Stroke. 2005;36:196-199
Published online before print January 6, 2005,
doi: 10.1161/01.STR.0000154559.03784.db
(Stroke. 2005;36:196.)
© 2005 American Heart Association, Inc.
Imaging
Jean-Claude Baron, MD
Steven Warach, MD
From the Department of Neurology (J.-C.B.), Cambridge University, Cambridge, UK; and the National Institute of Neurological Disorders and Stroke (S.W.), National Institutes of Health, Bethesda, Md.
Correspondence to Dr Jean-Claude Baron, Department of Neurology, Addenbrookes Hospital Hills Road, Box 83, Cambridge, CB2 2QQ, United Kingdom. E-mail jcb54{at}cam.ac.uk
Key Words: Advances in Stroke diffusion magnetic resonance imaging magnetic resonance imaging positron-emission tomography
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Acute Stroke
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The Melbourne group reported further application of the positron-emission
tomography (PET) hypoxia marker
18F-labeled fluoromisonidazole
(F-MISO).
13 In one article, they further developed and
validated their novel imaging methodology to map the penumbra
using this tracer.
1 Applying this method, they elegantly showed
that hypoxia affects white matter to a similar degree and extent
as gray matter, suggesting the former has at least as high a
resistance to ischemia than the latter and that its salvage
should help to maximize benefit of treatment.
2 In a third article,
3 they report that the impact of hypoxic tissue escaping infarction
on subsequent clinical recovery is similar whether the tissue
is identified within 12 hours of, or in the 12- and 48-hour
interval after stroke onset, documenting that F-MISO identifies
true penumbral tissue, and that, consistent with earlier evidence,
appropriate interventions should improve outcome even beyond
24 hours.
The year 2004 has seen the first, long-awaited, articles reporting direct PET and diffusion-weighted imaging (DWI)/perfusion-weighted imaging (PWI) comparisons.46 Using state-of-the-art diffusion tensor imaging (DTI) and fully quantitative PET as gold standard, Guadagno et al4 documented that the acute DWI lesion not only contains irreversibly damaged, but also penumbral tissue, in agreement with studies showing potential reversibility of the DWI lesion, while even severe apparent diffusion coefficient decreases can be found in either tissue category. One therapeutic implication is that a matched DWI/PWI lesion may still represent, at least in part, salvageable tissue. Comparing the predictive value of DWI and 11C-Flumazenil (FMZ) for final infarction, Heiss et al5 found that although both have similar overall predictive power (around 84% of the final infarct), false-positives occurred with DWI but not with FMZ, consistent with the Guadagno et al findings.4 Assessing the validity of PWI to assess the at-risk tissue by means of PET, Sobesky et al6 concluded that overall the simple DWI-PWI mismatch overestimates the penumbra, but the use of time-to-peak (TTP) delay maps helps toward solving this problem, with TTP delays >4 s being best suited. These results apply specifically to the TTP method of deriving magnetic resonance imaging (MRI) perfusion maps; other methods, such as mean transit time (MTT) maps, may be less prone to overestimate the region of symptomatic ischemia.7 Thijs et al8 found large variations in hypoperfusion lesion size with different arterial input function (AIF) locations used to derive MRI perfusion maps. They found that the AIF derived from the contralateral middle cerebral artery (MCA) gave ischemic volumes that most accurately predicted follow-up lesion volume.
