Ria Bogaerts

Effective dose range for dental cone beam computed tomography scanners

OBJECTIVE To estimate the absorbed organ dose and effective dose for a wide range of cone beam computed tomography scanners, using different exposure protocols and geometries. MATERIALS AND METHODS Two Alderson Radiation Therapy anthropomorphic phantoms were loaded with LiF detectors (TLD-100 and TLD-100 H) which were evenly distributed throughout the head and neck, covering all radiosensitive organs. Measurements were performed on 14 CBCT devices: 3D Accuitomo 170, Galileos Comfort, i-CAT Next Generation, Iluma Elite, Kodak 9000 3D, Kodak 9500, NewTom VG, NewTom VGi, Pax-Uni3D, Picasso Trio, ProMax 3D, Scanora 3D, SkyView, Veraviewepocs 3D. Effective dose was calculated using the ICRP 103 (2007) tissue weighting factors. RESULTS Effective dose ranged between 19 and 368 μSv. The largest contributions to the effective dose were from the remainder tissues (37%), salivary glands (24%), and thyroid gland (21%). For all organs, there was a wide range of measured values apparent, due to differences in exposure factors, diameter and height of the primary beam, and positioning of the beam relative to the radiosensitive organs. CONCLUSIONS The effective dose for different CBCT devices showed a 20-fold range. The results show that a distinction is needed between small-, medium-, and large-field CBCT scanners and protocols, as they are applied to different indication groups, the dose received being strongly related to field size. Furthermore, the dose should always be considered relative to technical and diagnostic image quality, seeing that image quality requirements also differ for patient groups. The results from the current study indicate that the optimisation of dose should be performed by an appropriate selection of exposure parameters and field size, depending on the diagnostic requirements.

Comparison between effective radiation dose of CBCT and MSCT scanners for dentomaxillofacial applications

OBJECTIVES To compare the effective dose levels of cone beam computed tomography (CBCT) for maxillofacial applications with those of multi-slice computed tomography (MSCT). STUDY DESIGN The effective doses of 3 CBCT scanners were estimated (Accuitomo 3D, i-CAT, and NewTom 3G) and compared to the dose levels for corresponding image acquisition protocols for 3 MSCT scanners (Somatom VolumeZoom 4, Somatom Sensation 16 and Mx8000 IDT). The effective dose was calculated using thermoluminescent dosimeters (TLDs), placed in a Rando Alderson phantom, and expressed according to the ICRP 103 (2007) guidelines (including a separate tissue weighting factor for the salivary glands, as opposed to former ICRP guidelines). RESULTS Effective dose values ranged from 13 to 82 microSv for CBCT and from 474 to 1160 microSv for MSCT. CBCT dose levels were the lowest for the Accuitomo 3D, and highest for the i-CAT. CONCLUSIONS Dose levels for CBCT imaging remained far below those of clinical MSCT protocols, even when a mandibular protocol was applied for the latter, resulting in a smaller field of view compared to various CBCT protocols. Considering this wide dose span, it is of outmost importance to justify the selection of each of the aforementioned techniques, and to optimise the radiation dose while achieving a sufficient image quality. When comparing these results to previous dosimetric studies, a conversion needs to be made using the latest ICRP recommendations.

Preservation of parotid function with uncomplicated conformal radiotherapy

BACKGROUND AND PURPOSE To evaluate (1) parotid function, (2) subjective xerostomia and (3) pattern of relapses after conformal parotid-sparing radiotherapy (RT) for head and neck cancer. (4) To study dose-response curves of parotid glands. MATERIAL AND METHODS From September 1999 to November 2000, 39 head and neck cancer patients requiring bilateral neck RT were treated with a fairly simple conformal RT technique (three-field set-up+anterior lower neck field; two opposed oblique boost fields). The contralateral parotid was spared. Parotid function was assessed by salivary gland scintigraphies performed before, early (median 4 weeks) and late (median 28 weeks) after RT. Xerostomia was monitored by visual analogue scales (VAS) and LENT SOMA scores. Location of locoregional recurrences was studied in relation to the radiation fields. A dose-response curve of parotids was created using logistic regression. RESULTS (1) Early after RT, on salivary gland scintigraphy, the mean loss of secretion function in the spared parotid was 67% and total in the non-spared. Late after RT, the mean loss remained 19% in the spared and total in the non-spared parotid. Normal excretion function was regained in 75% of the spared parotids. (2) Late after RT, 78% of patients had no, minimal or acceptable subjective xerostomia. (3) No recurrence was seen near the spared parotid (11/39 locoregional recurrences). (4) The dose-response curve of parotids showed that the mean parotid dose should preferentially be < or =20 Gy, to obtain a good chance (> or =70%) for preservation of its function on scintigraphy. CONCLUSIONS An easy conformal parotid-sparing RT technique prevents moderate or severe subjective xerostomia in 78% of patients. In the spared parotids, nearly complete to complete recovery is obtained after 6-12 months.

