Patent Publication Number: US-9414793-B2

Title: X-ray CT system

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to X-ray CT systems. 
     BACKGROUND ART 
     A conventional X-ray CT system detects X-rays emitted from an X-ray tube and transmitted through the subject, and reconstruct images based on detection results to obtain an X-ray tomographic image. In the X-ray CT system, the X-ray tube is provided inside an annular rotating body, whose center is a through-opening, into which a bed with the subject can be inserted. The annular rotating body is circumferentially shielded with a cover, which comprises a tubular case part for shielding the annular rotating body, the cover extending from the central side of the opening. Moreover, the tubular case part is provided with an X-ray transmission opening, where X-rays from the X-ray tube pass through. 
     The X-ray transmission opening is, however, closed with a sheet member for the purpose of ensuring safety. The sheet member prevents the subject from coming into contact with the annular rotating body, and it also protects the interior of the annular rotating body against penetration of blood or contrast agents. Furthermore, the sheet member prevents noise from coming out of the inside of the cover. The noise inside the cover includes a wind noise caused by the rotation of the annular rotating body and the sound of the motor driving it. 
     The sheet member comprises a thin film-like material having a high transmittance for X-rays and a laser beam that is used for marking the subject. This can suppress deterioration in the quality of images acquired by radiography. 
     As another type of medical diagnostic apparatus, for example, an MRI system, is provided with a sound absorption material that is applied as a lining to the internal surface of the cover. The silencing effect of the material reduces the noise leaking out of the machine see Japanese Laid-Open Patent Publication No. H8-257008) 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the simple closing of the X-ray transmission opening with the sheet member or with the sound absorption material, which is mentioned in Patent reference 1, does not sufficiently reduce the noise that leaks out from inside the cover. It has been a problem. 
     The object of this embodiment is to solve the above-mentioned problem and to provide an X-ray CT system that is capable of sufficiently reducing the noise leaking from inside the cover. 
     Means for Solving the Problems 
     To solve the above-mentioned problem, an X-ray CT system as the embodiment comprises an annular rotating body configured to accommodate an X-ray tube, the rotating body having a central opening, into which a bed can be inserted in the center thereof, a cover having a tubular case part, shaped in a tubuar form, which is fitted in the opening to shield the annular rotating body from a central side of the opening, the cover being provided with an X-ray transmission opening at the tubular case part, through which, X-rays from the X-ray tube are allowed to transmit, and two sheet-like soundproof members configured to sandwich a soundproof layer, the soundproof members being disposed to close the X-ray transmission opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an X-ray CT system as an embodiment. 
         FIG. 2  is a front view of the X-ray CT system. 
         FIG. 3  is a perspective view of the X-ray CT system looked at from a diagonal rear. 
         FIG. 4  is a sectional view of a soundproof construction that does not include a soundproof layer, presented as a contrastive example. 
         FIG. 5  is a graph showing relations between the thickness of a soundproof member and the value of sound transmission loss. 
         FIG. 6  is a sectional view of a soundproof construction that includes a soundproof layer. 
         FIG. 7  is an enlarged sectional view partially showing the soundproof member. 
         FIG. 8  is a sectional view of an elastic member, which is disposed to close the gap between the floor and the cover. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     An embodiment of this type of X-ray CT system is described with reference to  FIG. 1 .  FIG. 1  is a block diagram showing the X-ray CT system. 
     The X-ray CT system shown in  FIG. 1  is an example of X-ray CT system used for medical diagnosis. The X-ray CT system  10  comprises a gantry  11 , an annular rotating body  12 , a rotary mechanism  14 , a cover  16 , a cooler  40 , and a duct  50 . 
     The annular rotating body  12  and the rotary mechanism  14  are disposed at the inner side of the gantry  11 , and the annular rotating body  12  is driven and rotated by the rotary mechanism  14 . 
