On the computational assessment of white matter hyperintensity progression: difficulties in method selection and bias field correction performance on images with significant white matter pathology

Introduction Subtle inhomogeneities in the scanner’s magnetic fields (B0 and B1) alter the intensity levels of the structural magnetic resonance imaging (MRI) affecting the volumetric assessment of WMH changes. Here, we investigate the influence that (1) correcting the images for the B1 inhomogeneities (i.e. bias field correction (BFC)) and (2) selection of the WMH change assessment method can have on longitudinal analyses of WMH progression and discuss possible solutions. Methods We used brain structural MRI from 46 mild stroke patients scanned at stroke onset and 3 years later. We tested three BFC approaches: FSL-FAST, N4 and exponentially entropy-driven homomorphic unsharp masking (E2D-HUM) and analysed their effect on the measured WMH change. Separately, we tested two methods to assess WMH changes: measuring WMH volumes independently at both time points semi-automatically (MCMxxxVI) and subtracting intensity-normalised FLAIR images at both time points following image gamma correction. We then combined the BFC with the computational method that performed best across the whole sample to assess WMH changes. Results Analysis of the difference in the variance-to-mean intensity ratio in normal tissue between BFC and uncorrected images and visual inspection showed that all BFC methods altered the WMH appearance and distribution, but FSL-FAST in general performed more consistently across the sample and MRI modalities. The WMH volume change over 3 years obtained with MCMxxxVI with vs. without FSL-FAST BFC did not significantly differ (medians(IQR)(with BFC) = 3.2(6.3) vs. 2.9(7.4)ml (without BFC), p = 0.5), but both differed significantly from the WMH volume change obtained from subtracting post-processed FLAIR images (without BFC)(7.6(8.2)ml, p < 0.001). This latter method considerably inflated the WMH volume change as subtle WMH at baseline that became more intense at follow-up were counted as increase in the volumetric change. Conclusions Measurement of WMH volume change remains challenging. Although the overall volumetric change was not significantly affected by the application of BFC, these methods distorted the image intensity distribution affecting subtle WMH. Subtracting the FLAIR images at both time points following gamma correction seems a promising technique but is adversely affected by subtle WMH. It is important to take into account not only the changes in volume but also in the signal intensity

Three-dimensional time-of-flight MR angiography at 3 T compared to digital subtraction angiography in the follow-up of ruptured and coiled intracranial aneurysms: a prospective study

INTRODUCTION: Since digital subtraction angiography (DSA) carries a low risk of morbidity, and is associated with patient discomfort and higher cost, our objective was to determine whether high-resolution 3-D time-of-flight MR angiography (TOF-MRA) at 3 T may replace DSA in the follow-up of patients after coiling of an intracranial aneurysm. METHODS: This prospective study included 50 consecutive patients with a ruptured and subsequently coiled intracranial aneurysm. All patients were followed up at a mean of 14 months after coiling with DSA and high-resolution 3-D TOF-MRA at 3 T generating 0.02 mm3 isotropic voxels. One examiner used DSA and TOF-MR angiograms to assess the need for and risk of retreatment; these data were used to calculate intermodality agreement. Another two examiners independently assessed aneurysm occlusion by DSA and TOF-MRA according to the Raymond scale; these data were used to calculate interobserver agreement. RESULTS: Discrepancies between DSA and TOF-MRA were found in three patients (intermodality agreement kappa=0.86). While DSA indicated complete aneurysm occlusion, TOF-MRA showed small neck remnants in the three patients. Coils on all DSA projections obscured these three neck remnants. Interobserver agreement was higher for DSA (kappa=0.82) than for TOF-MRA (kappa=0.68), which was in part due to the complexity of the information provided by TOF source images and reconstructions. CONCLUSION: 3-D TOF-MRA at 3 T is not only an adjunctive tool but is ready to replace DSA in the follow-up of patients with previously coiled intracranial aneurysms. Additional DSA may only be performed in complex and not clearly laid out aneurysms