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Biomedical subjects

K Engelke

Publications and source records attributed to K Engelke.

At least 37 records · Page 2Linked to original sources
PubMed1995

Assessment of the skeletal status by peripheral quantitative computed tomography of the forearm: short-term precision in vivo and comparison to dual X-ray absorptiometry.

In order to assess precision of peripheral quantitative computed tomography (pQCT), duplicate bone mineral density (BMD) measurements at the radius were performed in 20 healthy premenopausal, 20 healthy postmenopausal, and 20 osteoporotic postmenopausal women using a Stratec XCT-960 system. The short-term reproductibility in vivo for the total, trabecular, and cortical regions of interest (ROI) was expressed as the absolute precision error (standard deviation, SD) and as the relative precision error (SD/mean x 100, or coefficient of variation, CV, in %). Reproducibility in vivo was good in all volunteers but was influenced by the study group and the ROI. The precision error for trabecular BMD was 3 mg/cm3, or about 1.6%. This is large relative to the aging decrease of 0.22%/year, or to the difference (12 mg/cm3, or 7%) between osteoporotic women and postmenopausal controls. In order to compare pQCT to dual X-ray absorptiometry (DXA) at the forearm and at the lumbar spine (L1-L4), 40 premenopausal healthy controls, 40 postmenopausal healthy controls, and 35 postmenopausal osteoporotic women were assessed. DXA measurements performed at the ultradistal, middistal, 1/3, and total ROI of the radius showed only moderate correlations between r = 0.38--0.75, r = 0.27--0.64, and r = 0.38--0.53 for the comparison versus pQCT total BMD, versus pQCT trabecular BMD, and versus pQCT cortical BMD, respectively. Correlations of DXA at the lumbar spine and pQCT were between r = 0.18 and 0.44. DXA at radius and spine was able to discriminate between post- menopausal controls and osteoporotic women (p = 0.001--.004),but BMD measurements by pQCT did not show this ability (p = 0.15--0.52). However, two nonstandard pQCT parameters, namely the surface area of the cortical bone and the cortical BMC were factors that discriminated well between these two groups (p = 0.002, p = 0.005, respectively). These pQCT parameters also yielded the highest relative annual changes in pre- and post-menopausal control subjects. The measurement of cortical bone in the distal radius proved to be a good predictor of vertebral fracture status and was a good indicator of age-related skeletal change. Our data emphasize the importance of cortical measurements when using pQCT of the radius to assess osteoporosis.

Absorptiometry, Photon↗
PubMed1995

Factors influencing short-term precision of dual X-ray bone absorptiometry (DXA) of spine and femur.

In this study we analyzed the effect of variations in bone area size, baseline soft tissue composition represented by the R-value, and bone region of interest positioning on the precision in vivo of bone mineral density (BMD) and content (BMC) as measured by dual X-ray absorptiometry (DXA). The posterior-anterior (PA) spine, decubitus lateral, and femur modes were evaluated. Eleven (PA-spine), 9 (dec-lat), and 14 (femur) postmenopausal women were scanned twice on a Norland XR-26 with repositioning to determine short-term precision of BMD, BMC, AREA, and the R-value. Phantom precisions (CV[%] of 10 consecutive scans) for BMD (BMC) were PA spine: 0.66% (0.57%), neck: 1.1% (1.2%), and trochanter: 0.55% (1.0%). Precisions in vivo (CV[%]; two consecutive scans averaged over all patients) were PA spine: 0.9% (1.0%), dec-lat: 7.1% (18%), neck: 1.3% (1.9%), and trochanter: 2.5% (4.9%). BMD precision could be fully explained by BMC and AREA variations. However, BMC alone was a particularly poor predictor of BMD in the dec-lat (r2 = 0.05) and in the neck (r2 = 0.13) modes. AREA was a strong predictor for BMC precision explaining between 41% and 88% of the BMC changes. Changes in soft tissue composition contributed significantly in explaining the BMC changes in the dec-lat projection. A higher dependence of BMC changes on AREA changes resulted in a larger difference between BMC and BMD precision.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗
PubMed1995

Which vertebrae should be assessed using lateral dual-energy X-ray absorptiometry of the lumbar spine.

