MULTICENTRE COMPARISON OF PATIENT AND DETECTOR DOSE FOR X-RAY-GUIDED EMBOLISATIONS OF ARTERIOVENOUS MALFORMATIONS IN THE BRAIN.

Dosimetric benchmarking at four hospitals was performed to investigate incident entrance dose and dose rate on a phantom, and entrance detector dose and dose rate for protocols that are used in routine clinical practice for complex neuroradiological treatment of arteriovenous malformations (AVMs). Measurements were performed with a head phantom that simulates the attenuation and scattering of the human head for the lateral and posteroanterior (PA) views. For fluoroscopy, the measured incident entrance dose rate and entrance detector dose rate were in the range of 44-172 and 0.3-1.3 µGy s(-1), respectively. The pulse rate in fluoroscopy varied between 6.3 and 15 frames per second (fps). For digital subtraction angiography (DSA), incident entrance dose per frame and entrance detector dose per frame were in the range of 744-2800 and 2.6-8.1 µGy/frame, respectively. Optimisation of acquisition parameters such as pulse rate in fluoroscopy, dose per frame in DSA, beam filtration and tube voltage may further improve imaging protocols and lower the patient dose in very complex X-ray-guided embolisations of AVMs in the brain. However, differences in these acquisition parameters observed in this study were relatively small, suggesting that a relatively high degree of optimisation has already been achieved.

COMPARISON OF WIRELESS DETECTORS FOR DIGITAL RADIOGRAPHY SYSTEMS: IMAGE QUALITY AND DOSE.

The purpose of this study was to compare dose and image quality of wireless detectors for digital chest radiography. Entrance dose at both the detector (EDD) and phantom (EPD) and image quality were measured for wireless detectors of seven different vendors. Both the local clinical protocols and a reference protocol were evaluated. In addition, effective dose was calculated. Main differences in clinical protocols involved tube voltage, tube current, the use of a small or large focus and the use of additional filtration. For the clinical protocols, large differences in EDD (1.4-11.8 µGy), EPD (13.9-80.2 µGy) and image quality (IQFinv: 1.4-4.1) were observed. Effective dose was <0.04 mSv for all protocols. Large differences in performance were observed between the seven different systems. Although effective dose is low, further improvement of imaging technology and acquisition protocols is warranted for optimisation of digital chest radiography.

MULTICENTRE COMPARISON OF IMAGE QUALITY FOR LOW-CONTRAST OBJECTS AND MICROCATHETER TIPS IN X-RAY-GUIDED TREATMENT OF ARTERIOVENOUS MALFORMATION IN THE BRAIN.

The treatment of brain arteriovenous malformations (AVMs) can be performed as a minimally invasive X-ray-guided procedure using a microcatheter for navigation to reach the target site. The performance of the interventional vascular surgery devices used for AVM was compared in four hospitals. The relation between image quality and the entrance surface air kerma (ESAK) was assessed for the default protocols for digital subtraction angiography (DSA) and fluoroscopy. A custom phantom, built with PMMA and aluminium plates was used to mimic the attenuation properties of the patient head. Image quality was assessed using low-contrast objects and catheters embedded in two phantoms. Differences were found in the ESAK values, especially for the fluoroscopy, whereas for DSA, the ESAK values were similar. The differences in image quality can be related to acquisition parameters, such as kV and filtration, and post-processing. The proposed method can be used to optimise the existing AVM protocols.

Low contrast detectability performance of model observers based on CT phantom images: kVp influence.

This paper studies low contrast detectability (LCD) performance of two model observers in CT phantom images acquired at different kVp levels and compares the results with humans in a 2-alternative forced choice experiment (2-AFC). Images of the Catphan phantom with objects of different contrasts (0.5 and 1%) and diameters (2-15 mm) were acquired in an Aquilion ONE 320-detector row CT (Toshiba Medical Systems, Tokyo, Japan), in two experiments, selecting (80-100-120-135 kV) with fixed mAs and varying the mAs to keep the dose constant, respectively. Four human observers evaluated the objects visibility obtaining a proportion correct (PC) for each case. LCD was also analyzed with two model observers (non-prewhitening matched filter with an eye filter, NPWE, and channelized Hotelling observer with Gabor channels, CHO). Object contrast was affected by kV, with differences up to 17% between the lowest and highest kV. Both models overestimated human performance and were corrected by efficiency and internal noise factors. The NPWE model reproduced better the human PC values trends showing Pearson's correlation coefficients ≥0.976 (0.954-0.987, 95% CI) for both experiments, whereas for CHO they were ≥0.706 (0.493-0.839). Bland-Altman plots showed better agreement between NPWE and humans being the average difference Δ and the range of the differences Δ±2σ (σ, standard deviation) of Δ=-0.3%, Δ±2σ = [-4.0%,4.5%]. For CHO, Δ=-1.2%, Δ± 2σ= [-10.7%,8.3%]. The NPWE model can be a useful tool to predict human performance in CT low contrast detection tasks in a standard phantom and be potentially used in protocol optimization based on kV selection.

