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Conversion of CBCT Gray Levels to Hounsfield Units
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Abstracts
to simulate a complete radiographic mouth series of x-rays. The experiment was repeated with a Planmeca (Roselle, IL) Dixi 3 sensor. Each radiographic image was captured using the manufacturer’s software and exported as a Tiff file into UTHSCSA ImageTool software. The number of discernible line pairs in the horizontal and vertical dimensions was determined and averaged for the 20 exposures. Results. In the static configuration, the image resolution for the Kodak RVG 6000 sensor was 14 line pairs/mm in the horizontal dimension and 11 line pairs/mm in the vertical dimension. This diminished to 10 line pairs/mm with manual positioning. With the Planmeca sensor the resolution was 8 line pairs/mm in both dimensions for both static and manual positioning. The same results were obtained for both classes of operators. Discussion. Our findings confirm our hypothesis that resolution may be lost when manual positioning is used. This effect is observable only with a high-resolution sensor such as the Kodak RVG 6000. In the case of the Planmeca Dixi 3 sensor, there was no significant detectable deterioration in motion blur, owing to the lower resolution of the sensor.
CONVERSION OF CBCT GRAY LEVELS TO HOUNSFIELD UNITS. P. Mah and W.D. McDavid, University of Texas Health Science Center, San Antonio (UTHSCSA). Background. Hounsfield units (HU) provide a standard scheme for scaling the reconstructed attenuation coefficients in medical computerized tomography (CT) systems. The manufacturers of dental cone-beam CT (CBCT) systems, on the other hand, have not agreed on a standard system for scaling the gray levels representing the reconstructed values. In the absence of such a system it is difficult to interpret the gray levels or to compare the values resulting from different machines. Presenting the gray levels as attenuation coefficients or HU would seem to offer a means for standardization. Objectives. Our purpose was to develop and test a methodology for converting CBCT gray levels to HU in a standardized format for analysis and comparison. Materials and methods. A radiographic phantom containing 3 standard materials (water, acrylic, and aluminum) for calibration and 1 additional substance (bone equivalent material) to act as a test of the method was constructed. The phantom was imaged with several of the CBCT machines currently on the market. The CBCT data was exported as a raw Dicom data set and transfered to UTHSCSA ImageTool for analysis. The average gray levels for the 3 standard materials were determined and plotted against the linear attenuation coefficients for the 3 materials at various photon energies. The energy resulting in the best linear fit was selected as the “effective energy” of the beam. The linear equation was used to transform the gray levels to linear attenuation coefficients. The method was tested by calculating the attenuation coefficient of the test material and comparing the result to the known value. Using the linear attenuation of water at the selected effective energy and the calculated attenuation coefficients, the entire image was converted to HU using the standard definition. Results. Using the method described above, the attenuation coefficient of the test material in the phantom was determined with less than 1% error. When the HU for the entire image were determined it was noted that the values corresponding to common materials were somewhat different from those commonly encountered in medical CT systems. This is to be expected, because of the significantly lower effective energy of dental CBCT machines.
OOOOE April 2008 Discussion. This study illustrates a simple method whereby HU can be derived from the gray levels of any CBCT machine possessing sufficient linearity. For clinical applications, a reference object containing the 3 standard materials can be placed in the field of view to provide data for calibration. Inaccuracies in the gray level values resulting from uncorrected beam hardening or the effects of interfaces between materials of greatly differing attenuation properties are unavoidably passed along to the transformed values.
