Cerebral perfusion CT: technique and clinical applications

Flat-panel Detector Perfusion Imaging and Conventional Multidetector Perfusion Imaging in Patients with Acute Ischemic Stroke : A Comparative Study

PURPOSE

Flat-panel detector computed tomography (FDCT) is increasingly used in (neuro)interventional angiography suites. This study aimed to compare FDCT perfusion (FDCTP) with conventional multidetector computed tomography perfusion (MDCTP) in patients with acute ischemic stroke.

METHODS

In this study, 19 patients with large vessel occlusion in the anterior circulation who had undergone mechanical thrombectomy, baseline MDCTP and pre-interventional FDCTP were included. Hypoperfused tissue volumes were manually segmented on time to maximum (Tmax) and time to peak (TTP) maps based on the maximum visible extent. Absolute and relative thresholds were applied to the maximum visible extent on Tmax and relative cerebral blood flow (rCBF) maps to delineate penumbra volumes and volumes with a high likelihood of irreversible infarcted tissue ("core"). Standard comparative metrics were used to evaluate the performance of FDCTP.

RESULTS

Strong correlations and robust agreement were found between manually segmented volumes on MDCTP and FDCTP Tmax maps (r = 0.85, 95% CI 0.65-0.94, p < 0.001; ICC = 0.85, 95% CI 0.69-0.94) and TTP maps (r = 0.91, 95% CI 0.78-0.97, p < 0.001; ICC = 0.90, 95% CI 0.78-0.96); however, direct quantitative comparisons using thresholding showed lower correlations and weaker agreement (MDCTP versus FDCTP Tmax 6 s: r = 0.35, 95% CI -0.13-0.69, p = 0.15; ICC = 0.32, 95% CI 0.07-0.75). Normalization techniques improved results for Tmax maps (r = 0.78, 95% CI 0.50-0.91, p < 0.001; ICC = 0.77, 95% CI 0.55-0.91). Bland-Altman analyses indicated a slight systematic underestimation of FDCTP Tmax maximum visible extent volumes and slight overestimation of FDCTP TTP maximum visible extent volumes compared to MDCTP.

CONCLUSION

FDCTP and MDCTP provide qualitatively comparable volumetric results on Tmax and TTP maps; however, direct quantitative measurements of infarct core and hypoperfused tissue volumes showed lower correlations and agreement.

Clinical Applications of Conebeam CTP Imaging in Cerebral Disease: A Systematic Review

BACKGROUND

Perfusion imaging with multidetector CT is integral to the evaluation of patients presenting with ischemic stroke due to large-vessel occlusion. Using conebeam CT perfusion in a direct-to-angio approach could reduce workflow times and improve functional outcome.

PURPOSE

Our aim was to provide an overview of conebeam CT techniques for quantifying cerebral perfusion, their clinical applications, and validation.

DATA SOURCES

A systematic search was performed for articles published between January 2000 and October 2022 in which a conebeam CT imaging technique for quantifying cerebral perfusion in human subjects was compared against a reference technique.

STUDY SELECTION

Eleven articles were retrieved describing 2 techniques: dual-phase (n = 6) and multiphase (n = 5) conebeam CTP.

DATA ANALYSIS

Descriptions of the conebeam CT techniques and the correlations between them and the reference techniques were retrieved.

DATA SYNTHESIS

Appraisal of the quality and risk of bias of the included studies revealed little concern about bias and applicability. Good correlations were reported for dual-phase conebeam CTP; however, the comprehensiveness of its parameter is unclear. Multiphase conebeam CTP demonstrated the potential for clinical implementation due to its ability to produce conventional stroke protocols. However, it did not consistently correlate with the reference techniques.

LIMITATIONS

The heterogeneity within the available literature made it impossible to apply meta-analysis to the data.

CONCLUSIONS

The reviewed techniques show promise for clinical use. Beyond evaluating their diagnostic accuracy, future studies should address the practical challenges associated with implementing these techniques and the potential benefits for different ischemic diseases.

The role of automated computed topography perfusion in prediction of hemorrhagic transformation after acute ischemic stroke

INTRODUCTION

The role of computed tomography perfusion (CTP) in prediction of hemorrhagic transformation (HT) has been evolving. We aimed to study the role of automated perfusion post-processing software in prediction of HT using the commercially available RAPID software.

METHODS

Two hundred eighty-two patients with anterior circulation ischemic stroke, who underwent CTP with RAPID automated post-processing, were retrospectively enrolled and divided into HT (n = 91) and non-HT groups (n = 191). The automated RAPID-generated perfusion maps were reviewed. Mismatch volume and ratio, time to maximum (Tmax) > 4-10s volumes, hypoperfusion index, cerebral blood flow (CBF) < 20-38% volumes, cerebral blood volume (CBV) < 34%-42% volumes, and CBV index were recorded and analyzed.

RESULTS

The volumes of brain tissues suffering from reduction of cerebral blood flow (CBF < 20%-38%), reduction in cerebral blood volumes (CBV < 34-42%), and delayed contrast arrival times (Tmax > 4-10s) were significantly higher in the HT group. The mismatch volumes were also higher in the HT group (p = .001). Among these parameters, the Tmax > 6s volume was the most reliable and sensitive predictor of HT (p = .001, AUC = 0.667). However, the combination of the perfusion parameters can slightly improve the diagnostic efficiency (AUC = 0.703). There was no statistically significant difference between the non-HT group and either the parenchymal or the symptomatic subtypes.

CONCLUSION

The RAPID automated CTP parameters can provide a reliable predictor of HT overall but not the parenchymal or the symptomatic subtypes. The infarct area involving the penumbra and core represented by the Tmax > 6s threshold is the most sensitive predictor; however, the combination of the perfusion parameters can slightly improve the diagnostic efficiency.

Effects of bolus injection duration on perfusion estimates in dynamic CT and dynamic susceptibility contrast MRI

Estimates of cerebral blood flow (CBF) and tissue mean transit time (MTT) have been shown to differ between dynamic CT perfusion (CTP) and dynamic susceptibility contrast MRI (DSC-MRI). This study investigates whether these discrepancies regarding CBF and MTT between CTP and DSC-MRI can be attributed to the different injection durations of these techniques. Five subjects were scanned using CTP and DSC-MRI. Region-wise estimates of CBF, MTT, and cerebral blood volume (CBV) were derived based on oscillatory index regularized singular value decomposition. A parametric model that reproduced the shape of measured time curves and characteristics of resulting perfusion parameter estimates was developed and used to simulate data with injection durations typical for CTP and DSC-MRI for a clinically relevant set of perfusion scenarios and noise levels. In simulations, estimates of CBF/MTT showed larger negative/positive bias and increasing variability for CTP when compared to DSC-MRI, especially for high CBF levels. While noise also affected estimates, at clinically relevant levels, the injection duration effect was larger. There are several methodological differences between CTP and DSC-MRI. The results of this study suggest that the injection duration is among those that can explain differences in estimates of CBF and MTT between these bolus tracking techniques.

Real-time assessment of individual optimal CT perfusion acquisition time in patients with ischemic stroke

BACKGROUND AND PURPOSE

This study aims to investigate the feasibility of a "real-time" estimate of the optimal CT perfusion (CTP) acquisition time (T_op ) in ischemic stroke patients.

METHODS

The arterial input function, the venous output function (VOF), and the time-attenuation curves of ischemic core and ischemic penumbra of 51 patients with acute ischemic stroke in anterior circulation were obtained. The curves were analyzed to determine for each patient the T_op value; additionally, several time parameters were derived from each waveform. The relationship between each of these parameters and T_op was investigated.

