Clinical Applications

Both TOF MRA and PC MRA have been used for non-invasive studies of cerebral vascular pathologies. New turbo (fast) TOF MRA sequences require at most 5-6 minutes to acquire panoramic images of the entire intracranial arterial district and, therefore, these sequences should be performed routinely to complete standard brain exams. As a general rule, 3D TOF MRA is indicated in all cases where non-invasive and selective studies of specific arterial districts are necessary, while 2D/3D PC or 2D TOF sequences are generally appropriate for selective visualization of the venous sinuses and major central veins.

Both techniques have limitations that reduce their diagnostic accuracy. The saturation phenomena and spin dephasing that occur in TOF MRA when the flow rate is greatly reduced or turbulent, results in an absence of signal on MIP reconstructions and, in consequence, an overestimation of stenosis and an increase in the number of false positives for occlusion. This is a well known problem for the carotid bifurcation, and can be seen in the intracranial circulation when 3D TOF MRA is used.

A decrease in flow velocity is also seen within cerebral aneurysms, especially in larger lesions in which flow separation and resulting spin saturation of slower blood leads to an underestimation of the size of the aneurysm. In smaller aneurysms this phenomenon may lead to the lesion not being seen. For the most part, these diagnostic pitfalls can be avoided when 3D CE MRA is used since saturation phenomena are no longer a problem. As a consequence, diagnostic accuracy is improved [34].

In cases of recent thrombosis, the presence of methemoglobin (a paramagnetic substance which, like gadolinium, is hyperintense on TOF images) in the coagulum may lead to an underestimation of cerebral artery occlusion which can result in false negative diagnoses.

The limitations of PC MRA are as well known as those of 3D TOF MRA. The possibility to visualize a specific venous or arterial vessel with PC MRA depends upon the VENC sampled for the specific blood vessel. The VENC values under normal conditions have, for the most part, already been determined and each of the producers of MR instruments provides pre-set sequences for the major arterial and venous districts. In pathological situations, however, the arterial and/or venous flow rates are modified and therefore the pre-de-termined VENC values lead to either an overesti-mation or an underestimation of vessel patency, thereby greatly reducing diagnostic accuracy. The possibility to utilize alternative 3D CE MRA techniques may avoid this diagnostic pitfall.

Both TOF and PC MRA have notably reduced the overall need for conventional digital subtraction angiography (DSA). On the other hand, DSA still has an important role in diagnostic work-up, especially in cases of vascular malformations (for example, arterial-venous malformations and arterial-venous fistulas) where multiphase dynamic images are required. Multiphase dynamic images can also be acquired using the most recent CE MRA techniques. Although the temporal resolution is not always sufficient with these newer CE MRA techniques, preliminary findings are promising [41].

The following sections summarize the state-of-the-art MRA techniques appropriate for various intracranial pathologies.

Cerebro-Vascular Diseases

Despite recent therapeutic and diagnostic advances, stroke remains the third leading cause of death in industrialized countries and, therefore, is an important social and economic concern. The most common cause of stroke is ischemic infarct of one or more intracranial arteries [42-44]. Cerebral ischemia is the result of a critical decrease in blood flow due to vessel stenosis/obstruction or systemic hemodynamic insufficiency. When the cerebral blood flow is reduced by more than 80% (<20 ml/100g/min) for a sufficiently long period of time, irreversible neuronal damage can occur leading to cerebral necrosis [45]. The causes of infarct include arterial thrombosis, cardiogenic or, more frequently, artery-to-artery embolus, reduced blood flow and venous thrombus [42].

MRA has good sensitivity for the detection and evaluation of stenoses of the major intracranial arteries [23,46-52]. Frequently, the technique is used in conjunction with other techniques to assess the acute phase of stroke and for patient work-up prior to endarterectomy.

3D TOF MRA is often used in the acute phase of stroke (within the first 12 hours after onset) to complete the basic MR exam in order to identify the cause and location of the occluded artery. For example, the identification of an arterial occlusion in a patient presenting with an acute neurological deficit of less than 3-4 hours' duration would tend to indicate ischemia even when the brain MRI is negative. In these cases, 3D TOF MRA, performed in conjunction with perfusion and diffusion studies, permits precise localization and evaluation of the extension of the ischemic areas (Fig. 24a-l). A roadmap of the intracranial arterial vessels previously obtained by means of 3D TOF MRA is necessary in order to correctly detect the arterial input factor (AIF). This factor must be calculated on a patent vessel in order to obtain reliable perfusion parameters: cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) [53].

