Fat fraction mapping using bSSFP Signal Profile Asymmetries for Robust multi-Compartment Quantification (SPARCQ)

Purpose: To develop a novel quantitative method for detection of different tissue compartments based on bSSFP signal profile asymmetries (SPARCQ) and to provide a validation and proof-of-concept for voxel-wise water-fat separation and fat fraction mapping. Methods: The SPARCQ framework uses phase-cycled bSSFP acquisitions to obtain bSSFP signal profiles. For each voxel, the profile is decomposed into a weighted sum of simulated profiles with specific off-resonance and relaxation time ratios. From the obtained set of weights, voxel-wise estimations of the fractions of the different components and their equilibrium magnetization are extracted. For the entire image volume, component-specific quantitative maps as well as banding-artifact-free images are generated. A SPARCQ proof-of-concept was provided for water-fat separation and fat fraction mapping. Noise robustness was assessed using simulations. A dedicated water-fat phantom was used to validate fat fractions estimated with SPARCQ against gold-standard 1H MRS. Quantitative maps were obtained in knees of six healthy volunteers, and SPARCQ repeatability was evaluated in scan rescan experiments. Results: Simulations showed that fat fraction estimations are accurate and robust for signal-to-noise ratios above 20. Phantom experiments showed good agreement between SPARCQ and gold-standard (GS) fat fractions (fF(SPARCQ) = 1.02*fF(GS) + 0.00235). In volunteers, quantitative maps and banding-artifact-free water-fat-separated images obtained with SPARCQ demonstrated the expected contrast between fatty and non-fatty tissues. The coefficient of repeatability of SPARCQ fat fraction was 0.0512. Conclusion: The SPARCQ framework was proposed as a novel quantitative mapping technique for detecting different tissue compartments, and its potential was demonstrated for quantitative water-fat separation.Comment: 20 pages, 7 figures, submitted to Magnetic Resonance in Medicin

Exploring configuration-based relaxometry and imaging with balanced steady-state free precession

Over the years, magnetic resonance (MR) imaging has become a fundamental part of the diagnostic process in hospitals worldwide. While the underlying physics dates back more than 60 years with the development of nuclear magnetic resonance, methods that aim to accurately measure the multitude of parameters governing the signal formation are still a topic of active research and developments today. The main aim of this work is to explore the possibilities of developing quantification techniques based on a particular type of MR acquisitions: balanced Steady-State Free Precession (bSSFP). The first chapter briefly introduces the most relevant basic concepts of MR physics that will serve as foundation for the development of the methods presented thereafter. In the second chapter, a new method using multiple bSSFP scans is presented that aim to achieve motion insensitive three-dimensional quantification of relaxation times and thereby improve a recently published technique based on unbalanced gradient echo acquisitions. The method is then evaluated both in phantoms and in vivo studies at 3 T and the results discussed. These include an interesting bias of the method that might provide useful insights into the underlying tissue micro-structure. One the major challenges with bSSFP imaging is the presence of dark regions inside the images, which are due to inhomogeneities in the main magnetic field. In the third chapter, a new approach to address those issues is proposed, termed trueCISS, which combines fast imaging using sparse sampling with compressed sensing reconstructions and multi-parametric fitting, which ultimately allows the synthesis of artefact-free images. Evaluation of the new method is done at 3 T for the human brain. Finally, an extension of the trueCISS technique is presented in chapter four, where the process used to model the data to the signal equation is replaced by a novel algorithm based on configuration theory, which is essentially a representation of the signal formation processes in the Fourier domain. The improved trueCISS imaging method is then successfully evaluated with measurements at ultra high field strengths such as 7 T and 9.4 T which demonstrate the advantages of the new approach