     The annular rotating body  12  is internally provided with an X-ray tube  17  and an X-ray detector  18 . Provided in the center of the gantry  11  and the annular rotating body  12  is an opening  15 , into which a subject P placed on the top plate  71  of a bed  70  is inserted from the front of the gantry. 
     The cover  16  is formed so as to shield the gantry  11  and the annular rotating body  12 . By the way, the details of the cover  16  are described later. 
     The X-ray tube  17  and the X-ray detector  18  are disposed facing each other, with the opening  15  being positioned in the middle. X-rays from the X-ray tube  17  are directed to irradiate the subject P, and X-rays that have transmitted through the subject P are detected by the X-ray detector  18  to be converted into electrical signals. The electrical signals are amplified by the data acquisition system (DAS)  19  and are converted into digital data. By the way, the mechanism provided for cooling the X-ray tube  17  (cooling mechanism) will be detailed later. 
     The X-ray detector  18  comprises, for example, an array of detector elements like a scintillator array or a photodiode array, and the elements are arranged along an arc whose center coincides with the focal point of the X-ray tube  17 . Furthermore, the digital data (projection data) from the DAS  19  are transmitted through a data transmitter  20  to the console  21 . 
     The data transmitter  20  is configured to transmit projection data from the annular rotating body  12  to the console  21  in a contact-less manner. The data transmitter  20  comprises a transmitter  201 , which is provided on the annular rotating body  12 , and a receiver  202 , which is provided on the stationary part of the gantry  11 . Data received by the receiver  202  is supplied to the console  21 . The transmitter  201  is attached on the annular rotating body, while the receiver  202  is attached on a annular stationary body. 
     In addition, the annular rotating body  12  includes a slip ring  22  and an X-ray controller  24 . The stationary part  23  is provided with a gantry controller  25 . 
     The console  21  constitutes a computer system. Projection data transmitted from the data transmitter  20  are supplied to a preprocessor  31 . The preprocessor  31  executes preprocessing such as data correction on the projection data and outputs the preprocessed data onto bus lines  32 . 
     Connected to the bus lines  32  are a system controller  33 , an input unit  34 , a data storage  35 , a reconstruction processor  36 , a data processor  37 , a display unit  38 , etc. The high voltage generator  39  is connected to the system controller  33 . 
     The system controller  33  functions as host controller, and it controls the actions of each part of the console  21  and controls the gantry controller  25  and the high voltage generator  39 . The data storage  35  stores data of tomographic images, and the reconstruction processor  36  reconstructs 3D image data from projection data. The data processor  37  processes image data stored in the data storage  35  or image data that have been reconstructed. The display unit  38  displays images that have been obtained by image-data processing. 
     The input unit  34  comprises a key-board, a mouse, etc. and is operated by the user (physician, operator, and others) for setting up various parameters for data processing. In addition, it is used for inputting variety of information regarding the state of the subject P or the procedure for the examination. 
     The high voltage generator  39  controls the X-ray controller  24  through the slip ring  22  and supplies electrical power to the X-ray tube  17 , i.e., the electrical power (tube voltage and tube current) necessary to radiate X-ray. The X-ray tube  17  generates an X-ray beam that spreads toward two directions, i.e., the slicing direction, in parallel with the axial direction of the subject P, and the channeling direction perpendicular to the slicing direction. The flare angle of X-ray beams in the slicing direction may be sometimes referred to as a “cone angle”, while the flare angle in the channeling direction may be referred to as a “fan angle”. 
     The system is provided with a laser projector (not shown), for the purpose of marking (the body surface). The marking is performed by irradiating colored laser beams for the body surface of the subject P from the side and above. As the colored laser beams, for example, red laser and green laser are desirable because they are easily recognized (visible) as marking. 
     [Cover] 
     The above description has been about the basic configuration of the X-ray CT system. Now, the details of the cover  16  are described with reference to  FIG. 2  and  FIG. 3 .  FIG. 2  is a front view of the X-ray CT system while  FIG. 3  is a perspective view of the X-ray CT system looked at from a diagonal rear. 
