Lori Jordan1, Melissa Gindville1, Allison Scott1, Meher Juttukonda1, Megan Strother1, Adetola Kassim1, Sheau-Chiann Chen1, Hanzhang Lu2, Sumit Pruthi1, Yu Shyr1, and Manus J. Donahue1
1Vanderbilt University Medical Center, Nashville, TN, United States, 2Johns Hopkins University, Baltimore, MD, United States
Synopsis
The
goal of this work is to apply hemo-metabolic MRI to evaluate relationships
between oxygen extraction fraction (OEF), cerebral blood flow (CBF), and
clinical impairment in adults with sickle cell anemia (SCA). Healthy (n=11) and
sickle cell anemia (n=34) participants received neurological evaluation, head/neck-angiography,
structural-MRI, CBF-weighted-MRI, and T2-relaxation-under-spin-tagging (TRUST)-MRI. CBF and OEF were
elevated (P<0.05) in SCA relative
to control participants; OEF (P<0.0001)
but not CBF was increased in SCA participants with higher clinical impairment. Data
provide support for TRUST-MRI being able to quickly and noninvasively detect
elevated OEF in SCA participants with high levels of clinical impairment.Purpose
The overall goal of this work is to apply novel
MRI-based measures of blood oxygenation to evaluate relationships between
oxygen extraction fraction (OEF; oxygen consumed/oxygen delivered), cerebral
blood flow (CBF; ml blood/100g tissue/min), and clinical impairment in adults
with sickle cell anemia (SCA). More specifically, unlike in children, no
screening procedures exist for evaluating stroke risk in adults with SCA.
Reduced oxygen carrying capacity is present in SCA, which may initially be
compensated for by an increase in CBF, after which an increasing gradient of OEF
may result in the presence of preserved cerebral metabolic rate of oxygen (CMRO2).
As such, we hypothesize that OEF and CBF can be measured noninvasively and
reproducibly with MRI in adults with SCA and that elevated OEF provides added
discriminatory capacity for clinical impairment relative to simple vasculopathy
extent and CBF.
Methods
Standard T1-weighted and T2-weighted
FLAIR MRI, intracranial and extracranial time-of-flight MRA, and CBF-weighted
(pCASL single-shot EPI; spatial resolution=3x3x7mm; post-labeling delay=1900
ms) MRI were applied in sequence with a noninvasive blood oxygenation-weighted T2-relaxation-under-spin-tagging
(TRUST)-MRI method [1] (TILT labeling; TI=1022 ms; effective TEs=0, 40, 80, 160
ms) in SCA adults (n=34) and race-matched controls without sickle trait (n=11).
Hematocrit and hemoglobin-S percent (HbS%) were determined by venipuncture on
the day of the scan. CBF was quantified from pCASL data [2] in gray matter of
major flow territories (Figure 1) and OEF from TRUST data [3] using established
models that accounted for hematocrit differences between patients and controls.
A Kruskal-Wallis test was applied to evaluate mean differences between SCA and
control parameters and between study parameters for SCA patients grouped by
less or more clinical impairment (defined by presence of infarct, vasculopathy,
or use of routine blood transfusion for SCA pain management). Two-sided p-values were corrected for
multiple-comparisons using the Holm method, and significance was defined as corrected-p<0.05.
Results and Discussion
CBF and OEF were elevated (p<0.05) in SCA participants not receiving monthly blood
transfusions (n=27; interquartile range CBF=46.2-56.8
ml/100g/min; OEF=0.39-0.50) relative to control participants
(interquartile range CBF=40.8-46.3 ml/100g/min; OEF=0.33-0.38)
(Figures 1 and 2). OEF (p<0.0001)
but not CBF was increased in patients with higher levels of clinical impairment
(Figure 3). Figure 4 shows examples of representative cases and images. OEF was weakly, inversely correlated with CBF
across all patients (R=-0.31; p=0.04).
Consistent with the primary hypothesis of this study, the correlation is
strongest for patients that are less impaired (R=-0.54; p=0.019) relative to those that are more impaired (R=-0.23; p=0.17). Ongoing work is focused on
improving quantitative pCASL and OEF data by incorporating refined blood
calibration models that incorporate knowledge of the blood hemoglobin-S
fraction.
Conclusion
OEF measured using TRUST MRI shows promise as a screening
tool for hemodynamic impairment and stroke risk in adults with SCA. A long-term
goal for this sort of methodology is to assess whether metabolic neuroimaging
predicts overt stroke in patients with sickle cell anemia, and if so to use
this method as a screening test for stroke risk in adults with sickle cell
anemia for whom no test is currently available.
Acknowledgements
No acknowledgement found.References
[1] Lu H and Ge Y. Magn
Reson Med.
2008 Aug;60(2):357-63. [2] Alsop DC
et al. Magn Reson Med. 2015 Jan;73(1). [3] Lu H et al. Magn
Reson Med. 2012 Jan;67(1):42-9.