Acupotomy Improves Synovial Hypoxia, Synovitis and Angiogenesis in KOA Rabbits

Purpose

Knee osteoarthritis (KOA) is a chronic inflammatory disease highly associated with intra-articular hypertension, hypoxia and angiogenesis of synovial tissue. Our previous studies showed that acupotomy could treat KOA in a variety of ways, including reducing cartilage deterioration and enhancing biomechanical qualities. However, the mechanism of hypoxia and angiogenesis induced by acupotomy in KOA synovium remains unclear. This study looked for the benign intervention of acupotomy in synovial pathology.

Methods

The rabbits were divided into 3 groups, Normal group, KOA group, and KOA + Acupotomy (Apo) group, with 11 rabbits in each group. The KOA rabbit model was established by the modified Videman method with six weeks. The KOA + Apo group performed the intervention. The tendon insertion of vastus medialis, vastus lateralis, rectus femoris, biceps femoris, and anserine bursa were selected as treatment points in rabbits. Rabbits were treated once every 7 days for 3 weeks. We observed the intra-articular pressure and oxygen partial pressure (BOLD MRI). The synovial morphology was monitored by Hematoxylin-Eosin Staining (HE Staining). The expression of hypoxia-inducible transcription factor-1α (HIF-1α), vascular endothelial growth factor (VEGF), interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α) was detected using Immunohistochemical (IHC), Western Blot and Enzyme-Linked Immunosorbent Assay (ELISA).

Results

Acupotomy reduced intra-articular hypertension and improved the synovial oxygen situation, synovial inflammatory and angiogenesis. HIF-1α, VEGF, IL-1β and TNF-α expression were downregulated by acupotomy.

Conclusion

Acupotomy may reduce inflammation and angiogenesis in KOA rabbit by reducing abnormally elevated intra-articular pressure and improving synovial oxygen environment. The above may provide a new theoretical foundation for acupotomy treatment of KOA.

Differential regional cerebrovascular reactivity to end-tidal gas combinations commonly seen during anaesthesia: A blood oxygenation level-dependent MRI observational study in awake adult subjects

BACKGROUND

Regional cerebrovascular reactivity (rCVR) is highly variable in the human brain as measured by blood oxygenation level-dependent (BOLD) MRI to changes in both end-tidal CO 2 and O 2 .

OBJECTIVES

We examined awake participants under carefully controlled end-tidal gas concentrations to assess how regional CVR changes may present with end-tidal gas changes seen commonly with anaesthesia.

DESIGN

Observational study.

SETTING

Tertiary care centre, Winnipeg, Canada. The imaging for the study occurred in 2019.

SUBJECTS

Twelve healthy adult subjects.

INTERVENTIONS

Cerebral BOLD response was studied under two end-tidal gas paradigms. First end-tidal oxygen (ETO 2 ) maintained stable whereas ETCO 2 increased incrementally from hypocapnia to hypercapnia (CO 2 ramp); second ETCO 2 maintained stable whereas ETO 2 increased from normoxia to hyperoxia (O 2 ramp). BOLD images were modeled with end-tidal gas sequences split into two equal segments to examine regional CVR.

MAIN OUTCOME MEASURES

The voxel distribution comparing hypocapnia to mild hypercapnia and mild hyperoxia (mean F I O 2  = 0.3) to marked hyperoxia (mean F I O 2  = 0.7) were compared in a paired fashion ( P  < 0.005 to reach threshold for voxel display). Additionally, type analysis was conducted on CO 2 ramp data. This stratifies the BOLD response to the CO 2 ramp into four categories of CVR slope based on segmentation (type A; +/+slope: normal response, type B +/-, type C -/-: intracranial steal, type D -/+.) Types B to D represent altered responses to the CO 2 stimulus.

RESULTS

Differential regional responsiveness was seen for both end-tidal gases. Hypocapnic regional CVR was more marked than hypercapnic CVR in 0.3% of voxels examined ( P  < 0.005, paired comparison); the converse occurred in 2.3% of voxels. For O 2 , mild hyperoxia had more marked CVR in 0.2% of voxels compared with greater hyperoxia; the converse occurred in 0.5% of voxels. All subjects had altered regional CO 2 response based on Type Analysis ranging from 4 ± 2 to 7 ± 3% of voxels.

CONCLUSION

In awake subjects, regional differences and abnormalities in CVR were observed with changes in end-tidal gases common during the conduct of anaesthesia. On the basis of these findings, consideration could be given to minimising regional CVR fluctuations in patients-at-risk of neurological complications by tighter control of end-tidal gases near the individual's resting values.

The physiological basis underlying functional connectivity differences in older adults: A multi-modal analysis of resting-state fMRI

The purpose of this study was to determine if differences in functional connectivity strength (FCS) with age were confounded by vascular parameters including resting cerebral blood flow (CBF₀), cerebrovascular reactivity (CVR), and BOLD-CBF coupling. Neuroimaging data were collected from 13 younger adults (24 ± 2 years) and 14 older adults (71 ± 4 years). A dual-echo resting state pseudo-continuous arterial spin labeling sequence was performed, as well as a BOLD breath-hold protocol. A group independent component analysis was used to identify networks, which were amalgamated into a region of interest (ROI). Within the ROI, FC strength (FCS) was computed for all voxels and compared across the groups. CBF₀, CVR and BOLD-CBF coupling were examined within voxels where FCS was different between young and older adults. FCS was greater in old compared to young (P = 0.001). When the effect of CBF₀, CVR and BOLD-CBF coupling on FCS was examined, BOLD-CBF coupling had a significant effect (P = 0.003) and group differences in FCS were not present once all vascular parameters were considered in the statistical model (P = 0.07). These findings indicate that future studies of FCS should consider vascular physiological markers in order to improve our understanding of aging processes on brain connectivity.

Chronic hypoxia in pregnant mice impairs the placental and fetal vascular response to acute hypercapnia in BOLD-MRI hemodynamic response imaging

INTRODUCTION

Brief hypercapnic challenge causes acute placental hypoperfusion with fetal brain sparing on BOLD-MRI. We hypothesize that this non-invasive imaging strategy can distinguish between normal pregnancy and chronic placental hypoperfusion (using the maternal hypoxia model).

METHODS

Eighteen pregnant female ICR mice were randomized to three groups: normoxia, late-onset hypoxia (12%O₂;E13.5-17.5) and early-onset hypoxia (12%O₂;E10.5-17.5). On E17.5, animals were imaged in a 4.7-T Bruker-Biospec MRI scanner. Fast coronal True-FISP was performed to identify organs of interest (placenta and fetal heart, liver and brain). BOLD-MRI was performed at baseline and during a 4-min hypercapnic challenge (5%CO₂). %-change in placental and fetal signal was analyzed from T2*-weighted gradient echo MR images. Following MRI, fetuses and placentas were harvested, weighed and immuno-stained.

RESULTS

In normoxic mice, hypercapnia caused reduction in BOLD-MRI signal in placenta (-44% ± 7%; p < 0.0001), fetal liver (-32% ± 7%; p < 0.0001) and fetal heart (-54% ± 12%; p < 0.002), with relative fetal brain sparing (-12% ± 5%; p < 0.0001). These changes were markedly attenuated in both hypoxia groups. Baseline fetal brain/placenta SI ratio was highest in normoxic mice (1.14 ± 0.017) and reduced with increasing duration of hypoxia (late-onset hypoxia: 1.00 ± 0.026; early-onset hypoxia: 0.91 ± 0.016; p = 0.02). Both hypoxic groups exhibited fetal growth restriction with prominent placental glycogen-containing cells, particularly in early-onset hypoxia. There was increased fetal neuro- and intestinal-apoptosis in early-onset hypoxia only.

CONCLUSIONS

BOLD-MRI with brief hypercapnic challenge distinguished between normoxia and both hypoxia groups, while fetal neuroapoptosis was only observed after early-onset hypoxia. This suggests that BOLD-MRI with hypercapnic challenge can identify chronic fetal asphyxia before the onset of irreversible brain injury.

