Patent Publication Number: US-10327721-B2

Title: Imaging apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. non-provisional patent application is a continuation of U.S. application Ser. No. 14/963,568 filed on Dec. 9, 2015, which claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2015-195851 filed on Oct. 1, 2015, in the Japanese Patent Office (JPO), and Japanese Patent Application No. 2014-248934 filed on Dec. 9, 2014, in the JPO, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     Example embodiments relate to an imaging apparatus such as computed tomography enabling high spatial, timing and energy resolution with smaller form factor and lower power consumption. 
     2. Related Art 
     A computed tomographic (CT) system or, a CT system is now widely used for medical imaging and diagnostics.  FIG. 1 ( a )  shows a conventional CT system  100 . The CT system  100  includes an imaging unit  1  including a set of sensor elements and data acquisition systems (DAS), an X-ray source  3 , and a bed  5  on which an imaged object or patient  7  lies down. The X-ray source  3  projects an X-ray beam  8 , for example, through the object  7  to be detected by sensor elements inside the imaging unit  1 . The imaging unit  1  and the X-ray source  3  are located in an opposite position inside their housing called a gantry. The imaging unit  1  and the X-ray source  3  rotate around the object  7  taking a slice image, and the object  7  on the bed  5  is moved in a direction of Z-axis to get additional slice images. As shown in  FIGS. 1( a ), 1( b ) and 1( c ) , a Cartesian coordinate system is defined. The X-ray source  3  and the imaging unit  1  rotate on the X-Y plane, and the bed  5  and/or the object  7  moves in a direction of Z-axis which is perpendicular to the X-Y plane. The CT system  100  also includes an image processing unit  2  which performs a reconstruction of a slice image. Electrical communication between the imaging unit  1  and the image processing unit  2  is performed by a slip-ring  4 . Image data obtained by the imaging unit  1  is transferred to a network interface circuit  15  in the image processing unit  2  through the slip-ring  4 . A central processing unit (CPU)  16 , a disk controller circuit  17 , and a memory disk unit  18  reconstruct a slice image, which is displayed on an image output device  19 . Moreover, a cooling system and a regulated power supply system (not illustrated), for example, should be equipped with a CT system. 
     Consequently, such a CT system becomes heavy, large sized and expensive machinery, which may be suitable mainly to large scale hospitals in metropolises, and less suitable for healthcare providers with smaller facilities. 
     SUMMARY 
     At least some example embodiments provide a computed tomographic (CT) system that enables the discovery of a disease or other condition harmful to the health of a patient in the early stages of the condition thereby reducing medical expenses. 
     As shown in  FIGS. 1( c ) and 1( d ) , the conventional CT system  100  uses plural elongated (e.g., long and/or slender) area image sensors  6 , on each of which a two dimensional pixel array  9  is formed. These image sensors  6  are mounted in the imaging unit  1  such that the longer direction of the image sensor  6  is parallel to the direction of the Z-axis around the object  7 , and each pixel array  9  is opposed to a fan beam X-ray source  3 . The pixel array  9 , however, has a flat surface formed on a silicon substrate, and it may be difficult or, alternatively, impossible to have a smooth curved surface to oppose a fan beam X-ray in the direction of the rotation in the X-Y plane. Further, with an increase in the pixel or slice number of the image sensor  6  in the direction of Z-axis, the cone angle of X-ray also increases, which may degrade image quality due to an artifact caused by the X-ray attenuations with different incident X-ray angles to the flat surface of the pixel array  9 . 
     Sensitivity variations between the image sensors  6  or between the pixels are desirably reduced, or alternatively, minimized in the rotating direction of the imaging unit  1  on the X-Y plane, because the worst sensor or pixel limits dominate the overall system sensitivity and reconstructed image quality. Otherwise, it may be difficult to reduce total X-ray dose, especially human exposure. As shown in  FIG. 1( d ) , the plural image sensors  6  and multi-channel analog-to-digital convertor (ADC) circuit boards  10  of CT system  100  should be fixed firmly being opposed to the X-ray source  3  inside the imaging unit  1 . It should be also noted that the imaging unit  1  moves around the object, and it may be necessary to replace parts like the image sensors  6  with a new parts, regularly. With the above-referenced use environment and maintenance requirements, it may be necessary to fix the positions of each image sensor  6  and each multi-channel ADC circuit board  10  with a high degree of accuracy in such a manner that each image sensor  6  and each multi-channel ADC circuit board  10  are still removable by precision machining tools, when replacement is necessary, in order to maintain an initial reconstructed image quality. As a result, efforts for downsizing, weight reduction and cost reduction of the CT system  100  may all be made more difficult by the above-referenced precision tools, in addition to other significant factors associated with at least some conventional systems like the CT system  100  including, for example, a large installation space, heavy maintenance loads, a large-scale regulated power supply, and a large-scale cooling system of the conventional CT system  100 . 
     Further, from an electrical point of view, peripheral circuits and input or output terminals which will not detect incident light (e.g., elements other than the pixel array  9  on the image sensor  6 ) may be damaged by radiation, and thus, an area for such peripheral circuits and input or output terminals should be reduced or minimized. In at least some conventional systems like the CT system  100 , vertical and horizontal scanning circuits, and input and output terminals besides the pixel array  9  are integrated on the image sensor  6  in order to reduce a blind or dead area on the image sensor  6 . As a result, the pixel array  9  is located close to the chip side edges which are not flat or smooth, and have a lot of crystal defects due to a wafer dicing process. Accordingly, the location of the pixel array  9  may affect the image quality because the photo-diodes and peripheral circuits are formed close to these edges, and thus, may be exposed to incoming contaminations of metal impurities or reactive chemicals, for example. 
     A high speed multi-channel analog-to-digital convertor (ADC) with a 16 bit or higher resolution as a semiconductor discrete part, for example, is mounted on a print circuit board  10  which is attached to the backside of the image sensor  6 . As shown in  FIG. 1( d ) , input and output signals to or from the print circuit board  10  are connected to a later-stage signal processing circuit block, which includes a signal control circuit  11 , a multiplexer circuit  12 , a data buffer circuit  13  and a parallel to serial converter circuit  14 . In at least some conventional systems, a high speed line driver is needed between each print circuit board  10  and the later-stage signal processing circuit block, because of the long wiring length. Consequently, even more power consumption or heat generation may be caused around the DAS. 
     Further, regarding the reliability and the maintenance on at least some conventional systems like the CT system  100 , a semiconductor chip of the conventional CT system  100  may be damaged by high energy radiation like X-rays, for example. As for the Metal-Oxide-Semiconductor (MOS) device, charges trapped in a gate insulator of silicon dioxide (SiO2), for example, may cause threshold voltage shift in a MOS transistor. This shift in threshold voltage may deteriorate image quality obtained by either MOS type or CCD type image sensors, and eventually reduce a product life time. A scintillator, for example, a columnar crystal Cesium Iodide, is applied above the conventional image sensor to convert the X-ray to visible light, which is effectively absorbed by a silicon substrate within ten (10) micron meters in depth. The columnar crystal Cesium Iodide is, however, a deliquescent substance and needs an ambient humidity and temperature control. As a result, at least some conventional systems like CT system  100  require periodic or regular maintenance including, for example, image sensor replacements and sensitivity adjustments. 
     Besides sensitivity or lower X-ray dose and image quality, other functionalities of a CT system should be considered. These other functionalities of the CT system that should be considered include, for example, a higher data transfer rate with lower power consumption, a smaller or portable form factor, a reduced maintenance load, and a reduction of the influence of surroundings like temperature, humidity and mechanical vibrations should be considered. For example, it may be desirable for a CT system to provide high spatial, time and energy resolution in order to enable three dimensional (3D) imaging with a variety of valuable bio-medical information even from active or moving objects, for example. 
     Further, as is discussed above, a conventional CT system uses a number of detector elements arranged around and facing an object where the detector elements have very specific positioning and angles with respect to each other. As a result, a conventional CT system may become a system composed of heavy, large-sized and extremely expensive machinery. However, it may be desirable to reduce or solve such difficulties or inconveniences in order to create a CT system that is affordable and portable for medical professionals, and thus, useful all over the world including in developing countries. 
     According to at least one example embodiment of the CT system, an image sensor which is installed inside an imaging unit is mounted on the same plane as a slice image of an object, and the semiconductor surface of the image sensor, where integrated circuits are formed, is at angle of 45 to 90 degrees to the direction of a bed or an object to be moved, and at least one face of the semiconductor substrate of the image sensor faces an object and receives an incident light, the incident light being, for example, X-rays that travel through or from the object. With such a configuration, a CT system may be implemented with reduced size, lighter weight, parts that are easier to replace, and less influence from environmental factors like temperature, humidity and mechanical vibrations, in comparison to a conventional CT system. 
     According to at least one example embodiment of the CT system, the face of the semiconductor substrate of the image sensor, which receives an incident light or X-ray, may be concave in shape. With such a configuration, incident beam angle dependence on the X-ray beam attenuation, which may cause a false or alias image referred to as an artifact, is reduced or prevented. 
     According to at least one example embodiment of the CT system, the imaging unit may use plural image sensors inside an imaging unit around an object. With an increase in the number of the image sensor used in the imaging unit, an X-ray source may project a wider fan beam angle, and a larger number of transmitted X-ray signals can detected simultaneously in a short time. As a result of such an arrangement, a total dose of X-rays received by an object being analyzed by the CT system may be reduced. 
     According to at least one example embodiment of the CT system, the concave face of the semiconductor substrate of the image sensor is covered by a thin silicon dioxide layer. With such a configuration, crystal defects caused by a mechanical or a thermal stress may be reduced and contamination of the semiconductor substrate by incoming metal impurities or reactive chemicals may be prevented. 
