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

Abstract

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.

Bone mineral density in cone beam computed tomography: Only a few shades of gray

“…Thus, CBCT has been proposed as a diagnostic method for the determination of bone mineral density [10,11,18,[34][35][36] . Gray values obtained with CBCT are used in an analog way as the HU values for the determination of mineral density [16] and show a linear relationship with the attenuation coefficients of the materials [13,15] , HU values obtained with medical CT [11,12,37,38] , and density values from DEXA [14] . Despite the correlation between gray values obtained with MSCT and CBCT, errors are expected when CBCT images are used to define the density of scanned structures [39] because these images present with inconsistencies and arbitrariness of gray values [16,40] , especially when related to abrupt changes of density in the object [41,42] , X-ray beam hardening effect [39,43] , scattered radiation [43] and projection data discontinuity-related effect [16] , making the validity of the measurements obtained questionable (Table 1).…”

“…Once these artifacts exhibit a different color from that of the structure to be analyzed, they are responsible for the inconsistencies in the gray values in the areas where they are present [15,41,42] . Another source of artifacts in CBCT images is the phenomenon of X-ray beam hardening.…”

“…Yet, there is no consensus regarding the accuracy of CBCT for this type of analysis. While some studies advocate its use [10][11][12][13][14][15] , others advocate that CBCT is not an adequate tool for this type of evaluation because the intensity values of CBCT are influenced by the characteristics of the system [4,13,16] and by the scanned object [16][17][18] . This study aims to provide a literature review concerning the reliability of CBCT for the determination of bone mineral density of craniofacial structures.…”

Evaluating Maxilla Bone Quality Through Clinical Investigation of Voxel Grey Scale Values from Cone-Beam Computed Tomography for Dental Use

“…The difference between HU obtained from original attenuation coefficients (actual HU) with those calculated was very small and depended on the manufacturer of CBCT scanner and conditions in which the scan was taken. In vivo study conducted by the same authors showed the difference between the calculated and actual HU less than 3%, whereas the relationship between grey levels (voxel values) and HU was defined as linear [14]. It should be noted that the authors used a mathematical model only, without comparing CT and CBCT scans.…”

Arrested Pneumatization of Sphenoid Sinus mimicking intra-osseous lesions of the skull base

“…The gray-scale CBCT values of the hypodense portion of the mass were measured. Using the gray-─ 68 ─ Arrested pneumatization of the sphenoid sinus mimicking intraosseous lesions of the skull base Inferior concha Choana scale values of the dental CBCT, Hounsfield units were derived using the comprehensive method described in the studies performed by Mah et al 11 and Reeves et al 12 The corrected Hounsfield units in the CBCT acquisition were characteristic of fatty tissue, suggesting the presence of fat surrounding the central, scattered curvilinear calcifications. The margins of the lesion were sharp and sclerotic, suggesting non-aggressive growth.…”