Radiation Effective Doses to Adults Undergoing Modified Barium Swallow Studies.

Modified Barium Swallow Studies (MBSSs) are important tests to aid the diagnosis of swallowing impairment and guide treatment planning. Since MBSSs use ionizing radiation, it is important to understand the radiation exposure associated with the exam. This study reports the average radiation dose in routine clinical MBSSs, to aid the evidence-based decision-making of clinical providers and patients. We examined the MBSSs of 200 consecutive adult patients undergoing clinically indicated exams and used kilovoltage (kV) and Kerma Area Product to calculate the effective dose. While 100% of patients underwent the exam in the lateral projection, 72% were imaged in the upper posterior-anterior (PA) projection and approximately 25% were imaged in the middle and lower PA projection. Average kVs were 63 kV, 77 kV, 78.3 kV, and 94.3 kV, for the lateral, upper, middle, and lower PA projections, respectively. The average effective dose per exam was 0.32 ± 0.23 mSv. These results categorize a typical adult MBSS as a low dose examination. This value serves as a general estimate for adults undergoing MBSSs and can be used to compare other sources of radiation (environmental and medical) to help clinicians and patients assess the risks of conducting an MBSS. The distinction of MBSS as a low dose exam will assuage most clinician's fears, allowing them to utilize this tool to gather clinically significant information about swallow function. However, as an X-ray exam that uses ionizing radiation, the principles of ALARA and radiation safety must still be applied.

Radiation Risks to Adult Patients Undergoing Modified Barium Swallow Studies.

Modified Barium Swallow Studies (MBSSs) are a fluoroscopic exam that exposes patients to ionizing radiation. Even though radiation exposure from MBSSs is relatively small, it is necessary to understand the excess cancer risk to the patient, in order to ensure a high benefit-to-risk ratio from the exam. This investigation was aimed at estimating the excess radiation risks during MBSSs. We examined 53 adult MBSSs performed using the full Modified Barium Swallow Impairment Profile (MBSImP) protocol. For each exam, the radiation dose (in terms of dose area product), patient age, and sex was recorded. Using published methodology, we determined the effective dose and organ specific dose then used BEIR VII data to calculate the excess cancer incidence related to radiation exposure from MBSSs in adults. Excess cancer incidence risks due to MBSSs were 11 per million exposed patients for 20-year-old males, 32 per million exposed patients for 20-year-old females, 4.9 per million exposed patients for 60-year-old males, and 7.2 per million exposed patients for 60-year-old females. Radiation exposure to the thyroid, lung, and red bone marrow contributed over 90% of the total cancer incidence risk. For the 20-year-old males, the excess cancer incidence risk is 4.7%/Sv, which is reduced to 1.0%/Sv in the 80-year-olds. For the 20-year-old females, the excess cancer incidence risk is 14%/Sv, which is reduced to 1.3%/Sv for 80-year-olds. Overall, the risk per unit effective dose from MBSSs is lower than the risk estimates for uniform whole-body irradiation. Patient age is the most important determinant of patient cancer risk from MBSSs.

Radiation intensity (CTDIvol) and visibility of anatomical structures in head CT examinations.

The purpose of this study was to quantify how changing the amount of radiation used to perform routine head CT examinations (CTDIvol) affects visibility of key anatomical structures. Eight routine noncontrast head CT exams were selected from six CT scanners, each of which had a different CTDIvol setting (60 to 75 mGy). All exams were normal and two slices were selected for evaluation, one at the level of basal ganglia and the other at the fourth ventricle. Three experienced neuroradiologists evaluated the visibility of selected structures, including the putamen, caudate nucleus, thalamus, internal capsule, grey/white differentiation, and brainstem. Images were scored on a five-point scoring scheme (1, unacceptable, 3, satisfactory, and 5, excellent). Reader scores, averaged over the cases obtained from each scanner, were plotted as a function of the corresponding CTDIvol. Average scores for the fourth ventricle were 3.06 ± 0.83 and for the basal ganglia were 3.20 ± 0.86. No image received a score of 1. Two readers showed no clear trend of an increasing score with increasing CTDIvol. One reader showed a slight trend of increasing score with increasing CTDIvol, but the increase in score from a 25% increase in CTDIvol was a fraction of the standard deviation associated average scores. Collectively, results indicated that there were no clear improvements in visualizing neuroanatomy when CTDIvol increased from 60 to 75 mGy in routine head CT examinations. Our study showed no apparent benefit of using more than 60 mGy when performing routine noncontrast head CT examinations.

