Description: Purpose To integrate, optimize, and evaluate a three-dimensional (3D) contrast-enhanced sparse MRA technique with iterative reconstruction on a standard clinical MR system. Methods Data were acquired using a highly undersampled Cartesian spiral phyllotaxis sampling pattern and reconstructed directly on the MR system with an iterative SENSE technique. Undersampling, regularization, and number of iterations of the reconstruction were optimized and validated based on phantom experiments and patient data. Sparse MRA of the whole head (field of view: 265 × 232 × 179 mm 3 ) was investigated in 10 patient examinations. Results High-quality images with 30-fold undersampling, resulting in 0.7 mm isotropic resolution within 10 s acquisition, were obtained. After optimization of the regularization factor and of the number of iterations of the reconstruction, it was possible to reconstruct images with excellent quality within six minutes per 3D volume. Initial results of sparse contrast-enhanced MRA (CEMRA) in 10 patients demonstrated high-quality whole-head first-pass MRA for both the arterial and venous contrast phases. Conclusion While sparse MRI techniques have not yet reached clinical routine, this study demonstrates the technical feasibility of high-quality sparse CEMRA of the whole head in a clinical setting. Sparse CEMRA has the potential to become a viable alternative where conventional CEMRA is too slow or does not provide sufficient spatial resolution. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose High-resolution MRI combined with phospholipid detection may improve breast cancer grading. Currently, configurations are optimized for either high-resolution imaging or 31 P spectroscopy. To be able to perform both imaging as well as spectroscopy in a single session, we integrated a 1 H receiver array into a 1 H- 31 P transceiver at 7T. To ensure negligible signal loss due to coupling between elements, we investigated the use of a floating decoupling loop to enable bilateral MRI and 31 P MRS. Methods Two quadrature double-tuned radiofrequency coils were designed for bilateral breast MR with active detuning at the 1 H frequency. The two coils were placed adjacent to each other and decoupled for both frequencies with a single resonant floating loop. Sensitivity of the bilateral configuration, facilitating space for a 26-element 1 H receive array, was compared with a transceiver configuration. Results The floating loop was able to decouple the elements over 20 dB for both frequencies. Enlargement of the elements, to provide space for the receivers, and the addition of detuning electronics altered the 31 P sensitivity by 0.4 dB. Conclusion Dynamic contrast-enhanced scans of 0.7 mm isotropic, diffusion-weighted imaging, and 31 P MR spectroscopic imaging can be acquired at 7T in a single session as demonstrated in a patient with invasive ductal carcinoma. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose Manganese (Mn) is an effective contrast agent and biologically active metal, which has been widely used for Mn-enhanced MRI (MEMRI). The purpose of this study was to develop and test a Mn binding protein for use as a genetic reporter for MEMRI. Methods The bacterial Mn-binding protein, MntR was identified as a candidate reporter protein. MntR was engineered for expression in mammalian cells, and targeted to different subcellular organelles, including the Golgi Apparatus where cellular Mn is enriched. Transfected HEK293 cells and B16 melanoma cells were tested in vitro and in vivo, using immunocytochemistry, MR imaging and relaxometry. Results Subcellular targeting of MntR to the cytosol, endoplasmic reticulum and Golgi apparatus was verified with immunocytochemistry. After targeting to the Golgi, MntR expression produced robust R1 changes and T1 contrast in cells, in vitro and in vivo. Co-expression with the divalent metal transporter DMT1, a previously described Mn-based reporter, further enhanced contrast in B16 cells in culture, but in the in vivo B16 tumor model tested was not significantly better than MntR alone. Conclusion This second-generation reporter system both expands the capabilities of genetically encoded reporters for imaging with MEMRI and provides important insights into the mechanisms of Mn biology which create endogenous MEMRI contrast. