CT Angiography (CTA): Carotid, Aortic & Peripheral Runoff Protocols

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Neurovascular CT Angiography: Carotid & Intracranial Circulation

Computed tomography angiography (CTA) of the supra-aortic and intracranial vessels is a critical frontline imaging examination in emergent neurology. Rapid volumetric acquisition synchronizes submillimeter multidetector helical scanning with a tightly compacted intravenous iodinated contrast bolus, visualizing arterial lumina from the aortic root to the cortical pial branches at the skull vertex.

Indications & Stroke Protocol Integration

• Acute Ischemic Stroke: Detecting emergent Large Vessel Occlusions (LVO) in the proximal anterior circulation (internal carotid artery terminus, middle cerebral artery M1 and M2 segments, anterior cerebral artery A1 segment) and posterior circulation (vertebral arteries, basilar artery). Rapid vascular assessment helps the stroke team select patients under the applicable time, vessel, symptom and imaging criteria; an occlusion does not automatically establish thrombectomy eligibility.
• Extracranial Carotid Stenosis Staging: Quantifying atherosclerotic plaque burden and luminal narrowing to determine candidacy for carotid endarterectomy (CEA) or carotid artery stenting (CAS).
• Transient Ischemic Attack (TIA): Identifying unstable ulcerated plaques, arterial webs, or high-grade stenoses causing microembolization.
• Blunt Cerebrovascular Injury (BCVI) & Carotid Dissection: Traumatic intimal tears, dissections, and pseudoaneurysms resulting from hyperflexion, seatbelt sign, or basilar skull fractures.
• Intracranial Aneurysm Screening & Subarachnoid Hemorrhage: Detecting saccular (berry) aneurysms and defining dome-to-neck morphology for endovascular coiling versus neurosurgical clipping.
• Vascular Malformations: Arteriovenous malformations (AVMs), dural arteriovenous fistulas (dAVFs), and developmental venous anomalies.

Anatomical Coverage, Injection Dynamics & Saline Chaser Mechanics

• Scan Range: Helical acquisition begins at the aortic arch (using the localizer to encompass the takeoff of the brachiocephalic / innominate, left common carotid, and left subclavian arteries) and extends continuously superiorly through the skull vertex (caudocranial direction).
• Contrast Media Delivery: High-concentration non-ionic iodinated contrast ($350\text{ to }370\text{ mg I/mL}$) administered at a volume of $60\text{ to }80\text{ mL}$ through an 18- or 20-gauge antecubital intravenous catheter at a delivery rate of $4.0\text{ to }5.0\text{ mL/s}$.
• The Dual-Syringe Saline Chaser ($40\text{--}50\text{ mL}$ at $4.0\text{--}5.0\text{ mL/s}$):
  • Pushes the tail end of the contrast column out of the peripheral arm veins into the central circulation, utilizing the full contrast volume for arterial opacification.
  • Helps clear dense contrast from the superior vena cava, right atrium, and brachiocephalic (innominate) veins. Without a saline flush, hyperdense pooling ($>1500\text{ HU}$) in the right brachiocephalic vein produces severe beam hardening and photon starvation streak artifacts across the origin of the right common carotid and right subclavian arteries, obscuring vascular pathology at the aortic arch takeoff.
  • Prolongs and compacts the bolus geometry, generating a homogeneous contrast plateau ($>300\text{ HU}$) throughout the circle of Willis.
• Bolus Tracking Protocol: An automated Region of Interest (ROI) is positioned in the aortic arch or mid-common carotid artery. Scanning automatically triggers when contrast density reaches $100\text{ to }150\text{ HU}$, utilizing a brief transit delay ($3\text{ to }5\text{ seconds}$) to account for arterial flow into intracranial vessels.

NASCET Measurement Criteria for Internal Carotid Artery Stenosis

The North American Symptomatic Carotid Endarterectomy Trial (NASCET) measurement methodology is the internationally accepted standard for grading internal carotid artery (ICA) stenosis on CTA and digital subtraction angiography (DSA):

$$\text{Percent Stenosis} = \left[ 1 - \left( \frac{A}{B} \right) \right] \times 100\%$$

where:

• $A$ is the residual luminal diameter at the narrowest portion of the stenosis.
• $B$ is the normal luminal diameter of the internal carotid artery well distal to the bulb and post-stenotic dilation, where the vessel walls run parallel.

Clinical Severity Stratification:

• Mild Stenosis: $<50\%$
• Moderate Stenosis: $50\%\text{ to }69\%$
• Severe Stenosis: $70\%\text{ to }99\%$
• Complete Occlusion: $100\%$ (absence of patent lumen)
• Near-Occlusion ("String Sign"): Collapsed, thread-like distal ICA lumen resulting from critical flow reduction; surgical trials treat near-occlusion as a distinct entity where surgical benefit is limited.
• Contrast with the ECST Method: The European Carotid Surgery Trial (ECST) compares the residual lumen ($A$) against the estimated original luminal diameter of the carotid bulb at the site of pathology. Because the normal bulb is naturally dilated relative to the distal ICA, the ECST method yields a numerically higher percentage stenosis than NASCET for the exact same physical lumen ($70\%$ NASCET $\approx 82\%$ ECST).