The purpose of this study was to determine precision and diagnostic capability of bone mineral density measurements using lateral dual-energy X-ray absorptiometry (DXA) of the lumbar spine in supine position. Duplicate postero-anterior (PA) and lateral DXA measurements were performed in 60 women. Precision errors of the single vertebral levels using lateral DXA ranged from 3.3% to 4.9%. The combination of all levels improved the precision errors to 2.0%. Paired PA and lateral DXA measurements (Hologic QDR 2000) including the vertebral levels L2 to L4 were performed in 331 postmenopausal women. In 42 women an overlap of L4 by the pelvis was suspected on the lateral DXA images. Vertebral fractures were assessed as a fracture/non-fracture dichotomy. L4 and combinations of vertebrae including L4 showed the best discriminatory capabilities with respect to vertebral fractures in receiver operating characteristic (ROC) analyses, t-tests and Z-scores, with smaller variability of the results when multiple vertebral levels were used. The areas under the ROC curves were 0.662 and 0.639 for lateral and PA measurements of L2 to L4, respectively when all women were included. Excluding the women with pelvic overlap on lateral DXA scans improved the ROC area for lateral scans to 0.686 while that for PA scans remained almost constant (0.641). The differences between PA and lateral measurements were not statistically significant. In 162 women of our study cohort an additional quantitative computed tomography (QCT) measurement of the vertebral levels L2 to L4 was performed and overlapping bony structures at the three levels were studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗
PubMed1994

Dual x-ray absorptiometry forearm software: accuracy and intermachine relationship.

An ash study was performed to analyze the accuracy of forearm measurements at the one-third site of three dual x-ray absorptiometry (DXA) systems using 20 cadavers. A Hologic QDR-2000, a Hologic QDR-1000/W, and a Lunar DPX-L system were used. The correlations between ash weight and DXA BMC were excellent for the three instruments (r > 0.97, p < 0.001), with accuracy errors < 5.2%. To perform a forearm cross-calibration of bone mineral content (BMC) and density (BMD) between the scanners, 10 healthy volunteers were additionally scanned at the distal one-third radius. The correlations among the DXA machines were excellent (r > 0.95); the absolute BMC and BMD values were significantly different between the two Hologics and the Lunar machine. The slope and intercept of both the BMC and BMD between the two Hologic systems were close to unity. In conclusion, the DXA forearm software packages provide accurate methods for assessing bone mineral content and density. The conversion of data among different manufacturers should be performed by careful cross-calibration measurements.

Absorptiometry, Photon↗
PubMed1994

Universal standardization for dual x-ray absorptiometry: patient and phantom cross-calibration results.

The comparison of patient data among different dual x-ray absorptiometry (DXA) scanners is complicated because no universally accepted cross-calibration procedure or standard currently exists. This study was performed under the auspices of the International DXA Standardization Committee to establish appropriate cross-calibration parameters. Posteroanterior (PA) lumbar spine measurements of 100 women, ages 20-80 years (mean 52.6 +/- 16, range of BMD = 0.4-1.6 g/cm2) were obtained on a Norland XR26 Mark II, a Lunar DPX-L, and a Hologic QDR 2000 densitometer using standard procedures (pencil beam mode for all three scanners). Area, BMC, and BMD results from the different scanners were compared for all patients. In addition, the European spine phantom (ESP) and the European spine phantom prototype (ESP prototype), as well as standard phantoms from all three manufacturers, were evaluated on the three systems. To achieve universal scanner calibration, we used the intercept and slope of the patient's correlations and the value of the middle vertebra of the ESP as a reference point in a series of standardization formulas, and we have expressed the results as sBMD (mg/cm2). The correlations of the patients' spinal BMD values were excellent for each of the three scanner pairs. The average absolute difference in patient spinal BMD values (L2-4) between Hologic and Norland was 0.012 g/cm2 (1.3%); it was 0.113 g/cm2 (11.7%) between Hologic and Lunar and 0.118 g/cm2 (12.2%) between Norland and Lunar. The phantoms' regression lines approximated those of the patient regression lines, and the phantoms with only one measurement point were very close to the patients' regression lines. After applying the standardization formulas, the average absolute differences for the 100 patients were 28 mg/cm2 (2.7%) for Hologic/Norland, 23 mg/cm2 (2.2%) for Hologic/Lunar, and 29 mg/cm2 (2.8%) for Norland/Lunar. Average BMD results for the patients before correction were 0.972 mg/cm2 for Hologic, 1.100 g/cm2 for Lunar, and 0.969 g/cm2 for Norland. After correction, sBMD results for patients were 1045 mg/cm2 for Hologic, 1047 mg/cm2 for Lunar, and 1043 mg/cm2 for Norland. The standardization approach as performed in our study provided compatibility of DXA results obtained on different scanners.

