Myocardial Perfusion Imaging 2008 Zoll Firm Lecture Series

Basic principles of myocardial perfusion imaging Inject radioactive tracers whose distribution in the heart are proportional to the amount of blood flow to the area Gamma camera detects the photons given off by the radioactive tracers and thus the relative blood flow to the different areas during rest and stress Detects RELATIVE, not ABSOLUTE blood flow May miss “balanced ischemia” if blood flow to all areas are reduced in the case of 3 vessel disease. 2008 Zoll Firm Lecture Series

Example of a normal scan (uniform uptake during rest and stress) Short axis base apex Vertical long axis anterior base apex inferior Septum lateral Horizontal long axis apex septal lateral base Posterior Anterior 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Infarct Decreased uptake at rest Decreased uptake during stress Defects are described by Size (this is large) Severity (this is severe) Reversibility (fixed) Location (inferior and lateral wall) Can infer coronary distribution (dom LCx or LCx and RCA lesion) 2008 Zoll Firm Lecture Series

Example of a reversible defect Uniform uptake at rest Inferior defect during stress Implies ischemia This defect is Medium-sized Moderate in intensity Reversible Located in the inferior wall Probable RCA lesion 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Commonly used tracers Technetium 99m Sestamibi Technetium 99m is a radioactive compound, sestamibi is a small cation that has been labeled with Technetium 99m. Binds the mitochondrial membrane ½ life of 6 hours Better imaging characteristics (intrinsically better and also because can inject higher dose since ½ life is shorter) Does not show “redistribution” Thallium Radioactive K analog. Enters/exits cell via Na/K ATPase ½ life of 3 days (more radioactive exposure to patients) Worse imaging characteristics (intrinsically worse and also because need to limit dose to limit radioactive exposure) Shows “redistribution”- after injection, the tracer moves to viable parts of the myocardium that initially shows less uptake 2008 Zoll Firm Lecture Series

Commonly used protocols Low dose, high dose Tc99m sestamibi same day protocol: Inject low dose of the tracer at rest Acquire images wait several hours for the tracers to decay (so that the rest pictures wouldn’t interfere with the interpretation of the stress pictures) stress the patient, iinject a higher dose of the tracer at peak stress Acquire images again Compare the pictures Most commonly used here. Dual isotope (thallium, Tc99m sestamibi) same day protocol: Inject thallium 201 at rest Acquire images Stress the patient, inject Tc99m sestamibi Advantage is that thallium and Tc99m gives off slightly different energy of photons so pictures from the rest wouldn’t “contaminate” the picture at stress Problems includes more radioactive exposure and difficulty in comparing thallium with Tc99m images. 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Other protocols Two days Tc99m sestamibi protocol -same sequence as the 1 day protocol except Inject high dose of tracer for rest and stress (don’t have to worry about high dose at rest contaminating the stress pictures since there is adequate time for the tracer to decay)- gets better quality rest pictures Wait 1 day between the stress and rest images Better for obese patients Obviously, time delay can be inconvenient. Thallium-redistribution protocol: Stress the patient and inject the tracer at peak stress Image the patient immediately Wait 3-4 hours and image the patient again (because tracer shows redistribution, it will move to areas that are alive there will be a difference between the stress and rest image if there is ischemia) Can wait 24 hours to assess viability (areas with little life will light up much later) 2008 Zoll Firm Lecture Series

Dypyridamole/Adenosine Adenosine is a small molecule with the following effects: Activation of the A2A receptors causes coronary vasodilatation- making this a useful agent for stress testing. Activation of A1 receptors causes atrioventricular (AV) conduction delay, explaining its use in the management of supraventricular arrhythmia Activation of A2B, A3, A4 receptors can mediate bronchospasm Methyxanthines (like caffeine, theophylline, aminophylline) are adenosine antagonists. Dypyridamole blocks the metabolism and therefore causes the build up of adenosine. Half life of 40-80 mintues, reversed with aminophylline. Cautious use in patients with hepatic dysfunction 2008 Zoll Firm Lecture Series

