Sickle cell anemia: reference values of cerebral blood flow determined by continuous arterial spin labeling MRI

Reduced oxygen extraction efficiency in sickle cell anemia patients with evidence of cerebral capillary shunting

Arterial spin labeling (ASL) magnetic resonance imaging (MRI) utilizes arterial blood water as an endogenous contrast agent to provide a quantitative measure of cerebral blood flow (CBF). Recently, hyperintense signal within dural venous sinuses in ASL images of sickle cell anemia (SCA) patients has been shown to be consistent with elevated flow velocities and may indicate capillary shunting and reduced oxygen extraction. Here, we performed oxygen extraction fraction (OEF) and CBF measurements in adults (cumulative n = 114) with (n = 69) and without (n = 45) SCA to test the hypothesis that hyperintense venous ASL signal is associated with reduced OEF. Higher categorical scores of shunting on ASL MRI were associated with lower OEF in participants with silent cerebral infarcts or white matter hyperintensities (p = 0.003), but not in those without lesions (p = 0.551). These findings indicate that venous hyperintense signal in ASL images in SCA patients may represent a marker of capillary-level disturbances in oxygen exchange efficiency and small vessel pathology.

The incidence of obesity, venous sinus stenosis and cerebral hyperaemia in children referred for MRI to rule out idiopathic intracranial hypertension at a tertiary referral hospital: a 10 year review

BACKGROUND

Children referred to a tertiary hospital for the indication, "rule out idiopathic intracranial hypertension (IIH)" may have an increased risk of raised venous sinus pressure. An increase in sinus pressure could be due to obesity, venous outflow stenosis or cerebral hyperemia. The purpose of this paper is to define the incidence of each of these variables in these children.

METHODS

Following a data base review, 42 children between the ages of 3 and 15 years were found to have been referred over a 10 year period. The body mass index was assessed. The cross sectional areas and circumferences of the venous sinuses were measured at 4 levels to calculate the hydraulic and effective diameters. The arterial inflow, sagittal and straight sinus outflows were measured. Automatic cerebral volumetry allowed the brain volume and cerebral blood flow (CBF) to be calculated. The optic nerve sheath diameter was used as a surrogate marker of raised intracranial pressure (ICP). The sagittal sinus percentage venous return was used as a surrogate marker of elevated venous pressure. Age and sex matched control groups were used for comparison.

RESULTS

Compared to controls, the obesity rates were not significantly different in this cohort. Compared to controls, those at risk for IIH had a 17% reduction in transverse sinus and 14% reduction in sigmoid sinus effective cross sectional area (p = 0.005 and 0.0009). Compared to controls, the patients at risk for IIH had an arterial inflow increased by 34% (p < 0.0001) with a 9% larger brain volume (p = 0.02) giving an increase in CBF of 22% (p = 0.005). The sagittal and straight sinus venous return were reduced by 11% and 4% respectively (p < 0.0001 and 0.0009) suggesting raised venous sinus pressure. Forty five percent of the patients were classified as hyperemic and these had optic nerve sheath diameters 17% larger than controls (p < 0.0002) suggesting raised ICP.

CONCLUSION

In children with the chronic headache/ IIH spectrum, the highest associations were with cerebral hyperemia and mild venous sinus stenosis. Obesity was not significantly different in this cohort. There is evidence to suggest hyperemia increases the venous sinus pressure and ICP.

Preliminary evidence for cerebral capillary shunting in adults with sickle cell anemia

