Dual-energy Quantitative Computed Tomography Research Articles

Quantification of the volume fraction of fat, water and bone mineral in spongiosa for red marrow dosimetry in molecular radiotherapy by using a dual-energy (SPECT/)CT

A patient-specific absorbed dose calculation for red marrow dosimetry requires quantifying patient-specific volume fractions of the red marrow, yellow marrow, and trabecular bone in the spongiosa of several skeletal sites. This quantification allows selecting appropriate S values calculated from the parameterized radiation transport models for bone and bone marrow dosimetry. Currently, no comprehensive, individualized, and non-invasive procedure is available for quantifying the volume fractions of red marrow, yellow marrow, and trabecular bone in the spongiosa. This study aims to provide a new quantitative method based on dual-energy computed tomography to fill this gap in red marrow dosimetry using a (SPECT/)CT system. MethodsFirst, a method for parametrizing the photon attenuation coefficients relative to water was implemented. Next, a method to calculate the effective atomic number (Zeff) and effective mass density (ρeff) using dual-energy CT (DECT) was employed. Lastly, two- and three-material decomposition using a dual-energy quantitative CT method (DEQCT) was performed in an anthropomorphic spine phantom and two bone samples of a boar, respectively.The measurements of Zeff and ρeff were compared with the syngo.CT DE Rho/Z tool (Siemens Healthineers). Furthermore, the DEQCT method implemented in this study (DEQCT-I) was compared with a second DEQCT method based on the use of external material standards (DEQCT-II). DEQCT-II was used as reference method for calculating relative errors. ResultsThe two-material decomposition in the anthropomorphic spine phantom presented a maximum relative error of −10% for the bone mineral density quantification. Furthermore, Zeff and ρeff calculated by DEQCT-I differed from syngo.CT DE Rho/Z tool by less than 4.4% and 1.9%, respectively. The three-material decomposition in the two bone samples showed a maximum relative error of 21%, −17%, and 15% for the quantification of the volume fractions of fat, water, and bone mineral equivalent materials. Lastly, Zeff and ρeff calculated by DEQCT-I differed from syngo.CT DE Rho/Z tool by less than 8.2% and 7.0%, respectively. ConclusionThis study shows that quantifying the volume fraction of fat, water, and bone mineral using a phantom-independent and post-reconstruction DEQCT method is feasible. DEQCT-I has the advantage of not requiring prior information about the X-ray spectra or the detector sensitivity function, as is the case with spectral-based DEQCT methods. Instead, DEQCT-I, similar to other DEQCT methods depends on the chemical description of reference materials and a beam hardening correction function.DEQCT-I method provides an individualized and non-invasive procedure using a (SPECT/)CT system to apply S values based on the patient-specific volume fractions of yellow marrow, red marrow, and bone mineral in red marrow dosimetry.

Independent changes in bone mineralized and marrow soft tissues following acute knee injury require dual-energy or high-resolution computed tomography for accurate assessment of bone mineral density and stiffness

Musculoskeletal injuries often induce local accumulation of blood and/or fluid within the bone marrow, which is detected on medical imaging as edema-like marrow signal intensities (EMSI). In addition to its biological effects on post-injury recovery, the displacement of low-attenuating, largely adipocytic marrow by EMSI may introduce errors into quantitative computed tomography (QCT) measurements of bone mineral density (vBMD) and resulting bone stiffness estimates from image-based finite element (FE) analysis. We aimed to investigate the impact of post-injury changes in marrow soft tissue composition on CT-based bone measurements by applying CT imaging at multiple spatial resolutions. To do so, dual energy QCT (DECT) material decomposition was used to detect EMSI in the tibiae of nineteen participants with a recent anterior cruciate ligament tear. We then measured bone density and FE-based apparent modulus within the EMSI region and in a matched volume in the uninjured contralateral knee. Three measurement methods were applied: 1.) standard, QCT density calibration and density-based FEM; 2.) a DECT density calibration that provides density measurements adjusted for marrow soft tissues; and 3.) high-resolution peripheral QCT (HR-pQCT) density and microFE analyses. When measured using standard, single-energy QCT, vBMD and apparent modulus were elevated in the EMSI compared to the contralateral. After adjusting for marrow soft tissue composition using DECT, these measurements were no longer different between the two regions. By allowing for high-resolution, localized density analysis, HR-pQCT indicated that trabecular tissue mineral density was 9 mgHA/cm3 lower, while density of marrow soft tissues was 18 mgHA/cm3 higher, in the EMSI than the contralateral region, suggesting that EMSI have opposite effects on the measured density of trabecular bone and the underlying soft marrow. Thus, after an acute injury, altered composition of marrow soft tissues may artificially inflate overall measurements of bone density and apparent modulus obtained using standard QCT. This can be corrected by accounting for marrow soft tissue attenuation, either by using DECT-based density calibration or HR-pQCT microFE and measurements of local density of trabeculae.

