Juan J. Touya

Single photon emission computed tomography in the diagnosis of pulmonary thromboembolism

The use of single photon emission computed tomography (SPECT) as the procedure for screening, assessing the size and number of embolized areas, and evaluating the follow-up of patients with pulmonary thromboembolism (PTE) is reviewed. Details of the technique for acquiring and processing perfusion and ventilation lung SPECTs are presented. The tomographic images produced by SPECT allow the application of a less-expensive diagnostic algorithm than that classically used. This is due to the fact that the SPECT images contain more anatomical and physiological information than similar planar images. The signs of PTE in perfusion SPECT and the significance of mismatch, match, and paradoxical mismatch are discussed and illustrated with examples.

The Lung as a Metabolic Organ

Recently, the lung has received increasing attention as a metabolic organ. In this role, the lung modulates the composition of the arterial blood by several mechanisms: removing active substances from the plasma, releasing substances into the plasma, temporarily holding substances from circulation, and activating or inactivating substances that pass through the lungs. In this report, the procedures proposed by different investigators for in vivo noninvasive assessment of the lung metabolic functions are reviewed. Most procedures are based on an estimation of the clearance of plasma amines by the lung endothelial cells. This clearance is assessed by measuring the lung uptake or the extraction fraction of an intravenously (IV) injected radiolabeled amine. Our own procedure, which assesses the number of free pulmonary endothelial amine receptors, is discussed in detail. In our procedure, the number of receptors was computed using the number of injected molecules of amine and determining the lung extraction fraction of the amine during its first pass through the lungs. In goats, using N-isopropyl-p-iodoamphetamine labeled with 123I as the radiopharmaceutical, the total number of endothelial lung amine receptors was found to be 1.589 X 10(20). The methods for studying the lung metabolic functions, which are discussed in this report can be applied in humans to evaluate either physiological or pathological conditions.

A NONINVASIVE PROCEDURE TO MEASURE IN VIVO LUNG ENDOTHELIAL RECEPTORS

We have designed a method based on radioimmunoassay principles and digital image processing to assay quantitatively the amount of free amine receptors in the pulmonary circulation. In previous publications we described the technique to measure the extraction of amines based on the analysis of the first pass of test and reference tracers injected as boli in the jugular vein. The present study was done in West African pigmy goats. Amine receptors were assayed using as ligand N-isopropyl-piodoamphetamine (IMP) mixed with I-123-IMP. Extraction fraction of the amine during its first pass through the lung was determined for different concentrations of ligand in the test bolus. The amount of ligand bound to the receptors was calculated as the product of the measured extraction fraction and the total amount of injected ligand. The dissociation constant (K) of the receptorligand reaction was measured from the slope of the Scatchard plot of the bound to free ligand ratio versus the bound ligand. K was found to be 11.7 mg. The amount of receptors in the lung was computed determining the halfsaturation point and it was found to be equivalent to 30 mg. of IMP. Considering that a single IMP molecule binds to a single receptor the total number of lung receptors was found to be 6.59E+19. In a computer simulation the sensitivity of the method to determine variations in the population of free receptors was studied. The optimal amount of ligand was determined to be 5 mg. of IMP. It is true that further work is necessary but the method promises to be a valuable research tool for in vivo study of lung amine receptors.