Contrast-free dynamic susceptibility contrast using sinusoidal and bolus oxygenation challenges

Deoxygenation-based dynamic susceptibility contrast (dDSC) MRI uses respiratory challenges as a source of endogenous contrast as an alternative to gadolinium injection. These gas challenges induce T2*-weighted MRI signal losses, after which tracer kinetics modeling was applied to calculate cerebral perfusion. This work compares three gas challenges, desaturation (transient hypoxia), resaturation (transient normoxia), and SineO2 (sinusoidal modulation of end-tidal oxygen pressures) in a cohort of 10 healthy volunteers (age 37 ± 11 years; 60% female). Perfusion estimates consisted of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). Calculations were computed using a traditional tracer kinetics model in the time domain for desaturation and resaturation and in the frequency domain for SineO2. High correlations and limits of agreement were observed among the three deoxygenation-based paradigms for CBV, although MTT and CBF estimates varied with the hypoxic stimulus. Cross-modality correlation with gadolinium DSC was lower, particularly for MTT, but on a par with agreement between the other perfusion references. Overall, this work demonstrated the feasibility and reliability of oxygen respiratory challenges to measure brain perfusion. Additional work is needed to assess the utility of dDSC in the diagnostic evaluation of various pathologies such as ischemic strokes, brain tumors, and neurodegenerative diseases.

Decreased erythrocyte aggregation in Glenn and Fontan: univentricular circulation as a rheologic disease model

BACKGROUND

In the Fontan palliation for single ventricle heart disease (SVHD), pulmonary blood flow is non-pulsatile/passive, low velocity, and low shear, making viscous power loss a critical determinant of cardiac output. The rheologic properties of blood in SVHD patients are essential for understanding and modulating their limited cardiac output and they have not been systematically studied. We hypothesize that viscosity is decreased in single ventricle circulation.

METHODS

We evaluated whole blood viscosity, red blood cell (RBC) aggregation, and RBC deformability to evaluate changes in healthy children and SVHD patients. We altered suspending media to understand cellular and plasma differences contributing to rheologic differences.

RESULTS

Whole blood viscosity was similar between SVHD and healthy at their native hematocrits, while viscosity was lower at equivalent hematocrits for SVHD patients. RBC deformability is increased, and RBC aggregation is decreased in SVHD patients. Suspending SVHD RBCs in healthy plasma resulted in increased RBC aggregation and suspending healthy RBCs in SVHD plasma resulted in lower RBC aggregation.

CONCLUSIONS

Hematocrit corrected blood viscosity is lower in SVHD vs. healthy due to decreased RBC aggregation and higher RBC deformability, a viscous adaptation of blood in patients whose cardiac output is dependent on minimizing viscous power loss.

IMPACT

Patients with single ventricle circulation have decreased red blood cell aggregation and increased red blood cell deformability, both of which result in a decrease in blood viscosity across a large shear rate range. Since the unique Fontan circulation has very low-shear and low velocity flow in the pulmonary arteries, blood viscosity plays an increased role in vascular resistance, therefore this work is the first to describe a novel mechanism to target pulmonary vascular resistance as a modifiable risk factor. This is a novel, modifiable risk factor in this patient population.

Postacute Sequelae of SARS-CoV-2 in Children

The coronavirus disease 2019 (COVID-19) pandemic has caused significant medical, social, and economic impacts globally, both in the short and long term. Although most individuals recover within a few days or weeks from an acute infection, some experience longer lasting effects. Data regarding the postacute sequelae of severe acute respiratory syndrome coronavirus 2 infection (PASC) in children, or long COVID, are only just emerging in the literature. These symptoms and conditions may reflect persistent symptoms from acute infection (eg, cough, headaches, fatigue, and loss of taste and smell), new symptoms like dizziness, or exacerbation of underlying conditions. Children may develop conditions de novo, including postural orthostatic tachycardia syndrome, myalgic encephalomyelitis/chronic fatigue syndrome, autoimmune conditions and multisystem inflammatory syndrome in children. This state-of-the-art narrative review provides a summary of our current knowledge about PASC in children, including prevalence, epidemiology, risk factors, clinical characteristics, underlying mechanisms, and functional outcomes, as well as a conceptual framework for PASC based on the current National Institutes of Health definition. We highlight the pediatric components of the National Institutes of Health-funded Researching COVID to Enhance Recovery Initiative, which seeks to characterize the natural history, mechanisms, and long-term health effects of PASC in children and young adults to inform future treatment and prevention efforts. These initiatives include electronic health record cohorts, which offer rapid assessments at scale with geographical and demographic diversity, as well as longitudinal prospective observational cohorts, to estimate disease burden, illness trajectory, pathobiology, and clinical manifestations and outcomes.

Iron restriction in sickle cell disease: When less is more

Primum non nocere! Can iron deficiency, an abnormality that causes anemia, benefit people with sickle cell disease (SCD) who already have an anemia? The published literature we review appears to answer this question in the affirmative: basic science considerations, animal model experiments, and noncontrolled clinical observations all suggest a therapeutic potential of iron restriction in SCD. This is because SCD's clinical manifestations are ultimately attributable to the polymerization of hemoglobin S (HbS), a process strongly influenced by intracellular HbS concentration. Even small decrements in HbS concentration greatly reduce polymerization, and iron deficiency lowers erythrocyte hemoglobin concentration. Thus, iron deficiency could improve SCD by changing its clinical features to those of a more benign anemia (i.e., a condition with fewer or no vaso-occlusive events). We propose that well-designed clinical studies be implemented to definitively determine whether iron restriction is a safe and effective option in SCD. These investigations are particularly timely now that pharmacologic agents are being developed, which may directly reduce red cell hemoglobin concentrations without the need for phlebotomies to deplete total body iron.

Cerebral hemodynamics and oxygenation in adult patients with sickle cell disease after stem cell transplantation

Sickle cell disease (SCD) is characterized by chronic hemolytic anemia associated with impaired cerebral hemodynamics and oxygen metabolism. Hematopoietic stem cell transplantation (HSCT) is currently the only curative treatment for patients with SCD. Whereas normalization of hemoglobin levels and hemolysis markers has been reported after HSCT, its effects on cerebral perfusion and oxygenation in adult SCD patients remain largely unexplored. This study investigated the effects of HSCT on cerebral blood flow (CBF), oxygen delivery, cerebrovascular reserve (CVR), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2 ) in 17 adult SCD patients (mean age: 25.0 ± 8.0, 6 females) before and after HSCT and 10 healthy ethnicity-matched controls (mean age: 28.0 ± 8.8, 6 females) using MRI. For the CVR assessment, perfusion scans were performed before and after acetazolamide as a vasodilatory stimulus. Following HSCT, gray and white matter (GM and WM) CBF decreased (p < .01), while GM and WM CVR increased (p < .01) compared with the baseline measures. OEF and CMRO2 also increased towards levels in healthy controls (p < .01). The normalization of cerebral perfusion and oxygen metabolism corresponded with a significant increase in hemoglobin levels and decreases in reticulocytes, total bilirubin, and LDH as markers of hemolysis (p < .01). This study shows that HSCT results in the normalization of cerebral perfusion and oxygen metabolism, even in adult patients with SCD. Future follow-up MRI scans will determine whether the observed normalization of cerebral hemodynamics and oxygen metabolism prevents new silent cerebral infarcts.

Cerebrovascular responses to a 90° tilt in healthy neonates

BACKGROUND

Tilts can induce alterations in cerebral hemodynamics in healthy neonates, but prior studies have only examined systemic parameters or used small tilt angles (<90°). The healthy neonatal population, however, are commonly subjected to large tilt angles (≥90°). We sought to characterize the cerebrovascular response to a 90° tilt in healthy term neonates.

METHODS

We performed a secondary descriptive analysis on 44 healthy term neonates. We measured cerebral oxygen saturation (rcSO2), oxygen saturation (SpO2), heart rate (HR), breathing rate (BR), and cerebral fractional tissue oxygen extraction (cFTOE) over three consecutive 90° tilts. These parameters were measured for 2-min while neonates were in a supine (0°) position and 2-min while tilted to a sitting (90°) position. We measured oscillometric mean blood pressure (MBP) at the start of each tilt.

RESULTS

rcSO2 and BR decreased significantly in the sitting position, whereas cFTOE, SpO2, and MBP increased significantly in the sitting position. We detected a significant position-by-time interaction for all physiological parameters.

CONCLUSION

A 90° tilt induces a decline in rcSO2 and an increase in cFTOE in healthy term neonates. Understanding the normal cerebrovascular response to a 90° tilt in healthy neonates will help clinicians to recognize abnormal responses in high-risk infant populations.

