Device for computed tomography

Claim 1

In a device for determining the radiation absorption distribution in a plane of examination which comprises radiation source means for irradiating the plane of examination with penetrating radiation from a large number of directions; detector means including a plurality of detector elements for measuring intensity values of the radiation which passes through the plane of examination; measuring means for measuring the intensity of the radiation before it passes through the plane of examination; first arithmetic means for determining the absorption distribution in the plane of examination from the intensity values; and adjusting means for influencing the intensity of radiation emitted by the source means so that said intensity is larger in first regions of the plane then it is in second regions of the plane, said first regions absorbing radiation more strongly then said second regions; wherein, as an improvement, the device further comprises additional arithmetic means for producing, for measuring values along all measuring paths at each angular position, adjusting signals for controlling the adjusting means, which additional arithmetic means include: second arithmetic means for determining a mean noise value (S(θ)) over all measuring values of the intensity (I(θ), p)) obtained from a detector element at position p within the detector means and at an angular position (θ), which is proportional to the sum of the quantum noise of all measuring values; third arithmetic means for determining a ratio of the mean noise value determined by the second arithmetic means and a predetermined, constant, maximum noise value; and fourth arithmetic means for producing the adjusting signal from the ratio, the adjusting signal being the λ power of the ratio, λ being greater than zero and less than one, the adjusting signal being applied to control the adjusting means.