Imaging in cerebrovascular disease

Characterizing spatiotemporal progression and prediction of infarct lesion volumes in experimental acute ischemia using quantitative perfusion and diffusion imaging

PURPOSE

This study was conducted to explore the diagnostic value of arterial spin labeling (ASL) combined with diffusion weighted imaging (DWI) in characterizing the spatiotemporal progression of infarct lesions in a rabbit middle cerebral artery occlusion (MCAO) model and predicting the acute cerebral infarction (ACI) volume.

MATERIALS AND METHODS

Forty-two male rabbits (2.9 ± 0.2 kg body weight) were used in this experimental study. Animals were initially anesthetized by intravenous injection of uratan. There were seven experimental groups with six rabbits in each group. The apparent diffusion coefficient (ADC) and cerebral blood flow (CBF) thresholds were established in the control group (n = 6), which were sacrificed at 12 h, stained for infarct volume, and imaged at each time point.

RESULTS

The normal ADC and CBF were estimated as 0.90 ± 0.03 × 10^-3 mm²/s and 0.68 ± 0.06 mL g^-1 min^-1, respectively. The viability thresholds of ADC and CBF yielding the lesion volumes (LVs) at 3 h, which best approximated the 2,3,5-triphenyltetrazolium chloride (TTC) infarct volumes at 12 h, were 0.52 ± 0.02 × 10^-3 mm²/s (42.2 ± 3% reduction) and 0.33 ± 0.09 mL g^-1 min^-1 (51.0 ± 11% reduction), respectively. The temporal evolution of the ADC- and CBF-defined LVs showed a significant perfusion/diffusion mismatch up to 1 h (p = 0.001).

CONCLUSION

ADC values and ACI volumes were positively correlated, while CBF was negatively correlated, which is supposed to be a reference for predicting ACI volume.

In vivo two-photon excited fluorescence microscopy reveals cardiac- and respiration-dependent pulsatile blood flow in cortical blood vessels in mice

Subtle alterations in cerebral blood flow can impact the health and function of brain cells and are linked to cognitive decline and dementia. To understand hemodynamics in the three-dimensional vascular network of the cerebral cortex, we applied two-photon excited fluorescence microscopy to measure the motion of red blood cells (RBCs) in individual microvessels throughout the vascular hierarchy in anesthetized mice. To resolve heartbeat- and respiration-dependent flow dynamics, we simultaneously recorded the electrocardiogram and respiratory waveform. We found that centerline RBC speed decreased with decreasing vessel diameter in arterioles, slowed further through the capillary bed, and then increased with increasing vessel diameter in venules. RBC flow was pulsatile in nearly all cortical vessels, including capillaries and venules. Heartbeat-induced speed modulation decreased through the vascular network, while the delay between heartbeat and the time of maximum speed increased. Capillary tube hematocrit was 0.21 and did not vary with centerline RBC speed or topological position. Spatial RBC flow profiles in surface vessels were blunted compared with a parabola and could be measured at vascular junctions. Finally, we observed a transient decrease in RBC speed in surface vessels before inspiration. In conclusion, we developed an approach to study detailed characteristics of RBC flow in the three-dimensional cortical vasculature, including quantification of fluctuations in centerline RBC speed due to cardiac and respiratory rhythms and flow profile measurements. These methods and the quantitative data on basal cerebral hemodynamics open the door to studies of the normal and diseased-state cerebral microcirculation.

Therapeutic hypothermia for stroke: do new outfits change an old friend?

Clinically significant neuroprotection for the brain continues to be an elusive quest. All attempts at developing effective pharmacologic agents have failed in clinical trials. Hypothermia has been thought to confer protection after brain injury for many years, but has recently regained interest as a neuroprotectant for focal ischemic stroke in the basic science and clinical literature. The failure to develop safe and efficacious pharmacologic agents along with promising clinical data on the efficacy of hypothermia for cardiac arrest patients have raised a great interest in hypothermia as a neuroprotectant for ischemic stroke. As a clinically meaningful neuroprotectant for stroke, hypothermia confers several theoretical advantages over pharmacologic agents. A major problem with neuroprotectant therapy is instituting therapy within a narrow time window. This obstacle may be easier for hypothermia to overcome as emergency medical service personnel can theoretically initiate it in the field. Additionally, pharmacologic agents are usually restricted to one aspect of the pathophysiologic cascade triggered by focal ischemia, whereas hypothermia acts on several of these pathways simultaneously. The recent advances and future directions in the utilization of hypothermia as a potential therapy for focal ischemic stroke are reviewed.