     According to at least one example embodiment of the CT system, the image sensor chip has a circular hollow inside the chip, where the inside and outside substrate sides (e.g., faces) are covered by a thin silicon dioxide layer. According to at least one example embodiment, the image sensor may have a donut shape. With such a configuration, all or, alternatively, substantially all of the photo-diodes and peripheral circuits of an image sensor chip are formed in a single die on the same plane, which may result in a sensitivity of the image sensor chip to variations around the object being reduced or, alternatively, minimized. The CT system using the single die image sensor can be smaller in size than a conventional CT system and portable. 
     According to at least one example embodiment of the CT system, an imaging unit may include plural image sensors which are stacked in the direction perpendicular to a slice image of an object. With such a configuration, a pixel number or so-called slice number can be increased. 
     According to at least one example embodiment of the CT system, the distance between adjacent pixels in a horizontal direction may be equal to the distance between adjacent pixels in a vertical direction. According to at least one example embodiment, the horizontal distance may be larger than the semiconductor substrate thickness. With such a configuration, a high spatial resolution with an accuracy of, for example, 0.1 micron meter or less can be realized. 
     According to at least one example embodiment of the CT system, the stacked image sensor includes a metal optical shield between the image sensors. For example, according to at least one example embodiment, a metal optical shield may include heavier metal atoms like tungsten or lead in order to attenuate unexpected incident light with wrong directions or scattered X-ray beams. With such a configuration, even higher image qualities may be achieved relative to a conventional CT system due to less cross-talk noise and fewer artifacts. 
     According to at least one example embodiment of the CT system, a color filter, or near infrared filter is laminated on the concave face of the image sensor used in the stacked image sensor. With such a configuration, the image quality of the CT system is improved and the CT system has a hybrid imaging capabilities which enable multi-diagnoses using different energy X-rays and/or near infrared (NIR) of multiple wavelengths. According to at least one example embodiment, a scintillator film is laminated on the concave face of the image sensor used in the stacked image sensor. With such a configuration, X-rays that pass through the scintillator without emitting a light may also be detected by the image sensor. 
     According to at least one example embodiment of the CT system, the image sensor is placed along the trace of a helix extending in the direction perpendicular to the slice image of an object. According to at least one example embodiment, more than two image sensors are located approximately along the helix. With such a configuration, the CT system according to at least one example embodiment may have advantageous effects of both helical and multi-scan systems. 
     According to at least one example embodiment of the CT system, the image sensor has a slit, which makes it easy to deform the substrate of the image sensor such that a sidewall of the substrate faces the helix. According to at least one example embodiment, two or more image sensors with slits are stacked by a holding member having a screw thread inside. With such a configuration, the image sensor or image sensor module including multiple image sensors stacked on each other may be accurately fixed helically inside the imaging unit. 
     According to at least one example embodiment of the CT system, the photo-diodes on the silicon substrate extend radially toward the incident light beam around the object. With such a configuration, sensitivity dependence on the incident light or X-ray angle may be reduced or, alternatively, minimized. 
     According to at least one example embodiment of the CT system, the peripheral circuits including the analog-to-digital converters (ADCs) and other CMOS circuits are integrated on a same image sensor chip. With such a configuration, high speed signal processing and lower device temperature due to reduced clock frequencies and lower power consumptions may be achieved. 
     According to at least one example embodiment of the CT system, on chip buffer memory and a data compression circuit are integrated on the image sensor. With such a configuration, the image data after analog to digital conversion is compressed down to, for example, between one third (⅓) and one fifth (⅕) without data loss, and thus the data transfer bit rate may be also decreased. 
     According to at least one example embodiment of the CT system, an image processing chip is stacked on the top of the image sensor. With such a configuration, compressed image data obtained from the image sensor is sent directly to the upper image processing chip with a reduced or, alternatively, minimum number of electrical paths by through silicon vias (TSVs), which results in higher speed data transfer and a data processing with lower power consumption relative to a conventional CT system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of example embodiments will become more apparent by describing in detail example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
         FIG. 1( a )  is a diagram illustrating a perspective view of a conventional computed tomographic (CT) system. 
         FIG. 1( b )  is a diagram illustrating a plan view of a conventional CT system with a view direction parallel to the Z-axis as illustrated in  FIG. 1( a ) . 
         FIG. 1( c )  is a diagram illustrating a perspective view of a conventional image sensor used in a conventional CT system. 
         FIG. 1( d )  is a diagram illustrating a plan view of an imaging unit and an image processing unit used in a conventional CT system. 
         FIG. 2( a )  is a diagram illustrating a perspective view of a CT system according to at least one example embodiment. 
         FIG. 2( b )  is a diagram illustrating a plan view of a CT system with a view direction parallel to the Z-axis as illustrated in  FIG. 2( a )  according to at least one example embodiment. 
         FIG. 2( c )  is a diagram illustrating a plan view of an arrangement of an image sensor in an imaging unit from a view point above the CT system  110  (i.e., looking down upon the bed  5 ) and a view direction parallel to the X-axis according to at least one example embodiment. 
         FIG. 3( a )  is a diagram illustrating a plan view and a block diagram of an image sensor used in the CT system according to at least one example embodiment. 
         FIG. 3( b )  is a diagram illustrating a plan view of an arrangement of an image sensor in an imaging unit from a top side of the CT system according to at least one example embodiment with a view direction parallel to the X-axis. 
         FIG. 3( c )  is a diagram illustrating an example of tracks of scanned images made by moving an image sensor around an object according to at least one example embodiment. 
         FIG. 4( a )  is a diagram illustrating a plan view of a CT system with a view direction parallel to the Z-axis according to at least one example embodiment. 
         FIG. 4( b )  is a diagram illustrating a plan view of a CT system with a view direction parallel to the Z-axis according to at least one example embodiment. 
         FIG. 4( c )  is a perspective view of a CT system according to at least one example embodiment. 
         FIG. 5( a )  is a diagram illustrating a plan view and a block diagram of an image sensor used in the CT system according to at least one example embodiment. 
         FIG. 5( b )  is a diagram illustrating a plan view and a block diagram of an image sensor used in the CT system according to at least one example embodiment. 
         FIG. 6( a )  is a diagram illustrating a perspective view of an image sensor module used in the CT system according to at least one example embodiment. 
         FIG. 6( b )  is a diagram illustrating a perspective view of an image sensor module used in the CT system according to at least one example embodiment. 
         FIG. 7( a )  is a diagram illustrating a side view of an image sensor module used in the CT system with a view direction parallel to the Y-axis, and a view direction parallel to the X-axis, according to at least one example embodiment. 
         FIG. 7( b )  is a diagram illustrating a side view of an image sensor module used in the CT system with a view direction parallel to a direction of a beam of light incident on the image sensor module according to at least one example embodiment. 
         FIG. 7( c )  is a diagram illustrating a side view of an image sensor module used in the CT system with a view direction parallel to the X-axis according to at least one example embodiment. 
         FIG. 8( a )  is a diagram illustrating a perspective view of a helical path along which an image sensor moves when traveling around an object being analyzed by a CT system including the image sensor, according to at least one example embodiment. 
         FIG. 8( b )  is a diagram illustrating a plan view and a block diagram of an image sensor used in the CT system according to at least one example embodiment, where the image sensor has a gap. 
         FIG. 8( c )  is a diagram illustrating a cross sectional view of an imaging unit used in the CT system according to at least one example embodiment. 
         FIG. 9( a )  is a diagram illustrating a cross sectional view of an image sensor used in the CT system according to at least one example embodiment. 
         FIG. 9( b )  is a diagram illustrating an enlarged portion of the cross sectional view of the photo-electrical conversion region illustrated in  FIG. 9( a ) , according to at least one example embodiment. 
         FIG. 10( a )  is a diagram illustrating a plan view and a block diagram of an image sensor used in the CT system according to at least one example embodiment. 
         FIG. 10( b )  is a diagram illustrating a perspective view of an image sensor module used in the CT system according to at least one example embodiment. 
         FIG. 10( c )  is a diagram illustrating a block diagram of an image processing chip stacked on the image sensor used in the CT system according to at least one example embodiment. 
