Abstract:
The invention relates to an imaging method for simultaneously determining in vivo distributions of bioluminescent and/or fluorescent markers and radioactive markers at identical projection angles, the distribution of the bioluminescent and/or fluorescent markers being determined by separate detection of photons having a first average energy, which are emitted by the bioluminescent and/or fluorescent markers, by means of at least one first detector and the distribution of the radioactive markers being determined by simultaneous separate detection of photons having a second average energy, which are emitted by the radioactive markers, by means of at least one second detector. Furthermore, it also relates to an apparatus for carrying out the imaging method, containing at least one micro lens array optical tomographic imaging system as first detector, at least one single photon emission computer tomography (SPECT) detector as second detector.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the present invention concerns an integrated, highly sensitive, and non-invasive SPECT and OT imaging system. 
       BACKGROUND 
       [0002]    Single photon emission computed tomography (SPECT) is a non-invasive imaging system for detecting photons that are emitted from a radioactive substance, such as a molecular probe, that has been administered to an individual. Clinically, SPECT is useful in oncology for determining, grading, and locating tumor mass and evaluating its malignancy before and after treatment. Radio labeled probes produce a continuous signal, independent of any underlying molecular interaction via radioisotope decay. 
         [0003]    Optical planar imaging/tomography (OT) is an alternative noninvasive and nonhazardous molecular imaging system, which detects light that is propagated through a tissue at single or multiple projections. Fluorescence mediated optical imaging can localize and quantify fluorescent probes present in tissues at high sensitivities and at millimeter resolutions, which make it a very useful tool for imaging breast cancer, brain function and gene expression in vivo. Optical imaging uses activatable probes that produce detectable signals upon interaction with a target. 
         [0004]    The present invention is drawn to a combination of SPECT and OT systems whereby the SPECT system is equipped with any type of known collimator (in case of small animal imaging a multi-pinhole type is used most effectively and used in the following exemplarily as one possible embodiment) and the OT system is of very thin extension and allocated within the field-of-view of the SPECT camera and between the imaged object and the SPECT collimator. 
         [0005]    Since both regional distribution and time variation of the underlying multivariate photon distributions are acquisition and subject specific and diversified by variations thereof, and imaging procedures cannot be performed repeatedly at short time intervals on the same living object in many cases, combined and simultaneous imaging is needed and possible with this novel device carrying clearly advantageous potential. Further advantages are simultaneous recording of tracer kinetics, less subject encumbrance, and identical imaging geometries. The proposed nuclearoptical tomographic imaging system has the potential to accurately quantify fluorescence and bioluminescence in deep heterogeneous media in vivo. The inventive apparatus supports the development of generalized reporter probes. 
       SUMMARY 
       [0006]    An aspect of the present invention is a dual-modality imaging system, wherein at least one single photon emission computed tomography (SPECT) camera for acquiring SPECT data and at least one optical imaging detector for acquiring optical imaging data are arranged to acquire the SPECT data and the optical imaging data of an imaged object simultaneously and from the same projection angle, the at least one optical imaging detector being a non-contact optical imaging detector for bioluminescence, fluorescence, and reflectance imaging and wherein the SPECT subsystem apparatus comprises a SPECT-detector to which, be way of example, a multipinhole collimator is attached for high-resolution/high-sensitivity radio-nuclide
       imaging and at least one optical imaging detector being arranged within the imaging volume to detect photons emitted by the imaged object, characterized in that the at least one optical imaging detector comprises a micro-lens array with a plurality of micro-lenses, the optical detector being attached onto the surface of the multi-pinhole collimator of the SPECT system.       
 
         [0008]    An another aspect of the present invention is a dual-modality imaging system, comprising (1) at least one single photon emission computed tomography (SPECT) camera for acquiring SPECT data, which comprises a SPECT-detector to which a collimator is attached and (2) at least one optical imaging detector for acquiring optical imaging data which is placed between the imaged object and the SPECT camera. 
         [0009]    In one embodiment, the SPECT camera collimator is a single-pinhole type, or is a multi-pinhole type, or is a parallel beam type, or is a fan-beam type, or is a conbeam type, or is an astigmatic collimator type, or is any parallel, diverging, or converging multi-hole type, or is an converging type with a single or multitude of focal points or lines. 
         [0010]    In another embodiment, the optical imaging detector is closely attached at the imaged object facing front of the collimator of the SPECT apparatus, the collimator preferably being a multi-pinhole type (see  FIG. 2 ). 