RESULTS

We found a strong linear correlation between each time parameter derived from VOF curve and T_op , suggesting that the VOF waveform is rescaled from patient to patient without significant change in shape.

CONCLUSIONS

The linear correlation between T_op and the VOF time to peak is well suited to implement a new technique to automatically customize the patient's CTP acquisition time. The method does not require an additional dose of contrast medium and does not increase the overall study time, so its use would be desirable to decrease the average radiation dose.

Developing new quantitative CT image markers to predict prognosis of acute ischemic stroke patients

BACKGROUND

Endovascular mechanical thrombectomy (EMT) is an effective method to treat acute ischemic stroke (AIS) patients due to large vessel occlusion (LVO). However, stratifying AIS patients who can and cannot benefit from EMT remains a clinical challenge.

OBJECTIVE

To develop a new quantitative image marker computed from pre-intervention computed tomography perfusion (CTP) images and evaluate its feasibility to predict clinical outcome among AIS patients undergoing EMT after diagnosis of LVO.

METHODS

A retrospective dataset of 31 AIS patients with pre-intervention CTP images is assembled. A computer-aided detection (CAD) scheme is developed to pre-process CTP images of different scanning series for each study case, perform image segmentation, quantify contrast-enhanced blood volumes in bilateral cerebral hemispheres, and compute features related to asymmetrical cerebral blood flow patterns based on the cumulative cerebral blood flow curves of two hemispheres. Next, image markers based on a single optimal feature and machine learning (ML) models fused with multi-features are developed and tested to classify AIS cases into two classes of good and poor prognosis based on the Modified Rankin Scale. Performance of image markers is evaluated using the area under the ROC curve (AUC) and accuracy computed from the confusion matrix.

RESULTS

The ML model using the neuroimaging features computed from the slopes of the subtracted cumulative blood flow curves between two cerebral hemispheres yields classification performance of AUC = 0.878±0.077 with an overall accuracy of 90.3%.

CONCLUSIONS

This study demonstrates feasibility of developing a new quantitative imaging method and marker to predict AIS patients' prognosis in the hyperacute stage, which can help clinicians optimally treat and manage AIS patients.

Low-dose CT reconstruction with Noise2Noise network and testing-time fine-tuning

PURPOSE

Deep learning-based image denoising and reconstruction methods demonstrated promising performance on low-dose CT imaging in recent years. However, most existing deep learning-based low-dose CT reconstruction methods require normal-dose images for training. Sometimes such clean images do not exist such as for dynamic CT imaging or very large patients. The purpose of this work is to develop a low-dose CT image reconstruction algorithm based on deep learning which does not need clean images for training.

METHODS

In this paper, we proposed a novel reconstruction algorithm where the image prior was expressed via the Noise2Noise network, whose weights were fine-tuned along with the image during the iterative reconstruction. The Noise2Noise network built a self-consistent loss by projection data splitting and mapping the corresponding filtered backprojection (FBP) results to each other with a deep neural network. Besides, the network weights are optimized along with the image to be reconstructed under an alternating optimization scheme. In the proposed method, no clean image is needed for network training and the testing-time fine-tuning leads to optimization for each reconstruction.

RESULTS

We used the 2016 Low-dose CT Challenge dataset to validate the feasibility of the proposed method. We compared its performance to several existing iterative reconstruction algorithms that do not need clean training data, including total variation, non-local mean, convolutional sparse coding, and Noise2Noise denoising. It was demonstrated that the proposed Noise2Noise reconstruction achieved better RMSE, SSIM and texture preservation compared to the other methods. The performance is also robust against the different noise levels, hyperparameters, and network structures used in the reconstruction. Furthermore, we also demonstrated that the proposed methods achieved competitive results without any pre-training of the network at all, that is, using randomly initialized network weights during testing. The proposed iterative reconstruction algorithm also has empirical convergence with and without network pre-training.

CONCLUSIONS

The proposed Noise2Noise reconstruction method can achieve promising image quality in low-dose CT image reconstruction. The method works both with and without pre-training, and only noisy data are required for pre-training.

Experimental examination of radiation doses from cardiac and liver CT perfusion in a phantom study as a function of organ, age and sex

Cardiac and liver computed tomography (CT) perfusion has not been routinely implemented in the clinic and requires high radiation doses. The purpose of this study is to examine the radiation exposure and technical settings for cardiac and liver CT perfusion scans at different CT scanners. Two cardiac and three liver CT perfusion protocols were examined with the N1 LUNGMAN phantom at three multi-slice CT scanners: a single-source (I) and second- (II) and third-generation (III) dual-source CT scanners. Radiation doses were reported for the CT dose index (CTDI_vol) and dose-length product (DLP) and a standardised DLP (DLP_10cm) for cardiac and liver perfusion. The effective dose (ED_10cm) for a standardised scan length of 10 cm was estimated using conversion factors based on the International Commission on Radiological Protection (ICRP) 110 phantoms and tissue-weighting factors from ICRP 103. The proposed total lifetime attributable risk of developing cancer was determined as a function of organ, age and sex for adults. Radiation exposure for CTDI_vol, DLP/DLP_{10 cm}and ED_{10 cm}during CT perfusion was distributed as follows: for cardiac perfusion (II) 144 mGy, 1036 mGy·cm/1440 mGy·cm and 39 mSv, and (III) 28 mGy, 295 mGy·cm/279 mGy·cm and 8 mSv; for liver perfusion (I) 225 mGy, 3360 mGy·cm/2249 mGy·cm and 54 mSv, (II) 94 mGy, 1451 mGy·cm/937 mGy·cm and 22 mSv, and (III) 74 mGy, 1096 mGy·cm/739 mGy·cm and 18 mSv. The third-generation dual-source CT scanner applied the lowest doses. Proposed total lifetime attributable risk increased with decreasing age. Even though CT perfusion is a high-dose examination, we observed that new-generation CT scanners could achieve lower doses. There is a strong impact of organ, age and sex on lifetime attributable risk. Further investigations of the feasibility of these perfusion scans are required for clinical implementation.

Learning non-local perfusion textures for high-quality computed tomography perfusion imaging

Background. Computed tomography perfusion (CTP) imaging plays a critical role in the acute stroke syndrome assessment due to its widespread availability, speed of image acquisition, and relatively low cost. However, due to its repeated scanning protocol, CTP imaging involves a substantial radiation dose, which might increase potential cancer risks.Methods. In this work, we present a novel deep learning model called non-local perfusion texture learning network (NPTN) for high-quality CTP imaging at low-dose cases. Specifically, considering abundant similarities in the CTP images, i.e. latent self-similarities within the non-local region in the CTP images, we firstly search the most similar pixels from the adjacent frames within a fixed search window to obtain the non-local similarities and to construct non-local textures vector. Then, both the low-dose frame and these non-local textures from adjacent frames are fed into a convolution neural network to predict high-quality CTP images, which can help better characterize the structure details and contrast variants in the targeted CTP image rather than simply utilizing the targeted frame itself. The residual learning strategy and batch normalization are utilized to boost the performance of the convolution neural network. In the experiment, the CTP images of 31 patients with suspected stroke disease are collected to demonstrate the performance of the presented NPTN method.Results. The results show the presented NPTN method obtains superior performance compared with the competing methods. From numerical value, at all dose levels, the presented NPTN method has achieved around 3.0 dB improvement of average PSNR, an increase of around 1.4% of average SSIM, and a decrease of around 4.8% of average RMSE in the low-dose CTP reconstruction task, and also has achieved an increase of around 3.4% of average SSIM and a decrease of around 61.1% of average RMSE in the cerebral blood flow (CBF) estimation task.Conclusions. The presented NPTN method can obtain high-quality CTP images and estimate high-accuracy CBF map by characterizing more structure details and contrast variants in the CTP image and outperform the competing methods at low-dose cases.

