Abstract:
A method of cooling a medical imaging system that includes a gantry is provided. The method includes supplying air to the gantry. The supplied air is conditioned to reduce variation of the temperature within the gantry. After the air is conditioned it is channeled throughout the gantry to a plurality of heat producing electronic devices within the gantry.

Description:
CROSS REFERENCE TO RELATED PATENTS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/630,796 filed Nov. 24, 2004, which is herein incorporated in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to methods and apparatus for computed tomography (CT), and more particularly to methods and apparatus that provide for thermal control in CT systems.  
         [0003]     Air cooling of CT systems offers a good combination of simplicity and functionality when the factors of design time, cost and system sitting are considered. One problem with air cooling in a CT system is that there is a local temperature control solution for controlling the photodiode that is dependent and effected by the global system air temperature and flow control. Typical systems simply rely on controlling the bulk air temperature of the gantry (system) air temperature as measured at some convenient location in the gantry. One problem currently being addressed is that with CT system electronic power consumption on the rise it may be no longer feasible to have independent local and global temperature control systems. A further issue for air cooled systems with increased power consumption is to remove audible noise as more power consumption typically uses more and/or larger fans. Lastly, as more fans are added to a higher wattage CT system reliability concerns and field service to replace failed fans or clean air filters become larger more expensive long term problems.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In one aspect, a method of cooling a medical imaging system that includes a gantry is provided. The method includes supplying air to the gantry. The supplied air is conditioned to reduce variation of the temperature within the gantry. After the air is conditioned it is channeled throughout the gantry to a plurality of heat producing electronic devices within the gantry.  
         [0005]     In another aspect, a medical imaging system is provided. The medical imaging system includes a gantry having a controller configured to receive signals indicative of a plurality of temperatures within the gantry. A local thermal control system is further provided to maintain a predetermined temperature at a plurality of electronic devices within the gantry. Finally, a bulk thermal control system is provided to maintain a predetermined temperature within the gantry. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a pictorial view of a CT imaging system embodiment.  
         [0007]      FIG. 2  is a block schematic diagram of the system illustrated in  FIG. 1 .  
         [0008]      FIG. 3  illustrates the allowable temperature variations for the CT system illustrated in  FIGS. 1 and 2 .  
         [0009]      FIG. 4  illustrates a system level thermal control system.  
         [0010]      FIG. 5  illustrates a system level thermal control system.  
         [0011]      FIG. 6  illustrates a system level thermal control system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     There are herein provided thermal control methods and apparatus useful for imaging systems such as, for example, but not limited to a Computed Tomography (CT) System. The apparatus and methods are illustrated with reference to the figures wherein similar numbers indicate the same elements in all figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate explanation of an exemplary embodiment of the apparatus and methods of the invention.  
         [0013]     In some known CT imaging system configurations, a radiation source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The radiation beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of a radiation beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.  
         [0014]     In third generation CT systems, the radiation source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that an angle at which the radiation beam intersects the object constantly changes. A group of radiation attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object includes a set of views made at different gantry angles, or view angles, during one revolution of the radiation source and detector.  
         [0015]     In an axial scan, the projection data is processed to reconstruct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display device.  
         [0016]     To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.  
         [0017]     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.  
         [0018]     Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated but a viewable image is not. Therefore, as used herein the term, “image,” broadly refers to both viewable images and data representing a viewable image. However, many embodiments generate (or are configured to generate) at least one viewable image.  
         [0019]      FIG. 1  is a pictorial view of a CT imaging system  10 .  FIG. 2  is a block schematic diagram of system  10  illustrated in  FIG. 1 . In the exemplary embodiment, a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT imaging system. Gantry  12  has a radiation source  14  that projects a cone beam  16  of X-rays toward a detector array  18  on the opposite side of gantry  12 .  
         [0020]     Detector array  18  is formed by a plurality of detector rows (not shown in  FIGS. 1 and 2 ) including a plurality of detector elements  20  which together sense the projected X-ray beams that pass through an object, such as a medical patient  22 . Each detector element  20  produces an electrical signal that represents the intensity of an impinging radiation beam and hence the attenuation of the beam as it passes through object or patient  22 . An imaging system  10  having a multislice detector  18  is capable of providing a plurality of images representative of a volume of object  22 . Each image of the plurality of images corresponds to a separate “slice” of the volume. The “thickness” or aperture of the slice is dependent upon the thickness of the detector rows.  
