Phase Correction Procedure Research Articles

Highly accelerated subtractive femoral non-contrast-enhanced MRA using compressed sensing with k-space subtraction, phase and intensity correction.

To develop an improved reconstruction method, k-space subtraction with phase and intensity correction (KSPIC), for highly accelerated, subtractive, non-contrast-enhanced MRA. The KSPIC method is based on k-space subtraction of complex raw data. It applies a phase-correction procedure to restore the polarity of negative signals caused by subtraction and an intensity-correction procedure to improve background suppression and thereby sparsity. Ten retrospectively undersampled data sets and 10 groups of prospectively undersampled data sets were acquired in 12 healthy volunteers. The performance of KSPIC was compared with another improved reconstruction based on combined magnitude subtraction, as well as with conventional k-space subtraction reconstruction and magnitude subtraction reconstruction, both using quantitative metrics and using subjective quality scoring. In the quantitative evaluation, KSPIC had the best performance in terms of peak SNR, structural similarity index measure, contrast-to-noise ratio of artery-to-background and sharpness, especially at high acceleration factors. The KSPIC method also had the highest subjective scores for all acceleration factors in terms of vessel delineation, image noise and artifact, and background contamination. The acquisition can be accelerated by a factor of 20 without significant decreases of subjective scores. The optimal size of the phase-correction region was found to be 12-20 pixels in this study. Compared with combined magnitude subtraction and conventional reconstructions, KSPIC has the best performance in all of the quantitative and qualitative measurements, permitting good image quality to be maintained up to higher accelerations. The KSPIC method has the potential to further reduce the acquisition time of subtractive MRA forclinicalexaminations.

CLUSIM: A Synthetic Aperture Radar Clutter Simulator for Planetary Exploration

AbstractIn this paper we present the CLUtter SIMulator (CLUSIM), a special program simulating radar side echoes from rough planetary surfaces using realistic topography data sets. A numerical model of realistic topography of the Martian surface, based on Mars Orbiter Laser Altimeter data, is developed. A specially developed computer routine for evaluation of wideband radar echoes reflected from rough surfaces, capable of aperture synthesis simulation, is described. A synthetic radargram for a portion of Mars Express (MEX) orbit 9466 is computed and validated against experimental data obtained by the MARSIS radar instrument. Finally, a previously developed ionospheric phase correction procedure is numerically tested with new simulated echo signals. Impact of the surface clutter on the ionospheric correction procedure is investigated with a direct numerical comparison to a known benchmark result, which shows robustness of the correction algorithm with respect to the surface clutter.

Joint correction of Nyquist artifact and minuscule motion-induced aliasing artifact in interleaved diffusion weighted EPI data using a composite two-dimensional phase correction procedure

Diffusion-weighted imaging (DWI) obtained with interleaved echo-planar imaging (EPI) pulse sequence has great potential of characterizing brain tissue properties at high spatial-resolution. However, interleaved EPI based DWI data may be corrupted by various types of aliasing artifacts. First, inconsistencies in k-space data obtained with opposite readout gradient polarities result in Nyquist artifact, which is usually reduced with 1D phase correction in post-processing. When there exist eddy current cross terms (e.g., in oblique-plane EPI), 2D phase correction is needed to effectively reduce Nyquist artifact. Second, minuscule motion induced phase inconsistencies in interleaved DWI scans result in image-domain aliasing artifact, which can be removed with reconstruction procedures that take shot-to-shot phase variations into consideration. In existing interleaved DWI reconstruction procedures, Nyquist artifact and minuscule motion-induced aliasing artifact are typically removed subsequently in two stages. Although the two-stage phase correction generally performs well for non-oblique plane EPI data obtained from well-calibrated system, the residual artifacts may still be pronounced in oblique-plane EPI data or when there exist eddy current cross terms. To address this challenge, here we report a new composite 2D phase correction procedure, which effective removes Nyquist artifact and minuscule motion induced aliasing artifact jointly in a single step. Our experimental results demonstrate that the new 2D phase correction method can much more effectively reduce artifacts in interleaved EPI based DWI data as compared with the existing two-stage artifact correction procedures. The new method robustly enables high-resolution DWI, and should prove highly valuable for clinical uses and research studies of DWI.

Dual spectral-band interferometry for spatio-temporal characterization of high-power femtosecond lasers.

We present and demonstrate a technique called RED-SEA TADPOLE for the spatio-temporal characterization of high peak power femtosecond lasers. It retains the basic principle of an existing method, where a scanning monomode fiber is utilized in an interferometric scheme to measure the spectral amplitude and phase at all points across an ultrashort laser beam. We combine this approach with dual spectral-band interferometry, to correct for all phase errors occurring in this interferometer, thus allowing for the simultaneous measurement of the beam wavefront and pulse front in a collimated beam of large diameter. The generic phase correction procedure implemented here can also be extended to other fiber optic device applications sensitive to phase fluctuations.

