Cone‐beam CT dose and imaging performance evaluation with a modular, multipurpose phantom

Abstract

Purpose A modular phantom for dosimetry and imaging performance evaluation in cone‐beam computed tomography (CBCT) is reported, providing a tool for quantitative technical assessment that can be adapted to a broad variety of CBCT imaging configurations and clinical applications. Methods The phantom presents a set of modules that can be ordered in various configurations suitable to a particular CBCT system. Modules include slabs containing a uniform medium, low‐contrast inserts, line‐spread features, and disk features suitable to measurement of image uniformity, noise, noise‐power spectrum (NPS), contrast, contrast‐to‐noise ratio (CNR), Hounsfield (HU) accuracy, linearity, spatial resolution modulation transfer function (MTF), and magnitude of cone‐beam artifact. Automated software recognizes the phantom configuration in DICOM images and provides structured reporting of such test measures. In any modular configuration, the phantom permits measurement of air kerma in central and peripheral locations with an air ionization chamber (e.g., Farmer chamber). The utility and adaptability of the phantom were demonstrated across a spectrum of CBCT systems, including scanners for orthopaedic imaging (Carestream OnSight 3D, Rochester NY), breast imaging (Doheny prototype, UC Davis), image‐guided surgery (IGS, Medtronic O‐arm, Littleton MA), angiography (Siemens Artis Zeego, Forcheim Germany), and image‐guided radiation therapy (IGRT, Elekta Synergy XVI, Stockholm Sweden). Results The phantom provided a consistent platform for quantitative assessment of dose and imaging performance compatible with a broad spectrum of CBCT systems. The purpose of the survey was not to obtain head‐to‐head performance comparison of systems designed for such distinct clinical applications. Rather, the survey demonstrated the suitability of the phantom to a broad spectrum of systems in a manner that provides characterization pertinent to disparate applications and imaging tasks. For example: the orthopaedic CBCT system (pertinent clinical tasks relating to high‐resolution bone imaging) was shown to achieve MTF consistent with imaging of high‐contrast trabecular bone structures (i.e., the MTF reduced to 10% at spatial frequency, f10 = 1.2 mm−1); the breast system (even higher‐resolution imaging of microcalcifications) exhibited f10 = 2.2 mm−1; the IGS system (tasks including both bone and soft‐tissue contrast resolution) provided f10 = 0.9 mm−1 and soft‐tissue CNR = 1.64; the angiography system (soft‐tissue body interventions) demonstrated CNR = 1.2 in soft tissues approximating liver lesions; and the IGRT system (pertinent tasks emphasizing HU linearity and image uniformity) showed linear response with HU values (R2 = 1), with a cupping artifact (tcup = 5.8%) due to x‐ray scatter. Conclusions The phantom provides an adaptable, quantitative basis for CBCT dosimetry and imaging performance evaluation suitable to a broad variety of CBCT systems. The dosimetry and image quality metrics are consistent with up‐to‐date methods for rigorous, quantitative,...

Theory, method, and test tools for determination of 3D MTF characteristics in cone‐beam CT

“…For axial mode MDCT, the null cone as described above should be recognized, although the measurement direction () may extend to larger angles (e.g., up to ~88° for a scanner with SAD = 1085 mm covering 32 mm in z ) and remain within the fully sampled region of the Fourier domain. Moreover, because MDCT scanners include automatic exposure control (AEC) that can strongly affect performance for small phantoms, the MTF test tool (e.g., the wedge or sphere phantom) should likely be placed within a larger attenuating medium — for example, a 16‐ or 32‐cm diameter cylinder — to better reflect operation in clinical use 14 . Furthermore, helical pitch is a parameter of considerable importance in MDCT image quality, and while it was not considered in the results reported above for CBCT, the test tools may provide a useful probe of the effects of pitch on spatial resolution and sampling effects.…”

“…Most of these are well known and established in textbooks and decades of modeling and measurement of the spatial resolution for 2D imaging systems 33–36 . Moreover, these principles are reflected in decades of MTF measurement for 2D projection imaging systems (e.g., flat‐panel detectors) 2–8 and 3D imaging systems (CT or CBCT) 9–18 . However, the systematic delineation of response functions and careful distinction of 2D or 3D domains make explicit a number of important points and clarify some potential ambiguities:…”

“…[33][34][35][36] Moreover, these principles are reflected in decades of MTF measurement for 2D projection imaging systems (e.g., flat-panel detectors) [2][3][4][5][6][7][8] and 3D imaging systems (CT or CBCT). [9][10][11][12][13][14][15][16][17][18] However, the systematic delineation of response functions and careful distinction of 2D ðu, vÞ or 3D ðx, y, zÞ domains make explicit a number of important points and clarify some potential ambiguities:…”

Validation of synthesized normal‐resolution image data generated from high‐resolution acquisitions on a commercial CT scanner

“…These include the estimation of technical measures such as the modulation transfer function (MTF) and noise power spectrum (NPS), dose relationships, scan time, etc. [1][2][3][4][5][6] Other key components in the development of a scanner include the evaluation of its impact on specific diagnostic tasks. [7][8][9] While the technical evaluations are typically phantom based, these clinical evaluations require patient imaging, comparing the new system with some existing, standard-of-care technology.…”

“…In the initial stages, engineering properties of the new scanner are established. These include the estimation of technical measures such as the modulation transfer function (MTF) and noise power spectrum (NPS), dose relationships, scan time, etc 1–6 . Other key components in the development of a scanner include the evaluation of its impact on specific diagnostic tasks 7–9 .…”