High-resolution human cervical spinal cord imaging at 7 T.

We present high-resolution anatomical imaging of the cervical spinal cord in healthy volunteers at the ultrahigh field of 7 T with a prototype four-channel radiofrequency coil array, in comparison with 3-T imaging of the same subjects. Signal-to-noise ratios at both field strengths were estimated using the rigorous Kellman method. Spinal cord cross-sectional area measurements were performed, including whole-cord measurements at both fields and gray matter segmentation at 7 T. The 7-T array coil showed reduced sagittal coverage, comparable axial coverage and the expected significantly higher signal-to-noise ratio compared with equivalent 3-T protocols. In the cervical spinal cord, the signal-to-noise ratio was found by the Kellman method to be higher by a factor of 3.5 with the 7-T coil than with standard 3-T coils. Cervical spine imaging in healthy volunteers at 7 T revealed not only detailed white/gray matter differentiation, but also structures not visualized at lower fields, such as denticulate ligaments, nerve roots and rostral-caudal blood vessels. Whole-cord cross-sectional area measurements showed good agreement at both field strengths. The measurable gray/white matter cross-sectional areas at 7 T were found to be comparable with reports from histology. These pilot data demonstrate the use of higher signal-to-noise ratios at the ultrahigh field of 7 T for significant improvement in anatomical resolution of the cervical spinal cord, allowing the visualization of structures not seen at lower field strength, particularly for axial imaging.

Intravoxel incoherent motion imaging of tumor microenvironment in locally advanced breast cancer.

Diffusion-weighted imaging plays important roles in cancer diagnosis, monitoring, and treatment. Although most applications measure restricted diffusion by tumor cellularity, diffusion-weighted imaging is also sensitive to vascularity through the intravoxel incoherent motion effect. Hypervascularity can confound apparent diffusion coefficient measurements in breast cancer. We acquired multiple b-value diffusion-weighted imaging at 3 T in a cohort of breast cancer patients and performed biexponential intravoxel incoherent motion analysis to extract tissue diffusivity (D(t)), perfusion fraction (f(p)), and pseudodiffusivity (D(p)). Results indicated significant differences between normal fibroglandular tissue and malignant lesions in apparent diffusion coefficient mean (±standard deviation) values (2.44 ± 0.30 vs. 1.34 ± 0.39 µm(2)/msec, P < 0.01) and D(t) (2.36 ± 0.38 vs. 1.15 ± 0.35 µm(2)/msec, P < 0.01). Lesion diffusion-weighted imaging signals demonstrated biexponential character in comparison to monoexponential normal tissue. There is some differentiation of lesion subtypes (invasive ductal carcinoma vs. other malignant lesions) with f(p) (10.5 ± 5.0% vs. 6.9 ± 2.9%, P = 0.06), but less so with D(t) (1.14 ± 0.32 µm(2)/msec vs. 1.18 ± 0.52 µm(2)/msec, P = 0.88) and D(p) (14.9 ± 11.4 µm(2)/msec vs. 16.1 ± 5.7 µm(2)/msec, P = 0.75). Comparison of intravoxel incoherent motion biomarkers with contrast enhancement suggests moderate correlations. These results suggest the potential of intravoxel incoherent motion vascular and cellular biomarkers for initial grading, progression monitoring, or treatment assessment of breast tumors.

Double-oblique free-breathing high resolution three-dimensional coronary magnetic resonance angiography.

OBJECTIVES: The goal of the present study was to develop a strategy for three-dimensional (3D) volume acquisition along the major axes of the coronary arteries. BACKGROUND: For high-resolution 3D free-breathing coronary magnetic resonance angiography (MRA), coverage of the coronary artery tree may be limited due to excessive measurement times associated with large volume acquisitions. Planning the 3D volume along the major axis of the coronary vessels may help to overcome such limitations. METHODS: Fifteen healthy adult volunteers and seven patients with X-ray angiographically confirmed coronary artery disease underwent free-breathing navigator-gated and corrected 3D coronary MRA. For an accurate volume targeting of the high resolution scans, a three-point planscan software tool was applied. RESULTS: The average length of contiguously visualized left main and left anterior descending coronary artery was 81.8 +/- 13.9 mm in the healthy volunteers and 76.2 +/- 16.5 mm in the patients (p = NS). For the right coronary artery, a total length of 111.7 +/- 27.7 mm was found in the healthy volunteers and 79.3 +/- 4.6 mm in the patients (p = NS). Comparing coronary MRA and X-ray angiography, a good agreement of anatomy and pathology was found in the patients. CONCLUSIONS: Double-oblique submillimeter free-breathing coronary MRA allows depiction of extensive parts of the native coronary arteries. The results obtained in patients suggest that the method has the potential to be applied in broader prospective multicenter studies where coronary MRA is compared with X-ray angiography.

