Torque free asymmetric gradient coils for echo planar imaging

In this work, we present a new torque-free asymmetric gradient coil that is capable of generating high quality axial, sagittal, and coronal echo planar images of the human head. This gradient set was calculated using the Biot-Savart law and least square approaches to optimize the field in the region of interest and to minimize net torques. The resulting structure has excellent shoulder-to-coil clearance and, as such, has the advantage of providing good patient access to the linear portion of the gradient while minimizing patient discomfort from claustrophobia.     

Concomitant magnetic-field-induced artifacts in axial echo planar imaging

When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre-ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.     

Concomitant gradient terms in phase contrast MR: Analysis and correction

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient G_z is simultaneously active with a transverse gradient (G_x or G_y). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G²/B₀, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.     

Correction of concomitant magnetic field-induced image artifacts in nonaxial echo-planar imaging.

Echo-planar images acquired in nonaxial planes are often distorted. Such image distortion has limited the applications of the echo-planar imaging (EPI) technique. In this article, it is demonstrated that a considerable amount of the distortion is caused by the higher-order magnetic field concomitant with the linear magnetic field gradient, or the concomitant magnetic field. The image distortion caused by the concomitant magnetic field is more prominent when a higher gradient amplitude is used for readout. It is also shown that the concomitant magnetic field can cause ghosting and blurring. A theoretical analysis is performed for the concomitant field effect in nonaxial EPI images. A point-by-point (or line-by-line) phase correction algorithm is developed to correct the image distortion, ghosting, and blurring. A postreconstruction processing algorithm is also developed to correct image distortion with much higher computational efficiency. Experimental results show that both correction methods effectively reduce the image distortion in coronal or sagittal images.     

Single-shot magnetic field mapping embedded in echo-planar time-course imaging.

A technique for acquiring magnetic field maps simultaneously with gradient-recalled echo-planar time-course data is described. This technique uses a trajectory in which the central part of k-space is collected twice. For a 64 x 64 image acquired with a 125-kHz bandwidth, a field map suitable for geometric correction can be collected simultaneously with the echo-planar time-course data in <70 ms. The field maps generated by this technique are registered with the magnitude images because they are calculated using the same data. They do not suffer from errors due to subject motion, or from different geometric distortions that can result from using different pulse sequences. In addition to correcting geometric distortions that resulted from dynamic magnetic field perturbations, this method was used to measure field shifts arising from respiration and jaw motion across five subjects. Values ranged from 0.035 to 0.165 parts per million (ppm).     

In vivo measurement of the apparent diffusion coefficient in normal and malignant prostatic tissues using echo-planar imaging

PURPOSE: To measure for the first time the apparent diffusion coefficient (ADC) values in anatomical regions of the prostate for normal and patient groups, and to investigate its use as a differentiating parameter between healthy and malignant tissue within the patient group. MATERIALS AND METHODS: Single-shot diffusion-weighted echo-planar imaging (DW-EPI) was used to measure the ADC in the prostate in normal (N = 7) and patient (N = 19) groups. The spin-echo images comprised 96 x 96 pixels (field of view of 16 cm, TR/TE = 4000/120 msec) with six b-factor values ranging from 64 to 786 seconds/mm(2). RESULTS: The ADC values averaged over all patients in non-cancerous and malignant peripheral zone (PZ) tissues were 1.82 +/- 0.53 x 10(-3) (mean +/- SD) and 1.38 +/- 0.52 x 10(-3) mm(2)/second, respectively (P = 0.00045, N = 17, paired t-test). The ADC values were found to be higher in the non-cancerous PZ (1.88 +/- 0.48 x 10(-3)) than in healthy or benign prostatic hyperplasia central gland (BPH-CG) region (1.62 +/- 0.41 x 10(-3)). For the normal group, the mean values were 1.91 +/- 0.46 x 10(-3) and 1.63 +/- 0.30 x 10(-3) mm(2)/second for the PZ and CG, respectively (P = 0.011, N = 7). Significant overlap exists between individual values among all tissue types. Furthermore, ADC values for the same tissue type showed no statistically significant difference between the two subject groups. CONCLUSION: ADC is quantified in the prostate using DW-EPI. Values are lower in cancerous than in healthy PZ in patients, and in BPH-CG than PZ in volunteers.     

Eddy current correction in diffusion-weighted imaging using pairs of images acquired with opposite diffusion gradient polarity.

In echo-planar-based diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion-sensitizing gradients. Correction methods that coregister diffusion-weighted and non-diffusion-weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. In this approach, only diffusion-weighted images with identical contrast are included for correction. That is, non-diffusion-weighted images are not needed as a reference for registration. Furthermore, the acquisition of an additional dataset with moderate diffusion-weighting as suggested by Haselgrove and Moore (Magn Reson Med 1996;36:960-964) is not required. With phantom data it is shown that the theoretically expected symmetry of distortions is preserved in the images to a very high degree, demonstrating the practicality of the new method. Results from human brain images are also presented.     

