Volume-/perfusion-ratio from Lung-SPECT/CT: Normal values and clinical use for pre interventional identification of pathologically altered lung lobes in severe pulmonary emphysema

Objectives In pulmonary emphysema volume reduction procedures using lobectomy (surgically or functionally by valves or coils) are established to optimize the respiratory pump function. It is of upmost importance to individually identify the most affected lobe to plan the procedure. Identification of the most emphysematically altered lobe can be obtained using relative lobar volume (relVol), relative lobar perfusion (relPerf) or using a parameter describing the volume-/perfusion-ratio (VPR). Differentiations between normal and pathological findings should be based on normal values to reduce subjectivity. The aim of the study was therefore 1.) to establish normal and threshold values for relVol, relPerf and VPR and 2.) the exemplary classification of data from patients suffering from severe pulmonary emphysema using the threshold values for relVol, relPerf and VPR.

Methods For normal ranges Ventilation-/Perfusion(V/P)SPECT-CT scans from 12 patients with suspected pulmonary embolism but normal findings were used. For exemplary classification V/P-SPECT-CT scans from 20 patients with severe pulmonary emphysema investigated before lung volume reduction procedure were used. All 32 patients underwent a lung V/P-SPECT-CT (Siemens Symbia T) after ventilation and injection of about 150 MBq 99m-Tc-MAA. Quantification was performed using “HERMES Hybrid 3D - Lung Lobe Quantification”, a software package that allows efficient and accurate segmentation of lobes from the (ld)-CTs. The resulting lobar VOIs were automatically transferred to the SPECT perfusion study to calculate the relative lobar volume and perfusion measurements. Normal ranges (mean±2sd) of relVol, relPerf and VPR were calculated from the PE suspected patients. For the 20 patients with severe pulmonary emphysema relVol, relPerf and VPR were also calculated. Using the threshold values (mean+2sd for relVol and VPR; mean-2sd for relPerf) all lobes were classified as normal or abnormal to identify pathological lung lobes as potential targets for interventional procedures.

Results Mean±SD (12 ,,normals“): right upper lobe 22.3±2.4% (relVol), 20.8±4.0% (relPerf), 1.09±0.10 (VPR), middle lobe 13.4±3.4% (relVol), 10.8±3.9% (relPerf), 1.31±0.31 (VPR); right lower lobe 18.9±3.9% (relVol), 22.1±5.8% (relPerf), 0.87±0.08 (VPR); left upper lobe 28.3±3.4% (relVol), 26.5±5.2% (relPerf), 1.09±0.11 (VPR); left lower lobe 16.8±2.7% (relVol), 19.8±4.5% (relPerf), 0.87±0.12 (VPR). For the 20 patients (100 lobes) with severe pulmonary emphysema there were only 7 lobes with pathological values (above 2 SDs) for relVol (23-36%), 14 lobes with pathological values (below 2 SDs) for relPerf (3-14%) but 31 lobes with pathological (above 2 SDs) VPR (1.14-6.66). According to the clinical experience there were most of the pathological findings in the right upper lobe.

Conclusions The estimation of the volume-/perfusion-ratio from Lung-SPECT/CT enables a combined view on relative volume and relative perfusion with one parameter. In patients with severe pulmonary emphysema the VPR allows - when using threshold values from normals -identifying possible target structures with much higher sensitivity than when using relative lobar perfusion or relative lobar volume alone. The specificity and the prognostic value of this new parameter have to be tested in a larger prospectively designed clinical trial.