S Larson

Accuracies of the synthesized monochromatic CT numbers and effective atomic numbers obtained with a rapid kVp switching dual energy CT scanner

PURPOSE This study was performed to investigate the accuracies of the synthesized monochromatic images and effective atomic number maps obtained with the new GE Discovery CT750 HD CT scanner. METHODS A Gammex-RMI model 467 tissue characterization phantom and the CT number linearity section of a Phantom Laboratory Catphan 600 phantom were scanned using the dual energy (DE) feature on the GE CT750 HD scanner. Synthesized monochromatic images at various energies between 40 and 120 keV and effective atomic number (Z(eff)) maps were generated. Regions of interest were placed within these images/maps to measure the average monochromatic CT numbers and average Z(eff) of the materials within these phantoms. The true Z(eff) values were either supplied by the phantom manufacturer or computed using Mayneords equation. The linear attenuation coefficients for the true CT numbers were computed using the NIST XCOM program with the input of manufacturer supplied elemental compositions and densities. The effects of small variations in the assumed true densities of the materials were also investigated. Finally, the effect of body size on the accuracies of the synthesized monochromatic CT numbers was investigated using a custom lumbar section phantom with and without an external fat-mimicking ring. RESULTS Other than the Z(eff) of the simulated lung inserts in the tissue characterization phantom, which could not be measured by DECT, the Z(eff) values of all of the other materials in the tissue characterization and Catphan phantoms were accurate to 15%. The accuracies of the synthesized monochromatic CT numbers of the materials in both phantoms varied with energy and material. For the 40-120 keV range, RMS errors between the measured and true CT numbers in the Catphan are 8-25 HU when the true CT numbers were computed using the nominal plastic densities. These RMS errors improve to 3-12 HU for assumed true densities within the nominal density +/- 0.02 g/cc range. The RMS errors between the measured and true CT numbers of the tissue mimicking materials in the tissue characterization phantom over the 40-120 keV range varied from about 6 HU-248 HU and did not improve as dramatically with small changes in assumed true density. CONCLUSIONS Initial tests indicate that the Z(eff) values computed with DECT on this scanner are reasonably accurate; however, the synthesized monochromatic CT numbers can be very inaccurate, especially for dense tissue mimicking materials at low energies. Furthermore, the synthesized monochromatic CT numbers of materials still depend on the amount of the surrounding tissues especially at low keV, demonstrating that the numbers are not truly monochromatic. Further research is needed to develop DE methods that produce more accurate synthesized monochromatic CT numbers.

Digital breast tomosynthesis: studies of the effects of acquisition geometry on contrast-to-noise ratio and observer preference of low-contrast objects in breast phantom images

The effect of acquisition geometry in digital breast tomosynthesis was evaluated with studies of contrast-to-noise ratios (CNRs) and observer preference. Contrast-detail (CD) test objects in 5 cm thick phantoms with breast-like backgrounds were imaged. Twelve different angular acquisitions (average glandular dose for each ~1.1 mGy) were performed ranging from narrow angle 16° with 17 projection views (16d17p) to wide angle 64d17p. Focal slices of SART-reconstructed images of the CD arrays were selected for CNR computations and the reader preference study. For the latter, pairs of images obtained with different acquisition geometries were randomized and scored by 7 trained readers. The total scores for all images and readings for each acquisition geometry were compared as were the CNRs. In general, readers preferred images acquired with wide angle as opposed to narrow angle geometries. The mean percent preferred was highly correlated with tomosynthesis angle (R = 0.91). The highest scoring geometries were 60d21p (95%), 64d17p (80%), and 48d17p (72%); the lowest scoring were 16d17p (4%), 24d9p (17%) and 24d13p (33%). The measured CNRs for the various acquisitions showed much overlap but were overall highest for wide-angle acquisitions. Finally, the mean reader scores were well correlated with the mean CNRs (R = 0.83).

Community‐specific impacts of exotic earthworm invasions on soil carbon dynamics in a sandy temperate forest