         [0021]     During a scan to acquire radiation projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 .  FIG. 2  shows only a single row of detector elements  20  (i.e., a detector row). However, multislice detector array  18  includes a plurality of parallel detector rows of detector elements  20  such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan.  
         [0022]     Rotation of gantry  12  and the operation of radiation source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes a radiation controller  28  that provides power and timing signals to radiation source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized radiation data from DAS  32  and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
         [0023]     Computer  36  also receives commands and scanning parameters from an operator via a console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , radiation controller  28 , and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 .  
         [0024]     In one embodiment, computer  36  includes a device  50 , for example, a floppy disk drive, CD-ROM drive, DVD drive, magnetic optical disk (MOD) device, or any other digital device including a network connecting device such as an Ethernet device for reading instructions and/or data from a computer-readable medium  52 , such as a floppy disk, a CD-ROM, a DVD or an other digital source such as a network or the Internet, as well as yet to be developed digital means. In another embodiment, computer  36  executes instructions stored in firmware (not shown). Generally, a processor in at least one of DAS  32 , reconstructor  34 , and computer  36  shown in  FIG. 2  is programmed to execute the processes described below. Of course, the method is not limited to practice in CT system  10  and can be utilized in connection with many other types and variations of imaging systems. In one embodiment, Computer  36  is programmed to perform functions described herein, accordingly, as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits. Although the herein described methods are described in a medical setting, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning CT system for an airport or other transportation center.  
         [0025]      FIG. 3  illustrates the allowable temperature variations for the CT system illustrated in  FIGS. 1 and 2 .  FIG. 4  illustrates one thermal control system  100  useable with the system illustrated in  FIGS. 1 and 2  and other CT systems and Volume CT systems (VCT). System  100  includes a plurality of rear exhaust fans  102  and a plurality of booster fans  104 . Although illustrated with a particular number of fans, it is contemplated that the benefits of the invention accrue to systems with various fans including a single exhaust fan or a single booster fan. System  100  also includes at least one controller  106 . An analog to digital board (AD Board)  108  is situated in an AD Board plenum  110  and operationally coupled to a detector rail(s)  112 . One controller  106  controls a heater  114  positioned in an inlet blower heater box  116 . A heater  118  may be positioned in a flow path with detector rails  112  and controlled by controller  106 . As illustrated in the figures and as described below a plurality of temperature sensors  120  are situated in various places.  
         [0026]     The herein described methods and apparatus provide, in one embodiment, for an integrated thermal system design which uses variable speed fans with PID (Proportional, Integral, Derivative) feedback control for both local diode temperature control and global system level air temperature and flow control. The local variable speed temperature control system uses the temperature of the digital modules and temperature control of the detector rail to maintain substantially optimal diode temperature control such as is described in co-pending application Ser. No. 10/710,213, filed Jun. 25, 2004, titled Variable Speed Fans for Advanced DAS/Detector Thermal Management, and is hereby incorporated in its entirety. The global thermal control system includes an inlet air blower with heaters used to provide controlled air into the front side of the CT system for consumption by the local digital module control system. Further, the inlet blower heater system provides controlled airflow to provide continuous air mixing. By providing inlet air to the front side of the gantry and the air mixing, the effects of air recirculation from the exhaust of the electronics and tube heat exchanger are minimized. The continuous flow also prevent local pockets of hot air to be trapped and mixing when gantry rotation begins thus preventing temperature spikes during scanning. One effect of the global thermal control system is to reduce the variation in air temperature that is seen by the local digital module temperature control system.  FIG. 3  illustrates how the room the temperature control ranges vary from room ambient down to the photo diode of the detector. As can be seen the room ambient graph has the largest temperature range which the global control system reduces to a level that the local system can compensate for to ultimately control the photo diode down to a very narrow range as required for proper performance. A further feature of the herein described methods and apparatus provide substantially optimized audible noise and fan reliability both of which increase with fan speed.  
         [0027]     Thus, by using variable speed fans at the local level, a wider range of inlet air temperatures can be accommodated by modifying air velocity to compensate for temperature difference between cooling air and target electronic components. Similarly, by using variable speed gantry exhaust fans, both a wide variation in room ambient air and gantry rotational speeds can be accommodated. Therefore, by using the inlet air temperature of local temperature control system as feedback to control the gantry exhaust fans the overall system audible noise and fan reliability can be substantially optimized as there will be multiple design points that satisfy the room ambient, system power output, and gantry rotational speed.  
         [0028]     The following equations illustrate relationships. 
 
