The role of nonlinear gradients in parallel imaging: A k‐space based analysis

Galiana, Gigi; Stockmann, Jason P.; Tam, Leo; Peters, Dana C.; Tagare, Hemant D.; Constable, R. Todd

doi:10.1002/cmr.a.21243

Cited by 19 publications

(23 citation statements)

References 27 publications

(50 reference statements)

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“…If too few orientations are acquired because of a slow nonlinear gradient rotation rate, then we can still only solve for average values of relatively large collections of k-space points. As shown in Galiana et al (43), a typical result of this is that in the final image, certain frequency components will not be represented across the entire FOV (which often suggests poor resolution at the center for 2 nd order fields). This explains the lack of resolution at the center of the image at the slowest rotation speeds.…”

Section: Methodsmentioning

confidence: 97%

Fast rotary nonlinear spatial acquisition (FRONSAC) imaging

Wang

Tam

Constable

et al. 2015

Magnetic Resonance in Med

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Purpose Nonlinear spatial encoding magnetic fields (SEMs) have been studied to reconstruct images from a minimum number of echoes. Previous work has also explored single shot trajectories in nonlinear SEMs. However, the search continues for optimal schemes that apply nonlinear SEMs to improve spatial encoding efficiency and image quality. Theory and Methods We enhance the encoding efficiency of standard linear gradient trajectories by adding a rapidly rotating nonlinear SEM of moderate amplitude, the so called FRONSAC (Fast ROtary Nonlinear Spatial ACquisition) imaging. This additional gradient greatly improves the image quality of highly undersampled single-shot trajectories, including EPI, Spiral, and Rosette trajectories. Results Our simulations, including noise and dephasing effects, test the effect of adding FRONSAC gradients, demonstrating the applicability of this approach. Performance is explained by demonstrating the additional k-space sampling the nonlinear gradient provides. Studies of the optimal amplitude and frequency of the additional FRONSAC field are presented, and the role of enhanced sampling during the readout demonstrated. Dynamic field mapping in a second-order gradient system shows the proposed gradient waveforms are feasible. Conclusions Images resulting from highly undersampled existing k-space trajectories, such as EPI, Spiral and Rosette, are greatly enhanced simply by adding a rotating nonlinear SEM field.

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Section: Methodsmentioning

confidence: 97%

Fast rotary nonlinear spatial acquisition (FRONSAC) imaging

Wang

Tam

Constable

et al. 2015

Magnetic Resonance in Med

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“…Images are reconstructed via a Kaczmarz algorithm (15, 16, 22, 23, 27–33, 39, 40). Generally, in a traditional Kaczmarz reconstruction, the update step is normally performed with the following equation:

I_{i} false(x, y false) = I_{i - 1} false(x, y false) - λ \cdot β_{i} \cdot A_{i} false(x, y false),

where I i −1 is the previous image estimate; I i is the new image estimate, updated to incorporate A i , the i th line of the encoding matrix.…”

Section: Theorymentioning

confidence: 99%

“…More recently, scan time has been reduced by spatial encoding with nonlinear magnetic fields, such as in Pat-Loc imaging (14), O-space imaging (15,16), null space imaging (17), four-dimensional radial in/out(4D-RIO) (18), echo planar imaging (EPI)-PatLoc (19)(20)(21), and others (22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). Here we focus on O-space imaging, which encodes spatial information through a radially varying magnetic gradient field B i ¼ z 2 -1/2((x -x i ) 2 þ (y À y i ) 2 ) centered at different center placements (x i , y i ), which form a ring about the center of the field of view (15).…”

Section: Introductionmentioning

confidence: 99%

Experimental O‐space turbo spin echo imaging

Wang

Tam

Kopanoğlu

et al. 2015

Magnetic Resonance in Med

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Purpose Turbo Spin Echo (TSE) imaging reduces imaging time by acquiring multiple echoes per repetition (TR), requiring fewer TRs. O-Space also can require fewer TRs by using a combination of nonlinear magnetic gradient fields and surface coil arrays. Although to date, O-Space has only been demonstrated for gradient echo imaging, it’s valuable to combine these two techniques. However, collecting multiple O-Space echoes per TR is difficult because of the different local k-space trajectories and variable T2-weighting. Theory and Methods A practical scheme is demonstrated to combine the benefits of TSE and O-Space for highly accelerated T2-weighted images. The scheme uses a modified acquisition order and filtered projection reconstruction to reduce artifacts caused by T2-decay, while retaining T2 contrast that corresponds to a specific echo time. Results The experiments illustrate the proposed method can produce highly accelerated T2-weighted images. Moreover, the proposed method can generate multiple images with different T2 contrasts from a single dataset. Conclusions The proposed O-Space TSE imaging method requires fewer echoes than conventional TSE and fewer repetitions than conventional O-Space imaging. It retains resilience to undersampling, clearly outperforming Cartesian SENSE at high levels of undersampling, and can generate undistorted images with a range of T2 contrast from a single acquired dataset.

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“…Provided the system is well characterized, gradient and magnet nonlinearities can be included directly in the reconstruction process. 83 The ability to obtain distortion-free images even in the presence of non/lesslinear gradients could allow the use of, for example, monopolar gradients 84 that are suitable for some of the magnet designs used in low-field and portable MRI.…”

Section: Future Avenuesmentioning

confidence: 99%

Low‐field MRI: An MR physics perspective

Marques

Simonis

Webb

2019

Magnetic Resonance Imaging

232

266

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Historically, clinical MRI started with main magnetic field strengths in the ∼0.05–0.35T range. In the past 40 years there have been considerable developments in MRI hardware, with one of the primary ones being the trend to higher magnetic fields. While resulting in large improvements in data quality and diagnostic value, such developments have meant that conventional systems at 1.5 and 3T remain relatively expensive pieces of medical imaging equipment, and are out of the financial reach for much of the world. In this review we describe the current state‐of‐the‐art of low‐field systems (defined as 0.25–1T), both with respect to its low cost, low foot‐print, and subject accessibility. Furthermore, we discuss how low field could potentially benefit from many of the developments that have occurred in higher‐field MRI. In the first section, the signal‐to‐noise ratio (SNR) dependence on the static magnetic field and its impact on the achievable contrast, resolution, and acquisition times are discussed from a theoretical perspective. In the second section, developments in hardware (eg, magnet, gradient, and RF coils) used both in experimental low‐field scanners and also those that are currently in the market are reviewed. In the final section the potential roles of new acquisition readouts, motion tracking, and image reconstruction strategies, currently being developed primarily at higher fields, are presented. Level of Evidence : 5 Technical Efficacy Stage : 1 J. Magn. Reson. Imaging 2019.

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The role of nonlinear gradients in parallel imaging: A k‐space based analysis

Cited by 19 publications

References 27 publications

Fast rotary nonlinear spatial acquisition (FRONSAC) imaging

Fast rotary nonlinear spatial acquisition (FRONSAC) imaging

Experimental O‐space turbo spin echo imaging

Low‐field MRI: An MR physics perspective

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