Magnetic resonance is an exceptionally powerful and versatile measurement technique. The basic structure of a magnetic resonance experiment has remained largely unchanged for almost 50 years, being mainly restricted to the qualitative probing of only a limited set of the properties that can in principle be accessed by this technique. Here we introduce an approach to data acquisition, post-processing and visualization—which we term ‘magnetic resonance fingerprinting’ (MRF)—that permits the simultaneous non-invasive quantification of multiple important properties of a material or tissue. MRF thus provides an alternative way to quantitatively detect and analyse complex changes that can represent physical alterations of a substance or early indicators of disease. MRF can also be used to identify the presence of a specific target material or tissue, which will increase the sensitivity, specificity and speed of a magnetic resonance study, and potentially lead to new diagnostic testing methodologies. When paired with an appropriate pattern-recognition algorithm, MRF inherently suppresses measurement errors and can thus improve measurement accuracy. A new approach to magnetic resonance, ‘magnetic resonance fingerprinting', is reported, which combines a data acquisition scheme with a pattern-recognition algorithm that looks for the ‘fingerprints’ of interest within the data. Although nuclear magnetic resonance is a powerful analytical tool for many scientific and medical disciplines, usually only a fraction of its potential power is harnessed. Most implementations are qualitative, and restricted in the range of properties that are probed. Dan Ma and colleagues introduce a new approach — termed magnetic resonance fingerprinting — aimed at greatly enhancing the amount of quantitative information that can be obtained in one measurement. Their approach combines a data-acquisition scheme that is indiscriminate in the material properties that it probes with pattern-recognition algorithms that look for the 'fingerprints' of interest within the data. Magnetic resonance fingerprinting has the potential to detect and analyse early indicators of disease or complex changes in materials, as well as increasing the sensitivity, specificity and speed of magnetic resonance studies.

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Magnetic resonance fingerprinting

SUMMARY: Susceptibility-weighted imaging (SWI) is a new neuroimaging technique, which uses tissue magnetic susceptibility differences to generate a unique contrast, different from that of spin density, T1, T2, and T2*. In this review (the first of 2 parts), we present the technical background for SWI. We discuss the concept of gradient-echo images and how we can measure local changes in susceptibility. Armed with this material, we introduce the steps required to transform the original magnitude and phase images into SWI data. The use of SWI filtered phase as a means to visualize and potentially quantify iron in the brain is presented. Advice for the correct interpretation of SWI data is discussed, and a set of recommended sequence parameters for different field strengths is given.

http://www.ajnr.org/content/ajnr/30/1/19.full.pdf

Susceptibility-weighted imaging: technical aspects and clinical applications, part 1.

Quantitative imaging of intrinsic magnetic tissue properties using MRI signal phase: an approach to in vivo brain iron metabolism?

In MRI, the main magnetic field polarizes the electron cloud of a molecule, generating a chemical shift for observer protons within the molecule and a magnetic susceptibility inhomogeneity field for observer protons outside the molecule. The number of water protons surrounding a molecule for detecting its magnetic susceptibility is vastly greater than the number of protons within the molecule for detecting its chemical shift. However, the study of tissue magnetic susceptibility has been hindered by poor molecular specificities of hitherto used methods based on MRI signal phase and T2* contrast, which depend convolutedly on surrounding susceptibility sources. Deconvolution of the MRI signal phase can determine tissue susceptibility but is challenged by the lack of MRI signal in the background and by the zeroes in the dipole kernel. Recently, physically meaningful regularizations, including the Bayesian approach, have been developed to enable accurate quantitative susceptibility mapping (QSM) for studying iron distribution, metabolic oxygen consumption, blood degradation, calcification, demyelination, and other pathophysiological susceptibility changes, as well as contrast agent biodistribution in MRI. This paper attempts to summarize the basic physical concepts and essential algorithmic steps in QSM, to describe clinical and technical issues under active development, and to provide references, codes, and testing data for readers interested in QSM. Magn Reson Med 73:82–101, 2015. © 2014 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance. This is an open access article under the terms of the Creative commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited.

Quantitative susceptibility mapping (QSM): Decoding MRI data for a tissue magnetic biomarker.

