Low-dose CT with metal artifact reduction in arthroplasty imaging: a cadaveric and clinical study.

OBJECTIVE: To determine whether a simulated low-dose metal artifact reduction (MAR) CT technique is comparable with a clinical dose MAR technique for shoulder arthroplasty evaluation. MATERIALS AND METHODS: Two shoulder arthroplasties in cadavers and 25 shoulder arthroplasties in patients were scanned using a clinical dose (140 kVp, 300 qrmAs); cadavers were also scanned at half dose (140 kVp, 150 qrmAs). Images were reconstructed using a MAR CT algorithm at full dose and a noise-insertion algorithm simulating 50% dose reduction. For the actual and simulated half-dose cadaver scans, differences in SD for regions of interest were assessed, and streak artifact near the arthroplasty was graded by 3 blinded readers. Simulated half-dose scans were compared with full-dose scans in patients by measuring differences in implant position and by comparing readers' grades of periprosthetic osteolysis and muscle atrophy. RESULTS: The mean difference in SD between actual and simulated half-dose methods was 2.42 HU (95% CI [1.4, 3.4]). No differences in streak artifact grades were seen in 13/18 (72.2%) comparisons in cadavers. In patients, differences in implant position measurements were within 1° or 1 mm in 149/150 (99.3%) measurements. The inter-reader agreement rates were nearly identical when readers were using full-dose (77.3% [232/300] for osteolysis and 76.9% [173/225] for muscle atrophy) and simulated half-dose (76.7% [920/1200] for osteolysis and 74.0% [666/900] for muscle atrophy) scans. CONCLUSION: A simulated half-dose MAR CT technique is comparable both quantitatively and qualitatively with a standard-dose technique for shoulder arthroplasty evaluation, demonstrating that this technique could be used to reduce dose in arthroplasty imaging.

Combined Dual-Energy and Single-Energy Metal Artifact Reduction Techniques Versus Single-Energy Techniques Alone for Lesion Detection Near an Arthroplasty.

OBJECTIVE. The purpose of this study was to compare a combined dual-energy CT (DECT) and single-energy CT (SECT) metal artifact reduction technique with a SECT metal artifact reduction technique for detecting lesions near an arthroplasty in a phantom model. MATERIALS AND METHODS. Two CT phantoms with a cobalt chromium sphere attached to a titanium rod, simulating an arthroplasty, within a background of soft-tissue attenuation containing spherical lesions (range, 10-20 mm) around the head and stem of different attenuations from the background (range of attenuation, 10-70 HU) were scanned with a single CT scanner individually (unilateral) and together (bilateral) with the following three dose-equivalent techniques: the currently used clinical protocol (140 kVp, 300 Reference mAs); 100 kVp; and DECT (100 kVp and 150 kVp with a tin filter). Three radiologists reviewed the datasets to identify lesions. Nonparametric AUC was estimated for each reader with each technique. Multireader ANOVA was performed to compare AUCs. Multiple-variable logistic regression analysis was used to identify factors affecting sensitivity and specificity. RESULTS. Accuracy was lower (p < 0.001) for the DECT 130-keV technique than for the 100-, 70-, and 140-kVp techniques. Sensitivity was higher with unilateral arthroplasties (p = 0.037), with greater contrast differences from background (p < 0.001), and with the SECT 100-kVp technique versus other techniques (p < 0.001). The difference in specificities of modalities was not statistically significant (p = 0.148). CONCLUSION. Combining DECT and SECT techniques does not provide additional benefits for lesion detection as opposed to using SECT alone.

Motion Artifact Reduction From High-Pitch Dual-Source Computed Tomography Pulmonary Angiography.

PURPOSE: The purpose of this study was to compare quantitative and qualitative measures of aortic, cardiac, and respiratory motion artifact between high-pitch dual-source (DS) and single-source (SS) computed tomography pulmonary angiography (CTPA) protocols. METHODS: This institutional review board-approved, Health Insurance Portability and Accountability Act-compliant study retrospectively reviewed 80 non-electrocardiogram-gated CTPA examinations acquired with a second-generation DS system at 100 kVp following 50 mL iodinated contrast injection - 40 consecutive SS and 40 consecutive DS studies. Quantitative measures of aortic, left ventricular, and diaphragmatic motion were recorded as the maximal excursion of a structure's "double image," and 3 independent readers performed qualitative motion assessments. Pulmonary arterial contrast enhancement, image noise, and radiation dose metrics were recorded. Statistical analyses were performed with 1-way analysis of variance and Fisher exact test. RESULTS: Dual source outperformed SS technique in both quantitative and qualitative measures of motion. Mean distances between motion-artifact double images were reduced with DS protocol at each location (all P ≤ 0.004), and DS examinations were more likely to receive an assessment of no motion in all locations (all P < 0.0001). The DS protocol demonstrated increases in contrast enhancement, although increased image noise resulted in lower enhancement to noise ratio. Mean radiation dose was 60% lower using the DS protocol. CONCLUSION: High-pitch DS CTPA significantly reduces artifacts resulting from ascending aortic, cardiac, and diaphragmatic motion.

