| 摘要 |
<p>Alternate modes of digital subtraction angiography are described. In each, a sequence of alternate low and high energy x-ray exposures are made extending over an interval including periods before. during and after an x-ray contrast medium has entered and departed from the blood vessel of interest. In one mode the first low energy exposure or mask image is subtracted from the respective subsequent low energy images and the sequence of difference images are stored on disk. The high energy images subsequent to the first one or mask image are subtracted from the high energy mask and stored. The plot of the projected intensity of the contrast medium as a function of time is determined and the values of the bolus plot at any time are the matched filter coefficients by which the sequences of low energy and high energy difference images are multiplied in the matched filtering process. The matched filtered low and high energy sequence of difference images are summed in separate channels and the results are multiplied by weighting factors that result in bone being cancelled when the summed images are subtracted to produce the hybrid difference image in the alternative mode, the signals representative of the alternate low and high energy exposures are subtracted during acquisition and the sequence of energy subtracted difference images are stored on disk. Before subtraction they are multiplied by respective weighting coefficients for cancelling bone when they are subtracted. The sequence of energy-subtracted images are then accessed from disk and subjected to matched filtering and summed to produce the hybrid difference image. In both modes, negative matched filter coefficient values are used which produce negative areas below the abscissa of the plot equal to the positive area of the bolus plot above the abscissa which, in effect, produces a dc component in the image signal that is equal to zero so everything that is constant in the sequence of images cancels out by subtraction.</p> |
