Nonlinear three-dimensional noise filter with low-dose CT angiography: effect on the detection of small high-contrast objects in a phantom model.

PURPOSE: To study the effect of a nonlinear noise filter on the detection of simulated endoleaks in a phantom with 80- and 100-kVp multidetector computed tomographic (CT) angiography. MATERIALS AND METHODS: An aortic aneurysm phantom, including iodinated endoleaks, was constructed. Multidetector CT angiography with use of 80-, 100-, and 120-kVp tube voltages was performed for simulated intermediate-sized and large patients (estimated body weights, 72-85 kg and 118-142 kg, respectively). Images obtained with 80 and 100 kVp were postprocessed by using a nonlinear noise filter. CT images containing 1152 endoleaks and images with no endoleaks were randomized and independently analyzed by three radiologists blinded to the location of the endoleaks. Diagnostic confidence and image quality were rated by using subjective scales. Analysis of variance was used for statistical assessment. RESULTS: In simulated intermediate-sized patients, energy reduction from 120 to 100 kVp and from100 to 80 kVp did not decrease image quality when images with reduced kilovoltage were filtered (P = .2692 and P > .99, respectively). Readers detected more endoleaks on the filtered 100-kVp images than on the nonfiltered images in simulated large patients (83 vs 75 lesions, P = .041). The number of detected endoleaks and the confidence rate were similar at 100 kVp with a filter and at 120 kVp in simulated large patients (P = .339 and P = .211, respectively). CONCLUSION: In a phantom, the nonlinear noise filter can prevent decreased image quality with use of 80- and 100-kVp abdominal multidetector CT angiography at a wide range of simulated body weights and may facilitate a better detection rate of endoleaks in heavy patients.

Decreased detection of hypovascular liver tumors with MDCT in obese patients: a phantom study.

OBJECTIVE: The purpose of this article is to assess the impact of large patient size on the detection of hypovascular liver tumors with MDCT and the effect of a noise filter on image quality and lesion detection in obese patients. MATERIALS AND METHODS: A liver phantom with 45 hypovascular tumors (diameters of 5, 10, and 15 mm) was placed into two water containers mimicking intermediate and large patients. The containers were scanned with a 64-MDCT scanner. The CT dataset from the large phantom was postprocessed using a noise filter. The image noise was measured and the contrast-to-noise ratio (CNR) of the tumors was calculated. Tumor detection was independently performed by three radiologists in a blinded fashion. RESULTS: The application of the noise filter in the large phantom yielded a reduction of image noise by 42% (p < 0.0001). The CNR values of the tumors in the nonfiltered and filtered large phantom were lower than that in the intermediate phantom (p < 0.05). In the non-filtered and filtered large phantom, 25% and 19% fewer tumors, respectively, were detected on average compared with the intermediate phantom (p < 0.01). CONCLUSION: The risk of missing hypovascular liver tumors with CT is substantially increased in large patients. A noise filter improves image quality in obese patients.

CT screening and follow-up of lung nodules: effects of tube current-time setting and nodule size and density on detectability and of tube current-time setting on apparent size.

OBJECTIVE: The purpose of the study was to quantify and compare the effect of CT dose and of size and density of nodules on the detectability of lung nodules and to quantify the influence of CT dose on the size of the nodules. MATERIALS AND METHODS: From 50 patients a total of 125 cuboidal regions of interest (3 × 3 × 1.5 cm volumes) showing a single nodule (≤ 8 mm) and 27 normal cuboids were selected. Image sets were reconstructed with the software from raw data simulating different dose levels: 300 (original dose), 220, 180, 140, 100, 80, 60, 50, 40, 30, 20, 10, and 5 reference mAs. A logistic regression model was used to analyze detectability for three blinded readers. Odds ratios were calculated for nodule size smaller than 3 mm versus 3 mm and larger and for nodule attenuation of -300 HU and greater versus less than -300 HU. RESULTS: Tube current-time settings of 10 mAs and greater were not associated with a significant difference in individual reader sensitivity compared with the standard setting of 300 mAs. At 5 mAs only one reader had a significant decrease in sensitivity, from 82% to 77% (p = 0.0035). According to the odds ratios and logistic regression results, the strongest negative effect on sensitivity can be assumed for low nodule density followed by small nodule size and dose level. The mean nodule volume measurement error between 5 and 300 mAs was 2.2% ± 18% (SD) and much lower than the interobserver volume measurement error rate of 38% ± 45%. CONCLUSION: The results show the feasibility of a low-dose CT protocol at 10 mAs for follow-up of lung nodules. Computer-aided volume measurement in follow-up of lung nodules decreases interobserver variability.

