Clinical Examples

Atherosclerosis

The most common indication for cervical MRA is for evaluation of atherosclerotic disease of the ex-tracranial cerebral vasculature. Cerebrovascular insufficiency, whether symptomatic or asymptomatic, is generally diagnosed and its severity expressed as the degree of (percent) stenosis within the proximal internal carotid artery at the carotid bifurcation. This is the most common site of stenotic atherosclerotic plaque formation although it should be noted that plaque formation and stenosis can occur anywhere along the extracra-nial cerebral circulation between the common carotid origin from the aortic arch and the distal internal carotid artery at the skull base. In addition, significant intracranial internal carotid stenosis secondary to atherosclerotic plaque formation occurs less commonly but these will be discussed in the intracranial MRA section.

MRA is a non-invasive alternative technique to invasive diagnostic catheter angiography. Catheter angiography has a long history of utilization for diagnosis of vascular pathology and has an established track record of high accuracy for the diagnosis of vessel stenosis. Thus, in evaluating MRA, catheter angiography provides the reference gold standard against which MRA is generally compared. It should be noted, however, that since catheter angiography is an invasive procedure there are measurable risks and complications associated with it. Clinical series have shown that reversible complications occur in 1 - 14% of catheter angiograms and that significant and often irreversible complications with severe morbidity or mortality occur in between 0.5 and 1% of cases [ 20, 21]. This should be kept in mind when comparing efficacy of catheter angiography with non-invasive techniques such as MRA and CT angiog-raphy (CTA). Although there may be slight differences or discrepancies in diagnosis between invasive versus noninvasive vascular imaging, minor decreases in sensitivity, specificity or accuracy may be tolerated in the case of noninvasive vessel imaging if the number and types of misdiagnoses potentially cause less harm than the number of patients who would have been significantly injured due to the known complication rate of invasive catheter angiography [22].

Mri Carotid Angiography Vent Patients

Fig. 16a, b. Overestimation of stenosis on 2D MRA. a 2D TOF MRA of the carotid bifurcation shows what appears to be severe narrowing at the origin of the internal carotid artery (arrow). b CE MRA of the same patient obtained in approximately the same projection now shows only moderate narrowing (measuring less than 50% in degree of stenosis) of the internal carotid artery origin. Both images are viewed from the projection that shows maximum degree of narrowing

Fig. 16a, b. Overestimation of stenosis on 2D MRA. a 2D TOF MRA of the carotid bifurcation shows what appears to be severe narrowing at the origin of the internal carotid artery (arrow). b CE MRA of the same patient obtained in approximately the same projection now shows only moderate narrowing (measuring less than 50% in degree of stenosis) of the internal carotid artery origin. Both images are viewed from the projection that shows maximum degree of narrowing

There have been a large number of studies comparing sensitivity and specificity of MRA with that of catheter angiography. Variations in results can be attributed to differences in the type of MRA technique used for carotid imaging. In the case of 2D TOF the major discrepancy is that of overestimation of vessel stenosis (Fig. 16a, b). This overes-timation in stenosis becomes more significant with increasing levels of stenosis [8]. With increased stenosis that approaches, but does not reach, the critical stenosis level of 70% determined by the NASCET study to be the threshold for treatment of patients with surgical endarterectomy, there is an increasing possibility of miscategoriz-ing the lesion. Miscategorizing a lesion due to overestimation could result in a patient undergoing invasive endarterectomy when in actuality the degree of stenosis was somewhat less than 70% if evaluated by catheter angiography. Another potential pitfall would involve patients with severe highgrade stenosis that result in a flow void on the MRA that is misdiagnosed as total vessel occlusion when in reality the vessel is patent [23,24]. Despite this limitation, however, a number of studies have shown that when a flow void at the carotid bifurcation was present on 2D TOF MRA this generally was associated with a stenosis of 70 % or greater, thus indicating the potential need for carotid en-darterectomy. Furthermore, with careful evaluation of the MRA with multiple different projections, in nearly all cases, the patency of the distal internal carotid artery could be established by identification of flow within the ICA beyond the flow void.-

Evaluation of carotid stenosis using 3D TOF MRA technique shows better correlation with catheter angiography [25]. Thus, several studies have shown that the sensitivity and specificity for quantification of stenosis with 3D TOF is in the range of 90% or better when compared with carotid catheter angiography [26, 27]. This prompted recommendation for utilizing both techniques for complete neck MRA evaluation. The 2D TOF MRA is utilized for full neck coverage of the vascular anatomy and the 3D TOF MRA yields a limited FOV or a "targeted" evaluation of the carotid artery bifurcation that also provides a more accurate determination of the percent carotid stenosis.