Cerebrovascular Reactivity Measurement Using Magnetic Resonance Imaging: A Systematic Review

Cerebrovascular reactivity (CVR) magnetic resonance imaging (MRI) probes cerebral haemodynamic changes in response to a vasodilatory stimulus. CVR closely relates to the health of the vasculature and is therefore a key parameter for studying cerebrovascular diseases such as stroke, small vessel disease and dementias. MRI allows in vivo measurement of CVR but several different methods have been presented in the literature, differing in pulse sequence, hardware requirements, stimulus and image processing technique. We systematically reviewed publications measuring CVR using MRI up to June 2020, identifying 235 relevant papers. We summarised the acquisition methods, experimental parameters, hardware and CVR quantification approaches used, clinical populations investigated, and corresponding summary CVR measures. CVR was investigated in many pathologies such as steno-occlusive diseases, dementia and small vessel disease and is generally lower in patients than in healthy controls. Blood oxygen level dependent (BOLD) acquisitions with fixed inspired CO₂ gas or end-tidal CO₂ forcing stimulus are the most commonly used methods. General linear modelling of the MRI signal with end-tidal CO₂ as the regressor is the most frequently used method to compute CVR. Our survey of CVR measurement approaches and applications will help researchers to identify good practice and provide objective information to inform the development of future consensus recommendations.

Cerebrovascular Reactivity Mapping Without Gas Challenges: A Methodological Guide

Cerebrovascular reactivity (CVR) is defined as the ability of vessels to alter their caliber in response to vasoactive factors, by means of dilating or constricting, in order to increase or decrease regional cerebral blood flow (CBF). Importantly, CVR may provide a sensitive biomarker for pathologies where vasculature is compromised. Furthermore, the spatiotemporal dynamics of CVR observed in healthy subjects, reflecting regional differences in cerebral vascular tone and response, may also be important in functional MRI studies based on neurovascular coupling mechanisms. Assessment of CVR is usually based on the use of a vasoactive stimulus combined with a CBF measurement technique. Although transcranial Doppler ultrasound has been frequently used to obtain global flow velocity measurements, MRI techniques are being increasingly employed for obtaining CBF maps. For the vasoactive stimulus, vasodilatory hypercapnia is usually induced through the manipulation of respiratory gases, including the inhalation of increased concentrations of carbon dioxide. However, most of these methods require an additional apparatus and complex setups, which not only may not be well-tolerated by some populations but are also not widely available. For these reasons, strategies based on voluntary breathing fluctuations without the need for external gas challenges have been proposed. These include the task-based methodologies of breath holding and paced deep breathing, as well as a new generation of methods based on spontaneous breathing fluctuations during resting-state. Despite the multitude of alternatives to gas challenges, existing literature lacks definitive conclusions regarding the best practices for the vasoactive modulation and associated analysis protocols. In this work, we perform an extensive review of CVR mapping techniques based on MRI and CO2 variations without gas challenges, focusing on the methodological aspects of the breathing protocols and corresponding data analysis. Finally, we outline a set of practical guidelines based on generally accepted practices and available data, extending previous reports and encouraging the wider application of CVR mapping methodologies in both clinical and academic MRI settings.

Effect of wearing a face mask on fMRI BOLD contrast

International spread of the coronavirus SARS-CoV-2 has prompted many MRI scanning facilities to require scan subjects to wear a facial covering ("mask") during scanning as a precaution against transmission of the virus. Because wearing a mask mixes expired air with the subject's inspired air stream, the concentration of inspired carbon dioxide [CO₂] is elevated, resulting in mild hypercapnia. Changes in the inspired gas mixture have been demonstrated to alter R2*-weighted Blood Oxygen Dependent (BOLD) contrast. In this study, we investigate a potential for face masking to alter BOLD contrast during a sensory-motor task designed to activate visual, auditory, and sensorimotor cortices in 8 subjects. We utilize a nasal cannula to supply air to the subject wearing a surgical mask in on-off blocks of 90s to displace expired CO₂, while the subject performs the sensory-motor task. While only a small fraction (2.5%) of the sensory-motor task activation is related to nasal air modulation, a 30.0% change in gray matter BOLD signal baseline is found due to air modulation. Repeating the scan with mask removed produces a small subject-specific bias in BOLD baseline signal from nasal air supply, which may be due to cognitive influence of airflow or cannula-induced hypoxia. Measurements with capnography demonstrate wearing a mask induces an average increase in ETCO₂ of 7.4%. Altogether, these results demonstrate that wearing a face mask during gradient-echo fMRI can alter BOLD baseline signal but minimally affects task activation.

Blood oxygenation level-dependent cardiovascular magnetic resonance of the skeletal muscle in healthy adults: Different paradigms for provoking signal alterations

PURPOSE

Stress blood oxygenation level-dependent (BOLD) cardiovascular magnetic resonance allows for quantitative evaluation of blood flow reserve in skeletal muscles. This study aimed to prospectively compare three commonly used skeletal BOLD cardiovascular magnetic resonance paradigms in healthy adults: gas inhalation, cuff compression-induced ischemia and postocclusive reactive hyperemia, and exercise.

METHODS

Twelve young (22 ± 0.9 years) and 10 elderly (58 ± 5.0 years) healthy subjects underwent BOLD cardiovascular magnetic resonance under the three paradigms. T 2 ∗ signal intensity time curves were generated and quantitative parameters were calculated. Meanwhile, stress transcutaneous oxygen pressure measurements were obtained as comparison. Measurement reproducibility was assessed with intraclass correlation coefficients. Differences in the T 2 ∗ BOLD variation, the correlation with transcutaneous oxygen pressure, and the age-related change between paradigms were statistically analyzed.

RESULTS

Minimum ischemic value and maximum hyperemic peak value showed the highest interobserver and interscan reproducibilities (intraclass correlation coefficient >0.90). The plantar dorsiflexion exercise paradigm elicited the largest T 2 ∗ BOLD variation (15.48% ± 10.56%), followed by ischemia (8.30% ± 6.33%). Negligible to weak changes were observed during gas inhalation. Correlations with transcutaneous oxygen pressure measurements were found in the ischemic phase (r = 0.966; P < .001) and in the postexercise phase (r = -0.936; P < .001). Minimum ischemic value, maximum hyperemic peak value, maximum postexercise value, and slope of postexercise signal decay showed significant differences between young and elderly subjects (P < .01).

CONCLUSION

Ischemia and reactive hyperemia have superior reproducibility, and exercise could induce the largest T 2 ∗ variation. Key parameters from the two paradigms show age-related differences.

Calibrated fMRI for dynamic mapping of CMRO2 responses using MR-based measurements of whole-brain venous oxygen saturation

Functional MRI (fMRI) can identify active foci in response to stimuli through BOLD signal fluctuations, which represent a complex interplay between blood flow and cerebral metabolic rate of oxygen (CMRO₂) changes. Calibrated fMRI can disentangle the underlying contributions, allowing quantification of the CMRO₂ response. Here, whole-brain venous oxygen saturation (Y_v) was computed alongside ASL-measured CBF and BOLD-weighted data to derive the calibration constant, M, using the proposed Y_v-based calibration. Data were collected from 10 subjects at 3T with a three-part interleaved sequence comprising background-suppressed 3D-pCASL, 2D BOLD-weighted, and single-slice dual-echo GRE (to measure Y_v via susceptometry-based oximetry) acquisitions while subjects breathed normocapnic/normoxic, hyperoxic, and hypercapnic gases, and during a motor task. M was computed via Y_v-based calibration from both hypercapnia and hyperoxia stimulus data, and results were compared to conventional hypercapnia or hyperoxia calibration methods. Mean M in gray matter did not significantly differ between calibration methods, ranging from 8.5 ± 2.8% (conventional hyperoxia calibration) to 11.7 ± 4.5% (Y_v-based calibration in response to hyperoxia), with hypercapnia-based M values between (p = 0.56). Relative CMRO₂ changes from finger tapping were computed from each M map. CMRO₂ increased by ∼20% in the motor cortex, and good agreement was observed between the conventional and proposed calibration methods.

