An automated iterative algorithm for water and fat decomposition in three-point Dixon magnetic resonance imaging

Improved nerve conspicuity with water-weighting and denoising in two-point Dixon magnetic resonance neurography

BACKGROUND

T₂-weighted, two-point Dixon fast-spin-echo (FSE) is an effective technique for magnetic resonance neurography (MRN) that can provide quantitative assessment of muscle denervation. Low signal-to-noise ratio and inadequate fat suppression, however, can impede accurate interpretation.

PURPOSE

To quantify effects of principal component analysis (PCA) denoising on tissue signal intensities and fat fraction (FF) and to determine qualitative image quality improvements from both denoising and water-weighting (WW) algorithms to improve nerve conspicuity and fat suppression.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-one subjects undergoing MR neurography evaluation (11/10 male/female, mean age = 46.3±13.7 years) with 60 image volumes. Twelve subjects (23 image volumes) were determined to have muscle denervation based on diffusely elevated T₂ signal intensity.

FIELD STRENGTH/SEQUENCE

3 T, 2D, two-point Dixon FSE.

ASSESSMENT

Qualitative assessment included overall image quality, nerve conspicuity, fat suppression, pulsation and ringing artifacts by 3 radiologists separately on a three-point scale (1 = poor, 2 = average, 3 = excellent). Quantitative measurements for FF and signal intensity relative to normal muscle were made for nerve, abnormal muscle and subcutaneous fat.

STATISTICAL TESTS

Linear and ordinal regression models were used for quantitative and qualitative comparisons, respectively; 95% confidence intervals (CIs) and p-values for pairwise comparisons were adjusted using the Holm-Bonferroni method. Inter-rater agreement was assessed using Gwet's agreement coefficient (AC₂).

RESULTS

Simulations showed PCA-denoising reduced FF error from 2.0% to 1.0%, and from 7.6% to 3.1% at noise levels of 10% and 30%, respectively. In human subjects, PCA-denoising did not change signal levels and FF quantitatively. WW decreased fat signal significantly (-83.6%, p < 0.001). Nerve conspicuity was improved by WW (odds ratio, OR = 5.8, p < 0.001). Fat suppression was improved by both PCA (OR = 3.6, p < 0.001) and WW (OR = 2.2, p < 0.001). Overall image quality was improved by PCA + WW (OR = 1.7, p = 0.04).

CONCLUSIONS

WW and PCA-denoising improved nerve conspicuity and fat suppression in MR neurography. Denoising can potentially provide improved accuracy of FF maps for assessing fat-infiltrated muscle.

Measurement of liver fat fraction and iron with MRI and MR spectroscopy techniques

Diffuse liver disease is a widespread global healthcare burden, and the abnormal accumulation of lipid and/or iron is common to important disease processes. Developing the improved methods for detecting and quantifying liver lipid and iron is an important clinical need. The inherent risk, invasiveness, and sampling error of liver biopsy have prompted the development of noninvasive imaging methods for lipid and iron assessment. Ultrasonography and computed tomography have the ability to detect diffuse liver disease, but with limited accuracy. The purpose of this review is to describe the current state-of-the-art methods for quantifying liver lipid and iron using magnetic resonance imaging and spectroscopy, including their implementation, benefits, and potential pitfalls. Imaging- and spectroscopy-based methods are naturally suited for lipid and iron quantification. Lipid can be detected and decomposed from the inherent chemical shift between lipid and water signals, whereas iron imparts significant paramagnetic susceptibility to tissue, which accelerates proton relaxation. However, measurements of these biomarkers are confounded by technical and biological effects. Current methods must address these factors to allow a precise correlation between the lipid fraction and iron concentration. Although this correlation becomes increasingly challenging in the presence of combined lipid and iron accumulation, advanced techniques show promise for delineating these quantities through multi-lipid peak analysis, T2 water mapping, and fast single-voxel water-lipid spectroscopy.

