The present invention relates to an optical bioimaging technique for living body samples such as small animals.
A technique for imaging the distribution of molecular species in a living body is an important tool used in medical and biological research. Imaging of molecular species at the cellular level has been widely performed using a microscope and a molecular probe such as a molecular probe labeled with a fluorescence pigment or a chemiluminescence molecular probe. However, recently, there is a growing demand for devices for observing in vivo the distribution of molecular species of interest at the organ or whole-body level rather than the cellular level. For example, such an observation device allows the imaging of the distribution of target cancer cells labeled with a fluorescence probe in the body of a small living animal, such as a mouse, to monitor the growth of the target cancer cells over a fixed period of time, such as every day or every week. In a case where the growth of cancer cells in the body of an animal is monitored using a conventional device for cellular-level imaging, the animal is killed to stain or fluorescently-label cancer cells in a predetermined part of the body of the animal. In this case, the growth of cancer cells in the same individual cannot be monitored over a long period of time. For this reason, there is a demand for the development of a device capable of observing the distribution of molecular species in the body of a small living animal to obtain internal information about the body of the small animal.
Near-infrared light can relatively easily pass through a living body, and therefore, devices for observing small animals generally use light having a wavelength in the range of about 650 to 900 nm. However, a conventional observation method has a problem in that a sample cannot be observed from multiple directions simultaneously. Therefore, for example, there is a case where when a mouse is observed from a certain direction, cancer is not detected, but when the mouse is observed from the opposite direction, cancer is detected. In a case where a mouse is observed using a device which can observe a sample from only one direction, an operator has to, by necessity, observe the mouse approximately from multiple directions by picking up images from different angles by rotating the mouse in small-angular increments about the body axis of the mouse. However, reproducible data cannot be obtained by such a method, and simultaneous detection from different directions cannot be achieved.
As another method for acquiring images picked up from multiple directions, a method in which images picked up from multiple angles are successively acquired in a time-division manner by using a rotating reflector is known (see US Patent Application No. 20050201614). According to this method, the sample can be observed from multiple directions by rotating a mirror during observation, without rotating the sample or a two-dimensional detector, but with slight parrarel sample movement.
However, the method using a rotating reflector disclosed in US Patent Application No. 20050201614 is disadvantageous in that a sample is observed from different directions not simultaneously but in a time-division manner, and therefore, it takes a lot of time to observe the sample and, in addition, the structure of a device for carrying out the method becomes complicated. It is therefore an object of the present invention to provide a living body imaging device which has a simple structure and which can observe a living body from multiple directions simultaneously in a short time.
As a method for observing a sample from multiple directions, a method in which the images of light emitted from a sample in multiple directions are formed on a common two-dimensional detector by a common image formation lens can be mentioned. That is, the living body image acquiring device of the present invention includes a sample holder for placing a living body sample thereon; one two-dimensional detector for picking up an image of light emitted from a sample placed on the sample holder; an image display device for displaying an image picked up by the two-dimensional detector;
a light guide optical system for observing a sample placed on the sample holder from multiple directions and guiding images of light emitted from the sample in different directions to the two-dimensional detector; and one main image formation lens provided between the two-dimensional detector and the light guide optical system to form a plurality of images guided by the light guide optical system on the two-dimensional detector.
An example of the light emitted from a sample is fluorescence emitted from a sample irradiated with excitation light. Another example of the light emitted from a sample is chemiluminescence or bioluminescence emitted from a sample itself without irradiation with excitation light.
The light guide optical system may include a multi-reflector assembly having two or more reflectors for reflecting images of a sample observed from different directions and guiding the images to the two-dimensional detector. More specifically, light beams emitted from a sample in multiple directions covering 360° are bent by the multi-reflector assembly and guided to different positions on the common two-dimensional detector. A reflector for bending light is generally used as a light guide optical system having the function of guiding light beams emitted in different directions to several positions spaced at appropriate intervals on one detector.
