MRI SYSTEM WITHOUT RF-AMPLIFIER
Publication Date: 2015-Jun-01
The IP.com Prior Art Database
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MRI system without RF-amplifier
The invention relates to an RF transmit-receive system for a magnetic resonance examination system that is configured for zero-echo time (ZTE) acquisition.
According to the invention, the RF transmit system comprises an LC-type transmit loop and a supply circuit which includes a simple DC power source and a resistor arranged in parallel to the capacitance of the transmit loop. Further, a discharge tube is provided in the transmit loop. Further, the RF receive system is configured to sample continuously and also to withstand RF-pulses by passive detuning.
The invention enables to generated RF transmit pulses each time the discharge tube discharges itself. Although this generates the RF transmit pulses irregularly, the continuously sampling RF receiver system is able to acquire the magnetic resonance signals in a ZTE acquisition approach.
The invention allows a very inexpensive RF transmits system. This adds to further cost reduction of the magnetic resonance examination system for ZTE imaging.
Zero-TE (ZTE) technologies allow, in principle, for a design of a dedicated cheap MR system. Yet, ZTE-technologies typically have high requirements on RF-amplifiers.
An MR system without an RF amplifier is proposed, but with a system that exploits the Q-factor of the transmit-coil system.
Schematically, the classical transmit circuit can be drawn as follows:
Figure 1: This contains an RF-amplifier that is able to provide RF-envelopes of every required shape.
However, for ZTE, the pulse is preferred to be as short and "hard" as possible. So, strictly speaking, the control of the RF-pulse envelope is not required anymore.
The following alternative circuit is proposed:
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Figure 2: Here, the DC source may be a pretty dumb power source. The resistor is dimensioned such that it loads the capacitor in a millisecond time frame. Then, at a given moment in time, the switch is closed. For a 1.5T system, the LC-circuit is tuned to 64MHz; it typically has a Q of 100 in case of the body coil providing this circuit directly. By using a separate resonator, this Q increases easily by a factor around 10. Closing the switch causes an exponentially decaying sinusoidal current, decaying with a time
constant of 25
Figure 3: the switch is replaced by a discharge tube. That one is supposed to be in a non-conducting state by default; so the RC-circuit slowly charges the capacitor, with a time-constant of roughly 1ms. During that time, the voltage over the discharge tube follows that over the capacitor. At a somewhat unpredictable moment, an arc occurs in the tube, bringing it into an effectively conducting state. The plasma does not disappear in the nanosecond timeframe, so the conducting state persists for a number of cycles, e.g. a few times 100/2π cycles, which is approximately 1µs. After that, the remaining current cannot support the arc anymore, and the tube returns to a non-conducting state.