Arjen Lindenholz

87 Comparison of 3T Intracranial Vessel Wall MRI Sequences 4 Image analysis The mean signal and standard deviation of the noise (SD noise ) were obtained from an axial plane of the vessel wall images and noise images (magnitude), respectively. Regions of interest (ROIs) for the mean signal (Mean ROI ) were manually drawn as follows: CSF was marked in the suprasellar and pontine cistern for the intracranial internal carotid arteries and basilar artery, respectively; blood was marked in the lumen of the middle cerebral artery; brain tissue was marked in the left orbital gyrus; and the vessel wall was represented by the circumferential of the intracranial internal carotid arteries (left and right) and basilar artery (within the same slice number as the ROIs for CSF ; Figure 1 ). In the 10 subjects who received contrast agent injection, a homogeneous hyperintense part of the center of the pituitary stalk was used as ROI to compute the mean as a consistent marker for contrast-enhancement ( Figure 1 ). To compute the SD of the noise images (SD noise ), a larger circular ROI was drawn on the noise images, encompassing the ROIs used to compute the mean. The SNR was calculated as SNR = Mean ROI / SD noise for each subject and subsequently averaged over all subjects included in the comparison. The CNRs were calculated as CNR x-y = SNR x - SNR y . As a measure of motion, vessel wall sequences were coregistered to the prior acquired sequence to calculate the registration parameters ( ∆ Rotation and ∆ Translation), using the elastix toolbox in MeVisLab (version 2.7; MeVis Medical Solutions, Bremen, Germany). 25 Figure 1. ROIs used in the magnitude images for calculating the signal-to-noise-ratios. ( A ) The circumferential of both carotid arteries ( I ) was used as marker for the carotid vessel wall, that runs through the suprasellar cistern which was used as marker for CSF ( II ). ( B ) The circumferential of the basilar artery ( I ) was used as marker for the basilar vessel wall that run through the pontine cistern, which was used as second marker for CSF ( II ). For the pituitary stalk, a homogenous hyperintense part of the center was used ( III ). In the lumen of the middle cerebral artery a ROI was drawn as a marker for blood ( IV ), and in the left orbital gyrus a ROI was drawn as marker for brain tissue ( V ). The ROIs are drawn for illustrative purposes and the exact contours may differ in the real measurements depending on patients’ specific anatomy. ROI = region of interest, CSF = cerebrospinal fluid.