WRITE A NOTE ON ROLE OF RADIOIMAGING IN MANAGEMENT OF STROKE ?
A 1 INTRODUCTION
1 objectives of imaging include prompt accurate diagnosis, treatment triage, prognosis prediction, and secondary preventative precautions.
2 Stroke is the fourth leading cause of death
3 Neuroimaging has become a critical tool in the evaluation and management of patients in whom acute ischemic stroke is suspected.
4 In addition to displaying anatomical structures, the latest neuroimaging techniques can elucidate the underlying hemodynamics and pathophysiology.
5 The goals of comprehensive imaging in patients with acute stroke are to provide prompt accurate diagnosis, facilitate triage, expand treatment cohort, optimize individual outcome, and strategize secondary precautions.
ROLE OF NEUROIMAGING
1 Ruling Out Intracranial Hemorrhage
A ) The initial step in the evaluation of patients with symptoms of acute stroke is to differentiate between hemorrhagic and ischemic stroke.
B ) Since intracranial hemorrhage is an absolute contraindication for reperfusion therapies, most stroke protocols begin with noncontrast head CT (NCHCT).
C ) NCHCT has been widely accepted as the standard method for the detection of acute intracranial hemorrhage
D ) Furthermore, T2*-weighted sequences have superior accuracy in the detection of small hemosiderin deposits from chronic microhemorrhages,which are often undetected on NCHCT due to insufficient signal contrast and limited spatial resolution.
2 In diagnosing ischemic stroke
A ) Early ischemic changes in NCHCT include loss of gray-white distinction, indistinct insular cortex and obscured basal ganglia, and hyperattenuated clot in the proximal vessels
B ) The hyperdense vessel sign is most specific but has low sensitivity.
C ) Multiple classification systems based on imaging features have been developed to offer reliable, reproducible grading of the extent of ischemic changes on NCHCT. One of them is the Alberta Stroke Program Early CT Score (ASPECTS), a 10-point scoring system
D ) Studies have shown that baseline ASPECTS correlates inversely with severity as assessed by the NIHSS within the first 3 hours of middle cerebral artery (MCA) stroke onset.
E ) ASPECTS of 7 or lower has been shown to predict poor functional outcome (78% sensitivity and 96% specificity) and symptomatic hemorrhage (90% sensitivity and 62% specificity)
F ) Diffusion-weighted imaging (DWI) is approximately 4 to 5 times more sensitive in detecting acute stroke than NCHCT
G ) Its sensitivity in detecting ischemia is reported as 99% with a high specificity of 92%
H ) Within minutes of vessel occlusion, failure of the sodium potassium pump leads to an influx of water from the extracellular space to the intracellular space.
I ) Cytotoxic edema restricts diffusion of water molecules and appears as increased signal intensity on DWI.
J ) About 95% of hyperacute infarcts are positive on DWI.
K ) DWI can detect acute brain infarction within 1 to 2 hours, while NCHCT may be negative for the first 24 to 36 hours.
L ) DWI can distinguish acute from chronic ischemia, thereby delineating new lesions even when located in proximity to prior ischemic injury
M ) Obscure lesions indiscernible on CT scans, such as lacunar infarcts, particularly those located in the posterior fossa, are better visualized on DWI
N ) Despite strong evidence supporting DWI as superior to NCHCT for confirming diagnosis of acute stroke within the first 24 hours, logistical issues limits its use in emergent setting. Most institutions find it challenging to obtain emergent MRI without delaying treatment.
3 In Determining Eligibility for IV tPA
A ) There is strong evidence supporting the use of IV tPA as a recanalization therapy to improve clinical outcomes when patients present within the standard 0- to 3-hour time window and the extended 3- to 4.5-hour time window.
B ) There is also strong evidence supporting the timely use of imaging to exclude hemorrhage in stroke patients before initiating IV thrombolytic therapy.