IMPACT

Healthy term neonates (≤14 days old) had decreased cerebral oxygen saturation (~1.1%) and increased cerebral oxygen extraction (~0.01) following a 90° tilt. We detected a significant position-by-time interaction with all physiological parameters measured, suggesting the effect of position varied across consecutive tilts. No prior study has characterized the cerebral oxygen saturation response to a 90° tilt in healthy term neonates.

Venous Blood Oxygenation Measurements Using TRUST and T2-TRIR MRI During Hypoxic and Hypercapnic Gas Challenges

BACKGROUND

Oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2 ) may serve as biomarkers in several diseases. OEF and CMRO2 can be estimated from venous blood oxygenation (Yv ) levels, which in turn can be calculated from venous blood T2 values (T2b ). T2b can be measured using different MRI sequences, including T2-relaxation-under-spin-tagging (TRUST) and T2-prepared-blood-relaxation-imaging-with-inversion-recovery (T2-TRIR). The latter measures both T2b and T1 (T1b ) but was found previously to overestimate T2b compared to TRUST. It remained unclear, however, if this bias is constant across higher and lower oxygen saturations.

PURPOSE

To compare TRUST and T2-TRIR across a range of O2 saturations using hypoxic and hypercapnic gas challenges.

STUDY TYPE

Prospective.

POPULATION

Twelve healthy volunteers (four female, age 36 ± 10 years).

FIELD STRENGTH/SEQUENCE

A 3T; turbo-field echo-planar-imaging (TFEPI), echo-planar-imaging (EPI), and fast-field-echo (FFE).

ASSESSMENT

TRUST- and T2-TRIR-derived T2b , Yv , OEF, and CMRO2 were compared across different respiratory challenges. T1b from T2-TRIR was used to estimate Hct (HctTRIR ) and compared with venipuncture (HctVP ).

STATISTICAL TESTS

Shapiro-Wilk, one-sample and paired-sample t-test, repeated measures ANOVA, Friedman test, Bland-Altman, and correlation analysis. Bonferroni multiple-comparison correction was performed. Significance level was 0.05.

RESULTS

A significant bias was observed between TRUST- and T2-TRIR-derived T2b , Yv , and OEF values (-13 ± 11 msec, -5.3% ± 3.5% and 5.9 ± 4.1%, respectively). For Yv and OEF, this bias was constant across the range of measured values. T1b was significantly lower during severe hypoxia and hypercapnia compared to baseline (1712 ± 86 msec and 1634 ± 79 msec compared to 1757 ± 90 msec). While no significant bias was found between HctVP and HctTRIR (0.02% ± 0.06%, P = 0.20), the correlation between these Hct values was significant but weak (r = 0.19).

DATA CONCLUSION

Given the constant bias, TRUST- and T2-TRIR-derived venous T2b values can be used interchangeably to estimate Yv , OEF, and CMRO2 across a broad range of oxygen saturations. Hct from T2-TRIR-derived T1-values only weakly correlated with Hct from venipuncture.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Cardiac complications in thalassemia throughout the lifespan: Victories and challenges

Thalassemias are among the most common hereditary diseases in the world because heterozygosity offers protection against malarial infection. Affected individuals have variable expression of alpha or beta chains that lead to their unbalanced utilization during hemoglobin formation, oxidative stress, and apoptosis of red cell precursors prior to maturation. Some individuals produce sufficient hemoglobin to survive but suffer the vascular stress imposed by chronic anemia and ineffective erythropoiesis. In other patients, mature red cell formation is insufficient, and chronic transfusions are required-suppressing anemia and ineffective erythropoiesis but at the expense of iron overload. The cardiovascular consequences of thalassemia have changed dramatically over the previous five decades because of evolving treatment practices. This review summarizes this evolution, focusing on complications and management pertinent to modern patient cohorts.

Splenic iron decreases without change in volume or liver parameters during luspatercept therapy

Splenic iron decreased whereas liver iron was stable during luspatercept therapy in some individuals with thalassemia. This suggests a reduction of ineffective erythropoiesis changes the organ distribution of iron and demonstrates that liver iron concentration alone may not accurately reflect total body iron content. This article describes data from subjects enrolled in BELIEVE (NCT02604433) and BEYOND (NCT03342404).

Brain BOLD and NIRS response to hyperoxic challenge in sickle cell disease and chronic anemias

PURPOSE

Congenital anemias, including sickle cell anemia and thalassemia, are associated with cerebral tissue hypoxia and heightened stroke risks. Recent works in sickle cell disease mouse models have suggested that hyperoxia respiratory challenges can identify regions of the brain having chronic tissue hypoxia. Therefore, this work investigated differences in hyperoxic response and regional cerebral oxygenation between anemic and healthy subjects.

METHODS

A cohort of 38 sickle cell disease subjects (age 22 ± 8 years, female 39%), 25 non-sickle anemic subjects (age 25 ± 11 years, female 52%), and 31 healthy controls (age 25 ± 10 years, female 68%) were examined. A hyperoxic gas challenge was performed with concurrent acquisition of blood oxygen level-dependent (BOLD) MRI and near-infrared spectroscopy (NIRS). In addition to hyperoxia-induced changes in BOLD and NIRS, global measurements of cerebral blood flow, oxygen delivery, and cerebral metabolic rate of oxygen were obtained and compared between the three groups.

RESULTS

Regional BOLD changes were not able to identify brain regions of flow limitation in chronically anemic patients. Higher blood oxygen content and tissue oxygenation were observed during hyperoxia gas challenge. Both control and anemic groups demonstrated lower blood flow, oxygen delivery, and metabolic rate compared to baseline, but the oxygen metabolism in anemic subjects were abnormally low during hyperoxic exposure.

CONCLUSION

These results indicated that hyperoxic respiratory challenge could not be used to identify chronically ischemic brain. Furthermore, the low hyperoxia-induced metabolic rate suggested potential negative effects of prolonged oxygen therapy and required further studies to evaluate the risk for hyperoxia-induced oxygen toxicity and cerebral dysfunction.

Researching COVID to Enhance Recovery (RECOVER) adult study protocol: Rationale, objectives, and design

IMPORTANCE

SARS-CoV-2 infection can result in ongoing, relapsing, or new symptoms or other health effects after the acute phase of infection; termed post-acute sequelae of SARS-CoV-2 infection (PASC), or long COVID. The characteristics, prevalence, trajectory and mechanisms of PASC are ill-defined. The objectives of the Researching COVID to Enhance Recovery (RECOVER) Multi-site Observational Study of PASC in Adults (RECOVER-Adult) are to: (1) characterize PASC prevalence; (2) characterize the symptoms, organ dysfunction, natural history, and distinct phenotypes of PASC; (3) identify demographic, social and clinical risk factors for PASC onset and recovery; and (4) define the biological mechanisms underlying PASC pathogenesis.

METHODS

RECOVER-Adult is a combined prospective/retrospective cohort currently planned to enroll 14,880 adults aged ≥18 years. Eligible participants either must meet WHO criteria for suspected, probable, or confirmed infection; or must have evidence of no prior infection. Recruitment occurs at 86 sites in 33 U.S. states, Washington, DC and Puerto Rico, via facility- and community-based outreach. Participants complete quarterly questionnaires about symptoms, social determinants, vaccination status, and interim SARS-CoV-2 infections. In addition, participants contribute biospecimens and undergo physical and laboratory examinations at approximately 0, 90 and 180 days from infection or negative test date, and yearly thereafter. Some participants undergo additional testing based on specific criteria or random sampling. Patient representatives provide input on all study processes. The primary study outcome is onset of PASC, measured by signs and symptoms. A paradigm for identifying PASC cases will be defined and updated using supervised and unsupervised learning approaches with cross-validation. Logistic regression and proportional hazards regression will be conducted to investigate associations between risk factors, onset, and resolution of PASC symptoms.

DISCUSSION

RECOVER-Adult is the first national, prospective, longitudinal cohort of PASC among US adults. Results of this study are intended to inform public health, spur clinical trials, and expand treatment options.

REGISTRATION

NCT05172024.

Low-field 0.55 T MRI evaluation of the fetus

Fetal magnetic resonance imaging (MRI) is an important adjunct modality for the evaluation of fetal abnormalities. Recently, low-field MRI systems at 0.55 Tesla have become available which can produce images on par with 1.5 Tesla systems but with lower power deposition, acoustic noise, and artifact. In this article, we describe a technical innovation using low-field MRI to perform diagnostic quality fetal MRI.

