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Neuroradiology |
1 From the Departments of Diagnostic Imaging (G.A.T., M.V.J., W.W.M.S., R.A.H., J.M.R., N.R.M.) and Neurosurgery (C.E.D.), Rhode Island Hospital/Brown Medical School, Providence. From the 2002 RSNA scientific assembly. Received November 8, 2002; revision requested January 16, 2003; final revision received May 30; accepted June 18. Supported in part by a grant from GE Medical Systems. Address correspondence to M.V.J., Department of Radiology, Stanford University Medical Center, 300 Pasteur Dr, Room S-047, Stanford, CA 94305-5105 (e-mail: maheshj@stanford.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Thirty-five consecutive adult patients with acute subarachnoid hemorrhage were recruited into the institutional review board–approved study and gave informed consent. All patients underwent both multi–detector row CT angiography and DSA no more than 12 hours apart. CT angiography was performed with a multi–detector row scanner (four detector rows) by using collimation of 1.25 mm and pitch of 3. Images were interpreted at computer workstations in a blinded fashion. Two radiologists independently reviewed the CT images, and two other radiologists independently reviewed the DSA images. The presence and location of aneurysms were rated on a five-point scale for certainty. Sensitivity and specificity were calculated independently for image interpretation performed by the two CT image readers and the second DSA image reader by using the first DSA reader’s interpretation as the reference standard.
RESULTS: A total of 26 aneurysms were detected at DSA in 21 patients, and no aneurysms were detected in 14 patients. Sensitivity and specificity for CT angiography were, respectively, 90% and 93% for reader 1 and 81% and 93% for reader 2. The mean diameter of aneurysms detected on CT angiographic images was 4.4 mm, and the smallest aneurysm detected was 2.2 mm in diameter. Aneurysms that were missed at initial interpretation of CT angiographic images were identified at retrospective reading.
CONCLUSION: Multi–detector row CT angiography has high sensitivity and specificity for detection of intracranial aneurysms, including small aneurysms, in patients with nontraumatic acute subarachnoid hemorrhage.
© RSNA, 2003
Index terms: Aneurysm, intracranial, 17.73 • Computed tomography (CT), angiography, 17.12116 • Digital subtraction angiography, comparative studies, 17.12333, 17.12434 • Hemorrhage, CT, 17.367
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Until recently, magnetic resonance (MR) angiography was the preferred modality for noninvasive intracranial imaging at most centers. MR angiography enables visualization of the circle of Willis without the use of ionizing radiation or intravenous contrast material. The performance of MR angiography in the evaluation of acute subarachnoid hemorrhage also has been compared favorably with that of DSA (3); although some aneurysms were missed, all were less than 5 mm in diameter. In addition, MR angiography can be technically challenging to perform in the acutely ill patient. Since 1978, there has been increasing interest in computed tomography (CT) as a noninvasive imaging modality for detection of intracranial aneurysms (4). Study results have shown that angiography performed with single–detector row helical CT scanners compares favorably with DSA in the detection of intracranial aneurysms (5,6). Recently published data indicate that single–detector row CT angiography has sensitivity and specificity equivalent to those of DSA in the evaluation of aneurysms smaller than 5 mm in diameter (7). The accurate depiction of aneurysms of this size is a key goal at imaging.
