Intracranial stenosis

Theoretically, 3D TOF-MRA can reach a spatial resolution of up to 0.2 mm voxels. With 1- to 1.5-T magnets this would not provide an acceptable signal-to-noise ratio and therefore in clinical routine the spatial resolution is realistically in a range of approximately 0.5 mm. 3D TOF-MRA is widely accepted as a standard for depiction of intracranial stenosis (Dagirmanjian et al. 1995 Oelerich et al. 1998). Intracranial TOF-MRA should depict the ves-... 

Drawbacks of PC-MRA are seen in its susceptibility for patient movements and its impairment by pulsatile or turbulent flow patterns. The diagnostic sensitivity of 3D PC-MRA in intracranial stenosis is markedly lower than with 3D TOF-MRA (Oelerich et al. 1998). Advantages are the good suppression of stationary tissue signal with the lack of interference with short T1 tissues, and the depiction of large volumes without limitations due to spin saturation effects. 

In the diagnosis of intracranial stenosis, TOF-MRA is definitely superior to PC-MRA protocols. Oelerich et al. (1998) found a sensitivity of 87% for 3D TOF-MRA in intracranial stenoses. The correlation with DSA was 78%, and other authors found a correlation with MRA up to 88% (Dagirmanjian et al. 1995). While sensitivity is relatively high, false negative findings are rare, so that with sufficient examination quality, a good negative prediction is reached. 

Patients who have had a stroke or transitory ischemic attack associated with intracranial artery stenosis (>50%) have a 12-14% risk of subsequent stroke in the 2-year period after the initial event, regardless of treatment with antithrombotic medications. Atherosclerosis in the intracranial portion of the ICA and the MCA is more common in the African-American, Hispanic, and Asian populations for unknown reasons. The proportion of patients hospitalized for ischemic strokes with symptomatic intracranial stenosis ranges from 1% in non-Hispanic whites to as high as 50% in Asian populations [21, 22]. Atherosclerosis in the intracranial portion of the carotid and in the MCA often causes multiple strokes in the same vascular territory. It may also cause slow stroke syndrome, in which there is progressive worsening of focal cortical ischemic symptoms over days or weeks. In addition, the penetrator arteries flowing to the deep white matter and striatum originate from the MCA stem (Ml) and may be occluded in patients with severe MCA stenosis [20]. 

The treatment of patients with symptomatic intracranial atherosclerotic disease can be summarized into prevention of occurrence of intraluminal thrombosis, plaque stabilization, and control of risk factors for atherosclerosis. Anticoagulation (compared with aspirin) has not shown to be beneficial in patients with intracranial atherosclerotic disease [24]. Current guidelines recommend that aspirin alone, the combination of aspirin and extended release dipyridamole, and clopidogrel monotherapy (rather than oral anticoagulants) are aU acceptable options [24]. hi patients with hemodynamically sigifificant intracranial stenosis who have symptoms despite medical therapies (antithrombotics, statins, and other treatments for risk factors), the usefulness of endovascular therapy (angioplasty and/or stent placement) is uncertain and is considered investigational [22, 25],... 

Suri MFK, Johnston SC Epidemiology of intracranial stenosis. J Neuroimaging 2009 19 Suppl 1 pp. 11S-6S. 

Despite its frequency, the prognosis for patients with symptomatic intracranial stenosis is not well defined. A retrospective study with symptomatic intracranial vertebrobasilar stenosis found that 14% of the patients had another stroke over the 15-month follow-up interval. There has been increasing enthusiasm for endovascular treatment of intracranial artery stenosis (Fig. 40.12). 

Heiserman JE, Drayer BP, Keller PJ, Fram EK. Intracranial vascular stenosis and occlusion evaluation with three-dimensional time-of-flight MR angiography. Radiology 1992 185 667-673. 

Dissection of the internal carotid and vertebral arteries is a common cause of stroke, particularly in young patients. Although many occur due to trauma, it is estimated that over half occur spontaneously. The mechanism of stroke following arterial dissection is either by artery-to-artery embolism, by thrombosis in situ, or by dissection-induced lumenal stenosis with secondary cerebral hypoperfusion and low-flow watershed infarction. Occasionally, dissection may lead to the formation of a pseudoaneurysm as a source of thrombus formation. Vertebrobasilar dissections that extend intracranially have a higher risk of rupture leading to subarachnoid hemorrhage (SAH). ° ... 

Transcranial Doppler can determine the presence of intracranial sclerosis (e.g., middle cerebral artery stenosis). 

In Caucasians, the vast majority of MCA occlusions are of embolic origin with emboli arising from a carotid stenosis, the aortic arch or the heart (Heinsius et al. 1998) or from the venous side in case of a patent foramen ovale. In black or Asian patients a higher prevalence of intracranial occlusive disease is found with subsequent thrombotic arterial occlusion or stenosis (Feldmann et al. 1990). 

The depiction of intracranial arterial stenosis by MRA requires a high sensitivity resolution technique, while the importance of an exact grading by MRA is not defined yet. A preventive anti-calculation for stenosis of proximal intracranial segments exceeding 50% is still a matter of debate (Sherman 2002). 

Fig.5.18a-c. TOF-MRA versus DSA correlation in intracranial stenoses. Occlusion of the right MCA and only mild stenosis of the left one on DSA (b,c), while signal loss on CE-MRA (a) should not lead to an overestimation of stenosis grade... 

Fig. 5.19a,b. Multiple intracranial stenoses TOF-MRA (a,b) with Doppler ultrasound correlation. High grade stenosis of left MCA (Ml) was confirmed (>400 cm/s), suspected right M2 stenosis was estimated as mild by ultrasound and a left PCA stenosis was suspected only on MRA... 

Fig. 5.25a,b. Moya Moya syndrome. Due to limited resolution as well as turbulence, TOF-MRA (a) and CE-MRA (b) do not enable definitive quantification of obstructions at the level of the distal intracranial carotids TOF might be diagnosed as left occlusion and right high grade stenosis, while CE-MRA demonstrates bilateral enhancement of MCA branches... 

Farb RI, Vanek I, Scott JN,Mikulis DJ,Willinski RA,Tomlinson G, terBrugge KG (2003) Idiopathic intracranial hypertension. The prevalence and morphology of sinovenous stenosis. Neurology 60 1418-1424... 

The main clinical indications for imaging the cerebral circulation are TIA (e.g. to identify arterial stenosis), acute ischemic stroke (e.g. to identify vessel occlusion), intracerebral hemorrhage (e.g. to identify an underlying vascular malformation) and possible arterial dissection, flbromuscular dysplasia or other arteriopathies, cerebral aneurysm, intracranial venous thrombosis or cerebral vasculitis. 

Chimowitz MI, Lynn MJ, Howlett-Smith H et al. (2005). Comparison of warfarin and aspirin for symptomatic intracranial arterial stenosis. New England Journal of Medicine 352 1305-1316... 

Bypass or endarterectomy of proximal vertebral artery stenosis Release of the vertebral artery from compressive fibrous bands or osteophytes Extracranial-to-intracranial procedures to bypass vertebral artery stenosis or occlusion Angioplasty and stenting of the vertebral and basilar arteries... 

The evaluation of acute stroke patients with CTA is being performed to evaluate for the underlying vascular etiology (Smith et al., 2006). CTA can nicely demonstrate areas of stenosis or thrombosis in the arteries. The presence of a large vessel intracranial occlusion in an acute stroke patient has been found to even be an independent predictor of poor outcome (Smith et al., 2006). Also, computed tomographic venography has been found to be beneficial for evaluating patients for the presence of venous thrombosis. 

---