Elevated global cerebral blood flow, oxygen extraction fraction and unchanged metabolic rate of oxygen in young adults with end-stage renal disease: an MRI study

Changes in Cerebral Blood Flow in Patients Who Receive Different Durations of Hemodialysis: An Arterial Spin Labeling MRI Study

BACKGROUND AND PURPOSE

This study aimed to determine the changes in cerebral blood flow (CBF) in patients who received different durations of hemodialysis (HD) using arterial spin labeling magnetic resonance imaging.

METHODS

The study included 46 patients who received HD and 24 demographically similar healthy controls (HCs). Patients who received HD were divided into three subgroups based on its duration: HD-1 (n=15, dialysis duration ≤24 months), HD-2 (n=16, dialysis duration >24 and ≤72 months), and HD-3 (n=15, dialysis duration ≥73 months). All subjects completed the Mini Mental State Examination and Montreal Cognitive Assessment tests, and the patients who received HD underwent laboratory tests. Group-level differences in the global and regional CBFs between patients who received HD and HCs were assessed. Correlation analysis was performed to evaluate the associations among CBF, clinical variables, and cognitive function.

RESULTS

Compared with HCs, global and regional CBFs were significantly increased in the HD-1 and HD-2 groups (p<0.05), but there was no significant difference in the HD-3 group (p>0.05). However, compared with the HD-1 group, the HD-3 group had significantly decreased global and regional CBFs (p<0.05). The cognitive function was worse in patients who received long-term HD than in HCs. Increased dialysis duration and hemoglobin level were predictive risk factors for decreased CBF in patients who received long-term HD.

CONCLUSIONS

Patients who received long-term HD with normal CBF had worse cognitive function, which may be related to increased dialysis duration.

Cerebral hemodynamic and metabolic dysregulation in the postradiation brain

Technological advances in the delivery of radiation and other novel cancer therapies have significantly improved the 5-year survival rates over the last few decades. Although recent developments have helped to better manage the acute effects of radiation, the late effects such as impairment in cognition continue to remain of concern. Accruing data in the literature have implicated derangements in hemodynamic parameters and metabolic activity of the irradiated normal brain as predictive of cognitive impairment. Multiparametric imaging modalities have allowed us to precisely quantify functional and metabolic information, enhancing the anatomic and morphologic data provided by conventional MRI sequences, thereby contributing as noninvasive imaging-based biomarkers of radiation-induced brain injury. In this review, we have elaborated on the mechanisms of radiation-induced brain injury and discussed several novel imaging modalities, including MR spectroscopy, MR perfusion imaging, functional MR, SPECT, and PET that provide pathophysiological and functional insights into the postradiation brain, and its correlation with radiation dose as well as clinical neurocognitive outcomes. Additionally, we explored some innovative imaging modalities, such as quantitative blood oxygenation level-dependent imaging, susceptibility-based oxygenation measurement, and T2-based oxygenation measurement, that hold promise in delineating the potential mechanisms underlying deleterious neurocognitive changes seen in the postradiation setting. We aim that this comprehensive review of a range of imaging modalities will help elucidate the hemodynamic and metabolic injury mechanisms underlying cognitive impairment in the irradiated normal brain in order to optimize treatment regimens and improve the quality of life for these patients.

Oxygen extraction fraction (OEF) assesses cerebral oxygen metabolism of deep gray matter in patients with pre-eclampsia

OBJECTIVES

The objective of this study was to compare oxygen extraction fraction (OEF) values in the deep gray matter (GM) of pre-eclampsia (PE) patients, pregnant healthy controls (PHCs), and non-pregnant healthy controls (NPHCs) to explore their brain oxygen metabolism differences in GM.

METHODS

Forty-seven PE patients, forty NPHCs, and twenty-one PHCs were included. Brain OEF values were computed from quantitative susceptibility mapping (QSM) plus quantitative blood oxygen level-dependent magnitude (QSM + qBOLD = QQ)-based mapping. One-way ANOVA was used to compare mean OEF values in the three groups. The area under the curve of the mean OEF value in each region of interest was estimated using a receiver operating characteristic curve analysis.

