Optical movement sensor

An optical movement sensor for clearly accurately detecting deformations of a human body. The movement sensor consists of a belt with a buckle made up of two parts which are joined together mechanically by a light-transmitting sensor element. Transmitting and a receiving fibre optic light guides are connected to one part of the buckle in such a way that light emitted by the transmitting light guide passes through the sensor element by way of a polarizer and is coupled to the receiving light guide via an analyzer.

BACKGROUND OF THE INVENTION 
The invention relates to an optical movement sensor for the detection of a 
human body. 
In computerized tomography, particularly nuclear spin tomography, 
intestinal, swallowing and muscular movements of a human body as well as 
movements originating from respiratory or heart activities produce 
so-called artefacts (i.e. shadows on the images of cross-sections lying at 
right angles to the body axis). The artefacts can falsify the examination 
result. To obtain perfect sectional images it is necessary therefore to 
clearly detect deformations of a human body under examination and take 
these into account in the production of the sectional images. 
German Pat. No. 8012386 (corresponding to U.S. Pat. No. 4,559,953) 
describes a capsule-shaped sensor for the detection of deformations of a 
human body. The sensor contains a hollow space sealed off with an elastic 
membrane and filled with air. The hollow space has an aperture which is 
connected via a hose to a volume transducer. 
The sensor is fixed by adhesive strips to the human body under examination 
in such a way that the elastic membrane lies on the body and is deflected 
with deformations of the body. The change which this brings about in the 
volume of the hollow space is detected and indicated by the volume 
transducer. 
Because the membrane has a very thin wall, air in the sensor is heated up. 
The heated air expands causing the measured results supplied by the volume 
transducer to exhibit a measuring error which depends the temperature of 
the capsule-shaped sensor. Thus, the measured results depend on both the 
shape of the body being examined and on the temperature of the sensor. The 
measured results cannot therefore unequivocally indicate the state of 
deformation of the body. 
SUMMARY OF THE INVENTION 
It is an object of the invention to create an optical movement sensor which 
unequivocally indicates independent of the body temperature, the state of 
deformation of the human body under examination at particular points in 
time. 
This object is achieved in an optical movement sensor of the type described 
above by a belt to be placed around the body of a patient. The belt 
comprises a buckle made up of two parts which are mechanically connected 
to one another by way of a light-transmitting sensor element. In one part 
of the buckle transmitting and receiving fiber optic light guides are 
connected to the sensor element by way of a polarizer or analyzer in such 
a way that light emitted by the transmitting light guide passes through 
the sensor element by way of the polarizer and then arrives in the 
receiving light guide by way of the analyzer. 
If the human body undergoes deformation, the stress on the belt and 
therefore on the light-transmitting sensor element changes. As a result, 
the state of polarization of polarized light is changed when it passes 
through the sensor element. An analyzer filters out plane-polarized light 
from the polarized beam. The intensity of the filtered light varies with 
the state of polarization of the light, and thus unequivocally provides 
and largely temperature-independent information on the stress state of the 
belt and, therefore, on the state of deformation of the body. 
A sensor which is simple and inexpensive to make is obtained if the 
receiving light guide is mounted on the opposite side of the sensor 
element to the transmitting light guide. 
To increase the sensitivity of the movement sensor it is advantageous if 
the transmitting and receiving light guides are mounted on the same side 
of the sensor element and a mirror is mounted on the opposite side of the 
sensor element to the light guides. The mirror reflects the light which is 
emitted by the transmitting light guide and which passes through the 
sensor element back through the sensor element and passes it onto the 
receiving light guide. In this way, the light passes through the sensor 
element twice so that the polarization of the light is altered twice in 
the same direction as a function of the stress on the movement sensor. 
In one advantageous configuration of the movement sensor, one buckle part 
has a socket for a fiber optical plug. The socket is connected to the 
transmitting and receiving light guides. Between the ends of the light 
guides and the sensor element, the socket contains a polarization foil 
which functions as a polarizer for the light emitted from the transmitting 
light guide. The foil functions as an analyzer for the light which is 
passed from the mirror to the receiving light guide. In this case, only a 
single fiber optical plug is required to connect the transmitting and 
receiving light guides to the movement sensor. Furthermore, the two light 
guides can be accommodated in one fiber optical cable so that the patient 
is not so inconvenienced by individual light guides. 
In order to avoid measuring errors in nuclear spin tomography as the result 
of magnetic fields caused by eddy currents in electrically conductive 
parts of the movement sensor, it is essential that the optical movement 
sensor be made from nonmetallic materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sectional view of the buckle consisting of buckle parts 1 and 
2. The two parts 1 and 2 of the buckle are each mechanically connected to 
one end of a belt 3. Belt 3 is placed around the upper body or stomach of 
a patient undergoing examination. In addition, the two parts 1 and 2 of 
the buckle are mechanically connected to one another by way of a 
light-transmitting sensor element 4 in the form of an oblong plate. In 
this case the sensor element 4 is fixed to buckle parts 1 and 2 by cotter 
pins 5 and 6. 
