Giant intracranial aneurysms - Imaging

General considerations

According to the conventional classification, giant intracranial aneurysms include saccular, fusiform, dissecting and serpentine aneurysms. This is a very heterogeneous group of vascular pathologies, with their etiologies, imaging characteristics and clinical manifestations often being very different.

The clinical findings leading to discovery are rarely that of a subarachnoid hemorrhage. It is much more usual that presenting symptoms and neurological signs suggest tumors, occlusive arterial disease etc. Their only common morphological feature is the size of the aneurysmal sac (or of the dilated segment), which by definition exceeds 25 mm.

A. Giant saccular aneurysms

Morphologically and pathologically giant saccular aneurysms are virtually identical to berry aneurysms of small size. The reason why certain saccular aneurysms develop into giant ones, while the majority either rupture at a smaller size or fail to grow and do not rupture, is still unclear.

Compared to the other types of giant cerebral aneurysms, a very important and specific feature of these aneurysms is that their neck constitutes the single point of entrance and exit of blood flow inside the aneurysmal lumen. Imaging evaluation of the neck of giant aneurysms can however be very difficult in many cases.

Giant saccular aneurysms are characterized by different, sometimes complex flow patterns.

1. In the "side-wall type" of aneurysms (typically seen in the carotid siphon or basilar artery), a peripheral circular rapid flow zone is sometimes very clearly distinguished from a very slow flowing central area, with hardly any mixing between the two.

Image 1. DSA images (lateral views) of a giant saccular aneurysm of the internal carotid artery siphon.

2. In the "end type" of these aneurysms (typically seen at bifurcations) flow patterns can be markedly turbulent and more complex.

Part of the lumen of a giant saccular aneurysm may also thrombose spontaneously.

Image 2. Partially thrombosed giant saccular aneurysm at the left internal carotid artery siphon on coronal non-enhanced (left) and Gadolinium-enhanced (right) T1-weighted spin-echo images.

This may be the cause of local and distant ischemic lesions (by occlusion of small perforators arising from the aneurysm wall or by dislodgment of thrombotic emboli respectively).

Due to their often considerable size, these aneurysms may also exert mass effect on adjacent parenchyma or cranial nerves. This mass effect often leads to the first clinical manifestations (signs of brainstem compression, oculomotor nerve palsy etc.). Rupture of these aneurysms is most frequently fatal.

Image 3. Giant saccular aneurysm of the basilar artery on thin T1-weighted gradient-echo images exhibiting a significant mass effect on the adjacent brainstem.

B. Giant dissecting, fusiform and serpentine aneurysms

Clear-cut distinction between each of these entities is not always easy. By definition, in all of these subgroups, the points of entrance and exit of their lumen are distinct and separated by a variable length of pathological vessel. Proximal and distal to the affected segment, the parent vessel can be normal and can give rise to normal cerebral branches.

From the hemodynamic point of view, these are generally slow flow lesions. Their clinical presentations are sometimes very insidious and misleading (headaches, progressive neurological deficit, epileptic seizures), suggesting tumors, slowly progressing ischemic cerebrovascular disease etc.

1. Dissecting aneurysms typically occur in the posterior cerebral circulation. Involvement of the vertebral arteries is frequent, whereas the isolated basilar artery form is rare. Their clinical manifestation typically suggests brain stem infarction, but several cases of spontaneous dissecting aneurysms of the basilar artery presenting with subarachnoid hemorrhage have been also reported.

Angiographically, the most characteristic feature is fusiform dilation with proximal irregular narrowing corresponding to the site of wall dissection. The intimal flap and intramural hematoma are often seen by conventional MRI and are considered to be pathognomonic.