RESULTS

We found that the mean OEF values in the thalamus, putamen, caudate nucleus, pallidum, and substantia nigra were significantly different in these three groups (F = 5.867, p = 0.004; F = 5.142, p = 0007; F = 6.158, p = 0.003; F = 6.319, p = 0.003; F = 5.491, p = 0.005). The mean OEF values for these 5 regions were higher in PE patients than in NPHCs and in PHCs (p < 0.05). The AUC of these ROIs ranged from 0.673 to 0.692 (p < 0.01) and cutoff values varied from 35.1 to 36.6%, indicating that the OEF values could discriminate patients with and without PE. Stepwise multivariate analysis revealed that the OEF values correlated with hematocrit in pregnant women (r = 0.353, p = 0.003).

CONCLUSION

OEF values in the brains of pregnant women can be measured in clinical practice using QQ-based OEF mapping for noninvasive assessment of hypertensive disorders.

KEY POINTS

• Pre-eclampsia is a hypertensive disorder associated with abnormalities in brain oxygen extraction. • Oxygen extraction fraction (OEF) is an indicator of brain tissue viability and function. QQ-based mapping of OEF is a new MRI technique that can noninvasively quantify brain oxygen metabolism. • OEF values in the brains of pregnant women can be measured for noninvasive assessment of hypertensive disorders in clinical practice.

Cerebral oxygen extraction fraction MRI: Techniques and applications

The human brain constitutes 2% of the body's total mass but uses 20% of the oxygen. The rate of the brain's oxygen utilization can be derived from a knowledge of cerebral blood flow and the oxygen extraction fraction (OEF). Therefore, OEF is a key physiological parameter of the brain's function and metabolism. OEF has been suggested to be a useful biomarker in a number of brain diseases. With recent advances in MRI techniques, several MRI-based methods have been developed to measure OEF in the human brain. These MRI OEF techniques are based on the T₂ of blood, the blood signal phase, the magnetic susceptibility of blood-containing voxels, the effect of deoxyhemoglobin on signal behavior in extravascular tissue, and the calibration of the BOLD signal using gas inhalation. Compared to ¹⁵ O PET, which is considered the "gold standard" for OEF measurement, MRI-based techniques are non-invasive, radiation-free, and are more widely available. This article provides a review of these emerging MRI-based OEF techniques. We first briefly introduce the role of OEF in brain oxygen homeostasis. We then review the methodological aspects of different categories of MRI OEF techniques, including their signal mechanisms, acquisition methods, and data analyses. The strengths and limitations of the techniques are discussed. Finally, we review key applications of these techniques in physiological and pathological conditions.

Cognitive Dysfunction and Its Risk Factors in Patients Undergoing Maintenance Hemodialysis

OBJECTIVE

Cognitive impairment (CI) in Maintenance hemodialysis (MHD) is attracting increasing attention. This study aims to clarify the prevalence and risk factors for cognitive dysfunction in patients on MHD who have no history of stroke.

METHODS

A total of 99 patients with no history of stroke undergoing MHD were enrolled. Global cognitive function was evaluated using the Montreal Cognitive Assessment scale. Attention and executive functions were evaluated by the Digital Span (DS) test and the Color Trail Test (CTT). The Hamilton Depression and Anxiety scales were used to assess depression and anxiety status. The effects of patient background factors, laboratory indicators, anxiety, and depression on cognitive dysfunction were examined by regression analysis.

RESULTS

There were 69.70% of the patients had general CI, 65.65% had depression, and 57.57% had anxiety. The forward and backward DS in the cognitively impaired (CI) group were shorter than in the normal cognitive function (NCF) group (P<0.05). Times required for CTT-I, CTT-II, and CTT II - CTT I were longer in the CI group than in the NCF group (P<0.05). Hemoglobin levels were lower, and parathyroid hormone (PTH) and uric acid levels were higher in the CI group than in the NCF group (P<0.05). Hemoglobin levels were negatively correlated with CI in these patients (odds ratio [OR] 0.634, P<0.05) and PTH, and uric acid levels were positively correlated with CI (OR 1.028, P<0.05; and OR 1.011, P<0.05). The proportions of patients with diabetes and depression were higher in the CI group (P<0.05).

CONCLUSION

There was a high prevalence of CI with significant impairment of attention and executive ability in MHD patients who had no stroke history. Hemoglobin may protect cognitive function, while diabetes, PTH, and uric acid levels may be risk factors. Depressive and anxiety states may aggravate CI in MHD patients.

