Magnetic Resonance Imaging of the Fibrotic Kidney.

Magnetic resonance imaging (MRI) has been used for many years for anatomic evaluation of the kidney. Recently developed methods attempt to go beyond anatomy to give information about the health and function of the kidneys. Several methods, including diffusion-weighted MRI, renal blood oxygen level-dependent MRI, renal MR elastography, and renal susceptibility imaging, show promise for providing unique insight into kidney function and severity of fibrosis. However, substantial limitations in accuracy and practicality limit the immediate clinical application of each method. Further development and improvement are necessary to achieve the ideal of a noninvasive image-based measure of renal fibrosis. Our brief review provides a short explanation of these emerging MRI methods and outlines the promising initial results obtained with each as well as current limitations and barriers to clinical implementation.

Psoas Muscle Cross-sectional Area as a Measure of Whole-body Lean Muscle Mass in Maintenance Hemodialysis Patients.

OBJECTIVE: We investigate whether psoas or paraspinous muscle area measured on a single L4-L5 image is a useful measure of whole lean body mass (LBM) compared to dedicated midthigh magnetic resonance imaging (MRI). DESIGN: Observational study. SETTING: Outpatient dialysis units and a research clinic. SUBJECTS: One hundred five adult participants on maintenance hemodialysis. No control group was used. INTERVENTION: Psoas muscle area, paraspinous muscle area, and midthigh muscle area (MTMA) were measured by magnetic resonance imaging. MAIN OUTCOME MEASURE: LBM was measured by dual-energy absorptiometry scan. RESULTS: In separate multivariable linear regression models, psoas, paraspinous, and MTMA were associated with increase in LBM. In separate multivariate logistic regression models, C statistics for diagnosis of sarcopenia (defined as <25th percentile of LBM) were 0.69 for paraspinous muscle area, 0.81 for psoas muscle area, and 0.89 for MTMA. With sarcopenia defined as <10th percentile of LBM, the corresponding C statistics were 0.71, 0.92, and 0.94. CONCLUSIONS: We conclude that psoas muscle area provides a good measure of whole-body muscle mass, better than paraspinous muscle area but slightly inferior to midthigh measurement. Hence, in body composition studies a single axial MR image at the L4-L5 level can be used to provide information on both fat and muscle and may eliminate the need for time-consuming measurement of muscle area in the thigh.

Science to practice: Renal hypoxia and fat deposition in diabetic neuropathy--new insights with functional renal MR imaging.

Despite being a valuable tool for evaluation of the kidneys, renal magnetic resonance (MR) imaging in clinical practice has been limited to depiction of anatomy and provides little diagnostic information about the health and function of the kidney in patients with chronic kidney disease (CKD) and diabetic nephropathy. In this issue, Peng et al (1) have used two MR imaging methods that go beyond depiction of anatomy to show renal function: renal blood oxygen level-dependent (BOLD) MR imaging, which shows oxygen levels in the kidney, and chemical shift-selective imaging, which shows the relative content of fat in the kidney parenchyma. In a mouse model of diabetes, Peng et al have shown higher fat and lower oxygen levels in kidneys of mice with diabetes than in those of normal controls. These MR imaging methods may help clarify the role of fat deposition and hypoxia in the progression of CKD. As the factors that contribute to the progression of CKD are better understood, ultimately more widespread clinical use for functional renal MR imaging protocols such as renal BOLD and chemical shift-selective imaging may be found to evaluate the severity of CKD and monitor the efficacy of clinical interventions, altering the course of disease progression.

Simultaneous fat suppression and band reduction with large-angle multiple-acquisition balanced steady-state free precession.

Balanced steady-state free precession (bSSFP) MRI is a rapid and signal-to-noise ratio-efficient imaging method, but suffers from characteristic bands of signal loss in regions of large field inhomogeneity. Several methods have been developed to reduce the severity of these banding artifacts, typically involving the acquisition of multiple bSSFP datasets (and the accompanying increase in scan time). Fat suppression with bSSFP is also challenging; most existing methods require an additional increase in scan time, and some are incompatible with bSSFP band-reduction techniques. This work was motivated by the need for both robust fat suppression and band reduction in the presence of field inhomogeneity when using bSSFP for flow-independent peripheral angiography. The large flip angles used in this application to improve vessel conspicuity and contrast lead to specific absorption rate considerations, longer repetition times, and increased severity of banding artifacts. In this work, a novel method that simultaneously suppresses fat and reduces bSSFP banding artifact with the acquisition of only two phase-cycled bSSFP datasets is presented. A weighted sum of the two bSSFP acquisitions is taken on a voxel-by-voxel basis, effectively synthesizing an off-resonance profile at each voxel that puts fat in the stop band while keeping water in the pass band. The technique exploits the near-sinusoidal shape of the bSSFP off-resonance spectrum for many tissues at large (>50°) flip angles.

Phase-sensitive sodium B1 mapping.

