GRASE imaging at 3 Tesla with template interactive phase–encoding

A new method for ordering the phase-encoding gradient is proposed, and an application for short effective TE gradient-and spin-echo (GRASE) imaging is demonstrated. The proposed method calculates the phase-encoding order from the signal decay of a template scan (hence “template interactive phase-encoding” or TIPE). Computer simulations are used to compare the point spread functions of different phase-encoding orders giving short effective echo times (kb centric GRASE, centric GRASE, centric TIPE). The conventional centric phase-encoding order is also considered for GRASE. The conventional centric method is sensitive to both amplitude and phase modulation of the signal in K-space. The centric TIPE method gives the least amplitude modulation artifacts but is vulnerable to phase artifacts. The TIPE experiment was implemented on a 3 Tesla system. To the best of our knowledge, we present the first in vivo GRASE images at this field strength.     

Magnetic resonance imaging of periosteum with ultrashort TE pulse sequences

PURPOSE: To assess the values of pulse sequences with ultrashort echo times (0.08 msec) for detecting and characterizing periosteum. MATERIALS AND METHODS: Two normal volunteers aged 33 and 58 years and 12 patients aged seven to 55 years were studied. A total of 10 of the patients had contrast enhancement with intravenous Gadodiamide. Two ovine tibias were examined before and after the periosteum was stripped from the bone. RESULTS: High signal regions were observed adjacent to cortical bone in all parts of the skeleton imaged. They were generally more conspicuous after fat suppression and contrast administration. In the ovine tibia there was a reduction in the high signal normally seen at the surface of the bone after periosteal stripping. The detached periosteum produced a high signal. Mean T(2)* values for adult human periosteum ranged from 5.3 to 11.4 msec. After enhancement the signal intensity increased. In two patients with tibial fractures, increased periosteal signal was seen and this showed marked enhancement. Signals from periosteum could be simulated by fat, contrast-enhanced blood and artifacts. CONCLUSION: The periosteum can be visualized with ultrashort echo time pulse sequences in health and disease.     

Body MRI artifacts in clinical practice: A physicist's and radiologist's perspective

The high information content of MRI exams brings with it unintended effects, which we call artifacts. The purpose of this review is to promote understanding of these artifacts, so they can be prevented or properly interpreted to optimize diagnostic effectiveness. We begin by addressing static magnetic field uniformity, which is essential for many techniques, such as fat saturation. Eddy currents, resulting from imperfect gradient pulses, are especially problematic for new techniques that depend on high performance gradient switching. Nonuniformity of the transmit radiofrequency system constitutes another source of artifacts, which are increasingly important as magnetic field strength increases. Defects in the receive portion of the radiofrequency system have become a more complex source of problems as the number of radiofrequency coils, and the sophistication of the analysis of their received signals, has increased. Unwanted signals and noise spikes have many causes, often manifesting as zipper or banding artifacts. These image alterations become particularly severe and complex when they are combined with aliasing effects. Aliasing is one of several phenomena addressed in our final section, on artifacts that derive from encoding the MR signals to produce images, also including those related to parallel imaging, chemical shift, motion, and image subtraction. J. Magn. Reson. Imaging 2013;38:269–287. © 2013 Wiley Periodicals, Inc.     

An alternative to GRASE: Toward spin-echo-like contrast with independent reconstruction of gradient-echo images

Multiple gradient echoes are generated for each RF echo of a Carr-Purcell-Meiboom-Gill (CPMG) train. Independently, phase-encoded fast spin-echo images are obtained from the different gradient echoes. Presently, three images are formed from three gradient-echoes from each of four RF echoes. The two peripheral gradient echo images are encoded for a late effective TE, then summed after reconstruction: this image has decreased fat intensity and increased susceptibility contrast compared with fast spin echo. The central gradient echoes yielded another image of intermediate contrast useful for neuroimaging. Raw data from the variously timed gradient echoes are not combined as they are in GRASE.     

