Method for tracking X-ray markers in serial CT projection images
A method and system of rapidly detecting and mapping a plurality of marker points identified in a sequence of projection images to a physical marker placed on a patient or other scanned object. Embodiments of the invention allow a marker physical mapping module executable by an electronic processing unit to obtain a sequence of images based on image data generated by a scanner. Each image in the sequence of images represents an angle of rotation by the scanner and includes a plurality of marker points each having a U position and a V position. Expected U and V positions for the physical marker are calculated and a list of candidate marker points is created based on the expected positions. The images are processed and selected candidate marker points are added to a good point list.
1. A method of associating marker projections with radiopaque markers affixed to a patient, the method executed by an imaging system including a scanner, a computer with an electronic processing unit, and a memory module storing a module executable by the electronic processing unit, the method comprising:
generating, with the scanner, a sequence of computer tomography projection frames comprising at least a first projection frame, wherein the first projection frame includes a first marker projection;
determining, with the electronic processing unit, an approximate 3D physical location in relation to the scanner of a first radiopaque marker affixed to a patient;
based at least on the approximate 3D physical location of the first radiopaque marker, calculating, with the electronic processing unit, a first expected marker projection location in the first projection frame, wherein the first marker projection and the first expected marker projection location are separated from each other by a first distance in at least one dimension;
based at least on the first distance being within an expected range, determining, with the electronic processing unit, that the first marker projection is a projection of the first radiopaque marker; and
correcting for patient motion based at least on determining that the first marker projection is a projection of the first radiopaque marker.
2. The method of claim 1 , wherein the step of determining the approximate 3D physical location of the first radiopaque marker comprises:
selecting a plurality of candidate 3D locations of the first radiopaque marker;
computing an error value of each of the plurality of candidate 3D locations, the step of computing an error value comprising:
for each frame of a plurality of frames in the sequence, determining an error between an expected projection location of the candidate 3D location and an actual projection location of the first radiopaque marker in the frame, and
computing a sum of the errors determined in the step of determining an error; and
selecting a candidate 3D location having a lowest sum error value of the computed error values.
3. The method of claim 1 , wherein the first distance is in a first dimension, wherein the first marker projection and the first expected marker projection location are further separated from each other by a second distance in a second dimension, the method further comprising:
determining the first distance in the first dimension;
determining the second distance in the second dimension, wherein the step of determining whether the first marker projection is a projection of the first radiopaque marker is based on the first and second distances.
4. The method of claim 1 , wherein the step of determining whether the first marker projection is a projection of the first radiopaque marker comprises:
if both the first marker projection and a second marker projection are near the first expected marker projection location, performing the steps of:
a. computing a trajectory of projections of the first radiopaque marker in a plurality of projection frames;
b. identifying which of the first and second marker projections is closer to the trajectory; and
d. associating the identified marker projection with the first radiopaque marker.
5. The method of claim 1 , wherein the radiopaque markers are affixed to the patient in a manner permitting relative motion between the radiopaque markers.