Brain BOLD MRI O2 and CO2 stress testing: implications for perioperative neurocognitive disorder following surgery

Background

Mechanical ventilation to alter and improve respiratory gases is a fundamental feature of critical care and intraoperative anesthesia management. The range of inspired O₂ and expired CO₂ during patient management can significantly deviate from values in the healthy awake state. It has long been appreciated that hyperoxia can have deleterious effects on organs, especially the lung and retina. Recent work shows intraoperative end-tidal (ET) CO₂ management influences the incidence of perioperative neurocognitive disorder (POND). The interaction of O₂ and CO₂ on cerebral blood flow (CBF) and oxygenation with alterations common in the critical care and operating room environments has not been well studied.

Methods

We examine the effects of controlled alterations in both ET O₂ and CO₂ on cerebral blood flow (CBF) in awake adults using blood oxygenation level-dependent (BOLD) and pseudo-continuous arterial spin labeling (pCASL) MRI. Twelve healthy adults had BOLD and CBF responses measured to alterations in ET CO₂ and O₂ in various combinations commonly observed during anesthesia.

Results

Dynamic alterations in regional BOLD and CBF were seen in all subjects with expected and inverse brain voxel responses to both stimuli. These effects were incremental and rapid (within seconds). The most dramatic effects were seen with combined hyperoxia and hypocapnia. Inverse responses increased with age suggesting greater risk.

Conclusions

Human CBF responds dramatically to alterations in ET gas tensions commonly seen during anesthesia and in critical care. Such alterations may contribute to delirium following surgery and under certain circumstances in the critical care environment.

Trial registration

ClincialTrials.gov NCT02126215 for some components of the study. First registered April 29, 2014.

Reproducibility of a ramping protocol to measure cerebral vascular reactivity using functional magnetic resonance imaging

Though individual differences in arterial carbon dioxide and oxygen levels inherently exist, the degree of their influence on cerebral vascular reactivity (CVR) is less clear. We examined the reproducibility of BOLD signal changes to an iso-oxic ramping P_et CO₂ protocol. CVR changes were induced by altering P_et CO₂ while holding P_et O₂ constant using a computer-controlled sequential gas delivery (SGD) device. Two MRI scans, each including a linear change in P_et CO₂ , were performed using a 3-Tesla (3T) scanner. This ramp sequence consisted of 1 min at 30 mmHg followed by 4 min period during where P_et CO₂ was linearly increased from 30 to 50 mmHg, 1 min at 51 mmHg, and concluded with 4 min at baseline. The protocol was repeated at a separate visit with 3 days between visits (minimum). Intraclass correlation coefficients (ICC) and coefficients of variation (CV) were used to verify reproducibility. Eleven subjects (6 females; mean age 26.5 ± 5.7 years) completed the full testing protocol. Good reproducibility was observed for the within-visit ramp sequence (Visit 1: ICC = 0.82, CV = 6.5%; Visit 2: ICC = 0.74, CV = 6.4%). Similarly, ramp sequence were reproducible between visits (Scan 1: ICC = 0.74, CV = 6.5%; Scan 2: ICC = 0.66, CV = 6.1%). Establishing reproducible methodologies for measuring BOLD signal changes in response to P_et CO₂ alterations using a ramp protocol will allow researchers to study CVR functionality. Finally, adding a ramping protocol to CVR studies could provide information about changes in CVR over a broad range of P_et CO₂ .

Multi-modal normalization of resting-state using local physiology reduces changes in functional connectivity patterns observed in mTBI patients

Blood oxygenation level dependent (BOLD) resting-state functional magnetic resonance imaging (rs-fMRI) may serve as a sensitive marker to identify possible changes in the architecture of large-scale networks following mild traumatic brain injury (mTBI). Differences in functional connectivity (FC) measurements derived from BOLD rs-fMRI may however be confounded by changes in local cerebrovascular physiology and neurovascular coupling mechanisms, without changes in the underlying neuronally driven connectivity of networks. In this study, multi-modal neuroimaging data including BOLD rs-fMRI, baseline cerebral blood flow (CBF0) and cerebrovascular reactivity (CVR; acquired using a hypercapnic gas breathing challenge) were collected in 23 subjects with reported mTBI (14.6±14.9 months post-injury) and 27 age-matched healthy controls. Despite no group differences in CVR within the networks of interest (P > 0.05, corrected), significantly higher CBF0 was documented in the mTBI subjects (P < 0.05, corrected), relative to the controls. A normalization method designed to account for differences in CBF0 post-mTBI was introduced to evaluate the effects of such an approach on reported group differences in network connectivity. Inclusion of regional perfusion measurements in the computation of correlation coefficients within and across large-scale networks narrowed the differences in FC between the groups, suggesting that this approach may elucidate unique changes in connectivity post-mTBI while accounting for shared variance with CBF0. Altogether, our results provide a strong paradigm supporting the need to account for changes in physiological modulators of BOLD in order to expand our understanding of the effects of brain injury on large-scale FC of cortical networks.

Cerebrovascular Resistance: The Basis of Cerebrovascular Reactivity

The cerebral vascular network regulates blood flow distribution by adjusting vessel diameters, and consequently resistance to flow, in response to metabolic demands (neurovascular coupling) and changes in perfusion pressure (autoregulation). Deliberate changes in carbon dioxide (CO₂) partial pressure may be used to challenge this regulation and assess its performance since CO₂ also acts to change vessel diameter. Cerebrovascular reactivity (CVR), the ratio of cerebral blood flow (CBF) response to CO₂ stimulus is currently used as a performance metric. However, the ability of CVR to reflect the responsiveness of a particular vascular region is confounded by that region’s inclusion in the cerebral vascular network, where all regions respond to the global CO₂ stimulus. Consequently, local CBF responses reflect not only changes in the local vascular resistance but also the effect of changes in local perfusion pressure resulting from redistribution of flow within the network. As a result, the CBF responses to CO₂ take on various non-linear patterns that are not well-described by straight lines. We propose a method using a simple model to convert these CBF response patterns to the pattern of resistance responses that underlie them. The model, which has been used previously to explain the steal phenomenon, consists of two vascular branches in parallel fed by a major artery with a fixed resistance unchanging with CO₂. One branch has a reference resistance with a sigmoidal response to CO₂, representative of a voxel with a robust response. The other branch has a CBF equal to the measured CBF response to CO₂ of any voxel under examination. Using the model to calculate resistance response patterns of the examined branch showed sigmoidal patterns of resistance response, regardless of the measured CBF response patterns. The sigmoid parameters of the resistance response pattern of examined voxels may be mapped to their anatomical location. We show an example for a healthy subject and for a patient with steno-occlusive disease to illustrate. We suggest that these maps provide physiological insight into the regulation of CBF distribution.

Comparison of Blood Oxygenation Level-Dependent fMRI and Provocative DSC Perfusion MR Imaging for Monitoring Cerebrovascular Reserve in Intracranial Chronic Cerebrovascular Disease

BACKGROUND AND PURPOSE

Loss of hemodynamic reserve in intracranial cerebrovascular disease reduces blood oxygenation level-dependent activation by fMRI and increases asymmetry in MTT measured by provocative DSC perfusion MR imaging before and after vasodilation with intravenous acetazolamide. The concordance for detecting hemodynamic reserve integrity has been compared.

MATERIALS AND METHODS

Patients (n = 40) with intracranial cerebrovascular disease and technically adequate DSA, fMRI and provocative DSC perfusion studies were retrospectively grouped into single vessels proximal to and distal from the circle of Willis, multiple vessels, and Moyamoya disease. The vascular territories were classified as having compromised hemodynamic reserve if the expected fMRI blood oxygenation level-dependent activation was absent or if MTT showed increased asymmetry following vasodilation. Concordance was examined in compromised and uncompromised vascular territories of each group with the Fischer exact test and proportions of agreement.

RESULTS

Extensive leptomeningeal collateral circulation was present in all cases. Decreased concordance between the methods was found in vascular territories with stenosis distal to but not proximal to the circle of Willis. Multivessel and Moyamoya diseases also showed low concordance. A model of multiple temporally displaced arterial inputs from leptomeningeal collateral flow demonstrated that the resultant lengthening MTT mimicked compromised hemodynamic reserve despite being sufficient to support blood oxygenation level-dependent contrast.

