الفهرس 
Normal Vascular AnatomyOnly 14 pages are availabe for public view
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Abstract
Sub-arachinoid hemorrhage refers to a leakage of blood into the subarachnoid space which is a continuous space between the supratentorial and infratentorial compartments.SAH originates from a focal source can be more diffuse and spread throughout wider aspects of the subarachnoid space. (Kirkpatrick,2008)
Non traumatic cases of SAH usually are caused by extravasation of blood from abnormal blood vessels onto the surface of the brain ,this is the result of aneurysmal rupture.There is evidence that aneurysms arising from the posterior circulation (i.e. intracranial vertebral arteries and basilar artery and its branches, including the posterior cerebral arteries) are at increased risk of rupture compared with aneurysms arising from the anterior circulation (i.e. from the internal carotid arteries or its branches, including the posterior communicating arteries) and that if they do rupture , the outcome is poorer .The detection of the subgroup of posterior circulation aneurysms is therefore particularly important. (Newton et al,2005)
The distribution of subarachnoid blood may be a strong clue to the presence and to the site of an aneurysm as where a local hematoma or the presence of hydrocephalus can do. Furthermore ,the results of a recent theoretic analysis suggest that increasing disease prevalence can lead to an apparent improvement in the sensitivity and specificity of an examination. (Tomandl et al, 2003)
Sudden onset of vigorous headache typically the leading symptom in patients with SAH caused by the rupture of an intracranial aneurysm . CT is the first step in the examination of these patients .Once SAH is confirmed , it is paramount to detect the source of bleeding in order to initiate therapy. (Tomandl et al, 2003)
Patients were selected by the referring physicians for DSA on the basis of clinical or radiological findings ,including presentation with acute SAH confirmed by non-enhanced CT or lumber puncture ; symptoms and signs suggestive of aneurysm such as headache or cranial neuropathy ; or a previous routine CT scan or MRangiogram suggesting the presence of an intracranial aneurysm. (Yanaka et al, 2007)
The possibility of using non-invasive modalities to screen an aneurysm may be attractive to physicians who must deal with worried relatives of patients with SAH .The diagnostic performance of CTA and MRA for detection of aneurysms as a most common cause is excellent in patients with acute SAH. (Wintermark et al ,2003)
CT scanning is used as a diagnostic tool for the numerous patients admitted to the emergency department for headaches mimicking SAH .Conventional cerebral CT scanning and MSCT angiography are the first investigation performed in emergency setting in every patient for whom the clinician has a suspicion of SAH. ( Wintermark et al ,2003)
Multislice CT angiography has the major advantage of being easily performed in emergency settings provided by expert hands. 2D and 3D reconstructions are performed by the resident in general radiology who is on duty. The learning process for manipulating data on the workstation is very rapid, even for junior radiologists. In every case , the data are reviewed by an on-call neuroradiologists . Early depiction and precise 3D characterization of an intracranial aneurysm are thus available after a post processing time averaging 10 to 15 minutes, aiding the medical staff in decision making and in the co-ordination and timing of treatment options. (Wintermark et al ,2003) .
During conventional arterial angiography ,identification of the projection that allows visualization of the aneurysm neck can be time consuming because of the superimposition of several arterial branches .MSCT angiography overcome this limitation by offering the opportunity to create any desired projection from the original data set and ,thus to obtain the best orientation to view the aneurysm neck. ( Wintermark et al ,2003)
Multi-section CT scanners are fundamentally improved over single section CT scanners in 3 ways: shorter acquisition times which allows a significant reduction in the administration dose of contrast material, despite an increased overlap. This feature in combination with a cranio-caudal scanning direction led to a significant reduction in the enhancement of overlapping intracranial venous structures , which potentially could obscure the arteries. In this way , aneurysms situated at the origin of PCoA were not obscured by the vein of Rosenthal and those located at the level of the cavernous segment of the ICA were not masked by the cavernous sinus and the short acquisition time facilitates the examination of unco-operative or comatose patients who need only a short acting sedative to complete CTA and a complete diagnostic work up of SAH can be performed without the need of a general anesthetic in most cases which is often required during an emergency angiographic work up. Besides , it is a non-invasive quick and reliable method and reduce the period of time between the diagnosis of SAH and the diagnosis of the reason and thus make it possible to operate rapidly in case of emergency.
(Teksam et al ,2004).
The volume of contrast media injected for a single CTA examination is only one-third to one-half of that used for a four-vessel DSA examination.In addition, radiation dosage is less than that of intraarterial angiography .Also CTA , due to its post-processing ability , proved to be superior to DSA in demonstrating the orientation and relationships of the aneurysm sac. It is also highly sensitive in demonstrating mural calcifications and thrombosis. In the patient with an ophthalmic carotid aneurysm ,CTA demonstrated calcifications within the wall and the neck of the aneurysm. This information is useful to the neurosurgeons because neck and wall calcifications may pose complications or difficulties during operation, i.e: incomplete occlusion of the aneurysm during clipping or clip slipping. The true size of the aneurysm is also mandatory for the neurosurgeons and CTA can provide this information by demonstrating both the lumen and the thrombosed part of the aneurysm.( Kowskouras et al,2004)
In addition to the ability to create a thinner or thicker sections from the raw data and improving 3D rendering with diminished artifacts and the high gantry speed of the multi-section CT scanners permits helical scanning to be performed with a small section thickness than is possible with conventional helical CT.
