Magnetic Resonance Imaging of the Human Body at 4T

Introduction:

Magnets with field strengths to 7 tesla (T) and higher are now available for human body imaging. Imaging signal, speed and resolution are benefited by these highest of field strengths. Until recently however, a magnetic field strength of 3 or 4 tesla was considered by many to be a practical limit for human head imaging. Body imaging wasn't given serious consideration for fields much above 1.5 T. These limitations to imaging field strength were imposed by technology limitations, primarily with radio frequency (RF) coils. RF coils are the antenna like devices inside the magnet that are used for stimulating and receiving the image signal in the body. One of our aims at the Center for Magnetic Resonance Research is to reach the inherent potential of high field MRI by overcoming the limitations imposed by conventional RF coil technology. We accomplish this goal by designing coils by transmission line (TEM) methods. A few examples of our coils and body images acquired with them at 4T follow. See also Research Highlights: "Magnetic Resonance Imaging of the Human Head at 7T".

Methods and Materials

A body coil is often needed for imaging the human body. A body coil can be used as a transmitter, together with separate receiver coils for better imaging of a region of interest in the body, next to the receiver coil. A complex system is required to operate the transmit coil and receive coils. The system developed for this purpose at the CMRR is shown in Figure 1. below.

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Figure 1. Body Coil System. The system is composed of a body coil transmitter and a multi-channel array coil receiver together with RF interface and control circuits. Following the same plan, a head coil system has also been developed for use with a variety of phased array and parallel array receivers tested to 7T. See "MRI of the Human Head at 7T".

Coil System Description:

An active body coil system has been developed for efficient head and body imaging at field strengths from 1.5T to 9.4T (1,2). This complete RF front-end system employs an actively detuned TEM head or body coil for NMR signal excitation, together with local receiver coils of the phased array or parallel array type. Included in the system as shown above are the homogeneous transmit coil (a), the multi-channel receiver coil (b), the power supply and control unit (c), the optically triggered, nonmagnetic PIN diode driver unit (d), the non-magnetic, multi-channel preamplifier (e), and all of the necessary fiber optic control lines (f), power supply lines (g) and RF signal cables (h). Not shown are the high power, non-magnetic transmit / receive (TR) switches and filters for RF signal and power supply lines. The system as depicted is currently in use for head, body, and laboratory animal applications on 3T, 4T, 4.7T and 7T MRI systems at the CMRR and at other high field laboratories.

Results:

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Figure 2. 4T Head Images. This multi slice gradient echo image set was acquired using a body coil transmitter and a 4-channel, strip line, parallel array receiver. The images were assembled by simple sum-of-squares magnitude addition without image intensity correction.

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Figure 3. 4T Breast Images. These high resolution spin echo breast images were acquired by the following parameters: T1-weighted, TE 21 ms, TR 400 ms, Slice 3 mm, FOV 20 cm, Matrix 512x512. A body coil transmitter was used together with a two channel, quadrature phased array receiver.

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Figure 4. 4T Abdomen Images. This multi-slice body image set was acquired from a 95kg male at 4T, using a TEM body coil for both transmission and reception. Acquisition parameters for this data set are: 2D FLASH, TR/TE = 250/5.2 ms, slice = 5 mm, FOV = 48 x 36 cm, Acq. Matrix 256 x 128, NEX = 2, 800W peak RF for low tip pulse. A 1kW power amplifier was used. A 1kW, 1.7ms hard pulse was required to achieve a 90o calibrated slice through the abdomen of a 95kg male subject (Figure). On the other end of the range, a 1kW, 1ms square pulse was required for a 120lb female subject. Fairly uniform RF field penetration is demonstrated. No signal intensity correction is applied.

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Figure 5. 4T Abdomen Images. In contrast to method of Figure 4, these images were acquired with a body coil transmitter, and a four channel phased array receiver. The breath-held, T1-weighted, spoiled gradient, recalled echo acquisition sequence parameters were: TE = 3.9 ms TR = 200 ms Flip = 90 Slice = 5 mm FOV = 36x27 cm Matrix = 256x96. The image intensity shading is a common artifact the phased array receivers. No intensity correction is applied to the combined image above, as would be typically applied to clinical phased array images.

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Figure 6. 4T Heart Images. This pair of breath held, gradient echo, cardiac images was acquired with EKG cardiac gating from a 160 lb. male. A body coil (Figure 1) was used for the transmitter. A four loop phased array was used for the receiver. Attention in called to the RF artifact causing signal drop out in the right atrium, in the image to the left above. This artifact has appeared consistently in four of four subjects imaged. The cause of this artifact in prominently featured in Maxwell models of the human chest at 170 MHz. This model can be consistently removed by changing the relative currents (magnitude and phase) in a process we refer to as "RF Shimming". The image to the right above, from the same subject, has been RF shimmed. No intensity correction is applied.

Conclusions:

MR Images from the whole human body look good at 4T! These initial images will certainly improve toward "clinical" standards with further refinements to technique and technology. As they are, this collection of images should help to relieve some of the remaining concerns of excessive RF power requirements and RF field penetration formerly accorded to 4T body imaging.

References:

• Vaughan JT, Adriany G, Garwood M, et.al. Dutuneable Transverse Electromagnetic (TEM) Volume Coil for High-Field NMR. Magn. Reson. Med. 47:990-1000 (2002)
• Vaughan JT, Adriany G, Bolinger L, Waks M, DelaBarre L, Garwood M, Andersen P, Ugurbil K. A body coil for high field MR. In: Proceedings of the 10th Annual Meeting of ISMRM, Honolulu, 2002.

Acknowledgements: (collaborators in high field body imaging)

• Lizann Bolinger, Ph.D., University of Iowa
• Haiying Liu, Ph.D., University of Minnesota
• Alan Stolpen, M.D., Ph.D., University of Iowa