Individualized Brain Tissue Oxygen-Monitoring Probe Placement Helps to Guide Therapy and Optimizes Outcome in Neurocritical Care

Background/Objective

In order to monitor tissue oxygenation in patients with acute neurological disorders, probes for measurement of brain tissue oxygen tension (ptO₂) are often placed non-specifically in a right frontal lobe location. To improve the value of ptO₂ monitoring, placement of the probe into a specific area of interest is desirable. We present a technique using CT-guidance to place the ptO₂ probe in a particular area of interest based on the individual patient’s pathology.

Methods

In this retrospective cohort study, we analyzed imaging and clinical data from all patients who underwent CT-guided ptO₂ probe placement at our institution between October 2017 and April 2019. Primary endpoint was successful placement of the probe in a particular area of interest rated by two independent reviewers. Secondary outcomes were complications from probe insertion, clinical consequences from ptO₂ measurements, clinical outcome according to the modified Rankin Scale (mRS) as well as development of ischemia on follow-up imaging. A historical control group was selected from patients who underwent conventional ptO₂ probe placement between January 2010 and October 2017.

Results

Eleven patients had 16 CT-guided probes inserted. In 15 (93.75%) probes, both raters agreed on the correct placement in the area of interest. Each probe triggered on average 0.48 diagnostic or therapeutic adjustments per day. Only one infarction within the vascular territory of a probe was found on follow-up imaging. Eight out of eleven patients (72.73%) reached a good outcome (mRS ≤ 3). In comparison, conventionally placed probes triggered less diagnostic and therapeutic adjustment per day (p = 0.007). Outcome was worse in the control group (p = 0.024).

Conclusion

CT-guided probe insertion is a reliable and easy technique to place a ptO₂ probe in a particular area of interest in patients with potentially reduced cerebral oxygen supply. By adjusting treatment aggressively according to this individualized monitoring data, clinical outcome may improve.

The Updated Role of the Blood Brain Barrier in Subarachnoid Hemorrhage: From Basic and Clinical Studies

Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke associated with high mortality and morbidity. The blood-brain-barrier (BBB) is a structure consisting primarily of cerebral microvascular endothelial cells, end feet of astrocytes, extracellular matrix, and pericytes. Post-SAH pathophysiology included early brain injury and delayed cerebral ischemia. BBB disruption was a critical mechanism of early brain injury and was associated with other pathophysiological events. These pathophysiological events may propel the development of secondary brain injury, known as delayed cerebral ischemia. Imaging advancements to measure BBB after SAH primarily focused on exploring innovative methods to predict clinical outcome, delayed cerebral ischemia, and delayed infarction related to delayed cerebral ischemia in acute periods. These predictions are based on detecting abnormal changes in BBB permeability. The parameters of BBB permeability are described by changes in computed tomography (CT) perfusion and magnetic resonance imaging (MRI). K_ep seems to be a stable and sensitive indicator in CT perfusion, whereas K^trans is a reliable parameter for dynamic contrast-enhanced MRI. Future prediction models that utilize both the volume of BBB disruption and stable parameters of BBB may be a promising direction to develop practical clinical tools. These tools could provide greater accuracy in predicting clinical outcome and risk of deterioration. Therapeutic interventional exploration targeting BBB disruption is also promising, considering the extended duration of post-SAH BBB disruption.

Abnormalities on Perfusion CT and Intervention for Intracranial Hypertension in Severe Traumatic Brain Injury

The role of invasive intracranial pressure (ICP) monitoring in patients with severe traumatic brain injury (STBI) remain unclear. Perfusion computed tomography (CTP) provides crucial information about the cerebral perfusion status in these patients. We hypothesised that CTP abnormalities would be associated with the severity of intracranial hypertension (ICH). To investigate this hypothesis, twenty-eight patients with STBI and ICP monitors were investigated with CTP within 48 h from admission. Treating teams were blind to these results. Patients were divided into five groups based on increasing intervention required to control ICH and were compared. Group I required no intervention above routine sedation, group II required a single first tier intervention, group III required multiple different first-tier interventions, group IV required second-tier medical therapy and group V required second-tier surgical therapy. Analysis of the results showed demographics and injury severity did not differ among groups. In group I no patients showed CTP abnormality, while patients in all other groups had abnormal CTP (p = 0.003). Severe ischaemia observed on CTP was associated with increasing intervention for ICH. This study, although limited by small sample size, suggests that CTP abnormalities are associated with the need to intervene for ICH. Larger scale assessment of our results is warranted to potentially avoid unnecessary invasive procedures in head injury patients.

Radiological predictors of hemorrhagic transformation after acute ischemic stroke: An evidence-based analysis

Hemorrhagic transformation (HT) is one of the most common adverse events related to acute ischemic stroke (AIS) that affects the treatment plan and clinical outcome. Identification of a sensitive radiological marker may influence the controversial thrombolytic decision in the setting of AIS and may at a minimum indicate more intensive monitoring or further prophylactic interventions. In this article we summarize possible radiological biomarkers and the role of different radiological modalities including computed tomography (CT), magnetic resonance imaging, angiography, and ultrasound in predicting HT. Different radiological indices of early ischemic changes, large ischemic lesion volume, severe blood flow restriction, blood-brain barrier disruption, poor collaterals and high blood flow velocities have been reported to be associated with higher risk of HT. The current levels of evidence of the available studies highlight the role of the different CT perfusion parameters in predicting HT. Further large standardized studies are recommended to compare the sensitivity and specificity of the different radiological markers combined and delineate the most reliable predictor.

Computed Tomography Perfusion Imaging Quality Affected by Different Input Arteries in Patients of Internal Carotid Artery Stenosis

BACKGROUND The aim of this study was to explore the influence of different input arteries on the parameters of computed tomography (CT) perfusion imaging for patients with different degree of stenosis of internal carotid artery (ICA). MATERIAL AND METHODS Forty patients were enrolled in the present study and divided into mild, moderate, severe stenosis and occlusion groups respectively with each 10 patients in each group. In reconstruction of cerebral CT perfusion (CTP) images, each raw perfusion image was reconstructed 3 times based on different reference input artery, including bilateral middle cerebral artery (MCA) and basilar arteries (BA). Region of interest (ROI) was drawn in the central territories of bilateral anterior cerebral artery, middle cerebral artery and posterior cerebral artery. And regional cerebral blood flow (rCBF) regional cerebral blood volume (rCBV), mean transit time (MTT), time to peak (TTP) and delay time (DT) were obtained from those ROI corresponding perfusion images. RESULTS In patients with mild and moderate ICA stenosis, there was no significant difference of perfusion parameters based on different input arteries (P>0.05). However, in severe ICA stenosis and occlusion CBF, MTT, and DT were significant different in affect side of the MCA group compared to the others (P<0.05). CONCLUSIONS Large intracranial artery can be selected as the input artery for patients with mild to moderate ICA stenosis, while for patients with severe stenosis and occlusion of ICA, the contra lateral middle cerebral artery or basilar artery would be better choice.

