WO2000064335A1 - Wireless physiological monitor for magnetic resonance imaging - Google Patents

Abstract

A wireless transmitter (180) is used in a physiological monitoring system for use with an MRI scanner (130). Sensor data corresponds cardiac data to respiratory motion data using a plurality of sensors on a patient (170).

Description

TITLE OF THE INVENTION

Wireless Physiological Monitor for Magnetic Resonance Imaging CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of earlier filed U.S. Provisional Application No. 60/131,261, filed April 27, 1999, which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION The present invention relates generally to a wireless physiological monitoring system that transmits patient data out of an MRI scanner without corrupting the data.

BACKGROUND OF THE INVENTION Magnetic Resonance Imaging (MRI) is a medical diagnostic technique that creates images of the body using nuclear magnetic resonance. A versatile, powerful, and sensitive tool, MRI can generate thin-section images of any part of the body from any angle, without surgical invasion and in a relatively short period of time. MRI gives biomedical and anatomical information that may allow early diagnosis of many diseases.

While undergoing MRI, a patient is placed in a cylindrical magnet that surrounds the body with a magnetic field. MRI next stimulates the body with radio waves and then "listens" to the body's electromagnetic transmissions. The transmitted signal is used to construct internal images of the body. The patient is placed in a magnetic field with a strength usually between 0.5 - 1.5 T. This field aligns the atoms with a magnetic moment, such as hydrogen atoms. The hydrogen atoms are then perturbed by an RF signal . As the atoms return to their equilibrium position, they emit their own RF signal, with a frequency that depends on the local magnetic field strength. By introducing variable gradients in the applied fields, and taking phase shifts in the signal into account, the hydrogen density within a particular volume can be reconstructed from the RF signals. The information obtained in this way is a measure of the amount of magnetization of the hydrogen in each volume element. This value differs for different types of tissues due to differences in the hydrogen density and its chemical bonds .

In current medical practice, MRI is preferred for diagnosing most diseases of the brain and central nervous system as well as having cardiac applications. MRI scanners also provide imaging supplementary to X-ray images because MRI can distinguish soft tissue in both normal and diseased states.

While MRI technology has seen many advancements in its twenty year history, there is room for improvement with respect to obtaining MRI data in a more efficient less cumbersome manner. Currently patients are required to be outfitted with several wires ending in leads positioned strategically about the body of the patient. Each wire is then routed out of the MRI tube to an information gathering device (typically a special purpose computer) . The number of leads, and therePfore wires is non-trivial and results in an arrangement that is time consuming to attach.

Hardwiring is used because an MRI environment is highly magnetic and does not lend itself to traditional modes of wireless transmission. Ferrous materials which are common to RF transmission devices are significantly affected by the strong magnetic fields associated with MRI . If a wireless physiological monitor could be developed, however, a simplified device attachable to a patient and containing a plurality of leads could be developed. Such a device would be easily attached to a patient saving a considerable amount of time and energy spent by both patient and staff.

What is needed is a wireless physiological monitor that includes multiple leads. Such a system would greatly reduce the amount of wiring attached to the patient resulting in more efficient and more accurate MRI gating algorithms and techniques .

SUMMARY OF THE INVENTION

The present invention comprises a wireless physiological monitor for use within an MRI scanner. Sensor data corresponding to cardiac and respiratory motion is gathered using a plurality of physiological sensors placed on the patient. The physiological sensor data is used to develop more accurate gating algorithms to compensate for patient motion during an MRI test. More accurate gating algorithms lead to better MRI images that have fewer and less significant image artifacts. The physiological sensors are connected to a wireless transmitter via sensor leads and wires. The wireless transmitter multiplexes the physiological sensor data into a single signal and transmits it out of the MRI scanner to a receiver still within the magnet room. The data is then relayed to a patch panel connecting the magnet room with the scanner electronics room where it is processed and interpreted accordingly.

The chief benefit of the present invention is the reduction of wiring required to monitor a patient. A wireless transmission system is employed to drastically reduce the amount of wiring required. The reduced wiring also reduces the time it takes to prepare a patient for an MRI test making the entire process more efficient. Moreover, the reduction in wiring provides an opportunity to use more physiological sensors in order to collect more data yielding more accurate motion data and better gating algorithms .