Optimization of parameter values for complex pulse sequences by simulated annealing: Application to 3D MP-RAGE imaging of the brain

A number of pulse sequence techniques, including magnetization-prepared gradient echo (MP-GRE), segmented GRE, and hybrid RARE, employ a relatively large number of variable pulse sequence parameters and acquire the image data during a transient signal evolution. These sequences have recently been proposed and/or used for clinical applications in the brain, spine, liver, and coronary arteries. Thus, the need for a method of deriving optimal pulse sequence parameter values for this class of sequences now exists. Due to the complexity of these sequences, conventional optimization approaches, such as applying differential calculus to signal difference equations, are inadequate. We have developed a general framework for adapting the simulated annealing algorithm to pulse sequence parameter value optimization, and applied this framework to the specific case of optimizing the white matter-gray matter signal difference for a T₁-weighted variable flip angle 3D MP-RAGE sequence. Using our algorithm, the values of 35 sequence parameters, including the magnetization-preparation RF pulse flip angle and delay time, 32 flip angles in the variable flip angle gradient-echo acquisition sequence, and the magnetization recovery time, were derived. Optimized 3D MP-RAGE achieved up to a 130% increase in white matter-gray matter signal difference compared with optimized 3D RF-spoiled FLASH with the same total acquisition time. The simulated annealing approach was effective at deriving optimal parameter values for a specific 3D MP-RAGE imaging objective, and may be useful for other imaging objectives and sequences in this general class.     

FID-based lung MRI at 0.5 T: Theoretical considerations and practical implications

Pulse sequences based on FID signals and projection reconstruction (PR) were investigated for lung MRI at 0.5 T and evaluated for artifacts caused by: (1) k-space mismapping due to either delay or distortion of the readout gradient waveform, (2) cardiac motion and pulsatile flow, and (3) respiratory motion. Nonstructured artifacts were described, simulated, and experimentally confirmed for the first time. Nonstructured artifacts did not impair the demonstration of structures of high signal-to-noise ratio (SNR) but generated quantitative errors in the image intensity analysis over the lung parenchyma. The use of FID-based PR techniques for lung MRI is not justified at 0.5 T.     

Halving imaging time of whole brain diffusion spectrum imaging and diffusion tractography using simultaneous image refocusing in EPI

Purpose

To increase the efficiency of densely encoded diffusion imaging of the brain, such as diffusion spectrum imaging (DSI), we time‐multiplex multiple slices within the same readout using simultaneous image refocusing echo‐planar imaging (SIR‐EPI).

Materials and Methods

Inefficiency in total scan time results from the long time of diffusion encoding gradient pulses which must be repeated for each and every image. We present a highly efficient multiplexing method, simultaneous image refocusing (SIR), for reducing the total scan time of diffusion imaging by nearly one‐half. SIR DSI is performed in 10 minutes rather than 21 minutes, acceptable for routine clinical application.

Results

Two identical studies were completed, comparing conventional single‐slice EPI DSI and SIR‐EPI DSI, showing equal signal‐to‐noise ratio (SNR) and contrast and small differences in registration, likely due to typical subject motion. Comparison of DSI and DTI tractographs showed matching quality and detection of white matter tracts.

Conclusion

The net reduction to nearly half the number of diffusion encoding gradient pulses in SIR‐EPI significantly reduces acquisition times of DSI and DTI. J. Magn. Reson. Imaging 2009;29:517–522. © 2009 Wiley‐Liss, Inc.     

Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field.

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.     

Reduction of motion artifacts in cine MRI using variable-density spiral trajectories

Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signalto-noise ratio (SNR), single-shot imaging is not sufficient Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of k-space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable-density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable-density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.     

The diminishing variance algorithm for real-time reduction of motion artifacts in MRI

A technique has been developed whereby motion can be detected in real time during the acquisition of data. This enables the implementation of several algorithms to reduce or eliminate motion effects from an image as it is being acquired. One such algorithm previously described is the acceptance/rejection method. This paper deals with another real-time algorithm called the diminishing variance algorithm (DVA). With this method, a complete set of preliminary data is acquired along with information about the relative motion position of each frame of data. After all the preliminary data are acquired, the position information is used to determine which data frames are most corrupted by motion. Frames of data are then reacquired, starting with the most corrupted one. The position information is continually updated in an iterative process; therefore, each subsequent reacquisition is always done on the worst frame of data. The algorithm has been implemented on several different types of sequences. Preliminary in vivo studies indicate that motion artifacts are dramatically reduced.     

Imaging time reduction through multiple receiver coil data acquisition and image reconstruction

A technique is described for the simultaneous acquisition of MRI data using two independent receiver coils surrounding the same region of tissue, which enables the collection of data necessary for image reconstruction in a reduced number of phase-encoded acquisitions. This results in a 50% reduction in minimum scan time and may be useful in time-critical procedures. The algorithm and imaging procedures are described, and example images are shown that illustrate the reconstruction. Signal to noise is decreased by the square root of the time savings, making this technique applicable to cases in which the need to decrease minimum scan time outweighs the signal to noise penalty.     

