In-Plane Shielding for CT: Effect of Off-Centering, Automatic Exposure Control and Shield-to-Surface Distance

OBJECTIVE: To assess effects of off-centering, automatic exposure control, and padding on attenuation values, noise, and radiation dose when using in-plane bismuth-based shields for CT scanning. MATERIALS AND METHODS: A 30 cm anthropomorphic chest phantom was scanned on a 64-multidetector CT, with the center of the phantom aligned to the gantry isocenter. Scanning was repeated after placing a bismuth breast shield on the anterior surface with no gap and with 1, 2, and 6 cm of padding between the shield and the phantom surface. The "shielded" phantom was also scanned with combined modulation and off-centering of the phantom at 2 cm, 4 cm and 6 cm below the gantry isocenter. CT numbers, noise, and surface radiation dose were measured. The data were analyzed using an analysis of variance. RESULTS: The in-plane shield was not associated with any significant increment for the surface dose or CT dose index volume, which was achieved by comparing the radiation dose measured by combined modulation technique to the fixed mAs (p > 0.05). Irrespective of the gap or the surface CT numbers, surface noise increased to a larger extent compared to Hounsfield unit (HU) (0-6 cm, 26-55%) and noise (0-6 cm, 30-40%) in the center. With off-centering, in-plane shielding devices are associated with less dose savings, although dose reduction was still higher than in the absence of shielding (0 cm off-center, 90% dose reduction; 2 cm, 61%) (p < 0.0001). Streak artifacts were noted at 0 cm and 1 cm gaps but not at 2 cm and 6 cm gaps of shielding to the surface distances. CONCLUSION: In-plane shields are associated with greater image noise, artifactually increased attenuation values, and streak artifacts. However, shields reduce radiation dose regardless of the extent of off-centering. Automatic exposure control did not increase radiation dose when using a shield.

Multidetector CT Urography in Imaging of the Urinary Tract in Patients with Hematuria

This review article comprehensively discusses multidetector CT urography protocols and their role in imaging of the urinary tract in patients with hematuria.

Techniques, Clinical Applications and Limitations of 3D Reconstruction in CT of the Abdomen

Enhanced z-axis coverage with thin overlapping slices in breath-hold acquisitions with multidetector CT (MDCT) has considerably enhanced the quality of multiplanar 3D reconstruction. This pictorial essay describes the improvements in 3D reconstruction and technical aspects of 3D reconstruction and rendering techniques available for abdominal imaging. Clinical applications of 3D imaging in abdomen including liver, pancreaticobiliary system, urinary and gastrointestinal tracts and imaging before and after transplantation are discussed. In addition, this article briefly discusses the disadvantages of thin-slice acquisitions including increasing numbers of transverse images, which must be reviewed by the radiologist.

Correlation of Patient Weight and Cross-Sectional Dimensions with Subjective Image Quality at Standard Dose Abdominal CT

OBJECTIVE: We evaluated the association between patients' weight and abdominal cross-sectional dimensions and CT image quality. MATERIALS AND METHODS: We prospectively evaluated 39 cancer patients aged more than 65 years with multislice CT scan of abdomen. All patients underwent equilibrium phase contrast-enhanced abdominal CT with 4 slices (from top of the right kidney) obtained at standard tube current (240 280 mA). All other scanning parameters were held constant. Patients' weight was measured just prior to the study. Cross-sectional abdominal dimensions such as circumference, area, average anterior abdominal wall fat thickness and, anteroposterior and transverse diameters were measured in all patients. Two subspecialty radiologists reviewed randomized images for overall image quality of abdominal structures using 5-point scale. Non-parametric correlation analysis was performed to determine the association of image quality with patients' weight and cross-sectional abdominal dimensions. RESULTS: A statistically significant negative linear correlation of 0.46, 0.47, 0.47, 0.58, 0.56, 0.54, and 0.56 between patient weight, anterior abdominal fat thickness, anteroposterior and transverse diameter, circumference, cross-sectional area and image quality at standard scanning parameters was found (p< 0.01). CONCLUSION: There is a significant association between image quality, patients' weight and cross-sectional abdominal dimensions. Maximum transverse diameter of the abdomen has the strongest association with subjective image quality.

Detection of Malignant Primary Hepatic Neoplasms with Gadobenate Dimeglumine (Gd-BOPTA) Enhanced T1-Weighted Hepatocyte Phase MR Imaging: Results of Off-site Blinded Review in a Phase-II Multicenter Trial

OBJECTIVE: To investigate the efficacy of gadobenate dimeglumine (Gd-BOPTA) enhanced MR imaging for the detection of liver lesions in patients with primary malignant hepatic neoplasms. MATERIALS AND METHODS: Thirty-one patients with histologically proven primary malignancy of the liver were evaluated before and after administration of Gd-BOPTA at dose 0.05 or 0.10 mmol/kg. T1-weighted spin echo (T1W-SE) and gradient echo (T1W-GRE) images were evaluated for lesion number, location, size and confidence by three off-site independent reviewers and the findings were compared to reference standard imaging (intraoperative ultrasound, computed tomography during arterial portography or lipiodol computed tomography). Results were analyzed for significance using a two-sided McNemar's test. RESULTS: More lesions were identified on Gd-BOPTA enhanced images than on unenhanced images and there was no significant difference in lesion detection between either concentration. The largest benefit was in detection of lesions under 1 cm in size (7 to 21, 9 to 15, 16 to 18 for reviewers A, B, C respectively). In 68% of the patients with more than one lesion, Gd-BOPTA increased the number of lesions detected. CONCLUSION: Liver MR imaging after Gd-BOPTA increases the detection of liver lesions in patients with primary malignant hepatic neoplasm.