Principles of nuclear cardiology
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Principles of nuclear cardiology. 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

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Principles of nuclear cardiology

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Principles of nuclear cardiology


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


  • 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


SPECT single photon emission computed tomography


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


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


SPECT


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


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


  • Differential washout-clearance is more rapid from normal myocardium

  • Hyperinsulinemic states reduce blood conc.&slow redistribution.so fasting recommended


  • 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


  • 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


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


  • Tc99m tracers bound by mitochondria.limiyed washout occurs.so imaging can commence later and can be repeated


  • 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


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


Radionuclide Properties


Stress protocols


  • Dipyridamole infusion for 4 min-isotope injection 3 min after infusion

  • Adenosine infusion for 6 min-isotope given 3 min into infusion


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


Visual Analysis of Perfusion SPECT

  • 0-normal uptake,

  • 1-mildly reduced uptake,

  • 2-moderately reduced uptake,

  • 3-severely reduced uptake, and

  • 4-no uptake


  • bull̒s eye polar plot-two dimensional compilation of all three dimensional short axis perfusion data


Ant

Stress

Apex

Inf

Rest

Septum  Lateral

Stress

Apex

Rest

Lat

Inferior  Anterior

Sep

Ant

Stress

Lat

Sep

Rest

Inf

Apex  Base

Normal


Ant

Apex

Stress

Inf

Rest

Septum  Lateral

Stress

Apex

Rest

Lat

Inferior  Anterior

Sep

Ant

Stress

Lat

Sep

Inf

Rest

Apex  Base

Reversible Ischeamia, defect appears

at stress and disappears during rest


Ant

Apex

Stress

Inf

Rest

Septum  Lateral

Stress

Apex

Rest

Lat

Inferior  Anterior

Sep

Ant

Stress

Lat

Sep

Inf

Rest

Apex  Base

Fixed Scar, defect is seen in both stress and rest


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 septalwall), 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)


Additional signs

  • Lung uptake of thallium

  • Transient ischemic dilatation of left ventricle


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


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


Variations

  • Dropout of the upper septum

  • Apical thinning

  • Lateral wall may appear brighter than septum

  • Minimised by review of series of normal volunteers


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


  • 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


  • Combined SPECT/CT or PET/CT scanners-complementary anatomical and functional information


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.


  • 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


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


  • 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


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


Perfusion tracers

  • Diffusible tracers-O-15-accumulate and wash out.

  • Non diffusible-Rb82,N13ammonia

  • Rb82-generator produced,t½76s.


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


Limitations

  • High cost

  • Requirement of cyclotron

  • Short half life-pharmacological stress only


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


  • 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


  • Enhanced FDG uptake relative to blood flow referred to as PET mismatch pattern indicative of viable myocardium


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)

Cellular membrance integrity

Glucose metabolism


*


PET ViabilityScan Patterns

ContractilityPerfusionMetabolism

Normal NNN

Stunning- NN -

Hibernation

Scar


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


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


  • 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


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


  • 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 coronaryartery), post-stress LV (left ventricular) dysfunction (LV dilatation, abnormal wall motion, decreased LVEF, lung uptake)


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