Absolute quantification of cerebral blood flow with magnetic resonance, reproducibility of the method, and comparison with H2(15)O positron emission tomography

Investigations of hypoxia-induced deoxyhemoglobin as a contrast agent for cerebral perfusion imaging

The assessment of resting perfusion measures (mean transit time, cerebral blood flow, and cerebral blood volume) with magnetic resonance imaging currently requires the presence of a susceptibility contrast agent such as gadolinium. Here, we present an initial comparison between perfusion measures obtained using hypoxia-induced deoxyhemoglobin and gadolinium in healthy study participants. We hypothesize that resting cerebral perfusion measures obtained using precise changes of deoxyhemoglobin concentration will generate images comparable to those obtained using a clinical standard, gadolinium. Eight healthy study participants were recruited (6F; age 23-60). The study was performed using a 3-Tesla scanner with an eight-channel head coil. The experimental protocol consisted of a high-resolution T1-weighted scan followed by two BOLD sequence scans in which each participant underwent a controlled bolus of transient pulmonary hypoxia, and subsequently received an intravenous bolus of gadolinium. The resting perfusion measures calculated using hypoxia-induced deoxyhemoglobin and gadolinium yielded maps that looked spatially comparable. There was no statistical difference between methods in the average voxel-wise measures of mean transit time, relative cerebral blood flow and relative cerebral blood volume, in the gray matter or white matter within each participant. We conclude that perfusion measures generated with hypoxia-induced deoxyhemoglobin are spatially and quantitatively comparable to those generated from a gadolinium injection in the same healthy participant.

Stuttering: A Disorder of Energy Supply to Neurons?

Stuttering is a disorder characterized by intermittent loss of volitional control of speech movements. This hypothesis and theory article focuses on the proposal that stuttering may be related to an impairment of the energy supply to neurons. Findings from electroencephalography (EEG), brain imaging, genetics, and biochemistry are reviewed: (1) Analyses of the EEG spectra at rest have repeatedly reported reduced power in the beta band, which is compatible with indications of reduced metabolism. (2) Studies of the absolute level of regional cerebral blood flow (rCBF) show conflicting findings, with two studies reporting reduced rCBF in the frontal lobe, and two studies, based on a different method, reporting no group differences. This contradiction has not yet been resolved. (3) The pattern of reduction in the studies reporting reduced rCBF corresponds to the regional pattern of the glycolytic index (GI; Vaishnavi et al., 2010). High regional GI indicates high reliance on non-oxidative metabolism, i.e., glycolysis. (4) Variants of the gene ARNT2 have been associated with stuttering. This gene is primarily expressed in the brain, with a pattern roughly corresponding to the pattern of regional GI. A central function of the ARNT2 protein is to act as one part of a sensor system indicating low levels of oxygen in brain tissue and to activate appropriate responses, including activation of glycolysis. (5) It has been established that genes related to the functions of the lysosomes are implicated in some cases of stuttering. It is possible that these gene variants result in a reduced peak rate of energy supply to neurons. (6) Lastly, there are indications of interactions between the metabolic system and the dopamine system: for example, it is known that acute hypoxia results in an elevated tonic level of dopamine in the synapses. Will mild chronic limitations of energy supply also result in elevated levels of dopamine? The indications of such interaction effects suggest that the metabolic theory of stuttering should be explored in parallel with the exploration of the dopaminergic theory.

Multi-parametric MRI (mpMRI) for treatment response assessment of radiation therapy

Magnetic resonance imaging (MRI) plays an important role in the modern radiation therapy (RT) workflow. In comparison with computed tomography (CT) imaging, which is the dominant imaging modality in RT, MRI possesses excellent soft-tissue contrast for radiographic evaluation. Based on quantitative models, MRI can be used to assess tissue functional and physiological information. With the developments of scanner design, acquisition strategy, advanced data analysis, and modeling, multiparametric MRI (mpMRI), a combination of morphologic and functional imaging modalities, has been increasingly adopted for disease detection, localization, and characterization. Integration of mpMRI techniques into RT enriches the opportunities to individualize RT. In particular, RT response assessment using mpMRI allows for accurate characterization of both tissue anatomical and biochemical changes to support decision-making in monotherapy of radiation treatment and/or systematic cancer management. In recent years, accumulating evidence have, indeed, demonstrated the potentials of mpMRI in RT response assessment regarding patient stratification, trial benchmarking, early treatment intervention, and outcome modeling. Clinical application of mpMRI for treatment response assessment in routine radiation oncology workflow, however, is more complex than implementing an additional imaging protocol; mpMRI requires additional focus on optimal study design, practice standardization, and unified statistical reporting strategy to realize its full potential in the context of RT. In this article, the mpMRI theories, including image mechanism, protocol design, and data analysis, will be reviewed with a focus on the radiation oncology field. Representative works will be discussed to demonstrate how mpMRI can be used for RT response assessment. Additionally, issues and limits of current works, as well as challenges and potential future research directions, will also be discussed.

Non-invasive methods for measuring vascular changes in neurovascular headaches

Vascular changes during spontaneous headache attacks have been studied over the last 30 years. The interest in cerebral vessels in headache research was initially due to the hypothesis of cerebral vessels as the pain source. Here, we review the knowledge gained by measuring the cerebral vasculature during spontaneous primary headache attacks with the use of single photon emission tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRA) and transcranial Doppler (TCD). Furthermore, the use of near-infrared spectroscopy in headache research is reviewed. Existing TCD studies of migraine and other headache disorders do not provide solid evidence for cerebral blood flow velocity changes during spontaneous attacks of migraine headache. SPECT studies have clearly shown cortical vascular changes following migraine aura and the differences between migraine with aura compared to migraine without aura. PET studies have shown focal activation in brain structures related to headache, but whether the changes are specific to different primary headaches have yet to be demonstrated. MR angiography has shown precise changes in large cerebral vessels during spontaneous migraine without aura attacks. Future development in more precise imaging methods may further elucidate the pathophysiological mechanisms in primary headaches.

A Quantitative Comparison of Clinically Employed Parameters in the Assessment of Acute Cerebral Ischemia Using Dynamic Susceptibility Contrast Magnetic Resonance Imaging

Purpose: Perfusion magnetic resonance imaging (P-MRI) is part of the mismatch concept employed for therapy decisions in acute ischemic stroke. Using dynamic susceptibility contrast (DSC) MRI the time-to-maximum (Tmax) parameter is quite popular, but its inconsistently defined computation, arterial input function (AIF) selection, and the applied deconvolution method may introduce bias into the assessment. Alternatively, parameter free methods, namely, standardized time-to-peak (stdTTP), z_f-score, and standardized-z_f (stdZ) are also available, offering consistent calculation procedures without the need of an AIF or deconvolution.

Methods: Tmax was compared to stdTTP, z_f-, and stdZ to evaluate robustness of infarct volume estimation in 66 patients, using data from two different sites and MR systems (i.e., 1.5T vs. 3T; short TR (= 689 ms) vs. medium TR (= 1,390 ms); bolus dose 0.1 or 0.2 ml/kgBW, respectively).

