Optimal duration of acquisition for dynamic perfusion CT assessment of blood-brain barrier permeability using the Patlak model

Novel advanced imaging techniques for cerebral oedema

Cerebral oedema following acute ischemic infarction has been correlated with poor functional outcomes and is the driving mechanism of malignant infarction. Measurements of midline shift and qualitative assessment for herniation are currently the main CT indicators for cerebral oedema but have limited sensitivity for small cortical infarcts and are typically a delayed sign. In contrast, diffusion-weighted (DWI) or T2-weighted magnetic resonance imaging (MRI) are highly sensitive but are significantly less accessible. Due to the need for early quantification of cerebral oedema, several novel imaging biomarkers have been proposed. Based on neuroanatomical shift secondary to space-occupying oedema, measures such as relative hemispheric volume and cerebrospinal fluid displacement are correlated with poor outcomes. In contrast, other imaging biometrics, such as net water uptake, T2 relaxometry and blood brain barrier permeability, reflect intrinsic tissue changes from the influx of fluid into the ischemic region. This review aims to discuss quantification of cerebral oedema using current and developing advanced imaging techniques, and their role in predicting clinical outcomes.

Whole-Brain Permeability Analysis on Admission Improves Prediction of Delayed Cerebral Ischemia Following Aneurysmal Subarachnoid Hemorrhage

PURPOSE

To evaluate the changes of blood-brain barrier permeability (BBBP) after aneurysmal subarachnoid hemorrhage (aSAH) and find out whether BBBP within 24 h after onset can further improve prediction of delayed cerebral ischemia (DCI).

METHODS

CT perfusion (CTP) was performed within 24 h after onset and in the DCI time window (DCITW). Whole brain average values of flow extraction product (mK^trans), qualitative and quantitative CTP parameters, and clinical data were compared between DCI and non-DCI groups. The changes of mK^trans were analysed using a Paired t test. Multivariate logistic regression analysis and ROC analyses were performed to identify predictors of DCI and evaluate the predictive performance.

RESULTS

One hundred and forty of 179 consecutive patients were included, 45 of whom (32%) developed DCI. mK^trans was higher in the DCI group both on admission and in the DCITW (P<0.001). mK^trans decreased significantly in the non-DCI group (P=0.003), but not in DCI group (P=0.285). Multivariate logistic regression analysis showed that mK^trans (OR=1.07, 95%CI: 1.03-1.11, P<0.001), World Federation of Neurosurgery Scale (OR=6.73, 95%CI: 1.09-41.41, P=0.040), Hunt-Hess grade (OR=0.16, 95%CI: 0.02-1.19, P=0.073), modified Fisher Score (OR=3.74, 95%CI: 1.30-10.75, P=0.014), and qualitative CTP (OR=4.31, 95%CI: 1.49-12.47, P=0.007) were independent predictors of DCI. The model with K^trans produced a larger AUC of 0.88 (95%CI: 0.81-0.95), with corresponding sensitivity and specificity of 84% and 86%, respectively.

CONCLUSION

BBBP measurement within 24 h after onset can improve the prediction of DCI. Early moderate BBB disruption may be reversible, whereas severe BBBP disruption indicates the risk of DCI.

Neuroimaging Prediction of Hemorrhagic Transformation for Acute Ischemic Stroke

BACKGROUND

Hemorrhagic transformation (HT) is a common complication of acute ischemic stroke, often resulting from reperfusion therapy. Early prediction of HT can enable stroke neurologists to undertake measures to avoid clinical deterioration and make optimal treatment strategies. Moreover, the trend of extending the time window for reperfusion therapy (both for intravenous thrombolysis and endovascular treatment) further requires more precise detection of HT tendency.

SUMMARY

In this review, we summarized and discussed the neuroimaging markers of HT prediction of acute ischemic stroke patients, mainly focusing on neuroimaging markers of ischemic degree and neuroimaging markers of blood-brain barrier permeability. This review is aimed to provide a concise introduction of HT prediction and to elicit possibilities of future research combining advanced technology to improve the accessibility and accuracy of HT prediction under emergent clinical settings. Key Messages: Substantial studies have utilized neuroimaging, blood biomarkers, and clinical variables to predict HT occurrence. Although huge progress has been made, more individualized and precise HT prediction using simple and robust imaging predictors combining stroke onset time should be the future goal of development.