Absorptiometry, Photon↗
PubMed1994

Phantom studies simulating the impact of trabecular structure on marrow relaxation time, T2'.

Phantom studies were conducted to investigate the impact of trabecular structure on the T2' signal measured by MRI. For a separation of density from structural effects, several phantoms were built. They consisted of parallel polyethylene strings arranged in a variety of different patterns to simulate a) a constant uniform trabecular distribution with increasing trabecular thickness and b) different structures with identical overall trabecular density. An asymmetric spin echo sequence was used to determine the apparent relaxation time T2'. Changes in T2' are induced by susceptibility differences between the polyethylene strings simulating trabeculae and Gd-DTPA doped saline simulating bone marrow. The results showed an increasing T2' decay rate with a) decreasing spacing while the string density was constant and b) with increasing string density while the string arrangement was constant. The results demonstrate that the T2' signal is affected not only by density but also by spatial distribution. However, the results also indicate that a separation of the two effects is not possible from a T2' measurement alone, but that e.g., a matching CT slice that would provide purely density information is additionally needed. Theoretical simulations confirmed these results.

Bone Marrow↗
PubMed1994

Radiographic absorptiometry for bone mineral measurement of the phalanges: precision and accuracy study.

PURPOSE: To evaluate the accuracy and precision of a radiographic absorptiometry (RA) method for assessment of bone mineral of the middle phalanges. MATERIALS AND METHODS: Nineteen cadaveric hands were radiographed with an aluminum wedge, once at 50 kVp and 400 mA and once at 60 kVp and 300 mA. Bone mineral content (BMC) and bone mineral density (BMD) of the second to fourth middle phalanges, expressed in arbitrary units (BMC-AU and BMD-AU), were analyzed and averaged in each hand. RESULTS: The precision error of this method was 1.0% for BMC-AU and 0.6% for BMD-AU. A 2.0%-2.4% reduction in BMD-AU seen on radiographs obtained through ethanol thicknesses of 5 and 6 mm compared with that seen on controls was statistically significant (P < .01). The correlation between BMC-AU and forearm BMC determined with dual x-ray absorptiometry was good (r = .887), and that between BMC-AU and ash weight in the phalanges was excellent (r = .983). CONCLUSION: The RA method is precise and accurate for bone mineral assessment of the peripheral appendicular skeleton.

Absorptiometry, Photon↗
PubMed1993

Quality assurance for bone densitometry research studies: concept and impact.

A concept for quality assurance (QA) in bone densitometry has been developed for clinical multicenter studies. Major elements provided by a coordinating center comprise (1) consulting services and certification of participating centers in the start-up phase of the study, (2) review of scan data acquired on QA standards for cross-calibration and longitudinal assessment of scanner stability, (3) review of selected patient data as well as of problem cases during the study, and (4) comprehensive review and correction of patient results based on QA data after conclusion of the study. Limitations of phantom-based QA data should be acknowledged. Typical problems encountered during research studies and guidelines for solutions are presented. Successful implementation of QA measures may yield substantial enhancement of statistical power. Depending on the study design and the variability of response within patient groups, improvement in precision due to QA measures may reduce the smallest detectable difference between subject groups or, alternatively, sample size by a few to more than 50%, and thus may contribute to a substantial reduction in study cost. Formulae for calculation of the magnitude of these effects are presented. To maximize the net benefit, QA efforts have to be limited to levels that assure reliability of the data at acceptable QA cost. While QA programs at individual clinical sites or for local practitioners may not need to be as extensive as for multicenter clinical trials, awareness of the potential problems and implementation of basic QA measures will help in obtaining high-quality bone densitometry results.

