Evaluation of the iterative kilovoltage cone-beam computed tomography reconstruction method for the Varian TrueBeam system

Abstract

Introduction: Radiotherapy is used to treat patients with various types of cancer, where linear accelerators produce the radiation beam used for the treatment. Before each treatment fraction, the patient needs to be correctly positioned according to the treatment plan. Patient setup is often performed using a build-in kV cone beam computed tomography (CBCT) system. Recently Varian has implemented a new CBCT reconstruction algorithm, based on iterative reconstruction (iCBCT). The purpose of this work was to evaluate the Varians iCBCT reconstruction method and compare it with the standard CBCT recontruction method.

Method: In this study, three types of phantom were used: Alderson Rando phantom, Kyoto Kagaku phantom and Catphan 604 phantom, and eight existing patients images. The image quality was evaluated using Catphan phantom in DoseLab. The image quality was evaluated by calculating contrast, HU constancy, noise, uniformity and spatial resolution. The impact of various reconstruction filters: smooth, standard and sharp and noise suppression parameters (for iCBCT) were studied using Kyoto Kagaku phantom for head and pelvis mode. The exposure mAs was decreased with iCBCT for both head and pelvis mode with Alderson Rando and Kyoto Kagaku phantom to achieve similar image quality as with standard CBCT visually. The CT dose index-volume (CTD Ivol) was measured for head and pelvis mode using CTDI phantoms and an ion chamber. The CBCT acquisition and reconstruction time for head and pelvis mode were measured using Kyoto Kagaku phantom.

Results: Phantom and patient images showed that iCBCT significantly improves the image quality by reducing the image noise and artifacts on the images. The adjustable parameters of the iterative reconstruction have much less impact on the image quality than the reconstruction method. To visually achieve similar image quality for iCBCT and standard CBCT mAs was reduced. This resulted in a decrease of measured CTD Ivol of about 70% for a pelvis scan and about 50% for a head scan. The reconstruction time for iCBCT is somewhat longer than the standard reconstruction method. For the phantom used in this work, the measured difference is 15 s for pelvis and 12 s for head.