Comparison of blind deconvolution- and Patlak analysis-based methods for determining vascular permeability

This study describes a computational algorithm to determine vascular permeability constants from time-lapse imaging data without concurrent knowledge of the arterial input function. The algorithm is based on "blind" deconvolution of imaging data, which were generated with analytical and finite-element models of bidirectional solute transport between a capillary and its surrounding tissue. Compared to the commonly used Patlak analysis, the blind algorithm is substantially more accurate in the presence of solute delay and dispersion. We also compared the performance of the blind algorithm with that of a simpler one that assumed unidirectional transport from capillary to tissue [as described in Truslow et al., Microvasc. Res. 90, 117-120 (2013)]. The algorithm based on bidirectional transport was more accurate than the one based on unidirectional transport for more permeable vessels and smaller extravascular distribution volumes, and less accurate for less permeable vessels and larger extravascular distribution volumes. Our results indicate that blind deconvolution is superior to Patlak analysis for permeability mapping under clinically relevant conditions, and can thus potentially improve the detection of tissue regions with a compromised vascular barrier.

Permeability-surface area product of the penumbra as a predictor of outcome after endovascular treatment of anterior circulation acute ischemic stroke

Background

Permeability-surface product is a predictor of blood–brain barrier disruption, a condition that may be related to higher likelihoods of hemorrhagic transformations in acute stroke.

Purpose

To investigate whether permeability-surface product can be used as a parameter for predicting outcome after mechanical thrombectomy in patients with anterior circulation acute ischemic stroke.

Material and Methods

We retrospectively identified patients with acute middle cerebral artery stroke who underwent successful mechanical thrombectomy between November 2009 and July 2015. Multiple parameters (including age) and CT perfusion-related parameters (including permeability-surface product) were compared between patients with favorable (modified Rankin Scale [mRS] = 0–2) and unfavorable (mRS > 2) outcome.

Results

Thirty patients were included, 50% having favorable and 50% having unfavorable outcome. Younger age was significantly associated with favorable outcome ( P < 0.03). Other baseline characteristics, such as size of CT perfusion core infarction, perfusion abnormality, and presentation of subcortical infarction were not significantly different between groups. No significant difference was observed between groups for permeability-surface product in the ipsilateral penumbra or for the ratio between permeability-surface product penumbra value and contralateral normal brain (permeability-surface product ratio).

Conclusions

No significant difference was observed between patients with and without favorable outcome after mechanical thrombectomy for either permeability-surface product value or permeability-surface product ratio. Although permeability-surface product is a good predictor of blood–brain barrier disruption, this study revealed no evidence that either permeability-surface product value or permeability-surface product ratio is associated with future change in the penumbra.

Diagnostic accuracy of computed tomography perfusion in the prediction of haemorrhagic transformation and patient outcome in acute ischaemic stroke: A systematic review and meta-analysis

Purpose

The aim of this systematic review and meta-analysis is to determine the diagnostic accuracy of computed tomography brain perfusion in the prediction of haemorrhagic transformation and patient outcome in acute ischaemic stroke.

Method

Electronic databases and grey literature published over the last 10 years related to healthcare and radiology were searched using the key terms: 'computed tomography perfusion', 'haemorrhagic transformation', 'acute ischaemic stroke', 'functional outcome' and their synonyms using both UK and American spellings. Inclusion criteria were: sample size at least 30 patients, original research, evaluate ability of computed tomography perfusion to predict haemorrhagic transformation, reports diagnostic accuracy or provide relevant data for a 2 × 2 contingency table, use follow-up non-contrast computed tomography (NCCT) or magnetic resonance imaging as reference standard.

Findings

Twelve studies were included in the review; studies cover a total of 808 patients. Haemorrhagic transformation occurred in 30.2% of patients. Pooled sensitivity and specificity were 85.9% (95% CI; 65-97%), 73.9% (95% CI; 45-92%) and accuracy of 79.1% (95% CI; 57-98%). Pooled NPV was 92.9% with a high false positive rate (19.8%), which could be explained in terms of outcome classification, acquisition artefact and computed tomography perfusion processing algorithms.

Discussion

This review evaluated the importance of using pre-defined threshold measurement for optimal prediction of HT, the relevance of patient pre-treatment clinical parameters to HT occurrence, the CTP parameters and the measurements that are independent predictors of HT, the significance of rtPA rather as an exacerbator of HT and the impact of both minor and major HT/PH on patient 2⁰ functional outcome.