CONCLUSIONS

Decreased concordance between the 2 methods for assessment of hemodynamic reserve for vascular disease distal to the circle of Willis is posited to be due to well-developed leptomeningeal collateral circulation providing multiple temporally displaced arterial input functions that bias the perfusion analysis toward hemodynamic reserve compromise while blood oxygenation level-dependent activation remains detectable.

Patient-Specific Alterations in CO2 Cerebrovascular Responsiveness in Acute and Sub-Acute Sports-Related Concussion

Background

Preliminary studies suggest that sports-related concussion (SRC) is associated with alterations in cerebral blood flow (CBF) regulation. Here, we use advanced magnetic resonance imaging (MRI) techniques to measure CBF and cerebrovascular responsiveness (CVR) in individual SRC patients and healthy control subjects.

Methods

15 SRC patients (mean age = 16.3, range 14–20 years) and 27 healthy control subjects (mean age = 17.6, range 13–21 years) underwent anatomical MRI, pseudo-continuous arterial spin labeling (pCASL) MRI and model-based prospective end-tidal targeting (MPET) of CO₂ during blood oxygenation level-dependent (BOLD) MRI. Group differences in global mean resting CBF were examined. Voxel-by-voxel group and individual differences in regional CVR were examined using statistical parametric mapping (SPM). Leave-one-out receiver operating characteristic curve analysis was used to evaluate the utility of brain MRI CO₂ stress testing biomarkers to correctly discriminate between SRC patients and healthy control subjects.

Results

All studies were tolerated with no complications. Traumatic structural findings were identified in one SRC patient. No significant group differences in global mean resting CBF were observed. There were no significant differences in the CO₂ stimulus and O₂ targeting during BOLD MRI. Significant group and patient-specific differences in CVR were observed with SRC patients demonstrating a predominant pattern of increased CVR. Leave-one-out ROC analysis for voxels demonstrating a significant increase in CVR was found to reliably discriminate between SRC patients and healthy control subjects (AUC of 0.879, p = 0.0001). The optimal cutoff for increased CVR declarative for SRC was 1,899 voxels resulting in a sensitivity of 0.867 and a specificity of 0.778 for this specific ROC analysis. There was no correlation between abnormal voxel counts and Postconcussion Symptom Scale scores among SRC patients.

Conclusion

Acute and subacute SRCs are associated with alterations in CVR that can be reliably detected by brain MRI CO₂ stress testing in individual patients.

Multiparametric measurement of cerebral physiology using calibrated fMRI

The ultimate goal of calibrated fMRI is the quantitative imaging of oxygen metabolism (CMRO₂), and this has been the focus of numerous methods and approaches. However, one underappreciated aspect of this quest is that in the drive to measure CMRO₂, many other physiological parameters of interest are often acquired along the way. This can significantly increase the value of the dataset, providing greater information that is clinically relevant, or detail that can disambiguate the cause of signal variations. This can also be somewhat of a double-edged sword: calibrated fMRI experiments combine multiple parameters into a physiological model that requires multiple steps, thereby providing more opportunity for error propagation and increasing the noise and error of the final derived values. As with all measurements, there is a trade-off between imaging time, spatial resolution, coverage, and accuracy. In this review, we provide a brief overview of the benefits and pitfalls of extracting multiparametric measurements of cerebral physiology through calibrated fMRI experiments.

Iterative analysis of cerebrovascular reactivity dynamic response by temporal decomposition

OBJECTIVE

To improve quantitative cerebrovascular reactivity (CVR) measurements and CO ₂ arrival times, we present an iterative analysis capable of decomposing different temporal components of the dynamic carbon dioxide- Blood Oxygen-Level Dependent (CO ₂-BOLD) relationship.

EXPERIMENTAL DESIGN

Decomposition of the dynamic parameters included a redefinition of the voxel-wise CO ₂ arrival time, and a separation from the vascular response to a stepwise increase in CO ₂ (Delay to signal Plateau - DTP) and a decrease in CO ₂ (Delay to signal Baseline -DTB). Twenty-five (normal) datasets, obtained from BOLD MRI combined with a standardized pseudo-square wave CO ₂ change, were co-registered to generate reference atlases for the aforementioned dynamic processes to score the voxel-by-voxel deviation probability from normal range. This analysis is further illustrated in two subjects with unilateral carotid artery occlusion using these reference atlases.

PRINCIPAL OBSERVATIONS

We have found that our redefined CO ₂ arrival time resulted in the best data fit. Additionally, excluding both dynamic BOLD phases (DTP and DTB) resulted in a static CVR, that is maximal response, defined as CVR calculated only over a normocapnic and hypercapnic calibrated plateau.

CONCLUSION

Decomposition and novel iterative modeling of different temporal components of the dynamic CO ₂-BOLD relationship improves quantitative CVR measurements.

The role of vascular resistance in BOLD responses to progressive hypercapnia

The ability of the cerebral vasculature to regulate vascular diameter, hence resistance and cerebral blood flow (CBF), in response to metabolic demands (neurovascular coupling), and perfusion pressure changes (autoregulation) may be assessed by measuring the CBF response to carbon dioxide (CO₂ ). In healthy individuals, the CBF response to a ramp CO₂ stimulus from hypocapnia to hypercapnia is assumed sigmoidal or linear. However, other response patterns commonly occur, especially in individuals with cerebrovascular disease, and these remain unexplained. CBF responses to CO₂ in a vascular region are determined by the combined effects of the innate vascular responses to CO₂ and the local perfusion pressure; the latter ensuing from pressure-flow interactions within the cerebral vascular network. We modeled this situation as two vascular beds perfused in parallel from a fixed resistance source. Our premise is that all vascular beds have a sigmoidal reduction of resistance in response to a progressive rise in CO₂ . Surrogate CBF data to test the model was provided by magnetic resonance imaging of blood oxygen level-dependent (BOLD) signals. The model successfully generated all the various BOLD-CO₂ response patterns, providing a physiological explanation of CBF distribution as relative differences in the network of vascular bed resistance responses to CO₂ . Hum Brain Mapp 38:5590-5602, 2017. © 2017 Wiley Periodicals, Inc.

Cerebrovascular reactivity measured by functional magnetic resonance imaging during breath-hold challenge: A systematic review

Cerebrovascular reactivity (CVR) is the cerebral hemodynamic response to a vasoactive substance. Breath-hold (BH) induced CVR has the advantage of being non-invasive and easy to implement during magnetic resonance imaging (MRI). We systematically reviewed the literature regarding MRI measurement of BH induced CVR. The literature was searched using MEDLINE with the search terms breath-hold; and MRI or cerebrovascular reactivity. The search yielded 2244 results and 54 articles were included. Between-group comparisons have found that CVR was higher among healthy controls than patients with various pathologies (e.g. sleep apnea, diabetes, hypertension etc.). However, counter-intuitive findings have also been reported, including higher CVR among smokers, sedentary individuals, and patients with schizophrenia vs.

CONTROLS

Methodological studies have highlighted important measurement characteristics (e.g. normalizing signal to end-tidal CO₂), and comparisons of BH induced CVR to non-BH methods. Future studies are warranted to address questions about group differences, treatment response, disease progression, and other salient clinical themes. Standardization of CVR and BH designs is needed to fully exploit the potential of this practical non-invasive method.

Invalidation of fMRI experiments secondary to neurovascular uncoupling in patients with cerebrovascular disease

PURPOSE

Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is a technique used to infer neuronal activity from the observed changes in blood flow. Cerebrovascular reactivity (CVR) is the ability of arterioles to increase blood flow in response to vasodilatory stimulus. We hypothesize that in areas of disease where there is exhausted vascular reserve and impaired CVR there will be diminished blood flow response following neuronal activation, and that these areas would appear as false-negative tests on BOLD fMRI.

MATERIALS AND METHODS

Patients with steno-occlusive disease and unilateral hemodynamic impairment received a standardized hypercapnic stimuli while being imaged with BOLD fMRI to generate CVR maps. These were compared to traditional BOLD fMRI maps of neuronal activation in the motor cortex in response to a motor task.

