Abstract:
Time consuming calibration of a multi-element x-ray detector for an x-ray computed tomography machine that has multi-sample rate capabilities is reduced by determining through the use of air-scans, a scalar relationship between sensitivity of detector elements as a function of sampling rate. This scalar relationship is in vector form and may be applied to independently obtain calibration vectors at a base scan rate to provide effective calibration vectors at a variety of scan rates without the need for time consuming daily calibration scans at each of those sample rates.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to x-ray computed tomography machines (CT) and specifically to a method of calibrating CT data when acquired at different sampling rates. 
     X-ray computed tomography is a well known procedure for creating cross-sectional images from computer processed x-ray projections taken along the plane of the cross section. In a typical CT machine, an x-ray tube is mounted on a rotatable gantry to project the fan beam of x-rays at a patient through a “slice” from a variety of angles. The x-rays are received after passing through the patient by a multi-element detector to provide a measurement of x-ray attenuation along a variety of rays of the fan beam (“projections”). The attenuation signals from the elements of the multi-element detector are sampled and digitized by a data acquisition system. 
     Digitized projections collected at a range of angles about the patient, typically no less than 180° plus half the fan beam angle, are collected in a “tomographic projection set” and reconstructed according to well known techniques in the art, such as filtered back projection, into an image of a cross section of the patient along that slice. 
     The mathematics of computed tomography reconstruction require that each detector be extremely stable so that attenuation signals over time are the same when identical x-ray flux is received by those detectors. To realize this stability, the detector elements are manufactured to have similar electrical characteristics and remaining variations are accommodated by means of one or more “correction vectors”. 
     The correction vectors provide a value for each detector element which may be subtracted from or multiplied by corresponding attenuation values (“scan values”) acquired by the detectors to correct the attenuation values for detector-to-detector variation. The correction vectors are updated at different intervals. Prior to every scan, an “offset vector” is measured that corrects signal offsets such as from “dark currents” that occur in detectors in the absence of any received x-rays and contains values subtracted from the attenuation values to remove offset. At the time of the scan, a “reference normalize vector” is produced based on a signal received at a reference detector. The vector corrects for variations caused by changes in x-ray tube current. 
     On a daily basis, an “air calibration vector” is measured which corrects signal scaling from a variety of possible sources including changes in x-ray tube voltage, aperture, focal spot size, filtration and sampling rate. The air calibration vector is measured with nothing in the x-ray beam, prior to scanning patients. Far less frequently, “beam hardening” and “primary speed” correction vectors are measured, the latter which is a function of the detector and does not change for a give detector. These correction vectors are typically measured rarely, once at the time of manufacture and thereafter only at major service intervals, for example, when the x-ray tube or filters are replaced. 
     Current CT machines allow for selection from a variety of scanning speeds. High scanning speeds may be desired for images where organ or patient movement can be a problem and low signal to noise ratio can be tolerated. Slower scanning speeds are used where motion is less of a problem and high signal to noise ratio images are needed. Each of these scanning speeds may require the use of a different sampling rate of the attenuation signals from the elements of the multi-element detector. 
     Variations in the sampling rate can significantly affect the air calibration vector. Accordingly, the calibration vector must be measured for each possible sampling rate, significantly increasing the time required to do this daily calibration procedure. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventors have recognized that a simple relationship may be developed between the values of the calibration vectors at different sampling rates. This relationship, which may be determined by executing a series of stationary air-scans at different sampling rates, may be used to modify a limited set of calibration vectors taken at a base sampling rate, for use with any sampling rate. 
     Generally, the present invention provides a method of calibrating attenuation signals obtained from a multi-element x-ray detector used in an x-ray computed tomography machine where the attenuation signals indicate the strength of x-rays received from an x-ray source after the x-rays pass through a measurement volume. The signals are sampled by a digital acquisition system at different sampling rates. For each of a plurality of different sampling rates including a base rate, the multi-element detector is used to acquire an air-scan vector of signals when the measurement volume is empty of an object to be imaged. The multi-element detector is then used to acquire at a given sampling rate, a tomographic projection set of signals when the measurement volume includes an object to be imaged. A sampling rate correction vector is generated being a function of the air-scan vector for the base rate and the air-scan vector for the given sampling rate and this is used to modify a calibration vector. The modified calibration vector is applied to the tomographic projection set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified diagram of the principal elements of a commercial tomography machine showing an opposed x-ray source ( 18 ) and multi-element detector ( 22 ) and a processing system ( 26 ) receiving attenuation signals from the multi-element detector and communicating with an operator console; 
     FIG. 2 is a detailed block diagram of the detector ( 22 ) and processing system ( 26 ) of FIG. 1 showing a data acquisition system ( 30 ) such as may acquire data from the multi-element detector at different sampling rates, an associated memory ( 36 ) for storing data including air-scan vector ratios ( 46 ) and a processor ( 38 ) for executing a program ( 48 ) to execute the method of the present invention; and 
     FIG. 3 is a data flow chart showing the reconstructing a projection set ( 42 ) using the air-scan ratios ( 46 ) of FIG. 2 to create an image. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, computed tomography machine  10  includes an annular gantry  12  having a central opening  14 . The gantry  12  is supported for rotation about an axis  16  centered within the opening  14  and generally perpendicular to a broad face of the gantry  12 . 
     Attached on the face at one edge of the gantry  12  is an x-ray tube  18  directing a fan beam of x-rays  20  across the opening  14  to a multi-element detector  22  attached at an opposite edge of the gantry  12 . Elements  24  of the multi-element detector  22  extend along the face of the gantry  12  about a radius centered on the x-ray source. Each element  24  measures attenuation of x-rays  20  caused by an imaged object (not shown) within the opening  14 . 
