B1 field-insensitive transformers for RF-safe transmission lines.
OBJECTIVE
Integration of transformers into transmission lines suppresses radiofrequency (RF)-induced heating. New figure-of-eight-shaped transformer coils are compared to conventional loop transformer coils to assess their signal transmission properties and safety profile.


METHODS
The transmission properties of figure-of-eight-shaped transformers were measured and compared to transformers with loop coils. Experiments to quantify the effect of decoupling from the B1 field of the MR system were conducted. Temperature measurements were performed to demonstrate the effective reduction of RF-induced heating. The transformers were investigated during active tracking experiments.


RESULTS
Coupling to the B1 field was reduced by 18 dB over conventional loop-shaped transformer coils. MR images showed a significantly reduced artifact for the figure-of-eight- shaped coils generated by local flip-angle amplification. Comparable transmission properties were seen for both transformer types. Temperature measurements showed a maximal temperature increase of 30 K/3.5 K for an unsegmented/segmented cable. With a segmented transmission line a robotic assistance system could be successfully localized using active tracking.


CONCLUSIONS
The figure-of-eight-shaped transformer design reduces both RF field coupling with the MR system and artifact sizes. Anatomical structure close to the figure-of-eight-shaped transformer may be less obscured as with loop-shaped transformers if these transformers are integrated into e.g. intravascular catheters.
Krafft
Axel
A
Deutsches Krebsforschungszentrum (dkfz), Abt. Medizinische Physik in der Radiologie (E020), Im Neuenheimer Feld 280, 69120, Heidelberg, Germany.
Müller
Sven
S
Umathum
Reiner
R
Semmler
Wolfhard
W
Bock
Michael
M
D004867:Equipment Design; D019544:Equipment Failure Analysis; D006358:Hot Temperature; D019047:Phantoms, Imaging; D011832:Radiation Injuries / Q000209:etiology / Q000517:prevention & control*; D011835:Radiation Protection / Q000295:instrumentation* / Q000379:methods; D011837:Radiation-Protective Agents; D011846:Radio Waves / Q000009:adverse effects*; D013696:Temperature; D013997:Time Factors
D011837:Radiation-Protective Agents