Multi–detector row (multisection) CT scanners provide increased spatial resolution and decreased scanning time, which should increase the sensitivity of the technique in depicting aneurysms of less than 5 mm in diameter. The purpose of our study was to prospectively compare the effectiveness of multi–detector row CT angiography with that of conventional intraarterial DSA used to detect intracranial aneurysms in patients with nontraumatic subarachnoid hemorrhage.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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18 years of age) undergoing DSA for nontraumatic acute subarachnoid hemorrhage indicated either by imaging findings at nonenhanced CT or by xanthochromia at lumbar puncture were considered eligible. Patients who had undergone prior surgical clipping or endovascular coiling for treatment of an aneurysm were excluded, as were patients with contraindication to intravenously administered iodinated contrast material. Informed consent was obtained in all instances either from the patient or, if the patient was unable to provide legal consent, from a family member. Between January and September 2002, 40 eligible patients were identified. Of these, 35 were enrolled in the study. Reasons for nonenrollment included inadequate peripheral intravenous access (two patients), renal insufficiency (one patient), and refusal to participate (two patients). Twenty-seven women and eight men were enrolled in the study. Mean patient age was 54 years (range, 26–79 years). Subarachnoid hemorrhage was seen at nonenhanced CT in 32 (91%) of the 35 patients and was deemed present in the other three patients because of xanthochromia observed at lumbar puncture. Among the 35 patients, 26 (74%) were graded I or II on the Hunt and Hess scale for neurologic condition, and the other nine (26%) were graded III (three patients), IV (three patients), or V (three patients).
Imaging
All 35 patients underwent CT with a multi–detector row scanner (LightSpeed QX/i; GE Medical Systems, Milwaukee, Wis). All CT images were diagnostic, and there were no technical failures or complications during scanning. Nonenhanced CT was performed first and was followed by contrast material–enhanced CT angiography. Parameters for the CT angiographic acquisition were 1.25-mm section thickness, 0.5-mm section interval, pitch of 3 (high-quality mode), 140 kVp, 200 mAs, and 14.0-cm field of view. Scanning time was approximately 31 seconds. The scanning volume extended from the superior aspect of the ring of the first cervical vertebra to a point 1 cm above the level of the lateral ventricles, as determined on the nonenhanced study. A total of 120 mL of iohexol (Omnipaque 300; Nycomed, Princeton, NJ), a low-osmolar iodinated contrast material, was administered intravenously with a power injector at a rate of 4 mL/sec via an 18- or 20-gauge catheter positioned in a peripheral vein. Contrast material administration was followed by a delay of 18 seconds before CT angiography was initiated. The transverse source images were reformatted as maximum intensity projection (MIP) images with 10-mm section thickness and 9-mm overlap in the transverse, coronal, and sagittal planes. The angiographic studies were interpreted at a workstation (Advantage for Windows; GE Medical Systems) by using MIP images and multiplanar reformatted images from the source image data set in the coronal and sagittal planes. The CT angiographic images were viewed with a window width of 650 HU and a window level of 160 HU. All patients underwent DSA within 12 hours of CT angiography. In 27 of 35 patients, CT angiography was performed prior to DSA, with the longest interval between the two examinations being 12 hours. In the remaining eight patients, CT angiography was performed after DSA, with the longest interval between the two examinations being 3 hours.
Standard DSA was performed by using a biplane DSA unit (Integris BN3000; Philips Medical Systems, Bothell, Wash) with a matrix of 1,024 x 1,024 pixels. DSA was performed with bilateral selective common carotid artery injections and either unilateral or bilateral vertebral artery injections, as necessary. Additional angiographic views were acquired at the discretion of the angiographer. DSA images were reviewed at a workstation (EasyVision; Philips Medical Systems).
Image Interpretation
The angiographic studies were interpreted by four radiologists: Two (W.W.M.S., G.A.T.) read the CT images, and two others (R.A.H., J.M.R.) read the DSA images. Three of the four readers (G.A.T., R.A.H., J.M.R.) have certificates of additional qualification in neuroradiology. The two readers of the CT images had 12 and 10 years experience in MR and CT image interpretation but had no practice in interpreting CT angiographic images of the circle of Willis, because such examinations had not previously been performed at their institution. The DSA readers had 17 years (R.A.H.) and 14 years (J.M.R.) of experience in interpreting cerebral angiographic images. DSA reader 1 (R.A.H.) was the angiographer who performed DSA, and DSA reader 2 (J.M.R.) reviewed the DSA images retrospectively. The four radiologists performed their readings independently, each being blinded to the results of the others’ readings and, in particular, to the findings on images acquired with the other modality. A standardized scoring sheet developed in conjunction with our department statistician was used by readers for both modalities to rate their certainty about aneurysm presence and location on a five-point scale (0 = definitely not present, 1 = probably not present, 2 = equivocal, 3 = probably present, 4 = definitely present) and at 14 standard locations: the left and right posterior inferior cerebellar arteries, vertebral artery–basilar artery junctions, ophthalmic arteries, distal internal carotid arteries, posterior communicating arteries, middle cerebral artery trifurcations, and anterior communicating artery and basilar apex. Space was available on the form also to describe aneurysms found at other locations. If an aneurysm was considered probably or definitely present, the aneurysm dome and neck were measured on CT angiographic images, and the ratio of the neck to the dome was measured on DSA images (Fig 1). This procedure was used to determine which patients would be potential candidates for endovascular therapy. Because information about magnification factors and catheter calibrations was not available, accurate measurement was not possible at DSA. After the results were tabulated, the CT angiographic images were reviewed in conjunction with the DSA images by the CT angiographic readers working jointly and in consensus.
