- Email: [email protected]
Acute myocardial perfusion imaging for the technologist
TECHNOLOGISTS' SECTION Acute myocardial perfusion imaging for the technologist April M a n n , CNMT, a RT(N), M i c h a e l P. W h i t e , CNMT, a a n d G a r y V. Heller, MD, PhD a,b
It is estimated that more than 5 million patients are evaluated annually in US emergency departments for chest pain syndromes perceived to be cardiac in origin.1 Of these patients, approximately 15% harbor acute ischemic heart disease. 2 Typically, the determination between cardiac or noncardiac causes is based on the evaluation of symptoms (typical versus atypical), the electrocardiogram (ECG), and serial enzyme testing within 24 hours. Typical Versus Atypical Symptoms. Often it is difficult to obtain a reliable history from the patient. His or her interpretation of symptoms may not be what is considered typical chest pain associated with ischemic changes. Age and ethnic background, especially if there is a language barrier, can make it difficult to evaluate symptoms. Electrocardiogram. There are several limitations to the value of the standard ECG. The ECG may be normal in many patients with unstable ischemic syndromes, particularly if the tracing is obtained during a pain-free interval. 2 In addition, various areas in the myocardium, such as circumflex artery distribution, are electrocardiographically silent. 2 Finally, certain baseline patterns and waveforms can make the ECG difficult to interpret. Creatine Kinase. There is a high sensitivity (100%) for serial creatine kinase (CK) and CK-MB isoenzyme subunit measurements within the first 24 hours of presentation to the emergency department for acute myocardial infarction. However, in patients with acute ischemia not involving necrosis, the CK and CKMB values may be normal. 2 Also, the sensitivity for early (<6 hours) and late (>72 hours) levels are very poor.
From the aNuclear Cardiology Laboratory, Division of Cardiology, Hartford, Connecticut, and the bDivisionsof Medicine and Nuclear Medicine, University of Connecticut School of Medicine, Farmington, Connecticut. Reprint requests: April Mann, CNMT, RT(N), Hartford Hospital, Nuclear Cardiology Laboratory, 80 Seymour St, Hartford, CT 06102; [email protected]. J Nucl Cardiol 1998;5:622-5. Copyright © 1998 by the AmericanSociety of Nuclear Cardiology. 1071-3581/98/$5.00 + 0 43/1/94152 622
These standard tests are used in the evaluation of more than 50% of patients seen in the emergency department who are hospitalized and in whom ischernic heart disease or myocardial infarction subsequently is excluded. 3 Despite low admission thresholds, even in patients with a low risk of coronary disease, 5% to 8% who are not hospitalized have a myocardial infarction within 48 hours. Thus current evaluation strategies for triage of acute chest pain syndromes are inadequate and not cost-efficient. It would be very helpful if emergency physicians had another diagnostic tool to use in conjunction with these standard evaluations that could enable them to make a more definitive diagnosis. The rising costs of health care, combined with the downsizing of many cardiac care units, are making such a tool a necessity. Acute myocardial perfusion imaging may offer such an alternative option. Background. Ischemia is defined as a decrease in blood flow to an organ, usually as a result of constriction or obstruction of an artery. 4 Similar to what occurs during standard exercise or pharmacologic stress myocardial perfusion imaging, if a patient is having true acute myocardial ischemic symptoms, a decrease in blood flow occurs in the location of the ischemic area. If a radioactive tracer is injected during these symptoms, a relative decrease in tracer uptake should be noted in the affected location of the myocardium. To achieve maximal results, it is important to inject the radionuclide during or as close to or the onset of symptoms as possible. Although it has not yet been determined how long residual ischemic effects last, it is believed to be 1 to 2 hours after the onset of symptoms. Previously studies have been performed with 201T1 for acute imaging. 5 However, certain limitations are associated with the use of this radioisotope. Because of its low energy (80 keV) and long half-life (72 hours), an injection of only 3 to 4 mCi can be given, which results in a low count rate, reducing image quality. Because 201T1 redistribution occurs within 30 minutes, imaging should begin almost immediately after injection. However, often time is required to stabilize the patient's condition and a delay of up to 2 hours before imaging is not infrequent. In addition, 2°1T1 is cyclotron produced and it is impractical to
Journal of Nuclear Cardiology Volume 5, Number 6;622-5
keep doses available for an unknown number of patients who may first be seen in the emergency department. In contrast, technetium-based agents have a higher energy (140 keV) and shorter half-life (6 hours). Thus a higher dose of 25 to 30 mCi can be given, which results in a higher count rate and better image quality. Several technetium-based imaging agents (sestamibi, tetrofosmin) also have very stable myocardial retention. 6-8 Thus an injection can be given at the time of, or as close to, the onset of chest pain symptoms as possible and imaging performed 1 to 2 hours later when the patient's condition has been stabilized.