A generalized approach to parallel magnetic resonance imaging.

Parallel magnetic resonance (MR) imaging uses spatial encoding from multiple radiofrequency detector coils to supplement the encoding supplied by magnetic field gradients, and thereby to accelerate MR image acquisitions beyond previous limits. A generalized formulation for parallel MR imaging is derived, demonstrating the relationship between existing techniques such as SMASH and SENSE, and suggesting new algorithms with improved performance. Hybrid approaches combining features of both SMASH-like and SENSE-like image reconstructions are constructed, and numerical conditioning techniques are described which can improve the practical robustness of parallel image reconstructions. Incorporation of numerical conditioning directly into parallel reconstructions using the generalized approach also removes a cumbersome and potentially error-prone sensitivity calibration step involving division of two distinct in vivo reference images. Hybrid approaches in combination with numerical conditioning are shown to extend the range of accelerations over which high-quality parallel images may be obtained.

SMASH imaging.

SMASH imaging is a new MR imaging technique that can be used to multiply the speed of existing imaging sequences. It operates by using an array of radiofrequency (RF) detection coils to perform some of the spatial encoding normally accomplished with magnetic field gradients. The speed of the SMASH technique results from appropriate combinations of coil array RF signals in which multiple lines of image data are gathered simultaneously, rather than one after another. SMASH can be used in conjunction with most rapid imaging sequences, including EPI, resulting in multiplicative gains in imaging speed. This article reviews the basic principles of SMASH imaging, outlines requirements for practical implementation, and presents a variety of in vivo results, highlighting ways in which SMASH may be used to increase imaging speed and to improve image quality for clinical MR imaging applications.

Tailored SMASH image reconstructions for robust in vivo parallel MR imaging.

The simultaneous acquisition of spatial harmonics (SMASH) imaging technique uses spatial information from an array of RF coils to substitute for omitted encoding gradient steps and thereby to accelerate MR image acquisition. Since SMASH image reconstructions rely on the accurate generation of sinusoidally varying composite sensitivity functions to emulate the spatial modulations produced by gradients, the technique was originally believed to be limited to certain image planes or coil array configurations which were particularly suited to the generation of spatial harmonics. Several key improvements to the SMASH reconstruction procedure are described, taking advantage of various degrees of freedom in the spatial harmonic fit. The use of tailored fitting procedures, in combination with a numerical conditioning approach based on new observations about noise propagation in the fit, are shown to allow high-quality SMASH image reconstructions in oblique and double-oblique image planes, both in phantoms and in high-resolution cardiac MR images. Magn Reson Med 44:243-251, 2000.

Simultaneous acquisition of spatial harmonics (SMASH): fast imaging with radiofrequency coil arrays.

SiMultaneous Acquisition of Spatial Harmonics (SMASH) is a new fast-imaging technique that increases MR image acquisition speed by an integer factor over existing fast-imaging methods, without significant sacrifices in spatial resolution or signal-to-noise ratio. Image acquisition time is reduced by exploiting spatial information inherent in the geometry of a surface coil array to substitute for some of the phase encoding usually produced by magnetic field gradients. This allows for partially parallel image acquisitions using many of the existing fast-imaging sequences. Unlike the data combination algorithms of prior proposals for parallel imaging, SMASH reconstruction involves a small set of MR signal combinations prior to Fourier transformation, which can be advantageous for artifact handling and practical implementation. A twofold savings in image acquisition time is demonstrated here using commercial phased array coils on two different MR-imaging systems. Larger time savings factors can be expected for appropriate coil designs.

Improved spatial harmonic selection for SMASH image reconstructions.

The fitting of coil sensitivity functions to spatial harmonics is central to image reconstructions using the simultaneous acquisition of spatial harmonics (SMASH) technique. It has previously been shown that the selection of the set of spatial harmonics used in a SMASH reconstruction can have a noticeable effect on the quality of the reconstructed image. However, a mechanism for automatic selection of the best set of harmonics in any particular situation has not been provided. In this work, a modification to the SMASH reconstruction procedure is introduced that allows the use of a weighted average of all possible harmonics in a reconstruction. The new reconstruction procedure is shown to allow automatic selection of the spatial harmonics and substantially improve SNR for both phantom and in vivo images.