     Here, the parts of the gantry  11  that correspond, respectively, to the front part, the rear part, both the lateral parts, the upper part, and the lower part of the annular rotating body  12  are sometimes called to, respectively, as front part, rear part, lateral parts, ceiling part, and bottom part. In addition, the right and left directions (both lateral directions), the up and down direction (height direction), and the axial direction (front and rear direction) are sometimes called to, respectively, as X-axis direction, Y-axis direction, and Z-axis direction. The rear part of the gantry  11  may be referred to as frame  13 . 
     Furthermore, in  FIGS. 2 and 3 , the frontward and rearward directions of the annular rotating body  12  are indicated, respectively, with arrows Z 1  and Z 2 ; the rightward and leftward directions of the annular rotating body  12  are indicated, respectively, with arrows X 1  and X 2 ; and the upward and downward directions of the to annular rotating body  12  are indicated, respectively, with arrows Y 1  and Y 2 . 
     As shown in  FIG. 2  and  FIG. 3 , the cover  16  comprises a bottom cover  161 , which shields the bottom of the gantry  11 , a front cover  162 , which shields the front part of the gantry  11 , a rear cover  163 , which shields the rear part of the gantry  11 , a ceiling cover  164 , which shields the ceiling part of the gantry  11 , and lateral covers  165 , which shield the lateral parts of the gantry  11 . 
     The front cover  162  includes a tubular opening front part  162   a . The tubular opening front part  162   a , which is shaped in a tubular form, is fitted into the opening (central opening)  15  from the front, so as to shield approximately the front half of the opening  15  from the Z-axis direction (axial direction). 
     The rear cover  163  includes a tubular opening rear part  163   a . The tubular opening rear part  163   a , which is shaped in a tubular form, is fitted into the opening  15  from the rear, so as to shield approximately the rear half of the opening  15  from the Z-axis direction. The tubular case part is configured with the tubular opening front part  162   a  and the tubular opening rear part  163   a.    
     The rear cover  163  is provided with a vent  163   b  at the upper part to release heat from a radiator out of the cover  16 . The radiator will be described later. The vent  163   b  is located at the position that corresponds to the clock position of twelve o′clock in  FIGS. 2 and 3 . Since the heat from the radiator rises inside the cover  16 , the heat is efficiently released through the vent  163   b , which is located at the upper part of the rear cover  163 . The noise coming through the vent  163   b  from inside the cover  16  to the front side of the X-ray CT system is much reduced compared to other configurations in which the vent  163   b  is provided at the front cover  162  or at the front part of the bottom cover  161 . 
     It is preferable that the vent  163   b  is arranged at the upper part of the cover  16 , for the purpose of efficiently releasing heat from the radiator. The vent  163   b  may be also provided, for example, through the ceiling cover  164 . 
     The rear cover  163  shields part of a duct  50 , which will be described later. In addition, the lateral covers  165  shield other part of the duct  50  and a fan  41 , which will be also described later. The other part of the duct  50  and the fan  41  may be shielded by another part of the cover  16 , for example, the ceiling cover  164 . 
     [Cooler and Duct] 
     The cooler has a fan  41 . The fan  41  is disposed in the vicinity of the radiator  26  and ejects heat from the radiator  26  into the duct  50 . The duct  50  is disposed between the gantry  11  and the cover  16 , and it receives the air from the fan  41  and leads the air to the vent  163   b  . The noise coming from inside the cover includes a wind noise caused by the rotating fan  41  and the sound of the motor driving it. 
     [Soundproof Construction] 
     Described below is a soundproof construction that reduces the noise coming from inside the cover  16 . 
     (Soundproof Construction as Contrastive Example) 
     At first, a contrastive example of a soundproof construction is described with reference to  FIG. 4 .  FIG. 4  is a sectional view of a soundproof construction configured to include one soundproof member, depicted with the opening  15  along the axial axis (Z-axis). 