Exotic earthworm introductions can alter above- and belowground properties of temperate forests, but the net impacts on forest soil carbon (C) dynamics are poorly understood. We used a mesocosm experiment to examine the impacts of earthworm species belonging to three different ecological groups (Lumbricus terrestris [anecic], Aporrectodea trapezoides [endogeic], and Eisenia fetida [epigeic]) on C distributions and storage in reconstructed soil profiles from a sandy temperate forest soil by measuring CO2 and dissolved organic carbon (DOC) losses, litter C incorporation into soil, and soil C storage with monospecific and species combinations as treatments. Soil CO2 loss was 30% greater from the Endogeic x Epigeic treatment than from controls (no earthworms) over the first 45 days; CO2 losses from monospecific treatments did not differ from controls. DOC losses were three orders of magnitude lower than CO2 losses, and were similar across earthworm community treatments. Communities with the anecic species accelerated litter C mass loss by 31-39% with differential mass loss of litter types (Acer rubrum > Populus grandidentata > Fagus grandifolia > Quercus rubra > or = Pinus strobus) indicative of leaf litter preference. Burrow system volume, continuity, and size distribution differed across earthworm treatments but did not affect cumulative CO2 or DOC losses. However, burrow system structure controlled vertical C redistribution by mediating the contributions of leaf litter to A-horizon C and N pools, as indicated by strong correlations between (1) subsurface vertical burrows made by anecic species, and accelerated leaf litter mass losses (with the exception of P. strobus); and (2) dense burrow networks in the A-horizon and the C and N properties of these pools. Final soil C storage was slightly lower in earthworm treatments, indicating that increased leaf litter C inputs into soil were more than offset by losses as CO2 and DOC across earthworm community treatments.

Assessment of calibration methods for estimating bone mineral densities in trauma patients with quantitative CT: an anthropomorphic phantom study.

RATIONALE AND OBJECTIVES Osteoporosis may contribute to the increased morbidity and mortality of elderly persons involved in motor vehicle accidents. Such patients commonly undergo whole-body computed tomographic (CT) studies that may be analyzed with quantitative CT. Various quantitative CT calibration techniques were investigated for use with patients who have suffered trauma, who are typically scanned on a backboard. MATERIALS AND METHODS Lumbar simulator phantoms were used to simulate small and large patients. Vertebral spongiosa inserts with a wide range of bone and fat compositions were placed in the phantoms, and their bone mineral densities (BMDs) were measured by using calibration lines derived from the CT numbers of a calibration standard. Four calibration techniques were tested. In three the lumbar simulator and the calibration standard were scanned simultaneously, with the standard placed beneath the backboard (method 1), on top of the backboard adjacent to the lumbar simulator (method 2), or on top of the abdomen region of the lumbar simulator (method 3). The fourth technique employed a single calibration line derived from a separate scan of the calibration standard beneath the small lumbar simulator without the backboard, with correction for patient body size. RESULTS The best overall results were obtained with the single calibration line method. The root mean square errors of the BMD values were 2.9-18.4, 2.5-7.5, 2.5-14.9, and 0.3-2.8 mg/cm3 for methods 1, 2, 3, and 4, respectively (ranges represent variations in the errors of the measured BMDs of the inserts due to changes in scanner table height and lumbar simulator phantom size). CONCLUSION The single calibration line method is an accurate means of measuring BMD in trauma patients.

Quantitative CT of lung nodules: Dependence of calibration on patient body size, anatomic region, and calibration nodule size for single- and dual-energy techniques

Calcium concentration may be a useful feature for distinguishing benign from malignant lung nodules in computer-aided diagnosis. The calcium concentration can be estimated from the measured CT number of the nodule and a CT number vs calcium concentration calibration line that is derived from CT scans of two or more calcium reference standards. To account for CT number nonuniformity in the reconstruction field, such calibration lines may be obtained at multiple locations within lung regions in an anthropomorphic phantom. The authors performed a study to investigate the effects of patient body size, anatomic region, and calibration nodule size on the derived calibration lines at ten lung region positions using both single energy (SE) and dual energy (DE) CT techniques. Simulated spherical lung nodules of two concentrations (50 and 100 mg/cc CaCO3) were employed. Nodules of three different diameters (4.8, 9.5, and 16 mm) were scanned in a simulated thorax section representing the middle of the chest with large lung regions. The 4.8 and 9.5 mm nodules were also scanned in a section representing the upper chest with smaller lung regions. Fat rings were added to the peripheries of the phantoms to simulate larger patients. Scans were acquired on a GE-VCT scanner at 80, 120, and 140 kVp and were repeated three times for each condition. The average absolute CT number separations between the calibration lines were computed. In addition, under- or overestimates were determined when the calibration lines for one condition (e.g., small patient) were used to estimate the CaCO3 concentrations of nodules for a different condition (e.g., large patient). The authors demonstrated that, in general, DE is a more accurate method for estimating the calcium contents of lung nodules. The DE calibration lines within the lung field were less affected by patient body size, calibration nodule size, and nodule position than the SE calibration lines. Under- or overestimates in CaCO3 concentrations of nodules were also in general smaller in quantity with DE than with SE. However, because the slopes of the calibration lines for DE were about one-half the slopes for SE, the relative improvement in the concentration estimates for DE as compared to SE was about one-half the relative improvement in the separation between the calibration lines. Results in the middle of the chest thorax section with large lungs were nearly completely consistent with the above generalization. On the other hand, results in the upper-chest thorax section with smaller lungs and greater amounts of muscle and bone were mixed. A repeat of the entire study in the upper thorax section yielded similar mixed results. Most of the inconsistencies occurred for the 4.8 mm nodules and may be attributed to errors caused by beam hardening, volume averaging, and insufficient sampling. Targeted, higher resolution reconstructions of the smaller nodules, application of high atomic number filters to the high energy x-ray beam for improved spectral separation, and other future developments in DECT may alleviate these problems and further substantiate the superior accuracy of DECT in quantifying the calcium concentrations of lung nodules.

