1. g Whole Body, Transmit-Receive Coil for Clinical 3 Tesla MRI
Ronald D. Watkins1, John F. Schenck1, Ken Rohling1, Joe Piel1,Randy Giaquinto, Daniel Rosenfeld2,
Avram Montag2, James Hugg2, Douglas Kelly3, Herbert Kressel4,Robert Lenkinski4, David Alsop4,Robert Greenman4
1 GE R&D, Schenectady NY, 2GE Med. Systems Israel,3MGH Hosp. Boston Ma.,4 Beth Israel Deaconess Boston Ma.
In recent years there has been an increase in usage of MRI systems at field strengths above the typical 1.5 Tesla. Research systems have been built with magnets as high as 8 Tesla. Systems are now commercially available at 3 tesla and 4 tesla. These systems are primarily used for research in fMRI, human head imaging, and MRS studies. The addition of a large volume whole body transmit receive coil would allow new clinical applications to be developed in other parts of the human body outside the head as well
as supporting the use of local receive only surface coils. Several technical issues must be resolved such as the increase in RF power required to produce the B1 field at higher frequencies. The shorter wavelength due to the higher frequency must also be considered. Attempts to retune standard 1.5T body coils are difficult due to the coil inductance. Steps must be taken to reduce this inductance and have a coil that uses capacitors of reasonably high value.
CONCLUSIONS
MATERIALS & METHODS
The addition of this whole body RF coil in a 3T system will allow the use of virtually all whole body clinical application facilitated by a standard whole body 1.5T system. Despite the aggressively short length of the coil the limitation in FOV was primarily due to the Z gradient coil in the sagittal and coronal images. The peak and average power required to run standard image protocols although higher than 1.5T systems is very reasonable and within current limits set by the FDA. 3 tesla whole body systems provide a potential to have significant increase in image quality and resolution in high resolution applications such as cardiac, spine and extremity imaging as well as the potential for reducing exam time . Whole body imaging with the described system has now received 510K clearance by the FDA.
A 55cm ID shielded highpass birdcage coil (1-4) (Figure 1.) was designed for a whole body 3 tesla magnet. (GE 3T94) fitted with gradient coils capable of 40mT/m and slew of 150T/m/S. The length of the birdcage was kept relatively short to reduce RF drive power, minimize rung inductance,and avoid wavelength problems in the rungs. The rungs were also made very wide to further reduce inductance and yield higher capacitor values, allowing the higher frequency operation. The coil was placed inside a segmented copper RF shield.The distance from the coil to the RF shield is very close resulting in higher coil currents but further reducing rung inductance and yielding reasonable capacitor values. The coil has an unloaded Q of ~300 and loaded Q of and 90 deg. cables were attached without the need for baluns(5). The coil was connected to a 90 deg. quadrature hybrid power splitter and driven by a 25KW RF amplifier(Astex Inc.). Pin diode disable circuits were employed in 8 locations for deresonating the coil during headcoil and receive surface coil operation. After installing the body coil no performance decrease was observed in headcoil operation. The coil was extensively tested to 27KW peak and 500W average power on phantoms to assure mechanical and electrical robustness.
T-Spine
Knee
F-Pelvic
F-Pelvic
C-Spine
C-Spine
M-Pelvic
M-lower abdomen
REFERENCES
Head-Neck T1
Head-Neck T2
Pancreas
Liver Fat-Water out of Phase
1.C. E. Hayes, W. A. Edelstein, J. F. Schenck, O. M. Mueller, and M. Eash, ``An efficient, highly homo- geneous radiofrequency coil for whole-body NMR imaging at 1.5 T,'' J Mag Res vol. 63, p
2. J. Tropp, ``The theory of the bird-cage resonator,'' J Mag Res Vol 83 p
3. US Patent # :Radio frequency field coil for NMR Edelstein et. al
4. US Patent # :Radio frequency field coil for NMR Hayes et al.
5. US Patent # :Method for providing multiple coaxial cable connections to a radio-frequency antenna without baluns Roemer et. al.
Heart long axis
Heart Short Axis
40cm T2 Spine
Wrist
Figure 2.
RESULTS & DISCUSSIONS
ACKNOWLEDGEMENTS
The coil produced a B1 field with a uniformity of +-10% over 20cm DSV and +-30% over 35cm DSV as measured in air with a field probe.Load impedance varied between 40 and 80 ohms for various patient weights from 120 to 310 lbs. B1 mapping in human subjects is further explored in a poster by David Alsop at this meeting.
Images of the spine, heart, wrist, prostate, and other various body parts were acquired with good results using prototype phased array coils as well as body images using the bodycoil for receive (Figure 2). The coil has an inherent efficiency of approximately 80%.
The author gratefully acknowledge significant contributions by Lindsey Carver,
Jeff McGovern, Glen Reynolds, Chuck Rossi, David Keren, Haim Rozenboim, Yoram Goren, David Weber, Saul Stokar, Benny Assif, Hillel Lisitzen, Zeev Harel, Joe Schaeffer, Clyde Krumrai, Tom Dixon, Xuli Zong, William Edelstein, Rich Mallozzi Nadeem Ishaque, Sayed El-Hamamsy, Anton Mushovic, Tsuri Bernstein and William Barber
Figure 1. Whole body 3 Tesla shielded birdcage coil