"Code-Stroke" CT Perfusion; Challenges and Pitfalls

RATIONALE AND OBJECTIVES

Regarding the most recent ischemic stroke treatment guideline, perfusion imaging has been recommended up to 24 hours after initial symptoms of brain infarction. Patients with a significant amount of salvageable peri-infarct ischemia and no contraindications benefit from delayed thrombolysis and intra-arterial thrombectomy. This approach causes increasingly more CT perfusion to be done in the subacute phase of ischemic stroke. CT perfusion findings in this "subacute phase" are slightly different from "hyper-acute" ischemic stroke. The interpreting radiologist must be confident in reporting the CT perfusion study in an urgent setting since these studies are under the umbrella of "code-stroke" and should be read in minutes. In addition, results of the CT perfusion have a critical effect on the patient's outcome and misinterpretation can be fatal in that underestimation of the salvageable ischemia excludes the patient from potential effective treatment. Underestimation of infarct volume may cause unnecessary thrombolysis/thrombectomy and potentially fatal intracranial hemorrhage.

MATERIALS AND METHODS

In this review, we are trying to explain the basic concept of "code-stroke" CT perfusion, typical findings, and pitfalls in a practical way.

Neurovascular Unit: Basic and Clinical Imaging with Emphasis on Advantages of Ferumoxytol

Physiological and pathological processes that increase or decrease the central nervous system's need for nutrients and oxygen via changes in local blood supply act primarily at the level of the neurovascular unit (NVU). The NVU consists of endothelial cells, associated blood-brain barrier tight junctions, basal lamina, pericytes, and parenchymal cells, including astrocytes, neurons, and interneurons. Knowledge of the NVU is essential for interpretation of central nervous system physiology and pathology as revealed by conventional and advanced imaging techniques. This article reviews current strategies for interrogating the NVU, focusing on vascular permeability, blood volume, and functional imaging, as assessed by ferumoxytol an iron oxide nanoparticle.

Statistical properties of cerebral CT perfusion imaging systems. Part I. Cerebral blood volume maps generated from nondeconvolution-based systems

PURPOSE

The development and clinical employment of a computed tomography (CT) imaging system benefit from a thorough understanding of the statistical properties of the output images; cerebral CT perfusion (CTP) imaging system is no exception. A series of articles will present statistical properties of CTP systems and the dependence of these properties on system parameters. This Part I paper focuses on the signal and noise properties of cerebral blood volume (CBV) maps calculated using a nondeconvolution-based method.

METHODS

The CBV imaging chain was decomposed into a cascade of subimaging stages, which facilitated the derivation of analytical models for the probability density function, mean value, and noise variance of CBV. These models directly take CTP source image acquisition, reconstruction, and postprocessing parameters as inputs. Both numerical simulations and in vivo canine experiments were performed to validate these models.

RESULTS

The noise variance of CBV is linearly related to the noise variance of source images and is strongly influenced by the noise variance of the baseline images. Uniformly partitioning the total radiation dose budget across all time frames was found to be suboptimal, and an optimal dose partition method was derived to minimize CBV noise. Results of the numerical simulation and animal studies validated the derived statistical properties of CBV.

CONCLUSIONS

The statistical properties of CBV imaging systems can be accurately modeled by extending the linear CT systems theory. Based on the statistical model, several key signal and noise characteristics of CBV were identified and an optimal dose partition method was developed to improve the image quality of CBV.

Imaging of Moyamoya Disease and Moyamoya Syndrome: Current Status

Moyamoya disease (MMD) and Moyamoya syndrome (MMS) are referring to a progressive steno-occlusive vasculopathy at terminal portions of the bilateral internal carotid arteries and their proximal branches with prominent collateral artery formation. They can be found throughout the world and cause irreversible damage to the cerebral hemodynamics due to the progressive nature. Prompt diagnosis and accurate assessment could significantly improve the prognosis of MMD and MMS. Some imaging modalities could be used for diagnosis and nonquantitative evaluation of MMD and MMS, such as conventional computed tomography (CT) and magnetic resonance imaging (MRI), digital subtraction angiography, CT angiography (CTA), and magnetic resonance angiography. Some could quantitatively evaluate the cerebral hemodynamics of MMD and MMS, such as single-photon emission CT, positron emission tomography, xenon-enhanced CT, perfusion CT, dynamic susceptibility contrast MRI, arterial spin labeling MRI, and the hemodynamic parameters measured by those imaging methods could guide treatment of MMD and MMS. All the imaging modalities have their merits and demerits, and they can play a part in certain situation. We need establish standardized protocols for preoperative and postoperative evaluation with different imaging techniques in the further science for MMD and MMS.

Indirect methods for improving parameter estimation of PET kinetic models

PURPOSE

Parametric images obtained from kinetic modeling of dynamic positron emission tomography (PET) data provide a new way of visualizing quantitative parameters of the tracer kinetics. However, due to the high noise level in pixel-wise image-driven time-activity curves, parametric images often suffer from poor quality and accuracy. In this study, we propose an indirect parameter estimation framework which aims to improve the quality and quantitative accuracy of parametric images.

METHODS

Three different approaches related to noise reduction and advanced curve fitting algorithm are used in the proposed framework. First, dynamic PET images are denoised using a kernel-based denoising method and the highly constrained backprojection technique. Second, gradient-free curve fitting algorithms are exploited to improve the accuracy and precision of parameter estimates. Third, a kernel-based post-filtering method is applied to parametric images to further improve the quality of parametric images. Computer simulations were performed to evaluate the performance of the proposed framework.

RESULTS AND CONCLUSIONS

The simulation results showed that when compared to the Gaussian filtering, the proposed denoising method could provide better PET image quality, and consequentially improve the quality and quantitative accuracy of parametric images. In addition, gradient-free optimization algorithms (i.e., pattern search) can result in better parametric images than the gradient-based curve fitting algorithm (i.e., trust-region-reflective). Finally, our results showed that the proposed kernel-based post-filtering method could further improve the precision of parameter estimates while maintaining the accuracy of parameter estimates.

Iterative quality enhancement via residual-artifact learning networks for low-dose CT

Radiation exposure and the associated risk of cancer for patients in computed tomography (CT) scans have been major clinical concerns. The radiation exposure can be reduced effectively via lowering the x-ray tube current (mA). However, this strategy may lead to excessive noise and streak artifacts in the conventional filtered back-projection reconstructed images. To address this issue, some deep convolutional neural network (ConvNet) based approaches have been developed for low-dose CT imaging inspired by the recent development of machine learning. Nevertheless, some of the image textures reconstructed by the ConvNet could be corrupted by the severe streaks, especially in ultra-low-dose cases, which could be close to prostheses and hamper diagnosis. Therefore, in this work, we propose an iterative residual-artifact learning ConvNet (IRLNet) approach to improve the reconstruction performance over the ConvNet based approaches. Specifically, the proposed IRLNet estimates the high-frequency details within the noise and then removes them iteratively; after eliminating severe streaks in the low-dose CT images, the residual low-frequency details can be processed through the conventional network. Moreover, the proposed IRLNet scheme can be extended for robust handling of quantitative dual energy CT/cerebral perfusion CT imaging, and statistical iterative reconstruction. Real patient data are used to evaluate the proposed IRLNet, and the experimental results demonstrate that the proposed IRLNet approach outperforms the previous ConvNet based approaches in reducing the image noise and streak artifacts efficiently at the same time as preserving edge details well, suggesting that the proposed IRLNet approach can be used to improve the CT image quality, especially in ultra-low-dose cases.

Dislocation of a cerebral protection device component during carotid stenting: A case report of favorable outcome from conservative management after failure of retrieval

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Though effective, CPDs may cause complications resulting in serious outcomes.

