Clinical evaluation of reduced field-of-view diffusion-weighted imaging of the cervical and thoracic spine and spinal cord.

BACKGROUND AND PURPOSE: DWI has the potential to improve the detection and evaluation of spine and spinal cord pathologies. This study assessed whether a recently described method (rFOV DWI) adds diagnostic value in clinical patients. MATERIALS AND METHODS: Consecutive patients undergoing clinically indicated cervical and/or thoracic spine imaging received standard anatomic sequences supplemented with sagittal rFOV DWI by using a b-value of 500 s/mm(2). Two neuroradiologists blinded to clinical history evaluated the standard anatomic sequences only for pathology and provided their level of confidence in their diagnosis. These readers then rescored the examinations after reviewing the rFOV DWI study and indicated whether this sequence altered findings or confidence levels. RESULTS: Two hundred twenty-three patients were included in this study. One hundred eighty patient scans (80.7%) demonstrated at least 1 pathologic finding. Interobserver agreement for identifying pathology (κ = 0.77) and in assessing the added value of the rFOV DWI sequence (κ = 0.77) was high. In pathologic cases, the rFOV DWI sequence added clinical utility in 33% of cases (P < .00001, Fisher exact test). The rFOV DWI sequence was found to be helpful in the evaluation of acute infarction, demyelination, infection, neoplasm, and intradural and epidural collections (P < .001, χ(2) test) and provided a significant increase in clinical confidence in the evaluation of 11 of the 15 pathologic subtypes assessed (P < .05, 1-sided paired Wilcoxon test). CONCLUSIONS: rFOV diffusion-weighted imaging of the cervical and thoracic spine is feasible in a clinical population and increases clinical confidence in the diagnosis of numerous common spinal pathologies.

Reduced field-of-view diffusion imaging of the human spinal cord: comparison with conventional single-shot echo-planar imaging.

BACKGROUND AND PURPOSE: DWI of the spinal cord is challenging because of its small size and artifacts associated with the most commonly used clinical imaging method, SS-EPI. We evaluated the performance of rFOV spinal cord DWI and compared it with the routine fFOV SS-EPI in a clinical population. MATERIALS AND METHODS: Thirty-six clinical patients underwent 1.5T MR imaging examination that included rFOV SS-EPI DWI of the cervical spinal cord as well as 2 comparison diffusion sequences: fFOV SS-EPI DWI normalized for either image readout time (low-resolution fFOV) or spatial resolution (high-resolution fFOV). ADC maps were created and compared between the methods by using single-factor analysis of variance. Two neuroradiologists blinded to sequence type rated the 3 DWI methods, based on susceptibility artifacts, perceived spatial resolution, signal intensity-to-noise ratio, anatomic detail, and clinical utility. RESULTS: ADC values for the rFOV and both fFOV sequences were not statistically different (rFOV: 1.01 ± 0.18 × 10(-3) mm(2)/s; low-resolution fFOV: 1.12 ± 0.22 × 10(-3) mm(2)/s; high-resolution fFOV: 1.10 ± 0.21 × 10(-3) mm(2)/s; F = 2.747, P > .05). The neuroradiologist reviewers rated the rFOV diffusion images superior in terms of all assessed measures (P < 0.0001). Particular improvements were noted in patients with metal hardware, degenerative disease, or both. CONCLUSIONS: rFOV DWI of the spinal cord overcomes many of the problems associated with conventional fFOV SS-EPI and is feasible in a clinical population. From a clinical standpoint, images were deemed superior to those created by using standard fFOV methods.

Anisotropic field-of-views in radial imaging.

Radial imaging techniques, such as projection-reconstruction (PR), are used in magnetic resonance imaging (MRI) for dynamic imaging, angiography, and short-T(2) imaging. They are robust to flow and motion, have diffuse aliasing patterns, and support short readouts and echo times. One drawback is that standard implementations do not support anisotropic field-of-view (FOV) shapes, which are used to match the imaging parameters to the object or region-of-interest. A set of fast, simple algorithms for 2-D and 3-D PR, and 3-D cones acquisitions are introduced that match the sampling density in frequency space to the desired FOV shape. Tailoring the acquisitions allows for reduction of aliasing artifacts in undersampled applications or scan time reductions without introducing aliasing in fully-sampled applications. It also makes possible new radial imaging applications that were previously unsuitable, such as imaging elongated regions or thin slabs. 2-D PR longitudinal leg images and thin-slab, single breath-hold 3-D PR abdomen images, both with isotropic resolution, demonstrate these new possibilities. No scan time to volume efficiency is lost by using anisotropic FOVs. The acquisition trajectories can be computed on a scan by scan basis.

Real-time interactive coronary MRA.