Researching COVID to enhance recovery (RECOVER) pediatric study protocol: Rationale, objectives and design

Importance

The prevalence, pathophysiology, and long-term outcomes of COVID-19 (post-acute sequelae of SARS-CoV-2 [PASC] or "Long COVID") in children and young adults remain unknown. Studies must address the urgent need to define PASC, its mechanisms, and potential treatment targets in children and young adults.

Observations

We describe the protocol for the Pediatric Observational Cohort Study of the NIH's RE searching COV ID to E nhance R ecovery (RECOVER) Initiative. RECOVER-Pediatrics is an observational meta-cohort study of caregiver-child pairs (birth through 17 years) and young adults (18 through 25 years), recruited from more than 100 sites across the US. This report focuses on two of five cohorts that comprise RECOVER-Pediatrics: 1) a de novo RECOVER prospective cohort of children and young adults with and without previous or current infection; and 2) an extant cohort derived from the Adolescent Brain Cognitive Development (ABCD) study ( n =10,000). The de novo cohort incorporates three tiers of data collection: 1) remote baseline assessments (Tier 1, n=6000); 2) longitudinal follow-up for up to 4 years (Tier 2, n=6000); and 3) a subset of participants, primarily the most severely affected by PASC, who will undergo deep phenotyping to explore PASC pathophysiology (Tier 3, n=600). Youth enrolled in the ABCD study participate in Tier 1. The pediatric protocol was developed as a collaborative partnership of investigators, patients, researchers, clinicians, community partners, and federal partners, intentionally promoting inclusivity and diversity. The protocol is adaptive to facilitate responses to emerging science.

Conclusions and Relevance

RECOVER-Pediatrics seeks to characterize the clinical course, underlying mechanisms, and long-term effects of PASC from birth through 25 years old. RECOVER-Pediatrics is designed to elucidate the epidemiology, four-year clinical course, and sociodemographic correlates of pediatric PASC. The data and biosamples will allow examination of mechanistic hypotheses and biomarkers, thus providing insights into potential therapeutic interventions.

Clinical Trialsgov Identifier

Clinical Trial Registration: http://www.clinicaltrials.gov . Unique identifier: NCT05172011.

Quantification of Liver Iron Overload with MRI: Review and Guidelines from the ESGAR and SAR

Accumulation of excess iron in the body, or systemic iron overload, results from a variety of causes. The concentration of iron in the liver is linearly related to the total body iron stores and, for this reason, quantification of liver iron concentration (LIC) is widely regarded as the best surrogate to assess total body iron. Historically assessed using biopsy, there is a clear need for noninvasive quantitative imaging biomarkers of LIC. MRI is highly sensitive to the presence of tissue iron and has been increasingly adopted as a noninvasive alternative to biopsy for detection, severity grading, and treatment monitoring in patients with known or suspected iron overload. Multiple MRI strategies have been developed in the past 2 decades, based on both gradient-echo and spin-echo imaging, including signal intensity ratio and relaxometry strategies. However, there is a general lack of consensus regarding the appropriate use of these methods. The overall goal of this article is to summarize the current state of the art in the clinical use of MRI to quantify liver iron content and to assess the overall level of evidence of these various methods. Based on this summary, expert consensus panel recommendations on best practices for MRI-based quantification of liver iron are provided.

Sinusoidal CO2 respiratory challenge for concurrent perfusion and cerebrovascular reactivity MRI

Introduction: Deoxygenation-based dynamic susceptibility contrast (dDSC) has previously leveraged respiratory challenges to modulate blood oxygen content as an endogenous source of contrast alternative to gadolinium injection in perfusion-weighted MRI. This work proposed the use of sinusoidal modulation of end-tidal CO2 pressures (SineCO 2 ), which has previously been used to measure cerebrovascular reactivity, to induce susceptibility-weighted gradient-echo signal loss to measure brain perfusion. Methods: SineCO 2 was performed in 10 healthy volunteers (age 37 ± 11, 60% female), and tracer kinetics model was applied in the frequency domain to calculate cerebral blood flow, cerebral blood volume, mean transit time, and temporal delay. These perfusion estimates were compared against reference techniques, including gadolinium-based DSC, arterial spin labeling, and phase contrast. Results: Our results showed regional agreement between SineCO 2 and the clinical comparators. SineCO 2 was able to generate robust CVR maps in conjunction to baseline perfusion estimates. Discussion: Overall, this work demonstrated feasibility of using sinusoidal CO2 respiratory paradigm to simultaneously acquire both cerebral perfusion and cerebrovascular reactivity maps in one imaging sequence.

Imputation models and error analysis for phase contrast MR cerebral blood flow measurements

Cerebral blood flow (CBF) supports brain metabolism. Diseases impair CBF, and pharmacological agents modulate CBF. Many techniques measure CBF, but phase contrast (PC) MR imaging through the four arteries supplying the brain is rapid and robust. However, technician error, patient motion, or tortuous vessels degrade quality of the measurements of the internal carotid (ICA) or vertebral (VA) arteries. We hypothesized that total CBF could be imputed from measurements in subsets of these 4 feeding vessels without excessive penalties in accuracy. We analyzed PC MR imaging from 129 patients, artificially excluded 1 or more vessels to simulate degraded imaging quality, and developed models of imputation for the missing data. Our models performed well when at least one ICA was measured, and resulted in R ² values of 0.998-0.990, normalized root mean squared error values of 0.044-0.105, and intra-class correlation coefficient of 0.982-0.935. Thus, these models were comparable or superior to the test-retest variability in CBF measured by PC MR imaging. Our imputation models allow retrospective correction for corrupted blood vessel measurements when measuring CBF and guide prospective CBF acquisitions.

In Situ Measurement of Electron Energy Evolution in a Laser-Plasma Accelerator

We report on a novel, noninvasive method applying Thomson scattering to measure the evolution of the electron beam energy inside a laser-plasma accelerator with high spatial resolution. The determination of the local electron energy enabled the in-situ detection of the acting acceleration fields without altering the final beam state. In this Letter we demonstrate that the accelerating fields evolve from (265±119)  GV/m to (9±4)  GV/m in a plasma density ramp. The presented data show excellent agreement with particle-in-cell simulations. This method provides new possibilities for detecting the dynamics of plasma-based accelerators and their optimization.

Intramyocardial hemorrhage drives fatty degeneration of infarcted myocardium

Sudden blockage of arteries supplying the heart muscle contributes to millions of heart attacks (myocardial infarction, MI) around the world. Although re-opening these arteries (reperfusion) saves MI patients from immediate death, approximately 50% of these patients go on to develop chronic heart failure (CHF) and die within a 5-year period; however, why some patients accelerate towards CHF while others do not remains unclear. Here we show, using large animal models of reperfused MI, that intramyocardial hemorrhage - the most damaging form of reperfusion injury (evident in nearly 40% of reperfused ST-elevation MI patients) - drives delayed infarct healing and is centrally responsible for continuous fatty degeneration of the infarcted myocardium contributing to adverse remodeling of the heart. Specifically, we show that the fatty degeneration of the hemorrhagic MI zone stems from iron-induced macrophage activation, lipid peroxidation, foam cell formation, ceroid production, foam cell apoptosis and iron recycling. We also demonstrate that timely reduction of iron within the hemorrhagic MI zone reduces fatty infiltration and directs the heart towards favorable remodeling. Collectively, our findings elucidate why some, but not all, MIs are destined to CHF and help define a potential therapeutic strategy to mitigate post-MI CHF independent of MI size.

Impaired Neurovascular Function Underlies Poor Neurocognitive Outcomes and Is Associated with Nitric Oxide Bioavailability in Congenital Heart Disease

We use a non-invasive MRI proxy of neurovascular function (pnvf) to assess the ability of the vasculature to supply baseline metabolic demand, to compare pediatric and young adult congenital heart disease (CHD) patients to normal referents and relate the proxy to neurocognitive outcomes and nitric oxide bioavailability. In a prospective single-center study, resting-state blood-oxygen-level-dependent (BOLD) and arterial spin labeling (ASL) MRI scans were successfully obtained from 24 CHD patients (age = 15.4 ± 4.06 years) and 63 normal referents (age = 14.1 ± 3.49) years. Pnvf was computed on a voxelwise basis as the negative of the ratio of functional connectivity strength (FCS) estimated from the resting-state BOLD acquisition to regional cerebral blood flow (rCBF) as estimated from the ASL acquisition. Pnvf was used to predict end-tidal CO2 (P_ETCO2) levels and compared to those estimated from the BOLD data. Nitric oxide availability was obtained via nasal measurements (nNO). Pnvf was compared on a voxelwise basis between CHD patients and normal referents and correlated with nitric oxide availability and neurocognitive outcomes as assessed via the NIH Toolbox. Pnvf was shown as highly predictive of P_ETCO2 using theoretical modeling. Pnvf was found to be significantly reduced in CHD patients in default mode network (DMN, comprising the ventromedial prefrontal cortex and posterior cingulate/precuneus), salience network (SN, comprising the insula and dorsal anterior cingulate), and central executive network (CEN, comprising posterior parietal and dorsolateral prefrontal cortex) regions with similar findings noted in single cardiac ventricle patients. Positive correlations of Pnvf in these brain regions, as well as the hippocampus, were found with neurocognitive outcomes. Similarly, positive correlations between Pnvf and nitric oxide availability were found in frontal DMN and CEN regions, with particularly strong correlations in subcortical regions (putamen). Reduced Pnvf in CHD patients was found to be mediated by nNO. Mediation analyses further supported that reduced Pnvf in these regions underlies worse neurocognitive outcome in CHD patients and is associated with nitric oxide bioavailability. Impaired neuro-vascular function, which may be non-invasively estimated via combined arterial-spin label and BOLD MR imaging, is a nitric oxide bioavailability dependent factor implicated in adverse neurocognitive outcomes in pediatric and young adult CHD.

