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Using of eye Bismuth shields in CT scan: its influence on ESD and image quality

P. A. T. Gamboa1, A. P. N. Batista2, M. M. C. P. Ribeiro2, A. L. C. F. Vieira2, H. N. Sousa3; 1 Vila Franca de Xira/PT 2 Lisbon/PT 3 Torres Vedras/PT


Purpose: Evaluate the impact of using the eye Bismuth shields in ESD (Entrance Skin Dose) and the effect on image quality in brain CT scans.
Methods and Materials: Two CT routine acquisitions of the brain were performed using a CT Activation 16 scanner and an anthropomorphic phantom RSD: with and without Bismuth shields placed over the orbital region. The ESD was evaluated by four TLD-100(LiF:Mg) dosimeters, two for each acquisition (one placed on the nasion -frontal bone - and the other on the inion - occipital bone). The acquisition parameters were fixed. The image quality was quantitatively analysed by five ROI's (two placed in the anterior, one in the middle and two in the posterior region of the brain), and qualitatively by three experienced neuroradiologists through a survey based on a comparative visual analysis test, under the criterias: image contrast; noise; anatomical evaluation; pathological evaluation, dose reduction and global image quality.
Results: The ESD values ​​show a reduction of 61% in the crystalline, with the use of these shields (34,13mGy vs 58,03mGy in average). The observers considered their beneficial use in selected cases. The quantitative results ​​show that the Bismuth shields cause artefacts and increase image noise in the frontal brain parenchyma.
Conclusion: The Bismuth shields are advantageous in certain areas/pathologies, and disadvantageous for studies of the anterior brain parenchyma. Radiographers and doctors should consider the advantages and disadvantages of these shields application in each specific case.


A multi-site head CT topogram acquisition protocol analysis based on DICOM metadata

M. Santos, L. Bastião, N. Rocha, A. Silva; Aveiro/PT


Purpose: Several international initiatives and scientific studies emphasise the need to promote a greater focus on patient safety when choosing Computed Tomography (CT) scan parameters. This work aimed to analyse the CT Topogram dimension and scan parameters used in Head CT studies conducted in three distinct CT equipment and based on DICOM metadata stored at PACS facilities from two health institutions.
Methods and Materials: The metadata produced by three different Computed Tomography (CT) equipment and stored in two health care PACS facilities were systematically analysed using a DICOM Metadata mining approach. For studies characterisation several DICOM tags were analysed (e.g. mAs, KvP, Exposure Time, Modality, Patient Age) within different queries. The CT Topogram dimension was obtained from the Exposure Time value and the table feed during the topograma acquisition.
Results: The DICOM metadata from 1.274.927 images, bellowing to 19.180 studies performed on 16.169 patients (including 768 paediatric patients) were analysed. The number of CT Topogram images performed per patient ranged from 1 to 2,33 and its dimension between 20,76 cm and 25,16 cm depending on the equipment (CT1=20,8cm±0,04; CT2=24,01cm±0,4; CT3=25,01cm±0,5), namely in paediatrics studies (CT1=20,78cm±0,01; CT2=23,61cm±0,71; CT3=24,98cm±0,01). With regard to CT Topogram scan parameters, no significant changes were identified when the x-ray Tube Current (CT1=10mA±0,02; CT2=49,98 mA±0,03; CT3=39,47 mA±2,29) and KvP values were analysed (CT1= 119,54KvP±1,65; CT2=120,02KvP±0,04; CT3=90KvP).
Conclusion: The age groups (including the paediatric patients) are not factors determining significant changes in the values of the exposure factors and size of the area exposed to radiation upon topogram acquisition.


