Absorbed and effective doses from cone beam volumetric imaging for implant planning.

OBJECTIVES: Volumetric CT using a cone beam has been developed by several manufacturers for dentomaxillofacial imaging. The purpose of this study was to measure doses for implant planning with cone beam volumetric imaging (CBVI) in comparison with conventional multidetector CT (MDCT). METHODS: The two CBVI systems used were a 3D Accuitomo (J. Morita), including an image-intensifier type (II) and a flat-panel type (FPD), and a CB MercuRay (Hitachi). The 3D Accuitomo operated at 80 kV, 5 mA and 18 s. The CB MercuRay operated at 120 kV, 15 mA, 9.8 s. The MDCT used was a HiSpeed QX/i (GE), operated at 120 kV, 100 mA and 0.7 s, and its scan length was 77 mm for both jaws. Measurement of the absorbed tissue and organ doses was performed with an Alderson phantom, embedding the radiophotoluminescence glass dosemeter into the organs/tissues. The values obtained were converted into the absorbed dose. The effective dose as defined by the International Commission on Radiological Protection was then calculated. RESULTS: The absorbed doses of the 3D Accuitomo of the organs in the primary beam ranged from 1-5 mGy, and were several to ten times lower than other doses. The effective dose of the 3D Accuitomo ranged from 18 muSv to 66 muSv, and was an order of magnitude smaller than the others. In conclusion, these results show that the dose in the 3D Accuitomo is lower than the CB MercuRay and much less than MDCT.

Characteristics of a newly developed dentomaxillofacial X-ray cone beam CT scanner (CB MercuRay): system configuration and physical properties.

OBJECTIVE: The purpose of this paper is to present the system configuration and physical properties of a new dentomaxillofacial X-ray cone beam CT system (CB MercuRay) being developed. METHODS: The system consists of an image intensifier and a cone beam X-ray source. There are two different models of this system, each with a different size image intensifier, 9" or 12". Each system has three field of view (FOV) modes. The 12" system has facial (F), panoramic (P) and implant (I) FOV modes. The 9" system has P, I and dental (D) modes. Images produced by these systems consist of 512 x 512 x 512 isotropic voxels. Physical properties such as resolution, noise and distortion of the images were evaluated in this study. Modulation transfer function (MTF) was measured using Boone's method. Image noise was measured as the standard deviation of the CT value in water. Circularity of the axial images yielded by the two models was measured using an 8 mm diameter acrylic pipe phantom. RESULTS: The resolving power at a MTF of 0.1 in the D mode was over 2.0 lp mm(-1), suggesting that this system yields images of high resolution. The standard deviation of the CT value in water was approximately 80, which is thought to be greater than that of conventional CT. The circularity of images of the pipe phantom was 99% of the ideal value. CONCLUSION: This study shows that our newly developed cone beam CT system produces high resolution three-dimensional volumetric images that will be useful for the examination of dentomaxillofacial disorders.

A practical method for position-dependent Compton-scatter correction in single photon emission CT.

A new method is proposed to subtract the count of scattered photons from that acquired with a photopeak window at each pixel in each planar image of single-photon emission computed tomography (SPECT). The subtraction is carried out using two sets of data: one set is acquired with a main window centered at photopeak energy and the other is acquired with two subwindows on both sides of the main window. The scattered photons included in the main window are estimated from the counts acquired with the subwindows and then they are subtracted from the count acquired with the main windows. Since the subtraction is performed at each pixel in each planar image, the proposed method has the potential to be more precise than conventional methods. For three different activity distributions in cylinder phantoms, simulation tests gave good agreement between the activity distributions reconstructed from unscattered photons and those from the corrected data.

[Estimation of scatter component in SPECT planar image using a Monte Carlo method].

A Monte Carlo simulation was performed to estimate quantitatively the scattered photons in planar images of SPECT. As a phantom we used a water-filled cylinder with a line source and calculated the energy spectra of primary and multiple scattered photons separately at each pixel of the planar images. The energy spectra of primary and scattered photons were studied on following parameters: the size of the phantom; the location of the source; the width of the energy window centered at 72.3 (Tl-201), 141 (Tc-99m), and 159 keV (I-123); and the view angle of the planar images. Obtained results were; (1) the energy spectra of Compton scattered photon varied with the phantom size, the source location, and the photopeak energy, (2) the scattered photons within energy windows of 10-30% centered at the photopeak energy were mainly composed by Compton scatter of the first order, (3) the higher order scattering component of the Compton photons did not represent the location of the line source on the planar image, and (4) the scatter fraction defined by the ratio of the scattered photons to the primary photons increased with increasing the size of the phantom and the width of energy window at the low photopeak energy. From the results, we discussed on the scatter subtraction methods.