Peter Mah

Deriving Hounsfield units using grey levels in cone beam CT: a clinical application

OBJECTIVE To present a clinical study demonstrating a method to derive Hounsfield units from grey levels in cone beam CT (CBCT). METHODS An acrylic intraoral reference object with aluminium, outer bone equivalent material (cortical bone), inner bone equivalent material (trabecular bone), polymethlymethacrylate and water equivalent material was used. Patients were asked if they would be willing to have an acrylic bite plate with the reference object placed in their mouth during a routine CBCT scan. There were 31 scans taken on the Asahi Alphard 3030 (Belmont Takara, Kyoto, Japan) and 30 scans taken on the Planmeca ProMax 3D (Planmeca, Helsinki, Finland) CBCT. Linear regression between the grey levels of the reference materials and their linear attenuation coefficients was performed for various photon energies. The energy with the highest regression coefficient was chosen as the effective energy. The attenuation coefficients for the five materials at the effective energy were scaled as Hounsfield units using the standard Hounsfield units equation and compared to those derived from the measured grey levels of the materials using the regression equation. RESULTS In general, there was a satisfactory linear relation between the grey levels and the attenuation coefficients. This made it possible to calculate Hounsfield units from the measured grey levels. Uncertainty in determining effective energies resulted in unrealistic effective energies and significant variability of calculated CT numbers. Linear regression from grey levels directly to Hounsfield units at specified energies resulted in greater consistency. CONCLUSIONS The clinical application of a method for deriving Hounsfield units from grey levels in CBCT was demonstrated.

Quality assurance phantom for digital dental imaging

OBJECTIVE The purpose of this research was to develop a simple quality assurance phantom that could be used for the initial calibration and follow-up testing of commercially available intraoral digital imaging systems. STUDY DESIGN A radiographic phantom was constructed that contains a calibrated step wedge for measuring dose response, an etched pattern of slits in a metallic background for measuring the spatial resolution in line pairs per millimeter, and 2 rows of wells of varying diameter and depth in an acrylic background for contrast-detail analysis. Quality assurance protocols were developed and validated. RESULTS The quality assurance phantom provides a method of assessing a digital intraoral imaging system by measuring the sensitivity and dynamic range, the contrast/detail detectability and the spatial resolution. CONCLUSIONS This quality assurance phantom can serve as an effective means to calibrate and monitor the performance characteristics of a digital dental intraoral imaging system.

Characterization of atherosclerotic plaque: a contrast-detail study using multidetector and cone-beam computed tomography

A Hindmarsh‐Rose model perceptibility phantom containing inserts with various in vitro atherosclerotic plaque compositions was constructed and imaged on a clinical 64 slice multidetector (MDCT) system using 80 and 120 kVp settings and two other cone‐beam (CBCT) systems at 80 kVp. Perceptibility of the simulated lipid‐fibrotic plaque solutions in the images was evaluated by six observers. The effective doses of the protocols employed were estimated using phantom CTDI‐vol measurements placed at identical settings. The CBCT system allowed reduction in effective dose in comparison with the conventional MDCT system for imaging of the carotid plaque phantoms without degrading image quality. The CBCT dose was less than MDCT, with a mean dose of 1.14±0.01 mSv and 1.11±0.02 mSv for MDCT using two measuring techniques vs. 0.35±0.01 mSv for CBCT. The image quality analysis showed no significant differences in the contrast‐detail (C‐D) curves of the best performing CBCT vs. clinical MDCT (p>0.05) using a Mann‐Whitney U test. Results indicate that low‐tube‐potential CBCT may produce comparable C‐D resolution for phantom‐based representations of soft plaque types with respect to MDCT systems. This study suggests that the utility of low kVp CT techniques for evaluating carotid vulnerable atherosclerotic plaque merits further study. PACS numbers: 87.53.Bn, 87.57.N‐, 87.57.Q‐, 87.57.cj