Measuring absorptive two-dimensional infrared spectra using chirped-pulse upconversion detection

Chirped-pulse upconversion (CPU) enables the detection of mid-infrared spectra using a silicon CCD camera by sum-frequency mixing a mid-IR field with a highly chirped near-IR (800 nm) field. Although the substantial chirp limits the spectral broadening and phase distortions caused by the inherent cross-phase modulation, the exquisite phase fidelity needed to measure a fully absorptive two-dimensional IR spectrum demands the correction of the phase distortions. We demonstrate how to correct all of the phase distortions involved in recording an absorptive 2DIR spectrum including delay stage calibrations as well as the requisite nonlinear signals. Besides the extra experimental step of the upconversion process itself, the phase correction procedure requires only the knowledge of the spectral or temporal phase of the chirped pulse. The method is demonstrated in a metal carbonyl complex, Mn2(CO)10, in n-hexane and methanol solvents showing that the method operates well in cases of both homogeneous and inhomogeneous broadening.

Refined structure of afwillite from the northern Baikal region

The crystal structure of the mineral afwillite [Ca12(H2O)8][SiO4]4[SiO2(OH)2]4 from the northern Baikal region was refined in a triclinic unit cell with the following parameters: a = 16.330(2) A, b = 5.6389(6) A, c = 11.685(1) A, α = 90.08(1)°, β = 126.446(2)°, γ = 89.95(1)°, and sp. gr. P1. The triclinic unit cell is related to the monoclinic unit cell determined earlier (sp. gr. Cc, a = 16.278(1) A, b = 5.6321(4) A, c = 13.236(1) A, and β = 134.898(3)°) by the transformation matrix [−100/010/101]. The structural model was determined using the phase-correction procedure and refined anisotropically to R = 2.8% based on 3270 independent reflections with F > 3σ(F). The crystal-chemical formula of afwillite was revised. The characteristic features of the IR spectrum of afwillite were explained based on the structural data.

Focal beam distortion and treatment planning in abdominal focused ultrasound surgery

Recent clinical trials show promising results in using MRI and MRI-based thermometry to guide focused ultrasound surgery to treat uterine fibroids. During treatment, large variation in the focal temperature distribution has been observed. It is possible that some of this variation is due to abdominal tissue inhomogeneity, which might be causing focal beam distortion, and might largely decrease the focusing ability in deep-seated tissues. The purpose of this study was to numerically demonstrate this effect and also show the feasibility of restoring the focal beam patterns by employing the phase correction procedure for phased arrays. Abdominal MR data from four uterine fibroid patients were obtained to reconstruct the three-dimensional meshes of interfaces used in simulations, and one patient was selected to perform the analysis of key parameters in focused ultrasound surgery. Results show that, without phase correction, the focused beam can be severely distorted while using a frequency above 1 MHz or delivering ring-shape focal patterns. Different focal positions at the same depth may require a different power to induce the same ultrasonic intensity level (up to 179% among the different focal patterns). After adding a phase correction procedure, the distorted focal beams can be restored, and the peak intensity can be largely recovered (up to 85% among the different focal patterns). This study may offer important implications and information for treatment planning toward optimizing focused ultrasound surgery in uterine fibroid or other abdominal tumor treatments.

Phase Correction Procedure for the IMG Interferogram Data and Effects of the Phase Correction Error on the Retrieval Results

Phase Correction Procedure for the IMG Interferogram Data and Effects of the Phase Correction Error on the Retrieval Results

Conditions of measurement of infrared absorption band intensities by fourier transform spectrometry

The influence of infrared spectra of some different conditions required in the use of modern FTIR spectrometers has been studied. This work has been performed in two ways: firstly, the influence of factors such as apodization function, level of zero filling, number of data points and phase correction procedure is determined from experimental data, and, secondly, numerical simulation of the whole process has been carried out in order to separate experimental error from that deu to the mathematical treatment.

The performance of a double-beam fourier transform spectrometer and its application to the measurement of weak i.r. absorption

A double-beam, optically compensated, Fourier transform spectrometer is described. Its superiority in the investigation of weak spectral features has been demonstrated by the detection of 0.5 to ∼1% peak absorption in a narrow line due to small amounts of gaseous NH 3. Moreover, the instrument has been applied to measure the disorder induced phonon absorption of a thin amorphous Ge film. The phase correction procedure, which has been developed for the compensating mode, is discussed.

Measurement of Acoustic Impedance of Several Artificial Ears

The driving point acoustic impedance of five artificial ears was measured over the frequency range 100–7000 Hz. An acoustic impedance measuring device was constructed, and consisted of a small pistonphone, equipped with an accelerometer within the piston and a 18-in. insert microphone. Operational amplifier integration and summing, together with an XY recorder, allowed the continuous plotting of measured acoustic impedance as the frequency range was swept. Since acceleration and pressure were not measured at precisely the same point in the system, a computer program was used to perform the required phase correction and thus obtain true driving point impedance values. The impedance measuring device and phase correction procedure are described, and the results of the measurements are presented. Artificial ears included are the NBS-9A coupler, the ANSI type 1 coupler, the B&K type 4153 artificial ear, a TRACOR experimental coupler, and an artificial ear constructed for this study. Measured values of acoustic impedance are compared with design values.