3D TOF MRA is routinely used for the preliminary and noninvasive study of the intracranial circulation because it is faster and more accurate than 3D PC MRA. As highlighted above, the major limitation of 3D TOF MRA is signal saturation in the more distal branches of the intracranial arteries which decreases both the sensitivity and specificity of the technique for the detection and characterization of vascular occlusion/stenosis.

The positive predictive value for correctly grading the degree of intracranial stenosis has been shown to be greater for 3D TOF MRA than for 3D PC MRA, due to the high dependance of signal intensity on mean blood flow velocity in the PC MRA acquisition [19]. Various studies have been performed to evaluate the diagnostic accuracy of 3D TOF MRA (Table 9) [ 19,48,49,54-58].The results indicate that the major drawback of the technique is its tendency to overestimate the degree of stenosis. Conversely, it has a high negative predictive value and is a completely noninvasive technique meaning that there is no risk of neurological complications in patients with cerebral vascular pathologies. In comparison, there is 1% to

Carotid Artery Velocity Table

Fig. 24a-i. Acute left middle cerebral artery infarction. 81-year-old female with sudden left hemiplegia. CT performed 4 hours later was almost negative (a). The MR with Apparent Coefficient Diffusion (ADC) map (b) and the T2-weighted image (c) demonstrate the presence of acute right fron-to-insular infarction. The Mean Transit Time (d) reveals perfusion impairment in a larger area and the 3D TOF MRA (e) demonstrates the presence of flow reduction in the right internal carotid artery and absence of flow in the middle cerebral artery. 24 hours later the 3D TOF MRA (f) revealed partial reperfusion of the right middle cerebral artery although occlusion of a posterior branch was still apparent; Mean Transit Time (g) revealed reduction of perfusion impairment although this was still present posteriorly. A week later the MR FLAIR image (h) revealed the definite size of the infartion and 3D TOF MRA (i) showed complete reperfusion of the right middle cerebral artery b

Table 9. Accuracy of MRA versus DSA for diagnostic imaging of stenosis of the intracranial arterial vessels

Author

MRA Technique

# Pts

Arteries

Sensitivity

Specificity

Heiserman, 1992 [48]

3D TOF

29

stenosis occlusion normal occlusion

100%

97% 100%

Korogi, 1994 [49]

2DFT and 3DFT TOF MRA

133

Internal carotid artery Middle cerebral artery

85.1% 88.3%

95.6% 96.8%

Wentz, 1994 [11]

T1and T2 weighted SE MRA

284

Intracranial cerebro-basilar system

Normal 100%

Stenotic

76%

100% 100%

Stock, 1995 [54]

MTS VFAE MRA

50

86%

86%

3D FT TOF with MIP

103

stenosis occlusion stenosis occlusion

100% 100%

Oelerich, 1998 [19]

3D TOF MRA PC MRA

18

87% 63%

91% 92%

Hirai, 2002 [55]

3D TOF MRA MIP and MPR

498

92%

91%

Mallouhi, 2002 [57]

VR-TOF MIP-TOF VR-CE MIP-CE

82

100%

Nederkoorn, 2003 [58]

MRA

62

Occlusion Overall

98% 95%

100% 90%

MTS VFAE - magnetic transfer suppression and variable flip angle excitation FT - Fourier transform

3% incidence of complications with DSA [55,59].

Paramagnetic contrast agents can be used to overcome saturation phenomena and thereby improve the visualization of the more distal portions of the cerebral arteries. As early as 1995 it was reported that CE MRA permitted the visualization of an occlusion of the ICA or MCA in an additional 17% of patients compared to nonenhanced MRA [60].

In patients with severe or pre-occlusive stenosis of the carotid bifurcation, the markedly re duced blood flow determines a signal saturation of the intracranial district of the ICA which often involves the ipsilateral MCA. The result is that these vessels are either not seen on 3D TOF MRA or are visualized with drastic reduction of the signal. This also occurs in response to a dissection of the ICA. If CE MRA is used, these drawbacks are greatly reduced or are completely eliminated. Several studies have compared unenhanced MRA and 3D CE MRA in acute stroke and have demonstrated that contrast agent application leads to improved

Ipsilateral LabiaIpsilateral Labia

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