Results: Quality factors (QF) for Tmax were 0.54 ± 0.18 (sensitivity), 0.90 ± 0.06 (specificity), and 0.87 ± 0.05 (accuracy). Though not significantly different, best specificity (0.93 ± 0.05) and accuracy (0.90 ± 0.04) were found for stdTTP with a sensitivity of 0.56 ± 0.17. Other tested parameters performed not significantly worse than Tmax and stdTTP, but absolute values of QFs were slightly lower, except for z_f showing the highest sensitivity (0.72 ± 0.16). Accordingly, in ROC-analysis testing the parameter performance to predict the final infarct volume, stdTTP and z_f showed the best performance. The odds for stdTTP to obtain the best prediction of the final infarct size, was 6.42 times higher compared to all other parameters (odds-ratio test; p = 2.2^*10–16).

Conclusion: Based on our results, we suggest to reanalyze data from large cohort studies using the parameters presented here, particularly stdTTP and zf-score, to further increase consistency of perfusion assessment in acute ischemic stroke.

Low-dose T1W DCE-MRI for early time points perfusion measurement in patients with intracranial tumors: A pilot study applying the microsphere model to measure absolute cerebral blood flow

BACKGROUND

Previous studies have measured cerebral blood flow (CBF) with DSC-MRI using an "early time points" (ET) method based on microsphere theory.

PURPOSE

To develop and assess a new ET method for absolute CBF estimation using low-dose high-temporal (LDHT) T1W-DCE-MRI.

STUDY TYPE

Retrospective cohort study.

SUBJECTS

Seven patients with sporadic vestibular schwannoma (VS) who underwent test-retest imaging; one patient with glioblastoma multiforme (GBM) imaged pretreatment; and 12 neurofibromatosis type 2 (NF2) patients undergoing bevacizumab treatment, imaged pre- and 90 days posttreatment.

FIELD STRENGTH/SEQUENCE

LDHT-DCE-MRI was performed at 1.5 and 3.0T, using 3D spoiled gradient echo with phase cycling. DSC-MRI performed in one patient, using 3D echo-shifted multi-shot echo-planar imaging (PRESTO) at 3T.

ASSESSMENT

Through Monte Carlo simulations, CBF estimation using three newly developed average contrast agent concentration (AC) -based methods (ACrPK, ACrMG, ACcomb), was compared against conventional maximum gradient (MG) approaches, at varying Rician noise levels. Reproducibility and applicability of the ACcomb method was assessed in our sporadic-VS/GBM/NF2 patient cohort, respectively.

STATISTICAL TESTS

Reproducibility was measured using test-retest coefficient of variation (CoV). Pre- and posttreatment CBF values were compared using paired t-test with Bonferroni correction.

RESULTS

Monte Carlo stimulations demonstrated that AC-based methods, particularly ACcomb, offered superior accuracy to conventional MG approaches. Overall test-retest CoV using the ACcomb method was 5.76 in normal-appearing white matter (NAWM). The new ACcomb method produced gray matter/white matter CBF estimates in the NF2 patient cohort of 55.9 ± 13.9/25.8 ± 3.5 on day 0; compared with 155.6 ± 17.2/128.4 ± 29.1 for the classical MG method. There was a moderate (10% using ACcomb and ACrPK) increase in CBF of NAWM 90 days post therapy (P = 0.03 and 0.005).

DATA CONCLUSION

Our new AC-based method of CBF estimation offers excellent reproducibility, and displays more accuracy in both Monte Carlo analysis and clinical data application, than conventional MG-based approaches.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 4 J. MAGN. RESON. IMAGING 2018;48:543-557.

Dynamic Susceptibility Contrast Perfusion Magnetic Resonance Imaging Demonstrates Reduced Periventricular Cerebral Blood Flow in Dogs with Ventriculomegaly

The nature of ventriculomegaly in dogs is still a matter of debate. Signs of increased intraventricular pressure and atrophy of the cerebral white matter have been found in dogs with ventriculomegaly, which would imply increased intraventricular pressure and, therefore, a pathological condition, i.e., to some extent. Reduced periventricular blood flow was found in people with high elevated intraventricular pressure. The aim of this study was to compare periventricular brain perfusion in dogs with and without ventriculomegaly using perfusion weighted-magnetic-resonance-imaging to clarify as to whether ventriculomegaly might be associated with an increase in intraventricular pressure. Perfusion was measured in 32 Cavalier King Charles spaniels (CKCS) with ventriculomegaly, 10 CKCSs were examined as a control group. Cerebral blood flow (CBF) was measured using free-hand regions of interest (ROI) in five brain regions: periventricular white matter, caudate nucleus, parietal cortex, hippocampus, and thalamus. CBF was significantly lower in the periventricular white matter of the dogs with ventriculomegaly (p = 0.0029) but not in the other ROIs. Reduction of periventricular CBF might imply increase of intraventricular pressure in ventriculomegaly.

Fast analytical spectral filtering methods for magnetic resonance perfusion quantification

The deconvolution in the perfusion weighted imaging (PWI) plays an important role in quantifying the MR perfusion parameters. The PWI application to stroke and brain tumor studies has become a standard clinical practice. The standard approach for this deconvolution is oscillatory-limited singular value decomposition (oSVD) and frequency domain deconvolution (FDD). The FDD is widely recognized as the fastest approach currently available for deconvolution of MR perfusion data. In this work, two fast deconvolution methods (namely analytical fourier filtering and analytical showalter spectral filtering) are proposed. Through systematic evaluation, the proposed methods are shown to be computationally efficient and quantitatively accurate compared to FDD and oSVD.

Comparison of Quantitative Cerebral Blood Flow Measurements Performed by Bookend Dynamic Susceptibility Contrast and Arterial Spin-Labeling MRI in Relapsing-Remitting Multiple Sclerosis

BACKGROUND AND PURPOSE

Quantitative CBF usage as a biomarker for cognitive impairment and disease progression in MS is potentially a powerful tool for longitudinal patient monitoring. Dynamic susceptibility contrast perfusion with bookend T1-calibration (bookend technique) and pseudocontinuous arterial spin-labeling have recently been used for CBF quantification in relapsing-remitting MS. The noninvasive nature of pseudocontinuous arterial spin-labeling is advantageous over gadolinium-based techniques, but correlation between the techniques is not well-established in the context of MS.

MATERIALS AND METHODS

We compared pseudocontinuous arterial spin-labeling CBF with the bookend technique in a prospective cohort of 19 healthy controls, 19 subjects with relapsing-remitting MS without cognitive impairment, and 20 subjects with relapsing-remitting MS with cognitive impairment on a voxelwise and Brodmann region basis. The linear Pearson correlation, SNR, and coefficient of variation were quantified.

RESULTS

Voxelwise paired t tests revealed no significant CBF differences between techniques after normalization of global mean intensities. The highest Pearson correlations were observed in deep GM structures (average r = 0.71 for the basal ganglia and r = 0.65 for the thalamus) but remained robust for cortical GM, WM, and white matter lesions (average r = 0.51, 0.53, 0.54, respectively). Lower Pearson correlations were observed for cortical lesions (average r = 0.23). Brodmann region correlations were significant for all groups. All correlations were maintained in healthy controls and in patients with relapsing-remitting multiple sclerosis. The highest SNR was present in bookend perfusion, while the highest coefficient of variation was present in white matter lesions.

CONCLUSIONS

Agreement between pseudocontinuous arterial spin-labeling and bookend technique CBF measurements is demonstrated in healthy controls and patients with relapsing-remitting MS.