Quantification of the trabecular bone volume fraction for bone marrow dosimetry in molecular radiotherapy by using a dual-energy (SPECT/)CT

A complete characterization of spongiosa (bone marrow plus trabecular bone) is required to calculate the absorbed dose to active bone marrow. Due to the complex microanatomy, it is necessary to apply non-conventional imaging methods in nuclear medicine. The aim of this study is validating a phantomless quantification method using dual-energy quantitative computed tomography (DEQCT) for the quantification of trabecular bone volume fraction for bone marrow dosimetry in molecular radiotherapy.First, a phantomless quantification method (mass fraction method) based on x-ray beam and detector sensitivity was validated in an integrated dual energy SPECT/CT and in a dual source computed tomography (DSCT) system for comparison. The validation was performed in a phantom consisting of different water, fat and hydroxyapatite compositions. Moreover, the European spine phantom (ESP) was used to simulate the spine geometry. Bone mineral content (BMC) of the whole vertebra and bone mineral density (BMD) in the spongiosa region of each phantom vertebra were measured using DEQCT and dual energy x-ray absorptiometry (DEXA). Lastly, BMC was measured in a patient using DEQCT and DEXA.Measured values of hydroxyapatite fraction and nominal values in the homemade phantom showed a good correlation. The relative error remained below 14.2%. Quantification of BMC (in whole vertebra) and BMD (in spongiosa) in the ESP showed a good agreement between measured values and nominal values. The relative error remained between 0.7% and 7.5% for BMCSPECT/CT, 1.1% and 7.7% for BMCDSCT, 5.4% and 32.0 for BMDSPECT/CT, and 59.4% and 10.0% for BMDDSCT. Quantification of BMC in lumbar vertebrae 1 and 2 of a patient showed relative errors of 7.6% and −8.4% between DEXA and DSCT.Our study shows that DEQCT using a mass fraction method (phantomless) enables quantification of hydroxyapatite in a clinical nuclear medicine setting. An overestimation of the hydroxyapatite volume fraction was observed in all quantified regions, in particular in the spongiosa region of ESP. This result might be related to insufficient information about the x-ray spectra and the detector sensitivity function.

Correction of QCT vBMD using MRI measurements of marrow adipose tissue.

