Artifacts and Pitfalls in Myocardial Perfusion Imaging* Steven Burrell, MD, and Anita MacDonald, BHSc Department of Diagnostic Imaging, Queen Elizabeth II Health Sciences Centre, and Dalhousie University, Halifax, Nova Scotia, Canada several potential pitfalls and artifacts that can limit the Myocardial perfusion imaging (MPI) is an important imaging utility of the study. The overwhelming majority of MPI modality in the management of patients with cardiovascular dis- studies are now performed using SPECT and electrocar- ease. MPI plays a key role in diagnosing cardiovascular disease, diographic (ECG) gating, which further add to the com- establishing prognosis, assessing the effectiveness of therapy, plexity of the study. Artifacts and pitfalls can arise at any and evaluating viability. However, MPI is a complex process, subject to a variety of artifacts and pitfalls, which may limit its stage in the MPI process and can be grouped into issues clinical utility. These factors may be related to the patient (includ- related to the patient, the equipment, or the technologist. As ing unique aspects of the patient’s heart), the nuclear medicine depicted in Figure 1, there is considerable overlap. For equipment, or the actions of the technologist. After reviewing example, patient motion clearly originates with the patient, this article, the reader should be familiar with the causes and but the technologist has a role to recognize it and, where the effects of these potential artifacts and pitfalls. The reader appropriate, to use the motion correction capabilities of the should develop an understanding of steps to limit these factors, equipment to minimize its effect on the study. Some prob- actions to correct them if they do arise and, when necessary, how to incorporate their influence into the interpretation of the lems, such as motion and gating errors, are truly considered study. artifacts. These must be minimized in preparation for and Key Words: myocardial perfusion imaging; artifacts; cardiac during the study and, if necessary, recognized and corrected imaging; SPECT after the fact. The technologist plays a key role in these J Nucl Med Technol 2006; 34:193–211 steps. Other problems, including cardiac abnormalities such as balanced ischemia and hypertrophic cardiomyopathy, are more properly classified as interpretation pitfalls. These aspects do not arise from any limitation of the procedure itself. Interpreting physicians must be familiar with these to Cardiovascular disease is the number one cause of death ensure a proper evaluation of the study. in North America. It also presents an enormous societal This article will review the preimaging, technical, patient- burden with respect to morbidity, health care expense, and related, and heart-related artifacts and pitfalls that may personal hardship. Myocardial perfusion imaging (MPI) is compromise the performance and interpretation of MPI a valuable tool in the management of patients with cardio- studies. It is essential to be aware of these factors, to limit vascular disease. With its unique ability to evaluate per- them wherever possible, and to recognize them when they fusion at the cellular level and to assess perfusion at peak do arise in clinical situations. exercise stress, MPI plays an important role in diagnosing cardiovascular disease, establishing prognosis, assessing the PREIMAGING ISSUES effectiveness of therapy, and evaluating viability. The clini- Patient Preparation cal importance of MPI is in part reflected in its use, with The first step in ensuring an optimal study is patient annual double-digit growth (1). preparation. To limit gut activity adjacent to the heart, pa- Whereas MPI is a valuable diagnostic tool, it is also a tients should have nothing by mouth or have only a light complex physiologic imaging process, which exposes it to meal, depending on the institution’s preference. Patients should wear comfortable clothing and footwear for the exer- cise stress portion of the examination. If the study is being Received Apr. 11, 2006; revision accepted Sep. 5, 2006. For correspondence or reprints contact: Steven Burrell, MD, Department done for the primary diagnosis of coronary artery disease, of Diagnostic Imaging, Queen Elizabeth II Health Sciences Centre, 1796 then sensitivity will be maximized if certain cardiac med- Summer St. Halifax, Nova Scotia, Canada B3H 3A7. E-mail: [email protected] ications are withheld (2), though this should be done only if *NOTE: FOR CE CREDIT, YOU CAN ACCESS THIS ACTIVITY THROUGH approved by the referring physician. THE SNM WEB SITE (http://www.snm.org/ce_online) THROUGH DECEMBER 2007. If a pharmacologic stress test with a vasodilator such as COPYRIGHT ª 2006 by the Society of Nuclear Medicine, Inc. adenosine or dipyridamole is to be performed, the patient MYOCARDIAL PERFUSION IMAGING ARTIFACTS • Burrell and MacDonald 193 FIGURE 2. (A and B) Two different views from raw data of a MPI study reveal focal attenuation (arrowheads) from an un- recognized telemetry monitor on the patient’s