Novel imaging approaches to cerebrovascular disease

Imaging techniques available to the physician treating neurovascular disease have substantially grown over the past several decades. New techniques as well as advances in imaging modalities continuously develop and provide an extensive array of modalities to diagnose, characterize, and understand neurovascular pathology. Modern noninvasive neurovascular imaging is generally based on computed tomography (CT), magnetic resonance (MR) imaging, or nuclear imaging and includes CT angiography, CT perfusion, xenon-enhanced CT, single-photon emission CT, positron emission tomography, magnetic resonance angiography, MR perfusion, functional magnetic resonance imaging with global and regional blood oxygen level dependent imaging, and magnetic resonance angiography with the use of the noninvasive optional vessel analysis software (River Forest, Ill). In addition to a brief overview of the technique, this review article discusses the clinical indications, advantages, and disadvantages of each of those modalities.

Comparison of cerebral blood flow measurement with [15O]-water positron emission tomography and arterial spin labeling magnetic resonance imaging: A systematic review

Noninvasive imaging of cerebral blood flow provides critical information to understand normal brain physiology as well as to identify and manage patients with neurological disorders. To date, the reference standard for cerebral blood flow measurements is considered to be positron emission tomography using injection of the [(15)O]-water radiotracer. Although [(15)O]-water has been used to study brain perfusion under normal and pathological conditions, it is not widely used in clinical settings due to the need for an on-site cyclotron, the invasive nature of arterial blood sampling, and experimental complexity. As an alternative, arterial spin labeling is a promising magnetic resonance imaging technique that magnetically labels arterial blood as it flows into the brain to map cerebral blood flow. As arterial spin labeling becomes more widely adopted in research and clinical settings, efforts have sought to standardize the method and validate its cerebral blood flow values against positron emission tomography-based cerebral blood flow measurements. The purpose of this work is to critically review studies that performed both [(15)O]-water positron emission tomography and arterial spin labeling to measure brain perfusion, with the aim of better understanding the accuracy and reproducibility of arterial spin labeling relative to the positron emission tomography reference standard.

Fiber-optic Monitoring of Spinal Cord Hemodynamics in Experimental Aortic Occlusion

BACKGROUND

Spinal cord ischemia occurs frequently during thoracic aneurysm repair. Current methods based on electrophysiology techniques to detect ischemia are indirect, non-specific, and temporally slow. In this article, the authors report the testing of a spinal cord blood flow and oxygenation monitor, based on diffuse correlation and optical spectroscopies, during aortic occlusion in a sheep model.

METHODS

Testing was carried out in 16 Dorset sheep. Sensitivity in detecting spinal cord blood flow and oxygenation changes during aortic occlusion, pharmacologically induced hypotension and hypertension, and physiologically induced hypoxia/hypercarbia was assessed. Accuracy of the diffuse correlation spectroscopy measurements was determined via comparison with microsphere blood flow measurements. Precision was assessed through repeated measurements in response to pharmacologic interventions.

RESULTS

The fiber-optic probe can be placed percutaneously and is capable of continuously measuring spinal cord blood flow and oxygenation preoperatively, intraoperatively, and postoperatively. The device is sensitive to spinal cord blood flow and oxygenation changes associated with aortic occlusion, immediately detecting a decrease in blood flow (-65 ± 32%; n = 32) and blood oxygenation (-17 ± 13%, n = 11) in 100% of trials. Comparison of spinal cord blood flow measurements by the device with microsphere measurements led to a correlation of R = 0.49, P < 0.01, and the within-sheep coefficient of variation was 9.69%. Finally, diffuse correlation spectroscopy is temporally more sensitive to ischemic interventions than motor-evoked potentials.

CONCLUSION

The first-generation spinal fiber-optic monitoring device offers a novel and potentially important step forward in the monitoring of spinal cord ischemia.

Quantitative Magnetic Resonance Angiography in Internal Carotid Artery Occlusion with Primary Collateral Pathway

Background and Purpose

Quantitative magnetic resonance angiography (Q-MRA) enables direct measurement of volume flow rate (VFR) of intracranial arteries. We aimed to evaluate the collateral flows in internal carotid artery (ICA) occlusion with primary collateral pathway via circle of Willis using Q-MRA, and to compare them between patients who recently developed ipsilateral symptomatic ischemia and those who did not.

Methods

Between 2012 and 2014, 505 patients underwent Q-MRA in our institution. Among these, 33 patients who had unilateral ICA occlusion with primary collateral pathway were identified, and grouped into asymptomatic patients, stable patients with chronic infarction, and symptomatic patients with acute/subacute infarction. Mean VFR (mVFR) in intracranial arteries was measured and compared between the patients’ groups. Kruskal-Wallis test was used for statistical analysis.

Results

Six patients were asymptomatic, fifteen with chronic infarction were stable, and twelve with acute/subacute infarction were symptomatic. The mVFR of ipsilateral middle cerebral artery in symptomatic patients was significantly lower than those in stable or asymptomatic patients (73.7±45.6 mL/min vs. 119.9±36.1 mL/min vs. 121.8±42.0 mL/min; P = 0.04). Total sum of the mVFR of ipsilateral anterior, middle, and posterior cerebral arteries was significantly lower in symptomatic patients than those in other groups (229.3 ± 51.3 mL/min vs. 282.0±68.6 mL/min vs. 314.0±44.4 mL/min; P = 0.02).

Conclusions

Q-MRA could be helpful to demonstrate the difference in the degree of primary collateral flow in ICA occlusion between the patients with recent symptomatic ischemia and those without.

Improved quantification of cerebral hemodynamics using individualized time thresholds for assessment of peak enhancement parameters derived from dynamic susceptibility contrast enhanced magnetic resonance imaging

Purpose

Assessment of cerebral ischemia often employs dynamic susceptibility contrast enhanced magnetic resonance imaging (DSC-MRI) with evaluation of various peak enhancement time parameters. All of these parameters use a single time threshold to judge the maximum tolerable peak enhancement delay that is supposed to reliably differentiate sufficient from critical perfusion. As the validity of this single threshold approach still remains unclear, in this study, (1) the definition of a threshold on an individual patient-basis, nevertheless (2) preserving the comparability of the data, was investigated.

Methods

The histogram of time-to-peak (TTP) values derived from DSC-MRI, the so-called TTP-distribution curve (TDC), was modeled using a double-Gaussian model in 61 patients without severe cerebrovascular disease. Particular model-based z_f-scores were used to describe the arterial, parenchymal and venous bolus-transit phase as time intervals I_a,_p,v. Their durations (delta I_a,p,v), were then considered as maximum TTP-delays of each phase.

Results

Mean-R² for the model-fit was 0.967. Based on the generic z_f-scores the proposed bolus transit phases could be differentiated. The I_p-interval reliably depicted the parenchymal bolus-transit phase with durations of 3.4 s–10.1 s (median = 4.3s), where an increase with age was noted (∼30 ms/year).

Conclusion

Individual threshold-adjustment seems rational since regular bolus-transit durations in brain parenchyma obtained from the TDC overlap considerably with recommended critical TTP-thresholds of 4 s–8 s. The parenchymal transit time derived from the proposed model may be utilized to individually correct TTP-thresholds, thereby potentially improving the detection of critical perfusion.