         FIG. 10( d )  is a diagram illustrating a plan view of an imaging unit and an image processing unit used in the CT system according to at least one example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected to”, “coupled to”, or “on” another element, it may be directly connected to, directly coupled to, or directly on the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly connected to”, “directly coupled to”, or “directly on” another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
       FIG. 2( a )  is a diagram illustrating a perspective view of a computed tomographic (CT) system  110 , which includes an imaging unit  1   a  in which an image sensor  40   a  is installed. A CT system includes an X-ray source  3 , and a bed  5  on which an imaged object or patient  7  lies down. The X-ray source  3  projects an X-ray beam  8 , for example, through the object  7  to be detected by the image sensor  40   a . X-ray beam  8  may be collimated to form a defined fan beam angle by a collimator (not illustrated), for example. The imaging unit  1   a  and the X-ray source  3  are located in opposing positions inside a housing called a gantry (not illustrated). According to at least some example embodiments, the X-ray source  3  and imaging unit  1   a  may be diametrically opposed to each other with respect to a diameter that passes through the object  7 . The imaging unit  1   a  and the X-ray source  3  rotate around the object  7  taking a slice image about a rotation axis  26 , and the object  7  on the bed is moved to get additional slice images in a direction of the Z-axis illustrated in  FIG. 2( a ) . In the example shown in  FIG. 2( a ) , the rotation axis  26  is parallel to the Z-axis. The CT system  110  may also include an image processing unit (an example of which is explained in greater detail below with reference to at least  FIGS. 10( b ) and 10( c ) ) which performs a reconstruction of a slice image. As shown in  FIG. 2( a ) , a Cartesian coordinate system is defined. As shown in  FIG. 2 ( b ) , according to at least one example embodiment, the X-ray source  3  and the image sensor  40   a  may rotate on the X-Y plane, and the bed  5  and/or the object  7  may move in a direction of Z-axis which is perpendicular to the X-Y plane. A semiconductor surface of the image sensor  40   a  where integrated circuits are formed may also be perpendicular to the Z axis. One face of the image sensor  40   a  faces an object  7  in order to detect X-ray beam  8 , which is incident on the face of the image sensor  40   a  that faces the object  7 . For example, according to at least some example embodiments, the image sensor  40   a  is placed at an angle theta (θ) of 90 degrees to the Z-axis, as shown in  FIG. 2 ( c ) . In the case of the conventional CT system, the conventional area image sensor is placed at an angle theta (θ) of zero degree to the Z-axis. For example, in the example illustrated in  FIG. 2( b ) , a plan view of the image sensor  40   a  includes a long face  46  and a concave face  44   a  that opposes the long face  46 .  FIG. 2( c )  shows the long face  46  of the image sensor  40   a . As in shown in  FIG. 2( c ) , the long face  46  may be rectangular in shape, and thus, may have two parallel short sides and two parallel long sides. As is shown in  FIG. 2( c ) , according to at least one example embodiment, the image sensor  40   a  may be arranged such that a first line  36  is perpendicular to the Z-axis. For the purpose of simplicity, the first line  36  is illustrated in the present disclosure as a line that passes through a center of the long face  46  and is parallel to the long sides of the first face  46 . However, as will be discussed in further detail below with reference to  FIG. 3( c ) , the first line  36  is a reference line that indicates a direction in which one or more rows of pixels  20  extend along a face of an image sensor that upon which X-ray beams  8  are incident (e.g., the concave face  44   a  for sensor  40   a ). 
     For the purpose of simplicity, the term “X-ray source” is used in the present disclosure in the description of several example embodiments. However, according to at least some example embodiments, elements described in the present disclosure as X-ray sources may be embodied sources capable of providing other types of radiation instead of, or in addition to, X-rays including, but not limited to, gamma rays and near infrared (NIR) light). 
     As is discussed in greater detail below, the concave face  44   a  of the image sensor  40   a  used in at least some example embodiments is exposed to incident X-ray beams  8 , thus allowing X-ray beams  8  to be incident on the photodiodes  33  of the pixels  20 , for example, after the X-ray beams pass through or emerge from an object being analyzed (e.g., the object  7 ). With a narrow fan beam angle or a pencil beam, the face exposed to the X-ray beams  8  could be flat in shape. However, according to at least one example embodiment, as shown in  FIGS. 2 ( a )  and  2  ( b ), the concave face  44   a  may be concave in shape, and thus, angles of beams that are incident on the concave face  44   a  may become even or equal in length. With such a configuration, incident beam angle dependence on the X-ray beam attenuation, which may cause false or alias images (e.g., artifacts), is reduced or, alternatively, prevented. Unlike the conventional arrangement of a number of image sensors with mechanical constructions around the object, no mechanical and angle alignment with high accuracy is required. For example, because the exposed surfaces of the individual sensors  6  of the conventional CT system  100  illustrated in FIGS.  1 ( a )-( d ) are flat, the sensors  6  each need to be arranged with precise angles relative to the X-ray source  3  when being installed into the CT system  100  initially or as replacements. To the contrary, the size of the image sensor  40   a  and the curvature of the concave face  44   a  may be controlled and determined, instead, prior to installation of the image sensor  40   a  in a CT system, during the image sensor manufacturing process, which may include a photo-lithography step. Thus, the process of installing one or more image sensors  40   a  in a CT system initially or as replacements may be substantially simplified relative to the process of installing the sensors  6 . The CT system, using the image sensor  40   a  with the exposed concave face  44   a  and the arrangement, described herein, of the concave face  44   a  relative to an object being analyzed (e.g., the object  7 ), may realize a small or portable form factor, relatively low weight, relatively easy parts replacement, relatively low maintenance load, and a relatively low amount of influence by environmental factors including, for example, temperature, humidity and mechanical vibrations, as a result of the limited number of parts and mechanical constructions used. 
       FIG. 3( a )  shows a plan view and a block diagram of the image sensor  40   a  used in the CT system according to at least one example embodiment. On the image sensor  40   a , a plurality of photo-diodes  33 , input or output terminals  31 , and peripheral circuits are formed. As used in the present disclosure, the term peripheral circuits may refer to, for example, a signal read-out and scanning circuit  21 , a timing pulse generator circuit  23 , an analog-to-digital converter (ADC) circuit  25 , a digital signal processing circuit  27 , and/or an interface circuit  29 . According to at least some example embodiments, the plurality of photo-diodes  33 , input or output terminals  31 , and peripheral circuits are formed on a single die, a single chip, or formed as a single-die image sensor chip. 
     According to at least some example embodiments, the peripheral circuits may be formed at locations on the semiconductor substrate  22  of the image sensor  40   a  that are spaced away (i.e., separated) from edges of the image sensor  40   a . For example, as is illustrated in  FIG. 3( a ) , peripheral circuits (e.g., the signal read-out and scanning circuit  21 , the timing pulse generator circuit  23 , the analog-to-digital converter (ADC) circuit  25 , the digital signal processing circuit  27 , and the interface circuit  29 ) are separated from edges of the image sensor  40   a  (i.e., edges of the semiconductor substrate  22 ) by input or output terminals  31 . 
     Each photo-diode from among the plurality of photo-diodes  33  may be a part of a corresponding pixel circuit  20 . Pixel circuits  20  are also referred to, in the present disclosure, as pixels  20 , and will be discussed in greater detail below with reference to  FIG. 3( c ) . According to at least some example embodiments, the concave face  44   a  of the image sensor  40   a  (illustrated in  FIG. 3( a )  as a concave upper side of the plan-view of the image sensor  40   a ), may be covered by a thin silicon dioxide layer  35 . The silicon dioxide layer  35  may reduce crystal defects caused by a mechanical or a thermal stress, and may protect the portions of the image sensor under the silicon dioxide layer  35  from contaminants like, for example, metal impurities or reactive chemicals. Each photo-diode  33  extends inside the same silicon substrate  22  to the signal read-out and scanning circuit  21  (with a length of, for example, 5 to 500 microns (μm)) to detect infrared light or X-ray beams of less than 50 kiloelectronvolts (Key) where photo-electric effect is dominant, for example. Longer lengths like 500 μm to 10 mm photo-diodes may be used for detecting X-ray energy of 50 Kev or higher where Compton scattering is dominant, for example. A thin silicon dioxide layer (e.g., layer  35 ) is discussed with reference to a number of embodiments in the present disclosure. According to at least some example embodiments, with respect to the thin silicon dioxide layer (e.g., layer  35 ) discussed in the present disclosure, the thickness of the thin silicon dioxide layer may be in the range of, for example, 100 angstroms (A) (i.e., 0.01 microns) to 10 microns (μm). According to at least some example embodiments, it may be desirable for the thickness of the thin silicon dioxide layer (e.g., layer  35 ) discussed in the present disclosure to be in the range of, for example, 1000 (A) to 1 (μm), for example. Depending on the film growing speed, thinner layers (e.g., films) less than 1000 (A) may be prepared by a thermal oxidation of silicon, which, in at least some cases, may be a relatively slow process. Further, thicker layers (e.g., films) more than 1000 (A) (i.e., 0.1 microns) may be prepared by chemical vapor deposition (CVD), for example, which, in at least some cases, may be a relatively fast process. According to at least some example embodiments, the thin silicon dioxide layer (e.g., layer  35 ) discussed in the present disclosure can be formed by a combination of the two above-referenced processes by, for example, growing a thin thermally grown silicon dioxide layer first, and adding and/or stacking the thick CVD silicon dioxide layer on the first layer (i.e., the thin thermally grown silicon dioxide layer). 
     For the purpose of simplicity, the term “silicon substrate” is used in the present disclosure in the description of several example embodiments. However, according to at least some example embodiments, elements described in the present disclosure as silicon substrates may alternatively be embodied by substrates composed of semiconductor materials other than (or, alternatively, in addition to) silicon. 
     According to at least some example embodiments, sensor architectures used to implement the pixels (e.g., pixels  20 ), photo-diodes (e.g., photo-diodes  33 ) and/or scanning circuits (e.g., scanning circuits  21 ) described in the present disclosure may follow, for example, known charged coupled device (CCD)-type sensor architectures or metal oxide semiconductor (MOS)-type sensor architectures. 
     With such a configuration, either light (i.e., electromagnetic radiation) with longer wavelengths like infrared light or light with shorter wavelengths like X-ray beams can be effectively detected by a single crystalline silicon substrate without using toxic or chemically unstable materials like amorphous selenium (a-Se), cadmium telluride (Cd—Te), or a needle crystalline cesium iodide (Cs—I) scintillator. In addition, peripheral circuits like AD convertors and other CMOS circuits can be integrated on a same image sensor chip, which enables high speed signal processing and lower device temperature due to reduced clock frequencies and lower power consumptions. Further, sensitivity variations or other non-uniformities in pixel characteristics, for example, can be reduced or, alternatively, minimized because all the pixels, being in line along the revolutions around the object, may be prepared on the same chip. Still further, some or all peripheral circuits in the image sensor  40   a  may be formed away from the face upon which the X-ray beams are incident (e.g., the concave face  44   a ) and most of X-rays may be adsorbed or generate photo-carriers in the photo-diodes  33 . As a result, radiation damage in MOS elements of the image sensor  40   a  (e.g., MOS field-effect transistors (MOSFETS)) or white defective spots on reproduced images generated by the image sensor  40   a  may be reduced. Further, a product life time of the image sensor  40   a  and/or a CT system including the image sensor  40   a  may be extended, and a maintenance frequency of the image sensor  40   a  and/or a CT system including the image sensor  40   a  may be decreased. 
