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W Kalender

Publications and source records attributed to W Kalender.

At least 19 recordsLinked to original sources
PubMed2000

Dose reduction in computed tomography by attenuation-based on-line modulation of tube current: evaluation of six anatomical regions.

This study investigated the potential of attenuation-based on-line modulation of tube current to reduce the dose of computed tomography (in milliamperes) without loss in image quality. The dose can be reduced for non-circular patient cross-sections by reducing the tube current at the angular positions at which the diameter through the patient diameter is smallest. We investigated a new technical approach with attenuation-based on-line modulation of tube current. Computed tomographic projection data were analyzed to determine the optimal milliampere values for each projection angle in real time, instead of performing prior measurements with localizer radiographs. We compared image quality, noise pattern, and dose for standard scans and for scans with attenuation-based on-line modulation of tube current in a group of 30 radiation therapy patients. Six different anatomical regions were examined: head, shoulder, thorax, abdomen, pelvis, and extremities (knee). Image quality was evaluated by four radiologists in a blinded fashion. We found the dose to be reduced typically by 15-50 %. In general, no deterioration in image quality was observed. Thus the dose in computed tomography be reduced substantially by technical measures without sacrificing image quality. Attenuation-based on-line modulation of tube current is an efficient and practical means for this.

Humans↗
PubMed1999

[Micro-CT. Technology and application for assessing bone structure].

The strength and fracture resistance of bone is determined by the structure of the trabecular network and the cortical shell. While standard 2D techniques like histomorphometry are inadequate to assess the 3D nature of the trabecular network, isotropic 3D datasets of this network can be acquired with the new imaging modality of microCT. However, so far the quantitative analysis of the generated datasets, in particular the extraction of appropriate parameters describing the bone structure, has not been finally solved. In this article we describe the technology and applications of microCT systems relevant in the field of osteology. The most important technical features of current microCT systems in this context are: 1. A spatial resolution down to 5-10 microns can be achieved. 2. The maximum sample size is related to the desired resolution by a factor of approximately 1000, that is, a resolution of 10 microns limits the maximum sample size to approximately 1 cm. 3. Scan times for microCT systems vary between minutes and hours. Currently five areas for the application of microCT systems in osteology can be identified: 1. The search of parameters characterizing the 3D trabecular structure. 2. The application of finite element models to determine the biochemical competence of the structural parameters. 3. The use of microCT in preclinical trials to study drug effects in small animals. 4. The validation of analysis methods used in high-resolution in-vivo imaging systems. 5. The 3D quantification of modeling and remodeling processes.

Adult↗
PubMed1996

[CT osteodensitometry in the area immediate to hip endoprostheses].

UNLABELLED: The value of metal artifact reduction in quantitative bone density determination in the vicinity of metallic implants was studied. METHOD: Ten cadaver femora with hip endoprostheses implanted intra vitam were examined in CT with varying slice thicknesses, tube voltage, scan time and magnification factor. The artifact-laden images were then processed using a metal artifact reduction programme (MAR). The metallic implant was removed, allowing the bone slices to be examined again with the same imaging parameters, but without the bothersome metal artifacts. This allowed comparison of bone density values obtained before and after application of MAR with data obtained after implant removal. RESULTS: A comparison of the bone density measurements between the sample group without implants and the group with indwelling implants prior to MAR showed an unexpected correlation coefficient r = 0.975-0.977. The correlation coefficients after MAR rose to r = 0.987-0.992 in 125 kV and r = 0.989-0.991 in 85 kV modes. The reproducibility of the density measurements for the groups without implants and with implants following MAR showed variation coefficients (VC) of 1.16% and 0.75% respectively. The reproducibility of the manual definition of the region of interest (ROI) in bone showed a VC = 2.19%. CONCLUSIONS: Bone density in the vicinity of metallic implants can be reliably and reproducibly determined using MAR.

Artifacts↗
PubMed1995

European semi-anthropomorphic spine phantom for the calibration of bone densitometers: assessment of precision, stability and accuracy. The European Quantitation of Osteoporosis Study Group.