The Role of Limited Head Computed Tomography in the Evaluation of Pediatric Ventriculoperitoneal Shunt Malfunction.

BACKGROUND: The evaluation of children with suspected ventriculoperitoneal shunt (VPS) malfunction has evolved into a diagnostic dilemma. This patient population is vulnerable not only to the medical risks of hydrocephalus and surgical complications but also to silent but harmful effects of ionizing radiation secondary to imaging used to evaluate shunt efficacy and patency. The combination of increased medical awareness regarding ionizing radiation and public concern has generated desire to reduce the reliance on head computed tomography (CT) for the evaluation of VPS malfunction. Many centers have started to investigate the utility of low-dose CT scans and alternatives, such as fast magnetic resonance imaging for the investigation of VP shunt malfunction in order to keep radiation exposure as low as reasonably achievable. This pilot study hopes to add to the armamentarium available to the clinician charged with evaluating this challenging patient population by testing the feasibility of a limited CT protocol as an alternative to a full head CT examination. OBJECTIVE: To evaluate the efficacy of a limited head CT protocol compared with a complete head CT for the evaluation of children presenting to the pediatric emergency department with suspected shunt malfunction. METHODS: We retrospectively reviewed all pediatric patients who received a head CT for suspected VPS malfunction evaluation at a tertiary care children's hospital from January 2001 through January 2013. Children were included in the pilot study if they had at least 2 CT scans in this study period interpreted by a specific senior attending neuroradiologist. For each patient enrolled, a limited series was generated from the most recent CT scan by selecting four representative axial slices based on the sagittal scout image. These 4 slices where selected at the level of the fourth ventricle, third ventricle, basal ganglia level, and lateral ventricles, respectively. A blinded, senior attending neuroradiologist first reviewed the limited 4-slice CT data set and was asked to determine if the ventricular system had increased, decreased, or remained stable. Subsequently, the neuroradiologist compared their interpretation of the limited examination with the official report from the full CT data set as the standard of reference as well as the interpretation of the most recent prior scan. RESULTS: Forty-six patients (age range, 2 months to 18 years; average age, 6.4 years (SD, 4.2), 54% male) were included in the study. Forty-four of 46 (95.7%) limited CT scans matched the official report of the full CT scan. No cases of increased ventricular size were missed (100% positive predictive value for increased ventricular size). The use of a limited head CT (4 axial images) instead of a complete head CT (average of 31 axial images in our studied patients) confers a radiation dose reduction of approximately 87%. CONCLUSIONS: Our pilot study demonstrates that utilization of limited head CT scan in the evaluation of children with suspected VP shunt malfunction is a feasible strategy for the evaluation of the ventricular size. Further prospective and multidisciplinary studies are needed to evaluate the reliability of limited head CT for the clinical evaluation of VP shunt malfunction.

Thyroid doses and risk to paediatric patients undergoing neck CT examinations.

OBJECTIVE: To estimate thyroid doses and cancer risk for paediatric patients undergoing neck computed tomography (CT). METHODS: We used average CTDI(vol) (mGy) values from 75 paediatric neck CT examinations to estimate thyroid dose in a mathematical anthropomorphic phantom (ImPACT Patient CT Dosimetry Calculator). Patient dose was estimated by modelling the neck as mass equivalent water cylinder. A patient size correction factor was obtained using published relative dose data as a function of water cylinder size. Additional correction factors included scan length and radiation intensity variation secondary to tube-current modulation. RESULTS: The mean water cylinder diameter that modelled the neck was 14 ± 3.5 cm. The mathematical anthropomorphic phantom has a 16.5-cm neck, and for a constant CT exposure, would have thyroid doses that are 13-17% lower than the average paediatric patient. CTDI(vol) was independent of age and sex. The average thyroid doses were 31 ± 18 mGy (males) and 34 ± 15 mGy (females). Thyroid cancer incidence risk was highest for infant females (0.2%), lowest for teenage males (0.01%). CONCLUSIONS: Estimated absorbed thyroid doses in paediatric neck CT did not significantly vary with age and gender. However, the corresponding thyroid cancer risk is determined by gender and age. KEY POINTS: • Thyroid doses can be estimated from the CTDI(vol) in paediatric neck CT . • Scan length, neck size, and radiation intensity variation should be accounted for. • Estimated absorbed thyroid doses did not significantly vary with age and gender. • Thyroid cancer incidence risk is primarily determined by gender and age.

