A system for automated noise parameter measurements on MR preamplifiers and application to high B₀ fields

Abstract

A noise figure and noise parameter measurement system was developed that consists of a combination spectrum and network analyzer, preamplifier, programmable power supply, noise source, tuning board, and desktop computer. The system uses the Y-factor method for noise figure calculation and allows calibrations to correct for a decrease in excess noise ratio between the noise source and device under test, second stage (system) noise, ambient temperature variations, and available gain of the device under test. Noise parameters are extracted by performing noise figure measurements at several source impedance values obtained by adjusting an electronically controlled tuner. Results for several amplifiers at 128 MHz and 200 MHz agree with independent measurements and with the corresponding datasheets. With some modifications, the system was also used to characterize the noise figure of MRI preamplifiers in strong static magnetic fields up to 9.4 T. In most amplifiers tested the gain was found to be reduced by the magnetic field, while the noise figure increased. These changes are detrimental to signal quality (SNR) and are dependent on the electron mobility and design of the amplifier's semiconductor devices. Consequently, gallium arsenide (GaAs) field-effect transistors are most sensitive to magnetic fields due to their high electron mobility and long, narrow channel, while silicon-germanium (SiGe) bipolar transistor amplifiers are largely immune due to their very thin base.

Experimental verification of SNR and parallel imaging improvements using composite arrays

“…Noise correlation (Fig. ) was calculated from simulated and measured Z ‐parameters both with and without the effect of the additional noise added by the matching network and preamplifier (a noise figure of 0.9 dB measured in Reference ), which typically reduces the noise correlation by increasing coil variance. For comparison, noise data were acquired by scanning without RF excitation and the noise covariance between the acquisition channels was calculated from the covariance according to the method in Appendix A of Reference .…”

“…The coils were connected using λ /2 coaxial cables to eight low‐impedance preamplifiers (input impedance Z pre = 6 ± 3 Ω, minimum noise figure 0.90 dB at 43 + 13i Ω impedance ) housed within a connector box (Philips Healthcare, Cleveland, OH, USA).…”

A 64‐channel 3T array coil for accelerated brain MRI

“…The need to align the preamplifier to z ‐direction (11–13) reduced the choices for preamplifier positioning. The layout was further constrained by the need to avoid passing an output coax of a preamplifier near (<2 cm) the input of other preamps to avoid positive feedback and possible oscillation.…”

“…Each preamplifier channel also contains a common mode rejection transformer between the first and second stages (10). All preamplifiers were carefully oriented in z ‐direction to minimize Hall effect issues within the field effect transistors used in the preamplifiers (11–13). Preamp outputs passed through an additional cable trap prior to the coil plug to suppress common mode currents and avoid interaction with the radiofrequency (RF) transmit system.…”

A patient‐friendly 16‐channel transmit/64‐channel receive coil array for combined head–neck MRI at 7 Tesla

“…The preamplifier's daughterboards were mechanically mounted on the helmet via 3D‐printed plastic standoffs. All preamplifiers (Siemens Healthineers AG, Erlangen, Germany) were carefully oriented in the z ‐direction to minimize Hall effect issues 40–42 . We used the impedance change of the series matching capacitor to transform the input impedance of the preamplifier to a parallel inductance across C TM1 of the capacitive voltage divider.…”