Pre-amplifiers for a 15-Tesla magnetic resonance imager

Transcription

1 From the SelectedWorks of Chin-Leong Lim December, 2013 Pre-amplifiers for a 15-Tesla magnetic resonance imager Chin-Leong Lim Peter Serano, Massachusetts General Hospital Jerome L Ackerman, Massachusetts General Hospital Available at:

2 Pre-amplifiers for a 15-Tesla magnetic resonance imager Chin-Leong Lim Avago Technologies Penang, Malaysia C.L.Lim@ieee.org Abstract High-field magnetic resonance imagers (MRI) give better signal-to-noise ratio (SNR) and image contrast. However clinical MRIs are currently limited to 3 Tesla (T) magnetic field strength. To create an uncommon 15 T scanner for research use, we evaluated several low-cost, intended for wireless communication, GaAs enhancement-mode pseudomorphic high electron mobility transistors (ephemt) in the critical preamplifier slot. This paper reports the experimental results that were obtained at both module and system levels. When evaluated in our prototype 15 T scanner front-end s preamplifier slot, the candidate devices sub 1dB noise figures enabled image SNR ~ 110 in a water phantom (test object) with a standard pulse sequence. Crisp and artifact-free images could be obtained with these candidate devices. In conclusion, high performance pre-amplifiers for 15 T MRIs can be realized using low cost ephemts originally marketed for wireless communications. Keywords - pre-amplifier; magnetic resonance imaging; MRI; 15 Tesla; ephemt I. INTRODUCTION Increasing the static magnetic field strength in a magnetic resonance imaging (MRI) scanner can improve its signal-tonoise ratio and image contrast. Scanners in current clinical use have static magnetic field strength B 0 of 3 Tesla or less, while 7 T is available in the research/pre-clinical stage [1]. Raising B 0 requires that the radio frequency (RF) front-end be scaled up in frequency as well because for every Tesla increase in the static field, the spin Larmor frequency is correspondingly linearly incremented by ~42 MHz [2]. The exact magnetic field of our scanner is T, resulting in an operating frequency of 619 MHz. The explosive growth of cellular communications in recent years has made available extremely high-performance but low-cost semiconductors that are potentially useful in high-field MRIs; one candidate device is the low-noise GaAs enhancement-mode pseudomorphic high electron mobility transistor (ephemt) which has already dropped below USD1. To fill the preamplifier (preamp) slot in an experimental 15 T scanner, we evaluated two candidate devices. The device selection is partially influenced by the availability of application notes, device models and printed circuit boards (PCB) from the manufacturer. II. 15-T MRI FRONT END The magnet, which is intended for small animal (mouse) research, has an available diameter of 60 mm inside the Peter Serano and Jerome L. Ackerman Martinos Ctr. for Biomedical Imaging, Dept. of Radiology Massachusetts General Hospital Boston, MA, USA magnetic field gradient coil. The RF coil which detects the MRI signal is matched to 50Ω in the magnet, and the signal is brought outside with a short length of 50Ω coax (Fig. 1). This physical separation between coil and preamp is not optimal, as there are a few tenths of a db loss in the coax connection but it facilitates the evaluation of the preamp. However, future versions will integrate the coil and preamp on one PCB. When the sample is excited by a high-power RF pulse, the preamp is temporarily disconnected from the common coil by the PIN diode switch. The switch will not be able to fully prevent saturating the preamp during the RF pulse. In most cases, except for short T2 signals, there is no problem if the preamp takes several tens of microseconds to recover from the transmit pulse. A commercial, moderate-field scanner console (Siemens), serving as the system back-end, provides the control signals for the switch. Following the preamp stage, a Mini-circuits ZMY-2, level 23 passive mixer [3] converts 619 MHz to 102 MHz for input to the Siemens receiver. Figure 1. Simplified block diagram of the 15-T MRI front-end III. PREAMP REQUIREMENTS AND DESIGN Preamp components should have low magnetism to prevent image distortion. This requirement is met by using device packaging which is mainly epoxy plastic and copper alloy (97% Cu: 2% Fe) lead-frame. The preamps should also be sufficiently rugged to withstand the transmit pulse and gradient pulse electromagnetic fields. Because we plan to fit 8 or 16 coil-preamp assemblies inside the magnet bore, compactness is desirable. A. Discrete pre-amp The first candidate device, ATF-54143, is a 0.25 x 800 µm ephemt in a 4-lead SOT-343 package [4]. Our 619 MHz preamp is created by re-tuning the manufacturer s 900 MHz reference design [ 5 ]. The ephemt and 14 passive components required for impedance matching, biasing and The 15T MRI project is supported by U.S. National Institutes of Health grant S10-RR

