Mitchell M. Goodsitt

Evaluation of a new set of calibration standards for the measurement of fat content via DPA and DXA

A simulation study was performed to evaluate a new set of calibration standards for estimating the fat content of the body via dual-photon absorptiometry (DPA) and dual-energy x-ray absorptiometry (DXA). The standards, proposed by Nord and Payne [presented at the 2nd meeting of The Bath Conference on Bone Mineral Measurement (1990)] consist of stearic acid (100% fat) and 0.6% NaCl in water (100% lean). They were compared with other standards consisting of average composition adipose/muscle tissues and fatty adipose/lean muscle tissues. Source and detector properties of a Gd-153 DPA system and three commercial DXA systems were modeled. For each system and calibration set, rms errors in the calculated fat contents of simulated tissues having fat mass percentages that ranged from about 4%-44% and thicknesses that ranged from 5-20 cm were determined. Beam hardening errors for the systems were evaluated as was a calibration technique employed by one of the manufacturers to correct for such errors. In general, the smallest rms errors (2% or less when the calibration standards and tissues were of equal thickness) were obtained with the average adipose/muscle standards. Equivalent results were obtained with standards consisting of stearic acid and 0.8% NaCl. The latter is a higher salt content than proposed by Nord and Payne and results from differences in the x-ray attenuation coefficients that were employed in calculating the fat equivalence of water. Other, more convenient standards, such as lucite and water may be employed by using appropriate fat equivalences (approximately 69% for lucite and approximately 10% for water). Beam hardening errors for the DXA systems are considerable, and the simulated correction technique was shown to be effective.

Effect of CT scanning parameters on volumetric measurements of pulmonary nodules by 3D active contour segmentation: a phantom study

The purpose of this study is to investigate the effects of CT scanning and reconstruction parameters on automated segmentation and volumetric measurements of nodules in CT images. Phantom nodules of known sizes were used so that segmentation accuracy could be quantified in comparison to ground-truth volumes. Spherical nodules having 4.8, 9.5 and 16 mm diameters and 50 and 100 mg cc(-1) calcium contents were embedded in lung-tissue-simulating foam which was inserted in the thoracic cavity of a chest section phantom. CT scans of the phantom were acquired with a 16-slice scanner at various tube currents, pitches, fields-of-view and slice thicknesses. Scans were also taken using identical techniques either within the same day or five months apart for study of reproducibility. The phantom nodules were segmented with a three-dimensional active contour (3DAC) model that we previously developed for use on patient nodules. The percentage volume errors relative to the ground-truth volumes were estimated under the various imaging conditions. There was no statistically significant difference in volume error for repeated CT scans or scans taken with techniques where only pitch, field of view, or tube current (mA) were changed. However, the slice thickness significantly (p < 0.05) affected the volume error. Therefore, to evaluate nodule growth, consistent imaging conditions and high resolution should be used for acquisition of the serial CT scans, especially for smaller nodules. Understanding the effects of scanning and reconstruction parameters on volume measurements by 3DAC allows better interpretation of data and assessment of growth. Tracking nodule growth with computerized segmentation methods would reduce inter- and intraobserver variabilities.

Two postprocessing CT techniques for determining the composition of trabecular bone.

Two dual-energy CT techniques have been developed to analyze the mineral and fat content of trabecular bone. Both are postprocessing techniques that employ calibration standards. Experiments were performed to test these techniques against conventional single-energy techniques and two other dual-energy techniques. As expected, all of the dual-energy methods estimate the mineral content more accurately when fat is present. In contrast to the other dual-energy methods, the new methods described in this article are unique because they make a separate estimate of the fat content of the bone. The results of preliminary tests of these techniques in estimating fat content have been encouraging. Although not exact, the estimates show the correct trend in increasing proportionately as the fat content increases. Possible applications of the techniques in the study of osteoporosis and other bone diseases are described.

THE COMPOSITION OF BONE MARROW FOR A DUAL-ENERGY QUANTITATIVE COMPUTED TOMOGRAPHY TECHNIQUE : A CADAVER AND COMPUTER SIMULATION STUDY

RATIONALE AND OBJECTIVES.The authors have been developing a dual-energy quantitative computed tomography (DEQCT) 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. METHODS.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. RESULTS AND CONCLUSIONS.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.

Conversion relations for quantitative CT bone mineral densities measured with solid and liquid calibration standards.

