Nuclear Cardiology: Myocardial Perfusion Studies and Ventriculography

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Nuclear CardiologyMyocardial Perfusion Studies
and Ventriculography

• Croft Stone
• Nuclear Medicine

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Basics of MPI

• Radionuclide injected at rest and/or stress
• Radionuclide taken up by myocardium and gamma
  rays emitted
• Rest images compared with stress images
• Decreased perfusion stress and rest MI
• Decreased perfusion at stress, normal with rest
  ischemia
• Area indicates the coronary artery, size
  correlates with severity of CAD

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Basics

• Two conditions necessary for blood flow deficit
  measurement
• 1) coronary flow must be elevated to near maximal
  levels
• 2) radiotracer whose myocardial extraction is
  proportional to coronary artery blood flow must
  be used

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Stress

• Exercise and pharmacologic agents are used to
  achieve maximal coronary dilation and flow.
• Exercise stress gives additional information
• Degree of exercise tolerance
• Time to maximal heart rate
• Blood pressure response

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Pharmacologic stress

• Agents
• Dipyridamole (persantine)
• Dobutamine
• Adenosine

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Indications for Pharmacologic Stress Imaging

• Inability to perform adequate exercise
• Left bundle branch block
• Ventricular pacemaker
• CCBs or Beta blockers
• Evaluation of patients very early after acute MI
  (lt3 days) or very early after stenting (lt2 weeks)

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Physics

• Radioactive decay
• Alpha particles (ionized helium nuclei)
• Beta particles (high energy electrons or
  positrons)
• Gamma rays (photons)
• Electron capture (x-rays)

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Gamma Camera
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Camera

• Multiple images taken at different rotation
  angles to obtain 3-D information
• Lead collimator excludes photons not traveling in
  direction of holes in the collimator
• 3-D picture can be reconstructed using a
  mathematical model
• Projection system modeled as system of
  simultaneous linear equations matrix is then
  inverted to reveal the source distribution.

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Protocols
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Radionuclide Properties
Property Thallous Chloride Tc-Sestamibi
Chemistry 1 cation, hydrophilic 1 cation, lipophilic
Shelf life 6 days 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
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Comparison of 201Tl and 99mTc for Myocardial
Perfusion Imaging
Property 201Tl 99mTc
Photon energy 69-80 keV Scatter and absorption Low resolution 140 keV Less scatter and absorption High resolution
Half life 73 hours Low dosage (2-3 mCi) Low count densities High dosage (20-30 mCi) High count densities
Effective dose 1.3 rad 1.1 rad
Availability Cyclotron-commercial mfr Generator - local
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Tc-99m

• Technetium chelated to to a molecule that will be
  absorbed by the myocardium
• Tc-99m-methoxyisobutyl (setamibi)
• Tc-99m 1,2bisbis (2-ethoxyethyl)phosphinoethane
  (tetrofosmin)
• During stress, metabolism changes polarization of
  cell membrane, driving agent into cell
• Also readily absorbed by liver and bowel

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Tc-99m preparation
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Tc-99m
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Quality control

• 1. Motion -- There is no evidence of patient
  motion.2. Alignment --The alignment is very
  good.3. Count Increase --The myocardial max
  counts increases in the stress study as
  expected.4. Normalization --Both studies are
  normalized to the portion within the myocardium
  with the highest uptake.5. Extra-Cardiac
  Activity --There is no significant extra cardiac
  activity.6. Soft tissue attenuation minimized
• 7. Protocol consistency

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ECG Gated SPECT imaging(MUGA multi gated
acquisition)

• Simultaneous assessment of perfusion and function
  in a single injection, single acquisition
  sequence.
• Tc-99m permits evaluation of regional myocardial
  wall motion and wall thickening throughout the
  cardiac cycle
• Quantitates LV volume and EF

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Technique

• Stannous pyrophosphate injected
• Tc-sodium pertechnetate injected
• Pertechnetate enters RBCs, becomes reduced by
  the intracellular stannous ion, and is bound to
  hemoglobin
• RBCs now tagged with radioisotope hence
  blood pool image

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Technique

• Images of heart are triggered (gated) on the R
  wave of the ECG
• 32 or more frames taken per cardiac cycle
• Many cardiac cycles imaged and stored for
  statistical significance
• Total amount of activity stored in frames at each
  gated time point plotted vs. total time cycle

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Technique

• Heart rate variations can result in temporal
  blurring (mixing of counts in adjacent frames).
• Beat rejection window usually set at 20

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Sources

• Cerqueira, Manuel D. Nuclear Cardiology, 1994, pp
  93 100, 103-109.
• Crean, Andrew and Coulden, Richard. Cardiac
  Imaging using nuclear medicine and positron
  emission tomography. Radiologic Clinics of North
  America, 42 (2004) 619-634
• Heller,Gary V. and Hendel, Robert C. Nuclear
  Cardiology Practical Applications, 2004. pp
  1-312.
• Kowalsky, Richard J. and Falen, Steven W.
  Radiopharmaceuticals in Nuclear Medicine and
  Nuclear Pharmacy, 2004 pp 515 555.