Elevated flow velocities in adults with sickle cell anemia (SCA) may cause rapid erythrocyte transit through capillaries. This phenomenon could present as dural venous sinus hyperintensity on arterial spin labeling (ASL)-MRI and could be indicative of capillary shunting. Here, the prevalence of ASL venous hyperintensities and association with relevant physiology in adults with SCA was investigated. SCA ( n = 46) and age-matched control ( n = 16) volunteers were recruited for 3.0 T MRI. Pseudo-continuous ASL-MRI was acquired for cerebral blood flow (CBF) calculation and venous hyperintensity determination; venous signal intensity and a categorical venous score (three raters; 0 = no hyperintensity, 1 = focal hyperintensity, and 2 = diffuse hyperintensity) were recorded. Flow velocity in cervical internal carotid artery segments was determined from phase contrast data (v_enc = 40 cm/s) and whole-brain oxygen extraction fraction (OEF) was determined from T₂-relaxation-under-spin-tagging MRI. Cerebral metabolic rate of oxygen was calculated as the product of OEF, CBF, and blood oxygen content. ASL venous hyperintensities were significantly ( p < 0.001) more prevalent in SCA (65%) relative to control (6%) participants and were associated with elevated flow velocities ( p = 0.03). CBF ( p < 0.001), but not OEF, increased with increasing hyperintensity score. Prospective trials that evaluate this construct as a possible marker of impaired oxygen delivery and stroke risk may be warranted.

Differential cerebral hemometabolic responses to blood transfusions in adults and children with sickle cell anemia

BACKGROUND

Blood transfusions are administered to children and adults with sickle cell anemia (SCA) for secondary stroke prevention, or as treatment for recurrent pain crises or acute anemia, but transfusion effects on cerebral hemodynamics and metabolism are not well-characterized.

PURPOSE

To compare blood transfusion-induced changes in hemometabolic parameters, including oxygen extraction fraction (OEF) and cerebral blood flow (CBF), within and between adults and children with SCA.

STUDY TYPE

Prospective, longitudinal study.

SUBJECTS

Adults with SCA (n = 16) receiving simple (n = 7) or exchange (n = 9) transfusions and children with SCA (n = 11) receiving exchange transfusions were scanned once when hematocrit was near nadir and again within 7 days of transfusion. Adult controls without SCA or sickle trait (n = 7) were scanned twice on separate days.

FIELD STRENGTH/SEQUENCE

3.0T T₁ -weighted, T₂ -weighted, and T₂ -relaxation-under-spin-tagging (TRUST) imaging, and phase contrast angiography.

ASSESSMENT

Global OEF was computed as the relative difference between venous oxygenation (from TRUST) and arterial oxygenation (from pulse oximetry). Global CBF was computed as total blood flow to the brain normalized by intracranial tissue volume.

STATISTICAL TESTS

Hemometabolic variables were compared using two-sided Wilcoxon signed-rank tests; associations were analyzed using two-sided Spearman's correlation testing.

RESULTS

In adults with SCA, posttransfusion OEF = 0.38 ± 0.05 was lower (P = 0.001) than pretransfusion OEF = 0.45 ± 0.09. A change in OEF was correlated with increases in hematocrit (P = 0.02; rho = -0.62) and with pretransfusion hematocrit (P = 0.02; rho = 0.65). OEF changes after transfusion were greater (P = 0.002) in adults receiving simple versus exchange transfusions. Posttransfusion CBF = 77.7 ± 26.4 ml/100g/min was not different (P = 0.27) from pretransfusion CBF = 82.3 ± 30.2 ml/100g/min. In children with SCA, both posttransfusion OEF = 0.28 ± 0.04 and CBF = 76.4 ± 26.4 were lower than pretransfusion OEF = 0.36 ± 0.06 (P = 0.004) and CBF = 96.4 ± 16.5 (P = 0.004).

DATA CONCLUSION

Cerebral OEF reduces following transfusions in adults and children with SCA. CBF reduces following transfusions more often in children compared to adults, indicating that vascular reserve capacity may remain near exhaustion posttransfusion in many adults.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage 5 J. Magn. Reson. Imaging 2019;49:466-477.

Cerebral Blood Flow and Marrow Diffusion Alterations in Children with Sickle Cell Anemia after Bone Marrow Transplantation and Transfusion

BACKGROUND AND PURPOSE

Hematopoietic marrow hyperplasia and hyperperfusion are compensatory mechanisms in sickle cell anemia. We have observed marrow diffusion and arterial spin-labeling perfusion changes in sickle cell anemia following bone marrow transplantation. We aimed to compare arterial spin-labeling perfusion and marrow diffusion/ADC values in patients with sickle cell anemia before and after bone marrow transplantation or transfusion.