In accordance with a first embodiment of the invention is a wireless physiological monitoring system for use in an MRI magnet room. The system includes a plurality of sensors for acquiring and relaying physiological data. The sensors are attached to a patient undergoing an MRI test. A wireless transmitter, designed to be operable within an MRI scanner, receives physiological data from the sensors, multiplexes the acquired physiological data of each sensor into a multiplexed physiological data signal and transmits the multiplexed physiological data signal to a receiver outside the MRI scanner. The wireless transmitter is comprised of a non-ferrous casing and non-ferrous circuit components housed within the non-ferrous casing. The non-ferrous circuit components draw relatively low current in order to maximize the service time of the battery that provides the power for the transmitter. The circuit components also include an isolation circuit for countering the effects of magnetic fields present in the MRI scanner.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES FIGURE 1 illustrates a typical MRI environment that incorporates a wireless transmitter.

FIGURE 2 illustrates the functions of the wireless transmitter.

FIGURE 3 illustrates a wireless transmitter according to an embodiment of the present invention.

DETAILED DISCLOSURE OF THE INVENTION High quality images of the heart and coronary arteries obtained by magnetic resonance imaging (MRI) are adversely affected by motion. In order to minimize the resulting image artifacts, gating to the heart and respiratory cycle is required. Cardiac gating is a method used to synchronize the MRI test to a certain phase in the cardiac cycle of the patient. Similarly, respiratory gating is a method used to synchronize the MRI test to a certain phase in the respiratory cycle of the patient. In this way, the MRI test is only delivered during a reproducible period of time in which any motion of the patient is known and accounted for in the treatment thereby leading to results that are more accurate. Greater accuracy results from a reduction in image artifacts. Gating algorithms are used to determine the timing of the MRI test.

Typically, the heart cycle is monitored by an electrocardiograph (EKG) , which measures changes in electrical potential occurring during the heartbeat, and the respiratory cycle is monitored using a pressure transducer ("bellows") affixed around the abdomen. Use of a plurality of EKG leads, however, would improve system performance by providing more information with respect to heart position and motion. A plurality of leads necessitates multi-channel information gathering and subsequent transmission.

The present invention describes means for wirelessly transmitting multi-channel patient data acquired from a Data Acquisition System (DAS) from within a hostile MRI environment to a special purpose processing device for analysis. A Data Acquisition System (DAS) acquires data from multiple channels simultaneously. Multi-channel capability allows analysis of the data off-line for possible correlations and patterns among the various sensors and navigator MRI data. Forwarding acquired MRI data from the patient to specialized processors for analysis is no small task.

One of the problems with obtaining MRI data is the cumbersome nature of the equipment that is attached to a patient undergoing the MRI . The present invention significantly reduces the amount of wiring associated with MRI by converting the wires previously used to carry acquired MRI sensor data out of the MRI chamber to a wireless physiological monitor. FIGURE 1 shows a typical MRI environment. There is a magnet room 110 and a scanner electronics room 120. Magnet room 110 includes an MRI scanner 130. Scanner electronics room 120 includes a signal processor 140 and a computer 150. Magnet room 110 and scanner electronics room 120 are linked by a patch panel 160. Patch panel 160 receives a multiplexed data signal from a receiver 190 and passes it to signal processor 140 in scanner electronics room 120. Magnet room 110 is inundated with magnetic fields of varying strength. These fields are necessary to perform imaging but are incompatible with the electronics used to process and display MRI images. Therefore, magnet room 110 is shielded in isolation and patch panel 160 provides the only link between magnet room 110 and scanner electronics room 120. In a typical MRI environment a patient 170 lays on a table within MRI scanner 130. Multiple physiological sensors are attached to patient 170. These physiological sensors gather cardiac and respiratory motion data pertaining to the patient. Cardiac and respiratory motion occur naturally as patient 170 breathes and his or her heart beats. Such motion must be compensated for, however, in order to provide the clearest most accurate MRI images that have the least amount of image artifacts. Compensation is achieved by measuring patient motion and developing gating algorithms that compensate for the motion. Thus, when actual MRI data is processed, it is first filtered through a gating algorithm to remove as much noisy data as possible that could cause image artifacts to appear on the MRI image. Gating algorithms are developed based on cardiac and respiratory physiological sensor information.