Contrast enhanced MRA is now the most commonly utilized technique for cervical vessel evaluation due to its speed, its high detail of the neck vasculature combined with relatively lower incidence of artifacts such as from minor patient motion. The contrast enhanced technique also shows good visualization of carotid stenosis that does not appear to have the problem with a flow void at relatively high degrees of stenosis that is associated with the 2D TOF technique. Nevertheless, a recent report by Townsend et al. comparing CE MRA with 3D TOF MRA suggests that contrast enhanced MRA overestimates the degree of stenosis in approximately 40% of cases compared with 3D TOF MRA [28]. They noted, however, that the degree of overestimation was greatest in vessels with less than 70% stenosis and was less critical in vessels with greater than 70% stenosis (in contrast to the experience documented with 2D TOF). Based on this, the authors recommended using 3D TOF MRA as a supplement to CE MRA to most accurately determine the degree of carotid stenosis. It should be noted, however, that they did not have catheter angiography correlation in the cases they reported. A number of other authors have noted a high degree of correlation between determination of degree of stenosis with contrast enhanced MRA versus DSA [29-34]. There may be several reasons for this discrepancy in results. The first is that Townsend compared contrast enhanced MRA with 3D TOF MRA and had no direct correlation with DSA. They did employ phantom study measurements but this may also be limited by the fact that it is not truly physiologic. Since neither 3D MRA or CE MRA is 100% percent accurate, inherent discrepancies in each of these two techniques might, in combination, overestimate projected inaccuracy of CE MRA relative to the "gold standard" of carotid angiography. Secondly, there are a number of variables in CE MRA technique used among the various studies including the type of MR scanner used, the specific pulse sequence, the timing of the contrast bolus and the rate of injection that might also alter results of these comparisons. Finally, it has been shown that, in addition to looking at the discrepancies between cross sectional stenosis, it is also important to realize that stenosis grading with catheter angiography is based on two or three different projections while MRA utilizes multiple different projections together with cross sectional views of the vessel to determine maximum stenosis. Neidercorn et al. demonstrated that when similar projections were compared between catheter angiography and 3D TOF MRA the correlation was much better than when minimum stenosis alone was graded which tended to show an overestima-tion of stenosis on the MRA that averaged about 7.5 percent compared with DSA [35].

Doppler ultrasound has for many years been a major diagnostic technique for evaluation of carotid stenosis. The technique is noninvasive, rapid and relatively low cost and is reasonably sensitive and accurate in the evaluation of the degree of carotid bifurcation stenosis. It has several limitations, however. The technique does not provide anatomical detail of the vessels within the neck. It has a limited area of coverage and thus cannot see tandem lesions or even isolated lesions within the distal internal carotid artery near the skull base. Thus, while doppler ultrasound is valuable in screening patients with carotid vessel disease, its limitations require complementary studies to provide information in instances where doppler ultrasound has major limitations. Early in the 1990's it was recognized that MRA provided good complementary information that increased the diagnostic accuracy of noninvasive carotid vascular evaluation. Anderson et al. have shown that this combination misses very little significant disease and provides a good noninvasive technique for screening as well as for surgical decision making [36]. Improvements in MRA, especially with evolution of modern contrast enhanced MRA, have further strengthened the robustness of this approach. The degree of stenosis is well evaluated with doppler ultrasound as well as with contrast enhanced MRA [37] (Fig. 17a, b). In addition, recent papers have shown that the use of contrast enhanced MRA not only provides detection of tandem vascular lesions proximal or distal to the carotid bifurcation but also improves the diagnostic confidence level in interpretation of the degree of stenosis and the presence of high grade stenosis versus total carotid occlusion at the carotid bifurcation [38].

In addition to carotid artery atherosclerotic disease, CE MRA is also well suited to evaluate vertebral artery disease (Fig. 18a, b). Segmented stenosis within a vertebral artery is easily demonstrated and diagnosed. However, because vertebral arteries in the neck may be asymmetric in size as

Vertebral Artery Diseases

Fig. 18a, b. Right vertebral artery origin stenosis. a Severe stenosis of the RVA is demonstrated in this rotated MIP view of CE MRA. Note the presence of a pseudo occlusion (arrow) due to high velocity-related signal loss through the stenotic RVA. b Another rotational view from the same MRA shows normal origin of LVA (arrow)

already discussed, it is important to distinguish a hypoplastic vertebral artery that represents a normal variant from a small vertebral artery secondary to proximal stenosis and reduced flow. This is readily distinguished by observation of the proximal vertebral arteries and the vertebral artery origins. In the case of a diseased vertebral artery with reduced flow there will be a change in caliber at the level of stenosis while the primary hypoplastic variant will demonstrate uniform small lumen size that extends to the vertebral artery origin.

