Material and methods

OSL dosimeters (Nanodot; Landauer, Glenwood, Ill) are plastic disks infused with aluminum oxide doped with carbon (Al ₂ O ₃ :C). The trace amounts of carbon in the aluminum oxide crystal lattice create imperfections that act as traps (F centers) for electrons and positively charged areas called “holes.” After exposure to ionizing radiation, free electrons and holes are generated and trapped at the F centers in proportion to the amount of energy in the exposure. Energy captured by the F centers is reemitted as light when electrons and holes recombine. Recombination occurs when the crystal is optically stimulated with a controlled exposure of 540-nm light from a light-emitting diode. The energy released from the F centers can be distinguished from the stimulating light because it is emitted in the form of 420-nm photons. The intensity or quantity of the emitted photons depends on the dose absorbed by the OSL dosimeter and the controlled intensity of the stimulating light. The intensity of the emitted light is proportional to the stored dose and is recorded by a photomultiplier tube that incorporates a filter that screens out photons from the stimulating light source. Each dosimeter is encased in a light-tight plastic holder measuring approximately 1 × 10 × 10 mm. This case prevents loss of energy through stimulation by ambient light. Dosimeters used in this study were read with a portable reader (MicroStar; Landauer). The reader was calibrated before use. After calibration, photon counts from dosimeters can be recorded with an accuracy of about ±2%. Photon counts are converted to doses using an energy-specific conversion factor. Doses reported by the reader were adjusted for energy response using a third-order polynomial calibration curve derived from side-by-side comparisons of recorded doses from an ion chamber and OSL dosimeters over a range of 80 to 120 kV(p) using an adjustable kilovolt peak source. For this study, the OSL sensitivity at 90 kV(p) was estimated at .94 (mean, 60 kV). OSL sensitivity at 120 kV(p) was estimated at 0.78 (mean, 80 kV).

A software and hardware upgraded model of the i-CAT Next Generation dental CBCT unit (Imaging Sciences) was investigated in this study. It was updated to meet specifications that were named “i-CAT FLX.” Volume scans for this unit ranged from 8 × 8 cm to 23 × 17 cm. The unit operates at 90 or 120 kV(p) and 3 or 5 mA with a pulsed exposure time from 2.0 to 7.4 seconds. The unit rotates through 360° over 8.6 or 26 seconds. A 180° rotation is also available with a rotation time of 4.8 seconds. The scan parameters used for scans of the child and adult phantoms are given in Table I .

Adult dosimetry was acquired using a tissue-equivalent phantom simulating the anatomy of an average adult male (ATOM Max model 711 HN; CIRS, Norfolk, Va). The phantom includes detailed 3-dimensional anthropomorphic anatomy including brain, bone, larynx, trachea, sinus, nasal cavities, and teeth. The bones contain both cortical and trabecular separations. The phantom was modified by machining slots to accept nanodot dosimeters at sites corresponding to the internal tissues of interest. A skin surface dosimeter in the back of the neck was positioned at the vertical center of the designated slice level and taped in position. Lens of eye dosimeters were centered over and inset in the anatomic location for the lens and taped in position. Internal dosimeters were positioned vertically with the upper edge of the dosimeter slot, flush with the surface of the selected slice level, and held in position by friction of the dosimeter case and the phantom material at the sampled anatomic location. Adult dosimeter anatomic locations and phantom levels are shown in Figure 1 . During scanning, the phantom was oriented with its section planes approximately parallel to the scan rotation plane (horizon). A phantom position simulating the positioning of a patient on the chin rest was used. With the exception of the 13 × 16-cm cephalometric and 6 × 16-cm maxillary FOVs, centered FOVs were positioned to capture approximately 5 mm of soft tissue below the lower cortical border of the chin. The anteroposterior position of the phantom was established to capture approximately 5 mm of soft tissue anterior to the facial surface of the maxillary incisor crowns. For the 6 × 16-cm maxillary views, the lower border of the FOV was positioned approximately 5 mm below the maxillary central incisor edge. For the 13 × 16-cm cephalometric views, the tip of the nose and the lower soft tissue border of the chin were included in the FOV ( Fig 2 ).

Locations of optically stimulated luminescent dosimeters ( OSLD ) in adult ATOM Max model 711 phantom.

Adult phantom demonstrating positioning principles for phantom scanning. A, The lower border of volume is 5 mm below the bony chin; the anterior border of volume is 5 mm anterior to the central incisors. This positioning was used on dental (8 × 8-cm high-resolution scan here), mandible, both arches, and arches + temporomandibular joint scans. B, The lower border of volume is 5 mm below the central incisal edge; the anterior border of volume is 5 mm anterior to the central incisors. This positioning was used on maxilla volumes (6 × 16-cm high-resolution scan here). C, The tip of the nose and the lower soft-tissue border of the chin are included in the FOV for standard cephalometric volumes (13 × 16-cm high-resolution scan here) and extended FOV volumes.