One buckle part 1 has a socket 7 on one side and a socket 8 on the other 
side of the sensor element. Into these sockets 7 and 8 can be inserted 
fiber optical plugs 9 and 10 for transmitting and receiving light guides 
11 and 12, respectively. The fiber optical plugs 9 and 10 each contain, 
between the ends of the transmitting and receiving light guides 11 and 12 
and the sensor element 4, an polarization foil 13 and 14, respectively. In 
fiber optical plug 9, foil 13 acts as a polarizer. In fiber optical plug 
10, foil 14 acts as an analyzer. The transmitting directions (the 
directions of polarization of light passing through the polarizer or 
analyzer) of the polarizer and the analyzer are parallel to one another 
and form an angle of 45.degree. with the longitudinal axis of the sensor 
element 4. 
The entire optical movement sensor is made from nonmetallic materials. 
Thus, the belt 3 may be made of leather, the two buckle parts 1 and 2, the 
cotter pins 5 and 6, and the fiber optical plug of plastic, and the sensor 
element 4 of glass or transparent plastic. 
FIG. 2 is a sectional view along the line II--II in FIG. 1. The two ends of 
the belt 3 are connected to T-shaped fixing devices 15 and 16. As FIG. 1 
also shows, devices 15 and 16 can be slid into the two buckle parts 1 and 
2. 
FIG. 2 shows the two cotter pins 5 and 6 and socket 7 in section. FIG. 2 
also indicates that the front side of the buckle is covered by a plate 17 
which is fixed to buckle part 1 by plastic screws 18 and 19. The plate 17 
and a projection 20 of buckle part 1 form a guide for buckle part 2 which 
is thereby secured against lateral deviations by silicone washers 21 and 
22. The elastic silicone washers 21 and 22 also prevent static and sliding 
friction losses due to the relative movements of the two buckle parts 1 
and 2. 
FIG. 3 is a sectional view of the buckle along the line III--III in FIG. 1. 
In FIG. 3, a possible configuration of the sensor element 4 is 
illustrated. Both the width and thickness of the plate-shaped sensor 
element 4 depend in this case on the required sensitivity of the optical 
movement sensor. The choice of material also affects the deformability 
and, therefore, the sensitivity of the sensor element 4. A movement sensor 
with a sensor element 4 of light-transmitting plastic has a greater 
sensitivity than a movement sensor with a sensor element 4 made of glass 
because plastic deforms more readily than glass. 
FIG. 4 is a sectional view of a part of the buckle in which the 
transmitting and receiving light guides 11 and 12 are arranged on one side 
of the sensor element 4. The transmitting and receiving light guides 11 
and 12 are connected in this case to buckle part 1 by a single fiber 
optical plug 26. Mounted on the opposite side of the sensor element 4 to 
the ends of transmitting and receiving light guides 11 amd 12 is a mirror 
23. 
Mirror 23 may be, for example, a dielectric laminated mirror which 
comprises several thin, transparent .lambda./4 layers of alternatingly 
high and low refractive index. Such mirrors can have a reflectivity of 
approximately 99.9%. 
Between the ends of transmitting and receiving light guides 11 and 12 the 
fiber optical plug 26 has a polarization foil 24. Foil 24 functions as a 
polarizer for the light emitted from the transmitting light guide and an 
an analyzer for the light coupled from the mirror 23 to the receiving 
light guide 12. The transmitting direction (the direction of polarization 
of light passing through the polarizer) of the polarization foil 24 forms 
an angle of 45.degree. with respect to the longitudinal axis of the sensor 
element 4. 
The mirror can be mounted at the bottom of a dummy plug 25 which has the 
same cross-section as a fiber optical plug and which therefore can be 
inserted in socket 8. The stress on the belt placed around the upper body 
or stomach of the patient is altered by every deformation caused by heart 
and respiratory activity but also by intestinal, swallowing and muscle 
movements such that the tensile stress acting on the sensor element 4 is 
also changed thereby. The elliptical polarization of the light emitted 
from transmitting light guide 11 and plane-polarized by polarizer 13 or 24 
increases as the tensile stress acting on the sensor element 4 increases. 
After passing through sensor element 4, plane-polarized light is filtered 
from the elliptically polarized light by analyzer 14. The intensity of 
this plane polarized light when polarizer 13 and analyzer 14 have parallel 
transmitting directions decreases as the ellipticity of the light in 
sensor element 4 increases (i.e. the greater the tensile stress 
transmitted from the belt to the sensing element 4). The intensity of the 
light coupled to the receiving light guide 12 provides information, 
therefore, on the tensile stress on the belt-shaped movement sensor and 
thus on variations in the girth of the human body being examined. 
In the case of the embodiment illustrated in FIG. 4, the light emitted by 
the transmitting light guide 11 is plane-polarized by the polarization 
foil 24 acting initially as a polarizer. The light by sensor element 4 is 
elliptically polarized as a function of the tensile stress acting on the 
sensing element 4. This elliptically polarized light is reflected by the 
mirror 23 through the sensing element 4 which, in addition, elliptically 
polarizes the light in such a way that the small axis of the polarization 
ellipse is reduced and the large axis is increased. The light then passes 
through the same polarization foil 24, this time acting as an analyzer and 
is coupled to the receiving light guide 12. 
During this process, the analyzer filters from the elliptically polarized 
light plane-polarized light. The intensity of the plane polarized light 
decreases very considerably with an increase in the tensile stress acting 
on the sensor element 4 such that even small deformations of the human 
body under examination can be detected by measurement of this intensity.