Slow gantry rotation acquisition technique for on-board four-dimensional digital tomosynthesis

Four-dimensional cone-beam computed tomography (4D CBCT) has been investigated for motion imaging in the radiotherapy treatment room. The drawbacks of 4D CBCT are long scan times and high imaging doses. The aims of this study were to develop and investigate a slow gantry rotation acquisition protocol for four-dimensional digital tomosynthesis (4D DTS) as a faster, lower dose alternative to 4D CBCT.

This technique was implemented using an On-Board Imager kV imaging system (Varian Medical Systems, Palo Alto, CA) mounted on the gantry of a linear accelerator. The general procedure for 4D DTS imaging using slow gantry rotation acquisition consists of the following steps: (1) acquire projections over a limited gantry rotation angle in a single motion with constant frame rate and gantry rotation speed; (2) generate a respiratory signal and temporally match projection images with appropriate points from the respiratory signal; (3) use the respiratory signal to assign phases to each of the projection images; (4) sort projection images into phase bins; and (5) reconstruct phase images. Phantom studies were conducted to validate theoretically derived relationships between acquisition and respiratory parameters. Optimization of acquisition parameters was then conducted by simulating lung scans using patient data. Lung tumors with approximate volumes ranging from 0.12 to 1.53 cm3 were studied.

A protocol for slow gantry rotation 4D DTS was presented. Equations were derived to express relationships between acquisition parameters (frame rate, phase window, and angular intervals between projections), respiratory cycle durations, and resulting acquisition times and numbers of projections. The phantom studies validated the relationships, and the patient studies resulted in determinations of appropriate acquisition parameters. The phase window must be set according to clinical goals. For 10% phase windows, we found that appropriate frame rates ranging from 2 to 5 frames/s, gantry rotation speeds ranging from 0.44 to 1.03 degrees/s, and aiming for an approximate maximum angular interval of 3.4 degrees between projections in phase bins were appropriate for dose, scan time, and tumor visibility optimization. Adequate tumor visibility was achieved for coronal 4D DTS images of all three lung tumors with acquisition times ranging from 0.45 to 2.12 min. vs. 1.84 to 4.24 min for 4D CBCT. 4D DTS imaging doses ranged from 0.12 to 0.72 times the dose of a standard CBCT scan vs 0.48 to 1.44 times the dose of a standard CBCT scan for 4D CBCT.

A slow gantry rotation acquisition technique for 4D DTS was developed and investigated. Study results indicated that 4D DTS is a feasible technique for imaging lung tumor motion in the treatment room and requires shorter acquisition times and less imaging dose than 4D CBCT for larger tumors that do not require large scan angles for sagittal views and for situations where only coronal views are needed to meet clinical needs.