         [0011]    In another embodiment, the optical imaging detector is placed at a certain distance to the imaged object facing front of the collimator of the SPECT apparatus, the collimator preferably being a fan-beam or cone-beam type (see  FIG. 7 ). 
         [0012]    In another embodiment, the optical imaging detector is placed at a certain shorter distance to the imaged object independently of the SPECT apparatus while the SPECT apparatus is placed at a certain longer distance to the optical imaging detector. 
         [0013]    In another embodiment, an optical imaging detector comprises at least one photo detector. 
         [0014]    In another embodiment, an optical imaging detector comprises a position-sensitive photo detector. 
         [0015]    In another embodiment, an optical imaging detector comprises a micro-lens array and the position-sensitive photo-detector is positioned at the focal plane of a micro-lens array. 
         [0016]    In one embodiment, the position-sensitive photo-detector is at least one sensor selected from the group of charge-coupled device (CCD) based detector, avalanche photo diode (APD) array, photo diode array or complementary metal-oxide semiconductor (CMOS) sensor. 
         [0017]    In further embodiment, the SPECT-detector and the optical imaging detector are mounted on a common gantry which is rotatable around 360 degrees to allow for arbitrary radial positioning of the optical detector and of the SPECT camera and to allow for tomographic imaging. 
         [0018]    In one embodiment, an apparatus of the present invention comprises a single or plurality of light sources for illuminating the imaged object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1 : schematic of an integrated optical detector and a SPECT camera with a multi-pinhole collimator. 
           [0020]      FIG. 2 : schematic of a plurality of combined SPECT-OT detectors on a common gantry, the collimator of the SPECT system being a multi-pinhole type. Laser scanning and large-field light sources are integrated into the inventive apparatus to facilitate fluorescence and bioluminescence imaging. 
           [0021]      FIG. 3 : shows the apparatus and results of a study illustrating the improvement of the performance of the SPECT-detector if multi-pinhole collimators are used. Shown are the results for 1, 4, and 6 pinholes with regard to sensitivity. 
           [0022]      FIG. 4 : shows a rendering of the cross-sectional view of the optical detector along with a photograph of the various parts of the detector. 
           [0023]      FIG. 5 : shows the results of a phantom data acquisition study performed to assess the imaging characteristics of the optical detector as used in this implementation of dual modality SPECT-OT imaging. 
           [0024]      FIG. 6 : shows the result of Monte Carlo simulations performed to investigate the multi-pinhole setup and possible effects the optical sensor might have on the SPECT photons. 
           [0025]      FIG. 7 : schematic of a plurality of combined SPECT-OT detectors on a common gantry, the collimator of the SPECT system being a fan-beam type. Laser scanning and large-field light sources are integrated into the inventive apparatus to facilitate fluorescence and bioluminescence imaging. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    An aspect of the present invention is a highly sensitive instrumentation system for identifying the location, magnitude, and time variation of specific functional or molecular events by simultaneously detecting optical and radioactive tracer and marker types in vivo. The inventive instrument is useful for monitoring functional events associated with, for instance, metabolism, physiological changes, and receptor binding, as well as for monitoring molecular events, such as gene expression and enzyme activity, which can be imaged and detected using the inventive SPECT/OT combination instrument. 
         [0027]    An innovative aspect of the present invention is that it makes it possible to perform unified simultaneous acquisition, reconstruction, and tracer/probe-kinetic modeling for dual-modality optical and radiotracer small animal imaging without the drawbacks such as explained above. Use of this device in academic or research environments has the potential to foster interdisciplinary research that leads to new approaches of molecular imaging techniques, to the development of additional reporter constructs, and to a better understanding of the mechanisms of disease and response to therapy. The inventive application to small animal imaging can also be extended to imaging cancerous tissues, such as breast cancer, and melanoma by simple changes to the overall geometry of the systems. 
         [0028]    An aspect of the present invention is the use of a plurality of specific detection cameras that are mounted on a common gantry to acquire images around a single axis of rotation with axially un-shifted, i.e. identical, spatially over-lapping field-of-views (FOV) of the involved sub-modalities. Thus, the overall sensitivity of the inventive imaging apparatus, which is one of the two critical parameters of in vivo imaging, is improved. 