Reduced regional cerebral venous oxygen saturation is a risk factor for the cognitive impairment in hemodialysis patients: a quantitative susceptibility mapping study

The purpose of this study was to noninvasively evaluate the changes of regional cerebral venous oxygen saturation (rSvO₂) in hemodialysis patients using quantitative susceptibility mapping (QSM) and investigate the relationship with clinical risk factors and neuropsychological testing. Fifty four (54) hemodialysis patients and 54 age, gender and education matched healthy controls (HCs) were recruited in this prospective study. QSM data were reconstructed from the original phase data of susceptibility weighted imaging to measure the susceptibility of cerebral regional major veins in all subjects and calculate their rSvO₂. The differences in rSvO₂ between hemodialysis patients and HCs were investigated using analysis of covariance adjusting for age and gender as covariates. Stepwise multiple regression and correlation analysis were performed between the cerebral rSvO₂ and clinical factors including neuropsychological testing. The SvO₂ of the bilateral cortical, thalamostriate, septal, cerebral internal and basal veins in hemodialysis patients was significantly lower than that in HCs (p < 0.001, Bonferroni corrected). The cerebral rSvO₂ in all these veins was reduced by 1.67% to 2.30%. The hematocrit, iron, glucose, pre-and post-dialysis diastolic blood pressure (DBP) were independent predictive factors for the cerebral rSvO₂ (all P < 0.05). The Mini-Mental State Examination and Montreal Cognitive Assessment (MoCA) scores were both lower in patients than those in HCs (both P < 0.05). The SvO₂ of the left cerebral internal vein correlated with MoCA scores (r = 0.492; P = 0.02, FDR corrected). In conclusion, our study indicated that the cerebral rSvO₂ was reduced in hemodialysis patients, which was the risk factor for neurocognitive impairment. The hematocrit, iron, glucose, pre-and post-dialysis DBP were independent risk factors for the cerebral rSvO₂.

Validation of T2 -based oxygen extraction fraction measurement with 15 O positron emission tomography

PURPOSE

To evaluate the accuracy of T₂ -based whole-brain oxygen extraction fraction (OEF) estimation by comparing it with gold standard ¹⁵ O-PET measurements.

METHODS

Sixteen healthy adult subjects underwent MRI and ¹⁵ O-PET OEF measurements on the same day. On MRI, whole-brain OEF was quantified by T₂ -relaxation-under-spin-tagging (TRUST) MRI, based on subject-specific hematocrit. The TRUST OEF was compared to the whole-brain averaged OEF produced by ¹⁵ O-PET. Agreement between TRUST and ¹⁵ O-PET whole-brain OEF measurements was examined in terms of intraclass correlation coefficient (ICC) and in absolute OEF values. In a subset of 10 subjects, test-retest reproducibility of whole-brain OEF was also evaluated and compared between the two modalities.

RESULTS

Across the 16 subjects, the mean whole-brain OEF of TRUST and ¹⁵ O-PET were 36.44 ± 4.07% and 36.45 ± 3.65%, respectively, showing no difference between the two modalities (P = .99). TRUST whole-brain OEF strongly correlated with that of ¹⁵ O-PET (N = 16, ICC = 0.90, P = 4 × 10^-7 ). The coefficient-of-variation of TRUST and ¹⁵ O-PET whole-brain OEF measurements were 1.79 ± 0.67% and 2.06 ± 1.55%, respectively, showing no difference between the two modalities (N = 10, P = .64). Further analyses on the effect of hematocrit revealed that correlation between PET OEF and TRUST OEF with assumed hematocrit remained significant (ICC = 0.8, P < 2 × 10^-5 ).

CONCLUSION

Whole-brain OEF measured by TRUST was in excellent agreement with gold standard ¹⁵ O-PET, with highly comparable accuracy and reproducibility. These findings suggest that TRUST MRI can provide accurate quantification of whole-brain OEF noninvasively.

How Do You Feel Now? The Salience Network Functional Connectivity in End-Stage Renal Disease

Objective

The network connectivity basis of cognitive declines in end-stage renal disease (ESRD) remains unclear. A triple-network model of the salience (SN), executive control, and default mode networks has been suggested to be critical for efficient cognition. Here, we aimed to test the hypothesis that SN may play a role in cognitive impairment in patients with ESRD.

Materials and Methods

We investigated functional connectivity (FC) alterations within the SN between 43 ESRD patients (19 females/24 males, 46 ± 10 years) and 43 healthy controls (HC) (19 females/24 males, 47 ± 10 years), and performed linear support vector machine (LSVM) analysis on significant FC pairs within the SN to discriminate the two groups, and tested the accuracy of the classifier. Association and mediation analyses were conducted among the significant FC pairs within the SN nodes, clinical indicators, and neuropsychological tests scores.