C ) Admission NCHCT is recommended prior to thrombolysis, because intracranial hemorrhage is an absolute contraindication to IV thrombolysis.
D ) Ischemia involving more than one-third of the MCA territory on images obtained within the 0- to 6-hour window constitutes a relative contraindication to IV thrombolysis
4 In Detecting Large Artery Occlusion
A ) Imaging of the intracranial and extracranial vessels is needed to determine whether an embolus/thrombus is present and whether it is amenable to endovascular revascularization.
B ) The outcome after endovascular therapy depends on the location of the thrombus, with better recanalization rates associated with more proximal thrombus and poorer outcomes with occlusion of the carotid terminus.
C ) Digital subtraction angiography (DSA) is considered the reference standard for detection of vascular occlusions and stenoses.
D ) CTA has reportedly high sensitivity (97%–100%) and specificity (98%–100%) for detecting intracranial occlusions and stenoses.
E ) CTA also performs well in characterizing extracranial occlusions and stenoses, with high sensitivity (95%–97%) and specificity (90%–99%).
F ) In addition to evaluating the degree of stenosis in the extracranial arteries, CTA provides valuable information of the plaque—such as morphology and composition.
G ) Magnetic resonance angiography (MRA) can also be used to detect and localize the occlusion.
H ) Threedimensional (3D) time-of-flight (TOF) MRA is the standard MR technique for the examination of intracranial vessels.
I ) TOF MRA is, however, especially susceptible to motion artifact and tends to overestimate the degree of stenosis.
J ) Traditional contrast-enhanced MRA (boluschase MRA) is the MR technique of choice for the study of the extracranial arteries. It is relatively independent of flow dynamics and is less susceptible to motion artifacts .
K ) Peak arterial enhancement is obtained by estimating the arrival of the contrast bolus. This technique is often limited by inaccurate timing of the bolus, which causes venous contamination.
L ) Newer time-resolved contrast-enhanced MRA repeatedly acquires images of a volume during the passage of the contrast material.
M ) Delineation of arterial vasculature from venous vasculature is more accurate.
N ) Additionally, the dynamic acquisition allows visualization of complex flow patterns
O ) The combined use of 3D TOF and contrast-enhanced MRA can improve diagnostic ability However, CTA has been shown to be slightly superior to MRA for this purpose, typically for distal vascular lesions
5 In Assessing Viable Ischemic Brain Tissue
A ) Following an arterial occlusion, brain tissue that cannot sustain the sudden drop in perfusion rapidly undergoes infarction.
B ) The surrounding hypoperfused tissue that can autoregulate to sustain metabolic needs remains viable
C ) The former is labeled “ischemic infarct core,” and the injury to this tissue is irreversible.
D ) The latter is known as “ischemic penumbra.” Differentiation of the ischemic core and ischemic penumbra relies on the tissue’s ability to carry out vascular autoregulation.
E ) The penumbra is in danger of proceeding to infarction, but represents potentially salvageable tissue if recanalization is achieved quickly
F ) The evolution of the penumbra is a dynamic process and can be indirectly visualized by various advanced imaging techniques.
G ) Perfusion-CT (PCT) can characterize cerebral perfusion by dynamically tracing an iodinated contrast bolus from its arrival to its departure
H ) PCT allows quantitative assessment of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT).
I ) PCT delineation of the ischemic core and penumbra is typically based on the concept of cerebral vascular autoregulation.
J ) In the penumbra, where autoregulation is preserved, MTT is prolonged, but CBV is maintained.
K ) Quantitatively, a relative increase in MTT of 145% most accurately represents the penumbra.
L ) Within the ischemic core, where autoregulation is lost, there is a matched reduction in CBF and CBV with prolonged MTT.
M ) A prospective multicenter study reported an absolute CBV < 2 mL/100 g optimally best delineated the ischemic core.
N ) However, more recent studies found relative CBF to be more predictive of the ischemic core
6 In establishing collaterals to check for any damage