         [0029]    The invention solves the problem of single-procedural, simultaneous projection data acquisition and image reconstruction of (i) time-resolved in vivo distributions of single- or multi labeled low-energy (light) fluorescent or bioluminescence optical probes (OT), and (ii) high-energy (various radioisotopes) photon emitting (SPECT) molecular markers in a small object, particularly in mice and rats, but also in a specific human organs and tissues such as breast and skin, whereby both data types are acquired from identical projection angles, either with a single or, preferable, a multitude (most optimal four) of rotating dual-modality detector heads. 
         [0030]    Fully integrated modalities (i) and (ii) employ specific detection cameras that are mounted on a common gantry (explained below) whereby projection images are acquired around a single axis of rotation with axially un-shifted (i.e. identical), spatially over-lapping field-of-views (FOV) of the involved sub-modalities. 
         [0031]    Thus present invention resolves issues associated with separately imaging modalities (i) and (ii) (i.e. sequentially imaging the object with different devices) as for instance the direct study of tracer/marker kinetics, image registration, time resolved concurrent data analysis and animal handling which are inaccessible (kinetics) or become crucial (registration, animal management). Accordingly, the present invention proposes an instrumentation system that is highly sensitive in identifying location, magnitude, and time variation of specific functional (metabolism, physiological information, receptor binding, etc.) or molecular events (e.g., gene expression and enzyme activity) by simultaneously detecting optical and radioactive tracer and marker types in vivo. 
         [0032]    The dual-modality instrument has been designed so that the optical modality detector is located in front of a SPECT modality collimator. The optical detector is separating and detecting low energy optical photons from the multi-energetic photon flux while high-energy SPECT photons are unaffected by this and, hence, are transmitted through the optical detector to be detected by the SPECT-detector which is located in radial extension to the optical detector. 
         [0033]    In application, the invention performs non-invasive fully tomographic simultaneous image acquisition of dual-labeled (near-infrared) fluorescent, bioluminescence and high-energy photon emitting molecular and functional markers in small objects, particularly mice and rats, but also in specific human organs and tissues such as breast and skin. The invention solves problems connected to separately imaging targets with different devices, as for instance the direct study of tracer/marker kinetics, image registration, time-resolved concurrent data analysis and animal handling which are inaccessible (kinetics) or become crucial (registration, animal management). The invention assesses visual representation, characterization, and quantification of biological processes at the cellular and sub-cellular levels within intact living organisms by means of simultaneously performed image acquisition procedures. The invention proposes an instrumentation system that is highly sensitive in identifying location, magnitude, and time variation of specific molecular events (e.g., gene expression and enzyme activity) by simultaneously detecting above listed tracer and marker types in vivo. 
         [0034]    The inventive imaging system is versatile and may be useful for a number of applications, including, but not limited to:
   (i) imaging specific cellular and molecular processes, e.g. gene expression, or more complex molecular interactions such as protein-protein interactions,   (ii) monitoring multiple molecular events simultaneously;   (iii) tracking single or dual-labeled cells;   (iv) optimizing drug and gene therapy, to image drug effects at a molecular and cellular level;   (v) assessing disease progression at a molecular pathological level; and   (vi) creating the possibility of achieving all of the above goals of imaging in a single, rapid, reproducible, and quantitative manner.   
 
         [0041]    Further application-specific use is anticipated to monitor time-dependent experimental, developmental, environmental, and therapeutic influences on gene products in the same animal (or patient), to study the interaction of tumor cells and the immune system, to study viral infections by marking the virus of interest with a reporter gene, and others. There is also an enormous clinical potential for the noninvasive assessment of endogeneous and exogeneous gene expression in vivo (gene (DNA), message (RNA), protein, function), for imaging receptors, enzymes, transporters, for novel applications in basic and translational research (gene therapy, etc.), for early detection of disease, for guidance of therapeutic choices, for monitoring drug action, for aid of pre-clinical drug development, for non-invasive and repetitive monitoring of gene therapy, and for optimizing clinical trials of human gene therapy. 