Xenon CT cerebral blood flow in acute stroke

Acute stroke therapy is evolving rapidly as research moves toward extending the time window for treatment so that more patients can benefit. As physiology-based imaging increasingly is used in patient selection, it is becoming evident that rigid time windows are not applicable to individual patients. Xenon CT has an important role in acute stroke therapeutic intervention as a quantitative, reproducible, rapid, and safe modality, which can provide valuable physiologic data that can optimize patient triage and aid in management.

Modern Emergent Stroke Imaging: Pearls, Protocols, and Pitfalls

Stroke remains one of the most important clinical diagnoses for which patients are referred to the radiologist for emergent imaging. Timely and accurate imaging guides admission from the emergency department or transfer to a hospital with a dedicated stroke service, triage to the intensive care unit, anticoagulation, thrombolysis, and many other forms of treatment and management. It is important to approach each patient's imaging needs logically and tailor each work-up, and constantly to review the entire process for potential improvements. Time saved in getting an accurate diagnosis of stroke may indeed decrease morbidity and mortality. This article discusses the current management of stroke imaging and reviews the relevant literature.

Brain tissue salvage in acute stroke

Thrombolysis is the only effective medical therapy of ultra-acute (<3 hours) cerebral ischemia, and it is moving from academic centers to community-based standard therapy in experienced centers. Despite intensive experimental and clinical research, the salvage of brain cells through a host of neuroprotective strategies has not been demonstrated to be efficient. As the imaging and other patient selection methods continue to develop, it may be possible eventually to identify patients who still have viable penumbral brain tissue even after the 3-h window. This review focuses on the possibilities of salvaging acutely ischemic brain tissue and potential reasons for differences in the efficacies of the thrombolytic and neuroprotective therapies.

The resistance to ischemia of white and gray matter after stroke

A contributing factor to the failure of trials of neuroprotectants in acute ischemic stroke may be the differing vulnerability to ischemia of white compared with gray matter. To address this issue, we determined to establish the existence of potentially viable tissue in white matter and its evolution to infarction or salvage in both gray and white matter compartments in patients with ischemic stroke. Twenty-seven patients (mean age, 73.4 years) at a median of 16.5 hours after symptom onset were studied using the hypoxic marker 18F-misonidazole with positron emission tomography (PET). Tissue was segmented using an magnetic resonance probabilistic map. Although there was a greater volume of initially "at-risk tissue" in gray matter (58.3 cm3, 29.9-93.0 cm3 than white matter (42.0 cm3, 15.8-74.0 cm3; p <0.001) at the time of PET imaging, a higher proportion of this was still potentially viable in white matter (41.4%, 4.6-74.5%) than in gray matter (23.6%, 3.2-61.1%; p <0.05). However, a similar proportion in each compartment spontaneously survived. These data provide evidence for the existence of potentially salvageable tissue in human white matter and is consistent with it having a similar or even greater resistance to ischemia than gray matter. For the latter possibility, alternative therapeutic strategies may be required for its salvage.

