Acute Stroke Imaging for Thrombolytic Therapy – An Update

Establishment and Evaluation of a Monkey Acute Cerebral Ischemia Model

OBJECTIVES

Cerebral ischemia seriously threatens human health and is characterized by high rates of incidence, disability and death. Developing an ideal animal model of cerebral ischemia that reflects the human clinical features is critical for pathological studies and clinical research. The goal of this study is to establish a local cerebral ischemia model in rhesus macaque, thereby providing an optimal animal model to study cerebral ischemia.

METHODS

Eight healthy rhesus monkeys were selected for this study. CT scans were performed before the operation to exclude cerebral vascular and intracranial lesions. Under guidance and monitoring with digital subtraction angiography (DSA), a microcatheter was inserted into the M1 segment of the middle cerebral artery (MCA) via the femoral artery. Then, autologous white thrombi were introduced to block blood flow. Immediately following embolization, multisequence MRI was used to monitor cerebrovascular and brain parenchymal conditions. Twenty-four hours after embolization, 2 monkeys were sacrificed and subjected to perfusion, fixation and pathological examination.

RESULTS

The cerebral ischemia model was established in 7 rhesus monkeys; one animal died during intubation. DSA and magnetic resonance angiography (MRA) indicated the presence of an arterial occlusion. MRI showed acute local cerebral ischemia. HE staining revealed infarct lesions formed in the brain tissues, and thrombi were present in the cerebral artery.

CONCLUSION

We established a rhesus macaque model of local cerebral ischemia by autologous thrombus placement. This model has important implications for basic and clinical research on cerebral ischemia. MRI and DSA can evaluate the models to ensure accuracy and effectiveness.

Extravasation into brain and subsequent spread beyond the ischemic core of a magnetic resonance contrast agent following a step-down infusion protocol in acute cerebral ischemia

Background

Limiting expansion of the ischemic core lesion by reinstating blood flow and protecting the penumbral cells is a priority in acute stroke treatment. However, at present, methods are not available for effective drug delivery to the ischemic penumbra. To address these issues this study compared the extravasation and subsequent interstitial spread of a magnetic resonance contrast agent (MRCA) beyond the ischemic core into the surrounding brain in a rat model of ischemia-reperfusion for bolus injection and step-down infusion (SDI) protocols.

Methods

Male Wistar rats underwent middle cerebral artery (MCA) occlusion for 3 h followed by reperfusion. Perfusion-diffusion mismatched regions indicating the extent of spread were identified by measuring cerebral blood flow (CBF) deficits by arterial spin-labeled magnetic resonance imaging and the extent of the ischemic core by mapping the apparent diffusion coefficient (ADC) of water with diffusion-weighted imaging. Vascular injury was assessed via MRCA, gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) penetration, by Look-Locker T₁-weighted MR imaging after either a bolus injection (n = 8) or SDI (n = 6). Spatial and temporal expansion of the MRCA front during a 25 min imaging period was measured from images obtained at 2.5 min intervals.

Results

The mean ADC lesion was 20 ± 7% of the hemispheric area whereas the CBF deficit area was 60 ± 16%, with the difference between the areas suggesting the possible presence of a penumbra. The bolus injection led to MRCA enhancement with an area that initially spread into the ischemic core and then diminished over time. The SDI produced a gradual increase in the area of MRCA enhancement that slowly enlarged to occupy the core, eventually expanded beyond it into the surrounding tissue and then plateaued. The integrated area from SDI extravasation was significantly larger than that for the bolus (p = 0.03). The total number of pixels covered by the SDI at its maximum was significantly larger than the pixels covered by bolus maximum (p = 0.05).

Conclusions

These results demonstrate that the SDI protocol resulted in a spread of the MRCA beyond the ischemic core. Whether plasma-borne acute stroke therapeutics can be delivered to the ischemic penumbra in a similar way needs to be investigated.

Acute ischemic stroke--from symptom recognition to thrombolysis

OBJECTIVES

The understanding of stroke has changed in the recent years from rehabilitation to an emergency approach. We review existing data from symptom recognition to thrombolysis and identify challenges in the different phases of patient treatment.

RESULTS

Implementation of treatment in dedicated stroke units with a multidisciplinary team exclusively treating stroke patients has led to significant reduction of stroke morbidity and mortality. Yet, first the introduction of treatment with intravenous rtPA (IVT) has led to the 'time is brain' concept where stroke is conceived as an emergency. As neuronal death in stroke is time dependent, all effort should be laid on immediate symptom recognition, rapid transport to the nearest hospital with a stroke treatment facility and diagnosis and treatment as soon as possible. The main cause of prehospital delay is that patients do not recognize that they suffered a stroke or out of other reasons do not call the Emergency Medical Services immediately. Educational stroke awareness campaigns may have an impact in increasing the number of patients eligible for rtPA treatment and can decrease the prehospital times if they are directed both to the public and to the medical divisions treating stroke. Stroke transport times can be shortened by the use of helicopter and a stroke mobile--an ambulance equipped with a CT scanner--may be helpful to decrease time from onset to treatment start in the future. Yet, IVT has several limitations such as a narrow time window and a weak effect in ischemic strokes caused by large vessel occlusions. In these cases, interventional procedures and the concept of bridging therapy, a combined approach of IVT and intraarterial thrombolysis or mechanical thrombectomy, might improve recanalization rates and patient outcome.

