Need for Contrast-enhanced MR Imaging Protocols and Quantitative Assessment of Wall Enhancement for Vessel Wall Imaging in Various Intracranial Arterial Diseases

To the editor,

We read with interest the review by Gomyo et al., “Vessel Wall Imaging of Intracranial Arteries: Fundamentals and Clinical Applications.”¹ They reviewed the findings of vessel wall imaging (VWI) for various intracranial arterial diseases. We would like to discuss the need for contrast-enhanced MRI protocols and quantitative assessment of contrast-enhanced VWI in various intracranial arterial diseases.

In the review by Gomyo et al., dynamic contrast-enhanced perfusion was shown to be a useful technique for quantitatively evaluating the enhancement effect of plaque in atherosclerosis. Ktrans, which corresponds to the constant of transfer speed from the plasma to the extracellular matrix, was reduced by hyperlipidemia treatment. Further, Gomyo et al. presented a case of active vasculitis in which vessel wall enhancement was observed before treatment and improved after steroid treatment. The contrast-enhanced VWI is useful in determining the treatment effect in intracranial arterial diseases. In intracranial aneurysms, aneurysms with vessel wall enhancement are associated with an estimated 5-year risk of rupture that is more than three times that of intracranial aneurysms without vessel wall enhancement. The contrast-enhanced VWI might identify evolving aneurysms among unruptured aneurysms or might be used for the grading of aneurysms. Since these previous studies have shown that contrast-enhanced VWI is useful for determining treatment effects and grading intracranial arterial disease, segmentation of the arterial wall on contrast-enhanced VWI and quantitative assessment of the degree of vessel wall enhancement will be essential. The walls of these vessels are extremely thin, and the segmentation of such thin vessel walls was difficult; however, several studies attempted to automatically perform the segmentation of intracranial aneurysms and arterial walls. In our study of intracranial aneurysm with contrast-enhanced VWI, we manually segmented the intracranial aneurysm lumen, and then one voxel of the outer and inner layer of the contour of the aneurysm lumen was semiautomatically segmented to calculate the enhancement effect of the outer and inner layers of the aneurysm.² The enhancement effect of the union of outer and inner layers was effective for differentiating between stable and evolving unruptured intracranial aneurysms. Other study groups reported fully automated deep-learning segmentation for arterial vessel walls and plaques.³ For quantitative assessment of the degree of vessel wall enhancement, we created the voxel-based enhancement maps by dividing the post-contrast images by the pre-contrast images voxel by voxel.² If quantitative enhancement rates are used, it may be necessary to standardize the timing of post-contrast images, i.e., the delay time after the administration of contrast media. Standardization of MRI protocols, automatic segmentation of arterial or aneurysmal walls, and establishment of a calculation method for quantitative enhancement rates will be necessary for the extensive use of contrast-enhanced VWI in various types of intracranial arterial diseases.

Once again, we appreciate the review of Gomyo et al. on the VWI in various intracranial arterial diseases.