Introduction and historical background 13 sensitivity were sought in the absence of paramagnetic methaemoglobin in the first days after the bleed,59 or in the restrictions on the generation of paramagnetic deoxyhaemoglobin due to the local oxygen tension of cerebrospinal fluid.60 In the same period however, superiority of MRI over CT in the detection of SAH in the later phase (after 4 days) was reported in literature, especially with proton density weighted sequences.61 Later publications also reported a high sensitivity in the acute phase: most worked with fluid attenuated inversion recovery (FLAIR) sequences62-64 while others found T2* gradient-echo sequences to be the most sensitive.56 However, doubt about the practical value of MRI for the detection of subarachnoid blood remains,55,57 and even if MRI were to be equally sensitive to CT in detecting SAH in the acute phase, MRI would be impracticable: MRI systems are less readily available than CT scanners, and the lengthier MRI acquisitions make MRI images more sensitive to movement artefacts than CT.36,65 Detection and evaluation of intracranial aneurysms: DSA Once the diagnosis of SAH has been established a causative aneurysm must be detected or excluded. Currently, three main diagnostic tools exist: digital subtraction angiography (DSA), CTA and MRA. Until recently, the standard tool used for the identification of the source of a bleed was conventional (catheter) angiography. This technique has evolved to the currently used DSA techniques using non-ionic contrast material.16,66-69 The ‘Guidelines for the Management of Aneurysmal Subarachnoid Hemorrhage : A Statement for Healthcare Professionals From a Special Writing Group of the Stroke Council, American Heart Association’ of 2009 give the following recommendations: • Selective cerebral angiography should be performed in patients with SAH to document the presence and anatomic features of aneurysms (Class I, Level of Evidence B). • MRA and CTA may be considered when conventional angiography cannot be performed in a timely fashion (Class IIb, Level of Evidence B).16 DSA is an invasive, labor-intensive and relatively expensive technique. It involves the placement of a catheter in the cervical internal carotid arteries and in the vertebral arteries. Therefore it carries a small but potentially significant risk of neurological complications. Permanent neurologic complication is reported to occur in 0.12 to 2.3 % of cases.70-77 Before the introduction of rotational DSA, the sensitivity of DSA for cerebral aneurysm detection was estimated to be greater than 90%; in the setting of acute SAH sensitivity decreased to slightly over 80%.41,78,79 Since then, the introduction of rotational DSA with 3D reconstruction has increased diagnostic sensitivity even further. This technique has been used for detecting and evaluating cerebral aneurysms80-83 ever since the first reports of rotational roentgenography84,85 and the application of rotational angiography to imaging of the cerebral vessels86-89 were first published . The 3D reconstructions provided by rotational angiography have consistently shown a strong correlation with intraoperative findings90 and have proven helpful in evaluating possibilities for endovascular treatment.91-94 There are several advantages to 3D rotational angiography over conventional 2D angiography: first, the spatial resolution which can be achieved using 3D DSA is 0.15mm, whereas this is limited to ca. 0.2mm for 2D DSA.95 Thus 3D DSA allows the detection of more aneurysms, especially those of smaller size.96-100 Second, in 2D DSA multiple acquisitions are usually required for detailed evaluation of the cerebral vessels, whereas in 3D DSA a single acquisition per vessel usually suffices.
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