Korean Journal of Cerebrovascular Surgery 2011;13(4):315-323.
Published online December 1, 2011.
Pitfalls in the use of Multidetector Row CT Angiography for Identification of Intracranial Vascular Abnormalities : Focus on the Various Radiological Findings.
Kim, Yong Sang , Cho, Byung Moon , Cho, Sung Min , Park, Se Hyuck , Oh, Sae Moon
Department of Neurosurgery, Kangdong Sacred Heart Hospital, Hallym University College of Medicine, Seoul, Korea. nschbm@hanmail.net
Abstract
OBJECTIVE
Multidetector-Row computed tomographic angiography (MDCTA) is a promising method for detection and surgical planning of intracranial vascular abnormalities. However, there are several problems, such as image degradation due to inevitable patients movement, venous contamination, kissing vessel artifact, demonstration of venous structures mimicking aneurysm and bone artifacts. The purpose of our study is to review our recent experience with false negative or positive cases on MDCTA. METHODS: Between May 2007 and May 2010, 259 consecutive patients, who were diagnosed with intracranial aneurysms or other vascular abnormalities by MDCTA, were retrospectively reviewed. Among the 259 patients, 172 patients who underwent digital subtraction angiography (DSA), which was considered as the standard of reference, were included in the study. Two neuroradiologists and two neurosurgeons evaluated independently and separately all of the MDCTA images. RESULTS: A total 26 cases (15.3%) were revealed abnormal vascular findings on MDCTA. There were 11 false negatives on MDCTA including incomplete scanning range of lesion site (n=3), a blood blister aneurysm (n=1), severe vasospasm (n=4) and bone artifacts (n=3). Also there were 15 false positives on MDCTA; venous contamination over the lesion site (n=6), focal dilation of the bifurcation or branching site of major vessels (n=6) and poor quality of the images due to inevitable patients movement (n=3). CONCLUSIONS: MDCTA is clearly not the total answer for aneurysm diagnosis. We recommend that MDCTA scanning range is planned to encompass the whole intracerebral vasculature. Close attention to image acquisition and interpretation are required to reduce errors in MDCTA of intracranial aneurysms.
Key Words: Multidetector-Row CT angiography, Cerebral aneurysm, Conventional angiography

Introduction
Traditionally, digital subtraction angiography (DSA) has been considered the gold standard among diagnostic modalities available for intracranial aneurysm detection.23)27) However, DSA is time consuming, invasive and relatively expensive.2)7)
Therefore, computed tomographic angiography (CTA) is playing a major role in the screening of patients who are suspected of having intracranial aneurysms. CTA is relatively inexpensive and can be performed rapidly and immediately after routine unenhanced CT of the brain in patients with suspected aneurysmal subarachnoid hemorrhage (SAH). Lastly, CTA had shown potential in the minimally invasive detection of intracranial aneurysms.4)15)29)33) At some institutions CTA has replaced DSA in the preoperative evaluations of patients with intracranial aneurysms.3)6) As CT technology has evolved and various subtraction and post processing techniques have been developed, CTA with good image quality for detection of intracranial vascular abnormalities has become possible. However, there are several problems, such as image degradation due to inevitable patients' movement, venous contamination, kissing vessel artifacts and demonstration of venous structures mimicking aneurysm and bone artifacts. There have been several reports on the pitfalls of CTA, but most previously published reports did not descried clinical and radiological findings.1-3)11) The purpose of our study was to review our recent experience with false negative or positive cases on multidetector-row CT angiography (MDCTA).
Methods
1. Patients
Between May 2007 and May 2010, 259 consecutive patients diagnosed with intracranial aneurysms or other vascular abnormalities detected by MDCTA were retrospectively reviewed. Among the 259 patients, 172 (83 men and 89 women; median age, 54.6 years; age range, 23-76years) patients who underwent DSA, which was considered as the standard of reference, were included in the study.MDCTA and DSA findings were compared with regard to negative and positive findings.
