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Subacutely, an additional 5 patients also had DSA, for a total of 14/54. CTA vessel occlusion was diagnosed in all 9 of the patients in whom it was diagnosed in acute DSA exams. Of the 14 patients overall who had DSA performed, CTA detected the same lesions in 12 of the patients (sensitivity 82%, specificity 100%). (Verro et al., 2002) Two other studies, both by the same group of investigators, compared CT technologies with magnetic resonance imaging (MRI), including MRA, perfusion-weighted MRI (PWI), and diffusion-weighted MRI (DWI), in patients with acute ischemic stroke. The authors were particularly interested in the performance of the combination of the three CT Computed Tomography (CT) Angiography of the Head - Commercial Medical Management Guideline technologies as an alternative to MRI because many emergency departments may have acute access to CT but not to MRI. Both of these studies found that a combination of unenhanced CT, CTA, and CTA source images (CTA-SI) can provide diagnostic assessment of acute stroke patients at a level similar to that of MRI and may be more feasible than MRI for some patients and in some healthcare settings. (Schramm et al., 2002; Schramm et al., 2004) CTA was compared with DSA and MRA in 28 patients with suspected cerebrovascular lesions. CTA had higher sensitivity than MRA for stenosis (98% versus 70%) and occlusion (100% versus 87%) and was superior to DSA in identification of vessel patency.
Wintermark et al. (2003) investigated the accuracy of multi-slice CTA in 50 consecutive patients. CTA detected ICAs with high per-patient sensitivity (99%), specificity (95.2%), accuracy (98.3%), positive predictive value (PPV) (99%), and negative predictive value (NPV) (95.2%). However, CTA missed ICAs in 3 patients; all of the missed lesions were less than 4 mm in size. Statistical analysis demonstrated a cutoff value of 2 mm for which the sensitivity of CTA was significantly reduced. CTA and DSA correlated well with respect to detection and definition of size and shape of ICAs.
CTA was somewhat less accurate for the detailed characterization of ICAs, such as in estimation of sac/neck ratios.
Teksam et al. (2004) used multi-slice CTA to evaluate 100 patients with suspected SAH and/or ICA, using DSA and surgery as the reference standards. CTA had a high sensitivity (97% to 100%) for aneurysms that were 4 mm or more in diameter but lower sensitivity (84%) if the lesions were less than 4 mm. These findings suggest that helical CTA may miss smaller lesions and may not provide sufficient information to replace DSA.
A 2004 study explored the repercussions of replacing DSA investigation of ICAs (ruptured and unruptured) with CTA as the only diagnostic and pretreatment planning study. They reasoned that the use of both DSA and CTA (as in a number of previous studies) negates the advantages of CTA, yet exposes a patient to the risks and disadvantages of both imaging modalities. A CTA protocol was established and, from January 2002 to January 2003, performed work-ups on 223 Computed Tomography (CT) Angiography of the Head - Commercial Medical Management Guideline patients with ICA using CTA alone as the initial diagnostic evaluation. Of the 223 patients, 109 had confirmed SAH and 114 did not. ICA treatment was initiated in 182 patients (82%), including 93 of 109 patients with SAH (86%) and 89 of 114 patients without SAH (78%). The ICA detection rate by CTA was 100% for the presenting ICA and 97% for total cerebral ICAs, including incident multiple ICAs. (Hoh et al., 2004) A prospective study evaluated 57 patients presenting with subarachnoid hemorrhage who underwent CTA followed by conventional angiography. The specificity of CT angiography for diagnosing intracranial aneurysms was 100% with a sensitivity of 86%. (Kadri et al., 2006) The usefulness of 16-row multi-slice CTA was evaluated in 57 patients with suspected intracranial aneurysms who underwent DSA and CTA. DSA or intraoperative findings confirmed 53 aneurysms in 44 patients. Sensitivity and specificity per aneurysm for DSA and CTA were the same ( 96.2% and 100%, respectively). For aneurysms of less than 3 mm, CTA had a sensitivity of 91.7%. The investigators concluded that CTA appears to be equivalent to DSA for the detection and evaluation of intracranial aneurysms. (Tipper et al., 