Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact.

The fast acquisition interleaved spin-echo (FAISE) method is a partial RF echo-planar technique which utilizes a specific phase-encode reordering algorithm to manipulate image contrast (Melki et al., J. Magn. Reson. Imaging 1:319, 1991). The technique can generate "spin-echo" like images up to 16 times faster than conventional spin-echo methods. However, the presence of T2 decay throughout the variable k-space trajectories used to manipulate T2 contrast ensures the presence of image artifacts, especially along the phase-encode direction. In this work, we experimentally and theoretically examine the type and extent of artifacts associated with the FAISE technique. We demonstrate the existence of well-defined minima of phase-encode ghost noise for selected k-space trajectories, examine the extent of blurring and edge enhancement artifacts, demonstrate the influence of matrix size and number of echoes per train on phase-encode artifact, and show how proper choice of FAISE sequence parameters can lead to proton density brain images which are practically indistinguishable from conventional spin-echo proton density images. A comparison of contrast between FAISE and standard spin-echo methods is presented in a companion article referred to as II.     

Sampling and reconstruction effects due to motion in diffusion-weighted interleaved echo planar imaging.

Subject motion during diffusion-weighted interleaved echo-planar imaging causes k-space offsets which lead to irregular sampling in the phase-encode direction. For each image, the k-space shifts are monitored using 2D navigator echoes, and are shown to lead to a frequent violation of the Nyquist condition when an ungated sequence is used on seven subjects. Combining data from four repeat acquisitions allows the Nyquist condition to be satisfied in all but 1% of images. Reconstruction of the irregularly-sampled data can be performed using a matrix inversion technique. The repeated acquisitions make the inversion more stable and additionally improve the signal-to-noise ratio. The resultant isotropic diffusion-weighted images and average apparent diffusion coefficient (ADC) maps show high resolution and enable clear localization of a stroke lesion. Residual ADC artifacts with a slow spatial variation are observed and assumed to originate from non-rigid pulsatile brain motion. Magn Reson Med 44:101-109, 2000.     

Dual-Echo Interleaved Echo-Planar Imaging of the Brain

An interleaved echo-planar imaging (EPI) technique is described that provides images from 20 sections of the brain at two echo times (27 and 84 ms) in 1:05. Six echoes per image per repetition are collected in 24 repetitions of the pulse sequence. MR images of the brain obtained from five volunteers using the dual-echo EPI sequence, fast spin-echo (FSE), and conventional dual-echo spin-echo were evaluated qualitatively for diagnostic use and quantitatively for relative signal-to-noise ratio (SNR), contrast, and contrast-to-noise ratios (CNR).     

Decoupled automated rotational and translational registration for functional MRI time series data: The dart registration algorithm

A rapid, in-plane image registration algorithm that accurately estimates and corrects for rotational and translational motion is described. This automated, one-pass method achieves its computational efficiency by decoupling the estimation of rotation and translation, allowing the application of rapid cross-correlation and cross-spectrum techniques for the determination of displacement parameters. k-space regridding and modulation techniques are used for image correction as alternatives to linear interpolation. The performance of this method was analyzed with simulations and echo-planar image data from both phantoms and human subjects. The processing time for image registration on a Hewlett-Packard 735/125 is 7.5 s for a 128 × 128 pixel image and 1.7 s for a 64 × 64 pixel image. Imaging phantom data demonstrate the accuracy of the method (mean rotational error, ?0.09°; standard deviation = 0.17°; range, ?0.44° to + 0.31°; mean translational error = ?0.035 pixels; standard deviation = 0.054 pixels; range, ?0.16 to + 0.06 pixels). Registered human functional imaging data demonstrate a significant reduction in motion artifacts such as linear trends in pixel time series and activation artifacts due to stimulus-correlated motion. The advantages of this technique are its noniterative one-pass nature, the reduction in image degradation as compared to previous methods, and the speed of computation.     