Quantitative sodium MRI requires accurate knowledge of factors affecting the sodium signal. One important determinant of sodium signal level is the transmit B(1) field strength. However, the low signal-to-noise ratio typical of sodium MRI makes accurate B(1) mapping in reasonable scan times challenging. A new phase-sensitive B(1) mapping technique has recently been shown to work better than the widely used dual-angle method in low-signal-to-noise ratio situations and over a broader range of flip angles. In this work, the phase-sensitive B(1) mapping technique is applied to sodium, and its performance compared to the dual-angle method through both simulation and phantom studies. The phase-sensitive method is shown to yield higher quality B(1) maps at low signal-to-noise ratio and greater consistency of measurement than the dual-angle method. An in vivo sodium B(1) map of the human breast is also shown, demonstrating the phase-sensitive method's feasibility for human studies.

Superelliptical insert gradient coil with a field-modifying layer for breast imaging.

Many MRI applications such as dynamic contrast-enhanced MRI of the breast require high spatial and temporal resolution and can benefit from improved gradient performance, e.g., increased gradient strength and reduced gradient rise time. The improved gradient performance required to achieve high spatial and temporal resolution for this application may be achieved by using local insert gradients specifically designed for a target anatomy. Current flat gradient systems cannot create an imaging volume large enough to accommodate both breasts; further, their gradient fields are not homogeneous, dropping off rapidly with distance from the gradient coil surface. To attain an imaging volume adequate for bilateral breast MRI, a planar local gradient system design has been modified into a superellipse shape, creating homogeneous gradient volumes that are 182% (Gx), 57% (Gy), and 75% (Gz) wider (left/right direction) than those of the corresponding standard planar gradient. Adding an additional field-modifying gradient winding results in an additional improvement of the homogeneous gradient field near the gradient coil surface over the already enlarged homogeneous gradient volumes of the superelliptical gradients (67%, 89%, and 214% for Gx, Gy, and Gz respectively). A prototype y-gradient insert has been built to demonstrate imaging and implementation characteristics of the superellipse gradient in a 3 T MRI system.

Pharmacokinetic mapping for lesion classification in dynamic breast MRI.

PURPOSE: To prospectively investigate whether a rapid dynamic MRI protocol, in conjunction with pharmacokinetic modeling, could provide diagnostically useful information for discriminating biopsy-proven benign lesions from malignancies. MATERIALS AND METHODS: Patients referred to breast biopsy based on suspicious screening findings were eligible. After anatomic imaging, patients were scanned using a dynamic protocol with complete bilateral breast coverage. Maps of pharmacokinetic parameters representing transfer constant (K(trans)), efflux rate constant (k(ep)), blood plasma volume fraction (v(p)), and extracellular extravascular volume fraction (v(e)) were averaged over lesions and used, with biopsy results, to generate receiver operating characteristic curves for linear classifiers using one, two, or three parameters. RESULTS: Biopsy and imaging results were obtained from 93 lesions in 74 of 78 study patients. Classification based on K(trans) and k(ep) gave the greatest accuracy, with an area under the receiver operating characteristic curve of 0.915, sensitivity of 91%, and specificity of 85%, compared with values of 88% and 68%, respectively, obtained in a recent study of clinical breast MRI in a similar patient population. CONCLUSION: Pharmacokinetic classification of breast lesions is practical on modern MRI hardware and provides significant accuracy for identification of malignancies. Sensitivity of a two-parameter linear classifier is comparable to that reported in a recent multicenter study of clinical breast MRI, while specificity is significantly higher.

Uncertainty in T(1) mapping using the variable flip angle method with two flip angles.

Propagation of errors, in conjunction with the theoretical signal equation for spoiled gradient echo pulse sequences, is used to derive a theoretical expression for uncertainty in quantitative variable flip angle T(1) mapping using two flip angles. This expression is then minimized to derive a rigorous expression for optimal flip angles that elucidates a commonly used empirical result. The theoretical expressions for uncertainty and optimal flip angles are combined to derive a lower bound on the achievable uncertainty for a given set of pulse sequence parameters and signal-to-noise ratio (SNR). These results provide a means of quantitatively determining the effect of changing acquisition parameters on T(1) uncertainty.

HASTE sequence with parallel acquisition and T2 decay compensation: application to carotid artery imaging.

T2-weighted carotid artery images acquired using the turbo spin-echo (TSE) sequence frequently suffer from motion artifacts due to respiration and blood pulsation. The possibility of using HASTE sequence to achieve motion-free carotid images was investigated. The HASTE sequence suffers from severe blurring artifacts due to signal loss in later echoes due to T2 decay. Combining HASTE with parallel acquisition (PHASTE) decreases the number of echoes acquired and thus effectively reduces the blurring artifact caused by T2 relaxation. Further improvement in image sharpness can be achieved by performing T2 decay compensation before reconstructing the PHASTE data. Preliminary results have shown successful suppression of motion artifacts with PHASTE imaging. The image quality was enhanced relative to the original HASTE image, but was still less sharp than a non-motion-corrupted TSE image.

A phase-sensitive method of flip angle mapping.