Up to now it has not been possible to reliably cross-calibrate dual-energy X-ray absorptiometry (DXA) densitometry equipment made by different manufacturers so that a measurement made on an individual subject can be expressed in the units used with a different type of machine. Manufacturers have adopted various procedures for edge detection and calibration, producing various normal ranges which are specific to each individual manufacturer's brand of machine. In this study we have used the recently described European Spine Phantom (ESP, prototype version), which contains three semi-anthropomorphic "vertebrae" of different densities made of stimulated cortical and trabecular bone, to calibrate a range of DXA densitometers and quantitative computed tomography (QCT) equipment used in the measurement of trabecular bone density of the lumbar vertebrae. Three brands of QCT equipment and three brands of DXA equipment were assessed. Repeat measurements were made to assess machine stability. With the large majority of machines which proved stable, mean values were obtained for the measured low, medium and high density vertebrae respectively. In the case of the QCT equipment these means were for the trabecular bone density, and in the case of the DXA equipment for vertebral body bone density in the posteroanterior projection. All DXA machines overestimated the projected area of the vertebral bodies by incorporating variable amounts of transverse process. In general, the QCT equipment gave measured values which were close to the specified values for trabecular density, but there were substantial differences from the specified values in the results provided by the three DXA brands. For the QCT and Norland DXA machines (posteroanterior view), the relationships between specified densities and observed densities were found to be linear, whereas for the other DXA equipment (posteroanterior view), slightly curvilinear, exponential fits were found to be necessary to fit the plots of observed versus specified densities. From these plots, individual calibration equations were derived for each machine studied. For optimal cross-calibration, it was found to be necessary to use an individual calibration equation for each machine. This study has shown that it is possible to cross-calibrate DXA as well as QCT equipment for the measurement of axial bone density. This will be of considerable benefit for large-scale epidemiological studies as well as for multi-site clinical studies depending on bone densitometry.

Absorptiometry, Photon↗
PubMed1995

Dual X-ray absorptiometry--cross-calibration and normative reference ranges for the spine: results of a European Community Concerted Action.

Bone density measurements by dual X-ray absorptiometry (DXA) of the spine can now be made precisely, but there is no uniformity in reporting results and in presenting reference data. A European Union Concerted Action therefore devised a uniform procedure for cross-calibrating and standardizing instruments, using the European spine phantom (ESP) prototype. This phantom differs in a number of respects from the final version of the ESP. Eighteen centers in nine countries obtained 1619 records (1035 women) from Caucasian subjects, aged 20-80 years, drawn from normal populations. The DXA machines used were made by the Hologic, Lunar, and Norland companies. Highly statistically significant differences were evident between populations, both in apparent rates of bone loss with age and in the spread of values about the age-adjusted means. There were small residual differences in the results obtained with the three machine brands which could have been due to the relatively large between-center population differences we observed. The alternative or additional explanation that they were attributable, in part, to the design differences between the ESP prototype and the definitive ESP, which became available after this study was completed, was shown to be a valid possibility. Results from postmenopausal women reported in relation to the years that have elapsed since menopause showed reduced population variance when compared with conventional reporting in relation to age. After cross-calibration, the center with the highest age-adjusted normal density value averaged 23% more than the center with the lowest. It is therefore crucially important to select appropriate reference data in clinical and epidemiological studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗
PubMed1994

European semi-anthropomorphic phantom for the cross-calibration of peripheral bone densitometers: assessment of precision accuracy and stability.

