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Detection of Heart Shunts by Means of ¹²⁵I¹

Santiago, Chile

ABSTRACT

External scintillation detection of a pure dilution curve originating in the right ventricle has been pursued by investigators for many years. The use of the soft photons of ¹²⁵I offers an improved method of determination by allowing excellent collimation through two mechanisms: (1) A small half value layer of 2 cm in tissue, and (2) Unidirectionality, in part due to all-or-none photoelectric absorption of these photons as compared with the pluridirectionality due to Compton scatter in the case of ¹³¹I.

In 30 normal individuals, externally detected dilution curves from the right heart ventricle and arterial dilution curves obtained by arterial puncture are compared. Five hundred microcuries of ¹²⁵I are injected into an antecubital vein. The amount of radiation absorbed from a 500 µc dose in an adult of 70 kilos is 1000 to 2000 mrads when injected in the form of ¹²⁵I iodide. The use of ¹²⁵I orthoiodohippurate is advised due to its short biological half-period and the integrated whole body radiation exposure is decreased 100 fold in comparison with the ¹²⁵I iodide. The shape of the right heart curve is very similar to the arterial dilution curve with a 13 ± 5 per cent excess of counts originating outside the right ventricle area. The descending segment of the right ventricle curve has a minimum count rate of only 12 ± 2 per cent of the peak of the curve as compared with the arterial curve of 10 ± 2.5 per cent. A mean T of the descending down slope of the right ventricle is 1.52 ± 0.45 seconds for the right heart curve and 2.4 ± 0.65 seconds for the arterial curve.

The clinical usefulness of this procedure in 20 patients with atrial septal defects and 9 patients with ventricular septal defects is analyzed.

In summary, it seems possible to perform selective right-heart radiocardiography by means of external precordial detection of ¹²⁵I. The method is simple and reproducible.

FOOTNOTES

¹ Supported in part by Rockefeller Grant No. RF-61139, and grant-in-aid from The Kellogg Foundation.

² Department of Medicine, Divisions of Nuclear Medicine and Cardiology. Catholic University Hospital. Santiago, Chile.