LOGISTICS OF ACUTE MYOCARDIAL PERFUSION IMAGING The ability to provide acute myocardial perfusion imaging as an effective diagnostic tool requires a great deal of preparation. Education, planning for proper staff and radioisotope availability, time of injection, imaging, and interpretation all play an important role in offering a quality service. Education. It is necessary to educate the emergency department staff and the cardiology and internal medicine staffs about the benefits of acute myocardial perfusion imaging. This procedure is not for all patients who come to the emergency department with chest pain. Rather, it is to aid in the evaluation of patients with acute chest pain syndromes believed to be cardiac in origin but who have normal or nondiagnostic ECGs, normal serial enzyme evaluations, with or without suspicious but not definitive symptoms. 2 An algorithm can be developed to help physicians recognize and understand how acute myocardial imaging can be best used and aid them in the care of the patient (Figure 1). Planning. Adequate planning should be done to allow for appropriate staff and availability of radiopharmaceutical agents. Appropriate staff members may include a technologist, a nurse to monitor patients in potentially unstable condition, and physician for interpretation. It is also necessary to arrange for adequate radioisotope availability. The 2 options available include a prepared vial or unit dose. Many radiopharmacies now offer programs for acute use. These programs involve having a precalibrated dose available at all times with credit issued for those doses that are not used. There is a minimal monthly charge associated with this service, but it may be cost-effective for laboratories that want to offer 24-hour service. If 24-hour service is offered, an area should be set aside for storage of the isotope; most importantly, someone should be made responsible for handling, record keeping, and disposal of isotopes. Depending on state
Mann, White, and Heller Acute myocardialperfusion imaging
623
regulatory agencies or the nuclear regulatory commission, the technologist may need to be solely responsible for these tasks. injection. It is very important that the injection be given as close to the onset of symptoms as possible and before any therapy is administered. Many laboratories choose to have the covering technologist on call, outside the facility, instead of on site. To ensure the injection is given at the appropriate time, it may be necessary to train emergency department staff members to perform the injection. When 24-hour coverage is desired this can be especially important. Stowers et al 9 presented an abstract in 1995 that showed the dramatic decrease in sensitivity (97% to 33%) and specificity (78% to 40%) when radioisotopes were injected after symptoms had resolved. Imaging. When acute myocardial perfusion imaging is performed, physicians are required to make a diagnosis from only 1 image set. It is very important to provide the best-quality images and information available. Superior image quality is dependent on the proper count rate, which can be achieved by injecting 25 to 30 mCi of a Tc-labeled perfusion agent. To avoid artifact caused by increased tracer uptake in the liver, it is important to note that acute injections are given under rest conditions and imaging should not be performed for at least 45 minutes after injection. Since evaluation of ventricular function in addition to myocardial perfusion is important and useful, a gated single photon emission computed tomography (SPECT) acquisition should be performed on all patients. Images should be acquired for 64 projections, for 30 seconds per projection, with a 64 x 64 matrix and a 180degree arc, with a window of no more than 40% for gated images. These parameters should be used for all types of equipment including single-, dual-, and triple-head cameras to ensure an optimal study. Also, if available, attenuation correction should be performed. Processing. Processing of the images should be per~ formed according to the same parameters as a stress/rest gated myocardial perfusion study. It is important to use a slice thickness of 1.0 pixel when processing slices to ensure defects are not missed and a slice thickness of 2.0 pixels is used for the gated cine loop to ensure optimal image quality and proper evaluation of wall motion. Interpretation. Images should be interpreted by a qualified nuclear medicine physician, cardiologist, or radiologist. All pieces of information available should be used including slices, gated cine loop, and unprocessed projection data. This allows the physician to make the most accurate diagnosis possible. It is important to have the acute images interpreted and a report given as soon as possible after imaging is completed. This allows emergency physicians to make
624
Mann, White, and Heller Acute myocardialperfusion imaging
Journal of Nuclear Cardiology November/December1998
" Patientpres;ntsto ED with Gh;st Pain
IRapidPainAssessment/HistorylECGITriageII Acute M'
"
I
' nC n" 'e I I Equivocal I i
ii
I ou'e' 'no I I '1 ,,
--]Admit
1
I
I F°ll°w'up Testing ~---~
I
Discharge I
Figure 1. Sample algorithm for patients in the emergency department with chest pain.
