Investigating respiratory motion induced changes on EM fields and SAR in UHF body MRI

• Applicant: Dr. Sebastian Schmitter
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405363511

• Final Report Year: 2024

Final Report Abstract

An essential part of the imaging process in magnetic resonance imaging (MRI) consists in the coherent excitation of the spins within the head/body by an external radiofrequency (RF) field that is generated by the MR coil. This RF field has an electric field component (E) and a magnetic component (B1+). While B1+ causes the required spin excitation to generate an MR image, the E-field component generates unwanted tissue heating, which is characterized by the (local) specific absorption rate (SAR). In ultra-high field (UHF) MRI, where scanners operate at field strengths of ≥7T, two problems arise: first, the B1+ field can be highly spatially heterogeneous such that the image intensity can become usable for diagnosis. Second, the SAR becomes increasingly focused, thus, localized "hot spots" may be generated in UHF MRI, which creates safety concerns and requires to limit the RF power. To address both problems, a technique termed 'parallel transmission' (pTx) that makes use of an RF coil consisting of multiple elements has successfully been applied. This technique allows reducing the undesired spatial B1+ heterogeneity while reducing the SAR. However, it has been shown that B1+ can strongly vary throughout the respiratory cycle, thus, a homogeneous pTx excitation during exhale may yield signal dropouts in the image during inhale. The central hypothesis of this proposal was, that respiration should impact not only the B1+-field but also the resulting SAR. We could show for the first time, that this hypothesis is true. Both, B1+ and SAR change substantially throughout the respiratory cycle and it depends on several factors. For this investigation electromagnetic (EM) field simulations were performed using a virtual human body model that exists for arbitrary phases of the respiratory cycle, i.e. exhale, inhale and intermediate states. SAR and B1+ was retrieved in the simulations for different coil setups, for different breathing patterns and for different RF pulses. An up to 2.5-fold increase in local SAR has been found between exhale and deep inhale, whereas shallow breathing had only a minor impact on SAR as well as on the excitation homogeneity. Interestingly, the respiration also changed SAR hotspot location for some excitations. A coil identically to the simulation has been built physically, and verification measurements have been performed at 7T and at 10.5T in phantoms and in-vivo, with similar results as in the simulations. Furthermore, different types of RF pulses have been generated to investigate the pulses' dependence on respiration. The results provide an important contribution for future UHF studies in the body, published in six peer-reviewed papers and several conference abstracts.

Publications

• 3D Free‐breathing multichannel absolute Mapping in the human body at 7T. Magnetic Resonance in Medicine, 85(5), 2552-2567.
  Dietrich, Sebastian; Aigner, Christoph S.; Kolbitsch, Christoph; Mayer, Johannes; Ludwig, Juliane; Schmidt, Simon; Schaeffter, Tobias & Schmitter, Sebastian
  
• Impact of respiration on B1+ field and SAR distribution at 7 T using a novel EM simulation setup. in Proc. Intl. Soc. Mag. Reson. Med 28 1120 (2020).
  Schön, N.
  
• Three‐dimensional static and dynamic parallel transmission of the human heart at 7 T. NMR in Biomedicine, 34(3).
  Aigner, Christoph Stefan; Dietrich, Sebastian & Schmitter, Sebastian
  
• Calibration‐free pTx of the human heart at 7T via 3D universal pulses. Magnetic Resonance in Medicine, 87(1), 70-84.
  Aigner, Christoph Stefan; Dietrich, Sebastian; Schaeffter, Tobias & Schmitter, Sebastian
  
• Investigation of respiration-induced changes of the scattering matrix by EM simulations and a breathing body model. in Proc. Intl. Soc. Mag. Reson. Med. 29 3342 (2021).
  Schön, N., Seifert, F., Metzger, G. J., Ittermann, B. & Schmitter, S.
  
• Motion‐compensated fat‐water imaging for 3D cardiac MRI at ultra‐high fields. Magnetic Resonance in Medicine, 87(6), 2621-2636.
  Dietrich, Sebastian; Aigner, Christoph Stefan; Mayer, Johannes; Kolbitsch, Christoph; Schulz‐Menger, Jeanette; Schaeffter, Tobias & Schmitter, Sebastian
  
• Respiration induced B1+ changes and their impact on universal and tailored 3D kT‐point parallel transmission pulses for 7T cardiac imaging. Magnetic Resonance in Medicine, 87(6), 2862-2871.
  Aigner, Christoph Stefan; Dietrich, Sebastian & Schmitter, Sebastian
  
• Respiration-Induced Variations in Upper SAR Boundary Appear to Increase with Field Strength: A Comparison of UHF Cardiac Imaging at 7T &10.5T. in ISMRM Workshop on Ultra-High Field MR (2022).
  Schön, N.
  
• The impact of respiratory motion on electromagnetic fields and specific absorption rate in cardiac imaging at 7T. in Proc. Intl. Soc. Mag. Reson. Med 30 2547 (2022).
  Schön, N.
  
• The impact of respiratory motion on electromagnetic fields and specific absorption rate in cardiac imaging at 7T. Magnetic Resonance in Medicine, 88(6), 2645-2661.
  Schoen, Natalie; Seifert, Frank; Petzold, Johannes; Metzger, Gregory J.; Speck, Oliver; Ittermann, Bernd & Schmitter, Sebastian