RESULTS

Neuronal activation from the motor task was found to be linearly correlated with CVR (n = 11 patients, R = 0.82). Regions with positive (normal) CVR showed positive activation on BOLD fMRI, while regions with negative CVR had attenuated neuronal activation on BOLD fMRI.

CONCLUSION

In areas with cerebrovascular disease where CVR is impaired, there is uncoupling of neuronal activation and blood flow that confounds traditional BOLD fMRI. CVR mapping is a noninvasive MRI-based imaging technique that can provide information about the vascular reactivity of the brain that is important to consider when interpreting traditional BOLD fMRI studies.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2017;46:1448-1455.

Vascular Reactivity Maps in Patients with Gliomas Using Breath-Holding BOLD fMRI

BACKGROUND AND PURPOSE

To evaluate whether breath-holding (BH) blood oxygenation level-dependent (BOLD) fMRI can quantify differences in vascular reactivity (VR), as there is a need for improved contrast mechanisms in gliomas.

METHODS

16 patients (gliomas, grade II = 5, III = 2, IV = 9) were evaluated using the BH paradigm: 4-second single deep breath followed by 16 seconds of BH and 40 seconds of regular breathing for five cycles. VR was defined as the difference in BOLD signal between the minimal signal seen at the end of the deep breath and maximal signal seen at the end of BH (peak-to-trough). VR was measured for every voxel and compared for gray versus white matter and tumor versus normal contralateral brain. VR maps were compared to the areas of enhancement and FLAIR/T2 abnormality.

RESULTS

VR was significantly lower in normal white matter than gray matter (P < .05) and in tumors compared to the normal, contralateral brain (P < 0.002). The area of abnormal VR (1103 ± 659 mm²) was significantly greater (P = .019) than the enhancement (543 ± 530 mm²), but significantly smaller (P = .0011) than the FLAIR abnormality (2363 ± 1232 mm²). However, the variability in the areas of gadolinium contrast enhancement versus VR abnormality indicates that the contrast mechanism elicited by BH (caused by abnormal arteriolar smooth muscles) appears to be fundamentally different from the contrast mechanism of gadolinium enhancement (caused by the presence of "leaky" gap junctions).

CONCLUSIONS

BH maps based on peak-to-trough can be used to characterize VR in brain tumors. VR maps in brain tumor patients appear to be caused by a different mechanism than gadolinium enhancement.

Reproducibility of Brachial Vascular Changes with Alterations in End-Tidal Carbon Dioxide

The purpose of this study was to examine the reproducibility of the peripheral vascular response to hypercapnia. Healthy college-aged men (n = 7) and women (n = 10) underwent an iso-oxic 10-mm Hg increase in PetCO2 for 12 min. Brachial artery diameter changes were measured using ultrasound imaging. Two tests were completed on day 1 with 15 min of rest between tests. Tests were repeated on day 2. Paired t-tests, Bland-Altman plots and intra-class correlations (ICCs) determined reproducibility. There were no significant differences in peak dilation within day (5.33 ± 3.73% vs. 4.52 ± 2.49%, p = 0.378). The within-day ICC was poor (0.213). Within-day time-to-peak dilation did not significantly differ (660.0 ± 231.8 s vs. 602.7 ± 259.9 s, p = 0.379), and the ICC was fair (0.416, p = 0.113). Between-day peak dilation did not significantly differ (5.24 ± 3.84% vs. 4.71 ± 3.17%, p = 0.123), and the ICC was fair (0.419). Hypercapnia-induced brachial artery dilation is similar within day and between days. The ICC for peak dilation suggests the methodology is not reproducible.

Quantitative mapping of cerebrovascular reactivity using resting-state BOLD fMRI: Validation in healthy adults

In conventional neuroimaging, cerebrovascular reactivity (CVR) is quantified primarily using the blood-oxygenation level-dependent (BOLD) functional MRI (fMRI) signal, specifically, as the BOLD response to intravascular carbon dioxide (CO2) modulations, in units of [%ΔBOLD/mmHg]. While this method has achieved wide appeal and clinical translation, the tolerability of CO2-related tasks amongst patients and the elderly remains a challenge in more routine and large-scale applications. In this work, we propose an improved method to quantify CVR by exploiting intrinsic fluctuations in CO2 and corresponding changes in the resting-state BOLD signal (rs-qCVR). Our rs-qCVR approach requires simultaneous monitoring of PETCO2, cardiac pulsation and respiratory volume. In 16 healthy adults, we compare our quantitative CVR estimation technique to the prospective CO2-targeting based CVR quantification approach (qCVR, the "standard"). We also compare our rs-CVR to non-quantitative alternatives including the resting-state fluctuation amplitude (RSFA), amplitude of low-frequency fluctuation (ALFF) and global-signal regression. When all subjects were pooled, only RSFA and ALFF were significantly associated with qCVR. However, for characterizing regional CVR variations within each subject, only the PETCO2-based rs-qCVR measure is strongly associated with standard qCVR in 100% of the subjects (p≤0.1). In contrast, for the more qualitative CVR measures, significant within-subject association with qCVR was only achieved in 50-70% of the subjects. Our work establishes the feasibility of extracting quantitative CVR maps using rs-fMRI, opening the possibility of mapping functional connectivity and qCVR simultaneously.

Neuroimaging Assessment of Cerebrovascular Reactivity in Concussion: Current Concepts, Methodological Considerations, and Review of the Literature

Concussion is a form of traumatic brain injury (TBI) that presents with a wide spectrum of subjective symptoms and few objective clinical findings. Emerging research suggests that one of the processes that may contribute to concussion pathophysiology is dysregulation of cerebral blood flow (CBF) leading to a mismatch between CBF delivery and the metabolic needs of the injured brain. Cerebrovascular reactivity (CVR) is defined as the change in CBF in response to a measured vasoactive stimulus. Several magnetic resonance imaging (MRI) techniques can be used as a surrogate measure of CBF in clinical and laboratory studies. In order to provide an accurate assessment of CVR, these sequences must be combined with a reliable, reproducible vasoactive stimulus that can manipulate CBF. Although CVR imaging currently plays a crucial role in the diagnosis and management of many cerebrovascular diseases, only recently have studies begun to apply this assessment tool in patients with concussion. In order to evaluate the quality, reliability, and relevance of CVR studies in concussion, it is important that clinicians and researchers have a strong foundational understanding of the role of CBF regulation in health, concussion, and more severe forms of TBI, and an awareness of the advantages and limitations of currently available CVR measurement techniques. Accordingly, in this review, we (1) discuss the role of CVR in TBI and concussion, (2) examine methodological considerations for MRI-based measurement of CVR, and (3) provide an overview of published CVR studies in concussion patients.

Reproducibility of blood oxygen level-dependent signal changes with end-tidal carbon dioxide alterations

Hypercapnia has been utilized as a stimulus to elicit changes in cerebral blood flow (CBF). However, in many instances it has been delivered in a non-controlled method that is often difficult to reproduce. The purpose of this study was to examine the within- and between-visit reproducibility of blood oxygen level-dependent (BOLD) signal changes to an iso-oxic square wave alteration in end-tidal carbon dioxide partial pressure (P_et CO₂ ). Two 3-Tesla (3T) MRI scans were performed on the same visit, with two square wave alterations administered per scan. The protocol was repeated on a separate visit with minimum of 3 days between scanning sessions. P_et CO₂ was altered to stimulate changes in cerebral vascular reactivity (CVR), while P_et O₂ was held constant. Eleven subjects (six females; mean age 26·5 ± 5·7 years) completed the full testing protocol. Excellent within-visit square wave reproducibility (ICC > 0·75) was observed. Similarly, square waves were reproducible between scanning sessions (ICC > 0·7). This study demonstrates BOLD signal changes in response to alterations in P_et CO₂ are reproducible both within- and between-visit MRI scans.