Dose Reduction With Dedicated CT Metal Artifact Reduction Algorithm: CT Phantom Study.

OBJECTIVE: The objective of this study was to compare reader accuracy detecting lesions near hardware in a CT phantom model at different radiation exposures using an advanced metal artifact reduction (MAR) algorithm and standard filtered back projection (FBP) techniques and to determine if radiation exposure could be decreased using MAR without compromising lesion detectability. MATERIALS AND METHODS: A CT phantom manufactured with spherical lesions of various sizes (10-20 mm) and attenuations (20-50 HU) embedded around cobalt-chromium spheres attached to titanium rods, simulating an arthroplasty, was scanned on a single CT scanner (FLASH, Siemens Healthcare) at 140 kVp and 0.6-mm collimation using clinical-dose (300 Quality Reference mAs [Siemens Healthcare]), low-dose (150 Quality Reference mAs), and high-dose (600 Quality Reference mAs) protocols. Images reconstructed with iterative MAR, advanced modeled iterative reconstruction (ADMIRE), and FBP with identical parameters were anonymized and independently reviewed by three radiologists. Accuracies for detecting lesions, measured as AUC, sensitivity, and specificity, were compared. RESULTS: Accuracy using MAR was significantly higher than that using FBP at all exposures (p values ranged from < 0.001 to 0.021). Sensitivity was also higher for MAR than for FBP at all exposures. Specificity was very high for both reconstruction techniques at all exposures with no significant differences. Accuracy of low-dose MAR was higher than and not inferior to standard-dose and high-dose FBP. MAR was significantly more sensitive than FBP in detecting smaller lesions (p = 0.021) and lesions near high streak artifact (p < 0.001). CONCLUSION: MAR improves reader accuracy to detect lesions near hardware and allows significant reductions in radiation exposure without compromising accuracy compared with FBP in a CT phantom model.

Extending the concept of weighted CT dose index to elliptical phantoms of various aspect ratios.

The purpose of this study was to extend the concept of weighted CT dose index ([Formula: see text]) to the elliptical phantoms. Based on the published body dimension data, eight body aspect ratios were chosen between 1 (perfectly circular) and 1.72 (extremely elliptical). For each aspect ratio, two elliptical cylinders were created digitally to represent adult and pediatric bodies. Their cross-sectional areas were identical to the standard 32- and 16-cm CTDI phantoms. For each phantom, [Formula: see text] at center and periphery were simulated for tube voltages between 70 and 140 kVp using a validated Monte Carlo program. The simulations also provided the average dose over the cross-sectional area, [Formula: see text]. Values of [Formula: see text] and [Formula: see text] allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved. Regardless of phantom shape, only two weighting coefficients were needed: [Formula: see text] for the central [Formula: see text] and [Formula: see text] for the average of the four peripheral [Formula: see text]'s. Over the full range of aspect ratios, [Formula: see text] increased linearly from 0.37 to 0.46, whereas [Formula: see text] decreased linearly from 0.63 to 0.54, allowing the concept of [Formula: see text] to be readily extended to the elliptical phantoms. When cross-sectional area (hence volume) was kept constant, all phantoms had the same [Formula: see text] regardless of shape.

Analysis of uncertainties in Monte Carlo simulated organ and effective dose in chest CT: scanner- and scan-related factors.

In Monte Carlo simulation of CT dose, many input parameters are required (e.g. bowtie filter properties and scan start/end location). Our goal was to examine the uncertainties in patient dose when input parameters were inaccurate. Using a validated Monte Carlo program, organ dose from a chest CT scan was simulated for an average-size female phantom using a reference set of input parameter values (treated as the truth). Additional simulations were performed in which errors were purposely introduced into the input parameter values. The effects on four dose quantities were analyzed: organ dose (mGy/mAs), effective dose (mSv/mAs), CTDIvol-normalized organ dose (unitless), and DLP-normalized effective dose (mSv/mGy · cm). At 120 kVp, when spectral half value layer deviated from its true value by ±1.0 mm Al, the four dose quantities had errors of 18%, 7%, 14% and 2%, respectively. None of the dose quantities were affected significantly by errors in photon path length through the graphite section of the bowtie filter; path length error as large as 5 mm produced dose errors of ⩽2%. In contrast, error of this magnitude in the aluminum section produced dose errors of ⩽14%. At a total collimation of 38.4 mm, when radiation beam width deviated from its true value by ± 3 mm, dose errors were ⩽7%. Errors in tube starting angle had little impact on effective dose (errors ⩽ 1%); however, they produced organ dose errors as high as 66%. When the assumed scan length was longer by 4 cm than the truth, organ dose errors were up to 137%. The corresponding error was 24% for effective dose, but only 3% for DLP-normalized effective dose. Lastly, when the scan isocenter deviated from the patient's anatomical center by 5 cm, organ and effective dose errors were up 18% and 8%, respectively.