Child dosimetry was acquired using a tissue-equivalent phantom simulating the anatomy of a 10-year-old child (ATOM model 706 HN; CIRS). Tissues simulated in this phantom were average soft tissue, average bone tissue, spinal cord, spinal discs, brain, and sinus. Simulated bone tissue matches age-related density. Dosimeter anatomic locations and child phantom levels are shown in Figure 3 . With the exception of the 11 × 16-cm, 13 × 16-cm, and 6 × 16-cm maxillary FOVs, centered FOVs were positioned to capture approximately 5 mm of soft tissue below the lower cortical border of the chin. The anteroposterior position of the phantom was established to capture approximately 5 mm of soft tissue anterior to the facial surface of the maxillary incisor crown. For the 6 × 16-cm maxillary views, the lower border of the FOV was positioned approximately 5 mm below the maxillary central incisor edge. For the 11 × 16-cm and 13 × 16-cm cephalometric views, the tip of the nose and the lower soft tissue border of the chin were included in the FOV ( Fig 4 ).

Ten-year-old child phantom dosimeter locations.

Child phantom demonstrating positioning principles for phantom scanning. A, The lower border of volume is 5 mm below the bony chin; the anterior border of volume is 5 mm anterior to the central incisors. This positioning was used on dental (8 × 8-cm high-resolution scan here), mandible, and both arches. B, The lower border of volume is 5 mm below the central incisal edge; the anterior border of volume is 5 mm anterior to the central incisors. This positioning was used on maxilla volumes (6 × 16-cm high-resolution scan here). C, The tip of the nose and lower soft-tissue border of the chin are included in the FOV for standard cephalometric volumes (13 × 16-cm high-resolution scan here) and arches + temporomandibular joint scans.

Two to 12 exposures were used for each dosimeter run to provide a more reliable measure of radiation in the dosimeters. Smaller FOVs required more exposure repetitions because more dosimeters are outside the field of direct exposure and absorb only small quantities of scatter radiation. For every scan, a scout view was also acquired. Doses recorded by the OSL dosimetry reader were divided by the number of scans to determine the exposure per examination for each dosimeter.

Doses recorded by OSL dosimeters at different positions in a tissue or an organ were averaged to express the average tissue-absorbed dose in micrograys. The products of these values and the percentage of a tissue or an organ irradiated in a radiographic examination ( Table II ) were used to calculate the equivalent dose in microsieverts.

For bone, the equivalent dose to the whole-body bone surface was calculated using the summation of the individual equivalent doses to the calvarium, mandible, and cervical spine. The determination of these equivalent doses was based on the distribution of bone throughout the body: the mandible contains 1.3%, the calvaria, 11.8%, and the cervical spine, 3.4%. Distribution of adult bone marrow was calculated using an average of data from Christy for 25- and 40-year-olds. The mandible contains 0.8%, the calvaria, 7.7%, and the cervical spine, 3.8% of the adult marrow distribution. The 10-year-old child’s marrow distribution was calculated as 1.1% for the mandible, 11.6% for the cranium, and 2.7% for the cervical spine, for a total of 15.4% of the total body marrow. With the technique of Underhill et al, 3 locations in the calvarium were averaged to determine the calvarial dose. For bone, a correction factor based on experimentally determined mass energy attenuation coefficients for bone and muscle irradiated with mono-energetic photons was applied. An effective beam energy estimated to be two thirds of the peak beam energy of the x-ray unit was used to determine the bone-to-muscle attenuation ratios. A linear fit (R ² = 0.996) of ratios from 40 to 80 kV from published data produced the following equation: bone-to-muscle attenuation ratio = −0.0618 × kV(p) × 2/3 + 6.9406. Values calculated from this equation provided bone-to-muscle attenuation ratios of 3.21 at 60 kV (90 kV[p]) and 1.97 at 80 kV (120 kV[p]).

The proportion of skin surface area in the head and neck region directly exposed during maxillofacial CBCT imaging is estimated at 5% of the total body to calculate the radiation-weighted dose to the skin following the procedure Ludlow et al. Similarly, muscle and lymphatic nodes exposures are estimated to represent 5% of the total body complement for these tissues. The proportion of the esophageal tract that is exposed was set at 10%. Other tissues of interest were calculated at 100%.

The effective dose is a calculation that permits comparison of the detriment of different exposures to ionizing radiation to an equivalent detriment produced by a full-body dose of radiation. Effective dose, expressed in microsieverts, is calculated with the equation: E = ∑ w _T × H _T , where E is the summation of the products of the tissue weighting factor ( w _T ), which represents the relative contribution of that organ or tissue to the overall risk, and the radiation weighted dose H _T . The whole-body risk is determined by the summation of the radiation-weighted doses to all tissues or organs exposed. International Commission on Radiological Protection weighting factors in Table II were used to calculate the effective doses.