The association between cerebrovascular reactivity and resting-state fMRI functional connectivity in healthy adults: The influence of basal carbon dioxide

Although widely used in resting-state fMRI (fMRI) functional connectivity measurement (fcMRI), the BOLD signal is only an indirect measure of neuronal activity, and is inherently modulated by both neuronal activity and vascular physiology. For instance, cerebrovascular reactivity (CVR) varies widely across individuals irrespective of neuronal function, but the implications for fcMRI are currently unknown. This knowledge gap compromises our ability to correctly interpret fcMRI measurements. In this work, we investigate the relationship between CVR and resting fcMRI measurements in healthy young adults, in both the motor and the executive-control networks. We modulate CVR within each individual by subtly increasing and decreasing resting vascular tension through baseline end-tidal CO2 (PETCO2), and measure fcMRI during these hypercapnic, hypocapnic and normocapnic states. Furthermore, we assess the association between CVR and fcMRI within and across individuals. Within individuals, resting PETCO2 is found to significantly influence both CVR and resting fcMRI values. In addition, we find resting fcMRI to be significantly and positively associated with CVR across the group in both networks. This relationship is potentially mediated by concomitant alterations in BOLD signal fluctuation amplitude. This work clearly demonstrates and quantifies a major vascular modulator of resting fcMRI, one that is also subject and regional dependent. We suggest that individualized correction for CVR effects in fcMRI measurements is essential for fcMRI studies of healthy brains, and can be even more important in studying diseased brains.

Fourier modeling of the BOLD response to a breath-hold task: Optimization and reproducibility

Cerebrovascular reactivity (CVR) reflects the capacity of blood vessels to adjust their caliber in order to maintain a steady supply of brain perfusion, and it may provide a sensitive disease biomarker. Measurement of the blood oxygen level dependent (BOLD) response to a hypercapnia-inducing breath-hold (BH) task has been frequently used to map CVR noninvasively using functional magnetic resonance imaging (fMRI). However, the best modeling approach for the accurate quantification of CVR maps remains an open issue. Here, we compare and optimize Fourier models of the BOLD response to a BH task with a preparatory inspiration, and assess the test-retest reproducibility of the associated CVR measurements, in a group of 10 healthy volunteers studied over two fMRI sessions. Linear combinations of sine-cosine pairs at the BH task frequency and its successive harmonics were added sequentially in a nested models approach, and were compared in terms of the adjusted coefficient of determination and corresponding variance explained (VE) of the BOLD signal, as well as the number of voxels exhibiting significant BOLD responses, the estimated CVR values, and their test-retest reproducibility. The brain average VE increased significantly with the Fourier model order, up to the 3rd order. However, the number of responsive voxels increased significantly only up to the 2nd order, and started to decrease from the 3rd order onwards. Moreover, no significant relative underestimation of CVR values was observed beyond the 2nd order. Hence, the 2nd order model was concluded to be the optimal choice for the studied paradigm. This model also yielded the best test-retest reproducibility results, with intra-subject coefficients of variation of 12 and 16% and an intra-class correlation coefficient of 0.74. In conclusion, our results indicate that a Fourier series set consisting of a sine-cosine pair at the BH task frequency and its two harmonics is a suitable model for BOLD-fMRI CVR measurements based on a BH task with preparatory inspiration, yielding robust estimates of this important physiological parameter.

Comparison of brachial dilatory responses to hypercapnia and reactive hyperemia

Flow-mediated dilation (FMD) relies on reactive hyperemia to stimulate the endothelium to release nitric oxide, causing smooth muscle relaxation. Hypercapnia also produces vasodilation, which is thought to be nitric oxide-independent. The purpose of this study was to compare and contrast the effects of hypercapnia and reactive hyperemia as stimuli for brachial artery dilation. On separate days, twenty-five participants underwent vasodilation studies via reactive hyperemia or hypercapnia (i.e. 10 mmHg increase in end-tidal carbon dioxide [PetCO2)]). During both studies changes in brachial artery diameter were recorded using continuous ultrasound imaging. Heart rate (HR) was measured throughout both tests. Resting HR (63  ±  11 versus 68  ±  14 beats min(-1), p  =  0.0027) and baseline brachial artery diameter measurements (4.57  ±  1.51 versus 5.28  ±  1.86 mm, p  =  0.022) were significantly different between reactive hyperemia and hypercapnia, respectively. HR at peak dilation (65  ±  11 versus 76  ±  14 beats min(-1), p  <  0.0001), peak vessel dilation (8.68  ±  4.50 versus 5.28  ±  1.86%, p  =  0.002), and time to peak dilation (90.8  ±  120.1 versus 658.3  ±  226.6 s, p  <  0.0001) were also significantly different between reactive hyperemia and hypercapnia. The dynamics by which reactive hyperemia and hypercapnia stimulate vasodilation appear to differ. Hypercapnia produces a smaller and slower vasodilatory effect than reactive hyperemia. Further research is necessary to better understand the mechanisms of vasodilation under hypercapnic conditions.

Hyperoxia and Functional MRI

Oxygen plays a fundamental role in functional magnetic resonance imaging (FMRI). Blood oxygenation level-dependent (BOLD) imaging is the foundation stone of all FMRI and is still the essential workhorse of the vast majority of FMRI procedures. Hemoglobin may provide the magnetic properties that allow the technique to work, but it is oxygen that allows the contrast to effectively be switched on or off, and it is oxygen that we are interested in tracking in order to observe the oxygen metabolism changes. In general the changes in venous oxygen saturation are observed in order to infer changes in the correlated mechanisms, which can include changes in cerebral blood flow, metabolism, and the fraction of inspired oxygen. By independently manipulating the fraction of inspired oxygen it is possible to alter the amount of dissolved oxygen in the plasma, the venous saturation, or even the blood flow. The effects that these changes have on the observed MRI signal can be either a help or a hindrance depending on how well the changes induced are understood. The administration of supplemental inspired oxygen is in a unique position to provide a flexible, noninvasive, inexpensive, patient-friendly addition to the MRI toolkit to enable investigations to look beyond statistics and regions of interest, and actually produce calibrated, targeted measurements of blood flow, metabolism or pathology.

Vascular autorescaling of fMRI (VasA fMRI) improves sensitivity of population studies: A pilot study

The blood oxygenation level-dependent (BOLD) signal is widely used for functional magnetic resonance imaging (fMRI) of brain function in health and disease. The statistical power of fMRI group studies is significantly hampered by high inter-subject variance due to differences in baseline vascular physiology. Several methods have been proposed to account for physiological vascularization differences between subjects and hence improve the sensitivity in group studies. However, these methods require the acquisition of additional reference scans (such as a full resting-state fMRI session or ASL-based calibrated BOLD). We present a vascular autorescaling (VasA) method, which does not require any additional reference scans. VasA is based on the observation that slow oscillations (<0.1Hz) in arterial blood CO2 levels occur naturally due to changes in respiration patterns. These oscillations yield fMRI signal changes whose amplitudes reflect the blood oxygenation levels and underlying local vascularization and vascular responsivity. VasA estimates proxies of the amplitude of these CO2-driven oscillations directly from the residuals of task-related fMRI data without the need for reference scans. The estimates are used to scale the amplitude of task-related fMRI responses, to account for vascular differences. The VasA maps compared well to cerebrovascular reactivity (CVR) maps and cerebral blood volume maps based on vascular space occupancy (VASO) measurements in four volunteers, speaking to the physiological vascular basis of VasA. VasA was validated in a wide variety of tasks in 138 volunteers. VasA increased t-scores by up to 30% in specific brain areas such as the visual cortex. The number of activated voxels was increased by up to 200% in brain areas such as the orbital frontal cortex while still controlling the nominal false-positive rate. VasA fMRI outperformed previously proposed rescaling approaches based on resting-state fMRI data and can be readily applied to any task-related fMRI data set, even retrospectively.

