Peak enhancement of the liver in children using power injection and helical CT.

OBJECTIVE: Our objective was to determine the level and timing of peak hepatic enhancement in children using power injection of contrast media, helical CT, and computer-automated scan technology. SUBJECTS AND METHODS: Forty-nine abdominal CT studies were performed using computer-automated scan technology. Patients were divided into four groups on the basis of body weight and contrast dose (group 1A, < or = 20 kg and 2 ml/kg; group 1B, < or = 20 kg and 3 ml/kg; group 2, 21-40 kg and 2 ml/kg; group 3, > 40 kg and < or = 2 ml/kg). Contrast injection rates were based on body weight (groups 1A and 1B, 1 ml/sec; group 2, 1.5 ml/sec; and group 3, 2 ml/sec). The peak hepatic enhancement level in Hounsfield units and the time to reach peak enhancement were determined for each patient. RESULTS: The mean peak hepatic enhancement and time to peak enhancement after completion of contrast injection were group 1A, 45 H and 11 sec; group 1B, 62 H and 3 sec; group 2, 52 H and 12 sec; and group 3, 45 H and 10 sec. CONCLUSION: The level and timing of peak hepatic enhancement in pediatric patients can be obtained using computer-automated scan technology. These data may then be used to optimize hepatic enhancement when obtaining helical abdominal CT scans of pediatric patients.

Photon-absorbed fractions for cylindrical geometry: a TLD photon-absorbed fraction model.

The purpose of this study is to develop a mathematical model and calculate photon-absorbed fractions in a homogeneous nonradioactive cylinder placed inside off-center and outside a cylindrical homogeneous distribution of activity. In the second case, both the radioactive cylinder and the nonradioactive one are placed in a tissue-equivalent nonradioactive medium. The values of the photon-absorbed fractions are investigated for various geometrical configurations using water as the material filling the cylinders and the medium in between and an isotope commonly used in Nuclear Medicine, 99mTc. The calculations for off-center cylinders allows for modeling inhomogeneous distributions of activity within a tumor by placing several "cold" cylinders of various sizes in a radioactive finite cylinder. This three-dimensional model calculates photon-absorbed fractions for inhomogeneous activity distributions that can be used in quantitative nuclear medicine for self-absorption correction, thus introducing a more realistic correction than the one-dimensional corrections. These calculations are also used to model the response of a cylindrical TLD (thermoluminiscent dosimeter) placed inside a homogeneous radioactive cylinder and outside the homogeneous radioactive cylinder, in an absorbing nonradioactive surrounding medium. The purpose of these calculations is to evaluate the photon-absorbed fraction in the TLD as an instrument of measuring the time-integrated activity of a homogeneous radioactive source versus an inhomogeneous one. The dependence of the TLD-absorbed fraction on the position of the TLD with respect to the radioactive cylinder is investigated.

Photon absorbed fractions for cylindrical geometry: homogeneous nonradioactive cylinder containing a homogeneous cylindrical distribution of activity.

The purpose of this study was to calculate photon absorbed fractions in tissue surrounding a radioactive source where both the source and the surrounding tissue are assumed to have cylindrical geometry. Specifically, we treated two cases: the case of a cylindrical source of homogeneous activity placed in air, and second, the case of a cylindrical source of homogeneous activity placed in a cylindrical nonradioactive absorbing material. In this study we offer an analytical solution to these problems followed by numerical integration. The computer program allowed for very general calculations, e.g., different tissues, different geometrical setups. Tables of absorbed fractions have been developed for commonly used radionuclide energies and tissue-equivalent material. A comparison between our results and the results of other related studies showed the advantages and limitations of this approach.

Efficacy of chest radiography in pediatric intensive care.

We prospectively evaluated the efficacy and clinical usefulness of bedside chest radiography in a pediatric intensive-care unit. Seven hundred ninety-five radiographs were evaluated in 126 patients over a 10-week period. Eighty-one percent of all radiographs showed one or more cardiopulmonary abnormalities, and 25% of routine radiographs had findings that altered management of patients. Nineteen percent of radiographs, including 17% of routine radiographs, showed a malpositioned tube or catheter. Thirty-five percent of endotracheal tubes shown on postintubation radiographs and 41% of central venous catheters shown on post-catheter placement radiographs were malpositioned. Forty-five percent of radiographs with a previous reading showed a significant interval change. Radiographs in patients 1 year old or younger showed more cardiopulmonary abnormalities (p less than .04), tube or catheter malpositions (p less than .03), and significant interval changes (p less than .03), and they elicited more changes in clinical management (p less than .01) than did radiographs in patients over 1 year old. The frequency of management changes dictated by radiographs increased with increasing amounts of respiratory support (p less than .01). Our data indicate that bedside radiography in the pediatric intensive-care setting has a high efficacy and clinical utility.

Averaging error in NMR slice profile measurements.