Estimation of paediatric organ and effective doses from dental cone beam CT using anthropomorphic phantoms

OBJECTIVES Cone beam CT (CBCT) is an emerging X-ray technology applied in dentomaxillofacial imaging. Previous published studies have estimated the effective dose and radiation risks using adult anthropomorphic phantoms for a wide range of CBCT units and imaging protocols. METHODS Measurements were made five dental CBCT units for a range of imaging protocols, using 10-year-old and adolescent phantoms and thermoluminescent dosimeters. The purpose of the study was to estimate paediatric organ and effective doses from dental CBCT. RESULTS The average effective doses to the 10-year-old and adolescent phantoms were 116 μSv and 79 μSv, respectively, which are similar to adult doses. The salivary glands received the highest organ dose and there was a fourfold increase in the thyroid dose of the 10-year-old relative to that of the adolescent because of its smaller size. The remainder tissues and salivary and thyroid glands contributed most significantly to the effective dose for a 10-year-old, whereas for an adolescent the remainder tissues and the salivary glands contributed the most significantly. It was found that the percentage attributable lifetime mortality risks were 0.002% and 0.001% for a 10-year-old and an adolescent patient, respectively, which are considerably higher than the risk to an adult having received the same doses. CONCLUSION It is therefore imperative that dental CBCT examinations on children should be fully justified over conventional X-ray imaging and that dose optimisation by field of view collimation is particularly important in young children.

Quantification of metal artifacts on cone beam computed tomography images

OBJECTIVES To quantify metal artifacts obtained from a wide range of cone beam computed tomography (CBCT) devices and exposure protocols, to compare their tolerance to metals of different densities, and to provide insights regarding the possible implementation of metal artifact analysis into a QC protocol for CBCT. MATERIALS AND METHODS A customized polymethyl methacrylate (PMMA) phantom, containing titanium and lead rods, was fabricated. It was scanned on 13 CBCT devices and one multi-slice computed tomography (MSCT) device, including high-dose and low-dose exposure protocols. Artifacts from the rods were assessed by two observers by measuring the standard deviation of voxel values in the vicinity of the rods, and normalizing this value to the percentage of the theoretical maximum standard deviation. RESULTS For CBCT datasets, artifact values ranged between 6.1% and 27.4% for titanium, and between 10.% and 43.7% for lead. Most CBCT devices performed worse than MSCT for titanium artifacts, but all of them performed better for lead artifacts. In general, no clear improvement of metal artifacts was seen for high-dose protocols, although certain devices showed some artifact reduction for large FOV or high exposure protocols. CONCLUSIONS Regions in the vicinity of the metal rods were moderately or gravely affected, particularly in the area between the rods. In practice, the CBCT user has very limited possibilities to reduce artifacts. Researchers and manufacturers need to combine their efforts in optimizing exposure factors and implementing metal artifact reduction algorithms.

Variability of dental cone beam CT grey values for density estimations

OBJECTIVE The aim of this study was to investigate the use of dental cone beam CT (CBCT) grey values for density estimations by calculating the correlation with multislice CT (MSCT) values and the grey value error after recalibration. METHODS A polymethyl methacrylate (PMMA) phantom was developed containing inserts of different density: air, PMMA, hydroxyapatite (HA) 50 mg cm(-3), HA 100, HA 200 and aluminium. The phantom was scanned on 13 CBCT devices and 1 MSCT device. Correlation between CBCT grey values and CT numbers was calculated, and the average error of the CBCT values was estimated in the medium-density range after recalibration. RESULTS Pearson correlation coefficients ranged between 0.7014 and 0.9996 in the full-density range and between 0.5620 and 0.9991 in the medium-density range. The average error of CBCT voxel values in the medium-density range was between 35 and 1562. CONCLUSION Even though most CBCT devices showed a good overall correlation with CT numbers, large errors can be seen when using the grey values in a quantitative way. Although it could be possible to obtain pseudo-Hounsfield units from certain CBCTs, alternative methods of assessing bone tissue should be further investigated. ADVANCES IN KNOWLEDGE The suitability of dental CBCT for density estimations was assessed, involving a large number of devices and protocols. The possibility for grey value calibration was thoroughly investigated.

