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Abstract PurposeBreath‐held fat‐suppressed volumetric T1‐weighted MRI is an important and widely‐used technique for evaluating the abdomen. Both fat‐saturation and Dixon‐based fat‐suppression methods are used at conventional field strengths; however, both have challenges at lower field strengths (<1.5T) due to insufficient fat suppression and/or inadequate resolution. Specifically, at lower field strengths, fat saturation often fails due to the short T1 of lipid; and Cartesian Dixon imaging provides poor spatial resolution due to the need for a long ∆TE, due to the smaller ∆f between water and lipid. The purpose of this work is to demonstrate a new approach capable of simultaneously achieving excellent fat suppression and high spatial resolution on a 0.55T whole‐body system. MethodsWe applied 3D stack‐of‐spirals Dixon imaging at 0.55T, with compensation of concomitant field phase during reconstruction. The spiral readouts make efficient use of the requisite ∆TE. We compared this with 3D Cartesian Dixon imaging. Experiments were performed in 2 healthy and 10 elevated liver fat volunteers. ResultsStack‐of‐spirals Dixon imaging at 0.55T makes excellent use of the required ∆TE, provided high SNR efficiency and finer spatial resolution (1.7 × 1.7 × 5 mm3) compared Cartesian Dixon (3.5 × 3.5 × 5 mm3), within a 17‐s breath‐hold. We observed successful fat suppression, and improved definition of structures such as the liver, kidneys, and bowel. ConclusionWe demonstrate that high‐resolution single breath‐hold volumetric abdominal T1‐weighted imaging is feasible at 0.55T using spiral sampling and concomitant field correction. This is an attractive alternative to existing Cartesian‐based methods, as it simultaneously provides high‐resolution and excellent fat‐suppression.
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This content will become publicly available on February 1, 2026
Multidimensional RF pulse design with consideration of concomitant field effects
Abstract PurposeTo develop a small‐tip multidimensional RF pulse design procedure that incorporates linear time‐invariant gradient imperfections and concomitant field effects. This could be particularly important for contemporary low‐field MRI systems with high‐performance gradients. Theory and MethodsWe developed an extension of the small‐tip excitation k‐space formalism, where concomitant fields were approximated as a Bloch‐Siegert shift in the rotating frame. This was evaluated using realistic simulations of 2D selective excitation at various field strengths (0.2T, 0.55T, 1.5T, 3T, and 7T) with single and parallel transmit. Simulated excitation profiles from the original and extended k‐space formalisms were compared. Experimental validations were performed at 0.55T with a single‐channel transmit. ResultsThe extended formalism provides improved 2D excitation profiles in all scenarios simulated, compared against the original formalism. The proposed method corrects the concomitant field effects on 2D selective excitations forB0 > 0.2T when the magnitude of theB0is far larger than that of nonrotating concomitant fields. Simulation and phantom experiments at 0.55T match well for both original and proposed methods, with the proposed method providing sharper and more accurate excitation profiles at off‐isocenter distances up to 15 cm. The impact of the proposed method is greatest in scenarios where concomitant fields are substantial, such as low field strengths and off‐isocenter. ConclusionConcomitant fields can be modeled as a Bloch‐Siegert shift in the rotating frame during multidimensional RF pulse design, resulting in improved excitation profiles with sharp edges. This is important to consider for off‐isocenter excitations and imaging at low field strengths with strong gradients.
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- Award ID(s):
- 1828736
- PAR ID:
- 10572560
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Magnetic Resonance in Medicine
- Volume:
- 93
- Issue:
- 2
- ISSN:
- 0740-3194
- Page Range / eLocation ID:
- 718 to 729
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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