Synopsis
There is growing research interest in the development of QSM techniques for extra-cranial applications. These techniques are faced with additional challenges beyond those typically encountered in brain QSM. By addressing important challenges such as the presence of motion, fat, and large susceptibility shifts, these techniques may enable novel QSM applications for research and clinical applications in multiple organs, including heart, liver, kidney, pancreas, breast as well as whole-body applications. Educational Objectives
* Understand the potential applications of quantitative susceptibility mapping (QSM) outside the brain
* Appreciate the major challenges associated with QSM outside the brain
* Become familiar with the techniques designed to overcome these challenges
Motivation for QSM outside the brain
Magnetic susceptibility is a fundamental property of all materials(1). Within human tissue, the presence of certain biomaterials (e.g. iron, gadolinium) causes a significant change in the magnetic susceptibility of the tissue. Importantly, this change in susceptibility is directly proportional to the concentration of that biomaterial. Therefore, knowledge of the magnetic susceptibility of a particular tissue, like iron-overloaded tissue, can be used to determine the precise concentration of the biomaterial (e.g. iron) within that tissue.
MRI is inherently sensitive to the magnetic susceptibility of tissue. The susceptibility of the tissue being imaged causes a distortion of the main magnetic field in MRI. The mathematical relationship between the 3D magnetic susceptibility spatial distribution (i.e. magnetic susceptibility map) and the resultant magnetic field distortion is well understood(2-4). Thus, by measuring the magnetic field distortion using MRI, it is possible to determine the magnetic susceptibility map of the tissue.
MRI-based methods for magnetic susceptibility mapping of the brain have been developed(5,6). These methods have been proposed for multiple applications, including quantification of the concentration of iron deposits in focal hemorrhages and deep nuclei(7,8), characterization of calcifications in the brain(8,9), and quantification of the concentration of gadolinium-based contrast agents in cerebral perfusion imaging(10,11). As in the brain, magnetic susceptibility mapping of the body has the potential for broad clinical impact including the quantification of iron overload in the liver(12,13), pancreas(14), and heart(15-17), the improved detection and characterization of calcifications in the breast(17,18), and the absolute quantification of contrast agent concentration in dynamic perfusion imaging(19), lymph node imaging(20,21), and cell labeling(22).
In summary, the potential of QSM to enable tissue characterization based on a fundamental property (magnetic susceptibility) makes it an appealing technique for research and clinical applications outside the brain. In some of these applications, QSM may be able to replace current qualitative and relaxometry (eg: R2, R2*) based techniques.
Challenges for QSM outside the brain
* Motion: Several types of motion, including cardiac, respiratory and peristaltic motion, are typically present when imaging outside the brain (particularly in cardiovascular and abdominal imaging applications). Motion introduces artifacts in the measured image magnitude and phase, and therefore motion compensation techniques are often required for QSM outside the brain. This presentation will cover different approaches to address the presence of motion during QSM acquisitions, including breath-held and free-breathing techniques.
* Fat: Multiple depots of fat, including subcutaneous, visceral as well as within other organs (eg: intra-hepatic) are relevant when performing QSM outside the brain. Fat acts as a confounding factor when estimating the B0 field map. If fat is not properly accounted for, the B0 field map will be estimated inaccurately, and these errors will propagate to the estimate of the magnetic susceptibility map. This presentation will discuss techniques for “fat-corrected” QSM, which account for the different resonance frequencies of water and fat protons.
* Large susceptibility shifts: Magnetic susceptibility can often vary significantly throughout the body due to the presence of organ-specific contrast agents or large amounts of iron deposits in certain organs like the liver or pancreas. These large susceptibility shifts significantly increase the R2* relaxation. These large variations do not typically occur in the brain. This presentation will discuss techniques to optimize the acquisition (eg: choice of echo times), as well as the QSM reconstruction in order to optimize the accuracy and precision of QSM outside the brain.
In addition to these challenges, QSM outside the brain faces several challenges which also appear in brain applications. In particular, the choice of background field removal approach and regularized QSM inversion technique will be discussed.
Acknowledgements
We
acknowledge the support of NIH (research grants R01DK083380, R01DK088925,
R01DK100651, K24 DK102595, UL1TR00427), and GE Healthcare.References
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