Development and applicability of a quality control phantom for dental cone‐beam CT

Cone‐beam CT (CBCT) has shown to be a useful imaging modality for various dentomaxillofacial applications. However, optimization and quality control of dental CBCT devices is hampered due to the lack of an appropriate tool for image quality assessment. To investigate the application of different image quality parameters for CBCT, a prototype polymethyl methacrylate (PMMA) cylindrical phantom with inserts for image quality analysis was developed. Applicability and reproducibility of the phantom were assessed using seven CBCT devices with different scanning protocols. Image quality parameters evaluated were: CT number correlation, contrast resolution, image homogeneity and uniformity, point spread function, and metal artifacts. Deviations of repeated measurements were between 0.0% and 3.3%. Correlation coefficients of CBCT voxel values with CT numbers ranged between 0.68 and 1.00. Contrast‐to‐noise ratio (CNR) values were much lower for hydroxyapatite (0<CNR<7.7) than for air and aluminum (5.0<CNR<32.8). Noise values ranged between 35 and 419. The uniformity index was between 3.3% and 11.9%. Full width at half maximum (FWHM) measurements varied between 0.43 mm and 1.07 mm. The increase of mean voxel values surrounding metal objects ranged between 6.7% and 43.0%. Results from preliminary analyses of the prototype quality control phantom showed its potential for routine quality assurance on CBCT. Large differences in image quality performance were seen between CBCT devices. Based on the initial evaluations, the phantom can be optimized and validated. PACS numbers: 87.57.C‐, 87.57.N‐, 87.57.Q‐

A pragmatic approach to determine the optimal kVp in cone beam CT: balancing contrast-to-noise ratio and radiation dose

OBJECTIVES To determine the optimal kVp setting for a particular cone beam CT (CBCT) device by maximizing technical image quality at a fixed radiation dose. METHODS The 3D Accuitomo 170 (J. Morita Mfg. Corp., Kyoto, Japan) CBCT was used. The radiation dose as a function of kVp was measured in a cylindrical polymethyl methacrylate (PMMA) phantom using a small-volume ion chamber. Contrast-to-noise ratio (CNR) was measured using a PMMA phantom containing four materials (air, aluminium, polytetrafluoroethylene and low-density polyethylene), which was scanned using 180 combinations of kVp/mA, ranging from 60/1 to 90/8. The CNR was measured for each material using PMMA as background material. The pure effect of kVp and mAs on the CNR values was analysed. Using a polynomial fit for CNR as a function of mA for each kVp value, the optimal kVp was determined at five dose levels. RESULTS Absorbed doses ranged between 0.034 mGy mAs(-1) (14 × 10 cm, 60 kVp) and 0.108 mGy mAs(-1) (14 × 10 cm, 90 kVp). The relation between kVp and dose was quasilinear (R(2) > 0.99). The effect of mA and kVp on CNR could be modelled using a second-degree polynomial. At a fixed dose, there was a tendency for higher CNR values at increasing kVp values, especially at low dose levels. A dose reduction through mA was more efficient than an equivalent reduction through kVp in terms of image quality deterioration. CONCLUSIONS For the investigated CBCT model, the most optimal contrast at a fixed dose was found at the highest available kVp setting. There is great potential for dose reduction through mA with a minimal loss in image quality.

Effective radiation dose and eye lens dose in dental cone beam CT: effect of field of view and angle of rotation

OBJECTIVE To quantify the effect of field of view (FOV) and angle of rotation on radiation dose in dental cone beam CT (CBCT) and to define a preliminary volume-dose model. METHODS Organ and effective doses were estimated using 148 thermoluminescent dosemeters placed in an anthropomorphic phantom. Dose measurements were undertaken on a 3D Accuitomo 170 dental CBCT unit (J. Morita, Kyoto, Japan) using six FOVs as well as full-rotation (360°) and half-rotation (180°) protocols. RESULTS For the 360° rotation protocols, effective dose ranged between 54 µSv (4 × 4 cm, upper canine) and 303 µSv (17 × 12 cm, maxillofacial). An empirical relationship between FOV dimension and effective dose was derived. The use of a 180° rotation resulted in an average dose reduction of 45% compared with a 360° rotation. Eye lens doses ranged between 95 and 6861 µGy. CONCLUSION Significant dose reduction can be achieved by reducing the FOV size, particularly the FOV height, of CBCT examinations to the actual region of interest. In some cases, a 180° rotation can be preferred, as it has the added value of reducing the scan time. Eye lens doses should be reduced by decreasing the height of the FOV rather than using inferior FOV positioning, as the latter would increase the effective dose considerably. ADVANCES IN KNOWLEDGE The effect of the FOV and rotation angle on the effective dose in dental CBCT was quantified. The dominant effect of FOV height was demonstrated. A preliminary model has been proposed, which could be used to predict effective dose as a function of FOV size and position.

Study of the demagnetization and optimization of the magnetic field of perpendicular ferromagnetic thin films usingesub-μ m lithography

Abstract The succesful fabrication using holographic lithography of a submicron grating reducing the demagnetizing field of a perpendicular ferromagnetic thin film is reported. An increase in the extraordinary Hall effect by a factor 3 is measured. The presence of holes in the film lowers the coercive field and causes a stepwise switching.