         [0042]    According to one aspect, the inventive apparatus therefore integrates an optical detector  10  and a SPECT camera  28  which employs a multi-pinhole collimator  18 .  FIG. 1  shows an optical detector  10  having a field of view which is identified by reference numeral  12 . Behind the optical detector  10 , a multi-pinhole mask  18  is arranged. A read-out electronics is labeled with reference numeral  14 . In  FIG. 1 , a view  16  in the drawing plane is shown in which the optical detector  10  is arranged in front of said multi-pinhole mask labeled with reference numeral  18 . Still further, the inventive apparatus comprises a SPECT-detector  28  shown in the plane  34 . Rectangular areas  38  of the surface of said SPECT-detector  28  are separated from one another by thin spaces identified with reference numeral  32  in  FIG. 1 . An overlap  36  shows single areas  38  of said SPECT-detector  28  covering respective edges of other single areas  38  on the surface of the SPECT-detector  28 . That optical detector  10  is arranged above said SPECT-detector  28 . For instance, in one embodiment of the present invention, said optical detector  10  is arranged on top of a shielding  26 , whereas said SPECT-detector  28  is arranged on the bottom of said shielding  26 . Said shielding  26  defines a hollow interior  30 . Below said optical detector  10 , the surface of which is labeled with reference numeral  24 , said multi-pinhole mask  18  is arranged in this part integrated into the ceiling of said shielding  26 . Above said surface  24  of the optical detector  10 , an imaged object  22  is shown, the radiation beams are given in dotted lines. Reference numeral  100  depicts the entire optical detector unit. 
         [0043]    Laser scanning and large-field light sources are integrated to facilitate fluorescence imaging in addition to bioluminescence imaging.  FIG. 2  shows a schematic of a plurality of combined SPECT-OT detectors on a common gantry, a collimator of a SPECT system being a multi-pinhole type. According to this arrangement, laser scanning and large field light sources are integrated into the inventive apparatus to facilitate fluorescence and bioluminescence imaging. As can be seen in  FIG. 2 , four optical detector units  100  are arranged on a common gantry  42 , which is rotatable into sense of rotation indicated by arrow  40 . Four optical detector units  100  each having a SPECT-detector  28 , said shielding  26  and a multi-pinhole mask  18  arranged below the optical detectors  10  are provided. According to the embodiment given in  FIG. 2 , said optical detector units  100  are arranged within an angle of 90° with respect to one another, thus, completely surrounding said imaged object  22 . Four light sources A, B, C, D are provided between the respective shielding  26  of two neighboring optical detector units  100  arranged adjacent to one another. As can be seen in  FIG. 2 , said second light source B and said first light source A each emit light directed to said imaged object  22  being surrounded by the optical detectors  10 . Corresponding to  FIG. 1 , below each of said optical detectors  10  said multi-pinhole mask  18  is arranged within the shielding  26  of each of the optical detector units  100 . Said SPECT-detectors  28  are preferably dual modality detector heads being a part of each optical detector unit  100 . Each optical detector unit  100  comprises four dual modality heads labeled  44 ,  46 ,  48 ,  50  according to  FIG. 2 . Each optical detector unit consists of a large-area photo sensor for light detection, a micro lens array for field of-view definition, a septum mask for cross-talk suppression, and a transferable filter for wavelength selection. 
         [0044]    In one design, optical photons are separated from the photon flux and collimated onto a detector by a cylindrical lattice of micro lens arrays  92  (see  FIG. 4 ) (MLAs), which form an inner optical detection ring while a SPECT-detector  28  is mounted in radial extension. In one embodiment of the present invention, an optical detector  10  may be mounted onto the surface of the SPECT collimator  94  (see  FIG. 4 ). The inventive apparatus can accommodate the incorporation of different collimators  94 , and in that set-up the optical detector  10  may be located most effectively close to the imaged object  22  while the SPECT camera is be positioned further away from the imaged object  22 , depending on field-of-view geometry  114 . See, for instance, an example of such an arrangement in  FIG. 7 . Thus, multiple cameras can be mounted on a common gantry  42  as illustrated in  FIGS. 2 and 7 , respectively. In addition to the four detector heads  44 ,  46 ,  48 ,  50  there also are included, for instance, four light sources, which are necessary for fluorochrome excitation, labeled A to D in  FIGS. 2 and 7 , respectively. By way of example, sources A and B are bright-field sources, source C is not producing light, and source D is producing a light beam for focal point illumination. 
