the resolution of a computer monitor screen and the flexibility in adjusting a variety of parameters, including contrast, thresholds, and colors, makes this the medium that is greatly preferred. The practice of interpreting only “hard copy” images is discouraged, especially in view of the dynamic data, which is available by use of a workstation.

The patient’s body habitus should be considered when interpreting images, as this information may support the artifactual nature of apparent perfusion defects. Therefore, data regarding height, weight, and gender should be provided to the interpreting physician. Additional details, such as chest and bra size and the presence of a mastectomy or breast prostheses, may also be useful.

A linear color table is recommended for the interpretation of perfusion images. While linear gray scale is preferred and is recommended by many imaging guidelines, other continuous, linear color tables such as hot body/hot iron revised may also be used effectively (Fig. 12-1). A great variety of other color tables are available. It is critical that the interpreter understands the workings of these various color tables. It is usually recommended to avoid color tables with an abrupt transition between each color, for <10% change in tracer activity. Furthermore, it is critical that when a specific color table is used, the scaling should be linear, not exponential, as this will further enhance the appearance of artifacts potentially leading to false-positive results. Irrespective of the color table selected, the most important aspect of the use of these displays is that the operator be very familiar with the one selected. A bar delineating the color table should also be displayed on screen.

Care should be taken in the review of images to ensure that the particular tomographic slices are aligned. By convention, the stress study is placed in the top row with the resting study below. It is now well accepted that the display of images should be in a specific format, as more than 10 years ago, the Joint Guidelines from the American Heart Association, the American College of Cardiology, and the Society of Nuclear Medicine stated the manner in which SPECT images should be displayed.⁶

The top row should present the short-axis views, which are obtained by slicing perpendicular to the long-axis of the lower left ventricle. By convention, the septum is on the left, with the lateral wall on the right. The slices should be displayed from apex to base (left to right). The long-axis should also be presented, demonstrating the data by slicing in a vertical plane (vertical long-axis) and a horizontal plane (horizontal long-axis). The vertical long-axis views should be displayed with the septal slices positioned on the left and progressing to the lateral wall on the right. The horizontal long-axis should be displayed with the inferior slices on the left and moving to the anterior location on the right. All images on the SPECT study should be normalized usually to the brightest pixel in the entire left ventricle within a series of slices (stress or rest). This is called series normalization.⁴ Flexibility in display software should permit such scaling. Individual frame normalization may optimize image quality but lead to erroneous interpretation.

Artifact Recognition

Although image artifacts may be recognized in several ways, the raw (unprocessed) projection images often provide the most useful information. Therefore, a critical aspect of the interpretation of tomographic data is the review of these rotating planar images.³ All modern camera systems permit such a review often on the same screen as the SPECT slices.

Count Density

Adequate count data are essential to avoid “splotchy” data, which may easily be confused with perfusion defects. The reasons for low count density include: the dose and type of radiotracer, mode of stress, energy window, collimation, and attenuation. Beyond the visual assessment of rotating planar images or tomographic slices, quantitative assessment of the anterior planar project may be performed, with the requirement for peak pixel count of 100 and 200 for thallium-201 and Tc-99m, respectively.⁴

Patient Motion

While a sinogram (Fig. 12-2) or linogram has frequently been used to demonstrate motion and is composed of a sum of the planar data, this method for quality assessment is not ideal and is not recommended. The preferred approach is a review of a cine loop of both the stress and rest planar images usually simultaneously with the tomographic slices. It is often helpful to place a horizontal line beneath the inferior margin of the left ventricle for both rest and stress to better assess subtle but perhaps significant motion.

A patient motion-related artifact may be present in up to 15% of all SPECT studies.⁷ A review of the rotating images provides clear evidence when there is patient motion, in either a superior–inferior manner or laterally. If substantial motion is present (≥2 pixels), repeating the image acquisition is recommended. Motion correction algorithms may also be successfully applied,^8,9 but usually only when the patient motion is superior–inferior. Correction of lateral or rotational motion is challenging, but has been successfully incorporated into many software packages.

Multidetector systems may demonstrate abrupt motion on review of the rotating images. This is usually caused by temporal factors associated with a two-detector system and the fact that the last frame of acquisition for detector 1 is substantially later than the first frame obtained from detector 2. Thus, gradual motion throughout the acquisition is therefore accentuated when reviewing the rotating images.