Search: Dissecting aneurysm of the basilar artery

2. Fusiform aneurysms are most frequently seen in the vertebrobasilar system as well. This type of giant intracranial aneurysm is probably a variant of the dolichoectasia, representing a focally evolving specific form of the diffuse, systemic degenerative arterial wall disease. Fusiform aneurysms are in fact particularly tortuous and dilated dolichoectatic arteries. Mural thrombus reducing the lumen of the artery to a narrow residual channel is common. Complete occlusion by progressive thrombosis is a rare but extremely serious complication. The typical clinical presentation is usually related to mass effect, global territorial circulatory disturbances, and local or distant ischemia, as noted previously. This pathological condition is most frequently encountered in the basilar artery and more rarely in the internal carotid arteries but never in second or third order cerebral branches.

Search: Fusiform aneurysm of the basilar artery

3. Giant serpentine aneurysms. This is a very unusual and rare subgroup of giant intracranial aneurysms, characterized by a wavy, sinusoidal vessel and, as almost a rule, partial intraluminal thrombosis. It is suggested that this is a congenital malformation of the vessel wall with progressive expansion due to repetitive episodes of hemorrhage and subsequent partial thrombosis with attempts at recanalization. This hypothesis is supported by evaluation of pathological specimens which show the presence of intraluminal laminated organized clot of different age with calcification. This also explains the complex and often ambiguous appearance of these lesions on CT images. Differentiation between fusiform and serpentine aneurysms can be difficult by imaging criteria alone. Location can be in certain cases a distinctive feature, as the most frequent locations of serpentine aneurysms include the terminal segment of the vertebral artery or branches of the middle or posterior cerebral arteries and also the supraclinoid internal carotid artery, the latter being a frequent site of dolichoectasia as well. Fusiform aneurysms (or dolichoectasia) usually result in a more diffuse appearance with other manifestations of diffuse degenerative arterial disease, whereas serpentine aneurysms are typically isolated.

Search: Serpentine aneurysm

Imaging strategies

Imaging strategies have to be defined according to the previously mentioned specific features of giant intracranial aneurysms.

Conventional catheter based angiography of these vascular pathologies is limited solely to the visualization of the lumen or the residual channel, if previous thrombosis has occurred. This is a considerable drawback and explains why MR imaging of these lesions is always mandatory for complete evaluation.

A. Conventional MR imaging (MRI)

1. Without intravenous contrast injection

a. Sagittal T1-weighted spin-echo sequence for general evaluation, but an additional (coronal or transverse) sequence might serve as a reference image for subsequent comparison with post-contrast images.

Image 4. Sagittal T1-weighted spin-echo images of a giant saccular aneurysm of the basilar artery.

Image 5. Sagittal T1-weighted spin-echo images of a giant saccular aneurysm at the internal carotid artery siphon.

Image 6. Partially thrombosed giant saccular aneurysm at the left internal carotid artery siphon on coronal non-enhanced (left) and Gadolinium-enhanced (right) T1-weighted spin-echo images.

Image 7. Partially thrombosed giant saccular aneurysm at the left internal carotid artery siphon on transverse Gadolinium-enhanced T1-weighted spin-echo images.

b. Transverse proton-density and T2-weighted fast spin-echo sequences for better characterization of the aneurysm and detection of associated parenchymal lesions. In cases of partially thrombosed aneurysms a T2-weighted gradient-echo sequence can also be useful, being more sensitive to hemoglobin degradation products, especially hemosiderin (search: MR signal of static blood and hemorrhage).

Image 8. Coronal proton density-weighted fast spin-echo images of a partially thrombosed giant saccular aneurysm of the left internal carotid artery. The typical laminated appearance of the staged intraaneurysmal thrombus is well seen.

Image 9. Coronal T2-weighted gradient-echo images of a partially thrombosed giant saccular aneurysm of the left internal carotid artery. The hemosiderin containing layers within the laminated intraaneurysmal thrombus are better delineated on these images.

c. For the demonstration of small or barely detectable ischemic parenchymal lesions, the newly available fluid-attenuated inversion recovery (FLAIR) sequences can be used successfully to improve sensitivity.

Image 10. Transverse FLAIR images of the posterior fossa clearly demonstrating ischemic lesions in the brain stem parenchyma adjacent to a dissecting basilar artery aneurysm.

