A radiographic and tomographic imaging system integrated into a medical linear accelerator for localization of bone and soft-tissue targets

Purpose:Dose escalation in conformal radiation therapy requires accurate fi eld placement. Electronic portal imaging devices are used to verify field p lacement but are limited by the low subject contrast of bony anatomy at meg avoltage (MV) energies, the large imaging dose, and the small size of the r adiation fields. In this article, we describe the in-house modification of a medical linear accelerator to provide radiographic and tomographic locali zation of bone and soft-tissue targets in the reference frame of the accele rator. This system separates the verification of beam delivery (machine set tings, field shaping) from patient and target localization. Materials and Methods: A kilovoltage (kV) x-ray source is mounted on the dr um assembly of an Elekta SL-20 medical linear accelerator, maintaining the same isocenter as the treatment beam with the central axis at 90 degrees to the treatment beam axis. The x-ray tube is powered by a high-frequency gen erator and can be retracted to the drum-face. Two CCD-based fluoroscopic im aging systems are mounted on the accelerator to collect MV and kV radiograp hic images. The system is also capable of cone-beam tomographic imaging at both MV and kV energies. The gain stages of the two imaging systems have be en modeled to assess imaging performance. The contrast-resolution of the kV and MV systems was measured using a contrast-detail (C-D) phantom, The dos imetric advantage of using the kV imaging system over the MV system for the detection of bone-like objects is quantified for a specific imaging geomet ry using a C-D phantom. Accurate guidance of the treatment beam requires re gistration of the imaging and treatment coordinate systems. The mechanical characteristics of the treatment and imaging gantries are examined to deter mine a localizing precision assuming an unambiguous object. MV and kV radio graphs of patients receiving radiation therapy are acquired to demonstrate the radiographic performance of the system. The tomographic performance is demonstrated on phantoms using both the MV and the kV imaging system, and t he visibility of soft-tissue targets is assessed. Results and Discussion: Characterization of the gains in the two systems de monstrates that the MV system is x-ray quantum noise-limited at very low sp atial frequencies; this is not the case for the kV system. The estimates of gain used in the model are validated by measurements of the total gain in each system. Contrast-detail measurements demonstrate that the MV system is capable of detecting subject contrasts of less than 0.1% (at 6 and 18 MV). A comparison of the kV and MV contrast-detail performance indicates that e quivalent bony object detection can be achieved with the kV system at signi ficantly lower doses (factors of 40 and 90 lower than for 6 and 18 MV, resp ectively). The tomographic performance of the system is promising; soft-tis sue visibility is demonstrated at relatively low imaging doses (3 cGy) usin g four laboratory rats. Conclusions: We have integrated a kV radiographic and tomographic imaging s ystem with a medical linear accelerator to allow localization of bone and s oft-tissue structures in the reference frame of the accelerator. Modeling a nd experiments have demonstrated the feasibility of acquiring high-quality radiographic and tomographic images at acceptable imaging doses. Full integ ration of the kV and MV imaging systems with the treatment machine will all ow on-line radiographic and tomographic guidance of field placement. (C) 19 99 Elsevier Science Inc.