Intracranial Aneurysms & Circle of Willis Anatomy

The Circle of Willis provides crucial collateral pathways between the anterior and posterior circulations. Saccular (berry) intracranial aneurysms arise at arterial bifurcations due to congenital internal elastic lamina defects and hemodynamic shear stress:

• Distribution of Saccular Aneurysms:
  • Anterior Communicating Artery (ACom): Most common site ($30\%\text{ to }35\%$).
  • Posterior Communicating Artery (PCom): Junction with internal carotid artery ($20\%\text{ to }25\%$); new third-nerve palsy with pupil involvement warrants urgent assessment, but does not itself prove imminent rupture.
  • Middle Cerebral Artery (MCA) Bifurcation: Main trunk division ($20\%$).
  • Basilar Artery Tip & Posterior Circulation: Basilar apex, PICA, and vertebral junctions ($10\%$).
• High-Resolution CTA Evaluation: Thin-slice ($0.625\text{ mm}$) multiplanar reconstructions, maximum intensity projections (MIP, $5\text{--}10\text{ mm}$ slabs), and 3D volume rendering show dome dimensions, neck width, dome-to-neck ratio, parent-vessel origin and mural calcification for specialist planning. One ratio does not mandate a particular procedure.

Cervical Carotid Dissection: Pathophysiology & Imaging Biomarkers

Carotid dissection occurs when a tear in the intima allows high-pressure arterial blood to enter the media, creating a false lumen that compresses the true lumen or ruptures the adventitia.

• Clinical Presentation: Unilateral neck or facial pain, headache, cerebral ischemia / TIA, and ipsilateral partial Horner syndrome (ptosis and miosis without anhidrosis, caused by compression of sympathetic fibers traveling along the internal carotid wall).
• Characteristic CT Signs:
  • Crescentic Intramural Hematoma: An eccentric, hyperattenuating (on unenhanced CT) or hypoattenuating (on contrast-enhanced CTA) crescent within the thickened arterial wall.
  • Eccentric Luminal Stenosis: Compression of the true lumen into a narrow crescent or circle.
  • Sparing of the Carotid Bulb: Atherosclerosis characteristically centers on the carotid bifurcation and bulb. In contrast, cervical dissections characteristically begin $1\text{ to }2\text{ cm}$ distal to the bulb, extending superiorly toward the petrous carotid canal.
  • The "Flame-Shaped" Occlusion: Smooth, tapered, concentric or eccentric tapering ending in total occlusion.
  • Intimal Flap & Double Lumen: Direct visualization of the thin linear filling defect separating true and false channels.
  • Dissecting Pseudoaneurysm: Outpouching of the false lumen beyond the normal vessel perimeter.

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Aortic CT Angiography: Thoracic & Abdominal Protocols

Acute aortic syndrome and dissection classification

Aortic dissection separates layers of the aortic wall and can create true and false lumens with an intimal flap. Trace the involved segments and branch vessels on multiplanar data. True and false lumen enhancement varies with timing and flow; morphology and continuity matter more than one brightness rule.

Intramural hematoma is blood within the aortic wall, often apparent as high-attenuation thickening on unenhanced CT. It is not always caused solely by vasa-vasorum rupture without an intimal defect. Penetrating atherosclerotic ulcer extends through an atherosclerotic plaque into the wall. The approved acute aortic protocol may include an unenhanced phase to assess wall blood and arterial CTA to assess lumen, branches and complications.

Stanford type A involves the ascending aorta, regardless of where the dissection began. Type B spares the ascending aorta and can involve the arch; it is not limited to dissections wholly distal to the left subclavian artery. The finding requires urgent communication. The clinical team determines operative, endovascular or medical management according to type, complications and patient circumstances.

In the DeBakey scheme, type I begins in the ascending aorta and extends beyond it; type II is confined to the ascending aorta; type III begins in the descending aorta, with IIIa confined above the diaphragm and IIIb extending below. Keep DeBakey III subtypes separate from endoleak III subtypes.

Electrocardiographic (ECG) Gating: Reducing Ascending Aorta Motion

During cardiac systole, the contraction of the left ventricle imparts vigorous rotational and translational motion to the aortic root. On non-gated helical thoracic CT scans, this cardiac pulsation produces severe blurring and double-contour artifacts across the ascending aorta that mimic a false intimal dissection flap (leading to false-positive surgeries) or obscure true tears:

• Prospective ECG Triggering (Step-and-Shoot): The scanner detects the R-wave of the patient's ECG and acquires data in a selected cardiac phase, often late diastole ($70\%\text{ to }75\%$ of the R-R interval) when the heart and aortic root are quiescent. Delivers substantial radiation dose savings compared to retrospective gating.
• Retrospective ECG Gating: Continuous helical acquisition throughout the entire cardiac cycle while simultaneously recording the ECG trace. Post-processing reconstructs images at selectable phases of the cycle (typically mid-to-late diastole) and enables dynamic cine evaluation of intimal flap mobility and aortic valve function.

