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

Abstract

Purpose: The 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.

Method: Dose 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.

Result: Nominal 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.

Conclusion: The 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.

Keywords: brachytherapy; dose; image guidance; image quality; mobile CT.

Figure 1

Experimental setup. (a) Mobile CT scanner (Brainlab Airo) with custom brachytherapy phantom. (b) Vaginal cylinder. (c) Interstitial ring and tandem.

Figure 2

CT number linearity and accuracy. (a) Measured CT number for tissue‐equivalent inserts of varying relative electron density. Linear fits are superimposed: solid (helical, standard filter); variations of dashed and dotted (axial, all filters). (b) Measured CT number for tissue‐equivalent inserts of varying relative electron density vs reference HU values on a diagnostic CT scanner (CT/i, GE Healthcare).

Figure 3

Image uniformity. (a) Magnitude of cupping in the transverse plane for helical and axial mode, averaged over three reconstruction filters. (b) Transverse slice of a 20–cm‐diameter water phantom positioned 5 cm below isocenter, showing nonuniformity introduced by positioning off isocenter. (c) Coronal slice of the water phantom, helical and axial mode, illustrating stitching artifacts for the latter.

Figure 4

Spatial resolution. (a) MTF for helical and axial mode (standard filter). (b) MTF for helical scan mode for various reconstruction filters. (c) Images of line‐pair patterns in the ACR accreditation phantom for helical scan protocols from the Airo (left) and Philips Big Bore (right). MTF, modulation transfer function.

Figure 5

Image noise. (a) Noise measured as a function of mAs (or mAs_eff) in a 20‐cm‐diameter water phantom. (b) Noise measured for various reconstruction filters for axial mode. The spatial distribution “map” of image noise is shown in transverse and coronal planes in (c,e) helical and (d,f) axial modes (each for the standard filter).

Figure 6

NPS for various scan modes and reconstruction filter. (a) Transverse plane NPS in helical and axial modes with standard reconstruction filter. (b) Transverse plane NPS for helical mode and three reconstruction filters. (c) Transverse (f_x,f_y) and coronal (f_x,f_z) plane NPS are shown in (c‐f) for helical and axial modes (each with standard reconstruction filter). NPS, noise power spectrum.

Figure 7

Image NPS for the mobile CT (Airo) in comparison to diagnostic CT (Philips Big Bore). (a) Transverse plane NPS in helical mode, each with “standard” reconstruction filter and comparable dose (10.1 and 9.9 mGy, respectively), illustrating a factor of ~2 in spatial‐frequency cutoff between the two systems. (b) Transverse (f_x,f_y) and (c) coronal (f_x,f_z) plane NPS for the Airo (top) and Big Bore CT (bottom), each with “standard” reconstruction filter and comparable dose. NPS, noise power spectrum.

Figure 8

Low‐contrast performance: CNR and simulated soft‐tissue visualization. (a) CNR between simulated adipose and polypropylene background measured in a 16‐cm phantom. (b) Example helical and axial mode images of low contrast resolution inserts (Module 2) in ACR 464 CT accreditation phantom. CNR, contrast‐to‐noise ratio.

Figure 9

Soft‐tissue visualization and artifacts in an anthropomorphic phantom. Example helical and axial mode images of (a) coronal slices in regions of the pelvis, including an interstitial brachytherapy catheter (standard filter), (b) sagittal slices in pelvic region (soft filter), and (c) transverse slices of an abdomen phantom in regions containing a variety of low‐ and high‐contrast spheres in the liver (soft filter). Note the windmill artifacts evident about high‐contrast, high‐frequency structures such as the vertebrae for helical mode (c) and the stitching artifacts evident in coronal and sagittal planes for axial mode (a, b).

Figure 10

Images of an anthropomorphic phantom containing surgical instruction. (A) Helical mode: although the MAR algorithm reduces shading and streak artifacts associated with beam hardening (arrows (a) and (b)), helical mode is susceptible to windmill artifacts about high‐contrast, high‐frequency structures (i.e., surgical instrumentation) that are not addressed by MAR. (B) Axial mode: the MAR algorithm is seen to reduce shading and streaks associated with beam hardening (arrows (a) and (b)). MAR, metal artifact reduction.

Figure 11

Brachytherapy phantom: vaginal cylinder. (A) 3D rendering of (a) vaginal cylinder, (b) low‐density tissue‐simulating inserts, (c) higher density bone‐simulating inserts, (d) plastic support rod, (e) Delrin rods (simulating femurs), (f) Superflab, and (g) stainless steel vaginal tube. (B) Coronal slice showing the vaginal cylinder and surrounding low‐density inserts as well as an air pocket (h) visible at the tip of the cylinder. (C) Transverse slice showing the vaginal cylinder and surrounding tissue‐simulating inserts (each labeled according to relative electron density).

Figure 12

Brachytherapy phantom: interstitial ring and tandem. (A) 3D rendering of (a) interstitial ring, (b) low‐density tissue‐simulating inserts, (c) higher density bone‐simulating insert, (d) plastic support rod, (e) Superflab, (f) Delrin rods (simulating femurs), (g) interstitial needles, (h) first dwell position marker, and (i) tandem rod. (B) Coronal slice showing the interstitial ring, low‐density inserts, high‐density inserts, first dwell position, and tandem rod. (C) Transverse slice showing interstitial needles, tandem rod, and tissue‐simulating inserts (each labeled according to relative electron density).