Abstract ▼ AI-Helper

The primary advantages of magnetic resonance imaging (MRI) are its excellent soft tissue contrast and the fact that it does not use ionizing radiation, allowing repeated measurements for longitudinal studies. In addition, dedicated open-type MRI units have become more attractive for image-guided rad...

The primary advantages of magnetic resonance imaging (MRI) are its excellent soft tissue contrast and the fact that it does not use ionizing radiation, allowing repeated measurements for longitudinal studies. In addition, dedicated open-type MRI units have become more attractive for image-guided radiotherapy and musculoskeletal imaging owing to their easy-access configuration for patients, versatility, and low-cost efficiency compared to the restricted tunnel-type systems that are conventionally used. However, open and short bore systems are limited to the use of a large FOV and severe geometric distortion occurs around the outer regions far from the magnet center. Moreover, the geometric image distortion caused by intrinsic magnetic field inhomogeneity and gradient nonlinearity hinders accurate decomposition of the tissue of interest in MRI. The purpose of this study was to investigate the use of isocentric imaging for correcting the geometric distortion for off-center anatomy and for elongating the effective field of view (FOV) through assembly of the reduced FOV without distortion and without the wraparound artifact using object sliding and spatially selective excitation. We first demonstrated the features of the geometric distortion model characteristics and the correction performance for MRI. The known physical dimensions of the phantom were used for detection and modeling of the geometric distortion. Results confirmed that a larger error of the global polynomial function with respect to the different FOV was generated by the local deformation near the center of the gradient coil. Moreover, image distortion was more severely affected by gradient nonlinearity with an increasing FOV. We then proposed an image stitching algorithm to merge the non-distorted regions of images obtained at the isocenter of the magnet and produce an undistorted FOV using the proposed geometric distortion correction technique. The composite image produced through the sequential generation of the same object at different table positions was generated using an image stitching technique. This image stitching technique consisted of configuring tiles using optimized global registration and then seamlessly blending the aligned tiles. The results showed that geometric distortion caused by imaging gradient nonlinearity was system-induced distortion and that the stitching of segmented isocenter images with a defined distortion free area efficiently corrects hardware imperfection. We proposed reduced FOV imaging without aliasing through combining two-dimensional radio frequency (2DRF) excitations and sensitivity encoding (SENSE)-type reconstruction without phase oversampling and spatial pre-saturation. A 2D spatially selective pulse sequence requires gradients along two independent axes and the resultant frequency response in 2DRF is linearly associated with the spatial profile in the presence of two independent gradient waveforms. Reduced FOV imaging with 2D spatially selective RF excitations was weighted by the component of the RF and gradient waveform. This spatial weighting can be exploited to significantly undersample the k-space for SENSE-type reconstruction. 2DRF excitations and the SENSE-type reconstruction method were implemented for reduced FOV MRI. This approach suppressed the wraparound artifact in images without phase oversampling and spatial pre-saturation. In conclusion, isocentric imaging can be used for correcting the geometric distortion for off-center anatomy and for elongating the effective field of view (FOV) through assembly of the reduced FOV without distortion and without the wraparound artifact using object sliding and spatially selective excitation.

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