Abstract: A spectral correction algorithm for correcting dense object-induced spectral artifacts is described. In one embodiment, a calibration object, representative of typical head scanning conditions is scanned and the data reconstructed to provide an image. A water or water-equivalent cylinder of about the same diameter also is scanned and reconstructed, on the same display field of view (DFOV). These two images are designated respectively by BWEQ and WEQ. The ratio of images BWEQ and WEQ is then evaluated, and a region of interest extracted by multiplying the ratio by a function II(r), to obtain a calibration pattern CP. The calibration pattern is then averaged in azimuth to obtain a calibration vector. This calibration vector is fitted with low--order polynomial, and then divided by the fitting polynomial, to take out from the vector the low frequency components that, for instance, would be introduced on an "ideal" scanner. By subtracting 1.

Abstract: Methods and apparatus for performing channel dependent and gain dependent smoothing filter (across channels) to compensate for noise in a multislice imaging system are described. The smoothing mainly affects the radial resolution. The filter can be combined with the matrix deconvolution filter typically used in multi-slice scanners. The filter is channel and DAS gain dependent and provides that an image generated from data collected in a multi-slice scan has about the same image quality, e.g., noise reduction, as images generated by other types of scanners.

Abstract: The present invention, in one form, is a system which, in one embodiment, determines an x-ray beam z-axis profile and properly positions a detector array. Specifically, in one embodiment, signals intensities from the detector array are used to determine the optimal position of the detector array. In addition, the signal intensities are utilized to adjust an aperture of a pre-patient collimator. By adjusting the collimator aperture an x-ray beam umbra is aligned with the edges of the detector array. As a result, an optimal x-ray beam is radiated toward the detector array.

Abstract: An X-ray tube comprises an anode assembly having an associated anode and a cathode assembly for providing a focused electron beam directed to the anode. An anode target, associated with the anode assembly, accelerates the electron beam and produces X-rays upon electron impact with the anode target. Off-focal radiation tends to be most intensely emitted by the top surface of the target. In conventional X-ray tubes, the off-focal radiation as seen by the detector encompasses almost the entire top surface of the target assembly. In accordance with the present invention, the anode target comprises a cut-out top surface for reducing off-focal radiation, by blocking the off-focal radiation from the top surface from reaching the detector. Blocking the radiation produced at the top surface becomes more important as the ratio of the track area visible at the detector decreases with respect to the top surface area.

Abstract: Methods and apparatus for dose reduction in a computed tomography (CT) system are described. In one embodiment, the CT system includes a configurable multislice detector array and an adjustable source collimator. The detector array includes a photodiode cell array optically coupled to a scintillator array. The photodiode array includes a plurality of photodiodes arranged in rows and columns that may be combined to collect slice data from a number of inner and outer slices. The CT system also includes an x-ray source and a collimator. The x-ray source generates an x-ray beam that is collimated by the collimator to define the thickness of each outer slice. In operation, an operator determines the quantity and thickness of inner slices and the thickness of each outer slice. After altering the detector and collimator configuration, as defined by an operator, slice data for each inner slice and each outer slice is gathered.

Abstract: The present invention, in one form, is an x-ray CT system that modulates x-ray tube current as a function of gantry angle and reconstruction algorithm weighting coefficients. View indexes are stored in a table, and during scanning, values are periodically read from this table to determine weighting coefficients. An algorithm is applied to the weighting coefficients to generate an x-ray tube modulating factor. This modulating factor is then applied to the x-ray tube current to generate modulated x-ray tube current.

Abstract: The present invention, in one form, is a method for determining and applying a ZCAL correction vector to the channel outputs from the detector array in a multi-slice CT system. More specifically, and in accordance with one embodiment of the present invention, the signal gain for each detector cell, or channel, at multiple z-axis locations is measured. For each channel, a correction polynomial Q.sub.n is fitted to the signal variation curve as a function of z-axis position so that the correction polynomial value multiplied by its channel signal produces a constant value over the operational range of z-axis positions. Beam positions (Z.sub.r) are measured as a function of each position reference signal (D.sub.r) and the Z position of the beam at the sensor channel and the location of the focal spot are determined. The z-axis location for each channel and the ZCAL correction vector are then determined. The ZCAL correction vector is then multiplied by the channel signal to calibrate the channel signal.