Stroke: imaging and differential diagnosis

Structural and vascular imaging helps to differentiate haemorrhagic from acute ischemic stroke (AIS) and rule out non-stroke causes, as well as identify specific subtypes of stroke such as carotid dissection and venous thrombosis. However, it is negative in most AIS patients within 3-6 hrs of onset and thus does not allow efficient patient classification for management purposes. Physiologic neuroimaging with PET, SPECT and combined diffusion- and perfusion-weighted MR gives access to tissue perfusion and cell function/homeostasis. It has near 100% sensitivity in AIS, even in small cortical or brainstem strokes. In middle-cerebral artery (MCA) stroke, physiologic imaging also allows pathophysiological differentiation into four tissue subtypes: i) already irreversibly damaged ("core"); ii) severely hypoperfused ("penumbra"), which represents the main target for therapy; iii) mildly hypoperfused ("oligaemia"), not at risk of infarction unless secondary complications arise; and iv) reperfused/hyperperfused. PET studies have evidenced the penumbra in man, shown its largely cortical topography, documented its anticipated impact on both acute-stage neurological deficit and recovery therefrom, and shown its persistence up to 16 hrs after stroke onset in some patients. However, some patients acutely exhibit extensive irreversible damage, which places them at considerable risk of malignant MCA infarction, and others early spontaneous reperfusion, which is almost invariably associated with rapid and complete recovery. Thrombolytics and/or neuroprotective agents would therefore be expected to benefit, and hence should ideally be reserved to, only those patients in whom a substantial penumbra is documented by physiologic neuroimaging, even perhaps beyond the 3 to 6 hrs rule. In addition, excluding from thrombolytic therapy those patients with substantial necrotic core should avoid many instances of symptomatic haemorrhagic transformations. Finally, patients with extensive core might benefit from early decompressive surgery, and those with early extensive reperfusion from anti-inflammatory agents. Overall, therefore, the pathophysiologic heterogeneity underlying AIS may account for both the complications from thrombolysis and the limited success of clinical trials of neuroprotective agents, despite apparent benefit in the laboratory. Pathophysiological diagnosis as afforded by neuroimaging should now be incorporated in the design of clinical trials as well as in the routine management of stroke.

Inflammation and infection in clinical stroke

Stroke has enormous clinical, social, and economic implications, and demands a significant effort from both basic and clinical science in the search for successful therapies. Atherosclerosis, the pathologic process underlying most coronary artery disease and the majority of ischemic stroke in humans, is an inflammatory process. Complex interactions occur between the classic risk factors for atherosclerosis and its clinical consequences. These interactions appear to involve inflammatory mechanisms both in the periphery and in the CNS. Central nervous system inflammation is important in the pathophysiologic processes occurring after the onset of cerebral ischemia in ischemic stroke, subarachnoid hemorrhage, and head injury. In addition, inflammation in the CNS or in the periphery may be a risk factor for the initial development of cerebral ischemia. Peripheral infection and inflammatory processes are likely to be important in this respect. Thus, it appears that inflammation may be important both before, in predisposing to a stroke, and afterwards, where it is important in the mechanisms of cerebral injury and repair. Inflammation is mediated by both molecular components, including cytokines, and cellular components, such as leukocytes and microglia, many of which possess pro- and/or antiinflammatory properties, with harmful or beneficial effects. Classic acute-phase reactants and body temperature are also modified in stroke, and may be useful in the prediction of events, outcome, and as therapeutic targets. New imaging techniques are important clinically because they facilitate dynamic evaluation of tissue damage in relation to outcome. Inflammatory conditions such as giant cell arteritis and systemic lupus erythematosus predispose to stroke, as do a range of acute and chronic infections, principally respiratory. Diverse mechanisms have been proposed to account for inflammation and infection-associated stroke, ranging from classic risk factors to disturbances of the immune and coagulation systems. Considerable opportunities therefore exist for the development of novel therapies. It seems likely that drugs currently used in the treatment of stroke, such as aspirin, statins, and modulators of the renin-angiotensin-aldosterone system, act at least partly via antiinflammatory mechanisms. Newer approaches have included antimicrobial and antileukocyte strategies. One of the most promising avenues may be the use of cytokine antagonism, for example, interleukin-1 receptor antagonist.

Imaging the cerebrovascular tree in the cadaveric head for planning surgical strategy

OBJECTIVE

The objective of the study was to identify whether an integration of cadaveric dissections with preoperative imaging information may enable a better understanding of pathological anatomy, especially vascular lesions, and thus allow for greater precision in surgical planning.

METHODS

We selected a computed tomographic contrast agent and experimentally determined the proportion of it that could mix compatibly with the silicone compound. The resultant mixture was injected into the cerebrovascular systems of six fresh human cadaveric heads. The specimens underwent computed tomography for the purpose of digital virtual exposures in parallel with laboratory dissections performed on these specimens.