Early initiation of disease-modifying therapy can impede or prevent diffuse myocardial fibrosis in sickle cell anemia

Cardiovascular disease is a major cause of mortality in patients with sickle cell disease (SCD). Niss et al previously reported that cardiac magnetic resonance in 25 patients showed universal myocardial fibrosis, which they correlated with increased extracellular volume fraction (ECV). In the current study, they compared patients with SCD who were treated with hydroxyurea or transfusion at age &lt;6 years to a group of patients with SCD without therapy. They documented that patients treated early had ECV levels comparable to normal controls, potentially preventing subsequent myocardial fibrosis.

Chronic anemia: The effects on the connectivity of white matter

Chronic anemia is commonly observed in patients with hemoglobinopathies, mainly represented by disorders of altered hemoglobin (Hb) structure (sickle cell disease, SCD) and impaired Hb synthesis (e.g. thalassemia syndromes, non-SCD anemia). Both hemoglobinopathies have been associated with white matter (WM) alterations. Novel structural MRI research in our laboratory demonstrated that WM volume was diffusely lower in deep, watershed areas proportional to anemia severity. Furthermore, diffusion tensor imaging analysis has provided evidence that WM microstructure is disrupted proportionally to Hb level and oxygen saturation. SCD patients have been widely studied and demonstrate lower fractional anisotropy (FA) in the corticospinal tract and cerebellum across the internal capsule and corpus callosum. In the present study, we compared 19 SCD and 15 non-SCD anemia patients with a wide range of Hb values allowing the characterization of the effects of chronic anemia in isolation of sickle Hb. We performed a tensor analysis to quantify FA changes in WM connectivity in chronic anemic patients. We calculated the volumetric mean of FA along the pathway of tracks connecting two regions of interest defined by BrainSuite's BCI-DNI atlas. In general, we found lower FA values in anemic patients; indicating the loss of coherence in the main diffusion direction that potentially indicates WM injury. We saw a positive correlation between FA and hemoglobin in these same regions, suggesting that decreased WM microstructural integrity FA is highly driven by chronic hypoxia. The only connection that did not follow this pattern was the connectivity within the left middle-inferior temporal gyrus. Interestingly, more reductions in FA were observed in non-SCD patients (mainly along with intrahemispheric WM bundles and watershed areas) than the SCD patients (mainly interhemispheric).

Anemia Increases Oxygen Extraction Fraction in Deep Brain Structures but Not in the Cerebral Cortex

Sickle cell disease (SCD) is caused by a single amino acid mutation in hemoglobin, causing chronic anemia and neurovascular complications. However, the effects of chronic anemia on oxygen extraction fraction (OEF), especially in deep brain structures, are less well understood. Conflicting OEF values have been reported in SCD patients, but have largely attributed to different measurement techniques, faulty calibration, and different locations of measurement. Thus, in this study, we investigated the reliability and agreement of two susceptibility-based methods, quantitative susceptibility mapping (QSM) and complex image summation around a spherical or a cylindrical object (CISSCO), for OEF measurements in internal cerebral vein (ICV), reflecting oxygen saturation in deep brain structures. Both methods revealed that SCD patients and non-sickle anemia patients (ACTL) have increased OEF in ICV (42.6% ± 5.6% and 30.5% ± 3.6% in SCD by CISSCO and QSM respectively, 37.0% ± 4.1% and 28.5% ± 2.3% in ACTL) compared with controls (33.0% ± 2.3% and 26.8% ± 1.8%). OEF in ICV varied reciprocally with hematocrit (r ² = 0.92, 0.53) and oxygen content (r ² = 0.86, 0.53) respectively. However, an opposite relationship was observed for OEF measurements in sagittal sinus (SS) with the widely used T₂-based oximetry, T₂-Relaxation-Under-Spin-Tagging (TRUST), in the same cohorts (31.2% ± 6.6% in SCD, 33.3% ± 5.9% in ACTL and 36.8% ± 5.6% in CTL). Importantly, we demonstrated that hemoglobin F and other fast moving hemoglobins decreased OEF by TRUST and explained group differences in sagittal sinus OEF between anemic and control subjects. These data demonstrate that anemia causes deep brain hypoxia in anemia subjects with concomitant preservation of cortical oxygenation, as well as the key interaction of the hemoglobin dissociation curve and cortical oxygen extraction.

Assessment of functional shunting in patients with sickle cell disease

Silent cerebral infarcts (SCI) are common in patients with sickle cell disease (SCD) and are thought to be caused by a mismatch between oxygen delivery and consumption. Functional cerebrovascular shunting is defined as reduced oxygen offloading due to the rapid transit of blood through the capillaries caused by increased flow and has been suggested as a potential mechanism underlying reduced oxygenation and SCI. We investigated the venous arterial spin labeling signal (VS) in the sagittal sinus as a proxy biomarker of cerebral functional shunting, and its association with hemodynamic imaging and hematological laboratory parameters. We included 28 children and 38 adults with SCD, and ten healthy racematched adult controls. VS, cerebral blood flow (CBF), velocity in the brain feeding arteries, oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) were measured before and after acetazolamide administration. VS was higher in patients with SCD compared to controls (P<0.01) and was increased after acetazolamide administration in all groups (P<0.01). VS was primarily predicted by CBF (P<0.01), but CBF-corrected VS was also associated with decreased CMRO2 (P<0.01). Additionally, higher disease severity defined by low hemoglobin and increased hemolysis was associated with higher CBF-corrected VS. Finally, CMRO2 was negatively correlated with fetal hemoglobin, and positively correlated with lactate dehydrogenase, which could be explained by changes in oxygen affinity. These findings provide evidence for cerebral functional shunting and encourage future studies investigating the potential link to aberrant capillary exchange in SCD.

Dynamic MR imaging of cerebral perfusion during bicycling exercise

Habitual physical activity is beneficial for cerebrovascular health and cognitive function. Physical exercise therefore constitutes a clinically relevant cerebrovascular stimulus. This study demonstrates the feasibility of quantitative cerebral blood flow (CBF) measurements during supine bicycling exercise with pseudo-continuous arterial spin labeling (pCASL) magnetic resonance imaging (MRI) at 3 Tesla. Twelve healthy volunteers performed a steady-state exercise-recovery protocol on an MR-compatible bicycle ergometer, while dynamic pCASL data were acquired at rest, during moderate (60% of the age-predicted supine maximal heart rate (HR_max)) and vigorous (80% of supine HR_max) exercise, and subsequent recovery. These CBF measurements were compared with 2D phase-contrast MRI measurements of blood flow through the carotid arteries. Procedures were repeated on a separate day for an assessment of measurement repeatability. Whole-brain (WB) CBF was 41.2 ± 6.9 mL/100 g/min at rest (heart rate 63 [57-71] beats/min), remained similar at moderate exercise (102 [97-107] beats/min), decreased by 10% to 37.1 ± 5.7 mL/100 g/min (p = 0.001) during vigorous exercise (139 [136-142] beats/min) and decreased further to 34.2 ± 6.0 mL/100 g/min (p < 0.001) during recovery. Hippocampus CBF decreased by 12% (p = 0.001) during moderate exercise, decreased further during vigorous exercise (-21%; p < 0.001) and was even lower during recovery (-31%; p < 0.001). In contrast, motor cortex CBF increased by 12% (p = 0.027) during moderate exercise, returned to resting-state values during vigorous exercise, and decreased by 17% (p = 0.006) during recovery. The inter-session repeatability coefficients for WB CBF were approximately 20% for all stages of the exercise-recovery protocol. Phase-contrast blood flow measurements through the common carotid arteries overestimated the WB CBF because of flow directed to the face and scalp. This bias increased with exercise. We have demonstrated the feasibility of dynamic pCASL-MRI of the human brain for a quantitative evaluation of cerebral perfusion during bicycling exercise. Our spatially resolved measurements revealed a differential response of CBF in the motor cortex as well as the hippocampus compared with the brain as a whole. Caution is warranted when using flow through the common carotid arteries as a surrogate measure for cerebral perfusion.