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No unique solution (conservative/surgical) for risk-free management of entrapment.

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Improving standard of care and patient safety is a top priority.

Introduction

Cerebral-protection devices (CPDs) are a well-established system for reduction of embolic risk in carotid artery angioplasty and stenting (CAS). Although rare, adverse events with CPDs are unpredictable and can be associated with serious outcomes and iatrogenic sequelae.

Presentation of case

We describe the unique case of dislocation of a FilterWire EX™ filter loop during right CAS. On trying to recapture the CPD filter at the end of the procedure, the filter loop suddenly detached from the guidewire and dislocated to the proximal middle cerebral artery. Attempted retrieval of the loop failed and the patient developed a transient neurological deficit caused by an acute ischemic infarction in the lenticular nucleus. No further retrieval attempt was pursued. No further dislocation of the loop or clinical event have been reported during the 16-year follow up.

Discussion

This case reported a favorable outcome of conservative management for entrapped material from a CPD after iatrogenic damage from failed retrieval. No similar reports are available in the literature, and conservative management is generally not a recommended approach because of the potential complications. However, rescue retrieval attempts are as well a potential source of serious events, and no clear guidelines exist on the management of mechanical complications from CPD.

Conclusion

Entrapment of CPD components constitutes an adverse event with no unique solution for risk-free management. The potential risks associated with the use of protection devices are still to be fully explored, and improving the standard of care and patient safety needs to be a top priority.

Traumatic brain injury and intracranial hemorrhage–induced cerebral vasospasm: a systematic review

OBJECTIVE

Little is known regarding the natural history of posttraumatic vasospasm. The authors review the pathophysiology of posttraumatic vasospasm (PTV), its associated risk factors, the efficacy of the technologies used to detect PTV, and the management/treatment options available today.

METHODS

The authors performed a systematic review in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines using the following databases: PubMed, Google Scholar, and CENTRAL (the Cochrane Central Register of Controlled Trials). Outcome variables extracted from each study included epidemiology, pathophysiology, time course, predictors of PTV and delayed cerebral ischemia (DCI), optimal means of surveillance and evaluation of PTV, application of multimodality monitoring, modern management and treatment options, and patient outcomes after PTV. Study types were limited to retrospective chart reviews, database reviews, and prospective studies.

RESULTS

A total of 40 articles were included in the systematic review. In many cases of mild or moderate traumatic brain injury (TBI), imaging or ultrasonographic studies are not performed. The lack of widespread assessment makes finding the true overall incidence of PTV a difficult endeavor. The clinical consequences of PTV are important, given the morbidity that can result from it. DCI manifests as new-onset neurological deterioration that occurs beyond the timeframe of initial brain injury. While there are many techniques that attempt to diagnose cerebral vasospasm, digital subtraction angiography is the gold standard. Some predictors of PTV include SAH, intraventricular hemorrhage, low admission Glasgow Coma Scale (GCS) score (< 9), and young age (< 30 years).

CONCLUSIONS

Given these results, clinicians should suspect PTV in young patients presenting with intracranial hemorrhage (ICH), especially SAH and/or intraventricular hemorrhage, who present with a GCS score less than 9. Monitoring and regulation of CNS metabolism following TBI/ICH-induced vasospasm may play an important adjunct role to the primary prevention of vasospasm.

The cerebral collateral circulation: Relevance to pathophysiology and treatment of stroke

The brain's collateral circulation consists of arterial anastomotic channels capable of providing nutrient perfusion to brain regions whose normal sources of flow have become compromised, as occurs in acute ischemic stroke. Modern CT-based neuroimaging is capable of providing detailed information as to collateral extent and sufficiency and is complemented by magnetic resonance-based methods. In the present era of standard-of-care IV thrombolysis for acute ischemic stroke, and following the recent therapeutic successes of randomized clinical trials of acute endovascular intervention, the sufficiency of the collateral circulation has been convincingly established as a key factor influencing the likelihood of successful reperfusion and favorable clinical outcome. This article reviews the features of the brain's collateral circulation; methods for its evaluation in the acute clinical setting; the relevance of collateral circulation to prognosis in acute ischemic stroke; the specific insights into the collateral circulation learned from recent trials of endovascular intervention; and the major influence of genetic factors. Finally, we emphasize the need to develop therapeutic approaches to augment collateral perfusion as an adjunctive strategy to be employed along with, or prior to, thrombolysis and endovascular interventions, and we highlight the possible potential of inhaled nitric oxide, albumin, and other approaches. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Application of Dendrimer-based Nanoparticles in Glioma Imaging

Dendritic polymers or dendrimers present an alternate template for the development of nanoparticulate-based drug delivery and imaging systems. The smaller size (~7-12 nm) of dendrimers have the advantage over the other particles, because its smaller size can possibly improve tumor penetration and the inclusion of tumor specific drug release mechanisms. A Paramagnetic Chemical Exchange Saturation Transfer (PARACEST) MRI contrast agent, Eu-DOTA-Gly4 or a clinical relevant Gd-DOTA was conjugated on the surface of a G5 PAMAM dendrimer. To create a dual mode MRI-optical imaging nanoparticle, Dylight680 was also incorporated on the amines surface of a G5 dendrimer. The particle was detected with in vivo MRI in preclinical glioma animal model. Furthermore, noninvasive imaging results were validated with in vivo and ex-vivo optical imaging.

CT-based Techniques for Brain Perfusion

Recent rapid advances in endovascular treatment for acute ischemic stroke highlight the crucial role of neuroimaging especially multimodal computed tomography (CT) including CT perfusion in stroke triage and management decisions. With an increasing focus on changes in cerebral physiology along with time-based matrices in clinical decisions for acute ischemic stroke, CT perfusion provides a rapid and practical modality for assessment and identification of salvageable tissue at risk and infarct core and provides a better understanding of the changes in cerebral physiology. Although there are challenges with the lack of standardization and accuracy of quantitative assessment, CT perfusion is evolving as a cornerstone for imaging-based strategies in the rapid management of acute ischemic stroke.

Alterations of cerebral perfusion in asymptomatic internal carotid artery steno-occlusive disease

Patients with asymptomatic occlusion in the internal carotid arteries (ICA) have been shown to have a better preserved hemodynamic status of the brain as compared to patients with symptoms. This study was aimed to explore the cerebral perfusion alterations in asymptomatic patients using multi-parametric arterial spin-labeling (ASL) magnetic resonance (MR) imaging. Forty-two patients diagnosed with asymptomatic ICA stenosis/occlusion were prospectively included and divided into high-grade (ultrasonographic stenosis ≥70%, N = 20) and low-grade groups (N = 22). On a 3-Tesla clinical MR scanner, pseudo-continuous ASL was performed to measure cerebral blood flow CBF, arterial transit time ATT, and flow territory. Fisher's exact test indicates that the high-grade group has higher frequency in asymmetric ATT (p < 10-3) and asymmetric flow territory (p < 10-3) as compared to the low-grade group. The between-group difference in CBF asymmetry is marginal (p = 0.062). Logistic regression further reveals that hemispherical asymmetry in ATT and flow territory is associated with the existence of high-grade ICA stenosis (odds ratio = 12 and 21, respectively), whereas hemispherical asymmetry in CBF is not. Our data suggest that ATT and flow territory may be better predictors of asymptomatic high-grade ICA stenosis diagnosed by carotid ultrasonography than CBF.