chest. terminated before reaching the optimal level, the sensitivity for identifying ischemia will be reduced (2). It is also im- portant to have the patient continue exercising at the maxi- mal level for a minimum of 1–2 min after the injection of the radiopharmaceutical if they can safely do so. This al- lows for adequate circulation and uptake, ensuring that the distribution of radiopharmaceutical within the heart reflects the state of perfusion at maximal stress. FIGURE 1. Causes of, and potential corrections for, MPI The sensitivity of pharmacologic stress is believed to be artifacts and pitfalls may be related to the patient, the roughly equivalent to that of an exercise stress test in which equipment, or the technologist, often with overlap among these the maximal 85% of age-predicted heart rate is reached, categories. LBBB 5 left bundle branch block; GI 5 gastroin- provided, of course, that caffeine is avoided. testinal. Radiopharmaceutical Injection should abstain from medications containing methylxan- Commonly used single-photon MPI radiopharmaceuti- thines and beverages, food (such as chocolate), and medica- cals include 99mTc-sestamibi, 99mTc-tetrofosmin, and 201Tl. tions containing caffeine for 12 h (2)to24h(3,4). Caffeine Dose recommendations for MPI are provided in the SNM and methylxanthines block the adenosine receptors on arte- guidelines (2). Doses may be adjusted according to patient rial smooth muscle cells, thus limiting the effectiveness of weight depending on institutional practice. these vasodilator agents (4). Many institutions require that The placement of an intravenous line is recommended caffeine be avoided even if an exercise stress is planned, in for radiopharmaceutical injection for the rest and stress case there is a need to switch to a pharmacologic stress. portions of MPI. The insertion of an intravenous line will Note that some decaffeinated coffee contains up to 13 mg reduce the possibility of an infiltrated dose. If there is any of caffeine per 240-mL (8-oz) serving, so it may be best to question with regard to an infiltrated dose, a static image of have the patient avoid alleged decaffeinated beverages as the injection site should be obtained. Figure 3A shows the well (4). raw data from a same-day rest–stress MPI study. A dose of Before imaging, metal and other potential attenuators 407 MBq of 99mTc-sestamibi was used for the rest study, must be removed from the patient if they will project into and 1,295 MBq was used for the stress study. It was noticed the imaging field of view and potentially interfere with the that the stress study was noisier than usual for a patient of study. Figure 2 shows the raw data from a MPI study of a this size and, indeed, on the raw data images the signal-to- patient with a cardiac telemetry device on her chest, which noise ratio was similar to that of the rest study. An infil- projects over the heart during the SPECT acquisition, trated injection was suspected, and this was confirmed with resulting in focal attenuation. a static image of the injection site (Fig. 3B). A repeat-stress study was performed on the next day, with a dose of 1,184 Cardiac Stressing MBq. The raw data from this study are also shown in Figure The test sensitivity for ischemia is optimized by having 3A, with a much better signal-to-noise ratio than that on the the patient perform the exercise portion of the test to a study with the infiltrated injection. On the repeat-stress maximal safe level. This is generally taken to be 85% of perfusion images, there was a defect in the anteroseptum, the patient’s age-predicted maximal heart rate (3). Of course, not present on the stress images associated with the infil- not all patients can safely reach this level, and the test may trated injection (Fig. 3C), as that study reflected primarily need to be stopped for other reasons, such as moderate-to- the perfusion from the earlier rest study. severe angina, significant blood pressure drop, sustained An infiltrated injection may compromise the study in 3 ventricular tachycardia, and others (5). If the stress test is ways. First, because less radiopharmaceutical is taken up 194 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY • Vol. 34 • No. 4 • December 2006 FIGURE 3. (A) Raw data images from same-day rest and stress images demonstrate low counts in stress image, similar to low- dose rest image, resulting from an infiltrated injection during stress study. Raw data from a repeat-stress study (right panel) on the following day demonstrate expected better counts from high-dose stress study. (B) Static image of injection site in right arm confirms infiltrated injection. (C) Short-axis views from infiltrated stress study (top row) and repeat-stress study (bottom row). Repeat-stress study demonstrates a stress-induced defect in anteroseptum (arrowheads) not present on the study with infiltrated injection. by the myocardium, counting statistics are lowered, result- which revealed an infiltrated injection, and, hence, a workup ing in a poorer-quality study.