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TRANSMISSION LINE COIL MATCHING AND BLOCKING

IP.com Disclosure Number: IPCOM000197945D
Publication Date: 2010-Jul-23
Document File: 6 page(s) / 48K

Publishing Venue

The IP.com Prior Art Database

Abstract

A technique to match and block magnetic resonance (MR) coils such as, receive coil with a preamplifier is disclosed. The technique described herein uses a quarter-wavelength of coaxial cable for matching the block MR coils with a preamplifier. Further, characteristic impedance of the coaxial cable is chosen in such a way that the coaxial cable matches the coil to the impedance desired by the preamplifier.

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Page 1 of 6

RP13439

TRANSMISSION LINE COIL MATCHING AND BLOCKING

BRIEF ABSTRACT

    A technique to match and block magnetic resonance (MR) coils such as, receive coil with a preamplifier is disclosed. The technique described herein uses a quarter-wavelength of coaxial cable for matching the block MR coils with a preamplifier. Further, characteristic impedance of the coaxial cable is chosen in such a way that the coaxial cable matches the coil to the impedance desired by the preamplifier.

KEYWORDS

    Quarter-wavelength of coaxial cable(s), matching network, impedance, preamplifier, transmission line coil, receive coil, coil match, coil block, magnetic resonance imaging and MRI

DETAILED DESCRIPTION

    In general, magnetic resonance imaging (MRI) receive coils need to be matched to a preamplifier, decoupled from other receive coils via the preamplifier's low input impedance, and switched off during the imaging cycle's transmit phase. In order to achieve the matching of the receive coil, the matching network transforms resonant series coil impedance (typically a small resistance) to the impedance required by the preamplifier for minimum noise figure (typically 50 Ohm). Further, the small preamplifier input impedance (typically 2 Ohm) needs to be transformed with few losses to a high impedance at the coil, in order to suppress currents and inductive coupling to other coils.

    A similar mechanism is required to block currents during receive. Here, a pair of crossed diodes and / or a PIN diode are typically turned on during transmit. The

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Page 2 of 6

RP13439

low impedance is transformed by the network to high impedance at the coil. In some instances, the network allows the preamplifier to be mounted off the coil. In such configurations, mounting is typically achieved by placing an additional half- wavelength coaxial cable between the preamplifier and the lumped element matching/blocking network. Some conventional methods address the matching issue by using lumped element T-sections comprising two inductors and a capacitor. Alternatively, balanced bridge method comprising two capacitors and two inductors is also used. Figure 1 illustrates a typical receive coil where a T-section matches the coil to the characteristic impedance of the coaxial cable. The T-section is comprised of the blocking inductor, the matching capacitor and part of the inductance of the coil.

    However, the conventional methods suffer disadvantages. At increased magnetic field strengths, the required inductors for small coils become smaller, as the resistance of the coils drop. Further, the higher quality factor (Q) of small coils signifies that the requirements on blocking and preamplifier decoupling become more stringent. The matching or blocking circuits are highly resonant and have large standing waves, thus making the circuits difficult to tune. In addition, at high powers, small inductors may not be able to sustain high resonance resulting from high reflection coefficients. The T-sec...