Structure of the water resonance in small voxels in rat brain detected with high spectral and spatial resolution MRI

PURPOSE: To acquire high spectral and spatial resolution (HiSS) MR images of the water resonance in rat brain, evaluate the lineshape of the water resonance in small voxels, and compare images derived from HiSS data with conventional images. MATERIALS AND METHODS: Spectroscopic images of rat brain were obtained at 4.7 Tesla using phase encoding gradients only. Spectral resolution in each voxel was approximately 8 Hz and bandwidth was 1,000 Hz. Spatial resolution was approximately 250 microns in 1-mm slices. Images were synthesized to show the water signal integral, peak height, linewidth, resonance frequency, and asymmetry. RESULTS: Two or more resolved components of the water resonance were detected in approximately 14% +/- 6% of voxels in the brains of eight rats. The water resonances in approximately 20% +/- 10% of voxels (n = 8) were highly asymmetric. Images with intensity proportional to water signal peak height, T(2)*, or to selected components of the water resonance showed features that were not evident in conventional images. CONCLUSIONS: The complexity of the water signal reflects the anatomy and physiology of the sub-voxelar environment, and may be a useful source of image contrast. HiSS imaging of brain provides accurate anatomic information, and may improve image contrast and delineation of subtle anatomic features.     

Echo‐planar spectroscopic imaging (EPSI) of the water resonance structure in human breast using sensitivity encoding (SENSE)

High spectral and spatial resolution MRI, based on echo‐planar spectroscopic imaging, has been applied successfully in diagnostic breast imaging, but acquisition times are long. One way of increasing acquisition speed is to apply the sensitivity encoding algorithm for complex high spectral and spatial resolution data. We demonstrate application of a complex sensitivity encoding algorithm to high spectral and spatial resolution MRI data, in a phantom and human breast, with 7‐ and 16‐channel dedicated breast phased‐array coils. Very low g factors are obtained using the breast coils, and the signal‐to‐noise ratio (SNR) penalty for water resonance peak height and water resonance asymmetry images is small at acceleration factors of up to 6 and 4, respectively, as evidenced by high Pearson correlation factors between fully sampled and accelerated data. This is the first application of the sensitivity encoding algorithm to characterize the structure of the water resonance at high spatial resolution. Magn Reson Med 63:1557–1563, 2010. © 2010 Wiley‐Liss, Inc.     

The effect of varying spectral resolution on the quality of high spectral and spatial resolution magnetic resonance images of the breast

PURPOSE: To evaluate the effect of varying spectral resolution on image quality of high spectral and spatial resolution (HiSS) images. MATERIALS AND METHODS: Eight women with suspicious breast lesions and six healthy volunteers were scanned using echo-planar spectroscopic imaging (EPSI) at 1.5 Tesla with 0.75- to 1-mm in-plane resolution and 2.3- to 2.6-Hz spectral resolution. Time domain data were truncated to obtain proton spectra in each voxel with varying (2.6-83.3 Hz) resolution. Images with intensity proportional to water signal peak heights were synthesized. Changes in water signal line shape following contrast injection were analyzed. RESULTS: Fat suppression is optimized at approximately 10-Hz spectral resolution and is significantly improved by removal of wings of the fat resonance. This was accomplished by subtracting a Lorentzian fit to the fat resonance from the proton spectrum. The water resonance is often inhomogeneously broadened, and very high spectral resolution is necessary to resolve individual components. High spectral resolution is required for optimal contrast in anatomic features with very high T(2)* (e.g., within a lesion) and for detection of often subtle effects of contrast agents on water signal line shape. CONCLUSION: Despite a trade-off between the spectral resolution and signal-to-noise ratio, it is beneficial to acquire data at the highest spectral resolution currently attainable at 1.5 Tesla.     

Uptake of a superparamagnetic contrast agent imaged by MR with high spectral and spatial resolution.

Conventional MRI implicitly treats the proton signal as a single, narrow Lorentzian. However, water signals in vivo are often in homogeneously broadened and have multiple resolvable components. These components represent discrete populations of water molecules within each pixel which are affected differently by physiology and contrast agents. Accurate measurement of each component of the water resonance can improve anatomic and functional MR images and provide insight into the structure and dynamics of subpixelar microenvironments. This report describes high spectral and spatial resolution (HiSS) MR imaging of rodent prostate tumors before and after injection of a superparamagnetic contrast agent. HiSS datasets were used to synthesize images in which intensity is proportional to peak height, peak frequency, and linewidth. These images showed anatomic features which were not clearly delineated in conventional T(2) and gradient echo images. HiSS images obtained after injection of the contrast agent showed T *(2) and T(1) changes which were not seen in conventional images. These changes are associated with microvessel density and permeability. The results suggest HiSS with superparamagnetic contrast agents has the potential to improve characterization of tumors.     

