| 摘要 |
An in-line 4D cone beam CT reconstruction algorithm, i.e. one that works in parallel with image acquisition, comprises an imaging system for a object exhibiting internal periodic motion comprising a source of penetrating radi ation and a two-dimensional detector for the radiation, the source and the d etector being rotateable around an axis lying on the beam path from the sour ce to the detector, a storage means for images obtained from the detector, a control means for initiating rotation of the source and the detector and fo r obtaining images from the detector at a plurality of rotation angles over time, a processing means for (i) condensing the images in a direction transv erse to an axis to produce a one-dimensional image, (ii) collating the one-d imensional images obtained up to that point side-by-side into a two dimensio nal image, (iii) analysing the two-dimensional image thus obtained to identi fy periodic patterns, (iv) allocating phase information to the images in the storage means on the basis of that analysis, (v) selecting images in the st orage means having like phase information, and (vi) backprojecting the selec ted images, the control means being adapted to invoke the processing means a fter a plurality of images have been placed in the storage means, and then p lace further images in the storage means and further invoke the processing m eans. Thus, we queue a limited number of projection images such that the pha se determination algorithm can look-ahead. At regular intervals, the queue i s scanned and those images which have enough look-ahead to obtain phase info rmation are filtered and back-projected. The algorithm thus keeps up with th e image acquisition speed and produces a 4D reconstruction within a few seco nds of the end of scanning. A local rigid registration algorithm is then use d to match the tumor region defined in the mid-ventilation frame of our 4D p lanning CT with each of the phases of the 4D CBCT. An animation technique pr ovides rapid visual verification; the mean position of the tumor is computed and used for correction, while the amplitude is reviewed to validate the ma rgin.
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