Investigating optimal abdominal CT protocol for obese patients

A. A. Qurashi1, L. A. Rainford1, A. Ajlan2, K. Khashoggi2, L. Ashkar2, M. Alraddadi3, M. Alghamdi2, M. Althobaiti2, S. J. Foley1; 1 Dublin/IE 2 Jeddah/SA 3 Medina/SA


Purpose: To investigate optimal CT parameters when scanning obese patients.
Methods and Materials: With ethical permission, abdominal CT scans for 47 obese patients (AP diameter>26cm, BMI>30kgm-2) were retrospectively collected from 3 different CT models from a single manufacturer over three-month period. Patients were scanned using 5 different protocols, due to various settings used locally (kV: 100/120, mAs: ATCM reference mA: 150, 190, 218, 250, 300). When available, images were retrospectively reconstructed using various SAFIRE strengths (2,3,4 and 5) and FBP. Subjective image quality was determined by three experienced radiologists using visual grading analysis in accordance to EUR 16262. Comparison between radiation dose and image quality was performed using one-way-anova test.
Results: Patients ranged in weight from 65-190 kg with DLP ranging from 345-1525mGy.cm. Mean dose was lowest (up to 60%) for the protocol using 100 kVp technique with statistical differences noted between this protocol and all other scanning techniques (p<0.05). All mean image quality scores for all the protocols being examined were deemed suitable for diagnostic interpretation. Images reconstructed with FBP for the protocol with 100 kVp were given the lowest score (p<0.05) compared to 120 kVp protocols, especially those with higher reference mAs. However, when the images for the low dose protocol were reconstructed using SAFIRE no significant difference was seen.
Conclusion: Numerous CT protocols are being used clinically for obese patients, with wide variations in dose and image quality. 100kVp protocols produced acceptable image quality for obese patients with up to 60% dose reduction than 120 kVp protocols when IR is used.


Optimisation of CT via analysis of computational fluid dynamics in cerebral aneurysm

M. Ogasawara1, T. Hirano1, T. Yagi2, M. Nakamoto2, H. Nagahama1; 1 Sapporo/JP 2 Tokyo/JP


Purpose: Computational fluid dynamics (CFD) in a cerebral aneurysm (CA) is a recently reported method for analyses of CA haemodynamics. However, the reliability of analytical data and optimal CT scan conditions remain unclear. The objective of this study was to investigate optimal scan conditions and contrast medium concentrations for CFD.
Methods and Materials: We performed scans using a 320-slice CT scanner (Aquilion ONE; Toshiba Medical Corporation) and CFD analysis software (hemoscope; EBM Co., Ltd). The hand-made CA phantom contained enclosed concentrations of contrast medium (CT values: 200-500 HU) and silicon (straight line and clinical model). We obtained images using varying noise index values and reconstruction FOVs and used these images to investigate wall shear stress (WSS).
Results: The CFD analytical results indicated a stable maximum WSS (WSSmax) at CT values of 300-500 HU and scan doses of SD20, SD15, and SD10 [straight line model: 2.63 ± 0.42 (16.1%) Pa, clinical model: 16.82 ± 3.80 (22.6%) Pa]. Our results indicated a tendency of the reconstruction FOV (image resolution) and WSSmax-WSSmin (exponential) to converge. Therefore, this technique seems to improve analytical accuracy by setting a smaller reconstruction FOV.
Conclusion: We determined that for CT-based CFD analysis of CA, the optimal scan conditions include a CT value >300 HU, scan dose >SD20, and smaller reconstruction FOV. Under these conditions, all users will be able to obtain a stable, accurate WSS analysis.


Evaluate the effect of contrast media on patient dose in CT examination of abdomen-pelvic performed with AEC

