1	"Lim" Lim, S; Shankar, J; Deus-Silva, L; Torres, C; dos Santos, MP; Lum, C; Chakraborty, S. Division of Neuroimaging, The Ottawa Hospital, University of Ottawa, 1053, Carling Avenue, Ottawa, Ontario, Canada
2	Disclosure Nothing to disclose.
3	"Purpose" Purpose. Introduction. Approach and methods. Radiation dose comparison of different studies. Results. Cases. Pitfalls and limitations of dsCTA. Conclusion.
4	Purpose To demonstrate the clinical utility of dynamic subtracted computed tomography (dsCTA) using a 320 multislice volume CT in depicting various neurovascular diseases.
5	Introduction dsCTA offers a non invasive technique to acquire a time series CT angiography images of the whole brain, thus removing timing uncertainties found in typical static CTA images and also provides temporal flow information. The state of the art 320 multislice volume CT allows better spatial and temporal resolution in these time resolved images.
6	Approach/Methods We prospectively recruited 30 patients from May 2008- November 2008 with various neurovascular diseases as illustrated in *Table I. All cases were performed on a 320 slice Toshiba CT scanner.
7	Acquisition technique with 320 slice CT An unenhanced CT head was performed (mask) first. 50 ml of nonionic iodinated contrast at the rate of 4 cc/sec. Acquisition parameters: 80 KV and 100 mA with a rotation time of 1 second. 20 volumes of the brain were acquired at the rate of one volume every 2 seconds for 24 seconds followed by one volume every 5 seconds for another 20 seconds with an acquisition delay of 7 seconds. Post-processing done on Vitrea Fx workstation using singular value decomposition (SVD) plus deconvolution method. All patients with vascular malformations also had catheter digital subtraction angiography.
8	Table I
9	Radiation dose comparison of different studies.
10	Results Maximum intensity projection (MIP) and 3D surface rendering images of the time resolved volumetric data were of diagnostic quality. This technique gives us dynamic flow information (contrast wash-in/wash out) in a non invasive way with reasonable radiation and contrast dose. For assessment of arteriovenous malformation (AVM ) or high flow vascular pathology, timing of the scan is crucial and ‘test bolus’ assessment should be used to improve the quality of the scan and keep the radiation dose to minimum.
11	Cases Case 1: Arteriovenous malformation. Case 2: Periorbital arteriovenous fistula. Case 3: Stroke related to occlusion in distal intracranial ICA. Case 4: Developmental venous anomaly. Case 5: Moya-moya disease. Internal carotid artery (ICA). Case 6: Total occlusion. Case 7: High grade stenosis. Case 8: Left ICA dissection. Case 9: Subclavian artery stenosis. Case 10: STA-MCA bypass- assessment of patency. Case 11: Braindeath.
12	Case 1: Arteriovenous malformation (AVM)
13	Arteriovenous malformation (AVM) dsCTA demonstrates arterial feeders and venous drainage patterns and allows for lesion localization, enabling hemodynamic and morphologic appreciation of these lesions.
14	Arteriovenous malformation (AVM) ds CTA enables one to appreciate vascular flow characteristics over time and functional temporal information of these lesions. The image volumes could be rotated for 3D flow assessment in the workstation.
15	Case 2: Periorbital arteriovenous fistula (AVF) ds CTA (sagittal view) demonstrates a right periorbital AVF with arterial supplies from a frontal branch of the right superficial temporal artery and a lacrimal branch of the right middle meningeal artery. The fistula drains into the right facial vein. No cortical venous reflux or choroidal blush.
16	Periorbital arteriovenous fistula Selective right external carotid artery (ECA) angiogram confirms the ds CTA findings of a right periorbital AVF with arterial supplies and venous drainage as described. DSA however demonstrates arterial branches from the right internal maxillary artery and venous drainage into the superior ophthalmic vein which are not appreciated on ds CTA.
17	Case 3: Stroke related to occlusion in the distal intracranial right ICA.
18	Stroke related to occlusion in the distal intracranial right ICA. On the static CTA images, contrast peters off within the right internal carotid artery (ICA) at C2/3 level and becomes unopacified beyond C2 level , suspicious for right ICA occlusion at C2 level.