A semi-anthropomorphic 'distal radius like' phantom, developed by Kalender and Ruegsegger for use in peripheral bone densitometry using single photon (DPA) dual X-ray (DXA) and quantitative computed tomography (QCT) machines, has been studied with a view to cross-calibrating different types and brands of densitometers in current use. In the context of an EU 'Concerted Action' (second Framework Programme) the phantom was repeatedly measured on six SPA machines, three DXA machines and nine QCT machines (545 measurements). Linear regression equations were derived, individual to each machine, which allowed the derivation of 'standardized densities'. In this way we converted measurements made by machines of the same modality to a common scale of measurements. Two machines (one DXA, one SPA) showed statistically significant instability over time emphasising the need for rigorous quality control in the application of densitometry. In other respects these results provide an encouraging basis for the derivation of standardized normative ranges and the more effective use of peripheral densitometry in future clinical and epidemiological studies.

Absorptiometry, Photon↗
PubMed1991

Low vertebral bone density values in young non-elite female runners.

Thirty-five female runners (26.6 +/- 0.9 years, range 17-35) were scheduled for bone mineral density evaluation, using quantitative computed tomography of the lumbar spine. In 17 women with oligo-amenorrhea, vertebral bone mineral density was under the normal range (defined from a control group of 46 sedentary healthy females in the same age range), while it was within the normal range in all runners with regular menses (n = 18). When age classes were considered, all runners aged 17-21 (11/11) were found to have oligo-amenorrhea and low bone mineral density values, the difference in mineral density with the controls of the same age quartile being highly significant (p less than 0.001). Runners from the two youngest age classes (17-21 and 22-26) had started training early after menarche (0.9 +/- 0.6 and 2.5 +/- 1.6 years, respectively). These results show that very young female runners with oligo-amenorrhea may have impressively low bone mineral density values. The possibility that early onset of training, close to menarcheal age, might be a risk factor for low mineral density, deserves further investigation.

Adolescent↗
PubMed1989

Bone loss in young hypoestrogenic women due to primary ovarian failure: spinal quantitative computed tomography.

Vertebral bone mineral density was investigated in 22 young (29.3 +/- 1 years) women with primary ovarian failure, attending for infertility. The lumbar spine was studied, using a preprocessing dual-energy quantitative computed tomography technique and assessing vertebral bone mineral density from the reconstructed 125-kV image. Previously untreated women had significantly (P less than 0.001) lower values than a group of age-matched healthy controls, while the difference between those women having received estrogen therapy and the same controls was not significant. Sixteen patients were restudied after 6 to 21 months of estrogen therapy, part of a regimen intended to prepare the endometrium for oocyte donation, and vertebral bone mineral density increased in 12 of them. The authors conclude that primary ovarian failure is a cause for bone loss in these young women and that estrogen therapy is effective.

Adult↗
PubMed1988

[Quantitative CT studies of the lung in an animal experiment model of silicosis].

High resolution, narrow section CT examinations of the lungs were carried out on 21 Javanese macaques (five controls) shortly before the conclusion of 27 months exposure to quartz and/or high pressure. The aim was to establish morphometric CT data for differentiating and quantifying normal and abnormal findings. The results show that the technique has a high sensitivity for demonstrating the pathological lung changes. Final validation depends upon the use of respiratory physiology and biochemical, cytological, and histomorphometric data.

Animals↗
PubMed1988

Reproducibility of CT bone densitometry: operator versus automated ROI definition.

Intrasubject reproducibility with repeated determination of vertebral mineral density from a given set of CT images was investigated. The region of interest (ROI) in 10 patient scans was selected by four independent operators either manually or with an automated procedure separating cortical and spongeous bone, the operators being requested to interact in ROI selection. The mean intrasubject variation was found to be much lower with the automated process (0.3 to 0.6%) than with the conventional method (2.5 to 5.2%). In a second study, 10 patients were examined twice to determine the reproducibility of CT slice selection by the operator. The errors were of the same order of magnitude as in ROI selection.

Algorithms↗
PubMed1987

[Material selective imaging and density measurement with the dual energy method. III. Determination of bone mineral of the spine with CT].