Vasodilator stress test Causes hyperemia but does not increase workload! Can only be performed with nuclear perfusion imaging Can be safely performed soon after uncomplicated acute coronary syndrome (safety tested 2 days after ACS) Brown KA et al. Circulation 100:2060, 1999 May decrease the amount of false positive anterior-septal defects in patients with LBBB compared to an exercise stress test. The absence of EKG changes does not indicate the absence of CAD. However, the presence of EKG changes is a very bad prognostic factor. 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Diagnosis Thallium SPECT- sensitivity of 88% and specificty of 77% Ann Intern Med 1999 May 4;130(9):719-28 EKG stress test- sensitivity 68%, specificity 77% Circulation 1989 Jul;80(1):87-98. Stress echocardiography- sensitivity of 76% and specificity of 88% In general, nuclear myocardial stress imaging considered more sensitive and less specific then stress echocardiography. Referral bias- only the people with positive ETT goes on to have the gold standard test. Hence, true sensitivity is probably lower, true specificity higher. 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Prognosis More important than diagnosis is prognosis! The more severe, more numerous the defects, the worse the prognosis. Patients with a normal scan have a very good short to intermediate term prognosis. Among 5183 patients who underwent myocardial perfusion imaging, the presence and severity of an abnormal scan was predictive of cardiac death or a myocardial infarction during a two year follow-up. Data from Hachamovitch, R, Berman, DS, Shaw, LJ, et al, Circulation 1998; 97:535. 2008 Zoll Firm Lecture Series

Doesn’t erase exercise performance, symptoms, EKG changes…etc Patients with a normal scan have a good prognosis but NOT if they have a high duke treadmill score! Patients with a mildly abnormal scan does not have a good prognosis if they have an intermediate to high duke treadmill score. Event rate (death/MI) in 2200 conse- Cutive patients during 1.5 yrs f/u Data from Hachamovitch, R, Berman, S, Kiat, H, et al, Circulation 1996; 93:905. 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Gated Imaging Divides the cardiac cycle into phases Data collected during each phase is pooled to form a single image Images from each phase are put together to compose a series of images called a cine Further information can then be obtained from this data by applying computer algorithms 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series Gated Imaging 3D images allow for accurate quantification of volumes in each phase of the cardiac cycle Calculated by using computerized edge detection to determine the endocardial border LVEF = 1-(ESV/EDV)- good correlation with echo and CMR. To get accurate quantification, the computer must be able to accurately detect the endocardium Needs regular rhythm Motion or other artifacts that significantly affect the perfusion images can reduce accuracy Severe defects (real or attenuation) is a problem No counts, no border Small hearts- resolution not high enough to be accurate 2008 Zoll Firm Lecture Series

Attenuation artifacts diagphragmatic attenuation or infarct? Where there is overlying soft tissue, less photons from the heart penetrate these tissue and get to the camera, creating an apparent defect. Fixed defects can represent either myocardial infarction or an artifact due to soft tissue attenuation Inferior defects often due to diagphragm, anterior defect due to breast Difficult to distinguish between attenuation and infarct Soft tissue attenuation is very common Major limitation in the specificity of SPECT imaging for the detection of CAD Breast attenuation or infarct? 2008 Zoll Firm Lecture Series

How to deal with attenuation? Tend to be of mild intensity, but can be moderate Usually follow one of these typical patterns Usually evidence of attenuation on the projection images or the attenuation map Attenuation correction software available Works better for inferior attenuation defects Prone Imaging Inferior attenuation decreased (gets the diagphragm out of the way) Look at Gated Images The wall motion with fix defect should be abnormal if it’s an infarct, normal if it is attenuation Bottom line, interpret mild inferior defects and mild anterior defects in woman with caution 2008 Zoll Firm Lecture Series

Zoll Firm Lecture Series High risk features TID in a patient with multiple severe defects Transient ischemic dilatation RV uptake post stress Increased pulmonary tracer uptake Decrease in post-stress LVEF Increase RV uptake post-stress 2008 Zoll Firm Lecture Series