     As shown in  FIG. 4 , the X-ray transmission opening S 1 , which lets X-rays pass through, is positioned between the rear end  162   d  of the tubular opening front part  162   a  of the front cover  162  and the front end  163   d  of the tubular opening rear part  163   a  of the rear cover  163 . The circumferential width of the X-ray transmission opening S 1  corresponds to the fan angle of X-ray beams. The width in the Z-axis direction of the X-ray transmission opening S 1  corresponds to the cone angle of X-ray beams. 
     As shown in  FIG. 4 , the X-ray transmission opening S 1  is closed with a sheet-like soundproof member  61 . Therefore, it is possible to ensure safety so that the subject P never comes into contact with the annular rotating body  12 . It is also possible to protect intrusion of the interior of the annular rotating body  12  against accidental penetration of blood or contrast agents. Furthermore, it prevents noise leaking out through the cover  16 . 
     (Relation between Thickness of Soundproof Member and Sound Transmission Loss) 
     Here, the relation between the thickness of the soundproof member  61  and sound transmission loss is explained with reference to  FIG. 5 .  FIG. 5  is a graph showing the relation between the thickness of the soundproof member and the value of sound transmission loss. Here, the value of sound transmission loss is the amount that is obtained by multiplying the logarithm of the reciprocal of acoustic transmission coefficient by  10 , and it is expressed in decibels [dB]. The acoustic transmission coefficient is the ratio of transmitted sound intensity to incident sound intensity. 
       FIG. 5  is a graph whose horizontal axis represents the thickness [mm] of the soundproof member and whose vertical axis represents the value of sound transmission loss, along with frequencies [Hz] of coincidence effect. Here, the term “coincidence effect” means a phenomenon that the value of sound transmission loss decreases at a particular frequency. 
     In  FIG. 5 , the solid line indicates the frequency at which a coincidence effect occurs for the thickness T of the soundproof member  61  without a soundproof layer  62 ; the dash-dot line indicates the frequency at which a coincidence effect occurs for the thickness T of the soundproof member  61  with a soundproof layer  62 ; and the dashed line indicates the value of sound transmission loss for the thickness T of the soundproof member  61 . 
     The value of sound transmission loss rises as the thickness T of the soundproof member  61  increases, resulting in an improvement in acoustic insulation. In addition, the frequency at which a coincidence effect occurs moves to the side of lower tone as the thickness T of the soundproof member  61  increases. The frequency when the thickness T is “t 1 ” is indicated by “f 1 ” in  FIG. 5 . However, depending on the frequency characteristics of noises, the increased thickness T does not always lead to an improvement in acoustic insulation. For example, if the noise includes the frequency f 1  at which a coincidence effect takes place, then the value of sound transmission loss drops and the acoustic insulation drops. For this reason, it is necessary to make the thickness T of the soundproof member  61  thinner than an upper limit t 1  (T &lt;t 1 ). 
     The above description has been about a soundproof construction that is configured with one soundproof member  61 . It should be understood here that there is a difficulty in improving the sound insulation property of the system by applying only one soundproof member  61 . 
     (Soundproof Construction with Soundproof Layer) 
     Now, the soundproof construction of this embodiment is described with reference to  FIG. 6  and  FIG. 7 .  FIG. 6  is a sectional view of the soundproof construction, which includes a soundproof layer, depicted with the central opening  15  along the axial axis (Z-axis). 
     As shown in  FIG. 6 , the soundproof construction of the present embodiment comprises two soundproof members  61  and a soundproof layer  62  for improving the sound insulation property. The soundproof layer  62  is an air layer. Note that the soundproof construction may comprise two or more sets of the combination of two soundproof members  61  and one soundproof layer  62 . Furthermore, the soundproof layer  62  may include an acoustic absorption member and/or an acoustic reflection member. 
