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C D Stockham

Publications and source records attributed to C D Stockham.

5 recordsLinked to original sources
PubMed1989

Precise measurement of vertebral bone density using computed tomography without the use of an external reference phantom.

Bone density measurement by quantitative computed tomography (QCT) commonly uses an external reference phantom to decrease scan-to-scan and scanner-to-scanner variability. However, the peripheral location of these phantoms and other phantom variables is also responsible for a measurable degradation in accuracy and precision. Due to non-uniform artifacts such as beam hardening, scatter, and volume averaging, the ideal reference phantom should be as close to the target tissue as possible. This investigation developed and tested a computer program that uses paraspinal muscle and fat tissue as internal reference standards in an effort to eliminate the need for an external phantom. Because of their proximity, these internal reference tissues can be assumed to reflect more accurately the local changes in the x-ray spectra and scatter distribution at the target tissue. A user interactive computerized histogram plotting technique enabled the derivation of reproducible CT numbers for muscle, fat, and trabecular bone. Preliminary results indicate that the use of internal reference tissues with the histogram technique may improve reproducibility of scan-to-scan measurements as well as inter-scanner precision. Reproducibility studies on 165 images with intentional region-of-interest (ROI) mispositioning of 1.5, 2.5, or 3.5 mm yielded a precision of better than 1% for normals and 1% to 2% for osteoporotic patients--a twofold improvement over the precision from similar tests using the standard technique with an external reference phantom. Such improvements in precision are essential for QCT to be clinically useful as a noninvasive modality for measurement of the very small annual changes in bone mineral density.

Bone Density↗
PubMed1982

The influence of modulation transfer function shape on computed tomographic image quality.

The influence of modulation transfer function (MTF) shape on computed tomographic (CT) image quality was studied by computer simulation of a recently proposed x-ray detector with a nonuniform, shaped response to radiation. The shaped detector, while maintaining a high value of limiting spatial resolution, was shown to depress MTF and signal-to-noise ratio values at moderate spatial frequencies. This led to a significant loss of sharpness on noise-free images. Despite a superior radiation dose capture efficiency in the shaped detector, the MTF suppression was shown to produce inferior visibility of 1.0-mm objects in an image with typical CT noise. The significance for clinical imaging of the MTF suppression was demonstrated using a scan of a human head. The implication is that better methods than those currently used are needed for evaluating CT image quality.

Computers↗
PubMed1979

A simulation study of aliasing in computed tomography.

Aliasing in CT images is the result of discrete sampling in the data-taking process. Artifacts associated with aliasing, while object-dependent, also depend on fundamental parameters such as beam width and the number of rays and views. This dependence has been studied in simulation with a phantom designed to represent a transaxial section through the lower head. Although the simulation program assumes a stationary detector system, the results do not depend strongly on geometry. Aliasing has also been compared with other sources of artifact which are often more significant, such as beam hardening, machine alignment and noise.

Head↗