When a multi-reflector assembly is used, differences in focus positions of an image formation lens generally arise for respective light paths. Insertion of a reflector generally increases light path lengths and changes the focusing point, therefore, focus correction is performed by inserting auxiliary image formation lenses having different curvatures into the light paths of light beams emitted in respective directions. That is, in a preferred embodiment of the present invention, the light guide optical system includes light paths different in light path length from a sample to the main image formation lens, and at least one light path of the light guide optical system has an auxiliary image formation lens for correcting, according to a light path length difference, an image formed on the two-dimensional detector by the main image formation lens. An example of the auxiliary image formation lenses is a mosaic lens for different fields of view provided on the light paths of the light guide optical system, respectively. Insertion of appropriate auxiliary image formation lenses such as a mosaic lens eases restrictions on the layout of the light guide optical system, thereby making it possible to relatively flexibly design the bending of light beams. In this way, light beams emitted from a sample in multiple directions can be introduced into the common detector, and therefore, the sample can be observed from multiple directions simultaneously in a short time. Further, an observation device having no moving parts can be achieved. As will be described later, the auxiliary image formation lens for focus correction can have a small curvature (i. e. , a long focal length), and therefore, a single lens such as an eyeglass is sufficiently effective as the auxiliary image formation lens, which prevents the observation device from having a complex structure.
The present invention will be more specifically described with reference to
The images formed on the two-dimensional detector 14 are shown in
The image display device can display images obtained by subjecting images formed on the two-dimensional detector to correction for a size difference resulting from a difference in light path length between the light paths of the light guide optical system. The image display device can also display image information obtained by changing the orientation and sequence of images formed on the two-dimensional detector.
It is preferred that the light guide optical system allows a sample placed on the sample holder to be observed from four or more evenly spaced directions covering 360° around the sample.
In a preferred embodiment, the main image formation lens and the two-dimensional detector are placed in one direction perpendicular to the axial direction of a sample and each of the reflectors of the light guide optical system has a plane containing a straight line parallel to the axial direction of a sample as a reflection plane.
In another preferred embodiment, the main image formation lens and the two-dimensional detector are placed on an extended line of the axis of a sample, and the reflectors are arranged so that the principal rays emitted from the sample in different directions lie in n evenly divided planes (n is an integer of 3 or more) of which central axis is the axis of the sample.
Further, another modified example will be described. That is, in either case where the main image formation lens and the two-dimensional detector are placed in one direction perpendicular to the axial direction of a sample or a case where the main image formation lens and the two-dimensional detector are placed on an extended line of the axis of a sample, a device for controlling image pickup operation can be added to observe a sample from n evenly spaced directions and the controlling device can rotate the detector and the image formation lens relative to the sample in increments of an angle of 1/(n×m) of 360° (m is an integer of 2 or more) around the sample to perform an operation of acquiring images of the sample observed from n evenly spaced directions m times every 1/(n×m) of 360-degree rotation so that images observed from n×m evenly spaced directions covering 360° are acquired.
When the living body image acquiring device acquires a fluorescence image as an image of the light emitted from a sample, an excitation optical system for irradiating the sample with excitation light to generate fluorescence may be placed in a space between light paths of the light guide optical system. The excitation optical system preferably includes, as excitation light sources, light-emitting devices, each having a laser diode or a light-emitting diode. In such a case, the irradiation direction of a sample with excitation light can be changed by changing the lighting on/off pattern of the light-emitting devices. Further, each of the excitation light sources of the excitation optical system may have two or more light-emitting devices emitting light of different wavelengths, and each of the light-emitting devices may have an interference filter to remove an unnecessary wavelength component that might be incidentally included in the excitation light sources. In this case, it is possible to change the wavelength of excitation light by selecting the lighting on/off pattern of the light-emitting devices.
The living body image acquiring device according to the present invention can easily and simultaneously obtain the images of a living body sample observed from multiple directions covering 360° around the sample because the light guide optical system guides the images of light emitted from the living body sample in different directions to the common two-dimensional detector through the common main image formation lens.
