O‐space imaging: Highly efficient parallel imaging using second‐order nonlinear fields as encoding gradients with no phase encoding

Abstract

Recent improvements in parallel imaging have been driven by the use of greater numbers of independent surface coils placed so as to minimize aliasing along the phase encode direction(s). However, gains from increasing the number of coils diminish as coil coupling problems begin to dominate and the ratio of acceleration gain to expense for multiple receiver chains becomes prohibitive. In this work we redesign the spatial encoding strategy in order to gain efficiency, achieving a gradient encoding scheme that is complementary to the spatial encoding provided by the receiver coils. This approach leads to “O-Space” imaging, wherein the gradient shapes are tailored to an existing surface coil array, making more efficient use of the spatial information contained in the coil profiles. In its simplest form, for each acquired echo the Z2 spherical harmonic is used to project the object onto sets of concentric rings, while the X and Y-gradients are used to offset this projection within the imaging plane. The theory is presented, an algorithm is introduced for image reconstruction, and simulations reveal that O-Space encoding achieves high encoding efficiency compared to SENSE, radial projection imaging, and PatLoc imaging, suggesting that O-Space imaging holds great potential for accelerated scanning.

Simultaneously driven linear and nonlinear spatial encoding fields in MRI

“…Using a custom-built ultra-low field (0.0192 T) system, a nonlinear SEM has been used directly in conjunction with linear SEMs to produce Fresnel encoding as an alternative to Fourier encoding (6). A preliminary result demonstrating experimental data collected with nonlinear SEMs in simultaneous combination with conventional linear SEMs using a commercially available MR system is presented in (7). In that study, an SEM in the form of a paraboloid of revolution was manipulated using hardware designed for real-time shimming, concurrent to the conventional gradients.…”

Ultrafast multi-slice spatiotemporally encoded MRI with slice-selective dimension segmented

“…Due to its unique characteristic, the quadratic phase distribution recently has received renewed interest. Some other recent applications of quadratic phase in MRI include RASER, PatLoc, O-space imaging and spread spectrum MRI [25][26][27][28].…”

Cyclic generalized projection MRI

“…More recently, specialized non-linear gradient field coils [11] have been used with time varying gradient methods, like PatLoc [12][13][14], to reduce the ΔB max /Δt responsible for producing painful peripheral nerve stimulation. The efficiency of imaging with nonlinear B 0 encoding fields may be increased, for example in methods like O-space imaging [15] and null space imaging [16], by designing the B 1 fields to encode information complementary to that encoded by the SEM. For these SEM methods, the concept of generalized projection is useful and methods that use backprojection as part of the image reconstruction process have been developed [17].…”