Lisette van Dam

Chapter 7 118 image quality without significantly increasing overall radiation dose. 46,47 Currently 320-MDCT techniques are used that may acquire brain volumes within a single second. Several techniques can be used for removing unwanted overlying (bone) structures from the vascular (venous) structures to improve the diagnostic accuracy of CT venography, which is especially needed for 3D interpretation. Previous studies have used the ‘graded subtraction’ technique, a non-automated post-processing technique which is time-consuming and operator dependent. 42,43,48 State of the art techniques include mask-subtraction and dual-energy. With mask-subtraction a low-dose non-enhanced CT is subtracted from the contrast-enhanced vascular (CT venography) acquisition. 48,49 With dual-energy techniques bone removal is obtained by postprocessing a dual-energy CT data set that is simultaneously acquired with a low- and a high kilovoltage, where difference in x-ray absorption of different materials depend on x-ray energy. 50 New spectral CT techniques such as photon-counting CT hold promise for further improved visualization of CVT. 51,52 Magnetic resonance imaging Various MR techniques are available to visualize cerebral vascular structures and/or thrombosis. The MRI techniques can be divided into three groups based on how the thrombosis is depicted: 1) non-contrast-enhanced flow related MRI, 2) native contrast thrombus MRI and 3) contrast-enhanced MRI ( Table 2 ). Non- contrast-enhanced flow relatedMRI, often called non-contrast-enhancedmagnetic resonance venography (MRV), includes time-of-flight (TOF) MRV and phase- contrast (PC) MRV ( Table 3 ). With this MR technique a thrombus can be depicted by absence of normal flow patterns ( Figure 2 ). A thrombus can also be directly visualized with native contrast thrombus MRI techniques. These sequences are used to visualize thrombus by presence of paramagnetic deoxyhaemoglobin, methaemoglobin, or hemosiderin. 53 Moreover, MRI after the administration of an intravenous gadolinium-based agent can be used (contrast-enhanced MRI), which allows direct luminal visualization that is comparable to that of CT venography, where a thrombus can be identified as a filling defect. 5 Compared to CT venography, MRI is more sensitive for the detection of small parenchymal lesions and cerebral edema and has the advantage of not exposing the patient to ionizing radiation. 5,12,54 On the other hand, advantages of CT venography over MRI are the fast acquisition times and the possibility to scan more patients, since many MRI contraindications exist.