The sensitivity of MRI relative to computed tomography (CT) has now been established for acute hemorrhage diagnosis in patients with focal stroke symptoms of less than 6 hours duration. Susceptibility-weighted MRI, most commonly the gradient-recalled echo (GRE) sequence, is used for that purpose. Fiebach et al9 found near perfect discrimination of hemorrhagic from ischemic stroke on MRI in a sample containing 62 cases of each, obtained in <6 hours: 100% sensitivity among experts; 95% sensitivity among medical students after a brief tutorial. Kidwell et al10 prospectively investigated a broad sample of 200 stroke patients, in which MRI followed by CT was obtained in <6 hours. The consensus of 4 experts independent, blinded reads found MRI superior for detecting any hemorrhage (because of MRI sensitivity to micro- and other chronic bleeds) and equivalent for acute hemorrhage, which was diagnosed by both modalities in 25 patients. There were 8 discrepant reads, 4 in either direction, for acute hemorrhage. Three of the discrepant cases of acute hemorrhage on CT were also diagnosed by MRI but classified incorrectly as chronic hemorrhage. However, 4 cases of acute hemorrhagic transformation on MRI were missed on CT. Smaller retrospective series have also reported cases of hemorrhagic transformation evident on susceptibility-weighted MRI but not CT following thrombolytic therapy,11,12 including cases where CT findings were equivocal because of residual angiography contrast.11
As evidence continues to confirm that prethrombolysis severity of clinical or MRI parameters predict outcome with recanalization, so does evidence that resolution of perfusion deficits is predictive of clinical recovery. Singer et al13 reported that greater amounts of at-risk tissue did not progress to infarct among patients who had recanalized relative to those who had not in a sample of 17; 80% of the MTT defect and 78% of the TTP
2 s delayed region did not progress to infarct on follow-up imaging. Chalela et al14 reported in a sample of 42 patients that resolution of at least 30% of the volume of MTT defect by 2 hours after standard IV tissue plasminogen activator treatment was associated with excellent clinical outcome (modified Rankin score of 0 or 1). This degree of early reperfusion was a stronger predictor of outcome than pretreatment clinical severity or the volume of pretreatment diffusion or hypoperfusion lesion. This 30% early reperfusion criterion associated with 90-day clinical recovery in thrombolytic therapy was confirmed in the Desmoteplase In Acute Stroke (DIAS) Trial,15 a randomized placebo controlled trial that found a similar dose positive response on both early reperfusion (using 30% or greater resolution of MTT volume) and excellent 90-day clinical outcome. Significant clinical benefits at the highest dose were observed when this thrombolytic therapy was initiated 3 to 9 hours from onset. The DIAS trial also illustrated that the simple diffusion-perfusion mismatch, although an overestimate of true penumbra, may effectively select the target population for intravenous thrombolytic trials beyond the 3-hour time window.
There is growing interest in understanding the potential role of tissue inflammation after stroke. Using single photon emission computed tomography (SPECT) and 111In-troponolate-labeled neutrophils, Price et al16 longitudinally studied cerebral neutrophil recruitment after MCA stroke. Significant neutrophil recruitment was demonstrated within 24 hours of onset and shown to attenuate over time. Neutrophil accumulation appeared to correlate significantly with infarct expansion. PET studies using the activated microglia-specific ligand 11C-PK11 195 are now awaited.
Three novel applications of MRI contrast material show promise for clinical application. Invasion of macrophages into the evolving infarction has been demonstrated in patients with T1-weighted MRI by Saleh and colleagues following injection of ultrasmall superparamagnetic iron oxide (USPIO) particles, which are taken up by macrophages 1 week after ischemic stroke.17 Increasing contrast, distinct from the pattern of parenchymal gadolinium enhancement, was found from 24 to 48 hours after injection, indicative of increasing accumulation of labeled macrophages.
Latour and colleagues18 identified gadolinium enhancement on fluid-attenuated inversion recovery MRI of the intrasulcal and other hemispheric CSF spaces indicative of blood brain barrier disruption <12 hours after onset of ischemia in acute stroke patients. This enhancement pattern was associated with reperfusion, risk of hemorrhagic transformation and worse clinical outcome and was exacerbated by treatment with thrombolytics, suggesting this marker may have utility in evaluating strategies to decrease hemorrhagic risk of thrombolytics.
Barber and colleagues19 demonstrated binding of a novel MRI contrast agent, gadolinium-DTPA-sLe(x) A, to activated endothelium in a mouse ischemic stroke model. Such targeted contrast agents might one day find clinical application in developing therapeutic strategies addressing inflammatory response to ischemia/reperfusion.
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Carotid Disease
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It is well established that in patients with symptomatic internal
carotid artery (ICA) occlusion the presence of misery perfusion
or severely impaired vasodilatory reserve considerably increases
the risk of subsequent ipsilateral stroke, justifying ongoing
trials of extracranial/intracranial bypass on selected patients.
