CT and MR evaluation of a wooden foreign body in an in vitro model of the orbit.

We made an in vitro model of a wooden foreign body using both fresh and dry pine wood in both fat and soft-tissue background mediums. Air/wood/background medium interfaces were studied with MR and CT to determine which method provided the best image contrast for detecting a wooden foreign body. CT was demonstrated to be superior to MR in the evaluation of the orbit in the in vitro model.

Technologic advances in computed tomography.

Technologic improvements in CT continue. As always, emphasis in the past year has been on reducing scan times. Units employing slip-ring technology are now available. This technology makes possible spiral scanning, which promises to further reduce the effects of image-degrading patient motion, as well as opening new possibilities for quantitative and dynamic CT studies. These applications are discussed here and an update on the advances on cine-CT and the Dynamic Spatial Reconstructor unit is provided. Three-dimensional reconstructions obtained at very short scan times promise to be of particular benefit for scanning the bronchial tree. Finally, patient doses of radiation in CT are discussed.

Generators, x-ray tubes, and exposure geometry in mammography.

The author reviews different types of generators used in mammography, x-ray tubes, and physical principles affecting exposure geometry. Target and filter combinations, focal spots, and unsharpness are discussed in detail.

Quality assurance breast phantoms for screen-film mammography: design and use.

Used properly, well-designed breast phantoms are a very useful part of the mammography QA program. However, they should be designed specifically for the purpose. The principles involved are discussed and illustrated by the design of the unit shown in figure 1. QA phantoms also must be applied appropriately. Procedures are described for checking the consistency of film density, evaluating image quality, and checking phototimer operation. The simple film density measurement test (table 2) also provides a useful check on the constancy of patient dose levels. However, errors may result when QA phantoms are used for dosimetry, primarily because their composition differs from that of the mixture BR12, which is usually taken as the standard for dosimetry measurement. Procedures are described for computing average glandular dose, as well as measuring the two required quantities: the beam HVL and the exposure in air required for proper film density.

Fundamental physics of magnetic resonance imaging.

Although similar to computerized tomography, in that cross-sectional images are produced, the physical principles underlying magnetic resonance are entirely different. The MRI process, as commonly implemented, involves the excitation of hydrogen nuclei and the analysis of how these nuclei recover to the original equilibrium steady states that they had prior to excitation. This article discusses that process, that is, preparatory alignment, RF excitation, relaxation and signal measurement, and spatial localization.

Screen-film mammographic technique for breast cancer screening.

The values of dose and signal-to-noise ratio (SNR) for many techniques and breast thicknesses were computed and compared with a reference technique and breast thickness to provide a valid basis upon which to select a screening technique for screen-film mammography. The reference consisted of a Min-R/OM1 combination (or its equivalent) exposed through a 4.5-cm thick breast via 28 kV and a molybdenum target-beryllium window tube with a 0.03-mm Mo filter. Radiographs of an improved breast phantom were used to relate computed relative SNR values of techniques to diameters of the smallest calcific and soft-tissue objects demonstrated in mammograms. Without use of a grid, four Mo target/filtration combinations yielded similar computed dose and SNR levels, as did a tungsten target tube operated at 23 kV without a filter. Operation of the latter tube at 27.5 kV with a 0.051-mm rhodium filter should reduce dose by half but also SNR by 22%. However, such operation with a grid should greatly improve imaging moderate to large breasts without increasing dose over reference values.

The retrolenticular afterglow: an echo enhancement artifact.

A mathematical model for a sonogram containing a circular object with a sonic speed less than that of its surroundings was constructed. The circular object acts as a lens to the ultrasound beam, which explains the refractive artifacts often observed. An unexpected artifact is also predicted, a region of increased echogenicity, which the authors named the retrolenticular afterglow, since it occurs behind a lens-like structure. An experiment was designed to show the appearance of this artifact occurring distal to an ethyl alcohol-filled cylindrical well in a tissue equivalent phantom.

Dosage evaluation in mammography.

