MR image reconstruction from generalized projections

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.

Section: Discussionmentioning

confidence: 99%

Experimental O‐space turbo spin echo imaging

Wang

Tam

Kopanoğlu

et al. 2015

“…To acquire data, the magnet is physically rotated around the sample in discrete steps. At each rotation step, generalized projections onto the nonlinear field are acquired (similar to those described in Schultz et al) . Examples of these projections are shown in Figure for a simple two‐sphere phantom.…”

Section: Methodsmentioning

confidence: 99%

Two‐dimensional imaging in a lightweight portable MRI scanner without gradient coils

Cooley

Stockmann

Armstrong

et al. 2014

150

162

Purpose As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as Intensive Care Units, physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods We construct and validate a truly portable (<100kg) and silent proof-of-concept MRI scanner which replaces conventional gradient encoding with a rotating lightweight cryogen-free, low-field magnet. When rotated about the object, the inhomogeneous field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections which encode the iteratively reconstructed 2D image. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but is shown to be likely sufficient for many medical applications. Conclusion The presented MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating the gradient coil. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology.

“…Other researchers have taken more empirical approaches to designing nonlinear encoding trajectories, such as the MDE work by Lin (20) and optimization schemes being pursued by Layton et al (21). Much effort has also gone into improving the reconstruction algorithms for data obtained with nonlinear SEMs (22)(23)(24)(25)(26)(27)(28).…”

Section: Introductionmentioning

confidence: 99%

Fast rotary nonlinear spatial acquisition (FRONSAC) imaging

Wang

Tam

Constable

et al. 2015

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.