DETECTABILITY OF ADHESION IN TMJ SPACE—A COMPARATIVE STUDY OF MDCT AND CBCT. M.A. Momin, M.M. Alkhader, J. Sakamoto, H. Watanabe, N. Ohbayashi, and T. Kurabayashi, Tokyo Medical and Dental University, Japan. Background. Computerized tomography (CT)–arthrography is a combination of the injection of contrast media into the temporomandibular joint (TMJ) space and simultaneous CT scanning. It provides better definition of the adhesion of the TMJ disk than conventional arthrography. However, the optimal methods of this technique have not been established. Objectives. The purpose of the present study was to optimize the concentration of contrast media and to compare the ability of multidetector CT (MDCT) and cone-beam CT (CBCT) in detecting the adhesion of the TMJ disk by CT-arthrography. Materials and methods. Phantom: A square-shaped phantom was made by nylon thread to simulate adhesive change in TMJ space. The 8 types of threads (diameters 0.128, 0.148, 0.165, 0.205, 0.235, 0.330, 0.405, and 0.500 mm) were joined inside the phantom in parallel at 1 mm intervals. Contrast medium: The contrast medium (Omnipaque 350 mg I/mL) was diluted with water, and 7 types of the medium with different concentrations (5%, 10%, 20%, 30%, 40%, 50%, and 100%) were examined. The thread phantom was kept in a rubber balloon filled with contrast medium and then placed in the TMJ space of the cadaver, so that all of the threads paralleled the F-H plane as much as possible. Imaging: MDCT was performed using a Sensation 64 (Siemens Medical System, Erlangen, Germany) operated at 120 kV and 140 mA. After axial CT scan at 0.6 mm slice thickness, all images were calculated using small FOV (50 ⫻ 50 mm), and the voxel size was approximately 0.1 ⫻ 0.1 ⫻ 0.6 mm. Then the coronal images reconstructed at 0.6 mm intervals vertical to the nylon threads were obtained. CBCT was performed using a 3DX (J. Morita Co., Tokyo, Japan) operated at 80 kV and 5.0 mA. Each examination was done at the same phantom positioning of MDCT. The voxel size was 0.125 ⫻ 0.125 ⫻ 0.125 mm. Then the coronal images at 0.5 mm intervals vertical to the nylon threads were obtained. Evaluation method: Four observers independently assessed whether the defect of contrast media by thread was present or not in 12 consecutive slices in the middle of the thread. The detectability of the thread was defined by the following formula: number of slices with detected nylon thread / 12 slices. The mean values of the detectability among the 4 observers were obtained and used for the analysis. The MDCT images were examined on printed film and CBCT on computer display. Results. The defect of contrast media, simulating the adhesion of the TMJ disk, could be observed in 4 out of the 8 threads—with diameter of 0.235 mm or more. In the remaining 4, defect could not be detected regardless of contrast media con-
Abstracts
to simulate a complete radiographic mouth series of x-rays. The experiment was repeated with a Planmeca (Roselle, IL) Dixi 3 sensor. Each radiographic image was captured using the manufacturer’s software and exported as a Tiff file into UTHSCSA ImageTool software. The number of discernible line pairs in the horizontal and vertical dimensions was determined and averaged for the 20 exposures. Results. In the static configuration, the image resolution for the Kodak RVG 6000 sensor was 14 line pairs/mm in the horizontal dimension and 11 line pairs/mm in the vertical dimension. This diminished to 10 line pairs/mm with manual positioning. With the Planmeca sensor the resolution was 8 line pairs/mm in both dimensions for both static and manual positioning. The same results were obtained for both classes of operators. Discussion. Our findings confirm our hypothesis that resolution may be lost when manual positioning is used. This effect is observable only with a high-resolution sensor such as the Kodak RVG 6000. In the case of the Planmeca Dixi 3 sensor, there was no significant detectable deterioration in motion blur, owing to the lower resolution of the sensor.
CONVERSION OF CBCT GRAY LEVELS TO HOUNSFIELD UNITS. P. Mah and W.D. McDavid, University of Texas Health Science Center, San Antonio (UTHSCSA). Background. Hounsfield units (HU) provide a standard scheme for scaling the reconstructed attenuation coefficients in medical computerized tomography (CT) systems. The manufacturers of dental cone-beam CT (CBCT) systems, on the other hand, have not agreed on a standard system for scaling the gray levels representing the reconstructed values. In the absence of such a system it is difficult to interpret the gray levels or to compare the values resulting from different machines. Presenting the gray levels as attenuation coefficients or HU would seem to offer a means for standardization. Objectives. Our purpose was to develop and test a methodology for converting CBCT gray levels to HU in a standardized format for analysis and comparison. Materials and methods. A radiographic phantom containing 3 standard materials (water, acrylic, and aluminum) for calibration and 1 additional substance (bone equivalent material) to act as a test of the method was constructed. The phantom was imaged with several of the CBCT machines currently on the market. The CBCT data was exported as a raw Dicom data set and transfered to UTHSCSA ImageTool for analysis. The average gray levels for the 3 standard materials were determined and plotted against the linear attenuation coefficients for the 3 materials at various photon energies. The energy resulting in the best linear fit was selected as the “effective energy” of the beam. The linear equation was used to transform the gray levels to linear attenuation coefficients. The method was tested by calculating the attenuation coefficient of the test material and comparing the result to the known value. Using the linear attenuation of water at the selected effective energy and the calculated attenuation coefficients, the entire image was converted to HU using the standard definition. Results. Using the method described above, the attenuation coefficient of the test material in the phantom was determined with less than 1% error. When the HU for the entire image were determined it was noted that the values corresponding to common materials were somewhat different from those commonly encountered in medical CT systems. This is to be expected, because of the significantly lower effective energy of dental CBCT machines.
