Organ and effective doses in newborns and infants undergoing voiding cystourethrograms (VCUG): a comparison of stylized and tomographic phantoms.

The time-sequence videotape-analysis methodology, developed [Sulieman et al., Radiology 178, 653-658 (1991)] for use in tissue dose estimations in adult fluoroscopy examinations and utilized [Bolch et al., Med. Phys. 30, 667-680 (2003)] for analog fluoroscopy in newborn patients, has been extended to the study of digital fluoroscopic examinations of the urinary bladder in newborn and infant female patients. Individual frames of the fluoroscopic and radiographic video were analyzed with respect to unique combinations of field size, field center, projection, tube potential, and tube current (mA), and integral tube current (mAs), respectively. The dosimetry study was conducted on five female patients of ages ranging from four-days to 66 days. For each patient, three different phantoms were utilized: a stylized computational phantom of the reference newborn (3.5 kg), a tomographic computational phantom of the reference newborn (3.5 kg), and (3) a tomographic computational phantom uniformly rescaled to match patient total-body mass. The latter phantom set circumvented the need for mass-dependent rescaling of recorded technique factors (kVp, mA, mAs, etc.), and thus represented the highest degree of patient specificity in the individual organ dose assessment. Effective dose values for the voiding cystourethrogram examination ranged from 0.6 to 3.2 mSv, with a mean and standard deviation of 1.8+/-0.9 mSv. The ovary and colon equivalent doses contributed in total approximately 65%-80% of the effective dose in these fluoroscopy studies. Percent differences in the effective dose assessed using the two tomographic phantoms (one fixed at 3.5 kg with rescaled technique factors rescaled and one physically rescaled to individual patient masses with no adjustment of recorded technique factors) ranged for -49% to +15%. Percent differences in effective dose found using the 3.5 kg stylized phantom and the 3.5 kg tomographic phantom, both with patient-specific rescaling of technique factors, ranged from -10% to +17%. These differences are due in part to a reduced ovary dose in the tomographic phantom for right posterior oblique (RPO) views when compared to those seen in the stylized phantom.

Organ and effective doses in infants undergoing upper gastrointestinal (UGI) fluoroscopic examination.

To provide more detailed data on organ and effective doses in digital upper gastrointestinal (UGI) fluoroscopy studies of newborns and infants, the present study was conducted employing the time-sequence videotape-analysis technique used in a companion study of newborn and infant voiding cystourethrograms (VCUG). This technique was originally pioneered [O. H. Suleiman, J. Anderson, B. Jones, G. U. Rao, and M. Rosenstein, Radiology 178, 653-658 (1991)] for adult UGI examinations. Individual video frames were analyzed to include combinations of field size, field center, x-ray projection, image intensifier, and magnification mode. Additionally, the peak tube potential and the mA or mAs values for each segment/subsegment or digital photospot were recorded for both the fluoroscopic and radiographic modes of operation. The data from videotape analysis were then used in conjunction with a patient-scalable newborn tomographic computational phantom to report both organ and effective dose values via Monte Carlo radiation transport. The study includes dose estimates for five simulated UGI examinations representative of patients ranging from three to six months of age. Effective dose values for UGI examinations ranged from 1.17 to 6.47 mSv, with a mean of 3.14 mSv and a large standard deviation of 2.15 mSv. The colon, lungs, stomach, liver, and esophagus absorbed doses in sum were found to constitute between 63 and 75% of the effective dose in these UGI studies. Representing 23-30% of the effective dose, the lungs were found to be the most significant organ in the effective dose calculation. Approximately 80-95% of the effective dose is contributed by the dynamic fluoroscopy segments with larger percentages found in longer studies. The mean effective dose for newborn UGI examinations was not found to be statistically different from that seen in newborn VCUG examinations.

Organ and effective doses in pediatric patients undergoing helical multislice computed tomography examination.

