Method for reconstructing a 3D image from 2D X-ray images

1. A method for reconstructing a 3D image from 2D X-ray images acquired with an X-ray imaging system, said method comprising the steps of:

a) receiving a set of 2D X-ray images of a region of a patient with said X-ray imaging system, wherein a calibration phantom comprising at least one radiopaque marker having a known 3D position in a coordinate system of said calibration phantom is placed on said patient during the acquisition of the set of 2D X-ray images, such that said acquired set comprises:

at least two 2D X-ray images containing each at least one detectable radiopaque marker of the calibration phantom; and

at least one 2D X-ray image wherein no radiopaque marker of the calibration phantom is automatically detectable;

a1) selecting, from said set of 2D X-ray images, reference images in which at least one radiopaque marker of the calibration phantom is detected automatically, and determining the 2D position of each detected marker in each of said reference images, all remaining images being classified as non-reference images;

a2) computing an optimal rigid transformation between a coordinate system of the X-ray imaging system and the coordinate system of the calibration phantom by optimizing the registration between the known 3D position of at least one marker of the calibration phantom and the corresponding 2D position of said marker detected in at least two different reference images using nominal projective geometry data of said X-ray imaging system, such that the distance between the projections of the 3D position of the marker in each reference image and the corresponding 2D position in said reference images is minimized;

a3) applying said optimal rigid transformation to the 3D position of said at least one marker of the calibration phantom to determine its respective transformed 3D position in the coordinate system of the X-ray imaging system;

a4) for each of said reference images, computing adjusted projective geometry data from the 2D position of the at least one marker detected in said reference image and said transformed 3D phantom marker position, such that the projection of said transformed 3D position using the adjusted projective geometry data fits optimally with the 2D position of corresponding marker;

a5) calculating a reconstructability criterion that characterizes the ability to reconstruct a 3D image with sufficient quality from the reference images only;

b) computing an initial 3D image within the coordinate system of the X-ray imaging system by using at least part of said 2D X-ray images with their respective projective geometry data; said computation being implemented by using:

if the reconstructability criterion calculated in step (a5) is met, the reference images with their respective adjusted projective geometry data; or

if the reconstructability criterion calculated instep (a5) is not met, the reference images with their respective adjusted projective geometry data and the non-reference images with their respective nominal projective geometry data

c) projecting said initial 3D image on the non-reference images and adjusting the respective projective geometry data of said non-reference images by registration of said non-reference images with the projection of the initial 3D image using an image-to-image registration technique;

d) computing an updated 3D image using the complete set of 2D X-ray images with their respective adjusted projective geometry data.

2. The method of claim 1 wherein one or several steps from (a1) to (d) are iterated until a registration quality measure is below a predetermined threshold or a predetermined number of iterations has been reached.

3. The method of claim 1 wherein a new iteration of selection step (a1) uses the projection of the at least one marker of the phantom whose position is known in the coordinate system of the X-ray imaging system after the previous registration step (a2) has been performed in order to improve the number of markers that are automatically detected.

4. The method of claim 1 , wherein said at least one radiopaque marker is ball-shaped or needle-shaped.

5. The method of claim 1 , wherein the calibration phantom comprises radiopaque markers covered by reflective material detected by an optical localizer.

6. The method of claim 1 , wherein the calibration phantom comprises surgical implants.

7. The method of claim 1 , wherein said at least one radiopaque marker comprises an electro-magnetic coil usable as a transmitter or a receiver of an electromagnetic localization devices embedded in a surgical navigation system.

8. The method of claim 1 , wherein the calibration phantom comprises a combination of ball-shaped radiopaque markers, needle-shaped radiopaque markers and electro-magnetic coils.

9. The method of claim 1 , wherein step (a2) is achieved by computing said transformation resulting in a best fit between 3D phantom marker position and 2D position of marker detected in said reference images back projected according to said nominal projective geometry data.

10. The method of claim 1 , wherein only one marker of said calibration phantom is detected on at least two 2D images and wherein the transformation in step (a2) is a translation.

11. The method of claim 1 , wherein in step (a4) the modifications applied to the nominal projective geometry data for computation of said adjusted projective geometry data for each reference 2D image consist of modifications of the position of the X-ray source in a plane parallel to the image detector.

12. The method of claim 1 , wherein in step (c) the adjustments applied to the nominal projective geometry data of said non-reference images are limited to translation and rotation in a plane parallel to the image detector.

13. The method of claim 1 , wherein in step (a), at least one radiopaque marker is automatically detected in at least two of said acquired 2D X-ray images which yield an angle of at least 15° to each other.

14. The method of claim 1 , wherein the calibration phantom comprises only one or two radiopaque marker(s).

15. The method of claim 1 , wherein the calibration phantom comprises localizing elements that enable the navigation of a surgical instrument in the reconstructed 3D image.