Technical assessment of a mobile CT scanner for image‐guided brachytherapy

Abstract

PurposeThe imaging performance and dose of a mobile CT scanner (Brainlab Airo®, Munich, Germany) is evaluated, with particular consideration to assessment of technique protocols for image‐guided brachytherapy.MethodDose measurements were performed using a 100‐mm‐length pencil chamber at the center and periphery of 16‐ and 32‐cm‐diameter CTDI phantoms. Hounsfield unit (HU) accuracy and linearity were assessed using materials of specified electron density (Gammex RMI, Madison, WI), and image uniformity, noise, and noise‐power spectrum (NPS) were evaluated in a 20‐cm‐diameter water phantom as well as an American College of Radiology (ACR) CT accreditation phantom (Model 464, Sun Nuclear, Melbourne, FL). Spatial resolution (modulation transfer function, MTF) was assessed with an edge‐spread phantom and visually assessed with respect to line‐pair patterns in the ACR phantom and in structures of interest in anthropomorphic phantoms. Images were also obtained on a diagnostic CT scanner (Big Bore CT simulator, Philips, Amsterdam, Netherlands) for qualitative and quantitative comparison. The manufacturer’s metal artifact reduction (MAR) algorithm was assessed in an anthropomorphic body phantom containing surgical instrumentation. Performance in application to brachytherapy was assessed with a set of anthropomorphic brachytherapy phantoms — for example, a vaginal cylinder and interstitial ring and tandem.ResultNominal dose for helical and axial modes, respectively, was 56.4 and 78.9 mGy for the head protocol and 17.8 and 24.9 mGy for the body protocol. A high degree of HU accuracy and linearity was observed for both axial and helical scan modes. Image nonuniformity (e.g., cupping artifact) in the transverse (x,y) plane was less than 5 HU, but stitching artifacts (~5 HU) in the longitudinal (z) direction were observed in axial scan mode. Helical and axial modes demonstrated comparable spatial resolution of ~5 lp/cm, with the MTF reduced to 10% at ~0.38 mm−1. Contrast‐to‐noise ratio was suitable to soft‐tissue visualization (e.g., fat and muscle), but windmill artifacts were observed in helical mode in relation to high‐frequency bone and metal. The MAR algorithm provided modest improvement to image quality. Overall, image quality appeared suitable to relevant clinical tasks in intracavitary and interstitial (e.g., gynecological) brachytherapy, including visualization of soft‐tissue structures in proximity to the applicators.ConclusionThe technical assessment highlighted key characteristics of dose and imaging performance pertinent to incorporation of the mobile CT scanner in clinical procedures, helping to inform clinical deployment and technique protocol selection in brachytherapy. For this and other possible applications, the work helps to identify protocols that could reduce radiation dose and/or improve image quality. The work also identified areas for future improvement, including reduction of stitching, windmill, and metal artifacts.

Cone‐beam CT imaging with laterally enlarged field of view based on independently movable source and detector

“…Thus, it is in general possible to image the whole patient as in conventional CT, but without table motion and with significantly extended scan time. Similar procedures are already feasible also with other devices, particularly for image-guided brachytherapy 38 or dentistry. 39,40 A large benefit of this technology with mobile imaging is that particularly in radiotherapy patients can be imaged direct in the treatment room and position, without the needs for additional patient transfer.…”

Intraoperative radiation therapy with 50 kV x‐rays: A multi‐institutional review

“…There are several imaging systems available, for example, Brain-Lab Mobius imaging (Airo 32), Neurologica CT on wheels, Ziehm Imaging, Medtronic CT, and Paracelsus. 66 Each of these systems is capable of providing in-room CT imaging.Next,images must be imported into the TPS for planning purposes. To make the process efficient, a treatment plan may be created on simulation imaging (pre-OR) and then deformed to in-room CT images.…”

Robotic Systems in Radiotherapy and Radiosurgery

“…Of particular interest for BT and proton therapy are mobile CT scanners due to their increased flexibility in positioning and a smaller footprint in the treatment room compared to conventional CT scanners. While the indication of mobile CT scanners is primarily in image-guided spine surgery, Chernavsky et al assessed the Airo scanner (Brainlab, Munich, Germany) for image-guided BT [76]. Furthermore, Oliver et al compared its performance to existing non-robotic CT scanners [77].…”