As multidetector computed tomography (CT) serves as an increasingly frequent diagnostic modality, radiation risks to patients became a greater concern, especially for children due to their inherently higher radiosensitivity to stochastic radiation damage. Current dose evaluation protocols include the computed tomography dose index (CTDI) or point detector measurements using anthropomorphic phantoms that do not sufficiently provide accurate information of the organ-averaged absorbed dose and corresponding effective dose to pediatric patients. In this study, organ and effective doses to pediatric patients under helical multislice computed tomography (MSCT) examinations were evaluated using an extensive series of anthropomorphic computational phantoms and Monte Carlo radiation transport simulations. Ten pediatric phantoms, five stylized (equation-based) ORNL phantoms (newborn, 1-year, 5-year, 10-year, and 15-year) and five tomographic (voxel-based) UF phantoms (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) were implemented into MCNPX for simulation, where a source subroutine was written to explicitly simulate the helical motion of the CT x-ray source and the fan beam angle and collimator width. Ionization chamber measurements were performed and used to normalize the Monte Carlo simulation results. On average, for the same tube current setting, a tube potential of 100 kVp resulted in effective doses that were 105% higher than seen at 80 kVp, and 210% higher at 120 kVp regardless of phantom type. Overall, the ORNL phantom series was shown to yield values of effective dose that were reasonably consistent with those of the gender-specific UF phantom series for CT examinations of the head, pelvis, and torso. However, the ORNL phantoms consistently overestimated values of the effective dose as seen in the UF phantom for MSCT scans of the chest, and underestimated values of the effective dose for abdominal CT scans. These discrepancies increased with increasing kVp. Finally, absorbed doses to select radiation sensitive organs such as the gonads, red bone marrow, colon, and thyroid were evaluated and compared between phantom types. Specific anatomical problems identified in the stylized phantoms included excessive pelvic shielding of the ovaries in the female phantoms, enhanced red bone marrow dose to the arms and rib cage for chest exams, an unrealistic and constant torso thickness resulting in excessive x-ray attenuation in the regions of the abdominal organs, and incorrect positioning of the thyroid within the stylized phantom neck resulting in insufficient shielding by clavicles and scapulae for lateral beam angles. To ensure more accurate estimates of organ absorbed dose in multislice CT, it is recommended that voxel-based phantoms, potentially tailored to individual body morphometry, be utilized in any future prospective epidemiological studies of medically exposed children.

A tomographic physical phantom of the newborn child with real-time dosimetry. II. Scaling factors for calculation of mean organ dose in pediatric radiography.

Following the recent completion of a tomographic physical newborn dosimetry phantom with incorporated metal-oxide-semiconductor field effect transistor (MOSFET) dosimetry system, it was necessary to derive scaling factors in order to calculate organ doses in the physical phantom given point dose measurements via the MOSFET dosimeters (preceding article in this issue). In this study, we present the initial development of scaling factors using projection radiograph data. These point-to-organ dose scaling factors (SF(POD)) were calculated using a computational phantom created from the same data set as the physical phantom, but which also includes numerous segmented internal organs and tissues. The creation of these scaling factors is discussed, as well as the errors associated when using only point dose measurements to calculate mean organ doses and effective doses in physical phantoms. Scaling factors for various organs ranged from as low as 0.70 to as high as 1.71. Also, the ability to incorporate improvements in the computational phantom into the physical phantom using scaling factors is discussed. An comprehensive set of SF(POD) values is presented in this article for application in pediatric radiography of newborn patients.

Organ and effective doses in newborn patients during helical multislice computed tomography examination.

In this study, two computational phantoms of the newborn patient were used to assess individual organ doses and effective doses delivered during head, chest, abdomen, pelvis, and torso examinations using the Siemens SOMATOM Sensation 16 helical multi-slice computed tomography (MSCT) scanner. The stylized phantom used to model the patient anatomy was the revised ORNL newborn phantom by Han et al (2006 Health Phys.90 337). The tomographic phantom used in the study was that developed by Nipper et al (2002 Phys. Med. Biol. 47 3143) as recently revised by Staton et al (2006 Med. Phys. 33 3283). The stylized model was implemented within the MCNP5 radiation transport code, while the tomographic phantom was incorporated within the EGSnrc code. In both codes, the x-ray source was modelled as a fan beam originating from the focal spot at a fan angle of 52 degrees and a focal-spot-to-axis distance of 57 cm. The helical path of the source was explicitly modelled based on variations in collimator setting (12 mm or 24 mm), detector pitch and scan length. Tube potentials of 80, 100 and 120 kVp were considered in this study. Beam profile data were acquired using radiological film measurements on a 16 cm PMMA phantom, which yielded effective beam widths of 14.7 mm and 26.8 mm for collimator settings of 12 mm and 24 mm, respectively. Values of absolute organ absorbed dose were determined via the use of normalization factors defined as the ratio of the CTDI(100) measured in-phantom and that determined by Monte Carlo simulation of the PMMA phantom and ion chamber. Across various technique factors, effective dose differences between the stylized and tomographic phantoms ranged from +2% to +9% for head exams, -4% to -2% for chest exams, +8% to +24% for abdominal exams, -16% to -12% for pelvic exams and -7% to 0% for chest-abdomen-pelvis (CAP) exams. In many cases, however, relatively close agreement in effective dose was accomplished at the expense of compensating errors in individual organ dose. Per cent differences in organ dose between the stylized and tomographic phantoms at 120 kVp and 12 mm collimator setting ranged from -25% (skin) to +164% (muscle) for head exams, -92% (thyroid) to +98% (ovaries) for chest exams, -144% (uterus) to +112% (ovaries) for abdominal exams, -98% (SI wall) to +20% (thymus) for pelvic exams and -60% (extrathoracic airways) to +13% (ovaries) for CAP exams. Better agreement was seen between the two phantom types for organs entirely within the scan field. In these cases, corresponding per cent differences in organ absorbed dose did not vary more than 17%. For all scans, the effective dose was found to range approximately 1-13 mSv across the scan parameters and scan regions. The largest effective dose occurred for CAP scans at 120 kVp.