decisions based on the results of the acute images to either admit the patient for further evaluation or to discharge the patient. TECHNICAL CHALLENGES A potential source of error in the interpretation of acute myocardial perfusion imaging involves artifacts related to soft tissue attenuation, increased liver activity, and motion. It is the role of the technologist to ensure optimal image quality each time acute imaging is performed. Soft T i s s u e A t t e n u a t i o n . Soft tissue attenuation may result in an apparent localized decrease in count density. 10 The location, size, and severity of the defect depends on the attenuation in relation to the myocardium and its size and density. The most common types of soft tissue attenuation are caused by breast tissue, lateral chest wall fat, and the diaphragm. Soft tissue attenuation is often the cause of a false-positive interpretation because of deficient counts misinterpreted as scar or ischemia. To avoid artifact from soft tissue attenuation, many companies have developed attenuation correction packages. If such technology is available and validated, its use is recommended in combination with other information to allow for optimal image interpretation. Liver Activity. As mention earlier, it is necessary t o allow adequate time after injection for liver clearance of the radiopharmaceutical. No hemodynamic changes occur during acute myocardial ischemia; hence the injection is given under rest conditions. Image acquisition after an acute injection should begin no sooner than 45 minutes after injection, similar to a rest injection with any Tc-labeled myocardial perfusion agent. 6-8 Because
of the location of the liver in relation to the inferior wall of the myocardium, artifacts can result during back-filter projection processing, quantitative analysis, and polar plot generation. 11-13 N o t i o n . Motion can be another source of error in the interpretation of acute myocardial perfusion imaging. Several sources of motion can be related to the camera; however, the most common is patient motion as many patients are still experiencing chest pain or are anxious in a hospital setting. When cardiac motion occurs, misalignment of data results in telltale artifacts in the tomographic slices. 10 This appears as misalignment of the anterior and posterior walls with "tails" of activity curving into the background. To avoid motion-related artifact the technologist should make sure the patient is as comfortable as possible before starting the acquisition. If necessary, pillows should be placed under the patient's knees or arms for comfort, proper arms rest should be used, and most importantly, technologists need to thoroughly explain the procedure to the patient before imaging begins. The technologist should be alert and aware of the patient throughout the acquisition. Unprocessed projection data should be reviewed on completion of the acquisition and if motion is seen the acquisition should be repeated. If the patient's condition is unstable or the patient is not able to withstand another acquisition, motion correction should be applied before processing the study. VALIDATION Several studies have been performed to demonstrate the validity of acute myocardial perfusion imaging. Varetto et al 3 performed a study to determine the role of acute imaging in patients with chest pain. Sixty-four patients seen in the emergency department with chest pain suspected to be cardiac in origin and who had nondiagnostic ECGs were injected with 99mTc-labeled sestamibi. The results of this study demonstrated 100% sensitivity, 92% specificity, 100% negative predictive value, and 90% positive predictive value for acute imaging compared with the final coronary artery disease diagnosis. Other studies performed by Hilton et al t3 and Gregoire and Theroux ]4 concluded that acute myocardial perfusion imaging with 99mTc-labeled sestamibi, when used in conjunction with other standard evaluations, offers a high accuracy rate and can be used as a reliable diagnostic tool in the evaluation of chest pain syndromes. C O S T ANALYSIS Emergency department evaluation for acute chest pain syndromes exceeds $5 million in the United States