     A filter/collimator  19  which may include multiple interchangeable filter elements and collimators is placed between the x-ray tube  18  and the opening  14  according to techniques well known in the art. 
     Attenuation signals from each of the elements  24  is received by a processing system  26  which also controls rotation of the gantry  12 , selection of filtration and collimation of the filter/collimator  19  and activation of the x-ray tube  18 . A console  28  is also connected to the processing system  26  and provides for input of scanning parameters (e.g., scan speed) from an operator and the output of reconstructed tomographic images to the operator. 
     Referring now to FIG. 2, detector elements  24  of the multi-element detector  22  each provide independent attenuation signals to a multichannel data acquisition system  30  which samples the independent signals on each of the detector elements  24  at a sampling rate determined by a sample rate clock  32 . Generally, the sample rate clock  32  will be adjusted so that the sample rate of the attenuation signals provides a desired angular separation between the projections of an acquired tomographic projection set with different gantry speeds of rotation. As mentioned, the gantry speed may be changed to control the scanning time. 
     Each of the sampled attenuation signals are digitized and transferred over an internal bus  34  as raw attenuation data. The internal bus  34  also communicating with a memory  36  and processing unit  38 . The bus  34  may also communicate via a port (not shown) with the console  28 . The processing unit  38  operating through the bus  34  may control the speed of the sample rate clock  32  according the desired scan speed entered by the operator through console  28 . 
     One sampling of the full set of attenuation signals from data elements  24  of the multi-element detector  22  produces a projection vector  39  of values where the vector elements correspond with raw attenuation data of particular detector elements  24 . A tomographic projection set  42  will be a set of projection vectors  39  corresponding to different angles of gantry rotation. 
     Generally, the memory  36  may store a tomographic projection set  42  of vectors for processing as well as a calibration vector  40  and an offset vector  44 . The calibration vector  40  includes values that when multiplied by the raw attenuation data of the tomographic projection set (the multiplication being between corresponding elements of the vectors) corrects the raw attenuation data of the projection set  42  for variations in measurements caused by factors other than the attenuation of x-rays so as to reduce artifacts in the reconstructed image. As such, the calibration vector  40  may include calibrations for beam hardening and primary speed, as described above. 
     The offset vector  44  provides values that when subtracted from the raw attenuation data of the projection set  42  eliminate offsets unrelated to attenuation of x-ray energy. The offset vector  44  is normally applied prior to the calibration vector  40 . 
     Per the present invention, the memory  36  also stores a set of vectors of stationary air-scan ratios  46 A through  46 C, each related to a different speed of the sample rate clock  32 . Vector air-scan ratios  46 A through  46 C represent multiple attenuation measurement (for each detector element) without gantry rotation and without a patient in the opening  14  of the gantry, taken at different speeds of the sample rate clock  32 , averaged and referenced ratiometrically to similarly acquired and averaged attenuation measurements at a reference base sample rate. The measurements at the reference base sample rate form the numerator and the measurements at the different sample rates form the denominators of the air-scan ration  46 . The air-scan ratios  46 A through  46 C provide a measure of interdetector sensitivity differences as a function of different scan rates. The base scanning rate is typically the middle scanning rate. 
     On a daily basis, air calibrations are done only at the base scanning rate to produce a base scanning rate air scan  45 . The air calibration ratios  46  taken earlier can be multiplied by the daily base scanning rate air scan  45  to produce a suitable calibration vector for different sampling rates. While only three different such vectors of stationary air-scan ratios are shown, generally one vector of ratios will be stored for each possible sampling rate however many. 
     Also included in memory  36  is a program  48  executed by the processing unit  38  to provide machine control and reconstruction as is understood in the art and the calibration process described hereafter being part of the present invention. 
     Referring now to FIG. 3, a projection set  42  of data may be acquired including reference channel data  52  used to generate the reference normalize vector. Each of the projection vectors  39  of the projections set  42  is then corrected by the offset vector  44  which is subtracted from each of the projection vectors  39 , the subtraction being performed on an element by element basis to produce offset corrected projections  54  by subtractor  55 . 
     The offset corrected projections  54  are provided to a multiplier  56  to be multiplied by the primary speed correction portion of vector  40 . The resulting offset corrected projections  57  are then provided to divider  58  to be divided by a reference normalize vector formed from the reference channel data  52  as is understood in the art. Generally, the reference data  52  for a given projection vector  59  divides the other elements for that projection vector  39 . 
     The thus produced reference corrected data  60  is then provided to multiplier  61  to be multiplied by the product of (1) one of the air-scan ratios  46 A- 46 B as dictated by the sampling rate at which the projection set  42  was acquired and (2) the base scanning rate air scan  45 . The thus modified air-scan ratio  46 A- 46 C is applied on an element by element basis by multiplier  61  to produce air scan corrected data  62 . 
     This air scan corrected data  62  is then provided to pre-processor  63  which applies a negative log (reflecting the exponential attenuation of x-rays) and the beam hardening correction of vector  40  according to methods will known in the art. This corrected data  70  is applied to a reconstructor  72  for production of the tomographic image (according to a well known technique) such as is provided to the console  28 . 
     The above described various embodiments of the invention provide for different features. With the invention, the storage space needed for calibration vectors can be reduced and daily acquisitions of air-scans to is limited to a single sampling rate. 
     It should be noted that the present invention allows for the acquisition of multiple air-scan data vectors for at least one given scan rate and averaging them together and in this way an arbitrary precision may be obtained in the generation of the scaling factor that relates sampling rates to adjustments in the calibration vector. 
     When the tomography machine may include an x-ray tube mounted in opposition to the multi-element x-ray detector on a rotatable gantry, the air-scans are may be taken without gantry movement, thus a scaling factor can be obtained that isolates the effects of sampling from ancillary effects such as those that may arise with gantry movement. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.