         [0045]      FIG. 3  shows the apparatus and results of a study illustrating the improvement of the performance of the SPECT-detector  28  if multi-pinhole collimators  18  are used. Shown are the results for 1, 4, and 6 pinholes with regard to efficiency (sensitivity). Said optical detector unit  100  comprises the SPECT-detector  28 , the surface of which is labeled by reference numeral  32 . Said optical detector unit  100  comprises the shielding  26  on top of which the multi-pinhole mask  18  is arranged. A distance b shows the distance between the surface of said multi-pinhole mask  18  and the imaged object  22 . In the diagram according to  FIG. 3 , the efficiency is shown with respect to the spatial resolution given in mm. According to the diagram, a poor third efficiency  80  is achieved by a multi-pinhole mask  18  having one pinhole only. An increase in the number of pinholes results in an improvement of efficiency such as the second efficiency  70  when four pinholes are provided in said multi-pinhole mask  18 . A good first efficiency is obtained, see reference numeral  60 , if said multi-pinhole mask  18  comprises six pinholes. The larger the number of pinholes the better the efficiency is. That is the result of the diagram according to  FIG. 3 . 
         [0046]    According to  FIG. 4 , a single optical detector system comprises four parts: (I) a micro lens array  92  intended for field-of-view definition, (2) a large area complementary metal oxide semiconductor (CMOS) chip for light detection, (3) a septum mask for cross-talk suppression, and (4) exchangeable filters  90  for wavelength selection. 
         [0047]    The assembly of all parts allows for a very thin detector design yielding an effective complete detector thickness of about 4.0 mm. Optical imaging using micro lens arrays  92  (MLAs) also referred to as microlenticular or lens let arrays are used primarily in optical data communication applications, e.g. to interconnect optical fiber bundles, or as light collection elements to increase the optical fill factor in CCDs. See  FIG. 4 , which shows a rendering of the cross-sectional view of the optical detector  10  along with a photograph of the various parts of the detector  10 . See also WO2006111486, which is incorporated herein by reference. According to the cross-sectional view of the optical detector  10  according to  FIG. 4 , said optical detector  10  comprises an exchangeable filter  90 , said micro lens array  92 , previously being mentioned as well as a collimator  94 . Still further, a photosensor  96  and a corresponding electronics  98  are comprised within the optical detector  10 . Since all components of said optical detector  10  are very thin, the effective complete detector thickness can be minimized and as previously mentioned amounts to about 4 mm. 
         [0048]    To date, MLAs  92  are produced from fused silica, silicon, or other materials, depending on application wavelength, and are typically available in array sizes up to 120 mm×120 mm and with lens diameters in the range of 10 microns to 2 mm. In the field of optical imaging, micro lens arrays  92  have been used for instance to realize an artificial apposition compound eye, as described recently by Duparr&#39;e and colleagues. In the device concept presented herein, the MLA  92  is used exclusively for field-of-view definition allowing for non-contact in vivo optical imaging and possibly optical tomography. An analogy for illustrating the purpose of the micro lens array in this application might be seen in the use of multi-hole collimators in high-energy detector physics such as in SPECT. 
         [0049]    To allow imaging of whole mice, it was found that planar optical projection images should cover an effective area of about 10 cm axially×5 cm transaxially. Tomographic data can potentially be acquired either by rotating a single detector  10  around the object  22  or by mounting a multitude of detectors  10  on a common gantry  42 . Four detectors  10 , as shown in  FIG. 2 , i.e. reference numerals  44 ,  46 ,  48 ,  50 , having the above mentioned dimensions potentially allow for simultaneous tomographic optical imaging. 
         [0050]    In order to study intrinsic spatial resolution and sensitivity of the optical detector unit  100  an imaging apparatus was used in two different experimental settings. Intrinsic spatial resolution  108 . 1  to  108 . 8  according to  FIG. 5  as a function of object-detector distance d was measured using an electro-luminescent light screen (El-Light, Strausberg, Germany), 10 cm×10 cm in size, transilluminating a pattern of circles, ranging in diameter from 1.0 mm up to 2.0 mm in 0.2 mm steps, compared in whole diameters  104 . 0  to  104 . 5  according to  FIG. 5 , similar to the well-known Derenzo-like phantom geometry. Detector sensitivity with respect to d was investigated using a red light emitting diode (Kingbright DLA2/6ID, Kingbright Elec. Co., Ltd., Taipei Hsien, Taiwan) powered by a constant current source in the pico-Ampere range. 