An interactive real-time imaging system capable of rapid coronary artery imaging is described. High-resolution spiral and circular echo planar trajectories were used to achieve 0.8 x 1.6 mm2 resolution in 135 ms (CEPI) or 1.13 x 1.13 mm2 resolution in 189 ms (spirals), over a 20-cm FOV. Using a sliding window reconstruction, display rates of up to 37 images/sec were achieved. Initial results indicate this technique can perform as a high-quality 2D coronary localizer and with SNR improvement may enable rapid screening of the coronary tree.

Rapid evaluation of left ventricular volume and mass without breath-holding using real-time interactive cardiac magnetic resonance imaging system.

OBJECTIVES: The purpose of this study was to validate cardiac measurements derived from real-time cardiac magnetic resonance imaging (MRI) as compared with well-validated conventional cine MRI. BACKGROUND: Although cardiac MRI provides accurate assessment of left ventricular (LV) volume and mass, most techniques have been relatively slow and required electrocardiogram (ECG) gating over many heart beats. A newly developed real-time MRI system allows continuous real-time dynamic acquisition and display without cardiac gating or breath-holding. METHODS: Fourteen healthy volunteers and nine patients with heart failure underwent real-time and cine MRI in the standard short-axis orientation with a 1.5T MRI scanner. Nonbreath-holding cine MRI was performed with ECG gating and respiratory compensation. Left ventricular end-diastolic volume (LVEDV), left ventricular endsystolic volume (LVESV), ejection fraction (EF) and LV mass calculated from the images obtained by real-time MRI were compared to those obtained by cine MRI. RESULTS: The total study time including localization for real-time MRI was significantly shorter than cine MRI (8.6 +/- 2.3 vs. 24.7 +/- 3.5 min, p < 0.001). Both imaging techniques yielded good quality images allowing cardiac measurements. The measurements of LVEDV, LVESV, EF and LV mass obtained with real-time MRI showed close correlation with those obtained with cine MRI (LVEDV: r = 0.985, p < 0.001; LVESV: r = 0.994, p < 0.001; EF: r = 0.975, p < 0.001; LV mass: r = 0.977, p < 0.001). CONCLUSIONS: Real-time MRI provides accurate measurements of LV volume and mass in a time-efficient manner with respect to image acquisition.

Characterization and reduction of the transient response in steady-state MR imaging.

Refocused steady-state free precession (SSFP) imaging sequences have recently regained popularity as faster gradient hardware has allowed shorter repetition times, thereby reducing SSFP's sensitivity to off-resonance effects. Although these sequences offer fast scanning with good signal-to-noise efficiency, the "transient response," or time taken to reach a steady-state, can be long compared with the total imaging time, particularly when using 2D sequences. This results in lost imaging time and has made SSFP difficult to use for real-time and cardiac-gated applications. A linear-systems analysis of the steady-state and transient response for general periodic sequences is shown. The analysis is applied to refocused-SSFP sequences to generate a two-stage method of "catalyzing," or speeding up the progression to steady-state by first scaling, then directing the magnetization. This catalyzing method is compared with previous methods in simulations and experimentally. Although the second stage of the method exhibits some sensitivity to B(1) variations, our results show that the transient time can be significantly reduced, allowing imaging in a shorter total scan time. Magn Reson Med 46:149-158, 2001.

Real-time black-blood MRI using spatial presaturation.

A real-time interactive black-blood imaging system is described. Rapid blood suppression is achieved by exciting and dephasing slabs outside the imaging slice before each imaging excitation. Sharp-profiled radio frequency saturation pulses placed close to the imaging slice provide good blood suppression, even in views containing slow through-plane flow. In vivo results indicate that this technique improves endocardial border definition during systole in real-time cardiac wall-motion studies. Phantom and animal results indicate that this technique nearly eliminates flow artifacts in real-time intravascular studies. J. Magn. Reson. Imaging 2001;13:807-812.

Rapid ventricular assessment using real-time interactive multislice MRI.

A multislice real-time imaging technique is described which can provide continuous visualization of the entire left ventricle under resting and stress conditions. Three dynamically adjustable slices containing apical, mid, and base short axis views are imaged 16 times/sec (48 images/sec), with each image providing 3.12 mm resolution over a 20 cm field of view. Initial studies indicate that this technique is useful for the assessment of LV function by providing simultaneous real-time visualization of all 16 wall segments. This technique may also be used for stress LV function and, when used in conjunction with contrast agents, myocardial perfusion imaging. Magn Reson Med 45:371-375, 2001.

Efficient off-resonance correction for spiral imaging.

