MRI resolution enhancement: How useful are shifted images obtained by changing the demodulation frequency?

Abstract

Super-resolution reconstruction is a process by which a set of different low resolution images of the same object are used to create an enhanced, higher resolution image of that object. Recently there has been debate amongst researchers whether it is possible to obtain in-plane image enhancement using a set of low resolution magnetic resonance images, acquired by making small, independent changes to the demodulation frequency. We show that shifted low-resolution images contain different information that can be used to obtain denser sampling, leading to image enhancement. We conclude this from specific phantom experiments, applying signal processing sampling theory and taking into consideration the relative sampling of the point spread function with respect to the location of signal sources. Furthermore, the maximum achievable resolution for Fourier encoded MRI data at a boundary or object feature is governed by the effective width of the point spread function or the Fourier pixel size determined by the extent of k-space; this is verified experimentally. Magn Reson Med 65:664-672,

MRI demodulation frequency changes provide different information

“…Uecker and coworkers claim that our conclusion in Ref 1. was supported by neither the theoretical arguments nor the experimental data presented.…”

“…Furthermore, comparison of Fig. 4(a) to (b) in Ref 1. shows that enhancement is most observable at object features, where dimensions are near the effective width of the point spread function (PSF).…”

“…We tested our method with and without the addition of noise, and showed that improvements were present in both cases (see Figs. 4 and 5 in Ref 1…”

Single Anisotropic 3-D MR Image Upsampling via Overcomplete Dictionary Trained From In-Plane High Resolution Slices

“…In [6] variable demodulation frequency was used to obtain shifted sampling in the image space for in-plane SR. In [7] the authors compared the combination of images acquired at the same sample points in k-space but shifted afterwards, and the combination of the same number of images acquired at shifted positions through changes in demodulation frequency. They found that the HR images obtained by the latter method contained additional details in the form of image features, suggesting that new information was added through a denser sampling of the point-spread function (PSF).…”

Reply to: MRI resolution enhancement: How useful are shifted images obtained by changing the demodulation frequency?

“…In a recent article, Tieng et al studied a super-resolution MRI technique, which relies on the combination of several low-quality images (same field-of-view and resolution) that are shifted against each other by a fraction of a pixel (1). Because additional (new) information is required in these images to improve the in-plane resolution by super-resolution principles, the authors investigated whether small changes in the demodulation frequency of the MRI acquisition may lead to images with different information content.…”