2. MDCTA protocol
All MDCTA scans were obtained by using a 16-detector row CT scanner (MX 8000 Infinite Detector Technology;Philips, Haifa, Israel) with the following parameters: 1-mm section thickness; 0.5-second gantry rotation time; pitch of 0.35; 512 L 512 matrix; 20- to 22- cm FOV; 120 kV; and 200-280 mA. For optimal intraluminal contrast enhancement,the delay time between the start of contrast material administration and the start of scanning was determined for each patient individually by using a bolus-tracking technique, as described in detail elsewhere.5)6) A total of 100–120 mL of iohexol (Omnipaque 300; GE Healthcare, Princeton,NJ), a low-osmolar iodinated contrast material, was administered intravenously via an 18- or 20- gauge catheter positioned in an antecubital vein. The volumetric data so obtained were transferred to a workstation with commercially available software (Rapidia 3D; Infinitt, Seoul, Korea) for further processing. Transverse sections were reconstructed with a section width of 0.5 mm. MDCTA images were processed from the obtained source images by using 3 different methods: 1) sagittal and coronal multiplanar reformations (MPRs), 2) volume-rendered technique (VRT) algorithm, and 3) VRT after automatic segmentation of a precontrast scan dataset (ie, overlapping bony structures). All standardized reconstructions were completed by experienced technicians; image-processing time did not exceed 15 minutes. Two neuroradiologists and two neurosurgeons evaluated independently and separately all of the MDCTA images. Each reader recorded the presence and location of lesions.
Results
A total 26 cases (15.3%) were revealed abnormal vascular findings on MDCTA including false negative (n=11) and false positive (n=15). Among them, four were located in the anterior cerebral artery (ACA), five in the internal carotid artery (ICA), nine in the middle cerebral artery (MCA) and eight in the posterior circulation. Fourteen patients presented with SAH. The causes of the false negative findings on MDCTA were incomplete scanning range of lesion site (n =3), a blood blister ICA aneurysm (n=1), severe vasospasm (n=4), and bone artifacts (n=3) (Table 1). On MDCTA, causes of the false positive findings were venous contamination over the lesion site (n=6), focal dilation of the bifurcation or branching site of major vessels (n=6), and poor quality of the images due to inevitable patients' movement (n=3) (Table 2).
Illustrative cases
Case 1: Anterior wall blood blister-like aneurysm of the ICA A 37-year-old man presented with a Hunt-Hess grade V. The initial CT scan showed hyperdense intracerebral hematoma (ICH) in the left inferior frontal lobe and diffuse SAH in the both basal cistern, interhemispheric fissure and sylvian fissure (Fig. 1A). In contrast to the CT finding, the initial MDCTA and DSA revealed a suspicious tiny blister-like bulge at the anteromedial wall of the right ICA (Fig. 1B, C). Therefore, we believed that the left cerebral artery occult aneurysm was ruptured. Because the aneurysm was questionable, we had carefully observed meticulous control of blood pressure. Repeated MDCTA and DSA obtained on nine days after the first study showed the aneurysm had grown into saccular configuration from the previous suspicious tiny bulge at the right ICA (Fig. 1D, E).
The aneurysm originated from the non-branching site of the anteromedial wall of the ICA, just above to the lateral aspect of the right optic nerve.
Case 2: Incomplete scanning range of lesion site : A tiny distal MCA aneurysm A 46-year-old man came to the department of neurology with a week long history of severe headache. The CT scan showed hyperdense ICH in the right parietal lobe and SAH in the right sylvian fissure, and MRI revealed same findings (Fig. 2A). Initial MDCTA was reported as normal finding (Fig. 2B). The aneurysm on distal MCA was excluded from the initial MDCTA because of incomplete scanning range of lesion site. DSA and repeated MDCTA with more appropriate
scan levels, confirmed a small saccular aneurysm (2 mm) on the corticomedullary junction of the right distal MCA (Fig. 2C, D).
Case 3: Hypoplastic stump, Right A1 segment A 28-year -old man presented with a Hunt-Hess grade II. The CT scan showed SAH in the bilateral sylvian cistern (Fig. 3A). MDCTA and DSA revealed a suspicious tiny bulge at the anterior communicating artery (Fig. 3B, C). Therefore, we believed that the anterior communicating artery aneurysm was ruptured. A right pterional approach was performed. At operation, prominent hypoplastic stump of right A1 segment. The SAH existed around the vessels, and there was no definite focus of hemorrhage.