Fat suppression with spectrally selective inversion vs. high spectral and spatial resolution MRI of breast lesions: qualitative and quantitative comparisons

PURPOSE: To compare conventional fat-suppressed MR images of the breast to images derived from high spectral and spatial resolution MR data. Image quality and the level of fat suppression are compared qualitatively and quantitatively. MATERIALS AND METHODS: Women with suspicious breast lesions found on X-ray mammography were imaged on 1.5 Tesla GE SIGNA scanners. High spectral and spatial resolution (HiSS) data were acquired using echo-planar spectroscopic imaging. Images with intensity proportional to the water signal peak height in each voxel were synthesized. Conventional fat-suppressed images were acquired using a frequency selective inversion method. The experimental (HiSS) and conventional images were compared by experienced radiologists to evaluate the quality of fat suppression. In addition, fat suppression and image quality were evaluated quantitatively. RESULTS: Fat suppression, tumor edge delineation, lesion conspicuity, and image texture were improved in the peak height images derived from HiSS data. CONCLUSION: The results demonstrate that the water peak height images obtained from HiSS data potentially could improve the quality of fat suppression, detection and diagnosis of breast cancer. HiSS allowed detection of lesions and evaluation of lesion morphology prior to contrast media injection. J. Magn. Reson. Imaging 2006. (c) 2006 Wiley-Liss, Inc.     

2D locally focused MRI: Applications to dynamic and spectroscopic imaging

Conventional magnetic resonance images have uniform spatial resolution across the entire field of view. A method of creating MR images with user-specified spatial resolution along one dimension of the field of view was described recently by the authors. This paper presents the 2D generalization of this technique, which allows the user to specify arbitrary spatial resolution in arbitrary 2D regions. These images are reconstructed from signals that sparsely sample the k-space representation of the image. Therefore, locally focused images can be acquired in less time than that required by Fourier imaging with uniformly high resolution. In this paper the authors show how to increase the temporal resolution of dynamic imaging (e.g., interventional imaging) by using high resolution in areas of expected change and lower resolution elsewhere. Alternatively, by matching the local spatial resolution to the expected edge content of the image, it is possible to avoid the localized truncation artifacts that mark Fourier images reconstructed from the same number of signals. For example, the authors show how proton spectroscopic images of the head may be improved by using high resolution in the neighborhood of scalp lipids that might otherwise cause truncation artifacts.     

High spectral and spatial resolution MRI of breast lesions: preliminary clinical experience

OBJECTIVE: In previous research, high spectral and spatial resolution (HiSS) echo-planar spectroscopic imaging (EPSI) was successfully applied to the human breast, obtaining improved contrast, anatomic detail, and sensitivity to contrast agents. To test HiSS in the clinical setting, we used HiSS MRI to image 30 women with suspicious breast lesions. SUBJECTS AND METHODS: Women with suspicious breast lesions were scanned before and after contrast administration using EPSI at 1.5 T (0.63-mm in-plane resolution, 2.6-Hz spectral resolution). Images with intensity proportional to the water signal peak height in each voxel were synthesized and compared with standard clinical fat-saturated and early dynamic subtraction images. Pre- and postcontrast HiSS images were compared to assess the effect of the contrast agent on water resonance structure. RESULTS: HiSS images scored significantly better than standard clinical images in lesion conspicuity, margin definition, and internal definition, even though they were acquired before contrast agent injection. Fat suppression was more complete and uniform and detail was shown on HiSS images more clearly than on conventional fat-saturation images. Thus, HiSS images often allowed easier evaluation of the lesion. Contrast agent-affected changes were often spatially and spectrally inhomogeneous. CONCLUSION: HiSS scans were successfully integrated into standard clinical examinations and provided diagnostically useful images before contrast agent injection. Thus, it might be possible to characterize suspicious lesions on the basis of precontrast high-resolution spectral information. This information and information about the effect of contrast agents could potentially improve the specificity of breast MRI.     