       FIG. 3( b )  is a plan view of an arrangement of an image sensor  40   a  in an imaging unit  1   a  with a view direction parallel to the X-axis from a view point above the CT system  120  (i.e., looking down upon the bed  5 ) according to at least one example embodiment. The CT system  120  may have the same structure and operation as the CT system  110 , with the following exceptions. The image sensor  40   a  is placed in the imaging unit  1   a  at a tilt angle theta (θ), which is less than 90 degrees but larger than 45 degrees (45&lt;θ&lt;90) to the Z-axis. For example, as is shown in  FIG. 3( b ) , the image sensor  40   a  may be arranged such that the tilt angle theta (θ) exists between the rotation axis  26  (illustrated in  FIG. 3( b )  the Z-axis) and the first line  36 . According to at least some example embodiments, the image sensor  40   a  may include an array of pixels  20  arranged into a plurality of columns and one or more rows such that the number of rows in the array of pixels  20  is less than the number of columns in the array of pixels  20 . For example, in  FIG. 3( c ) , the array of pixels  20  is arranged in six columns and one row. Further, as is illustrated in  FIG. 3( c )  and discussed above with respect to  FIG. 2( c ) , the first line  36  indicates a direction in which the one or more rows of the array of pixels  20  extend (i.e., the first line  36  is parallel to a direction in which the one or more rows of the array of pixels  20  extend). Thus, as is shown in  FIG. 3( b ) , the image sensor  40   a  may be arranged such that the tilt angle theta (θ) exists between the Z-axis and the first line  36  (i.e., such that the tilt angle theta (θ) exists between the rotation axis  26  and a direction in which the one or more rows of the array of pixels  20  extend). 
     Further, according to at least some example embodiments, the image sensor  40   a  may be arranged such that the tilt angle theta (θ) exists between a direction in which the object  7  and or bed  5  moves (illustrated in  FIGS. 3( b ) and 3( c )  as the Z-axis) and the first line  36 . 
     Though, for the purpose of simplicity, the array of pixels  20  in  FIG. 3( c )  is illustrated as having only six columns and one row of pixels, according to at least some example embodiments, the array of pixels  20  may have any number of pixels arranged in any number of columns and any number of rows that is less than the number of columns. Further, the array of pixels  20  may be formed monolithically on a single semiconductor chip. 
     As is shown in  FIG. 3( c ) , when the image sensor is tilted in the manner shown in  FIG. 3( b ) , the paths of pixels  20  (illustrated in  FIG. 3( c )  as bars extending from the pixels  20 ) do not overlap as the image sensor  40   a  moves around an object in the CT system  120 , and thus pixels  20  of a single image sensor  40   a  may follow, respectively, different paths simultaneously as the image sensor  40   a  rotates around an object by analyzed by the CT system  120 . As is noted above, each pixel  20  may include a corresponding photodiode from among the plurality of photodiodes  33 . According to at least some example embodiments, the tracks or traces (i.e., paths) of the pixels  20  may become helical shape during continuous image capturing when an object being analyzed by the CT system  120  moves in the direction of Z-axis. According to at least example embodiments, by changing the tilt angle theta (θ) between 45 and 90 degrees, spatial resolution of the image reproduced by the CT system  120  and correlation between the neighboring pixels can be set to a desirable level or, alternatively, optimized in accordance with the nature of the object. According to at least some example embodiments, the tilt angle theta (θ) may be changed between 45 and 90 degrees in accordance with the desires of a user and/or manufacturer of the CT system  120 . According to at least some example embodiments, the space or distance between the pixels  20  (e.g., a pitch of the pixels  20 ) is determined not by the mechanical assembly but the photo-mask design and semiconductor device manufacturing process capable of performing highly accurate alignment even for distances less than 0.1 micron-meter (μm), for example. 
       FIG. 4( a )  is a diagram illustrating a plan view of a CT system  130  with respect to view direction parallel to the Z-axis, which includes an imaging unit  1   b , an X-ray source  3 , a bed  5  on which an imaged object or patient  7  lies down. The X-ray source  3  projects an X-ray fan beam  8 , for example, through the object  7  to be detected by the imaging unit  1   b . The imaging unit  1   b  uses three image sensors  40   b - 1 ,  40   b - 2  and  40   b - 3 , for example, which encircle the imaged object  7 . Similar to image sensor  4   a , each image sensor  40   b - 1 - 40   b - 3  has a concave face. According to at least some example embodiments, for each of image sensors  40   b - 1 - 40   b - 3 , the surface of the concave face is parallel to the view direction of  FIG. 4( a ) , and is covered by a thin silicon dioxide layer (not shown in this figure). For each of image sensors  40   b - 1 - 40   b - 3 , the curved (i.e., concave) face of the sensor, which is exposed to the incident fan beam  8 , may form a part of a circumference around an object being analyzed by the CT system  130 . According to at least one example embodiment, each of the image sensors  40   b  may have the same structure and operation as that described above with reference to the image sensor  40   a , with the exception that the curvature of the concave faces of at least some of the image sensors  40   b  may differ from the curvature of the concave face  44   a  of the image sensor  40   a , as is illustrated in  FIG. 4( a ) . 
     With the configuration described above with reference to  FIG. 4( a ) , which includes increasing the number of the image sensors used in the imaging unit  1   b  relative to, for example, imaging unit  1   a  of CT system  110 , in addition to the above advantageous effects, an X-ray source  3  projecting a wider fan beam angle may be used to detect a larger number of transmitted X-ray signals simultaneously in a short time. Consequently, the total dose of an X-ray received by a patient may be reduced. The size of each image sensor  40   b  may depend on the size of a silicon wafer used; however, as explained above, the plural image sensors  40   b  placed in the imaging unit  1   b  may cover a wider field of vision in the X-Y plane. The plural image sensors  40   b  placed in the imaging unit  1   b  may be manufactured from the same silicon wafer or the same process lot, which may result limited non-uniformities of pixel characteristics such as sensitivity variations among the image sensors  40   b . As explained above, peripheral circuits like AD convertors and other CMOS circuits can be integrated on each chip with a relatively small, or alternatively, minimum wiring length, which may result in reduced clock frequencies and lower power consumption in addition to higher speed signal processing and a higher signal to noise ratio (S/N) relative to, for example, at least some conventional CT systems. 
       FIG. 4( b )  is a diagram illustrating a plan view of a CT system  140  with a view direction parallel to the Z-axis. A donut shaped imaging unit  1   c  includes image sensors  40   c , which surround the object  7  covering 360 degrees. Each image sensor  40   c  has a concave face to form a circular form. The concave face, the surface of which is parallel to the view direction of  FIG. 4( b ) , is covered by a thin silicon dioxide layer (not shown in this figure). In the example illustrated in  FIG. 4( b ) , a movable X-ray source  3  is not used but a donut shaped target ring may be placed near the imaging unit  1   c , for example. An example of a CT system including a target ring will now be discussed in greater detail with reference to  FIG. 4( c ) .  FIG. 4( c )  is a diagram illustrating a perspective view of a CT system  140 . The CT system  140  includes an electron beam gun  3   a , a focusing and deflection coil  3   b , and the imaging system  1   c . The imaging system  1   c  includes the image sensor  40   c  and a target ring  3   d  which, according to at least some example embodiments, may arranged near, or alternatively, adjacent to the image sensor  40   c  in the imaging unit  1   c , as is illustrated in  FIG. 4( c ) . As is illustrated in  FIG. 4( c ) , according to at least some example embodiments, the electron beam gun  3   a  generates an electron beam  3   c , which is focused and deflected by a focusing and deflection coil  3   b  towards the target ring  3   d , such that the electron beam  3   c  is incident on the target ring  3   d . As is also illustrated in  FIG. 4( c ) , as a result of electron beam scanning (e.g., using electron beam  3   c ) and a high energy electron bombardment to the target ring  3   d , X-ray beams  8  are generated. X-ray beams  8  may travel through the object  7 , and can be detected by the image sensor  40   c  located at a side of the object  7  opposite to a side of the object  7  upon which each of X-ray beams  8  are incident. Returning to  FIG. 40( b ) , as is illustrated in  FIG. 4( b ) , X-ray beams  8  may be incident on several different sides of the object  7 . According to at least some example embodiments, some or all of image sensors  40   c  may be implemented by the image sensor  40   a  illustrated in  FIG. 3( a ) . According to at least some example embodiments, as is discussed in greater detail below, the sensors  40 ( c ) may include photo-diodes (not shown in this figure) that are formed inside the silicon substrate of the sensors  40 ( c ) radially from the center of an X-ray or a gamma-ray, for example. For example, according to at least some example embodiments, some or all of image sensors  40   c  may be implemented by the image sensor  40   m , which will be discussed in greater detail below with reference to  FIG. 10( a ) . In addition, all the sides of the image sensors  40   c  may be concave in shape forming a circle around the object  7 , where incident beam angles on each concave side may become even or nearly equal. As a result, incident beam angle dependence on the X-ray beam attenuation, which may cause false or alias images (e.g., artifacts), may be reduced or effectively suppressed by an error correction algorithm, for example. 
