Browsing by Subject "Reduced field-of-view imaging"

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Open Access
2D RF pulse design for optimized reduced field-of-view imaging at 1.5T and 3T
(Elsevier Inc., 2021-10-22) Eren, Orhun Caner; Barlas, Bahadır Alp; Sarıtaş, Emine Ülkü
Two-dimensional spatially selective radiofrequency (2DRF) excitation pulses are widely used for reduced field-of-view (FOV) targeted high-resolution diffusion weighted imaging (DWI), especially for anatomically small regions such as the spinal cord and prostate. The reduction in FOV achieved by 2DRF pulses significantly improve the in-plane off-resonance artifacts in single-shot echo planar imaging (ss-EPI). However, long durations of 2DRF pulses create a sensitivity to through-plane off-resonance effects, especially at 3 T where the off-resonance field doubles with respect to 1.5 T. This work proposes a parameter-based optimization approach to design 2DRF pulses with blips along the slice-select axis, with the goal of maximizing slab sharpness while minimizing off-resonance effects on 1.5 T and 3 T MRI scanners, separately. Extensive Bloch simulations are performed to evaluate the off-resonance robustness of 2DRF pulses. Three different metrics are proposed to quantify the similarity between the actual and ideal 2D excitation profiles, based on the signals within and outside the targeted reduced-FOV region. In addition, simulations on a digital brain phantom are performed for visual comparison purposes. The results show that maintaining a sharp profile is the primary design requirement at 1.5 T, necessitating the usage of relatively high slab sharpness with a time-bandwidth product (TBW) around 8–10. In contrast, off-resonance robustness is the primary design requirement at 3 T, requiring the usage of a moderate slap sharpness with TBW around 5–7.
Open Access
Robust high-resolution reduced field-of-view MRI with sheared 2D RF excitation
(2022-07) Barlas, Bahadır Alp
Reduced field-of-view (FOV) single-shot echo-planar imaging (ssEPI) is a widely applied imaging technique for diffusion-weighted magnetic resonance imaging (MRI), due to its robustness against in-plane off-resonance artifacts. Two-dimensional echo-planar RF (2D RF) excitation is a popular approach for reduced-FOV imaging due to its fat suppression capability and sharp slab profiles. However, long pulse durations render 2D RF pulses sensitive to through-plane off-resonance effects, causing local signal losses in reduced-FOV images. The standard 2D RF pulses also generate excitation replicas along the slice stack, limiting the slice coverage during multislice imaging. This thesis proposes a sheared 2D RF design for reduced-FOV imaging for significant reduction in pulse duration, leading to significant improvement in through-plane off-resonance ro-bustness. The proposed design also provides unlimited slice coverage and high fidelity fat suppression. Sheared k-space trajectories are designed such that the excitation replicas are positioned outside the slice stack to guarantee unlimited slice coverage, while ensuring identical k-space coverage as that of a standard 2D RF pulse. The efficacy of the sheared design is demonstrated by extensive simulations in terms of pulse duration, fat suppression capability, and signal com-parisons under off-resonance effects for a range of design parameters and hardware limits. The sheared and standard 2D RF pulses are then compared via imaging experiments on a custom head and neck phantom, and in vivo imaging experi-ments in the spinal cord at 3 T. The results show that in regions with high off-resonance effects, the sheared 2D RF pulse improves the signal by more than 50%when compared to the standard 2D RF pulse while preserving profile sharpness. Lastly, the benefits of the sheared design are demonstrated for low-cost low-field MRI systems via simulations and phantom experiments, making reduced-FOV imaging applicable on these systems. The proposed sheared 2D RF design will be especially beneficial in regions suffering from a variety of off-resonance effects, such as spinal cord and breast.
Open Access
Sheared two-dimensional radiofrequency excitation for off-resonance robustness and fat suppression in reduced field-of-view imaging
(Wiley, 2022-09-30) Barlas, Bahadır Alp; Bahadır, Çağla Deniz; Kafalı, Sevgi Gökçe; Yılmaz, Uğur; Sarıtaş, Emine Ülkü
Purpose: Two-dimensional (2D) echo-planar radiofrequency (RF) pulses are widely used for reduced field-of-view (FOV) imaging in applications such as diffusion-weighted imaging. However, long pulse durations render the 2D RF pulses sensitive to off-resonance effects, causing local signal losses in reduced-FOV images. This work aims to achieve off-resonance robustness for 2D RF pulses via a sheared trajectory design. Theory and Methods: A sheared 2D RF pulse design is proposed to reduce pulse durations while covering identical excitation k-space extent as a standard 2D RF pulse. For a given shear angle, the number of sheared trajectory lines is minimized to obtain the shortest pulse duration, such that the excitation replicas are repositioned outside the slice stack to guarantee unlimited slice coverage. A target fat/water signal ratio of 5% is chosen to achieve robust fat suppression. Results: Simulations, imaging experiments on a custom head and neck phantom, and in vivo imaging experiments in the spinal cord at 3 T demonstrate that the sheared 2D RF design provides significant improvement in image quality while preserving profile sharpnesses. In regions with high off-resonance effects, the sheared 2D RF pulse improves the signal by more than 50% when compared to the standard 2D RF pulse. Conclusion: The proposed sheared 2D RF design successfully reduces pulse durations, exhibiting significantly improved through-plane off-resonance robustness, while providing unlimited slice coverage and high fidelity fat suppression. This method will be especially beneficial in regions suffering from a variety of off-resonance effects, such as spinal cord and breast. © 2022 International Society for Magnetic Resonance in Medicine.