Radiation risks: what is to be done?

OBJECTIVE: What is currently known about radiologic risks is reviewed, policies that should be adopted based on our current knowledge are proposed, and how these policies can be applied to adequately protect patients in everyday clinical practice is described. CONCLUSION: All activities in life (e.g., driving automobiles) are associated with risks, and medical imaging is no different, so the most important message to convey to patients is whether a proposed examination is worthwhile. Our collective goal should be ensuring that all radiologic examinations are justified and are as low as reasonably achievable (ALARA), which maximizes the benefits of medical imaging for our patients.

Thyroid Doses and Risks to Adult Patients Undergoing Neck CT Examinations.

OBJECTIVE: The purpose of this study was to estimate absorbed thyroid dose and consequent cancer risks in adult patients undergoing neck CT examinations. MATERIALS AND METHODS: We used data from neck CT examinations of 68 consecutive adult patients to calculate the thyroid dose and estimate the corresponding cancer risk. Age and sex were recorded along with the volume CT dose index (CTDIvol) that was used to perform the examination. CTDIvol values were used to estimate thyroid doses in the mathematic phantom used in the ImPACT patient CT dosimetry calculator. Corresponding doses in patients were estimated by modeling each patient's neck as an equivalent cylinder of water and applying correction factors for varying neck size and scanning length and the variation of radiation intensity due to automatic exposure control. RESULTS: The mean (± SD) adult patient age was 59 ± 16 years, and the mean equivalent water cylinder diameter used for modeling the patient neck was 19.4 ± 4.2 cm. The average adult patient neck size was about 3 cm larger than the mathematic anthropomorphic phantom (16.5 cm), decreasing the estimated thyroid doses by 15%. Thyroid doses were independent of age and sex, with an average of 50 ± 23 mGy. The average cancer risk for a 20-year-old woman was six times higher than the corresponding risk for a 20-year-old man. Increasing patient age of either sex from 40 to 60 years reduced the cancer risk by approximately an order of magnitude. CONCLUSION: Patient sex and age are the most important factors in determining thyroid cancer risk, with the thyroid dose being secondary.

ECG-synchronized CT angiography in 324 consecutive pediatric patients: spectrum of indications and trends in radiation dose.

The aim of the study is to describe the spectrum of indications for pediatric ECG-synchronized CT angiography (CTA), the main determinants of radiation exposure, and trends in radiation dose over time at a single, tertiary referral center. The study was IRB approved and HIPAA compliant with informed consent waived. Between 2005 and 2013, 324 pediatric patients underwent ECG-synchronized CTA to evaluate known or suspected cardiovascular abnormalities (109 female, median age 8.1 years). The effective dose (ED) was calculated using age-specific correction factors. Univariate and multivariate regression analyses were performed to identify predictors of radiation dose. The most common primary indications for the CTA examinations included known or suspected coronary pathologies (n = 166), complex congenital heart disease (n = 73), and aortic pathologies (n = 41). Median radiation exposure decreased from 12 mSv for patients examined in the years 2005-2007 to 1.2 mSv for patients examined in the years 2011-2013 (p < 0.001). Patients scanned using a tube potential of 80 kV (n = 259) had a significantly lower median radiation dose (1.4 mSv) compared to patients who were scanned at 100 kV (n = 46, median 6.3 mSv) or 120 kV (n = 19, median 19 mSv, p < 0.001). Tube voltage, followed by tube current and the method of ECG-synchronization were the strongest independent predictors of radiation dose. Growing experience with dose-saving techniques and CTA protocols tailored to the pediatric population have led to a tenfold reduction in radiation dose over recent years and now allow routinely performing ECG-synchronized CTA in children with a radiation dose on the order of 1 mSv.

Reducing radiation dose at CT colonography: decreasing tube voltage to 100 kVp.

