CTA Head and Neck: Choosing the Trigger, Direction, and Response to Motion

• ROI in the aortic arch (or ascending aorta, depending on the level at which the arch is best seen on the scout). It is the largest, most reliably opacified structure in the path of the bolus and gives the earliest clean signal that arterial filling has begun.
• Trigger threshold around 100-150 HU, with a short post-trigger delay of roughly 4-6 seconds so that the column has time to travel from the arch to the skull base by the time the table reaches the cervical vessels. A straight 150 HU trigger with no delay tends to arrive early at the arch and late at the bifurcation.
• Why not the carotid? It is small, moves, and may not be in the plane during the monitoring phase; the ROI will drop off the vessel and the trigger fires on noise or on nothing.
• Why not the internal jugular? It opacifies on the venous return, so triggering on it guarantees a venous-timed acquisition with poor arterial contrast and heavy perivenous artifact.
• Note the irregular rhythm: the arterial curve will be less predictable, so keep the monitoring interval short (~1 s) and the post-trigger delay modest rather than long.

• Scan caudocranial, from the top of the aortic arch to the vertex.
• Why: the left brachiocephalic vein and superior vena cava carry dense contrast at the moment the arch opacifies. Scanning the thoracic inlet early - before venous return has built up in the neck - keeps the streak and perivenous artifact out of the vessel origins. If you scan craniocaudally you arrive at the arch last, with the veins maximally opacified and the arteries already washing out.
• Coverage: superior margin of the arch through the vertex, so that the great vessel origins, the carotid bifurcations (usually near C4), the vertebral arteries along their entire cervical course, and the circle of Willis are all included. A study that stops at C1 cannot answer a posterior circulation question.
• Technique: thin collimation (0.6-0.75 mm), pitch near 0.9-1.0, reconstructions at ~1 mm axial plus coronal and sagittal MPRs and MIPs of the neck and circle of Willis. A saline chaser at the same flow rate reduces artifact and clears the vein.
• Because his symptoms were right-sided, the left hemisphere (left ICA/MCA territory) is the side of interest - make sure the reconstructions of the left carotid siphon and left MCA are of good quality, not lost to artifact.

Do not reflexively re-scan the whole volume - a repeat is a full additional dose and needs a reason.

1. Judge diagnostic adequacy first. Motion at the thoracic inlet that obscures the vertebral origins may not matter if the intracranial vessels and the bifurcations - the territories that explain his symptoms - are clean. Review before deciding.
2. Fix the cause before the repeat. Coach him: arms down and still, no swallowing during the acquisition, a slow breath-hold if he can tolerate it, and a pillow under the knees to unload the back. Consider a faster gantry rotation and a reduced overall scan time.
3. Repeat only the degraded segment if the scanner and protocol permit a limited second acquisition, rather than re-injecting and re-scanning the entire arch-to-vertex length.
4. If a second contrast injection is required, recognize that the additional volume matters in a patient with a borderline renal profile; discuss with the radiologist, and consider whether the motion-affected portion is essential to the clinical question.
5. Explain the delay honestly - patients who understand why they must hold still cooperate better than patients who are told to 'just hold still.'