Conclusion

Computed tomography perfusion has a high sensitivity and moderately high specificity for prediction of haemorrhagic transformation in acute ischaemic stroke. Pre-treatment clinical decision making requires consideration of clinical factors in addition to imaging findings. This systematic review and meta-analysis highlights that pre-treatment computed tomography perfusion adds to clinical confidence by predicting potential for haemorrhage, both in thrombolysed and un-thrombolysed patients, and also influences decisions about alternative treatments for acute ischaemic stroke patients.

Relative Patlak plot for dynamic PET parametric imaging without the need for early-time input function

The Patlak graphical method is widely used in parametric imaging for modeling irreversible radiotracer kinetics in dynamic PET. The net influx rate of radiotracer can be determined from the slope of the Patlak plot. The implementation of the standard Patlak method requires the knowledge of full-time input function from the injection time until the scan end time, which presents a challenge for use in the clinic. This paper proposes a new relative Patlak plot method that does not require early-time input function and therefore can be more efficient for parametric imaging. Theoretical analysis proves that the effect of early-time input function is a constant scaling factor on the Patlak slope estimation. Thus, the parametric image of the slope of the relative Patlak plot is related to the parametric image of standard Patlak slope by a global scaling factor. This theoretical finding has been further demonstrated by computer simulation and real patient data. The study indicates that parametric imaging of the relative Patlak slope can be used as a substitute of parametric imaging of standard Patlak slope for tasks that do not require absolute quantification, such as lesion detection and tumor volume segmentation.

Severe Blood-Brain Barrier Disruption in Cardioembolic Stroke

Background

Previous studies demonstrated that cardioembolism (CE) was prone to develop hemorrhagic transformation (HT), whereas hyper-permeability of blood–brain barrier (BBB) might be one reason for the development of HT. We, thus, aimed to investigate whether the BBB permeability (BBBP) was higher in CE stroke than other stroke subtypes in acute ischemic stroke (AIS) patients.

Methods

This study was a retrospective review of prospectively collected clinical and imaging database of AIS patients who underwent CT perfusion. Hypoperfusion was defined as Tmax >6 s. The average relative permeability-surface area product (rPS), reflecting the BBBP, was calculated within the hypoperfusion region (rPS_hypo). CE was diagnosed according to the international Trial of Org 10172 in Acute Stroke Treatment criteria. Receiver operating characteristics (ROC) curve analysis was used to determine predictive value of rPS_hypo for CE. Logistic regression was used to identify independent predictors for CE.

Results

A total of 187 patients were included in the final analysis [median age, 73 (61–80) years; 75 (40.1%) females; median baseline National Institutes of Health Stroke Scale score, 12 (7–16)]. Median rPS_hypo was 65.5 (35.8–110.1)%. Ninety-seven (51.9%) patients were diagnosed as CE. ROC analysis revealed that the optimal rPS_hypo threshold for CE was 86.71%. The value of rPS_hypo and the rate of rPS_hypo>86.71% were significantly higher in patients with CE than other stroke subtypes (p < 0.05), after adjusting for the potential confounds.

Conclusion

The extent of BBB disruption is more severe in CE stroke than other stroke subtypes during the hyperacute stage.

Carotid Artery Stenting and Blood-Brain Barrier Permeability in Subjects with Chronic Carotid Artery Stenosis

Failure of the blood-brain barrier (BBB) is a critical event in the development and progression of diseases such as acute ischemic stroke, chronic ischemia or small vessels disease that affect the central nervous system. It is not known whether BBB breakdown in subjects with chronic carotid artery stenosis can be restrained with postoperative recovery of cerebral perfusion. The aim of the study was to assess the short-term effect of internal carotid artery stenting on basic perfusion parameters and permeability surface area-product (PS) in such a population. Forty subjects (23 males) with stenosis of >70% within a single internal carotid artery and neurological symptoms who underwent a carotid artery stenting procedure were investigated. Differences in the following computed tomography perfusion (CTP) parameters were compared before and after surgery: global cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time to peak (TTP) and PS. PS acquired by CTP is used to measure the permeability of the BBB to contrast material. In all baseline cases, the CBF and CBV values were low, while MTT and TTP were high on both the ipsi- and contralateral sides compared to reference values. PS was approximately twice the normal value. CBF was higher (+6.14%), while MTT was lower (−9.34%) on the contralateral than on the ipsilateral side. All perfusion parameters improved after stenting on both the ipsilateral (CBF +22.66%; CBV +18.98%; MTT −16.09%, TTP −7.62%) and contralateral (CBF +22.27%, CBV +19.72%, MTT −14.65%, TTP −7.46%) sides. PS decreased by almost half: ipsilateral −48.11%, contralateral −45.19%. The decline in BBB permeability was symmetrical on the ipsi- and contralateral sides to the stenosis. Augmented BBB permeability can be controlled by surgical intervention in humans.

