NMR pickup device delivering a signal representative of breathing of a patient

A detector device delivering a signal representative of the respiration of patient, adapted to be used in a charged and sensitive electromagnetic environment. The device is principally constituted by at least two non-metallic electrodes (1) and by a unit for the processing of electrical signals emitted by the heart, comprising a first module (5) for acquiring and shaping cardiac electric signals, a second module (6) for extraction of the respiratory signal, and a third module (7) for electro-optical conversion of this latter, the modules being disposed in a shielded casing (8) forming a Faraday cage and, finally, by an optical connection (9) connecting the casing (8) to at least one other apparatus or device.

FIELD OF THE INVENTION 
The present invention relates to the field of picking up and measuring 
biological signals and the surveillance of patients, particularly of 
patients under NMR (Nuclear Magnetic Resonance) examination for example in 
a magnetic resonance imager (MRI), and has for its object a pickup device 
delivering particularly a signal representative of the breathing of a 
patient. 
BACKGROUND OF THE INVENTION 
At present, the respiratory movements and/or the respiratory flow of a 
patient are principally picked up by means either of specific electronic 
detectors, measuring the air flow or temperature at the level of the 
nostrils, or by dedicated devices registering the deformations of the 
thorax by means of a transducer associated with a mechanical, hydraulic or 
pneumatic detector and delivering an electrical signal as a function of 
said deformations. 
However, in an MRI environment, the known devices do not permit picking up 
in a precise, reliable and reproducible manner the respiratory rhythm 
because particularly of the nature itself of the physical magnitudes 
measured, the electromagnetic disturbances and the modes of acquisition 
and measurement used, and of the primary importance of the positioning of 
the detectors, which are often difficult to install correctly and 
frequently involuntarily displaced in the course of the measuring cycle by 
the patient. 
More particularly, the production of electrical signals in a charged and 
sensitive electromagnetic environment, such as that of an NMR apparatus, 
gives rise to a harmful disturbance of said environment and, conversely, a 
disturbance of the electrical measurement signals produced, falsifying 
their meaning. 
Moreover, the cables and other electrical conductors, forming antennae, 
disturb the electromagnetic environment of the NMR apparatus and falsify 
the measurements and, in the case of an imager, the virtual reconstruction 
(images) obtained by this latter, even when said cables are sheathed and 
twisted. 
Conversely, field gradients, the radiofrequency fields and the phenomena 
connected with the commutations between emitting and receiving windings in 
the course of a test of the NMR type, strongly disturb the transmission of 
the picked up signals and can, by generation of important artificial 
signals, render these latter totally useless, the patient being disposed 
within the interior itself of the principal magnet of the NMR apparatus. 
Moreover, possible movements of the patient (particularly respiration 
itself) give rise to movements of said electrical transmission cables in 
the ambient field, from which automatically results an induction of 
potential generators of false signals. 
Moreover, the harmful phenomena recited above are strongly amplified when 
the transmission cables have one or several loops. 
Furthermore, the solution consisting in gaining and transmitting 
information or signals in pneumatic or hydraulic form outside the 
sensitive electromagnetic environment is no longer satisfactory because of 
the lack of reliability and precision of these modes of transmission and 
the important losses which they involve, of the problems of positioning 
the detector and of the inconvenience to the patient. 
Moreover, pneumatic or mechanical detectors are all subject to strong 
shunting which it is necessary continuously to compensate. 
Still further, the pneumatic or mechanical detectors react only to a given 
type of respiration, namely, thoracic or diaphragmatic, and are 
semi-insensitive as to the other type, this as a function of the 
positioning of said detectors on the patient. 
However, in certain patients only one of the two types of mentioned 
respiration is substantially predominant, the other type being of 
insufficient or even negligible amplitude. 
SUMMARY OF THE INVENTION 
The problem to be solved by the present invention consists accordingly in 
conceiving a less cumbersome detector device, picking up in a precise and 
reliable manner the respiratory signal or the signal corresponding to the 
respiratory movements of a patient and converting it to a form 
transmissible without disturbance in a charged electromagnetic 
environment, said detector device being able to be used with complete 
safety and without reciprocal influence on the patient disposed in the NMR 
apparatus. 
Moreover, said detector device should also, according to a second object of 
the invention, be able to pick up and transmit independently of other 
physiological signals or other control apparatus, without substantially 
increasing its size or the complexity of its construction. 
