Surgery – Diagnostic testing – Temperature detection
Reexamination Certificate
1999-11-23
2001-08-07
Lacyk, John P. (Department: 3736)
Surgery
Diagnostic testing
Temperature detection
Reexamination Certificate
active
06270463
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to instruments for making a temperature measurement in a strong electromagnetic field environment such as is found in magnetic resonance imaging (MRI) machines.
BACKGROUND OF THE INVENTION
Strong pulsed radio frequency (RF) fields in the imaging cavity of an MRI machine may induce very high RF currents in the conventional long, low resistance, electrical cables typically used with standard diagnostic electronic instruments, such as temperature measuring devices. These cables and the associated sensors can become very hot due to RF I
2
R heating, and if in contact with a patient's skin, can cause severe burns. Apart from the possibility of burning the patient, RF heating of a temperature sensor would nullify its usefulness as a temperature measuring device. In addition, the RF pulses, which are picked up on the long, low resistance, electrical cables normally used when measuring temperature, often enter the data monitoring equipment where they can degrade measurement accuracy. Conversely, many data monitoring instruments generate high frequency noise signals. These signals are often radiated from the instrument cables in the form of electromagnetic waves and are picked up by the MRI machine, which can cause artifacts in the MRI image.
U.S. Pat. No. 5,730,134 to Dumoulin et al. (“the '134 Patent”) discloses a temperature monitoring system which employs a temperature sensor that is incorporated into a device
150
that is inserted into a body during an MRI scan. The inserted device is used in conjunction with the MRI machine to position itself within the body or for localized magnetic resonance imaging within the body. As shown in
FIG. 2
, the inserted device
150
comprises a a small RF coil
200
which is electrically coupled to the MRI system via conductors
210
and
220
. The temperature sensor used to monitor the temperature of inserted device
150
could undergo localized heating due to induced RF currents. If a temperature rise in excess of a predetermined threshold is detected, power to the MRI machine is either reduced or cut off. In the preferred embodiment shown in
FIG. 2
, the temperature sensor consists of a flouresescent substance
275
located at the remote end of fiber optic cable
270
, which is connected to a light sensor
207
at the local end thereof. The '234 Patent indicates that in other embodiments the temperature sensor may comprise other temperature measurement devices such as a thermistor or thermocouple. However, the '134 Patent does not disclose any system or method which is used to prevent RF heating from occurring in the first instance. In addition, the temperature measurement system of the '134 Patent is used to monitor changes in the
10
temperature of the inserted device, not to monitor the patients temperature.
U.S. Pat. No. 5,209,233 to Holland et al. (“the '233 Patent”) discloses a system which monitors the temperature of ECG electrodes
40
mounted on a patient
8
undergoing an MRI scan (
FIG. 2
) to ensure that ECG electrode
40
does not undergo “localized heating” during an MRI examination and to allow monitoring of the patient's skin temperature. Although the '233 Patent does disclose impedances
64
and
82
connected in series with the RTD or thermistor
60
located at the ECG electrode to prevent excessive currents from being induced in lead
62
by either radio frequency signals or changing magnetic field gradients, the '233 Patent does not disclose the particular structure used in the temperature measurement system of the present invention in which a thermistor probe connected in the resistive mode of operation is driven by a constant current source through a first pair of leads and an instrument amplifier is connected to the temperature sensor through a second pair of leads combine to generate a signal representative of the measured temperature. In addition, the '233 Patent does not disclose another aspect of the present invention, namely, a low battery sensing circuit which provides an indication of the status of a battery used to provide power to the temperature measurement system.
It is therefore an objective of the present invention to provide an improved system for measuring temperature of a patient within an MRI machine which eliminates thermal heating of the cables and associated sensors due to RF I
2
R heating. is yet another objective of the present invention to provide an improved system for measuring temperature of a patient within an MRI machine which prevents RF noise from entering the data monitoring equipment.
It is a further objective of the present invention to provide a system for measuring temperature of a patient within an MRI machine which prevents high frequency noise signals from the data monitoring equipment from causing artifacts in an MRI image.
It is a still further object of the present invention to provide a system for measuring temperature within an MRI machine which also includes a system for indicating a low battery condition in the monitoring equipment.
SUMMARY
It has now been found that these and other objects of the invention are realized by the temperature measurement system of the present invention. In the presently preferred embodiment, a temperature sensor having output terminals is attached to a patient. An electronics sensor module coupled to the termpreature sensor generates an output signal responsive to the temperature measured by the temperature sensor. The electronic sensor module comprises a constant current source and a differential amplifier each having terminals coupled to the output terminals of the temperature sensor, and a transmitter having an input coupled to the output of the differential amplifier. Inteposed between the temperature sensor and the electronics module is an RF current reduction network. A temperature monitor processes a signal received from the electronics module and provides an indication of the measured temperature.
Preferably, the temperature monitor displays the measured temperature and comprises a receiver having an input coupled to a communications medium (which is also coupled to an output of the electronics module transmitter) and an output coupled to a controller for processing information received from the electronics sensor module.
Preferably, the temperature sensor comprises a dual-thermistor temperature probe and a linearizing network connected in a resistance mode. Preferably, the RF current reduction network comprises first and second series-connected resistors coupled between each of the output terminals of the temperature sensor and each of the connections to the constant current source and the differential amplifier. Preferably, the first resistor has a value of approximately 10 kohms and the second resistor has a value of approximately 1 kohms. Still further, preferably, the electronics sensor module is mounted within a Faraday housing and RF filters are mounted in the Faraday housing and interposed between the output of the RF current reduction network and the connections to the differential amplifier and to the constant current source. Finally, preferably, the transmitter comprises a pulse-width modulator coupled to a fiber optic transmitter, the communications medium comprises a fiber optic cable and the receiver comprises a fiber optic receiver coupled to a pulse-width demodulator.
In another aspect of the invention, the electronics sensor module includes a low battery sensing circuit which generates a low battery signal when the voltage of the battery providing power to the module falls below a predetermined threshold. The low battery signal is provided to the temperature monitor for display. In this way, the status of the battery can be monitored and a low battery condition detected so that the battery can be promptly replaced before the electronics sensor module becomes inoperative.
REFERENCES:
patent: 4558279 (1985-12-01), Ackerman et al.
patent: 4914608 (1990-04-01), LeBihan et al.
patent: 5109853 (1992-05-01), Taicher et al.
patent:
Morris, Sr. G. Ronald
Valentine James W.
Amster Rothstein & Ebenstein
Lacyk John P.
Medrad Inc.
Szmal Brina
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