Portal imaging using modulated treatment beam

1. A system for producing a portal image from a treatment beam produced by a radiation therapy system, the combination comprising:

a chopper mounted on the radiation therapy system and positioned to intercept the treatment beam, the chopper being operable to modulate the intensity of low energy components in the treatment beam;

a detector mounted on the radiation therapy system and positioned to receive the treatment beam after it passes through the chopper and a subject being treated, the detector producing signals indicative of the treatment beam components passing through the subject;

means for separating the modulated signal components from the detected signal; and

means for producing an image with the separated signal components.

2. The system as recited in claim 1 in which the chopper includes two segments that are alternately moved into the treatment beam, wherein one of the segments is made of a material that absorbs a substantial portion of the low energy components in the treatment beam and an insubstantial portion of the higher energy components in the treatment beam.

3. The system as recited in claim 2 in which the chopper is a rotatable wheel.

4. The system as recited in claim 1 in which the chopper is positioned to intercept the treatment beam before it passes through the subject.

5. The system as recited in claim 1 in which the detector produces said signals in the form of a series of image frames and each signal indicates the intensity of the detected treatment beam at one pixel location in the image frames.

6. The system as recited in claim 5 in which each image frame is a digitized representation of the signal values at each pixel location at a point in time; and

the means for separating the modulated signal components includes a programmed processor that receives the image frames and processes them.

7. The system as recited in claim 6 in which the processing includes producing difference image frames by subtracting successive pairs of received image frames.

8. The system as recited in claim 7 in which the rate at which the detector produces image frames is twice the rate at which the lower energy components in the treatment beam are modulated.

9. The system as recited in claim 1 in which the means for separating includes a lock-in circuit.

10. The system as recited in claim 1 in which the means for separating includes a demodulator circuit.

11. The system as recited in claim 10 in which the demodulator circuit includes a multiplier circuit and a low pass filter circuit.