Jack W. Lambert

Techniques and Tactics for Optimizing CT Dose in Adults and Children: State of the Art and Future Advances

With growing concern over radiation exposure from CT, dose reduction and optimization have become important considerations. Many protocol factors and CT technologies influence this dose reduction effort, and as such, users should maintain a working knowledge of developments in the field. Individual patient factors and scanner-specific details also require care and expertise, which are vital to the success of any dose reduction effort. The authors review the content of the Virtual Symposium on Radiation Safety in Computed Tomography (University of California Dose Optimization and Standardization Endeavor), specifically that pertaining to the more practical aspects of dose optimization. These range from prescan tips to postscan factors, as well as protocol definition itself. Topics discussed include localizer radiograph acquisition, tube current modulation, reconstruction methods, and pediatric considerations, with the content biased toward a CT technologist or protocol manager. Near-term innovations, including new iterative reconstruction methods, tube potential modulation, and dual-energy CT, are presented, and their capability for dose reduction is briefly discussed.

CT Image Contrast of High-Z Elements: Phantom Imaging Studies and Clinical Implications

PURPOSE To quantify the computed tomographic (CT) image contrast produced by potentially useful contrast material elements in clinically relevant imaging conditions. MATERIALS AND METHODS Equal mass concentrations (grams of active element per milliliter of solution) of seven radiodense elements, including iodine, barium, gadolinium, tantalum, ytterbium, gold, and bismuth, were formulated as compounds in aqueous solutions. The compounds were chosen such that the active element dominated the x-ray attenuation of the solution. The solutions were imaged within a modified 32-cm CT dose index phantom at 80, 100, 120, and 140 kVp at CT. To simulate larger body sizes, 0.2-, 0.5-, and 1.0-mm-thick copper filters were applied. CT image contrast was measured and corrected for measured concentrations and presence of chlorine in some compounds. RESULTS Each element tested provided higher image contrast than iodine at some tube potential levels. Over the range of tube potentials that are clinically practical for average-sized and larger adults-that is, 100 kVp and higher-barium, gadolinium, ytterbium, and tantalum provided consistently increased image contrast compared with iodine, respectively demonstrating 39%, 56%, 34%, and 24% increases at 100 kVp; 39%, 66%, 53%, and 46% increases at 120 kVp; and 40%, 72%, 65%, and 60% increases at 140 kVp, with no added x-ray filter. CONCLUSION The consistently high image contrast produced with 100-140 kVp by tantalum compared with bismuth and iodine at equal mass concentration suggests that tantalum could potentially be favorable for use as a clinical CT contrast agent.

Opportunities for new CT contrast agents to maximize the diagnostic potential of emerging spectral CT technologies.

The introduction of spectral CT imaging in the form of fast clinical dual-energy CT enabled contrast material to be differentiated from other radiodense materials, improved lesion detection in contrast-enhanced scans, and changed the way that existing iodine and barium contrast materials are used in clinical practice. More profoundly, spectral CT can differentiate between individual contrast materials that have different reporter elements such that high-resolution CT imaging of multiple contrast agents can be obtained in a single pass of the CT scanner. These spectral CT capabilities would be even more impactful with the development of contrast materials designed to complement the existing clinical iodine- and barium-based agents. New biocompatible high-atomic number contrast materials with different biodistribution and X-ray attenuation properties than existing agents will expand the diagnostic power of spectral CT imaging without penalties in radiation dose or scan time.

Pairwise comparison versus Likert scale for biomedical image assessment.

OBJECTIVE Biomedical imaging research relies heavily on the subjective and semi-quantitative reader analysis of images. Current methods are limited by interreader variability and fixed upper and lower limits. The purpose of this study was to compare the performance of two assessment methods, pairwise comparison and Likert scale, for improved analysis of biomedical images. MATERIALS AND METHODS A set of 10 images with varying degrees of image sharpness was created by digitally blurring a normal clinical chest radiograph. Readers assessed the degree of image sharpness using two different methods: pairwise comparison and a 10-point Likert scale. Reader agreement with actual chest radiograph sharpness was calculated for each method by use of the Lin concordance correlation coefficient (CCC). RESULTS Reader accuracy was highest for pairwise comparison (CCC, 1.0) and ranked Likert (CCC, 0.99) scores and lowest for nonranked Likert scores (CCC, 0.83). Accuracy improved slightly when readers repeated their assessments (CCC, 0.87) or had reference images available (CCC, 0.91). CONCLUSION Pairwise comparison and ranked Likert scores yield more accurate reader assessments than nonranked Likert scores.