Results

We identified significant between-group FC pairs within the SN and fairly good classification efficiency with significant accuracy (72.09%, p < 0.001). We found that FC between the right supramarginal gyrus and right anterior insula (AISL) was positively correlated with MoCA (r = 0.4010, p = 0.008); FC between the dorsal anterior cingulate cortex (dACC) and left AISL was positively correlated with the level of hemoglobin (r = 0.4979, p < 0.001). Mediation analysis found that the indirect effect of hemoglobin on forward digit span test scores via the FC between the dACC and right AISL (p < 0.05).

Conclusion

Disrupted SN connectivity may help explain cognitive declines in ESRD patients and act as a potential early biomarker. Moreover, the SN connectivity may interact with anemia to promote cognitive impairment.

Acute-stage MRI cerebral oxygen consumption biomarkers predict 24-hour neurological outcome in a rat cardiac arrest model

Brain injury following cardiac arrest (CA) is thought to be caused by a sudden loss of blood flow resulting in disruption in oxygen delivery, neural function and metabolism. However, temporal trajectories of the brain's physiology in the first few hours following CA have not been fully characterized. Furthermore, the extent to which these early measures can predict future neurological outcomes has not been determined. The present study sought to perform dynamic measurements of cerebral blood flow (CBF), oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂ ) with MRI in the first 3 hours following the return of spontaneous circulation (ROSC) in a rat CA model. It was found that CBF, OEF and CMRO₂ all revealed a time-dependent increase during the first 3 hours after the ROSC. Furthermore, the temporal trajectories of CBF and CMRO₂ , but not OEF, were different across rats and related to neurologic outcomes at a later time (24 hours after the ROSC) (P < .001). Rats who manifested better outcomes revealed faster increases in CBF and CMRO₂ during the acute stage. When investigating physiological parameters measured at a single time point, CBF (ρ = 0.82, P = .004) and CMRO₂ (ρ = 0.80, P = .006) measured at ~ 3 hours post-ROSC were positively associated with neurologic outcome scores at 24 hours. These findings shed light on brain physiological changes following CA, and suggest that MRI measures of brain perfusion and metabolism may provide a potential biomarker to guide post-CA management.

Cerebral blood flow characteristics following hemodialysis initiation in older adults: A prospective longitudinal pilot study using arterial spin labeling imaging

PURPOSE

To investigate cerebral blood flow (CBF) characteristics before and after hemodialysis initiation and their longitudinal associations with global cognitive function in older adults.

METHODS

A cohort of 17 older end-stage renal disease patients anticipating standard thrice-weekly hemodialysis and a group of 11 age- and sex-matched healthy control volunteers were recruited for brain perfusion imaging studies using arterial spin labeling. Hemodialysis patients participated in a prospective longitudinal study using brain magnetic resonance imaging and global cognitive assessment using the Modified Mini-Mental State Examination (3MS) at two time points: baseline, 2.9 ± 0.9 months before, and follow-up, 6.4 ± 2.4 months after hemodialysis initiation. Healthy controls were imaged once using the same protocol. CBF analyses were performed globally in grey and white matter and regionally in the hippocampus and orbitofrontal cortex. Covariate-adjusted linear mixed-effects models were used for statistical analyses (significance: p < 0.05; marginal significance: p < 0.1).

RESULTS

At baseline, global and regional CBF was significantly higher in hemodialysis patients than in healthy controls. However, after approximately 6 months of hemodialysis, CBF declined substantially in hemodialysis patients, and became comparable to those in healthy controls. Specifically, in the hemodialysis patients, CBF declined non-significantly globally for grey and white matter and significantly regionally in the hippocampus and orbitofrontal cortex. Marginally significant associations were observed between 3MS scores and regional CBF measurements in the hippocampus and orbitofrontal cortex at baseline and follow-up, and between longitudinal changes.

CONCLUSION

The significant decline in CBF after hemodialysis initiation and the observed association between longitudinal changes in regional CBF and 3MS scores suggest that decreased brain perfusion may contribute to the observed cognitive decline.

