Improved cortical surface reconstruction using sub-millimeter resolution MPRAGE by image denoising

Automatic cerebral cortical surface reconstruction is a useful tool for cortical anatomy quantification, analysis and visualization. Recently, the Human Connectome Project and several studies have shown the advantages of using T₁-weighted magnetic resonance (MR) images with sub-millimeter isotropic spatial resolution instead of the standard 1-mm isotropic resolution for improved accuracy of cortical surface positioning and thickness estimation. Nonetheless, sub-millimeter resolution images are noisy by nature and require averaging multiple repetitions to increase the signal-to-noise ratio for precisely delineating the cortical boundary. The prolonged acquisition time and potential motion artifacts pose significant barriers to the wide adoption of cortical surface reconstruction at sub-millimeter resolution for a broad range of neuroscientific and clinical applications. We address this challenge by evaluating the cortical surface reconstruction resulting from denoised single-repetition sub-millimeter T₁-weighted images. We systematically characterized the effects of image denoising on empirical data acquired at 0.6 mm isotropic resolution using three classical denoising methods, including denoising convolutional neural network (DnCNN), block-matching and 4-dimensional filtering (BM4D) and adaptive optimized non-local means (AONLM). The denoised single-repetition images were found to be highly similar to 6-repetition averaged images, with a low whole-brain averaged mean absolute difference of ~0.016, high whole-brain averaged peak signal-to-noise ratio of ~33.5 dB and structural similarity index of ~0.92, and minimal gray matter–white matter contrast loss (2% to 9%). The whole-brain mean absolute discrepancies in gray matter–white matter surface placement, gray matter–cerebrospinal fluid surface placement and cortical thickness estimation were lower than 165 μm, 155 μm and 145 μm—sufficiently accurate for most applications. These discrepancies were approximately one third to half of those from 1-mm isotropic resolution data. The denoising performance was equivalent to averaging ~2.5 repetitions of the data in terms of image similarity, and 1.6–2.2 repetitions in terms of the cortical surface placement accuracy. The scan-rescan variability of the cortical surface positioning and thickness estimation was lower than 170 μm. Our unique dataset and systematic characterization support the use of denoising methods for improved cortical surface reconstruction at sub-millimeter resolution.

Coronary high-signal-intensity plaques on T1-weighted magnetic resonance imaging reflect intraplaque hemorrhage

Coronary high-signal-intensity plaques (HIPs) detected by T₁-weighted magnetic resonance imaging are associated with future cardiovascular events. This study aimed to identify pathological findings reflecting HIPs in coronary arteries obtained from autopsy cases. Formalin-fixed hearts were imaged with noncontrast T₁-weighted imaging with a 1.5-T magnetic resonance system. We defined HIPs or non-HIPs as a coronary plaque to myocardial signal intensity ratio (PMR) of ≥1.4 or <1.4, respectively. We found HIPs in 4 of 37 (10.8%) hearts and analyzed 7 hearts in detail. The corresponding sections to HIPs (n=11) or non-HIPs (n=25) were histologically and immunohistochemically analyzed. We calculated the T₁ relaxation time of human venous blood in vitro. Plaque and necrotic core areas, and the frequency of intraplaque hemorrhage in HIPs were significantly larger/higher than those in non-HIPs. HIPs were immunopositive for CD68 (11/11), glycophorin A (10/11), and fibrin (11/11). Glycophorin-A-, matrix metalloprotease 9 (MMP9)-, and tissue factor-immunopositive areas were larger in HIPs than in non-HIPs. The PMR was positively correlated with glycophorin-A-, fibrin-, MMP9-, and tissue factor-immunopositive areas. Blood coagulation shortened the T₁ relaxation time of the blood and plasma, and the T₁ relaxation times in coagulated whole blood and erythrocyte-rich blood were significantly shorter than those in plasma. Coronary HIPs may reflect intraplaque hemorrhage and may be a novel marker for plaque instability and thrombogenic potential.