Imaging of Arthroplasties: Improved Image Quality and Lesion Detection With Iterative Metal Artifact Reduction, a New CT Metal Artifact Reduction Technique.

OBJECTIVE: The purpose of this study was to compare iterative metal artifact reduction (iMAR), a new single-energy metal artifact reduction technique, with filtered back projection (FBP) in terms of attenuation values, qualitative image quality, and streak artifacts near shoulder and hip arthroplasties and observer ability with these techniques to detect pathologic lesions near an arthroplasty in a phantom model. MATERIALS AND METHODS: Preoperative and postoperative CT scans of 40 shoulder and 21 hip arthroplasties were reviewed. All postoperative scans were obtained using the same technique (140 kVp, 300 quality reference mAs, 128 × 0.6 mm detector collimation) on one of three CT scanners and reconstructed with FBP and iMAR. The attenuation differences in bones and soft tissues between preoperative and postoperative scans at the same location were compared; image quality and streak artifact for both reconstructions were qualitatively graded by two blinded readers. Observer ability and confidence to detect lesions near an arthroplasty in a phantom model were graded. RESULTS: For both readers, iMAR had more accurate attenuation values (p < 0.001), qualitatively better image quality (p < 0.001), and less streak artifact (p < 0.001) in all locations near arthroplasties compared with FBP. Both readers detected more lesions (p ≤ 0.04) with higher confidence (p ≤ 0.01) with iMAR than with FBP in the phantom model. CONCLUSION: The iMAR technique provided more accurate attenuation values, better image quality, and less streak artifact near hip and shoulder arthroplasties than FBP; iMAR also increased observer ability and confidence to detect pathologic lesions near arthroplasties in a phantom model.

Diagnostic Accuracy of CT Enterography for Active Inflammatory Terminal Ileal Crohn Disease: Comparison of Full-Dose and Half-Dose Images Reconstructed with FBP and Half-Dose Images with SAFIRE.

Purpose To compare the diagnostic accuracy and image quality of computed tomographic (CT) enterographic images obtained at half dose and reconstructed with filtered back projection (FBP) and sinogram-affirmed iterative reconstruction (SAFIRE) with those of full-dose CT enterographic images reconstructed with FBP for active inflammatory terminal or neoterminal ileal Crohn disease. Materials and Methods This retrospective study was compliant with HIPAA and approved by the institutional review board. The requirement to obtain informed consent was waived. Ninety subjects (45 with active terminal ileal Crohn disease and 45 without Crohn disease) underwent CT enterography with a dual-source CT unit. The reference standard for confirmation of active Crohn disease was active terminal ileal Crohn disease based on ileocolonoscopy or established Crohn disease and imaging features of active terminal ileal Crohn disease. Data from both tubes were reconstructed with FBP (100% exposure); data from the primary tube (50% exposure) were reconstructed with FBP and SAFIRE strengths 3 and 4, yielding four datasets per CT enterographic examination. The mean volume CT dose index (CTDIvol) and size-specific dose estimate (SSDE) at full dose were 13.1 mGy (median, 7.36 mGy) and 15.9 mGy (median, 13.06 mGy), respectively, and those at half dose were 6.55 mGy (median, 3.68 mGy) and 7.95 mGy (median, 6.5 mGy). Images were subjectively evaluated by eight radiologists for quality and diagnostic confidence for Crohn disease. Areas under the receiver operating characteristic curves (AUCs) were estimated, and the multireader, multicase analysis of variance method was used to compare reconstruction methods on the basis of a noninferiority margin of 0.05. Results The mean AUCs with half-dose scans (FBP, 0.908; SAFIRE 3, 0.935; SAFIRE 4, 0.924) were noninferior to the mean AUC with full-dose FBP scans (0.908; P < .003). The proportion of images with inferior quality was significantly higher with all half-dose reconstructions than with full-dose FBP (mean proportion: 0.117 for half-dose FBP, 0.054 for half-dose SAFIRE 3, 0.054 for half-dose SAFIRE 4, and 0.017 for full-dose FBP; P < .001). Conclusion The diagnostic accuracy of half-dose CT enterography with FBP and SAFIRE is statistically noninferior to that of full-dose CT enterography for active inflammatory terminal ileal Crohn disease, despite an inferior subjective image quality. (©) RSNA, 2016 Online supplemental material is available for this article.

