Abstract
PurposeRelative cerebral blood volume (rCBV) from dynamic susceptibility contrast (DSC)-MRI is a valuable biomarker in patients with glioblastoma for assessing treatment response and predicting overall survival. DSC-MRI based on echo planar images (EPI) may possess severe geometric distortions from magnetic field inhomogeneities up to the order of centimeters. The aim of this study is to assess how much two readily available EPI-based geometric distortion correction methods, FSL TOPUP and EPIC, affect rCBV values from DSC-MRI in patients with confirmed glioblastoma. MethodWe used a combined single-shot 2D gradient-echo (T2*), spin-echo (T2) EPI sequence to estimate both T2* and T2-weighted rCBV from the same contrast agent injection. Effects of distortion correction on the positive phase-encoded T2- and T2*-images were assessed in healthy anatomical brain regions in terms of Wilcoxon signed rank tests on median rCBV change and on Dice coefficients, as well as in tumor lesions in terms of Wilcoxon signed rank tests on median rCBV change. ResultsOur results show that following distortion correction, both gradient-echo and spin-echo rCBV increased in cortical areas of the frontal, temporal and occipital lobe, including the posterior orbital gyri in the frontal lobe and middle frontal gyri (p < 0.0008). Similar, improved Dice coefficients were observed for gradient-echo EPI in temporal, occipital and frontal lobe. Only spin-echo rCBV in enhancing lesion increased with correction (p = 0.0002). ConclusionOur study sheds light on the importance of performing geometric distortion correction on EPI-based MRI data before assessing functional information such as rCBV values. Our findings may indicate that uncorrected rCBV values can be underestimated from positive phase-encoding EPI and that geometric distortion correction is warranted when comparing EPI-based data to conventional MRI.
Highlights
Mapping tissue perfusion by dynamic susceptibility contrast mag netic resonance imaging (DSC-MRI) is a well-established and invasive technique proven to give valuable information for both grading and prediction of glioma status [1,2]
The kernel density estimates of median Relative cerebral blood volume (rCBV) for all regions with a significant (p < 0.0008) change following TOPUP and EPIC distortion correction are shown for Gradient Echo (GE) rCBV in Fig. 3, and for Spin Echo (SE) rCBV in Fig. 4, respectively
Their median quartiles indicate a general increase in esti mated rCBV after distortion correction
Summary
Mapping tissue perfusion by dynamic susceptibility contrast mag netic resonance imaging (DSC-MRI) is a well-established and invasive (because of the use of an exogenous contrast agent) technique proven to give valuable information for both grading and prediction of glioma status [1,2]. Besides using alternative sequences such as multi-shot EPI or parallel acquisition methods that minimizes geometric distortionsthe most established methods for correction of geometric distortions in EPI estimates the B0 field inhomogeneity from either multiple GE images with different echo times (field-mapping approach) or from pairs of opposite phase-encoded EPI (reverse gradient approach) [6].; DSC-MRI-based biomarkerssuch as rCBV, are typically used in combination with other higher resolution structural non-EPI images as overlays for treatment assessment, including planning of biopsies and surgery In this context, the correction of EPI related distortions is relevant to ensure the spatial accuracy and correct EPI and rCBV values. Our study highlights the need for careful assessment when using uncorrected DSC-MRI data from these brain regions
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