Complementary contrast media for metal artifact reduction in dual-energy computed tomography

Abstract. Metal artifacts have been a problem associated with computed tomography (CT) since its introduction. Recent techniques to mitigate this problem have included utilization of high-energy (keV) virtual monochromatic spectral (VMS) images, produced via dual-energy CT (DECT). A problem with these high-keV images is that contrast enhancement provided by all commercially available contrast media is severely reduced. Contrast agents based on higher atomic number elements can maintain contrast at the higher energy levels where artifacts are reduced. This study evaluated three such candidate elements: bismuth, tantalum, and tungsten, as well as two conventional contrast elements: iodine and barium. A water-based phantom with vials containing these five elements in solution, as well as different artifact-producing metal structures, was scanned with a DECT scanner capable of rapid operating voltage switching. In the VMS datasets, substantial reductions in the contrast were observed for iodine and barium, which suffered from contrast reductions of 97% and 91%, respectively, at 140 versus 40 keV. In comparison under the same conditions, the candidate agents demonstrated contrast enhancement reductions of only 20%, 29%, and 32% for tungsten, tantalum, and bismuth, respectively. At 140 versus 40 keV, metal artifact severity was reduced by 57% to 85% depending on the phantom configuration.

Model of structural damage to carbon fibre composites due to thermo-electric effects of lightning strikes

The study concentrates on structural damage caused by the thermal effects of lightning strike to carbon fibre composites (CFC). The main objectives are to construct a model of the major physical effects involved, and to understand the correlation between the damage mechanisms and the damage witnessed in modern CFC. Model verification will be done by experimental decoupling of damage mechanisms, e.g. the real Joule heating from a lightning strike is replaced by a high power laser beam acting on composite surface. Outcomes from this study can then be used for further investigations and optimisation of lightning strike protection methods.

Shadow imaging of GEO satellites

Analyses show that astronomical occultation methods may be used to determine the silhouettes of satellites at geostationary distances, a result few other techniques can achieve. Specifically, an array of photon-counting detectors is positioned in the path of the target shadow from one star. Reduction of the received star intensity vs. time can yield silhouette resolution of less than a meter. In this paper, we address the critical issues of a) the limited density of useable stars, b) positioning of the detector array into the path of the shadow, and c) undoing the effects of diffraction. A conceptual design for an imaging station is presented.

Accessory spleen versus lymph node: Value of iodine quantification with dual-energy computed tomography

OBJECTIVES To evaluate whether iodine quantification with Dual-Energy Computed Tomography (DECT) improves the differentiation of accessory spleens (AS) from lymph nodes (LN) compared to CT number measurements. METHODS Abdominal DECT images of 75 patients with either AS (n=35) or LN (n=48) (benign entity) were retrospectively evaluated. Hounsfield Units (HU) and iodine concentrations of AS, LN and the main spleen were measured. Receiver operating characteristics (ROC) were performed to calculate an optimal threshold for distinguishing AS from LN. Sensitivity, specificity, and accuracy were calculated for distinguishing AS from LN by iodine concentration measurements. RESULTS Mean CT numbers and iodine concentrations were higher for AS (148±29 HU and 48.2±11×100μg/cc) than LN (83±19 HU and 31.5±6.2×100μg/cc, respectively, P<0.001 each). Mean CT numbers were lower for AS compared to the main spleen (161±29HU, P<0.01), whereas mean iodine concentrations (47.7±10×100μg/cc) were not significantly different (P=0.095). An iodine concentration greater than 38×100μg/cc suggested AS with a sensitivity, specificity and accuracy of 91%, 85%, and 88%, respectively (Area under ROC curve 0.941). CONCLUSIONS Iodine measurements might contribute to the differentiation of AS from LN. Iodine concentrations similar to that of the main spleen may help to confirm the diagnosis of AS.

