Radiation Safety Concerns Related to PET/Computed Tomography Imaging for Assessing Pediatric Diseases and Disorders.

The Image Gently Nuclear Medicine Working Group published a 10-year update in 2019. One of the future goals of this working group is to continue the efforts started in 2014 to harmonize the North American guidelines with the European Association of Nuclear Medicine pediatric dosing guidelines, continuing to publicize the use of the North American guidelines. The update also acknowledged the need for standardization of CT parameters in hybrid imaging and also will seek to tackle this issue as one of its future goals.

Radiation dose reduction using novel size 1 and size 0 rectangular collimators in pediatric dental imaging.

OBJECTIVES: Commercial intraoral rectangular collimators are available for collimating to size 2 image receptor. The benefits of reducing the x-ray beam to match the area of the image detector in adult intraoral radiography are endorsed internationally. However, in pediatric dentistry the image receptor can be further decreased to size 1 and 0. METHOD AND MATERIALS: For this study size 1 and 0 rectangular collimators were fabricated using 1.65-mm lead sheets (Rotometals). The custom-fabricated collimators were fixed to the plastic body of a Rinn (Dentsply) Universal Collimator attachment. Aperture sizes were extrapolated based on the active imaging area of size 1 and 0 digital image receptors. A dose area product (DAP) measuring device was used to determine the change in radiation absorbed dose as a function of the imaging field of view. RESULTS: DAP measurements were evaluated in the 31.7 cm2 conventional round collimation, Rinn 12.0 cm2 Universal rectangular collimator, and in the manufactured size 1 (8.25 cm2) and size 0 (5.72 cm2) rectangular collimators. The size 1 collimator had a 32% DAP reduction from the size 2, and a 53% reduction for the size 0. CONCLUSION: Size 1 and size 0 rectangular collimators can be independently manufactured and utilized in pediatric dentistry. This study suggests that a considerable radiation dose reduction is possible in pediatric intraoral imaging when using the size 1 and 0 matched collimation. Since the pediatric population is vulnerable to radiation exposure, any measurable reduction has a potential for long-term health benefits and is therefore clinically significant.

Assessment of dental radiation dose reduction utilizing mathematical pediatric phantom models: applications in clinical practice.

OBJECTIVES: Replacing conventional round intraoral collimators with rectangular collimators provides a considerable radiation dose reduction in adult patients. This study aimed to determine the radiation dose reduction via mathematical phantom when converting from round to appropriately sized rectangular collimation in children ages 5 to 15 years. METHOD AND MATERIALS: Virtual full mouth series (FMX) were simulated using a commercially available radiation dose software. This software is designed to calculate patient radiation doses from x-ray exams for various age pediatric and adult mathematical phantoms. For this pediatric study an 18-image FMX was simulated for the 15-year-old and a 12-image FMX was simulated for the 5-year-old and 10-year-old pediatric phantoms. An area of 12.0 to 16.8 cm2 represented rectangular collimation, while a 20.4 to 31.7 cm2 area represented typical round collimation. RESULTS: Effective doses decreased in all ages by nearly 60% when switching from 31.7 cm2 round to 12.0 cm2 rectangular collimation. Reduction in absorbed doses to the thyroid (70% to 73%), salivary glands (62% to 78%), and active bone marrow (60% to 62%) were also noted when switching from the largest to smallest collimation. CONCLUSION: This study suggests the use of rectangular collimators provides clinically relevant dose reduction for pediatric patients, even when altering from smaller round to rectangular collimation with equivalent beam quality, and this information can be utilized in all dental practices.

Kerma area product (KAP) and scatter measurements for intraoral X-ray machines using three different types of round collimation compared with rectangular beam limiter.

