Browsing by Subject "Fat suppression"

Now showing 1 - 3 of 3

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.
Open Access
Spectrally selective imaging with wideband balanced steady-state free precession MRI
(John Wiley and Sons Inc., 2016) Çukur, Tolga
Purpose Unwanted, bright fat signals in balanced steady-state free precession sequences are commonly suppressed using spectral shaping. Here, a new spectral-shaping method is proposed to significantly improve the uniformity of stopband suppression without compromising the level of passband signals. Methods The proposed method combines binomial-pattern excitation pulses with a wideband balanced steady-state free precession sequence kernel. It thereby increases the frequency separation between the centers of pass and stopbands by π radians, enabling improved water-fat contrast. Simulations were performed to find the optimal flip angles and subpulse spacing for the binomial pulses that maximize contrast and signal efficiency. Results Comparisons with a conventional binomial balanced steady-state free precession sequence were performed in simulations as well as phantom and in vivo experiments at 1.5 T and 3 T. Enhanced fat suppression is demonstrated in vivo with an average improvement of 58% in blood-fat and 68% in muscle-fat contrast (P < 0.001, Wilcoxon signed-rank test). Conclusion The proposed binomial wideband balanced steady-state free precession method is a promising candidate for spectrally selective imaging with enhanced reliability against field inhomogeneities.