Normal variations in brain oxygen extraction fraction are partly attributed to differences in end-tidal CO2

Cerebral oxygen extraction fraction is an important physiological index of the brain's oxygen consumption and supply and has been suggested to be a potential biomarker for a number of diseases such as stroke, Alzheimer's disease, multiple sclerosis, sickle cell disease, and metabolic disorders. However, in order for oxygen extraction fraction to be a sensitive biomarker for personalized disease diagnosis, inter-subject variations in normal subjects must be minimized or accounted for, which will otherwise obscure its interpretation. Therefore, it is essential to investigate the physiological underpinnings of normal differences in oxygen extraction fraction. This work used two studies, one discovery study and one verification study, to examine the extent to which an individual's end-tidal CO₂ can explain variations in oxygen extraction fraction. It was found that, across normal subjects, oxygen extraction fraction is inversely correlated with end-tidal CO₂. Approximately 50% of the inter-subject variations in oxygen extraction fraction can be attributed to end-tidal CO₂ differences. In addition, oxygen extraction fraction was found to be positively associated with age and systolic blood pressure. By accounting for end-tidal CO₂, age, and systolic blood pressure of the subjects, normal variations in oxygen extraction fraction can be reduced by 73%, which is expected to substantially enhance the utility of oxygen extraction fraction as a disease biomarker.

Hemodynamic mechanisms underlying elevated oxygen extraction fraction (OEF) in moyamoya and sickle cell anemia patients

Moyamoya is a bilateral, complex cerebrovascular condition characterized by progressive non-atherosclerotic intracranial stenosis and collateral vessel formation. Moyamoya treatment focuses on restoring cerebral blood flow (CBF) through surgical revascularization, however stratifying patients for revascularization requires abilities to quantify how well parenchyma is compensating for arterial steno-occlusion. Globally elevated oxygen extraction fraction (OEF) secondary to CBF reduction may serve as a biomarker for tissue health in moyamoya patients, as suggested in patients with sickle cell anemia (SCA) and reduced oxygen carrying capacity. Here, OEF was measured (TRUST-MRI) to test the hypothesis that OEF is globally elevated in patients with moyamoya (n = 18) and SCA (n = 18) relative to age-matched controls (n = 43). Mechanisms underlying the hypothesized OEF increases were evaluated by performing sequential CBF-weighted, cerebrovascular reactivity (CVR)-weighted, and structural MRI. Patients were stratified by treatment and non-parametric tests applied to compare study variables (significance: two-sided P < 0.05). OEF was significantly elevated in moyamoya participants (interquartile range = 0.38-0.45) compared to controls (interquartile range = 0.29-0.38), similar to participants with SCA (interquartile range = 0.37-0.45). CBF was inversely correlated with OEF in moyamoya participants. Elevated OEF was only weakly related to reductions in CVR, consistent with basal CBF level, rather than vascular reserve capacity, being most closely associated with OEF.

Oxygen metabolism in acute ischemic stroke

Gaining insights into brain oxygen metabolism has been one of the key areas of research in neurosciences. Extensive efforts have been devoted to developing approaches capable of providing measures of brain oxygen metabolism not only under normal physiological conditions but, more importantly, in various pathophysiological conditions such as cerebral ischemia. In particular, quantitative measures of cerebral metabolic rate of oxygen using positron emission tomography (PET) have been shown to be capable of discerning brain tissue viability during ischemic insults. However, the complex logistics associated with oxygen-15 PET have substantially hampered its wide clinical applicability. In contrast, magnetic resonance imaging (MRI)-based approaches have provided quantitative measures of cerebral oxygen metabolism similar to that obtained using PET. Given the wide availability, MRI-based approaches may have broader clinical impacts, particularly in cerebral ischemia, when time is a critical factor in deciding treatment selection. In this article, we review the pathophysiological basis of altered cerebral hemodynamics and oxygen metabolism in cerebral ischemia, how quantitative measures of cerebral metabolism were obtained using the Kety-Schmidt approach, the physical concepts of non-invasive oxygen metabolism imaging approaches, and, finally, clinical applications of the discussed imaging approaches.