A new spiral imaging technique incorporates the acquisition of a field map into imaging interleaves. Variable density spiral trajectories are designed to oversample the central region of k-space, and interleaves are acquired at two different echo times. A field map is extracted from this data and multifrequency reconstruction is used to form an off-resonance corrected image using the entire dataset. Simulation, phantom, and in vivo results indicate that this technique can be used to achieve higher image and/or field map spatial resolution compared to conventional techniques. Magn Reson Med 45:521-524, 2001.

Reducing off-resonance distortion by echo-time interpolation.

Off-resonant spins, produced by chemical shift, tissue-susceptibility differences, or main-field inhomogeneity, can cause blurring or shifts, severely compromising the diagnostic value of magnetic resonance images. To mitigate these off-resonance effects, the authors propose a technique whereby two images are acquired at different echo times (TEs) and interpolated to produce a single image with dramatically-reduced blurring. The phase difference of these two images is not used to produce a field map; instead, the weighted complex-valued average of the two images is used to produce a single image. Previously-described methods reconstruct a set of preliminary images, each at a different off-resonant frequency, and then assemble these into one final image, choosing the best off-resonant frequency for each voxel. Compared to these methods, the proposed technique requires the same or less processing time and is much less sensitive to errors in the field map. This technique was applied to centric-ordered EPI but it can be applied to any imaging trajectory, including one-shot EPI, spiral imaging, projection-reconstruction imaging, and 2D GRASE. Magn Reson Med 45:269-276, 2001.

On the optimality of the gridding reconstruction algorithm.

Gridding reconstruction is a method to reconstruct data onto a Cartesian grid from a set of nonuniformly sampled measurements. This method is appreciated for being robust and computationally fast. However, it lacks solid analysis and design tools to quantify or minimize the reconstruction error. Least squares reconstruction (LSR), on the other hand, is another method which is optimal in the sense that it minimizes the reconstruction error. This method is computationally intensive and, in many cases, sensitive to measurement noise. Hence, it is rarely used in practice. Despite their seemingly different approaches, the gridding and LSR methods are shown to be closely related. The similarity between these two methods is accentuated when they are properly expressed in a common matrix form. It is shown that the gridding algorithm can be considered an approximation to the least squares method. The optimal gridding parameters are defined as the ones which yield the minimum approximation error. These parameters are calculated by minimizing the norm of an approximation error matrix. This problem is studied and solved in the general form of approximation using linearly structured matrices. This method not only supports more general forms of the gridding algorithm, it can also be used to accelerate the reconstruction techniques from incomplete data. The application of this method to a case of two-dimensional (2-D) spiral magnetic resonance imaging shows a reduction of more than 4 dB in the average reconstruction error.

Combined connectivity and a gray-level morphological filter in magnetic resonance coronary angiography.

A connectivity algorithm combined with a new gray-level morphological filter dramatically improves the segmentation of tortuous coronary arteries from 3D MRI. Small coronary arteries are segmented from the larger ventricles with a new filter. These blood vessels are segmented from the noise background with connectivity. Coronary angiograms were computed in nine datasets acquired on volunteers with 3D stack of spirals and contrast-enhanced navigator sequences by both a maximum intensity projection and surface rendering. Surface images provided depth information needed to distinguish branching arteries from crossing veins. Magn Reson Med 43:892-895, 2000.

The real-time interactive 3-D-DVA for robust coronary MRA.

A graphical user interface (GUI) has been developed which enables interactive feedback and control to the real-time diminishing variance algorithm (DVA). This interactivity allows the user to set scan parameters, view scan statistics, and view image updates during the course of the scan. In addition, the DVA has been extended to simultaneously reduce motion artifacts in three dimensions using three orthogonal navigators. Preliminary in vivo studies indicate that these improvements to the standard DVA allow for significantly improved consistency and robustness in eliminating respiratory motion artifacts from MR images, particularly when imaging the coronary arteries.

Partial-FOV reconstruction in dynamic spiral imaging.

In many applications of dynamic MR imaging, only a portion of the field-of-view (FOV) exhibits considerable variations in time. In such cases, a prior knowledge of the static part of the image allows a partial-FOV reconstruction of the dynamic section using only a fraction of the raw data. This method of reconstruction generally results in higher temporal resolution, because the scan time for partial-FOV data is shorter. The fidelity of this reconstruction technique depends, among other factors, on the accuracy of the prior information of the static section. This information is usually derived from the reconstructed images at previous time frames. This data, however, is normally corrupted by the motion artifact Because the temporal frequency contents of the motion artifact is very similar to that of the dynamic section, a temporal low-pass filter can efficiently remove this artifact from the static data. The bandwidth of the filter can be obtained from the rate of variations inside and outside the dynamic area. In general, when the temporal bandwidth is not spatially uniform, a bank of low-pass filters can provide a proper suppression of the motion artifact outside the dynamic section. This reconstruction technique is adapted for spiral acquisition and is successfully applied to cardiac fluoroscopy, doubling the temporal resolution.