The diagnosis of many neurologic diseases benefits from the ability to quantitatively assess iron in the brain. Paramagnetic iron modifies the magnetic susceptibility causing magnetic field inhomogeneity in MRI. The local field can be mapped using the MR signal phase, which is discarded in a typical image reconstruction. The calculation of the susceptibility from the measured magnetic field is an ill-posed inverse problem. In this work, a bayesian regularization approach that adds spatial priors from the MR magnitude image is formulated for susceptibility imaging. Priors include background regions of known zero susceptibility and edge information from the magnitude image. Simulation and phantom validation experiments demonstrated accurate susceptibility maps free of artifacts. The ability to characterize iron content in brain hemorrhage was demonstrated on patients with cavernous hemangioma. Additionally, multiple structures within the brain can be clearly visualized and characterized. The technique introduces a new quantitative contrast in MRI that is directly linked to iron in the brain.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.22187

Quantitative susceptibility map reconstruction from MR phase data using bayesian regularization: validation and application to brain imaging.

The molecular basis for gray and white matter contrast in phase imaging.

Magnetic properties of materials for MR engineering, micro-MR and beyond.

To investigate the feasibility of multiecho balanced steady-state free precession (bSSFP)-based fast chemical shift mapping hyperpolarized 13C metabolites. The overall goal was to reduce total imaging time and to increase spatial resolution compared to common chemical shift imaging (CSI). A multiecho bSSFP sequence in combination with an iterative reconstruction algorithm was implemented. 1H experiments were performed on phantoms and on a human volunteer in order to investigate the feasibility of the method on a system with metabolite maps that are known beforehand. 13C experiments were performed in vivo on pigs, where CSI images were acquired also for comparison. Chemical shift images of three and four distinct 1H resonance frequencies as well as chemical shift images of up to five hyperpolarized 13C metabolites were successfully obtained. Fast metabolite mapping based on multiecho balanced SSFP in combination with an iterative reconstruction approach could successfully separate several 1H resonances and hyperpolarized 13C metabolites.

Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds

A novel balanced SSFP technique for the separation or suppression of different resonance frequencies (e.g., fat suppression) is presented. The method is based on applying two alternating and different repetition times, TR(1) and TR(2). This RF scheme manipulates the sensitivity of balanced SSFP to off-resonance effects by a modification of the frequency response profile. Starting from a general approach, an optimally broadened stopband within the frequency response function is designed. This is achieved with a TR(2) being one third of TR(1) and an RF-pulse phase increment of 90 degrees . With this approach TR(2) is too short ( approximately 1 ms) to switch imaging gradients and is only used to change the frequency sensitivity. Without a significant change of the spectral position of the stopband, TR(1) can be varied over a range of values ( approximately 2.5-4.5 ms) while TR(2) and phase cycling is kept constant. On-resonance spins show a magnetization behavior similar to balanced SSFP, but with maximal magnetization at flip angles about 10 degrees lower than in balanced SSFP. The total scan time is increased by about 30% compared to conventional balanced SSFP. The new technique was applied on phantoms and volunteers to produce rapid, fat suppressed images.

Alternating repetition time balanced steady state free precession

Purpose:

To evaluate the ability of conventional and ultra-short or zero echo time MRI for imaging of soft and solid dental components in and ex vivo.



Materials and Methods:

Turbo spin echo (TSE), ultra-short echo time (UTE), and zero echo time (ZTE) MRI were performed on extracted (human and equine) teeth and in vivo using whole-body and small-bore MR systems at 3 T, 7T, and 9.4T, respectively.



Results:

At an isotropic resolution of (600 μm)3, strong signal of soft-tissue, e.g., mucosa and nerves with excellent contrast was achieved using TSE at 3T in vivo. No signal, though, was obtained from solid components, e.g., teeth (due to short T2). In contrast, dentin, cementum as well as enamel of extracted teeth were readily depicted using UTE and ZTE at a resolution of ≈ (150 μm)3 at 7T and 9.4T. In particular, ZTE provided higher signal in enamel.



Conclusion:

As an alternative to X-ray based methods like cone-beam computed tomography (CT) or conventional CT, the presented results demonstrate the potential of ZTE and UTE MRI as a radiation-free imaging modality, delivering contrast of soft and solid components at the same time. J. Magn. Reson. Imaging 2012;36:841–846. © 2012 Wiley Periodicals, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.23712

Jochen Leupold

Papers

The molecular basis for gray and white matter contrast in phase imaging.

Magnetic properties of materials for MR engineering, micro-MR and beyond.

Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds

Alternating repetition time balanced steady state free precession

Dental MRI: imaging of soft and solid components without ionizing radiation.