Chemical-shift imaging: a hybrid approach

The hybrid technique of projection-reconstruction echo-planar (PREP) imaging for obtaining chemical-shift images is demonstrated experimentally using a fluorine sample. The technique which is a variation on echo-planar imaging (EPI) relies on a multipass procedure. It is nevertheless quite efficient and consequently chemical-shift images may be produced in a few minutes. The method produces images in 64 chemical-shift regions, each region mapped spatially by 64 X 64 pixels. The imaging time was just over 4 min. These 64 chemical-shifted images can be straightforwardly added together to form an undistorted image of the complete object. In addition the chemical-shift spectrum can be extracted and the various chemical-shift images can be unambiguously assigned to the spectral peaks.     

Principles and applications of echo-planar imaging: a review for the general radiologist.

Echo-planar imaging is a very fast magnetic resonance (MR) imaging technique capable of acquiring an entire MR image in only a fraction of a second. In single-shot echo-planar imaging, all the spatial-encoding data of an image can be obtained after a single radio-frequency excitation. Multishot echo-planar imaging results in high-quality images comparable to conventional MR images. However, echo-planar imaging offers major advantages over conventional MR imaging, including reduced imaging time, decreased motion artifact, and the ability to image rapid physiologic processes of the human body. The use of echo-planar imaging has already resulted in significant advances in clinical diagnosis and scientific investigation, such as in evaluation of stroke and functional imaging of the human brain, respectively. The clinical indications for echo-planar imaging are expanding rapidly, and it can now be applied to many parts of the body, including the brain, abdomen, and heart. Today, with the availability of echo-planar imaging-capable MR imagers at many sites, the general radiologist can benefit from echo-planar imaging and its clinical applications.     

Large field-of-view real-time MRI with a 32-channel system.

The emergence of parallel MRI techniques and new applications for real-time interactive MRI underscores the need to evaluate performance gained by increasing the capability of MRI phased-array systems beyond the standard four to eight high-bandwidth channels. Therefore, to explore the advantages of highly parallel MRI a 32-channel 1.5 T MRI system and 32-element torso phased arrays were designed and constructed for real-time interactive MRI. The system was assembled from multiple synchronized scanner-receiver subsystems. Software was developed to coordinate across subsystems the real-time acquisition, reconstruction, and display of 32-channel images. Real-time, large field-of-view (FOV) body-survey imaging was performed using interleaved echo-planar and single-shot fast-spin-echo pulse sequences. A new method is demonstrated for augmenting parallel image acquisition by independently offsetting the frequency of different array elements (FASSET) to variably shift their FOV. When combined with conventional parallel imaging techniques, image acceleration factors of up to 4 were investigated. The use of a large number of coils allowed the FOV to be doubled in two dimensions during rapid imaging, with no degradation of imaging time or spatial resolution. The system provides a platform for evaluating the applications of many-channel real-time MRI, and for understanding the factors that optimize the choice of array size.     

Clinical multishot DW-EPI through parallel imaging with considerations of susceptibility, motion, and noise.

Geometric distortions and poor image resolution are well known shortcomings of single-shot echo-planar imaging (ss-EPI). Yet, due to the motion immunity of ss-EPI, it remains the most common sequence for diffusion-weighted imaging (DWI). Moreover, both navigated DW interleaved EPI (iEPI) and parallel imaging (PI) methods, such as sensitivity encoding (SENSE) and generalized autocalibrating parallel acquisitions (GRAPPA), can improve the image quality in EPI. In this work, DW-EPI accelerated by PI is proposed as a self-calibrated and unnavigated form of interleaved acquisition. The PI calibration is performed on the b = 0 s/mm2 data and applied to each shot in the rest of the DW data set, followed by magnitude averaging. Central in this study is the comparison of GRAPPA and SENSE in the presence of off-resonances and motion. The results show that GRAPPA is more robust than SENSE against both off-resonance and motion-related artifacts. The SNR efficiency was also investigated, and it is shown that the SNR/scan time ratio is equally high for one- to three-shot high-resolution diffusion scans due to the shortened EPI readout train length. The image quality improvements without SNR efficiency loss, together with motion tolerance, make the GRAPPA-driven DW-EPI sequence clinically attractive.     