MATERIALS AND METHODS

We reviewed brain MRIs from patients with sickle cell anemia obtained during 6 consecutive years at a children's hospital. Quantitative marrow diffusion values were procured from the occipital and sphenoid bones. Pseudocontinuous arterial spin-labeling perfusion values (milliliters/100 g of tissue/min) of MCA, anterior cerebral artery, and posterior cerebral artery territories were determined. Territorial CBF, whole-brain average CBF, and marrow ADC values were compared for changes before and after either bone marrow transplantation or transfusion. Bone marrow transplantation and transfusion groups were compared. Two-tailed paired and unpaired Student t tests were used; P < .05 was considered significant.

RESULTS

Fifty-three examinations from 17 patients with bone marrow transplantation and 29 examinations from 9 patients with transfusion were included. ADC values significantly increased in the sphenoid and occipital marrow following bone marrow transplantation in contrast to patients with transfusion (P > .83). Whole-brain mean CBF significantly decreased following bone marrow transplantation (77.39 ± 13.78 to 60.39 ± 13.62 ml/100 g tissue/min; P < .001), without significant change thereafter. CBF did not significantly change following the first (81.11 ± 12.23 to 80.25 ± 8.27 ml/100 g tissue/min; P = .47) or subsequent transfusions. There was no significant difference in mean CBF between groups before intervention (P = .22).

CONCLUSIONS

Improved CBF and marrow diffusion eventuate following bone marrow transplantation in children with sickle cell anemia in contrast to transfusion therapy.

Large-Vessel Vasculopathy in Children With Sickle Cell Disease: A Magnetic Resonance Imaging Study of Infarct Topography and Focal Atrophy

BACKGROUND

Large-vessel vasculopathy (LVV) increases stroke risk in pediatric sickle cell disease beyond the baseline elevated stroke risk in this vulnerable population. The mechanisms underlying this added risk and its unique impact on the developing brain are not established.

METHODS

We analyzed magnetic resonance imaging and angiography scans of 66 children with sickle cell disease and infarcts by infarct density heatmaps and Jacobian determinants, a metric utilized to delineate focal volume change, to investigate if infarct location, volume, frequency, and cerebral atrophy differed among hemispheres with and without LVV.

RESULTS

Infarct density heatmaps demonstrated infarct "hot spots" within the deep white matter internal border zone region in both LVV and non-LVV hemispheres, but with greater infarct density and larger infarct volumes in LVV hemispheres (2.2 mL versus 0.25 mL, P < 0.001). Additional scattered cortical infarcts in the internal carotid artery territory occurred in LVV hemispheres, but were rare in non-LVV hemispheres. Jacobian determinants revealed greater atrophy in gray and white matter of the parietal lobes of LVV compared with non-LVV hemispheres.

CONCLUSION

Large-vessel vasculopathy in sickle cell disease appears to increase ischemic vulnerability in the borderzone region, as demonstrated by the increased frequency and extent of infarction within deep white matter, and increased risk of focal atrophy. Scattered infarctions across the LVV-affected hemispheres suggest additional stroke etiologies of vasculopathy (i.e., thromboembolism) in addition to chronic hypoxia-ischemia.

Cerebral hemodynamics and pseudo-continuous arterial spin labeling considerations in adults with sickle cell anemia

Sickle cell anemia (SCA) is a genetic disorder resulting in reduced oxygen carrying capacity and elevated stroke risk. Pseudo-continuous arterial spin labeling (pCASL) measures of cerebral blood flow (CBF) may have relevance for stroke risk assessment; however, the effects of elevated flow velocity and reduced bolus arrival time (BAT) on CBF quantification in SCA patients have not been thoroughly characterized, and pCASL model parameters used in healthy adults are often applied to patients with SCA. Here, cervical arterial flow velocities and pCASL labeling efficiencies were computed in adults with SCA (n = 19) and age- and race-matched controls without sickle trait (n = 7) using pCASL in sequence with phase contrast MR angiography (MRA). Controls (n = 7) and a subgroup of patients (n = 8) also underwent multi-post-labeling-delay pCASL for BAT assessment. Mean flow velocities were elevated in SCA adults (velocity = 28.3 ± 4.1 cm/s) compared with controls (velocity = 24.5 ± 3.8 cm/s), and mean pCASL labeling efficiency (α) was reduced in SCA adults (α = 0.72) relative to controls (α = 0.91). In patients, mean whole-brain CBF from phase contrast MRA was 91.8 ± 18.1 ml/100 g/min, while mean pCASL CBF when assuming a constant labeling efficiency of 0.86 was 75.2 ± 17.3 ml/100 g/min (p < 0.01), resulting in a mean absolute quantification error of 23% when a labeling efficiency appropriate for controls was assumed. This difference cannot be accounted for by BAT (whole-brain BAT: control, 1.13 ± 0.06 s; SCA, 1.02 ± 0.09 s) or tissue T₁ variation. In conclusion, BAT variation influences pCASL quantification less than elevated cervical arterial velocity and labeling efficiency variation in SCA adults; thus, a lower labeling efficiency (α = 0.72) or subject-specific labeling efficiency should be incorporated for SCA patients.