Pelvic Beam-Hardening Artifacts in Dual-Energy CT Image Reconstructions: Occurrence and Impact on Image Quality.

OBJECTIVE The purpose of this study was to describe the frequency and appearance of beam-hardening artifacts on rapid-kilovoltage-switching dual-energy CT (DECT) image reconstructions of the pelvis. MATERIALS AND METHODS Monochromatic (70, 52, and 120 keV) and material decomposition CT images (iodine-water and water-iodine) from consecutive pelvic rapid-kilovoltage-switching DECT scans were retrospectively evaluated. We recorded the presence, type (high versus low attenuation), and severity of beam-hardening artifacts (Likert scale from 0, barely seen, to 4, severe), clarity of anatomic delineation (Likert scale from 0, unimpaired, to 4, severely impaired) and SD of CT numbers, iodine and water concentrations, and gray-scale values for artifact-affected regions and corresponding unaffected reference tissue. A pelvic phantom was scanned and evaluated in a similar manner. Wilcoxon signed rank and paired t tests were used to compare results between the image reconstructions. RESULTS Beam-hardening artifacts were seen in all image reconstructions in all 41 patients (22 men, 19 women; mean age, 57 years; range 22-86 years) who met the inclusion criteria. The median artifact severity score was worse for water-iodine and iodine-water images (score of 3 for each) than for 70-keV (score 1), 52-keV (score 2), and 120-keV (score 1) images (all p < 0.001). The anatomic delineation was worse (p < 0.001) for water-iodine and iodine-water images than for monochromatic images. Higher CT number SD values, material concentrations, and gray-scale values were found for areas affected by artifacts than for reference tissues in all datasets (all p < 0.001). Similar results were seen in the phantom study. CONCLUSION Beam-hardening artifacts are prevalent in pelvic rapid-kilovoltage-switching DECT and more severe in material decomposition than monochromatic image reconstructions.

Axial or Helical? Considerations for wide collimation CT scanners capable of volumetric imaging in both modes

Purpose: To determine whether axial or helical mode is more appropriate for a 16 cm collimation CT scanner capable of step‐and‐shoot volumetric axial coverage, in terms of radiation dose, image quality, and scan duration. Methods: All scans were performed with a Revolution CT (GE Healthcare) operating at 120 kV and 100 mAs. Using calibrated optically stimulated luminescence detectors, radiation dose along the axial scan profile was evaluated at the isocenter, including the overlap region between two axial sections. This overlap region measures 3 cm in the z‐axis at the isocenter and is required to obtain sufficient projection data from the relatively large cone‐beam angles. Using an image quality phantom (Gammex Model 464), spatial resolution, CT number uniformity, image noise, and low contrast detectability (LCD) were evaluated under five different conditions: in the middle of a helical acquisition, in the middle of a 16 cm axial section, at both ends of an axial section and in the overlap region between two axial sections. Scan durations and dose length products (DLP) were recorded for prescribed scan lengths of 2.5–100 cm. Results: The overlap region between two axial sections received a dose 83% higher than the single‐exposure region at the isocenter. Within a single axial section, the dose at the anode end was 37% less than at the cathode end due to the anode heel effect. Image noise ranged from a low of 13 HU for the cathode end of an axial section up to 14.7 HU for the anode end (P < 0.001). The LCD was at lower at the anode end of the axial section compared to both the cathode end (P < 0.05) and the overlap location (P < 0.02). The spatial resolution and CT number uniformity were consistent among all conditions. Scan durations were shorter (0.28 s) for the axial mode compared to the helical mode at scan lengths ≤ 16 cm, and longer at scan lengths ≥ 16 cm where more than one table position was required, up to a difference of 13.9 s for a the 100 cm scan length (3.8 s for helical compared to 17.6 s for axial). DLPs were consistent between scan modes; slightly lower in axial mode at shorter scan lengths due to helical overranging, and slightly higher in axial mode at longer scan lengths due to the axial overlap regions. Conclusions: To ensure the most consistent radiation dose and image quality along the scan length, we recommend helical mode for scans longer than the 16 cm coverage of a single axial section. For scan lengths ≤ 16 cm, axial scanning is the most practical option, with a shorter scan duration and higher dose efficiency.