David J. Goodenough

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.

Development of a Phantom for Evaluation and Assurance Of Image Quality in CT Scanning

The development and use of a phantom for evaluation, comparison, and quality assurance of CT scanners will be discussed. Examples of measurements on seven CT scanners using early prototypes of the phantom will be presented along with measurements on several scanners using the final phantom configuration. The phantom contains four modular sections which are removable to allow for future fabrication and replacement of individual sections for specialized applications. Section I is used to measure contrast sensitivity and scan slice geometry of the system. Section II is used to measure the sensitometric response of the system. Section III is used to determine the spatial resolution of the system at various contrast levels. Section IV is used to determine the noise, spatial uniformity, alignment, and MTF of the system. In addition, it contains a part with fittings where items may be placed such as in vitro samples, dosimeters, or a motion phantom.

Factors related to low contrast resolution in CT scanners

The concept of low contrast resolution in CT scanners is examined. Several questions are raised, not only about limitations to interpretation of figures of merit of low contrast resolution, such as contrast-detail diagrams; but also, interpretation of related CT physical performance parameters, such as: noise, dose, spatial resolution and slice width (or sensitivity profile). Caveats are raised concerning interpretation of contrast-detail studies and the degree to which first-order observer studies are reflective of the actual clinical situation. Particular care is suggested in interpretating contrast-detail diagrams in which sensitivity, specificity and the number of target signals may be varying.

A new software correction approach to volume averaging artifacts in CT

The nature of artifacts in computed tomography, due to nonlinear partial volume effects, is briefly reviewed. A methodology for correction of these artifacts proposed. This methodology utilizes a correction algorithm for post-processing the original reconstructed CT image. The algorithm utilizes local CT values to predict the probability of volume averaging of bone, air and tissue, and incorporates information from the original image on the spatial extent of the averaging, to correct the nonlinear effects. The algorithm is successfully demonstrated on mathematical phantoms, wherein volume averaging can be introduced over central targets and/or peripheral annuli of tissue, bone, and air. The algorithm is also demonstrated on actual CT brain scans but only for its qualitative effects. It is pointed out that higher order corrections, to be reported in future publications, will further correct the actual clinical scans.

Phantoms for specifications and quality assurance of mr imaging scanners

The accompanying paper examines the subject of NMR phantoms. The paper reports on initial experience with existing phantoms and reviews proposals from various standards groups and professional organizations. Many image tests are illustrated by existing vendor phantoms. The paper concludes that many phantoms already meet or exceed most of the suggestions for tests of classical imaging parameters, which can be pursued by first order adaptations of CT phantoms. The paper does, however, point out the limitations of existing phantoms and raise the possibility of developing phantoms that more accurately reflect human shapes and cavities, and which present more realistic resistive losses, and T1 and T2 values.

Theoretical Limitations of Tumor Imaging

The question of limitations of tumor imaging needs to be examined against the characteristics of current medical imaging systems. Moreover, the examination needs to be carried out against the backdrop of physical imaging properties of the imaging systems, as well as the physiological and chemical aspects of the underlying biological signals (tumors) and background (nontumor) areas. In particular, the important possibilities for enhanced image performance using labeled monoclonal antibodies need to be explored.

Overview of Computed Tomography

The evolution of computed tomography (CT) as a viable medical tool will be considered from the time of the introduction of the original EMI CT Scanner, to the status of current and planned CT x-ray scanners. Changes in physical and engineering parameters, such as spatial resolution, noise levels, radiation dose, scan time, and reconstruction time, will be discussed. Limitations imposed by finite radiation dose on certain of these parameters will also be discussed. Differences between x-ray CT and conventional radiography will be illustrated by use of appropriate physical models and performance levels. A brief examination of the development of computed tomography in other medical diagnostic areas, such as nuclear medicine, ultrasound and heavy particle utilization, will be presented.

Image Levels and Accuracy in Computed Tomography

There are three basic sources of resolution degradation or unsharpness in screen/film radiography: motion unsharpness -- the blurring of a point due to the motion of the patient during the exposure time of the procedure; receptor unsharpness -- the inherent blurring operation caused by the transducing or conversion of x-rays into light within the intensifying screen; and geometric unsharpness (focal spot unsharpness) -- the blurring due to the finite size and x-ray intensity distribution of the x-ray focal spot. Combinations of these sources generally lead to an overall blurring or defocusing effect. This blurring or unsharpness may be characterized by its spatial extent, which is generally the blur size or point spread function, or by the Modulation Transfer Function (MTF), which describes how the spatial frequencies that make up an object are transferred by the image recording system.

Sensitometry In Computerized Tomography

This paper will present a definition of Sensitometry in Computerized Tomography and discuss practical techniques used to determine relative CT sensitometry. Several factors that influence CT sensitometry are: (1) the x-ray beam spectral distribution, (2) the composition of the sample materials, (3) volume averaging, and (4) spatial nonuniformity of the scan field including the x-ray intensity distribution within the sample volume. Examples of comparative sensitometry among several CT units is presented as well as discussion of a technique for obtaining a continuous rather than discrete output.

Development of a Phantom for Evaluation and Assurance of Image Quality in CT Scanning

The development and use of a phantom for evaluation, comparison, and quality assurance of CT scanners will be discussed. Examples of measurements on seven CT scanners using early prototypes of the phantom will be presented along with measurements on several scanners using the final phantom configuration. The phantom contains four modular sections which are removable to allow for future fabrication and replacement of individual sections for specialized applications. Section I is used to measure contrast sensitivity and scan slice geometry of the system. Section II is used to measure the sensitometric response of the system. Section III is used to determine the spatial resolution of the system at various contrast levels. Section IV is used to determine the noise, spatial uniformity, alignment, and MTF of the system. In addition, it contains a part with fittings where items may be placed such as in vitro samples, dosimeters, or a motion phantom.