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Principles of nuclear cardiology

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Principles of nuclear cardiology

  1. 1. Principles of nuclear cardiology
  2. 2. History • Hermann blumgart-1927-injected radon to measure circulation time • Liljestrand-1939-normal blood volume • Myron prinzmetal-1948- radiolabelled albumin • Hal anger-1952-gamma camera-beginning of clinical nuclear cardiology • 1976-thallium201-two dimensional planar imaging
  3. 3. • 1980s-SPECT using rotating anger camera • 1990-technetium99m based agents and gated SPECT • 90% of SPECT in U.S use technetium and 90% are gated SPECT
  4. 4. SPECT single photon emission computed tomography
  5. 5. Basic concept • Intravenously injected radiotracer distributes to myocardium proportional to blood flow • Gamma camera captures the photons, converts to digital data and displays it as a scintillation event • Parallel hole collimator-better localisation of source • Photomultiplier tubes-conversion of signals • Final result-multiple tomograms of radiotracer distribution
  6. 6. SPECT image display • Short axis images-perpendicular to long axis of the heart,displayed from apex to base • Vertical long axis-parallel to long axis of heart and parallel to long axis of body • Horizontal long axis-parallel to long axis of heart,perpendicular to VLA slice
  7. 7. SPECT
  8. 8. SPECT perfusion tracers • Thallium 201 • Technetium–99m – Sestamibi (Cardiolyte) – Tetrafosmin (Myoview) – Teboroxime • Dual Isotope – Thallium injected for resting images – Tech -99m injected at peak stress
  9. 9. Thallium-201 • Monovalent cation,property similar to potassium • Half life 73 hours,emits 80keV photons,t½ 73hrs,85% first pass extraction • Peak myocardial concentration in 5 min, rapid clearance from intravascular compartment • Redistribution of thallium-begins 10-15 min.after ,related to conc.gradient of thallium between myocyte and blood
  10. 10. • Differential washout-clearance is more rapid from normal myocardium • Hyperinsulinemic states reduce blood conc.&slow redistribution.so fasting recommended
  11. 11. • Thallium protocols- – Stress protocols-injected at peak stress and images taken at peak stress and at 4 hrs,24hrs – Reversal of a thallium defect marker of reversible ischemia – Rest protocols-thallium defect reversibility from initial rest images to delayed redistribution images reflect viable myocardium with resting hypoperfusion – Initial defect persists-irreversible defect
  12. 12. • Stress/redistribution/reinjection method commonly used • Reinjection if fixed defects seen at 4 hrs • Timing of stress image-early • Rest redistribution image for resting ischemia/viability
  13. 13. Technetium-99m labelled tracers • Half life 6 hrs,140keV photons,60% extraction • Uptake by passive distribution by gradient • Minimal redistribution-require two separate injections-one at peak stress and one at rest • Single day study-first injected dose is low • Two day study-higher doses injected both rest and stress-optimise myocardial count rate- larger body habitus
  14. 14. • Tc99m tracers bound by mitochondria.limiyed washout occurs.so imaging can commence later and can be repeated
  15. 15. • 2 day image protocol better for image quality • Most common-same day low dose rest/high dose stress-disadvantage is reduction in stress defect contrast. • Viability assessment improved by NTG prior to rest study
  16. 16. Dual isotope protocol • Anger camera can collect image in different energy windows • Thallium at rest followed by Tc 99m tracer at peak stress • If there is rest perfusion defect,redistribution imaging taken either 4 hrs prior or 24hrs after Tc99m injection