Quantitative computed tomography (QCT) measurements of volumetric bone mineral density (vBMD) are subject to errors due to variations in the amount of bone marrow adipose tissue (BMAT). The purpose of our study was to describe and validate a novel method to correct lumbar spine trabecular vBMD measurements for BMAT using chemical shift-encoded magnetic resonance imaging (CSE-MRI). CSE-MRI measurements of proton density fat fraction (PDFF) were used to correct QCT spine vBMD measurements for BMAT based on the H2O and K2HPO4 basis set equivalent densities of bone, red and yellow bone marrow. BMAT corrected and uncorrected vBMD measurements of the L1 vertebra were compared with dual-energy QCT (DEQCT) measurements in 18 subjects (mean age: 68 y, range 60 to 93 y). A further 400 subjects (mean age: 53 y, range 21 to 82 y) had 120 kVp single-energy QCT and CES-MRI scans of L2-L4 and the data used to simplify the adipose tissue correction by deriving a linear equation between the CSE-MRI vBMD correction and fractional BMAT content. Application of the CSE-MRI derived vBMD correction changed the bias (95% limits of agreement) compared with DEQCT from 26.7 (11.0 to 42.4) mg/cm3 to 2.2 (-9.5 to 13.9) mg/cm3 at 80 kVp, and from 22.4 (3.3 to 41.6) mg/cm3 to 2.9 (-12.6 to 18.4) mg/cm3 at 120 kVp. Data for the 400 subjects gave the following relationship valid at 120 kVp: vBMD correction (mg/cm3) = -12.96 + 75.76 × BMAT. CSE-MRI measurements of PDFF can be used to correct for BMAT content and improve the accuracy of lumbar spine QCT vBMD measurements calibrated using a K2HPO4 phantom.

A novel dual energy method for enhanced quantitative computed tomography

Accurate assessment of bone mineral density (BMD) is critically important in clinical practice, and conveniently enabled via quantitative computed tomography (QCT). Meanwhile, dual-energy QCT (DEQCT) enables enhanced detection of small changes in BMD relative to single-energy QCT (SEQCT). In the present study, we aimed to investigate the accuracy of QCT methods, with particular emphasis on a new dual-energy approach, in comparison to single-energy and conventional dual-energy techniques. We used a sinogram-based analytical CT simulator to model the complete chain of CT data acquisitions, and assessed performance of SEQCT and different DEQCT techniques in quantification of BMD. We demonstrate a 120% reduction in error when using a proposed dual-energy Simultaneous Equation by Constrained Least-squares method, enabling more accurate bone mineral measurements.

The measurement of liver fat from single-energy quantitative computed tomography scans.

Studies of soft tissue composition using computed tomography (CT) scans are often semi-quantitative and based on Hounsfield units (HU) measurements that have not been calibrated with a quantitative CT (QCT) phantom. We describe a study to establish the water (H2O) and dipotassium hydrogen phosphate (K2HPO4) basis set equivalent densities of fat and fat-free liver tissue. With this information liver fat can be accurately measured from any abdominal CT scan calibrated with a suitable phantom. Liver fat content was measured by comparing single-energy QCT (SEQCT) HU measurements of the liver with predicted HU values for fat and fat-free liver tissue calculated from their H2O and K2HPO4 equivalent densities and calibration data from a QCT phantom. The equivalent densities of fat were derived from a listing of its constituent fatty acids, and those of fat-free liver tissue from a dual-energy QCT (DEQCT) study performed in 14 healthy Chinese subjects. This information was used to calculate liver fat from abdominal SEQCT scans performed in a further 541 healthy Chinese subjects (mean age 62 years; range, 31-95 years) enrolled in the Prospective Urban Rural Epidemiology (PURE) Study. The equivalent densities of fat were 941.75 mg/cm3 H2O and -43.72 mg/cm3 K2HPO4, and for fat-free liver tissue 1,040.13 mg/cm3 H2O and 21.34 mg/cm3 K2HPO4. Liver fat in the 14 subjects in the DEQCT study varied from 0-17.9% [median: 4.5%; interquartile range (IQR): 3.0-7.9%]. Liver fat in the 541 PURE study subjects varied from -0.3-29.9% (median: 4.9%; IQR: 3.4-6.9%). We have established H2O and K2HPO4 equivalent densities for fat and fat-free liver tissue that allow a measurement of liver fat to be obtained from any abdominal CT scan acquired with a QCT phantom. Although radiation dose considerations preclude the routine use of QCT to measure liver fat, the method described here facilitates its measurement in patients having CT scans performed for other purposes. Further studies comparing the results with magnetic resonance (MR) measurements of liver fat are required to validate the method as a useful clinical tool.

Agreement and precision of periprosthetic bone density measurements in micro-CT, single and dual energy CT.