O. Seraydarmansour; Tehran/IR


Purpose: The aim of our study was to investigate the relative dose increase when an abdomen-pelvic CT examination is carried out after administration of oral and IV contrast medium with respect to the same CT scan without contrast medium.
Methods and Materials: Forty patients who underwent both unenhanced and contrast-enhanced abdomen-pelvic CT scan. All examinations were performed using an automatic tube potential selection tool on a 16MDCT scanner (Ingenuity, Philips) with the following protocol: Collimation 16X1.5mm, 120Kvp, pitch 1.37, rotation time 0.5sec. All CT images were reconstructed at 5-millimeter section thickness with a standard reconstruction algorithm. The oral contrast agent was made up of 40ml of meglumine compound (370mgI/ml ) mixes in 1 litre of water 3 hours before scanning. Iodinated contrast medium (Ultravist 350) containing 350mgI/ml was administered IV at a dose of 1.2ml/kg of patient weight. Contrast-enhanced CT examination was performed 60 seconds after the start of injection. Two abdominal radiologists evaluated the images for diagnostic confidence.
Results: CTDI vol values obtained for each patient on dose report at the end of CT examinations. An increase in patient dose was noted in all CT examinations with IV and oral contrast compared to unenhanced CT examinations. Average dose increase was 11%.
Conclusion: Our evaluation showed an increase in patient dose when iodinated contrast medium is used in CT examination of abdomen-pelvic basing on the increment in Hounsfield units on the organs.


Impact of acquisition protocol variations on effective dose for CT head examinations

M. R. Benhalim, A. England; Salford/UK


Purpose: For range of acquisition protocols, determine the most dominant acquisition factors which impact on effective dose for paediatric CT brain examinations.
Methods and Materials: An ATOM phantom (1-year old child) was implanted with metal oxide semiconductor field effect transducer (MOSFET) dosimeters; these were inserted into 24 organs (167 predrilled holes). Helical CT brain scanning was undertaken using 54 protocols; protocol variations included changes in rotation times, gantry angulation, tube current/potential and fixed slice thickness (0.5x16). ED was calculated for each acquisition protocol.
Results: ED ranged from 1.01 mSv and 4.81 mSv. Organs located within the scan volume consistently received the highest absorbed doses; organs at the periphery and outside of the scan volume still received a radiation dose. ED trends were as follows: ED increased consistently by around 33% when increasing kVp from 100 to 120 (mean ED 2.35 SD 0.75 mSv versus 3.52 SD 1.21 mSv, respectively; P<0.05). Changes in gantry angle had minimal effect on ED, when moving from a zero gantry angle to +27 degrees the mean ED decreased by 0.04 mSv (P>0.05). Changing the tube current from 120 to 200 mAs lead to increase in ED by 27.7% (mean ED 1.50 SD 0.40 mSv vice versa 2.23 SD 0.65 mSv, respectively; P<0.05). Increasing the rotation time from 0.5 seconds to 1 seconds showed increase in ED by 33.4% (mean 1.52 SD 0.46 mSv versus 2.25 SD 0.80 mSv; P<0.05).
Conclusion: The most dominant factors affecting ED are tube current, tube potential and scan rotation time.


Effective dose comparison between fixed tube current FTC and automatic tube current ATC methods for abdominal CT examinations

M. Alrowily, A. England, P. Hogg; Manchester/UK


Purpose: Fixed tube current (FTC) and automatic tube current modulation ATCM are options used during abdominal CT exams. The aim of this study was to evaluate the effective radiation dose E when comparing FTC and ATCM techniques, using different detector configurations and pitch factors, for abdominal CT scans.
Methods and Materials: Using a Toshiba Aquilion 16 CT scanner 120 kVp, 0.5 seconds tube rotation an adult ATOM dosimetry phantom was exposed to a series of FTC and ATCM CT protocols with variation in detector configuration 0.5mmx16; 1.0mmx16 and 2.0mmx16 and pitch factors 0.688, 0.938 and 1.438. Effective dose E estimations were undertaken using two methods, one using mathematical modelling with k-factors and DLP and the other using Metal Oxide Semiconductor Field Effect Transistor MOSFET dosimeters and tissue weighting factors.
Results: The mean E for ATCM and FTC methods using MOSFET were 5.9mSv and 4.7mSv, respectively. The mean E using DLP and k-factors were 6.3mSv and 5.1mSv, respectively. Using both E estimation methods the radiation doses from ATCM techniques were around 20% higher P<0.006. Good correlation existed between ATCM and FTC doses R2>0.82.
Conclusion: In this study, ATCM effective doses were higher than FTC using both dosimetry techniques. E estimated by DLP and k-factors were higher than when directly measured by MOSFET. Differences between the two methods could be due to the limitations of Monte Carlo modelling used in the DLP/k factors approach.


