| abstract |
Differential x-ray images can be produced by sub-tracting two different x-ray images which are produced by using two different quasi-monoenergetic x-ray spectra. Two different x-ray filters may be employed alternately to produce such spectra. For example, two different filters containing iodine and cerium may be employed alternately to produce two different x-ray spectra having peaks at different energies. The x-rays to be filtered may be derived from an ordinary x-ray tube which produces a continuous spectrum of x-rays over a wide band of energies. The purpose of producing the differential x-ray images is to subtract or cancel out the portions of the x-ray images which are due to the ordinary tissues of the patient, particularly the soft tissues, so that the presence of certain contrast substances in the patient''s body will be emphasized or enhanced. Such contrast substances include iodine, xenon or barium, introduced into the bloodstream, the lungs or the food canal of the patient. The ordinary soft tissues of the patient transmit x-rays of different energies to different extents. While the two different x-ray images can be balanced for any particular thickness of the patient, variations in such thickness over the field of view tend to upset such balance. Thus, without compensation for patient thickness variations, cancellation of the x-ray images due to the ordinary tissues of the patient can be achieved at only one value of patient thickness. In accordance with the present invention, such patient thickness compensation is achieved by adjusting the composition and density of the two x-ray filters, and by adjusting the high voltage supplied to the x-ray tube, so that the ratio of the two different x-ray images produced by soft tissues remains nearly constant over a wide range of patient thickness. Thus, substantial cancellation of the soft tissue images can be achieved over a wide range of variations in the patient thickness. The x-ray filtration is adjusted so that one of the x-ray filters produces a spectrum of transmitted x-rays having two peaks at energies below and above the energy of the peak produced by the other x-ray filter. Thus, the sum of the images produced by the two peaks tends to remain in a constant relationship to the image produced by the other spectrum, despite variations in patient thickness. The supply voltage to the x-ray tube is varied as an inverse function of the average patient thickness. Thus, the voltage is reduced when the average patient thickness increases. The adjustment of the supply voltage to the x-ray tube changes the two different quasi-monoenergetic x-ray spectra so as to optimize the compensation for patient thickness variations. |