Discussion
Aneurysms are one of the most important causes of SAH with a fatality rate from 40 to 60%, whereas misdiagnosis is associated with further increased morbidity and mortality.13)16) Traditionally, DSA has been considered the “gold standard” for aneurysm detection; currently, three-dimensional rotational DSA (3DRA; obtained via the catheter angiogram) may offer increased aneurysm detection, with improved visualization of an aneurysm’s configuration and contour compared with DSA alone.8)20)22)26) However, the combination of DSA/3DRA is invasive, time consuming and may involve neurologic complications from 1~2%.5)30) For these reasons, an accurate and noninvasive test would be invaluable in the emergent screening for SAH. CTA is recognized as less-invasive, simple and fast imaging modality in comparison to DSA. Also CTA can be useful to undergo emergency surgery when endovascular therapy is not feasible. Over the past 10~12 years, the sensitivity and accuracy of CTA in detecting intracranial aneurysms have progressively improved during the evolution from single-section CTA to 4MDCTA to the current routine use of 16MDCTA and 64MDCTA.13) In the study by Zouaoui, CTA demonstrated sensitivity and specificity rates of up to 97% and 100% respectively.35) However, MDCTA is clearly not the total answer for aneurysm diagnosis. MDCTA has some shortcomings and pitfalls when compared with conventional angiography. As Napel et al.17) suggested problems can arise if the time of scanning is not the same as that of peak intra-arterial contrast-medium concentration. Yoon et al.33) described that a scan delay of 15 seconds after the end of the injection of contrast into a peripheral vein is usually suitable for adults; this short delay optimizes arterial opacification, with little or no venous contamination. Also small perforating arteries with a diameter less than 1 mm are not visible on CTA.10) In addition, kissing vessel artifact makes it impossible to see a clear margin between an aneurysm and adjacent arteries.25) Therefore, the main difficulties in CTA lie with the detection of small aneurysms (< 4 mm), differentiating the infundibular dilatation at the origin of an artery from an aneurysm,1) aneurysms of the proximal ICA and ophthalmic artery close to bone, such as the skull base,14)19) inability to identify thrombosis and calcification on three-dimensional images, and loss of information due to artifact from surgical clips.18)21)
Of the 15 false positives detailed in Table 2, most of our false positives occurred with focal dilation of the bifurcation or branching site of major vessels (n = 6) and venous contamination (Fig. 4) over the lesion site (n = 6). Venous contamination will be impossible to see a clear margin between an aneurysm and adjacent arteries, which results in the erroneous impression that a connection between these vascular structures might exist. Kim et al.12) reported a false positive case because of the prominent sylvian vein superimposed on the bifurcation of the left MCA. The time that elapses between the arterial and the venous phase of a contrast material bolus flowing through the intracranial circulation is about 5-6 seconds. Therefore, even with multisection scanners, it is difficult to produce pure arterial phase CT angiography. Thus, the depiction of venous structures cannot be avoided, and when they appear adjacent to arteries they can sometimes be mistaken for aneurysms (Fig. 5). We have found that a scan delay of 15 seconds after the end of the injection of contrast into a peripheral vein is usually suitable for adults; this short delay optimizes arterial opacification, with little or no venous contamination.33) Most of false negatives occurred with aneurysms 3 mm or less in maximum dimension,34) severe vasospasm and bone artifact. Assessment of the axial source images is an essential part of this technique. Vieco et al.28) documented that examination of the axial source images is needed in order to detect or confirm small aneurysms close to branching vessels. Previous literatures have shown a propensity to miss aneurysms in both periophthalmic ICA and the cavernous ICA due to their proximity to other attenuated structures, such as bone or enhancing cavernous sinus.24)31) In our cases, four aneurysms were not detected by severe vasospasm on MDCTA (Fig. 6). One was blood blister ICA aneurysm and three were bone artifact. Hwang et al.9) demonstrated that 3D CTA with bone subtraction showed significantly higher diagnostic accuracy for the detection of intracranial aneurysms as compared to 3D CTA without bone subtraction. Some 5~10% of intracranial aneurysms arise outside the circle of Willis. Our studies are normally limited to the circle of Willis and adjacent major arteries, because otherwise the data acquisition and manipulation with our current equipment would require an exorbitant amount of time. As the illustrative case 2 shows, aneurysms which located outside from the scan field of view will be overlooked.21)32) Although the pitfalls enumerated cause problems in the imaging and interpretation of MDCTA, the technique has several major advantages in diagnosis of intracranial aneurysms: it is simple and quick for the patient; it is safer than angiography; it is useful for screening of selected patient groups, particularly relatives and past patients.
Conclusion
Clearly, CTA is not the total answer for aneurysm diagnosis. We recommend that CTA scanning range is planned to encompass the whole intracerebral vasculature. Close attention to image acquisition and interpretation is required to reduce errors in CTA of intracranial aneurysms.
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