Furthermore, the detector device should not have drift, or only a drift of 
small amplitude and which is controllable, and should be able to pick up 
signals generated by diaphragmatic breathing, as well as those generated 
by thoracic breathing. 
To this end, the present invention has for its object a detector device 
delivering particularly a signal representative of the breathing of a 
patient, adapted to be used in a charged and sensitive electromagnetic 
environment, particularly adjacent or within a nuclear magnetic resonance 
apparatus, and more particularly on a patient within the tunnel of the 
magnet of an MRI, characterized that it is principally constituted, on the 
one hand, by at least two non-metallic electrodes, mounted on a base of a 
support body of a non-magnetic material and adapted to be applied against 
the skin of a patient in the cardiac region, on the other hand, by a 
process unit for the electrical signals emitted by the heart, comprising a 
first acquisition and shaping module for the cardiac electric signals, a 
second module for the extraction of the respiratory signal and a third 
module for electrooptic conversion of this latter, disposed in a shielded 
casing forming a Faraday cage and carried by the base and, finally, by 
optical connection means connecting said casing to at least one other 
apparatus or device located as the case may be within the charged and 
sensitive electromagnetic environment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the invention, and as shown in FIGS. 1, 2 and 3 of the 
accompanying drawings, the detector device is principally constituted, on 
the one hand, by at least two non-metallic electrodes 1, mounted on a base 
2 of a support body 3 of a non-magnetic material and adapted to be applied 
against the skin of the patient 4 in the cardiac region, on the other 
hand, by a processing unit for the electric signals emitted by the heart, 
comprising a first acquisition and shaping module 5 for the electric 
cardiac signals, a second module 6 for the extraction of the respiratory 
signal and a third module 7 for electro-optical conversion of this latter, 
disposed in a shielded casing 8 forming a Faraday cage and carried by the 
base 2, and, finally, by optical connection means 9 connecting said casing 
8 to at least one other apparatus or device 10 located as the case may be 
outside the charged and sensitive electromagnetic environment. 
The present invention is accordingly based on the extraction of the 
respiratory signal from the electrocardiograph signals picked up in a very 
precise way by the electrodes 1 and immediately processed after their 
pickup, without transmission, in a suitable unit totally insulated from 
the external electromagnetic environment. 
To this end, as shown particularly in FIG. 4 of the accompanying drawings, 
the electrocardiogram signal, constructed from electrical signals emitted 
by the heart and comprised by a sequence of QRS complexes, is subjected to 
periodical amplitude variation whose phase and period correspond to the 
respiratory cycle. 
This phenomenon is explained by the fact that, in the course of the 
respiratory cycle of the patient, the heart is displaced periodically by 
the successive movements of expansion and contraction of the thoracic cage 
and the resulting movements of the other organs which are disposed within 
it, which gives rise directly to a displacement of the electrical axis of 
the heart. 
It is therefore this direct checking of the cyclic respiratory movements by 
the electrocardiogram signal which is used to the profit of the invention 
to extract from this latter the respiratory signal. 
According to a first characteristic of the invention, the first module 5 
comprises on the one hand an acquisition stage of the electrocardiac 
signals picked up by the electrodes 1, constituted essentially by an 
instrumentation amplifier 11 each of whose inlets is connected to a 
corresponding electrode 1 by means of a radiofrequency filter 12 and a 
limit resistance 12', and, on the other hand, a shaping stage comprised by 
a pass band filter 13 and a commutator circuit 13'. 
The filters 12 and the resistances 12', disposed between the 
instrumentation amplifier 11 and the electrodes 1, serve respectively to 
eliminate parasites induced by the environmental radiofrequency emissions 
and to reduce the possible currents induced by these latter, the 
connections between the electrodes 1 and the casing 8 being moreover 
provided by rigid wires. 
Preferably, said electrodes 1 are three in number and are of a conductive 
material selected from the group comprised by carbon, carbon compounds and 
loaded plastic materials, one of said electrodes 1 being used to increase 
the rejection size of the common mode. 
Moreover, the electrodes 1 being secured to the base 2, the relative 
positions of them to each other are fixed, by being spaced a distance 
which can be a function of the size of the patient and it suffices to 
position said base 2 adjacent the heart so that the electrodes 1 will be 
placed in a satisfactory manner. 
The only external metallic element of the detector device, namely the 
shielded casing 8, will as a result never be in direct contact with the 
patient 4, because it will be at least separated from this latter by said 
base 2, which avoids any risk of burning. 