10 living body sample
14 two-dimensional detector
20 light source mounting base
M2 to M5, M2′ , R1 to R8 reflectors
L main image formation lens
L0 to L5 auxiliary image formation lenses
S1 to S5 excitation light sources
FEM fluorescence filter
LDλ1A, LDλ1B, LDλ2A, LDλ2B laser diodes
Fexλ1A, Fexλ1B, Fexλ2A, Fexλ2B excitation light filters
(Description of Method for Simultaneous Observation from Five Directions)
Simultaneous observation from five directions will be described by way of example with reference to
The main image formation lens L and the two-dimensional detector 14 are placed in one direction perpendicular to the direction of the axis (in a case where the sample 10 is a small animal, the body axis extending from its head to tail) of the sample 10 placed on the sample holder. The reflectors M2 to M5 of the light guide optical system are arranged in five directions evenly spaced around the sample 10 so that each of the reflectors M2 to M5 has a plane containing a straight line parallel to the axial direction of the sample 10 as a reflection plane.
As described above, the light guide optical system includes the reflectors M2 to M5, and therefore has light paths different in light path length from the sample 10 to the main image formation lens L. Therefore, an auxiliary image formation lens for correcting, according to a light path length difference, an image formed on the two-dimensional detector 14 by the main image formation lens L is provided between the main image formation lens L and the light guide optical system on at least one light path of the light guide optical system. In the embodiment shown in
The point of this embodiment has been already described in “MEANS FOR SOLVING THE PROBLEM” with reference to
The images A, A2′, A3′, A4′, and A5′ can be seen in five directions below the image formation lens L. In this case, the image A is a real image and the other four images A2′, A3′, A4′, and A5′ are virtual images. As can be seen from
As shown in
A typical focal length of the image formation lens L is about 15 to 20 mm (for example, when the distance from the image formation lens L to the virtual image A3′ of the sample 10 is 300 mm and the magnification of the image of the sample 10 formed on the two-dimensional detector 14 is 1/15, the distance between the center of the image formation lens L and the two-dimensional detector 14 becomes 20 mm, which is calculated by multiplying 300 mm by a magnification of 1/15, and therefore, the focal length of the image formation lens L is a little less than 20 mm). On the other hand, a typical focal length of each of the auxiliary image formation lenses L1, L2, and L5 determined by calculation is about 500 mm to 1500 mm. The reason for this is as follows. Let us define the distance between the sample 10 and the lens L as “a” , and the distance between the virtual image A3′ and the lens L as “b”. The focal length of the auxiliary image formation lens L1 (defined as “f”) is determined so that the light from the distance “a” (for example, “a”=200 mm), proceeds as if it comes from the distance “b” (for example, “b”=300 mm), i.e., the distance 200 mm is transformed to the distance 300 mm by the lens L1. So the focal length “f” can be determined by the following simple image formation formula: (1/f)=(1/a)−(1/b). In this case, the focal length “f” determined by this image formation formula is 600 mm. On the other hand, the focal length of the auxiliary image formation lens L2 (L5) is set so that a distance between the virtual image A2′ (A5′) and the lens L of about 250 mm is transformed to 300 mm which is the distance between the virtual image A3′ and the lens L. Therefore, after the similar calculation, the focal length of the lens L2 (L5) becomes 1500 mm, which is much longer than that of the lens L1. As described above, lenses having focal lengths longer than that of the lens L, that is, lenses having extremely small curvatures suffice as the auxiliary image formation lenses L1, L2, and L5.
It is to be noted that, in this embodiment, the lens L is focused on the farthest images A3′ and A4′, and therefore, it is not necessary to provide auxiliary image formation lenses for the images A3′ and A4′. Alternatively, a simple plane-parallel glass plate may be placed instead of an auxiliary image formation lens at the position of each of the auxiliary image formation lenses L3 and L4.
The image formation lens L may be focused on the position of intermediate distance between the image A and the images A3′ and A4′, ie., on the vicinity of the images A2′ and A5′. In this case, weak concave lenses having a long focal length of about 1000 mm may be used as auxiliary image formation lenses for the images A3′ and A4′, and a weak convex lens having a focal length of about 1000 mm may be used as an auxiliary image formation lens for the front real image A.
