Abstract: The effective range of an analog linear-log converter is extended above the number of output decades over which this converter is reasonably linear to cover several additional decades of log conversion. This is made possible by adding a variable gain amplifier having several decades of switchable gain to the input of the converter. Switching between decades to utilize the added decades of output is controlled by a microcomputer which monitors the output level. This switching restores the signal level of each of the added decades to the same level as that of an earlier decade thus maintaining the same slope accuracy and noise levels of the earlier decades throughout the whole conversion range. The microcomputer also provides slope corrections from a lookup table for each of the different output decades as well as also applying to the output corrections for offsets due to the circuit components. Ambient temperature compensation is also provided following the linear-log converter for increased precision.

Abstract: Computerized tomographic X-ray apparatuses direct X-rays, usually in a fan-shaped distribution, through the body of a patient. In some examples the fan distribution may subtend angles of 50.degree. or more at the source. It has been found that such tubes, particularly of the rotating anode type, tend to have X-ray emission from regions surrounding the focal spot and it is not convenient to use anode shields to eliminate it. This off-focus radiation can present problems, especially when attenuators are used to equalize absorbing paths through the patient, since the off-focus radiation can sometimes be less attenuated than the main beam. It is proposed to correct the signals produced by the X-ray detectors, for off-focus radiation. A profile of off-focus or halo radiation is produced and the attenuation imposed on it is estimated.