Intramyocardial Hemorrhage and the "Wave Front" of Reperfusion Injury Compromising Myocardial Salvage

BACKGROUND

Reperfusion therapy for acute myocardial infarction (MI) is lifesaving. However, the benefit of reperfusion therapy can be paradoxically diminished by reperfusion injury, which can increase MI size.

OBJECTIVES

Hemorrhage is known to occur in reperfused MIs, but whether hemorrhage plays a role in reperfusion-mediated MI expansion is not known.

METHODS

We studied cardiac troponin kinetics (cTn) of ST-segment elevation MI patients (n = 70) classified by cardiovascular magnetic resonance to be hemorrhagic (70%) or nonhemorrhagic following primary percutaneous coronary intervention. To isolate the effects of hemorrhage from ischemic burden, we performed controlled canine studies (n = 25), and serially followed both cTn and MI size with time-lapse imaging.

RESULTS

CTn was not different before reperfusion; however, an increase in cTn following primary percutaneous coronary intervention peaked earlier (12 hours vs 24 hours; P < 0.05) and was significantly higher in patients with hemorrhage (P < 0.01). In hemorrhagic animals, reperfusion led to rapid expansion of myocardial necrosis culminating in epicardial involvement, which was not present in nonhemorrhagic cases (P < 0.001). MI size and salvage were not different at 1 hour postreperfusion in animals with and without hemorrhage (P = 0.65). However, within 72 hours of reperfusion, a 4-fold greater loss in salvageable myocardium was evident in hemorrhagic MIs (P < 0.001). This paralleled observations in patients with larger MIs occurring in hemorrhagic cases (P < 0.01).

CONCLUSIONS

Myocardial hemorrhage is a determinant of MI size. It drives MI expansion after reperfusion and compromises myocardial salvage. This introduces a clinical role of hemorrhage in acute care management, risk assessment, and future therapeutics.

Perfusion MRI using endogenous deoxyhemoglobin as a contrast agent: Preliminary data

PURPOSE

To demonstrate the feasibility of mapping cerebral perfusion metrics with BOLD MRI during modulation of pulmonary venous oxygen saturation.

METHODS

A gas blender with a sequential gas delivery breathing circuit was used to implement rapid isocapnic changes in the partial pressure of oxygen of the arterial blood. Partial pressure of oxygen was initially lowered to a baseline of 40 mmHg. It was then rapidly raised to 95 mmHg for 20 s before rapidly returning to baseline. The induced cerebral changes in deoxyhemoglobin concentration were tracked over time using BOLD MRI in 6 healthy subjects and 1 patient with cerebral steno-occlusive disease. BOLD signal change, contrast-to-noise ratio, and time delay metrics were calculated. Perfusion metrics such as mean transit time, relative cerebral blood volume, and relative cerebral blood flow were calculated using a parametrized method with a mono-exponential residue function. An arterial input function from within the middle cerebral artery was used to scale relative cerebral blood volume and calculate absolute cerebral blood volume and cerebral blood flow.

RESULTS

In normal subjects, average gray and white matter were: BOLD change = 6.3 ± 1.2% and 2.5 ± 0.6%, contrast-to-noise ratio = 4.3 ± 1.3 and 2.6 ± 0.7, time delay = 2.3 ± 0.6 s and 3.6 ± 0.7 s, mean transit time = 3.9 ± 0.6 s and 5.5 ± 0.6 s, relative cerebral blood volume = 3.7 ± 0.9 and 1.6 ± 0.4, relative cerebral blood flow = 70.1 ± 8.3 and 20.6 ± 4.0, cerebral blood flow volume = 4.1 ± 0.9 mL/100 g and 1.8 ± 0.5 mL/100 g, and cerebral blood flow = 97.2 ± 18.7 mL/100 g/min and 28.7 ± 5.9 mL/100 g/min.

CONCLUSION

This study demonstrates that induced abrupt changes in deoxyhemoglobin can function as a noninvasive vascular contrast agent that may be used for cerebral perfusion imaging.

Effects of B1 + Heterogeneity on Spin Echo-Based Liver Iron Estimates

BACKGROUND

Liver iron concentration (LIC) measured by MRI has become the clinical reference standard for managing iron overload in chronically transfused patients. Transverse relaxivity (R₂ or R₂ ^* ) measurements are converted to LIC units using empirically derived calibration curves.

HYPOTHESIS

That flip angle (FA) error due to B₁ ⁺ spatial heterogeneity causes significant LIC quantitation error. B₁ ⁺ scale (b₁ , [FA_actual /FA_specified ]) variation is a major problem at 3 T which could reduce the accuracy of transverse relaxivity measurements.

STUDY TYPE

Prospective.

POPULATION

Forty-seven subjects with chronic transfusional iron overload undergoing clinically indicated LIC assessment.

FIELD STRENGTH/SEQUENCE

5 T/3 T dual-repetition time B₁ ⁺ mapping sequence ASSESSMENT: We quantified the average/standard deviation b₁ in the right and left lobes of the liver from B₁ ⁺ maps acquired at 1.5 T and 3 T. The impact of b₁ variation on spin echo LIC estimates was determined using a Monte Carlo model.

STATISTICAL TESTS

Mean, median, and standard deviation in whole liver and right and left lobes; two-sided t-test between whole-liver b₁ means.

RESULTS

Average b₁ within the liver was 99.3% ± 12.3% at 1.5 T versus 69.6% ± 14.6% at 3 T and was independent of iron burden (P < 0.05). Monte Carlo simulations demonstrated that b₁ systematically increased R₂ estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R₂ -LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R₂ -LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5 T and 3 T (for the same R₂ value). The R₂ ^* -LIC relationship was independent of b₁ .

CONCLUSION

Spin echo R₂ measurement of LIC at 3 T is error-prone without correction for b₁ errors. The impact of b₁ error on current 1.5 T spin echo-based techniques for LIC quantification is large enough to introduce measurable intersubject variability but the in vivo effect size needs a dedicated validation study.

LEVEL OF EVIDENCE

1.

TECHNICAL EFFICACY STAGE

2.

Reduced global cerebral oxygen metabolic rate in sickle cell disease and chronic anemias

Anemia is the most common blood disorder in the world. In patients with chronic anemia, such as sickle cell disease or major thalassemia, cerebral blood flow increases to compensate for decreased oxygen content. However, the effects of chronic anemia on oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂ ) are less well understood. In this study, we examined 47 sickle-cell anemia subjects (age 21.7 ± 7.1, female 45%), 27 non-sickle anemic subjects (age 25.0 ± 10.4, female 52%) and 44 healthy controls (age 26.4 ± 10.6, female 71%) using MRI metrics of brain oxygenation and flow. Phase contrast MRI was used to measure resting cerebral blood flow, while T₂ -relaxation-under-spin-tagging (TRUST) MRI with disease appropriate calibrations were used to measure OEF and CMRO₂ . We observed that patients with sickle cell disease and other chronic anemias have decreased OEF and CMRO₂ (respectively 27.4 ± 4.1% and 3.39 ± 0.71 ml O₂ /100 g/min in sickle cell disease, 30.8 ± 5.2% and 3.53 ± 0.64 ml O₂ /100 g/min in other anemias) compared to controls (36.7 ± 6.0% and 4.00 ± 0.65 ml O₂ /100 g/min). Impaired CMRO₂ was proportional to the degree of anemia severity. We further demonstrate striking concordance of the present work with pooled historical data from patients having broad etiologies for their anemia. The reduced cerebral oxygen extraction and metabolism are consistent with emerging data demonstrating increased non-nutritive flow, or physiological shunting, in sickle cell disease patients.

Improving CPMG liver iron estimates with a T 1 -corrected proton density estimator

PURPOSE

CPMG spin echo acquisitions are attractive for diagnosing and monitoring liver iron concentration in iron overload disorders due to their time efficiency and potential to reveal unique information about tissue iron distribution. Clinical adoption remains low due to the insensitivity of CPMG-based R 2 estimates to liver iron concentration (LIC) when common fitting techniques are applied. In this work, we demonstrate that the inclusion of a proton density estimator (PDE) derived from the CPMG acquisition increase the sensitivity of CPMG R 2 estimates to LIC in both simulated and in-vivo human data.