Comparison of cerebral blood flow acquired by simultaneous [15O]water positron emission tomography and arterial spin labeling magnetic resonance imaging

Until recently, no direct comparison between [(15)O]water positron emission tomography (PET) and arterial spin labeling (ASL) for measuring cerebral blood flow (CBF) was possible. With the introduction of integrated, hybrid magnetic resonance (MR)-PET scanners, such a comparison becomes feasible. This study presents results of CBF measurements recorded simultaneously with [(15)O]water and ASL. A 3T MR-BrainPET scanner was used for the simultaneous acquisition of pseudo-continuous ASL (pCASL) magnetic resonance imaging (MRI) and [(15)O]water PET. Quantitative CBF values were compared in 10 young healthy male volunteers at baseline conditions. A statistically significant (P<0.05) correlation was observed between the two modalities; the whole-brain CBF values determined with PET and pCASL were 43.3±6.1 mL and 51.9±7.1 mL per 100 g per minute, respectively. The gray/white matter (GM/WM) ratio of CBF was 3.0 for PET and 3.4 for pCASL. A paired t-test revealed differences in regional CBF between ASL and PET with higher ASL-CBF than PET-CBF values in cortical areas. Using an integrated, hybrid MR-PET a direct simultaneous comparison between ASL and [(15)O]water PET became possible for the first time so that temporal, physiologic, and functional variations were avoided. Regional and individual differences were found despite the overall similarity between ASL and PET, requiring further detailed investigations.

fMRI paradigm designing and post-processing tools

In this article, we first review some aspects of functional magnetic resonance imaging (fMRI) paradigm designing for major cognitive functions by using stimulus delivery systems like Cogent, E-Prime, Presentation, etc., along with their technical aspects. We also review the stimulus presentation possibilities (block, event-related) for visual or auditory paradigms and their advantage in both clinical and research setting. The second part mainly focus on various fMRI data post-processing tools such as Statistical Parametric Mapping (SPM) and Brain Voyager, and discuss the particulars of various preprocessing steps involved (realignment, co-registration, normalization, smoothing) in these software and also the statistical analysis principles of General Linear Modeling for final interpretation of a functional activation result.

Effects of short-term environmental hyperthermia on patterns of cerebral blood flow

Environmental hyperthermia is a very common risk factor for many occupations, however, its potential influences on cerebral circulation remain obscure. In this study, 20 participants underwent two simulated environmental thermal conditions (50 °C/25 °C, 1 h), and their cerebral blood flows (CBFs) were quantified using a pseudo-continuous arterial spin labeling (ASL) MR imaging. During the experiment, the physiological parameters, including rectal temperature, arterial blood pressure and weight loss, heart rate and respiration rate, were recorded, and a visual analog scale (VAS) test was performed during both conditions to evaluate the psychological state including vigilance, anxiety, vigor, confidence, anger, nervousness, drowsiness, and loquacity. After scanning, a highly-demanding attentional task--the psychomotor vigilance test (PVT) was performed for behavioral performance evaluation. Compared with that during normothermic condition, the global CBF (gCBF) during hyperthermic condition showed a tendency of decrease, but no significant differences. Regional CBFs (rCBFs) were significantly altered mainly in the prefrontal cortex, somatosensory areas and limbic system. Physiological detection revealed significantly decreased diastolic pressure and systolic pressure and accelerated respiration rate. Furthermore, linear multivariate regression analysis showed that altered rCBFs in several regions could be predicted by physiological (systolic pressure, rectal temperature) and psychological (vigilance, drowsiness, nervousness, anger) changes. And PVT revealed significantly slower attentional reaction during hyperthermia, and the longer reaction time was correlated with the altered rCBF in the left dorsolateral prefrontal cortex (DLPFC). These findings suggested that during short-term hyperthermia gCBF might remain relatively stable under the integrated effect of physiological changes and cerebral auto-regulation, rather than decreased solely dependently on hyperthermia-induced physiological changes. Furthermore, altered regional blood distribution might be accounted for neural activity of thermal sensation and regulation, mood state and cognitive changes.

Fluorodeoxyglucose positron emission tomography/magnetic resonance imaging: current status, future aspects

Simultaneous positron emission tomography (PET)/magnetic resonance (MR) imaging is a promising novel technology for oncology diagnosis and staging and neurologic and cardiac applications. Our institution's current research protocol results in a total imaging time of approximately 45 to 70 minutes with simultaneous PET/MR imaging, making this a feasible total body imaging protocol. Further development of MR-based attenuation correction will improve PET quantification. Quantitatively accurate multiparametric PET/MR data sets will likely improve diagnosis of disease and help guide and monitor the therapies for individualized patient care.

Imaging Biomarkers for Intra-arterial Stroke Therapy

Despite high rates of early revascularization with intra-arterial stroke therapy, the clinical efficacy of this approach has not been clearly demonstrated. Neuroimaging biomarkers will be useful in future trials for patient selection and for outcomes evaluation. To identify patients who are likely to benefit from intra-arterial therapy, the combination of vessel imaging, infarct size quantification and degree of neurologic deficit appears critical. Perfusion imaging may be useful in specific circumstances, but requires further validation. For measuring treatment outcomes, surrogate biomarkers that appear suitable are angiographic reperfusion as measured by the modified Thrombolysis in Cerebral Infarction scale and final infarct volume.

Cerebral blood flow quantification in swine using pseudo-continuous arterial spin labeling

PURPOSE

To develop quantitative cerebral blood flow (CBF) imaging using pseudo-continuous arterial spin labeling (PCASL) in swine, accounting for their cerebrovascular anatomy and physiology.

MATERIALS AND METHODS

Five domestic pigs (2.5-3 months, 25 kg) were used in these studies. The orientation of the labeled arteries, T1bl , M0bl , and T1gm were measured in swine. Labeling parameters were tuned with respect to blood velocity to optimize labeling efficiency based on the data collected from three subjects. Finally, CBF and arterial transit time (ATT) maps for two subjects were created from PCASL data to determine global averages.

RESULTS

The average labeling efficiency over measured velocities of 5-18 cm/s was 0.930. The average T1bl was 1546 ms, the average T1gm was 1224 ms, and the average blood-to-white matter ratio of M0 was 1.25, which was used to find M0bl . The global averages over the subjects were 54.05 mL/100 g tissue/min CBF and 1261 ms ATT.

CONCLUSION

This study demonstrates the feasibility of PCASL for CBF quantification in swine. Quantification of CBF using PCASL in swine can be further developed as an accessible and cost-effective model of human cerebral perfusion for investigating injuries that affect blood flow.

Brain metabolism in patients with hepatic encephalopathy studied by PET and MR

We review PET- and MR studies on hepatic encephalopathy (HE) metabolism in human subjects from the point of views of methods, methodological assumptions and use in studies of cirrhotic patients with clinically overt HE, cirrhotic patients with minimal HE, cirrhotic patients with no history of HE and healthy subjects. Key results are: (1) Cerebral oxygen uptake and blood flow are reduced to 2/3 in cirrhotic patients with clinically overt HE but not in cirrhotic patients with minimal HE or no HE compared to healthy subjects. (2) Cerebral ammonia metabolism is enhanced due to increased blood ammonia in cirrhotic patients but the kinetics of cerebral ammonia uptake and metabolism is not affected by hyperammonemia. (3) Recent advantages in MR demonstrate low-grade cerebral oedema not only in astrocytes but also in the white matter in cirrhotic patients with HE.