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Statistical Analysis
Data from image interpretations by all four readers were tabulated with a software application (Access; Microsoft, Redmond, Wash). Angiographic studies in which at least one aneurysm was identified as "probably present" or "definitely present" were considered positive; all others were tabulated as negative for aneurysm. Sensitivity and specificity were calculated separately for each angiographic study (each patient) and for each CT angiographic reader and DSA reader 2 by using the interpretation of DSA reader 1 (R.A.H.) as the reference standard. Findings by DSA reader 1 were considered the standard because he had performed the examinations with DSA and had the option of obtaining additional projections at his discretion, whereas DSA reader 2 reviewed the images retrospectively.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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A number of research studies have been performed previously in which single–detector row CT angiography has been compared with DSA in the depiction of intracranial aneurysms. In one prospective study of patients with subarachnoid hemorrhage (9), investigators detected 131 (91%) of 144 aneurysms with CT angiography. Of the 13 aneurysms missed, 11 were visible retrospectively, one was outside the scanned image volume, and one was not visible retrospectively. Investigators in another study (10) of 79 known aneurysms in 50 patients found a sensitivity of 97% and specificity of 98% for single–detector row CT angiography. However, other study results have not been as promising as these. White et al (11) found a sensitivity of only 62% (51 of 82 aneurysms) for single–detector row CT angiography in a comparison with DSA. In their study, small (<3 mm) aneurysms were especially poorly depicted at CT angiography, with a sensitivity of only 40%. Villablanca et al (7), by contrast, found a sensitivity of 98% and 100%, respectively, for each of two readers in the detection of small (<5 mm) aneurysms at single–detector row CT angiography. The mean diameter of the aneurysms in our study was only 4.4 mm, and only six (23%) were larger than 5 mm.
Advantages of multi–detector row CT angiography over DSA and MR angiography include the ability to depict bone landmarks and the proximity of aneurysms to vascular structures, which may play an important role in treatment planning. Although prior investigators of multi–detector row CT angiography reported a high rate of missed aneurysms adjacent to bone (9), the readers in this study reliably differentiated bone from vascular structures by adjusting the window and level settings at the workstation during image interpretation in multiple planes. Since all of the aneurysms in our study were retrospectively identifiable on multi–detector row CT angiographic images, the proximity of an aneurysm to bone had no effect on the results of image interpretation.
Arterial and venous structures have nearly equivalent attenuation on multi–detector row CT angiographic images, with resultant benefits and drawbacks. The major benefit is in the delineation of venous anatomy (more variable than arterial anatomy), which has been cited as an advantage of multi–detector row CT angiography in patients for whom surgery is being considered (12). However, the overlap of arterial structure with venous structures on CT angiographic images can make their interpretation more challenging than that of MR angiographic images or DSA images. In our study, during image interpretation at the workstation, it was often necessary to trace the proximal and distal portions of a vessel to clearly identify whether it was an artery or a vein. Our study protocol included no monitoring or timing of the intravenous bolus during transit; in all patients, contrast material administration was followed by a delay of 18 seconds prior to initiation of scanning, with no adjustments being made for cardiac output, heart rate, or irregular sinus rhythm. The protocol used by Villablanca et al (7) at single–detector row CT angiography included intravenous bolus timing and a mean delay of 18 seconds. The key to obtaining good results at image interpretation is to select the appropriate window and level settings. As in the study reported by Korogi et al (6), the presence of subarachnoid blood did not affect image interpretation in our study, because blood has a lower attenuation than intravascular contrast material and because appropriate window and level settings (width of 650 HU, level of 160 HU) were used.