Image Noise, CNR, and Detectability of Low-Contrast, Low-Attenuation Liver Lesions in a Phantom: Effects of Radiation Exposure, Phantom Size, Integrated Circuit Detector, and Iterative Reconstruction.

Purpose To assess image noise, contrast-to-noise ratio (CNR) and detectability of low-contrast, low-attenuation liver lesions in a semianthropomorphic phantom by using either a discrete circuit (DC) detector and filtered back projection (FBP) or an integrated circuit (IC) detector and iterative reconstruction (IR) with changes in radiation exposure and phantom size. Materials and Methods An anthropomorphic phantom without or with a 5-cm-thick fat-mimicking ring (widths, 30 and 40 cm) containing liver inserts with four spherical lesions was scanned with five exposure settings on each of two computed tomography scanners, one equipped with a DC detector and the other with an IC detector. Images from the DC and IC detector scanners were reconstructed with FBP and IR, respectively. Image noise and lesion CNR were measured. Four radiologists evaluated lesion presence on a five-point diagnostic confidence scale. Data analyses included receiver operating characteristic (ROC) curve analysis and noninferiority analysis. Results The combination of IC and IR significantly reduced image noise (P < .001) (with the greatest reduction in the 40-cm phantom and at lower exposures) and improved lesion CNR (P < .001). There was no significant difference in area under the ROC curve between detector-reconstruction combinations at fixed exposure for either phantom. Reader accuracy with IC-IR was noninferior at 50% (100 mAs [effective]) and 25% (300 mAs [effective]) exposure reduction for the 30- and 40-cm phantoms, respectively (adjusted P < .001 and .04 respectively). IC-IR improved readers' confidence in the presence of a lesion (P = .029) independent of phantom size or exposure level. Conclusion IC-IR improved objective image quality and lesion detection confidence but did not result in superior diagnostic accuracy when compared with DC-FBP. Moderate exposure reductions maintained comparable diagnostic accuracy for both detector-reconstruction combinations. Lesion detection in the 40-cm phantom was inferior at smaller exposure reduction than in the 30-cm phantom. (©) RSNA, 2016 Online supplemental material is available for this article.

Adult abdomen-pelvis CT: Does equilibrium dose-pitch product better account for the kVp dependence of organ dose than conventional CTDI?

PURPOSE: In CT imaging, a desirable quality assurance dose quantity should account for the dose variability across scan parameters and scanner models. Recently, AAPM Task Group 111 proposed to use equilibrium dose-pitch product (Dˆeq), in place of CT dose index (CTDI100), for scan modes involving table translation. The purpose of this work is to investigate whether this new concept better accounts for the tube voltage (kVp) dependence of organ dose than the conventional CTDI100. METHODS: Three extended cardiac-torso patient models were included in this study. They represented normal-weight, overweight, and obese patients with abdomen-pelvis diameters ranging between 23 and 36 cm and body mass indices ranging between 20 and 42. A Monte Carlo program developed and validated for a 128-slice CT system was used to simulate organ dose for abdomen-pelvis scans at five tube voltages (70, 80, 100, 120, 140 kVp) with a pitch of 0.8 and a collimation of 38.4 mm. The same Monte Carlo program was also used to obtain CTDI100 and Dˆeq as well as their volume-averaged values, CTDIvol and Dˆeq,vol. RESULTS: With other scan parameters kept constant, organ dose itself depended strongly on kVp. For the normal-weight patient model, the coefficient of variation (COV) across the five kVp values ranged between 72% and 75% for nine organs (liver, spleen, stomach, pancreas, kidneys, colon, small intestine, bladder, and ovaries) fully encompassed by the primary radiation beam. The COV generally increased with increasing patient size, ranging between 72%-77% and 76%-81% for the overweight and obese patient models, respectively. One-way analysis of variance for the effect of kVp was highly significant for all patient models (p<2×10(-26)). When organ dose was normalized by CTDIvol, the COV across kVps reduced to 5%-11%, 6%-15%, 12%-22% for the normal-weight, overweight, and obese patients, respectively. The effect of kVp was still highly significant (p=0.0001-0.004). When organ dose was normalized by Dˆeq,vol, the COV further reduced to 1%-8%, 3%-11%, 9%-19%, respectively. The effect of kVp was still significant for the obese patient model (p=0.004), but no longer significant for the normal-weight and overweight patient models (p=0.4 and 0.09, respectively). Finally, if organ dose conversion factors obtained at 120 kVp were used to approximate the values at 70 and 80 kVp, the resulting errors in the estimated organ dose were significantly reduced when the conversion factors were based on Dˆeq,vol instead of CTDIvol. CONCLUSIONS: In adult abdomen-pelvis CT, equilibrium dose-pitch product better accounts for the kVp dependence of organ dose than CTDI100.