The presence of patient motion may produce artifacts and therefore reduce diagnostic accuracy.¹⁰ Not only do these artifacts resemble ischemic heart disease, but also patient motion may create the appearance of multivessel disease. “Upward creep” may also be detected by reviewing the rotating images. This phenomenon occurs when imaging is performed soon after strenuous exercise and results from the repositioning of the cardiac structures as the respiratory excursion decreases following exercise. The “upward creep” artifact may be avoided if imaging is delayed for about 15 minutes after exercise. Prominent patient motion may also be noted on the tomographic slices, producing a characteristic defect such as the “hurricane” sign and “flame” that occurs near the apex, as shown in Figure 12-3.¹¹

Extracardiac Activity

The rotating planar images should be reviewed for the presence of abnormal activity beyond the boundaries of the myocardial structures. Skin or clothing contamination may mask or mimic a true perfusion abnormality but should be identifiable on the rotating images. The presence of intense subdiaphragmatic activity, emanating either from the liver or from the gastrointestinal (GI) tract, may confound image interpretation. Once such activity is present, it may cause a negative lobe artifact, also known as a ramp filter artifact. Intense adjacent activity may cause this reconstruction artifact, for which there is no reliable correction, although iterative reconstruction (as opposed to filtered backprojection) may help.⁴ This type of abnormality may create an artifactual perfusion abnormality or may mask the presence of a true abnormality (Fig. 12-4). Ideally, when substantial activity is noted especially in the liver or adjacent bowel loop, image acquisition should be repeated to eliminate this type of artifact.

The interpretation of SPECT images should not be restricted only to the myocardium. A variety of neoplastic lesions may also be detected with commonly used radiopharmaceuticals.^3,12 These may reflect either primary or metastatic tumors and include the following types of neoplastic growths: lung, breast, sarcoma, lymphoma, thymoma, parathyroid tumor, thyroid abnormality, and kidney and hepatic tumors. Incidentally discovered clinical thyrotoxicosis can be detected by careful evaluation of the rotating planar images and may aid in the early detection of thyroid disease.¹³

Finally, the rotating images may reveal contamination by the radiopharmaceutical, occurring on either the skin or clothing, which once again may confound the SPECT image interpretation. In addition, it is usually possible to distinguish between a neoplastic growth and contamination by reviewing the rotating images.

Attenuation

Soft tissue, overlying cardiac structures, may confound image interpretation. Breasts/chest soft tissue, or that related to subdiaphragmatic structures, may reduce the specificity for coronary artery disease (CAD) detection and is present in up to 40% of all studies.

Photopenic areas may be noted from overlapping breast tissue even when the size of the breast is relatively small. It is often possible to appreciate where the reduction of photons may occur on the SPECT slices by reviewing the cine images (Fig. 12-5).

In addition, soft tissue attenuation from the diaphragm may obscure the inferior wall, causing a false impression of an inferior wall abnormality. This occurs most commonly in men. Recognition of the superior-placed diaphragm is helpful in the interpretation of images and the enhanced recognition of a potential artifact. When such an abnormality is present, prone imaging may be helpful. In patients with large BMI, whose defect in the apical-inferior region may be diagnostically challenged, the addition of prone acquisition has been shown to improve diagnostic confidence, and when applied to stress-only MPI can reduce the need for unnecessary rest scans.^14,15

Obviously, gated SPECT^16,17 and attenuation correction¹⁸ methodologies may also be of substantial use in the correct interpretation of soft tissue abnormalities. Differential soft tissue attenuation may occur when the overlying soft tissue is present in different positions on the rest and stress images, thereby leading to the appearance of an apparently reversible perfusion defect. This challenging scenario is not helped by gated SPECT, as true reversible defects (ischemia) may demonstrate normal left ventricular (LV) function.

Analysis of Planar Images

Although tomographic MPI is considered the preferred modality for assessment of myocardial perfusion, planar imaging may be an alternative option in certain circumstances, such as in claustrophobic or critically ill patients where rapid acquisition is required, or in morbidly obese patients that do not qualify for the SPECT camera table. ECG-gated planar images can also be acquired. Regardless of whether the tomographic MPI is performed, planar images should always be inspected first preferably on a linear gray scale. Soft tissue attenuation by breast tissue, diaphragm, or other sources should be noted. Breast marker has been shown to be useful in identifying true perfusion defects from breast attenuation on planar images.