Yamauchi et al
20 found evidence that by resetting the oxygen
needs of the tissue the occurrence of cortical metabolic depression
(secondary to diaschisis or selective neuronal damage in patients
with striatocapsular infarction) might "mask" misery perfusion
in ICA disease, a confounder that will be important to consider
in future studies. In the same vein, Kuroda et al
21 found that
patients with ICA disease and reduced cerebral blood flow but
normal vasodilatory reserve had cortical metabolic depression
and proportionally reduced cortical FMZ binding, suggesting
selective neuronal damage. Thus, cortical metabolic depression
may afford protection from further ischemic events distal to
ICA disease.
In 2002, 18F-2-fluorodeoxyglucosePET was shown to be able to detect inflammation within carotid plaques in vivo. This year, a novel tracer to detect plaque inflammation was reported in a preliminary form. Kietselaer et al22 used SPECT and 99mTc-annexin A5 to label apoptotic cells. They report increased uptake in symptomatic carotid bifurcations subsequently shown to exhibit evidence of plaque instability at histology, whereas stable plaques did not show increased uptake before endarterectomy. This type of approach may allow in the future the detection of patients most at risk of ischemic event perhaps independently of degree of stenosis. Using USPIO-enhanced MRI with histological correlation, Trivedi et al23 found areas of signal intensity reduction within the plaque in 7 of 8 symptomatic patients, corresponding to USPIO particles accumulation in macrophages, with imaging being optimal 24 to 36 hours after contrast infusion.
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Plasticity
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A substantial number of functional MRI (fMRI) studies have addressed
the neural processes underlying motor recovery after stroke.
2433 Although difficult to achieve, several longitudinal studies
assessed both clinical recovery and fMRI patterns over time.
2429 Expanding on earlier studies, Ward et al
25 showed that as recovery
proceeds, there is correlated decrease in the amount of activation
in widespread motor areas bilaterally, indicating less neural
recruitment needed to perform the same task over time. Across
patients, the amount of activation in these areas correlated
with the severity of motor deficit at each time point, but a
few cortical areas showed a significant change in this relationship,
29 suggesting different rehabilitation strategies might be required
as recovery proceeds. Excessive contralesional M1 activation
is present during hand movement from the early stages after
stroke,
26,30 but does not appear to contribute directly (ie,
via the uncrossed corticospinal tract) to recovery of affected
hand as shown by single-pulse transcranial magnetic stimulation
applied in the same patients;
30 it might be part of a widespread
top-down recruitment in an effort by the stroked brain to perform
the task. In parallel with recovery under regular physiotherapy
however, the activation pattern tends to return toward a more
ipsilesional, ie, physiological, pattern.
26 Consistent with
this, intensive gait training is associated with shifts of activation
toward the ipsilesional hemisphere, which correlate with the
amount of gait recovery.
31 In a randomized controlled trial,
Luft et al
32 found that relative to regular physiotherapy, additional
bilateral arm training was associated with increased activation
of bilateral motor areas, more so contralesionally, during affected
elbow flexion-extension movements. This suggests that contralesional
M1 activation may be useful for enhanced motor function after
stroke, which may however relate to the proximal limb movement
or the bilateral training used, or more likely to the fact that
the patients were all severely affected. Studying patients with
sensorimotor cortex infarcts during tactile exploration, Binkofski
and Seitz
26 found foci of activation in the cortex adjacent
to the infarcted area as early as a few days from stroke, consistent
with earlier studies indicating that survival of the peri-infarct
penumbra offers opportunities for cortical map reorganization.
In addition to fMRI, more studies using DTI in stroke are beginning to appear.34,35 Fractional anisotropy mapping was used to assess the presence and severity of corticospinal tract disruption, be it by direct damage33 or secondary Wallerian degeneration.34 Combining fractional anisotropy mapping or full DTI-derived tractography with fMRI should help to better understand the mechanisms underlying recovery. New methods to map progressive focal or extensive atrophy following stroke36 may also find similar applications.
Received December 14, 2004;
accepted December 14, 2004.
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