Regarding the imaging characteristics and differential diagnosis of acute ischemic stroke on CT and MRI, all of the following are true EXCEPT:

Explanation

## Advanced Imaging Features in Acute Ischemic Stroke ### Early CT Hypodensity — The Fogging Effect **Key Point:** Non-contrast CT may appear falsely normal in hyperacute stroke due to limited gray-white differentiation. - **Hyperacute phase (0–6 hours):** Hypodensity often **not visible** on CT - **Acute phase (6–24 hours):** Hypodensity becomes apparent - **Fogging effect (1–2 weeks):** Infarcted tissue becomes **isodense** to normal brain due to: - Edema resolution - Petechial hemorrhage - Gliosis - Can **mimic normal brain** and delay diagnosis if clinical correlation is missed **Clinical Pearl:** This is why MRI (DWI) is preferred in the first 24 hours — it detects ischemia when CT appears normal. ### Hemorrhagic Transformation Detection **High-Yield:** Gradient echo (GRE) and susceptibility-weighted imaging (SWI) are exquisitely sensitive to blood products: - Detect **hemosiderin** and **deoxyhemoglobin** (paramagnetic) - More sensitive than conventional T2 or FLAIR - Essential for assessing **post-thrombolysis hemorrhage** ### CTA for Arterial Occlusion — The Trap **Warning:** The statement about coronal and sagittal reformats being **superior** to axial imaging is misleading. **Correct approach for CTA visualization:** - **Axial imaging** is the **primary and most sensitive** plane for detecting arterial occlusion - Thrombus appears as **hypodense** material within the artery on arterial phase - **Coronal and sagittal reformats** are **supplementary** — useful for: - Anatomical orientation - Assessing vessel course - Communicating findings to clinicians - **NOT superior** to axial for thrombus detection - **Maximum intensity projection (MIP)** reconstructions are often used for vessel visualization **Mnemonic:** **AXIAL FIRST** — Axial imaging is the standard for acute vascular imaging; reformats support but do not replace it. ### Comparison Table: Imaging Modalities in Acute Stroke | Modality | Best For | Timing | Limitation | | --- | --- | --- | --- | | **Non-contrast CT** | Rule out hemorrhage | Any time | Insensitive to early ischemia | | **CTA (axial)** | Arterial occlusion, thrombus | Hyperacute | Contrast-dependent | | **DWI-MRI** | Acute ischemia | Minutes–days | Contraindicated if metallic implants | | **GRE/SWI-MRI** | Hemorrhagic transformation | Any time | Sensitive to artifact | | **CTP** | Penumbra, extended window** | Any time | Radiation, contrast load | **High-Yield:** Option 4 incorrectly prioritizes coronal/sagittal over axial imaging for CTA — axial is the standard.