Quantifying disease activity in fatty-infiltrated skeletal muscle by IDEAL-CPMG in Duchenne muscular dystrophy

The purpose of this study was to explore the use of iterative decomposition of water and fat with echo asymmetry and least-squares estimation Carr-Purcell-Meiboom-Gill (IDEAL-CPMG) to simultaneously measure skeletal muscle apparent fat fraction and water T2 (T2,w) in patients with Duchenne muscular dystrophy (DMD). In twenty healthy volunteer boys and thirteen subjects with DMD, thigh muscle apparent fat fraction was measured by Dixon and IDEAL-CPMG, with the IDEAL-CPMG also providing T2,w as a measure of muscle inflammatory activity. A subset of subjects with DMD was followed up during a 48-week clinical study. The study was in compliance with the Patient Privacy Act and approved by the Institutional Review Board. Apparent fat fraction in the thigh muscles of subjects with DMD was significantly increased compared to healthy volunteer boys (p <0.001). There was a strong correlation between Dixon and IDEAL-CPMG apparent fat fraction. Muscle T2,w measured by IDEAL-CPMG was independent of changes in apparent fat fraction. Muscle T2,w was higher in the biceps femoris and vastus lateralis muscles of subjects with DMD (p <0.05). There was a strong correlation (p <0.004) between apparent fat fraction in all thigh muscles and six-minute walk distance (6MWD) in subjects with DMD. IDEAL-CPMG allowed independent and simultaneous quantification of skeletal muscle fatty degeneration and disease activity in DMD. IDEAL-CPMG apparent fat fraction and T2,w may be useful as biomarkers in clinical trials of DMD as the technique disentangles two competing biological processes

Body MRI artifacts in clinical practice: a physicist\u27s and radiologist\u27s perspective.

The high information content of MRI exams brings with it unintended effects, which we call artifacts. The purpose of this review is to promote understanding of these artifacts, so they can be prevented or properly interpreted to optimize diagnostic effectiveness. We begin by addressing static magnetic field uniformity, which is essential for many techniques, such as fat saturation. Eddy currents, resulting from imperfect gradient pulses, are especially problematic for new techniques that depend on high performance gradient switching. Nonuniformity of the transmit radiofrequency system constitutes another source of artifacts, which are increasingly important as magnetic field strength increases. Defects in the receive portion of the radiofrequency system have become a more complex source of problems as the number of radiofrequency coils, and the sophistication of the analysis of their received signals, has increased. Unwanted signals and noise spikes have many causes, often manifesting as zipper or banding artifacts. These image alterations become particularly severe and complex when they are combined with aliasing effects. Aliasing is one of several phenomena addressed in our final section, on artifacts that derive from encoding the MR signals to produce images, also including those related to parallel imaging, chemical shift, motion, and image subtraction

Acquisition and Reconstruction Techniques for Fat Quantification Using Magnetic Resonance Imaging

Quantifying the tissue fat concentration is important for several diseases in various organs including liver, heart, skeletal muscle and kidney. Uniquely, MRI can separate the signal from water and fat in-vivo, rendering it the most suitable imaging modality for non-invasive fat quantification. Chemical-shift-encoded MRI is commonly used for quantitative fat measurement due to its unique ability to generate a separate image for water and fat. The tissue fat concentration can be consequently estimated from the two images. However, several confounding factors can hinder the water/fat separation process, leading to incorrect estimation of fat concentration. The inhomogeneities of the main magnetic field represent the main obstacle to water/fat separation. Most existing techniques rely mainly on imposing spatial smoothness constraints to address this problem; however, these often fail to resolve large and abrupt variations in the magnetic field. A novel convex relaxation approach to water/fat separation is proposed. The technique is compared to existing methods, demonstrating its robustness to resolve abrupt magnetic field inhomogeneities. Water/fat separation requires the acquisition of multiple images with different echo-times, which prolongs the acquisition time. Bipolar acquisitions can efficiently acquire the required data in shorter time. However, they induce phase errors that significantly distort the fat measurements. A new bipolar acquisition strategy that overcomes the phase errors and provides accurate fat measurements is proposed. The technique is compared to the current clinical sequence, demonstrating its efficiency in phantoms and in-vivo experiments. The proposed acquisition technique is also applied on animal models to achieve higher spatial resolution than the current sequence. In conclusion, this dissertation describes a complete framework for accurate and precise MRI fat quantification. Novel acquisitions and reconstruction techniques that address the current challenges for fat quantification are proposed