Improved image registration by using fourier interpolation

This paper presents a technique for performing two-dimensional rigid-body image registration for functional magnetic resonance images (fMRI). The method provides accurate motion correction without local distortion. The approach is to perform the translation and rotation in the Fourier domain. For images sampled on a grid, such as in echo-planar imaging (EPI), one potential stumbling block to this approach is the computational burden of reconstruction, since the rotated image will no longer be on the Cartesian grid. A method of approximating rotations via local translations (shearing) is presented, which keeps the data on the Cartesian grid. This can provide quite accurate approximations with only a moderate amount of computation. A mean squared error (MSE) criterion is used for determining the registration parameters. This method is tested on several sets of simulated images and shown to have an accuracy ranging from 0.02 to 0.3 pixels for images with SNRs ranging from 100 to 10, respectively. They techniques have been tested on several sets of images. They are shown to work well on real subjects, for both echo-planar and spiral data acquisition schemes. The techniques are used in an activation study in which the subject moved his head during image collection. After use of this registration technique, the activation is easily detected.     

Importance of k-space trajectory in echo-planar myocardial tagging at rest and during dobutamine stress.

Hybrid fast gradient echo/echo-planar imaging (FGRE-EPI) can be used to increase temporal resolution, enhance tag contrast, and/or decrease scan time for breathhold myocardial tagging. However, off-resonance effects and motion can lead to local phase discontinuities in FGRE-EPI raw data when a conventional interleaved bottom-up k-space trajectory is used. These discontinuities can be particularly problematic for myocardial tagging, where the image energy is not only concentrated near the k-space origin, but is also concentrated in multiple spectral peaks centered throughout k-space. In this study, tag distortion artifacts in FGRE-EPI tagging due to off-resonance and velocity-induced phase discontinuities were characterized at rest and dobutamine stress, and the flyback and gradient moment smoothing (GMS) methods were shown to reduce these artifacts. For the specific parameters used in this study, flyback and GMS resulted in improved image quality at rest and stress, increased myocardium-tag contrast-to-noise ratio (11.4 +/- 2.1 vs. 10.0 +/- 2.9, P < 0.01 at rest; 11.1 +/- 1.8 vs. 8.1 +/- 2.4, P < 0.01 at stress), and reduced full width at half maximum of the tag profile (3.6 vs. 3.8 pixels at rest; 4.0 vs. 5.1 pixels at stress) compared to the conventional trajectory. A limitation of the improved trajectory is a parameter-dependent decrease in data acquisition efficiency. For the specific imaging protocol used, the repetition time of the improved trajectory increased by 36% compared to the conventional trajectory.     

Optimization of spiral‐based pulse sequences for first‐pass myocardial perfusion imaging

Although spiral trajectories have multiple attractive features such as their isotropic resolution, acquisition efficiency, and robustness to motion, there has been limited application of these techniques to first‐pass perfusion imaging because of potential off‐resonance and inconsistent data artifacts. Spiral trajectories may also be less sensitive to dark‐rim artifacts that are caused, at least in part, by cardiac motion. By careful consideration of the spiral trajectory readout duration, flip angle strategy, and image reconstruction strategy, spiral artifacts can be abated to create high‐quality first‐pass myocardial perfusion images with high signal‐to‐noise ratio. The goal of this article was to design interleaved spiral pulse sequences for first‐pass myocardial perfusion imaging and to evaluate them clinically for image quality and the presence of dark‐rim, blurring, and dropout artifacts. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Magnetic field shift due to mechanical vibration in functional magnetic resonance imaging.

Mechanical vibrations of the gradient coil system during readout in echo-planar imaging (EPI) can increase the temperature of the gradient system and alter the magnetic field distribution during functional magnetic resonance imaging (fMRI). This effect is enhanced by resonant modes of vibrations and results in apparent motion along the phase encoding direction in fMRI studies. The magnetic field drift was quantified during EPI by monitoring the resonance frequency interleaved with the EPI acquisition, and a novel method is proposed to correct the apparent motion. The knowledge on the frequency drift over time was used to correct the phase of the k-space EPI dataset. Since the resonance frequency changes very slowly over time, two measurements of the resonance frequency, immediately before and after the EPI acquisition, are sufficient to remove the field drift effects from fMRI time series. The frequency drift correction method was tested "in vivo" and compared to the standard image realignment method. The proposed method efficiently corrects spurious motion due to magnetic field drifts during fMRI.     