Changes in cerebral vascular reactivity and structure following prolonged exposure to high altitude in humans

Although high‐altitude exposure can lead to neurocognitive impairment, even upon return to sea level, it remains unclear the extent to which brain volume and regional cerebral vascular reactivity (CVR) are altered following high‐altitude exposure. The purpose of this study was to simultaneously determine the effect of 3 weeks at 5050 m on: (1) structural brain alterations; and (2) regional CVR after returning to sea level for 1 week. Healthy human volunteers (n = 6) underwent baseline and follow‐up structural and functional magnetic resonance imaging (MRI) at rest and during a CVR protocol (end‐tidal PCO₂ reduced by −10, −5 and increased by +5, +10, and +15 mmHg from baseline). CVR maps (% mmHg⁻¹) were generated using BOLD MRI and brain volumes were estimated. Following return to sea level, whole‐brain volume and gray matter volume was reduced by 0.4 ± 0.3% (P < 0.01) and 2.6 ± 1.0% (P < 0.001), respectively; white matter was unchanged. Global gray matter CVR and white matter CVR were unchanged following return to sea level, but CVR was selectively increased (P < 0.05) in the brainstem (+30 ± 12%), hippocampus (+12 ± 3%), and thalamus (+10 ± 3%). These changes were the result of improvement and/or reversal of negative CVR to positive CVR in these regions. Three weeks of high‐altitude exposure is reflected in loss of gray matter volume and improvements in negative CVR.

Measuring cerebrovascular reactivity: the dynamic response to a step hypercapnic stimulus

We define cerebral vascular reactivity (CVR) as the ratio of the change in blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal (S) to an increase in blood partial pressure of CO2 (PCO2): % Δ S/Δ PCO2 mm Hg. Our aim was to further characterize CVR into dynamic and static components and then study 46 healthy subjects collated into a reference atlas and 20 patients with unilateral carotid artery stenosis. We applied an abrupt boxcar change in PCO2 and monitored S. We convolved the PCO2 with a set of first-order exponential functions whose time constant τ was increased in 2-second intervals between 2 and 100 seconds. The τ corresponding to the best fit between S and the convolved PCO2 was used to score the speed of response. Additionally, the slope of the regression between S and the convolved PCO2 represents the steady-state CVR (ssCVR). We found that both prolongations of τ and reductions in ssCVR (compared with the reference atlas) were associated with the reductions in CVR on the side of the lesion. τ and ssCVR are respectively the dynamic and static components of measured CVR.

The dynamics of cerebrovascular reactivity shown with transfer function analysis

Cerebrovascular reactivity (CVR) is often defined as the increase in cerebral blood flow (CBF) produced by an increase in carbon dioxide (CO2) and may be used clinically to assess the health of the cerebrovasculature. When CBF is estimated using blood oxygen level dependent (BOLD) magnetic resonance imaging, CVR values for each voxel can be displayed using a color scale mapped onto the corresponding anatomical scan. While these CVR maps therefore show the distribution of cerebrovascular reactivity, they only provide an estimate of the magnitude of the cerebrovascular response, and do not indicate the time course of the response; whether rapid or slow. Here we describe transfer function analysis (TFA) of the BOLD response to CO2 that provides not only the magnitude of the response (gain) but also the phase and coherence. The phase can be interpreted as indicating the speed of response and so can distinguish areas where the response is slowed. The coherence measures the fidelity with which the response follows the stimulus. The examples of gain, phase and coherence maps obtained from TFA of previously recorded test data from patients and healthy individuals demonstrate that these maps may enhance assessment of cerebrovascular pathophysiology by providing insight into the dynamics of cerebral blood flow control and distribution.

Comparing cerebrovascular reactivity measured using BOLD and cerebral blood flow MRI: The effect of basal vascular tension on vasodilatory and vasoconstrictive reactivity

Cerebrovascular reactivity (CVR) is an important metric of cerebrovascular health. While the BOLD fMRI method in conjunction with carbon-dioxide (CO2) based vascular manipulation has been the most commonly used, the BOLD signal is not a direct measure of vascular changes, and the use of arterial-spin labeling (ASL) cerebral blood flow (CBF) imaging is increasingly advocated. Nonetheless, given the differing dependencies of BOLD and CBF on vascular baseline conditions and the diverse CO2 manipulation types currently used in the literature, knowledge of potential biases introduced by each technique is critical for the interpretation of CVR measurements. In this work, we use simultaneous BOLD-CBF acquisitions during both vasodilatory (hypercapnic) and vasoconstrictive (hypocapnic) stimuli to measure CVR. We further imposed different levels of baseline vascular tension by inducing hypercapnic and hypocapnic baselines, separately from normocapnia by 4mmHg. We saw significant and diverse dependencies on vascular stimulus and baseline condition in both BOLD and CBF CVR measurements: (i) BOLD-based CVR is more sensitive to basal vascular tension than CBF-based CVR; (ii) the use of a combination of vasodilatory and vasoconstrictive stimuli maximizes the sensitivity of CBF-based CVR to vascular tension changes; (iii) the BOLD and CBF vascular response delays are both significantly lengthened at predilated baseline. As vascular tension can often be altered by potential pathology, our findings are important considerations when interpreting CVR measurements in health and disease.

In vivo quantification of cerebral r2*-response to graded hyperoxia at 3 tesla

Objectives:

This study aims to quantify the response of the transverse relaxation rate of the magnetic resonance (MR) signal of the cerebral tissue in healthy volunteers to the administration of air with step-wise increasing percentage of oxygen.

Materials and Methods:

The transverse relaxation rate (R2*) of the MR signal was quantified in seven volunteers under respiratory intake of normobaric gas mixtures containing 21, 50, 75, and 100% oxygen, respectively. End-tidal breath composition, arterial blood saturation (SaO₂), and heart pulse rate were monitored during the challenge. R2* maps were computed from multi-echo, gradient-echo magnetic resonance imaging (MRI) data, acquired at 3.0T. The average values in the segmented white matter (WM) and gray matter (GM) were tested by the analysis of variance (ANOVA), with Bonferroni post-hoc correction. The GM R2*-reactivity to hyperoxia was modeled using the Hill's equation.

Results:

Graded hyperoxia resulted in a progressive and significant (P < 0.05) decrease of the R2* in GM. Under normoxia the GM-R2* was 17.2 ± 1.1 s^-1. At 75% O₂ supply, the R2* had reached a saturation level, with 16.4 ± 0.7 s^-1 (P = 0.02), without a significant further decrease for 100% O₂. The R2*-response of GM correlated positively with CO₂ partial pressure (R = 0.69 ± 0.19) and negatively with SaO₂ (R = -0.74 ± 0.17). The WM showed a similar progressive, but non-significant, decrease in the relaxation rates, with an increase in oxygen intake (P = 0.055). The Hill's model predicted a maximum R2* response of the GM, of 3.5%, with half the maximum at 68% oxygen concentration.

Conclusions:

The GM-R2* responds to hyperoxia in a concentration-dependent manner, suggesting that monitoring and modeling of the R2*-response may provide new oxygenation biomarkers for tumor therapy or assessment of cerebrovascular reactivity in patients.

Cerebral vasomotor reactivity: steady-state versus transient changes in carbon dioxide tension

New Findings

What is the central question of this study?

The relationship between changes in cerebral blood flow and arterial carbon dioxide tension is used to assess cerebrovascular function. Hypercapnia is generally evoked by two methods, i.e. steady-state and transient increases in carbon dioxide tension. In some cases, the hypercapnia is immediately preceded by a period of hypocapnia. It is unknown whether the cerebrovascular response differs between these methods and whether a period of hypocapnia blunts the subsequent response to hypercapnia.

What is the main finding and its importance?

The cerebrovascular response is similar between steady-state and transient hypercapnia. However, hyperventilation-induced hypocapnia attenuates the cerebral vasodilatory responses during a subsequent period of rebreathing-induced hypercapnia.