Quantitative computed tomography for measuring vertebral bone mineral content offers high sensitivity and reproducibility. However, several systematic errors of the method have to be considered, such as the influence of fat present in the spongiosa in varying concentrations as well as beam hardening effects and different calibration methods. The unknown fat content of the spongiosa of the vertebral bodies can be responsible for errors up to 30%. Beam hardening effects result in comparatively minor errors which can moreover be easily corrected via reference phantoms. The calibration method, on the other hand, results in potentially major systematic deviations due to the necessarily arbitrary determination of the reference points. These deviations must be taken into account especially in cross-comparisons of measured data. Dual energy methods enable a highly selective determination of the density of calcified tissue and soft tissue and decisively reduces the systematic errors mentioned above. The order of magnitude of the individual errors for different CT techniques was assessed by means of phantom studies and clinical studies.

Adult↗
PubMed1987

[Material-selective imaging and density measurement using the dual-energy method. I. Principles and methodology].

The principles of the Dual Energy method are discussed from two perspectives: a clear illustration of the method and an exact presentation of the principles involved. The principles apply to both radiography and computed tomography. The method was implemented on a CT scanner (SOMATOM DR, Siemens AG). Dual Energy images enable highly selective imaging based on the determination of density values for calcified and soft tissue. Images may also be reconstructed that are based on other parameters such as electron density and effective atomic number. The degree to which selective imaging is possible is discussed. Examples of Dual Energy clinical applications for radiography and computed tomography are presented. More detailed information will be available in future contributions.

Humans↗
PubMed1984

[Computerized tomographic demonstration of the nerves and blood vessels of the cerebellopontile angle. Results of pneumocisternomeatography in comparison with intraoperative findings].

Disorders of the 8th cranial nerve such as sensori-neural hearing loss, tinnitus or vertigo may be of vascular origin. Pulsation of arteries can cause compression of, or traction on the nerve, and surgical "decompression" using a modified transtemporal approach may be helpful for these cases. The investigations are based on modern computerized tomography techniques combined with air-cisterno-meatography. The nerves can be demonstrated routinely, while the imaging of vessels is not always possible. This different appearance of arteries may be caused by their pulsatile motion, as shown by phantom studies. The effect of this motion is discussed on the basis of a comparison between preoperative CT-findings and the topography of the vessels verified at operation.

Angiography↗
PubMed1983

[Relevance of computed tomography to the fine-structure analysis of bone. A comparative radiological study].

Computed tomography, a relatively young X-ray imaging modality, has so far been mainly employed for the differentiation and densitometry of soft tissues. By modification of the image reconstruction algorithm spatial resolution was improved significantly such that imaging of fine structures and of high contrast boundaries became possible. The image characteristics of the high-resolution computed tomograms were investigated on petrous bones and lumbar vertebral bodies. In the petrous bone, an exact anatomic reconstruction of the bony structures of the inner ear is achieved. In vertebral bodies, imaging of spongiosa structures with 2 mm slice thickness and thereby an early diagnosis of osteoporosis are possible.

Bone and Bones↗
PubMed1982

[Initial clinical experiences with the fast digital picture of roentgen transmission profiles in the human (the chronogram)].

By means of suitable hardware and software changes it becomes possible to obtain transmission profiles of high time resolution (up to 50 profiles/sec) with the help of a CT device which is suitable for the recording of digital radiographs, in one transversal body layer. In this manner, it becomes possible to measure changes in the density profile from 6 up to 50 sec and to analyse them. This method called "Chronogram", enables high-resolution recording of density changes, similar to kymography. The possibilities of using contrast media are demonstrated by means of phantom measurements, and initial clinical experiences are outlined on the basis of practical examples.

Abdominal Neoplasms↗
PubMed1981

Monte Carlo calculations of x-ray scatter data for diagnostic radiology.

X-ray scatter data have been calculated by Monte Carlo methods for diagnostic radiology applications. The scatter intensities relative to the primary intensities are given for different detectors for various values of object thickness, field size, object-to-detector distance, and primary energy. The results are compared the those from previous investigations. The calculations made it possible to resolve contradictions in published measurements regarding the dependence of scatter intensities on primary X-ray energy and detector response.

Models, Theoretical↗