A flow diagram of the method to obtain CTA images from a CT perfusion series. The three main parts of the method are (a) registration, (b) weighted averaging, and (c) optionally masking of bone and vein structures. The numbers in the figure correspond to different processing steps and are explained in Sec. II.
Contrast in a CTP examination. (a) An axial CT image from a CT perfusion examination at the time of maximal arterial contrast enhancement with two VOIs indicated, an arterial VOI in the right middle cerebral artery (MCA) and a venous VOI in the right sigmoid sinus (Sig S) (window width 600 HU, window center 200 HU). (b) The contrast of the VOIs shown in (a) as a function of .
Creation of the vein mask, illustrated in an axial image from a CTP examination. (a) venous weighted average image made from a CTP examination, with the bone and voxels outside the skull masked (window width 280, window level 75). (b) Image (a) thresholded with , all selected voxels are white, [(c) and (d)] arterial and venous correlation coefficients and , white representing a high correlation and black a low one. For this illustration the correlation coefficients are shown for all voxels; in the calculation of the vein mask only the voxels shown in (b) are used. (e) Scatter plot of the arterial and venous correlation coefficients for the voxels shown in (b). (f) Final venous mask (black voxels) with arterial voxels colored white. In this final mask, the venous voxels were conditionally dilated (see Sec. ???).
CT values of tissue equivalent material within the skull of the Rando phantom as a function of the distance to the skull. CT values are indicated with a “x,” the experimental correction function with a continuous line.
Images of a region of interest containing the circle of Willis in an axial slice (slice width of ) of a CTP examination: (a) Weighted averaged, (b) plain averaged, and (c) image with maximal arterial enhancement, (d) axial image (slice width of ) from a registered CTA examination. In (c) window width is 600 HU and window level 75 HU, the window width and level in the other three images were chosen to obtain approximately the same gray values for the arteries and background of brain tissue as in (c).
MIP on the sagittal plane of a VOI of of a CTP examination: (a) Weighted averaged, (b) weighted averaged with vein masking, and (c) image with maximal arterial enhancement. For comparison (d) shows the MIP image of the same VOI in the registered CTA. In (c) window width is 500 HU and window level 230 HU, the window width and level in the other three images were chosen to obtain approximately the same gray values for the arteries and background of brain tissue as in (c).
Maximum intensity projections on the coronal plane of the complete scan volume of a weighted averaged CTP examination: (a) Without vein masking and (b) with vein masking. Window width 300 and window center 130. In the lower part of the MIPs some artifacts due to the presence of platinum coils are present.
SD of the CT values in a circular ROI (diameter of ) in a homogenous area in the Rando phantom as a function of number of images that is averaged. The theoretical values are indicated with the continuous line and the measured values with a “+” symbol.
The region of interest (diameter of ) in the head of the Rando phantom in which the standard deviation was measured. (a) One of the scans from the CTP series. (b) The average of the 15 scans in the CTP series. Window width 200 and window center 0 HU.
Translation in three directions applied to the center of 14 scan volumes relative to the center in the first scan volume in the registration procedure. Average and maximal values were taken over 14 registration of two volumes for eight patients. With the -axis horizontally and the -axis vertically in the cross-sectional plane, and the -axis in the direction of table movement.
Estimated effective doses for CTP and CTA examinations with a scan range of .
The CNR, relative to CNR in the image with maximal enhancement, for 16 arteries in patient 1. CNR: Contrast-to-noise ratio; SD: Standard deviation; L: Left; R: Right; ICA: Internal carotid artery; BA: Basilar artery; PCA: Posterior cerebral artery; MCA: Middle cerebral artery; MI–M3: The numbered anatomical segment (Ref. 29); PA: Pericallosal artery; sup.: Superior; inf: Inferior; prox: Proximal; and dist: Distal.
The average contrast-to-noise ratio (CNR), relative to CNR in the images with maximal arterial enhancement, for eight patients, with the standard deviation (SD) between parentheses.
The average contrast of regions of interest in three arteries, the standard deviation of the noise, and the CNR values in the images obtained with weighted averaging of a CTP examination, the images in the CTP series at maximal enhancement, and in the images of a CTA examination for three patients. CTP: CT perfusion; CTA: CT angiography; SD: standard deviation; CNR: contrast-to-noise ratio.
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