Tandem lesions of the carotid vasculature in the proximal common carotid artery or the distal

Fig. 17a, b. MRA is very sensitive for showing mild degrees of stenosis at the common carotid artery bifurcation using different techniques. a 2D TOF MRA of carotid bifurcation shows mild narrowing at origin of internal carotid artery (arrow). b CE MRA of carotid bifurcation in a different patient shows a similar mild degree of stenosis at the internal carotid artery origin (arrow). Note that in both of these cases the degree of stenosis is compared with the distal ICA according to the NASCET method

Fig. 18a, b. Right vertebral artery origin stenosis. a Severe stenosis of the RVA is demonstrated in this rotated MIP view of CE MRA. Note the presence of a pseudo occlusion (arrow) due to high velocity-related signal loss through the stenotic RVA. b Another rotational view from the same MRA shows normal origin of LVA (arrow)

internal carotid artery are well shown on the contrast enhanced MRA. The presence of intraluminal clot, which is generally considered to be an indication for urgent surgical intervention is also demonstrated using MRA techniques (Fig. 19a-c). Ulceration of the carotid vessels can be shown in many cases with MRA. (Fig. 20a-d) However, catheter angiography as well as high resolution MR imaging have both been shown to provide better information about carotid plaque rupture and ulceration [39].

High-resolution carotid plaque imaging in particular is currently an investigational technique

Ulcerated Mra Plaque

Fig. 19a-c. Intralumi-nal thrombus in left internal carotid artery bulb. a Edited MIP image of the left carotid artery from a CE MRA shows filling defect in the left carotid bulb. Note also the bovine origin of the left common carotid artery (arrow).

b Close up view and slightly different projection confirms the intralu-minal filling defect (arrow).

c Catheter carotid an-giogram confirms the presence of intraluminal thrombus, which was subsequently removed surgically

Mra Occlusion Carotid Bulb

Fig. 20a-d. Carotid stenosis with ulcerated plaque. a 2D TOF MRA of right common carotid bifurcation shows marked stenosis at the origin of the internal carotid artery with an ulceration (arrow). b and c Two axial source images show the origin of the right internal carotid as it bifurcates (b) and at a level 2 to 3 millimeters above this (c); the ulcer cavity (arrow) can be seen projecting medially. d CE MRA shows the area of stenosis and the ulcer cavity (arrow) to better advantage. The vessel detail is much improved with the CE MRA. However, note the overestimation of the right external carotid artery origin presumably due to turbulent flow when compared to the 2D TOF image in a)

Fig. 21a, b. aT1-weighted high-resolution axial image obtained just below CCA bifurcation shows flow void in carotid lumen (asterisk) together with bright signal from an intraplaque hemorrhage (arrow). Intraplaque hemorrhage is best shown on T1 weighted images when intraluminal flow signal is suppressed. b Histologic section of endarterectomy specimen of same patient sectioned at same level and orientation as in (a) confirms intraplaque hemorrhage (Mallory's Trichrome stain) [Case courtesy of Chun Yuan, Ph.D. and Thomas Hatsukami, M.D.]

Fig. 22. Axial high resolution MRA source image through the proximal right internal carotid artery. The lumen is seen as hyper-intense signal surrounded by a thick, complex atherosclerotic plaque. The dark band surrounding the carotid lumen represents a thick fibrous cap. The lateral portion of the fibrous cap line is absent indicating rupture of the fibrous cap [Case courtesy of Chun Yuan, Ph.D. and Thomas Hatsukami, M.D.]

Fig. 22. Axial high resolution MRA source image through the proximal right internal carotid artery. The lumen is seen as hyper-intense signal surrounded by a thick, complex atherosclerotic plaque. The dark band surrounding the carotid lumen represents a thick fibrous cap. The lateral portion of the fibrous cap line is absent indicating rupture of the fibrous cap [Case courtesy of Chun Yuan, Ph.D. and Thomas Hatsukami, M.D.]

Fig. 21a, b. aT1-weighted high-resolution axial image obtained just below CCA bifurcation shows flow void in carotid lumen (asterisk) together with bright signal from an intraplaque hemorrhage (arrow). Intraplaque hemorrhage is best shown on T1 weighted images when intraluminal flow signal is suppressed. b Histologic section of endarterectomy specimen of same patient sectioned at same level and orientation as in (a) confirms intraplaque hemorrhage (Mallory's Trichrome stain) [Case courtesy of Chun Yuan, Ph.D. and Thomas Hatsukami, M.D.]

being assessed to help better define the vulnerable carotid plaque in comparison to the presumed stable atherosclerotic plaque. Using these techniques high resolution MRI has been shown to reveal intra plaque hemorrhage (Fig. 21a, b) and fibrous cap rupture (Fig. 22) in patients with complex atherosclerotic plaque disease [40]. Detection of in-traplaque hemorrhage is a very important finding since it likely distinguishes the biologically active, fragile plaque that is more likely to lead to stroke from a more stable lesion [41]. Identification of fibrous cap rupture with MRI is another finding associated with an unstable plaque and it has been shown to be highly associated with recent transient ischemic attack or stroke [39]. In the future these techniques may become a major tool in the diagnosis and surgical decision making for evaluating patients with atherosclerotic cerebrovascular disease.

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