In Vivo T1 of Blood Measurements in Children with Sickle Cell Disease Improve Cerebral Blood Flow Quantification from Arterial Spin-Labeling MRI

BACKGROUND AND PURPOSE

Children with sickle cell disease have low hematocrit and elevated CBF, the latter of which can be assessed with arterial spin-labeling MR imaging. Quantitative CBF values are obtained by using an estimation of the longitudinal relaxation time of blood (T1blood). Because T1blood depends on hematocrit in healthy individuals, we investigated the importance of measuring T1blood in vivo with MR imaging versus calculating it from hematocrit or assuming an adult fixed value recommended by the literature, hypothesizing that measured T1blood would be the most suited for CBF quantification in children with sickle cell disease.

MATERIALS AND METHODS

Four approaches for T1blood estimation were investigated in 39 patients with sickle cell disease and subsequently used in the CBF quantification from arterial spin-labeling MR imaging. First, we used 1650 ms as recommended by the literature (T1blood-fixed); second, T1blood calculated from hematocrit measured in patients (T1blood-hematocrit); third, T1blood measured in vivo with a Look-Locker MR imaging sequence (T1blood-measured); and finally, a mean value from T1blood measured in this study in children with sickle cell disease (T1blood-sickle cell disease). Quantitative flow measurements acquired with phase-contrast MR imaging served as reference values for CBF.

RESULTS

T1blood-measured (1818 ± 107 ms) was higher than the literature recommended value of 1650 ms, was significantly lower than T1blood-hematocrit (2058 ± 123 ms, P < .001), and, most interesting, did not correlate with hematocrit measurements. Use of either T1blood-measured or T1blood-sickle cell disease provided the best agreement on CBF between arterial-spin labeling and phase-contrast MR imaging reference values.

CONCLUSIONS

This work advocates the use of patient-specific measured T1blood or a standardized value (1818 ms) in the quantification of CBF from arterial spin-labeling in children with SCD.

Risk factor analysis of cerebral white matter hyperintensities in children with sickle cell disease

Sickle cell disease (SCD) is complicated by silent cerebral infarcts, visible as white matter hyperintensities (WMHs) on magnetic resonance imaging (MRI). Both local vaso-occlusion, elicited by endothelial dysfunction, and insufficiency of cerebral blood flow (CBF) have been proposed to be involved in the aetiology. We performed an explorative study to investigate the associations between WMHs and markers of endothelial dysfunction and CBF by quantifying WMH volume on 3.0 Tesla MRI. We included 40 children with HbSS or HbSβ(0) thalassaemia, with a mean age of 12.1 ± 2.6 years. Boys demonstrated an increased risk for WMHs (odds ratio 4.5, 95% confidence interval 1.2-17.4), unrelated to glucose-6-phosphate dehydrogenase deficiency. In patients with WMHs, lower fetal haemoglobin (HbF) was associated with a larger WMH volume (regression coefficient = -0.62, R2 = 0.5, P = 0.04). Lower ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) levels were associated with lower CBF in the white matter (regression coefficient = 0.07, R2 = 0.15, P = 0.03), suggesting that endothelial dysfunction could potentially hamper CBF. The findings of our explorative study suggest that a high level of HbF may be protective for WMHs and that endothelial dysfunction may contribute to the development of WMHs by reducing CBF.