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Fig. 25a-e. Superior sagittal and right transverse sinus thrombosis. 52-year-old female with seizures. T1-weighted MR images reveal isointense signal of the right transverse sinus (a) (arrow) and superior sagittal sinus (b) (arrow). The T2-weighted (c) image reveals hyperin-tensity of the superior sagittal sinus (arrow) and the presence of small bilateral cortical infarction. The 3D PC MRA images (d, e) show the presence of thrombosis of the central and anterior portions of the superior sagittal sinus (arrows) and of the right transverse sinus (arrow c visualization of the remnant patency of the tributary arteries of the ischemic territory [60-62].

Atherosclerosis of the intra- and extracranial vessels is the most common cause of stroke [45]. There is a significant association between stenosis of the intracranial arteries and the risk of stroke [45, 63, 64]. Stenosis of the intracranial arteries is present in 15% of patients with stenosis of neck vessels [65] and in 20% of patients with stroke or transient ischemic attack (TIA) [66]. Therefore, evaluation of the supra-aortic and intracranial arterial segments in patients that undergo work-up for cerebrovascular degenerative disease is very important. MRA is particularly well indicated in these patients while DSA, which is considerably more invasive, should be used only in selected cases.

Turbo or fast CE MRA permits panoramic and high-resolution acquisitions of the arterial vessels of the neck, the carotid arch, and the intracranial circulation with just a single bolus of contrast agent. In order to complete the study of the in-tracranial circulation, high resolution 3D TOF MRA without contrast agent should be performed and the results compared to the CE MRA acquisi tions. Previous studies have shown that unen-hanced 3D TOF MRA combined with CTA has a diagnostic accuracy comparable to DSA for determining the degree of stenosis and the presence of occlusions in the intracranial circulation [55].

Venous Vessel Thrombosis

Basic MR combined with MRA has very high sensitivity for the detection of cerebral venous thrombosis. MRA can also be used to monitor the response to thrombolytic therapy. For accurate confirmation of the presence and extension of a venous thrombosis, 3D PC MRA techniques (Fig. 25a-e) are generally preferred over TOF MRA sequences since the latter are frequently unable to distinguish flow from subacute thrombi.

The use of 3D CE MRA and image subtraction minimizes the possibility of false positives due to flow problems. This approach has been proposed as a sensitive technique for evaluation of the dural sinuses, particularly in those regions (transverse sinuses, posterior part of superior sagittal sinus, transverse-sigmoid junction) in which TOF and

Fig. 26a-b. Small aneurysm (arrows) of the right middle cerebral artery bifurcation. (a) DSA and (b) 3D TOF MRA

Aneurysm Comparison With Healthy

Fig. 26a-b. Small aneurysm (arrows) of the right middle cerebral artery bifurcation. (a) DSA and (b) 3D TOF MRA

PC MRA techniques may be troublesome because of saturation or complex flow. 3D CE MRA enables more complete visualization of venous structures which is most apparent for large dural sinuses [67].At present this technique seems to be promising, although little has been reported as yet concerning its application in pathological series in comparison with unenhanced techniques.

While unenhanced MRA techniques are capable of distinguishing occlusion and focal stenosis of larger dural sinuses, a diagnostic problem is posed by normal variants or transverse sinus hy-poplasia. In these cases improved diagnoses may be achieved with the use of a contrast agent in conjunction with either a PC or TOF MRA technique or preferably a fast 3D CE MRA technique [67].

Another field of possible application in MR venography is in the presurgical evaluation of lesions involving the dural sinuses e.g. meningiomas of the vault. In these cases both 3D PC MRA and fast 3D CE MRA have been proposed.

Intracranial Aneurysms

The term "aneurysm" indicates a dilation of an arterial segment resulting from a defect of the elastic tunica of a blood vessel wall, delimited by just the intima and adventitia.

Asymptomatic (unruptured) cerebral aneurysms are found in 2-8% of autopsies and in 7% of DSA studies [68]. The worst complication for an intracranial aneurysm is a subarachnoid hemorrhage (SAH) which has an annual risk of 12% and a mortality rate of 50% [68]. The frequency with which aneurysms rupture should be directly proportional to the dimension of the aneurysm. In other words, the probability of rupture for an aneurysm that is greater than 5 mm in size is greater than that for an aneurysm that is less than 5 mm in size. Nevertheless, even the smallest aneurysms present a risk of rupture [69]. Symptomatic aneurysms are most frequently seen in the 40 to 60 year-old age range and only 2% are symptomatic in subjects under 20 years of age. The incidence of aneurysms is greater in women than in men by a ratio of 2 to 1, and there is a prevalence of 10% for asymptomatic familial intracranial aneurysms [70]. Intracranial aneurysms may be multiple in 14 to 45% of cases, with multiplicity more frequent among women [71].

An increased prevalence of intracranial aneurysms has been reported for a number of conditions including congenital intracranial vascular malformations, anatomical variations (e.g., persistence of the trigeminal artery, duplication of the MCA and variations of the OA) [72,73], polycystic renal disease, fibromuscular dysplasia, and Mar-fan's syndrome [72-75].

Intracranial aneurysms are most often located in the circle of Willis and the trifurcation of the MCA (Fig. 26a-b). One third of all aneurysms are located in the anterior communicating artery and 10% originate in the vertebral-basilar district (Fig. 27a-b) [71].

There are numerous types of aneurysm. Berry (saccular) aneurysms are the result of a defect of the arterial wall which can either be congenital or acquired after continuous local hemodynamic stress. In these aneurysms, the medial elastic tunica terminates in the neck of the aneurismal sac. There is a relatively high frequency of multiplicity for berry aneurysms [71]. Fusiform aneurysms, on the other hand, are localized dilations of an arterial tract caused by arteriosclerosis and are, therefore, more frequent among the elderly. They are typically localized in the basilar artery, vertebral artery or in the supraclinoidal tract of the ICA (Fig. 28a-c). Although they are frequently symptomatic due to compression of the adjacent cranial nerves, these aneurysms rarely rupture. Dissecting aneurysms have a different pathogenesis and can

Fig. 27a-b. Small aneurysm (arrows) at the origin of the posterior inferior cerebellar artery apparent behind the left middle cerebellar peduncle on the coronal T2-weighted image (a) and on the 3d TOF MRA targeted MIP reconstruction (b) of the vertebro-basilar circulation [courtesy of Prof. C. Colosimo; University of Chieti]

Fig. 27a-b. Small aneurysm (arrows) at the origin of the posterior inferior cerebellar artery apparent behind the left middle cerebellar peduncle on the coronal T2-weighted image (a) and on the 3d TOF MRA targeted MIP reconstruction (b) of the vertebro-basilar circulation [courtesy of Prof. C. Colosimo; University of Chieti]

Inferior Mesenteric Artery Function

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