Red cell exchange transfusions lower cerebral blood flow and oxygen extraction fraction in pediatric sickle cell anemia

Blood transfusions are the mainstay of stroke prevention in pediatric sickle cell anemia (SCA), but the physiology conferring this benefit is unclear. Cerebral blood flow (CBF) and oxygen extraction fraction (OEF) are elevated in SCA, likely compensating for reduced arterial oxygen content (CaO2). We hypothesized that exchange transfusions would decrease CBF and OEF by increasing CaO2, thereby relieving cerebral oxygen metabolic stress. Twenty-one children with SCA receiving chronic transfusion therapy (CTT) underwent magnetic resonance imaging before and after exchange transfusions. Arterial spin labeling and asymmetric spin echo sequences measured CBF and OEF, respectively, which were compared pre- and posttransfusion. Volumes of tissue with OEF above successive thresholds (36%, 38%, and 40%), as a metric of regional metabolic stress, were compared pre- and posttransfusion. Transfusions increased hemoglobin (Hb; from 9.1 to 10.3 g/dL; P < .001) and decreased Hb S (from 39.7% to 24.3%; P < .001). Transfusions reduced CBF (from 88 to 82.4 mL/100 g per minute; P = .004) and OEF (from 34.4% to 31.2%; P < .001). At all thresholds, transfusions reduced the volume of peak OEF found in the deep white matter, a location at high infarct risk in SCA (P < .001). Reduction of elevated CBF and OEF, both globally and regionally, suggests that CTT mitigates infarct risk in pediatric SCA by relieving cerebral metabolic stress at patient- and tissue-specific levels.

Reduced deep regional cerebral venous oxygen saturation in hemodialysis patients using quantitative susceptibility mapping

Cerebral venous oxygen saturation (SvO₂) is an important indicator of brain function. There was debate about lower cerebral oxygen metabolism in hemodialysis patients and there were no reports about the changes of deep regional cerebral SvO₂ in hemodialysis patients. In this study, we aim to explore the deep regional cerebral SvO₂ from straight sinus using quantitative susceptibility mapping (QSM) and the correlation with clinical risk factors and neuropsychiatric testing_. 52 hemodialysis patients and 54 age-and gender-matched healthy controls were enrolled. QSM reconstructed from original phase data of 3.0 T susceptibility-weighted imaging was used to measure the susceptibility of straight sinus. The susceptibility was used to calculate the deep regional cerebral SvO₂ and compare with healthy individuals. Correlation analysis was performed to investigate the correlation between deep regional cerebral SvO₂, clinical risk factors and neuropsychiatric testing. The deep regional cerebral SvO₂ of hemodialysis patients (72.5 ± 3.7%) was significantly lower than healthy controls (76.0 ± 2.1%) (P < 0.001). There was no significant difference in the measured volume of interests of straight sinus between hemodialysis patients (250.92 ± 46.65) and healthy controls (249.68 ± 49.68) (P = 0.859). There were no significant correlations between the measured susceptibility and volume of interests in hemodialysis patients (P = 0.204) and healthy controls (P = 0.562), respectively. Hematocrit (r = 0.480, P < 0.001, FDR corrected), hemoglobin (r = 0.440, P < 0.001, FDR corrected), red blood cell (r = 0.446, P = 0.003, FDR corrected), dialysis duration (r = 0.505, P = 0.002, FDR corrected) and parathyroid hormone (r = -0.451, P = 0.007, FDR corrected) were risk factors for decreased deep regional cerebral SvO₂ in patients. The Mini-Mental State Examination (MMSE) scores of hemodialysis patients were significantly lower than healthy controls (P < 0.001). However, the deep regional cerebral SvO₂ did not correlate with MMSE scores (P = 0.630). In summary, the decreased deep regional cerebral SvO₂ occurred in hemodialysis patients and dialysis duration, parathyroid hormone, hematocrit, hemoglobin and red blood cell may be clinical risk factors.

Quantitative assessment of cerebral venous blood T2 in mouse at 11.7T: Implementation, optimization, and age effect

PURPOSE

To develop a non-contrast-agent MRI technique to quantify cerebral venous T₂ in mice.

METHODS

We implemented and optimized a T₂ -relaxation-under-spin-tagging (TRUST) sequence on an 11.7 Tesla animal imaging system. A flow-sensitive-alternating-inversion-recovery (FAIR) module was used to generate control and label images, pair-wise subtraction of which yielded blood signals. Then, a T₂ -preparation module was applied to produce T₂ -weighted images, from which blood T₂ was quantified. We conducted a series of technical studies to optimize the imaging slice position, inversion slab thickness, post-labeling delay (PLD), and repetition time. We also performed three physiological studies to examine the venous T₂ dependence on hyperoxia (N = 4), anesthesia (N = 3), and brain aging (N = 5).