Abstract: Precise alignment of the focal spot position on an x-ray CT system is achieved using a deflection coil that produces a magnetic field which acts on the electron beam path in the x-ray tube. A variable current power supply drives the deflection coil and is controlled by input signals to align the focal spot at a static reference position, to correct for focal spot drift between scans, and to wobble the focal spot position during a scan or between scans.

Abstract: Image artifacts are reduced in a volumetric CT scanner system by decreasing the detector pitch along one or both dimensions of a 2D detector array. Four different embodiments for reducing detector pitch without reducing the size of each detector element are disclosed.

Abstract: An x-ray CT system acquires scout data over a region of a patient prior to performing a scan. The scout data is employed to locate the patient in a succession of slices over the region. Geometric scan parameters including display field of view (DFOV), scan field of view (SFOV) and patient centering offsets (X.sub.OFF,Y.sub.OFF) are calculated from the scout data, displayed to the operator, and used as default values in the subsequent scan of the region.

Abstract: An x-ray CT system modulates x-ray tube current as a function of gantry angle to reduce the total patient dose without significantly increasing image noise. A scout scan is performed to acquire attenuation data which enables an optimal current modulation profile to be calculated for each slice in the scan. The modulation profile is clipped when the modulation limit of the x-ray generator is reached.

Abstract: A 3rd generation CT system produces gain calibration factors (.gamma.) for each channel of a detector array while a scan is being performed. Redundant views are acquired during the scan but the x-ray intensity values in the second view are shifted to the adjacent detector channel by focal spot wobbling the x-ray source. Gain calibration factors (.gamma.) are calculated using the ratio (.beta.) of x-ray intensity values measured at adjacent detector channels.

Abstract: An x-ray CT system performs a multi-slice scan in which tube current is modulated to reduce the x-ray dose without increasing image noise. Scout data is acquired from which a relative attenuation function (RAF) is calculated to relate expected patient attenuation of the x-ray beam at each slice to a reference. Tube current commands (mA) for each slice are calculated from the RAF and used to acquire image data.

Abstract: An x-ray CT system modulates x-ray tube current as a function of gantry angle to reduce the total patient dose without significantly increasing image noise. A scout scan is performed to acquire attenuation data which enables an optimal current modulation profile to be calculated for each slice in the scan.

Abstract: A CT apparatus for reducing aliasing in reconstructed images uses an x-ray tube with a translatable focal spot to double the spatial sampling rate, over that achieved by a conventional CT machine. Radial resolution artifacts in the image, identified to the "bleeding through" of previous samples from different focal spot positions into the present sample are removed by a convolution process employing the inverse of the detector transfer function. Timing of the data sampling with respect to the changing of the wobble positions is also employed to minimize the bleed through and to improve signal-to-noise ratio.

Abstract: A CT apparatus reduces errors in projection data acquired in helical scanning. The imaged object moves concurrently along a translation axis and the x-ray beam is periodically translated with the imaged object so as to subtend a single predetermined volume element during the acquisition of one projection set of data for a first slice. The x-ray beam then returns to its starting position and tracks a second predetermined volume element within a next slice. The x-ray beam may be translated by moving the focal point or a collimator or a combination of both. Helical scans with a pitch requiring sweeping of the x-ray beam beyond the detector limits are accommodated by limiting the sweep to a lessor compliance distance. The angular rate of the sweep is held constant within this compliance distance during the sweep.

Abstract: A CT apparatus reduces errors in projection data acquired in helical scanning. The imaged object moves concurrently along a translation axis and the x-ray beam is periodically translated with the imaged object so as to subtend a single predetermined volume element during the acquisition of one projection set of data for a first slice. The x-ray beam then returns to its starting position and tracks a second predetermined volume element within a next slice. The x-ray beam may be translated by moving the focal point or a collimator or a combination of both.

Abstract: A computed tomography system corrects image errors resulting from misalignment of the fan beam as a result of placement of the x-ray tube, thermal drift of the x-ray tube anode of mechanical stresses of the gantry or x-ray tube. Misalignment is determined alternately by means of a z-axis offset detector which detects movement of the exposure area of the CT fan beam or a predictive model that calculates thermal drift of mechanical deflection of the focal spot of the x-ray tube based on previous use. The system includes a collimator with a controllable z-axis position which may be controlled to reduce z-axis offset of the fan beam exposure area or to make the fan beam more parallel with the imaging plane of the CT system.