According to this embodiment described above, it is possible to achieve a simple structure having no moving parts and to form images observed from different angles on the common two-dimensional detector 14 at one time.
(Description of Observation in Fluorescence Mode)
The above description illustrates observation in chemiluminescence mode or bioluminescence mode in which a sample containing a molecular probe which itself emits light is observed. Hereinbelow, a method for applying the first embodiment of the present invention to fluorescence mode in which a sample containing a molecular probe which emits fluorescence by irradiation with excitation light is observed will be described.
In the case of such fluorescence mode, as will be described later, the method using the multi-reflector assembly used in the present invention has an advantage in that positions for placing light sources for fluorescence excitation can be easily provided. Referring to
In the case of observation from five directions evenly spaced around the sample 10, the real image A and the virtual images A2′, A3′, A4′, and A5′ of the sample 10 are formed every 72°, and therefore, excitation light with which the sample 10 is irradiated forms an angle of +36° or −36° with a principal ray emitted from the sample and traveling directly toward the lens L1 or toward the center of the reflector M2, M3, M4, or M5. In the case of observation from six or seven directions evenly spaced around the sample 10, the angle which the direction of excitation light forms with the principal ray is ±30° or ±25. 714°, respectively, which is an irradiation angle suitable for measuring fluorescence.
In the case of fluorescence measurement, the wavelength of excitation light emitted from the light sources S1 to S5 is usually selected according to the absorption wavelength of a fluorescence probe having specificity to a molecular species or a tumor of interest. A fluorescence filter FEM is provided just before the image formation lens L to detect only the wavelength component within the spectral pass band of the FEM, separating from all the fluorescence light that comes from the sample 10 by irradiation with excitation light.
If some parts of the wavelength components of excitation light leak through the filter after being scattered with their wavelengths unchanged and then are detected, such wavelength components become background light and interfere with observation. Therefore, the selection of the wavelength of excitation light emitted from the light sources S1 to S5 and the selection of the transmission characteristics of the fluorescence filter FEM are important to completely prevent the passage of wavelength components of the excitation light through the fluorescence filter FEM.
In a case where semiconductor lasers are used as the excitation light sources S1 to S5, only the necessary light source (s) can be freely turned on and off by switching on and off their respective power supply circuits.
In this case, there are some choices of the lighting pattern of excitation light to excite fluorescence to observe the sample 10 from a plurality of angles over 360°. Hereinbelow, these choices will be described with reference to the case of observation from five directions described above.
A first choice is to turn on all the excitation light sources S1 to S5 at the same time. More specifically, five images which appear on the two-dimensional detector 14 as shown in
A second choice is as follows. Five images are picked up by the two-dimensional detector 14 in a state where a pair of two angularly adjacent excitation light sources (S1 and S2) out of the five excitation light sources S1 to S5 is turned on and the remaining three excitation light sources are turned off, and then five images are further picked up in a state where another pair of two adjacent excitation light sources (S2 and S3) is turned on and the remaining three excitation light sources are turned off, and such an operation is repeated changing the combination of two adjacent excitation light sources in turn, and finally, five images are picked up in a state where the final pair of two adjacent excitation light sources (S5 and S1) is turned on and the remaining three light sources are turned off.
Thus obtained pictures that contain 25 (5 different view×5 different irradiation angle) images include many fluorescence images with irradiation of, front direction or back direction or side direction and so on. Therefore summarizing once again, 25 images can be obtained in total by performing exposure 5 times because each of the five images picked up from five different directions around the animal has five variations picked up by changing the irradiation direction of the sample with excitation light.
From the 25 images, it can be estimated whether the depth of a fluorescence source present in the body of the animal is shallow or deep. More specifically, when a fluorescence source is present at a shallow depth, it can be supposed that a small extremely-bright spot appears in any one of the 25 images of the subject, and on the other hand, when a fluorescence source is present at a deep depth, it can be supposed that widely diffused light distribution appear in all the 25 images. In addition, the original distribution of a fluorescent material can be imaged by inverse operation using an appropriate algorithm.