A vast data base exists for QCT measurements of bone mineral density (BMD) referenced to K2HPO4 in water (liquid) standards. To effectively utilize the more stable hydroxyapatite in water-equivalent plastic (solid) standards that have recently been introduced, it will be necessary to derive conversion relations. A study was performed to investigate the dependence of these relations upon x-ray tube voltage, marrow composition, and patient body size. Test objects included five diverse composition vertebral marrow inserts within three different size lumbar simulators and an L1 vertebra within a Humanoid phantom. The calibration standards were manufactured by Image Analysis, and all data were acquired with a GE 9800 CT scanner operated at 80 kVp and 140 kVp. Least square fits to corresponding liquid versus solid referenced BMD measurements of the inserts all had rs > 0.999. SEEs, < 2 mg/ml, and intercepts of approximately 0. The slopes (BMDK2HPO4/BMDhydroxyapatite) for the various body sizes were all about the same with values of 0.86, 0.81, and 0.96 to 1.02 for the single-energy@80 kVp, single-energy@140 kVp, and dual-energy measurements, respectively. Corresponding ratios for the Humanoid vertebra were 0.86, 0.82, and 0.96. The conversion relations were essentially independent of marrow composition and body size but did depend upon kVp. Finally, although the solid standards are more stable, they may still exhibit problems, and these are discussed.

Quantitative computed tomography scanning for measurement of bone and bone marrow fat content. A comparison of single- and dual-energy techniques using a solid synthetic phantom.

Quantitative CT (QCT) has become a popular method for estimating bone mineral content. In addition, QCT can be used to estimate the fat content of trabecular bone. Although the latter has received little attention, it may prove to be clinically significant. Using a set of custom-built, tissue-mimicking plastic inserts in an anthropomorphic phantom, we tested a variety of methods for estimating mineral and fat content. We also investigated the influence of patient size, reconstruction circle size, and reference phantom choice on the accuracy of the results. Best estimates were obtained when there was a match between patient and reconstruction circle size. Single-energy methods yielded the best estimates of mineral content for inserts that did not contain fat, and dual-energy methods yielded the best estimates for inserts that contained fat. A dual-energy method that we developed was best in estimating the mineral and fat content of the latter inserts. We found that an external calibration reference phantom containing aqueous solutions of K2HPO4 could be used satisfactorily to estimate the mineral content of trabecular bone mimicking inserts; however, more representative materials must be used for accurate estimates of fat content.

Digital Breast Tomosynthesis: Observer Performance of Clustered Microcalcification Detection on Breast Phantom Images Acquired with an Experimental System Using Variable Scan Angles, Angular Increments, and Number of Projection Views

PURPOSE To investigate the dependence of microcalcification cluster detectability on tomographic scan angle, angular increment, and number of projection views acquired at digital breast tomosynthesis ( DBT digital breast tomosynthesis ). MATERIALS AND METHODS A prototype DBT digital breast tomosynthesis system operated in step-and-shoot mode was used to image breast phantoms. Four 5-cm-thick phantoms embedded with 81 simulated microcalcification clusters of three speck sizes (subtle, medium, and obvious) were imaged by using a rhodium target and rhodium filter with 29 kV, 50 mAs, and seven acquisition protocols. Fixed angular increments were used in four protocols (denoted as scan angle, angular increment, and number of projection views, respectively: 16°, 1°, and 17; 24°, 3°, and nine; 30°, 3°, and 11; and 60°, 3°, and 21), and variable increments were used in three (40°, variable, and 13; 40°, variable, and 15; and 60°, variable, and 21). The reconstructed DBT digital breast tomosynthesis images were interpreted by six radiologists who located the microcalcification clusters and rated their conspicuity. RESULTS The mean sensitivity for detection of subtle clusters ranged from 80% (22.5 of 28) to 96% (26.8 of 28) for the seven DBT digital breast tomosynthesis protocols; the highest sensitivity was achieved with the 16°, 1°, and 17 protocol (96%), but the difference was significant only for the 60°, 3°, and 21 protocol (80%, P < .002) and did not reach significance for the other five protocols (P = .01-.15). The mean sensitivity for detection of medium and obvious clusters ranged from 97% (28.2 of 29) to 100% (24 of 24), but the differences fell short of significance (P = .08 to >.99). The conspicuity of subtle and medium clusters with the 16°, 1°, and 17 protocol was rated higher than those with other protocols; the differences were significant for subtle clusters with the 24°, 3°, and nine protocol and for medium clusters with 24°, 3°, and nine; 30°, 3°, and 11; 60°, 3° and 21; and 60°, variable, and 21 protocols (P < .002). CONCLUSION With imaging that did not include x-ray source motion or patient motion during acquisition of the projection views, narrow-angle DBT digital breast tomosynthesis provided higher sensitivity and conspicuity than wide-angle DBT digital breast tomosynthesis for subtle microcalcification clusters.

