Integration of accelerated MRI and post-processing software: a promising method for studies of knee osteoarthritis.

OBJECTIVE: Magnetic resonance imaging (MRI) is a widely used imaging modality for studies of knee osteoarthritis (OA). Compared to radiography, MRI offers exceptional soft tissue imaging and true three-dimensional (3D) visualization. However, MRI is expensive both due to the cost of acquisition and evaluation of the images. The goal of our study is to develop a new method to address the cost of MRI by combining innovative acquisition methods and automated post-processing software. METHODS: Ten healthy volunteers were scanned with three different MRI protocols: A standard 3D dual-echo steady state (DESS) pulse sequence, an accelerated DESS (DESSAcc), acquired at approximately half the time compared to DESS, and a multi-echo time DESS (DESSMTE), which is capable of producing measurements of T2 relaxation time. A software tool was used to measure cartilage volume. Accuracy was quantified by comparing DESS to DESSAcc and DESSMTE and precision was measured using repeat readings and acquisitions. T2 precision was determined using duplicate DESSMTE acquisitions. Intra-class correlation coefficients (ICCs), root-mean square standard deviation (RMSSD), and the coefficient of variation (CoV) were used to quantify accuracy and precision. RESULTS: The accuracies of DESSAcc and DESSMTE were CoV = 3.7% and CoV = 6.6% respectively, while precision was 3.8%, 3.0%, and 3.1% for DESS, DESSAcc and DESSMTE. T2 repositioning precision was 5.8%. CONCLUSION: The results demonstrate that accurate and precise quantification of cartilage volume is possible using a combination of substantially faster MRI acquisition and post-processing software. Precise measurements of cartilage T2 and volume can be made using the same acquisition.

Computer simulations of the belousov-zhabotinsky reaction.

Morphological features of the two-dimensional Belousov-Zhabotinsky reaction were modeled with an algorithm involving only two simple parameters, one describing the productivity of the reaction on a local scale length and the other characterizing the delay or quiescent time after the localized reaction. Self-organizing wavelike structures, including single-and multiarmed spirals, were most easily generated.

Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave source.

On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.

Electromagnetic evidence that SSS17a is the result of a binary neutron star merger.

Eleven hours after the detection of gravitational wave source GW170817 by the Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers, an associated optical transient, SSS17a, was identified in the galaxy NGC 4993. Although the gravitational wave data indicate that GW170817 is consistent with the merger of two compact objects, the electromagnetic observations provide independent constraints on the nature of that system. We synthesize the optical to near-infrared photometry and spectroscopy of SSS17a collected by the One-Meter Two-Hemisphere collaboration, finding that SSS17a is unlike other known transients. The source is best described by theoretical models of a kilonova consisting of radioactive elements produced by rapid neutron capture (the r-process). We conclude that SSS17a was the result of a binary neutron star merger, reinforcing the gravitational wave result.

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger.

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after the merger. Over the first hour of observations, the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measured the photosphere cooling from [Formula: see text] to [Formula: see text] kelvin, and determined a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a began displaying broad features after 1.46 days and evolved qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high-velocity, lanthanide-free material.

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis.

On 17 August 2017, gravitational waves (GWs) were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB 170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days postmerger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in rapid neutron capture (r-process) nucleosynthesis in the universe.

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.

Velocity encoding using both phase and magnitude.

Imaging time constitutes a major limitation of phase-contrast (PC) angiography. It is possibly the main disadvantage of PC methods over the time-of-flight (TOF) methods that actually are used clinically. This relatively long imaging time comes from the fact that conventional PC methods require the acquisition of at least four images with different velocity sensitization to reconstruct a single angiogram (1, 2). However, more than one-half of the information gathered through the acquisition of these four images is either redundant or simply discarded. We propose a faster approach to making PC angiograms in which the quantity of data acquired is diminished by as much as a factor 2. This is made possible by encoding velocity information in both the phase and magnitude of the image. Due to the use of extra radiofrequency (RF) and gradient waveforms, decreases in data requirements do not translate in a direct manner into decreases in imaging time. Nevertheless, significant reductions in imaging time are achieved with the present approach.

Velocity encoding using ghost artifacts.

