nuclear cardiology
TRANSCRIPT
Nuclear Cardiology:Myocardial Perfusion Studies and
Ventriculography
Croft Stone
Nuclear Medicine
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
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
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
Pharmacologic stress
• Agents: – Dipyridamole (persantine)– Dobutamine– Adenosine
Indications for Pharmacologic Stress Imaging
• Inability to perform adequate exercise
• Left bundle branch block
• Ventricular pacemaker
• CCB’s or Beta blockers
• Evaluation of patients very early after acute MI (<3 days) or very early after stenting (<2 weeks)
Physics
• Radioactive decay– Alpha particles (ionized helium nuclei)– Beta particles (high energy electrons or
positrons)– Gamma rays (photons)– Electron capture (x-rays)
Gamma Camera
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.
Protocols
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
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
Tc-99m
• Technetium chelated to to a molecule that will be absorbed by the myocardium– Tc-99m-methoxyisobutyl (setamibi)– Tc-99m 1,2bis[bis (2-
ethoxyethyl)phosphinoethane (tetrofosmin)
• During stress, metabolism changes polarization of cell membrane, driving agent into cell
• Also readily absorbed by liver and bowel
Tc-99m preparation
Tc-99m
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
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
Technique
• Stannous pyrophosphate injected
• Tc-sodium pertechnetate injected
• Pertechnetate enters RBC’s, becomes reduced by the intracellular stannous ion, and is bound to hemoglobin
• RBC’s now “tagged” with radioisotope: hence “blood pool” image
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
Technique
• Heart rate variations can result in temporal blurring (mixing of counts in adjacent frames).
• Beat rejection window usually set at 20%
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.