Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-11-25
2001-03-06
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000
Reexamination Certificate
active
06198288
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the art of radio frequency interface circuits. The invention finds particular application in interfacing between the transmission and reception of magnetic resonance signals and will be described with particular reference thereto. It is to be appreciated however, that the invention may find further application in other fields in which high power, multiple-frequency radio signals are alternately transmitted and received through a common antenna.
Heretofore, magnetic resonance imagers have commonly been used to generate images based on hydrogen nuclei in a subject. Typically, a radio frequency generator generates a high powered RF signal at the resonance frequency of hydrogen which is passed through an interface circuit to an RF coil. The generated RF signals induce magnetic resonance in the hydrogen in an imaged volume. During the passing of the excitation signals, the interface circuit uses quarter wavelength cables, other inductive and capacitive elements, and PIN diodes to create narrow, but effective bandpass filters at the resonance frequency. After excitation, the bias on the PIN diodes is changed such that the narrow bandpass filter becomes a low impedance interconnection between the RF coil and the receiver.
However, there are many paramagnetic nuclei of potential diagnostic interest, such as helium 3, fluorine, phosphorous, carbon, and xenon. At a given magnetic field strength, each of these nuclei have a distinctly different resonance frequency. More particularly, the resonance frequencies are sufficiently different that the bandpass filter of the interface circuit is ineffective for any but one of the selected frequencies.
The present invention provides a new, multiple-frequency transmit-receive switch which overcomes the above-mentioned difficulties and others.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic resonance imaging apparatus for generating image representations of a volume of interest includes an RF signal generator for selectively generating one of a first transmit signal at a first transmit frequency and a second transmit signal at a second transmit frequency in accordance with a desired imaging profile. An RF coil is in electrical communication with the signal generator through an interface circuit. The RF coil transmits the transmit signal into an examination region thereby producing a magnetic resonance signal at a corresponding one of first and second resonance frequencies. An RF receiver is in electrical communication with the RF coil through the interface circuit, and receives the magnetic resonance signal. The interface circuit includes a first isolation circuit for selectively isolating the RF receiver from the transmit signals of the transmit frequency, and for selectively passing the magnetic resonance signals around the first frequency from the RF coil to the RF receiver. The interface circuit also includes a second isolation circuit for selectively isolating the RF receiver from the transmit signals of the second transmit frequencies and for selectively passing magnetic resonance signals around the second frequency from the RF coil to the RF receiver. An image reconstruction processor operatively connected with the RF receiver reconstructs the received resonance signals into an electronic image representation.
In accordance with another aspect of the present invention, the first isolation circuit includes a first PIN diode forming an effective short circuit between a first inductor and first capacitor in response to a forward bias. The first PIN diode forms an effective open circuit between the first inductor and first capacitor in response to a reverse bias. The reverse bias is associated with a receive cycle portion of the apparatus, and reduces the first isolation circuit to that of the first capacitor. The forward bias is associated with the transmit cycle portion of the apparatus and reduces the first isolation circuit to that of the first capacitor electrically parallel to the first inductor such that the first isolation circuit has a high impedance at the first transmit frequency.
In accordance with another aspect of the present invention, the second isolation circuit includes a second PIN diode forming an effective short circuit between a second capacitor and a second inductor in response to the forward bias. The second PIN diode forms an effective open circuit between the second capacitor and the second inductor in response to the reverse bias. The reverse bias is associated with the receive cycle and reduces the second isolation circuit to that of the second inductor. The forward bias is associated with the transmit cycle portion and reduces the second isolation circuit to that of the second conductor electrically parallel to the second capacitor, such that the second circuit has a high impedance at the second transmit frequency.
In accordance with another aspect of the present invention, the imaging apparatus includes a grounding PIN diode between the second isolation circuit and the RF receiver forming an effective short circuit between the RF receiver and ground in response to the forward bias.
In accordance with another aspect of the present invention, the imaging apparatus also includes a bandpass filter between the second isolation circuit and the RF receiver having a low impedance at the first and second magnetic resonance frequencies.
In accordance with another embodiment of the present invention, a high power, multiple-frequency transmit and receive interface circuit directs transmit signals at one of at least a first selected frequency and a second selected frequency from an RF source input to an RF coil node while isolating an RF receiver output. The interface circuit includes a first isolation circuit connected with the RF coil for selectively presenting a high impedance to the first frequency in a transmit mode and a low impedance to a first and second frequency in a receive mode. The interface circuit also includes a second isolation circuit connected between the first isolation circuit and the receiver output for selectively presenting a high impedance to the second frequency in the transmit mode and a low impedance to the first and second frequencies in the receive mode.
In accordance with another aspect of the present invention, the interface circuit further includes a source input isolation circuit connected between the RF source input and the RF coil for selectively presenting a high impedance to noise signals in the receive mode.
In accordance with another aspect of the present invention, the source input isolation circuit includes PIN diodes connected in series across a capacitor to ground. The PIN diodes selectively have a nonconductive state and a conductive state. An inductor is also provided in series with the diodes. In the conductive state of the PIN diodes, the PIN diodes, the inductor and the capacitor form a lowpass filter. In the nonconductive state of the PIN diodes, the diodes, the inductor and the capacitor form a voltage division between (1) of the diodes and the capacitor, and (2) the other of the diodes and the RF coil. The voltage division presents a high attenuation to a broad band noise signal from the RF source.
In accordance with the present invention, a method of magnetic resonance imaging in which radio frequency resonance excitation signals are generated at at least first and second frequencies includes during the excitation of resonance passing the radio frequency excitation signals substantially unattenuated from a signal generator to an RF coil. The method further includes electrically isolating a receiver from at least the first and second frequency signals.
In accordance with another aspect of the present invention, the method further includes chaining a bias on PIN diodes to change between (1) passing resonance signals at the first and second frequencies to the receiver during receiving of magnetic resonance signals, and (2) isolating the receiver from receiving the first and second fr
Burl Michael
Gauss Robert C.
Arana Louis
Fay Sharpe Fagan Minnich & McKee LLP
Picker International Inc.
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