The objective of this study was to test the precision and agreement between bone mineral density measurements performed in micro CT, single and dual energy computed tomography, to determine how the keV level influences density measurements and to assess the usefulness of quantitative dual energy computed tomography as a research tool for longitudinal studies aiming to measure bone loss adjacent to total hip replacements. Samples from 10 fresh-frozen porcine femoral heads were placed in a Perspex phantom and computed tomography was performed with two acquisition modes. Bone mineral density was calculated and compared with measurements derived from micro CT. Repeated scans and dual measurements were performed in order to measure between- and within-scan precision. Mean density difference between micro CT and single energy computed tomography was 72 mg HA/cm3 . For dual energy CT, the mean difference at 100 keV was 128 mg HA/cm3 while the mean difference at 110-140 keV ranged from -84 to -67 mg HA/cm3 compared with micro CT. Rescanning the samples resulted in a non-significant overall between-scan difference of 13 mg HA/cm3 . Bland-Altman limits of agreement were wide and intraclass correlation coefficients ranged from 0.29 to 0.72, while 95% confidence intervals covered almost the full possible range. Repeating the density measurements for within-scan precision resulted in ICCs >0.99 and narrow limits of agreement. Single and dual energy quantitative CT showed excellent within-scan precision, but poor between-scan precision. No significant density differences were found in dual energy quantitative CT at keV-levels above 110 keV. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1470-1477, 2017.

Evaluation of dual energy quantitative CT for determining the spatial distributions of red marrow and bone for dosimetry in internal emitter radiation therapy

To evaluate a three-equation three-unknown dual-energy quantitative CT (DEQCT) technique for determining region specific variations in bone spongiosa composition for improved red marrow dose estimation in radionuclide therapy. The DEQCT method was applied to 80/140 kVp images of patient-simulating lumbar sectional body phantoms of three sizes (small, medium, and large). External calibration rods of bone, red marrow, and fat-simulating materials were placed beneath the body phantoms. Similar internal calibration inserts were placed at vertebral locations within the body phantoms. Six test inserts of known volume fractions of bone, fat, and red marrow were also scanned. External-to-internal calibration correction factors were derived. The effects of body phantom size, radiation dose, spongiosa region segmentation granularity [single (∼17 × 17 mm) region of interest (ROI), 2 × 2, and 3 × 3 segmentation of that single ROI], and calibration method on the accuracy of the calculated volume fractions of red marrow (cellularity) and trabecular bone were evaluated. For standard low dose DEQCT x-ray technique factors and the internal calibration method, the RMS errors of the estimated volume fractions of red marrow of the test inserts were 1.2-1.3 times greater in the medium body than in the small body phantom and 1.3-1.5 times greater in the large body than in the small body phantom. RMS errors of the calculated volume fractions of red marrow within 2 × 2 segmented subregions of the ROIs were 1.6-1.9 times greater than for no segmentation, and RMS errors for 3 × 3 segmented subregions were 2.3-2.7 times greater than those for no segmentation. Increasing the dose by a factor of 2 reduced the RMS errors of all constituent volume fractions by an average factor of 1.40 ± 0.29 for all segmentation schemes and body phantom sizes; increasing the dose by a factor of 4 reduced those RMS errors by an average factor of 1.71 ± 0.25. Results for external calibrations exhibited much larger RMS errors than size matched internal calibration. Use of an average body size external-to-internal calibration correction factor reduced the errors to closer to those for internal calibration. RMS errors of less than 30% or about 0.01 for the bone and 0.1 for the red marrow volume fractions would likely be satisfactory for human studies. Such accuracies were achieved for 3 × 3 segmentation of 5 mm slice images for: (a) internal calibration with 4 times dose for all size body phantoms, (b) internal calibration with 2 times dose for the small and medium size body phantoms, and (c) corrected external calibration with 4 times dose and all size body phantoms. Phantom studies are promising and demonstrate the potential to use dual energy quantitative CT to estimate the spatial distributions of red marrow and bone within the vertebral spongiosa.