(Kowskouras et al,2004)
Moreover , MSCT angiography enables increased arterial opacification and accordingly , increased spatial resolution as well as improved coincidence between MSCT angiography data acquisition and passage of iodinated contrast material within the volume of interest. ( Teksam et al ,2004)
Volume rendering is used to view 3D data because this technique makes all the voxels available within a volume, avoiding extensive loss of information, potentially arbitrary vessel border definition, vessel-bone interface difficulties, and loss of perspective which are inherent to shaded surface display and maximum intensity pixel technique for CTA.The 3D images were most useful for detecting and characterizing intradural lesions that are obvious when viewed in a 3D environment but inconspicuous when viewed in a 2D mode. ( Villablanca et al ,2002)
The advantages of the slab-MIP images are the avoidance of time consuming-bone removal and the usefulness of the remaining bony landmarks for the neurosurgeon’s preoperative planning .Also MIP unlike SS display enables differentiation of vascular structures from bone. Although the SSD images provide valuable information regarding the relation of the cranial base to the blood vessels, the quality and the usefulness of these images depends on the level of threshold chosen (HU level for inclusion of contrast filled vessels), which ,if incorrect may result in loss of information such as the presence and location of small vessels when the threshold is too high or distortion of aneurysm and neck size if the threshold too low. (Velthuis et al, 1998)
Most authors used combinations of VRT,MIP and multiplanar reconstruction for the analysis of MDCTA axial data. The MIP method has an advantage over VRT for differentiating parietal calcification and intraluminal thrombus from the true vascular lumen . However, calcification at the wall or thrombus within the lumen of an aneurysm can be clearly shown on the careful view of the source images.( Yanaka et al, 2007)
The VRT method is far quicker and easier to produce than MIP algorithms and can give better anatomic detail of the intracranial vasculature. The 4D angio soft ware package allows rapid VRT images to be produced , which highlight the vessels over the bony landmarks. (Yanaka et al, 2007)
CTA can be performed immediately following confirmation of SAH with non-enhanced CT, so may be used as a tool for therapeutic decision making and therapy planning, in addition, a major advantage of CT angiography compared with DSA is the generation of 3D information on the exact anatomy of the intracranial arteries as well as the mere detection of aneurysms which are the most common cause of SAH , the location , size ,lobularity and aneurysm sac and neck dimensions, the sac/neck ratio and the absence or presence of arterial branches originating from the aneurysm were evaluated which is an important consideration generally for MCA aneurysm assessment and for anterior communicating artery (ACoMA) aneurysms particularly and so we note the combination of aneurysm neck location ,direction of aneurysm projection, and the relative size of the proximal anterior cerebral arteries as determined by CTA correctly predicted the particular anterior cerebral artery that preferentially ”filled” the aneurysm revealed by DSA.( Anderson,1997)
So these 3D models can be most helpful for therapy planning ,when intravascular coiling is the therapy chosen on the basis of the CT angiographic findings ,additional pretherapeutic DSA isnot necessary, as it is part of the coiling procedure, in addition, CT angiography can help predict the ideal angle for the endovascular approach .Information on the exact dimensions of an aneurysm can be used to determine the diameter of the first coil. For the neurosurgeons information about adjacent vascular structures and the possibility of simulating the intra-operative view prior to surgery are often helpful. (Yanaka et al, 2007 )
However there are several disadvantages of using CT angiography technique including that the use of any commercially available workstation for post processing requires intensive familiarization with both the techniques of 3D visualization as well as the different manipulation tools available on the workstation used, their actual value strongly depends on the individual target structures, correct adjustment of transfer functions as well as efficient interactive manipulation of volume data are necessary to make the aneurysms clearly visible. In addition the examination is limited to one phase of the circulation where as selective cerebral angiography can show alterations in cerebral blood flow such as collaterals and vasospasm. Small perforating arteries cannot be identified by CTA.The ICA was generally a difficult region to analyze by CTA as the curved vessel is caught in the axial slices in different parts so it is difficult to detect the aneurysm located within the cavernous sinus and near the skull base, however special post processing techniques that suppress irrelevant bone structures are required and these includes two solutions : (Tomandl et al, 2003)
a. The acquisition of two data sets ,one before and one after subtraction in the images in the cotext of CT-DSA. The main disadvantage of this strategy are that patients must lie absolutely still during scanning and they are exposed to increased radiation.
