Bruno Madore

Unaliasing by Fourier-Encoding the Overlaps Using the Temporal Dimension (UNFOLD), Applied to Cardiac Imaging and fMRI

In several applications, MRI is used to monitor the time behavior of the signal in an organ of interest; e.g., signal evolution because of physiological motion, activation, or contrast‐agent accumulation. Dynamic applications involve acquiring data in a k–t space, which contains both temporal and spatial information. It is shown here that in some dynamic applications, the t axis of k–t space is not densely filled with information. A method is introduced that can transfer information from the k axes to the t axis, allowing a denser, smaller k–t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames.

UNFOLD‐SENSE: A parallel MRI method with self‐calibration and artifact suppression

This work aims at improving the performance of parallel imaging by using it with our “unaliasing by Fourier‐encoding the overlaps in the temporal dimension” (UNFOLD) temporal strategy. A self‐calibration method called “self, hybrid referencing with UNFOLD and GRAPPA” (SHRUG) is presented. SHRUG combines the UNFOLD‐based sensitivity mapping strategy introduced in the TSENSE method by Kellman et al. ( 5 ), with the strategy introduced in the GRAPPA method by Griswold et al. ( 10 ). SHRUG merges the two approaches to alleviate their respective limitations, and provides fast self‐calibration at any given acceleration factor. UNFOLD‐SENSE further includes an UNFOLD artifact suppression scheme to significantly suppress artifacts and amplified noise produced by parallel imaging. This suppression scheme, which was published previously ( 4 ), is related to another method that was presented independently as part of TSENSE. While the two are equivalent at accelerations ≤ 2.0, the present approach is shown here to be significantly superior at accelerations > 2.0, with up to double the artifact suppression at high accelerations. Furthermore, a slight modification of Cartesian SENSE is introduced, which allows departures from purely Cartesian sampling grids. This technique, termed variable‐density SENSE (vdSENSE), allows the variable‐density data required by SHRUG to be reconstructed with the simplicity and fast processing of Cartesian SENSE. UNFOLD‐SENSE is given by the combination of SHRUG for sensitivity mapping, vdSENSE for reconstruction, and UNFOLD for artifact/amplified noise suppression. The method was implemented, with online reconstruction, on both an SSFP and a myocardium‐perfusion sequence. The results from six patients scanned with UNFOLD‐SENSE are presented. Magn Reson Med 52:310–320, 2004.

SMASH and SENSE : Experimental and numerical comparisons

Three parallel‐imaging methods were implemented and compared in terms of artifact and noise content: original SMASH, Cartesian SENSE, and an extremely simple method called here the “scissors method.” These methods represent very different approaches to the parallel‐imaging problem. The experimental and numerical comparisons presented here aim at shedding light on the whole spectrum of parallel‐imaging methods, not just the three methods actually implemented. In our results, SMASH images had an artifact level significantly higher than SENSE images for all acceleration factors. The SNR in SENSE images was nearly optimal at low acceleration factors. As acceleration was increased, the noise content in SENSE images eventually sharply departed from optimal values, while the artifact content remained low. Magn Reson Med 45:1103–1111, 2001.

Using UNFOLD to remove artifacts in parallel imaging and in partial‐Fourier imaging

In dynamic MRI, it is often difficult to achieve the acquisition speed required to resolve or freeze the temporal variations of the imaged object. Several MRI methods aim at speeding up the image acquisition process. Through assumptions and/or prior knowledge, these dynamic MRI methods allow part of the needed data to be calculated instead of acquired. For example, partial‐Fourier imaging assumes that phase varies smoothly within the object, and parallel imaging (e.g., simultaneous acquisition of spatial harmonics (SMASH) and sensitivity encoding (SENSE)) uses prior knowledge about receiver‐coil sensitivity. While these methods accelerate acquisition, they can introduce artifacts or amplify noise in doing so. The present work aims at accelerating image acquisition significantly, while introducing almost no artifacts or noise amplification. It is shown here that new, extra information is gained if dynamic MRI methods are modified so that the sampling function changes in specific ways from time‐frame to time‐frame. In other words, the set of k‐space locations that are acquired (instead of calculated) changes with time. The present temporal strategy, based on the UNaliasing by Fourier‐encoding the Overlaps in the temporaL Dimension (UNFOLD) method, can be incorporated into common dynamic MRI methods. Results with partial‐Fourier, SMASH, and SENSE imaging are presented here, where UNFOLDs contribution is to very significantly reduce the artifact and/or amplified noise content. Used in this way, UNFOLD contributes indirectly, rather than directly to the improvement in image acquisition speed, as it allows companion methods to operate properly at greater acceleration settings than would otherwise be feasible. Magn Reson Med 48:493–501, 2002.