Improvements in the quantitative assessment of cerebral blood volume and flow with the removal of vessel voxels from MR perfusion images

Objective. To improve the quantitative assessment of cerebral blood volume (CBV) and flow (CBF) in the brain voxels from MR perfusion images. Materials and Methods. Normal brain parenchyma was automatically segmented with the time-to-peak criteria after cerebrospinal fluid removal and preliminary vessel voxel removal. Two scaling factors were calculated by comparing the relative CBV and CBF of the segmented normal brain parenchyma with the absolute values in the literature. Using the scaling factors, the relative values were converted to the absolute CBV and CBF. Voxels with either CBV > 8 mL/100 g or CBF > 100 mL/100 g/min were characterized as vessel voxels and were excluded from the quantitative measurements. Results. The segmented brain parenchyma with normal perfusion was consistent with the angiographic findings for each patient. We confirmed the necessity of dual thresholds including CBF and CBV for proper removal of vessel voxels. The scaling factors were 0.208 ± 0.041 for CBV, and 0.168 ± 0.037, 0.172 ± 0.037 for CBF calculated using standard and circulant singular value decomposition techniques, respectively. Conclusion. The automatic scaling and vessel removal techniques provide an alternative method for obtaining improved quantitative assessment of CBV and CBF in patients with thromboembolic cerebral arterial disease.

The Massachusetts General Hospital acute stroke imaging algorithm: an experience and evidence based approach

The Massachusetts General Hospital Neuroradiology Division employed an experience and evidence based approach to develop a neuroimaging algorithm to best select patients with severe ischemic strokes caused by anterior circulation occlusions (ACOs) for intravenous tissue plasminogen activator and endovascular treatment. Methods found to be of value included the National Institutes of Health Stroke Scale (NIHSS), non-contrast CT, CT angiography (CTA) and diffusion MRI. Perfusion imaging by CT and MRI were found to be unnecessary for safe and effective triage of patients with severe ACOs. An algorithm was adopted that includes: non-contrast CT to identify hemorrhage and large hypodensity followed by CTA to identify the ACO; diffusion MRI to estimate the core infarct; and NIHSS in conjunction with diffusion data to estimate the clinical penumbra.

Assessing Cerebrovascular Reactivity in Carotid Steno-Occlusive Disease Using MRI BOLD and ASL Techniques

Impaired cerebrovascular reactivity (CVR), a predictive factor of imminent stroke, has been shown to be associated with carotid steno-occlusive disease. Magnetic resonance imaging (MRI) techniques, such as blood oxygenation level-dependent (BOLD) and arterial spin labeling (ASL), have emerged as promising noninvasive tools to evaluate altered CVR with whole-brain coverage, when combined with a vasoactive stimulus, such as respiratory task or injection of acetazolamide. Under normal cerebrovascular conditions, CVR has been shown to be globally and homogenously distributed between hemispheres, but with differences among cerebral regions. Such differences can be explained by anatomical specificities and different biochemical mechanisms responsible for vascular regulation. In patients with carotid steno-occlusive disease, studies have shown that MRI techniques can detect impaired CVR in brain tissue supplied by the affected artery. Moreover, resulting CVR estimations have been well correlated to those obtained with more established techniques, indicating that BOLD and ASL are robust and reliable methods to assess CVR in patients with cerebrovascular diseases. Therefore, the present paper aims to review recent studies which use BOLD and ASL to evaluate CVR, in healthy individuals and in patients with carotid steno-occlusive disease, providing a source of information regarding the obtained results and the methodological difficulties.

Imaging-based treatment selection for intravenous and intra-arterial stroke therapies: a comprehensive review

Reperfusion therapy is the only approved treatment for acute ischemic stroke. The current approach to patient selection is primarily based on the time from stroke symptom onset. However, this algorithm sharply restricts the eligible patient population, and neglects large variations in collateral circulation that ultimately determine the therapeutic time window in individual patients. Time alone is unlikely to remain the dominant parameter. Alternative approaches to patient selection involve advanced neuroimaging methods including MRI diffusion-weighted imaging, magnetic resonance and computed tomography perfusion imaging and noninvasive angiography that provide potentially valuable information regarding the state of the brain parenchyma and the neurovasculature. These techniques have now been used extensively, and there is emerging evidence on how specific imaging data may result in improved clinical outcomes. This article will review the major studies that have investigated the role of imaging in patient selection for both intravenous and intra-arterial therapies.

Test-retest reliability of arterial spin labeling with common labeling strategies

PURPOSE

To compare the test-retest reproducibility of three variants of arterial spin labeling (ASL): pseudo-continuous (pCASL), pulsed (PASL) and continuous (CASL).

MATERIALS AND METHODS

Twelve healthy subjects were scanned on a 3.0T scanner with PASL, CASL, and pCASL. Scans were repeated within-session, after 1 hour, and after 1 week to assess reproducibility at different scan intervals.

RESULTS

Comparison of within-subject coefficients of variation (wsCV) demonstrated high within-session reproducibility (ie, low wsCV) for CASL-based methods (gray matter [GM] wsCV for pCASL: 3.5% ± 0.02%, CASL: 4.1% ± 0.07%) compared to PASL (wsCV: 7.5% ± 0.06%), due to the higher signal-to-noise ratio (SNR) associated with continuous labeling, evident in the 20% gain in temporal SNR and 58% gain in raw SNR for pCASL relative to PASL. At the 1-week scan interval, comparable reproducibility between PASL (GM wsCV 9.2% ± 0.12%) and pCASL (GM wsCV 8.5% ± 0.14%) was observed, indicating the dominance of physiological fluctuations.

CONCLUSION

Although all three approaches are capable of measuring cerebral blood flow within a few minutes of scanning, the high precision and SNR of pCASL, with its insensitivity to vessel geometry, make it an appealing method for future ASL application studies.

Detecting misery perfusion in unilateral steno-occlusive disease of the internal carotid artery or middle cerebral artery by MR imaging

BACKGROUND AND PURPOSE

Elevated OEF is a surrogate for misery perfusion. Our aim was to detect misery perfusion in patients with unilateral steno-occlusive disease of the ICA or MCA by using T2*-based MR imaging and to determine the relationship between brain ischemia and OEF.

MATERIALS AND METHODS

Twenty-three patients with unilateral steno-occlusive disease of the ICA or MCA and 8 healthy volunteers were included in this study. Hemodynamic information was obtained in all subjects by MR imaging. Three regions of interest were placed in the anterior, middle, and posterior parts of the brain bilaterally to measure the OEF and CBF values. The OEFs of the regions of interest in the hemispheres ipsilateral and contralateral to the vascular lesions were compared. Brain regions with OEF greater than that in controls were determined as misery perfusion in patients. The association of vascular lesions, rCBF, and the presence of territory infarction with elevated OEF was investigated.

RESULTS

There was a statistically significant difference in OEF between the ipsilateral and contralateral hemispheres in the patients (t = 3.632, P = .001). Fourteen regions of interest with misery perfusion were determined in the ipsilateral hemispheres, while 3 regions with elevated OEFs were found in the contralateral hemispheres. In the ipsilateral hemispheres, decreased rCBF was associated with elevated OEF (r = -0.451, P < .001). Patients with territory infarction had more regions of interest with misery perfusion than patients without territory infarction (χ(2) = 3.889, P = .049).

CONCLUSIONS

By using the MR imaging technique, misery perfusion demonstrated as elevated OEF was detected in patients with severe atherosclerotic ICA or MCA disease. Identification of misery perfusion with MR imaging may be helpful in the evaluation of brain ischemia.