CT perfusion analysis by nonlinear regression for predicting hemorrhagic transformation in ischemic stroke

PURPOSE

Intravenous thrombolysis can improve clinical outcome in acute ischemic stroke patients but increases the risk of hemorrhagic transformation (HT). Blood-brain barrier damage, which can be quantified by the vascular permeability for contrast agents, is a potential predictor for HT. This study aimed to assess whether this prediction can be improved by measuring vascular permeability using a novel fast nonlinear regression (NLR) method instead of Patlak analysis.

METHODS

From a prospective ischemic stroke multicenter cohort study, 20 patients with HT on follow-up imaging and 40 patients without HT were selected. The permeability transfer constant K(trans) was measured in three ways; using standard Patlak analysis, Patlak analysis with a fixed offset, and the NLR method. In addition, the permeability-surface (PS) area product and the conventional perfusion parameters (blood volume, flow, and mean transit time) were measured using the NLR method. Relative values were calculated in two ways, i.e., by dividing the average in the infarct core by the average in the contralateral hemisphere, and by dividing the average in the ipsilateral hemisphere by the average in the contralateral hemisphere. Mann-Whitney U tests and receiver operating characteristic (ROC) analyses were performed to assess the discriminative power of each of the relative parameters.

RESULTS

Both the infarct-core and whole-hemisphere averaged relative K(trans) (rK(trans)) values, measured with the NLR method, were significantly higher in the patients who developed HT as compared with those who did not. The rK(trans) measured with standard Patlak analysis was not significantly different. The relative PS (rPS), measured with NLR, had the highest discriminative power (P = 0.002). ROC analysis of rPS showed an area under the curve (AUC) of 0.75 (95% confidence interval: 0.62-0.89) and a sensitivity of 0.75 at a specificity of 0.75. The AUCs of the Patlak rK(trans), the Patlak rK(trans) with fixed offset, and the NLR rK(trans) were 0.58, 0.66, and 0.67, respectively.

CONCLUSIONS

CT perfusion analysis may aid in predicting HT, but standard Patlak analysis did not provide estimates for rK(trans) that were significantly higher in the HT group. The rPS, measured in the infarct core with NLR, had superior discriminative power compared with K(trans) measured with either Patlak analysis with a fixed offset or NLR, and conventional perfusion parameters.

Application of Blood-Brain Barrier Permeability Imaging in Global Cerebral Edema

BACKGROUND AND PURPOSE

Blood-brain barrier permeability is not routinely evaluated in the clinical setting. Global cerebral edema occurs after SAH and is associated with BBB disruption. Detection of global cerebral edema using current imaging techniques is challenging. Our purpose was to apply blood-brain barrier permeability imaging in patients with global cerebral edema by using extended CT perfusion.

MATERIALS AND METHODS

Patients with SAH underwent CTP in the early phase after aneurysmal rupture (days 0-3) and were classified as having global cerebral edema or nonglobal cerebral edema using established noncontrast CT criteria. CTP data were postprocessed into blood-brain barrier permeability quantitative maps of PS (permeability surface-area product), K(trans) (volume transfer constant from blood plasma to extravascular extracellular space), Kep (washout rate constant of the contrast agent from extravascular extracellular space to intravascular space), VE (extravascular extracellular space volume per unit of tissue volume), VP (plasmatic volume per unit of tissue volume), and F (plasma flow) by using Olea Sphere software. Mean values were compared using t tests.

RESULTS

Twenty-two patients were included in the analysis. Kep (1.32 versus 1.52, P < .0001), K(trans) (0.15 versus 0.19, P < .0001), VP (0.51 versus 0.57, P = .0007), and F (1176 versus 1329, P = .0001) were decreased in global cerebral edema compared with nonglobal cerebral edema while VE (0.81 versus 0.39, P < .0001) was increased.

