Superresolution MRI with a Structured-Illumination Approach

Abstract

Unlike optical imaging methods, where the Fourier representation of the object is created by wave diffraction, magnetic resonance imaging (MRI) scans the Fourier domain by spatially modulating the nuclear magnetization with magnetic field gradients. Therefore, the resolution of MRI is limited by signal strength rather than the wavelength of the diffracted wave. However, in several important applications, such as the imaging of water diffusion or blood oxygenation in the human brain, MRI must collect all the Fourier data during a single evolution of the signal to avoid artifacts caused by random fluctuations of its phase. The Fourier domain coverage of the ``single-shot'' MRI strategy is limited by the lifetime of the signal and results in a resolution limit analogous to the diffraction limit in optical microscopy. Consequently, recently developed optical superresolution methods may appear helpful. This work employs the recently developed method of phaseless encoding, which implements the idea of linear structured illumination microscopy (SIM) to enable multishot data acquisition and triple the MRI resolution regardless of the presence of spin phase fluctuations, and develops it further to enhance the resolution in two dimensions and to push its gain above a factor of 3 using ``illumination patterns'' with multiple harmonics, a concept borrowed from saturated SIM (SSIM) methodology. Results obtained with enhanced phaseless encoding on a 3-Tesla clinical scanner in phantoms and human brain are presented. This method will be beneficial when conventional multishot MRI is impeded by signal instability due to motion or other factors.