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.