Motion artifacts represent a significant limitation of MRI, and an ideal solution to that problem has proved elusive. However, in this paper, motion artifacts are not considered as the usual enemy and are not suppressed; on the contrary, they are deliberately created to encode flow information. In MRI, velocity is encoded readily into the phase of a pixel. However, if the pixel contains overlapping signals, the phase of one of these signals now has consequences on both the magnitude and phase of the resulting pixel. It is shown here that an overlap of information may be used to encode velocity both in the phase and in the magnitude of an image, making the velocity-encoding process faster. The overlap of information is obtained by superposing ghosting artifacts of different orders and information is retrieved about complex intensity and velocity in two dimensions using the equivalent of two images instead of the usual three images. The price to pay to do so is some loss of simplicity in the equations involved, an increase in reconstruction computing time requirements, and a factor of 4 decrease in signal-to-noise ratio in the velocity measurements.

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.

Motion artifacts in fast spin-echo imaging.

The purpose of this work is to obtain a better understanding of motion artifacts in fast spin-echo imaging, in order to eventually identify efficient ways of suppressing them. To do so, the point spread function of a moving point was calculated for fast spin-echo imaging, and experimental data were acquired by imaging a moving liquid sphere with a diameter of 1.5 mm. The agreement of the experimental results with the calculated point spread function is shown to be excellent. It was found that motion artifacts in fast spin-echo imaging arise from the convolution of two distinct band patterns. One of these patterns may dominate the convolution, giving its own spacing to the resulting image. For other acquisition parameters, the convolution results in an intricate pattern that may appear to lack overall structure.

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. Results are presented for cardiac-triggered imaging and functional MRI (fMRI), and are compared with data obtained in a conventional way. The temporal resolution was increased by nearly a factor of two in the cardiac-triggered study, and by as much as a factor of eight in the fMRI study. This increase allowed the acquisition of fMRI activation maps, even when the acquisition time for a single full time frame was actually longer than the paradigm cycle period itself. The new method can be used to significantly reduce the acquisition time of the individual temporal frames in certain dynamic studies. This can be used, for example, to increase the temporal or spatial resolution, increase the spatial coverage, decrease the total imaging time, or alter sequence parameters e.g., repetition time (TR) and echo time (TE) and thereby alter contrast. Magn Reson Med 42:813-828, 1999.

A reduced field-of-view method to increase temporal resolution or reduce scan time in cine MRI.

In some dynamic applications of MRI, only a part of the field-of-view (FOV) actually undergoes dynamic changes. A class of methods, called reduced-FOV (rFOV) methods, convert the knowledge that some part of the FOV is static or not very dynamic into an increase in temporal resolution for the dynamic part, or into a reduction in the scan time. Although cardiac imaging is an important example of an imaging situation where changes are concentrated in a fraction of the FOV, the rFOV methods developed up to now are not compatible with one of the most common cardiac sequences, the so-called retrospective cine method. The present work is a rFOV method designed to be compatible with cine imaging. An increase by a factor n in temporal resolution or a decrease by n in scan time is obtained in the case where only one nth of the FOV is dynamic (the rest being considered static). Results are presented for both Cartesian and spiral imaging.

A distance to the galaxy NGC4258 from observations of Cepheid variable stars.

Cepheid variable stars pulsate in a way that is correlated with their intrinsic luminosity, making them useful as 'standard candles' for determining distances to galaxies; the potential systematic uncertainties in the resulting distances have been estimated to be only 8-10%. They have played a crucial role in establishing the extragalactic distance scale and hence the value of the Hubble constant. Here we report observations of Cepheids in the nearby galaxy NGC4258; the distance calculated from the Cepheids is 8.1 +/- 0.4 Mpc, where the uncertainty does not include possible systematic errors. There is an independently determined geometric distance to this galaxy of 7.2 +/- 0.5 Mpc, based on the observed proper motions of water masers orbiting the central black hole; the distances differ by 1.3sigma. If the maser-based distance is adopted and the Cepheid distance scale revised accordingly, the derived value of the Hubble constant would increase by 12 +/- 9%, while the expansion age of the Universe would decrease by the same amount.