Similarly to the analysis of tomographic slices, segmental analysis of myocardial perfusion can be performed. The standard views for imaging positions and standardized nomenclature for myocardial segmental perfusion evaluation on planar images have been described in the ASNC myocardial perfusion planar imaging guideline.¹⁹ For qualitative assessment, the severity of perfusion defect can be classified as mild, moderate, or severe and the extent of defect as small, medium, or large. A five-point segmental scoring system can be applied for semiquantitative evaluation, which is further described later in this chapter. If a quantitative analysis is to be performed on planar imaging, background subtraction must be applied to the images. Reversibility may also be reported from planar images.

Analysis of Tomographic Slices

Image Quality

The first task is to determine whether or not adequate count statistics are present. A number of quality assurance tools are available from most manufacturers that assist in this process. The study should be graded based on overall image quality (uninterpretable, poor, fair, good, and excellent). Factors related to body habitus should be considered.

Cardiac and Lung Activity

The projection data also provide an assessment of cardiac size. Left ventricular hypertrophy (LVH) may be suspected when a reduced LV cavity:wall thickness ratio is noted. Prominent right ventricular uptake may be noted on the raw images or tomographic slices and may indicate right ventricular hypertrophy such as seen in pulmonary hypertension. However, no criterion other than subjective visual impression is available for this diagnosis.

Prominent lung activity may be present, which frequently is present in the setting of severe LV dysfunction or extensive ischemia. However, while abnormal lung activity is an important finding associated with thallium-201 scintigraphy, there is no consensus as to its meaning with technetium-99m imaging.⁴

LV cavity size may be assessed first by reviewing the rotating planar images. However, the overall cavity-to-wall-thickness ratio may be qualitatively determined by looking at the SPECT slices. In addition, it should be noted if the poststress images reveal a larger LV cavity than noted on the resting study (Fig. 12-6). This would be consistent with transient cavity dilation (TCD) also known as transient ischemic dilation (TID) of the LV cavity. The presence of TCD is a marker of proximal LAD and/or multivessel disease and a worsened prognosis.²⁰ The upper limits of normal TID values vary by protocols and types of isotopes used in the study (Table 12-2). Usually, about a 20% increase is required when using the dual-isotope protocol.²⁰ When using a single-isotope study, a lesser amount of cavity enlargement (approximately 5–10%) is felt to be abnormal.^21,22 Pharmacological MPI typically results in higher upper normal TID ratios when compared to exercise MPI as illustrated in Table 12-2. Gender difference for TID thresholds has also been noted, and should be taken into consideration when interpreting TID ratios.²³ In addition to the visual assessment, quantitative analysis of the TID ratio is available on most software packages.

Perfusion Defect

Defect severity is often described in a qualitative fashion (mild, moderate, and severe). A mild abnormality is one in which the clinical significance of the defect is unknown. Such an abnormality may reflect an equivocal finding. This often represents only a 10% reduction of peak tracer activity for a particular study. Moderate and severe defects carry more important diagnostic and prognostic value. In addition, the extent of the perfusion abnormality may also be qualitatively described as small, medium, or large (Figs. 12-7 and 12-8). Although these descriptions are relative, they may be based on objective information from quantitative programs.

In an attempt to describe the severity and extent as a combined value, a variety of scoring systems have been designed, the most popular being the summed stress and summed rest scores. These scores are derived by adding the point value using the range of “0” for normal perfusion to “4” for absent activity for each segment of the 17-segment model.¹ A mild, moderate, or severe reduction in count should be scored as 1, 2, or 3, respectively. The difference between the summed stress score and the summed rest score is called the summed difference score and is a measure of reversibility. Usually, individual segments with a ≥2-grade improvement on the resting study are felt to represent substantial ischemia. The size of the defect should be noted, as small, moderate, or large (Table 12-3).

The type of perfusion abnormality should also be described. A fixed perfusion defect (i.e., one that is the same on both the post stress and rest images) is often equated to a myocardial scar, especially when the abnormality is of severe intensity. However, a fixed perfusion abnormality may also reflect severe myocardial ischemia and the presence of myocardial viability. A reversible abnormality is a perfusion abnormality noted on the poststress images, but largely normalizes on the resting images. In many cases, some interpreters may use the term partially reversible. It is critical to determine whether it is a predominantly reversible defect or only a minimally reversible abnormality. Quantitatively, reversibility has a variety of definitions, but is often associated with a 20% to 30% improvement in regional activity.