Quantification of Right-to-Left Shunting in Cyanotic Heart Disease by Isosensitive Scanning

Abstract

A new method for the quantification of shunting is discussed experimentally and clinically. Following quantitative injection of ^99mTc-MAA to a patient in a supine position, whole body counting was obtained by summing up the counts of linear scanning with our Medical Universal Human Counter. For linear scanning, two detectors with a slit-field collimator are placed symmetrically above and two below the table. The table of the scanning bed was moved automatically along the longitudinal axis at the speed of 40 cm/min. Isoresponse curves of this system in air and chest phantom were made with a window of 140 keV±2%. Whole body linear scanning was performed with the same window in the patients with cyanotic heart diseases.
Mean transmission rate (MTR) of ^99mTc-MAA from the lung was calculated in healthy subjects. Good correlation was obtained between ln (MTR) and chest thickness (X) of the subjects (γ=-0.95, p<0.001) . Linear regression was ln (MTR) =-0.035X+0.024 Absorption of γ-rays in the chest was corrected by multiplying 1/MTR
Right to left shunt rate was calculated as follows:
R to L shunt rate=N₀-N_a/MTR/N₀×100 (%)
N₀: Counts of the administered dose in air
N_a: Counts of the lung in the whole body linear scan
MTR: Mean transmission rate in the patient
Good correlation between linear scan method and cardiac catheterization was noted. This method is safe and can be simply performed at rest and even though on exertion.