What are the guidelines for using intravenous (IV) contrast for computed tomography (CT) scans in patients with suspected stroke, particularly those with impaired renal function?

IV Contrast for CT in Acute Stroke

In acute stroke evaluation, IV contrast should be administered for CT angiography (CTA) of the head and neck to rapidly detect large vessel occlusion (LVO), as this is the preferred and most time-sensitive imaging modality that directly impacts mortality and morbidity through enabling endovascular thrombectomy (EVT). 1

Primary Imaging Protocol

Non-Contrast CT First

• Always begin with non-contrast CT head to exclude hemorrhage before any contrast administration, as this is essential for determining thrombolytic eligibility and must be completed within 25 minutes of arrival 1, 2
• Non-contrast CT also identifies early ischemic changes using ASPECTS scoring, which guides treatment decisions for both IV thrombolysis and EVT 1

CTA Head and Neck With IV Contrast - The Standard of Care

• CTA of the head and neck is the most rapid means of detecting LVO and is strongly recommended as the initial vascular imaging study in acute stroke 1
• The American College of Radiology (ACR) 2024 guidelines emphasize that stroke due to LVO is a true medical emergency requiring the rapidity of diagnosis that only CTA provides 1
• CTA has high sensitivity and specificity for detecting intracranial LVO and is supported by multiple randomized controlled trials demonstrating improved outcomes with rapid LVO detection 1
• CTA of the neck provides critical information for endovascular surgical planning, including assessment of vascular tortuosity that directly impacts procedural success 1

Renal Function Considerations

Contrast Safety in Acute Stroke

• Do not delay CTA for renal function testing in acute stroke patients without known renal impairment, as the incidence of contrast-induced nephropathy (CIN) is extremely low (2-3%) in this population 3, 4
• Research demonstrates that prompt CTA/CTP imaging need not be delayed in patients with no history of renal impairment, even when baseline creatinine is unknown 3
• In a study of 162 acute stroke patients receiving 140 ml of contrast, only 2% developed CIN when appropriate fluid substitution was provided, and no patient required dialysis 4

Patients With Known Renal Impairment

• For patients with renal insufficiency or contrast allergy, use time-of-flight (TOF) MRA without contrast to identify arterial occlusions and guide therapeutic decisions 1
• The combination of TOF MRA and diffusion-weighted MRI (DWI) without IV contrast can identify patients eligible for EVT in the 6-24 hour window without contrast exposure 1
• However, recognize that MRI may delay treatment compared to CT, and this delay can worsen outcomes in the hyperacute setting 1

Metformin Management

• Stop metformin at the time of or prior to contrast administration in patients with eGFR 30-60 mL/min/1.73m², those with hepatic impairment, alcoholism, heart failure, or when intra-arterial contrast will be used 5
• Re-evaluate eGFR 48 hours after imaging before reinitiating metformin 5

CT Perfusion (CTP) With IV Contrast

Extended Window Imaging (6-24 Hours)

• CTP is indicated for determining EVT eligibility in the extended 6-24 hour window when clinical presentation time is uncertain or delayed 1
• Major randomized trials demonstrating EVT benefit in extended windows used either CTP or MR perfusion for patient selection 1
• The rapidity of CTP compared to MR perfusion makes it the preferred modality in most settings 1

Hyperacute Window (<6 Hours)

• CTP is generally not required in the hyperacute window when LVO is confirmed on CTA, as treatment should proceed immediately without perfusion imaging delays 1

Imaging Modalities NOT Recommended

Contrast-Enhanced CT Head

• There is no role for routine contrast-enhanced CT of the head (without angiography) in acute stroke evaluation 1

CT or MR Venography With Contrast

• CTV and MRV have no role in suspected ischemic stroke unless there is specific suspicion for cerebral venous thrombosis 1

MRI With Gadolinium Contrast

• Contrast-enhanced MRA of the head has no relevant literature supporting its use in acute ischemic stroke evaluation 1
• MRI with and without IV contrast may be helpful only in secondary workup when stroke mimics (brain tumors, other conditions) are suspected 1

Critical Pitfalls to Avoid

• Never delay IV thrombolysis while obtaining advanced imaging beyond non-contrast CT in eligible patients 2, 6
• Do not withhold CTA due to concerns about contrast nephropathy in patients without known severe renal impairment, as the risk is minimal and the benefit of rapid LVO detection is substantial 3, 4
• Avoid using MRI as the primary imaging modality in the hyperacute setting unless immediately available, as delays in LVO diagnosis directly worsen outcomes 1
• Ensure blood pressure is controlled below 185/110 mmHg before administering thrombolytics, but this does not preclude obtaining CTA 1, 2
• Do not proceed directly to catheter angiography without non-contrast CT first, except in highly selected cases with clear LVO indicators (e.g., hyperdense MCA sign with new atrial fibrillation) 1

Practical Algorithm

1. Non-contrast CT head immediately (within 25 minutes of arrival) 1, 2
2. If no hemorrhage: Proceed immediately to CTA head and neck 1
3. If LVO detected and <6 hours from onset: Proceed to EVT without perfusion imaging 1
4. If 6-24 hours from onset or unknown time: Add CTP to determine salvageable tissue 1
5. If renal insufficiency or contrast allergy: Use TOF MRA + DWI-MRI instead of CTA/CTP 1