A third choice is as follows. Five images are picked up in a state where one of the excitation light sources S1 to S5 is turned on and the remaining light sources are turned off, and then such an operation is repeated 4 times by changing the light source to be turned on in turn. Therefore, exposure is performed 5 times. The third choice is very similar to the second choice and becomes equivalent to the second choice if the principle of image superposition holds. When the above principle holds (i.e., the third choice is equivalent to the second choice), the second choice is more advantageous from the viewpoint of S/N ratio because the intensity of excitation light is higher. On the other hand, when the third choice is not equivalent to the second choice, both the second and third choices may be implemented. In this case, 50 images are obtained by 10 times exposure, and calculation for imaging the original distribution of a fluorescent material can be performed using these data. If necessary, other various lighting on/off patterns of the excitation light sources can be achieved.
The important point is that the fluorescence excitation method used in the present invention requires no moving parts, and therefore, can be flexibly changed simply by changing the lighting on/off pattern of excitation light so that the sample is irradiated with excitation light from the front, side, or back thereof. Therefore, observed images of the sample excited from different directions covering 360° can be easily obtained even in the case of fluorescence mode.
(More Detailed Description of Examples of Fluorescence Excitation Light Source)
Amore specific example of the fluorescence excitation light source to be used in the present invention such as the light sources S1 to S5 shown in
The four requirements of the fluorescence excitation light source are as follows: (1) light having a wavelength suitable for exciting a target fluorescence pigment can be produced; (2) excitation light contains no spectral energy in the spectral pass band of the filter for fluorescence detection (e. g. , the filter FEM shown in
Further, excitation light filters Fexλ1A, Fexλ2A, Fexλ1B, and Fexλ2B are attached to the four laser diodes, respectively. Therefore, pairs of one laser diode and one filter, i. e., (LDλ1A and Fexλ1A) , (LDλ2A and Fexλ2A) , (LDλ1B and Fexλ1B) , and (LDλ2B and Fexλ2B) each emit excitation light toward the sample. In general, a semiconductor laser emits one fixed wavelength, and therefore, it is often assumed that a semiconductor laser can sufficiently perform its function (i.e., excitation) by itself. However, when examined in more detail, excitation light emitted from a semiconductor laser often contains not only a main laser emission wavelength but also a broad and weak spectrum appearing at the foot of the main peak of laser emission. If some part of the weak light component passes through the fluorescence filter, it is detected as leaked light. It has been found that such a leaked light component contained in excitation light and overlapping with fluorescence can be reduced to a very low level by adding a suitable interference filter to an original laser diode. The interference filters Fexλ1A, Fexλ2A, Fexλ1B, and Fexλ2B are added to the laser diodes for this purpose. In the situation where the five light sources (S1 to S5) of the above structure are arranged around the sample, selection of the position(s) of necessary light source(s) and the wavelength of excitation light can be flexibly performed simply by electrically selecting (i. e. , by turning on) only the necessary laser diode(s) from the 20 laser diodes (4 laser diodes/one light source×5 light sources S1 to S5).
In the above-described case, each of the light sources S1 to S5 has two wavelengths, but as a matter of course, more laser diodes having different wavelengths may be provided if space permits. Further, the laser diode and the excitation light filter are mechanically fixed to each other. Therefore, it is very easy to design an appropriate mechanical light shield (not shown) that prevent the occurrence of light leakage through a gap between the laser diode and the filter, at the same time ensuring the necessary light emitted from the laser diode always pass through the filter.