     With the above-referenced configuration described with reference to  FIGS. 4( b ) and 4( c ) , in addition to the above advantageous effects, an imaging unit  1   c  and an X-ray source like the target ring  3   d  are fixed inside a gantry without mechanically moving parts, which enables the CT system  140  to have a structure that omits a slip ring, and thus, allows the CT system  140  to have downsized smaller size and a higher time-resolution, relative to at least some conventional CT systems. With the CT system  140  according to example embodiments, even higher speed 3D imaging for moving organs such as a beating heart called a cardiac angiography, may be realized. Further, additional example applications disclosed herein include, but are not limited to, a positron emission tomography (PET), a single photon emission CT (SPECT) using radioactive materials (e.g., radio isotopes (RI) or positron nuclide), and an optical topography using near-infrared (NIR) light source. In the case of PET apparatus, image sensors  40   c  placed all around the object so as to detect two photons generated when a positron is pair-annihilated with electrons by collapse of a radioactive isotope which doses test objects. Unlike a conventional CT system using X-ray source, according to the additional example applications, even higher time and space resolutions enable the detection of the temporally brief and microscopic phenomena of a positron or a single photon emission. 
       FIG. 5( a )  is a diagram illustrating a plan view of a single silicon die (i.e., chip) image sensor  40   d  being shaped so as to form a circular hole (e.g., a cylindrical opening) inside the die, thereby exposing a curved face  44   d  within the silicon substrate of the die such that the curved face  44   d  defines the circular hole and has the shape of an interior curved surface of a hollow right cylinder. According to at least some example embodiments, the curved face  44   d  is covered by a thin silicon dioxide layer  35  which may reduce crystal defects caused by a mechanical or a thermal stress, and may prevent contaminations from incoming metal impurities or reactive chemicals. As is illustrated in  FIG. 5( a ) , the external shape of the die of the image sensor  40   d  may be a rectangle. The four edges of the rectangular exterior of the image sensor  40   d  may not be flat or smooth, and may have a lot of crystal defects due to a wafer dicing process, which may not affect the image quality of images generated based on the image sensor  40   d  because, as will be discussed in greater detail below, the photo-diodes and peripheral circuits of the image sensor  40   d  may be formed away from the above-referenced four edges. An object or a patient (not shown in this figure) may be placed inside the hollow formed in the die of the image sensor  40   d  (i.e., inside an interior of the cylindrical shape formed by the curved face  44   d  such that the curved face  44   d  encircles the object of patient). An X-ray beam that travels through an object or a patient being analyzed, or a gamma-ray from the nuclides inside dosed into the object or the patient, for example, may be detected by the circularly arrayed photo-diodes  33 . The circularly arrayed photo-diodes  33  are formed inside the silicon substrate of the die of the image sensor  40   d  radially from the center of the circle. For example, according to at least some example embodiments, each photo-diode  33  of the image sensor  40   d  may be formed radially in the image sensor  40   d  so as to extend from the curved face  44   d  into the image sensor  40   d  in a direction parallel to a straight line between a center of the cylindrical opening and the photo-diode  33 . In the same manner discussed above with reference to  FIGS. 3( a ) and 3( c ) , the plurality of photo-diodes  33  of the image sensor  40   d  may be included in a plurality of corresponding pixel circuits  20  (not illustrated in  FIG. 5( a ) ), respectively, so as to form a monolithic array of pixels  20  configured to receive light that travels through or from an object located in a central portion of the cylindrical opening at the center of the image sensor  40   d . In addition, the curved surface  44   d  may be viewed as being concave in shape and encircling the object  7 , where incident beam angles on each concave side may become even or nearly equal in length. On the peripheral region of the die of the image sensor  40   d , a signal read-out and scanning circuit  21 , a timing pulse generator circuit  23 , an AD convertor circuit  25 , a digital signal processing circuit  27 , and an interface circuit  29  are formed. Input or output terminals  32 , in the image sensor  40   d , are micro pads formed above one or more through-silicon via(s) (TSVs) which may be referred to herein as TSV structure. As is discussed in greater detail below, multiple image sensor  40   d  may be stacked on each other successively to increase the number of pixels in a direction parallel to the view direction of  FIG. 5( a )  (i.e., a direction parallel to the Z-axis). Electrical connections between multiple image sensors  40   d  stacked on each other may be accomplished by the TSV structure with wiring lengths that are minimized or, alternatively, reduced with respect to at least some conventional CT systems, unlike image sensors mounted in at least some conventional CT systems where each image sensor is electrically connected by a large number of long metal wirings between image sensors and supporting circuits on each print-circuit board (PCB) of the convention CT systems. According to at least one example embodiment, the image sensor  40   d  may be included in an imaging unit of a CT system in a manner that is the same as (or similar to) that discussed above with respect to the circularly-arranged image sensors  40   c  included in the imaging unit  1   c  of CT system  140  illustrated in  FIG. 4( c ) . For example, according to at least some example embodiments, a CT system including the image sensor  40   d  may include the electron beam gun  3   a , focusing and deflection coil  3   b , and target ring  3   d , each of which may have the same arrangement and operation as those discussed above with respect to  FIG. 4( c ) . Accordingly, X-rays beams generated as a result of an electron beam (e.g., electron beam  3   c ) irradiating the target ring  3   d  may be incident on an object located in the hollow center of the image sensor  40   d , and resulting light that passes through or from the object may be incident on pixels  20  located on the curved face  44   d  of the image sensor  40   d.    
     The die size of the die of the image sensor  40   d  or the diameter of the circular hollow inside the die of the image sensor  40   d  may depend on the size of a silicon wafer being used to create the die, which is rectangular. Using a twelve inch silicon wafer, the length of a diagonal line between a first corner of the rectangular die and a second corner of the rectangular die that as cater-corner to the first corner may be less than twelve inches, and then the diameter of the circular hollow may be six to seven inches, for example. With such a configuration, in addition to the above advantageous effects, all the photo-diodes and peripheral circuits are formed in a single die which may reduce, or alternatively, minimize its sensitivity variations, signal delay times and power consumption or the die temperature. The CT system using the single die image sensor  40   d , according to the embodiment, can be relatively small and portable, and may be suitable for applications like analyzing smaller objects like a part of a body or breast, performing diagnoses for small animals, or industrial uses like a three dimensional (3D) scanner that provides 3D image data to a 3D printer, for example. A cross sectional view taken along a dotted line A-A′ illustrated on the image sensor  40   d  in  FIG. 5( a )  is explained below, with reference to  FIG. 9( a ) . 
       FIG. 5( b )  is a diagram illustrating a plan view of a single silicon die (chip) image sensor  40   e  having a donut shape, such that the silicon substrate of the die has a curved outer surface  43   e  and a curved inner surface  44   e , each of which is covered by a thin silicon dioxide layer  35 . The thin silicon dioxide layer  35  may reduce crystal defects caused by a mechanical or a thermal stress, and prevent contamination by incoming metal impurities or reactive chemicals. Like the image sensor  40   d  of  FIG. 5( a ) , the image sensor  40   e  of  FIG. 5( b )  may include photo-diodes  33  arranged in a circular array. In the same manner discussed above with reference to image sensor  40 ( d ) of  FIG. 5( a ) , the plurality of photo-diodes  33  of the image sensor  40   e  may be included in a plurality of corresponding pixel circuits  20  (not illustrated in  FIG. 5( b ) ), respectively. Further, the photo-diodes  33  may be formed radially in the same manner discussed above with respect to image sensor  40   d  of  FIG. 5( a ) . Unlike the image sensor  40   d  shown in  FIG. 5( a ) , the external shape of the die of the image sensor  40   e  is circular and covered by a thin silicon dioxide layer  35 . Further, input or output terminals  32  and peripheral circuits including, for example, a signal read-out and scanning circuit  21 , a timing pulse generator circuit  23 , an AD convertor circuit  25 , a digital signal processing circuit  27 , and an interface circuit  29 , are formed close to the outer edge. 
     Using a twelve inch silicon wafer, for example, the outer diameter of the donut shape may be less than twelve inches but the inner diameter of the donut shape may be more than ten inches. With such a configuration, in addition to the above advantageous effects, a CT system including the image sensor  40   e , according to at least some example embodiments, can be relatively small and portable, and may be suitable for applications like dental tomography and brain topography, analysis of smaller objects like a part of a body or breast, diagnoses for small animals, or industrial use like a three dimensional (3D) scanner that provides 3D image data to a 3D printer, for example. According to at least one example embodiment, the image sensor  40   e  may be included in an imaging unit of a CT system in a manner that is the same as (or similar to) that discussed above with respect to the circularly-arranged image sensors  40   c  included in the imaging unit  1   c  of CT system  140  illustrated in  FIG. 4( c ) . For example, according to at least some example embodiments, a CT system including the image sensor  40   e  may include the electron beam gun  3   a , focusing and deflection coil  3   b , and target ring  3   d , each of which may have the same arrangement and operation as those discussed above with respect to  FIG. 4( c ) . Accordingly, X-rays beams generated as a result of an electron beam (e.g., electron beam  3   c ) irradiating the target ring  3   d  may be incident on an object located in the hollow center of the image sensor  40   e , and resulting light that passes through or from the object may be incident on pixels  20  located on the curved face  44   e  of the image sensor  40   e.    
       FIG. 6( a )  is a diagram illustrating a perspective view of a sensor module  200  which includes three image sensors  40   n  stacked on each other, for example. Electrical communications between the dies of the image sensors  40   n  may be achieved, for example, by TSV structures. According to at least some example embodiments, some or all of the stacked image sensors  40   n  of the sensor module  200  may be implemented by image sensors having the structure of the image sensor  40   a  illustrated in  FIG. 3( a )  and/or image sensors having the structure of the image sensor  40   m , which will be discussed in greater detail below with reference to  FIG. 10( a ) . As is illustrated if  FIG. 6( a ) , the image sensor  40   n  may be stacked in such a manner as to form a concave face of the image sensor module  200 . The concave face of the image sensor module  200  may face an object to detect incident X-ray beams that pass through the object and are incident on the concave face. The plural image sensors  40   n  may be stacked in a direction parallel to the Z-axis in the CT system (e.g., a direction in which a patient or object being analyzed by the CT system is moved during analysis in a CT system), according to at least some example embodiments. With the configuration described above with reference to  FIG. 6( a ) , in addition to the above advantageous effects, a pixel number or so-called a slice number can be increased, which may be useful for implementing a multi-slice CT system or cone beam type CT system with a small form factor, a high image quality, and a high speed image recognition, for example. According to at least some example embodiments, image sensor module  220  described in greater detail below with reference to  FIG. 7( a )  is an example of the manner in which image sensors  40   n  of the image sensor module  200  may be stacked. 