According to a second embodiment of the detector device according to the 
invention, shown in FIG. 6 of the accompanying drawings, the electrodes 1 
are not positioned below the casing 8, but are spaced from this latter and 
secured to the end of a lateral prolongation 2', of small thickness, of 
the base 2, the connection between said electrodes 1 and the casing 8 
being effected by non-metallic conductors, for example of carbon fibers. 
Such a construction of the detector device permits spacing the casing 8, 
which has a substantial height or thickness, from the reception points of 
the EKG in the cardiac region, where it may be necessary, as the case may 
be, to apply an MRI measuring antenna as close as possible to the skin of 
the patient. 
Preferably, the pass-band filter 13 of the first module 5 permits ridding 
the signal delivered by the instrumentation amplifier 11 of its 
superfluous and useless components for the ultimate treatment and to 
isolate the portions of the signals (the successive QRS complexes) whose 
variation of amplitude with respiration is to be measured. 
This pass-band filter 13 can for example be constituted by the combination 
of a low-pass filter and a high-pass filter, together delimiting a 
pass-band of about 15 Hz to 35 Hz. 
Furthermore, the redresser circuit 13' permits obtaining at the outlet of 
said module 5 a signal which is always polarized in a known and 
predetermined direction. Thus, as a function of the position of the 
electrodes 1 relative to the body, the electrocardiogram signal delivered 
by the instrumentation amplifier 11 can be polarized different and 
oppositely. 
As shown in FIG. 2 of the accompanying drawings, the second module 6 
preferably comprises a maxima detector circuit 14, supplied by the shaped 
signal from the first module 5 and controlling a sampling-blocking circuit 
15 also supplied by the signal from said first module 5, the output signal 
of said sampling-blocking circuit 15 being processed by an averaging and 
smoothing circuit 16. 
Thus, the detector circuit 14 permits detecting the times of appearance of 
the signal shaped or controlled by the first stage of the first module 5, 
signals of which it is desired to measure the amplitude of a predetermined 
point, for example the maximum amplitude. 
This detection is used to control the sampling-blocking circuit 15 which 
carries out the measurement and ensures the maintenance of the picked up 
amplitudes which vary with the successive QRS complexes as a function of 
the respiratory cycles. The signal from said circuit 15 is in the form of 
a series of shoulders of amplitudes variable with the respiration and 
representing schematically the respiratory signal. 
This structure permits preventing the signal from the circuit 15 from being 
affected by false voltage signals which can exist in the signal from the 
instrumentation amplifier 11 between two successive measurements of 
amplitudes and which have no relation to the cardiac activity and whose 
amplitude is in no case connected to the respiratory activity. 
The circuit 16 calculates the mean value of the signal from the 
sampler-blocker 15 and suppresses in it the component of continuous 
voltage, the signal delivered by this circuit representing as a result 
precisely the respiratory signal. 
As a modification, the second module 6 can have the form of a digital 
integrated circuit or of several programmable digital integrated circuits 
permitting the numerical processing of the signals delivered by the first 
module 5 and supplying a signal representative of the breathing or a 
synchronization signal derived from the respiratory signal to the third 
module 7. 
According to another characteristic of the invention, shown also in FIG. 2 
of the accompanying drawings, the third module 7 is preferably comprised 
by a modulator circuit 17, for example for a frequency modulation or pulse 
width, receiving the signal delivered by the second module 6 and of an 
electro-optical transducer 18 connected to an optical conductor 9 
constituting the optical connection means. 
This latter can deliver the respiratory signal, for example, to a display 
and recordation device, to a control means for the NMR apparatus or of a 
supplemental device, or to several of these apparatus, devices or the like 
after derivation or multiplexing of said optical conductor. 
According to a preferred embodiment of the invention shown in FIG. 3 of the 
accompanying drawings and so as to ensure a firm positioning of the 
electrodes 1 and to guarantee pickup of cardiac electrical signals as 
close as possible to the heart, there is provided a belt 19 or a harness 
of a non-metallic material, if desired elastic, provided with quick 
closure means and length adjustment means and traversing the support body 
3 or at least one handle secured to said support body 3, said belt 19 or 
harness ensuring the positioning in translation and in rotation of said 
support body 3 and of said electrodes 1. 
Thus, the base 2 of the support body 3 and hence the electrodes 1 will be 
permanently applied forcibly against the skin of the patient adjacent the 
heart. 