Quantification of Perfusion Changes during a Motor Task Using Arterial Spin Labeling

Arterial spin labeling (ASL) is a non-invasive MRI technique that allows the quantitative measurement of perfusion, (regional cerebral blood flow (rCBF)). The ASL techniques use the labeling of the blood, by inverting or saturating the spins of water molecules of the blood supplying the imaged region. When reaching the capillary bed, these will be exchanged with tissue water giving rise to a perfusion-weighted signal. The subtraction of control (without label) from labeled images yields a signal difference that directly reflects the local perfusion. Being a non-invasive method, it can be repeated as many times as needed allowing the brain perfusion variation quantification associated with endogenous and exogenous stimuli. In this study, the authors have evaluated the CBF variation induced by the neural activity during a common motor task. The study was conducted on a Siemens Verio 3T system using a 12-channel head coil and a pulsed ASL Q2TIPS-PICORE sequence with a GE-EPI readout. The sequences were driven in 3D PACE mode for prospective motion correction. Fifteen healthy volunteers were studied using a simple motor task consisting in sequential thumb-digit opposition. Two different functional ASL protocols were used: #1 one perfusion scan was obtained during rest and another one during an equal period of motor task (total scan time ~8 min) (TI1 = 700 ms, TI1s = 1600 ms, TI2 =1800 ms; 91 Interleaved tag and control volumes were acquired; TR/TE = 2500/25 ms and flip angle = 90°; nine contiguous axial slices of 8 mm thickness acquired in-line with the AC-PC axis, positioned from the vertex of the brain to the top of cerebellum; FOV = 256 × 256 mm(2); matrix 64 × 64; gap between the labeling slab and the proximal 18.8 mm) and #2 a block design alternating five 25s periods of motor task with five 25s periods of rest (total scan time ~4 min) (TI1 = 700 ms, TI1s = 1600 ms, TI2 = 1800 ms; 101 interleaved tag and control volumes were acquired; TR/TE = 2500/11 ms and flip angle = 90°; nine contiguous axial slices of 6 mm thickness acquired in-line with the AC-PC axis, positioned from the vertex of the brain to the top of cerebellum; FOV = 256 × 256 mm(2) ; matrix 64 × 64; gap between the labeling slab and the proximal 18.8 mm). The post-processing was performed using FSL (www.fmrib.ox.uk/fsl). The mean CBF values obtained for protocols #1 / #2 were: CBFrest = 61.0 / 69.4 ml/100g/min; CBFactivation = 104.8 / 109.9 ml/100g/min; and CBFvariation = CBFactivation - CBFrest = 43.7 / 40.5 ml/100g/min. The relative perfusion changes during activation [defined as CBFvariation / CBFrest (%)] were 73±6 % and 62±7 % (mean±SE) for protocols #1 and #2, respectively. These results show that both activation vs rest and block design functional protocols were capable to detect consistent variations in perfusion associated with a simple motor task. However, the block design has the advantages of requiring shorter acquisitions, directly comparing rest and activation conditions and allowing the acquisition of simultaneous Blood oxygenation level dependent (BOLD) contrast information, while still providing comparable results with the more conventional activation vs rest protocol. In conclusion, our results indicate that a block design ASL-BOLD protocol may be a preferable approach for the evaluation of perfusion changes to endogenous stimuli.

A systematic review of the literature on the effects of dexamethasone on the brain from in vivo human-based studies: implications for physiological brain imaging of patients with intracranial tumors

BACKGROUND

Among glucocorticoids, dexamethasone is most widely used for treatment of cerebral edema because of its long biological half-life and its low mineralocorticoid activity (sodium retaining).

OBJECTIVE

A systematic review of the literature on the effects of dexamethasone on the brain from in vivo studies in humans.

METHODS

A MEDLINE database search (via the PubMed interface) and an EMBASE database search (via the Dialog interface) of the past 35 years was performed. Every article relating to human use reported in English was included. In addition, references of all eligible articles were searched to identify other possible sources.

RESULTS

Twenty-four articles matched the eligibility criteria. There were disparate methodologies and conflicting results, although they tended to indicate a decrease in blood-tumor barrier permeability, decreased tumoral perfusion, decreased tumoral diffusivity, and the possibility of decreased perfusion in contralateral normal-appearing brain tissue.

CONCLUSION

Treatment with dexamethasone may alter imaging parameters from cerebral perfusion studies used in the management of brain tumors. In adequately powered studies, it may be possible to assess the longer term effects of dexamethasone on normal brain tissue to help optimize use with longer term survivors that are emerging as improvements in glioma treatment are made.

Validation of VASO cerebral blood volume measurement with positron emission tomography

Cerebral blood volume (CBV) has been shown to be an important biomarker in a number of neurological disorders and in the quantitative interpretation of functional MRI. One approach to determine CBV in humans is vascular-space-occupancy MRI, and this technique has been applied to the studies of brain glioma, Schizophrenia, and Alzheimer's disease. However, validation of this technique with a gold standard method has not been reported. In this study, we compared vascular-space-occupancy MRI with a radiotracer-based positron emission tomography technique in a group of healthy subjects. It was found that regional CBV measured with vascular-space-occupancy MRI was highly correlated with that of the positron emission tomography data (R=0.79±0.10, N=8). Furthermore, absolute CBV values quantified by vascular-space-occupancy were also in excellent agreement with those by positron emission tomography (slope=1.00±0.15). Because of the differences in the labeling principles between the two modalities, systematic CBV differences were observed in large vessel and ventricle regions.

Automated macrovessel artifact correction in dynamic susceptibility contrast magnetic resonance imaging using independent component analysis

Dynamic susceptibility contrast-MRI is the most commonly used functional MRI-based method for studying changes in cerebral perfusion. However, several studies indicated a systematic overestimation of perfusion parameters compared with other imaging modalities related to the high sensitivity of dynamic susceptibility contrast-MRI for blood flow in large vessels. In this study, we therefore suggest an improved, automated, robust, and efficient method allowing for generating hemodynamic parameter maps where signal influence from large vessels is minimized. Based on independent component analysis, this fully automated approach corrects dynamic susceptibility contrast-MRI data without any user interaction, thus making a clinical applicability possible. The accuracy of the proposed method was tested in 10 patients with cerebrovascular disease. Application of our correction algorithm resulted in a significant reduction of the effect of macrovessel signal on hemodynamic parameters like the cerebral blood flow and the cerebral blood volume compared with uncorrected data. As desired, our method specifically corrected for macrovessel artifacts in cortical grey matter tissue, leaving white matter tissue parameters largely unaffected. This may increase sensitivity and reliability of detecting perfusion abnormalities in patient groups, in particular with regard to stroke and other cerebrovascular disorders.

Blood-brain barrier permeability for ammonia in patients with different grades of liver fibrosis is not different from healthy controls

Increased blood-brain barrier (BBB) permeability for ammonia is considered to be an integral part of the pathophysiology of hepatic encephalopathy (HE) in patients with liver cirrhosis. Increased glutamate-/glutamine-signal intensity in magnetic resonance spectroscopic studies of the brain in cirrhotic patients was explained as a consequence of increased cerebral ammonia uptake. As similar spectroscopic alterations are present in patients with liver fibrosis, we hypothesized that BBB permeability for ammonia is already increased in liver fibrosis, and thereby contributing to the development of HE. To test this hypothesis, cerebral perfusion and ammonia metabolism were examined through positron emission tomography with (15)O-water, respectively, (13)N-ammonia in patients with Ishak grades 2 and 4 fibrosis, cirrhosis, and healthy controls. There were neither global nor regional differences of cerebral blood flow, the rate constant of unidirectional transport of ammonia from blood into brain tissue, the permeability surface area product of the BBB for ammonia, the net metabolic clearance rate constant of ammonia from blood into glutamine in brain, or the metabolic rate of ammonia. The hypothesis that increased permeability of the BBB for ammonia in patients with liver fibrosis contributes to the later development of HE could not be supported by this study.