CONCLUSIONS

Extended CTP was used to evaluate blood-brain barrier permeability in patients with SAH with and without global cerebral edema. Kep is an important indicator of altered blood-brain barrier permeability in patients with decreased blood flow, as Kep is flow-independent. Further study of blood-brain barrier permeability is needed to improve diagnosis and monitoring of global cerebral edema.

Diagnosis of acute ischemia using dual energy CT after mechanical thrombectomy

BACKGROUND AND PURPOSE

To assess the performance of dual energy unenhanced CT in the detection of acute ischemia after mechanical thrombectomy.

METHODS

Retrospective study, approved by the local institutional review board, including all patients that underwent intra-arterial thrombectomy in our institution over a period of 2 years. The presence of acute ischemia and hemorrhage was evaluated by three readers. Sensitivity and specificity of the non-contrast CT weighted sum image (NCCT) and the virtual non-contrast reconstructed image (VNC) were estimated and compared using generalized estimating equations to account for the non-independence of regions in each patient.

RESULTS

58 patients (27 women and 31 men; mean age 70.4 years) were included in the study, yielding 580 regions of interest. Sensitivity and specificity in detecting acute ischemia were higher for all readers when using VNC, with a significant increase in sensitivity for two readers (p<0.001 and 0.01) and a significant increase in specificity in one reader (p<0.001). Specificity in detecting hemorrhage was excellent for all readers.

CONCLUSIONS

Dual energy unenhanced CT VNC images were superior in the identification of acute ischemia in comparison with NCCT.

Dynamic contrast-enhanced MRI and CT provide comparable measurement of blood-brain barrier permeability in a rodent stroke model

In the current management of acute ischemic stroke (AIS), clinical criteria are used to estimate the risk of hemorrhagic transformation (HT), which is a devastating early complication. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and computed tomography (DCE-CT) may serve as physiologically-based decision making tools to more reliably assess the risk of HT. Before these tools can be properly validated, the comparability of the blood-brain barrier (BBB) permeability measurements they generate should be assessed. Sixteen rats were subjected to a transient middle cerebral artery occlusion before successively undergoing DCE-CT and DCE-MRI at 24-hours. BBB permeability (K(trans)) values were generated from both modalities. A correlation of R=0.677 was found (p<0.01) and the resulting relationship was [DCE-CT=(0.610*DCE-MRI)+4.140]. A variance components analysis found the intra-rat coefficient of variation to be 0.384 and 0.258 for K(trans) values from DCE-MRI and DCE-CT respectively. Permeability measures from DCE-CT were 22% higher than those from DCE-MRI. The results of this study demonstrate for the first time comparability between DCE-CT and DCE-MRI in the assessment of AIS. These results may provide a foundation for future clinical trials making combined use of these modalities.

Blood-brain barrier permeability imaging using perfusion computed tomography

Background.

The blood-brain barrier represents the selective diffusion barrier at the level of the cerebral microvascular endothelium. Other functions of blood-brain barrier include transport, signaling and osmoregulation. Endothelial cells interact with surrounding astrocytes, pericytes and neurons. These interactions are crucial to the development, structural integrity and function of the cerebral microvascular endothelium. Dysfunctional blood-brain barrier has been associated with pathologies such as acute stroke, tumors, inflammatory and neurodegenerative diseases.

Conclusions.

Blood-brain barrier permeability can be evaluated in vivo by perfusion computed tomography - an efficient diagnostic method that involves the sequential acquisition of tomographic images during the intravenous administration of iodinated contrast material. The major clinical applications of perfusion computed tomography are in acute stroke and in brain tumor imaging.

Relation between stroke severity, patient characteristics and CT-perfusion derived blood-brain barrier permeability measurements in acute ischemic stroke

Purpose

Increased blood-brain barrier permeability (BBBP) can result from ischemia. In this study the relation between stroke severity, patient characteristics and admission BBBP values measured with CT-perfusion (CTP) was investigated in acute ischemic stroke patients.

Methods

From prospective data of the Dutch Acute Stroke Study 149 patients with a middle cerebral artery stroke and extended CTP were selected. BBBP values were measured in the penumbra and infarct core as defined by CTP thresholds, and in the contra-lateral hemisphere. The relation between stroke (severity) variables and patient characteristics, including early CT signs, dense vessel sign (DVS), time to scan and National Institute of Health Stroke Score (NIHSS), and BBBP parameters in penumbra and infarct core was quantified with regression analysis.

