Perfusion heterogeneity in human skeletal muscle: fractal analysis of PET data

Muscle blood flow has been shown to be heterogeneous at the voxel by voxel level in positron emission tomography (PET) studies using oxygen-15 labelle d water. However, the limited spatial resolution of the imaging device does not allow direct measurement of true vascular flow heterogeneity. Fractal dimension (D) obtained by fractal analysis describes the relationship betwe en the relative dispersion and the size of the region studied, and has been used for the assessment of perfusion heterogeneity in microvascular units. This study was undertaken to evaluate fractal characteristics of PET perfu sion data and to estimate perfusion heterogeneity in microvascular units. S keletal muscle blood flow was measured in healthy subjects using [O-15]wate r PET and the fractal characteristics of blood flow in resting and exercisi ng skeletal muscle were analysed. The perfusion heterogeneity in microvascu lar units was estimated using the measured heterogeneity (relative dispersi on, RD= SD/mean) and D values. Heterogeneity due to methodological factors was estimated with phantoms and subtracted from the flow data. The number o f aggregated voxels was inversely correlated with RD both in phantoms (Pear son r=-0.96-0.97) and in muscle (Pearson r=-0.94) when both parameters were expressed using a logarithmic scale. Fractal dimension was similar between exercising (1.13) and resting (1.14) muscles and significantly lower than the values in the phantoms with different activity levels (1.27-1.29). Meas ured flow heterogeneity values were 20%+/-6% (exercise) and 27%+/-5% (rest, P<0.001), whereas estimated flow heterogeneity values in microvascular uni ts (1 mm(3)) were 35%+/-14% (exercise) and 49%+/-14% (rest, P<0.01). In con clusion, these results show that it is feasible to apply fractal analysis t o PET perfusion data. When microvascular flow heterogeneity is estimated us ing fractals, perfusion appears to be more heterogeneous in microvascular u nits than when obtained by routine spatial analysis of PET data. Analysis o f flow heterogeneity using PET and fractals could provide new insight into physiological conditions and diseases associated with changes in peripheral vascular function.