         [0051]    In this setup, source intensity can be precisely controlled by altering the forward current. Note that in all experiments conducted the maximum forward current applied was 60 pA which yields a light output that is hardly human-perceivable. In order to further judge detector sensitivity images were additionally acquired using a highly sensitive CCD camera (Orca-AG, Hamamatsu Photonics K.K., Hamamatsu City, Japan). It employs a cooled advanced progressive scan interline CCD chip with 1344×1024 pixel resolution and is rated at 0.1 electrons per second dark current (at −20° C.), 8 electrons read noise, 18,000 electrons saturation, 12 bits digitization accuracy. A 4×4 binning was used in all experiments. The camera was equipped with a Cinegon 1.4/18 lens (Schneider Optische Werke GmbH, Bad Kreuznach, Germany). Exposure time was set to 30 sec for all measurements, involving the CCD as well as the CMOS sensor. 
         [0052]      FIG. 5  shows the results of a phantom data acquisition study which was performed to assess the imaging characteristics of the optical detector  10  as used in this implementation of dual modality SPECT-OT imaging. 
         [0053]    According to  FIG. 5 , the distance d between object and detector has been varied between a first detector-object distance  106 . 1  of 1 cm in 1 cm steps until an object-detector distance of 8 cm, see reference numerals  106 . 8 . The steps in between have been made within an 1 cm range, compare reference numerals  106 . 2 ,  106 . 3 ,  106 . 4 ,  106 . 5 ,  106 . 6  and  106 . 7 , respectively. Corresponding to said variations in the object-detector distance intrinsic spatial resolutions  108 . 1  to  108 . 8  have been measured. The arrangement of pinholes  102  on the multi-pinhole mask  18  according to  FIG. 5  has been unaltered, the variation between object and detector has varied. Compared with the first distance  106 . 1  of 1 cm between object and distance with the setting of a distance  106 . 8  of  8  cm and comparing both intrinsic spatial resolutions  108 . 1  and  108 . 8 , respectively, the intrinsic spatial resolution  108 . 8  is more trapezoidal as compared to the very distinct intrinsic spatial resolution  108 . 1  of the first set-up having a distance  106 . 1  of 1 cm between detector and object. 
         [0054]    In order to study the multi-pinhole geometry, a Monte Carlo simulation study was performed.  FIG. 6  shows the result of Monte Carlo simulations that have been performed in order to investigate the multi-pinhole setup as well as possible effects the optical sensor might have on the SPECT photons. There were no measurable degradations found as a result of the optical detector  10  being within the field-of-view  114  of the SPECT camera. 
         [0055]    A variety of suitable large-field photo-sensors are available commercially, most of them incorporating either CCD or CMOS sensors which are two different technologies for capturing images digitally by converting light into electric charge. In a CCD sensor, every pixel&#39;s charge is transferred through a single output node, voltage-converted, pre-amplified, buffered, and sent off-chip as an analog signal with comparatively low noise, high dynamic range, and high uniformity. In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and most CMOS sensors also include amplifiers, noise-correction, and digitization circuits. CMOS sensors have improved considerably in achievable dynamic range and signal-to-noise ratio and start to challenge CCDs in terms of spatial resolution and sensitivity. 
         [0056]    The assembly as presently described employed a Rad-Eye™ 1 large-area CMOS imaging sensor (Rad-icon Imaging Corp., Santa Clara, Calif.). Another favorable feature of this sensor is the lateral placement of the read-out electronics  14 ,  98  which can be placed outside the field-of-view  114  of the SPECT subsystem and such can be shielded to avoid radiation damage. The sensor chip is equipped with a 512×1024 silicon photodiode matrix at 48 μm pixel pitch, yielding a 24.6 mm×49.2 mm active area. These sensors can be positioned head-to-head leaving only a small gap of less than 1 mm between sensor fields such that active areas of 10 cm× a manifold of 2.5 cm can be constructed. According to the manufacturer&#39;s data sheet the sensor can be operated at frame rates of 0.01 to 4.5 per second and has a dynamic range of 85 dB (14 bits). Average dark current is stated with 4,000 electrons per second (at 23° C.) and the read noise with 150 electrons (at 1 frame per second). Saturation is attained at 2,800,000 electrons per pixel. The detector signal is transferred to a PXD1000 digital frame grabber (CyberOptics Semiconductor Inc., Beaverton, Oreg.). 
         [0057]    While sensitivity is of foremost concern for in vivo imaging applications the need for highest intrinsic spatial resolution of the optical detector  10  might be less important because spatial resolution is primarily limited by photon scattering in tissue and not so much by the intrinsic spatial resolution of the detector-an inverse situation as compared to SPECT. 
