Methods and apparatus for imaging in conjunction with radiotherapy

1. A method for imaging comprising:

generating a cone X-ray beam by directing a megavolt electron beam at a low-atomic-number target;

shaping the cone X-ray beam to match a shape of a volume of interest in a subject; and

detecting X-rays of the cone X-ray beam that have passed through the subject at an imaging X-ray detector.

2. A method according to claim 1 wherein shaping the cone X-ray beam comprises adjusting positions of leaves of a multi-leaf collimator.

3. A method according to claim 1 wherein the low-atomic-number target comprises a target of aluminum, carbon or beryllium.

4. A method according to claim 1 wherein the imaging is performed in combination with delivering radiation to the subject for radiotherapy and the method comprises switching from a high-atomic-number target used for radiotherapy to the low-atomic-number target.

5. A method according to claim 1 wherein the low-atomic-number target has a thickness sufficient to block passage of substantially all electrons in the electron beam.

6. A method according to claim 5 wherein the low-atomic-number target has a thickness of 3 mm or more.

7. A method according to claim 2 wherein the low-atomic-number target is supported on a gantry that is rotatable relative to the subject and the method comprises repeating:

shaping the cone X-ray beam to match a shape of a volume of interest in a subject; and

detecting X-rays of the cone X-ray beam that have passed through the subject at the imaging X-ray detector;

to obtain a plurality of images for a corresponding plurality of different angles of the gantry relative to the subject.

8. A method according to claim 7 comprising processing the plurality of images to provide one or more computed tomography images for at least in a plane passing through the volume of interest.

9. A method according to claim 7 wherein the volume of interest comprises a first volume of interest of a plurality of volumes of interest and the method comprises controlling the positions of the leaves of the multileaf collimator for each of the gantry angles to shape the cone X-ray beam to match the plurality of volumes of interest as viewed from a source of the cone beam.

10. A method according to claim 2 wherein the volume of interest is a first volume of interest of a plurality of volumes of interest and the method comprises controlling the positions of the leaves of the multileaf collimator to shape the cone X-ray beam to match the plurality of volumes of interest.

11. A method according to claim 1 wherein the volume of interest is a first volume of interest and the method further comprises:

shaping the cone X-ray beam to match a shape of a second volume of interest at least partially nested within the first volume of interest; and

detecting at the imaging X-ray detector X-rays of the cone X-ray beam shaped to match the shape of the second volume of interest that have passed through the subject.

12. A method according to claim 1 comprising processing the images by:

truncating the image at a boundary of the volume of interest projected onto the imaging X-ray detector; and

filling the truncated image outside of the boundary of the volume of interest with a fill image of an area surrounding the volume of interest.

13. A method according to claim 12 wherein the fill image comprises a digitally reconstructed radiograph of the subject.

14. A method according to claim 12 comprising acquiring the fill image by relaxing the shaping of the cone X-ray beam and detecting at the imaging X-ray detector X-rays of the cone X-ray beam with relaxed shaping that have passed through the subject.

15. A method according to claim 12 wherein relaxing the shaping of the cone X-ray beam comprises providing a full-field cone X-ray beam.

16. A method according to claim 1 wherein the imaging X-ray detector comprises an electronic portal imaging device.