       FIG. 7  is a partially enlarged sectional view of the soundproof members. As shown in  FIG. 6  and  FIG. 7 , as the soundproof construction, two soundproof members  61  are disposed so as to sandwich an air layer as a soundproof layer  62 . These two soundproof members  61  are disposed to bridge over the X-ray transmission opening S 1 . Although  FIG. 6  and  FIG. 7  show the two soundproof members  61  being disposed in parallel, the configuration is not restricted to this. The two soundproof members  61  may be disposed with a predetermined angle between them. 
     (Soundproof Member) 
     The soundproof member  61  comprises a thin film-like material that has a high sound transmission loss and a high transmittance for X-rays and laser beams used for marking. It is possible to limit deterioration to the quality of images acquired by radiography. 
     The types of materials for the soundproof member  61  include acoustic absorption material, which has a property of converting part of sound energy into heat energy and reducing sound reflection, and acoustic reflection material, which has a property of reflecting and refracting incident sound. 
     The examples of acoustic absorption material include a fibrous member and a sponge-like member that has a lot of small pores. It is preferable that a porous material such as glass-wool or urethane be used as a leading example of acoustic absorption material. 
     For example, the acoustic reflection member may be configured by enclosing, between two soundproof members, a gas like helium, which has a property of faster acoustic transmission than air. 
     In this embodiment, for example, polyethylene terephthalate (PET) is used for the soundproof member  61 . As a PET, it is preferable to choose Mylar (registered trademark). 
     The resin materials used for the soundproof member  61  are, however, not limited to PET, and they can be any materials that have a high transmittance for X-rays and laser beams and a low resistance to deterioration of X-ray. 
     It is preferable that the thickness of the soundproof member  61  be 0.5 mm -1.0 mm. The reason why the lower limit is set at 0.5 mm is that even if the subject P should come into contact with the soundproof member  61 , the thickness equal to or more than 0.5 mm can secure the safety of the subject. Furthermore, the reason why the upper limit is set at 1.0 mm is that the thickness equal to or less than 1.0 mm can ensure a good transmittance of X-rays and laser beams for the soundproof member. Conversely, if the thickness is more than 1.0 mm, then the soundproof member absorbs X-rays and laser beams. The X-ray absorption by the soundproof member causes to deteriorate the quality of resultant images, and the laser beam absorption causes to lost the easy recognition of the above-mentioned laser marking. 
     Now, more details are given of the soundproof member  61  that is made of the above-mentioned material and to the above-mentioned thickness. 
     The soundproof member  61  may be made of a transparent and colorless film. Here (as well as in the following), the term “film” includes a sheet. 
     The soundproof member  61  may also be made of a colored film that transmits colored laser beams. If the soundproof member  61  is made of a colored film, then the color of the film corresponds to the color of the laser beam to be used. For example, if the laser is red, then a red film is used; if green laser, a green film. If a colored film is used as the soundproof member  61  in such a way, then the laser beam used for marking is transmitted while the other colors of visible light are not. With the system in such a configuration, the subject P in position never sees the interior of the device through the soundproof member  61 . Thus, the appearance of the system can be improved without imposing a strange feeling to the subject P. 
     The soundproof member  61  is made of a colored film alone. The soundproof member  61  may also be made of a combination of a colored film and a colorless, transparent film. 
     The colorless, transparent film is formed from a transparent and colorless resin. The colored film is formed by coloring the transparent and colorless resin. 
     For the coloration of the resin, coloring agents such as pigments and dyes are used. As pigments, inorganic and organic pigments are used. Dyes are selected and used, in consideration of mutual solubility with the resin. Dyes dissolve and spread in water, solvent, oil, etc. and show hues as results of absorbing particular visible rays. Dyes are suitable for coloration of transparent plastics because the molecules of dyes disperse. 