OBJECTIVES:: The objective of the study was to determine the radiation dose reduction achieved when rectangular collimation was used on various round collimators. In addition, we evaluated the tissue doses imparted to various head and neck organs. METHODS:: To evaluate the variation in radiation output based on the variable geometric configurations, the kerma area product (KAP) was measured using a commercially available KAP-meter with an internal ion chamber capable of detecting both radiation dose (µGy) and the primary X-ray beam area. The KAP was measured using standard 20.4, 25.7, and 31.7 cm2 round collimators with and without rectangular X-ray field restrictors. To evaluate the potential change in patient scatter radiation dose, an adult head phantom was loaded with thin strips of gafchromic film. A full mouth X-ray series was acquired with various geometric configurations. The films were quantified using a calibration factor to yield absorbed organ doses for the eyes, thyroid, and salivary glands. RESULTS:: With the use of rectangular collimator, the KAP for a 31.7 cm2 round collimator was reduced by up to 60% while the 20.4 cm2 round collimator elicited a reduction from up to 40%. In the organ study, results of up to 81% reduction in scatter radiation dose were observed. CONCLUSIONS:: Although, US FDA regulations allow a maximum beam size of 38.5 cm2 on the patient skin, this study suggests that the use of rectangular collimators provide clinically relevant dose reduction for patients, even when using smaller round collimation, hence the use of rectangular collimation for all intraoral radiographic procedures is highly recommended.

METHOD FOR OCCUPATIONAL SKIN DOSE ESTIMATION IN UPPER EXTREMITY 131I-MIBG CONTAMINATION.

This study examines upper extremity skin contamination of nuclear medicine and radiation safety staff during 131I-Metaiodobenzylguanidine (MIBG) therapy. Utilizing retrospective data, a methodology for performing a rapid assessment of the radiation dose to the skin of the upper extremities is presented. Using the skin contamination measurements and calculated skin dose for each contamination incident at our facility, a conversion factor (XE) was derived that estimates skin dose (DE) based on the initial contamination measurement. This methodology yields an estimate of the final skin dose accounting for radioactive decay, decontamination and other factors, such as skin sloughing. As a standard practice multiple time-point measurements from initial contamination to background should be used to calculate the total attributable skin dose. However, to provide an early projection of the expected skin dose, the dose can be reasonably estimated to be <0.10% mSv cpm-1 (10% mrem cpm-1) of the initial contamination measurement.

Investigation of dental cone-beam CT pixel data and a modified method for conversion to Hounsfield unit (HU).

OBJECTIVES: To investigate the relationship in dental cone-beam CT (CBCT) between the manufacturer-reported image pixel data and a modified conversion to CT number densities in Hounsfield unit (HU). METHODS: A standardized CT phantom was imaged using typical clinical parameters on CBCT from three manufacturers (Carestream 9300®, Carestream Health, Rochester, NY; J Morita 3D Accutomo®, J. Morita Mfg. Corp., Kyoto, Japan; and Planmeca Promax 3D®, Planmeca Helsinki, OY, Finland). Reconstructed axial slices were evaluated using regions of interest to ascertain the mean pixel value in five materials in the phantom. The Digital Imaging and Communications in Medicine data were also evaluated to determine if raw pixel data had been adjusted during the image reconstruction. A modified version of the existing manual HU conversion technique was applied, and the resultant slope and y-intercept were used to scale the pixel values ultimately to HU for all images. RESULTS: The DICOM header data show that a modified rescale y-intercept was applied to both the Carestream and Planmeca image data yielding manufacturer-produced results in HU. The Morita pixel data were unmodified and report in shades of grey or grey values (GV). The Carestream manufacturer-derived HU measurements showed good correlation in air (-1000 HU), but all other materials ranged from 2.6 to 13.5 σ from the specified phantom value. Results in the modified conversion technique images fell within 1.0-2.4 σ from the specified phantom values. CONCLUSIONS: While more studies are needed to test for regularity, this study suggests that our modified technique could be a means of getting more accurate quantitative data from dental CBCTs.

Female gonadal shielding with automatic exposure control increases radiation risks.

BACKGROUND: Gonadal shielding remains common, but current estimates of gonadal radiation risk are lower than estimated risks to colon and stomach. A female gonadal shield may attenuate active automatic exposure control (AEC) sensors, resulting in increased dose to colon and stomach as well as to ovaries outside the shielded area. OBJECTIVE: We assess changes in dose-area product (DAP) and absorbed organ dose when female gonadal shielding is used with AEC for pelvis radiography. MATERIALS AND METHODS: We imaged adult and 5-year-old equivalent dosimetry phantoms using pelvis radiograph technique with AEC in the presence and absence of a female gonadal shield. We recorded DAP and mAs and measured organ absorbed dose at six internal sites using film dosimetry. RESULTS: Female gonadal shielding with AEC increased DAP 63% for the 5-year-old phantom and 147% for the adult phantom. Absorbed organ dose at unshielded locations of colon, stomach and ovaries increased 21-51% in the 5-year-old phantom and 17-100% in the adult phantom. Absorbed organ dose sampled under the shield decreased 67% in the 5-year-old phantom and 16% in the adult phantom. CONCLUSION: Female gonadal shielding combined with AEC during pelvic radiography increases absorbed dose to organs with greater radiation sensitivity and to unshielded ovaries. Difficulty in proper use of gonadal shields has been well described, and use of female gonadal shielding may be inadvisable given the risks of increasing radiation.