Local attenuation curve optimization framework for high quality perfusion maps in low-dose cerebral perfusion CT

PURPOSE

Cerebral perfusion x-ray computed tomography (PCT) is a powerful tool for noninvasive imaging of hemodynamic information throughout the brain. Conventional PCT requires the brain to be imaged multiple times during the perfusion process, and hence radiation dose is a major concern. The authors propose a PCT reconstruction algorithm that allows for lowering the dose while maintaining a high quality of the perfusion maps. It relies on an accurate estimation of the arterial input function (AIF), which in turn depends on the quality of the attenuation curves in the arterial region.

METHODS

The authors propose the local attenuation curve optimization (LACO) framework. It accurately models the attenuation curves inside the vessel and arterial regions and optimizes its shape directly based on the acquired x-ray projection data.

RESULTS

The LACO algorithm is extensively validated with simulation and real clinical experiments. Quantitative and qualitative results show that our proposed approach accurately estimates the vessel and arterial attenuation curves from only few x-ray projections. In contrast to conventional approaches, where the AIF is estimated based on the reconstructed images, our method computes an optimal AIF directly based on the projection data, resulting in far more accurate perfusion maps.

CONCLUSIONS

The LACO algorithm allows estimating high quality perfusion maps in low dose scanning protocols.

Cerebral perfusion computed tomography deconvolution via structure tensor total variation regularization

PURPOSE

Cerebral perfusion computed tomography (PCT) imaging as an accurate and fast acute ischemic stroke examination has been widely used in clinic. Meanwhile, a major drawback of PCT imaging is the high radiation dose due to its dynamic scan protocol. The purpose of this work is to develop a robust perfusion deconvolution approach via structure tensor total variation (STV) regularization (PD-STV) for estimating an accurate residue function in PCT imaging with the low-milliampere-seconds (low-mAs) data acquisition.

METHODS

Besides modeling the spatio-temporal structure information of PCT data, the STV regularization of the present PD-STV approach can utilize the higher order derivatives of the residue function to enhance denoising performance. To minimize the objective function, the authors propose an effective iterative algorithm with a shrinkage/thresholding scheme. A simulation study on a digital brain perfusion phantom and a clinical study on an old infarction patient were conducted to validate and evaluate the performance of the present PD-STV approach.

RESULTS

In the digital phantom study, visual inspection and quantitative metrics (i.e., the normalized mean square error, the peak signal-to-noise ratio, and the universal quality index) assessments demonstrated that the PD-STV approach outperformed other existing approaches in terms of the performance of noise-induced artifacts reduction and accurate perfusion hemodynamic maps (PHM) estimation. In the patient data study, the present PD-STV approach could yield accurate PHM estimation with several noticeable gains over other existing approaches in terms of visual inspection and correlation analysis.

CONCLUSIONS

This study demonstrated the feasibility and efficacy of the present PD-STV approach in utilizing STV regularization to improve the accuracy of residue function estimation of cerebral PCT imaging in the case of low-mAs.

Brain Vascular Imaging Techniques

Recent major improvements in a number of imaging techniques now allow for the study of the brain in ways that could not be considered previously. Researchers today have well-developed tools to specifically examine the dynamic nature of the blood vessels in the brain during development and adulthood; as well as to observe the vascular responses in disease situations in vivo. This review offers a concise summary and brief historical reference of different imaging techniques and how these tools can be applied to study the brain vasculature and the blood-brain barrier integrity in both healthy and disease states. Moreover, it offers an overview on available transgenic animal models to study vascular biology and a description of useful online brain atlases.

CT imaging selection in acute stroke

Acute stroke has become an increasingly treatable cause of acute neurological deficits. Indeed, over the last two decades, the introduction of first thrombolysis, and now thrombectomy has improved patient outcomes and extended the therapeutic window. Computed tomography has been established as the most simple and readily available technique for the diagnosis and management of patients with acute stroke. Indeed, CT allows easy confirmation or exclusion of acute hemorrhage on the one hand, and on the other hand the early signs are quite reliable in the detection of ischemia. In the early phase the clot can be seen as well as exchanges related to early changes in water concentration in ischemia and the surrounding penumbra. Additional techniques such as angio-CT show the location of the clot and perfusion techniques reveal local hemodynamics as well as potential tissue viability. Newer techniques such as double energy CT and late phase CT should provide information on collateral flow as well as on the presence of early hemorrhagic transformation. All these techniques should thus make available new information on tissue viability,that is indispensable in the choice of revascularization technique. Thus CT techniques allow a quick and reliable triage as well as a finer characterization of the ischemic process. The use of all these CT techniques in an optimal way should help improve patient triage and selection of the most adequate treatment with further improvements in clinical outcomes as a result.

Expanding the concept of neuroprotection for acute ischemic stroke: The pivotal roles of reperfusion and the collateral circulation

This review surveys the efforts taken to achieve clinically efficacious protection of the ischemic brain and underscores the necessity of expanding our purview to include the essential role of cerebral perfusion and the collateral circulation. We consider the development of quantitative strategies to measure cerebral perfusion at the regional and local levels and the application of these methods to elucidate flow-related thresholds of ischemic viability and to characterize the ischemic penumbra. We stress that the modern concept of neuroprotection must consider perfusion, the necessary substrate upon which ischemic brain survival depends. We survey the major mechanistic approaches to neuroprotection and review clinical neuroprotection trials, focusing on those phase 3 multicenter clinical trials for acute ischemic stroke that have been completed or terminated. We review the evolution of thrombolytic therapies; consider the lessons learned from the initial, negative multicenter trials of endovascular therapy; and emphasize the highly successful positive trials that have finally established a clinical role for endovascular clot removal. As these studies point to the brain's collateral circulation as key to successful reperfusion, we next review the anatomy and pathophysiology of collateral perfusion as it relates to ischemic infarction, as well as the molecular and genetic influences on collateral development. We discuss the current MR and CT-based diagnostic methods for assessing the collateral circulation and the prognostic significance of collaterals in ischemic stroke, and we consider past and possible future therapeutic directions.

The diagnosis and clinical management of the neuropsychiatric manifestations of lupus

Neuropsychiatric (NP) involvement in Systemic Lupus Erythematosus (SLE), can be a severe and troubling manifestation of the disease that heavily impacts patient's health, quality of life and disease outcome. It is one of the most complex expressions of SLE which can affect central, peripheral and autonomous nervous system. Complex interrelated pathogenetic mechanisms, including genetic factors, vasculopathy, vascular occlusion, neuroendocrine-immune imbalance, tissue and neuronal damage mediated by autoantibodies, inflammatory mediators, blood brain barrier dysfunction and direct neuronal cell death can be all involved. About NPSLE a number of issues are still matter of debate: from classification and burden of NPSLE to attribution and diagnosis. The role of neuroimaging and new methods of investigation still remain pivotal and rapidly evolving as well as is the increasing knowledge in the pathogenesis. Overall, two main pathogenetic pathways have been recognized yielding different clinical phenotypes: a predominant ischemic-vascular one involving large and small blood vessels, mediated by aPL, immune complexes and leuko-agglutination which it is manifested with more frequent focal NP clinical pictures and a predominantly inflammatory-neurotoxic one mediated by complement activation, increased permeability of the BBB, intrathecal migration of autoantibodies, local production of immune complexes and pro-inflammatory cytokines and other inflammatory mediators usually appearing as diffuse NP manifestations. In the attempt to depict a journey throughout NPSLE from diagnosis to a reasoned therapeutic approach, classification, epidemiology, attribution, risk factors, diagnostic challenges, neuroimaging techniques and pathogenesis will be considered in this narrative review based on the most relevant and recent published data.