In a case where the excitation light source has such a structure as described above, the wavelength of excitation light and the wavelength of fluorescence to be detected are selected in the following manner. The lighting on/off pattern of the five excitation light sources and the wavelength of excitation light are selected by an electric method, and the fluorescence filter FEM shown in
(Conversion from Images on Two-Dimensional Detector to Images on Display Device)
As described above, images observed from different angles are formed on the common two-dimensional detector 14, but there are problems that these images are different in magnification and are not arranged in proper sequence and some of these images are horizontally inverted by the mirrors. However, as shown in
In addition to the bioluminescence or fluorescence images of molecular species, the photographs of appearance of the sample can also be taken by the same two-dimensional detector 14 to superpose the images of molecular species onto the photographs. Image correction of different magnifications, left/right inversion and image order adjustment can be similarly achieved by performing transformations in the same manner as shown in
(Description of Modified Examples of First Embodiment)
In the above description, observation from five evenly spaced directions has been explained. Hereinbelow, observation of four evenly spaced directions will be described with reference to
Referring to
As can be seen from
A second embodiment of the present invention will be described with reference to
The advantages of the first and second embodiments of the present invention can be summarized as follows:
1) A sample can be observed from multiple directions simultaneously by using one two-dimensional detector.
2) A sample can be easily observed from multiple directions, and therefore, even when a sample (e.g., a small animal) has a tumor on its underside that cannot be seen from the observer' s side, the tumor can be detected.
3) In the case of fluorescence observation, positions for placing excitation light sources can be provided without conflict in spaces between reflectors used to observe a sample from multiple directions. Therefore, even in the case of fluorescence observation, a sample can be easily observed from multiple directions.
4) In the case of fluorescence observation, each of the excitation light sources may be formed by attaching a filter to a semiconductor laser or an. LED. In this case, the irradiation direction of a sample with excitation light and the wavelength of excitation light emitted from the excitation light sources can be selected by turning on and off only the necessary excitation light source(s) without using moving parts.
5) For fluorescence observation, combined data of multi-directional excitation and multi-directional observation are obtainable. Full set of these combined data will constitute the basis of reconstructing in-vivo fluorescence imaging. For example, in
According to a third embodiment, a sample (or a detection system) is tilted relative to a detection system (or a sample) in increments to obtain data every certain angle close to an angle achieved by continuously tilting the sample (or the detection system) relative to the detection system (or the sample). The picture of the third embodiment is not shown, and therefore, will be described with reference to
A sample (small animal) 10 placed at the center is held by one holder, and all other elements, such as mirrors, light sources, detector, and lenses are attached to another holding system different from the holder. The holding system is rotatably moved relative to the sample 10. For example, in a case where the sample 10 is observed from five evenly spaced directions, the holding system may be designed so as to be able to rotate 360°/5 (72°) relative to the sample 10. In this case, when the sample 10 is observed, for example, 6 times every 12° (72° divided by 6 equals 12°), images observed from 30 evenly spaced directions covering 360° can be obtained. It is not necessary to relatively rotate the sample 10 or the holding system 360°. The sample 10 or the holding system is relatively rotated by a relatively small angle because a rotation of 180° or 360° of a sample puts a heavy burden on a small animal as the sample, and it is difficult for the holder to even hold the sample. In addition, a rotation of 360° of the holding system complicates the handling of cables and the mechanical structure of the image acquiring device. However, as described above, a gentle rotation of the holder holding the sample 10 through an angle of, for example, one-fifth of 360° (72°) does not put a heavy burden on the small animal, and a rotation of the holding system through an angle of 72° is not difficult, either. Such a method used in the third embodiment, that is, a method in which the sample is observed from multiple directions evenly spaced with a smaller pitch such as a fraction of a pitch between the mirrors can be relatively easily achieved and is also useful.
The advantages of the third embodiment can be summarized as follows:
1) A plurality of images (e.g., 5 images) can be picked up at the same time, and therefore, the speed of observation is X times higher (X is the number of images picked up at the same time and is, for example, 5) even though the observation is performed in a time-division manner.
2) The angle of rotation of a sample (or a detector) is small, that is, at most one-fifth of 360°, and therefore, the structure of the image acquiring device can be simplified.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/322826 | 11/16/2006 | WO | 00 | 1/13/2010 |