Tissue weighting factors used in the International Commission on Radiological Protection calculation of effective dose include 14 independently weighted tissues and 14 remainder tissues. Because the uterus and the cervix are present only in females, and the prostate only in males, the number used in the weighted averaging of the remainder tissues was 13.

In previous studies, a radiation analog dosimetry system (RANDO) phantom (Nuclear Associates, Hicksville, NY) and thermoluminescent dosimeters (TLDs) have commonly been used for dosimetry. Because each RANDO phantom is formed around an actual human skull, the attenuation characteristics for each RANDO are variable. The precision-manufactured ATOM phantom was selected as a more reproducible model for comparative dosimetry. Effective doses were evaluated for standard and extended-field cephalometric data runs and for previously reported RANDO TLD data runs as well as for unreported OSL runs. These were compared with the adult ATOM phantom results of this study.

Analysis of variance (ANOVA) of effective dose results was used to assess data for significant differences caused by phantom type (adult or child), region of interest (dental, mandible, both arches, arches + temporomandibular joint, standard cephalometrics), and scan protocol (QuickScan+, QuickScan, standard, or high resolution). When significant differences were found, the Tukey HSD test was used to determine which factors were significantly different from other factors. An alpha value of 0.05 was chosen for all tests.

Image quality indicators associated with FOV, scanning angle, kilovolt (peak), and voxel size were acquired using a QUART DVT_AP phantom and QUART DVT_TEC software (QUART, Zorneding, Germany) ( Fig 5 ). The phantom consisted of 16-cm-diameter cylindrical slabs of acrylic plastic with polyvinyl chloride and air elements configured to permit measurements of polymethylmethacrylate voxel, polymethylmethacrylate noise, homogeneity, contrast, contrast-to-noise ratio, modulation transfer function 10%, modulation transfer function 50%, and Nyquist frequency. The measurements were calculated in a user-guided, semiautomatic manner from 2 DICOM slices selected from the volume. Each volume was measured 3 times, and averages and standard deviations of each parameter were calculated.

QUART DVT_AP CBCT image quality system: A, phantom; B, sample axial images of polyvinyl chloride and air elements ( top ) and acrylic plastic layer ( bottom ); C, analysis software window for calculation of the Nyquist frequency; D, analysis software window for calculating homogeneity.

Results

Volumes produced by ATOM and RANDO adult phantoms for the 13 × 16-cm standard cephalometric and 17 × 23-cm extended FOVs are shown in Figure 6 . An unbiased estimator of difference between pairs ([A-B]/[A+B]/2) demonstrated that OSL dosimeter readings were 1.0% less than thermoluminescent dosimetry readings for standard cephalometric fields and 2.5% less for extended FOV RANDO imaging. The adult ATOM phantom effective doses were 0.2% less than the RANDO doses for the standard cephalometric field and 0.8% less for the extended FOV.

Comparison of RANDO and ATOM phantoms, TLDs and OSL dosimeters, and standard cephalometric and extended FOVs.

ANOVA of the combined adult and child phantom effective dose data demonstrated significant differences caused by phantom ( P = 0.0026), region of interest ( P <0.0001), and exposure protocol ( P <0.0001). Considering QuickScan+, QuickScan, and standard protocols, child phantom doses were on average 36% greater than adult phantom doses. Table III highlights the effective dose results for the adult phantom. The Tukey HSD test demonstrated that the QuickScan+ protocols resulted in significantly lower doses than the standard or high-resolution protocols but were not statistically different from the QuickScan protocols. The QuickScan protocol also resulted in lower doses than the high-resolution protocols but was not statistically different from the standard protocols. The 13 × 16-cm cephalometric imaging resulted in statistically higher doses than all other FOVs. Similarly, the 6 × 16-cm maxillary imaging resulted in significantly lower doses than all other fields. Results for the 17 × 23-cm extended FOV are not included in Table III because only standard and enhanced modes are available for this field. The effective doses were 69 μSv for the standard extended FOV and 136 μSv for the enhanced extended FOV.

Table IV highlights the effective dose results for the child phantom. The Tukey HSD test demonstrated that the each scan protocol was significantly different from the other protocols with increasing doses in order of QuickScan+, QuickScan, and standard protocols. Maxillary FOVs produced statistically lower doses than the arch + temporomandibular joint and the 13 × 16-cm cephalometric views.

Figure 7 is a graphic representation of the data in Table III . Figure 8 is a graphic representation of the data in Table IV .

Adult phantom effective dose by scan type and FOV.

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