Synopsis

In MRI system it is important that whole body SAR is controlled accurately to obtain good scanning images and to keep patients' safety. A simple transmission antenna resonance Q value measurement system has been developed using conventional RF power monitors. The system has been validated comparing with the network analyzer measurements. Based on this validation, whole-body SAR management system was integrated. Using the integrated system, accurate whole-body SAR management has been made possible.

INTRODUCTION

METHODS

The antenna loss coefficient alpha is defined as; $$\alpha=\frac{Q_{loaded}}{Q_{unloaded}}$$ Total input power $$$P_{tot}$$$ is divided mainly into two parts; human body loss ($$$P_{obj}$$$) and antenna loss ($$$P_{coil}$$$). This relationship is expressed using $$$\alpha$$$ as; $$$P_{tot}=P_{obj}+P_{coil}=P_{tot}\times(1-\alpha)+P_{tot}\times\alpha$$$. The whole-body SAR is described using $$$P_{tot,ch}$$$ and $$$\alpha_{ch}$$$ as; $$WBSAR=\sum_{ch}^{}P_{tot,ch}(1-\alpha_{ch})/Weight$$ The curves of the absolute antenna reflection coefficient |S| differs with various loading conditions as described in figure 1. Using this different characteristics, the Q values can be calculated without information of phases of the reflection S parameters. The absolute antenna reflection coefficient |S| can be measured with forward power ($$$P_{fwd}$$$) and reflection power ($$$P_{ref}$$$) monitoring using following equation; $$|S|=\sqrt{\frac{P_{ref}}{P_{fwd}}}$$ The reflection coefficient |S| is fitted using LCR resonance circuit model impedance Z and S-Z conversion; $$Z=\frac{\frac{1}{2\pi f C i}(2\pi f L i+R)}{\frac{1}{2\pi f C i}+2\pi f L i+R}$$

After obtaining L, C, and R values by fitting, the Q was calculated using equation; $$$Q=\frac{1}{R}\sqrt{\frac{L}{C}}$$$. The $$$\frac{L}{C}$$$ values are fixed for each channel for specific values to stabilize the fitting process. Three Tesla whole body MRI system (Hitachi, Ltd., Tokyo, Japan) with four channel transmission antenna² is used for system integration. The RF configuration detail is described in figure 2. Finally, the software and the system was integrated. The eleven |S| point data for each four channel could be measured in four seconds. The measurement frequency span was set to be 2 MHz. Influences from long cables, transmit receive switching circuits, and the directional coupling circuits (figure 3) were compensated with calibration matrix³. The data fitting result example is shown in figure 4.

Validation of the system was made by comparing $$$\alpha$$$ measured using a network analyzer and those using the developed system. We assumed $$$\alpha$$$ obtained using the network analyzer has the correct value. Data from volunteers were obtained after receipt of written informed consent. This study was approved by the ethics committee of Hitachi group headquarters. With twelve volunteers, fifteen scanning positions, and total 119 data sets for each four channels were measured for validation (table in figure 5). The scanning positions include off-centered positions and lateral positions in which patient body is very close to one of the four channels of the transmission antenna. The fifteen-scanning position were head, C-spine, CT-spine, chest, liver, abdomen, pelvis, hip joint, long bone, shoulder, elbow, wrist, knee, ankle, and breast.

RESULTS

Measured Q values range from 47 to 271. The $$$\alpha$$$ measured by the network analyzer ranges from 0.142 to 1.033. The four channel transmission antenna has a slight mutual coupling between channels. The coupling leads $$$\alpha$$$ value greater than unity in some cases. The heaviest loaded condition was $$$\alpha=0.142$$$. The comparison plot of $$$\alpha$$$ is shown in figure 5. Good correlation coefficient (R=0.9524) is obtained between network analyzer measurements and the system measurements. Based on this correlation, whole body SAR management system was integrated. Using this integrated system, accurate whole body SAR management has been made possible.

DISCUSSION

CONCLUSION

A simple transmission antenna resonance Q value measurement system has been developed. The system uses conventional RF power monitors. The system has been validated comparing with the network analyzer measurements.

Acknowledgements

References

[1] P. Mansfield, et. al., "NMR imaging in biomedicine", Academic Press, (1982) p313.

[2] Y. Soutome, et. al., "Simulation Study of a 4-Channel Ladder-shape Body Coil at 3T", Proc. ISMRM 21 (2013) p2750.

[3] A. Ferrero, et. al., "Multiport Vector Network Analyzer Calibration: A General Formulation", IEEE Trans. microw. theory tech., vol. 42, 2455 (1994).