A spin echo chemical shift MR imaging technique

A new method is described that produces images of either the fat or water component in tissues in magnetic resonance imaging. Only a single scan is required, with scan times of a few minutes. Chemical shift selectivity is achieved in the spin echo process by controlling the spectral content of the 180 degree pulse that induces the spin echoes. A theoretical analysis of the selective spin echo process for the case of a radio frequency pulse of constant amplitude shows that spin echoes will be suppressed for certain values of offset frequency that are similar to, but different from, the frequencies at which the Fourier spectrum of the pulse vanishes. The theory was confirmed by experiment on a water phantom. The imaging technique was tested on both a phantom of oil and water and on a human forearm. Excellent suppression of the water signal was found in the fat images, and the small fat component seen in the water images is attributable to components of the triglyceride molecule for which spectral lines overlap those of water. The forearm images also showed blood flow effects in the water image that were not visible in the fat image. The relationship of this method to other proposed methods of chemical shift imaging is discussed.     

Normal shoulder: MR imaging

Relatively poor spatial resolution has been obtained in magnetic resonance (MR) imaging of the shoulder because the shoulder can only be placed in the periphery of the magnetic field. The authors have devised an anatomically shaped surface coil that enables MR to demonstrate normal shoulder anatomy in different planes with high spatial resolution. In the axial plane, anatomy analogous to that seen on computed tomographic (CT) scans can be demonstrated. Variations in scapular position (produced by patient positioning) may make reproducibility of sagittal and coronal plane images difficult by changing the relationship of the plane to the shoulder anatomy. Oblique planes, for which the angle is chosen from the axial image, have the advantage of easy reproducibility. Obliquely oriented structures and relationships are best seen in oblique plane images and can be evaluated in detail.     

Partial fourier reconstruction for three-dimensional gradient echo functional MRI: Comparison of phase correction methods

Partial Fourier (PF) methods take advantage of data symmetry to allow for either faster image acquisition or increased image resolution, Faster acquisition and increased spatial resolution are advantageous for fMRl because of increased temporal resolution and/or reduced partial volume effects, respectively. Standard PF methods, which use a phase reference obtained from a low resolution image, are adequate for the reconstruction of time-stationary images acquired using either spin echoes or short TE gradient echoes. In fMRI, however, multiple images are acquired using long TE gradient echoes, which introduces possible phase drifts in the fMRl data and high spatial frequencies in the phase reference. This work investigates several techniques developed to reconstruct fMRl data obtained with PF acquisitions. PF methods that account for both high-frequency spatial variations and time-dependent drifts in the phase reference are discussed and are quantitatively evaluated using receiver operator characteristic curve analysis.     

Single-point (constant-time) imaging in radiofrequency Fourier transform electron paramagnetic resonance.

This study describes the use of the single-point imaging (SPI) modality, also known as constant-time imaging (CTI), in radiofrequency (RF) Fourier transform (FT) electron paramagnetic resonance (EPR). The SPI technique, commonly used for high-resolution solid-state nuclear magnetic resonance (NMR) imaging, has been successfully applied to 2D and 3D RF-FT-EPR imaging of phantoms containing narrow-line EPR spin probes. The SPI scheme is essentially a phase-encoding technique that operates by acquiring a single data point in the free induction decay (FID) after a fixed delay (phase-encoding time), following the pulsed RF excitation, in the presence of static magnetic field gradients. Since the phase-encoding time remains constant for a given image data set, the spectral information is automatically deconvolved, providing well-resolved pure spatial images. Therefore, images obtained using SPI are artifact-free and the resolution is not significantly limited by the line width, compared to the images obtained using the conventional filtered back-projection (FBP) scheme, suggesting that the SPI modality may have advantages for EPR imaging of large objects. In this work the advantages and limitations of SPI as compared to FBP are investigated by imaging suitable phantom objects. Although SPI takes longer to perform than the FBP method, optimization of the data collection scheme may increase the temporal resolution, rendering this technique suitable for in vivo studies. Spectral information can also be extracted from a series of SPI images that are generated as a function of the delay from the excitation pulse.     