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose The aim of this study was to develop a technique for rapid collection of chemical exchange saturation transfer images with the saturation varied to modulate signal loss transfer and enhance contrast. Methods Multi-echo Length and Offset VARied Saturation (MeLOVARS) divides the saturation pulse of length T sat into N = 3–8 submodules, each consisting of a saturation pulse with length of T sat /N (∼0.3–1 s), one or more low flip-angle gradient-echo readout(s) and a flip back pulse. This results in N readouts with increasing saturation time from T sat /N to T sat without extra scan time. Results For phantoms, eight images with T sat incremented every 0.5 s from 0.5–4 s were collected simultaneously using MeLOVARS, which allows rapid determination of exchange rates for agent protons. For live mice bearing glioblastomas, the Z-spectra for five different T sat values from 0.5 to 2.5 s were acquired in a time normally used for one T sat . With the additional T sat -dependence information, LOVARS phase maps were produced with a more clearly defined tumor boundary and an estimated 4.3-fold enhanced contrast-to-noise ratio (CNR). We also show that enhancing CNR is achievable by simply averaging the collected images or transforming them using the principal component analysis. Conclusions MeLOVARS enables collection of multiple saturation-time-weighted images without extra time, producing a LOVARS phase map with increased CNR. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose To study how sensitivity encoding (SENSE) impacts periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) image quality, including signal-to-noise ratio (SNR), robustness to motion, precision of motion estimation, and image quality. Methods Five volunteers were imaged by three sets of scans. A rapid method for generating the g-factor map was proposed and validated via Monte Carlo simulations. Sensitivity maps were extrapolated to increase the area over which SENSE can be performed and therefore enhance the robustness to head motion. The precision of motion estimation of PROPELLER blades that are unfolded with these sensitivity maps was investigated. An interleaved R-factor PROPELLER sequence was used to acquire data with similar amounts of motion with and without SENSE acceleration. Two neuroradiologists independently and blindly compared 214 image pairs. Results The proposed method of g-factor calculation was similar to that provided by the Monte Carlo methods. Extrapolation and rotation of the sensitivity maps allowed for continued robustness of SENSE unfolding in the presence of motion. SENSE-widened blades improved the precision of rotation and translation estimation. PROPELLER images with a SENSE factor of 3 outperformed the traditional PROPELLER images when reconstructing the same number of blades. Conclusion SENSE not only accelerates PROPELLER but can also improve robustness and precision of head motion correction, which improves overall image quality even when SNR is lost due to acceleration. The reduction of SNR, as a penalty of acceleration, is characterized by the proposed g-factor method. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose To investigate the microscopic diffusion properties of formalin-fixed breast tissue. Methods Diffusion microimaging was performed at 16.4T with 40-μm isotropic voxels on two normal and two cancer tissue samples from four patients. Results were correlated with histology of the samples. Results Diffusion-weighted images and mean diffusivity maps demonstrated distinct diffusivity differences between breast tissue components. Mean diffusivity (MD) in normal tissue was 0.59 ± 0.24 μm 2 /ms for gland lobule (voxels containing epithelium and intralobular stroma) and 1.23 ± 0.34 μm 2 /ms for interlobular fibrous stroma. In the cancer samples, MD = 0.45 ± 0.23 μm 2 /ms for invasive ductal carcinoma (voxels contain epithelium and intralobular stroma) and 0.61 ± 0.35 μm 2 /ms for ductal carcinoma in situ. There were significant MD differences between all tissue components ( P  〈 0.005), except between gland lobule and ductal carcinoma in situ ( P  = 0.71). The low diffusivity of epithelium-rich cancer tissue and of normal epithelium relative to its supporting fibrous stroma was similar to that reported for prostate tissue and the esophageal wall. Conclusion Diffusion microimaging demonstrates distinct diffusivity differences between breast tissue glandular structures. Low diffusivity may be a distinctive feature of mammalian epithelia. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose This study explores the possibility of using gradient echo-based sequences other than balanced steady-state free precession (bSSFP) in the magnetic resonance fingerprinting (MRF) framework to quantify the relaxation parameters . Methods An MRF method based on a fast imaging with steady-state precession (FISP) sequence structure is presented. A dictionary containing possible signal evolutions with physiological range of T 1 and T 2 was created using the extended phase graph formalism according to the acquisition parameters. The proposed method was evaluated in a phantom and a human brain. T 1 , T 2 , and proton density were quantified directly from the undersampled data by the pattern recognition algorithm. Results T 1 and T 2 values from the phantom demonstrate that the results of MRF FISP are in good agreement with the traditional gold-standard methods. T 1 and T 2 values in brain are within the range of previously reported values. Conclusion MRF-FISP enables a fast and accurate quantification of the relaxation parameters. It is immune to the banding artifact of bSSFP due to B 0 inhomogeneities, which could improve the ability to use MRF for applications beyond brain imaging. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose High spatial-temporal four-dimensional imaging with large volume coverage is necessary to accurately capture and characterize liver lesions. Traditionally, parallel imaging and adapted sampling are used toward this goal, but they typically result in a loss of signal to noise. Furthermore, residual under-sampling artifacts can be temporally varying and complicate the quantitative analysis of contrast enhancement curves needed for pharmacokinetic modeling. We propose to overcome these problems using a novel patch-based regularization approach called Patch-based Reconstruction Of Under-sampled Data (PROUD). Theory and Methods PROUD produces high frame rate image reconstructions by exploiting the strong similarities in spatial patches between successive time frames to overcome the severe k-space under-sampling. To validate PROUD, a numerical liver perfusion phantom was developed to characterize contrast-to-noise ratio (CNR) performance compared with a previously proposed method, TRACER. A second numerical phantom was constructed to evaluate the temporal footprint and lag of PROUD and TRACER reconstructions. Finally, PROUD and TRACER were evaluated in a cohort of five liver donors. Results In the CNR phantom, PROUD, compared with TRACER, improved peak CNR by 3.66 times while maintaining or improving temporal fidelity. In vivo, PROUD demonstrated an average increase in CNR of 60% compared with TRACER. Conclusion The results presented in this work demonstrate the feasibility of using a combination of patch based image constraints with temporal regularization to provide high SNR, high temporal frame rate and spatial resolution four dimensional imaging. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose Diffusion-weighted (DW) hyperpolarized 129 Xe morphometry magnetic resonance imaging (MRI) can be used to map regional differences in lung tissue micro-structure. We aimed to generate absolute xenon concentration ([Xe]) and alveolar oxygen partial pressure (p A O 2 ) maps by extracting the unrestricted diffusion coefficient ( D 0 ) of xenon as a morphometric parameter. Methods In this proof-of-concept demonstration, morphometry was performed using multi b-value (0, 12, 20, 30 s/cm 2 ) DW hyperpolarized 129 Xe images obtained in four never-smokers and four COPD ex-smokers. Morphometric parameters and D 0 maps were computed and the latter used to generate [Xe] and p A O 2 maps. Xenon concentration phantoms estimating a range of values mimicking those observed in vivo were also investigated. Results Xenon D 0 was significantly increased ( P  = 0.035) in COPD (0.14 ± 0.03 cm 2 /s) compared with never-smokers (0.12 ± 0.02 cm 2 /s). COPD ex-smokers also had significantly decreased [Xe] (COPD = 8 ± 7% versus never-smokers = 13 ± 8%, P  = 0.012) and increased p A O 2 (COPD = 18 ± 3% versus never-smokers = 15 ± 3%, P  = 0.009) compared with never-smokers. Phantom measurements showed the expected dependence of D 0 on [Xe] over the range of concentrations anticipated in vivo. Conclusion DW hyperpolarized 129 Xe MRI morphometry can be used to simultaneously map [Xe] and p A O 2 in addition to providing micro-structural biomarkers of emphysematous destruction in COPD. Phantom measurements of D 0 ([Xe]) supported the hypotheses that differences in subjects may reflect differences in functional residual capacity. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Description: Purpose To characterize the fundamental limits of MRI near metallic implants on RF excitation, frequency encoding, and chemical shift–encoding water–fat separation. Methods Multicomponent three-dimensional (3D) digital models of a total hip and a total knee replacement were used to construct material-specific susceptibility maps. The fundamental limits and spatial relationship of imaging near metallic prostheses were investigated as a function of distance from the prosthetic surface by calculating 3D field map perturbations using a well-validated k-space based dipole kernel. Results Regions limited by the bandwidth of RF excitation overlap substantially with those fundamentally limited by frequency encoding. Rapid breakdown of water–fat separation occurs once the intravoxel off-resonance exceeds ∼6 ppm over a full range of fat fractions (0%–100%) and SNR (5–100). Conclusion Current 3D multispectral imaging methods would not benefit greatly from exciting spins beyond ±12 kHz despite the presence of signal that lies outside of this range from tissue directly adjacent to the metallic implants. Methods such as phase encoding in all three spatial dimensions are necessary to spatially resolve spins beyond an excitation bandwidth of ±12 kHz. The approach described in this study provides a benchmark for the capabilities of current imaging techniques to guide development of new MRI methods for imaging near metal. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.