         [0058]    MLA&#39;s  92  size and lens geometry was specified according to the parameters of the CMOS sensor as lens pitch g should be a multiple of pixel pitch, and also with respect to the desired intrinsic spatial resolution (ISR) of the optical imager. For the intended in vivo imaging application ISR is considered necessary to be in the range of about half a millimeter. Given 48 μm pixel pitch of the CMOS sensor we did chose MLA lens pitch at 480 μm. Hence, individual lenses on the MLA correspond to local fields of 10×10 sensor pixels. The overall size of the MLA is 24.6 mm×49.2 mm, matching the size of CMOS sensor. MLAs were manufactured according to our specifications by Advanced Microoptic Systems GmbH, Saarbruecken, Germany using S-TIH53 optical glass (Ohara Inc., Kanagawa, Japan) of 1 mm thickness. Focal lengths of all lenses have been defined at 2.2 mm forming a focal plane at that distance coplanar to the MLA at which the large-field CMOS sensor is aligned. 
         [0059]    In order to avoid cross-talk between the outputs of individual lenses, a light-tight, non-reflective septum mask (similar in structure to a parallel-hole collimator in SPECT) is placed between the CMOS sensor and the MLA  92 . Bore diameter is chosen at 400 μm with 480 μm pitch, which is identical to lens pitch. This mask was coaligned with the lens pattern. Having the same overall size as sensor and MLA its thickness is defined by the empty space (=focal distance minus some offset) between MLA and sensor, which is 2.1 mm. The final element of the arrangement is a removable filter  90  located in front of the MLA  92  used to filter the optical signal. In our realization, the filter  90  and its bearing are also used to protect the detector from fluid contamination. 
         [0060]    In connection with  FIG. 6  it has to be mentioned that said shielding  26  of the optical detector unit  100  comprises said SPECT-detector  28  and on its upper sealing below the multi-pinhole mask  18  the optical detector  10 . The surface  24  of said optical detector  10  is directed towards the imaged object  22 . As shown in  FIGS. 1 and 2 , respectively, said shielding  26  limits the hollow interior  30  of the optical detector unit  100 .  FIG. 6  shows the results of a Monte Carlo simulation 10,000,000 photons, 140.5 keV, rdi lesions: background (1:1:1): 0 (left hand side)/20 (right hand side). 
         [0061]    According to  FIG. 7 , a plurality of combined SPECT-OT detectors  28  are arranged on a common gantry  42  which is rotatable in the sense of rotation  40 . The collimators of this SPECT system each are of fan beam type. Laser scanning and large field light sources A, B, C and D, respectively are integrated to provide for fluorescence and bioluminescence imaging. 
         [0062]    The arrangement according to  FIG. 7  resembles the arrangement according to  FIG. 2 , the difference being that said SPECT-detectors  28  having surfaces  32  oriented to the imaged object  22  with shieldings  26 . Said SPECT-detectors  28  are of fan beam type, the respective field of view indicated by reference numeral  114 . Said beams overlap within the area of the imaged object  22  where the optical detectors  10  between which said light sources A, B, C and D, respectively, are arranged. 
       REFERENCE LIST 
       [0000]    
       
           10  Optical detector 
           12  Field of view of optical detector 
           14  Read-out electronics 
           16  Front-view 
           18  Multi-pinhole mask 
           20  Field of view mapping 
           22  Imaged object 
           24  Surface of optical detectors 
           26  Shielding 
           28  SPECT-detector 
           30  Hollow interior 
           32  Surface of SPECT-detector 
           34  Plane 
           36  Overlap 
           38  Area 
           40  Sense of rotation 
           42  Gantry 
           44  Dual modality head  1   
           46  Dual modality head  2   
           48  Dual modality head  3   
           50  Dual modality head  4   
         A 1. light source 
         B 2. light source 
         C 3. light source 
         D 4. light source 
         b Distance 
           60  1. efficiency (6 pinholes) 
           70  2. efficiency (4 pinholes) 
           80  3. efficiency (1 pinhole) 
           90  Filter 
           92  Microlens array 
           94  Collimator 
           96  Photosensor 
           98  Electrons 
           100  Optical detector unit 
           102  Pinholes 
           104 . 0 - 104 . 5  Pinhole diameter 
           106 . 1 - 106 . 8  Detector-object distance 
           108 . 1 - 108 . 8  Intrinsic spatial resolution 
           110  Layered structure 
           112  Pinhole area 
           114  Field of view of collimator