Quantification of pulmonary scintigraphy

Abstract

Modern nuclear medicine techniques, such as SPECT (single photon emission computed tomography), enable quantification and localisation of lung function in small lung elements, but appropriate methods are not yet available. The aim of the present project was therefore to develop and evaluate methods for mapping lung function in planar scintigrams and in SPECT.The analysing tools were developed in IDL (Interactive Data Language, Research Systems, USA) and the evaluation was based on studies of life long non-smokers, smokers and emphysematous patients. Ventilation distribution was assessed by the deposition of inhaled 99mTc-Technegas (Tetley Manufacturing Ltd., Australia) and perfusion distribution by intravenously injected 99mTc-macro-aggregated-human-albumin. In planar scintigrams, function was assessed in 1.6 cm-high stripes or 2 cm ´ 2 cm elements and in SPECT study, these elements were 2 cm ´ 2 cm ´ 1 cm. Inhomogeneity of ventilation distribution was quantified by the coefficient of variation (CV) of the pixel counts or among lung element counts. Ventilation (V) and perfusion (Q) matching was expressed by the V/Q index in 2 cm ´ 2 cm elements. The localisation and the degree of abnormality were visualised by parametric images of CV and V/Q distribution.CVs of ventilation counts intra- and inter-elements in emphysematous lungs were more widely dispersed and had higher values than those in healthy lungs. CVs of some smokers were inside the normal range, but others were higher and closer to emphysematous patients' range. Ventilation CVs correlated with conventional lung function test results. The distributions of V/Q indices in emphysematous lungs varied greater than those in healthy lungs. The standard deviations of log(V/Q) in patients were significantly higher than the ones in healthy volunteers. Parametric images were found useful regarding localisation of abnormal lung function and visualising the degree of abnormality.The clearance process of deposited 99mTc-Technegas was investigated. The time-activity curve of the whole lung was found to comprise two phases. The initial unexpected rapid clearance phase was shown to be due to the clearance of 99mTc-Pertechnegas contaminating the 99mTc-Technegas.We conclude that the present methods enable quantification and localisation of ventilation inhomogeneity in planar and SPECT images and mismatches of ventilation and perfusion in planar images.