AUTO-SMASH: a self-calibrating technique for SMASH imaging. SiMultaneous Acquisition of Spatial Harmonics.

Recently a new fast magnetic resonance imaging strategy, SMASH, has been described, which is based on partially parallel imaging with radiofrequency coil arrays. In this paper, an internal sensitivity calibration technique for the SMASH imaging method using self-calibration signals is described. Coil sensitivity information required for SMASH imaging is obtained during the actual scan using correlations between undersampled SMASH signal data and additionally sampled calibration signals with appropriate offsets in k-space. The advantages of this sensitivity reference method are that no extra coil array sensitivity maps have to be acquired and that it provides coil sensitivity information in areas of highly non-uniform spin-density. This auto-calibrating approach can be easily implemented with only a small sacrifice of the overall time savings afforded by SMASH imaging. The results obtained from phantom imaging experiments and from cardiac studies in nine volunteers indicate that the self-calibrating approach is an effective method to increase the potential and the flexibility of rapid imaging with SMASH.

SMASH imaging with an eight element multiplexed RF coil array.

SMASH (SiMultaneous Acquisition of Spatial Harmonics) is a technique which can be used to acquire multiple lines of k-space in parallel, by using spatial information from a radiofrequency coil array to perform some of the encoding normally produced by gradients. Using SMASH, imaging speed can be increased up to a maximum acceleration factor equal to the number of coil array elements. This work is a feasibility study which examines the use of SMASH with specialized coil array and data reception hardware to achieve previously unattainable accelerations. An eight element linear SMASH array was designed to operate in conjunction with a time domain multiplexing system to examine the effectiveness of SMASH imaging with as much as eightfold acceleration factors. Time domain multiplexing allowed the multiple independent array elements to be sampled through a standard single-channel receiver. SMASH-reconstructed images using this system were compared with reference images, and signal to noise ratio and reconstruction artifact power were measured as a function of acceleration factor. Results of the imaging experiments showed an almost constant SNR for SMASH acceleration factors of up to eight. Artifact power remained low within this range of acceleration factors. This study demonstrates that efficient SMASH imaging at high acceleration factors is feasible using appropriate hardware, and that time domain multiplexing is a convenient strategy to provide the multiple channels required for rapid imaging with large arrays.

A multicoil array designed for cardiac SMASH imaging.

Recently, several partially parallel acquisition (PPA) techniques have been presented which use spatial information inherent in an RF coil array to reconstruct an image from a reduced set of phase encoding steps. PPAs represent a change in paradigm for the RF coil designer since the focus for arrays to be used with PPAs is to optimize the spatial encoding that is provided by the array. One of the first practical implementations of PPA imaging was demonstrated using the SMASH technique. In this study, we present our results from the construction of the first array designed specifically for cardiac SMASH imaging. Additional design criteria are presented for SMASH arrays that are not considered in conventional array design. Using these design criteria, a four-element array was constructed and then tested in SMASH imaging experiments in the heart. This array has been used in all of our initial cardiac and head SMASH studies with good results.

Signal-to-noise ratio and signal-to-noise efficiency in SMASH imaging.

A general theory of signal-to-noise ratio (SNR) in simultaneous acquisition of spatial harmonics (SMASH) imaging is presented, and the predictions of the theory are verified in imaging experiments and in numerical simulations. In a SMASH image, multiple lines of k-space are generated simultaneously through combinations of magnetic resonance signals in a radiofrequency coil array. Here, effects of noise correlations between array elements as well as new correlations introduced by the SMASH reconstruction procedure are assessed. SNR and SNR efficiency in SMASH images are compared with results using traditional array combination strategies. Under optimized conditions, SMASH achieves the same average SNR efficiency as ideal pixel-by-pixel array combinations, while allowing imaging to proceed at otherwise unattainable speeds. The k-space nature of SMASH reconstructions can lead to oscillatory spatial variations in noise standard deviation, which can produce local enhancements of SNR in particular regions.

Accelerated cardiac imaging using the SMASH technique.

SMASH (SiMultaneous Acquisition of Spatial Harmonics) was recently introduced as a novel rapid-imaging technique. The SMASH technique uses a partially parallel acquisition strategy, using spatial information from a radiofrequency coil array to accelerate imaging. This study constitutes the first application of SMASH to cardiac magnetic resonance imaging. The increased imaging speed provided by SMASH was used to obtain images with reduced breathhold duration, enhanced spatial resolution, and increased temporal resolution in healthy volunteers. The results obtained demonstrate the feasibility and potential clinical utility of cardiac magnetic resonance imaging using the SMASH technique.