The supernova remnant CTA1 and the surrounding interstellar medium

Dominion Radio Astrophysical Observatory aperture synthesis observations, in the continuum at 1420 MHz and in the 21 cm line of H I, are presented of the supernova remnant (SNR) CTA 1. The angular resolution is 1 arcmin. Full sensitivity to structures of all angular sizes down to this scale has been achieved by combining the aperture synthesis observations with single-antenna data. In the continuum, in addition to the well-known bright arcs giving the SNR the appearance of an incomplete shell structure, diffuse emission can be traced to the north and northeast of the remnant and also in the southwest. Some of these faint features coincide with diffuse [O III] emission. A comparison with the 100 μm coadded IRAS image shows the presence of two cavities of low infrared emissivity adjacent to these regions

Combining Two-Dimensional Spatially Selective RF Excitation, Parallel Imaging, and UNFOLD for Accelerated MR Thermometry Imaging

MR thermometry can be a very challenging application, as good resolution may be needed along spatial, temporal, and temperature axes. Given that the heated foci produced during thermal therapies are typically much smaller than the anatomy being imaged, much of the imaged field‐of‐view is not actually being heated and may not require temperature monitoring. In this work, many‐fold improvements were obtained in terms of temporal resolution and/or 3D spatial coverage by sacrificing some of the in‐plane spatial coverage. To do so, three fast‐imaging approaches were jointly implemented with a spoiled gradient echo sequence: (1) two‐dimensional spatially selective RF excitation, (2) unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD), and (3) parallel imaging. The sequence was tested during experiments with focused ultrasound heating in ex vivo tissue and a tissue‐mimicking phantom. Temperature maps were estimated from phase‐difference images based on the water proton resonance frequency shift. Results were compared to those obtained from a spoiled gradient echo sequence sequence, using a t‐test. Temporal resolution was increased by 24‐fold, with temperature uncertainty less than 1°C, while maintaining accurate temperature measurements (mean difference between measurements, as observed in gel = 0.1°C ± 0.6; R = 0.98; P > 0.05). Magn Reson Med 66:112–122, 2011.

Reduced field-of-view MRI with two-dimensional spatially-selective RF excitation and UNFOLD.

When the region of interest (ROI) is smaller than the object, one can increase MRI speed by reducing the imaging field of view (FOV). However, when such an approach is used, features outside the reduced FOV will alias into the reduced‐FOV image along the phase‐encoding direction. Reduced‐FOV methods are designed to correct this aliasing problem. In the present study, we propose a combination of two different approaches to reduce the acquired FOV: 1) two‐dimensional (2D) spatially‐selective RF excitation, and 2) the unaliasing by Fourier‐encoding the overlaps using the temporal dimension (UNFOLD) technique. While 2D spatially‐selective RF excitation can restrict the spins excited within a reduced FOV, the UNFOLD technique can help to eliminate any residual aliased signals and thus relaxes the requirement for a long RF excitation pulse. This hybrid method was implemented for MR‐based temperature mapping, and resulted in artifact‐free images with a fourfold improvement in temporal resolution. Magn Reson Med 53:1118–1125, 2005.

Multipathway sequences for MR thermometry

MR‐based thermometry is a valuable adjunct to thermal ablation therapies as it helps to determine when lethal doses are reached at the target and whether surrounding tissues are safe from damage. When the targeted lesion is mobile, MR data can further be used for motion‐tracking purposes. The present work introduces pulse sequence modifications that enable significant improvements in terms of both temperature‐to‐noise‐ratio properties and target‐tracking abilities. Instead of sampling a single magnetization pathway as in typical MR thermometry sequences, the pulse‐sequence design introduced here involves sampling at least one additional pathway. Image reconstruction changes associated with the proposed sampling scheme are also described. The method was implemented on two commonly used MR thermometry sequences: the gradient‐echo and the interleaved echo‐planar imaging sequences. Data from the extra pathway enabled temperature‐to‐noise‐ratio improvements by up to 35%, without increasing scan time. Potentially of greater significance is that the sampled pathways featured very different contrast for blood vessels, facilitating their detection and use as internal landmarks for tracking purposes. Through improved temperature‐to‐noise‐ratio and lesion‐tracking abilities, the proposed pulse‐sequence design may facilitate the use of MR‐monitored thermal ablations as an effective treatment option even in mobile organs such as the liver and kidneys. Magn Reson Med, 2011.

New approach to 3D time-resolved angiography

TRICKS is an acquisition and reconstruction method capable of generating 3D time‐resolved angiograms. Arguably, the main problem with TRICKS is the way it handles the outer regions of the k‐space matrix, leading to artifacts at the edges of blood vessels. An alternative to the data‐ processing stage of TRICKS, designed to better represent edges and small vessels, is presented here. A weakness of the new approach is an increased sensitivity to motion compared to TRICKS. Since this method can use the same data as TRICKS, a hybrid reconstruction method could conceivably be developed where the advantages of both approaches are combined. Magn Reson Med 47:1022–1025, 2002.

A new way of averaging with applications to MRI.

Averaging is often used to increase the quality of an image degraded by noise or artifacts. A method is developed in which several degrees of freedom are introduced in the averaging process, this freedom making possible the choice of different weighting factors for different portions of the Fourier space. If a weighting factor is associated with each line of a magnetic resonance acquisition, we show that we obtain some freedom to eliminate motion artifacts. The process minimizes a quantity called the gradient energy over a region of interest in the image plane. A processed image is obtained from a mosaic of such regions of interest scanned over the whole image plane. The method is shown to yield greater motion artifact suppression in magnetic resonance images than that achieved with regular averaging. The main strength of the method is probably its ability to diminish the intensity of unstructured artifacts which are usually poorly managed by other postprocessing methods of artifacts suppression.