Independent component analysis of dynamic contrast-enhanced magnetic resonance images of breast carcinoma: a feasibility study

PURPOSE: To study the possibility of using independent component analysis (ICA) to identify breast lesions as separate hemodynamic sources on dynamic contrast-enhanced (DCE) MR images, as depicted by the passage of contrast medium. MATERIALS AND METHODS: Six patients who were histopathologically confirmed with breast carcinoma underwent DCE MRI with 5 precontrast and 60 postcontrast scans at a time-resolution of 8 s. A spatial ICA algorithm was applied on the DCE MRI data set to extract spatial component maps corresponding to source locations with different signal time-intensity patterns. To verify the present hypothesis of the ability of ICA to reveal tumor voxels as a separate hemodynamic phase, tumor margins were outlined by an experienced radiologist who was blinded from the ICA results, and the manual outlines were compared with the ICA maps. RESULTS: Consistently for each of the six patient study cases, it was found that ICA yields a tumor component map associated with typical tumor enhancement patterns of rapid enhancement with washout or plateau. Tumor outlines manually drawn by the radiologist were in good agreement with the tumor locations depicted in the tumor component maps. CONCLUSION: ICA may provide an objective method for identifying the outlines of enhancing breast tumors on DCE MR images and to automatically extract the tumor signal intensity-time curve for subsequent tracer kinetics analysis.     

Sensitivity to tumor microvasculature without contrast agents in high spectral and spatial resolution MR images

Contrast‐enhanced (CE)‐MRI is sensitive to cancers but can produce adverse reactions and suffers from insufficient specificity and morphological detail. This research investigated whether high spectral and spatial resolution (HiSS) MRI detects tumor vasculature without contrast agents, based on the sensitivity of the water resonance line shape to tumor blood vessels. HiSS data from AT6.1 tumors inoculated in the hind legs of rats (N = 8) were collected pre‐ and post–blood pool contrast agent (iron‐oxide particles) injection. The waterline in small voxels was significantly more asymmetric at the tumor rim compared to the tumor center and normal muscle (P < 0.003). Composite images were synthesized, with the intensity in each voxel determined by the Fourier component (FC) of the water resonance having the greatest relative image contrast at that position. We tested whether regions with high contrast in FC images (FCIs) contain vasculature by comparing FCIs with CE‐MRI as the “gold standard” of vascular density. The FCIs had 75% ± 13% sensitivity, 74% ± 10% specificity, and 91% ± 4% positive predictive value (PPV) for vasculature detection at the tumor rim. These results suggest that tumor microvasculature can be detected using HiSS imaging without the use of contrast agents. Magn Reson Med 61:291–298, 2009. © 2009 Wiley‐Liss, Inc.     

Total removal of unwanted harmonic peaks (TruHARP) MRI for single breath‐hold high‐resolution myocardial motion and strain quantification

Current MRI methods for myocardial motion and strain quantification have limited resolution because of Fourier space spectral peak interference. Methods have been proposed to remove this interference in order to improve resolution; however, these methods are clinically impractical due to the prolonged imaging times. In this paper, we propose total removal of unwanted harmonic peaks (TruHARP); a myocardial motion and strain quantification methodology that uses a novel single breath‐hold MR image acquisition protocol. In post‐processing, TruHARP separates the spectral peaks in the acquired images, enabling high‐resolution motion and strain quantification. The impact of high resolution on calculated circumferential and radial strains is studied using realistic Monte Carlo simulations, and the improvement in strain maps is demonstrated in six human subjects. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Magnetic resonance imaging in real time: Advances using radial FLASH

Purpose

To develop technical advances for real‐time magnetic resonance imaging (MRI) that allow for improved image quality and high frame rates.

Materials and Methods

The approach is based on a combination of fast low‐angle shot (FLASH) MRI sequences with radial data sampling and view sharing of successive acquisitions. Gridding reconstructions provide images free from streaking or motion artifacts and with a flexible trade‐off between spatial and temporal resolution. Immediate image reconstruction and online display is accomplished with the use of an unmodified 3 T MRI system. For receive coils with a large number of elements this process is supported by a user‐selectable channel compression that is based on a principal component analysis and performed during initial preparation scans.

Results

In preliminary applications to healthy volunteers, real‐time radial FLASH MRI visualized continuous movements of the temporomandibular joint during voluntary opening and closing of the mouth at high spatial resolution (0.75 mm in‐plane) and monitored cardiac functions at high temporal resolution (20 images per second) during free breathing and without synchronization to the electrocardiogram.