Journal of Nuclear Cardiology Volume 5, Number 6;622-5
annually. 15 More than 40% of the evaluations of these patients lead to costly hospital admissions with only a small percentage of patients sustaining acute myocardial infarction. 15 Performance of acute myocardial perfusion imaging may eliminate or reduce the length of hospital stay for patients seen in the emergency department with low to moderate risk of ischemic heart disease, resulting in lower overall expense for patients. The average cost of an acute imaging study is approximately $600. For a patient who is hospitalized for 1 night because of a nondiagnostic or equivocal ECG, the cost can exceed $2,000. A study by Heller et a116 evaluated the clinical use and cost analysis of acute imaging with 99mTc-labeled tetrofosmin. The results demonstrated that use of a normal acute SPECT image as a criterion for not admitting patients would result in a 57% reduction in hospital admissions and a mean cost savings of $4,258 per patient.
CONCLUSION Acute myocardial perfusion imaging involves careful planning from the nuclear medicine or nuclear cardiology laboratory to ensure optimal results are achieved. The role of the technologist is to ensure a high-quality study is performed on every patient who is referred to the laboratory. This is one of the most important roles because the decision for further evaluation can be based on the interpretation of the acute images. When acute myocardial perfusion imaging is used appropriately, in conjunction with standard methods of evaluation for patients presenting to the emergency department with chest pain syndromes perceived to be cardiac in origin, it can be of great benefit. It offers a more definitive diagnosis of chest pain syndromes and can be used to reduce the expense of otherwise costly hospital stays, even in patients with moderate risk of ischemic heart disease.
References 1. Dupont/Merck Rhadiopharmaceuticals. Cardiolite use for acute chest pain management. Implementation and logistical guidebook. Billerica, Mass.: 1995. 2. Selker HE Zalenski RI, Antman EM, et al. An evaluation of technologies for identifying acute cardiac ischemia in the emergency depart-
Mann, White, and Heller Acute myocardial perfusion imaging
625
ment: a report from a national heart attack alert program working group. Ann Emerg Med 1997;29:13-87. 3. Varetto T, Cantaietupi D, Altieri A, et al. Emergency room technetium99m sestamibi imaging to rule out acute myocardial ischemic events in patients with non-diagnostic electrocardiograms. J Am Coll Cardiol 1993;22:1804-8. 4. American Heart Association. Heart and stroke facts. Dallas (TX): AHA, 1994. 5. Wackers FJTh, Heller GV, Stowers SA, et al. Normal rest tetrofosmin SPECT imaging in patients with chest pain and normal or nondiagnostic ECG in the emergency department is associated with lower need for subsequent cardiac catheterization and revascularization [abstract]. J Am Coll Cardiol 1997;29:137A. 6. Jain D, Wackers FJTh, Mattera J, et ai. Biokinetics of technetium-99mtetrofosmin: Myocardial perfusion imaging agent: implications for a one-day imaging protocol. J Nucl Med 1993;34:1254-9. 