Perfusion computed tomography of intracranial meningiomas: In vivo correlation of cerebral blood volume and vascular permeability

BACKGROUND

A noninvasive method to predict the grade of a meningioma would be desirable since it would anticipate information about tumour nature, recurrence and improve tumour management and outcomes. The aim of the present study was to assess the ability of perfusion computed tomography (PCT) technique in predicting the meningioma grade before surgery. Data from PCT, such as cerebral blood volume (CBV) and permeability surface (PS), were correlated with immunohistolopathological information.

METHODS

Twenty-three patients with a diagnosis of intracranial meningioma underwent PCT for pre-surgical evaluation of CBV and PS. During surgery, samples from the centre and periphery of the tumour were obtained. Two correspondent regions of interest (ROIs) were drawn on CBV and PS maps. Central and peripheral CBV and PS mean values were calculated. PCT parameters were correlated to CD-34 and endoglin.

RESULTS

There was a positive correlation between PS and CD-34. No correlation was found between PS values and endoglin, CBV values and CD-34 and endoglin values.

CONCLUSION

Our findings suggest that PCT may support conventional morphological imaging in predicting meningioma grading before surgery.

CT perfusion with acetazolamide challenge in C6 gliomas and angiogenesis

Background

This study was performed to investigate the correlation between CT perfusion with acetazolamide challenge and angiogenesis in C6 gliomas.

Methods

Thirty-two male Sprague-Dawley rats were evaluated. The rats were divided randomly to four groups: eight rats with orthotopically implanted C6 gliomas at 10-days old (Group A), eight rats with gliomas at 14-days old (Group B), eight rats with gliomas at 18-days old (Group C), eight rats with orthotopically injected normal saline served as controls. CT perfusion was performed before and after administration of acetazolamide. Changes in perfusion parameters due to acetazolamide administration were calculated and analyzed.

Results

Elevated carbon dioxide partial pressure and decreased pH were found in all 32 rats post acetazolamide challenge (P<0.01). Cerebral blood flow_{pre-challenge} was increased in group C (95.0±2.5 ml/100g/min), as compared to group B (80.1±11.3 ml/100g/min) and group A (63.1±2.1 ml/100g/min). Cerebral blood flow percentage changes were detected with a reduction in group C (54.2±4.8%) as compared to controls (111.3±22.2%). Cerebral blood volume _{pre-challenge} was increased in group C (50.8±1.7ml/100g), as compared to group B (45.7±1.9 ml/100g) and group A (38.2±0.8 ml/100g). Cerebral blood volume percentage changes were decreased in group C (23.5±4.6%) as compared to controls (113.5±30.4%). Angiogenesis ratio = [(CD105-MVD) / (FVIII-MVD)] ×100%. Positive correlations were observed between CD105-microvessel density, angiogenesis ratio, vascular endothelial growth factor, proliferation marker and cerebral blood flow_{pre-challenge}, cerebral blood volume _{pre-challenge}. Negative correlations were observed between CD105-microvessel density and cerebral blood flow percentage changes (P<0.01, correlation coefficient r=-0.788), cerebral blood volume percentage changes (P<0.01, r=-0.703). Negative correlations were observed between angiogenesis ratio, vascular endothelial growth factor, proliferation marker and cerebral blood flow percentage changes, cerebral blood volume percentage changes.

Conclusion

Our findings suggest that CT perfusion with challenge can provide new insight into non-invasive assessment of rat C6 glioma angiogenesis.

Increased blood-brain barrier permeability on perfusion computed tomography predicts hemorrhagic transformation in acute ischemic stroke

BACKGROUND/PURPOSE

Perfusion computed tomography (CT) is capable of measuring the permeability surface product (PS). PS reflects the permeability of the blood-brain barrier, involved in the pathophysiology of hemorrhagic transformation (HT) of ischemic stroke. The aim of our study was to determine if an increased PS can predict HT.

METHODS

A total of 86 patients with ischemic stroke were included. They underwent multimodality CT, including the measurement of PS. We compared the clinical and radiological characteristics of patients who developed HT to those who did not, using univariate analysis. Multivariate regression analyses were then used to determine HT predictors.

RESULTS

HT was observed in 27 patients (31%). Infarct PS was significantly associated with HT (p = 0.047), as were atrial fibrillation (p = 0.03), admission National Institute of Health Stroke Scale score (p = 0.02), infarct volume (p = 0.0004), presence of large-vessel occlusion (p = 0.0005) and a poorer collateral status (p = 0.003). Using logistic regression modeling, an infarct PS >0.84 ml/100 g/min was an independent predictor of HT (OR 28, 95% CI 1.75-452.98; p = 0.02). Other independent predictors of HT were infarct volume and a history of atrial fibrillation.

CONCLUSIONS

Our findings suggest that infarct PS can be a predictor of HT and may help clinicians to improve patient care around thrombolysis decisions in the acute phase of ischemic stroke.

Interventional neuroradiology of stroke, still not dead

Since the National Institute of Neurological Disorders and Stroke trial, intravenous thrombolysis has been gaining wide acceptance as the modality of treatment for acute embolic stroke, with a current therapeutic window of up to 4.5 h. Both imaging [with either magnetic resonance imaging (MRI) or computed tomography (CT)] and interventional techniques (thrombolysis and/or thrombectomy) have since improved and provided us with additional imaging of the penumbra using CT or MRI and more advanced thrombolysis or thrombectomy strategies that have been embraced in many centers dealing with patients with acute cerebral ischemia. These techniques, however, have come under scrutiny due to their accrued healthcare costs and have been questioned following major recent studies. These studies basically showed that interventional techniques were not superior to the traditional intravenous thrombolysis techniques and that penumbra imaging could not determine what patients would benefit from more aggressive (i.e., interventional) treatment. We discuss this in the light of the latest developments in both diagnostic and interventional neuroradiology and point out why further studies are needed in order to define the right choices for patients with acute stroke. Indeed, these studies were in part conducted with suboptimal patient recruitment strategies and did not always use the latest interventional techniques available today. So, while these studies may have raised some relevant questions, at the same time, definitive answers have not been given, in our opinion.

Appropriate use of CT perfusion following aneurysmal subarachnoid hemorrhage: a Bayesian analysis approach

BACKGROUND AND PURPOSE

In recent years CTP has been used as a complementary diagnostic tool in the evaluation of delayed cerebral ischemia and vasospasm. Our aim was to determine the test characteristics of CTP for detecting delayed cerebral ischemia and vasospasm in SAH, and then to apply Bayesian analysis to identify subgroups for its appropriate use.

MATERIALS AND METHODS

Our retrospective cohort comprised consecutive patients with SAH and CTP performed between days 6 and 8 following aneurysm rupture. Delayed cerebral ischemia was determined according to primary outcome measures of infarction and/or permanent neurologic deficits. Vasospasm was determined by using DSA. The test characteristics of CTP and its 95% CIs were calculated. Graphs of conditional probabilities were constructed by using Bayesian techniques. Local treatment thresholds (posttest probability of delayed cerebral ischemia needed to initiate induced hypertension, hypervolemia, and hemodilution or intra-arterial therapy) were determined via a survey of 6 independent neurologists.

