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NEURORADIOLOGY HEAD AND NECK IMAGING Table of Contents   
Year : 2000  |  Volume : 10  |  Issue : 4  |  Page : 211-220
Neurovascular applications of CT angiography


Dept of Imaging, PD Hinduja Hosp, Mumbai - 400 016, India

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Keywords: CT angiography, carotid, brain, applications

How to cite this article:
Shetty PG, Jhaveri K S. Neurovascular applications of CT angiography. Indian J Radiol Imaging 2000;10:211-20

How to cite this URL:
Shetty PG, Jhaveri K S. Neurovascular applications of CT angiography. Indian J Radiol Imaging [serial online] 2000 [cited 2019 Sep 20];10:211-20. Available from: http://www.ijri.org/text.asp?2000/10/4/211/30565
Catheter angiography is the gold standard for diagnostic imaging of the neuro-vasculature. However it is invasive, expensive and has an associated risk of 1.5% to 2% of significant morbidity and mortality [1]. These drawbacks have led to the development of noninvasive or minimally invasive techniques. Magnetic resonance angiography (MRA) and/or color Doppler ultrasound (CDUS) have become established as the standard noninvasive techniques for this purpose. Over the last few years, with the advent of helical CT, CT angiography (CTA) has become an important adjunct technique for evaluation of the neuro-vasculature

MRA has several drawbacks relative to CTA which sometime include longer examination times with resultant motion artifacts, pulsation artifacts, turbulent flow and in plane flow causing apparent exaggerated stenosis, poor demonstration of calcium and bony landmarks. In addition, post operative status evaluation in patients with metallic clips and stents is not easy with MRA.

CDUS is operator dependent and can yield variable results. It also has limitations in the evaluation of the intracranial vasculature, though transcranial Doppler has been used for the same. The reproducibility of CDUS across many centres is poor.

Compared to MRA and CDUS, CTA is faster, less expensive, more widely available, more sensitive for mural calcium, can display bony landmarks well and also can be used in patients with aneurysm clips and other MR incompatible metallic hardware. Also CTA images are dependent on the volume of blood within the vessel as compared to MRA and CDUS where the image production is dependent on the velocity of blood in the vessel. Limitations of CTA are the use of intravenous contrast and radiation exposure. With the current helical CT technology, simultaneous assessment of both extracranial and intracranial vasculature at the same sitting is possible. However, catheter angiography provides better spatial and temporal resolution in comparison to CTA.


   Technique: Top


The fundamental principles of CT angiography include helical CT acquisition, image processing and image display.

Spiral CT Acquisition: The rapid developments in spiral CT technology over the last six years have resulted in faster data acquisition. The state of art spiral scanner today has a gantry rotation time of 0.7 to 0.75 second which can acquire a spiral length of 40-60 seconds and a second back to back spiral with only a 5-second interscan delay. This has enabled us to obtain much larger volumes of very high-resolution data in a single breath hold, which are ideal for 3D imaging. Subsecond helical gantry rotation time increases the distance covered and further reduces motion artifacts [2],[3 ]. With the introduction of multislice CT that can acquire images at a pitch of 3:1 to 6:1 it will be possible to evaluate the entire craniocervical circulation at a single sitting. This technique can acquire coverage of 20 cm in less than15 seconds with a collimation of 1.25 mm

The general optimal protocols for CTA of the carotid bifurcation and circle of Willis are shown in [Table 1]. The parameters may be varied depending upon the clinical application and region of interest.

Optimal CT angiographic technique requires rapid uniform intravenous injection of contrast material via pressure injector with flow rate of at least 3 ml/s. Higher rates of 5 to 7 ml/s have been used but do not offer any significant advantage [4]. Ideally, nonionic contrast medium should be used utilizing a 20 to 22 gauge angiocatheter planed in the medial antecubital vein. Automated techniques are available to measure the contrast circulation time in individual patients in order to calculate delay time, however an empiric delay of 12 to 15 seconds is good enough for most patients. Patients with low cardiac output need a longer pre-scan delay varying from 15-20 sec.