Comparison of the CT scatter fractions provided in NCRP report no. 147 to scanner-specific scatter fractions and the consequences for calculated barrier thickness

The new NCRP Report No. 147 includes methodology to determine x-ray protective shielding for CT scanner rooms. This methodology assumes fixed values of the scatter fraction per centimeter (&kgr;) for the peripheral axis of the head and body CT phantoms. An investigation was performed to determine &kgr; for different makes and models of CT scanner and examine the consequences of the differences between these and the fixed NCRP values on a typical shielding calculation. &kgr; values were calculated using an equation for the scattered air kerma at 1 m from NCRP 147 (Kermascatter = &kgr; × ScanLength × CTDI100 × pitch−1) and using scattered air kerma data provided by the manufacturers and measured CTDI100 (periphery) values. Typical barrier calculations, following NCRP 147 methodology, were performed for each CT scanner using the fixed &kgr; values and, separately, using the calculated scanner-specific values. Ten CT scanner models from three manufacturers were investigated. The calculated scanner-specific &kgr; values varied from the NCRP fixed values by as much as 82%. However, when these &kgr; values were used in typical barrier calculations, the final shielding requirements using the NCRP fixed values were 0.5 to 13% less than those using the scanner specific values. It is likely that such small underestimates in the shielding requirement due to using the NCRP fixed &kgr; values would be more than compensated by the conservative assumptions that are incorporated in a typical barrier calculation.

WE‐A‐103‐01: Ultrasound

Ultrasound quality control (QC) testing is often over-looked in ultrasound imaging practice because there are few regulations requiring regular QC of these systems. Many sites rely on the manufacturers preventive maintenance program to ensure these systems are functioning optimally. However, the regulatory climate is changing, placing emphasis on safe and effective imaging practice. Regular QC testing of ultrasound equipment is a valuable tool that helps ensure proper function and good image quality in ultrasound imaging. This two-hour session is organized in two parts. The first hour will cover the basic concepts of ultrasound performance measurement, including the tests that are recommended for acceptance and annual testing as well as routine QC, the rationale for these tests, and specific testing methods. It will also include what the physicist needs to know about ultrasound accreditation. The next hour includes two presentations related to quantitative assessment of ultrasound QC. First presented is the work of the AAPM working group on Quantitative B-mode Ultrasound QC Test Development. This group has designed software intended to aid in the evaluation of transducer element condition. The second presentation will demonstrate use of a user-friendly automation software, developed at the University of Wisconsin, for periodic rapid quality assurance (QA or QC) using the tissue-mimicking conical window phantom described last year. The phantom has been designed specifically for determination of three basic parameters. A method for organizing an electronic filing system for paperless recording of results will also be described. LEARNING OBJECTIVES 1. Understand what constitutes an effective ultrasound QC program. 2. Achieve familiarity with common ultrasound QC test methods and phantoms. 3. Understand what the physicist needs to know about ultrasound program accreditation. 4. Learn about a public software tool being developed by the AAPM Ultrasound Subcommittee to detect and assess transducer uniformity artifacts. 5. Learn about a user-friendly automation software tool to address the most-recommended ultrasound QC tests.

TH‐E‐217BCD‐10: The Effect of Model Based Iterative Reconstruction (GE‐VEO) on the CT Numbers and Noise of Both Small Lung Nodules and Large Homogeneous (heart and Spongiosa) Regions in an Anthropomorphic Chest Phantom