Rapid hyperpolarized 3He diffusion MRI of healthy and emphysematous human lungs using an optimized interleaved-spiral pulse sequence

PURPOSE: To develop and validate an interleaved-spiral diffusion pulse sequence capable of hyperpolarized (3)He MR imaging of the whole lung in less than 10 seconds. MATERIALS AND METHODS: Hyperpolarized (3)He diffusion measurements were performed in seven healthy volunteers and five patients with emphysema using an interleaved-spiral pulse sequence that provided 11 contiguous 15-mm thick coronal ADC maps, with an in-plane resolution of 3.9 mm, covering the whole lung in 5.5 seconds. The resulting means and SDs of ADC values were compared statistically to those from a gradient-echo pulse sequence with identical resolution and diffusion-weighting gradients that acquired five ADC maps in 10.5 seconds. RESULTS: High-quality diffusion-weighted interleaved-spiral images covering the whole lung were obtained, and showed no significant susceptibility-induced image degradation compared to corresponding gradient-echo images. On a subject-by-subject basis, the means and SDs of ADC values for the interleaved-spiral technique were not statistically different from those for the gradient-echo technique. The mean ADC values from the two techniques were highly correlated on a section-by-section basis (R = 0.99). CONCLUSION: The interleaved-spiral diffusion pulse sequence permits rapid acquisition of contiguous ADC maps covering the whole lung during a short breath-hold period, and provides ADC values that are statistically equivalent to those from standard gradient-echo techniques.     

Minimization of dead-periods in MRI pulse sequences for imaging oblique planes

With the advent of breath-hold MR cardiac imaging techniques, the minimization of TR and TE for oblique planes has become a critical issue. The slew rates and maximum currents of gradient amplifiers limit the minimum possible TR and TE by adding dead-periods to the pulse sequences. We propose a method of designing gradient waveforms that will be applied to the amplifiers instead of the slice, readout, and phase encoding waveforms. Because this method ensures that the gradient amplifiers will always switch at their maximum slew rate, it results in the minimum possible dead-period for given imaging parameters and scan plane position. A GRASS pulse sequence has been designed and ultra-short TR and TE values have been obtained with standard gradient amplifiers and coils. For some oblique slices, we have achieved shorter TR and TE values than those for nonoblique slices.     

Flow artifacts in steady-state free precession cine imaging.

Steady-state free precession (SSFP) cardiac cine images are frequently corrupted by dark flow artifacts, which can usually be eliminated by reshimming and retuning the scanner. A theoretical explanation for these artifacts is provided in terms of spins moving through an off-resonant point in the magnetic field, and the theory is validated using phantom experiments. The artifacts can be reproduced in vivo by detuning the center frequency by an amount in the range of half the inverse repetition time (TR). Since this offset is similar in magnitude to the frequency difference between the water and lipid peaks, a likely cause of the artifacts in vivo is that the center frequency is tuned incorrectly to the lipid peak rather than the water peak.     

Analysis and correction of motion artifacts in diffusion weighted imaging

For diffusion-weighted magnetic resonance imaging and under circumstances where patient movement can be modeled as rigid body motion, it is shown both theoretically and experimentally that translations and rotations produce phase errors which are zero- and first-order, respectively, in position. Whlile a navigator echo can be used to correct the imaging data for arbitrary translations, only when the diffusion gradient is applied in the phase encode direction is there sufficient information to correct for rotations around all axes, and therefore for general rigid body motion. Experiments in test objects and human brain imaging confirm theoretical predictions and demonstrate that appropriate corrections dramatically improve image quality in vivo.     

Hepatic T2-weighted MRI: A prospective comparison of sequences,including breath-hold,half-Fourier turbo spin echo (HASTE)