Diffusion-weighted imaging of the parotid gland: Influence of the choice of b-values on the apparent diffusion coefficient value

PURPOSE: To determine how the ADC value of parotid glands is influenced by the choice of b-values. MATERIALS AND METHODS: In eight healthy volunteers, diffusion-weighted echo-planar imaging (DW-EPI) was performed on a 1.5 T system, with b-values (in seconds/mm2) of 0, 50, 100, 150, 200, 250, 300, 500, 750, and 1000. ADC values were calculated by two alternative methods (exponential vs. logarithmic fit) from five different sets of b-values: (A) all b-values; (B) b=0, 50, and 100; (C) b=0 and 750; (D) b=0, 500, and 1000; and (E) b=500, 750, and 1000. RESULTS: The mean ADC values for the different settings were (in 10(-3) mm2/second, exponential fit): (A) 0.732+/-0.019, (B) 2.074+/-0.084, (C) 0.947+/-0.020, (D) 0.890+/-0.023, and (E) 0.581+/-0.021. ADC values were significantly (P <0.001) different for all pairwise comparisons of settings (A-E) of b-values, except for A vs. D (P=0.172) and C vs. D (P=0.380). The ADC(B) was significantly higher than ADC(C) or ADC(D), which was significantly higher than ADC(E). ADC values from exponential vs. logarithmic fit (P=0.542), as well as left vs. right parotid gland (P=0.962), were indistinguishable. CONCLUSION: The ADC values calculated from low b-value settings were significantly higher than those calculated from high b-value settings. These results suggest that not only true diffusion but also perfusion and saliva flow may contribute to the ADC.     

Ultrafast imaging: Principles,pitfalls, solutions,and applications

Ultrafast MRI refers to efficient scan techniques that use a high percentage of the scan time for data acquisition. Often, they are used to achieve short scan duration ranging from sub‐second to several seconds. Alternatively, they may form basic components of longer scans that may be more robust or have higher image quality. Several important applications use ultrafast imaging, including real‐time dynamic imaging, myocardial perfusion imaging, high‐resolution coronary imaging, functional neuroimaging, diffusion imaging, and whole‐body scanning. Over the years, echo‐planar imaging (EPI) and spiral imaging have been the main ultrafast techniques, and they will be the focus of the review. In practice, there are important challenges with these techniques, as it is easy to push imaging speed too far, resulting in images of a nondiagnostic quality. Thus, it is important to understand and balance the trade‐off between speed and image quality. The purpose of this review is to describe how ultrafast imaging works, the potential pitfalls, current solutions to overcome the challenges, and the key applications. J. Magn. Reson. Imaging 2010;32:252–266. © 2010 Wiley‐Liss, Inc.     

Results for diffusion‐weighted imaging with a fourth‐channel gradient insert

Diffusion‐weighted imaging suffers from motion artifacts and relatively low signal quality due to the long echo times required to permit the diffusion encoding. We investigated the inclusion of a noncylindrical fourth gradient coil, dedicated entirely to diffusion encoding, into the imaging system. Standard three‐axis whole body gradients were used during image acquisition, but we designed and constructed an insert coil to perform diffusion encodings. We imaged three phantoms on a 3‐T system with a range of diffusion coefficients. Using the insert gradient, we were able to encode b values of greater than 1300 s/mm² with an echo time of just 83 ms. Images obtained using the insert gradient had higher signal to noise ratios than those obtained using the whole body gradient: at 500 s/mm² there was a 18% improvement in signal to noise ratio, at 1000 s/mm² there was a 39% improvement in signal to noise ratio, and at 1350 s/mm² there was a 56% improvement in signal to noise ratio. Using the insert gradient, we were capable of doing diffusion encoding at high b values by using relatively short echo times. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Fast concomitant gradient field and field inhomogeneity correction for spiral cardiac imaging

Non‐Cartesian imaging provides many advantages in terms of flexibility, functionality, and speed. However, a major drawback to these imaging methods is off‐resonance distortion artifacts. These artifacts manifest as blurring in spiral imaging. Common techniques that remove the off‐resonance field inhomogeneity distortion effects are not sufficient, because the high order concomitant gradient fields are nontrivial for common imaging conditions, such as imaging 5 cm off isocenter in an 1.5 T scanner. Previous correction algorithms are either slow or do not take into account the known effects of concomitant gradient fields along with the field inhomogeneities. To ease the correction, the distortion effects are modeled as a non‐stationary convolution problem. In this work, two fast and accurate postgridding algorithms are presented and analyzed. These methods account for both the concomitant field effects and the field inhomogeneities. One algorithm operates in the frequency domain and the other in the spatial domain. To take advantage of their speed and accuracy, the algorithms are applied to a real‐time cardiac study and a high‐resolution cardiac study. Both of the presented algorithms provide for a practical solution to the off‐resonance problem in spiral imaging. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Diffusion tensor imaging (DTI) with retrospective motion correction for large-scale pediatric imaging

Purpose:

To develop and implement a clinical DTI technique suitable for the pediatric setting that retrospectively corrects for large motion without the need for rescanning and/or reacquisition strategies, and to deliver high‐quality DTI images (both in the presence and absence of large motion) using procedures that reduce image noise and artifacts.