Estimates of breast dose per view are needed for selection of mammographic techniques and verification of their proper use. However, accurate dosimetry requires standardization of both the methodology and the assumed breast composition. Because several different methods have been reported, the authors developed a simple and reproducible method using a reference "average breast" composition of 50/50% water/fat by weight. Working curves were derived for average glandular and whole-breast dose per unit of exposure in air vs. HVL and thickness. When these curves are combined with on-site measurements of exposure per view, one obtains values of dose per view for each technique. Factors were also computed to correct dosage from the reference composition to other breast compositions.

Measurements of diagnostic x-ray backscatter by a novel ion chamber method.

There is a major gap in backscatter information for diagnostic x-ray beams. Such information is increasingly needed for dose measurements and calculations, as well as for designing devices and techniques. We have therefore carried out measurements on both low Z materials and metals, using an ion chamber method designed specifically for the purpose. Lucite and two D.R. White tissue substitutes were studied extensively (BR 12 "average breast" and MS 11 "water"). Measured percent backscatter (BS) was greatest for Lucite and least for MS 11, with BR 12 in between. Backscatter buildup is rapid: 50% of full backscatter is achieved with 6 mm thickness for all three materials using mammographic beams and with about 12 mm using general diagnostic beams. A simple relationship between BS and field area permits close estimates of BS values for fields for which measured data is not available. Among metals tested, copper exhibited greatest backscatter (39% BS maximum), aluminum least, and lead in between--information of potential importance in cassette design and similar applications.

A study of mammographic exposure and detail visibility using three systems: Xerox 125, Min-R, and Xonics XERG.

A breast phantom of novel design has been used to measure visibility of simulated calcific and soft-tissue fibrillar details in mammography, as well as to determine the roentgen exposure vs. depth. Exposure data were combined with a model of the breast as compressed during mammography to compute the mean exposure to the ductal parenchyma (MDE). Three different imaging systems were compared over a wide range of x-ray beam energies and breast characteristics. "Dosage" criteria other than the MDE are discussed.

Rapid computation of diagnostic X-ray bremsstrahlung spectra.

A method is described for rapid and accurate computation of diagnostic x-ray spectra. Accuracy limitations of Kramers' equation are overcome by providing intensity correction factors derived from published measured data. Use of a parameter based on the energy of the Kramers spectrum intensity peak permits deriving master factor curves which are remarkably independent of kVp, waveform and filtration. Computed and measured spectra generally agree to better than +/- 1 keV for beams generated at 100 kVp and below. Possible application of the method to higher energy diagnostic beams is discussed.

A device to indicate anatomical level in computed tomography.

A device was designed that makes possible direct anatomical correlation between the computed tomography scan and radiographic scout films. The acrylic-air groove approach used here eliminates the possibility of producing image degrading artifacts as is common with catheter type markers.

A breast phantom method for evaluating mammography technique.

A new breast phantom has been designed for use in evaluating mammographic system performance. This phantom incorporates simulated calcifications and fibrillar objects in fat, of graded size, to permit measurements of detail visibility. A special methodology has been developed for measuring visible object size to achieve reproducible and clinically relevant results. Materials and construction of the phantom also permit carrying out dosimetry with an appropriate ionization chamber. Dosage and detail visibility measurements are reported for the Xerox 125, Min-R and Xonics systems. In addition to providing information regarding technique and image receptors, these results demonstrate the usefulness of the basic phantom design, and suggest possible improvements.

CT intracranial localization with a new marker system.

Application of a new CT localization system for intracranial lesions in pre- and intraoperative guidance is described. This system consists of air grooves within an acrylic plate for CT examinations and a duplicate plate with wires in place of air grooves for scout radiography. This new device can also be applied in stereotaxic surgery or in radiotherapy planning.

Temporal response of microdensitometers.

Microdensitometers have both spatial and temporal finite responses which may lead to degradation in images being analyzed. These responses may be quantitated in terms of spatial and temporal modulation transfer functions (MTFs). The temporal response of microdensitometers is studied here. Specifically, the technique of differentiating temporal step-function responses to determine the temporal MTF is presented. Experimental results illustrating the theory are described using a Baird Atomic microdensitometer.