Search

Regional pulmonary glucose metabolism (MRglu; mumol h-1 g-1), extravascular lung density (D(EV); g cm-3) and vascular volume (VB; ml cm-3) were measured in a single midthoracic transaxial slice (approximately 2 cm thick) using position emission tomography (PET) in seven patients with histologically proven sarcoidosis. The measurements were repeated 1-7 months later after steroid therapy (in two cases, no treatment) in order to assess MRglu as an index of inflammation and relate it to routine pulmonary function tests, chest radiography and serum angiotensin converting enzyme (SACE) levels. MRglu was computed from serial lung scans and peripheral venous blood samples for 60 min following an i.v. injection of 18F-2-fluoro-2-deoxy-D-glucose (18FDG). Both MRglu (which was increased in six of seven patients) and elevated SACE levels returned to normal in those patients treated with high-dose steroids. Regional vascular volume was normal in six of seven cases and did not change significantly with therapy. The high tissue density measured in all patients decreased significantly in two of three patients treated with 40 mg prednisolone daily. The abnormal MRglu observed in active sarcoidosis becomes normal pari passu with SACE levels during high-dose steroid therapy. We conclude that MRglu measured with 18FDG and PET may reflect "disease activity" in sarcoidosis in quantitative terms (per gram lung tissue) and in respect of disease distribution.

The adaptation to PET of the steady-state technique for the measurement of alveolar ventilation, based on the short-lived radionuclide 19Ne (T1/2 = 17.4 sec), allows the steady-state lung model to be analyzed in a quantitative way under well-defined geometrical conditions. The regional gas volume is essential to this analysis, and regional measurements of the pulmonary gas volume based on transmission tomography are presented and validated in this paper. The accuracy of the steady-state method rests largely with the validity of the lung model applied to describe the transport of tracer in the lung. This study considers tracer transport and mixing within individual lung regions. Blood flow and the alveolar-to-capillary exchange of gases do not significantly affect the values obtained, not even in regions with highly abnormal ventilation/perfusion ratios. A nonuniform intra-regional gas flow distribution results in an underestimation of the regional ventilation, determined by the dispersion of the ventilatory turnover rates of alveolar gas within the region. In the normal lung this underestimation is negligible. In disease, a mixing within the resolution volume of units that are almost non-ventilated and units that perform normally may result in an underestimation of alveolar ventilation by up to 60%.

Positron emission tomography is a major technological advance in the characterisation of structure-function relationships within and between regions in normal and abnormal lungs (Hughes et al. 1985). The measurements are noninvasive and relatively exact since the geometric conditions are precisely defined. Regional expansion, flow (ventilation, perfusion), oxygen concentration (from VA/Q) and glucose metabolism can be measured in absolute terms per cubic centimetre of thorax or per gram of extravascular lung. Examples of structure-function relationships in normal subjects, emphysema, bronchitis and sarcoidosis are briefly presented.