RESULTS

The 1:8 ratio of contrast agent to silicone rubber was determined to be appropriate for both computed tomography and subsequent laboratory dissection of the specimens. The blood vessels in computed tomographic scans demonstrated a higher attenuation than surrounding soft tissues. The opacity consistency of the injected vessels was a critical parameter for a clear three-dimensional rendering of the vascular structures in the natural surroundings of the skull base. Static and dynamic three-dimensional images of the cadaveric vascular tree were obtained as viewed through surgical corridors of various skull base approaches.

CONCLUSION

We demonstrated a new cadaveric preparation model for imaging and dissection. This model allows for static and dynamic three-dimensional examination of the surgical anatomy from a neurosurgeon's perspective. It may facilitate the study of cerebrovascular system morphology/pathology in relation to the skull base as a tool for surgical planning.

Neuroimaging, the ischaemic penumbra, and selection of patients for acute stroke therapy

Advances in neuroimaging have been central to the expansion of knowledge in the neurosciences over the past 20 years. One of the most important roles of brain imaging is in the selection of patients for acute stroke therapy. Currently, computed tomography (CT) is commonly used to select patients who have had strokes for thrombolytic therapy on the basis of the absence of haemorrhage and, more controversially, the presence of early CT changes of ischaemia. Since patients with ischaemic penumbra are more likely than those without to respond to therapy, identification of patients with this feature will become increasingly important. Although several imaging modalities can identify the penumbra, the most practical is magnetic resonance imaging (MRI) showing perfusion-weighted and diffusion-weighted imaging mismatch. Although uncertainties in image interpretation remain, surrogate MRI outcome measures are becoming an important component of translational research. Future developments in imaging technologies may provide other opportunities for surrogate outcome studies.

Functional neuroimaging in acute stroke

To the present day, the first and most widespread diagnostic approach in the assessment of acute stroke remains CT scan. Its sensitivity is very high (nearly 100%) in detecting intracerebral hemorrhage in the acute period, but its capability of revealing ischemic injury in the very first hours from symptom onset is relatively poor. Since the efficacy of thrombolytic treatment in acute stroke has been suggested by the ECASS and NINDS rt-PA trials, functional neuroimaging able to distinguish potentially salvageable tissue from irreversibly injured areas has acquired primary importance. The possibility to correctly identify the tissue of the ischemic penumbra within the first hours from symptom onset is essential for correct patient selection for thrombolitic treatment. Different imaging strategies are available for the definition of perfusion deficits within the acute time window; among these are positron emission tomography (PET), single photon emission computed tomography (SPECT), Xenon CT (XeCT), dynamic CT perfusion imaging (CTP), diffusion weighted magnetic resonance imaging (DW-MRI), and perfusion weighted magnetic resonance imaging (PW-MRI). Though each technique has its advantages and limitations to present day functional MRI remains the most widespread imaging technique in the assessment of acute stroke being more accessible than both SPECT and PET, and capable of giving information on both perfusion and tissue functional status in a single imaging session. In this paper we discuss the role of functional neuroimaging in acute stroke.

Ischaemic penumbra: highlights

The ischaemic penumbra was described for the first time in the late 1970s as a ring of hypoperfused zone surrounding the region of complete infarction. The penumbral zone is a functionally silent tissue which is able to regain its function if promptly reperfused. This implies that the ischaemic penumbra is not a static but a "dynamic" and "time-dependent" concept. In this paper we describe the role of neuroimmaging tecniques such as single photon emission tomography (SPET), positron emission tomography (PET), and diffusion-weighted and perfusion-weighted magnetic resonance imaging (DWI and PWI) in the study of ischaemic penumbra. These functional imaging techniques have the advantage of giving "in vivo" quantitative estimate of cerebral blood flow (CBF) as well as information on how the ischaemic tissue metabolic changes develop. It follows that, as therapeutic options for treating acute stroke evolve, neuroimaging strategies are assuming an increasingly important role in the initial evaluation and management of the acute ischaemic patient. In this regard, a wide range of therapeutic approaches have been investigated for either ameliorating the perfusion, or interfering with the pathobiochemical cascade leading to ischaemic neuronal damage, or improving endogenous neuroprotection pathways. The "time windows" required for these treatments to be effective varies being rather short for reperfusion and longer for neuroprotection. Salvaging more penumbra would enhance recovery and thereby allow the most appropriate candidate for therapeutic trials to be selected.