Chapter 3 Acute Stroke Imaging
There was a time, not too long ago, when acute brain imaging in patients with suspected stroke was thought to be useful only to exclude hemorrhage or obvious stroke mimickers, such as tumors. The introduction of effective acute stroke therapies changed this conception completely, however. Today emergency brain imaging is essential for the management of acute stroke patients. We have learned that computed tomography (CT) scans can offer valuable information even when obtained within the first few hours of the ischemic event (dispelling the notion that CT scans are not useful for ischemic strokes until 1 or 2 days after onset). New CT-based protocols, including CT perfusion (CTP) scans and CT angiograms, are rapidly gaining ground in clinical practice. Diffusion-weighted and perfusion-weighted (DWI and PWI) magnetic resonance imaging (MRI) provide the ability to depict the penumbra and promise expansion of the therapeutic window for vessel opening on individual cases based on the subsistence of salvageable tissue. Conventional angiography has been transformed from a purely diagnostic test into a means for therapeutic intervention. Even transcranial Doppler may have an important role in the emergent evaluation and management of acute ischemic stroke, providing proof of large intracranial vessel occlusion and possibly improving the chances of recanalization with thrombolysis when continuous insonation is employed.
The uses of various neuroimaging techniques in acute stroke are multiple and continue to expand. The most common current indications and purposes of acute neuroimaging in stroke patients are listed in Table 3-1.
Indication/purpose | Imaging modality |
---|---|
Confirmation of diagnosis (TIA vs. stroke vs. stroke mimics) | CT/MRI |
Differentiation of ischemia vs. hemorrhage | CT/MRI |
Visualization of established infarction (as contraindication for thrombolysis) | CT |
Localization of ischemia/stroke pattern (which may guide evaluation of stroke mechanism) | CT/MRI |
Evaluation of penumbra (which may extend therapeutic window for acute revascularization) | DWI-PWI/CTP |
Identification of early prognostic markers (e.g., HDMCA sign, extensive high ASPECTS score, large volume of DWI restriction) | CT/MRI |
Visualization of arterial site of occlusion | MRA/CTA/catheter angiography |
Documentation of recanalization | MRA/CTA/catheter angiography/TCD |
US-assisted intravenous thrombolysis | TCD |
Access and information to make endovascular treatment possible | Catheter angiography |
ASPECTS, Alberta Stroke Program Early CT Score; CT, computed tomography; CTA, CT angiography; CTP, CT perfusion; DWI, diffusion-weighted imaging; HDMCA, hyperdense middle cerebral artery; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging; TCD, transcranial Doppler; TIA, transient ischemic attack; US, ultrasound.
COMPUTED TOMOGRAPHY
CT Signs of Acute Ischemia
A 50-year-old man with a history of hypertension and rapid palpitations presented to the emergency department with acute left hemiparesis. Neurological examination showed right gaze preference, left homonymous hemianopia, left hemiparesis, and left hemineglect. Initial CT scan obtained 5 hours and 20 minutes after symptom onset revealed early signs of edema and infarction throughout the territory of the right middle cerebral artery (Figure 3-1, upper row). Because of the presence of these radiological findings, endovascular revascularization treatments were not attempted. On Day 3, he was more somnolent, and a repeat CT scan showed spontaneous hemorrhage in the area of infarction (Figure 3-1, lower row). The patient was diagnosed with atrial fibrillation, and anticoagulation was subsequently started for secondary stroke prevention. He survived his stroke but remained moderately disabled.
Sign | Significance |
---|---|
Hyperdense vessel sign | Intraluminal thrombus |
Loss of insular ribbon | Focal tissue edema |
Obscuration of the lenticular nucleus | Focal tissue edema |
Loss of gray–white matter distinction | Focal tissue edema |
Sulcal effacement | Focal tissue edema |
Areas of hypoattenuation | Tissue infarction |
CT Perfusion
CT perfusion |
Easier access |
Rapid acquisition of images |
Robust quantitative physiological measurements |
Feasible in patients with contraindication for MRI |
DWI-PWI MRI |
May be easier to visualize the penumbra |
Depiction of cellular edema |
Greater spatial resolution (whole brain imaging) |
Does not require iodine contrast |
CT, computed tomography; DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging.
CT Angiogram
MAGNETIC RESONANCE IMAGING
Furthermore, MRI (with DWI and susceptibility weighted sequence) has been proved superior to CT scanning for the detection of acute ischemia and chronic hemorrhage and at least comparable to CT for the diagnosis of acute hemorrhage.36 Thus solid arguments support the use of MRI as the primary imaging modality for the emergency evaluation of acute stroke patients if the study can be performed without delay.
Diffusion-Weighted and Perfusion-Weighted Imaging
Hyperacute and acute diagnostic confirmation of ischemic stroke |
Differentiation of acute vs. subacute vs. chronic ischemic lesions |
Assessment of ischemic penumbra (in combination with PWI) |
Acute differential diagnosis between TIA and minor stroke with reversible neurological deficits |
Distinction of cytotoxic and vasogenic edema (in conditions such as eclampsia or hyperperfusion syndrome) |
Identification of patients at risk of severe reperfusion hemorrhage |
DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging; TIA, transient ischemic attack.