Intussusception reduction: Effect of air vs. liquid enema on radiation dose.

BACKGROUND: Both air and radiopaque liquid contrast are used to reduce ileocolic intussusception under fluoroscopy. Some suggest air lowers radiation dose due to shorter procedure times. However, air enema likely lowers radiation dose regardless of fluoroscopy time due to less density over the automatic exposure control cells. OBJECTIVES: We test the hypothesis that air enema reduction of ileocolic intussusception results in lower radiation dose than liquid contrast enema independent of fluoroscopy time. We describe a role for automatic exposure control in this dose difference. MATERIALS AND METHODS: We retrospectively evaluated air and liquid intussusception reductions performed on a single digital fluoroscopic unit during a 26-month period. We compared patient age, weight, gender, exam time of day and year, performing radiologist(s), radiographic image acquisitions, grid and magnification use, fluoroscopy time and dose area product. We compared categorical and continuous variables statistically using chi-square and Mann-Whitney U tests, respectively. RESULTS: The mean dose area product was 2.7-fold lower for air enema, 1.3 ± 0.9 dGy·cm2, than for liquid, 3.5 ± 2.5 dGy·cm2 (P<0.005). The mean fluoroscopy time was similar between techniques. The mean dose area product/min was 2.3-fold lower for air, 0.6 ± 0.2 dGy·cm2/min, than for liquid, 1.4 ± 0.5 dGy·cm2/min (P<0.001). No group differences were identified in other measured dose parameters. CONCLUSION: Fluoroscopic intussusception reduction using air enema uses less than half the radiation dose of liquid contrast enema. Dose savings are independent of fluoroscopy time and are likely due to automatic exposure control interaction.

THE EFFECT OF RADIATION SHIELDS ON OPERATOR EXPOSURE DURING CONGENITAL CARDIAC CATHETERISATION.

Cardiac catheterisation personnel are exposed to occupational radiation and its health risks. Little data exist regarding the efficacy of radiation-protective equipment from congenital catheterisation laboratories (CLs). The authors retrospectively reviewed data in which CL operators wore a radiation dosemeter during catheterizations on patients of >20 kg. A leaded under-table skirt was present in all cases. Three additional radiation-protective devices were utilised at operator discretion: a top extension to the under-table skirt, a ceiling-mounted shield and a disposable patient drape. Case details, operator position, fluoroscopy time, incident air KERMA in the patient plane (K, mGy) and dose-area product (DAP, µGy·m2) were recorded. A total of 136 catheterizations over 8 months were included. Median operator dose (OpD) was 12 µSv (range 0-930) and indexed to K and DAP to correct for patient factors and case times. Indexed OpD decreased significantly with each additional shield used (14.8 vs. 1.3 nSv µGy-1 m-2 and 124 vs. 14 nSv mGy-1 with one and four shields, respectively, p < 0.001). This trend was not significant with operator at head-of-bed. Combinations that included the ceiling shield had the lowest indexed OpD. The patient drape did not further reduce OpD when all other shields were used (1.3 vs. 2.2 nSv µGy-1 m-2, p = 0.5; 14 vs. 17 nSv mGy-1, p = 0.4) and was associated with higher patient exposure indexed to weight and fluoroscopy time (4.5 vs. 3.1 µGy m2 kg-min-1, p = 0.009; and 0.51 vs. 0.38 mGy kg-min-1, p = 0.01). Supplemental radiation barriers can decrease operator-absorbed radiation. A ceiling-mounted shield may provide greatest benefit. The authors do not recommend routine use of disposable patient drapes.

Computation of thyroid doses and carcinogenic radiation risks to patients undergoing neck CT examinations.

