What is the protocol for a CT (Computed Tomography) perfusion scan of the head in suspected acute ischemic stroke?

CT Perfusion Protocol for Acute Ischemic Stroke

The optimal CT perfusion protocol for suspected acute ischemic stroke includes non-contrast CT head followed immediately by CT angiography (CTA) of the head and neck, with CT perfusion (CTP) performed next, requiring an additional 45-60 seconds of scanning time and 40-50 mL of contrast per slab. 1

Initial Imaging Sequence

1. Non-contrast CT head

   • Essential first step to rule out hemorrhage and assess for early ischemic changes
   • Sections should be 5mm thick (preferred) but not thicker than 10mm
   • Scan plane should be parallel to the canthal meatal line
   • Should be performed no longer than 1 hour before initiating thrombolytic therapy 1

2. CT Angiography (CTA)

   • Acquired immediately after non-contrast CT
   • Coverage from vertex to aortic arch
   • Uses threshold-based triggering with approximately 105 mL of low-osmolar, nonionic contrast agent
   • Infused at 4 mL/s with a saline push power injector 1

3. CT Perfusion (CTP)

   • Performed immediately after CTA
   • Requires an additional 45-60 seconds of scanning time
   • Uses an additional 40-50 mL of contrast per slab
   • Small contrast bolus administered at 4-7 mL/s during continuous cine imaging
   • Imaging begins 5 seconds after start of infusion
   • Typically covers 2-4 cm per bolus (5 or 10mm thick slices) 1

Technical Parameters

• CTP imaging parameters: 80 kilovolts (peak) [kVp], 200 mA, 1-second rotation time
• At least one imaged slice must include a major intracranial artery for CTP map construction
• Scan plane angled along the superior orbital roof
• Some centers routinely obtain 2 slabs to double coverage (requiring additional 40 mL contrast) 1

Timing Considerations

• The entire multimodal CT protocol (non-contrast CT, CTA, and CTP) typically adds no more than 5 minutes to the time required for a standard head CT 1
• This protocol should not delay intravenous thrombolysis, which can be administered directly at the CT scanner after completion of the non-contrast CT 1
• Substituting CT perfusion for MRI in acute stroke evaluation can reduce imaging time by approximately 49% (from 158 minutes to 81 minutes) 2

Clinical Applications

• 0-6 hour window: CTP may not be necessary for clinical decision-making if CTA shows clear large vessel occlusion 1
• 6-24 hour window: CTP is recommended to determine eligibility for mechanical thrombectomy in patients with anterior circulation large vessel occlusion 1
• CTP helps identify salvageable tissue (penumbra) versus unsalvageable core infarct, especially in cases with unknown time of onset 3, 4

Contrast Safety Considerations

• Modern iodinated CT contrast agents have been shown not to worsen stroke outcomes 1
• Most centers use low or iso-osmolar contrast to minimize risk of contrast-induced nephropathy 1
• The incidence of contrast-induced nephropathy in acute stroke patients is relatively low (approximately 2.9%) 5
• Adequate pre-procedure and post-procedure hydration (up to 12 hours before and after contrast administration) is recommended when possible 1

Common Pitfalls and Limitations

• Technical failures occur in approximately 1% of cases 6
• Non-diagnostic CTP can result from lacunar infarction (10%), infarct outside slab coverage (8%), or reperfusion (0.7%) 6
• Technical adequacy improves significantly with experience (from 56% to 86% after 6 months of implementation) 6
• CTP has limited coverage compared to whole-brain imaging, though newer scanners provide improved coverage 1

By following this protocol, healthcare providers can rapidly assess patients with suspected acute ischemic stroke and make timely decisions regarding eligibility for reperfusion therapies, which is critical for improving patient outcomes.