Nonlinear regularization for per voxel estimation of magnetic susceptibility distributions from MRI field maps

Magnetic susceptibility is an important physical property of tissues, and can be used as a contrast mechanism in magnetic resonance imaging (MRI). Recently, targeting contrast agents by conjugation with signaling molecules and labeling stem cells with contrast agents have become feasible. These contrast agents are strongly paramagnetic, and the ability to quantify magnetic susceptibility could allow accurate measurement of signaling and cell localization. Presented here is a technique to estimate arbitrary magnetic susceptibility distributions by solving an ill-posed inversion problem from field maps obtained in an MRI scanner. Two regularization strategies are considered: conventional Tikhonov regularization and a sparsity promoting nonlinear regularization using the l(1) norm. Proof of concept is demonstrated using numerical simulations, phantoms, and in a stroke model in a rat. Initial experience indicates that the nonlinear regularization better suppresses noise and streaking artifacts common in susceptibility estimation.

Noise considerations of three-point water-fat separation imaging methods

Separation of water from fat tissues in magnetic resonance imaging is important for many applications because signals from fat tissues often interfere with diagnoses that are usually based on water signal characteristics. Water and fat can be separated with images acquired at different echo time shifts. The three-point method solves for the unknown off-resonance frequency together with the water and fat densities. Noise performance of the method, quantified by the effective number of signals averaged (NSA), is an important metric of the water and fat images. The authors use error propagation theory and Monte Carlo simulation to investigate two common reconstructive approaches: an analytic-solution based estimation and a least-squares estimation. Two water-fat chemical shift (CS) encoding strategies, the symmetric (-theta, 0, theta) and the shifted (0, theta, 2theta) schemes are studied and compared. Results show that NSAs of water and fat can be different and they are dependent on the ratio of intensities of the two species and each of the echo time shifts. The NSA is particularly poor for the symmetric (-theta, 0, theta) CS encoding when the water and fat signals are comparable. This anomaly with equal amounts of water and fat is analyzed in a more intuitive geometric illustration. Theoretical prediction of NSA matches well with simulation results at high signal-to-noise ratio (SNR), while deviation arises at low SNR, which suggests that Monte Carlo simulation may be more appropriate to accurately predict noise performance of the algorithm when SNR is low.

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.

Interleaved water and fat dual-echo spin echo imaging with intrinsic chemical-shift elimination

A new technique was developed to simultaneously acquire water and fat dual-echo spin echo images in a single acquisition period. Chemical shifts between water and fat images are intrinsically eliminated, and the images are combined to form water-plus-fat image. In vivo water-only images show fat suppression superior to that of conventional spin echo images. This technique may be clinically useful for musculoskeletal imaging.

Bone marrow segmentation in leukemia using diffusion and T (2) weighted echo planar magnetic resonance imaging

Magnetic resonance images of leukemic bone marrow were acquired over large regions of the pelvis and lower abdomen with minimal interference from overlying tissues using diffusion and T(2) weighted echo planar imaging. Data acquisition times were on the order of 1 min for scanning volumes of up to 25 l at a spatial resolution of 31 microl. A survey of 21 patients with leukemia and eight healthy adult volunteers was undertaken to determine the magnitude of the observed effect and its dependence upon specific pathologies. The acquisition methods yielded high-quality segmentation of leukemic bone marrow prior to therapy in seven of seven patients with acute lymphocytic leukemia, chronic lymphocytic leukemia or chronic myelogenous leukemia, and who had hypercellular (>95%) bone marrow at the time of the study. The quality of the segmentation was sufficient to allow the use of maximum intensity projection images which afforded a convenient evaluation of both intra- and extramedullary disease. The measured signal-to-noise ratios agreed with a theoretical estimate that accounted for the percentage cellularity, T(2) relaxation time of water, and self-diffusion coefficient of water in iliac bone marrow. In addition, the mean signal-to-noise ratios from iliac marrow were strongly dependent upon the time after the initiation of chemotherapeutic regimens, implying that the methods may be useful for therapeutic monitoring.