Super resolution using sparse sampling at portable ultra-low field MR
Corinne Donnay, Serhat V. Okar, Charidimos Tsagkas, María I. Gaitán, Megan Poorman, Daniel S. Reich, Govind Nair
Abstract
Ultra-low field (ULF) magnetic resonance imaging (MRI) holds the potential to make MRI more accessible, given its cost-effectiveness, reduced power requirements, and portability. However, signal-to-noise ratio (SNR) drops with field strength, necessitating imaging with lower resolution and longer scan times. This study introduces a novel Fourier-based Super Resolution (FouSR) approach, designed to enhance the resolution of ULF MRI images with minimal increase in total scan time. FouSR combines spatial frequencies from two orthogonal ULF images of anisotropic resolution to create an isotropic T2-weighted fluid-attenuated inversion recovery (FLAIR) image. We hypothesized that FouSR could effectively recover information from under-sampled slice directions, thereby improving the delineation of multiple sclerosis (MS) lesions and other significant anatomical features. Importantly, the FouSR algorithm can be implemented on the scanner with changes to the k -space trajectory. Paired ULF (Hyperfine SWOOP, 0.064 tesla) and high field (Siemens, Skyra, 3 Tesla) FLAIR scans were collected on the same day from a phantom and a cohort of 10 participants with MS or suspected MS (6 female; mean ± SD age: 44.1 ± 4.1). ULF scans were acquired along both coronal and axial planes, featuring an in-plane resolution of 1.7 mm × 1.7 mm with a slice thickness of 5 mm. FouSR was evaluated against registered ULF coronal and axial scans, their average (ULF average) and a gold standard SR (ANTs SR). FouSR exhibited higher SNR (47.96 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mo>±</mml:mo></mml:math> 12.6) compared to ULF coronal (36.7 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mo>±</mml:mo></mml:math> 12.2) and higher lesion conspicuity (0.12 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mo>±</mml:mo></mml:math> 0.06) compared to ULF axial (0.13 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M4"><mml:mo>±</mml:mo></mml:math> 0.07) but did not exhibit any significant differences contrast-to-noise-ratio (CNR) compared to other methods in patient scans. However, FouSR demonstrated superior image sharpness (0.025 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M5"><mml:mo>±</mml:mo></mml:math> 0.0040) compared to all other techniques (ULF coronal 0.021 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M6"><mml:mo>±</mml:mo></mml:math> 0.0037, q = 5.9, p -adj. = 0.011; ULF axial 0.018 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M7"><mml:mo>±</mml:mo></mml:math> 0.0026, q = 11.1, p -adj. = 0.0001; ULF average 0.019 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M8"><mml:mo>±</mml:mo></mml:math> 0.0034, q = 24.2, p -adj. &lt; 0.0001) and higher lesion sharpness (−0.97 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M9"><mml:mo>±</mml:mo></mml:math> 0.31) when compared to the ULF average (−1.02 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M10"><mml:mo>±</mml:mo></mml:math> 0.37, t (543) = −10.174, p = &lt;0.0001). Average blinded qualitative assessment by three experienced MS neurologists showed no significant difference in WML and sulci or gyri visualization between FouSR and other methods. FouSR can, in principle, be implemented on the scanner to produce clinically useful FLAIR images at higher resolution on the fly, providing a valuable tool for visualizing lesions and other anatomical structures in MS.