THEORY AND METHODS

CPMG R 2 acquisitions from 50 clinically indicated MRI studies in patients with iron overload were analyzed with and without PDE constraints. Liver regions of interest were fit to monoexpontial and nonexponential signal decay equations. LIC by R 2 ∗ served as the reference standard. The observed calibration between CPMG R 2 values and LIC were compared to results predicted from a previously validated Monte Carlo model.

RESULTS

The sensitivity of CPMG-derived R 2 triples when a proton density constraint is applied. When compared with R 2 ∗ -LIC estimates, both monoexponential and nonexponential models were unbiased but demonstrated broad 95% confidence intervals particularly for LIC values below 12 mg/g. Absolute error did not increase with LIC.

CONCLUSION

A proton density constraint can increase the sensitivity of CPMG-based models to iron. CPMG acquisitions are time-efficient and could potentially improve the dynamic range of single spin echo techniques as well as providing insight into tissue iron distribution.

Cerebral oxygen saturation and cerebrovascular instability in newborn infants with congenital heart disease compared to healthy controls

Objective

Infants with Congenital Heart Disease (CHD) are at risk for developmental delays, though the mechanisms of brain injury that impair development are unknown. Potential causes could include cerebral hypoxia and cerebrovascular instability. We hypothesized that we would detect significantly reduced cerebral oxygen saturation and greater cerebrovascular instability in CHD infants compared to the healthy controls.

Methods

We performed a secondary analysis on a sample of 43 term infants (28 CHD, 15 healthy controls) that assessed prospectively in temporal cross-section before or at 12 days of age. CHD infants were assessed prior to open-heart surgery. Cerebral oxygen saturation levels were estimated using Near-Infrared Spectroscopy, and cerebrovascular stability was assessed with the response of cerebral oxygen saturation after a postural change (supine to sitting).

Results

Cerebral oxygen saturation was 9 points lower in CHD than control infants in both postures (β = -9.3; 95%CI = -17.68, -1.00; p = 0.028), even after controlling for differences in peripheral oxygen saturation. Cerebrovascular stability was significantly impaired in CHD compared to healthy infants (β = -2.4; 95%CI = -4.12, -.61; p = 0.008), and in CHD infants with single ventricle compared with biventricular defects (β = -1.5; 95%CI = -2.95, -0.05; p = 0.04).

Conclusion

CHD infants had cerebral hypoxia and decreased cerebral oxygen saturation values following a postural change, suggesting cerebrovascular instability. Future longitudinal studies should assess the associations of cerebral hypoxia and cerebrovascular instability with long-term neurodevelopmental outcomes in CHD infants.

Impairment of Cerebrovascular Hemodynamics in Patients With Severe and Milder Forms of Sickle Cell Disease

In patients with sickle cell disease (SCD), cerebral blood flow (CBF) is elevated to counteract anemia and maintain oxygen supply to the brain. This may exhaust the vasodilating capacity of the vessels, possibly increasing the risk of silent cerebral infarctions (SCI). To further investigate cerebrovascular hemodynamics in SCD patients, we assessed CBF, arterial transit time (ATT), cerebrovascular reactivity of CBF and ATT (CVR_CBF and CVR_ATT) and oxygen delivery in patients with different forms of SCD and matched healthy controls. We analyzed data of 52 patients with severe SCD (HbSS and HbSβ⁰-thal), 20 patients with mild SCD (HbSC and HbSβ⁺-thal) and 10 healthy matched controls (HbAA and HbAS). Time-encoded arterial spin labeling (ASL) scans were performed before and after a vasodilatory challenge using acetazolamide (ACZ). To identify predictors of CBF and ATT after vasodilation, regression analyses were performed. Oxygen delivery was calculated and associated with hemoglobin and fetal hemoglobin (HbF) levels. At baseline, severe SCD patients showed significantly higher CBF and lower ATT compared to both the mild SCD patients and healthy controls. As CBF_postACZ was linearly related to CBF_preACZ, CVR_CBF decreased with disease severity. CVR_ATT was also significantly affected in severe SCD patients compared to mild SCD patients and healthy controls. Considering all groups, women showed higher CBF_postACZ than men (p < 0.01) independent of baseline CBF. Subsequently, post ACZ oxygen delivery was also higher in women (p < 0.05). Baseline, but not post ACZ, GM oxygen delivery increased with HbF levels. Our data showed that baseline CBF and ATT and CVR_CBF and CVR_ATT are most affected in severe SCD patients and to a lesser extent in patients with milder forms of SCD compared to healthy controls. Cerebrovascular vasoreactivity was mainly determined by baseline CBF, sex and HbF levels. The higher vascular reactivity observed in women could be related to their lower SCI prevalence, which remains an area of future work. Beneficial effects of HbF on oxygen delivery reflect changes in oxygen dissociation affinity from hemoglobin and were limited to baseline conditions suggesting that high HbF levels do not protect the brain upon a hemodynamic challenge, despite its positive effect on hemolysis.

Calibration of T2 oximetry MRI for subjects with sickle cell disease

PURPOSE

Cerebral T₂ oximetry is a non-invasive imaging method to measure blood T₂ and cerebral venous oxygenation. Measured T₂ values are converted to oximetry estimates using carefully validated and potentially disease-specific calibrations. In sickle cell disease, red blood cells have abnormal cell shape and membrane properties that alter T₂ oximetry calibration relationships in clinically meaningful ways. Previous in vitro works by two independent groups established potentially competing calibration models.

METHODS

This study analyzed pooled datasets from these two studies to establish a unified and more robust sickle-specific calibration to serve as a reference standard in the field.

RESULTS

Even though the combined calibration did not demonstrate statistical superiority compared to previous models, the calibration was unbiased compared to blood-gas co-oximetry and yielded limits of agreement of (-10.1%, 11.6%) in non-transfused subjects with sickle cell disease. In transfused patients, this study proposed a simple correction method based on individual hemoglobin S percentage that demonstrated reduced bias in saturation measurement compared to previous uncorrected sickle calibrations.

CONCLUSION

The combined calibration is based on a larger range of hematocrit, providing greater confidence in the hematocrit-dependent model parameters, and yielded unbiased estimates to blood-gas co-oximetry measurements from both sites. Additionally, this work also demonstrated the need to correct for transfusion in T₂ oximetry measurements for hyper-transfused sickle cell disease patients and proposes a correction method based on patient-specific hemoglobin S concentration.

Loss of alpha-globin genes in human subjects is associated with improved nitric oxide-mediated vascular perfusion

Alpha thalassemia is a hemoglobinopathy due to decreased production of the α-globin protein from loss of up to four α-globin genes, with one or two missing in the trait phenotype. Individuals with sickle cell disease who co-inherit the loss of one or two α-globin genes have been known to have reduced risk of morbid outcomes, but the underlying mechanism is unknown. While α-globin gene deletions affect sickle red cell deformability, the α-globin genes and protein are also present in the endothelial wall of human arterioles and participate in nitric oxide scavenging during vasoconstriction. Decreased production of α-globin due to α-thalassemia trait may thereby limit nitric oxide scavenging and promote vasodilation. To evaluate this potential mechanism, we performed flow-mediated dilation and microvascular post-occlusive reactive hyperemia in 27 human subjects (15 missing one or two α-globin genes and 12 healthy controls). Flow-mediated dilation was significantly higher in subjects with α-trait after controlling for age (P = .0357), but microvascular perfusion was not different between groups. As none of the subjects had anemia or hemolysis, the improvement in vascular function could be attributed to the difference in α-globin gene status. This may explain the beneficial effect of α-globin gene loss in sickle cell disease and suggests that α-globin gene status may play a role in other vascular diseases.