Purpose: To investigate the effect of Model‐Based Iterative Reconstruction (MBIR) on CT#s and noise (standard deviation, SD) of both nodules and homogeneous regions (heart and vertebra spongiosa) in an anthropomorphic chest phantom. Methods: A Lungman phantom (Kyoto‐Kagaku Co. Japan) was imaged with a GE Discovery CT750 HD CT scanner using our clinical lung nodule protocol (120kVp,80mA,0.5s,1.375:1pitch,1.25mm‐slice,0.625mm‐interval,40%ASIR) and lower dose protocols (20mA and 10mA), different %ASIR (0%,70%), and MBIR (VEO). A variety of 5mm‐diameter simulated nodules (−630HU,100HU,water equivalent, etc.) were placed in the vasculature insert in the phantom. CT#s were measured within 7.9mm2 ROIs at the centers of the nodules and within larger ROIs in the heart and vertebra regions. Results: For nodules, on average, the CT#s are 18HU, 25HU, and 30HU greater for VEO than for 0%, 40% and 70%ASIR, respectively. The ratios of the SDs for VEO to those for 0%, 40% and 70%ASIR are 0.60, 0.77, and 0.93, respectively. For the heart region, the CT#s are independent of %ASIR at a given mA, and the CT#s for VEO are smaller than those for all ASIR blends by 10HU at 10mA, 3HU at 20mA, and 1HU at 80mA. The ratios of the SDs for VEO to those for ASIR blends increase with %ASIR and mA ranging from 0.14 for 0%ASIR/10 mA to 0.47 for 70%ASIR/80mA. Corresponding values in the spongiosa region show similar trends, with CT# differences of 35HU, 12HU, and 2HU, and SD ratios ranging from 0.13 to 0.40. Conclusions: The effect of MBIR on mean CT#s and noise depends on object size. The CT#s are greater for MBIR than ASIR blends in nodules; whereas, they are smaller for MBIR in large homogeneous regions. The noise is almost always lower for MBIR, but the degree of smoothing is much greater in large homogeneous regions than in small nodules

MO‐D‐220‐10: Automatic Quality Control Processing for Detection of Elements Dropout in Ultrasound Transducers

Purpose: To develop a simple and reliable method of detection of lost or reduced sensitivity elements in diagnostic ultrasound arrays through analysis of the systematic features evident in appropriately acquired cineloops Methods: An algorithm was developed to detect the systematic features produced by element/channel signal loss evident in a cineloop acquired with a random background signal: A temporal median of the cineloop is produced followed by construction of a profile defined as the mean over depth [column‐wise mean] within a horizontal strip of the median image. A modified anisotropic diffusion filter is applied to the profile to isolate outlying features from characteristic noise, and simple statistical methods are then used to identify regions of the profile which exhibit characteristics of reduced sensitivity elements such as well‐defined, symmetric troughs. With an element occluded by fishing line to simulate dropout, cineloops were acquired with a GE‐Logiq 9 scanner either by rapidly scanning a transducer over a tissue mimicking phantom or by placing the transducer in a liquid phantom composed of an agitated cornstarch and water suspension. Results: Analysis with the algorithm clearly revealed the occluded section of the array as well as previously unnoticed reduced‐sensitivity elements. The efficacy of a simple and inexpensive method of detecting lost or reduced sensitivity elements of diagnostic ultrasound arrays was demonstrated in a confined dataset. The algorithm has been developed into an ImageJ plugin which presently handles uncompressed DICOM data containing rectangular scan regions; this should soon be made available for use by the medicalultrasound community after testing by Ultrasound Subcommittee members. Supported in part by the Ultrasound Subcommittee of the AAPM Science Council andNIH Grant CA91713

TH‐E‐110‐11: The Effect of Region‐Of‐Interest Z‐Axis Collimation on the Image Quality of a Commercial Cone Beam CT Imaging System

Purpose: A study was performed to determine the degree to which region‐ of‐interest z‐axis collimation can improve the image quality of a cone‐beam CT system. Methods: The linearity and low and high‐contrast resolution modules of a 20‐cm diameter Catphan 600 phantom were imaged with a Siemens Zeego Angiography system using three “DynaCT” cone beam CT modes. These included (a) DR‐H Head (20‐sec), (b) DSA‐H ( 8‐sec), and (c) DR‐Body( 8‐sec). All modes employ automatic exposure control (AEC), which can vary the kVp, mA and pulse width throughout the scan. Image quality was compared in 5‐mm thick slices obtained with full‐field (19‐cm) collimation and minimal‐width (2.3‐cm) collimation. Body scans (c) were compared with and without saline bags added at the periphery of the Catphan to create a larger phantom. Results: The contrast‐to‐noise‐ratios (CNRs) of the 15‐mm diameter, nominal 1% (10HU) disk in the Catphan when imaged with full‐field collimation were 1.28, 1.93, and 1.23 for DynaCT modes a, b, and c, respectively. With 2.3‐cm collimation, the corresponding CNRs improved to 2.13, 2.46, and 1.45. The CNRs for images of the large phantom degraded to 0.51 for full‐field and 0.55 for 2.3‐ cm collimation. Plots of measured CT numbers of plastics in the linearity section as a function of mass density were linear (R‐squared >0.99) in all cases, with slopes that varied from 761 to 1006 HU/(g/cc) depending upon collimation, phantom size and the AEC selected kVp. Spatial resolution was 12 lp/cm for mode (a), and it was 8–;9 lp/cm for all other modes including scans with the large phantom. Conclusions: Collimation significantly improves CNR for a small patient/phantom, but is less effective for large patients/phantoms. Collimation and phantom size have minimal effect on spatial resolution. They affect the slope and intercept but not the degree of linearity of CT number with density.