Estimated Patient Dose Indexes in Adult and Pediatric MDCT: Comparison of Automatic Tube Voltage Selection With Fixed Tube Current, Fixed Tube Voltage, and Weight-Based Protocols.

OBJECTIVE: The purposes of this study were to determine the differences in estimated volumetric CT dose index (CTDIvol) obtained from the topogram before abdominal and pelvic MDCT in adult and pediatric patients using a scan type-based algorithm for selecting kilovoltage (CARE kV) and a fixed and a weight-based Quality Reference mAs for selecting tube (gmAs) current-exposure time product, in comparison with standard protocols, and to determine the bias and variability of estimated CTDIvol vis-à-vis actual CTDIvol using the standard protocols. MATERIALS AND METHODS: During a 14-month period, 312 adult and pediatric patients referred for abdominal and pelvic MDCT were included in the study. For all patients, the estimated CTDIvol based on the topogram was recorded: protocol A, CARE kV on and 210 gmAs; protocol B, CARE kV on and 1 gmAs times patient weight (in pounds); and protocol C (standard protocol), CARE kV off, 120 kVp, and 1 gmAs times patient weight (in pounds). For the pediatric patients, estimated CTDIvol for the standard protocol D was calculated with 120 kVp and 150 gmAs. All patients were scanned with the standard protocols, and the actual CTDIvol was recorded. Linear regression models compared the CTDIvol of the three protocols in adults and the fourth for children. The estimated and actual CTDIvol were compared using a t test. RESULTS: Protocol B yielded the lowest estimated CTDIvol (mean, 13.2 mGy for adults and 3.5 mGy for pediatric patients). The estimated CTDIvol overestimated the actual CTDIvol by, on average, 1.07 mGy for adults and 0.3 mGy for children. CONCLUSION: CARE kV appears to reduce estimated CTDIvol vis-à-vis standard protocols only when a weight-based gmAs is used. Prescan estimated CTDIvol calculations appear to generally overestimate actual CTDIvol.

Metal artifact reduction: standard and advanced magnetic resonance and computed tomography techniques.

An increasing number of joint replacements are being performed in the United States. Patients undergoing these procedures can have various complications. Imaging is one of the primary means of diagnosing these complications. Cross-sectional imaging techniques, such as computed tomography (CT) and MR imaging, are more sensitive than radiographs for evaluating complications. The use of CT and MR imaging in patients with metallic implants is limited by the presence of artifacts. This review discusses the causes of metal artifacts on MR imaging and CT, contributing factors, and conventional and novel methods to reduce the effects of these artifacts on scans.

Iterative metal artifact reduction: evaluation and optimization of technique.

OBJECTIVE: Iterative metal artifact reduction (IMAR) is a sinogram inpainting technique that incorporates high-frequency data from standard weighted filtered back projection (WFBP) reconstructions to reduce metal artifact on computed tomography (CT). This study was designed to compare the image quality of IMAR and WFBP in total shoulder arthroplasties (TSA); determine the optimal amount of WFBP high-frequency data needed for IMAR; and compare image quality of the standard 3D technique with that of a faster 2D technique. MATERIALS AND METHODS: Eight patients with nine TSA underwent CT with standardized parameters: 140 kVp, 300 mAs, 0.6 mm collimation and slice thickness, and B30 kernel. WFBP, three 3D IMAR algorithms with different amounts of WFBP high-frequency data (IMARlo, lowest; IMARmod, moderate; IMARhi, highest), and one 2D IMAR algorithm were reconstructed. Differences in attenuation near hardware and away from hardware were measured and compared using repeated measures ANOVA. Five readers independently graded image quality; scores were compared using Friedman's test. RESULTS: Attenuation differences were smaller with all 3D IMAR techniques than with WFBP (p < 0.0063). With increasing high-frequency data, the attenuation difference increased slightly (differences not statistically significant). All readers ranked IMARmod and IMARhi more favorably than WFBP (p < 0.05), with IMARmod ranked highest for most structures. The attenuation difference was slightly higher with 2D than with 3D IMAR, with no significant reader preference for 3D over 2D. CONCLUSIONS: IMAR significantly decreases metal artifact compared to WFBP both objectively and subjectively in TSA. The incorporation of a moderate amount of WFBP high-frequency data and use of a 2D reconstruction technique optimize image quality and allow for relatively short reconstruction times.