RESULTS

Ninety-seven patients with SAH were included in the study; 39% (38/97) developed delayed cerebral ischemia. Qualitative CTP deficits were seen in 49% (48/97), occurring in 84% (32/38) with delayed cerebral ischemia and 27% (16/59) without. The sensitivity, specificity, and positive and negative predictive values (95% CI) for CTP were 0.84 (0.73-0.96), 0.73 (0.62-0.84), 0.67 (0.51-0.79), and 0.88 (0.74-0.94), respectively. A subgroup of 57 patients underwent DSA; 63% (36/57) developed vasospasm. Qualitative CTP deficits were seen in 70% (40/57), occurring in 97% (35/36) with vasospasm and 23% (5/21) without. The sensitivity, specificity, and positive and negative predictive values (95% CI) for CTP were 0.97 (0.92-1.0), 0.76 (0.58-0.94), 0.88 (0.72-0.95), and 0.94 (0.69-0.99), respectively. Treatment thresholds were determined as 30% for induced hypertension, hypervolemia, and hemodilution and 70% for intra-arterial therapy.

CONCLUSIONS

Positive CTP findings identify patients who should be carefully considered for induced hypertension, hypervolemia, and hemodilution and/or intra-arterial therapy while negative CTP findings are useful in guiding a no-treatment decision.

Arterial input function in perfusion MRI: a comprehensive review

Cerebral perfusion, also referred to as cerebral blood flow (CBF), is one of the most important parameters related to brain physiology and function. The technique of dynamic-susceptibility contrast (DSC) MRI is currently the most commonly used MRI method to measure perfusion. It relies on the intravenous injection of a contrast agent and the rapid measurement of the transient signal changes during the passage of the bolus through the brain. Central to quantification of CBF using this technique is the so-called arterial input function (AIF), which describes the contrast agent input to the tissue of interest. Due to its fundamental role, there has been a lot of progress in recent years regarding how and where to measure the AIF, how it influences DSC-MRI quantification, what artefacts one should avoid, and the design of automatic methods to measure the AIF. The AIF is also directly linked to most of the major sources of artefacts in CBF quantification, including partial volume effect, bolus delay and dispersion, peak truncation effects, contrast agent non-linearity, etc. While there have been a number of good review articles on DSC-MRI over the years, these are often comprehensive but, by necessity, with limited in-depth discussion of the various topics covered. This review article covers in greater depth the issues associated with the AIF and their implications for perfusion quantification using DSC-MRI.

Long-term impact of perfusion CT data after subarachnoid hemorrhage

INTRODUCTION

Dynamic perfusion computed tomography (PCT) has been established as a diagnostic instrument for the detection of vasospasm after subarachnoid hemorrhage (SAH). The purpose of this study was to assess the prognostic impact of PCT parameters after SAH on the long-term outcome of patients.

METHODS

Three hundred twelve patients were retrospectively interrogated with a questionnaire 23.06 ± 14.33 months after spontaneous subarachnoid hemorrhage. The modified Rankin scale (mRS) was determined, respectively. Scheduled PCT data sets from the first days after ictus were available for all patients.

RESULTS

The maximum mean transit time over several examinations per hemisphere (MTTPEAK) values were significantly correlated (p ≤ 0.001, r = 0.422) with the clinical long-term outcome (mRS). Corresponding to our linear regression analysis, MTTPEAK is the second most important regressor (behind clinical severity of the initial hemorrhage) for the prediction of long-term mRS. An MTTPEAK threshold of 3.98 s (identified by receiver operating characteristic analysis, area under the curve = 0.75) predicted an unfavorable long-term outcome (mRS ≥ 2) with a sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of 67.3, 74.3, 84.5, 52.1, and 69.6 %, respectively.

CONCLUSION

The presented data corroborate the relevance of PCT data for the clinical long-term outcome of SAH patients. By identification of patients who are at risk for a bad outcome and may need escalation of therapy, risk-benefit analysis is supported.

Effects of magnetically labeled exogenous endothelial progenitor cells on cerebral blood perfusion and microvasculature alterations after traumatic brain injury in rat model

BACKGROUND

Increasing evidence suggests that endothelial progenitor cells (EPCs), a subgroup of bone marrow hematopoietic stromal cells, play a critical role in neovascularization and tissue repair.

PURPOSE

To explore the effect of exogenous EPCs on the cerebral blood perfusion and microvessels in the injured region in rat model with traumatic brain injury (TBI).

MATERIAL AND METHODS

EPCs were collected from the spleens of healthy Sprague-Dawley rats. Fifty-four Sprague-Dawley rats were randomly divided into six groups. The controlled cortical impact TBI was performed. Spleen-derived exogenous EPCs labeled with super-paramagnetic iron oxide (SPIO) (SPIO-EPCs) were transplanted into the blood by tail vein of rats at 6 and 12 h after TBI, respectively. Magnetic resonance imaging (MRI) and computed tomography perfusion imaging were performed at various time points. Microvascular density was determined by immunohistochemistry.

RESULTS

In SPIO-EPCs group, patchlike hypointensities were detected in the injured region at 24 h after transplantation, and the range of hypointensities tended to expand gradually over time on MRI, which was confirmed by Prussian blue staining. Computed tomography perfusion imaging parameters were gradually developed from hyperperfusion to normal, while, microvascular density was gradually increased during 72 to 168 h after injury. The values of these indices in SPIO-EPCs group were significantly lower than those in SPIO-alone group at the same time point, but no significant differences were found in different time groups.

CONCLUSION

The intravenously transplanted EPCs diminish the brain injury through restoring cerebral blood perfusion and increasing the cerebral microvasculature in the injured region in rat model with TBI.

In vitro evaluation of the imaging accuracy of C-arm conebeam CT in cerebral perfusion imaging

PURPOSE

The authors have developed a method to enable cerebral perfusion CT imaging using C-arm based conebeam CT (CBCT). This allows intraprocedural monitoring of brain perfusion during treatment of stroke. Briefly, the technique consists of acquiring multiple scans (each scan comprised of six sweeps) acquired at different time delays with respect to the start of the x-ray contrast agent injection. The projections are then reconstructed into angular blocks and interpolated at desired time points. The authors have previously demonstrated its feasibility in vivo using an animal model. In this paper, the authors describe an in vitro technique to evaluate the accuracy of their method for measuring the relevant temporal signals.

METHODS

The authors' evaluation method is based on the concept that any temporal signal can be represented by a Fourier series of weighted sinusoids. A sinusoidal phantom was developed by varying the concentration of iodine as successive steps of a sine wave. Each step corresponding to a different dilution of iodine contrast solution contained in partitions along a cylinder. By translating the phantom along the axis at different velocities, sinusoidal signals at different frequencies were generated. Using their image acquisition and reconstruction algorithm, these sinusoidal signals were imaged with a C-arm system and the 3D volumes were reconstructed. The average value in a slice was plotted as a function of time. The phantom was also imaged using a clinical CT system with 0.5 s rotation. C-arm CBCT results using 6, 3, 2, and 1 scan sequences were compared to those obtained using CT. Data were compared for linear velocities of the phantom ranging from 0.6 to 1 cm∕s. This covers the temporal frequencies up to 0.16 Hz corresponding to a frequency range within which 99% of the spectral energy for all temporal signals in cerebral perfusion imaging is contained.

RESULTS

The errors in measurement of temporal frequencies are mostly below 2% for all multiscan sequences. For single scan sequences, the errors increase sharply beyond 0.10 Hz. The amplitude errors increase with frequency and with decrease in the number of scans used.

CONCLUSIONS

Our multiscan perfusion CT approach allows low errors in signal frequency measurement. Increasing the number of scans reduces the amplitude errors. A two-scan sequence appears to offer the best compromise between accuracy and the associated total x-ray and iodine dose.