Volumetric cerebral perfusion imaging in healthy adults: regional distribution, laterality, and repeatability of pulsed continuous arterial spin labeling (PCASL)

The regional distribution, laterality, and reliability of volumetric pulsed continuous arterial spin labeling (PCASL) measurements of cerebral blood flow (CBF) in cortical, subcortical, and cerebellar regions were determined in 10 normal volunteers studied on two occasions separated by 3 to 7 days. Regional CBF, normalized for global perfusion, was highly reliable when measured on separate days. Several regions showed significant lateral asymmetry; notably, in frontal regions CBF was greater in the right than left hemisphere, whereas left was greater than right in posterior regions. There was considerable regional variability across the brain, whereby the posterior cingulate and central and posterior precuneus cortices had the highest perfusion and the globus pallidus the lowest gray matter perfusion. The latter may be due to iron-induced T1 shortening affecting labeled spins and computed CBF signal. High CBF in the posterior cingulate and posterior and central precuneus cortices in this task-free acquisition suggests high activity in these principal nodes of the "default mode network."

Cerebral blood flow measurement using fMRI and PET: a cross-validation study

An important aspect of functional magnetic resonance imaging (fMRI) is the study of brain hemodynamics, and MR arterial spin labeling (ASL) perfusion imaging has gained wide acceptance as a robust and noninvasive technique. However, the cerebral blood flow (CBF) measurements obtained with ASL fMRI have not been fully validated, particularly during global CBF modulations. We present a comparison of cerebral blood flow changes (ΔCBF) measured using a flow-sensitive alternating inversion recovery (FAIR) ASL perfusion method to those obtained using H₂¹⁵O PET, which is the current gold standard for in vivo imaging of CBF. To study regional and global CBF changes, a group of 10 healthy volunteers were imaged under identical experimental conditions during presentation of 5 levels of visual stimulation and one level of hypercapnia. The CBF changes were compared using 3 types of region-of-interest (ROI) masks. FAIR measurements of CBF changes were found to be slightly lower than those measured with PET (average ΔCBF of 21.5 ± 8.2% for FAIR versus 28.2 ± 12.8% for PET at maximum stimulation intensity). Nonetheless, there was a strong correlation between measurements of the two modalities. Finally, a t-test comparison of the slopes of the linear fits of PET versus ASL ΔCBF for all 3 ROI types indicated no significant difference from unity (P > .05).

Arterial transit time effects in pulsed arterial spin labeling CBF mapping: insight from a PET and MR study in normal human subjects

Arterial transit time (ATT), a key parameter required to calculate absolute cerebral blood flow in arterial spin labeling (ASL), is subject to much uncertainty. In this study, ASL ATTs were estimated on a per-voxel basis using data measured by both ASL and positron emission tomography in the same subjects. The mean ATT increased by 260 +/- 20 (standard error of the mean) ms when the imaging slab shifted downwards by 54 mm, and increased from 630 +/- 30 to 1220 +/- 30 ms for the first slice, with an increase of 610 +/- 20 ms over a four-slice slab when the gap between the imaging and labeling slab increased from 20 to 74 mm. When the per-slice ATTs were employed in ASL cerebral blood flow quantification and the in-slice ATT variations ignored, regional cerebral blood flow could be significantly different from the positron emission tomography measures. ATT also decreased with focal activation by the same amount for both visual and motor tasks (approximately 80 ms). These results provide a quantitative relationship between ATT and the ASL imaging geometry and yield an assessment of the assumptions commonly used in ASL imaging. These findings should be considered in the interpretation of, and comparisons between, different ASL-based cerebral blood flow studies. The results also provide spatially specific ATT data that may aid in optimizing the ASL imaging parameters.

Reliability and precision of pseudo-continuous arterial spin labeling perfusion MRI on 3.0 T and comparison with 15O-water PET in elderly subjects at risk for Alzheimer's disease

Arterial spin labeling (ASL) offers MRI measurement of cerebral blood flow (CBF) in vivo, and may offer clinical diagnostic utility in populations such as those with early Alzheimer's disease (AD). In the current study, we investigated the reliability and precision of a pseudo-continuous ASL (pcASL) sequence that was performed two or three times within one hour on eight young normal control subjects, and 14 elderly subjects including 11 with normal cognition, one with AD and two with Mild Cognitive Impairment (MCI). Six of these elderly subjects including one AD, two MCIs and three controls also received (15)O-water positron emission tomography (PET) scans 2 h before their pcASL MR scan. The instrumental reliability of pcASL was evaluated with the intraclass correlation coefficient (ICC). The ICCs were greater than 0.90 in pcASL global perfusion measurements for both the young and the elderly groups. The cross-modality perfusion imaging comparison yielded very good global and regional agreement in global gray matter and the posterior cingulate cortex. Significant negative correlation was found between age and the gray/white matter perfusion ratio (r = -0.62, p < 0.002). The AD and MCI patients showed the lowest gray/white matter perfusion ratio among all the subjects. The data suggest that pcASL provides a reliable whole brain CBF measurement in young and elderly adults whose results converge with those obtained with the traditional (15)O-water PET perfusion imaging method. pcASL perfusion MRI offers an alternative method for non-invasive in vivo examination of early pathophysiological changes in AD.

Characterization of gadolinium-based dynamic susceptibility contrast perfusion measurements in permanent and transient MCAO models with volumetric based validation by CASL

Perfusion imaging is crucial in imaging of ischemic stroke to determine 'tissue at risk' for infarction. In this study we compared the volumetric quantification of the perfusion deficit in two rat middle-cerebral-artery occlusion (MCAO) models using two gadolinium-based contrast agents (P1152 (Guerbet) and Magnevist (Bayer-Schering, Pittsburgh, PA, USA)) as compared with our well established continuous arterial spin labeling (CASL) perfusion imaging technique. Animals underwent either permanent MCAO or transient MCAO with 80-min reperfusion. Imaging was performed at four different time points after MCAO. A region-of-interest (ROI) analysis of the subregions of the ischemic zone (core, penumbra, transient reversal (TR), and sustained reversal (SR)) using P1152 showed significant reduction in blood flow in the core and TR subregions relative to the penumbral and SR subregions while occluded. After reperfusion, a significant increase in blood flow was recorded at all time points after reperfusion in all regions except TR. From the ROI analysis the threshold for the penumbra was determined to be -62+/-11% and this value was subsequently used for quantification of the volumetric deficit. The ischemic volume as defined by dynamic susceptibility contrast (DSC), was only statistically different from the CASL-derived ischemic volume when using Magnevist at post-reperfusion time points.

Improving dynamic susceptibility contrast MRI measurement of quantitative cerebral blood flow using corrections for partial volume and nonlinear contrast relaxivity: A xenon computed tomographic comparative study

PURPOSE

To test whether dynamic susceptibility contrast MRI-based CBF measurements are improved with arterial input function (AIF) partial volume (PV) and nonlinear contrast relaxivity correction, using a gold-standard CBF method, xenon computed tomography (xeCT).

MATERIALS AND METHODS

Eighteen patients with cerebrovascular disease underwent xeCT and MRI within 36 h. PV was measured as the ratio of the area under the AIF and the venous output function (VOF) concentration curves. A correction was applied to account for the nonlinear relaxivity of bulk blood (BB). Mean CBF was measured with both techniques and regression analyses both within and between patients were performed.