TH-A-103-01: Investigation of Factors Affecting the Accuracy of a Dual Energy Quantitative CT Method for Estimating Regional Bone Marrow Cellularity

Purpose: To evaluate a 3-equation 3-unknown Dual Energy Quantitative CT technique for determining region specific variations in bone marrow cellularity for improved red marrow dose estimation in radionuclide therapy. Methods: The DEQCT method was applied to 80/140kVp images of patient simulating lumbar phantoms of three different sizes (small, medium, large). External calibration rods of bone, marrow and fat simulating material were placed beneath the phantoms. Similar internal calibration inserts were placed at a vertebral location within the phantoms. Six test inserts of known volume fractions of bone, fat and red marrow (Red marrow range:0.15–0.74) were also scanned. External to internal calibration correction factors were derived. The effects of phantom size, slice thickness, dose, spongiosa region segmentation(1,4(2×2),9(3×3)) and calibration method on the accuracy of the calculated red marrow volume fractions (cellularity) were evaluated. The method was also applied to six patients. Results: For both the small and medium sized phantoms using internal calibration, RMS errors(RMSEs) in the estimated volume fractions of red marrow were 0.03, 0.04 and 0.05 for partitioning of a 23.4mmx23.4mm ROI into 1 region, 4 regions and 9 regions, respectively. Corresponding results for a large sized body phantom were significantly poorer (0.06,0.09,0.10). Reducing slice thickness to 2.5mm(vs. 5mm) increased RMSEs of the marrow volume fractions by 0.01 to 0.06. Similar increases in RMSEs were obtained for reduction in dose(mAs). Results for external calibrations exhibited large errors that were reduced to those of internal calibration by application of external to internal correction factors. Application of these factors to the 6 patient lumbar vertebrae yielded cellularity values that differed by 29% on average from biopsy values at the iliac crest, potentially due to regional variability. Conclusion: Phantom studies were promising, but show poorer results for large bodies, thinner slices, lower doses and smaller sub-regions. Future validation studies in human cadaver vertebrae are needed. National Institutes of Health (NIH) under Grant 2RO1 EB001994

The X-ray attenuation characteristics and density of human calcaneal marrow do not change significantly during adulthood

Changes in the material characteristics of bone marrow with aging can be a significant source of error in measurements of bone density when using X-ray and ultrasound imaging modalities. In the context of computed tomography, dual-energy computed techniques have been used to correct for changes in marrow composition. However, dual-energy quantitative computed tomography (DE-QCT) protocols, while increasing the accuracy of the measurement, reduce the precision and increase the radiation dose to the patient in comparison to single-energy quantitative computed tomography (SE-QCT) protocols. If the attenuation properties of the marrow for a particular bone can be shown to be relatively constant with age, it should be possible to use single-energy techniques without experiencing errors caused by unknown marrow composition. Marrow was extracted by centrifugation from 10 mm thick frontal sections of 34 adult cadaver calcanei (28 males, 6 females, ages 17–65 years). The density and energy-dependent linear X-ray attenuation coefficient of each marrow sample were determined. For purposes of comparing our results, we then computed an effective CT number at two GE CT/i scan voltages (80 and 120 kVp) for each specimen. The coefficients of variation for the density, CT number at 80 kVp and CT number at 120 kVp were each less than 1%, and the parameters did not change significantly with age ( p>0.2, r 2<0.02, power>0.8 where the minimum acceptable r 2=0.216). We could demonstrate no significant gender-associated differences in these relationships. These data suggest that calcaneal bone marrow X-ray attenuation properties and marrow density are essentially constant from the third through sixth decades of life.

