Peter Steiger

Quantitative computed tomography in assessment of osteoporosis

Computed tomography (CT) has been widely investigated and applied in recent years as a means for noninvasive quantitative bone mineral determination. The usefulness of computed tomography for measurement of bone mineral lies in its ability to provide a quantitative image and, thereby, measure trabecular, cortical, or integral bone, centrally or peripherally. For measuring the spine, the potential advantages of quantitative computed tomography (QCT) over dual-photon absorptiometry (DPA) are its capability for precise three-dimensional anatomic localization providing a direct density measurement, and its capability for spatial separation of highly responsive cancellous bone from less responsive compact bone. Currently, QCT vertebral mineral determination has been implemented at over 800 sites encompassing a wide geographic distribution and a wide array of commercial scanners. With a world-wide distribution of approximately 8,000 advanced CT body scanners, the capability now exists for widespread application of vertebral bone mineral determination by quantitative computed tomography. These QCT techniques for vertebral mineral determination have been used to study skeletal changes in osteoporosis and other metabolic bone diseases. Longitudinal and cross-sectional bone mass measurements have been obtained at the University of California at San Francisco (UCSF) in over 3,000 patients seen clinically or on research protocols. The results presented here illustrate the use of QCT spinal mineral measurement in the delineation of normal age-related bone loss, in the evaluation of estrogen effects on bone, in the assessment of fracture threshold and risk, and in the study of the effects of various exercise regimens on bone mineral and the determination of relationships to other techniques of bone mineral measurement. The laboratory and clinical results presented herein indicate that QCT provides a reliable means to evaluate and monitor the many forms of osteoporosis and the various interventions aimed at ameliorating this condition. The greatest advantages of spinal QCT for noninvasive bone mineral measurement lie in the high precision of the technique, the high sensitivity of the vertebral trabecular measurement site, and the potential for widespread application.

Morphometric X-ray absorptiometry of the spine: Correlation in vivo with morphometric radiography

The diagnosis of vertebral fractures has been based on lateral thoracic and lumbar spine films and entails the determination of crush, endplate and wedge deformities of the vertebral bodies, generally between T4 and L4. Accuracy and precision of X-ray based morphometry are limited by geometric distortion and other technique-related factors. This paper proposes the use of morphometric X-ray absorptiometry (MXA) as a method which overcomes many of the limitations of morphometric radiography. MXA generates paired anteroposterior and lateral images of the spine using a scanning fan beam geometry that significantly reduces distortions inherent in the cone beam geometry used in conventional X-rays. Intra-observer precision of MXA on 41 subjects aged 65 years and older was 1 mm for vertebral height assessments and 4.7% for vertebral wedge parameters. Linear correlation with vertebral heights and wedge parameters on 32 subjects evaluated by both MXA and morphometric radiography demonstrated a root of the mean squared error of 2.1–2.4 mm and 7.0%, respectively. Vertebral deformities could be identified by MXA. The study documents the feasibility of MXA for the assessment of vertebral deformities. However, further investigation is needed to document the ability of MXA to diagnose prevalent and incident fractures.

The index of radiographic area (IRA): a new approach to estimating the severity of vertebral deformity

To assess the overall severity of vertebral deformity for an individual, we developed the index of radiographic area (IRA), a computerized method for analyzing vertebral dimensions from lateral radiographs of the spine. Six coordinates for each vertebral body are recorded in a computer by means of a digitizer. A computer program identifies all abnormal vertebrae, computes the radiographic area of the remaining normal vertebrae, and then estimates the expected normal radiographic area for the abnormal vertebrae in that individual. Differences between the expected and observed radiographic area of all abnormal vertebrae are summed to produce the IRA score. The score correlated well with the quantitative score of vertebral deformity of an experienced radiologist (r = 0.85), and with the vertebral bone density by quantitative computed tomography (QCT) (r = 0.66). The IRA is a practical, standardized method for quantitating the overall severity of vertebral deformity in individuals who have vertebral fractures. Its reproducibility for longitudinal studies and clinical trials needs further study.

Overlap detection and contour-tracking algorithms for critical organs—application to kidney

An auto-contouring technique has been developed for critical structures on transverse Computer Tomographic (CT) images with one or two overlap regions where object and background have similar CT values. In this technique, those regions are identified and skipped during contour tracking. The contours thus obtained are discontinuous which are corrected afterwards by suitable interpolations. The overlap detection criterion is never satisfied in a nonoverlapping environment and the contour, in this case, is essentially tracked on the basis of CT value threshold alone. The entire process can be initiated by minimal operator intervention. The method has been successfully tested for kidneys and several examples are furnished. The success and concomitant limitations of this technique are also discussed.

A comparison of morphometric definitions of vertebral fracture

To compare the accuracy of several approaches for defining prevalent vertebral fractures from measurements of vertebral dimensions (morphometry), we measured the lateral dimensions of vertebral bodies of 115 normal premenopausal and 100 postmenopausal women. Of the postmenopausal women two observers agreed that 49 had definite vertebral fractures and 38 were definitely normal. Using these classifications as an independent reference, women were then classified as fractured or normal by several definitions based on vertebral morphometry. No morphometric definition of vertebral fracture agreed perfectly with the consensus classifications. In general, definitions that involved combinations of measurements of anterior (Ha), middle (Hm), and posterior (Hp) vertebral height classified women more accurately than did definitions based on a single measurement or ratio. The Ha/Hp ratio produced many false positives unless it was adjusted for normal variations in the shapes of different vertebral bodies. Definitions of fracture based on a greater than 15% reduction in heights or ratios had higher sensitivity but more false positives than definitions that used a more stringent (greater than 20%) criterion. All morphometric definitions of vertebral fracture separated the post-menopausal women into two groups (fractured and normal) that had significantly (P less than 0.001) different mean spine bone density by quantitative computed tomography. Definitions that had the lowest rates of false positives also produced the largest differences in bone density between those defined as fractured and those defined as normal.

90066043 Mild versus definite osteoporosis: Comparison of bone densitometry techniques using different statistical models

The purpose of this investigation was to determine the ability of three bone densitometry techniques to discriminate subjects with mild vertebral deformities from those with definite compression fractures. We determined bone mineral density (BMD) in 68 postmenopausal women by quantitative computed tomography (QCT) and dual-photon absorptiometry (DPA) of the spine, as well as single-photon absorptiometry (SPA) of the radius. Forty four individuals were classified as having mild deformities of the spine and 24 were considered to have definite vertebral compressions. Several statistical approaches were used to compare these subgroups and to estimate the relative risk of vertebral fracture. Included among these were percent decrements and zeta-scores, ROC curves, odds ratio estimations, and logistic regression analysis. Individuals with definite vertebral fractures had lower bone mineral density at all sites, but measurement of radial compact bone by SPA failed to reach significance. Using ROC analysis to distinguish mild deformities from true compressions, we found that measurement of spinal trabecular bone by QCT to be the most sensitive discriminator; although measurement of spinal integral bone by DPA also gave satisfactory discrimination, whereas assessment of radial compact bone did not adequately differentiate patients with mild deformities from those with definite compressions. Likewise, we found determination of spinal trabecular bone to be the most robust predictor of relative risk of definite fracture using either odds ratios or logistic regression analysis. Measurement of BMD in the peripheral cortical skeleton offered no predictive power for true vertebral fracture. We concluded that direct assessment of the spine, particularly of the trabecular portion, offered the strongest discrimination and relative risk prediction for definite osteoporotic fractures compared with milder forms of this condition.