Three-Dimensional Spectroscopic Imaging with Time-Varying Gradients

A spectroscopic imaging sequence with a time-varying readout gradient in the slice selection direction is used to image multiple contiguous slices. For a given voxel size, the imaging time and signal-to-noise ratio of the three-dimensional spectroscopic sequence are the same as for a single slice acquisition without the oscillating readout gradient. The data reconstruction employs a gridding algorithm in two dimensions to interpolate the nonuniformly sampled data onto a Cartesian grid, and a fast Fourier transform in four dimensions: three spatial dimensions and the spectral dimension. The method is demonstrated by in vivo imaging of NAA in human brain at 1.5 T with 10 slices of 16 x 16 pixels spectroscopic images acquired in a total scan time of 17 min.     

High temporal resolution phase contrast MRI with multiecho acquisitions.

Velocity imaging with phase contrast (PC) MRI is a noninvasive tool for quantitative blood flow measurement in vivo. A shortcoming of conventional PC imaging is the reduction in temporal resolution as compared to the corresponding magnitude imaging. For the measurement of velocity in a single direction, the temporal resolution is halved because one must acquire two differentially flow-encoded images for every PC image frame to subtract out non-velocity-related image phase information. In this study, a high temporal resolution PC technique which retains both the spatial resolution and breath-hold length of conventional magnitude imaging is presented. Improvement by a factor of 2 in the temporal resolution was achieved by acquiring the differentially flow-encoded images in separate breath-holds rather than interleaved within a single breath-hold. Additionally, a multiecho readout was incorporated into the PC experiment to acquire more views per unit time than is possible with the single gradient-echo technique. A total improvement in temporal resolution by approximately 5 times over conventional PC imaging was achieved. A complete set of images containing velocity data in all three directions was acquired in four breath-holds, with a temporal resolution of 11.2 ms and an in-plane spatial resolution of 2 mm x 2 mm.     

Diffusion-weighted interleaved echo-planar imaging with a pair of orthogonal navigator echoes

This work describes a diffusion-weighted (DW) interleaved echo-planar imaging (IEPI) method for use on either conventional whole-body scanners or scanners equipped with highspeed gradient and receiver hardware. In combination with cardiac gating and motion correction with a pair of orthogonal navigator echoes, the presented method is time-efficient, compensates for patient motions, and is less sensitive to image distortions than single-shot methods. The motion-correction scheme consists of correction for constant and linear phase terms found from the orthogonal navigator echoes. The correction for the linear phase term in the phase-encode direction includes gridding the data to the Cartesian grid. The DW IEPI was used to image a phantom rotating about the slice-select direction, and motion correction was performed to eliminate ghost artifacts arising from motion in either the readout- or phase-encoding directions. DW IEPI with motion correction was performed on a normal volunteer and on a patient with a 26-day-old region of ischemia over much of the right hemisphere.     

Centric ordering is superior to gradient moment nulling for motion artifact reduction in EPI

Echo-planar imaging (EPI) is sensitive to motion despite its rapid data acquisition rate. Compared with traditional imaging techniques, it is more sensitive to motion or flow in the phase-encode direction, which can cause image artifacts such as ghosting, misregistration, and loss of spatial resolution. Consequently, EPI of dynamic structures (eg, the cardiovascular system) could benefit from methods that eliminate these artifacts. In this paper, two methods of artifact reduction for motion in the phase-encode direction are evaluated. First, the k-space trajectory is evaluated by comparing centric with top-down ordered sequences. Next, velocity gradient moment nulling (GMN) of the phase-encode direction is evaluated for each trajectory. Computer simulations and experiments in flow phantoms and rabbits in vivo show that uncompensated centric ordering produces the highest image quality. This is probably due to a shorter readout duration, which reduces T2* relaxation losses and off-resonance effects, and to the linear geometry of phantoms and vessels, which can obscure centric blurring artifacts.     