According to a first modified embodiment of the invention, the shielded 
casing 4 contains also a battery 20 or a long-life rechargeable 
accumulator of the amagnetic type, an optical conductor, associated with 
an optical control switch disposed in the casing 8, which can permit 
controlling the operation and the supply of said processing unit 5, 6, 7 
and as the case may be the adjustment of the different circuits comprising 
its constituent modules (not shown). 
According to a second modified embodiment of the invention, the energy 
supply of the processing unit is constituted by means of an optical 
conductor coacting with a photovoltaic cell or like device disposed in the 
casing. 
According to a supplemental characteristic of the invention, shown in FIG. 
2 of the accompanying drawings, the detector device can also comprise an 
independent supplemental unit for processing signals delivered by the 
acquisition stage of the first module 5, comprised essentially of a 
low-pass filter 21, having a cut-off frequency at around 20 Hz, and a 
module 21' for electro-optic conversion, connected to a second optical 
connection means 9'. 
Such a supplemental processing unit is particularly described in French 
patent application No. 2 704 131 in the name of the applicant. 
Thus, thanks to this latter arrangement, it is possible to deliver 
independently two important physiological signals of different natures, 
elaborated from the same electrocardiac starting signals, by separate 
processing and particularly filtering adapted to each of the processing 
paths, carried out in immediate proximity to the place where the starting 
signals were picked up, 
According to another supplemental characteristic of the invention, the 
detector device can comprise moreover a circuit 22 for cyclic detection of 
a particular point or level of the repeating signal (electrocardiogram 
modulated in amplitude by respiration) delivered by the first module 5 and 
to provide a synchronization signal at each occurrence and detection of 
said predetermined point or level, said detection circuit 22 being 
followed by a corresponding electro-optical converter 23 connected to an 
optical connection means proper 9". 
As a modification, said detection circuit 22 could also be supplied by a 
signal from the sampling-blocking circuit 15 forming a part of the second 
module 6. 
The synchronization signals delivered by the detection circuit 22 will 
correspond to the approximate success of coincidences, repeated 
periodically, between a given point of a QRS complex (for example its 
maximum) of the electrocardiogram signal, and a given point of the 
respiratory signal. 
This signal, which constitutes a timed marker, accordingly permits 
determining timewise and to repeat in a repetitive manner a given 
condition of the heart in a given position of this latter, in the course 
of a respiratory cycle. 
The manner of operation of the circuit 22 and the meaning of the signal 
supplied by this latter, can be explained more precisely with reference to 
FIG. 5 of the accompanying drawings. 
Taking as a reference the respiratory signal, a time location A can be 
defined, coinciding with a QRS complex E1 and there can be sought during 
the period following the respiratory signal, at the same position in the 
cycle B, the QRS complex E2 which is the closest, and so on for the 
following cycles (cycle C and QRS E3, etc.). 
From the QRS's E1, E2, E3, etc., can be derived the synchronization pulses 
of the image (also known as "TRIGGER signal") such as S1, S2, S3, etc., 
taking directly into account the two cardiac and respiratory activities. 
The NMR images can also be produced with constant offsets of time relative 
to the time references corresponding to the pulses S1, S2, S3, etc. 
Such a synchronization signal, indicating the successive occurrences of a 
predetermined point or level in the respiratory signal, is thus 
particularly interesting in the framework of NMR imagery to trigger 
repetitive sequences of taking of images, forming by superpositions and 
virtual reconstruction of the final image. 
As a modification of the use of EKG signals and of respiratory movements 
such as described above in the case of production of an NMR image, there 
can be provided NMR images at each occurrence of a complex QRS and by 
using each of these latter as image synchronization pulses, the 
respiratory signal being picked up and registered simultaneously and 
separately by the detector device. 
Then, during ultimate use of the produced NMR images, the respiratory 
signal is used to extract and select the NMR images which correspond to a 
predetermined and repetitive point on the respiratory cycle (respiratory 
compensation). 
Thus, the detector device according to the invention permits synchronizing 
the taking of NMR images and/or the formation of NMR images on the basis 
of two physiologic signals, namely the EKG signal and the respiratory 
signal, and hence to take account of the two types of physiologic 
movements for the formation of said images. 
Of course, the invention is not limited to the embodiment described and 
shown in the accompanying drawings. Modifications remain possible, 
particularly as to the construction of the various elements or by 
substitution of technical equivalents, without thereby departing from the 
scope of protection of the invention.