The aim of the study was to investigate how differences in patient anatomy and CT technical factors in neck CT impact on thyroid doses and the corresponding carcinogenic risks. The CTDIvol and dose-length product used in 11 consecutive neck CT studies, as well as data on automatic exposure control (AEC) tube current variation(s) from the image DICOM header, were recorded. For each CT image that included the thyroid, the mass equivalent water cylinder was estimated based on the patient cross-sectional area and average relative attenuation coefficient (Hounsfield unit, HU). Patient thyroid doses were estimated by accounting for radiation intensity at the location of the patient's thyroid, patient size and the scan length. Thyroid doses were used to estimate thyroid cancer risks as a function of patient demographics using risk factors in BEIR VII. The length of the thyroid glands ranged from 21 to 54 mm with an average length of 42 ± 12 mm. Water cylinder diameters corresponding to the central slice through the patient thyroid ranged from 18 to 32 cm with a mean of 25 ± 5 cm. The average CTDIvol (32-cm phantom) used to perform these scans was 26 ± 6 mGy, but the use of an AEC increased the tube current by an average of 44 % at the thyroid mid-point. Thyroid doses ranged from 29 to 80 mGy, with an average of 55 ± 19 mGy. A 20-y-old female receiving the highest thyroid dose of 80 mGy would have a thyroid cancer risk of nearly 0.1 %, but radiation risks decreased very rapidly with increasing patient age. The key factors that affect thyroid doses in neck CT examinations are the radiation intensity at the thyroid location and the size of the patient. The corresponding patient thyroid cancer risk is markedly influenced by patient sex and age.

Technical note: estimating absorbed doses to the thyroid in CT.

PURPOSE: To describe a method for estimating absorbed doses to the thyroid in patients undergoing neck CT examinations. METHODS: Thyroid doses in anthropomorphic phantoms were obtained for all 23 scanner dosimetry data sets in the ImPACT CT patient dosimetry calculator. Values of relative thyroid dose [R(thy)(L)], defined as the thyroid dose for a given scan length (L) divided by the corresponding thyroid dose for a whole body scan, were determined for neck CT scans. Ratios of the maximum thyroid dose to the corresponding CTDI(vol) and [D'(thy)], were obtained for two phantom diameters. The mass-equivalent water cylinder of any patient can be derived from the neck cross-sectional area and the corresponding average Hounsfield Unit, and compared to the 16.5-cm diameter water cylinder that models the ImPACT anthropomorphic phantom neck. Published values of relative doses in water cylinders of varying diameter were used to adjust thyroid doses in the anthropomorphic phantom to those of any sized patient. RESULTS: Relative thyroid doses R(thy)(L) increase to unity with increasing scan length and with very small difference between scanners. A 10-cm scan centered on the thyroid would result in a dose that is, nearly 90% of the thyroid dose from a whole body scan when performed using the constant radiographic techniques. At 120 kV, the average value of D'(thy) for the 16-cm diameter was 1.17 +/- 0.05 and was independent of CT vendor and year of CT scanner, and choice of x-ray tube voltage. The corresponding average value of D'(thy) in the 32-cm diameter phantom was 2.28 +/- 0.22 and showed marked variations depending on vendor, year of introduction into clinical practice as well as x-ray tube voltage. At 120 kV, a neck equivalent to a 10-cm diameter cylinder of water would have thyroid doses 36% higher than those in the ImPACT phantom, whereas a neck equivalent to a 25-cm cylinder diameter would have thyroid doses 35% lower. CONCLUSIONS: Patient thyroid doses can be estimated by taking into account the amount of radiation used to perform the CT examination (CTDI(vol)) and accounting for scan length and patient anatomy (i.e., neck diameter) at the thyroid location.

CT effective dose per dose length product using ICRP 103 weighting factors.

PURPOSE: To generate effective dose per unit dose length product (E/DLP) conversion factors incorporating ICRP Publication 103 tissue weighting factors. METHODS: Effective doses for CT examinations were obtained using the IMPACT Dosimetry Calculator using all 23 dose data sets that are offered by this spreadsheet. CT examinations were simulated for scans performed along the patient long axis for each dosimetry data set using a 4 cm beam width ranging from the upper thighs to top of the head. Five basic body regions (head, neck, chest, abdomen, and pelvis), as well as combinations of the regions (head/neck, chest/abdomen, abdomen/ pelvis, and chest/abdomen/pelvis) and whole body CT scans were investigated. Correction factors were generated that can be applied to convert E/DLP conversion factors based on ICRP 60 data to conversion factors that are valid for ICRP 103 data (i.e., E103/E60). RESULTS: Use of ICRP 103 weighting factors increase effective doses for head scans by approximately 11%, for chest scans by approximately 20%, and decrease effective doses for pelvis scans by approximately 25%. Current E/DLP conversion factors are estimated to be 2.4 microSv/mGy cm for head CT examinations and range between 14 and 20 microSv/mGy cm for body CT examinations. CONCLUSIONS: Factors that enable patient CT doses to be adjusted to account for ICRP 103 tissue weighting factors are provided, which result in E/DLP factors that were increased in head and chest CT, reduced in pelvis CT, and showed no marked change in neck and abdomen CT.