Biomechanical compression tests with a new implant for thoracolumbar vertebral body replacement

The authors present an investigation into the biomechanical functioning of a new titanium implant for vertebral body replacement (Synex). Possible indications are fractures and/or dislocations with damage of the anterior column, posttraumatic kyphosis and tumors of the thoracolumbar spine. The construction must be supplemented by a stabilizing posterior or anterior implant. For best fit and contact with adjacent end-plates, Synex is distractable in situ. We performed comparative compression tests with Synex and MOSS ("Harms mesh cage") on human cadaveric specimens of intact vertebrae (L1). The aim of the study was to measure the compressive strength of the vertebral body end-plate in uniaxial loading via both implants to exclude collapse of Synex in vivo. Twelve human cadaveric specimens of intact vertebrae (L1) were divided into two identical groups (matched pairs) according to bone mineral density (BMD), determined using dual-energy quantitative computed tomography (DE-QCT). The specimens were loaded with an axial compression force at a constant speed of 5 mm/min to failure, and the displacement was recorded with a continuous load-displacement curve. The mean ultimate compression force (Fmax) showed a tendency towards a higher reading for Synex: 3396 N versus 2719 N (non-significant). The displacement until Fmax was 2.9 mm in the Synex group, which was half as far as in the MOSS group (5.8 mm). The difference was significant (P < 0.001). The compression force was twice as high, and significantly (P < 0.05) higher with Synex at displacements of 1 mm, 1.5 mm and 2 mm. A significant (P < 0.001) correlation (R = 0.89) between Fmax and BMD was found. Synex was found to be at least comparable to MOSS concerning the compressive performance at the vertebral end-plate. A possible consequence of the significantly higher mean compression forces between 1 and 2 mm displacement might be decreased collapse of the implant into the vertebral body in vivo.

上顎部へ移植された自家海綿骨のＱＣＴによる骨塩定量

Dual-energy quantitative computed tomography (DEQCT) is useful in the quantitative analysis of bone mineral content (BMC). This study describes the use of DEQCT in the quantitative evaluation of the BMC of particulate cancellous bone and marrow grafted to the basal maxillary sinus and alveolar cleft defects.Several regions of interest (ROI) were used for analysis. In all ROI, BMC increased after the graft in all patients. An irregular ROI may be best suited for the evaluation of whole grafted bone. If the placement of root form implants is planned, evaluation of a circular ROI is useful. On the other hand, a CT-profile permitted the localization of BMC.

The Composition of Bone Marrow for a Dual-Energy Quantitative Computed Tomography Technique

The authors have been developing a dual-energy quantitative computed tomography (DE-QCT) technique that requires calibration standards that mimic the x-ray attenuation properties of bone, red marrow, and yellow marrow. To resolve questions regarding the compositions of red and yellow marrow that appear in the literature, the authors performed chemical analyses of bone marrow samples. The newly derived compositions were used in a simulation study to test the accuracy of the DEQCT technique. Red marrow samples were extruded from the vertebrae of cadavers of young boys. Yellow marrow samples were removed directly from the femurs of cadavers of elderly women. The fat, protein, water, and mineral contents of these samples were determined. The compositions of 12 mixed marrow samples extruded from cadaver vertebrae also were measured. A computer simulation study was performed in which calibration standards with the new compositions were employed to estimate the fat and bone contents of spongiosas containing the 12 mixed marrows. The red marrow samples contained 3% to 6% fat, 6% to 8% protein, 82% to 86% water, and 0.5% to 1% mineral. The yellow marrow samples contained 71% to 92% fat, 1% to 2% protein, 7% to 26% water, and 0.2% to 0.4% mineral. The simulation study yielded good results in three cases and mediocre to poor results in nine cases. An alternative approach was tried in which an average fat-free marrow was derived from the compositions of the 12 mixed marrows, and this substance, fat, and bone were used as the calibration standards. The DEQCT technique with these standards was applied to simulated spongiosas containing the 12 original mixed marrows plus nine additional mixed marrows. All of the estimates were in good agreement with the true compositions. The rms error of the mass fractions of fat was 0.03, and the rms error of the bone concentrations was 3.7 mg/mL.