  17. 17. Radionuclide Properties Property Thallous Chloride Tc-Sestamibi Chemistry +1 cation, hydrophilic +1 cation, lipophilic half life 73 hrs 6 hours Photon energy 68-80 keV 140 keV Uptake Active: Na-K ATPase pump Passive diffusion (if intact membrane potentials) Extraction fraction 85% 66% Heart uptake 4% 1.2% Redistribution Redistributes Fixed
  18. 18. Stress protocols
  19. 19. • Dipyridamole infusion for 4 min-isotope injection 3 min after infusion • Adenosine infusion for 6 min-isotope given 3 min into infusion
  20. 20. Interpretation and reporting • Myocardium devided into 17 segments on the basis of 3 short axis and a long axis slice • Perfusion graded from 0(normal perfusion) to 4(no uptake) • SSS-summed stress score-stress perfusion abnormality • SRS –summed rest score-extent of infarction • SDS-summed difference score-stress induced ischemia
  21. 21. Visual Analysis of Perfusion SPECT • 0-normal uptake, • 1-mildly reduced uptake, • 2-moderately reduced uptake, • 3-severely reduced uptake, and • 4-no uptake
  22. 22. • bull̒s eye polar plot-two dimensional compilation of all three dimensional short axis perfusion data
  23. 23. Ant Inf LatSep Apex  Base Ant Inf Apex Septum  Lateral Apex Sep Lat Inferior  Anterior Stress Stress Stress Rest Rest Rest Normal
  24. 24. Ant Inf LatSep Apex  Base Ant Inf Apex Septum  Lateral Apex Sep Lat Inferior  Anterior Stress Stress Stress Rest Rest Rest Reversible Ischeamia, defect appears at stress and disappears during rest
  25. 25. Ant Inf LatSep Apex  Base Ant Inf Apex Septum  Lateral Apex Sep Lat Inferior  Anterior Stress Stress Stress Rest Rest Rest Fixed Scar, defect is seen in both stress and rest
  26. 26. Interpretation of the Findings-SPECT Stress Rest Interpretation • No defects No defects Normal • Defect No defect Ischemia • Defect Defect Scar/ hibernating • Defect location (anterior, posterior, lateral, or septal wall), size (small, medium, or big), severity (mild, moderate, absent), degree of reversibility at rest (completely reversible, partially reversible, irreversible) • Regional wall motion, EDV, ESV, EF (Stress-induced ischemia)
  27. 27. Additional signs • Lung uptake of thallium • Transient ischemic dilatation of left ventricle
  28. 28. Thallium-201 Lung Uptake • ↑ lung uptake of thallium following stress -marker of severe CAD,elevation of PCWP,↓EF • ↑PCWP-slow pulmonary transit-more extraction • Minimal splanchnic uptake,early image after stress-lung uptake more apparent in thallium • More liver uptake,delayed imaging-lung uptake missed with Tc99m
  29. 29. TID: transit Ischemic Dilation (Stress induced LV Cavity Dilation) • Severe, extensive CAD (usually with classic ischemic defect) Left Main Prox LAD MVD diffuse subendocardial ischemia
  30. 30. Variations • Dropout of the upper septum • Apical thinning • Lateral wall may appear brighter than septum • Minimised by review of series of normal volunteers
  31. 31. Technical artifacts • Breast attenuation- – Minimised by Tc99m agents,ecg gated SPECT – Presence of preserved wall motion and thickening • Inferior wall attenuation – Diaphragm overlapping inferior wall – Minimised by gated SPECT,prone position • Extracardiac tracer uptake – Repeat imaging,drink cold water to clear tracer from visceral organs
  32. 32. • LBBB- – isolated reversible perfusion defects of septum – Heterogeneity of flow b/w LAD &LCx due to delayed septal relaxation – Reduced O2 demand due to late septal contraction,when wall stress is less • HCM- – due to ASH,appearance of lateral perfusion defect
  33. 33. • Combined SPECT/CT or PET/CT scanners- complementary anatomical and functional information
  34. 34. Gated SPECT • Simultaneous assessment of LV function and perfusion • Each R-R interval is devided into prespecified number of frames • Frame one represent end diastole,middle frames end systole • An average of several hundred beats of a particular cycle length acquired over 8-15 min.