CONCLUSIONS

As neuronal death in stroke patients occurs in a time-dependent fashion, all effort should be made to decrease time from symptom onset to treatment start with rtPA: major challenges are stroke recognition in the public, transport times to hospital and an efficient stroke triage in the hospital.

Computed tomography perfusion–based selection of patients for endovascular recanalization

Intravenous and intraarterial recombinant tissue plasminogen activator remains underutilized in the treatment of acute ischemic stroke, largely due to strict adherence to the concept of the therapeutic time window for administration. Recent efforts to expand the number of patients eligible for thrombolysis have been mirrored by an evolution in endovascular recanalization technology and techniques. As a result, there is a growing need to establish efficient and reliable means by which to select candidates for endovascular intervention beyond the traditional criteria of time from symptom onset. Perfusion imaging techniques, particularly CT perfusion used in combination with CT angiography, represent an increasingly recognized means by which to identify those patients who stand to benefit most from endovascular recanalization. Additionally, CT perfusion and CT angiography appear to provide sufficient data by which to exclude patients in whom there is little chance of neurological recovery or a substantial risk of postprocedure symptomatic intracranial hemorrhage. The authors review the current literature as it pertains to the limitations of time-based selection of patients for intervention, the increasing utilization of endovascular therapy, and the development of a CT perfusion-based selection of acute stroke patients for endovascular recanalization. Future endeavors must prospectively evaluate the utility and safety of CT perfusion-based selection of candidates for endovascular intervention.

Prediction of infarct volume and neurologic outcome by using automated multiparametric perfusion-weighted magnetic resonance imaging in a primate model of permanent middle cerebral artery occlusion

By optimizing thresholds, we identified the perfusion-weighted magnetic resonance imaging (PWI) parameters that accurately predict final infarct volume and neurologic outcome in a primate model of permanent middle cerebral artery (MCA) occlusion. Ten cynomolgus monkeys underwent PWI and diffusion-weighted imaging (DWI) at 3 and 47 hours, respectively, after right MCA occlusion using platinum coils, and were killed at 48 hours. Volumes of the hypoperfused areas on PWI were automatically measured using different thresholds and 11 parametric maps to determine the optimum threshold (at which least difference was found between the average volumes on PWI and those determined using specimens or DWI). In the case of arrival time (AT), cerebral blood volume (CBV), time to peak (TTP), time to maximum (T(max)), and cerebral blood flow (CBF) determined using deconvolution techniques, the volume of the hypoperfused area significantly correlated with the infarct volumes and the neurologic deficit scores with small variations, whereas in the case of mean transit time and nondeconvolution CBF, relatively poor correlations with large variations were seen. At optimum threshold, AT, CBV, TTP, T(max), and deconvolution CBF can accurately predict the final infarct volume and neurologic outcome in monkeys with permanent MCA occlusion.

Case of “Slow” Stroke from Carotid Artery Occlusion Treated by Delayed but Cautious Endovascular Intervention

In a challenging case of carotid occlusion with slowly evolving stroke, we used brain imaging to facilitate endovascular revascularization resulting in the relief of the patient's symptoms. Patients with carotid occlusion and continued neurological worsening or fluctuations present enormous treatment challenges. These patients may present “slow” strokes with subacute infarcts that present significant challenges and risks during attempts at revascularization of the occluded artery. We present such a case in which we used multimodal imaging techniques, including MR-perfusion, to facilitate endovascular revascularization. Our approach of delayed but cautious intra-arterial thrombolytic therapy, guided by brain imaging, and followed by stent placement across the residual stenosis, enabled revascularization of the occluded artery without overt in-hospital complications.

A Retrospective Study on Intracerebral Haemorrhage Reduction by Mri versus Ct in Intravenous Thrombolysis for Acute Ischaemic Stroke

Introduction

Multimodal MRI may be an effective tool for selecting suitable acute ischaemic stroke patients for thrombolysis, reducing the risk of haemorrhage. In this study, we demonstrated the usefulness of our central alerting system to reduce door-to-needle time for thrombolysis following MRI. This system allowed timely intervention and reduced the rate of symptomatic haemorrhage.

Methods

We reviewed the records of 73 patients with hyperacute ischaemic stroke who received intravenous (IV) tissue plasminogen activator (t-PA) between January 2006 and December 2007 following the adoption of a central stroke alerting system in our hospital.

Results

Of the 73 patients who received IV t-PA, 44 were based on CT and 29 on MRI findings. The door-to-needle time was 10 minutes longer for the MRI group (49.9±23.2 min) compared to the CT group (39.6±19.7 min) but it was still within the recommended 60 minutes time frame. On the other hand, the rate of symptomatic haemorrhage was lower, though insignificantly, in the MRI group (0%) compared to the CT group (13.6%) (p=0.08).

Conclusions

In this study, we demonstrated that the combination of diagnostic MRI and a central alerting system might reduce the rate of symptomatic haemorrhage without compromising the door-to-needle time.