Effect of reduced radiation exposure and iterative reconstruction on detection of low-contrast low-attenuation lesions in an anthropomorphic liver phantom: an 18-reader study.

PURPOSE: To measure the effect of reduced radiation exposure on low-contrast low-attenuation liver lesion detection in an anthropomorphic abdominal phantom by using filtered back projection (FBP) and sinogram-affirmed iterative reconstruction. MATERIALS AND METHODS: Eighteen radiologists blinded to phantom and study design interpreted randomized image data sets that contained 36 spherical simulated liver lesions of three sizes and three attenuation differences (5-mm diameter: 12, 18, and 24 HU less than the 90-HU background attenuation of the simulated liver insert; 10- and 15-mm diameter: 6, 12, and 18 HU less than the 90-HU background attenuation) scanned with four discrete exposure settings and reconstructed by using FBP and sinogram-affirmed iterative reconstruction. Response assessment included region-level lesion presence or absence on a five-point diagnostic confidence scale. Statistical evaluation included multireader multicase receiver operating characteristic curve analysis, with nonparametric methods and noninferiority analysis at a margin of -0.10. RESULTS: Pooled accuracy at 75% exposure for both FBP and sinogram-affirmed iterative reconstruction was noninferior to 100% exposure (P = .002 and P < .001, respectively). Subsequent exposure reductions resulted in a significant decrease in accuracy. When the smallest (5-mm-diameter) lesions were excluded from analysis, sinogram-affirmed iterative reconstruction was superior to FBP at 100% exposure (P = .011), and sinogram-affirmed iterative reconstruction at 25% and 50% exposure reduction was noninferior to FBP at 100% exposure (P ≤ .013). Reader confidence was greater with sinogram-affirmed iterative reconstruction than with FBP for 10- and 15-mm lesions (2.94 vs 2.76 and 3.62 vs 3.52, respectively). CONCLUSION: In this low-contrast low-attenuation liver lesion model, a 25% exposure reduction maintained noninferior diagnostic accuracy. However, detection was inferior with each subsequent exposure reduction, regardless of reconstruction method. Sinogram-affirmed iterative reconstruction and FBP performed equally well at modest exposure reduction (25%-50%). Readers had higher confidence levels with sinogram-affirmed iterative reconstruction for the 10- and 15-mm lesions.

Scatter correction associated with dedicated dual-source CT hardware improves accuracy of lung air measures.

RATIONALE AND OBJECTIVES: Accurate assessment of air density used to quantitatively characterize amount and distribution of emphysema in chronic obstructive pulmonary disease (COPD) subjects has remained challenging. Hounsfield units (HU) within tracheal air can be considerably less negative than -1000 HU. This study has sought to characterize the effects of improved scatter correction used in dual-source pulmonary computed tomography (CT). MATERIALS AND METHODS: Dual-source dual-energy (DSDE) and single-source (SS) scans taken at multiple energy levels and scan settings were acquired for quantitative comparison using anesthetized ovine (n = 6), swine (n = 13), and a lung phantom. Data were evaluated for the lung, inferior vena cava, and tracheal segments. To minimize the effect of cross-scatter, the phantom scans in the DSDE mode were obtained by reducing the current of one of the tubes to near zero. RESULTS: A significant shift in mean HU values in the tracheal regions of animals and the phantom is observed, with values consistently closer to -1000 HU in DSDE mode. HU values associated with SS mode demonstrated a positive shift of up to 32 HU. In vivo tracheal air measurements demonstrated considerable variability with SS scanning, whereas these values were more consistent with DSDE imaging. Scatter effects in the lung parenchyma differed from adjacent tracheal measures. CONCLUSION: Data suggest that the scatter correction introduced into the dual-energy mode of imaging has served to provide more accurate CT lung density measures sought to quantitatively assess the presence and distribution of emphysema in COPD subjects. Data further suggest that CT images, acquired without adequate scatter correction, cannot be corrected by linear algorithms given the variability in tracheal air HU values and the independent scatter effects on lung parenchyma.

Dual-source dual-energy CT with additional tin filtration: Dose and image quality evaluation in phantoms and in vivo.