Evaluation of adipose tissue volume quantification with IDEAL fat-water separation

Purpose: To validate iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) for adipose tissue volume quantification. IDEAL allows MRI images to be produced only from adipose-containing tissues; hence, quantifying adipose tissue should be simpler and more accurate than with current methods. Materials and Methods: Ten healthy controls were imaged with 1.5 Tesla (T) Spin Echo (SE), 3.0T T1-weighted spoiled gradient echo (SPGR), and 3.0T IDEAL-SPGR. Images were acquired from the abdomen, pelvis, mid-thigh, and mid-calf. Mean subcutaneous and visceral adipose tissue volumes were compared between the three acquisitions for each subject. Results: There were no significant differences (P \u3e 0.05) between the three acquisitions for subcutaneous adipose tissue volumes. However, there was a significant difference (P = 0.0002) for visceral adipose tissue volumes in the abdomen. Post hoc analysis showed significantly lower visceral adipose tissue volumes measured by IDEAL versus 1.5T (P \u3c 0.0001) and 3.0T SPGR (P \u3c 0.002). The lower volumes given by IDEAL are due to its ability to differentiate true visceral adipose tissue from other bright structures like blood vessels and bowel content that are mistaken for adipose tissue in non-fat suppressed images. Conclusion: IDEAL measurements of adipose tissue are equivalent to established 1.5T measurement techniques for subcutaneous depots and have improved accuracy for visceral depots, which are more metabolically relevant. © 2011 Wiley-Liss, Inc

Fat suppression techniques in breast magnetic resonance imaging: a critical comparison and state of the art

Robust and accurate fat suppression is highly desirable in breast magnetic resonance imaging (MRI) because it can considerably improve the image quality and lesion conspicuity. However, fat suppression is also more challenging in the breast compared with other regions in the body. Technical advances have been made over time to make fat suppression more efficient and reliable. Combined with other innovations, breast MRI continues to be the most sensitive and comprehensive diagnostic modality in the detection and evaluation of breast lesions. This review offers a critical comparison of various fat suppression techniques in breast MRI including spectral-selective excitation and saturation techniques based on the chemical shift difference between fat and water, the inversion recovery techniques based on the T1 relaxation time difference, the hybrid spectral-selective inversion recovery techniques, and the new Dixon fat and water separation techniques based on the phase difference between fat and water signal at different echo times. This review will also cover less frequently used techniques such as slice-selective gradient reversal. For each fat suppression technique in breast MRI, a detailed explanation of the technical principle, the advantages and disadvantages, the approaches for optimization as well as the clinical examples are included. The additional challenges of fat suppression in breast MRI at higher field strength and in the presence of metallic and silicone implants are also discussed

Acquisition strategies for fat/water separated MRI

This thesis focuses on new ways to more efficiently acquire the signal for fat/water separated MRI, also known as Dixon methods. In paper I, the concept of dual bandwidths was introduced to improve SNR efficiency by removing dead times in a spin echo PROPELLER sequence. By correcting for the displacement of fat, we were able to improve the motion correction. This required additional considerations during reconstruction in order to avoid noise amplification, which was solved with a noise-whitening Tikhonov regularization. Paper II explores the combination of fat/water separation in k-space with partially acquired data, i.e. partial Fourier sampling. With reduced sampling coverage comes the ability of increased spatial resolution, which is often limited in fat/water imaging, particularly in gradient echo sequences. A modified POCS routine was also developed with real-valued estimates, exploiting Hermitian symmetry to improve the inverse problem conditioning in the fully sampled region. A single-TR dual-bandwidth RARE (fast/turbo spin echo) sequence without dead times was developed in Paper III, which uses partial Fourier sampling with late and early echoes to improve the chemical shift encoding. The proposed sequence can acquire images with 0.5 mm in-plane resolution without dead times, with image quality exceeding current state-of-the-art techniques. An automated selection of gradient waveforms based on Cramér-Rao bounds was implemented on the scanner. In Paper IV, the dual-bandwidth concept was generalized to continuous bandwidths. Instead of the conventional shift of a trapezoidal readout gradient, we describe a new method of encoding chemical shift by using asymmetric readout waveforms. Asymmetric readouts were implemented in a RARE sequence to completely remove dead times from multi-TR acquisitions, with typical scan time reductions of 25 %. The developed methods enable fat/water imaging with reduced scan times and increased spatial resolution, which has previously limited their use