Imaging Flashcards
Ddx: Brain Regional lesions
Dominant:
Lateral temporal cortex
Nondominant:
Parietal lobe
Dominant Lateral Temporal Cortex
Semantics: loss of the sense of what features make an animal distinct from other animals (ie, what features make a dog different from an elephant and a camel different from an owl)
Nondominant Parietal Lobe
Visuospatial: Draw a geometric shape accurately
CT
- CT is usually the first investigation and is mainly done to rule out hemorrhage.
- Acute infarcts are typically seen on CT scan six to 12 hours after onset of stroke symptoms.
Dense MCA sign
CT can sometimes show thromboembolic infarcts in the proximal MCA
Insular ribbon sign
Another subtle early sign of acute MCA infarct
hemorrhagic transformation
Infarcts can sometimes undergo hemorrhagic transformation, which can be life-threatening. Thrombolytic therapy for clot lysis increases the risk of hemorrhagic transformation. Currently intravenous tPA is given within three hours after onset of stroke symptoms for clot lysis if there is no evidence of intracranial hemorrhage on the noncontrast head CT. The ECASS-III trial found that intravenous tPA can be safely given up to four-and-a-half hours. However, use beyond three hours is not FDA approved. Once patients are outside of these time frames, the risk of hemorrhage is thought to outweigh the benefits of thrombolysis.
CT angiography (CTA)
Evaluates the intracranial vasculature and can detect intraluminal thrombus. Involves radiation exposure and may be obscured by artifact from calcium in atherosclerotic plaques. Can be performed at the same time as brain imaging (CT or MRI) to exclude occlusion.
Magnetic resonance angiography (MRA)
A noninvasive substitute for digital subtraction angiography and can detect extracranial carotid disease with high sensitivity and specificity. Can be performed at the same time as brain imaging (CT or MRI) to exclude occlusion.
Diffusion weighted imaging (DWI)[MRI]:
- DWI on MRI is the most sensitive imaging modality for detection of acute ischemia.
- The cytotoxic edema that results from an acute infarct results in restricted diffusion, which will appear bright on DWI.
- Diffusion weighted images (DWI) on MRI, can detect acute ischemia within 15 to 30 minutes of onset of symptoms.
Water molecules in healthy tissue are in constant random diffusion (Brownian motion). Diffusion weighted imaging (DWI) uses gradients to detect increases or decreases in the randomness, velocity, and direction of this diffusion. Most diffusion of water occurs in the extracellular space; the diffusion of water within cells is more restricted by the dense protein, glycans, etc., within healthy cells. When a cell dies, either by apoptosis or necrosis, one of the first events is loss of cell membrane pumps (e.g. Na-K pump). This results in swelling of the cell, and water moves from the extracellular space (relatively free diffusion) to the intracellular space (restricted diffusion). This net restriction of the ability of water to diffuse freely can be measured using DWI imaging, which helps very much to distinguish areas where cell death is occurring. Areas in the brain that demonstrate restricted diffusion will appear bright on DWI (indicating pathology). Areas showing restricted diffusion will appear bright on DWI (indicating pathology). DWI has revolutionized stroke imaging. Restricted diffusion in infarcts is seen as early as 20 minutes and usually lasts about 10 days.
Apparent diffusion coefficient (ADC) is a measure of the magnitude of diffusion (of water molecules) within tissue, and is commonly clinically calculated using MRI with diffusion weighted imaging (DWI)
FLAIR images:
FLuid-Attenuated Inversion-Recovery. We use these sequences to look for edema. FLAIR is the single most important sequence for the brain (other than to show hyperacute infarct, which is seen in as soon as 20 minutes on a DWI scan).
On these images, fluid such as edema in the brain will still appear bright. However, the bright CSF signal in the ventricles and subarachnoid spaces is saturated out and those spaces are darkened.
Gradient Recalled Echo (GRE):
Paramagnetic and diamagnetic substances (i.e., metal, calcification, most blood products except hyperacute blood ) cause loss of signal locally on this sequence, which blooms, making it easier to identify them.
GRE on MRI is very sensitive for detection of blood products rather than acute ischemia.
PET CT of the brain
Is mainly used for oncology applications such as differentiation of tumor recurrence from radiation necrosis following treatment of neoplasm in the brain with radiation therapy; or determining prognosis after treatment.
Catheter angiography
The gold standard for detection of vasospasm.
MR Venogram
Venous Infarcts: Caused by dural venous sinus thrombosis and seen in pregnancy, puerperium, dehydration, procoagulant states, and penetrating trauma. It almost always is hemorrhagic and does not conform to arterial territories. The GRE sequence of MRI best shows hemorrhages. MR Venogram can demonstrate the thrombosis non-invasively.