PURPOSE: To assess the effect of a decrease in tube voltage from 120 kVp to 100 kVp on dose, contrast-to-noise ratio (CNR), and three-dimensional (3D) image quality in patients undergoing computed tomographic (CT) colonography as well as to determine how these changes are affected by patient size. MATERIALS AND METHODS: This HIPAA-compliant and institutional review board-approved retrospective study included 63 consecutive patients who underwent CT colonography and who waived informed consent. Scanning was performed with patients in the supine (120 kVp) and prone (100 kVp) positions, with other parameters unchanged. Volume CT dose index (CTDI(vol)), dose-length product (DLP), image noise, attenuation of selected materials, and CNR were compared with the Wilcoxon matched-pairs signed rank test. Two readers blinded to tube voltage independently assessed 3D endoluminal image quality. The k coefficients were calculated for interobserver agreement. Average image quality ratings were compared with the Wilcoxon signed rank test. All recorded data were stratified by patient anteroposterior diameter to determine effects of patient size. RESULTS: Decreasing tube voltage from 120 to 100 kVp resulted in a 20% decrease in CTDI(vol) (P < .001) and a 16% decrease in DLP (P < .001). Image noise increased by 32% (P < .001). Mean attenuation of tagged fluid increased from 395 to 487 HU (P < .001). There was no change in mean CNR of tagged fluid (17.1 at 120 kVp, 16.8 at 100 kVp; P = .37), regardless of patient size. The 3D image quality decreased slightly from a median score of 5 out of 5 to 4 out of 5 (P < .001). There was substantial interobserver agreement. CONCLUSION: A decrease in tube voltage from 120 to 100 kVp results in a significant decrease in radiation dose but only a minimal decrease in 3D image quality at all patient sizes. © RSNA, 2012.

Accuracy of CT guidance of lumbar facet joint block.

OBJECTIVE: Lumbar facet joint block is generally performed under fluoroscopic guidance. The purpose of this study was to assess the technical success rate of facet joint block under CT guidance. The CT scanner was operated tableside with a step-and-shoot mode for intermittent needle visualization, and the amount of radiation used to perform the procedures was estimated. CONCLUSION: CT-guided facet joint block is safe and rapid. Use of CT ensures reliable needle guidance with extremely high procedural accuracy at an effective radiation dose comparable to that of a procedure performed with 1 minute of fluoroscopic guidance.

Effect of patient size on mean sterile water attenuation during multiphase CT examinations.

OBJECTIVE: The purpose of this study was to determine the variability of attenuation measurements in a water phantom included in the FOV during multiphase 64-MDCT. SUBJECTS AND METHODS: Ninety-seven consecutively registered patients undergoing multiphase kidney and liver protocol CT of the abdomen on the same 64-MDCT scanner had a sealed water bottle placed on their anterior abdomen during the examination. Region of interest (ROI) measurements of the mean attenuation (in HU) of the water bottle were made during the unenhanced, dynamic, and delayed phases of contrast enhancement. Generalized estimating equations were used to model mean attenuation in the ROI as a function of cross-sectional patient area, contrast phase, and protocol. Day of month and time of day were covariates. A phantom was created to model the patient study. RESULTS: The mean attenuation values in the water bottle ROIs were -13.1 HU for the kidney protocol and -9.1 HU for the liver protocol in the unenhanced phase, -11.7 HU for the kidney and -9.5 HU for the liver protocol in the dynamic phase, and -11.9 HU for the kidney and 11.0 HU for the liver protocol in the delayed phase. Kidney protocol water bottle ROI attenuation values were lower than the liver values (p = 0.04). In all phases with both protocols, the values differed from 0 HU (all p < 0.0001). Water bottle ROI attenuation decreased as patient cross-sectional area increased (-0.01 HU/cm(2), p < 0.0001). Three patients had absolute water bottle attenuation changes greater than 20 HU between phases. Day of the month (p = 0.08) and time of day (p = 0.93) were not significant factors. Phantom data supported the study findings. CONCLUSION: The mean attenuation of water decreased as patient diameter increased. Both artifactual enhancement and decrease in enhancement greater than 20 HU were found in three larger patients.

Effective radiation dose in computed tomographic angiography of the chest and diagnostic cardiac catheterization in pediatric patients.

Computed tomographic angiography (CTA) and cardiac catheterization are useful adjuncts to echocardiography for delineating cardiovascular anatomy in pediatric patients. These studies require ionizing radiation, and it is paramount to understand the amount of radiation pediatric patients receive when these tests are performed. Modern dosimetry methods facilitate the conversion of radiation doses of varying units into an effective radiation dose. To compare the effective radiation dose between nongated CTA of the chest and diagnostic cardiac catheterization in pediatric patients. This is a retrospective cohort study of patients of patients who underwent either nongated CTA of the chest or diagnostic cardiac catheterization between July 2009 and April 2010. Fifty patients were included in each group as consecutive samples at a single tertiary care center. An effective radiation dose (mSv) was formulated using conversion factors for each group. The median effective dose (ED) for the CTA group was 0.74 mSv compared with 10.8 mSv for the catheterization group (p < 0.0001). The median ED for children <1 year of age in the CTA group was 0.76 mSv compared with 13.4 mSv for the catheterization group (p < 0.0001). Nongated CTA of the chest exposes children to 15 times less radiation than diagnostic cardiac catheterization. Unless hemodynamic data are necessary, CTA of the chest should be considered in lieu of diagnostic cardiac catheterization in patients with known or presumed cardiac disease who need additional imaging beyond echocardiography.