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

Accelerated CMR using zonal, parallel and prior knowledge driven imaging methods

Accelerated imaging is highly relevant for many CMR applications as competing constraints with respect to spatiotemporal resolution and tolerable scan times are frequently posed. Three approaches, all involving data undersampling to increase scan efficiencies, are discussed in this review. Zonal imaging can be considered a niche but nevertheless has found application in coronary imaging and CMR flow measurements. Current work on parallel-transmit systems is expected to revive the interest in zonal imaging techniques. The second and main approach to speeding up CMR sequences has been parallel imaging. A wide range of CMR applications has benefited from parallel imaging with reduction factors of two to three routinely applied for functional assessment, perfusion, viability and coronary imaging. Large coil arrays, as are becoming increasingly available, are expected to support reduction factors greater than three to four in particular in combination with 3D imaging protocols. Despite these prospects, theoretical work has indicated fundamental limits of coil encoding at clinically available magnetic field strengths. In that respect, alternative approaches exploiting prior knowledge about the object being imaged as such or jointly with parallel imaging have attracted considerable attention. Five to eight-fold scan accelerations in cine and dynamic CMR applications have been reported and image quality has been found to be favorable relative to using parallel imaging alone

Development and Optimization of 19F-MRI for Tracking Cellular Therapeutics

Introduction: This thesis aims to advance magnetic resonance imaging (MRI) for imaging cellular therapeutics. Traditional, proton-based, MRI provides detailed anatomical images, particularly of soft tissue. However, in order to obtain information at a cellular level specialized imaging agents are required to detect the cells of interest. Perfluorocarbons containing non-radioactive fluorine-19 (19F) are both biologically safe and MR sensitive. Methods: Pre-clinical 19F-MRI was implemented on a Varian 9.4T MRI scanner, using a dual 19F/1H-tuned birdcage volume coil. Mesenchymal stromal cells (MSC) were pre-labeled with a commercial, FDA approved 19F-perfluorocarbon emulsion, then implanted intramuscularly into the mouse hindlimb. To track the inflammation resulting from transplantation, a dual-agent cellular MRI technique was developed. This technique utilizes 19F to track MSC and superparamagnetic iron oxide nanoparticles (SPIO) to image macrophages, through the presence of signal quenching. A clinical imaging protocol was developed to translate 19F-MRI on a 3T GE MR750 scanner with a dual 19F/1H-tuned surface coil. Peripheral blood mononuclear cells (PBMC) were labeled with a FDA-approved 19F-agent and injected into a ham shank phantom for protocol optimization. Results: The balanced steady-state free precession pulse sequence was chosen for all studies due to the high signal-to-noise per unit time. Image acquisition was optimized for 19F detection sensitivity, accuracy of quantification, and compatibility with isoflurane. In vivo quantification of MSC on the day of implantation was in strong agreement with the expected number of cells. The change in 19F-signal was quantified over time and compared between two murine transplantation models. When iron oxide was administered i.v., the migration of immune cells could be tracked to the injection site. The presence of SPIO decreased both the 1H and 19F signal, indicating that transplant rejection was occurring. On a clinical system, as few as 4x106 PBMC could be imaged following both surface and subcutaneous injection. The minimum number of detectable cells was strongly influenced by intracellular 19F uptake. Conclusions: 19F-MRI is a promising tool for imaging cellular therapeutics. By pre-labeling cells of interest, they can be localized and the change in signal can be quantified over time. The technique shows promise for both pre-clinical and clinical applications

Towards accurate and precise T1 and extracellular volume mapping in the myocardium: a guide to current pitfalls and their solutions

Mapping of the longitudinal relaxation time (T1) and extracellular volume (ECV) offers a means of identifying pathological changes in myocardial tissue, including diffuse changes that may be invisible to existing T1-weighted methods. This technique has recently shown strong clinical utility for pathologies such as Anderson- Fabry disease and amyloidosis and has generated clinical interest as a possible means of detecting small changes in diffuse fibrosis; however, scatter in T1 and ECV estimates offers challenges for detecting these changes, and bias limits comparisons between sites and vendors. There are several technical and physiological pitfalls that influence the accuracy (bias) and precision (repeatability) of T1 and ECV mapping methods. The goal of this review is to describe the most significant of these, and detail current solutions, in order to aid scientists and clinicians to maximise the utility of T1 mapping in their clinical or research setting. A detailed summary of technical and physiological factors, issues relating to contrast agents, and specific disease-related issues is provided, along with some considerations on the future directions of the field. Towards accurate and precise T1 and extracellular volume mapping in the myocardium: a guide to current pitfalls and their solutions. Available from: https://www.researchgate.net/publication/317548806_Towards_accurate_and_precise_T1_and_extracellular_volume_mapping_in_the_myocardium_a_guide_to_current_pitfalls_and_their_solutions [accessed Jun 13, 2017]