2. With intravenous contrast injection

T1-weighted spin-echo sequences can provide useful information about some specific features of the aneurysm (partial thrombosis, wall enhancement etc.). Due to slow flow, a contrast enhancement might be observed in the lumen (or residual channel) of the aneurysm, hence the appreciation of the different compartments of the aneurysm is facilitated.

B. Vascular MR imaging (MRA)

1. 2D Phase Contrast ("survey") sequence(s) are performed in one or several planes (sagittal, coronal or transverse), and if necessary, with several velocity encoding values (providing a kind of "pilot study" to optimize velocity encoding (Venc) of subsequent MRA sequences). Generally, as these are relatively slow or sometimes very slow flowing lesions, low Venc values are used (between 5 and 35 cm/s).

Image 11. Coronal 2D PC MR angiogram used for quick survey imaging in the follow-up of a dissecting basilar artery aneurysm.

Intravenous injection of contrast material has a positive effect on signal to noise ratio and thus can be considered as an effective tool to increasing image quality (search: Techniques for improvement of vessel contrast in MRA: Contrast media).

Image 12. Sagittal non-enhanced (above) and Gadolinium-enhanced (below) 2D PC MR angiograms of a giant saccular aneurysm of the basilar artery.

2. 3D Phase Contrast sequences (with thin partitions) provide high quality MIP images. They have several advantages over the 3D Time-of-Flight technique.

a. 3D Phase Contrast sequences can be used in any plane without the risk of signal loss due to progressive intravolume spin saturation.

Image 13. Coronal targeted 3D PC (above) and single slab 3D TOF MR angiograms of a very slow flowing, partially thrombosed fusiform aneurysm of the basilar artery. With the 3D TOF technique, the vertebro-basilary system is hardly seen due to intravascular signal loss related to progressive intravolume spin saturation.

b. Ability to evaluate gross aneurysm and adjacent brain morphology on the readily available "T1-weighted anatomical" averaged modulus source images (search: Phase Contrast MR angiography: Image types).

Image 14. Transverse "T1-weighted anatomical" source images of a giant saccular aneurysm of the basilar artery providing gross morphological information of both the aneurysm and the adjacent brain stem parenchyma.

c. Ability to evaluate the neck geometry on the magnitude of complex differences sources images (search: Phase Contrast MR angiography: Image types).

Image 15. Transverse averaged modulus (left) and corresponding magnitude of complex differences (right) type source images of the upper part of the giant basilar artery aneurysm from a Gadolinium-enhanced 3D PC MRA sequence. The uppermost magnitude of complex differences image shows the neck area (red arrows), evidenced by the direct communication between the parent artery and the aneurysm lumen. On the lowermost image the parent artery is seen at a distance from the aneurysm, therefore this level is above the neck already.

d. Ability to analyze inflow and outflow at the neck area and intrinsic flow patterns within the aneurysmal sac on the source magnitude of complex differences images.

Image 16. Sagittal averaged modulus (left) and magnitude of complex differences (right) type source images of a giant saccular aneurysm of the basilar artery from a 3D PC MRA acquisition. Clear identification of the inflow (red arrows) and outflow (blue arrows) pathways of the aneurysm.

e. Ability to obtain additional flow information by the creation of directional phase images from the saved raw data set (search: Phase Contrast MR angiography: Image types).

Image 17. Sagittal directional phase difference type source images (Gadolinium-enhanced thick 3D PC acquisition with retrospective cardiac gating). The circular intraaneurysmal flow pattern is well appreciated on both images with cranio-caudal (above) and with antero-posterior (below) encoding (compare with the known physiological flow directions of the venous structures of the midline).

3. The 2D or 3D Phase Contrast (the latter with relatively thick, therefore fewer partitions) sequences can also be adapted (by the additional use of retrospective gating during the acquisition) to provide hemodynamic information about flow within the aneurysmal sac.

a. Determination of intra-aneurysmal flow pattern (turbulent or circular-laminar).

b. Determination of intra-aneurysmal flow directions.