Diagnostic accuracy of pulmonary CT angiography at low tube voltage: intraindividual comparison of a normal-dose protocol at 120 kVp and a low-dose protocol at 80 kVp using reduced amount of contrast medium in a simulation study.

OBJECTIVE: The purpose of this study was to simulate pulmonary emboli (PE) and image quality at low tube energy and reduced contrast material volume in normal-dose pulmonary CT angiography (CTA) images and to analyze the diagnostic accuracy with normal- and low-dose pulmonary CTA. MATERIALS AND METHODS: Normal-dose pulmonary CTA examinations using 120 kVp and 100 mL of contrast material in 10 patients with no PE were retrospectively selected. The image characteristics of an 80-kVp low-dose pulmonary CTA protocol (patient exposure reduction, 57%) with 75 mL of contrast material were simulated. Four different sets of filling defects were computer simulated in identical locations in each normal-dose and corresponding low-dose examination, equaling 783 PE in 40 normal-dose and 40 low-dose datasets. Ten normal-dose and 10 low-dose examinations contained no emboli and were used as controls. The 100 pulmonary CTA studies were randomly assessed by three readers blinded to PE location and image quality. The results were assessed by nonparametric tests and Student t tests. RESULTS: No difference was found between the CT protocols in terms of sensitivity, specificity, and positive and negative diagnostic likelihood ratios at all ramification levels of the pulmonary arteries (p = 0.343-1). The overall sensitivity and specificity with the normal and simulated low-dose protocols were 79.9% versus 81.3% and 98.0% versus 98.2% (p = 0.444 and 0.702), respectively. The diagnostic confidence (2.81 ± 0.39 vs 2.77 ± 0.47; p = 0.297) and overall image quality (3.92 ± 0.52 vs 3.83 ± 0.54; p = 0.216) were similar at 120 kV and 80 kV. CONCLUSION: The intraindividual comparison of diagnostic accuracy with normal-dose and simulated low-dose pulmonary CTA protocols revealed no difference under experimental conditions.

Effect of tumor size and tumor-to-liver contrast of hypovascular liver tumors on the diagnostic performance of hepatic CT imaging.

OBJECTIVE: To assess the effect of tumor size and tumor-to-liver contrast of simulated hypovascular liver tumors on the diagnostic accuracy of hepatic computed tomography (CT). MATERIALS AND METHODS: This retrospective study was approved by the institutional review board, and informed consent was waived. A total of 153 simulated hypovascular liver tumors were embedded in 70 hepatic CT data sets that were acquired during the portal venous phase. The simulated tumors had 3 different diameters (6, 10, and 14 mm) and 3 different tumor-to-liver contrast values (20, 35, and 50 HU). There were also 30 hepatic CT data sets without liver tumors. Three radiologists independently performed tumor detection on the randomized 100 hepatic CT data sets. RESULTS: The lowest sensitivity was obtained for the 6-mm tumors with a tumor-to-liver contrast of 20 HU (4.1%), and the highest sensitivity was obtained for the 10- and 14-mm tumors with a tumor-to-liver contrast of 50 HU (100%). Increasing the contrast from 20 to 35 to 50 HU in the 6-mm tumors yielded a significant increase in sensitivity (4.1%, 48.8%, and 92.4%, respectively; P < 0.0001). The sensitivity for the 10- and 14-mm tumors also increased significantly as the tumor-to-liver contrast value increased from 20 to 35 HU (P < 0.01). However, no significant increase in sensitivity was seen for the 10- and 14-mm tumors as the tumor-to-liver contrast values increased from 35 to 50 HU (P = 0.733 and P = 1.0, respectively). CONCLUSIONS: Increasing the tumor-to-liver contrast from 20 to 35 HU results in a significant increase in the detection of hypovascular liver tumors ranging from 6 to 14 mm in diameter. Optimization of the tumor-to-liver contrast is necessary for improved detection of hypovascular liver tumors.