Variance reduction for monte carlo-based scatter estimation
A method rotates a radiation source and a detector over a sequence of acquisition angles about a subject and acquires, at each acquisition angle, a 2D projection image. A simulation subset is formed that contains some, but not all, of the acquired 2D projection images, wherein subset membership is determined according to relative signal change between successive 2D projection images. Scatter is characterized for the acquired sequence of projection images according to the formed simulation subset. One or more of the acquired sequence of projection images is corrected using the scatter characterization. An image volume is reconstructed and stored according to the sequence of scatter corrected projection images. One or more images of the reconstructed image volume are rendered to a display or transmitting the stored image volume data.
1. A method for scattered radiation characterization, the method comprising:
revolving a radiation source and a radiation detector over a sequence of acquisition angles about a subject and acquiring, at each acquisition angle, a 2D projection image of the subject;
forming a simulation subset that includes some, but not all, of the acquired 2D projection images based on measured signal level changes as between successive acquired 2D projection images;
for each 2D projection image in the simulation subset, defining a simulation region on the radiation detector for x-ray scatter estimation, including identifying a region on the radiation detector as a low exposure region based on exposure values of the low exposure region satisfying a preset exposure threshold and defining the simulation region as the low exposure region;
characterizing x-ray scatter for the acquired sequence of 2D projection images in the simulation subset using simulated x-ray scatter impinging the defined simulation region and ignoring simulated x-ray scatter impinging areas outside the defined simulation region;
correcting the acquired sequence of 2D projection images using the characterized scatter;
reconstructing a 3D image volume according to the sequence of scatter corrected projection images; and
rendering one or more images of the reconstructed 3D image volume to a display or transmitting the reconstructed 3D image volume; and
measuring a magnitude of signal level change for each acquired 2D projection image as compared to a signal level of a preceding acquired 2D projection image, accumulating an accumulated sum of the measured magnitudes of signal level changes until the accumulated sum exceeds a preset signal level threshold, adding to the simulation subset the acquired 2D projection image whose measured magnitude of signal level change caused the accumulated sum to exceed the preset signal level threshold, and resetting the signal level threshold.
2. The method of claim 1 , wherein the step of characterizing x-ray scatter comprises executing Monte Carlo simulation for photon energy incident on the defined simulation region and ignoring photon energy directed outside the simulation region.
3. The method of claim 1 , wherein reconstructing comprises ignoring 2D projection image data related to x-ray system components.
4. The method of claim 1 , further comprising acquiring 2D projection images of a human hand, elbow, knee, or ankle.
5. An apparatus for obtaining a volume image of a patient, the apparatus comprising:
a radiographic volume imaging apparatus having an x-ray source and at least one x-ray detector, the x-ray source and the at least one x-ray detector configured to revolve about the patient;
a computer programmed with instructions for causing the computer to:
acquire 2D projection images of the patient using the radiographic volume imaging apparatus;
define a simulation subset including some, but not all, of the acquired 2D projection images of the patient;
for each acquired 2D projection image in the simulation subset, define a simulation region on the x-ray detector by identifying a region on the x-ray detector as a low exposure region based on exposure values of the low exposure region satisfying a preset exposure threshold and defining the simulation region as the low exposure region;
measure a magnitude of signal level change for each acquired 2D projection image as compared to a signal level of a preceding acquired 2D projection image, accumulate an accumulated sum of the measured magnitudes of signal level changes until the accumulated sum exceeds a preset signal level threshold, add to the simulation subset the acquired 2D projection image whose measured magnitude of signal level change caused the accumulated sum to exceed the preset signal level threshold, and reset the signal level threshold;
characterize scatter according to simulated scatter impinging the defined simulation region for each acquired 2D projection image in the simulation subset and ignoring simulated scatter impinging outside the defined simulation region;
apply scatter compensation to the acquired 2D projection images of the patient according to the characterized scatter; and
reconstruct a 3D volume image of a volume of interest of the patient using the scatter compensated 2D projection images of the patient; and
a display in signal communication with the computer for displaying one or more images of the reconstructed 3D volume image.
6. The apparatus of claim 5 , wherein the computer is further programmed with instructions for causing the computer to execute Monte Carlo simulation for photon energy incident on the defined simulation region and to ignore photon energy directed outside the simulation region.
7. The method of claim 6 , wherein characterizing x-ray scatter comprises executing the Monte Carlo simulation for photon energy incident on a defined simulation region and ignoring photon energy directed outside the simulation region.
8. A method for scattered radiation characterization, the method comprising:
revolving a radiation source and a radiation detector over a sequence of acquisition angles about a subject and acquiring, at each acquisition angle, a 2D projection image of the subject;
forming a simulation subset that includes some, but not all, of the acquired 2D projection images, including measuring a magnitude of signal level change for each acquired 2D projection image as compared to a signal level of a preceding acquired 2D projection image, accumulating an accumulated sum of the measured magnitudes of signal level changes change until the accumulated sum exceeds a preset signal level threshold is satisfied, adding to the simulation subset the acquired 2D projection image whose measured magnitude of signal level change caused the accumulated sum to exceed the preset signal level threshold to be satisfied, and resetting the signal level threshold;
for each acquired 2D projection image in the simulation subset, defining a simulation region on the radiation detector for x-ray scatter estimation;
characterizing x-ray scatter for the acquired sequence of 2D projection images in the simulation subset using simulated x-ray scatter impinging the defined simulation region;
correcting the acquired sequence of 2D projection images using the characterized scatter;
reconstructing a 3D image volume according to the sequence of scatter corrected projection images; and
rendering one or more images of the reconstructed 3D image volume to a display or transmitting the reconstructed 3D image volume.
9. The method of claim 8 , further comprising identifying a region on the radiation detector as a low exposure region based on exposure values of the low exposure region satisfying a preset exposure threshold and defining the simulation region as the low exposure region.
10. An apparatus for obtaining a volume image of a patient, the apparatus comprising:
a radiographic volume imaging apparatus having an x-ray source and at least one x-ray detector, the x-ray source and the at least one x-ray detector configured to revolve about the patient;
a computer programmed with instructions for causing the computer to:
acquire 2D projection images of the patient using the radiographic volume imaging apparatus;
define a simulation subset including some, but not all, of the acquired 2D projection images of the patient, including measuring a magnitude of signal level change for each acquired 2D projection image as compared to a signal level of a preceding acquired 2D projection image, accumulating an accumulated sum of the measured magnitudes of signal level changes change until the accumulated sum exceeds a preset signal level threshold is satisfied, adding to the simulation subset the acquired 2D projection image whose measured magnitude of signal level change caused the accumulated sum to exceed the preset signal level threshold to be satisfied, and resetting the signal level threshold;
for each acquired 2D projection image in the simulation subset, define a simulation region on the x-ray detector for x-ray scatter estimation;
characterize x-ray scatter according to simulated scatter impinging the defined simulation region for each acquired 2D projection image in the simulation subset;
apply scatter compensation to the acquired 2D projection images of the patient according to the characterized x-ray scatter; and
reconstruct a 3D volume image of a volume of interest of the patient using the scatter compensated 2D projection images of the patient; and
a display in signal communication with the computer for displaying one or more images of the reconstructed 3D volume image.
11. The apparatus of claim 10 , wherein the computer is further programmed with instructions for causing the computer to identify a region on the radiation detector as a low exposure region based on exposure values of the low exposure region satisfying a preset exposure threshold and to define the simulation region as the low exposure region.