A novel approach to measure local cerebral haematocrit using MRI.

The percentage blood volume occupied by red blood cells is known as haematocrit. While it is straightforward to measure haematocrit in large arteries, it is very challenging to do it in microvasculature (cerebral haematocrit). Currently, this can only be done using invasive methods (e.g. PET), but their use is very limited. Local variations in cerebral haematocrit have been reported in various brain abnormalities (e.g. stroke, tumours). We propose a new approach to image cerebral haematocrit using MRI, which relies on combining data from two measurements: one that provideshaematocrit-weightedand other onehaematocrit-independentvalues of the same parameter, thus providing an easily obtainable measurement of this important physiological parameter. Four different implementations are described, with one illustrated as proof-of-concept using data from healthy subjects. Cerebral haematocrit measurements were found to be in general agreement with literature values from invasive techniques (e.g. cerebral/arterial ratios of 0.88 and 0.86 for sub-cortical and cortical regions), and showed good test-retest reproducibility (e.g. coefficient-of-variation: 15% and 13% for those regions). The method was also able to detect statistically significant haematocrit gender differences in cortical regions (p < 0.01). The proposed MRI technique should have important applications in various neurological diseases, such as in stroke and brain tumours.