It is axiomatic that the more accurate and plentiful patient motion data, the more accurate an MRI image will be. As mentioned earlier, cardiac and respiratory sensors are placed on patient 170 during MRI testing. Typically, these physiological sensors comprise sensor leads connected to wires. Prior to the present invention, the wires ran from patient 170 (within MRI scanner 130) to a physiological monitor (outside of MRI scanner 130 but still within magnet room 110) then to patch panel 160 before being fed to signal processor 140 in scanner electronics room 120. Such a configuration is clumsy, inefficient, and time consuming for the patient and MRI test administrator. The situation is exacerbated as the number of sensors increases .

A solution posed by the present invention is to wirelessly transmit multiplexed physiological sensor data from patient 170 to patch panel 160 via receiver 190. To do so requires a wireless transmitter 180 placed on or about patient 170 while inside MRI scanner 130. Wireless transmitter 180 must be capable of accepting all of the sensor leads as well as transmitting the acquired physiological sensor data out of MRI scanner 130. This is no easy task considering the high magnetic fields associated with magnet room 110 while an MRI test is in progress .

Once physiological sensor data is wirelessly transmitted out of MRI scanner 130 it is received by receiver 190. Receiver 190 would likely be attached to a wall in close proximity to patch panel 160. Once receiver 190 receives the multiplexed physiological sensor data it then forwards same to patch panel 160 via a shielded wire connection. An optically shielded connection between receiver 190 and patch panel 160 would likely provide the best shielding in magnet room 110.

The advantages of wireless transmission of physiological sensor data are clear. A device holding wireless transmitter 180 can easily be constructed to fit about patient 170. Multiple physiological sensors (analog or digital) can be attached (plugged into) wireless transmitter 180. The resulting decrease in wire lead lengths drastically reduces the cumbersome nature of the MRI test. The device can be put on by patient 170 prior to entry into MRI scanner 130 and the sensor leads can be positioned accordingly. The time required to prepare patient 170 for the MRI test can be significantly reduced. Moreover, the number of physiological sensors can be increased without a significant increase in wiring because the wire leads merely need to travel from their position on patient 170 to wireless transmitter 180. This is significant because an increase in physiological sensor data yields a better gating algorithm and, in turn, a more accurate MRI image .

FIGURE 2 illustrates the functions of the wireless transmitter. One of the chief benefits of using wireless technology is the ability to multiplex multiple data signals into a single information signal for transmission. The signal is de-multiplexed at some point downstream so that individual data signals can be processed. The electronics within wireless transmitter 180 are typical of most common RF transmitters from a functional standpoint. A plurality of signals (analog 205 and digital 210) are received into wireless transmitter 180. Analog signals 205 are initially multiplexed by a first multiplexer 215 before being converted to a digital signal by A/D converter 220. The multiplexed digitized signal is then fed to a second multiplexer 225. Second multiplexer 225 also receives sensor signals from the outside that are already in digital form. Second multiplexer 225 multiplexes the digitized multiplexed analog signals 205 and the digital signals 210 into a single digital signal that is then passed to a parallel to serial conversion circuitry 230. Once converted the signal is sent to a modulation and amplification stage. A frequency shift key modulator 235 operates on the signal under the guidance of a frequency controller 240. The modulated signal is then combined with an RF oscillation signal from an RF oscillator 245 and fed to an RF amplifier 250. The signal is amplified to the strength necessary to reach receiver 190. In a final step before transmission, the signal is filtered by a surface acoustic wave filter 255.

FIGURE 3 illustrates one possible implementation of wireless transmitter 180. Due to the high magnetic fields present in magnet room 110, special materials are used in the construction of wireless transmitter 180 in order for it to function within the hostile environment of MRI scanner 130.