Endoleak categories after endovascular repair

An endoleak is flow outside the graft lumen but within the treated aneurysm sac. Scan phases and reconstruction are selected to detect and localize the suspected leak while monitoring sac size. Not every follow-up requires the same phase count.

Types I and III can expose the sac to systemic pressure and require prompt specialist attention. Type II management depends on sac behavior and the complete assessment; do not assert that it is always low pressure or mandate intervention from one universal growth cutoff. A delayed phase may demonstrate slow flow missed on arterial data. Compare with prior sac measurements using a consistent method.

Reference: 2022 ACC/AHA aortic disease guideline.

Peripheral Lower Extremity CTA Runoff Protocols

Anatomical Coverage & Scanning Parameters

Peripheral CTA runoff evaluates severe peripheral artery disease (PAD), acute limb ischemia, bypass graft occlusion, and traumatic vascular injury. The scan volume extends continuously from the celiac axis / renal arteries ($T12\text{--}L1$) down to the plantar vascular arches of the toes, encompassing a z-axis anatomical length exceeding $1200\text{ to }1400\text{ mm}$.

• Collimation & Matrix: Thin slice collimation ($0.625\text{ to }1.25\text{ mm}$), reconstructed into axial, coronal, and curved planar reformations (CPR), alongside maximum intensity projections (MIP) that map the aortoiliac, femoropopliteal, and infrapopliteal (anterior tibial, posterior tibial, peroneal) runoff vessels.

Hemodynamic Mechanics: The Challenge of "Chasing the Bolus"

The fundamental technical difficulty in peripheral runoff CTA is synchronizing the table feed rate with the physiological transit speed of the contrast bolus through diseased, stenotic, or collateralized lower extremity vascular beds:

• The Risk of Outrunning the Bolus: In healthy individuals, arterial transit from the aorta to the feet requires $10\text{ to }15\text{ seconds}$. In elderly patients with critical limb ischemia, long-segment superficial femoral artery (SFA) occlusions, diabetic microangiopathy, or complex collateral bypasses, contrast transit can be delayed to $30\text{ to }40\text{ seconds}$. If the CT table moves too rapidly down the z-axis, the detectors "outrun" the advancing contrast front. The resulting distal images capture empty, unenhanced tibial vessels, falsely mimicking diffuse arterial occlusion.
• The Risk of Venous Contamination: If the table travels too slowly or a prolonged delay is used, contrast enters the capillary bed and opacifies the deep popliteal, tibial, and saphenous veins. Early venous return is accelerated by active cellulitis, ischemic ulcers, or arteriovenous shunting. Dense venous contrast superimposes on the tiny infrapopliteal arteries on MIP and 3D reformations, potentially impairing interpretation; inspect axial and selective thin-slab views.

Technical Strategies to Optimize Peripheral CTA

1. Contrast Volume & Flow Dynamics: A high-volume contrast bolus ($100\text{ to }120\text{ mL}$ of high-concentration iodinated contrast, $350\text{--}370\text{ mg I/mL}$) is injected at $4.0\text{ to }5.0\text{ mL/s}$, immediately followed by a $50\text{ mL}$ saline flush. The saline flush pushes the contrast column into the distal arterial tree, maintaining a continuous, elongated bolus geometry.
2. Variable Table Speed & Stepping Protocols: Some supported protocols use automated bolus-tracking triggers in the infrarenal aorta (triggering at $150\text{ HU}$), followed by a programmed variable table speed that accelerates through the wide, high-flow aortoiliac segment and deliberately decelerates across the thighs and calves to accommodate sluggish collateral runoff.
3. Test Bolus Transit Assessment: In patients with known critical limb ischemia or asymmetric pulse deficits, a low-volume test bolus ($15\text{--}20\text{ mL}$) can be monitored at the popliteal artery level to calculate the precise time-to-peak transit time, tailoring the helical delay to the patient's individual cardiovascular hemodynamics.

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A diameter calculation and intracranial landmarks

For a stipulated residual ICA lumen of 1.5 mm and normal distal reference lumen of 5.0 mm, NASCET stenosis is [1 − (1.5/5.0)] × 100 = 70%. Near-occlusion with distal collapse requires separate assessment rather than treating the collapsed reference as a normal denominator. Measure perpendicular to the vessel centerline and inspect source data because calcium blooming can distort the lumen.

Trace each intracranial ICA to the anterior and middle cerebral arteries. The anterior communicating artery connects the anterior circulation across the midline, while posterior communicating arteries link to posterior cerebral arteries arising from the basilar system. The basilar artery is formed by the vertebral arteries. Variants and hypoplastic segments are common; an incomplete circle is not automatically an acute occlusion. Evaluate continuity, caliber and enhancement on thin source images as well as MIP views.