C-Arm CT Measurement of Cerebral Blood Volume in Ischemic Stroke: An Experimental Study in Canines

Stroke Preparation

Under an institutionally approved animal protocol, 10 canines were included in the study. After induction with propofol (2–4 mg/kg), general endotracheal anesthesia was maintained with isoflurane 1%–3%. Constant monitoring of heart rate, O₂ saturation, and end-tidal CO₂ was performed throughout the experiments. For autologous clot formation, a series of 6F sheath dilators was filled with autologous blood mixed with bovine thrombin and incubated at room temperature for 1 hour. Depending on the variation in the individual animal's vascular anatomy, selective catheterization of either an ICA or a VA was performed by using access from 1 common femoral artery. In animals with large and relatively straight ICAs at the skull base, a 4F guiding catheter was used to first deliver short segments of thrombus directly into the ICA. After delivery of 4–6 pieces of the thrombus, the same number of pieces of 5.0 silk suture, cut to lengths of 3–5 mm, was injected through the same catheter. Serial DSAs were performed to monitor the status of the embolizations. In animals with small or extremely tortuous ICAs at the skull base, a 4F guiding catheter was placed into the cervical segment of 1 VA. Through this, a microcatheter was advanced over a guidewire until it was positioned near the origin of the ipsilateral MCA. Through this microcatheter, multiple aliquots of microfibrillar collagen (Avitene; Bio-Medicine, Houston, Texas) were injected until there was a significant decrease in flow through the ipsilateral MCA. In an attempt to prevent recanalization of the occluded arteries, a single detachable coil was placed into the origin of either the ICA or the VA at the end of the embolization procedure.

DWI

Animals were carefully monitored for 4 hours after embolization, after which each animal underwent DWI on a 1.5T system (Signa HDx; GE Healthcare, Waukesha, Wisconsin) to confirm the presence or absence of cerebral infarction. The parameters for the DWI sequence were the following: TR/TE, 10,000/99.8–120.8 ms; FOV, 18 × 18 cm²; matrix, 256 × 256; 34 images; thickness, 3 mm; b = 0, 1000, 2500 s/mm²; bandwidth, 250 kHz.

PCT Measurements

PCT was performed on a 64-section V scanner (GE Healthcare). Eight adjacent 5-mm thick sections from a level just anterior to the optic chiasm to just anterior to the torcula were selected from a scout image. The CBV map was acquired after injection of iopamidol, 370 mg I/mL (Isovue 370; Bracco Diagnostics, Princeton, New Jersey), by using a dual-syringe power injector (Medtron, Saarbrücken, Germany). A monophasic injection of 12 mL of contrast (370 mg I/mL) at 1.5 mL/s was followed by a 12-mL saline chase at 1.5 mL/s. Following the power injection of the contrast medium and after a 5-second prep delay, a continuous scanning was initiated with the following parameters: 80 kVp, 200 mA, 1 second per rotation for 50 seconds. The 1-second images were reformatted at 0.5-second intervals.

Postprocessing was performed by an experienced technologist (K.P.) in conjunction with a senior neuroradiologist (C.M.S.) by using a commercially available CT perfusion software (GE Healthcare). For arterial and venous input functions, extracranial vascular structures were selected manually because of the small size of the canine intracranial arteries and the proximity of the superior sagittal sinus to the thick skull base.

C-Arm CT

Immediately following the PCT examination, C-arm CT was performed on a biplane flat-detector angiographic system (Artis dBA, Siemens Healthcare, Forchheim, Germany). The same contrast agent used in PCT (Isovue 370; Bracco Diagnostics) was injected into a peripheral vein with the same dual-syringe power injector (Medtron) used in the prior PCT examination.

Two CBV maps were acquired with the C-arm CT, by using 2 different injection protocols (described in detail below). To determine the time when there would be a steady level of contrast medium in the brain parenchyma, we injected contrast medium into a peripheral vein at the rate of 3.0 mL/s, and simultaneously with the start of the injection, a 2D-DSA acquisition was performed. From this, the time of full opacification of the superior sagittal sinus was noted. (At this time, contrast medium is present in arteries, parenchyma, and veins.) This time then was used to determine when normal parenchyma would have a steady level of contrast. To be sure that any parenchyma that might have slow flow as the result of perfusion through the collateral circulation also had a steady state of contrast filling during the acquisitions, we set the x-ray delay at 15–17 seconds for 1 acquisition and at 23–25 seconds for the second acquisition. We thought that this was prudent: Because we were not able to measure the dynamic elements of perfusion (MTT and CBF) with the C-arm technique, we were unable to determine if there was tissue downstream from an obstruction that, while having reduced MTT and CBF, was still maintaining normal or elevated CBV because of adequate but slow supply through collaterals. To be as sure as possible that we had allowed any such tissue to reach a steady state of enhancement, we extended the x-ray delay and performed the second CBV acquisition. After a 10-minute delay, the second CBV acquisitions were obtained. Each acquisition protocol consisted of 2 rotations: an initial rotation (mask run) followed by the power injection of the contrast medium, an appropriate x-ray delay, and then a second rotation (fill run).

Each of the 2 fill runs was started after injection of 25 mL of contrast agent at 1.0 mL/s for 25 seconds and at 1.5 mL/s for 17 seconds, respectively. Two hundred seventy-five projection images were obtained during a 10-second rotation in both instances. A 10-minute interval was taken between each acquisition.

In all animals except 1, the C-arm CT examinations were performed within a half hour of the PCT measurements (in 1 instance, the angiography suite was not available until 7 hours after creation of the stroke). All canines were euthanized after the final C-arm CT examination.

Data Analysis

PCT measurements were processed by using commercially available software (GE Healthcare). C-arm CT measurements were processed by using prototype software installed on a dedicated research work station (Leonardo, Siemens Healthcare). The reconstruction algorithm has been described previously.¹⁰ In brief, after reconstruction and subtraction of the mask run and the fill run separately, an algorithm is applied to further segment out air and bone from the image volume. The steady-state arterial input function value is then calculated from an automated histogram analysis of the vessel tree. A final scaling is then applied to account for the arterial input value and for other physiologic values (eg, hematocrit), before a smoothing filter is applied to reduce pixel noise.

An experienced researcher (K.P.) placed a region of interest manually on the PCT and C-arm CT CBV maps as close as possible to the center of the infarct as depicted on the DWIs. A symmetric region of interest was automatically prescribed in the contralateral normal hemisphere. Each region of interest measured 112 mm² and contained both gray and white matter. The absolute values and the mean ratios (abnormal side to normal side) of these CBV values in the 2 regions of interest were calculated and compared.

Ten PCT CBV maps and 10 C-arm CT CBV maps from a previous study of normal canines were combined according to a computer-generated randomization scheme with the maps from the PCT and C-arm CT CBV studies of the 10 animals with ischemic stroke. The combined CBV maps (n = 50; 10 normal PCT, 10 normal C-arm CT, 10 stroke PCT, and 20 stroke C-arm CT) were then presented to 3 experienced observers (H.R., P.T., S.B.) who were blinded to the technique used to create the maps and to whether the maps were from normal or abnormal animals. The observers were asked to grade each map as being either normal or abnormal and, if abnormal, to designate the side of the abnormality.

A 2-sample paired t test was used to compare the mean values of the region of interest within the infarcted territory and the corresponding normal tissue of the PCT and the 2 C-arm CT injection protocols. Descriptive statistics were performed for assessment of the 3 readers' performances in identifying the infarcts.