RESULTS

Our technical studies suggested that, for efficient data acquisition with minimal bias in estimated T₂ , a preferred TRUST protocol was to place the imaging slice at the confluence of sagittal sinuses with an inversion-slab thickness of 2.5-mm, a PLD of 1000 ms and a repetition time of 3.5 s. Venous T₂ values under normoxia and hyperoxia (inhaling pure oxygen) were 26.9 ± 1.7 and 32.3 ± 2.2 ms, respectively. Moreover, standard isoflurane anesthesia resulted in a higher venous T₂ compared with dexmedetomidine anesthesia (N = 3; P = 0.01) which is more commonly used in animal functional MRI studies to preserve brain function. Venous T₂ exhibited a decrease with age (N = 5; P < 0.001).

CONCLUSION

We have developed and optimized a noninvasive method to quantify cerebral venous blood T₂ in mouse at 11.7 T. This method may prove useful in studies of brain physiology and pathophysiology in animal models. Magn Reson Med 80:521-528, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Three-dimensional mapping of brain venous oxygenation using R2* oximetry

PURPOSE

Cerebral venous oxygenation (Y_v ) is an important biomarker for brain diseases. This study aims to develop an R2*-based MR oximetry that can measure cerebral Y_v in 3D.

METHODS

This technique separates blood signal from tissue by velocity-encoding phase contrast and measures the R2* of pure blood by multi-gradient-echo acquisition. The blood R2* was converted to Y_v using an R2*-versus-oxygenation (Y) calibration curve, which was obtained by in vitro bovine blood experiments. Reproducibility, sensitivity, validity, and resolution dependence of the technique were evaluated.

RESULTS

In vitro R2*-Y calibration plot revealed a strong dependence of blood R2* on oxygenation, with additional dependence on hematocrit. In vivo results demonstrated that the technique can provide a 3D venous oxygenation map that depicts both large sinuses and smaller cortical veins, with venous oxygenation ranging from 57 to 72%. Intrasession coefficient of variation of the measurement was 3.0%. The technique detected an average Y_v increase of 10.8% as a result of hyperoxia, which was validated by global oxygenation measurement from T₂ -Relaxation-Under-Spin-Tagging (TRUST) MRI. Two spatial resolutions, one with an isotropic voxel dimension and the other with a nonisotropic dimension, were tested for full brain coverage.

CONCLUSIONS

This study demonstrated the feasibility of 3D brain oxygenation mapping without using contrast agent. Magn Reson Med 79:1304-1313, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Severity-specific alterations in CBF, OEF and CMRO2 in cirrhotic patients with hepatic encephalopathy

OBJECTIVES

To assess how the severity of hepatic encephalopathy (HE) affects perfusion and metabolic changes in cirrhotic patients and the association between severity and liver disease and anemia.

METHODS

The study groups comprised 31 healthy subjects and 33 cirrhotic patients who underwent MR examinations, and blood and neuropsychological tests. Of the cirrhotic patients, 14 were unaffected, and 11 had covert HE (CHE) and 8 overt HE (OHE). Global cerebral blood flow (CBF), oxygen extraction fraction (OEF), and metabolic rate of oxygen (CMRO2) were noninvasively measured by phase-contrast and T2-relaxation-under-spin-tagging MRI. Correlations were performed between MR measurements, hematocrits, ammonia levels, Child-Pugh scores and neuropsychological test scores.

RESULTS

Compared with the values in healthy subjects, CBF was higher in unaffected patients, the same in CHE patients and lower in OHE patients, OEF was higher in all patients, and CMRO2 was the same in unaffected and CHE patients and lower in OHE patients. Hematocrit was negatively correlated with CBF and OEF, but not with CMRO2. Ammonia level was negatively correlated with CBF and CMRO2, and Child-Pugh score was negatively correlated with CMRO2.

CONCLUSIONS

The severity-associated alterations in cirrhotic patients indicate that homeostasis of oxygen delivery and oxidative metabolism in HE is regulated by multiple mechanisms. These physiological alterations appeared to be associated with the degree of anemia, ammonia level, and liver function.

KEY POINTS

• CBF, OEF and CMRO2 did not change monotonically with HE progression. • Anemia possibly contributed to CBF and OEF changes in cirrhotic patients. • Liver dysfunction mainly contributed to changes in CMRO2 in cirrhotic patients.