   Image Processing: Top


The goal of image processing is to obtain images, which closely resemble conventional angiographic images and facilitate the assessment of the vascular abnormality in relation to anatomic landmarks. The display techniques include Multiplanar Reconstructions (MPR), Maximum Intensity Projection (MIP), Shaded Surface Display (SSD) and Volume Rendering Technique (VRT).

Multiplanar reconstructions (MPR) allow the user to view the raw data in any desired plane including curved planes along the course of the vessels. This is very effective for relatively straight vessels such as the carotid arteries and is less effective for complex vascular anatomy in areas such as the Circle of Willis and for evaluation of vascular malformations.

Maximum Intensity projection (MIP) images are generated by projection of imaginary rays through the image data and mapping the maximum attenuation value along each ray to a gray scale image. These projected images can be oriented in any anatomical plane. MIP has the property to distinguish high-density structures such as bone and calcium from contrast filled vessel lumens. The vessel lumen, calcium and thrombus are thus well delineated [Figure - 1]. MIP calculates degree of stenosis accurately as compared to SSD. The disadvantage of MIP is that vessels cannot be separately delineated.

Shaded surface display (SSD) uses a preset threshold and computes a surface connecting neighboring pixels above this threshold. SSD displays complex anatomic relationship in region of vessel overlap or vessel tortuosity [Figure - 2]. Its limitations are that the interior of the contrast opacified vessel lumen and wall calcification are not well demonstrated, stenosis is underestimated and thrombus is not well seen

Volume rendering (VR) is a more advanced and computer intensive 3D rendering algorithm that can incorporate all of the relevant data into the resulting 3D image and overcomes many of the problems seen with MIP and SSD. Improved accuracy is the primary reason for use of volume rendering over MIP and SSD. The limitations of VRT are the requirement of expensive computation technique software.


   Applications Top


(A) Extracranial Vasculature

1) Carotid artery stenosis

The current management of carotid atherosclerotic disease is based on the results of the North American Symptomatic Carotid Endarterectomy Trial (NASCET) which concluded that carotid endarterectomy, has a clear benefit in symptomatic patients with high grade stenosis (70-99%) [5]. Thus the detection and accurate quantification of carotid artery stenosis is of vital importance to facilitate appropriate treatment.

The technique of CTA is very crucial for accurate estimation of carotid artery stenosis. It has been shown in phantom studies than 1 mm collimation provides more accurate results than 2 mm collimation [6]. Also the pitch may be increased to two without significant loss of resolution. Studies using a collimation of 3 mm or more had poorer results and further emphasize the use of thin collimation [7,][8].

Clave et al [9] concluded from phantom studies that a luminal attenuation of 150 H is optimal for accurate estimation of carotid stenosis. A luminal attenuation of <100 H or >250 H significantly reduces the accuracy. This optimal luminal attenuation of 150 H is obtained with a flow rate of 3 ml/s. A recent study [10] concluded on the basis of hemodynamic and vascular anatomic data that for optimal image quality injection of intravenous contrast material should be made into the right arm and during deep inspiration. This is because the right brachiocephalic vein runs parallel to the ascending aorta while the left brachiocephalic vein differs in course, length and can be compressed between the sternum anteriorly and aortic arch posteriorly. Deep inspiration causes antegrade venous flow and prevents decrease in relative carotid contrast density. Image display with volume rendering technique (VRT) is the best as this has been shown to have an error rate of only 2% for stenosis quantification of vessels such as carotid which are perpendicular to the axial plane [11]. Stenosis measurements are perhaps best made from axial source images and in conjunction with three-dimensional models [12],[13 ]. Artefactual luminal eccentricity has significant implications for measuring percentage of stenosis revealed by CT angiography. Eccentricity increases with longer stenoses, thicker slices and vessels oriented parallel to the Z-axis of the patient [14].

Although conventional angiography is accepted as the gold standard for estimating carotid artery disease a number of recent studies have indicated that the associated risks with the procedure are equal to or even greater than those related to the surgical treatment itself [4]. An interobserver, variability of +/- 7% has been found among experts interpreting carotid stenosis with a tendency towards overestimation by an average of 6% [15]. Further, eccentric or irregular stenosis may not be optimally assessed due to limited number of views available with conventional angiography [14].