Contrast-enhanced 3D MR angiography with simultaneous acquisition of spatial harmonics: A pilot study.

A partially parallel image acquisition technique, simultaneous acquisition of spatial harmonics, or SMASH, was used to increase the spatial and/or temporal resolution in contrast material-enhanced three-dimensional magnetic resonance angiography of the abdominal aorta and renal arteries. In eight healthy subjects, the breath-hold duration was halved at constant spatial resolution, or the spatial resolution was doubled at fixed breath-hold duration, with a 30%-55% reduction in the signal-to-noise ratio but otherwise preserved or improved image quality.

Superiority of prone position in free-breathing 3D coronary MRA in patients with coronary disease.

Navigator-gated and corrected 3D coronary MR angiography (MRA) allows submillimeter image acquisition during free breathing. However, cranial diaphragmatic drift and relative phase shifts of chest-wall motion are limiting factors for image quality and scanning duration. We hypothesized that image acquisition in the prone position would minimize artifacts related to chest-wall motion and suppress diaphragmatic drift. Twelve patients with radiographically-confirmed coronary artery disease and six healthy adult volunteers were studied in both the prone and the supine position during free-breathing navigator-gated and corrected 3D coronary MRA. Image quality and the diaphragmatic positions were objectively compared. In the prone position, there was a 36% improvement in signal-to-noise ratio (SNR; 15.5 +/- 2.7 vs. 11.4 +/- 2.6; P < 0.01) and a 34% improvement in CNR (12.5 +/- 3.3 vs. 9.3 +/- 2.5, P < 0.01). The prone position also resulted in a 17% improvement in coronary vessel definition (P < 0.01). Cranial end-expiratory diaphragmatic drift occurred less frequently in the prone position (23% +/- 17% vs. 40% +/- 26% supine; P <0.05), and navigator efficiency was higher. Prone coronary MRA results in improved SNR and CNR with enhanced coronary vessel definition. Cranial end-expiratory diaphragmatic drift also was reduced, and navigator efficiency was enhanced. When feasible, prone imaging is recommended for free-breathing coronary MRA.

Coil-by-coil image reconstruction with SMASH.

The SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique uses linear combinations of undersampled datasets from the component coils of an RF coil array to reconstruct fully sampled composite datasets in reduced imaging times. In previously reported implementations, SMASH reconstructions were designed to reproduce the images that would otherwise be obtained by simple sums of fully gradient encoded component coil images. This strategy has left SMASH images vulnerable to phase cancellation artifacts when the sensitivities of RF coil array elements are not suitably phase-aligned. In fully gradient encoded imaging schemes these artifacts can be eliminated using a variety of methods for combining the individual coil images, including matched filter combinations as well as sum of squares combinations. Until now, these reconstruction schemes have been unavailable to SMASH reconstructions as SMASH produced a final composite image directly from the raw component coil k-space datasets. This article demonstrates a modification to SMASH that allows reconstruction of a full set of accelerated individual component coil images by fitting component coil sensitivity functions to a complete set of spatial harmonics tailored for each coil in the array. Standard component coil combinations applied to the individual reconstructed images produce final composite images free of phase cancellation artifacts.

Resolution enhancement in single-shot imaging using simultaneous acquisition of spatial harmonics (SMASH).

Spatial resolution in single-shot imaging is limited by signal attenuation due to relaxation of transverse magnetization. This effect can be reduced by minimizing acquisition times through the use of short interecho spacings. However, the minimum interecho spacing is constrained by limits on gradient switching rates, radiofrequency (RF) power deposition and RF pulse length. Recently, simultaneous acquisition of spatial harmonics (SMASH) has been introduced as a method to acquire magnetic resonance images at increased speeds using a reduced number of phase-encoding gradient steps by extracting spatial information contained in an RF coil array. In this study, it is shown that SMASH can be used to reduce the effects of relaxation, resulting in single-shot images with increased spatial resolution without increasing imaging time. After a brief theoretical discussion, two strategies to reduce signal attenuation and increase spatial resolution in single-shot imaging are introduced and their performance is evaluated in phantom studies. In vivo single-shot echoplanar imaging (EPI), BURST, and half-Fourier single-shot turbo spin-echo (HASTE) images are then presented demonstrating the practical implementation of these resolution enhancement strategies. Images acquired with SMASH show increased spatial resolution and improved image quality when compared with images obtained with the conventional acquisitions. The general principles presented for imaging with SMASH can also be applied to other partially parallel imaging techniques.