Radiation exposure time during MBSS: influence of swallowing impairment severity, medical diagnosis, clinician experience, and standardized protocol use.

Guidelines and preventive measures have been established to limit radiation exposure time during modified barium swallow studies (MBSS) but multiple variables may influence the duration of the exam. This study examined the influence of clinician experience, medical diagnosis category, swallowing impairment severity, and use of a standardized protocol on fluoroscopy time. A retrospective review of 739 MBSSs performed on 612 patients (342 males/270 females; age range = 18-96 years) completed in 1 year at the Medical University of South Carolina was performed with IRB approval. All studies were completed by speech-language pathologists trained in the data collection protocol, interpretation, and scoring of the MBSImP™(©). Medical diagnosis category, swallowing impairment severity (MBSImP™(©) score), clinician experience, and fluoroscopy time were the variables recorded for analysis. Fluoroscopy time was not significantly associated with medical diagnosis category (p = 0.10). The severity of the MBSImP™(©) Oral Total and Pharyngeal Total resulted in statistically significant increases in fluoroscopy time (p < 0.05). Studies by novice clinicians had longer exposure times when compared to those of experienced clinicians (p = 0.037). Average radiation exposure time using the MBSImP™(©) approach was 2.9 min, with a 95 % confidence interval of 2.8-3.0 min, which was well within the range of exposure times reported in the literature. This study provides preliminary information regarding the impact of medical diagnosis category, swallowing impairment severity, and clinician experience on fluoroscopy time. These findings also suggest that a thorough, standardized protocol for MBSSs did not cause unnecessary radiation exposure time during the MBSS.

Preliminary investigation of the effect of pulse rate on judgments of swallowing impairment and treatment recommendations.

Reducing fluoroscopic pulse rate, a method used to reduce radiation exposure from modified barium swallow studies (MBSSs), decreases the number of images available from which to judge swallowing impairment. It is necessary to understand the impact of pulse rate reduction on judgments of swallowing impairment and, consequentially, treatment recommendations. This preliminary study explored differences in standardized MBSS measurements [Modified Barium Swallow Impairment Profile (MBSImP™©) and Penetration Aspiration Scale (PAS) Scores] between two pulse rates: 30 and simulated 15 pulses per second (pps). Two reliable speech-language pathologists (SLPs) scored all five MBSSs. Five SLPs reported treatment recommendations based on those scores. Differences in judgments of swallowing impairment were found between 30 and simulated 15 pps in all five MBSSs. These differences were in six physiological swallowing components: initiation of pharyngeal swallow, anterior hyoid excursion, epiglottic movement, pharyngeal contraction, pharyngeal-esophageal segment opening, and tongue base retraction. Differences in treatment recommendations were found between 30 and simulated 15 pps in all five MBSSs. These findings suggest that there are differences in both judgment of swallowing impairment and treatment recommendations when pulse rates are reduced from 30 to 15 pps to minimize radiation exposure.

Quality assurance: a comparison study of radiographic exposure for neonatal chest radiographs at 4 academic hospitals.

BACKGROUND: Little is known about exposure differences among hospitals. Large differences might identify outliers using excessive exposure. OBJECTIVE: We used the newly described exposure index and deviation index to compare the difference in existing radiographic exposures for neonatal portable chest radiographs among four academic children's hospitals. MATERIALS AND METHODS: For each hospital we determined the mean exposure index. We also set target exposure indices and then measured the deviation from this target. RESULTS: There was not a large difference in exposure index among sites. No site had an exposure index mean that was more than twice or less than half that of any other site. For all four sites combined, 92% of exposures had a deviation index within the range from -3 to +3. Thus exposures at each hospital were consistently within a reasonable narrow spectrum. CONCLUSION: Mean exposure index differences are caused by operational differences with mean values that varied by less than 50% among four hospitals. Ninety-two percent of all exposures were between half and double the target exposure. Although only one vendor's equipment was used, these data establish a practical reference range of exposures for neonatal portable radiographs that can be recommended to other hospitals for neonatal chest radiographs.