19	Stroke related to occlusion in the distal intracranial right ICA. dsCTA images confirms the lack of flow in the right MCA territory and also shows slow flow and delayed filling of the right distal ICA. Hence, the site of occlusion is in the supraclinoid/ cavernous segment of the right ICA.
20	Case 4: Developmental venous anomaly (DVA) Routine MRI for headache shows serpiginous flow void (arrows), MRA is ‘normal’ (upper limit of scan range did not cover lesion). Click to view non dynamic CTA coronal image.
21	Developmental venous anomaly CTA confirms an abnormal penetrating vessel with suspicion for AV fistula (arrow), catheter angiogram was suggested
22	Developmental venous anomaly ds CTA confirms filling of this vessel in late venous phase (i.e. DVA) thus avoiding invasive angiogram.
23	Case 5: Moya moya disease Selective right internal carotid artery (ICA) angiogram (AP view) shows occlusion of the right supraclinoid ICA (thin arrow) and an area of ‘puff of smoke’ in the right basal ganglia region (bold arrow).
24	Moya moya disease 3D surface rendering dsCTA images shows occlusion of supraclinoid internal carotid artery bilaterally. ‘Puff of smoke’ appearance is beyond the resolution of dsCTA.
25	Case 6: Internal carotid artery (ICA) total occlusion Surface rendering images of ds CTA demonstrating right ICA total occlusion near its origin.
26	Case 7: Internal carotid artery (ICA) stenosis - high grade stenosis 67 year old male presented with left facial droop. CTA shows a complex atherosclerotic plaque involving the proximal right ICA with ~ 90% stenosis with no significant narrowing distally (arrow).
27	Internal carotid artery stenosis - high grade stenosis dsCTA demonstrates that the right ICA is not completely occluded by the complex atherosclerotic plaque and there is only mildly delayed filling compared to the contralateral side. Findings are consistent with a high grade stenosis in the proximal ICA which is not completely occluded.
28	Case 8: Left ICA dissection
29	Left ICA dissection dsCTA shows that the artery is reconstituted just after this contrast gap, however with a diameter smaller than the contralateral carotid. There is significantly slow flow noted distally.
30	Case 9: Subclavian artery stenosis
31	Subclavian artery stenosis There is more than 90% stenosis of left subclavian artery just distal to its origin from the arch of aorta. The surface rendering images of dsCTA of the neck shows delayed filling of left vertebral artery. The left vertebral artery is filling antegradely during the venous phase. No evidence of subclavian steal could be demonstrated.
32	Case 10: Right superior temporal artery (STA)- middle cerebral artery (MCA) bypass- demonstrates patency of the bypass CTA coronal reformat shows patent bypass at the level of burr hole (arrow).
33	Right STA-MCA bypass dsCTA images show patent bypass and also help assessing the flow through it.
34	Case 11: Brain death A temporal resolution of 30 sec was obtained from the arrival of contrast in the carotid arteries. During this time span, there is no evidence of flow in the intracranial right ICA, intradural portions of both vertebral arteries and basilar artery. There is flow to a severely narrowed left supraclinoid ICA with no flow beyond this into the A1 and M1 segments. No venous flow is seen. Given the temporal resolution, the appearance is consistent with absent cerebral blood flow.
35	Pitfalls and limitations of dsCTA Spatial resolution of dsCTA is inferior to DSA. Inability to obtain selective vessel study thus there is contamination from surrounding vessels. Limitation of computational power with 23 volumes of data could be processed at a time. The single volume acquisition takes 1 second giving a temporal resolution of 1/sec, however by manipulating the raw data (1/2 or 1/5^th) better temporal resolution of 5/sec is possible.
36	Conclusion dsCTA adds another dimension to routine CTA enabling one to appreciate vascular flow characteristics over time and is therefore a valuable tool in depicting various neurovascular diseases allowing for precise localization and temporal resolution of these lesions.