Results

Early CT signs were related to higher BBBP values in the infarct core (B = 0.710), higher ipsi- to contra-lateral BBBP ratios (B = 0.326) and higher extraction ratios in the infarct core (B = 16.938). Females were found to have lower BBBP values in penumbra and infarct core (B = − 0.446 and − 0.776 respectively) and lower extraction ratios in the infarct core (B = − 10.463). If a DVS was present the ipsi- to contra-lateral BBBP ratios were lower (B = − 0.304). There was no relation between NIHSS or time to scan and BBBP values.

Conclusion

Early CT signs are related to higher BBBP values in the infarct core, suggesting that only severe ischemic damage alters BBBP within the first hours after symptom onset.

Determination of vascular permeability coefficients under slow luminal filling

This Communication describes a method to obtain the permeability product (permeability coefficient normalized by vascular dimensions) from time-lapse intensity data for which the introduction of labeled solute into the vasculature does not occur at a sharply defined time. This method has an error of ~10% across a wide range of filling times and noise levels, and is particularly well-suited for situations in which the permeability coefficient is greater than 10(-6)cm/s. We show that it is applicable whether the increase in vascular solute concentration is sustained or transient.

Measurement of blood-brain barrier permeability in acute ischemic stroke using standard first-pass perfusion CT data

Background and purpose

Increased blood–brain barrier permeability is believed to be associated with complications following acute ischemic stroke and with infarct expansion. Measurement of blood–brain barrier permeability requires a delayed image acquisition methodology, which prolongs examination time, increasing the likelihood of movement artefacts and radiation dose. Existing quantitative methods overestimate blood–brain barrier permeability when early phase CT perfusion data are used. The purpose of this study is to develop a method that yields the correct blood–brain barrier permeability value using first-pass perfusion CT data.

Methods

We acquired 43 CT perfusion datasets, comprising experimental (n = 30) and validation subject groups (n = 13). The Gjedde–Patlak method was used to estimate blood–brain barrier permeability using first-pass (30–60 s after contrast administration) and delayed phase (30–200 s) data. In the experimental group, linear regression was used to obtain a function predicting first-pass blood–brain barrier permeability estimates from delayed phase estimates in each stroke compartment. The reliability of prediction with this function was then tested using data from the validation group.

Results

The predicted delayed phase blood–brain barrier permeability was strongly correlated with the measured delayed phase value (r = 0.67 and 0.6 for experimental and validation group respectively; p < 0.01). Predicted and measured delayed phase blood–brain barrier permeability in each stroke compartment were not significantly different in both experimental and validation groups.

Conclusion

We have developed a method of estimating blood–brain barrier permeability using first-pass perfusion CT data. This predictive method allows reliable blood–brain barrier permeability estimation within standard acquisition time, minimizing the likelihood of motion artefacts thereby improving image quality and reducing radiation dose.

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Delayed phase BBBP can be predicted from first-pass perfusion CT data.

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Predicted BBBP was not significantly different from delayed phase measurements.

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Prediction model allows reliable BBBP estimation within the standard acquisition time.

Can iterative reconstruction improve imaging quality for lower radiation CT perfusion? Initial experience

BACKGROUND AND PURPOSE

Initial results using IR for CT of the head showed satisfactory subjective and objective imaging quality with a 20-40% radiation dose reduction. The aim of our study was to compare the influence of IR and FBP algorithms on perfusion parameters at standard and lowered doses of CTP.

MATERIALS AND METHODS

Forty patients with unilateral carotid stenosis post-carotid stent placement referred for follow-up CTP were divided into 2 groups (tube currents were 100 mAs in group A and 80 mAs in group B). Datasets were reconstructed with IR and FBP algorithms; and SNRs of gray matter, white matter, and arterial and venous ROIs were compared. CBF, CBV, and MTT means and SNRs were evaluated by using linear regression, and qualitative imaging scores were compared across the 2 algorithms.

RESULTS

The mean effective radiation dose of group B (2.06 mSv) was approximately 20% lower than that of group A (2.56 mSv). SNRs for ROIs in the dynamic contrast-enhanced images were significantly higher than those for the FBP images. Correlations of the SNRs for CBF, CBV, and MTT across the 2 algorithms were moderate (R² = 0.46, 0.23, and 0.44, respectively). ROIs in gray matter rather than the IR algorithm predicted increasing SNRs in all CBF, CBV, and MTT maps. Two cases of significant restenosis were confirmed in both algorithms. CBV, CBF, and MTT imaging scores did not differ significantly across algorithms or groups.