       FIG. 6( b )  is a diagram illustrating a perspective view of an image sensor module  210 , where three image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3  are stacked, for example. The image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3  may be stacked in a direction parallel to the Z-axis in the CT system (e.g., a direction in which a patient or object being analyzed by the CT system is moved during analysis in a CT system), according to at least some example embodiments. Electrical communications between the dies of the image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3  may be achieved by the TSV structures. Each image sensor, among image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3 , may have different optical or electrical characteristics such that a CT system using image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3  can gather respectively different optical information from image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3 . Each image sensor among image sensors  40   f - 1 ,  40   f - 2  and  40   f - 3  may have different spectral characteristics against an incident light, or different spatial resolution which is inversely proportional to light sensitivity, for example. With the configuration described above with respect to  FIG. 6( b ) , in addition to the above advantageous effects, a CT system, according to the embodiment, may have a hybrid imaging capabilities which enable multi-diagnoses using different light sources like different energy X-rays and/or NIR lights of multiple wavelengths, for example. According to at least some example embodiments, some or all of the stacked image sensors  40   f - 1 ,  40   f - 2 , and  40   f - 3  of the sensor module  210  may be implemented by image sensors having the structure of the image sensor  40   d  illustrated in  FIG. 5( a )  and/or image sensors having the structure of the image sensor  40   e  illustrated in  FIG. 5( b ) . According to at least some example embodiments, image sensor module  220  described in greater detail below with reference to  FIG. 7( a )  is an example of the manner in which image sensors  40   f - 1 ,  40   f - 2 , and  40   f - 3  of the image sensor module  210  may be stacked. 
     According to at least one example embodiment, the image sensors  40   f - 1 - 3  may be included in an imaging unit of a CT system in a manner that is the same as (or similar to) that discussed above with respect to the circularly-arranged image sensors  40   c  included in the imaging unit  1   c  of CT system  140  illustrated in  FIG. 4( c ) . For example, according to at least some example embodiments, a CT system including the image sensors  40   f - 1 - 3  may include the electron beam gun  3   a , focusing and deflection coil  3   b , and target ring  3   d , each of which may have the same arrangement and operation as those discussed above with respect to  FIG. 4( c ) . Accordingly, X-rays beams generated as a result of an electron beam (e.g., electron beam  3   c ) irradiating the target ring  3   d  may be incident on an object located in the hollow center of the image sensors  40   f - 1 - 3 , and resulting light that passes through or from the object may be incident on pixels located on the curved faces of the image sensors  40   f - 1 - 3 . 
     According to at least some example embodiments, the implementation of portable CT systems may be facilitated when using image sensors like the image sensors  40   c ,  40   d ,  40   e , and  40   f - 1 - 3  of  FIGS. 4( b ), 4( c ), 5( a ), 5( b ), and 6( b ) , due to the relative ease with which a target ring (e.g., target ring  3   d ) can be attached to the flat, or alternatively, relatively flat faces of the image sensors  40   c ,  40   d ,  40   e  and  40   f - 1 - 3  (e.g., the flat faces, surfaces of which are perpendicular to a central axis of the hollow cylindrical spaces at the centers of the image sensors  40   c ,  40   d ,  40   e , and  40   f - 1 - 3 ). 
       FIG. 7( a )  is a diagram illustrating cross sectional views of an image sensor module  220  used in a CT system with respect to the X-Z plane and the Y-Z plane, according to at least one example embodiment. As is illustrated in  FIG. 7( a ) , the image sensor module  220  may include a plurality of image sensors  40   g  stacked on each other. In the example shown in  FIG. 7( a ) , the plurality of image sensors  40   g  includes three image sensors  40   g - 1 ,  40   g - 2 , and  40   g - 3  are stacked in a direction parallel to the Z-axis to form an image sensor module  220 . According to at least some example embodiments, each of image sensors  40   g - 1 ,  40   g - 2 , and  40   g - 3  of the image sensor module  220  may have the structure of one of image sensors  40   a ,  40   b ,  40   c ,  40   d ,  40   e  discussed above, or image sensors  40   k  or  40   m  which will be discussed in greater detail below with reference to  FIGS. 8( b ) and 10( b ) . The left hand side and right hand side of  FIG. 7( a )  show cross sectional views with respect to a view direction parallel to the Y-axis and a view direction parallel to the X-axis, respectively. As shown in the left hand side of  FIG. 7( a ) , pixels  20  form a pixel array two dimensionally in the X-Z plane. As shown in the right hand side of  FIG. 7( a ) , image sensor  40   g - 2  is stacked on image sensor  40   g - 1  on which a non-conductive interlayer  53  is coated. Similarly, the image sensor  40   g - 3  is stacked on image sensor  40   g - 2  on which a non-conductive interlayer  53  is coated. TSVs  51  are formed in each image sensor  40   g - 1 ,  40   g - 2 , and  40   g - 3 , in which conductive materials like aluminum, copper or doped poly-silicon, for example, may be embedded. Between each substrate (e.g., each silicon substrate of each silicon die of image sensors  40   g - 1 ,  40   g - 2 , and  40   g - 3 ), micro bumps  55  are sandwiched to connect neighboring TSVs  51  electrically. According to at least some example embodiments, micro-pads  32  (not shown in this figure) may be inserted between the TSVs  51  and the micro-bumps  55 . 
     It should be noted that, according to at least some example embodiments, the distances between the pixels  20  horizontally and vertically may be chosen so as to achieve desirable levels of quality for reconstructed images generated by a CT system including image sensor module  220 . According to at least some example embodiments, the horizontal distance Dh may be determined by a fine micro fabrication technology like a photo-lithographic process with the highest accuracy. The vertical distance Dv may depend on the thickness Dt of the silicon substrate of one or more of image sensors  40   g - 1 ,  40   g - 2 , and  40   g - 3  and the thickness of the non-conductive interlayer  53 . The silicon substrate thickness Dt may be controlled and determined by a chemical and mechanical polishing (CMP) process applied to the backsides of the silicon wafers upon which the dies of image sensors  40   g - 1 ,  40   g - 2 , and  40   g - 3  are created with an accuracy of 0.1 micron meter or less, for example. The thickness of the non-conductive interlayer  53  may be accurately controlled and determined by a chemical vapor deposition (CVD) technique during the image sensor manufacturing process with an accuracy of 0.1 micron meter or less, for example. There are at least two approaches to determine the values of the horizontal distance Dh and vertical distance Dv, respectively. With respect to the first approach, assuming a value of the horizontal distance Dh is fixed, the silicon substrate thickness Dt may be controlled by a CMP process to meet the requirement of the horizontal distance Dh being equal to vertical distance Dv, for example. According to at least one example embodiment, the horizontal distance Dh may be larger than the substrate thickness Dt. Using the CMP, the substrate thickness Dt may be thinned accurately down to 100 to several microns taking into account of the thickness of the non-conductive interlayer  53 . With respect to the second approach, assuming a value of the vertical distance Dv is fixed, the horizontal distance Dh may be determined by a photo-mask pattern to meet the requirement of horizontal distance Dh being equal to the vertical distance Dv, for example. Unlike at least some conventional CT systems that combine multiple image sensors mechanically, where the distance between the pixels inside an individual image sensor may be different from the distance of between adjacent one of the multiple image sensors in the X-Y plane. With such a configuration, in addition to the above advantageous effects, a high spatial resolution with an accuracy of 0.1 micron or less for example, in directions parallel to the X, Y, and Z-axis can be realized, and then a capillary vessel or a brain tissue images, for example, may be clearly reconstructed by a relatively smaller-sized CT system. 
     According to at least some example embodiments, an area of each pixel  20  may be 100 (10 μm by 10 μm) to 10000 (100 μm by 100 μm) μm 2 , for example. In the discussion of  FIG. 7( a )  above, according to at least some example embodiments, the vertical distance Dv is discussed as being equal to the horizontal distance Dh. Alternatively, the vertical distance Dv and horizontal distance Dh may be substantially equal to each other. According to at least some example embodiments, “substantially equal” refers to a difference between the vertical distance Dv and horizontal distance Dh that is, for example, less than 2 microns or less than 1 micron. According to at least some example embodiments, “substantially equal” refers to a difference between the vertical distance Dv and horizontal distance Dh that is, for example, no more than 0.02 microns or no more than 0.01 microns. 
       FIG. 7( b )  is a diagram illustrating cross sectional views of an image sensor module  230  used in a CT system with respect to the X-Z plane, according to at least one example embodiment. Three image sensors  40   h  are stacked in the direction parallel to the Z-axis, and metal optical shields  57  are inserted between each adjacent pair of stacked sensors  40   g  to form the image sensor module  230 . According to at least one example embodiment, each metal optical shield  57  may be composed of one or more materials, examples of which include, but are not limited to, metals including heavier metal atoms like tungsten or lead, in order to attenuate unexpected incident light with wrong directions or scattered X-ray beams. With such a configuration, in addition to the above advantageous effects, even higher image qualities of less cross-talk noise and artifact, for example, may be obtained. By inserting metal shield layers  57  between the image sensors  40   g  in the manner described above, the temperature of the image sensor module  230  may be kept lower in comparison to an image sensor module that does not include the metal shield layers in-between adjacent stacked image sensors, because the inserted metal shield layer may effectively release the heat generated inside the stacked image sensors  40   g  of the image sensor module  230 , which may reduce a dark noise and improve a signal to noise ratio (S/N), thus allowing for even lower X-ray doses for an object or a patient being analyzed by a CT system including the image sensor module  230 . According to at least some example embodiments, each of the image sensors  40   h  of the image sensor module  230  may have the structure of one of image sensors  40   a ,  40   b ,  40   c ,  40   d ,  40   e  discussed above, or image sensors  40   k  or  40   m  which will be discussed in greater detail below with reference to  FIGS. 8( b ) and 10( b ) . 