7. Wackers FJTh, Berman DS, Maddahi J, et al. Tecbnetium-99m hexakis 2-methoxisobutyl isonitrile: human biodistribution, dosimetry, safety, and preliminary comparison to thallium-201 for myocardial perfusion imaging. J Nucl Med 1989;30:301-11. 8. Higley B, Smith FW, Smith T, et al. Technetium-99m-l,2-bis[bis92ethoxyetbyl)phosphino]ethane: human biodistribution, dosimetry and safety of a new myocardial perfusion imaging agent. J Nucl Med 1993;34:30-8. 9. Stowers S, Abuan T, Szmanski T, et al. Technetium-99m sestamibi SPECT and technetium-99m tetrofosmin SPECT in prediction of cardiac events in patients injected during chest pain and following resolution of pain. J Nucl Med 1995;36:88P-89R 10. DePuey EG. Artifacts in SPECT myocardial perfusion imaging. In: DePuey EG, Berman DS, Garcia EV, eds. Cardiac SPECT imaging. Philadelphia: Lippincott-Raven, 1996:169-200. 11. Germano G, Chua T, Kiat H, et al. A quantitative analysis of artifacts due to hepatic activity in technetium-99m myocardial perfusion SPECT studies. J Nucl Med 1994;35:356-9. 12. Nuyts J, DuPont P, Van den Maegdenbergh V, et al. A study of the liver-heart artifact in emission tomography. J Nucl Med 1995;36:133-9. 13. Hilton T, Thompson R, Williams H, et al. Technetium-99m sestamibi myocardial perfusion imaging in the emergency room evaluation of chest pain. J Am Coll Cardiol 1994;23:1016-22. 14. Gregiore J, Theroux E Detection and assessment of unstable angina using myocardial perfnsion imaging: comparison between technetium99m sestamibi SPECT and 12-lead electrocardiogram. Am J Cardiol 1990;66:42E-46E. 15. Tatum JL, Jesse RL, Kontos MC, et al. Comprehensive strategy for the evaluation and triage of the chest pain patient. Ann Emerg Med 1997;29:116-25. 16. Heller GV, Stowers SA, Hendel RC, et al. Clinical value of acute rest technetium-99m tetrofosmin tomographic myocardial perfusion imaging in patients with acute chest pain and nondiagnostic electrocardiograms. J Am Coll Cardiol 1998;31:1011-7.
It is estimated that more than 5 million patients are evaluated annually in US emergency departments for chest pain syndromes perceived to be cardiac in origin.1 Of these patients, approximately 15% harbor acute ischemic heart disease. 2 Typically, the determination between cardiac or noncardiac causes is based on the evaluation of symptoms (typical versus atypical), the electrocardiogram (ECG), and serial enzyme testing within 24 hours. Typical Versus Atypical Symptoms. Often it is difficult to obtain a reliable history from the patient. His or her interpretation of symptoms may not be what is considered typical chest pain associated with ischemic changes. Age and ethnic background, especially if there is a language barrier, can make it difficult to evaluate symptoms. Electrocardiogram. There are several limitations to the value of the standard ECG. The ECG may be normal in many patients with unstable ischemic syndromes, particularly if the tracing is obtained during a pain-free interval. 2 In addition, various areas in the myocardium, such as circumflex artery distribution, are electrocardiographically silent. 2 Finally, certain baseline patterns and waveforms can make the ECG difficult to interpret. Creatine Kinase. There is a high sensitivity (100%) for serial creatine kinase (CK) and CK-MB isoenzyme subunit measurements within the first 24 hours of presentation to the emergency department for acute myocardial infarction. However, in patients with acute ischemia not involving necrosis, the CK and CKMB values may be normal. 2 Also, the sensitivity for early (<6 hours) and late (>72 hours) levels are very poor.