CONCLUSIONS

Lower dose CTP (20% below normal dose) without IR can effectively identify oligemic tissue in poststenting follow-up. IR does not alter the absolute values or increase the SNRs of perfusion parameters. Other methods should be attempted to improve SNRs in settings with low tube currents.

Quantification of blood-to-brain transfer rate in multiple sclerosis

Blood-brain barrier (BBB) disruption visualized in lesions by MRI is a major biomarker of disease activity in multiple sclerosis (MS). However, in MS, destruction occurs to a variable extent in lesions as well as in gray matter (GM) and in the normal appearing white matter (NAWM). A method to quantify the BBB disruption in lesions as well as in non-lesion areas would be useful for assessment of MS progression and treatments. The objective of this study was to quantify the BBB transfer rate (Ki) in WM lesions, in the NAWM, and in the full-brain of MS patients. Thirteen MS patients with active lesions and 10 healthy controls with age and gender matching were recruited for full-brain and WM Ki studies. Dynamic contrast-enhanced MRI (DCEMRI) scans were conducted using T1 mapping with partial inversion recovery (TAPIR), a fast T1 mapping technique, following administration of a quarter-dose of the contrast agent Gadolinium-DTPA (Gd-DTPA). The Patlak modeling technique was used to derive a voxel-based map of Ki. In all patients contrast-enhanced lesions, quantified by Ki maps, were observed. Compared with controls, patients with MS exhibited an increase in mean Ki of the full-brain (P-value<0.05) but no significant difference in mean Ki of NAWM. The identified increase in full-brain Ki of MS patients suggests a global vascular involvement associated with MS disease. The lack of observed significant decrease in Ki in NAWM suggests lower involvement of WM vasculature than full-brain vasculature in MS. Ki maps constructed from time series data acquired by DCEMRI provide additional information about BBB that could be used for evaluation of vascular involvement in MS and monitoring treatment effectiveness.

Comparison between acetazolamide challenge and 10% carbon dioxide challenge perfusion CT in rat C6 glioma

RATIONALE AND OBJECTIVES

The aim of this study was to investigate the effect of perfusion computed tomography (PCT) with acetazolamide (ACZ) challenge and compare it to 10% carbon dioxide (CO(2)) challenge in rat C6 glioma.

MATERIALS AND METHODS

PCT was performed on 32 rats, including 20 with orthotopically implanted C6 gliomas and 12 serving as controls. Ten rats with gliomas and six normal rats underwent PCT with ACZ challenge. The other 10 rats with gliomas and six normal rats underwent PCT with 10% CO(2) challenge. The raw data were processed using Philips computed tomographic brain perfusion software. Perfusion parameters before and after the challenge were recorded. Percentage changes due to ACZ administration and 10% CO(2) challenge were calculated. Pearson's correlation coefficients were used to investigate relationships between percentage changes in perfusion parameters and vascular endothelial growth factor and microvessel density.

RESULTS

In C6 gliomas, percentage change in cerebral blood flow was significantly different between ACZ (72.73%) and 10% CO(2) (28.47%) challenge (P < .01). Percentage change in cerebral blood volume was 37.85% with ACZ and 24.69% with 10% CO(2) challenge (P = .02). In controls, percentage change in cerebral blood flow was significantly different between ACZ (117.42%) and 10% CO(2) (65.86%) challenge (P < .01). For percentage change in cerebral blood volume, there was no significant difference between ACZ (107.51%) and 10% CO(2) (92.95%) challenge. Significant correlations were observed among percentage changes in vascular endothelial growth factor, microvessel density, and cerebral blood volume (P < .01). Percentage change in cerebral blood flow correlated well with vascular endothelial growth factor.

CONCLUSIONS

The results of this study indicate that PCT with ACZ challenge is a more reliable technique compared to 10% CO(2) challenge for the quantitative evaluation of microcirculation in gliomas.

The role of free radical generation in increasing cerebrovascular permeability

The brain endothelium constitutes a barrier to the passive movement of substances from the blood into the cerebral microenvironment, and disruption of this barrier after a stroke or trauma has potentially fatal consequences. Reactive oxygen species (ROS), which are formed during these cerebrovascular accidents, have a key role in this disruption. ROS are formed constitutively by mitochondria and also by the activation of cell receptors that transduce signals from inflammatory mediators, e.g., activated phospholipase A₂ forms arachidonic acid that interacts with cyclooxygenase and lipoxygenase to generate ROS. Endothelial NADPH oxidase, activated by cytokines, also contributes to ROS. There is a surge in ROS following reperfusion after cerebral ischemia and the interaction of the signaling pathways plays a role in this. This review critically evaluates the literature and concludes that the ischemic penumbra is a consequence of the initial edema resulting from the ROS surge after reperfusion.

