-
Today I'll talk about "intracranial aneurysm".
I want to begin with "subarachnoid hemorrhage".
subarachnoid hemorrhages are classified into two general categories: traumatic and spontaneous. A CT scan reveals blood in the sulcus and fissures near the surface of the brain when a traumatic SAH occurs; while we can see blood around the basal cistern when a spontaneous SAH happens.
What cause spontaneous subarachnoid hemorrhages? There are several reasons in the list below, of which intracranial aneurysms comprise the majority, about 80%.
Intracranial aneurysm is a bulging, or a dilating of the wall of an intracranial artery. It can be classified into several types, these pictures shows the most common ones: saccular(also called berry) and fusiform. Besides, there is another type: dissecting aneurysm, which often occurs after a traumar.
This figure shows the circle of Willis, the most common locations for berry aneurysms are at the bifurcation of ICA with PcomA, in the region of the AcomA and ACA, and at the trifurcation of the MCA. Aneurysms from internal carotid system comprise 90% of cases while less than 10% from vertebrobasilar artery system. Multiple aneurysms occur in about 20 to 30% of cases.
The etiology of cerebral aneurysms has not yet been fully clarified. Maybe both the congenital and aquired factors lead to changes in two aspects: one is degeneration of the vessle wall, the other is greater pressure at certain locations such as at a bifurcation. Many factors have been implicated, but not definitely linked to it. However famale and age between 40 to 60 seems more easily to suffer aneurysms.
Cerebral aneurysms lead to symptoms in two general ways:it's rupture cause intracranial hemorrhage; and it's enlargement cause compression on surrounding structures. Finally these symtoms appears, like headache, vomiting, oculomotor nerve paresis and so on.
We can see that there is an abnormal loss or absence of the muscular layer of the artery wall, so when growing larger, it will rupture. In the CT image we often see a SAH. different aneurysm locations will present different CT image...
Vasospasm will be triggered by SAH. 35 % of the patients with SAH have a secondary neurological deterioration due to vasospasms. DSA can image vasospasm. Transcranial Doppler sonography is an usefull tool in early detection of cerebral vasospasm, it can provide the flow velocity.
The oculomotor nerve is the most common structure compromised by a PCA aneurysm,or a PcoA aneurysm because of the anatomic relationship. This shows a ICA-PcoA aneurysm compressing the oculomotor nerve.
So, if one has a spontaneous SAH or a oculomotor nerve paresis, we suspect that he/she has intracranial anuerysms. Then how to clarify it? DSA, CT, MRI even ultrasound can be used for detecting intracranial anuerysm.
DSA has always been regarded as the gold standard for intracranial aneurysm detection. These images show different locations of aneurysms. DSA is considered as the gold standard due to its primary advantage of high-resolution images(0.3 mm resolution)and a relatively high sensitivity and specificity. However, a false negative rate of 5–10% is reported for DSA. This is mainly due to difficulty in obtaining the optimal projection, also omission of one side of VA angiography and spontaneous thrombosis in an aneurysm will cause false negetive results. False positive rate is low. As is known by us that invasive nature of DSA is an additional disadvantage carring a low but significant complication risk. Furthermore, it's expensive and time consuming. The inability to simultaneously image osseous and vascular structures is noted as a weakness of this technique compared with CT angiography.
This problem can be resolved by a 3D-DSA. 3D-DSA combines the anatomic resolution of DSA with 3D visualization abilities. It can better details the anatomic relationship.
A large aneurysm can be seen in a normal scan image of CT or MRI. These are showing the same left middle cerebral artery aneurysm in CT, MRI and DSA. However, more often than not, an angiography is required.
CTA is a well-known tool for detection of intracranial aneurysms. It is much less invasive and a more patient-friendly procedure. It has many sophisticated postprocessing tools such as MIP, MPR, VR to show leisions well. It can provide the true size of the aneurysm information by demonstrating both the lumen and the thrombosed part of the aneurysm. It demonstrated calcifications within the wall and the neck of the aneurysm, which is important in clipping surgery. It is more sensitive in visualizing hypoplastic A1 segments than DSA. However, CTA presented some difficulties in evaluating aneurysms located near bone structures.
MRA is based on three techniques: time of flight, phase contrast and contrast enhancement. Aneurysms 5 mm and larger are detected reliably on good quality studies. Aneurysms in the cavernous sinus or near the skull base may be more easy to detect with MRA than with CTA. However, MRA proved to be inferior to CTA in the demonstration of the aneurysm characteristics. More specifically, it underestimated the diameter of the aneurysm neck due to flow phenomena. The main disadvantages of this technique are the prolonged acquisition time and the artefacts due to flow phenomena and patient motion.
Now I want to show you some comparison figures.
These are showing a left posterior communicating artery aneurysm, MRA was superior to CTA in demonstrating the neck of the PComA aneurysm that was located close to the clinoid processes.
This case shows false positive results by CTA and MRA due to a vessle loop which is confirmed by DSA. The image resolution of CTA and MRA still can't catch up with that of DSA, and Distant mycotic aneurysms can be missed by both CTA and MRA, so maybe they can't succeed DSA as a gold standard. Furthermore, DSA is superior to CTA and MRA in visualizing the flow patterns within the aneurysm.
This case shows the mistake description of the aneurysm by imaging examination. CTA described this AComA aneurysm as 'bilobed aneurysm', while a 'double' AComA aneurysm was found at surgery.
A transcranial doppler sonography can also be used to show circle of Willis and detect intracranial aneurysms. In comparison with reported studies of CTA and MRA, TCDS appears to be inferior at aneurysm detection. But as just we said that it is an usefull tool in detection of vasospasm.
Doctors treat brain aneurysms in two basic ways. one is clipping and the other is coiling.
Surgical clipping isolates an aneurysm from the normal circulation without blocking off any small perforating arteries nearby. A postoperative examination is required to make sure the aneurysm is clipped completely. If it has been partially clipped, there is a risk of regrowth. Postoperative CTA can demonstrat clearly the clip and aneurysmal remnant.
Some saccular aneurysms and fusiform aneurysms can not be clipped. They need a occlusion of the artery, therefore another bypass should be constructed.
Endovascular embolization(also called coiling) is another main approch. It involves coiling a thin strand of wire inside the aneurysm, then blood clots in and around the coil instead of pushing on the weak walls of the aneurysm. After the endocascular embolization, there is a change in the DSA image.
收藏到：Del.icio.us
引用地址：
卑劣
闷骚之人