Bone Density↗
PubMed1993

Precision and accuracy for rat whole body and femur bone mineral determination with dual X-ray absorptiometry.

Precision and accuracy for rat whole body and excised femur bone mineral density (BMD) measurements were evaluated with two dual X-ray absorptiometry (DXA) systems. The 'small subject' mode on the Norland XR-26 (XR), and the 'ultra high resolution mode' on the Hologic QDR-1000/W(QDR) were used for the analysis. The whole body mode was only available on the XR. The lowest precision error for the whole body was found using a scan resolution of 1.0 x 1.0 mm and a scan speed of 15 mm/s. The scan spatial resolution of the femur measurement was approximately eight times higher on the QDR than on the XR. However, the XR allowed analysis of an arbitrary region of interest within the femur, which was not easily done with the QDR. Precision for the total femur measurement on the QDR (0.5-0.9%) was approximately two to four times superior to that of the XR (1.5-4.3%). The difference may be due to the superior scan resolution of the QDR. Bone mineral content and BMD on the QDR significantly declined with an increase of water depth (P < 0.001). No significant change was observed on the XR. Both DXA systems demonstrated an excellent correlation (r > or = 0.98) with ash weight under the scan conditions examined. The optimal scan condition for the excised femur measurement on the XR was obtained with 1.5-2.5 cm of perspex or water and a scan speed of 10 mm/s. For the QDR, we recommend scanning the rat femur with approximately 2.5 cm of perspex or water in terms of precision and accuracy.

Absorptiometry, Photon↗
PubMed1993

High spatial resolution imaging of bone mineral using computed microtomography. Comparison with microradiography and undecalcified histologic sections.

RATIONALE AND OBJECTIVES: The application of various high-resolution (< 100 microns) imaging techniques for in vitro bone mineral analysis is explored. METHODS: The techniques of contact microradiography and microtomography, using the x-ray spectrum filtered out of synchrotron radiation (SR) and conventional staining techniques, are compared to each other by presenting a variety of different samples. The relationship between radiation exposure and spatial resolution micro-computed tomography (CT) images of a finger bone is explored. The relevant properties of SR are explained. RESULTS: In CT images, a spatial resolution of 100 microns was obtained. New bone mineral induced by mechanical periosteal irritation in a rabbit tibia was quantified. In one case a microradiogram and a microtomogram of the same slice were taken for comparison. Histologic sections and microradiograms taken from a specimen of a human femur for comparison are presented. CONCLUSIONS: Microradiography and staining techniques require rather sophisticated sample preparation; quantitative image analysis is more difficult as the resulting image must be digitized. The CT technique requires almost no sample preparation and allows for accurate bone mineral quantification. However, CT images with a resolution of several microns limit the sample size to a few mm. Micro-CT and microradiography can be performed with conventional x-ray sources, but the use of SR is of particular interest in high resolution imaging, because its white spectrum allows for optimum x-ray energy selection and its high intensity for short scan times.

Absorptiometry, Photon↗
PubMed1993

Fetal ACTH and blood pressure responses to thromboxane mimetic U-46619.

