14 Chapter 1 Consequently, 3D DSA leads to a reduction in patient radiation and iodinated contrast material exposure.101 Due to these advantages, many now consider 3D DSA the gold standard for detection and evaluation of cerebral aneurysms.68,83,102 The standard 2D DSA images should not be dismissed entirely however: 3D reconstructed DSA images may cause the dome-to-neck ratio to be underestimated103 and pseudostenosis in the parent vessel to be observed.104 The true diagnostic accuracy of DSA for detecting intracerebral aneurysms is difficult to determine due to the lack of an alternate standard of reference. Indeed, initial DSA may be negative in 10-20% of cases, sometimes falsely-negative due to small aneurysm size, aneurysm thrombosis, local vasospasm, or simply due to incomplete imaging. Repeat angiography has traditionally been recommended within 1 to 2 weeks. If this repeat DSA is negative it is concluded that an aneurysm is not present and the cause of the SAH should be sought elsewhere. However, the estimated additional diagnostic yield of this procedure is only 1%-2%, and its cost effectiveness and benefit-risk balance are topics of debate.16 Repeat DSA may not be functional in several identifiable patient subpopulations. Different bleeding patterns in SAH, for example, may give an indication of the presence or absence of an aneurysm.105 It is known that a perimesencephalic or pretruncal distribution pattern is not usually caused by an aneurysm.106,107 And in patients exhibiting such patterns not only does repeat DSA have a very low diagnostic yield,108-111 but the clinical course is favorable with very few rebleeds.112,113 On the other hand, when the bleeding distribution pattern is not typical for a perimesencephalic SAH, repeat angiography reveals an aneurysm in up to 45% of cases.114-118 Another category is the group of patients with a clinical presentation of SAH, with a CT study negative for SAH but a positive CSF finding at LP. In all studies where this patient group was examined separately, repeat DSA did not reveal a single aneurysm.108,114,116,117,119,120 Introducing a second repeat DSA after a longer period proved of little value in the aforementioned subgroups.109,121 Based on the above information the following strategy appears to be most practical for patients with a DSA-negative SAH: • If the diagnosis of SAH is based on clinical presentation with positive CSF findings but a negative CT study, repeat DSA is not recommended • If the bleeding pattern in SAH has a typical perimesencephalic distribution on CT, repeat DSA is not recommended • If the SAH has a non-perimesencephalic distribution pattern on CT, repeat DSA within the first week is recommended Detection and evaluation of intracranial aneurysms: CTA CTA is an alternative to DSA for the detection of aneurysms in patients with a SAH. Since the first description of axial computed tomography was published by Hounsfield and Ambrose in 1973,122-125 CT has become a valuable tool in imaging of the brain. Early CT units produced crude images on a 64x64 matrix and early computers took all night to process these images.126 The main technological advance that led to the development of extended coverage CT and CTA was the introduction of spiral CT in 1989, which allowed continuous tube-detector rotation and transport of the
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