MAGNETIC RESONANCE IMAGING (MRI) is one of the best-known examples of RF technology applied to medicine. MRI scanning and imaging systems can provide early detection of cancerous tumors and arm medical personnel with knowledge to make differences in patients' lives. MRI systems have made great strides in recent years, to the point where sensitive full-body scans can now be performed within a reasonable time frame; much of the credit for this is due to advances in computers as well as in RF electronics.
One of the RF subsystems of note in modern MRI systems is a multichannel 32-kW amplifier system from Communication Power Corp. (CPC) which has made possible MRI platforms with 10.5-T field strength for high-resolution full-body scans. the amplifier system (Fig. 1) developed by CPC is the model 10.5T2000m-16/1c, designed for high-field MRI systems, in particular 10.5-t MRI systems. It drives as many as 16 amplifier channels, each with more than 2 kW output power from 40 to 450 mHz (see table). It can be switched via controller area network (CAN) interface bus from 16-channel mode to singlechannel operation with 30 kW output power by combining outputs via a switch matrix and radial power combiner.
The 10.5t2000m-16/1C power amplifier system brings improved MRI imaging quality, resolution, and speed to medical applications. Some early, lower-power MRI scans required nearly five hours (with a patient in a fixed position) for usable images. Early MRI scanners, such as pioneered by Paul Lauterbur, Sir Peter Mansfield, Raymond Damadian, and associates, can be traced to the late 1970s (Fig. 2a). modern systems (Fig. 2b) are more complex but capable of providing comprehensive metabolic information with anatomical scans.
An MRI system takes advantage of the nuclear magnetic resonance (NMR) property of materials on a molecular level. The nuclei of different atoms have different resonant frequencies, and can be made to precess within a large magnetic field. When they are subjected to a sufficiently large electromagnetic (EM) field, these nuclei will emit an NMR signal that can be ultimately converted to an image. In its simplest form, superconducting magnets create the large magnetic fields within an MRI enclosure. Vacuum tubes were typically used in early RF power amplifiers. Newer amplifiers, such as the model 10.5T2000M-16-/1C, are based on solid-state devices.
Many early and current MRI scans make use of the hydrogen (H) nuclei, since the human body is about 70% water. But magnetic resonance spectroscopy (MRS) has been used to produce images with other atoms and their nuclei, although this requires RF amplifiers operating at frequencies suitable to the nuclei of interest. Creating images based on multiple nuclei can help identify certain chemicals indicative of metabolic activity within tumors.
Figure 3 shows an anatomical MRI scan coupled with an MRS spectrum. The lesion in the center of the breast image may be cancerous. MRI researchers have learned that malignant breast tumors produce a metabolite called choline, which is used as a marker for angiogenesis. MRS can quantify choline concentration within a tumor, an indicator of tumor growth activity. Without MRS, a tumor might be treated by chemotherapy, with progress measured after several weeks by measuring the size of the tumor. MRS, which can detect a drop in choline concentration one day after the start of chemotherapy (see T. Vaughan et al., "7-T Whole Body Imaging Preliminary Results," Magnetic Resonance Medicine, Vol. 61, 2009, pp. 244-248).
MRI scanners are complex systems. In addition to the large magnets and high-power amplifiers, they incorporate RF componentssuch as low-noise amplifiers, modulators, and frequency synthesizersand high-current power supplies, computers, and software. All components contribute to success, with the power amps designed to deliver clean outputs for a wide range of input signals, including short modulated pulses.
In an MRI scanner, high field strengths, which are measured in Tesla (T), translate into higher system signal-to-noise ratios (SNRs) with improved imaging resolution. But high field strengths require large magnet structures and high-power RF amplifiers, along with the challenges of producing clear images at higher field strengths.
An increase in field strength corresponds to an increase in frequency (and shortening of wavelengths for higher resolution). It is relatively simple to produce a homogeneous RF field inside a patient at lower field strengths, such as 1.5 T. Above 7 T (about 300 MHz), the task of creating a uniform RF field becomes complex with a single antenna and amplifier. Standing waves within the body can produce uneven fields, yielding images with darkened areas and misleading visual information.