RESULTS

Mean xeCT CBF was 43.3 +/- 13.7 mL/100g/min (mean +/- SD). BB correction decreased CBF by a factor of 4.7 +/- 0.4, but did not affect precision. The least-biased CBF measurement was with BB but without PV correction (45.8 +/- 17.2 mL/100 g/min, coefficient of variation [COV] = 32%). Precision improved with PV correction, although absolute CBF was mildly underestimated (34.3 +/- 10.8 mL/100 g/min, COV = 27%). Between patients correlation was moderate even with both corrections (R = 0.53).

CONCLUSION

Corrections for AIF PV and nonlinear BB relaxivity improve bolus MRI-based CBF maps. However, there remain challenges given the moderate between-patient correlation, which limit diagnostic confidence of such measurements in individual patients.

Quantification of cerebral blood flow, cerebral blood volume, and blood-brain-barrier leakage with DCE-MRI

Dynamic susceptibility contrast MRI (DSC-MRI) is the current standard for the measurement of Cerebral Blood Flow (CBF) and Cerebral Blood Volume (CBV), but it is not suitable for the measurement of Extraction Flow (EF) and may not allow for absolute quantification. The objective of this study was to develop and evaluate a methodology to measure CBF, CBV, and EF from T1-weighted dynamic contrast-enhanced MRI (DCE-MRI). A two-compartment modeling approach was developed, which applies both to tissues with an intact and with a broken Blood-Brain-Barrier (BBB). The approach was evaluated using measurements in normal grey matter (GM) and white matter (WM) and in tumors of 15 patients. Accuracy and precision were estimated with simulations of normal brain tissue. All tumor and normal tissue curves were accurately fitted by the model. CBF (mL/100 mL/min) was 82 +/- 21 in GM and 23 +/- 14 in WM, CBV (mL/100 mL) was 2.6 +/- 0.8 in GM and 1.3 +/- 0.4 in WM. EF (mL/100 mL/min) was close to zero in GM (-0.009 +/- 0.05) and WM (-0.03 +/- 0.08). Simulations show an overlap between CBF values of WM and GM, which is eliminated when Contrast-to-Noise (CNR) is improved. The model provides a consistent description of tracer kinetics in all brain tissues, and an accurate assessment of perfusion and permeability in reference tissues. The measurement sequence requires optimization to improve CNR and the precision in the perfusion parameters. With this approach, DCE-MRI presents a promising alternative to DSC-MRI for quantitative bolus-tracking in the brain.

Diffusion and perfusion MR imaging of acute ischemic stroke

Diffusion and perfusion MR imaging have proven to be highly useful in the clinical description and understanding of acute and hyperacute ischemic stroke. In this article, the authors give a brief overview of the basic concepts of diffusion and perfusion imaging and describe some of the current developments, applications, challenges, and limitations of these techniques as applied to cerebral ischemia.

Arterial spin-label imaging in patients with normal bolus perfusion-weighted MR imaging findings: pilot identification of the borderzone sign

PURPOSE

To determine whether perfusion abnormalities are depicted on arterial spin-labeling (ASL) images obtained in patients with normal bolus perfusion-weighted (PW) magnetic resonance (MR) imaging findings.

MATERIALS AND METHODS

Institutional review board approval and written informed patient consent were obtained. This study was HIPAA compliant. Consecutive patients suspected or known to have cerebrovascular disease underwent 1.5-T brain MR imaging, including MR angiography, gradient-echo PW imaging, and pseudocontinuous ASL imaging, between October 2007 and January 2008. Patients with normal bolus PW imaging findings were retrospectively identified, and two neuroradiologists subsequently evaluated the ASL images for focal abnormalities. The severity of the borderzone sign-that is, bilateral ASL signal dropout with surrounding cortical areas of hyperintensity in the middle cerebral artery borderzone regions-was classified by using a four-point scale. For each group, the ASL-measured mean mixed cortical cerebral blood flow (CBF) at the level of the centrum semiovale was evaluated by using the Jonckheere-Terpstra test.

RESULTS

One hundred thirty-nine patients met the study inclusion criteria, and 41 (30%) of them had normal bolus PW imaging findings. Twenty-three (56%) of these 41 patients also had normal ASL imaging findings. The remaining 18 (44%) patients had the ASL borderzone sign; these patients were older (mean age, 71 years +/- 11 [standard deviation] vs 57 years +/- 16; P < .005) and had lower mean CBF (30 mL/100 g/min +/- 12 vs 46 mL/100 g/min +/- 12, P < .003) compared with the patients who had normal ASL imaging findings. Five patients had additional focal ASL findings that were related to either slow blood flow in a vascular structure or postsurgical perfusion defects and were not visible on the PW images.

CONCLUSION

Approximately half of the patients with normal bolus PW imaging findings had abnormal ASL findings-most commonly the borderzone sign. Results of this pilot study suggest that ASL imaging in patients who have this sign and are suspected of having cerebrovascular disease yields additional and complementary hemodynamic information.

The performance of MRI-based cerebral blood flow measurements in acute and subacute stroke compared with 15O-water positron emission tomography: identification of penumbral flow

BACKGROUND AND PURPOSE

Perfusion-weighted MRI-based maps of cerebral blood flow (CBF(MRI)) are considered a good MRI measure of penumbral flow in acute ischemic stroke but are seldom used in clinical routine due to methodical issues. We validated CBF(MRI) on quantitative CBF measurement by 15O-water positron emission tomography (CBF(PET)).

MATERIAL AND METHODS

Comparative CBF(MRI) and CBF(PET) were performed in patients with acute and subacute stroke. In a voxel-based seed-growing technique, predefined CBF(MRI) thresholds (<40, <30, <20, <10 mL/100 g/min) were applied and the resulting volumes were compared with the hypoperfusion volume detected by the penumbral threshold (<20 mL/100 g/min) on CBF(PET). The volumetric comparison was expressed as the C-ratio (volume CBF(MRI)/volume CBF(PET)) to identify the best MRI threshold. The influence of vessel pathology, hypoperfusion size, and time point of imaging was described. The proportion of voxels correctly classified as hypoperfused and the proportion of voxel correctly classified as nonhypoperfused of the best CBF(MRI) threshold was calculated and a Bland-Altman plot illustrated the method-specific differences.

RESULTS

In 24 patients (median time MRI to PET: 68 minutes; 16 patients imaged within 24 hours after stroke), the median volume of hypoperfusion <20 mL/100 g/min (CBF(PET)) was 78.5 cm(3). Median hypoperfusion volume on CBF(MRI) ranged from 245.9 cm(3) (<40 mL/100 g/min) to 35.5 cm(3) (<10 mL/10 g/min). On visual inspection, an excellent qualitative congruence was found. The quantitative congruence was best for the MRI-CBF threshold <20 mL/100 g/min (median C-ratio: 1.0), reaching a proportion of voxels correctly classified as hypoperfused of 76% and a proportion of voxel correctly classified as nonhypoperfused of 96%, but a wide interindividual range (C-ratio 0.3 to 3.5) was found. Ipsilateral vessel pathology, time point of imaging, and size of hypoperfusion did not significantly influence the C-ratio. The Bland-Altman analysis for the volumetric difference of CBF(MRI) and CBF(PET) found a good overall agreement but a large SD.

CONCLUSIONS

Hypoperfusion areas below the CBF(PET) penumbral threshold can be well identified by the CBF(MRI) threshold <20 mL/10 g/min at a group level, but a large individual variance (exceeding 20% of volume in nearly half of the patients) could not be explained. Our results support a prudent use of MRI-based quantitative CBF measurement in clinical routine.