Comparison of breath-hold multishot echo-planar and respiratory triggered fast-spin-echo sequences for T2-weighted MRI of liver lesions

The purpose of this study was to evaluate the usefulness of multishot echo-planar imaging in detecting liver tumors in comparison with respiratory triggered T2-weighted fast-spin-echo (FSE) imaging. Thirty-two patients with 70 focal liver lesions were imaged using a 1.5-T high speed MR imager. Eight-shot echo-planar images covering the whole liver were acquired during a single breath-hold period. FSE images were acquired with respiratory triggering in approximately 4 minutes. Lesion detectability and image quality of the two pulse sequences were analyzed qualitatively. Quantitative analysis was performed by means of signal-to-noise and tumor-liver contrast-to-noise analysis. Lesion detectability was comparable in both solid (86.3% vs 90.2%: .3 < P < .5) and nonsolid lesions (89.5% vs 100%: .3 < P < .5) between echo-planar and FSE images. Echo-planar imaging provided significantly reduced image artifact, better lesion conspicuity, and anatomic detail compared with FSE imaging. The signal-to-noise and contrast-to-noise ratios of echo-planar images were significantly higher than those of FSE images. Breath-hold eight-shot echo-planar imaging can be an alternative to T2-weighted FSE imaging because it can provide comparable image quality in a substantially decreased acquisition time.     

Diffusion-tensor MR imaging of the human brain with gradient- and spin-echo readout: technical note

Diffusion-tensor MR imaging of the brain is an objective method that can measure diffusion of water in tissue noninvasively. Five adult volunteers participated in this study that was performed to evaluate the potential of gradient- and spin-echo readout for diffusion-tensor imaging by comparing it with single-shot spin-echo echo-planar imaging. Gradient- and spin-echo readout provides comparable measures of water diffusion to single-shot spin-echo echo-planar readout with significantly less geometrical distortion at the expense of a longer imaging time.     

Reducing artifacts in one‐dimensional Fourier velocity encoding for fast and pulsatile flow

When evaluating the severity of valvular stenosis, the peak velocity of the blood flow is routinely used to estimate the transvalvular pressure gradient. One‐dimensional Fourier velocity encoding effectively detects the peak velocity with an ungated time series of spatially resolved velocity spectra in real time. However, measurement accuracy can be degraded by the pulsatile and turbulent nature of stenotic flow and the existence of spatially varying off‐resonance. In this work, we investigate the feasibility of improving the peak velocity detection capability of one‐dimensional Fourier velocity encoding for stenotic flow using a novel echo‐shifted interleaved readout combined with a variable‐density circular k‐space trajectory. The shorter echo and readout times of the echo‐shifted interleaved acquisitions are designed to reduce sensitivity to off‐resonance. Preliminary results from limited phantom and in vivo results also indicate that some artifacts from pulsatile flow appear to be suppressed when using this trajectory compared to conventional single‐shot readouts, suggesting that peak velocity detection may be improved. The efficiency of the new trajectory improves the temporal and spatial resolutions. To realize the proposed readout, a novel multipoint‐traversing algorithm is introduced for flexible and automated gradient‐waveform design. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Development of counting method of black spots caused by the radioactive contaminations on the imaging plate

Due to accidents of the nuclear power plants in Fukushima prefecture, a lot of radioisotopes were diffused into the environment. They adhered onto the surface of the X-ray detector (imaging plate; IP) and many black spots were seen on the medical images. The process to count them is important to evaluate the degree of contamination and/or removal. In this study, we aimed to develop a counting method for black spots. Based on the analysis of the medical images having black spots, we summarized that areas affected by the certain black spots were limited to the eight pixels surrounding the most intensive pixel. The newly developed counting method was applied to these nine pixels (3×3 pixels) and selection rules were based on the following two information: 1. differences between the digital value of the most intensive pixel and those of the surrounding eight pixels, and 2. total summation of the digital values in the nine pixels. The estimated image based on our method showed a good concordance with the original image. Therefore, we summarized that our counting method is a powerful tool for estimating numbers of black spots.