The purpose of this study was to quantitatively compare the hepatic contrast characteristics of conventional spin-echo (CSE) and fast spin-echo (FSE) sequences with breath-hold T2-weighted images acquired with half-Fourier turbo spin echo (HASTE). Forty-five patients were examined with a phased-array surface coil. Nineteen patients had focal hepatic lesions, including eight malignant tumors, 10 cavernous hemangiomas, and one hepatic adenoma. Twenty-six patients had no focal hepatic lesions. T2-weighted images with comparable TE were acquired with CSE, FSE, and HASTE pulse sequences. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for liver, spleen, and lesions were measured. FSE demonstrated significantly better quantitative performance than CSE for liver-spleen CNR (P = .0084). No statistically significant difference was demonstrated between FSE and CSE for liver or spleen SNR. FSE demonstrated clear scan time and resolution advantages over CSE. HASTE performed significantly poorer than CSE and FSE for liver-spleen CNR (P < .0001), liver SNR (P = .0002 for CSE and P < .0001 for FSE), and spleen SNR (P < .0001). Optimized FSE images with a short echo train length performed comparably to CSE images of equivalent TE. Liver-lesion CNR was suppressed on HASTE images, suggesting that long echo train length FSE sequences could diminish solid lesion detection compared to CSE and short echo train length FSE.     

Band artifacts due to bulk motion.

Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase- and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period.     

Parallel acquisition techniques in cardiac cine magnetic resonance imaging using TrueFISP sequences: comparison of image quality and artifacts

PURPOSE: To compare image quality, artifacts, and signal-to-noise ratio (SNR) in cardiac cine TrueFISP magnetic resonance imaging (MRI) with and without parallel acquisition techniques (PAT). MATERIALS AND METHODS: MRI was performed in 16 subjects with a TrueFISP sequence (1.5 T; Magnetom Sonata, Siemens): TR, 3.0 msec; TE, 1.5 msec; flip angle (FA), 60 degrees. Three axes were scanned without PAT (no PAT) and using the generalized autocalibrating partially parallel acquisition (GRAPPA) and modified sensitivity encoding (mSENSE) reconstruction algorithms with an autocalibration mode to reduce scan time. A conventional spine array and a body flex array were used. Artifacts, image noise, and overall image quality were classified on a 4-point scale by an observer blinded to the implemented technique; for quantitative comparison, SNR was measured. RESULTS: With a PAT factor of two, acquisition time could be reduced by 39%. No PAT did not show artifacts, and GRAPPA revealed fewer artifacts than mSENSE. PAT provided inferior-quality scores concerning image noise and overall image quality. In quantitative measurements, GRAPPA and mSENSE (20.1 +/- 6.2 and 15.6 +/- 6.2, respectively) yielded lower SNR than no PAT (30.6 +/- 20.1; P < 0.05) and P < 0.001). CONCLUSION: Time savings in PAT are accompanied by artifacts and an increase in image noise. The GRAPPA algorithm was superior to mSENSE concerning image quality, noise, and SNR.     

Spoiling of transverse magnetization in gradient-echo (GRE) imaging during the approach to steady state

The signal evolution behaviors and corresponding image appearances for different methods of spoiling or refocusing the transverse magnetization in short TR gradient-echo imaging during the approach to steady state were investigated experimentally and using computer simulations based on the Bloch equations. Specifically, ideally spoiled, gradient-spoiled, gradient-refocused, and RF-spoiled pulse sequence configurations were studied. This study showed that, for the gradient-spoiled configuration, the signal evolution is position and phase-encoding order-dependent and, under typical imaging conditions, can deviate substantially from the ideally spoiled signal evolution at some spatial positions, resulting in intensity banding image artifacts. For the gradient-refocused configuration, the signal evolution oscillates toward the steady state and, generally, does not closely approximate that of ideal spoiling, resulting in different image contrast or image blurring. Using RF spoiling, the signal evolution closely approximates the ideally spoiled case for flip angles less than approximately 20° and T₂ values of less than approximately 200 ms and results in relatively artifact-free images. Also, this study showed that, for RF spoiling, an RF-pulse phase-difference increment other than 117°, such as 84°, may be optimal for gradient-echo imaging during the approach to steady state.     

Reducing flow artifacts in echo-planar imaging

Echo-planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, “partial flyback” is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k-space, only the even echoes of the EPI echo-train are used. Partial flyback is shown to improve the readout-flow properties at the expense of a slight worsening of the phase-encode flow and off-resonance properties. We recommend that the flyback region acquire 95% of the energy in k-space. Second, “inside-out” EPI is used to reduce artifacts arising from flow in the phase-encode direction. Data collection begins at the center of k-space, with separate interleaves to acquire the top and bottom halves of k-space. Partial flyback is combined with partial-Fourier EPI and inside-out EPI. Partial-flyback inside-out EPI has worse off-resonance properties than partial-flyback partial-Fourier EPI but demonstrates better flow properties and does not require partial k-space reconstruction.