CTA has been shown to have a very good correlation with conventional angiography. Reported sensitivity for severe stenosis and occlusion has ranged from 88% to 100% [15],[16],[17],[18]. Also CTA can provide infinite views for accurate estimation of eccentric or irregular stenosis. CT also provides assessment of the brain parenchyma at the same time.

Preliminary studies comparing CTA, MRA and US with conventional angiography for estimating carotid artery stenosis found them to provide similar accuracy in reference to conventional angiography [18],[19]. A recent study by Mark et al [20] comparing CTA with MRA in reference to standard conventional angiography found CTA to be superior to MRA for the estimation of severe carotid stenosis and occlusions. MRA tended to overestimate stenosis. CTA does have superiority over MRA in estimating high grade stenosis and occlusions as the conspicuity of the vessel is dependent on the volume of blood within the vessel (as in conventional angiography) while in MRA the signal is dependent on the flow velocity. Also in MRA, turbulent flow and in plane flow can cause loss of signal leading to overestimation of stenosis. CTA can generally delineate mural calcium from luminal contrast and prevent inaccuracy in grading stenosis [Figure - 3],[Figure - 4]. 3D time-of-flight (TOF) MRA with IV contrast is more accurate in quantifying stenosis but is relatively expensive

In a recent study [24] comparing CTA to CDUS with conventional angiography as the reference, CTA was found to yield higher sensitivity, specificity and predictive valves than CDUS in assessing high grade stenosis and distinguishing it from complete occlusion. This is very crucial, as high grade stenosis is an indication for carotid endarterectomy whereas complete occlusion is a contraindication to surgery. (It also eliminates some of the artifacts that degrade 2D TOF MR angiography) [20],[21],[22].

Thus though CTA appears to be superior to both MRA and CDUS in the evaluation of carotid artery stenosis, larger multicentre studies are required to establish CTA as the non invasive modality of choice for estimation of carotid artery disease.

CTA also suffers from certain limitations in carotid stenosis estimation in that simultaneous evaluation of the intracranial vasculature for tandem stenosis is not possible at the same sitting, though the newer generation of CT scanners with multislice technology overcome this limitation. Also CTA cannot provide direct information of the directionality of blood flow which CDUS, phase contrast MRA and catheter angiography can do. Hemodynamic of blood flow is not possible with CTA.

(2) Carotid dissection and other non-atherosclerotic occlusive disease.

Dissections of the extracranial internal carotid artery are being increasingly recognized as a cause of stroke accounting for upto to 20% of ischemic strokes in young adults [25]. Non invasive techniques such as USG and MR have been used during the acute stage of ICA dissections [26],[27], Helical CT angiography was found to provide results in close agreement with those obtained at angiography [28]. The presence of a narrowed eccentric lumen in association with mural thickening and enlargement of the overall diameter of the ICA are considered good criteria for the diagnosis. Helical CT scan also correctly depicts stenosis, occlusions and pseudoaneurysms associated with dissection.

CTA can also be used as a noninvasive modality to assess the follow up of patients with ICA dissections and thus guide appropriate treatment. CT seems to be the most valuable method for imaging both the arterial wall and lumen [29]. Though CDUS can show recanalization and enable analysis of residual flow abnormalities [27], its reliability in the assessment of the superior portion of the ICA is limited. Helical CT is also theoretically superior to MR in the evaluation of aneurysm, associated with dissection because of its high resolution and absence of flow related artifact [28] but MR imaging may be better for evaluating the arterial segment at the skull base. CTA can also be used to assess abnormalities of vertebral arteries such as dissections and occlusions [30]. [Figure - 6],[Figure - 7].

Both CTA and MRA will have a higher sensitivity and specificity in the diagnosis of arterial dissections as subadventitial dissection and the presence of intramural hematoma can easily be assessed by these cross sectional modalities.

Polytrauma patients with a high index of vascular injury can also be evaluated by CTA. Helical CT provides simultaneous assessment of vascular, soft tissue and vertebral injury in this setting [30].

(B) Intracranial Vasculature .

(1) Aneurysms

DSA is the gold standard for the evaluation of intracranial aneurysms and patients with subarachnoid hemorrhage secondary to suspected aneurysmal rupture. Following acute subarachnoid hemorrhage, DSA is the method of choice for detection of aneurysm but it is time consuming, invasive and carries a 1% complication risk and 0.5% risk of persistent neurological deficit [34]. Also, angiography performed within the first six hours after the initial bleeding incident has been associated with an increase re-bleeding rate [35],[36].