Cerebral vasomotor reactivity (CVMR) to changes in arterial carbon dioxide tension () is assessed during steady-state or transient changes in . This study tested the following two hypotheses: (i) that CVMR during steady-state changes differs from that during transient changes in ; and (ii) that CVMR during rebreathing-induced hypercapnia would be blunted when preceded by a period of hyperventilation. For each hypothesis, end-tidal carbon dioxide tension () middle cerebral artery blood velocity (CBFV), cerebrovascular conductance index (CVCI; CBFV/mean arterial pressure) and CVMR (slope of the linear regression between changes in CBFV and CVCI versus) were assessed in eight individuals. To address the first hypothesis, measurements were made during the following two conditions (randomized): (i) steady-state increases in of 5 and 10 Torr above baseline; and (ii) rebreathing-induced transient breath-by-breath increases in . The linear regression for CBFV versus (P = 0.65) and CVCI versus (P = 0.44) was similar between methods; however, individual variability in CBFV or CVCI responses existed among subjects. To address the second hypothesis, the same measurements were made during the following two conditions (randomized): (i) immediately following a brief period of hypocapnia induced by hyperventilation for 1 min followed by rebreathing; and (ii) during rebreathing only. The slope of the linear regression for CBFV versus (P < 0.01) and CVCI versus (P < 0.01) was reduced during hyperventilation plus rebreathing relative to rebreathing only. These results indicate that cerebral vasomotor reactivity to changes in is similar regardless of the employed methodology to induce changes in and that hyperventilation-induced hypocapnia attenuates the cerebral vasodilatory responses during a subsequent period of rebreathing-induced hypercapnia.

Brain MRI CO2 stress testing: a pilot study in patients with concussion

BACKGROUND

There is a real need for quantifiable neuro-imaging biomarkers in concussion. Here we outline a brain BOLD-MRI CO2 stress test to assess the condition.

METHODS

This study was approved by the REB at the University of Manitoba. A group of volunteers without prior concussion were compared to post-concussion syndrome (PCS) patients--both symptomatic and recovered asymptomatic. Five 3-minute periods of BOLD imaging at 3.0 T were studied--baseline 1 (BL1--at basal CO2 tension), hypocapnia (CO2 decreased ∼5 mmHg), BL2, hypercapnia (CO2 increased ∼10 mmHg) and BL3. Data were processed using statistical parametric mapping (SPM) for 1st level analysis to compare each subject's response to the CO2 stress at the p = 0.001 level. A 2nd level analysis compared each PCS patient's response to the mean response of the control subjects at the p = 0.05 level.

RESULTS

We report on 5 control subjects, 8 symptomatic and 4 asymptomatic PCS patients. Both increased and decreased response to CO2 was seen in all PCS patients in the 2nd level analysis. The responses were quantified as reactive voxel counts: whole brain voxel counts (2.0±1.6%, p = 0.012 for symptomatic patients for CO2 response < controls and 3.0±5.1%, p = 0.139 for CO2 response > controls: 0.49±0.31%, p = 0.053 for asymptomatic patients for CO2 response < controls and 4.4±6.8%, p = 0.281 for CO2 response > controls).

CONCLUSIONS

Quantifiable alterations in regional cerebrovascular responsiveness are present in concussion patients during provocative CO2 challenge and BOLD MRI and not in healthy controls. Future longitudinal studies must aim to clarify the relationship between CO2 responsiveness and individual patient symptoms and outcomes.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

Comparison of CO2 in air versus carbogen for the measurement of cerebrovascular reactivity with magnetic resonance imaging

Measurement of cerebrovascular reactivity (CVR) can give valuable information about existing pathology and the risk of adverse events, such as stroke. A common method of obtaining regional CVR values is by measuring the blood flow response to carbon dioxide (CO2)-enriched air using arterial spin labeling (ASL) or blood oxygen level-dependent (BOLD) imaging. Recently, several studies have used carbogen gas (containing only CO2 and oxygen) as an alternative stimulus. A direct comparison was performed between CVR values acquired by ASL and BOLD imaging using stimuli of (1) 5% CO2 in air and (2) 5% CO2 in oxygen (carbogen-5). Although BOLD and ASL CVR values are shown to be correlated for CO2 in air (mean response 0.11±0.03% BOLD, 4.46±1.80% ASL, n=16 hemispheres), this correlation disappears during a carbogen stimulus (0.36±0.06% BOLD, 4.97±1.30% ASL). It is concluded that BOLD imaging should generally not be used in conjunction with a carbogen stimulus when measuring CVR, and that care must be taken when interpreting CVR as measured by ASL, as values obtained from different stimuli (CO2 in air versus carbogen) are not directly comparable.

Quantification of BOLD fMRI parameters to infer cerebrovascular reactivity of the middle cerebral artery

PURPOSE

To quantify the amplitude and temporal aspects of the blood oxygenation level-dependent (BOLD) response to an auditory stimulus during normocapnia and hypercapnia in healthy subjects in order to establish which BOLD parameters are best suited to infer the cerebrovascular reactivity (CVR) in the middle cerebral artery (MCA) territory.

MATERIALS AND METHODS

Twenty healthy volunteers (mean age: 23.6 ± 3.7 years, 11 women) were subjected to a functional paradigm composed of five epochs of auditory stimulus (3 sec) intercalated by six intervals of rest (21 sec). Two levels of hypercapnia were achieved by a combination of air and CO2 while the end-tidal CO2 (ETCO2 ) was continually measured. An autoregressive method was applied to analyze four parameters of the BOLD signal: onset-time, time-to-peak, full-width-at-half-maximum (FWHM), and amplitude.

RESULTS

BOLD onset time (P < 0.001) and full-width at half-maximum (FWHM) (P < 0.05) increased linearly, while BOLD amplitude decreased (P < 0.001) linearly with increasing levels of hypercapnia. Test-retest for reproducibility in five subjects revealed excellent concordance for onset time and amplitude.

CONCLUSION

The robust linear dependence of BOLD onset time, FWHM, and amplitude to hypercapnia suggest future application of this protocol in clinical studies aimed at evaluating CVR of the MCA territory.

Measuring cerebrovascular reactivity: what stimulus to use?

Cerebrovascular reactivity is the change in cerebral blood flow in response to a vasodilatory or vasoconstrictive stimulus. Measuring variations of cerebrovascular reactivity between different regions of the brain has the potential to not only advance understanding of how the cerebral vasculature controls the distribution of blood flow but also to detect cerebrovascular pathophysiology. While there are standardized and repeatable methods for estimating the changes in cerebral blood flow in response to a vasoactive stimulus, the same cannot be said for the stimulus itself. Indeed, the wide variety of vasoactive challenges currently employed in these studies impedes comparisons between them. This review therefore critically examines the vasoactive stimuli in current use for their ability to provide a standard repeatable challenge and for the practicality of their implementation. Such challenges include induced reductions in systemic blood pressure, and the administration of vasoactive substances such as acetazolamide and carbon dioxide. We conclude that many of the stimuli in current use do not provide a standard stimulus comparable between individuals and in the same individual over time. We suggest that carbon dioxide is the most suitable vasoactive stimulus. We describe recently developed computer-controlled MRI compatible gas delivery systems which are capable of administering reliable and repeatable vasoactive CO2 stimuli.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

Quantitative functional MRI biomarkers improved early detection of colorectal liver metastases

PURPOSE

To implement and evaluate the performance of a computerized statistical tool designed for robust and quantitative analysis of hemodynamic response imaging (HRI) -derived maps for the early identification of colorectal liver metastases (CRLM).

MATERIALS AND METHODS

CRLM-bearing mice were scanned during the early stage of tumor growth and subsequently during the advanced-stage. Three experienced radiologists marked various suspected-foci on the early stage anatomical images and classified each as either highly certain or as suspected tumors. The statistical model construction was based on HRI maps (functional-MRI combined with hypercapnia and hyperoxia) using a supervised learning paradigm which was further trained either with the advanced-stage sets (late training; LT) or with the early stage sets (early training; ET). For each group of foci, the classifier results were compared with the ground-truth.

RESULTS

The ET-based classification significantly improved the manual classification of the highly certain foci (P < 0.05) and was superior compared with the LT-based classification (P < 0.05). Additionally, the ET-based classification, offered high sensitivity (57-63%), accompanied with high positive predictive value (>94%) and high specificity (>98%) for suspected-foci.

CONCLUSION

The ET-based classifier can strengthen the radiologist's classification of highly certain foci. Additionally, it can aid in classifying suspected-foci, thus enabling earlier intervention which can often be lifesaving.