From the aNuclear Cardiology Laboratory, Division of Cardiology, Hartford, Connecticut, and the bDivisionsof Medicine and Nuclear Medicine, University of Connecticut School of Medicine, Farmington, Connecticut. Reprint requests: April Mann, CNMT, RT(N), Hartford Hospital, Nuclear Cardiology Laboratory, 80 Seymour St, Hartford, CT 06102; [email protected]. J Nucl Cardiol 1998;5:622-5. Copyright © 1998 by the AmericanSociety of Nuclear Cardiology. 1071-3581/98/$5.00 + 0 43/1/94152 622
These standard tests are used in the evaluation of more than 50% of patients seen in the emergency department who are hospitalized and in whom ischernic heart disease or myocardial infarction subsequently is excluded. 3 Despite low admission thresholds, even in patients with a low risk of coronary disease, 5% to 8% who are not hospitalized have a myocardial infarction within 48 hours. Thus current evaluation strategies for triage of acute chest pain syndromes are inadequate and not cost-efficient. It would be very helpful if emergency physicians had another diagnostic tool to use in conjunction with these standard evaluations that could enable them to make a more definitive diagnosis. The rising costs of health care, combined with the downsizing of many cardiac care units, are making such a tool a necessity. Acute myocardial perfusion imaging may offer such an alternative option. Background. Ischemia is defined as a decrease in blood flow to an organ, usually as a result of constriction or obstruction of an artery. 4 Similar to what occurs during standard exercise or pharmacologic stress myocardial perfusion imaging, if a patient is having true acute myocardial ischemic symptoms, a decrease in blood flow occurs in the location of the ischemic area. If a radioactive tracer is injected during these symptoms, a relative decrease in tracer uptake should be noted in the affected location of the myocardium. To achieve maximal results, it is important to inject the radionuclide during or as close to or the onset of symptoms as possible. Although it has not yet been determined how long residual ischemic effects last, it is believed to be 1 to 2 hours after the onset of symptoms. Previously studies have been performed with 201T1 for acute imaging. 5 However, certain limitations are associated with the use of this radioisotope. Because of its low energy (80 keV) and long half-life (72 hours), an injection of only 3 to 4 mCi can be given, which results in a low count rate, reducing image quality. Because 201T1 redistribution occurs within 30 minutes, imaging should begin almost immediately after injection. However, often time is required to stabilize the patient's condition and a delay of up to 2 hours before imaging is not infrequent. In addition, 2°1T1 is cyclotron produced and it is impractical to
Journal of Nuclear Cardiology Volume 5, Number 6;622-5
keep doses available for an unknown number of patients who may first be seen in the emergency department. In contrast, technetium-based agents have a higher energy (140 keV) and shorter half-life (6 hours). Thus a higher dose of 25 to 30 mCi can be given, which results in a higher count rate and better image quality. Several technetium-based imaging agents (sestamibi, tetrofosmin) also have very stable myocardial retention. 6-8 Thus an injection can be given at the time of, or as close to, the onset of chest pain symptoms as possible and imaging performed 1 to 2 hours later when the patient's condition has been stabilized.
LOGISTICS OF ACUTE MYOCARDIAL PERFUSION IMAGING The ability to provide acute myocardial perfusion imaging as an effective diagnostic tool requires a great deal of preparation. Education, planning for proper staff and radioisotope availability, time of injection, imaging, and interpretation all play an important role in offering a quality service. Education. It is necessary to educate the emergency department staff and the cardiology and internal medicine staffs about the benefits of acute myocardial perfusion imaging. This procedure is not for all patients who come to the emergency department with chest pain. Rather, it is to aid in the evaluation of patients with acute chest pain syndromes believed to be cardiac in origin but who have normal or nondiagnostic ECGs, normal serial enzyme evaluations, with or without suspicious but not definitive symptoms. 2 An algorithm can be developed to help physicians recognize and understand how acute myocardial imaging can be best used and aid them in the care of the patient (Figure 1). Planning. Adequate planning should be done to allow for appropriate staff and availability of radiopharmaceutical agents. Appropriate staff members may include a technologist, a nurse to monitor patients in potentially unstable condition, and physician for interpretation. It is also necessary to arrange for adequate radioisotope availability. The 2 options available include a prepared vial or unit dose. Many radiopharmacies now offer programs for acute use. These programs involve having a precalibrated dose available at all times with credit issued for those doses that are not used. There is a minimal monthly charge associated with this service, but it may be cost-effective for laboratories that want to offer 24-hour service. If 24-hour service is offered, an area should be set aside for storage of the isotope; most importantly, someone should be made responsible for handling, record keeping, and disposal of isotopes. Depending on state