     Resin can be colored by the following two methods: 1) a coloring agent is melted and mixed and kneaded with the resin before forming; and 2) after the resin is formed, the formed article is colored with a coloring agent. 
     (Air Layer) 
     As mentioned previously, an air layer as soundproof layer  62  is sandwiched between two soundproof members  61 . In this case, the air layer may be formed just by attaching the two soundproof members  61  to each other with pieces of double-stick tape  63  having a thickness of 0.5 mm-1.0 mm. 
     Providing an air layer can improve acoustic insulation. In addition, thickening the air layer lowers the frequency at which a coincidence effect takes place.  FIG. 5  indicates with “f 2 ” the frequency that has been decreased by the provision of the air layer.  FIG. 5  also indicates with “t 2 ” the upper limit of the thickness T of the soundproof member  61  for the frequency f 2 . As shown in  FIG. 5 , by increasing the upper limit of the thickness T of the soundproof member  61  to “t 2 ”, the thickness T of the soundproof member  61  can be increased (T &lt;t 2 ) to increase the value of acoustic transmission coefficient loss, thereby improving the acoustic insulation. 
     As shown in  FIG. 6  and  FIG. 7 , the rear end  162   d  of the tubular opening front part  162   a  is provided with a recessed shelf  166 , and the front end  163   d  of the tubular opening rear part  163   a  is also provided with a recessed shelf  166 . 
     The relation between the depth of the recessed shelf  166  and the thickness of the double-stick tape  63  is expressed by the following expression (2)
 
 D&gt; 2 T+t    (2)
 
     where D indicates the depth of the recessed shelf  166 , and t indicates the thickness of the double-stick tape  63 . 
     Furthermore, the relations among the length of the double-stick tape  63 , the width between both the recessed shelves  166 , and the width of the X-ray transmission opening S 1  in the Z-axis direction are expressed by the following expression (3)
 
 W 0 ≧W&gt;W 1  (3)
 
     where W indicates the length of the double-stick tape  63 , W 0  indicates the width between both the recessed shelves  166 , and W 1  indicates the width of the X-ray transmission opening Si in the Z-axis direction. 
     The stacking up of the two soundproof members  61  enhances the soundproof effect. In addition, the effect achieved with the thickness of the air layer improves the acoustic insulation (sound insulating properties) on in a range of low to middle tones. 
     The use of the double-stick tape  63  improves the sealability of the air layer, which, in turn, further improves the acoustic insulation. 
     The thickness of the air layer may be changed by modifying the thickness of the double-stick tape  63 . By doing so, the predetermined frequency where a coincidence effect occurs can be lowered. In addition, these two soundproof members  61  may be disposed so as to define a predetermined angle between them. In this case, two double-stick tapes  63  having differing thicknesses may be applied, respectively, one on the rear end  162   d  of the tubular opening front part  162   a  and the other on the front end  163   d  of the tubular opening rear part  163   a.    
     Described below is a procedure of closing the X-ray transmission opening S 1  with the soundproof members  61 . 
     At first, an adhesive is applied to the recessed shelves  166 . Then, with the adhesive, one piece of soundproof member  61  is applied to stick onto the recessed shelves  166 . 
     After that, a double-stick tape  63  is applied onto the piece of soundproof member  61  in position. Then, a second piece of soundproof member  61  is stuck on the double-stick tape  63 . 
     By this procedure, the X-ray transmission opening S 1  is closed with the soundproof members  61 . In addition, by using the double-stick tape  63 , the sticking of two pieces of soundproof member  61  is applied easily, and thus workability can be improved. 
     By the way, two pieces of soundproof member  61  that have been attached to each other with a double-stick tape  63  may be applied onto the recessed shelves  166 . Instead, a double-stick tape  63  may be applied onto the recessed shelves  166  first, and then a first piece of soundproof member  61  may be applied onto the double-stick tape  63 . 
