The principles of obtaining the physical quantity of magnetic susceptibility using MRI are being presented. Quantitative Susceptibility Mapping (QSM) reflects tissue susceptibility using the MR phase information acquired using a gradient echo sequence. To obtain susceptibility maps several steps are required in the reconstruction, including (i) phase measurement, (ii) field map estimation, (iii) background field removal, and (iv) susceptibility map calculation by solving the inverse problem. Examples and challenges of QSM are presented and discussed.
Highlights
• QSM reflects tissue susceptibility using the MR phase information acquired using a gradient echo sequence
• To obtain susceptibility maps several steps are required in the reconstruction
• QSM is believed to play an important role in the diagnosis and monitoring of neurodegenerative diseases, and new clinical applications are emerging
Target Audience
Researchers who want to understand and apply the method of QSM in research and clinical problems.Objectives
The audience will be able to understand the principles of obtaining the physical quantity of magnetic susceptibility using MRI, be able to make an informed decision on the QSM measurement and processing parameters, be able to assess the value of QSM for a certain application.Background
Certain tissue types in the human body contain components that have magnetic properties influencing the magnetic field in its vicinity, e.g. red blood cells contain oxy- and deoxyhemoglobin; the basal ganglia in the brain contain iron compounds. The magnetic susceptibility χ is the physical parameter that characterizes this tissue property and relates the magnetization induced in a material when it is exposed to an external magnetic field. Quantitative susceptibility mapping (QSM) is a relatively new method to obtain the physical quantity χ using the measured MR phase, as the phase image contains the information about the local magnetic field ΔB(r) from which χ can be derived. The reconstruction of susceptibility maps is complex and requires a number of steps that shall be described here.Methods
(1) Phase measurement: The MR phase is usually measured using a 3D gradient echo (GE) sequence. The maximum contrast-to-noise for the phase is close to the T2* of the tissue of interest, so to cover a range of T2* values for different tissue types a multiple echo (ME) GE sequence is considered beneficial. As the acquisition time of a ME-GE sequence can be quite long (in the order of several minutes), echo-planar techniques have been proposed to achieve measurement times below one minute. A complicating issue for accurate calculation of QSM can be the combination of the phase information of all receive coil elements of the commonly used multi-channel receive coils, and several methods have been proposed as a solution. QSM data are most commonly acquired at high magnetic field strength (3T and higher) as the phase contrast is increased in addition to the benefit of higher signal.
(2) Field map estimation: Unwrapping of phase data is necessary to obtain the field map, as the phase values are only defined in a 2π interval, which leads to “phase wraps” that have to be removed before further processing and many different methods have been published to solve this issue. To obtain the final field map the unwrapped phase map has then to be scaled with the echo time (TE) and the gyromagnetic ratio g.
(3) In a next step one has to remove the background field variation produced by sources outside the region of interest, e.g inhomogeneities of the main magnetic field, to obtain the magnetic field distribution of the tissue. To separate the magnetic field perturbation of tissue and background, several methods have been proposed, among which (i) sophisticated harmonic artifact reduction for phase data (SHARP) that solves the Laplacian equation by utilizing the spherical mean value property of the background field, and (ii) projection onto dipole fields (PDF) that estimates the background field by dipoles placed outside the region of interest. These methods have limitations near the boundary of the region of interest, and additional strategies can be used to reduce this effect.
(4)
Calculate susceptibility map: In a last step, the susceptibility
distribution caused by the tissue in the region of interest can determined by
deconvolving the magnetic field distribution with the point-dipole response.
The deconvolution in image space can be represented as a division in k-space
using a Fourier transform. However, the zeros contained in the point-dipole
response in Fourier space makes this division ill-posed and susceptible to
small noise contributions, which can cause severe streaking artifacts in the
reconstructed susceptibility maps. These artifacts can be reduced by using
measurements at multiple orientation or some form of multiple regularization.
Examples
Currently, most QSM applications have been shown in neuroimaging, particularly for the depiction of the basal ganglia that show a high susceptibility contrast due to iron storage. QSM has also been demonstrated for the differentiation between calcifications and bleeds, as well as estimation of venous oxygenation in the brain, ultimately leading to functional QSM, where the change in oxygenation during a functional task is estimated. Recently, QSM outside brain has been demonstrated in the characterization of cartilage, breast tissue, kidney, and prostate, and shows great promise for applications such as hepatic iron overload.Conclusion
QSM has developed from an abstract possibility
to a method that is being used for a range of clinical and research
applications. The main field of clinical application of QSM is believed to be
in the early diagnosis and monitoring of the development of neurodegenerative
diseases. But recently it has been shown that QSM can play a major role in
organs where iron metabolism and storage is relevant (cardiac and liver), as
well as for renal and musculoskeletal applications.
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