Mann, White, and Heller Acute myocardialperfusion imaging
623
regulatory agencies or the nuclear regulatory commission, the technologist may need to be solely responsible for these tasks. injection. It is very important that the injection be given as close to the onset of symptoms as possible and before any therapy is administered. Many laboratories choose to have the covering technologist on call, outside the facility, instead of on site. To ensure the injection is given at the appropriate time, it may be necessary to train emergency department staff members to perform the injection. When 24-hour coverage is desired this can be especially important. Stowers et al 9 presented an abstract in 1995 that showed the dramatic decrease in sensitivity (97% to 33%) and specificity (78% to 40%) when radioisotopes were injected after symptoms had resolved. Imaging. When acute myocardial perfusion imaging is performed, physicians are required to make a diagnosis from only 1 image set. It is very important to provide the best-quality images and information available. Superior image quality is dependent on the proper count rate, which can be achieved by injecting 25 to 30 mCi of a Tc-labeled perfusion agent. To avoid artifact caused by increased tracer uptake in the liver, it is important to note that acute injections are given under rest conditions and imaging should not be performed for at least 45 minutes after injection. Since evaluation of ventricular function in addition to myocardial perfusion is important and useful, a gated single photon emission computed tomography (SPECT) acquisition should be performed on all patients. Images should be acquired for 64 projections, for 30 seconds per projection, with a 64 x 64 matrix and a 180degree arc, with a window of no more than 40% for gated images. These parameters should be used for all types of equipment including single-, dual-, and triple-head cameras to ensure an optimal study. Also, if available, attenuation correction should be performed. Processing. Processing of the images should be per~ formed according to the same parameters as a stress/rest gated myocardial perfusion study. It is important to use a slice thickness of 1.0 pixel when processing slices to ensure defects are not missed and a slice thickness of 2.0 pixels is used for the gated cine loop to ensure optimal image quality and proper evaluation of wall motion. Interpretation. Images should be interpreted by a qualified nuclear medicine physician, cardiologist, or radiologist. All pieces of information available should be used including slices, gated cine loop, and unprocessed projection data. This allows the physician to make the most accurate diagnosis possible. It is important to have the acute images interpreted and a report given as soon as possible after imaging is completed. This allows emergency physicians to make
624
Mann, White, and Heller Acute myocardialperfusion imaging
Journal of Nuclear Cardiology November/December1998
" Patientpres;ntsto ED with Gh;st Pain
IRapidPainAssessment/HistorylECGITriageII Acute M'
"
I
' nC n" 'e I I Equivocal I i
ii
I ou'e' 'no I I '1 ,,
--]Admit
1
I
I F°ll°w'up Testing ~---~
I
Discharge I
Figure 1. Sample algorithm for patients in the emergency department with chest pain.
decisions based on the results of the acute images to either admit the patient for further evaluation or to discharge the patient. TECHNICAL CHALLENGES A potential source of error in the interpretation of acute myocardial perfusion imaging involves artifacts related to soft tissue attenuation, increased liver activity, and motion. It is the role of the technologist to ensure optimal image quality each time acute imaging is performed. Soft T i s s u e A t t e n u a t i o n . Soft tissue attenuation may result in an apparent localized decrease in count density. 10 The location, size, and severity of the defect depends on the attenuation in relation to the myocardium and its size and density. The most common types of soft tissue attenuation are caused by breast tissue, lateral chest wall fat, and the diaphragm. Soft tissue attenuation is often the cause of a false-positive interpretation because of deficient counts misinterpreted as scar or ischemia. To avoid artifact from soft tissue attenuation, many companies have developed attenuation correction packages. If such technology is available and validated, its use is recommended in combination with other information to allow for optimal image interpretation. Liver Activity. As mention earlier, it is necessary t o allow adequate time after injection for liver clearance of the radiopharmaceutical. No hemodynamic changes occur during acute myocardial ischemia; hence the injection is given under rest conditions. Image acquisition after an acute injection should begin no sooner than 45 minutes after injection, similar to a rest injection with any Tc-labeled myocardial perfusion agent. 6-8 Because