Conclusion

Real‐time radial FLASH MRI emerges as a simple and versatile tool for a large range of clinical applications. J. Magn. Reson. Imaging 2010. © 2009 Wiley‐Liss, Inc     

Adaptive bilateral breast MRI using projection reconstruction time-resolved imaging of contrast kinetics

PURPOSE: To introduce a bilateral implementation of an adaptive imaging technique in which both dynamic and high resolution breast MR images are acquired simultaneously. MATERIALS AND METHODS: Adaptive three-dimensional bilateral breast imaging in the sagittal plane was achieved by combining two elements: a projection reconstruction time-resolved imaging of contrast kinetics (PR-TRICKS) k-space trajectory and a slab interleaved sequence that imaged alternate breasts every TR. A pilot study was performed to evaluate image quality and contrast uptake behavior, using eight patients with previously identified benign lesions. RESULTS: Adaptive reconstruction demonstrated breast lesions in all eight women with similar image quality and signal-to-noise ratio (SNR) to Cartesian images with comparable imaging parameters. Contrast enhancement curves covering the entire postinjection time period were obtained from the dynamic images and in one case compared to previous enhancement profiles from a conventional Cartesian trajectory. CONCLUSION: Bilateral dynamic and high spatial resolution images with high SNR can be achieved in a clinically feasible manner, providing both kinetic and morphologic analysis with a single data set. This may obviate the need for multiple MRI examinations for a thorough breast MRI workup.     

Invited. Ultrafast T2-weighted imaging of the abdomen and pelvis: Use of single shot fast spin-echo imaging

Single shot (SS) rapid acquisition with relaxation enhancement (RARE) and half Fourier SS-RARE (HFSS-RARE, HASTE, or SS-FSE) sequences allow ultrafast imaging acquisition and generate high imaging quality. Images can be acquired within a very short time, without artifacts from physiologic motion. They are widely applied in the abdominal MRI. Clinical application of the ultrafast SS-RARE imaging techniques provide not only improved temporal resolution but better spatial resolution, higher SNR, and higher tissue contrast. Imaging parameters must be optimized for different MR scanners to obtain diagnostic images.     

MR imaging with spatially variable resolution.

In some situations it may be advantageous to produce "locally focused" magnetic resonance images that have nonuniform spatial resolution matching the expected local rate of spatial variation in the object. Because such an image has fewer pixels than a conventional image with uniformly high resolution, it can be reconstructed from fewer signals, acquired in less time. This can be done by using a highly convergent representation of the image as a sum of orthonormal functions with slow (fast) spatial variation in relatively homogeneous (heterogeneous) parts of the object. Since this series is shorter than a conventional truncated Fourier series, its terms can be calculated from a subset of the usual array of phase-encoded signals. The optimal choice of these phase encodings, which are usually scattered nonuniformly in k space, results in minimization of noise in the reconstructed image. The technique is illustrated by applying it to simulated data and to data from images of phantoms.     

Multispectral imaging with three-dimensional rosette trajectories.

Two-dimensional intersecting k-space trajectories have previously been demonstrated to allow fast multispectral imaging. Repeated sampling of k-space points leads to destructive interference of the signal coming from the off-resonance spectral peaks; on-resonance data reconstruction yields images of the on-resonance peak, with some of the off-resonance energy being spread as noise in the image. A shift of the k-space data by a given off-resonance frequency brings a second frequency of interest on resonance, allowing the reconstruction of a second spectral peak from the same k-space data. Given the higher signal-to-noise per unit time characteristic of a 3D acquisition, we extended the concept of intersecting trajectories to three dimensions. A 3D, rosette-like pulse sequence was designed and implemented on a clinical 1.5T scanner. An iterative density compensation function was developed to weight the 3D intersecting trajectories before Fourier transformation. Three volunteers were scanned using this sequence and separate fat and water images were reconstructed from the same imaging dataset.     

Removing undersampling artifacts in DCE-MRI studies using independent components analysis.

In breast MRI mammography both high temporal resolution and high spatial resolution have been shown to be important in improving specificity. Adaptive methods such as projection reconstruction time-resolved imaging of contrast kinetics (PR-TRICKS) allow images to be reconstructed at various temporal and spatial resolutions from the same data set. The main disadvantage is that the undersampling, which is necessary to produce high temporal resolution images, leads to the presence of streak artifacts in the images. We present a novel method of removing these artifacts using independent components analysis (ICA) and demonstrate that this results in a significant improvement in image quality for both simulation studies and for patient dynamic contrast-enhanced (DCE)-MRI images. We also investigate the effect of artifacts on two quantitative measures of contrast enhancement. Using simulation studies we demonstrate that streak artifacts lead to pronounced periodic oscillations in pixel concentration curves which, in turn, lead to increased errors and introduce bias into heuristic measurements. ICA filtering significantly reduces this bias and improves accuracy. Pharmacokinetic modeling was more robust and there was no evidence of bias due to the presence of streak artifacts. ICA filtering did not significantly reduce the errors in the estimated pharmacokinetic parameters; however, the chi-squared error was greatly reduced after ICA filtering.