Algorithms for segmenting cerebral time-of-flight magnetic resonance angiograms from volunteers and anemic patients

Purpose: To evaluate six cerebral arterial segmentation algorithms in a set of patients with a wide range of hemodynamic characteristics to determine real-world performance. Approach: Time-of-flight magnetic resonance angiograms were acquired from 33 subjects: normal controls ( N = 11 ), sickle cell disease ( N = 11 ), and non-sickle anemia ( N = 11 ) using a 3 Tesla Philips Achieva scanner. Six segmentation algorithms were tested: (1) Otsu's method, (2) K-means, (3) region growing, (4) active contours, (5) minimum cost path, and (6) U-net machine learning. Segmentation algorithms were tested with two region-selection methods: global, which selects the entire volume; and local, which iteratively tracks the arteries. Five slices were manually segmented from each patient by two readers. Agreement between manual and automatic segmentation was measured using Matthew's correlation coefficient (MCC). Results: Median algorithm segmentation times ranged from 0.1 to 172.9 s for a single angiogram versus 10 h for manual segmentation. Algorithms had inferior performance to inter-observer vessel-based ( p < 0.0001 , MCC = 0.65 ) and voxel-based ( p < 0.0001 , MCC = 0.73 ) measurements. There were significant differences between algorithms ( p < 0.0001 ) and between patients ( p < 0.0042 ). Post-hoc analyses indicated (1) local minimum cost path performed best with vessel-based ( p = 0.0261 , MCC = 0.50 ) and voxel-based ( p = 0.0131 , MCC = 0.66 ) analyses; and (2) higher vessel-based performance in non-sickle anemia ( p = 0.0002 ) and lower voxel-based performance in sickle cell ( p = 0.0422 ) compared with normal controls. All reported MCCs are medians. Conclusions: The best-performing algorithm (local minimum cost path, voxel-based) had 9.59% worse performance than inter-observer agreement but was 3 orders of magnitude faster. Automatic segmentation was non-inferior in patients with sickle cell disease and superior in non-sickle anemia.

Tract-specific analysis and neurocognitive functioning in sickle cell patients without history of overt stroke

INTRODUCTION

Sickle cell disease (SCD) is a hereditary blood disorder in which the oxygen-carrying hemoglobin molecule in red blood cells is abnormal. SCD patients are at increased risks for strokes and neurocognitive deficit, even though neurovascular screening and treatments have lowered the rate of overt strokes. Tract-specific analysis (TSA) is a statistical method to evaluate microstructural WM damage in neurodegenerative disorders, using diffusion tensor imaging (DTI).

METHODS

We utilized TSA and compared 11 major brain WM tracts between SCD patients with no history of overt stroke, anemic controls, and healthy controls. We additionally examined the relationship between the most commonly used DTI metric of WM tracts and neurocognitive performance in the SCD patients and healthy controls.

RESULTS

Disruption of WM microstructure orientation-dependent metrics for the SCD patients was found in the genu of the corpus callosum (CC), cortico-spinal tract, inferior fronto-occipital fasciculus, right inferior longitudinal fasciculus, superior longitudinal fasciculus, and left uncinate fasciculus. Neurocognitive performance indicated slower processing speed and lower response inhibition skills in SCD patients compared to controls. TSA abnormalities in the CC were significantly associated with measures of processing speed, working memory, and executive functions.

CONCLUSION

Decreased DTI-derived metrics were observed on six tracts in chronically anemic patients, regardless of anemia subtype, while two tracks with decreased measures were unique to SCD patients. Patients with WMHs had more significant FA abnormalities. Decreased FA values in the CC significantly correlated with all nine neurocognitive tests, suggesting a critical importance for CC in core neurocognitive processes.

Tricuspid regurgitant jet velocity and myocardial tissue Doppler parameters predict mortality in a cohort of patients with sickle cell disease spanning from pediatric to adult age groups - revisiting this controversial concept after 16 years of additional evidence

Sickle cell disease (SCD) is a monogenic hemoglobinopathy associated with significant morbidity and mortality. Cardiopulmonary, vascular and sudden death are the reasons for the majority of young adult mortality in SCD. To better understand the clinical importance of multi-level vascular dysfunction, in 2009 we assessed cardiac function including tricuspid regurgitant jet velocity (TRV), tissue velocity in systole(S') and diastole (E'), inflammatory, rheologic and hemolytic biomarkers as predictors of mortality in patients with SCD. With up to 9 years of follow up, we determined survival in 95 children, adolescents and adults with SCD. Thirty-eight patients (40%) were less than 21 years old at initial evaluation. Survival and Cox proportional-hazards analysis were performed. There was 19% mortality in our cohort, with median age at death of 35 years. In the pediatric subset, there was 11% mortality during the follow up period. The causes of death included cardiovascular and pulmonary complications in addition to other end-organ failure. On Cox proportional-hazards analysis, our model predicts that a 0.1 m/s increase in TRV increases risk of mortality 3%, 1 cm/s increase in S' results in a 91% increase, and 1 cm/s decrease in E' results in a 43% increase in mortality. While excluding cardiac parameters, higher plasma free hemoglobin was significantly associated with risk of mortality (p=.049). In conclusion, elevated TRV and altered markers of cardiac systolic and diastolic function predict mortality in a cohort of adolescents and young adult patients with SCD. These predictors should be considered when counseling cardiovascular risk and therapeutic optimization at transition to adult providers.

Kidney iron deposition by R2* is associated with haemolysis and urinary iron

Kidney iron deposition measured by R2* (magnetic resonance imaging) MRI is posited to result from tubular reabsorption of filtered haemoglobin due to intravascular haemolysis. In chronically transfused sickle cell disease (SCD), R2* is elevated and positively correlated with lactate dehydrogenase (LDH). To account for contributions to renal iron from systemic iron overload, we evaluated kidney R2*, urinary iron and haemolysis markers in 62 non-transfused SCD patients. On multivariate analysis, kidney R2* was associated with urinary iron and LDH (R²  = 0·55, P < 0·0001). Our study confirms that kidney R2* is associated with intravascular haemolysis and raises important questions regarding the role of iron in SCD nephropathy.

Progressive vasoconstriction with sequential thermal stimulation indicates vascular dysautonomia in sickle cell disease

Persons with sickle cell disease (SCD) exhibit subjective hypersensitivity to cold and heat perception in experimental settings, and triggers such as cold exposure are known to precipitate vaso-occlusive crises by still unclear mechanisms. Decreased microvascular blood flow (MBF) increases the likelihood of vaso-occlusion by increasing entrapment of sickled red blood cells in the microvasculature. Because those with SCD have dysautonomia, we anticipated that thermal exposure would induce autonomic hypersensitivity of their microvasculature with an increased propensity toward vasoconstriction. We exposed 17 patients with SCD and 16 control participants to a sequence of predetermined threshold temperatures for cold and heat detection and cold and heat pain via a thermode placed on the right hand. MBF was measured on the contralateral hand by photoplethysmography, and cardiac autonomic balance was assessed by determining heart rate variability. Thermal stimuli at both detection and pain thresholds caused a significant decrease in MBF in the contralateral hand within seconds of stimulus application, with patients with SCD showing significantly stronger vasoconstriction (P = .019). Furthermore, patients with SCD showed a greater progressive decrease in blood flow than did the controls, with poor recovery between episodes of thermal stimulation (P = .042). They had faster vasoconstriction than the controls (P = .033), especially with cold detection stimulus. Individuals with higher anxiety also experienced more rapid vasoconstriction (P = .007). Augmented vasoconstriction responses and progressive decreases in perfusion with repeated thermal stimulation in SCD are indicative of autonomic hypersensitivity in the microvasculature. These effects are likely to increase red cell entrapment in response to clinical triggers such as cold or stress, which have been associated with vaso-occlusive crises in SCD.

Quantitative perfusion mapping with induced transient hypoxia using BOLD MRI

PURPOSE

Gadolinium-based dynamic susceptibility contrast (DSC) is commonly used to characterize blood flow in patients with stroke and brain tumors. Unfortunately, gadolinium contrast administration has been associated with adverse reactions and long-term accumulation in tissues. In this work, we propose an alternative deoxygenation-based DSC (dDSC) method that uses a transient hypoxia gas paradigm to deliver a bolus of paramagnetic deoxygenated hemoglobin to the cerebral vasculature for perfusion imaging.

METHODS

Through traditional DSC tracer kinetic modeling, the MR signal change induced by this hypoxic bolus can be used to generate regional perfusion maps of cerebral blood flow, cerebral blood volume, and mean transit time. This gas paradigm and blood-oxygen-level-dependent (BOLD)-MRI were performed concurrently on a cohort of 66 healthy and chronically anemic subjects (age 23.5 ± 9.7, female 64%).

RESULTS

Our results showed reasonable global and regional agreement between dDSC and other flow techniques, such as phase contrast and arterial spin labeling.

CONCLUSION

In this proof-of-concept study, we demonstrated the feasibility of using transient hypoxia to generate a contrast bolus that mimics the effect of gadolinium and yields reasonable perfusion estimates. Looking forward, optimization of the hypoxia boluses and measurement of the arterial-input function is necessary to improve the accuracy of dDSC. Additionally, a cross-validation study of dDSC and DSC in brain tumor and ischemic stroke subjects is warranted to evaluate the clinical diagnostic utility of this approach.

