Megavoltage planar and cone‐beam imaging with low‐ Z targets: Dependence of image quality improvement on beam energy and patient separation

Purpose: The purpose of this study is to investigate the improvement of megavoltage planar and cone‐beam CT (CBCT) image quality with the use of low atomic number ( Z ) external targets in the linear accelerator. Methods: In this investigation, two experimental megavoltage imaging beams were generated by using either 3.5 or 7.0 MeV electrons incident on aluminum targets installed above the level of the carousel in a linear accelerator (2100EX, Varian Medical, Inc., Palo Alto, CA). Images were acquired using an amorphous silicon detector panel. Contrast‐to‐noise ratio (CNR) in planar and CBCT images was measured as a function of dose and a comparison was made between the imaging beams and the standard 6 MV therapy beam. Phantoms of variable diameter were used to examine the loss of contrast due to beam hardening. Porcine imaging was conducted to examine qualitatively the advantages of the low‐ Z target approach in CBCT. Results: In CBCT imaging CNR increases by factors as high as 2.4 and 4.3 for the 7.0 and 3.5 MeV ∕ Al beams, respectively, compared to images acquired with 6 MV . Similar factors of improvement are observed in planar imaging. For the imaging beams, beam hardening causes a significant loss of the contrast advantage with increasing phantom diameter; however, for the 3.5 MeV ∕ Al beam and a phantom diameter of 25 cm , a contrast advantage remains, with increases of contrast by factors of 1.5 and 3.4 over 6 MV for bone and lung inhale regions, respectively. The spatial resolution is improved slightly in CBCT images for the imaging beams. CBCT images of a porcine cranium demonstrate qualitatively the advantages of the low‐ Z target approach, showing greater contrast between tissues and improved visibility of fine detail. Conclusions: The use of low‐ Z external targets in the linear accelerator improves megavoltage planar and CBCT image quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.