One way to provide a more uniform distribution of power at high field strengths is by means of an array of amplifiers and antennas, with the amplitude and phase of each adjusted accordingly. This multichannel amplifier approach has been used in the model 10.5T2000M-16/1C RF power amplifier system to achieve high-quality MRI images at 10.5 T.1
The model 10.5T2000M-16/1C amplifier system's unique architecture (Fig. 4) allows it to seamlessly switch between multichannel and single-channel operation, using programming commands over the CANbus. The input/output switch matrices perform the signal routing. They either send signals to and from the 16 RF amplifier channels directly (for 16- channel operation) or split one input signal 16 ways, send these to the 16 amplifiers, then combine their outputs in a low-loss, high-power 16-channel radial power combiner for single-channel operation. Although multichannel mode is preferential for uniform RF fields, some applications benefit from single-channel operation.
Conventional MRI scanners are equipped with a frequency synthesizer as the RF source. For compatibility with older MRI scanners, the model 10.5T2000M-16/1C amplifier system has an integral digital gain/phase controller. It can divide a single RF input into multiple signals and provide amplitude and phase modulation control prior to amplification. Amplitude and phase can be changed between pulses, effectively emulating a 16-channel phase-locked synthesizer.
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The 10.5T2000M-16/1C's master system microcontroller monitors and controls overall system command/fault status and RF power output via an RS-485 serial bus routed to all subsystem assemblies. In multichannel mode, the 16 channel amplifiers operate independently: If one channel should fail, the other 15 channels continue operating. In single-channel mode, the system will shut down all amplifiers if one channel should fail.
The amplifier supports testing on five atomic nuclei from 40 to 450 MHz. The dual-mode amplifier produces 17 data sets (16 channels in multichannel mode and 1 channel in single-channel mode). To test all five key nuclei would result in 17 x 5 = 85 data sets. Figures 5 through 7 show sample data for one channel.
Amplifiers for MRI systems are generally characterized in the frequency, power, and time domains. The input VSWR (Fig. 5a) and gain/gain flatness (Fig. 5b) plots show input impedance match and small-signal gain from 40 to 450 MHz. The VSWR is 1.20:1 or better while the 63-dB gain is flat within 2.25 dB. Each channel can be driven to full output by a 0-dBm (3-dB) input signal source.
Measurements performed over a wide dynamic range, in the power domain, help define the channel gain and phase linearity performance of the high-power MRI amplifier. Tested over a 20-dB dynamic range from 20 W to 2 kW, the system's amplifiers maintain near constant insertion gain and phase even as the output power is varied over that wide range. The insertion phase remains linear within 6 deg. while the insertion gain is within 0.6 dB at 447 MHz (Fig. 6).
Because the 10.5T2000M-16/1C amplifier system will be subjected to a wide range of signal pulse conditions, it is helpful to stimulate the system during testing with a challenging input signal, such as a rectangular pulse train. Such a test signal was used to produce the plots in Figs. 7a-7c, where the rise time is 24 ns, the pulse tilt is -4.25% (Fig. 7b), and the fall time is 30 ns (Fig. 7c). The falling transition duration shows a few cycles of flywheel effect modulating the RF. This is due to energy in the decoupling inductors cycling over to coupling capacitors and vice versa following each RF pulse. While an aberration, the levels are within specified fall times for the system.
Because the final output stages of an MRI amplifier will generate noise when biased on, it must be shut off or "blanked" during signal acquisition. The unblank delay time is the amount of time needed to turn on and shut off the final amplification stages to reduce output noise. For the 32-kW MRI amplifier system, the noise output is reduced in 788 ns.
Since the model 10.5T2000M-16/1C is truly a system and not just a collection of amplifier stages, it is housed in multiple racks (Fig. 1). Each of the tall racks on the outer sides of the system hold eight 2-kW amplifier channels, with each channel a 3U chassis that contains a driver amplifier and two quadrature-combined power amplifier modules. Each 3U chassis can be removed for service or maintenance. The lower two chassis on the outer racks are high-current DC power supplies, each capable of more than 1000 A current. The center rack contains the CAN interface for the system, the phase/gain controller, the input/output switch matrices, the radial output combiner, and additional power supplies.
At present, 3-T MRI scanners are ubiquitous, although higher-field-strength systems promise improved image quality. An amplifier such as the 32-kW model 10.5T2000M-16/1C clears the way for 10.5-T MRI systems.
The author would like to acknowledge the technical contributions of Dr. J. Thomas Vaughan and Dr. Michael Garwood (University of Minnesota, CMRR), Dr. Hoby Hetherington (Yale University Medical Center), and Dr. Joel Stutman. Special thanks to Eleanor Roe Design for the article graphics.