OOOOE April 2008 Discussion. This study illustrates a simple method whereby HU can be derived from the gray levels of any CBCT machine possessing sufficient linearity. For clinical applications, a reference object containing the 3 standard materials can be placed in the field of view to provide data for calibration. Inaccuracies in the gray level values resulting from uncorrected beam hardening or the effects of interfaces between materials of greatly differing attenuation properties are unavoidably passed along to the transformed values.
DETECTABILITY OF ADHESION IN TMJ SPACE—A COMPARATIVE STUDY OF MDCT AND CBCT. M.A. Momin, M.M. Alkhader, J. Sakamoto, H. Watanabe, N. Ohbayashi, and T. Kurabayashi, Tokyo Medical and Dental University, Japan. Background. Computerized tomography (CT)–arthrography is a combination of the injection of contrast media into the temporomandibular joint (TMJ) space and simultaneous CT scanning. It provides better definition of the adhesion of the TMJ disk than conventional arthrography. However, the optimal methods of this technique have not been established. Objectives. The purpose of the present study was to optimize the concentration of contrast media and to compare the ability of multidetector CT (MDCT) and cone-beam CT (CBCT) in detecting the adhesion of the TMJ disk by CT-arthrography. Materials and methods. Phantom: A square-shaped phantom was made by nylon thread to simulate adhesive change in TMJ space. The 8 types of threads (diameters 0.128, 0.148, 0.165, 0.205, 0.235, 0.330, 0.405, and 0.500 mm) were joined inside the phantom in parallel at 1 mm intervals. Contrast medium: The contrast medium (Omnipaque 350 mg I/mL) was diluted with water, and 7 types of the medium with different concentrations (5%, 10%, 20%, 30%, 40%, 50%, and 100%) were examined. The thread phantom was kept in a rubber balloon filled with contrast medium and then placed in the TMJ space of the cadaver, so that all of the threads paralleled the F-H plane as much as possible. Imaging: MDCT was performed using a Sensation 64 (Siemens Medical System, Erlangen, Germany) operated at 120 kV and 140 mA. After axial CT scan at 0.6 mm slice thickness, all images were calculated using small FOV (50 ⫻ 50 mm), and the voxel size was approximately 0.1 ⫻ 0.1 ⫻ 0.6 mm. Then the coronal images reconstructed at 0.6 mm intervals vertical to the nylon threads were obtained. CBCT was performed using a 3DX (J. Morita Co., Tokyo, Japan) operated at 80 kV and 5.0 mA. Each examination was done at the same phantom positioning of MDCT. The voxel size was 0.125 ⫻ 0.125 ⫻ 0.125 mm. Then the coronal images at 0.5 mm intervals vertical to the nylon threads were obtained. Evaluation method: Four observers independently assessed whether the defect of contrast media by thread was present or not in 12 consecutive slices in the middle of the thread. The detectability of the thread was defined by the following formula: number of slices with detected nylon thread / 12 slices. The mean values of the detectability among the 4 observers were obtained and used for the analysis. The MDCT images were examined on printed film and CBCT on computer display. Results. The defect of contrast media, simulating the adhesion of the TMJ disk, could be observed in 4 out of the 8 threads—with diameter of 0.235 mm or more. In the remaining 4, defect could not be detected regardless of contrast media con-