of the location of the liver in relation to the inferior wall of the myocardium, artifacts can result during back-filter projection processing, quantitative analysis, and polar plot generation. 11-13 N o t i o n . Motion can be another source of error in the interpretation of acute myocardial perfusion imaging. Several sources of motion can be related to the camera; however, the most common is patient motion as many patients are still experiencing chest pain or are anxious in a hospital setting. When cardiac motion occurs, misalignment of data results in telltale artifacts in the tomographic slices. 10 This appears as misalignment of the anterior and posterior walls with "tails" of activity curving into the background. To avoid motion-related artifact the technologist should make sure the patient is as comfortable as possible before starting the acquisition. If necessary, pillows should be placed under the patient's knees or arms for comfort, proper arms rest should be used, and most importantly, technologists need to thoroughly explain the procedure to the patient before imaging begins. The technologist should be alert and aware of the patient throughout the acquisition. Unprocessed projection data should be reviewed on completion of the acquisition and if motion is seen the acquisition should be repeated. If the patient's condition is unstable or the patient is not able to withstand another acquisition, motion correction should be applied before processing the study. VALIDATION Several studies have been performed to demonstrate the validity of acute myocardial perfusion imaging. Varetto et al 3 performed a study to determine the role of acute imaging in patients with chest pain. Sixty-four patients seen in the emergency department with chest pain suspected to be cardiac in origin and who had nondiagnostic ECGs were injected with 99mTc-labeled sestamibi. The results of this study demonstrated 100% sensitivity, 92% specificity, 100% negative predictive value, and 90% positive predictive value for acute imaging compared with the final coronary artery disease diagnosis. Other studies performed by Hilton et al t3 and Gregoire and Theroux ]4 concluded that acute myocardial perfusion imaging with 99mTc-labeled sestamibi, when used in conjunction with other standard evaluations, offers a high accuracy rate and can be used as a reliable diagnostic tool in the evaluation of chest pain syndromes. C O S T ANALYSIS Emergency department evaluation for acute chest pain syndromes exceeds $5 million in the United States
Journal of Nuclear Cardiology Volume 5, Number 6;622-5
annually. 15 More than 40% of the evaluations of these patients lead to costly hospital admissions with only a small percentage of patients sustaining acute myocardial infarction. 15 Performance of acute myocardial perfusion imaging may eliminate or reduce the length of hospital stay for patients seen in the emergency department with low to moderate risk of ischemic heart disease, resulting in lower overall expense for patients. The average cost of an acute imaging study is approximately $600. For a patient who is hospitalized for 1 night because of a nondiagnostic or equivocal ECG, the cost can exceed $2,000. A study by Heller et a116 evaluated the clinical use and cost analysis of acute imaging with 99mTc-labeled tetrofosmin. The results demonstrated that use of a normal acute SPECT image as a criterion for not admitting patients would result in a 57% reduction in hospital admissions and a mean cost savings of $4,258 per patient.
CONCLUSION Acute myocardial perfusion imaging involves careful planning from the nuclear medicine or nuclear cardiology laboratory to ensure optimal results are achieved. The role of the technologist is to ensure a high-quality study is performed on every patient who is referred to the laboratory. This is one of the most important roles because the decision for further evaluation can be based on the interpretation of the acute images. When acute myocardial perfusion imaging is used appropriately, in conjunction with standard methods of evaluation for patients presenting to the emergency department with chest pain syndromes perceived to be cardiac in origin, it can be of great benefit. It offers a more definitive diagnosis of chest pain syndromes and can be used to reduce the expense of otherwise costly hospital stays, even in patients with moderate risk of ischemic heart disease.
References 1. Dupont/Merck Rhadiopharmaceuticals. Cardiolite use for acute chest pain management. Implementation and logistical guidebook. Billerica, Mass.: 1995. 2. Selker HE Zalenski RI, Antman EM, et al. An evaluation of technologies for identifying acute cardiac ischemia in the emergency depart-
Mann, White, and Heller Acute myocardial perfusion imaging
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