Computed tomography in acute ischemic stroke

Stroke remains the third most important cause of mortality in industrialized countries; this has prompted research for improvements in both diagnostic and therapeutic strategies for patients with signs of acute cerebral ischemia. Over the last decade, there has been a parallel in progress in techniques in both diagnostic and therapeutic options. While previously only used for excluding hemorrhage, imaging now has the possibility to detect ischemia, vascular occlusion, as well as detect tissue at risk in one setting. It should also allow to monitor treatment and predict/exclude therapeutic complications. Parallel to advances in magnetic resonance imaging of stroke, computed tomography has improved immensely over the last decade due to the development of CT scanners that are faster and that allow to acquire studies such as CT perfusion or CT angiography in a reliable way. CT can detect many signs that might help us detect impending signs of massive infarction, but we still lack the experience to use these alone to prevent a patient from benefitting from possible therapy.

Magnetic resonance imaging versus computed tomography for detection of acute vascular lesions in patients presenting with stroke symptoms

BACKGROUND

Magnetic resonance imaging (MRI) is increasingly used for the diagnosis of acute ischaemic stroke but its sensitivity for the early detection of intracerebral haemorrhage has been debated. Computed tomography (CT) is extensively used in the clinical management of acute stroke, especially for the rapid exclusion of intracerebral haemorrhage.

OBJECTIVES

To compare the diagnostic accuracy of diffusion-weighted MRI (DWI) and CT for acute ischaemic stroke, and to estimate the diagnostic accuracy of MRI for acute haemorrhagic stroke.

SEARCH STRATEGY

We searched MEDLINE and EMBASE (January 1995 to March 2009) and perused bibliographies of relevant studies for additional references.

SELECTION CRITERIA

We selected studies that either compared DWI and CT in the same patients for detection of ischaemic stroke or examined the utility of MRI for detection of haemorrhagic stroke, had imaging performed within 12 hours of stroke onset, and presented sufficient data to allow construction of contingency tables.

DATA COLLECTION AND ANALYSIS

Three authors independently extracted data on study characteristics and measures of accuracy. We assessed data on ischaemic stroke using random-effects and fixed-effect meta-analyses.

MAIN RESULTS

Eight studies with a total of 308 participants met our inclusion criteria. Seven studies contributed to the assessment of ischaemic stroke and two studies to the assessment of haemorrhagic stroke. The spectrum of patients was relatively narrow in all studies, sample sizes were small, there was substantial incorporation bias, and blinding procedures were often incomplete. Amongst the patients subsequently confirmed to have acute ischaemic stroke (161/226), the summary estimates for DWI were: sensitivity 0.99 (95% CI 0.23 to 1.00), specificity 0.92 (95% CI 0.83 to 0.97). The summary estimates for CT were: sensitivity 0.39 (95% CI 0.16 to 0.69), specificity 1.00 (95% CI 0.94 to 1.00). The two studies on haemorrhagic stroke reported high estimates for diffusion-weighted and gradient-echo sequences but had inconsistent reference standards. We did not calculate overall estimates for these two studies. We were not able to assess practicality or cost-effectiveness issues.

AUTHORS' CONCLUSIONS

DWI appears to be more sensitive than CT for the early detection of ischaemic stroke in highly selected patients. However, the variability in the quality of included studies and the presence of spectrum and incorporation biases render the reliability and generalisability of observed results questionable. Further well-designed studies without methodological biases, in more representative patient samples, with practicality and cost estimates are now needed to determine which patients should undergo MRI and which CT in suspected acute stroke.

Emergency MRI utilization trends at a tertiary care academic medical center: baseline data

RATIONALE AND OBJECTIVES

This study evaluates utilization trends of emergency department (ED)-ordered magnetic resonance imaging (MRI) examinations in an adult academic medical center over a 5-year period (2001-2005).

MATERIALS AND METHODS

MRI examinations from the ED-ordering location were identified by searching the radiology information system (RIS). Overall MRI volume and ED patient encounters were also assessed during the same period. Each examination was assessed by type of examination, examination completion time, and primary and secondary clinical indications by reviewing the RIS and medical record.

RESULTS

During the study period, 1,900 ED-ordered MRI examinations were performed on 1,020 patients. During the same period, 62,823 total MRI examinations were performed, and the total ED patient volume was 420,840. ED-ordered MRI volume increased 391% over the study period (125 to 614 examinations/year), whereas total MRI volume increased only 38.9% and ED patient volume in our institution actually decreased by -9%. MRI examinations of the brain, magnetic resonance angiography of the head and neck, and spinal MRI were the most commonly ordered tests. Clinical indications were analyzed. Peak times of ED-ordered examination completion were between 4:00 PM and 11:00 PM. A total of 15.5% of examinations required overtime or callback of MRI technologists for completion.

CONCLUSIONS

Multiple reasons are suggested that may increase utilization (perceived need for diagnostic certainty, as well as medico-legal and patient-driven factors). Whether this increase in MRI utilization resulted in improved patient outcomes is unclear and should be studied further. Implications for radiologist coverage and resident training are discussed.