Quantitative validation of MRI mapping of cerebral venous oxygenation with direct blood sampling: A graded-O2 study in piglets

Brain Frontal-Lobe Misery Perfusion in COVID-19 ICU Survivors: An MRI Pilot Study

Post-acute COVID-19 syndrome (PCS) is highly prevalent. Critically ill patients requiring intensive care unit (ICU) admission are at a higher risk of developing PCS. The mechanisms underlying PCS are still under investigation and may involve microvascular damage in the brain. Cerebral misery perfusion, characterized by reduced cerebral blood flow (CBF) and elevated oxygen extraction fraction (OEF) in affected brain areas, has been demonstrated in cerebrovascular diseases such as carotid occlusion and stroke. This pilot study aimed to examine whether COVID-19 ICU survivors exhibited regional misery perfusion, indicating cerebral microvascular damage. In total, 7 COVID-19 ICU survivors (4 female, 20-77 years old) and 19 age- and sex-matched healthy controls (12 female, 22-77 years old) were studied. The average interval between ICU admission and the MRI scan was 118.6 ± 30.3 days. The regional OEF was measured using a recently developed technique, accelerated T2-relaxation-under-phase-contrast MRI, while the regional CBF was assessed using pseudo-continuous arterial spin labeling. COVID-19 ICU survivors exhibited elevated OEF (β = 5.21 ± 2.48%, p = 0.047) and reduced relative CBF (β = -0.083 ± 0.025, p = 0.003) in the frontal lobe compared to healthy controls. In conclusion, misery perfusion was observed in the frontal lobe of COVID-19 ICU survivors, suggesting microvascular damage in this critical brain area for high-level cognitive functions that are known to manifest deficits in PCS. Physiological biomarkers such as OEF and CBF may provide new tools to improve the understanding and treatment of PCS.

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.

Red cell exchange transfusions increase cerebral capillary transit times and may alter oxygen extraction in sickle cell disease

Persons with sickle cell disease (SCD) suffer from chronic hemolytic anemia, reduced blood oxygen content, and lifelong risk of silent and overt stroke. Major conventional stroke risk factors are absent in most individuals with SCD, yet nearly 50% have evidence of brain infarcts by the age of 30 years, indicating alternative etiologies for ischemia. We investigated whether radiological evidence of accelerated blood water transit through capillaries, visible on arterial spin labeling (ASL) magnetic resonance imaging, reduces following transfusion-induced increases in hemoglobin and relates to oxygen extraction fraction (OEF). Neurological evaluation along with anatomical and hemodynamic imaging with cerebral blood flow (CBF)-weighted pseudocontinuous ASL and OEF imaging with T2 -relaxation-under-spin-tagging were applied in sequence before and after blood transfusion therapy (n = 32) and in a comparator cohort of nontransfused SCD participants on hydroxyurea therapy scanned at two time points to assess stability without interim intervention (n = 13). OEF was calculated separately using models derived from human hemoglobin-F, hemoglobin-A, and hemoglobin-S. Gray matter CBF and dural sinus signal, indicative of rapid blood transit, were evaluated at each time point and compared with OEF using paired statistical tests (significance: two-sided p < 0.05). No significant change in sinus signal was observed in nontransfused participants (p = 0.650), but a reduction was observed in transfused participants (p = 0.034), consistent with slower red cell transit following transfusion. The dural sinus signal intensity was inversely associated with OEF pretransfusion (p = 0.011), but not posttransfusion. Study findings suggest that transfusion-induced increases in total hemoglobin may lengthen blood transit times through cerebral capillaries and alter cerebral OEF in SCD.

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.

Dynamic Changes of Asymmetric Cortical Veins Relate to Neurologic Prognosis in Acute Ischemic Stroke

Background Cerebral oxygenation is closely related to neural function in acute ischemic stroke (AIS) and can be measured noninvasively from asymmetrically prominent cortical veins (APCVs) using quantitative susceptibility mapping (QSM). Purpose To quantify venous oxygen saturation (SvO₂) using brain MRI with QSM in patients with AIS, to analyze its change at 2-week follow-up, and to assess the influence of SvO₂ in clinical prognosis. Materials and Methods Between 2016 and 2020, consecutive patients with AIS who underwent brain MRI within 24 hours from symptom onset and 2 weeks after treatment were retrospectively enrolled. The SvO₂ of APCVs was quantified using QSM. The independent sample t test was used to compare the SvO₂ between patients with and patients without APCVs. The paired sample t test was used to assess the dynamic change in SvO₂. Pearson and Spearman correlation analysis was used to explore the relationship among dynamic change in SvO₂ and hypoperfusion, National Institutes of Health Stroke Scale (NIHSS) score change, and 90-day modified Rankin Scale (mRS) score. The independent sample t test was used to compare the dynamic change in SvO₂ between different clinical prognoses and outcome subgroups. Results APCVs were detected in 39 of 73 patients (mean age, 70 years ± 10 [standard deviation]; 49 men) at admission and disappeared in 35 patients at 2-week follow-up MRI. The mean SvO₂ increased from 35.0% ± 5.8 to 64.5% ± 10.0 (P < .001) in 39 patients. For the 35 patients with APCVs that disappeared, the dynamic change in SvO₂ negatively correlated with change in NIHSS score (r = -0.37, R² = 0.19, P = .03) and 90-day mRS score (r = -0.54, R² = 0.27, P = .001), and the dynamic change in SvO₂ in the subgroup with good 90-day outcomes (n = 19) was greater than that in the subgroup with poor 90-day outcomes (n = 16) (mean, 34.5% ± 5.8 vs 29.7% ± 6.3; 95% CI: 0.6, 8.9; P = .03). Conclusion Improved oxygen saturation of asymmetric cortical veins detected using brain MRI with quantitative susceptibility mapping corresponded with better acute ischemic stroke outcomes for patients with asymmetrically prominent cortical veins that disappeared at 2-week follow-up MRI. © RSNA, 2021 Online supplemental material is available for this article.