3 stabilization fit a FR4 PCB measuring x 0.75 x inch. The input and output networks, C1-L1 and C4-L4, are tuned for low noise and good matching, respectively. The supply voltage Vdd and current are 5 V and ~60 ma. Figure 3. Circuit (top) & photo (bottom) of the ephemt MMIC preamp Figure 2. Circuit & photo of the discrete ephemt preamp B. MMIC pre-amp The second preamp uses the MGA-633P8 microwave monolithic integrated circuit (MMIC) which integrates a 0.25 x 1600 µm ephemt, active bias and electrostatic discharge protection [6 ]. Because of the higher integration, it costs ~30% more than the discrete device. The metal content of its QFN2x2 package is insignificantly low at ~0.25mm 3 or 8.4% of the package volume. Although the preamp follows the manufacturer s recommended design for 900 MHz operation [7-8], it is broadband enough to perform well at 619 MHz. The PCB material is 10mil Rogers RO4350 (fig. 3). The device and 9 passive components fit an 8 x 10 mm PCB area. The supply requirements are 5 V and 50 ma. IV. RESULTS AND DISCUSSION A. Preamps standalone performance The two candidate devices substantially different gate sizes (800 µm vs µm) impact their noise, gain and linearity performances (Table 1). The discrete and MMIC preamps achieve 0.6dB and 0.4dB noise figures (NF) respectively. These results are referenced to the coaxial connectors, so they include PCB and connector losses. The MMIC preamp s lower NF can be explained by its lower S11 magnitude (0.25 vs at 500 MHz) which minimizes the matching network loss. The system benefit of a lower NF is a reduction in the averaging time required for a good image. The discrete and MMIC preamps exhibit 23dB and 21dB gain, respectively. The flip side of the MMIC s larger geometry is lower gain because of higher parasitic capacitances. A high gain in the preamp is desirable because it can overcome the mixer s ~7dB NF. The input return losses (IRL) are -9dB and -14dB for discrete and MMIC preamps, respectively. The discrete preamp has a poorer IRL because of noise matching. On the other hand, the MMIC s optimized geometry permits conjugate matching because it is very close to noise matching. The discrete and MMIC preamps third order intercept points (OIP3) are 33dBm and 36dBm, respectively. Although both preamps have almost equal DC consumption, the MMIC preamp achieves higher linearity because of its larger geometry. Extremely high linearity and dynamic range are

4 needed for 3D MRI because the time domain signal varies by several orders of magnitude. Although there is a single central frequency in the NMR spectrum, the actual received signal occurs over a finite bandwidth, in some cases up to 200 khz. Any nonlinearity will create distortion in the signal and reduce image quality. Table 1 Experimental RF performance at 619 MHz Preamp device NF (db) G (db) IRL (db) ORL (db) OIP3 (dbm) ATF MGA-633P Evaluation of the Rollett stability factor (k) indicates that both preamps are unconditionally stable (k 1) over DC-20 GHz (Fig. 4). The preamp s source represents an infinite mismatch during the pulsed excitation because the shunt PIN diode is biased to near 0Ω. Moreover, the high Q coil will emit a ringing tail to discharge the stored excitation energy. The preamp must not oscillate under these adverse conditions or else imaging artifacts will occur k vs f ATF Start: 50.0 MHz MGA-633P8 Stop: 20.1 GHz Figure 4. Both discrete and MMIC pre-amps are unconditionally stable from DC to 20 GHz also evaluated in the 15 T front end. The PMA is a discrete ephemt that has been marketed as a drop-in replacement for the ATF Its datasheet specifies a device-level NF of 0.6dB and a 22.6dB gain at 500 MHz; however, the NF of the assembled preamp is expected to be a few tenth of a db higher because of PCB, component, and connector losses. The ATF preamp achieves the highest signal to noise ratio (SNR), followed by the MGA-633P8 preamp and then, the PMA (Table 2). Based on the preamps NF results alone, we would have expected the MGA-633P8 preamp to achieve the highest SNR. So, we suspect the ATF s better SNR is due to its higher gain (+2dB) preventing the mixer s noise from dominating. Table 2 Experimental results of three pre-amp designs in the MRI Preamp device Mean image Noise SNR signal std. dev. ATF MGA-633P PMA All three preamps produced highly detailed and artifactfree images (Fig. 5). The images represent cross sections of a water-filled plastic sphere about 35 mm in diameter using the Siemens localizer pulse sequence. The dark areas to the left are air bubbles (up is toward left in these views). The out-ofroundness and non-uniform brightness are due to shimming (magnetic field uniformity) and eddy current (artifactual transient pulsed magnetic fields) effects that had not been compensated at the time these images were obtained. The overall system noise figure under operating conditions, measured with a noise source (HP 346B) replacing the RF coil (signal transducer) and using the processed image to measure the noise power, was about 1 db for all of the preamps; in this case a coaxial transmission line and the transmit/receive switch are between the preamp and noise source, increasing system NF over preamp NF. B. System-level performance In addition to the two previously described preamps, a third preamp using the Mini-Circuits PMA [9] was

5 Figure T MRI images of a water filled sphere produced by the following preamps (L-R): ATF-54143, MGA-633P8 and PMA V. CONCLUSION An MRI front-end with an industry-leading 15 Tesla magnetic field strength has been developed using low-cost devices intended for wireless communication in the preamplifier slot. Although demonstrated at the Larmor frequency of 619 MHz, these devices are capable of operating at even higher frequencies to support higher strength magnets. A possible extension of this work is to integrate the high-power PIN switch and the preamp into a multi-chip module similar to what we have developed for GHz wireless infrastructure [10] - [11]. ACKNOWLEDGMENT The author thanks M. D. Suhaiza and S. Punithevati for fabricating the preamplifiers, M. A. R. Rahimie for calculating the package metal content, and S. A. Asrul and the management of Avago Technologies for approving the publication of this work. APPENDIX: PREAMPS RESULTS Figure 7. The ATF discrete preamp achieves ~23dB gain and better than -9dB return loss at 619 MHz Figure 6. The ATF discrete and MGA-633P8 MMIC preamps have noise figures of <0.6dB and <0.4dB, respectively at 619 MHz Figure 8. The MGA-633P8 MMIC preamp achieves ~21dB gain and better than -15 return loss at 619 MHz

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