Stereomammography: Evaluation of depth perception using a virtual 3D cursor

We are evaluating the usefulness of stereomammography in improving breast cancer diagnosis. One area that we are investigating is whether the improved depth perception associated with stereomammography might be significantly enhanced with the use of a virtual 3D cursor. A study was performed to evaluate the accuracy of absolute depth measurements made in stereomammograms with such a cursor. A biopsy unit was used to produce digital stereo images of a phantom containing 50 low contrast fibrils (0.5 mm diam monofilaments) at depths ranging from 1 to 11 mm, with a minimum spacing of 2 mm. Half of the fibrils were oriented perpendicular (vertical) and half parallel (horizontal) to the stereo shift direction. The depth and orientation of each fibril were randomized, and the horizontal and vertical fibrils crossed, simulating overlapping structures in a breast image. Left and right eye images were generated by shifting the x-ray tube from +2.5 degrees to -2.5 degrees relative to the image receptor. Three observers viewed these images on a computer display with stereo glasses and adjusted the position of a cross-shaped virtual cursor to best match the perceived location of each fibril. The x, y, and z positions of the cursor were indicated on the display. The z (depth) coordinate was separately calibrated using known positions of fibrils in the phantom. The observers analyzed images of two configurations of the phantom. Thus, each observer made 50 vertical filament depth measurements and 50 horizontal filament depth measurements. These measurements were compared with the true depths. The correlation coefficients between the measured and true depths of the vertically oriented fibrils for the three observers were 0.99, 0.97, and 0.89 with standard errors of the estimates of 0.39 mm, 0.83 mm, and 1.33 mm, respectively. Corresponding values for the horizontally oriented fibrils were 0.91, 0.28, and 0.08, and 1.87 mm, 4.19 mm, and 3.13 mm. All observers could estimate the absolute depths of vertically oriented objects fairly accurately in digital stereomammograms; however, only one observer was able to accurately estimate the depths of horizontally oriented objects. This may relate to different aptitudes for stereoscopic visualization. The orientations of most objects in actual mammograms are combinations of horizontal and vertical. Further studies are planned to evaluate absolute depth measurements of fibrils oriented at various intermediate angles and of objects of different shapes. The effects of the shape and contrast of the virtual cursor and the stereo shift angle on the accuracy of the depth measurements will also be investigated.

Precision in quantitative CT: Impact of x-ray dose and matrix size

A study was performed to determine whether recommended technique factors for postprocessing dual-energy (DE) quantitative computed tomography are optimum in terms of precision and x-ray dose. In particular, possible dose reduction as a result of an upgrade of the CT scanner to more efficient detectors was explored. Series of images of an anthropomorphic phantom containing a human vertebra, a tissue-simulating lumbar simulator with various marrow inserts, and a polyethylene cylinder were generated. Recommended DE x-ray technique factors as well as factors resulting in about two times, one-half, and one-fourth the x-ray dose were employed. The effects of reconstruction with different matrix sizes was studied. Standard deviations of the CT numbers within regions of interest in individual images (noise) and standard deviations of mean CT numbers, single-energy (SE), and DE measurements for series of images (reproducibilities) were computed. It was found that the low-energy component of the DE technique was optimum, but the high-energy component could be reduced by a factor of 2 with negligible loss in precision. This translates into a dose reduction of 36% relative to the recommended DE technique. Vertebral inhomogeneities were responsible for more than 65% of the standard deviations in individual images of the vertebra even at the lowest doses. For all of the techniques, the noise in images of all objects decreased as the x-ray dose increased and as the matrix size decreased. Reproducibility of mean values, however, did not necessarily improve, and aberrant results such as improvement in reproducibility with a reduction in dose were sometimes observed. It is hypothesized that this may be due to variations in the actual kVp for each image in a series.

A new set of calibration standards for estimating the fat and mineral content of vertebrae via dual energy QCT

A new set of calibration standards has been developed for implementing a dual-energy (DE) quantitative CT technique for estimating the fat and bone content of vertebrae. The QCT technique is based upon a three-component model of bone and utilizes calibration materials that mimic those components in their X-ray attenuation properties. The three components we chose to simulate are bone (mineral plus collagen), fat and a fat-free red marrow. This choice was predicated upon our desire to employ materials that would facilitate later experimental verification of the method. The calibration standards and a set of test samples were manufactured of tissue-simulating epoxy resins. They were employed in studies of the accuracy (consistency) and precision of the technique and in a study of 21 normal postmenopausal women. Estimates of the bone and fat content of the test samples were consistent with the manufacturers specifications to within 13 mg/ml and 7 vol%, respectively. Long-term reproducibility (coefficient of variation) for both quantities was about 3%. The average bone content of the T12-L3 vertebrae of the human subjects was 262 +/- 32 mg/ml (152 +/- 18 mg/ml calcium hydroxyapatite or mineral) and the average fat content was 63 +/- 8%. Conventional single energy (SE) QCT measurements of these vertebrae were about 23% less than the DE mineral measurements, which is consistent with the differences between SEQCT and ash content that others have determined via chemical analysis. The DE fat content is, in absolute terms, about 15% greater than values reported in the literature and may be due to an error in the assumed composition of red marrow. The true accuracy of the bone and fat estimates is to be determined in a planned human vertebral specimen study.