  35. 35. • Normal regional systolic function-brightening of wall during systole • Quantitative analysis of LV function-three dimensional display representing global LV function created by information from all tomographic slices-EF and LV volumes calculated
  36. 36. Radionuclide ventriculography • MUGA scanning-multiple gated acquisition – Tc 99m labelled r.b.c or albumin – Image constructed over an average cardiac cycle by e.c.g gating,16-32 frames /cycle – Image acquired in antr.,LAO, left lateral projections – Size of chambers,RWMA,LV function – Time activity curve-LV volumes
  37. 37. • First pass RVG-i.v injected radioactive tracer passes through rt.chambers-lungs-lt.chambers • Tc99m DTPA preferred • RAO projection • 2-5 cycles summed for RV phase,5-7 for LV phase • Time activity curves generated-quantitative analysis
  38. 38. PET • Radiotracers labelled with positron emitting isotopes • Perfusion tracers-Rb82 and n13 ammonia • Metabolic tracer-F18 FDG • Beta decay-positron emission • Annihilation-collide with electron-give two gamma rays of 511keV-travel in opp.direction • PET scanner detects opposing photons in coincidence-spatial and temporal resolution
  39. 39. Perfusion tracers • Diffusible tracers-O-15-accumulate and wash out. • Non diffusible-Rb82,N13ammonia • Rb82-generator produced,t½76s.
  40. 40. Advantage of PET • Higher spatial resolution • Improved attenuation correction • Quantification regional blood flow – SPECT may fail to detect balanced ischemia in multivessel CAD – ↓blood flow reserve by PET –early identification of CAD • Higher sensitivity and specificity(95%)for detection of CAD
  41. 41. Limitations • High cost • Requirement of cyclotron • Short half life-pharmacological stress only
  42. 42. Metabolic tracers • C-11 palmitate • I-123 BMIPP-Ischemic memory-fatty acid metabolism suppressed for longer time after an ischemic event • F18 FDG-imaging myocardial glucose utilisation with PET – Phosphorylated and trapped in myocardium – Uptake may be increased in hibernating but viable myocardium
  43. 43. • FDG uptake in regions with reduced blood flow at rest –marker of hibernation • FDG studies performed after 50 to 75 gm glucose loading 1-2 hrs prior to injection – ↑glucose metabolism,FDG uptake and improves image quality
  44. 44. • Enhanced FDG uptake relative to blood flow referred to as PET mismatch pattern indicative of viable myocardium
  45. 45. Viability PET Study • Traditionally the gold standard • Two sets of resting images to detect viable and hibernating myocardium: – Perfusion image (usually with N-13 ammonia or rubidium-82) – Glucose metabolic image (with F-18 fluorodeoxyglucose = FDG)
  46. 46. *
  47. 47. PET Viability Scan Patterns Contractility Perfusion Metabolism Normal N N N Stunning - N N - Hibernation Scar
  48. 48. Guidelines • Acute syndromes – Assessment of patients presenting to ED with chest pain – Diagnosis of AMI when other measures non diagnostic-Tc99m – Risk assessment,prognosis in AMI – Risk assessment,prognosis in NSTEMI/UA
  49. 49. Chronic syndromes- recommendations Class1- • Exercise SPECT for identifying location ,severity of ischemia in pts without baseline ECG abnormalities that interfere with ST seg.analysis • Adenosine SPECT for LBBB,paced rhythem,unable to exercise • To assess functional significance of an intermediate coronary lesion(25-75%) • Intermediate duke TMT score • Rpt.MPI for recent change of symptoms
  50. 50. • Class 2a- – 3-5 yrs after revascularisation in asymptomatic patients – As initial test in high risk patients(>20% 10yr risk) • Class 2 b- – Pts with cor.calcium score more than 75 percentile – Asymptomatic pts.high risk occupation
  51. 51. Indications for PET for risk stratification of patients with intermediate likelihood of CAD CLASS1- – SPECT study equivocal • Class 2a- – As initial test in patients unable to exercise – As initial test in pts. With baseline ECG abnormalities
  52. 52. Risk Stratification • Normal perfusion imaging after adequate stress: very low cardiac event rate < 1% • Small fixed defect with normal global LV function: good prognosis • High risk: (reversible defects) more than one territory, LAD (most important coronary artery), post-stress LV (left ventricular) dysfunction (LV dilatation, abnormal wall motion, decreased LVEF, lung uptake)

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