Various subtraction and postprocessing algorithms have been proposed previously (13). Given the improved spatial resolution obtainable with multi–detector row scanners compared with single–detector row scanners, we believe that bone subtraction is unnecessary if appropriate window and level settings are used. However, as other investigators have suggested (14), postprocessed images such as MIPs should be compared carefully with the source images, because the vascular anatomy near the skull base may be obscured by higher-attenuating overlapping bone. Given the isotropic nature of the data set, we consider multiplanar reformation extremely useful for evaluation of aneurysms that are not well depicted in the transverse plane. We did not routinely use shaded-surface rendering or other postprocessing techniques in our study.
Multi–detector row CT angiography is not without limitations, however. Experience is critical in the interpretation of images acquired with this technique. According to Pedersen et al (9), observer experience noticeably affected observer findings, with nine (12%) of 75 aneurysms being missed during image interpretation in the 1st year and only four (6%) of 69 aneurysms being missed in the 2nd year. We noticed a similar learning curve, albeit on a smaller scale. The fact that all aneurysms detected at DSA in our study were also visible retrospectively on CT angiographic images supports the suggestion by Pedersen et al that radiologist experience both with the CT angiographic technique and with image review at the workstation plays a crucial role in image interpretation. Lacking such experience, radiologists might miss not only aneurysms but also small adjacent branch vessels—an oversight that could negatively affect treatment planning.
Our study design also had limitations. All patients included in the study manifested symptoms of acute subarachnoid hemorrhage, which significantly increased the pretest probability of positive findings in those patients at angiography. This fact might have introduced observer bias, in that a reader might be more confident about a borderline finding in our study population compared with the general population; however, the potential for observer bias would arguably be the same, regardless of the modality of images being interpreted. We consider DSA the diagnostic standard; small aneurysms, however, may not be depicted with this technique—especially when multiple branch vessels overlap the aneurysm on multiple projections—and yet may be depicted with CT angiography (7). In this study, we used a multi–detector row scanner with four detector rows and 1.25-mm section thickness to perform CT angiography. Newer scanners are now available that offer narrower collimation (0.5 mm) and the ability to acquire 16 images per tube rotation. These improvements in technology may decrease acquisition time, improve spatial resolution, and increase the accuracy and specificity of CT angiography for aneurysm detection.
In conclusion, multi–detector row CT angiography is a promising method for the radiologic detection of aneurysms in intracranial vessels. In addition to being noninvasive, CT angiography has the ability to accurately characterize aneurysms and delineate anatomic landmarks such as adjacent branch vessels and bones. All aneurysms in our study were visible retrospectively, including those with a diameter of less than 5 mm, which constituted the majority. With increasing reader experience, multi–detector row CT angiography may become the method of choice for aneurysm screening in patients with acute subarachnoid hemorrhage.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, M.V.J., W.W.M.S., G.A.T.; study concepts, M.V.J., W.W.M.S., G.A.T., J.M.R., R.A.H.; study design, M.V.J., W.W.M.S., G.A.T.; literature research, M.V.J.; clinical studies, M.V.J., W.W.M.S., R.A.H., G.A.T., J.M.R.; data acquisition, M.V.J., N.R.M.; data analysis/interpretation, M.V.J., G.A.T., W.W.M.S.; statistical analysis, M.V.J., W.W.M.S., G.A.T.; manuscript preparation, all authors; manuscript definition of intellectual content, M.V.J., W.W.M.S., G.A.T.; manuscript editing, revision/review, and final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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