Nuclear Cardiology PDF

Nuclear Cardiology Frank Meissner, MD, FACP, FACC, FACP Emergency Medicine VA Medical Center - Salt Lake City  General Approach IV administration of radiolabeled agents Scintillation or positron camera Computer processing Physiologic/functional data, rather than st...

Nuclear Cardiology Frank Meissner, MD, FACP, FACC, FACP Emergency Medicine VA Medical Center - Salt Lake City  General Approach IV administration of radiolabeled agents Scintillation or positron camera Computer processing Physiologic/functional data, rather than structural/anatomic e.g., Myocardial perfusion imaging is prognostically more important than classification by angiography  Types of Studies/Agents Myocardial Perfusion Thallium-201 (201Tl), technetium-99m sestamibi Blood Pool/First Pass Tc 99m pertechnetate Myocardial Infarct PET Myocardial Perfusion -Thallium 201 imaging Intracellular transport by passive and active mechanisms Early myocardial uptake directly proportional to regional myocardial blood flow and myocardial extraction fraction Myocardial Perfusion -Thallium 201 imaging After initial phase continuous exchange of myocardial 201Tl and extracardiac 201Tl This process of continuous exchange is the basis of 201Tl redistribution Thallium, metallic element group IIIA - periodic table  Thallium Redistribution Defined as total or partial resolution of initial postexercise defects Reimaging at 2.5 to 4 hrs after tracer injection Late reimaging is performed when defects are believed to be due to severe ischemia  Technical Considerations 2.5 to 3.0 mCi of 201Tl via IV cannula End-points: angina , dyspnea, fatigue, claudication, hypotension Exercise 30 to 45 s so that initial myocardial uptake reflects peak exercise Image within 5 minutes post exercise Thallium TMT interpretation Decreased 201Tl uptake ischemia or scar Reversible defects = ischemia 30% of persistent defects = severe ischemia rather than scar Reinjection protocols reveals reversibility in 40% of 4 hr ‘defects’ 24 hr redistribution imaging show reversibility in 20-25% of ‘fixed’ 4 hour Sensitivity/Specificity Considerations Qualitative visual 201Tl using planar imaging sensitivity and sensitivity of 84 & 87% respectively Quantatively analysis (computer assistance) 90% sensitivity & specificity Spect 201 Tl increased sensitivity with decreased specificity  SPECT ADVANTAGES Images free of background Lesion contrast higher Localization of defects is more precise and more clearly seen by the inexperienced eye Extent and size of defects better defined  Sensitivity Factors Left circumflex lesions difficult to ID Branch stenoses of arteries more difficult Sensitivity for single vessel disease.5) Exercise induced transient LV dilatation  PROGNOSTIC Characteristics Presence of reversible defects worse prognosis than fixed defects Total number of defects best prognositic indicator vs Presence of Lung uptake (reported as POORER prognosis than total segments) Chest Pain + TOTALLY normal 201Tl scans < 1% yrly risk of sudden death or  Resting Thallium Useful technique for case selection in patients with depressed LVEF & CAD ‘Hibernating’ myocardium preserved 201Tl uptake at rest IV 201Tl imaged at 20 mins and 4 hours Resting hypoperfusion will demonstrate initial defects that fill with redistribution Asynergy with preserved 201Tl uptake improved systolic function post-Bypass  201Tl Limitations Breast tissue attenuates tracer penetration Large RV blood pool overlying inferior wall on Anterior Projection => artifact High left Hemidiaphragm overlying post wall SPECT imaging relatively less than 201Tl activity in the inferobasilar  99mTc sestamibi Imaging I Lipophilic cationic 99mTc-complex whose myocardial uptake proportional to blood flow 140 keV photon energy peak is optimized for gamma camera imaging Produces higher quality images than those produced by 201Tl 99mTc sestamibi Imaging II Shorter half life than 201Tl permits administration of 10-15 X’s as high a dose of tracer than 201Tl Gated acquisition of SPECT allows for animation and rgn wall motion analysis First PASS acquisition can yield LVEF at rest or exercise with animation for MUGA like scans done prior to 99mTc-sestamibi Technical Characteristics Does NOT redistribute after injection Separate injections during stress and resting states Ideal protocol is 24 hours between rest and stress However, current practice is to inject and image rest followed by Stress images Increased photon