Cerebral Blood Volume

Cerebral blood volume (CBV) is the fraction of each imaged voxel comprising the intravascular space, and therefore the volume of the blood vessels within a volume of brain tissue. It is measured as millilitres of blood per 100 g of tissue; since brain tissue approximates water in density (1 g/millilitre), this can also be expressed as a percent value.

Typical values for the brain are between 3% and 5 %. Only a small fraction of CBV is arterial, most of it being divided between capillaries and veins, and its changes are related to autoregulatory vasodilatation of the capillaries and/or veins [6, 7, 38]. CBV is a potentially sensitive indicator of vascular endothelial response to changes in local CBF and tissue metabolism.

Relative CBV can be measured as the area under the curve of the voxel concentration versus time. Absolute CBV can be determined by dividing the area under the

Fig. 9.5. Intensity-time curves are correlated to cerebral perfusion, better visualized in the CBV map: perfusion is greater in grey than in white matter and is reduced or absent in the isch-aemic area
Fig. 9.6. Intensity-time curves and CBV maps in normal tissue and in focal ischaemic area

Normal Perfusion

curve by a „reference" voxel known to contain 100% blood, such as the superior sagittal sinus [7, 39].

Unfortunately several technical factors can impair the accuracy of this relatively simple measurement [1, 3, 41]. The first is that dynamic susceptibility contrast arises from spin diffusion in the space surrounding the blood vessels, making it difficult to determine a reference 100% blood-filled voxel [41]. Secondly, the effects of contrast recirculation must be eliminated or minimized. Thirdly, CBV measurement can be erroneous if the gadolinium chelate leaks through a disrupted BBB. This is of particular concern when imaging tumours, since several tumours cause BBB disruption, yielding a bright signal on post-contrast T1 images [7, 42].

New molecules, such as superparamagnetic iron-oxide particles, dendritic compounds saturated with gadolinium atoms, or reversible protein-binding gadolinium-based agents have a longer half-life in blood [7, 43-45]. These so-called „blood pool" agents offer higher SNR for measuring CBV using steady-state susceptibility contrast that circumvents many of the problems outlined earlier.

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