       FIG. 7( c )  is a diagram illustrating a side view of the image sensor module  240  used in a CT system, according to at least one example embodiment. As is illustrated in  FIG. 7( c ) , the image sensor module  240  includes four image sensors  40 - 4 ,  40 - 5 ,  40 - 6  and  40 - 7  stacked successively in a direction parallel to the Z-axis. As shown in  FIG. 7( c ) , incident x-ray beam  8  reaches upper faces of the image sensors  40 - 4 ,  40 - 5 ,  40 - 6  and  40 - 7 . In the example illustrated in  FIG. 7( c ) , a color filter  59 , which selectively transmits or blocks a certain range of light wavelengths to improve image quality like sharpness or energy resolutions, is laminated on the top of image sensor  40 - 4 , for example. In the example illustrated in  FIG. 7( c ) , nothing, except a surface thin silicon dioxide layer and a thin impurity doped layer (neither of which is illustrated in  FIG. 7( c ) ), is laminated on the top of image sensor  40 - 5 , which may detect incident beams like a soft X-ray or an alpha-ray, for example. In the example illustrated in  FIG. 7( c ) , a near-infrared (NIR) filter  61 , which selectively transmits NIR light and blocks visible light from the surroundings to improve spectral resolutions, is laminated on the top of image sensor  40 - 6 , for example. In the example illustrated in  FIG. 7( c ) , a scintillator  63 , which may effectively convert an incident X-rays to visible light of a known wavelength, is laminated on the top of image sensor  40 - 7 . According to at least one example embodiment, the scintillator  63  is embodied by an organic scintillator like anthracene or stilbene, for example, which may be laminated on the image sensor by a known coating and etching process. According to at least some example embodiments, X-rays, which penetrate through the scintillator  63  without emitting a light, may be also detected by the image sensor  40 - 7  as explained above. 
     With the configuration described above with reference to  FIG. 7( b ) , in addition to the above advantageous effects, a CT system using the image sensor module  240 , according to at least some example embodiments, may have a hybrid imaging capabilities which enable multi-diagnosis capabilities by using different light sources like different energy X-rays and/or NIR lights of multiple wavelengths, for example. 
       FIG. 8( a )  is a diagram illustrating a schematic drawing of an image sensor arrangement inside an imaging unit of a CT system. An image sensor  40   j  is placed along the trace of a helix extending in the direction of Z-axis as indicated an arrow of solid line which does not exist but is just for explanation. For example, the imagining unit in which the image sensor  40   j  is included may be structured such that the image sensor  40   j  follows the helical path illustrated by the solid arrow illustrated in  FIG. 8( a ) . As is illustrated in  FIG. 8( a ) , at least one face of the image sensor  40   j  may face a center of the helical path. The face of the image sensor  40   j  that faces the center of the helical path may have a modified concave in shape and some mechanical deformation. According to at least one example embodiment, more than two image sensors  40   j  may be located approximately along the helix. With such a configuration, according to the embodiment, CT system may have advantageous effects of both helical and multi-scan system capable of a high speed imaging and a low radiation exposure. The spatial resolution in the direction of Z-axis may be improved when a moving speed of an object being analyzed by a CT system in which the image sensor  40   j  is included with respect to a direction parallel to the Z-axis is equal to the moving speed of the image sensor  40   j  in the direction parallel to the Z-axis while an imaging unit in which the image sensor  40   j  is included (not illustrated in  FIG. 8( a ) ) is rotating around the object. 
       FIG. 8( b )  is a diagram illustrating a plan view of an image sensor  40   k  which, according to at least some example embodiments, may provide at least some benefits over helix structures used in imaging units of at least some CT systems. The photo-diodes  33 , peripheral circuits  21 ,  23 ,  25 ,  27 ,  29  and micro-pads  33  may be formed on a single die, for example, in the same manner discussed above with respect to image sensor  40   e  of  FIG. 5( b ) . According to at least some example embodiments, the image sensor  40   k  may have the same structure as that described above with respect to the image sensor  40   e , with the exception that the silicon substrate of the silicon die of the image sensor  44   e  includes a slit or gap  65 . The slit  65  may facilitate deformation the image sensor  40   e  into the shape of a helix or a section of a helix, for example. As is explained above with reference to the image sensor  40   e , both outer and inner faces  43   k  and  44   k  of the silicon substrate of the image sensor  40   k  may be covered (e.g., coated) by thin silicon dioxide layers  35 . The inner face  44   k  does not form a closed circle but may form a part of helix around an object located in a region central to the helical shape formed by the image sensor  40   k    
       FIG. 8( c )  is a diagram illustrating a side view of two stacked image sensor  40   k  engaged with edges of a holding member  67  inside an imaging unit  1   k . In the example shown in  FIG. 8( c ) , the image sensors  40   k  shown in  FIG. 8( b )  are stacked on each other and, together, form the shape of a helix that extends in a direction parallel to the Z-axis. The holding member  67  may be a structure formed such that interior edges of the holding member  67  have a shape of screw threads inside the cylindrical tube. The screw-thread-shaped edges of the holding member  67  may be shaped so as to mesh with edges of the helix-shaped image sensors  40   k , in the manner shown in  FIG. 8( c ) . According to at least some example embodiments, the relationship between the stacked image sensors  40   k  and the holding member  67  is like that of screw and nut. With the configuration described above with reference to  FIG. 8( c ) , according to at least some example embodiments, an image sensor  40   k  or a plurality of image sensors  40   k  stacked together may be are accurately fixed in a helical shape inside the imaging unit  1   k , and may be replaced with a new image sensor easily. As is shown in  FIG. 8( c ) , the image sensor  40   k  may have a shape of a helix that winds in such a manner as to define a cylindrical space about which the helix is wound. Further, the curved face  44   k  may face the cylindrical space such that an array of pixels  20  formed on the curved face  44   k  may face an object located in a central portion of the cylindrical space and receive light that travels through or from the object located in a central portion of the cylindrical space. According to at least one example embodiment, the image sensor  40   k  may be included in an imaging unit of a CT system in a manner that is similar to that discussed above with respect to the circularly-arranged image sensors  40   c  included in the imaging unit  1   c  of CT system  140  illustrated in  FIG. 4( c ) . For example, according to at least some example embodiments, the imaging unit  1   k  may include the target ring  3   d . Further, a CT system including the imaging unit  1   k  may include the electron beam gun  3   a  and focusing and deflection coil  3   b . The electron beam gun  3   a , focusing and deflection coil  3   b  and target ring  3   d  may each have the same arrangement and operation as those discussed above with respect to  FIG. 4( c ) . Accordingly, X-rays beams generated as a result of an electron beam (e.g., electron beam  3   c ) irradiating the target ring  3   d  may be incident on an object located in the hollow cylindrical center of the helical image sensors  40   k , and resulting light that passes through or from the object may be incident on pixels located on the curved faces  44   k  of the helical image sensors  40   k  of the imaging unit  1   k.    
       FIG. 9( a )  is a diagram illustrating a cross sectional view taken along a dotted line A-A′ of the image sensor  40   d  shown in  FIG. 5( a )  according to at least one example embodiment. An X-ray beam  8 , illustrated as entering from a left side of  FIG. 9( a ) , for example, may irradiate a face of a silicon substrate  69 . According to at least some example embodiments, the silicon substrate  69  may be a p-type silicon substrate. The X-ray beam  8  penetrates into a photo-diode  33 . The photo-diode  33  may be fully depleted inside the silicon substrate  69 . The X-ray beam  8  that penetrates into the photo-diode  33  may be read out in a photo-electrical conversion region  34 . As will be discussed in greater detail below with reference to  FIG. 9( b ) , the photo-electrical conversion region may include the photo-diode  33  and a floating gate amplifier  91 . The silicon dioxide layer  35 , which may be thermally grown in an oxidizing atmosphere, is formed on the face of the substrate  69  upon which the X-ray beam  8  may be incident. On the p-type substrate  69 , n-well  71  is formed, and p-well  73  is formed on the n-well  71 , where CMOS peripheral circuits may be formed. According to at least some example embodiments, the silicon substrate  69  is covered by a thin gate silicon dioxide  75  and thick nonconductive layer  81 . The backside of the substrate  69  may have a thin P +  doped region  83  adjacent to a thin silicon dioxide layer  85 . According to at least some example embodiments, a thin P +  doped region  79  is located between the thin silicon dioxide layer  35  and the photo-diode  33 . The thin P +  doped region  79  and the thin silicon dioxide layer  35  may prevent crystal defects and impurity contaminations. A metal optical shield  57  including, for example, tungsten atoms, is laminated on the backside of the substrate  69 . The advantageous effects of the metal optical shield  57  are already mentioned above with reference to  FIG. 7( b ) . A face of the substrate  59  near the micro-pad  32  may have a rough surface  77  which may be caused by a dicing saw, for example. The n-well  71  and the p-well  73 , where peripheral circuits may be formed, are separated from the rough surface  77  such that the micro pad  32  is between the rough surface  77  and n-well  71 /p-well  73 . With such a configuration, a conventional dicing saw may be used for the wafer dicing process without degrading the sensor characteristics caused by crystal defects or mechanical stress. 