A radiofrequency (RF) excitation scheme is presented in which flip angle is encoded in the phase of the resulting excitation. This excitation is implemented with nonselective hard pulses, and is used to give flip angle maps over three-dimensional volumes. This phase-sensitive B1 mapping excitation can be combined with various acquisition methods such as gradient recalled echo (GRE) and echo-planar (EP) readouts. Imaging time depends primarily on the readout method, and is roughly equivalent to the imaging time of conventional double-angle techniques for three-dimensional acquisition. The phase-sensitive method allows imaging over a much wider range of flip angles than double-angle methods. Phantom and in vivo results are presented comparing the phase-sensitive method with the conventional double-angle method, demonstrating the ability of the phase-sensitive method to measure a wider range of flip angles than double-angle methods.

A statistical analysis of the Bloch-Siegert B1 mapping technique.

A number of B1 mapping methods have been introduced. A model to facilitate choice among these methods is valuable, as the performance of each technique is affected by a variety of factors, including acquisition signal-to-noise ratio (SNR). The Bloch-Siegert shift B1 mapping method has recently garnered significant interest. In this paper, we present a statistical model suitable for analysis of the Bloch-Siegert shift method. Unlike previously presented models, the analysis is valid in both low SNR and high SNR regimes. We present a detailed analysis of the performance of the Bloch-Siegert shift B1 mapping method across a broad range of acquisition scenarios, and compare it to two other B1 mapping techniques (the dual angle method and the phase sensitive method). Further validation of the model is presented through both Monte Carlo simulations and experimental results. The simulations and experimental results match the model well, lending confidence to its accuracy. Each technique is found to perform well with high acquisition SNR. However, our results suggest that the dual angle method is not reliable in low SNR environments. Furthermore, the phase sensitive method appears to outperform the Bloch-Siegert shift method in these low-SNR cases, although variations of the Bloch-Siegert method may be possible that improve its performance at low SNR.

An analytic framework for the evaluation of coil configurations for parallel transmission MRI with subsampled cartesian excitation k-space.

The use of multiple independent simultaneous radio-frequency (RF) transmitters and coils, known as parallel transmission, has the potential to make multidimensional excitation applicable to a wide range of magnetic resonance imaging applications. The sensitivity profile of the RF coils in a parallel transmission system determines the performance of the system. We present a theoretical framework, allowing the evaluation of the performance of a coil array for parallel transmission. We show through theoretical analysis and Monte Carlo simulation that the proposed framework predicts the fidelity of excitation that can be achieved by a given coil configuration in the presence of noise in the measured coil sensitivity profiles. We evaluate the fidelity of excitation achieved by four candidate coil configurations for a four-channel parallel transmission system with noisy coil sensitivity estimates. Theoretical results are confirmed with Monte Carlo simulation. The results give insight into the design of coil configurations for parallel transmission. In particular, optimal fidelity of excitation for subsampled Cartesian excitation k -space is achieved with a coil sensitivity profile having uniform amplitude and increasing linear phase for each channel. Such sensitivity profiles may be achieved with twisted birdcage coil designs.

An analysis of the accuracy of magnetic resonance flip angle measurement methods.

Several methods of flip angle mapping for magnetic resonance imaging have been proposed. We evaluated the accuracy of five methods of flip angle measurement in the presence of measurement noise. Our analysis was performed in a closed form by propagation of probability density functions (PDFs). The flip angle mapping methods compared were (1) the phase-sensitive method, (2) the dual-angle method using gradient recalled echoes (GRE), (3) an extended version of the GRE dual-angle method incorporating phase information, (4) the AFI method and (5) an extended version of the AFI method incorporating phase information. Our analysis took into account differences in required imaging time for these methods in the comparison of noise efficiency. PDFs of the flip angle estimate for each method for each value of true flip angle were calculated. These PDFs completely characterize the performance of each method. Mean bias and standard deviation were computed from these PDFs to more simply quantify the relative accuracy of each method over its range of measurable flip angles. We demonstrate that the phase-sensitive method provides the lowest mean bias and standard deviation of flip angle estimate of the five methods evaluated over a wide range of flip angles.

Rapid fat suppression in MRI of the breast with short binomial pulses.

PURPOSE: To develop a faster method of fat suppression for use in dynamic contrast enhanced MRI of the breast. MATERIALS AND METHODS: A method of fast fat suppression is presented using spatially nonselective rapid binomial pulses. In contrast to conventional binomial frequency-selective pulses, these short pulses are applied without interpulse delay, allowing for very rapid spectrally selective excitation. RESULTS: Effective water excitation and fat suppression were achieved in breast MRI at 3.0 Tesla with total excitation time as low as 160 microsec, which is several times shorter than the excitation time of currently used fat suppression techniques. Rapid fat suppression comes at the expense of increased specific absorption rate (SAR) and mildly degraded quality of suppression. A flexible tradeoff of short imaging time vs. SAR can be made to optimize imaging speed for fat-suppressed breast MRI. CONCLUSION: Rapid binomial pulses can be used for dynamic contrast enhanced breast MRI with excitation times significantly shorter than currently used fat suppression pulses. Shorter excitation time allows more rapid imaging, allowing greater temporal and spatial resolution for characterization of breast lesions.