Densitometric Calculation of Volumes

Quantitative analysis techniques were developed for projection X-ray imaging techniques based on the principle that image density measured after opacification of ventricular cavities with contrast material is proportional to the volume of blood contained in a given cavity. These methods, referred to as densitometric measurement techniques, applied to angio-graphy after digital subtraction, have been validated for the calculation of left and right ventricular volumes and the evaluation of ventricular stroke volumes.

Evaluation of left ventricular volume can be accomplished using peripheral or central venous injection of contrast material or by selective injection of contrast material directly into the left ventricle [19,20]. Generally, the real-time subtracted image series is preprocessed by manually or automatically selecting images with the largest (end-diastolic) and smallest (end-systolic) left ventricular silhouette.

The development of digital subtraction angiography (DSA) provides an ideal setting for densitometric analysis as an alternative to the geometric method for the determination of either left ventricular ejection fraction or right ventricular ejection fraction. Densitometric measures utilize the relative brightness values of picture elements ( pixels) in regions of interest containing the left ventricular or the right ventricular area during systole and diastole to derive the ejection fraction. The technique is independent of left ventricular geometry and odd-shaped ventricles.

In practice, a number of technical problems make absolute densitometric measurements extremely difficult. These include nonlinearity of the image intensifier and video transfer function, beam scatter and energy modification, geometric

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