Radionuclide studies

Question 1

How does radionuclide study evaluate cardiac shunt

Answer 1

• 1st pass radionuclide angiocardiography
  o Records passage of radioactive bolus through central circulation
  o CrVC → RA → RV → PAs → lungs → PVs → LA → LV → Ao
• Normal dog: ↑ activity in lungs → rapidly ↓ to low level as bolus passes through pulmonary vasculature
  o Residual activity = systemic arterial blood supply to lung + thoracic wall activity

Question 2

What radionuclide is typically used

Answer 2

Pertechnetate bolus

Question 3

Abn w/ L to R shunt

Answer 3

• Prolonged retention of radioactivity in the lungs
  o Recirculation of blood in pulmonary vasculature
   1st peak + 2nd peak of activity
   Delayed washout phase as bolus continuously recirculates
• Area under curve of peak of time activity α to blood volume
  o Magnitude of shunt can be estimated from areas under 1st (A1) and 2nd (A2) peak
  o Qp/Qs = A1/A2 – A2
  o Qp/Qs >1.2 indicates L to R shunting
• Anatomic location cannot always be determined

Question 4

Abn w/ R to L shunt

Answer 4

• Simultaneous activity in PA and Ao
  o Shunted radioactive blood appear early in systemic circulation → bypass pulmonary circulation
  o Small shunt may obscure Ao by high activity in the lungs
• Injection of 99mTcmacroaggregated albumin (99mTc-MAA)
  o Large particle size → trapped in capillaries
  o Normal animals = all particles trapped in lungs
  o R → L shunt = bypass the lungs and go to systemic circulation
• % shunt = extrapulmonary activity/ extrapulmonary + pulmonary activity

Question 5

how does radionuclide can eval LV fct

Answer 5

• Gated equilibrium radionuclide angiography (gated blood pool ventriculography)
  o Imaging cardiac blood pool after injected tracer has mixed with IV space
   IV injection of blood pool agent
• 99mTc-labeled red blood cells (99mTc-RBC)
• 99mTc-labeled serum albumin (99mTc-HAS)
  o After equilibration w blood pool → activity of given qty of blood directly related to its volume
   Image acquisition (gamma camera, scintigraphic images) synchronized w R wave from ECG = various stages of cardiac cycle obtained at specific intervals
   Changes in ventricular activity = volume changes
• Derived functional indices from ventricular time activity
  o EF% → if MR, not indicator of myocardial function
  o Filling times
  o Peak filling rates
  o Time/rate of ventricular ejection
  o Regional wall motion

Question 6

Advantages of LV fct eval w/ radionuclide

Answer 6

independent images from R and L side → permit to better isolate/evaluate RV

data from several hundreds of cardiac cycles → more accurate average cardiac fct

Question 7

Disadvantages of LV fct eval w/ radionuclide

Answer 7

only 4-6 cardiac cycle can be evaluated

Question 8

Evaluation of myocardial perfusion, ischemia and infarction w/ radionuclide

Answer 8

• Myocardial uptake of Thalium-201 is α to myocardial blood flow
  o 85% of injected Thalium-201 removed in single pass
  o After several hours → redistributes to the heart to reflect distribution of K+ pool
• Myocardial defect → show extent of perfusion abnormality
  o Suggest fixed abnormality (permanent scar)
  o If improve after initial scan → indicate myocardial ischemia
• Attenuation: can happen with overlying soft tissue → false positive results
  o More likely with Technetium-99m

Question 9

Agent used for myocardial perfusion eval

Answer 9

Thalium-201
o Similar biologic properties to K+ → K+ analog

Question 10

Imaging agents

Answer 10

• Radioactive Thalium-201
• Technetium-99m markers
• Iodine-123 labeled fatty acid
• Iodine-123 metaiodobenzylguanidine

Question 11

What is eval by radionuclide ventriculography

Answer 11

• Evaluate ventricular function
  o 1st transit studies → beat to beat evaluation
   RV function
   Intracardiac shunt
  o Gated (ECG synchronized) blood pool imaging → multiple gated acquisition
   Nuclear medicine gated blood pool scan: labelling RBCs with radiopharmaceutical
   Measure amount of blood in heart during different part of cardiac cycle
   Images acquired from gamma camera (scintigraphy)
   Blood pool → analysis of amount of radioactive blood pooling in cardiac chambers at different times on cardiac cycle
   Gated → study performed in time with heart rhythm

Question 12

Viability studies

Answer 12

• Dysfunctional but viable myocardium → potential to recover function after reperfusion
• Assessment of myocardial viability with scintigraphic techniques
  o Intact perfusion
   Evaluated by Thalium-201/Technetium-m99 labeled tracers
  o  membrane integrity
   Evaluated by Thalium-201
  o Intact mitochondria
   Technetium-m99 labeled tracers
  o Preserved glucose/fatty acid metabolism
   FDG and radiolabelled fatty acids (Iodine-123)
  o Contractile reserve