Upload
toril
View
54
Download
1
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
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 PropertiesProperty 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
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
• bul s eye polar plot-two dimensional l̒ �
compilation of all three dimensional short axis perfusion data
Ant
Inf
LatSep
Apex Base
Ant
Inf
Apex
Septum Lateral
Apex
Sep Lat
Inferior Anterior
Stress
Stress
Stress
Rest
Rest
Rest
Normal
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
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
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, l̒ateral̒, or septal̒ wal̒l̒), size (smal̒l̒, medium, or big), severity (mil̒d,
moderate, absent), degree of reversibility at rest (compl̒etel̒y reversibl̒e, partial̒l̒y reversibl̒e, irreversibl̒e)
• 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 MainProx LADMVDdiffuse 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)
*
PET Viability Scan Patterns
Contractility Perfusion MetabolismNormal N N N
Stunning - N N -
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 coronary artery), post-stress LV (l̒eft ventricul̒ar) dysfunction (LV dilatation, abnormal wall motion, decreased LVEF, lung uptake)