Cine loop presentation of the image data set further enhances the value of this technique.

Video 6. Regular circular intraaneurysmal flow pattern in a giant saccular aneurysm of the basilar artery.

Video 8. Turbulent intraaneurysmal flow pattern in a partially thrombosed giant saccular aneurysm at the internal carotid artery siphon.

4. 3D Time-of-Flight sequences (in transverse orientation) can also be used to demonstrate morphology, in case the Phase Contrast techniques are not available. Non-enhanced 3D Time-of-Flight sequences can sometimes provide ambiguous MR angiograms due to intra-aneurysmal spin saturation, in which case contrast injection might be helpful. This can also enhance evaluation of the residual channel and the intra-aneurysmal thrombus (a potential problem on non-enhanced images).

Analysis of the thin source images for neck geometry can also be useful, as with 3D Phase Contrast sequences.

Image 18. Transverse source images from a non-enhanced multislab 3D TOF MRA acquisition. The aneurysm has a wide neck (arrows) at its origin from the basilar artery as evidenced by analysis of the relationship between the aneurysm and the parent artery on the adjacent individual partitions.

Image analysis

For all the above reasons diagnostic imaging of giant aneurysms should include:

A. Diagnosis and differential diagnosis

In the majority of cases conventional MR images will be highly suggestive of giant intracranial aneurysms. However, some are still difficult to diagnose and other non-vascular pathologies may mimic aneurysms. MR angiography offers the possibility to confirm or rule out in a rapid and reliable fashion the vascular nature of any dubious structure on conventional images. The Phase Contrast technique is particularly suitable for such purposes, due to its short acquisition time (in case of 2D PC technique) and- unlike the 3D Time-of-Flight technique- relative insensitivity to intravolume spin saturation problems.

Search: - Giant internal carotid artery aneurysm
               - Multiple giant cerebral saccular aneurysms

T1 contamination artifacts may cause differential diagnostic problems with the Time-of-Flight technique.

Search: Ethmoidal mucocele

B. Evaluation of morphology

This step is important for the precise description and subsequent classification of giant intracranial aneurysms.

1. Gross morphology

2. Intraluminal morphology (thrombosis, residual lumen, false lumen, intimal flap, intramural hematoma)

Image 19. Partially thrombosed fusiform aneurysm of the basilar artery. Coronal "anatomical" averaged modulus (left) and magnitude of complex differences (right) type source images from a Gadolinium-enhanced 3D PC MRA acquisition.

Image 20. Coronal averaged modulus (left) and magnitude of complex differences (right) type source images from a Gadolinium-enhanced 3D PC MRA acquisition (Venc:35 cm/s) of a dissecting aneurysm of the basilar artery. Demonstration of the intramural thrombosis (arrows) of the dissecting aneurysm, presumably indicating the site of the wall rupture, responsible for the subarachnoid hemorrhage. These high quality "anatomical" and "flow" images provide a unique insight into the intravascular space.

Image 21. Transverse source images of a dissecting aneurysm of the basilar artery from a Gadolinium-enhanced multislab 3D TOF MRA acquisition. These images provide more accurate delineation of the patent channel (yellow arrows) and the partially thrombosed false aneurysm (red arrows) within the ectatic basilar artery with clear visualization of the intimal flap between the two (blue arrows).

3. Neck morphology (in saccular aneurysms)

Some of the morphological data might greatly influence therapeutic considerations (occlusion by GDC coils, surgical clipping, endovascular or surgical sacrifice of the parent vessel). Intraluminal thrombosis and dissection suggest anticoagulant therapy, for example. The knowledge of neck morphology (geometry)- in cases of saccular aneurysms- is crucial before instituting therapy.

Multiprojectional MIP reconstructions and subsequent cine presentation of the data set often allow satisfactory delineation of the neck.