Mineral content of vertebral trabecular bone: Accuracy of dual energy quantitative computed tomography evaluated against neutron activation analysis and flame atomic absorption spectrometry

The goal of this study was to evaluate the accuracy of preprocessing dual energy quantitative computed tomography (QCT) for assessment of trabecular bone mineral content (BMC) in lumbar vertebrae. The BMC of 49 lumbar vertebrae taken from 16 cadavers was measured using dual energy QCT with advanced software and hardware capabilities, including an automated definition of the trabecular region of interest (ROI). The midvertebral part of each vertebral body was embedded in a polyester resin and, subsequently, an experimental ROI was cut out using a Scanjet image transmission procedure and a computer-assisted milling machine in order to mimic the ROI defined on QCT. After low temperature ashing, the experimental ROIs reduced to a bone powder were submitted to either nondestructive neutron activation analysis ( n = 49) or to flame atomic absorption spectrometry ( n = 45). BMC obtained with neutron activation analysis was closely related ( r = 0.896) to that derived from atomic absorption spectrometry, taken as the gold standard, with, however, a slight overestimation. BMC values measured by QCT were highly correlated with those assessed using the two reference methods, all correlation coefficients being > 0.841. The standard errors of the estimate ranged 47.4–58.9 mg calcium hydroxyapatite in the regressions of BMC obtained with reference methods against BMC assessed by single energy QCT, 47.1–51.9 in the regressions involving dual energy QCT. We conclude that the trabecular BMC of lumbar vertebrae can be accurately measured by QCT and that the superiority in accuracy of dual energy is moderate, which is possibly a characteristic of the preprocessing method.

Influence of temperature on QCT: implications for mineral densitometry.

Inaccuracies in quantitative CT (QCT) for vertebral bone mineral measurements may result from differences between the temperature of the vertebrae and the calibration standards. This study aims to quantify these effects by using scans of marrow-equivalent materials and computer simulation. The CT numbers of fat, water, gelatin suspension, and bone were measured within an anthropomorphic phantom at temperatures between -18 and 38 degrees C. A computer simulation was then performed using these materials to represent marrow fat, soft tissue, and bone in varying proportions over this temperature range. Postprocessing single and dual energy QCT calculations were then performed on the data acquired from the simulation. A change of 80 HU in the CT number of water on cooling from 38 to -18 degrees C was demonstrated. An increase of 95 HU in the CT number of fat occurred over the same temperature range. Dry cortical bone showed no change in CT number with temperature changes from 24 to -18 degrees C. In the computer simulation, the fat error associated with single energy QCT for trabecular bone mineral densitometry was 20% less for specimens at room temperature than at body temperature. In simulated frozen specimens, varying marrow fat/soft tissue composition had almost no effect on single energy QCT mineral densitometry. Dual energy QCT methods that use a fat-equivalent reference material were significantly influenced by the temperature of the specimen. The fat error of single energy QCT for mineral densitometry may have been underestimated in previous in vitro studies using vertebral specimens scanned at room temperature. In the simulation, the fat error diminished as the temperature of the specimen was reduced and was negligible when frozen. Fat-equivalent reference materials used for dual energy QCT in vivo should have similar X-ray-attenuating properties at room temperature to those of marrow fat at body temperature.

Mechanical and textural properties of pelvic trabecular bone

So far, virtually nothing is known about the mechanical properties of pelvic trabecular bone. In this study, several techniques have been used to establish some insight in these properties. Dual-energy quantitative computer tomography (DEQCT) was used to look at the distribution of bone densities throughout the pelvic bone and nondestructive mechanical testing was used to obtain Young's moduli and Poisson's ratios in three orthogonal directions for cubic speciments of pelvic trabecular bone. The same specimens were then used for stereological measurements to obtain volume fractions and the spatial orientations of the mean intercept lengths. The combined data on the mechanical tests and the stereological measurements made it possible to calculate Young's moduli and Poisson's ratios for the specimen's principal material axes. DEQCT showed that bone densities within a pelvic bone are significantly higher in the superior part of the acetabulum, extending to the sacroiliac joint area and, secondly, in the area of the pubic symphysis. Volume fractions found for the specimens did not exceed 20%. This may be considered rather low when compared to values reported in the literature for trabecular bone of femoral or tibial origin, but the values do lie in the same range as vertebral trabecular bone. With the volume fraction as its primary predictor, values of Young's moduli were also low. For most specimens these values were not higher than 100 MPa, with an occasional peak of 250 MPa. Looking at the ratio of the highest and lowest Young's modulus or at the components of the fabric tensor, it can be concluded that pelvic trabecular bone is not highly anisotropic. On an average, Poisson's ratio was found to be closer to 0.2 rather than 0.3, which is in accordance with other studies on Poisson's ratio of trabecular bone.