b. Automatic segmentation of bone structures was suggested ,this approach is technically even more challenging ,since a threshold –based elimination of bone is difficult because of partial –volume effects .As mentioned before, a prerequisite for 3D representation. Therefore ,3D imaging cannot replace careful analysis of the underlying source images. Moreover , MIP images have a particular disadvantages in postscan processing removal of the cranial bones for adequate visualization of the blood vessels. This task may be quite difficult in removing the cranial base while preserving the supraclinoid internal carotid artery and its branches. ( Anderson,1997)
MR angiography is another radiological method for non-invasive investigation of SAH. Its main advantage when compared to CT angiography is that the bone doesnot disturb the images .The reported sensitivity of MR angiography for the detection of intracranial aneurysms which are the most common cause of SAH is comparable to that of the CTA for aneurysms with a diameter greater than 3mm but is significantly lower for smaller aneurysms.( Tomandl et al, 2003)
One of the advantages of MR angiography is that it enables the visualization of the circle of Willis’ without the use of ionizing radiation or intravenous contrast material. ( Tomandl et al, 2003 )
Source images viewed in cine mode were useful for visualizing vessels close to bone ( e.g. ophthalmic and posterior communicating arteries) that were often poorly depicted by post processing techniques. The area close to the skull base and around sphenoid sinus is also problematic for MRA both because of susceptibility artifact due to adjacent bone and because of complex flow in the carotid siphon causing signal loss as a result of phase incoherence. In these locations , MPR or axial source data images can be used to focus on an aspect of a known aneurysm such as the neck-to fundus ratio , particularly if its orientation is known , without incurring loss or disorientation of data due to processing , MPR images have nearly the same spatial resolution as also the original axial images.( Adams,2000 ).
However , one of the disadvantages of time of flight MRA technique is that it doesnot depict some aneurysms. This is mainly because of spin saturation secondary to slow flow and / or phase dispersion effects due to turbulent flow in an aneurysm. In addition to the poor quality images obtained in sick, restless patients with acute SAH. The long acquisition time of 3D time of flight MRA makes these patients particularly susceptible to this problem.(Tomandl et al, 2003)
Advantage of phase contrast MRA technique over TOF-MRA , because of its variable sensitivity to velocity ,low sensitivity to saturation effects and superior background suppression. However, flow direction within an aneurysm can oscillate during the cardiac cycle. Pulsatile flow ,because of the necessity for subtraction , can results in artifacts. This contributes to the signal loss of the PC-MRA technique. Flow velocity within an aneurysm can be much slower than in the parent vessel. Setting the appropriate velocity encoding profile to accommodate both aneurysm and vessel is impractical. (Adams,2000)
Relatively few investigators have directly correlated MRA and CTA. CTA has the advantage that it can readily be performed immediately after the unenhanced CT has confirmed SAH . There is no contraindication to scanning these patients who would be unsuitable for MRA ,such as critically ill patients ,patients with claustrophobia and those with pacemakers, although CTA may be inadequate in those patients with left ventricular failure due to suboptimal opacification of the intracranial vasculature. (Adams,2000)
CTA is a non-invasive volumetric imaging technique that doesnot require arterial puncture or catheter manipulation. Furthermore, CTA images can be obtained in less than 1 min by a trained technologist and the CTA data can be viewed from unlimited projections in both 2D and 3D modes, thus facilitating aneurysm detection and characterization . MRA , on the other hand ,is also a non-invasive technique that is based on the detection of blood flow within the cerebral vessels . Its primary advantage is that it can provide us with very thin, almost sub-mm source images that can be viewed later on by using both 2D and 3D modes. The main disadvantage of this technique are the prolonged acquisition time and the artifacts due to flow phenomena and patient motion. ( Kowskouras et al, 2004)
Advantages of multi-detector row CT angiography over MR angiography include the ability to depict bone landmarks and the relation of aneurysms to vascular structures, which may play an important role in the treatment planning .( Jayaraman et al, 2004)
Turbulent flow within the aneurysm can produce poor visualization of the aneurysm with MRA , a problem which not true with CTA , which generates images based on contrast enhancement of the aneurysms lumen. The same is true in the case of giant aneurysm , where turbulent or slow flow and thrombosis can cause poor visualization of the aneurysm on MRA .
While small vascular lesions can be missed by both methods. Also artifacts from surgical clips preclude accurate postoperative evaluation for both CTA and MRA. (Kowskouras et al, 2004)
CTA proved to be superior to MRA in the demonstration of the aneurysms characteristics. More specifically, MRA underestimated the diameter of the aneurysm neck due to flow phenomena i.e. inflow jet. In addition false partial thrombosis of the aneurysm was due to turbulence flow within the aneurysm . Both methods were equal in demonstrating the location and the relationships of the aneurysm with adjacent vessels. On the other hand , CTA presented some difficulties in evaluating aneurysms located near bone structures. (Kowskouras et al, 2004)