MRI and MRA are currently the non-invasive modality of choice for screening patients in the high-risk group for aneurysms. MRA is also used for the follow up of incidental aneurysms, which are being managed conservatively.

Currently CTA plays a major role in the detection and characterization of giant aneurysms prior to surgical or endovascular treatment, in catastrophic SAH where the patient cannot undergo DSA due to time considerations and in the assessment of post operative / post intervention status of aneurysm.

CTA acts as a problem solving modality in the characterization of aneurysms that conventional angiographic techniques show to be poorly defined or inconclusive. CTA provides a more detailed analysis of the dome, neck, vessel of origin and surrounding anatomy which is helpful in determining appropriate treatment options (surgical or endovascular) [31]. Moreover when endovascular treatment is a viable option, meticulous assessment of aneurysm sac morphology neck size, presence of calcium, parent vessel calibre and their interrelationship is imperative, because these factors directly influence the selection of embolisation material [32],[33]. Large aneurysms arising in anatomically complex regions such as the paraclinoid internal carotid artery tend to distort surrounding vessels making identification of these vessels difficult. CTA helps the preoperative assessment by identifying the vessel of origin and relationship of the aneurysm to other vessels and bony and soft tissue landmarks [Figure - 8],[Figure - 9].

Though conventional angiography is more sensitive for small aneurysms, CTA in a few instances may aid in aneurysm detection by showing the presence of small thrombosed aneurysm not well delineated by conventional angiography [31]. It may also be useful for the detection of pseudoaneurysms of the intracranial vasculature.

CTA seems very suitable in the acute stage after SAH as it does not require intra-arterial catheterization, scanning time is around a minute and can immediately follow the initial unenhanced CT examination. Recent clinical studies have found CTA to be very useful in this clinical setting [37],[38]. Another recent study found CTA to detect 90% of all aneurysms associated with acute SAH. Neurosurgeons assessed CTA as equal or superior to DSA in 83% of cases and in 74% of patients surgery might have been based on CTA findings alone without the need for DSA [39].

Intracranial vasospasm following SAH in patients with ruptured intracranial aneurysms remains a leading cause of morbidity or mortality [40],[41]. CTA has been found to be very useful for screening for vasospasm following SAH and clearly reveals vasospasm in a manner similar to conventional angiography [42]. Transcranial Doppler is useful in the detection of intracranial vasospasm and its follow-up in the setting of SAH and in evaluation of intracranial emboli. However it is limited by operated dependence and limited acoustic window resulting in a 20% failure rate [43].

CTA can also be used as a screening technique to detect aneurysms in high risk group patients such as those with strong family histories, adult polycystic kidney disease, Marfan's and Ehler Danlos syndromes, fibromuscular dysplasia, Moya Maya, aortic coarctation, Takayasu's disease and neurofibromatosis, though MRA is currently the modality of choice. [Figure - 10],[Figure - 11].

CTA is also very useful for the evaluation of the post-clipping status of the aneurysms with regard to residual aneurysm filling, vascular patency and position of the clip relative to the desired placement site [44]. [Figure - 12],[Figure - 13],[Figure - 14]

CTA has sensitivity ranging from 87 to 100% and specificity of 50% to 100% for detection of intracranial aneurysms [45],[46],[47],[48],[49]. Aneurysm size and location are the most important determinants of sensitivity. CT Angiography has sensitivity of 87% to 97% for aneurysms > 5 mm in size. However, those < 5 mm in size can be detected with a sensitivity of only about 20% [49],[50]. Small aneurysms located at the skull base are also very difficult to detect with CT angiography.

CTA and MRA have been found to be generally equivalent in their ability to detect and characterize aneurysms (> 5mm) [51]. CTA, however is superior to MRA, as turbulent flow or slow flow may cause artifactual loss of signal in MRA. Also MR does not provide good delineation of calcium and bony landmarks and is not optimal for assessing post aneurysm clipping status. MRA is not feasible following SAH due to the longer examination time.