energy defeats  Sensitivity & Specificity Sensitivity is reported at 85-90% range Increased specificity with superior image quality and decreased image artifacts Some authors report more individually stenosed arteries are dected by sestabmibi SPECT than 201Tl SPECT  Pharmacologic Stress Imaging Useful for patients UNABLE to exercise to reasonable double products Adenosine vs dipyridamole protocols Critical coronary stenosis detected by reduced flow reserve in stenotic area Degree of vasodilatation is less relative to increase in flow in normal segments Sensitivity and specificity is comparable to that reported with exercise protocols Dipyridamole Stress Imaging: Cavets/Technique NO caffeinated beverages for 12 hrs prior to the test NO use of Theophylline compounds 0.56 mg/kg dipyridamole infused over 4 minutes 3.0 mCi 201Tl injected at 9 minutes, with intial images at 5 mins post injection Aminophylline (50-100 mg IV) for systemic hypertension, chest pain, Adenosine Stress Imaging: Technique IV adenosine 140ug/kg/min for 6 minutes 3 mins after starting infusion inject with 3.0 mCi dose of 201Tl in contralateral vein Additional 3 minute infusion of adenosine  USEFULNESS of Pharmacological stress testing Predominately for PreOp eval of vascular surgical patient Preoperative 201Tl defects experience a 7X > periorperative ischemic event rate PATIENTS WITH RECURRENT ANGINA AT REST should NOT receive pharmacological stress testing  Radionuclide angiography Blood pool imaging rather than myocardial avid tracers Information obtained is identical to that of contrast ventriculography In vivo labeling with 99mTc IV stannous pyrophosphate is injected 15-20 mins prior to 15-30 mCi of 99mTc  Uses I Differentiation of ischemic from nonischemic cardiomyopathy Cancer patient monitoring for doxorubicin by serial estimate of EF RV fxn and size with first pass for suspected RV infarction RV dynamics in COPD  Uses II Bicycle ergometry for detection of CAD Timing of valve replacement in regurgitant valvular disease - serial studies Post MI risk stratification  First Pass Imaging Single bolus of 99mTc is injected rapidly via IV (preferably central line) Analysis is limited to intial transit Multicrystal scintillation camera prefered to single-crystal Anger Camera, since high count Rates (up to 400,000 counts/sec) can be obtained with multiple crystal  Equilibration Imaging I Multiple Gated Acquisition Scan (MUGA) Equal subdivisions of the patients cardiac cycle Generally 30-50ms framing interval at rest or 20-30 ms for exercise study 200 successive cardiac cycles, with R-wave gating  Equilibration Imaging II Several algorithms and methods to deal with R-R wave variability DO NOT SEND A Fib or Bigemy/Trigemeny patient to MUGA Time Activity Curve - Relative volume curve Displayed with activity in LV vs time Change in activity is proportional to change in LV volume  Data Obtained LVEF RVEF Peak systolic ejection rate Regurgitant fraction Diastolic filling time Left to right intracardiac shunts Phase analysis  MUGA Advantages Highly reproducible EF Able to image patients NOT well seen with Echo NO geometric assumptions are made in the calculation of EF (MOST IMPORTANT ADVANTAGE) Wall motion analysis is VERY comparable to ECHO & LV-gram  Myocardial infarction imaging Radiopharmaceutical is preferentially sequestered in necrotic myocardial tissue Yields a hot spot on the scan 99mTc pyrophosphate first agent used Scan is abnormal 12-24 hours after MI Recent interest in indium-111(111In) antimyosin antibodies  Technique Fab fragments of antimyosin antibodies labeled with 111In Images obtained one hour after injection  Clinical Uses NOT useful for routine patients Late patients (>2 days from Chest pain with negative enzymes and equivocal EKG’s) Surgical MI’s, i.e., patient felt to have MI in OR Acute Myocarditis Acute and chronic allograft rejection  PET Positron emission tomography Imaged with short half life positron emitors such as carbon-11 (11C), nitrogen-13 (13N), oxygen-15 (15O), fluorine-18 (18F), and rubidium-82 (82Rb) Generator produced half lifes of 75 seconds to 2 minutes  Uses Research TOOL ONLY For testing purposes the hallmark of Myocardial Viability in ‘stunned’ or hibernating myocardium is increased FDG (18F, 2-fluoro-2-deoxyglucose) activity in myocardial tissue Diminished perfusion with increased FDG activity is due to glycolysis Diminished perfusion with diminished FDG activity implies NO viability THE END