Intensity non-uniformity correction using N3 on 3-T scanners with multichannel phased array coils

Area of Science:

• Neuroimaging
• Medical Physics
• Radiology

Background:

• Longitudinal magnetic resonance imaging (MRI) is crucial for tracking brain changes.
• Intensity non-uniformity in MRI can compromise the accuracy of these measurements, especially at higher field strengths (3-T) and with multichannel coils.
• This artifact can hinder the detection of subtle disease progression.

Purpose of the Study:

• To evaluate the effectiveness of the non-parametric non-uniform intensity normalization (N3) technique in correcting intensity non-uniformity in brain MRI scans.
• To determine optimal N3 parameters (brain masks and spline smoothing distances) for improving image quality and temporal consistency.
• To assess the impact of these corrections on data acquired for large-scale neurodegenerative disease studies like ADNI.

Main Methods:

• Analyzed 72 volumetric brain MRI scans from normal elderly subjects acquired using MP-RAGE and SPGR sequences on 3-T scanners.
• Applied the N3 algorithm with five different brain masks and four spline smoothing distances (50-200 mm).
• Quantitatively assessed correction by measuring normalized white matter intensity variance for individual scans and variance of normalized difference images for registered scan pairs.

Main Results:

• N3 significantly improved individual scan uniformity (p<0.01) and reduced temporal variance when using accurate brain tissue class masks.
• Smaller spline smoothing distances (50-100 mm) yielded optimal results for both MP-RAGE and SPGR sequences.
• These optimized settings demonstrated enhanced correction consistency over time.

Conclusions:

• The N3 technique, with optimized parameters, effectively corrects intensity non-uniformity in 3-T brain MRI.
• Accurate brain masks and shorter smoothing distances are key for optimal N3 performance.
• These findings support the use of optimized N3 in large studies like ADNI to ensure reliable detection of brain changes and disease progression.