Diaphragm position can be accurately estimated from the scattering of a parallel transmit RF coil at 7 T
Hess AT., Tunnicliffe EM., Rodgers CT., Robson MD., Hess AT.
Purpose To evaluate the use of radiofrequency scattering of a parallel transmit coil to track diaphragm motion. Methods Measurements made during radiofrequency excitation on an 8-channel parallel transmit coil by the directional couplers of the radiofrequency safety monitor were combined and converted into diaphragm position. A 30-s subject-specific calibration with an MRI navigator was used to determine a diaphragm estimate from each directional-coupler measure. Seven healthy volunteers were scanned at 7 T, in which images of the diaphragm were continuously acquired and directional couplers were monitored during excitation radiofrequency pulses. The ability to detect coughing was evaluated in one subject. The method was implemented on the scanner and evaluated for diaphragm gating of a free-breathing cardiac cine. Results Six of the seven scans were successful. In these subjects, the root mean square difference between MRI and scattering estimation of the superior–inferior diaphragm position was 1.4 ± 0.5 mm. On the scanner, the position was calculated less than 2 ms after every radiofrequency pulse. A prospectively gated (echocardiogram and respiration) high-resolution free-breathing cine showed no respiratory artifact and sharp blood-myocardium definition. Conclusions Transmit coil scattering is sensitive to diaphragm motion and provides rapid, quantitative, and accurate monitoring of respiration.