Off-centered spiral trajectories.

The quality of spiral images depends on the accuracy of the k-space sampling locations. Although newer gradient systems can provide more accurate gradient waveforms, the sampling positions can be significantly distorted by timing misregistration between data acquisition and gradient systems. Even after the timing of data acquisition is tuned, minor residual errors can still cause shading artifacts which are problematic for quantitative MR applications such as phase-contrast flow quantitation. These timing errors can ideally be corrected by measuring the actual k-space trajectory, but trajectory measurement requires additional data acquisition and scan time. Therefore, off-centered spiral trajectories which are more robust against timing errors are proposed and applied to the phase-contrast method. The new trajectories turn shading artifacts into a slowly varying linear phase in reconstructed images without affecting the magnitude of images.

Reduced aliasing artifacts using variable-density k-space sampling trajectories.

A variable-density k-space sampling method is proposed to reduce aliasing artifacts in MR images. Because most of the energy of an image is concentrated around the k-space center, aliasing artifacts will contain mostly low-frequency components if the k-space is uniformly undersampled. On the other hand, because the outer k-space region contains little energy, undersampling that region will not contribute severe aliasing artifacts. Therefore, a variable-density trajectory may sufficiently sample the central k-space region to reduce low-frequency aliasing artifacts and may undersample the outer k-space region to reduce scan time and to increase resolution. In this paper, the variable-density sampling method was implemented for both spiral imaging and two-dimensional Fourier transform (2DFT) imaging. Simulations, phantom images and in vivo cardiac images show that this method can significantly reduce the total energy of aliasing artifacts. In general, this method can be applied to all types of k-space sampling trajectories.

Real-time color flow MRI.

A real-time interactive color flow MRI system capable of rapidly visualizing cardiac and vascular flow is described. Interleaved spiral phase contrast datasets are acquired continuously, while real-time gridding and phase differencing is used to compute density and velocity maps. These maps are then displayed using a color overlay similar to what is used by ultrasound. For cardiac applications, 6 independent images/sec are acquired with in-plane resolution of 2.4 mm over a 20 cm field of view (FOV). Sliding window reconstruction achieves display rates up to 18 images/sec. Appropriate tradeoffs are made for other applications. Flow phantom studies indicate this technique accurately measures velocities up to 2 m/sec, and accurately captures real-time velocity waveforms (comparable to continuous wave ultrasound). In vivo studies indicate this technique is useful for imaging cardiac and vascular flow, particularly valvular regurgitation. Arbitrary scan planes can be quickly localized, and flow measured in any direction.

Linear combination steady-state free precession MRI.

A new, fast, spectrally selective steady-state free precession (SSFP) imaging method is presented. Combining k-space data from SSFP sequences with certain phase schedules of radiofrequency excitation pulses permits manipulation of the spectral selectivity of the image. For example, lipid and water can be resolved. The contrast of each image depends on both T1 and T2, and the relative contribution of the two relaxation mechanisms to image contrast can be controlled by adjusting the flip angle. Several potential applications of the technique, referred to as linear combination steady-state free precession (LCSSFP), are demonstrated: fast musculoskeletal, abdominal, angiographic, and brain imaging.

Automatic field map generation and off-resonance correction for projection reconstruction imaging.

A new projection reconstruction technique utilizes the oversampling of low spatial frequencies to estimate and correct for off-resonance effects. Interleaved spokes are acquired at one of two different echo times. From separated early-TE and late-TE raw data, two one-quarter resolution images are reconstructed and a one-quarter resolution field map is computed. Multifrequency reconstruction with all the data is then used to simultaneously correct for off-resonance and compensate for the difference in echo times. Resulting images obtained on phantoms and in vivo demonstrate significantly reduced off-resonance artifact without the acquisition of a separate field map.

Fluctuating equilibrium MRI.

A new fast, spectrally selective imaging method called fluctuating equilibrium magnetic resonance is presented. With all gradients refocused over a repetition interval, certain phase schedules of radiofrequency excitation pulses produce an equilibrium magnetization that fluctuates from excitation to excitation, thus permitting simultaneous acquisition of several images with different contrast features. For example, lipid and water images can be rapidly acquired. The effective echo time can be adjusted using the flip angle, thus providing control over the T(2) contribution to the contrast. Several applications of the technique are presented, including fast musculoskeletal, abdominal, breast, and brain imaging, in addition to MR angiography. A technique for combining lipid and water images generated with this sequence for angiography is described and other potential applications are suggested. Magn Reson Med 42:876-883, 1999.