 T   plenum-inlet =ƒ( Q   AD-Boards   ,Q   xray-tube   ,N   gentry   ,T   ambient   ,Q   inlet-blower   ,N   gantry-fans )  (eq 1) 
 
 illustrates that the temperature at the plenum inlet (T plenum-inlet ) is a function (f) of heat (Q) produced at the AD-board(s), the x-ray tube, and the inlet blower, the rotation speed of the gantry (N gantry ), the number (N) of gantry fans, and the ambient temperature (T ambient ). 
 
 T   AD =ƒ( N   plenum-fans   ,T   plenum-inlet )  (eq 2) 
 
 illustrates that the temperature at the AD board(s) and/or AD plenum is a function of the plenum inlet temperature and the number of plenum fans. 
 
 DB   system-fan =ƒ( N   plenum-fans   ,N   gantry-fans )  (eq 3) 
 
 illustrates that total noise (DB, decibels) is a function of the number of plenum fans and gantry fans. 
 
Life plenum-fans =ƒ( N   plenum-fans   ,T   plenum-inlet )  (eq 4) 
 
 illustrates factors (number of fans and temperature at plenum inlet) that affect plenum fan replacement times. 
 
Life gantry-fans =ƒ( N   gantry-fan   ,T   gantry-exhaust )  (eq 5) 
 
 illustrates factors (number of fans and temperature at gantry exhaust) that affect gantry fan replacement times. 
 
         [0029]     Based on nearly constant electronic power output and the typically low air temperatures encountered in a CT system eq1, 4 and 5 can be simplified to the following. 
 
 T   plenum-inlet =ƒ( Q   xray-tube   ,N   gantry   ,T   ambient   ,N   gantry-fans )  (eq 1a) 
 
Life plenum-fans =ƒ( N   plenum-fans )  (eq 4a) 
 
Life gantry-fans =ƒ( N   gantry-fans )  (eq 5a) 
 
         [0030]     Thus, based on the allowable inlet plenum air temperatures that can be accommodated by the local control system variable speed fans, there are multiple possible gantry fan speeds that will give an acceptable AD board temperature to provide the desired diode temperature. This means that the gantry and fan speeds if tied together in a common control system can be optimized or substantially optimized to provide substantially minimum noise and substantially maximum life for any combination of room ambient and x-ray tube output power.  
         [0031]     In one embodiment, the gantry exhaust fans are tied to a PID feedback controller that uses the inlet plenum air temperature as the PID controllers feed back variable. Further, the gantry exhaust fans are broken into banks of fans that can be controlled to run at different speeds to achieve optimal performance. One design of a system level control system is illustrated in  FIGS. 4, 5 , and  6 . One embodiment is as illustrated in all of  FIGS. 4, 5 , and  6 , while other embodiments utilize just some of the features illustrated in some of  FIGS. 4, 5 , and  6 .  
         [0032]     A further feature of the herein described methods and apparatus are the service feedback mechanisms built into the design. Below is a list of the service features enabled by the herein described system level control architecture: 
        1. Air filter maintenance—The air flow after each air filter is monitored via a velocity sensor (a pressure sensor system can also be used) to send out a message to a service or a user that at least one air filter is critically clogged and need to be cleaned. This will enable pre-emptive maintenance before the filters have clogged to the point of causing system performance degradation.     2. Redundant sensor error handling—Multiple sensors and error trapping are employed to detect a bad sensor and remove it from the feedback loop. By using multiple sensors, an average value can be used for temperature and flow sensing feedback (e.g., as illustrated in  FIG. 6 ). When one of the controllers detects a bad sensor the control software removes the sensor from the averaging scheme and sends an error message to field service to replace the sensor at the next available scheduled down time. When any feedback system is down to only 2 working sensors to average a request for immediate service is sent out.     3. Imminent fan failure—The control systems continually monitor for change in current required to run the fans at a given speed compared to baseline current draw (when fans where new). Once a certain level of degradation is seen (e.g., a rise in current required by a fan) an error message is sent to the system warning that a fan is failing and needs to be replaced at the next opportunity.        
 
         [0036]     Technical effects of the herein described methods and apparatus include a more precise local control while maintaining low audible noise. A further technical effect of an integrated system is the ability to provide preemptive field service data to minimize unscheduled maintenance and eliminate the need for fixed PM schedules. And another technical effect is the ability to optimize against audible noise and fan life to achieve a more robust CT system design.  
         [0037]     Additional methods and apparatus incorporate a system level variable speed fan temperature control system that is tied to the local electronic cooling air temperature. Such a system allows the temperature within a gantry to vary without degrading system performance. A substantially minimization in air recirculation via forced controlled inlet airflow at the front of the gantry is therefore enabled. And a pre-heating of the inlet air enables a reduced sensitivity to low room ambient conditions. In one embodiment, multiple banks of gantry fans phased to provide minimal audible noise and maximum fan life via optimized fan speed set points. In some embodiments, smart service algorithms eliminate the need for fixed maintenance schedules for air filter cleaning or fan replacement, smart service algorithms warn of impending performance degradation due to dirty air filters or imminent fan failure, and/or smart control algorithms that take advantage of multiple redundant sensors that allow for continued operation in the event of a sensor failure.  
         [0038]     Exemplary embodiments are described above in detail. The assemblies and methods are not limited to the specific embodiments described herein, but rather, components of each assembly and/or method may be utilized independently and separately from other components described herein.  
         [0039]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.