Aktueller Stand der Knochendensitometrie: II. Methodik der Quantitativen Computertomographie

Part II of this review presents several approaches of Quantitative Computed Tomography (QCT). Aside of absorptiometric methods QCT is the most widely used technique of bone densitometry. Primarily used as Single Energy QCT (SEQCT), Dual Energy QCT (DEQCT) has also been developed and applied. Based on a description of their physical and technical principles typical applications and fundamental limitations are being presented. In clinical routine QCT currently is only applied to measurements at the spine. It is, however, the only method that allows for an assessment of true volumetric bone mineral density. Hence, it is the only method available for a selective assessment of trabecular and cortical bone. Aside of the commonly used SEQCT approach several DEQCT techniques are being described and a comparative assessment of their respective physical principles is carried out.

Clinical evaluation of age-related changes of bone mineral content of cortical and trabecular bones by dual energy QCT

The potential capability of a new dual energy (DE) quantitative computed tomography (QCT) method, 4-equation 4-unknown method (DEQCT4E-4U), was evaluated for estimation of bone mineral density (BMD) as well as bone mineral content/cm width (BMC) in trabecular bone of lumbar vertebrae. Cortical BMD and BMC were also estimated by the single energy QCT method (SEQCT 80 kVp) and the age-related change of cortical BMD and the cortical BMC/trabecular BMC ratio was also studied. The result indicated that the estimation of the BMD combining analysis of BMC in trabecular bone obtained by the present method provides further knowledge concerning vertebral trabecular bone mineral status. The profiles of the age-related changes of the cortical BMD and cortical BMC/trabecular BMC ratio in females were markedly different from those in males. These results indicated that our method combining analysis of BMC (BMD) of cortical bone and trabecular bone was also found to provide valuable information to evaluate the vertebral bone mineral status.

Selection of fat-equivalent materials in postprocessing dual-energy quantitative CT.

Postprocessing dual-energy QCT is supposed to be able to predict the bone mineral more accurately than single-energy QCT. In addition, the fat content in the vertebral body can be determined. To this aim, some methods include fat-equivalent materials in the calibration device. However, the choice of an appropriate fat-equivalent material is difficult. To solve this selection problem, a method has been developed in which the x-ray interactions of tissue are characterized by three energy-independent parameters. For five different known constituents of anatomical fat, fat-equivalent materials are evaluated. It is shown that it is not possible to find one fat-equivalent material for all anatomical fat compositions. For this reason, the influence of a mismatch between the characterization parameters of anatomical fat compositions and fat-equivalent materials has been evaluated. It is shown that a mismatch in tissue characterization parameters can result in deviations of 10% in the bone mineral content and more than 300% in the estimated fat contents.

Influence of calibration materials in single- and dual-energy quantitative CT.

With single-energy (SE) quantitative computed tomography (CT), the density of bone mineral in the vertebral body can be estimated. With dual-energy (DE) quantitative CT, both bone-mineral density and fat content can be measured. The calibration device normally used contains materials mimicking trabecular bone, fat, and hematopoietic tissue. To evaluate the influence of different calibration materials on these estimates, theoretical CT numbers were calculated for the trabecular part of the vertebral body and for different calibration devices. Calibration devices were simulated; they contained either identical materials or various accepted tissue-mimicking materials. For all combinations, quantitative CT data were generated for the SE quantitative CT method and for two DE quantitative CT methods. Only one method provided accurate results under ideal circumstances. The selection of tissue-mimicking materials in the calibration devices is important for the interpretation of results of SE and DE quantitative CT. Errors of more than a factor 2 were found when different types of materials were used.