Validation of in vivo magnetic resonance imaging blood-brain barrier permeability measurements by comparison with gold standard histology

BACKGROUND AND PURPOSE

We sought to validate the blood-brain barrier permeability measurements extracted from perfusion-weighted MRI through a relatively simple and frequently applied model, the Patlak model, by comparison with gold standard histology in a rat model of ischemic stroke.

METHODS

Eleven spontaneously hypertensive rats and 11 Wistar rats with unilateral 2-hour filament occlusion of the right middle cerebral artery underwent imaging during occlusion at 4 hours and 24 hours after reperfusion. Blood-brain barrier permeability was imaged by gradient echo imaging after the first pass of the contrast agent bolus and quantified by a Patlak analysis. Blood-brain barrier permeability was shown on histology by the extravasation of Evans blue on fluorescence microscopy sections matching location and orientation of MR images. Cresyl-violet staining was used to detect and characterize hemorrhage. Landmark-based elastic image registration allowed a region-by-region comparison of permeability imaging at 24 hours with Evans blue extravasation and hemorrhage as detected on histological slides obtained immediately after the 24-hour image set.

RESULTS

Permeability values in the nonischemic tissue (marginal mean ± SE: 0.15 ± 0.019 mL/min 100 g) were significantly lower compared to all permeability values in regions of Evans blue extravasation or hemorrhage. Permeability values in regions of weak Evans blue extravasation (0.23 ± 0.016 mL/min 100 g) were significantly lower compared to permeability values of in regions of strong Evans blue extravasation (0.29 ± 0.020 mL/min 100 g) and macroscopic hemorrhage (0.35 ± 0.049 mL/min 100 g). Permeability values in regions of microscopic hemorrhage (0.26 ± 0.024 mL/min 100 g) only differed significantly from values in regions of nonischemic tissue (0.15 ± 0.019 mL/min 100 g).

CONCLUSIONS

Areas of increased permeability measured in vivo by imaging coincide with blood-brain barrier disruption and hemorrhage observed on gold standard histology.

Quantitative measurement of blood-brain barrier permeability in human using dynamic contrast-enhanced MRI with fast T1 mapping

Breakdown of the blood-brain barrier (BBB), occurring in many neurological diseases, has been difficult to measure noninvasively in humans. Dynamic contrast-enhanced magnetic resonance imaging measures BBB permeability. However, important technical challenges remain and normative data from healthy humans is lacking. We report the implementation of a method for measuring BBB permeability, originally developed in animals, to estimate BBB permeability in both healthy subjects and patients with white matter pathology. Fast T(1) mapping was used to measure the leakage of contrast agent Gadolinium diethylene triamine pentaacetic acid (Gd-DTPA) from plasma into brain. A quarter of the standard Gd-DTPA dose for dynamic contrast-enhanced magnetic resonance imaging was found to give both sufficient contrast-to-noise and high T(1) sensitivity. The Patlak graphical approach was used to calculate the permeability from changes in 1/T(1). Permeability constants were compared with cerebrospinal fluid albumin index. The upper limit of the 95% confidence interval for white matter BBB permeability for normal subjects was 3 × 10(-4) L/g min. MRI measurements were not [corrected] correlated strongly with levels of cerebrospinal fluid albumin in those subjects undergoing lumbar puncture. Dynamic contrast-enhanced magnetic resonance imaging with low dose Gd-DTPA and fast T(1) imaging is a sensitive method to measure subtle differences in BBB permeability in humans and may have advantages over techniques based purely on the measurement of pixel contrast changes.

Delay correction for the assessment of blood-brain barrier permeability using first-pass dynamic perfusion CT

SUMMARY

Hemorrhagic transformation is a serious potential complication of ischemic stroke with damage to the BBB as one of the contributing mechanisms. BBB permeability measurements extracted from PCT by using the Patlak model can provide a valuable assessment of the extent of BBB damage. Unfortunately, Patlak assumptions require extended PCT acquisition, increasing the risk of motion artifacts. A necessary correction is presented for obtaining accurate BBB permeability measurements from first-pass PCT.