Establishment of diagnostic reference levels in computed tomography

D. Castro, P. Sousa, S. Rodrigues, A. F. Abrantes, R. P. P. Almeida, K. Azevedo, L. P. V. Ribeiro; Faro/PT


Purpose: To measure radiation dose levels in computed tomography of two regional public hospitals (A and B) and to compare them with international guidelines.
Methods and Materials: A survey of technical parameters (type of exam, patient age, kV, mAs, CTDIvol and DLP) from 446 CT exams was used to establish the Diagnostic Reference Levels (DRLs). For viability measurements, the dose rate and quality control using the RF detector Unfors Xi and PMMA (polymethylmethacrylate) phantom were carried out.
Results: The Diagnostic Reference Levels established for the Hospital A were 883.22 mGy.cm (Head), 302.18 mGy.cm (Neck), 371.46 mGy.cm (Cervical spine), 406.97 mGy.cm (Lumbar spine), 225.08 mGy.cm (Chest), 274.82 mGy.cm (Abdomen), 463.62 mGy.cm (Abdomen/Pelvis) and 582.78 mGy.cm (Ches/Abdomen/Pelvis). For the Hospital B and considering the same CT exams, DRLs values were, respectively, 959.37 mGy.cm, 339.45 mGy.cm, 306.73 mGy.cm, 480.73 mGy.cm, 243.59 mGy.cm, 278.24 mGy.cm, 606.22 mGy.cm and 674.33 mGy.cm. Despite the DRLs of the Hospital B being generally higher than the Hospital A, they are both lower than all international guidelines considered.
Conclusion: DRLs of both regional hospitals are in general greatly lower than the international guidelines. However, it is important to optimise the protocols used and similar studies in other hospitals are necessary in order to establish a National Dose Reference Level.


Single- and dual-energy quantitative CT adjacent to acetabular prosthetic components: a reliability study

B. R. Mussmann, P. E. Andersen, T. Torfing, S. Overgaard; Odense/DK


Purpose: Density measurements adjacent to acetabular prosthetic components are challenged by artifacts and the complex anatomy. Three-dimensional ROIs are needed to assess the bone stock. The purpose was to test the intraobserver agreement and reliability of custom segmentation software and to compare BMD measurements in single and dual energy CT (SECT and DECT).
Methods and Materials: 10 male patients with uncemented hip prosthetics were scanned and rescanned using 120 kVp SECT and DECT with virtual monochromatic images reconstructed at 130 keV. Hemispherical ROIs were defined slice-by-slice and bone mineral density was calculated using a calibration phantom. Median time between ROI drawings was 6 days.
Results: The mean BMD for repeated SECT scans was 430 mg/ccm with a between scan difference of 18 mg/ccm, p=0.001. ICC was 0.98 (95%CI: 0.73 to 1). For DECT the mean BMD was 162 mg/ccm with a difference of 11 mg/ccm, p<0.0001. ICC was 0.94 (95%CI: 0.36 to 0.99). Bland-Altman Limits of Agreement were -51 to 15 in SECT and -30 to 7 in DECT. Repeatability coefficients were 33 and 18 mg/ccm for SECT and DECT respectively. The difference was not statistically significant.
Conclusion: The intraobserver reliability was high for both scan modes with statistically significant differences between the repeated measurements in SECT and DECT. The limits of agreement were slightly narrower in DECT, but with no statistically significant difference between repeatability coefficients. The results suggest that the intraobserver agreement of the scan modes is equal. BMD cannot be measured interchangeably with SECT and DECT.

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