The casing 300 of wireless transmitter 180 shields the electronics within as much as possible given the hostile environment of MRI scanner 130 and magnet room 110. Thus, non-ferrous materials make up casing 300 as well as the internal circuit components of wireless transmitter 180. In addition to using non-ferrous materials to shield the electronics of wireless transmitter 180, custom circuitry operating at specific frequencies is also employed to prevent, to the greatest extent possible, magnetic interference associated with MRI scanner 130 with respect to the transmission of patient sensor data.

The interior base of casing 300 includes a printed circuit board (PCB) 305. PCB 305 facilitates electrical connections between and among the various sub-modules that comprise the RF transmitter circuitry. Four sub-modules are shown including a battery 310, a power sub-module 315, a sensor sub-module 320, and an RF transmitter sub-module 325. Incoming sensor leads 330 connect to sensor lead connections 335. Sensor lead connections 335 connect to PCB 305 and PCB traces carry the physiological sensor signals to sensor sub-module 320 for pre-processing and multiplexing. The multiplexed sensor signal is then relayed via PCB 305 to RF transmitter sub-module 325 for signal modulation and amplification. Battery 305 provides the power necessary to drive the other circuitry while power sub-module 310 regulates the power to the other sub- modules and their circuitry. As previously mentioned, all components of wireless transmitter 180 are comprised of non-ferrous materials so that operation inside a huge magnetic field such as MRI scanner 130 is not substantially degraded or affected. The preferred modulation technique is an FSK type where the physiological sensor signals are sampled and their binary values are used to modulate between one of several different frequencies. A primary goal is to make the transmitter just powerful enough to transmit while keeping its current draw to a minimum. This allows the battery operated device to last a sufficient amount of time so that several MRI scan sessions can take place.

The two main system parameters for wireless transmitter 180 are available modulation bandwidth and average current draw. Shielding from gradient switching and isolation from the main MRI RF excitation source are paramount in establishing a robust design that works across a variety of MRI scans. To achieve sufficient shielding, a correctly layered set of different shielding materials is implemented for each internal component. Isolation refers to the ability to allow transmission in one direction while providing high isolation from energy (MRI scanner 130 in this case) in the reverse direction. Sufficient isolation is achieved with either a passive or an active isolation network.

In the following claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

CLAIMS :

1. A wireless physiological monitoring system for use in an MRI magnet room comprising: a plurality of sensors for acquiring and relaying physiological data, said sensors capable of being operatively attached to a patient undergoing an MRI test, each of said sensors having a magnetically shielded lead length; a wireless transmitter operable within an MRI scanner for receiving each of said magnetically shielded lead lengths, multiplexing the acquired physiological data of each sensor into a multiplexed physiological data signal and transmitting the multiplexed physiological data signal; and a receiver within said magnet room for receiving the transmitted multiplexed physiological data signal.

2. A wireless physiological monitoring system operable within an MRI scanner and magnet room, said system comprising: means for obtaining physiological data; means for wirelessly transmitting obtained physiological data; and means for receiving transmitted physiological data.

3. A wireless transmitter operable within an MRI scanner comprising: a magnetically shielded non-ferrous casing; and non-ferrous circuitry housed within said non-ferrous casing, said non-ferrous circuitry for: receiving electrical signals representing physiological data; multiplexing said electrical signals representing physiological data creating a multiplexed physiological data signal; and wirelessly transmitting the multiplexed physiological data signal .

4. A wireless transmitter operable within an MRI scanner comprising: a non-ferrous casing; non-ferrous circuit components housed within said non- ferrous casing; and a battery, wherein said non-ferrous circuit components draw relatively low current in order to maximize the service time of said battery; and said circuit components include an isolation circuit for countering the effects of a magnetic field present in said MRI scanner.

5. A wireless transmitter operable within an MRI scanner comprising: non-ferrous casing materials; non-ferrous circuit components housed within said casing; circuit means for minimizing current draw to said non- ferrous circuit components; and circuit means for isolating a signal to be transmitted from magnetic fields associated with the MRI scanner.