2005) Wermer et al. constructed a decision model to examine the use of CTA to screen for new ICAs in patients who had already suffered one SAH. The strategies of "screening" and "no screening" were compared, taking into account risks of ICA recurrence, complications of CTA, and retreatment, as reported in the literature. The findings were not supportive of screening, leading the authors to conclude that at the present time, screening for new aneurysms after SAH cannot be recommended. They noted that screening may prevent new episodes of SAH but with too high a cost in terms of complications from preventive treatment. (Wermer et al., 2004) Van der Schaaf et al. used CTA to evaluate 415 patients with previously ruptured aneurysms that had been treated using cobalt clips. The study results indicated that CTA has good feasibility with good positive predictive value and interobserver agreement. Disadvantages were that very small aneurysms were missed and clip artifacts hampered clip site evaluation. (van der Schaaf et al., 2006) The accuracy of multi-slice CTA was compared to DSA in the postoperative evaluation of 49 patients with clipped aneurysms. The sensitivity and specificity of CTA for the detection of neck remnants were both 100% and the sensitivity and specificity for assessing vessel patency were 80 and 100%, respectively. (Dehdashti et al., 2006) There were a number of analyses of the value of using CTA for screening for ICAs. Between 3.6% and 6% of the population harbor an unruptured ICA, but for those without a previous SAH, the likelihood of rupture is very small.
Depending on ICA size, this varies from 0% per year for ICAs less than 7 mm to 3% per year for ICAs 13 to 24 mm.
(White and Wardlaw, 2003) The American College of Radiology (ACR) Appropriateness Criteria for cerebrovascular disease state, "to date, individuals with a history of aneurysm or SAH in a first-degree relative have been considered candidates for screening.
Nevertheless, significant gaps in knowledge of the natural history (and thus risk of rupture) of intracranial aneurysms remain. Hence, while screening with MRA or CTA may be appropriate in patients with a positive family history, its impact on patient outcome is questionable." (ACR, 2006) Arteriovenous Malformation (AVM): Chen et al. evaluated the use of 16-layer 3D CTA and DSA for diagnosing AVM in 16 patients. Three dimensional CTA appeared to be equivalent to DSA except that CTA did not provide sufficient information for diagnosing the third feeding artery of AVM. The investigators concluded the CTA was a superior approach for the diagnosis and follow-up of AVM. (Chen et al., 2004) Another study that included 23 patients also found that 3D CTA is equivalent to DSA for diagnosing AVM. (Wu et al., 2000) Tanaka et al. compared CTA, MRA, and DSA in the evaluation of 12 AVMs. Nidus dimension was measured by CTA in Computed Tomography (CT) Angiography of the Head - Commercial Medical Management Guideline all AVMs but not possible on MRA in 4 AVMs. CTA reconstructed images depicted more veins but fewer arteries than MRA. (Tanaka et al., 1997)
The American College of Radiology (ACR): The ACR produces widely recognized Appropriateness Criteria for use of diagnostic imaging modalities and employs a rating scale ranging from 1 (least appropriate) to 9 (most appropriate). The ratings are based on the assumption that magnetic resonance (MR) imaging is not available. Under cerebrovascular
disease, CTA of the head received the following appropriateness ratings:
- asymptomatic structural lesion on physical exam and/or risk factors (rating of 3 for CTA, head)
- carotid territory or vertebrobasilar TIA, initial screening survey (rating of 8 for CTA, head; MR preferred)
- new focal neurologic defect, fixed or worsening, less than 3 hours (rating of 8 for CTA, head; MR preferred)
- new focal neurologic defect, fixed or worsening, 3 to 24 hours (rating of 8 for CTA, head; MR preferred)
- new focal neurologic defect, fixed or worsening, greater than 24 hours (rating of 8 for CTA, head; MR preferred)
- risk for unruptured aneurysm, positive family history (rating of 8 for CTA, head; MR preferred)
- suspected subarachnoid hemorrhage, not confirmed (rating of 2 for CTA, head)
- proven subarachnoid hemorrhage (rating of 8 for CTA, head)
- suspected parenchymal hemorrhage or hematoma (rating of 4 for CTA, head)