3 research outputs found
physics considerations and electromagnetic field simulations up to 23.5 Tesla (1GHz)
Background Glioblastoma multiforme is the most common and most aggressive
malign brain tumor. The 5-year survival rate after tumor resection and
adjuvant chemoradiation is only 10 %, with almost all recurrences occurring in
the initially treated site. Attempts to improve local control using a higher
radiation dose were not successful so that alternative additive treatments are
urgently needed. Given the strong rationale for hyperthermia as part of a
multimodal treatment for patients with glioblastoma, non-invasive radio
frequency (RF) hyperthermia might significantly improve treatment results.
Methods A non-invasive applicator was constructed utilizing the magnetic
resonance (MR) spin excitation frequency for controlled RF hyperthermia and MR
imaging in an integrated system, which we refer to as thermal MR. Applicator
designs at RF frequencies 300 MHz, 500 MHz and 1GHz were investigated and
examined for absolute applicable thermal dose and temperature hotspot size.
Electromagnetic field (EMF) and temperature simulations were performed in
human voxel models. RF heating experiments were conducted at 300 MHz and 500
MHz to characterize the applicator performance and validate the simulations.
Results The feasibility of thermal MR was demonstrated at 7.0 T. The
temperature could be increased by ~11 °C in 3 min in the center of a head
sized phantom. Modification of the RF phases allowed steering of a temperature
hotspot to a deliberately selected location. RF heating was monitored using
the integrated system for MR thermometry and high spatial resolution MRI. EMF
and thermal simulations demonstrated that local RF hyperthermia using the
integrated system is feasible to reach a maximum temperature in the center of
the human brain of 46.8 °C after 3 min of RF heating while surface
temperatures stayed below 41 °C. Using higher RF frequencies reduces the size
of the temperature hotspot significantly. Conclusion The opportunities and
capabilities of thermal magnetic resonance for RF hyperthermia interventions
of intracranial lesions are intriguing. Employing such systems as an
alternative additive treatment for glioblastoma multiforme might be able to
improve local control by “fighting fire with fire”. Interventions are not
limited to the human brain and might include temperature driven targeted drug
and MR contrast agent delivery and help to understand temperature dependent
bio- and physiological processes in-vivo
Thermal magnetic resonance: physics considerations and electromagnetic field simulations up to 23.5 Tesla (1GHz)
Lipid suppression for brain MRI and MRSI by means of a dedicated crusher coil
PURPOSE: Lipid suppression in MR brain imaging and spectroscopy has been a long-standing problem for which various techniques have been developed. Most methods are based on inversion recovery or spatially or spectrally selective excitation of the lipid signal followed by dephasing. All techniques require additional RF pulses, gradient crushers and delays, which increase the duration and complexity of sequences. In addition, the lipid signal is poorly shimmed, and is composed of different resonance frequencies that have different relaxation properties. METHODS: In this work, a novel approach for suppression of extra cranial lipids is presented, by means of an outer volume crusher coil. It is based on the principle of surface spoiling gradients, which generate a very local and inhomogeneous magnetic field in the outer layer of the head, and thereby destroys the phase coherence of the extra cranial signals. RESULTS: Dephasing of the signal can be incorporated in almost any sequence because it requires only a short pulse of the coil, and does not require additional RF pulses or delays. Examples of lipid suppression are shown in both gradient echo imaging and spectroscopic imaging. CONCLUSION: Outer volume crushing allows for simple fat suppression and boosts scanning efficiency, which is particularly beneficial at ultra-high field strengths