Transient Hypoxia Model Revealed Cerebrovascular Impairment in Anemia Using BOLD MRI and Near-Infrared Spectroscopy

BACKGROUND

Obstructive sleep apnea and nocturnal oxygen desaturations, which are prevalent in sickle cell disease (SCD) and chronic anemia disorders, have been linked to risks of stroke and silent cerebral infarcts (SCI). Cerebrovascular response to intermittent desaturations has not been well studied and may identify patients at greatest risk.

PURPOSE

To investigate the cerebral dynamic response to induced desaturation in SCD patients with and without SCI, chronic anemia, and healthy subjects.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-six SCD patients (age = 21 ± 8.2, female 46.2%), including 15 subjects without SCI and nine subjects with SCI, 15 nonsickle anemic patients (age = 22 ± 5.8, female 66.7%), and 31 controls (age = 28 ± 12.3, female 77.4%).

FIELD STRENGTH/SEQUENCE

3T, gradient-echo echo-planar imaging.

ASSESSMENT

A transient hypoxia challenge of five breaths of 100% nitrogen gas was performed with blood oxygen level-dependent (BOLD) MRI and near-infrared spectroscopy (NIRS) acquisitions. Hypoxia responses were characterized by desaturation depth, time-to-peak, return-to-baseline half-life, and posthypoxia recovery in the BOLD and NIRS time courses. SCI were documented by T₂ fluid-attenuation inversion recovery (FLAIR).

STATISTICAL TESTS

Univariate and multivariate regressions were performed between hypoxic parameters and anemia predictors. Voxelwise two-sample t-statistic maps were used to assess the regional difference in hypoxic responses between anemic and control groups.

RESULTS

Compared to controls, SCD and chronically anemic patients demonstrated significantly higher desaturation depth (P < 0.01) and shorter return-to-baseline timing response (P < 0.01). Patients having SCI had shorter time-to-peak (P < 0.01), return-to-baseline (P < 0.01), and larger desaturation depth (P < 0.01) in both white matter regions at risk and normal-appearing white matter than patients without infarcts. On multivariate analysis, desaturation depth and timing varied with age, sex, blood flow, white blood cells, and cell-free hemoglobin (r² = 0.25 for desaturation depth; r² = 0.18 for time-to-peak; r² = 0.37 for return-to-baseline).

DATA CONCLUSION

Transient hypoxia revealed global and regional response differences between anemic and healthy subjects. SCI was associated with extensive heterogeneity of desaturation dynamics, consistent with extensive underlying microvascular remodeling.

Cerebral oxygen metabolism in adults with sickle cell disease

In sickle cell disease (SCD), oxygen delivery is impaired due to anemia, especially during times of increased metabolic demand, and cerebral blood flow (CBF) must increase to meet changing physiologic needs. But hyperemia limits cerebrovascular reserve (CVR) and ischemic risk prevails despite elevated CBF. The cerebral metabolic rate of oxygen (CMRO₂ ) directly reflects oxygen supply and consumption and may therefore be more insightful than flow-based CVR measures for ischemic risk in SCD. We hypothesized that adults with SCD have impaired CMRO₂ at rest and that a vasodilatory challenge with acetazolamide would improve CMRO₂ . CMRO₂ was calculated from CBF and oxygen extraction fraction (OEF), measured with arterial spin labeling and T₂ -prepared tissue relaxation with inversion recovery (T₂ -TRIR) MRI. We studied 36 adults with SCD without a clinical history of overt stroke, and nine healthy controls. As expected, CBF was higher in patients with SCD versus controls (mean ± SD: 74 ± 16 versus 46 ± 5 mL/100 g/min, P < .001), resulting in similar oxygen delivery (SCD: 377 ± 67 versus controls: 368 ± 42 μmol O₂ /100g/min, P = .69). OEF was lower in patients versus controls (27 ± 4 versus 35 ± 4%, P < .001), resulting in lower CMRO₂ in patients versus controls (102 ± 24 versus 127 ± 20 μmol O₂ /100g/min, P = .002). After acetazolamide, CMRO₂ declined further in patients (P < .01) and did not decline significantly in controls (P = .78), indicating that forcing higher CBF worsened oxygen utilization in SCD patients. This lower CMRO₂ could reflect variation between healthy and unhealthy vascular beds in terms of dilatory capacity and resistance whereby dysfunctional vessels become more oxygen-deprived, hence increasing the risk of localized ischemia.

Action of iron chelator on intramyocardial hemorrhage and cardiac remodeling following acute myocardial infarction

Intramyocardial hemorrhage is an independent predictor of adverse outcomes in ST-segment elevation myocardial infarction (STEMI). Iron deposition resulting from ischemia-reperfusion injury (I/R) is pro-inflammatory and has been associated with adverse remodeling. The role of iron chelation in hemorrhagic acute myocardial infarction (AMI) has never been explored. The purpose of this study was to investigate the cardioprotection offered by the iron-chelating agent deferiprone (DFP) in a porcine AMI model by evaluating hemorrhage neutralization and subsequent cardiac remodeling. Two groups of animals underwent a reperfused AMI procedure: control and DFP treated (N = 7 each). A comprehensive MRI examination was performed in healthy state and up to week 4 post-AMI, followed by histological assessment. Infarct size was not significantly different between the two groups; however, the DFP group demonstrated earlier resolution of hemorrhage (by T2* imaging) and edema (by T2 imaging). Additionally, ventricular enlargement and myocardial hypertrophy (wall thickness and mass) were significantly smaller with DFP, suggesting reduced adverse remodeling, compared to control. The histologic results were consistent with the MRI findings. To date, there is no effective targeted therapy for reperfusion hemorrhage. Our proof-of-concept study is the first to identify hemorrhage-derived iron as a therapeutic target in I/R and exploit the cardioprotective properties of an iron-chelating drug candidate in the setting of AMI. Iron chelation could potentially serve as an adjunctive therapy in hemorrhagic AMI.

Group delay method for MRI aortic pulse wave velocity measurements in clinical protocols with low temporal resolution: Validation in a heterogeneous cohort

BACKGROUND

MRI assessment of aortic pulse wave velocity (PWV) helps predict the risk of vascular events, but the recommended phase contrast sampling rate is faster than what is utilized in most clinical sequences. There are many existing MRI databases obtained for assessment of cardiac output using lower temporal frequency sampling where information might be obtained about aortic stiffness (PWV). In this work, we sought to evaluate whether the Group Delay (GD) method can generate a reproducible measure of stiffness and describe expected age-related stiffening of the aortic arch using lower sampling rates in standard clinical sequences.

METHODS

Phase contrast (PC) MRI was obtained on the ascending and descending aortic arch in a heterogeneous adult cohort (n = 23; 9 women) spanning over a wide range of ages (ages 24-89, mean 49.4 ± 18.4). Data was collected with standard cardiac MRI protocols for cardiac output evaluation (repetition time = 7.8 ms, views-per-segment = 4, encoding velocity = 200 cm/s). Pulse wave transit times (TT) were computed using the GD method, two other validated automated approaches (cross correlation TT Algorithm by Gaddum and Segment by Medviso), and the manual tangent method. Pressure waveforms from tonometry and flow waveforms from PC MRI were used to assess wave reflections.

RESULTS

Group Delay and TT-Algorithm showed significant and high retest reproducibility (r = 0.86 for both) as well as high PWV correlation with age (r = 0.93, P-value < 0.00005 and r = 0.96, P-value < 0.00005 respectively) and with each other (r = 0.94, P-value < 0.00001, RMSE = 0.94 m/s). Arbitrary altering of the image acquisition trigger in the GD method introduced error of 10%-13%, but the TT-algorithm error range was 11%-25%.

CONCLUSION

Group Delay enables reproducible assessment of transit time to derive PWV from low temporal resolution clinical cardiac MRI sequences that can also identify age-related stiffening.

End points for sickle cell disease clinical trials: patient-reported outcomes, pain, and the brain

To address the global burden of sickle cell disease (SCD) and the need for novel therapies, the American Society of Hematology partnered with the US Food and Drug Administration to engage the work of 7 panels of clinicians, investigators, and patients to develop consensus recommendations for clinical trial end points. The panels conducted their work through literature reviews, assessment of available evidence, and expert judgment focusing on end points related to: patient-reported outcomes (PROs), pain (non-PROs), the brain, end-organ considerations, biomarkers, measurement of cure, and low-resource settings. This article presents the findings and recommendations of the PROs, pain, and brain panels, as well as relevant findings and recommendations from the biomarkers panel. The panels identify end points, where there were supporting data, to use in clinical trials of SCD. In addition, the panels discuss where further research is needed to support the development and validation of additional clinical trial end points.