OBJECTIVE: The objective of this study was to investigate the effect on radiation dose and image quality of the use of additional spectral filtration for dual-energy CT using dual-source CT (DSCT). MATERIALS AND METHODS: A commercial DSCT scanner was modified by adding tin filtration to the high-kV tube, and radiation output and noise were measured in water phantoms. Dose values for equivalent image noise were compared between the dual-energy mode with and without tin filtration and the single-energy mode. To evaluate dual-energy CT material discrimination, the material-specific dual-energy ratio for calcium and that for iodine were determined using images of anthropomorphic phantoms. Data were additionally acquired from imaging a 38-kg pig and an 87-kg pig, and the noise of the linearly mixed images and virtual noncontrast images was compared between dual-energy modes. Finally, abdominal dual-energy CT images of two patients of similar sizes undergoing clinically indicated CT were compared. RESULTS: Adding tin filtration to the high-kV tube improved the dual-energy contrast between iodine and calcium as much as 290%. Data from our animal study showed that tin filtration had no effect on noise in the dual-energy CT mixed images but decreased noise by as much as 30% in the virtual noncontrast images. Virtual noncontrast images of patients acquired using 100 and 140 kV with added tin filtration had improved image quality relative to those generated using 80 and 140 kV without tin filtration. CONCLUSION: Tin filtration of the high-kV tube of a DSCT scanner increases the ability of dual-energy CT to discriminate between calcium and iodine without increasing dose relative to single-energy CT. Furthermore, the use of 100- and 140-kV tube potentials allows improved dual-energy CT imaging of large patients.

Feasibility of dual-energy CT in the arterial phase: Imaging after endovascular aortic repair.

OBJECTIVE: The purpose of this study was to investigate replacing unenhanced and arterial single-energy CT acquisitions after endovascular aneurysm repair with one dual-energy CT arterial acquisition. SUBJECTS AND METHODS: Thirty patients underwent arterial dual-energy CT (80 and 140 kVp) and venous single-energy CT (120 kVp) after endovascular aneurysm repair, and the radiation doses were compared with those of a standard triple-phase protocol. Both virtual unenhanced and arterial images were generated with dual-energy CT. Images were reviewed clinically for detection of endoleaks and evaluation of stent and calcium appearance. The aortic luminal attenuation on virtual unenhanced CT images was compared with that on previously acquired true unenhanced images. Virtual unenhanced, arterial, and venous images were compared for thrombus attenuation. Single-energy CT and dual-energy CT images were compared for noise. RESULTS: Replacement of two (unenhanced, arterial) of three single-energy CT acquisitions with one dual-energy CT acquisition resulted in 31% radiation dose savings. All images were clinically interpretable. Thoracic (32 +/- 2 vs 35 +/- 4 HU) and abdominal (30 +/- 3 vs 35 +/- 5 HU) aortic attenuation was similar on virtual unenhanced and true unenhanced images. Thrombus attenuation was similar on virtual unenhanced (32 +/- 6 HU), arterial phase (33 +/- 7 HU), and venous phase (34 +/- 6 HU) images. Decreased stent and calcium attenuation was observed at some locations on virtual unenhanced images. Noise in the thoracic (10 +/- 1 HU) and abdominal (12 +/- 2 HU) aorta was lower on virtual unenhanced images than on true unenhanced images (13 +/- 4 HU, 19 +/- 5 HU). Noise was comparable for dual-energy and single-energy CT (thorax, 16 +/- 2 vs 13 +/- 2 HU; abdomen, 21 +/- 3 vs 23 +/- 5 HU). CONCLUSION: Virtual unenhanced and arterial phase images derived from dual-energy CT can replace true unenhanced and arterial phase single-energy CT images in follow-up after endovascular aneurysm repair (except immediately after the procedure), providing comparable diagnostic information with substantial dose savings.

Detectability of urinary stones on virtual nonenhanced images generated at pyelographic-phase dual-energy CT.