Materials and Methods:

We implemented an in‐house built generalized autocalibrating partially parallel acquisitions (GRAPPA)‐accelerated diffusion tensor (DT) echo‐planar imaging (EPI) sequence at 1.5T and 3T on 1600 patients between 1 month and 18 years old. To reconstruct the data, we developed a fully automated tailored reconstruction software that selects the best GRAPPA and ghost calibration weights; does 3D rigid‐body realignment with importance weighting; and employs phase correction and complex averaging to lower Rician noise and reduce phase artifacts. For select cases we investigated the use of an additional volume rejection criterion and b‐matrix correction for large motion.

Results:

The DTI image reconstruction procedures developed here were extremely robust in correcting for motion, failing on only three subjects, while providing the radiologists high‐quality data for routine evaluation.

Conclusion:

This work suggests that, apart from the rare instance of continuous motion throughout the scan, high‐quality DTI brain data can be acquired using our proposed integrated sequence and reconstruction that uses a retrospective approach to motion correction. In addition, we demonstrate a substantial improvement in overall image quality by combining phase correction with complex averaging, which reduces the Rician noise that biases noisy data. J. Magn. Reson. Imaging 2012;36:961–971. © 2012 Wiley Periodicals, Inc.     

Multiple-region gradient arrays for extended field of view, increased performance, and reduced nerve stimulation in magnetic resonance imaging.

This article presents a novel design for magnetic resonance imaging (MRI) gradient systems. This design may allow the development of MRI scanners that are capable of imaging large regions with high performance while minimizing the potential for nerve stimulation. The general concept of the gradient system is that spatial oscillation is incorporated such that each gradient coil creates multiple, approximately linear gradient regions that oscillate in gradient polarity. Separate radiofrequency (RF) coil arrays are designed to be sensitive to the signals within each linear region and thus allow signal measurements to be obtained separately from each region. Enabling image acquisition in the transition region that separates each pair of adjacent linear regions requires a second gradient system with imaging regions that overlap and coincide with the transition regions of the first gradient system. Imaging the extended field of view (FOV) is accomplished by interleaved operation of the two gradient systems. Simulated annealing is used to create designs for both longitudinal and transverse gradient systems with two imaging regions.     

Effect of shot number on the calculated apparent diffusion coefficient in phantoms and in human liver in diffusion‐weighted echo‐planar imaging

Purpose

To show the signal intensity varies with shot number in diffusion‐weighted (DW) echo‐planar imaging (EPI) and affects apparent diffusion coefficient (ADC) calculation.

Materials and Methods

This prospective study was performed on 35 adult patients and 20 volunteers. Measurements were made on a 3T scanner using a breathhold DW spin‐echo EPI (SE EPI) sequence. Three protocols were used: A) eight consecutive shots at a fixed b‐value of 0 seconds/mm² with TR = 1000 and 3000 msec; B) seven consecutive shots at b‐values = 0, 1000, 750, 500, 250, 100, 0 seconds/mm² (in that order) with TR = 3500 msec; and C) seven consecutive shots (as in B) with TR = 1000, 1750, and 7000 msec.

Results

For protocol A, signal intensity decreased significantly from the first to second shot (P<0.0001) and thereafter remained constant. For protocol B, the ADC depended on which b = 0 seconds/mm² image was used. Using the first b = 0 seconds/mm², the mean ADC was 15% higher than using the second b = 0 seconds/mm² (P<0.0001). For protocol C, the difference between ADC using the first b = 0 seconds/mm² and the second b = 0 seconds/mm² decreased as the TR increased.

Conclusion

The signal intensity can vary with shot number in SE EPI. For TR ≥ 3000 msec, steady‐state is attained after one shot. Using data acquired prior to steady‐state confounds the calculation of ADC values. J. Magn. Reson. Imaging 2009;30:547–553. © 2009 Wiley‐Liss, Inc.     

Analysis and generalized correction of the effect of spatial gradient field distortions in diffusion-weighted imaging.

Nonuniformities of magnetic field gradients can cause serious artifacts in diffusion imaging. While it is well known that nonlinearities of the imaging gradients lead to image warping, those imperfections can also cause spatially dependent errors in the direction and magnitude of the diffusion encoding. This study shows that the potential errors in diffusion imaging are considerable. Further, we show that retrospective corrections can be applied to reduce these errors. A general mathematical framework was formulated to characterize the contribution of gradient nonuniformities to diffusion experiments. The gradient field was approximated using spherical harmonic expansion, and this approximation was employed (after geometric distortions were eliminated) to predict and correct the errors in diffusion encoding. Before the corrections were made, the experiments clearly revealed marked deviations of the calculated diffusivity for fields of view (FOVs) generally used in diffusion experiments. These deviations were most significant farther away from the magnet's isocenter. For an FOV of 25 cm, the resultant errors in absolute diffusivity ranged from approximately -10% to +20%. Within the same FOV, the diffusion-encoding direction and the orientation of the calculated eigenvectors can be significantly altered if the perturbations by the gradient nonuniformities are not considered. With the proposed correction scheme, most of the errors introduced by gradient nonuniformities can be removed.