However, in difficult cases analysis of the source images represents the best method of accurate evaluation of the geometrical parameters of the aneurysm neck

C. Evaluation of intra-aneurysmal hemodynamics

Experience is still limited with respect to intra-aneurysmal flow patterns and hemodynamics and hence the practical significance of such information by MRA cannot be determined yet. However, it is reasonable to presume that the precise evaluation of inflow and outflow channels might have an impact on therapeutic decisions in certain situations. By providing a unique and unprecedented insight into this hemodynamic information, MRA might also contribute to a better understanding of their pathogenesis.

Video 1. Cine demonstration of the intraaneurysmal flow with averaged modulus type images (transverse thick-slice 3D Phase Contrast MRA acquisition with retrospective cardiac gating).

Video 2. Cine demonstration of the intraaneurysmal flow with magnitude of complex differences type images (transverse thick-slice 3D Phase Contrast MRA acquisition with retrospective cardiac gating).

Video 6. Cine demonstration of the inflow channel and the intraaneurysmal flow with magnitude of complex differences type images (sagittal thick-slice Gadolinium enhanced 3D Phase Contrast MRA acquisition with retrospective cardiac gating).

Video 12 Cine demonstration of the intraaneurysmal flow with directional phase differences type images (sagittal thick-slice Gadolinium enhanced 3D Phase Contrast MRA acquisition with retrospective cardiac gating, antero-posterior encoding).

Video 14. Cine demonstration of the intraaneurysmal flow with directional phase differences type images (sagittal thick-slice Gadolinium-enhanced 3D Phase Contrast MRA acquisition with retrospective cardiac gating, cranio-caudal encoding).

Video 8. sagittal thick slice 3D Phase Contrast MRA acquisition with retrospective cardiac gating. Turbulent intraaneurysmal flow pattern in a partially thombosed giant saccular aneurysm at the internal carotid artery siphon.

Video 11. coronal thick slice 3D Phase Contrast MRA acquisition with retrospective cardiac gating. Turbulent intraaneurysmal flow pattern in a partially thombosed giant saccular aneurysm at the internal carotid artery siphon.

D. Secondary aspects

It is particularly important to look for mass effect and ischemia, because clinical presentation is very often directly related to these secondary lesions, which are "complications" or "side effects" of the primary pathological process.

1. Mass effect

Mass effect may unfavorably influence the function of adjacent parenchyma provoking focal neurological deficit, seizures, etc. Cranial nerves might also be affected by mass effect, which is a special form of "neuro-vascular conflicts".

Search: - Multiple cerebral aneurysms
              - Giant aneurysm

2. Ischemic lesions

Ischemic lesions, if occurring locally, are generally induced by occlusion of perforators (due to mural thrombosis or dissection) issued from the aneurysm wall. This is typically seen in basilar artery aneurysms.

Image 22. Transverse proton density and T2-weighted fast spin-echo images of the posterior fossa. Ischemic lesions in the brain stem parenchyma adjacent to a dissecting basilar artery aneurysm, due to occlusion of the direct perforating arteries to the pons arising from the dissected arterial wall.

Ischemic lesions in distal vascular territories, presenting as hyperintense areas in the brain parenchyma on long TR or FLAIR images, are almost exclusively of embolic origin, the emboli being dislodged from intraluminal thrombi.

Conclusions

The combined MRI-MRA diagnostic approach to intracranial giant aneurysms has the advantage of providing parenchymal and angiographic MR images in the same session, hence allowing full understanding of all the clinically important aspects of the primary vascular lesion and associated parenchymal abnormalities.

The Phase Contrast technique appears to be superior to Time-of-Flight in the MRA evaluation of giant aneurysms.

Giant cerebral aneurysms represent a good indication for MRA. Diagnosis and follow-up can also be achieved with MRA and hence catheter angiography can be reserved for pre-therapeutic (in case of subsequent surgery) or therapeutic (in case of endovascular treatment) purposes.

Image 23.Fusiform aneurysm of the basilar artery.

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