Abstract:
A method is described for controlling x-ray exposure during gated cardiac scanning, including the steps of detecting a first cardiac signal; starting scanning after a pre-selected wait time after detecting the first cardiac signal; and stopping the scanning after a first to occur of passage of a pre-selected data collection time and detection of a second cardiac signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of United States Provisional Application Ser. No. 60/166,466, filed Nov. 19, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for controlling x-ray exposure during gated cardiac scanning. 
     In at least one known computed tomography (CT) imaging system configuration, an x-ray 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 the “imaging plane”. The x-ray beam passes through the 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 beam radiation received at the detector array is dependent upon the attenuation of the x-ray 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. 
     In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray 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 comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct 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 cathode ray tube display. 
     Methods are known for controlling patient exposure to x-rays during gated cardiac scanning. For example, it is known to control patient exposure based upon a prediction of when a heartbeat will occur. Heartbeat timing predictions often are inaccurate, and resulting image quality can be degraded by unpredicted cardiac motion. It would be desirable to provide a method for controlling x-ray exposure during cardiac scanning without sacrificing image quality. It also would be desirable to control patient exposure while scanning patients having irregular heart rates. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment, a method for controlling x-ray exposure during gated cardiac scanning, including the steps of detecting a first cardiac signal; starting scanning after a pre-selected wait time after detecting the first cardiac signal; and stopping the scanning after a first to occur of passage of a pre-selected data collection time and detection of a second cardiac signal. The above-described method allows scanning exposure to be controlled for patients having heart rates as high as 92 beats per minute without sacrificing image quality for patients having slower heart rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system; 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1; and 
     FIG. 3 is a flow diagram of an embodiment of a method for controlling x-ray exposure of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14 , for example an x-ray tube, that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  that together sense the projected x-rays that pass through an object  22 , for example a medical patient. Detector array  18  may be fabricated in a single slice or multi-slice configuration. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that receives cardiac signals from patient  22  and provides power and timing signals to x-ray source  14 . Control mechanism  26  also includes 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 x-ray data from DAS  32  and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer  36  that stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator (not shown) via 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 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  that controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . 
     Referring to FIG. 3, a method for controlling x-ray exposure during gated cardiac scanning includes verifying  102  that patient  22  has a total cardiac cycle at least as long as a time (t 2 +t 4 ). Time t 2  is a time pre-selected as sufficient for completion of cardiac systolic motion, e. g. approximately 150 milliseconds. Time t 4  is a pre-selected minimum image data collection time required for imaging by system  10 , e. g. 500 milliseconds. After verification, imaging system  10  is prepared for scanning  104  and is set to wait  106  for a first cardiac signal, for example, an R-wave signal, from patient  22 . Detection of an R-wave signal sets a timer  106  to start scanning after a pre-selected wait time (t 1 +t 2 ), where time t 1  is a pre-selected time from R-wave detection through cardiac contraction start, e. g. approximately 50 milliseconds. 
     After wait time (t 1 +t 2 ) has passed, scanning is started  108  and is timed to continue through a pre-selected data collection time (t 3 +t 4 ), where time t 3  is a pre-selected time estimated for completion of cardiac fast filling, e. g. approximately 250 milliseconds. Scanning continues  110  either until data collection time (t 3 +t 4 ) has passed or until a second cardiac signal is detected, for example, a second R-wave signal. Occurrence of either event results in a continuation of scanning  112  for an additional time t 1 . 
     After additional time t 1  has passed, scanning is stopped  112 . If data was collected over at least a minimum image data collection time t 4  ending at the conclusion of scanning, an image is reconstructed  114  using data collected over the most recent minimum image data collection time t 4 . If, for example, either of first or second cardiac signals was triggered by an irregular heartbeat, time over which data was collected may be less than minimum image data collection time t 4 . In this case, before repeating scanning, it may be advisable to verify again  116  that patient  22  total cardiac cycle is at least (t 2 +t 4 ) milliseconds long so that data sufficient for reconstructing an image can be collected. 
     The above-described method does not require a prediction of patient heart rate but uses a patient cardiac signal, e.g. an R-peak signal, to start scanning after a wait time selected to avoid scanning during most of cardiac motion associated with systole. By avoiding scanning during these times, the above-described method reduces x-ray exposure while scanning patients having irregular heart rates. Exposure also is controlled for patients having a cardiac cycle as fast as (t 2 +t 4 ), i. e. time for systolic motion completion plus minimum data collection time required for system  10  image reconstruction. Thus, for example, where (t 2 +t 4 ) is 650 milliseconds, x-ray exposure is controlled for patients having heart rates as high as 92 beats per minute. 
     Although particular embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. In addition, the CT system described herein is a “third generation” system in which both the x-ray source and detector rotate with the gantry. Many other imaging systems, including “fourth generation” CT systems wherein the detector is a full-ring stationary detector and only the x-ray source rotates with the gantry, may be used. Moreover, the system described herein performs an axial scan; however, the invention may be used with a helical scan although more than 360 degrees of data are required. 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.