Patent Number: 052280705
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A mechanical gantry portion 10 includes a stationary gantry portion 12 and a rotating gantry portion 14. The stationary gantry portion includes a stationary cylinder 16 in which a subject to be imaged is received. The rotatable gantry portion 14 is mounted on the stationary cylinder 16 by suitable bearings (not shown) to allow free rotational movement therearound. A motor 18 selectively rotates the rotatable gantry portion 14 around the cylinder 16. The motor may be a separate motor as shown in FIG. 1 connected by a chain or other suitable drive. Alternately, the cylinder 16 may be an integral portion of the "rotor" with the "stator" windings mounted to the rotatable gantry 14. An x-ray tube 20 is mounted to the rotatable gantry portion 14 to rotate therewith. Appropriate slip ring electrical connections (not shown) are mounted between the cylinder 16 and the rotatable gantry portion 14 to provide electrical operating power to the x-ray tube 20. More specifically, a tube voltage (kV) is provided to bias the anode and cathode and a filament current is provided to adjust the tube current (mA) between the cathode and the anode. At a higher tube current, the x-ray beam generated by the interaction of the tube current and the anode has a higher x-ray energy fluence. The x-ray tube has an outlet port 22 through which radiation is directed toward the cylinder 16. A collimator and shutter arrangement 24 shapes the emitted radiation into a thin, fan-shaped beam which spans a scan circle, i.e. a circular imaging region 26 within the cylinder 16. A shutter selectively gates the radiation beam on and off. Radiation from the fan beam which has traversed the scan circle 26 impinges upon an array of radiation detectors 28. In the preferred embodiment, the radiation detectors 28 are arranged in a complete circle on the stationary gantry portion. Alternately, an arc of radiation detectors can be mounted to the rotating gantry portion to rotate with the radiation source. An angular position monitoring means or resolver 30 monitors the angular position of the x-ray source 20 relative to a subject disposed in the scan circle 26. The angular position resolver 30 produces an indication of the current angular position of the x-ray source relative to the cylinder 16, hence the subject. Each of the detectors 28 is connected With a sampling means 40 which samples a group of the detectors 28 which are receiving incident radiation. Each time the x-ray source rotates a preselected angular increment relative to the subject, the sampled group of detectors is incremented around the circle. In this manner, electronic data is collected which represents radiation attenuation along a preselected multiplicity of paths through the subject. A reconstruction means 42 reconstructs the radiation attenuation data using a filtered backprojection or other conventional algorithm into n image representation which is stored in an image memory 44 for display on a video monitor 46. A digital motor speed controller 50 controls the motor 18 in order to control the angular velocity of the rotating gantry portion 14 relative to the stationary cylinder 16. A velocity versus angular position means 52, such as a look-up table, provides rotational speed information to the digital motor controller. More specifically to the preferred embodiment, the angular position resolver 30 addresses the angular position versus motor speed look-up table 52 with the current angular position to retrieve a corresponding motor speed designation which is communicated to the digital motor control 50. Each time the angular position resolver 30 senses a preselected increment f angular rotation, such as the angle spanned by one of detectors 28, the angular position resolver 30 indexes the address to the angular position versus speed look-up table 52 to provide the digital motor control 50 with an updated angular velocity. Of course, other means may be provided for converting angular position indications into speed control signals. The angular position versus velocity look-up table may be programmed various ways. For example, the height and width dimensions of the region of interest of the patient can be measured and compared with a plurality of preselected height to width dimensions. A preprogrammed look-up table corresponding to the most similar height and width dimensions is loaded into the look-up table memory 52 to control rotational speed during the upcoming scan. This table can be derived through manual calculations, trial and error, or the like. In the preferred embodiment, the angular position versus angular velocity look-up table 52 is derived empirically for each patient. An x-ray energy fluence measuring means 60 determines the average x-ray energy fluence which is being received by the arc of radiation detectors irradiated by the fan beam at each angular position of the x-ray tube 20 during a prediagnostic imaging scan. An x-ray energy fluence rate versus angular position table 62 correlates the measured x-ray energy fluence with angular position around the subject. An x-ray energy fluence rate reference means 64, such as a computer memory, stores one or more preselected reference x-ray energy or fluence rate at which the detectors 28 operate optimally. Preferably, the look up table includes a first reference x-ray energy fluence for diagnostic scanning and a second, lower reference x-ray energy fluence for screening. Optionally, additional higher and lower reference levels may be provided for other scanning procedures. A difference means 66 subtracts the x-ray energy fluence rate measured at each angle from the reference x-ray energy fluence rate to determine an x-ray energy fluence error or deviation for each angular position of the x-ray tube. An x-ray energy fluence error versus angular position memory means 68 correlates the x-ray energy fluence error or deviation corresponding to each angular position of the x-ray tube. An x-ray energy fluence correcting means 70 calculates a speed or speed change which is projected to eliminate the x-ray energy fluence deviation. For example, the x-ray energy fluence correction means 70 may determine the percentage by which the x-ray energy fluence deviation or error differs from the standard reference x-ray energy fluence and increase or decrease the selected rotation speed by the same percentage. Optionally, the x-ray energy fluence correction means may take other variables into account. Optionally, a maximum x-ray energy fluence detecting means 72 detects the maximum x-ray energy fluence detected by any detector at each angular position and stores it in a maximum x-ray energy fluence versus angular position look-up table 74. The correction means 70 then determines the effect which the determined speed correction should have on each maximum x-ray energy fluence. For example, if the x-ray energy fluence deviation is 5% of the reference x-ray energy fluence, slowing the rotational speed by 5% can be expected to increase both the maximum and the average x-ray energy fluence by 5%. The maximum speed change allowed may be limited by the percentage difference between the maximum x-ray energy fluence for each angular position and the reference. As another alternative, the technique discussed above may be used on the fly during a diagnostic scan to adjust speed, monitor for malfunctions, and the like. The average x-ray energy fluence means 60 can determine the currently measured x-ray energy fluence and use it to adjust the motor speed. This procedure is always one angular increment behind. When the motor speed corrections are calculated on the fly, a complete look-up table is not necessary. Rather, the speed indicating means 52 may be a counter, or the like, which is indexed up or down by the appropriate percentage to indicate the now desired motor speed to the digital motor control means 50. A rate of angular velocity change limiting means, such as an integrating or averaging circuit in the speed indicating means, may be used to assure that the gantry changes angular velocity smoothly without jerky movements. Of course, the x-ray energy fluence impinging on the subject at each angular position of the x-ray tube can be adjusted in ways other than changing the speed at which the x-ray tube rotates. For example, the x-ray energy fluence produced by the x-ray tube may be varied in accordance with the angular position. To this end, a tube current means 80 identifies a selected tube current for each angular position. This may again be a look-up table analogous to table 52 for which the tube currents or current variations are calculated as described above. An x-ray tube control circuit 82 controls the operating parameters of the x-ray tube 20. More specifically, the x-ray tube operating circuit includes a control circuit for controlling the tube current (mA). The x-ray tube control circuit 82 also controls the tube voltage applied between a filament or cathode 90 and an anode or target 92. Electrons boiled off from the heated filament are propelled by the tube voltage to travel from the filament to the anode to create the tube current, commonly measured in milliamps (mA). Preferably, the tube current is controlled by the bias on a grid disposed between the cathode and the anode. Optionally, the tube current can be adjusted by adjusting the current supplied to an x-ray tube filament to control its heating. The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.