PURPOSE: To evaluate the detectability of urinary stones on virtual nonenhanced images generated at pyelographic-phase dual-energy computed tomography (CT). MATERIALS AND METHODS: This retrospective HIPAA-compliant study was institutional review board approved. All included patients had previously consented to the use of their medical records for research. Sixty-two patients (38 men, 24 women; age range, 35-91 years) had undergone CT urography, which consisted of nonenhanced and pyelographic-phase dual-energy CT performed by using a dual-source scanner. Commercial software was used to create virtual nonenhanced images by suppressing the iodine signal from the pyelographic-phase dual-energy CT scans. Two radiologists, in consensus, evaluated the virtual nonenhanced images for the presence of stones. Sensitivity for detecting stones was calculated on a per-stone basis. Sensitivity, specificity, and accuracy were also calculated on a per-renal unit (defined as the intrarenal collecting system and ureter of one kidney) basis. The true nonenhanced scan was considered the reference standard. A jackknife method was used because any patient may have multiple stones. RESULTS: Of 62 patients with 122 renal units, 21 patients with 25 renal units had a total of 43 stones (maximal transverse diameter range, 1-24 mm; median, 3 mm). The overall sensitivity for detecting stones was 63% (27 of 43 stones) per stone. Sensitivities were 29% (four of 14 stones) for 1-2-mm stones, 64% (nine of 14 stones) for 3-4-mm stones, 83% (five of six stones) for 5-6-mm stones, and 100% (nine of nine stones) for 7-mm or larger (7, 7, 7, 8, 8, 9, 11, 15, and 24 mm) stones. All three ureteral stones (3, 4, and 8 mm) were correctly identified. The sensitivity, specificity, and accuracy for detecting stones on a per-renal unit basis were 65% (17 of 26 renal units), 92% (88 of 96 renal units), and 86% (105 of 122 renal units), respectively. CONCLUSION: Virtual nonenhanced images generated at pyelographic-phase dual-energy CT enabled the detection of urinary stones with moderate accuracy. The detection of small (1-2-mm) stones was limited.

CT arthrography: in vitro evaluation of single and dual energy for optimization of technique.

The purpose of this study was to optimize CT arthrography technique and determine if dual energy CT (DECT) can provide any benefit over single energy CT (SECT). Iodinated contrast attenuation at different concentrations was measured using DECT and SECT at different beam energies (140, 120, and 80 kVp). Dose and noise were measured on phantoms at different tube currents. Three bovine femoral condyles with artificially created cartilage defects were scanned with dose-equivalent protocols. Contrast-to-noise ratio (CNR) between cartilage and iodine was measured, and the appearance of cartilage defects was graded by two readers. DECT scans were post-processed for iodine quantification. The beam energy 80 kVp had the highest iodine signal, 50% greater than DECT, 75% greater than 120 kVp, and 100% greater than 140 kVp. Noise was nearly identical for all techniques when dose was matched. The 80 kVp level had the highest CNR, 25% higher than 120 kVp and DECT, and 33% greater than 140 kVp. The 80 kVp technique was also preferred by both readers. DECT iodine quantification was significantly limited by the post-processing application, noise, and beam hardening. In this in-vitro study, the SECT 80 kVp CT arthrography technique was superior to currently performed 120 and 140 kVP SECT techniques and DECT.

Evaluation of z-axis resolution and image noise for nonconstant velocity spiral CT data reconstructed using a weighted 3D filtered backprojection (WFBP) reconstruction algorithm.

PURPOSE: To determine the constancy of z-axis spatial resolution, CT number, image noise, and the potential for image artifacts for nonconstant velocity spiral CT data reconstructed using a flexibly weighted 3D filtered backprojection (WFBP) reconstruction algorithm. METHODS: A WFBP reconstruction algorithm was used to reconstruct stationary (axial, pitch=0), constant velocity spiral (pitch = 0.35-1.5) and nonconstant velocity spiral CT data acquired using a 128 x 0.6 mm acquisition mode (38.4 mm total detector length, z-flying focal spot technique), and a gantry rotation time of 0.30 s. Nonconstant velocity scans used the system's periodic spiral mode, where the table moved in and out of the gantry in a cyclical manner. For all scan types, the volume CTDI was 10 mGy. Measurements of CT number, image noise, and the slice sensitivity profile were made for all scan types as a function of the nominal slice width, table velocity, and position within the scan field of view. A thorax phantom was scanned using all modes and reconstructed transverse and coronal plane images were compared. RESULTS: Negligible differences in slice thickness, CT number, noise, or artifacts were found between scan modes for data taken at two positions within the scan field of view. For nominal slices of 1.0-3.0 mm, FWHM values of the slice sensitivity profiles were essentially independent of the scan type. For periodic spiral scans, FWHM values measured at the center of the scan range were indistinguishable from those taken 5 mm from one end of the scan range. All CT numbers were within +/- 5 HU, and CT number and noise values were similar for all scan modes assessed. A slight increase in noise and artifact level was observed 5 mm from the start of the scan on the first pass of the periodic spiral. On subsequent passes, noise and artifact level in the transverse and coronal plane images were the same for all scan modes. CONCLUSIONS: Nonconstant velocity periodic spiral scans can achieve z-axis spatial resolution, CT number accuracy, image noise and artifact level equivalent to those for stationary (axial), and constant velocity spiral scans. Thus, periodic spiral scans are expected to allow assessment of four-dimensional CT data for scan lengths greater than the detector width without sacrificing image quality.