Slice thickness is a primary operational parameter in the evaluation of the performance of an MRI system with regard to providing accurate imaging of a slice of tissue. Of the various methods that have been utilized to evaluate slice thickness, a continuous high-signal ramp has gained the most support as the method of choice. However, problems with the accuracy of the measurement method have been associated with the finite thickness of the ramp. This report considers the accuracy of the measurement method as a function of ramp width.

Scoliosis examinations: organ dose and image quality with rare-earth screen-film systems.

We determined the dose to the breast and evaluated the image quality when various high-speed, rare-earth screen-film systems were used in conjunction with breast dose reduction methods in children undergoing scoliosis examinations. In addition, normalized organ dose to the breast, active bone marrow, thyroid, eyes, ovaries, and testes were measured in a pediatric anthropomorphic phantom comparing the anteroposterior and posteroanterior projections. The average measured dose to the breast was 6.9 and 4.0 mrad (10(-5) Gy), respectively, for nominal 400- and 600-speed, rare-earth screen-film systems used in combination with breast dose reduction methods. The image quality of these systems as evaluated by three radiologists was rated as adequate. The dosimetry results with an anthropomorphic phantom showed that the posteroanterior projection provides approximately a threefold reduction in breast dose as compared with the anteroposterior view. However, the dose to the bone marrow is doubled. Rare-earth screen-film systems used in combination with simple dose-reduction methods can provide adequate image quality for scoliosis examination while significantly reducing the radiation dose to the breast.

Normalized pediatric organ-absorbed doses from CT examinations.

Absorbed doses to the bone marrow, gonads, thyroid, eyes, breast, and skin from CT examinations of the head, chest, abdomen, and torso were measured in a phantom mimicking an average 6-year-old child. The doses were normalized to the CT dose index measured at the central position of a standardized cylindrical phantom, to allow approximation of organ-absorbed doses for similar scanners for which the CT dose index has been measured. Estimating organ-absorbed doses associated with CT examinations of children is necessary to evaluate the relative risks of carcinogenesis associated with radiation exposure of a particular organ. The risks from a particular CT examination depends on the patient's age because the geometric configuration of the organs (e.g., marrow distribution) changes over time and will, therefore, affect the scatter dose to the organs.

A comparative evaluation of rare earth screen-film systems. Free-response operating characteristic analysis and anatomic criteria analysis.

Diagnostic accuracy was evaluated in a signal detection experiment that used low-contrast acute lesions in a living dog model. At the high levels of certainty normally used by radiologists, rare earth screen-film systems provided accuracy comparable to that of a reference calcium tungstate screen-film system. Additionally subjective image quality evaluation for a given imaging task (clinical pediatric anterior-posterior chest films) based on visualization of anatomic landmarks and physical parameters has been conducted for several rare earth and one calcium tungstate screen-film system. The correspondence of subjective physical ranking with physical and psychophysical measurements was investigated. Evaluating the visualization of anatomic parameters can provide a clear and objective distinction among systems with comparable physical and psychophysical properties.

Practical dosimetric aspects of blood and blood product irradiation.

The method of choice to reduce susceptibility to transfusion-transmitted graft-versus-host disease is irradiation of allogenic blood and blood products for transfusion to immunosuppressed recipients. Optimal irradiation requires delivery of a known and homogeneous absorbed dose. The use of absorbed dose in air measured at the center of the irradiation volume without proper compensation for sample absorption can lead to approximately 20 percent underexposure. A lucite cylinder was used to provide the delivery of a homogeneous irradiation dose to blood products of different volumes by allowing rotation of the product.

A comparative evaluation of rare-earth screen-film systems. System speed, contrast, sensitometry, RMS noise, square-wave response function, and contrast-dose-detail analysis.

We evaluated the physical characteristics and contrast-dose-detail performance of 11 rare-earth and three calcium tungstate screen-film systems. Measurements included system speed, contrast, sensitometry, RMS noise, square-wave response function, and contrast-dose-detail analysis. The major differences in physical characteristics among systems were system speed and RMS noise. Square-wave response differences were more subtle. For contrast-dose-detail analysis, the rare-earth screen-film systems and the calcium tungstate system responses were significant over a limited subject contrast range as a function of detail diameter. Relative dose efficiency in the noise-limited region is a function of the properties of the screen only and is independent of the film.

Pediatric patient exposures from CT examinations: GE CT/T 9800 scanner.

This report presents Computed Tomography Dose Index (CTDI) values for typical CT examinations of children for a GE CT/T 9800 scanner and compares them with measured entrance skin absorbed doses of pediatric patients under clinical situations. Pediatric entrance skin absorbed doses were 1.1-2.4 rad (cGy) for chest and abdomen examinations, 2.0-3.4 rad (cGy) for pediatric head examinations, and 3.2-4.2 rad (cGy) for infant (less than or equal to 6 months) head examinations. CTDI measurements in a cylindrical Lucite head phantom predicted typical pediatric absorbed doses to within about 5% for chest and abdomen examinations and to within about 15% for head examinations, when corrections for amperage differences are taken into account.