In this study, we evaluated the ability of different MR imaging protocols to detect and characterize intraluminal thrombus within aneurysms. However, we were not able to definitively establish that the intraluminal structure we were observing was, in fact, thrombus. The diagnosis of intraluminal thrombus was based on the presence of a non-enhancing intraluminal mass within an aneurysmal artery segment, as defined by a convex outer border of the vessel. Thrombus could be distinguished from atheroma, which exhibits a relatively linear or only moderately outwardly remodeled outer surface, based largely on these geometric considerations. Furthermore, we did not observe many of the other features that are generally present in atherosclerotic plaque,13
such as a fibrous cap overlying a necrotic core, the presence of large calcified regions, or the presence of luminal surface irregularity or frank ulceration. In 2 instances, we were also able to demonstrate that thrombus formation occurred relatively quickly due to serial monitoring that initially did not indicate the presence of this structure. Thus, while we cannot definitively establish that the observed intraluminal mass was truly thrombus, we have a high degree of confidence in this diagnostic evaluation.
The presence of thrombus may have a strong impact on the risk associated with an aneurysm and the manner in which it is likely to evolve. It has already been established for abdominal aortic aneurysms that a substantial clot burden is associated with higher aneurysm growth rates.14
Fusiform intracranial aneurysms are challenging to manage clinically.15-17
Watchful waiting exposes the patient to continued risk of aneurysm rupture, progressive mass effect on critical neural structures, local thrombosis causing small perforating artery infarcts, or distal embolization of thrombus causing large-vessel infarction. Patients are often placed on antiplatelet or anticoagulant agents to reduce the risk of local thrombosis or distal embolization, though such agents may increase the extent of hemorrhage if an aneurysm ruptures. Surgical intracranial arterial bypass procedures, selective arterial clipping, clip reconstruction, bypass/trapping, endovascular covered stent placement or flow diversion, and endovascular coil occlusion of fusiform aneurysms or their input arteries have all been used with variable success to reduce flow to fusiform aneurysms while maintaining perfusion of brain stem perforating arteries.18-23
The ability to accurately detect, delineate, and characterize aneurysm thrombus noninvasively, therefore, may have significant clinical relevance in the management of patients with challenging fusiform aneurysms.
SSFP techniques were found to offer several advantages over either T1-weighted or T2-weighted imaging for detecting and characterizing thrombus: They provide moderate signal intensity-to-noise ratio within thrombus, exhibit signal-intensity heterogeneity within thrombus that is similar to that of T1-weighted imaging, and provide contrast against the blood pool and surrounding CSF that is comparable with or superior to that of T2-weighted imaging. Collectively, these data support the hypothesis that SSFP is the superior method for delineating and characterizing intraluminal thrombus. SSFP imaging has been widely developed for numerous applications, including cardiac imaging, angiography, and other pseudo-T2-weighted applications.24
SSFP offers high inherent signal intensity-to-noise ratio, exhibits contrast that is proportional to the square root of the ratio of T2 to T1, and is relatively motion-insensitive. Intraluminal thrombus was found to be either homogeneous in appearance on MR images or heterogeneous with a characteristic peripheral rim hyperintensity on T1-sensitive images. The presence of hyperintensity toward the arterial wall, rather than the arterial lumen, was somewhat unexpected. While there has been little attention in the literature, an early MR imaging study reported the opposite effect, with hyperintensity toward the luminal surface.25
This report, however, focused on acute findings in 2 patients who were being evaluated for immediate intervention. Conversely, our study analyzed patients with clinically stable conditions who exhibited intra-aneurysmal thrombus chronically. A more recent study of the growth potential of partially thrombosed aneurysms of the posterior circulation26
revealed findings similar to those reported here, albeit only with postcontrast T1-weighted imaging. Another study of incidentally discovered vertebrobasilar fusiform aneurysms27
also showed evidence of peripheral T1-related hyperintensity, though the authors were unsure whether this indicated the presence of thrombus or was a flow effect.
The signal intensity within both the core and periphery of the thrombus was found to be relatively constant for long periods of a year or more. This implies that the thrombus is either stagnant or in some form of dynamic equilibrium. In general, areas where there has been a fresh bleed will appear as hyperintense on T1-weighted imaging, denoting the presence of methemoglobin, a transformation product of oxyhemoglobin that is thought to transiently appear as thrombus ages.12
In the aneurysm studies here, the presence and retention of T1 shortening toward the arterial wall suggests that relatively fresh thrombus may exist at this location. If this is the case, then it could be hypothesized that thrombus is continually being deposited on the arterial wall side of the thrombus and may be shedding from the luminal surface. Postcontrast enhancement of the arterial wall side of the thrombus provides additional support for this hypothesis. Thus, it is possible that thrombi that present with a hyperintense rim and whose appearance remains constant under MR imaging may, in fact, be undergoing continuous remodeling. However, this has not been established histologically and thus represents a limitation of the study.
We evaluated conventional measures of aneurysm size, including maximal diameter and cross-sectional area, and studied the extent to which the inclusion of the thrombosed region affected the reported values. We further studied these parameters with time to determine the extent to which they evolved. When thrombus is present, the free lumen diameter and cross-sectional area may well decrease or remain unchanged while the true aneurysm diameter and cross-sectional area continue to grow. Thus, the inclusion of intraluminal thrombus is important to accurately stage the geometric properties of an aneurysm and may provide insights into the risk of rupture or symptomatic progressive enlargement of thrombosis with time. Thrombus deposition and aneurysm morphometry, however, are inherently 3D, and a comprehensive evaluation of aneurysm geometric properties and how they evolve with time requires more sophisticated analysis.28
While this may not be possible at many centers, it is clearly important to include the thrombosed component in any measures that are made. This represents an additional advantage of steady-state techniques, which are fast and readily adapted to 3D volumetric coverage.