(2) Arteriovenous malformations:

CTA has been used to image intracranial AVMs though the role is limited [52],[53]. It can visualize the feeding arteries, nidus and draining veins [Figure - 15]. Optimal evaluation of AVMs needs detailed information about the feeding arteries, nidus and early draining veins which in turn needs temporal resolution of the arterial and venous phases. High spatial resolution is also required to detect associated findings such as aneurysm, stenosis and small feeding vessels. CTA clearly lacks this high degree of temporal and spatial resolution, requisites which conventional angiography provides. The major use of CTA today is in the evaluation of the AVM nidus during radiosurgery planning and in following up patients after radiosurgical treatment [54] [Figure - 16].

Studies comparing roles of CTA and MRA in AVMs have not been reported though each modality appears to have distinct advantages and disadvantages. CTA displays arterial feeders, nidus and draining veins together and it may be difficult to separate the nidus from the tangle of feeding arteries and early draining veins. PC MRA, however may be able to show arterial feeders with different velocities and it may be possible to saturate venous flow. On the other hand, MRA may fail to show a nidus with slow flow on TOF technique. Embolisation material can interfere with delineation of vasculature on MRA but usually is not a problem with CTA [Figure - 16].

While CTA can detect venous angiomas, it is not specifically indicated for this purpose as this can be documented on a routine contrast enhanced study. CTA is not useful in documentation of cavernous angiomas, owing to slow flow of blood in these lesions.

(3) Acute ishemic stroke and intracranial stenosis

Acute local intraarterial thrombolysis has recently shown promise in improving patient outcome. Thrombosis must be identified and treated promptly for optimal results. CTA is an accurate technique for evaluation of vascular patency in acute stroke. CTA provided important diagnostic information about the site of arterial or venous occlusion, pattern of collaterals and the pattern of unenhanced (poorly perfused) brain tissue. The information obtained from both conventional CT and CTA provides a rational basis by which to choose the optimal treatment for patients with acute stroke [55],[56],[57].

(4) CT venography

CT venography is performed using 1 mm collimated sections and a pitch of 2:1 from the vertex to the skull base. Ninety-hundred ml of non-ionic contrast is injected intravenously at 3 ml/sec through a pressure injection with a 40-sec pre scan delay.

Dural sinus thrombosis is often clinically unsuspected and has varied clinical presentation. MR venography is currently the technique of choice for diagnostic evaluation and follow up of dural sinus thrombosis [58],[59]. However a recent study has shown that cerebral CT venography is superior to MR venography in the identification of cerebral veins and dural sinuses and is at least equivalent in the diagnosis of dural sinus thrombosis [Figure - 17][60]. CT venography can also differentiate slow flow from thrombosis, which is sometimes difficult with TOF MR. CT venography also helps in distinction of tumors that partially obstruct dural sinuses, from those which occlude them [Figure - 18]. Thus CT venography is a reliable alternative to MR venography in the examination of patients with suspected dural sinus thrombosis, especially if the patient cannot undergo MR or MR venography is equivocal.

(5) Tumors

CTA can be used effectively in demonstrating vascular encasement by skull base tumors. It also provides for excellent preoperative assessment of the bony and vascular anatomy prior to tumor excision. It is useful for the surgeon to plan his/her surgical approach and the three dimensional evaluation before surgery can reduce intraoperative time.


   Conclusion Top


CT Angiography is an evolving technique with a vast potential in neurovascular applications. With the advent of the multislice technology, the craniocervical circulation can be evaluated in one study. CT angiography closely simulates catheter angiography as it depicts the volume of contrast opacified blood in vessels unlike MR angiography and CDUS, which are highly dependent on velocity of blood flow. Ongoing improvements in X-ray tube design with greater heat storage capacity will further expand the potential of CTA in terms of greater coverage and higher resolution. Post-processing techniques however require to be radiologist-friendly and faster. It however remains to be seen whether CTA would compete with contrast enhanced MR angiography and surpass it or remain its poor cousin. CTA can serve as an important adjunct in the diagnostic armamentarium of the radiologist studying the neurovascular tree.

 
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Correspondence Address:
Prashant G Shetty
Dept of Imaging, PD Hinduja Hosp, Mumbai - 400 016
India
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18]

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