       FIG. 9( b )  is a diagram illustrating an enlarged portion of the photo-electrical conversion region  34  illustrated in  FIG. 9( a ) . The photo-diode  33  may include an n-type impurity region on the silicon substrate  69  and may extend in a direction parallel to a longer side of the silicon substrate  69  as illustrated in  FIGS. 9( a ) and 9( b )  (i.e., a direction parallel to incident beam  8  or perpendicular to the Z-axis). The length of the photo-diode  33  in the direction parallel to the longer side of the silicon substrate  69  may have a range of one micron to ten millimeters, for example, which effectively enables photo-detections of infrared light and/or X-rays, for example. Further, according to at least some example embodiments, the photo-diode  33  including an n-type impurity region on the silicon substrate  69  may also extend radially toward the incident beam  8  in the X-Y plan view as shown in  FIGS. 5( a ), 5( b ) and 8( b ) , for example. According to at least some example embodiments, the photo-diode  33  illustrated in  FIG. 9( b )  may be one of a plurality of photo-diodes, and may be electrically separated from neighboring photo-diodes by a device isolation region including a highly P +  doped region (not illustrated in  FIG. 9( a )  or  FIG. 9( b ) ), for example. A wiring group of four-phase driving pulses  87  may be formed above the photo-diode  33  in order to transfer generated charges toward a floating gate amplifier  91  in a manner that is the same as or, alternatively, similar to, known CCD image sensor operations. In the example shown in  FIG. 9( b ) , the charge transfer gate electrodes  88  are formed on the thin gate silicon dioxide layer  75  in a direction parallel to the incident beam  8 . Conventional CCD or MOS image sensors, at least some of which require a thick silicon substrate of more than 500 micron-meters to detect incident lights like X-rays in the direction parallel to the incident X-rays, may need a high voltage of more than 100 volts or higher in order to deplete the silicon substrate almost down to the bottom of the substrate. Moreover, unlike conventional CCD or MOS image sensors, in the image sensor structures shown  FIGS. 9( a ) and 9( b )  neither an electrode nor a transistor (either of which may attenuate the incident light  8 ), exists in the optical path through which the incident light  8  travels to reach the photodiode  33 . According to at least some example embodiments, only the thin P +  doped region  79  and/or the thin silicon dioxide layer  35  are located along the optical path through which the incident light  8  travels to reach the photodiode  33 . As a result, an image sensor that has a higher sensitivity, has less variation, and generates fewer false signals (e.g., artifacts) may be obtained. The detection efficiency of an incident light may depend on the light energy and the distance that the light traveled through the silicon substrate in the direction parallel to a direction in which the array of the charge transfer gate electrodes  88  are arranged as shown in  FIG. 9( b ) . When operating in a CCD mode, for example, each charge packet data has information on each position above an array of photo-diode  33 , and thus, it may become possible to analyze the incident light energy as a function of the position or the distance from the side of the image sensor. 
     Initially, the electric potential of the floating gate may be reset to that of the reset drain  95  by switching the reset gate  93  (e.g., by allowing charge carriers to flow through a channel of the reset gate  93 , which may be an MOSFET). Light exposure may start immediately after the photo-diode  33  is fully depleted by the four-phase driving pulses  87 . Electric charges generated and collected in each packet may be transferred toward the floating gate amplifier  91 , and finally the collected charges are read out into the floating gate of the floating gate amplifier  91  by switching the signal read-out gate  89  (e.g., by allowing charge carriers to flow through a channel of the read-out gate  89 , which may be a MOSFET). With such the configuration described above with reference to  FIG. 9( b ) , the photo-diode  33  may be fully depleted along the incident light path without using a thick silicon substrate of more than 200 micron meters, and without applying a high voltage of more than 20 volts, for example. 
       FIG. 10( a )  is a diagram illustrating a plan view and a block diagram of an image sensor  40   m  used in the CT system according to at least one example embodiment. Similar to the configuration of the image sensor  40   a  illustrated in  FIG. 3( a ) , peripheral circuits  21 ,  23 ,  25 ,  27 ,  29  are located between an array of photo-diodes  33  and micro-pads  32 . The micro-pads  32  may be used in stacked sensor structure. The face of the sensor  40   m  being exposed to an incident light may be concave in shape like concave face  44   a  of image sensor  40   a  illustrated in  FIG. 3( a ) . As is illustrated in  FIG. 10( a ) , the photo-diodes  33  of the image sensor  40   m  may be formed radially in a fan-shaped array to detect radial incident lights. According to at least one example embodiment, the photo-diodes  33  of the image sensor  40   m  each extend from the curved face of the image sensor  40   m  into the image sensor in a direction parallel to a direction in which a beam of light traveling in a straight line from an X-ray source to the photo-diode would travel. The circuit blocks  37  and  39  may be an on chip buffer memory and a data compression circuit, respectively. With the configuration discussed above with reference to  FIG. 10( a ) , the image data after analog to digital conversion by the circuit block  25  may be compressed down to size between one third and one fifth of an original size of the image data without data loss, for example, and thus the data transfer bit rate may be decreased. 
       FIG. 10( b )  is a diagram illustrating a perspective view of an image sensor module used in the CT system according to at least one example embodiment. Similar to the configuration of the image sensor module  200  illustrated in  FIG. 6( a ) , the image sensor module  250  includes three image sensors  40   m  as shown in  FIG. 10( a ) , for example. In addition, an image processing chip  45  is stacked on the top of the uppermost image sensor  40   m  of the image sensor module  250 . Between the image processing chip  45  and the uppermost image sensor  40   m , micro-pads and micro-bumps (neither of which are illustrated) may be formed to enable electrical communication each other. Though,  FIG. 10( b )  is illustrates image sensor module  250  as including the processing chip  45  stacked atop a plurality of image sensors  40   m  of  FIG. 10( a )  stacked on each other, according to at least some example embodiments, the image sensor module  250  may include the processing chip  45  stacked atop a plurality of image sensors  40   a  of  FIG. 3( a )  stacked on each other. 
       FIG. 10( c )  is a diagram illustrating a block diagram of the image processing chip  45  according to at least one example embodiment. A signal control circuit  11 , a multiplexer circuit  12 , a data buffer circuit  13  and a parallel to serial converter circuit  14  are formed on the image processing chip  45 , for example. 
       FIG. 10( d )  is a diagram illustrating a plan view of an imaging unit  1   d  including the image sensor module  250  of  FIG. 10( b ) , and an image processing unit  2  used in the CT system  150  according to at least one example embodiment. The CT system  150  also includes an X-ray source  3 , and an object or patient  7 . The X-ray fan beam  8  may be collimated by a collimator (not illustrated) to form a defined fan beam angle (not illustrated), for example. The X-ray source  3  projects an X-ray fan beam  8 , for example, through the object  7  to be detected by the image sensor module  250 . The imaging unit  1   d  and the X-ray source  3  are located in an opposite position inside a gantry. The gantry rotates around the object  7  taking a slice image, and the object is moved in a direction parallel to the Z-axis in order to allow the CT system  150  to obtain additional slice images. The semiconductor surface of the image sensor module  250 , where integrated circuits are formed, may be perpendicular to the Z axis. As  FIGS. 10( d ) and 10( b )  illustrate, the image sensor module  250  may include a concave face. The concave face of the image sensor module  250  may face an object  7  in order to detect incident X-ray fan beam  8 . The imaging unit  1   d  including the image sensor module  250  which outputs compressed digital image data may be electrically connected with the image processing unit  2  via the slip-ring  4 , for example. When compressed image data is received through the slip-ring  4 , a network interface circuit  15  may assign the received data to other circuit blocks such as a central processing unit (CPU)  16  which may perform a reconstruction of a slice image, for example. Either raw or processed image data may be read or written to or from a memory disk unit  18  by a disk controller circuit  17 . Image data with other related information may be monitored by, for example, a doctor via an image output device  19  such as a LCD screen. With the configuration described above with respect to  FIG. 10( d ) , compressed image data obtained from the stacked image sensors may be directly sent to the upper image processing chip  45  using electrical paths (which may be reduced or, alternatively, minimized in terms of number and/or length) by TSVs and without discrete high speed line drivers or buffers between the chips. As a result, high speed data transfer and a data processing, and lower power consumption, relative to at least some conventional CT systems, may be achieved by the CT system  150 . In example illustrated in  FIG. 10( d ) , the circuit blocks  37  and  39  are integrated on each image sensor  40 . According to at least some example embodiments, the circuit blocks  37  and  39  may be integrated on the upper image processing chip  45 , which may allow die size and/or data management to be improved or, alternatively, optimized for each of the image sensors  40   m  of the image sensor module  250 . 
     As is explained above, the CT systems according to at least some example embodiments enable the discovery of a disease or other conditions harmful to the health of a patient in the early stages, and as a result, reduce medical expenses. The CT systems according to at least some example embodiments may provide not only increased sensitivity, lower X-ray dose to patients being analyzed by the CT systems and increased image quality, but also a high data transfer rate with low power consumption, small or portable form factor, reduced maintenance load, and a lower amount of influence by environmental factors experienced by the CT systems like temperature, humidity and mechanical vibrations are obtained. In addition, the CT system according to at least some example embodiments provide high spatial, time and energy resolution enabling three dimensional (3D) imaging with a variety of valuable biomedical information on carcinoma tissue and capillary vessels around a beating heart, for example. 
     Though example embodiments of CT systems are described above in the context of medical image systems, the CT systems according to at least some example embodiments are not limited to medical applications, and may also be suitable for applications including X-ray phase shift imaging using, for example, a micro-focused X-ray source, various industrial X-ray imaging systems, X-ray astronomy, and high energy physics applications. 
     Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.