X-ray tube current modulation and patient doses in chest CT.

The aim of the study was to investigate how patient effective doses vary as a function of X-ray tube projection angle, as well as the patient long axis, and quantify how X-ray tube current modulation affects patient doses in chest CT examinations. Chest examinations were simulated for a gantry CT scanner geometry with projections acquired for a beam width of 4 cm. PCXMC 2.0.1 was used to calculate patient effective doses at 15° intervals around the patient's isocentre, and at nine locations along the patient long axis. Idealised tube current modulation schemes were modelled as a function of the X-ray tube angle and the patient long axis. Tube current modulations were characterised by the modulation amplitude R, which was allowed to vary between 1.5 and 5. Effective dose maxima occur for anteroposterior projections at the location of the (radiosensitive) breasts. The maximum to minimum ratio of effective doses as a function of the patient long axis was 4.9, and as a function of the X-ray tube angle was 2.1. Doubling the value of R reduces effective doses from longitudinal modulation alone by ∼4% and from angular modulation alone by ∼2%. In chest CT, tube current modulation schemes currently have longitudinal R values of ∼2.2, and angular R values that range between 1.5 and 3.4. Current X-ray tube current modulation schemes are expected to reduce patient effective doses in chest CT examinations by ∼10%, with longitudinal modulation accounting for two-thirds and angular modulation for the remaining one-third.

Patient doses and projection angle in cone beam CT.

PURPOSE: To investigate how x-ray tube projection angle affects organ and effective doses to patients undergoing a CT examination on a cone beam scanner. METHODS: The authors investigated two cone beam CT systems that use a flat panel detector to capture the x-ray pattern transmitted through patients. One system had the flat panel detector and x-ray tube mounted on a conventional CT gantry (gantry CT), whereas the other CT scanner had the x-ray tube and flat panel detector mounted on a C-arm apparatus (C-arm CT). PCXMC software package (version 2.0.1) was used to compute absorbed doses at a constant x-ray beam output as a function of the x-ray tube projection angle. This software uses a mathematical hermaphroditic phantom with a weight of 73.2 kg and a height of 178.6 cm. Average absorbed doses were generated and recorded for five radiosensitive organs (i.e., breast, colon, lung, red bone marrow, and stomach), as well as the corresponding effective dose. Doses for both CT gantries were obtained every 15 degrees of the x-ray tube projection angle, at each of six locations in 10 cm increments along the patient long axis. The authors also investigated the effect on patient dose of filtrations ranging from 2.5 to 9.5 mm Al, with x-ray tube voltages ranging from 80 to 140 kV. RESULTS: There were substantial differences in organ doses as a function of the projection angle, with higher organ doses for anteroposterior projection and lower doses for lateral projections. The maximum to minimum ratios of organ dose as a function of the x-ray tube angle were approximately 2.2 for the lungs, approximately 3.7 for the colon, approximately 5.9 for the red bone marrow, approximately 19.8 for the breast, and approximately 36 for the stomach. At the same x-ray tube voltage (kV) and intensity (mA s), the ratio of dose maxima for the two cone beam CT geometries for the five organs investigated was 1.85 +/- 0.10, which is mainly attributed to differences in source to isocenter distances. Effective doses varied with x-ray tube angle by a factor of approximately 2.7 for chest CT examinations and by a factor of approximately 4.0 for abdomen/pelvis examinations. Changes in the x-ray tube voltage and beam filtration showed the expected changes in absolute organ doses, but had little effect on relative doses. CONCLUSIONS: There are major differences in organ and effective dose as the x-ray tube rotates around the patient. The results suggest that the use of x-ray tube current modulation could produce substantial reductions in organ and effective dose for body imaging with cone beam CT.