KERMA ratios in pediatric CT dosimetry.

BACKGROUND: Patient organ doses may be estimated from CTDI values. More accurate estimates may be obtained by measuring KERMA (Kinetic Energy Released in Matter) in anthropomorphic phantoms and referencing these values to free-in-air X-ray intensity. OBJECTIVE: To measure KERMA ratios (R(K)) in pediatric phantoms at CT. MATERIALS & METHODS: CT scans produce an air KERMA K in a phantom and an air KERMA K(CT) at isocenter. KERMA ratios (R(K)) are defined as (K/K(CT)), measured using TLD chips in phantoms representing newborns to 10-year-olds. RESULTS: R(K) in the newborn is approximately constant. For the other phantoms, there is a peak R(K) value in the neck. The median R(K) values for the GE scanner at 120 kV were 0.92, 0.83, 0.77 and 0.76 for newborns, 1-year-olds, 5-year-olds and 10-year-olds, respectively. Organ R(K) values were 0.91 ± 0.04, 0.84 ± 0.07, 0.74 ± 0.09 and 0.72 ± 0.10 in newborns, 1-year-olds, 5-year-olds and 10-year-olds, respectively. At 120 kV, a Siemens Sensation 16 scanner had R(K) values 5% higher than those of the GE LightSpeed Ultra. CONCLUSION: KERMA ratios may be combined with air KERMA measurements at the isocenter to estimate organ doses in pediatric CT patients.

Volume CT dose index and dose-length product displayed during CT: what good are they?

The average medical radiation effective dose to the U.S. population in 2006 was estimated at approximately 3.0 mSv, an increase of 600% in a single generation. Computed tomography (CT) alone accounts for approximately half of this medical radiation dose. Ongoing advances suggest that CT will continue to be the most important contributor, by far, to medical doses in the United States. The use of ionizing radiation in medical imaging, including CT, provides valuable diagnostic information that undoubtedly benefits many patients. Exposure to radiation, however, is currently believed to carry a small, but nonzero, risk. Accordingly, the medical imaging community must ensure that the benefits of a radiologic examination in any given patient exceed the corresponding risks. It is also the responsibility of the radiologist to ensure that no more radiation is used than needed for obtaining diagnostic information in any radiologic examination, especially CT.

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.

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.

In-patient to isocenter KERMA ratios in CT.

PURPOSE: To estimate in-patient KERMA for specific organs in computed tomography (CT) scanning using ratios to isocenter free-in-air KERMA obtained using a Rando phantom. METHOD: A CT scan of an anthropomorphic phantom results in an air KERMA K at a selected phantom location and air kerma K(CT) at the CT scanner isocenter when the scan is repeated in the absence of the phantom. The authors define the KERMA ratio (R(K)) as K∕ K(CT), which were experimentally determined in a Male Rando Phantom using lithium fluoride chips (TLD-100). R(K) values were obtained for a total of 400 individual point locations, as well as for 25 individual organs of interest in CT dosimetry. CT examinations of Rando were performed on a GE LightSpeed Ultra scanner operated at 80 kV, 120 kV, and 140 kV, as well as a Siemens Sensation 16 operated at 120 kV. RESULTS: At 120 kV, median R(K) values for the GE and Siemens scanners were 0.60 and 0.64, respectively. The 10th percentile R(K) values ranged from 0.34 at 80 kV to 0.54 at 140 kV, and the 90th percentile R(K) values ranged from 0.64 at 80 kV to 0.78 at 140 kV. The average R(K) for the 25 Rando organs at 120 kV was 0.61 ± 0.08. Average R(K) values in the head, chest, and abdomen showed little variation. Relative to R(K) values in the head, chest, and abdomen obtained at 120 kV, R(K) values were about 12% lower in the pelvis and about 58% higher in the cervical spine region. Average R(K) values were about 6% higher on the Siemens Sensation 16 scanner than the GE LightSpeed Ultra. Reducing the x-ray tube voltage from 120 kV to 80 kV resulted in an average reduction in R(K) value of 34%, whereas increasing the x-ray tube voltage to 140 kV increased the average R(K) value by 9%. CONCLUSIONS: In-patient to isocenter relative KERMA values in Rando phantom can be used to estimate organ doses in similar sized adults undergoing CT examinations from easily measured air KERMA values at the isocenter (free in air). Conversion from in-patient air KERMA values to tissue dose would require the use of energy-appropriate conversion factors.