ABSTRACT
Purpose: Ventilation (V) perfusion (Q) SPECT imaging is important in the diagnostics of lung diseases such as pulmonary embolism, chronic obstructive pulmonary disease and recently in COVID-19 lung disease. The combined assessment of ventilation and perfusion permits the identification of classical mismatched and reverse mismatched defects, especially when these two parameters are combined in one measure as the ventilation perfusion ratio. Unfortunately, the ratio is only linear for ventilation and the scale is not symmetrical regarding classical and reversed mismatches. Small perfusion values give rise to artifacts. The scaled ventilation perfusion difference is presented as an alternative. Material(s) and Method(s): VQ SPECT was performed on patients with and without defects. Kr-81m was used as a ventilation tracer and MAA labelled with Tc-99m as a perfusion tracer. Ratio is defined as V/kQ and difference as V-kQ, where k is a scaling factor. The iterative z-map method was applied for determining the scaling factor for ratio and difference images yielding a ratio of one and zero difference for matched ventilation and perfusion. Clinical thresholds for both the difference images are derived resulting in color maps of relevant mismatches with an absolute ratio larger than the arbitrary value of two. Result(s): The relative difference is in second order proportional to the logarithm of the ratio, and has a scale going from -1 to +1, where -1 is perfusion only and +1 is ventilation only. Still the diagnostic value is hampered by the fact that areas with both low perfusion and ventilation can have high ratios. Therefore, the scaled (functional) difference is presented. Ratio, relative difference and scaled difference have been investigated in ten patients and are shown for three patients (one without defects). Ratio and relative difference images suffer from a visual overexposure effect as is clearly visible for the respiratory tract. One patient with an area in a bottom lung with a very low perfusion and slightly decreased ventilation demonstrated overflow problems of the intensity in ratio images and an overestimation of the mismatched area in both the ratio and relative difference images. Conclusion(s): While the relative ventilation perfusion-difference is a promising improvement of the ventilation-perfusion ratio, because it has a symmetrical scale and is bound on a closed domain, a better diagnostic value and functionality might be obtained by utilizing the scaled functional difference. The latter one seems superior to both the relative difference and ratio.Copyright © 2023 Southern Society for Clinical Investigation.