Several imaging modalities are available for use imaging in the diagnosis of cerebral conditions such as stroke, AVM (arteriovenous malformation), or cancer. Imaging modalities include CT (computed tomography) and MRI (magnetic resonance imaging), and are able to obtain anatomical and physiological information about an organ such as the brain.
Perfusion, which is the circulation of blood through tissue, is one indicator used for diagnosing a region of interest in a patient affected by a vascular stenosis or blockage. By determining perfusion at the capillary level of the region of interest, the effect of the stenosis or blockage of an artery can be assessed and localized directly. The extent of the stenosis or blockage can also be determined. From this information, a therapeutic treatment can be planned, and its success can then be monitored after the intervention by subsequently re-determining the perfusion of the region of interest.
When using CT, for example, images may be obtained to that provide functional information. Non-enhanced CT (NECT, which may also be called non-contrast CT, NCCT) as well with contrast enhanced CT angiography (CTA) with conventional contrast agents may be used for initial diagnosis. The administered contrast agent may also be used to visualize and measure perfusion of the organ. Parameters which may be considered include, for example, perfusion (F), blood volume (CBV), mean transit time (MTT), time to peak (TTP). Important diagnostic information can also be derived from a comparison of the various variables with one another to ascertain inconsistencies.
CT imaging methods are generally used pre-interventionally; that is, during the diagnostic phase in an emergency room. But, CT and MRI equipment is generally not available in the angiography laboratory where interventional therapy is performed. However, a typical hospital interventional laboratory may have a C-arm-based X-ray system. Thus, at present, the functional evaluation of a patient during the interventional phase is done qualitatively using 2D projection images.
Clinical studies have shown that there is a significant correlation between, for example cerebral blood volume (CBV) and the prognosis for a patient with acute stroke. Recent studies indicate that severely reduced cerebral blood volume is an important predictor for essentially non-viable tissue, and that CBV-determined lesion volumes are significantly correlated with follow-up lesion volumes as measured using CT.
A procedure for measuring CBV includes the obtaining a CT or MRI imaging data set of the brain before and after the intravenous administration of contrast agent. The difference between the two data sets corresponds to the contrast agent present in the brain and can thus be converted into the blood volume.
The tissue blood volume may be determined by subtracting the measured density of tissue, measured in Hounsfield units (HU) for the data sets with and without contrast agent. The image density in HU is linearly proportional to the concentration of the contrast agent in the blood and brain tissue. This change in contrast agent may be interpreted as blood volume as the blood-brain barrier prevents the contrast agent from escaping the circulatory system, and the change in density may thus be interpreted as being localized to the vasculature.
Absolute values of the CBV may be computed on a global or local basis, or qualitative comparisons of bilateral CBV so as to identify volumes having impaired circulation.
When this procedure is performed diagnostically, the contrast agent is injected intravenously, rather than arterially. The level of contrast agent needs to be kept constant during the image data set acquisition so that normalized measurements can be obtained. This involves a complex injection protocol and a relatively high amount of contrast agent. Moreover, the contrast differences between vessels and bones in intravenous contrast agent injection are very slight, which leads to difficulties in reconstructing the larger vessels in a perfused area in the head or in the region of the base of the skull. However, increasing the amount of contrast agent is contraindicated by an association with poorer outcomes.
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