     [Another Soundproof Construction] 
     Now, an elastic member  65  is described with reference to  FIG. 1 ,  FIG. 3  and  FIG. 8 .  FIG. 8  is a sectional view of the elastic member, which is disposed so as to close the gap S 2  between the floor and the cover  16 . 
     As shown in  FIG. 1 ,  FIG. 3 , and  FIG. 8 , a gap S 2  is provided between the room (floor F) where the X-ray CT system is installed and the lower edge of the bottom cover  161 . The gap S 2  is uneven. Since the unevenness is partly caused by imprecision involved in the production and assembly of the cover  16 , it is not possible to install the system without a gap S 2 . With a gap S 2 , noises are leaked out from inside the cover  16 . On the other hand, if the lower edge of the bottom cover  161  were in contact with the floor F, abnormal noises would be generated by vibration during the operation of the system. 
     The elastic member  65  is formed in a belt from an elastic material (e.g., plastic rubber). One edge  651  of the elastic member  65  is attached along the lower edge of the bottom cover  161 . The other edge  652  of the elastic member  65  is in contact with the floor F, and this edge bends to the inside of the bottom cover  161 . In this state, the resilience of the elastic member enables the elastic member to elastically touch the floor F. As a result, the gap S 2  is banished. 
     By closing the gap S 2  with the elastic member  65 , the noise coming from inside the cover  16  can be reduced. Furthermore, since the edge of the elastic member  65  elastically touches the floor F, there is no abnormal noise which may be otherwise generated by vibration during the operation of the system. 
     The above-mentioned other edge  652  of the elastic member  65  is formed to bend to the inside of the bottom cover  161 . As a result, the other edge  652  of the elastic member  65  in position hides itself in the bottom cover  161 . This is an improvement to the appearance of the system. 
     Furthermore, the other edge  652  of the elastic member  65  is curved so as to bend to the inside of the bottom cover  161 . Moreover, the thickness of the other edge  652  is made thinner than that of the other part including the one edge  651  so that the edge  652  can be easily bent. 
     In this embodiment, an acoustic absorption member may be attached to the internal surface of the cover  16 . As an example of the acoustic absorption member, a porous material such as rock wool or glass wool is formed into a high-density plate to be used. It is preferable that a thin film of polyethylene or vinyl be used as an example of porous material. 
     In addition, in the embodiment, an acoustic reflection member, which has a property of reflecting and refracting incident sound entered into the internal surface of the cover  16 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 
     DESCRIPTIONS OF NUMBERED PARTS 
     Letter P designates a subject; 
     S 1 , X-ray transmission opening; 
     S 2 , gap; 
       10 , X-ray CT system; 
       11 , gantry; 
       111 , fan installation; 
       112 , through-opening; 
       12 , annular rotating body; 
       121 , body axis; 
       122 , air port; 
       13 , frame; 
       15 , opening; 
       16 , cover; 
       161 , bottom cover; 
       162 , front cover; 
       162   a , tubular opening front part; 
       163 , rear cover; 
       163   a , tubular opening rear part; 
       163   b  , vent; 
       164 , ceiling cover; 
       164   a , contact point; 
       165 , lateral cover; 
       166 , recessed shelf; 
       17 , X-ray tube; 
       18 , X-ray detector; 
       19 , data acquisition part (DAS); 
       20 , data transmitter; 
       21 , console; 
       22 , slip ring; 
       23 , stationary part; 
       24 , X-ray controller; 
       25 , gantry controller; 
       26 , radiator; 
       31 , preprocessor; 
       32 , bus line; 
       33 , system controller; 
       34 , input unit; 
       35 , data storage; 
       36 , reconstruction processor; 
       37 , data processor; 
       38 , display unit; 
       39 , high voltage generator; 
       41 , fan; 
       50 , duct; 
       61 , soundproof member; 
       62 , soundproof layer; 
       63 , double-stick tape; 
       65 , elastic member; 
       70 , bed; and 
       71 , top plate.