This study was performed to test the hypothesis that thromboxane A2 stimulates increases in fetal adrenocorticotropic hormone (ACTH), vasopressin, or renin secretion and affects fetal cardiovascular function by an action on the fetal central nervous system. We infused a stable synthetic analogue of thromboxane A2, U-46619, into one common carotid artery or inferior vena cava or infused saline into one common carotid artery in chronically catheterized fetal sheep between 127 and 140 days gestation. We found that intracarotid but not intravenous infusions of U-46619 at a rate of 750 ng/min stimulated increases in fetal plasma ACTH concentration. Infusions of U-46619 at both sites increased fetal blood pressure; the infusion into the carotid arterial blood produced a more rapid increase in blood pressure and a significant decrease in central venous pressure. None of the infusions altered plasma vasopressin concentration or plasma renin activity, blood gases, hematocrit, or plasma cortisol concentration. We conclude that thromboxane A2 stimulates fetal ACTH, but not vasopressin or renin, secretion via an action within the area perfused by carotid arterial blood. Thromboxane A2 increases blood pressure via an action at the fetal central nervous system, as well as via a direct vasoconstrictor action in the systemic circulation.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed1993

[Advances in the diagnosis of osteoporosis].

Bone densitometry allows for an assessment of osteoporotic fracture risk. Well-established methods include dual X-ray absorptiometry (DXA), quantitative computed tomography (QCT), and single X-ray absorptiometry (SXA). The advantages and disadvantages of these methods are discussed. Promising innovative techniques include three new X-ray-based approaches: lateral DXA and peripheral QCT for the determination of bone mineral density of the spine and radius, respectively, and morphometric X-ray absorptiometry (MXA) for computerized quantification of vertebral deformities. Quantitative ultrasound (QUS) and quantitative magnetic resonance (QMR), on the other hand, open up new ways of assessing bone microarchitecture in addition to bone mineral density--without the use of ionizing radiation. These new approaches promise to yield new information on skeletal status. Complementing existing bone densitometry approaches, this may improve fracture risk assessment.

Absorptiometry, Photon↗
PubMed1993

[Osteoporosis diagnosis by broad-band ultrasound attenuation (BUA): its correlation with established measurement procedures of bone densitometry].

Broadband ultrasound attenuation (BUA) measurements of the os calcis of 54 women were correlated to single-energy x-ray absorptiometry (SXA) of the os calcis, dual-energy x-ray absorptiometry (DXA) of the lumbar spine, age and weight. The reproducibility of the BUA measurements was 2.1%. BUA was significantly correlated with SXA (r = 0.7), DXA (r = 0.5) and age (r = 0.6). BUA values ranged from 42 to 117 dB/MHz. Our results confirm BUA's potential as a simple, inexpensive, radiation free, and precise alternative technique for osteoporosis diagnostics of the os calcis. However, the correlation observed with bone mineral density of the spine is too weak to use BUA for predicting the lumbar density status. The ability of BUA to predict the risk of osteoporotic fractures has to be determined directly by assessing the association of BUA and the prevalence or preferably the incidence of fractures.

Absorptiometry, Photon↗
PubMed1992

Site-matched calcaneal measurements of broad-band ultrasound attenuation and single X-ray absorptiometry: do they measure different skeletal properties?

Because of the differences in the interactions of ultrasound and x-ray waves with bone, quantitative ultrasound (QUS) techniques may yield information about skeletal status not accessible by regular bone densitometry (BD) techniques. However, relatively strong correlations have been reported between broad-band ultrasound attenuation (BUA) and several x-ray-based BD methods. We assessed the precision and association of single x-ray absorptiometry (SXA) and BUA of the calcaneus. We examined both BUA and SXA at the calcaneus using special software for matching the regions of interest. An algorithm was derived and applied to correct the observed correlation coefficients for the attenuation effect caused by the precision errors for BUA and SXA. In a group of 33 volunteers covering a wide range of age and calcaneal bone mineral densities, the site-matched and precision-adjusted correlation coefficient between BUA and SXA was r = 0.58, with a standard error of the estimate (SEE) of 14.41 dB/MHz, or 17.08%. For the subgroup of 25 women the correlation was stronger, with r = 0.72 and SEE = 11.53 dB/MHz, or 14.33%. SXA precision was 0.79% for the regular region of interest (ROI) and 1.22% for the site-matched ROI. BUA precision was 2.76% for the entire subject group and 1.63% for women of age 40 or older. The observed correlation coefficient between ultrasound and x-ray based techniques of the order of 0.7 is significant, but it leaves about 50% of the variability unexplained.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