Monte Carlo simulation of the dose distribution of ICRP adult reference computational phantoms for acquisitions with a 320 detector-row cone-beam CT scanner.

PURPOSE: The purpose of this study was to develop and validate a Monte Carlo (MC) simulation tool for patient dose assessment for a 320 detector-row CT scanner, based on the recommendations of International Commission on Radiological Protection (ICRP). Additionally, the simulation was applied on four clinical acquisition protocols, with and without automatic tube current modulation (TCM). METHODS: The MC simulation was based on EGS4 code and was developed specifically for a 320 detector-row cone-beam CT scanner. The ICRP adult reference phantoms were used as patient models. Dose measurements were performed free-in-air and also in four CTDI phantoms: 150 mm and 350 mm long CT head and CT body phantoms. The MC program was validated by comparing simulations results with these actual measurements acquired under the same conditions. The measurements agreed with the simulations across all conditions within 5%. Patient dose assessment was performed for four clinical axial acquisitions using the ICRP adult reference phantoms, one of them using TCM. RESULTS: The results were nearly always lower than those obtained from other dose calculator tools or published in other studies, which were obtained using mathematical phantoms in different CT systems. For the protocol with TCM organ doses were reduced by between 28 and 36%, compared to the results obtained using a fixed mA value. CONCLUSIONS: The developed simulation program provides a useful tool for assessing doses in a 320 detector-row cone-beam CT scanner using ICRP adult reference computational phantoms and is ready to be applied to more complex protocols.

Comparison between human and model observer performance in low-contrast detection tasks in CT images: application to images reconstructed with filtered back projection and iterative algorithms.

OBJECTIVE: To compare low-contrast detectability (LCDet) performance between a model [non-pre-whitening matched filter with an eye filter (NPWE)] and human observers in CT images reconstructed with filtered back projection (FBP) and iterative [adaptive iterative dose reduction three-dimensional (AIDR 3D; Toshiba Medical Systems, Zoetermeer, Netherlands)] algorithms. METHODS: Images of the Catphan® phantom (Phantom Laboratories, New York, NY) were acquired with Aquilion ONE™ 320-detector row CT (Toshiba Medical Systems, Tokyo, Japan) at five tube current levels (20-500 mA range) and reconstructed with FBP and AIDR 3D. Samples containing either low-contrast objects (diameters, 2-15 mm) or background were extracted and analysed by the NPWE model and four human observers in a two-alternative forced choice detection task study. Proportion correct (PC) values were obtained for each analysed object and used to compare human and model observer performances. An efficiency factor (η) was calculated to normalize NPWE to human results. RESULTS: Human and NPWE model PC values (normalized by the efficiency, η = 0.44) were highly correlated for the whole dose range. The Pearson's product-moment correlation coefficients (95% confidence interval) between human and NPWE were 0.984 (0.972-0.991) for AIDR 3D and 0.984 (0.971-0.991) for FBP, respectively. Bland-Altman plots based on PC results showed excellent agreement between human and NPWE [mean absolute difference 0.5 ± 0.4%; range of differences (-4.7%, 5.6%)]. CONCLUSION: The NPWE model observer can predict human performance in LCDet tasks in phantom CT images reconstructed with FBP and AIDR 3D algorithms at different dose levels. ADVANCES IN KNOWLEDGE: Quantitative assessment of LCDet in CT can accurately be performed using software based on a model observer.

A national survey on radiation dose in CT in The Netherlands.

OBJECTIVES: To assess radiation exposure due to CT in the Netherlands. METHODS: Twenty-one hospitals participated in a dose survey for the 21 most frequently used CT protocols. Hospitals completed a Web survey with detailed parameters for one patient per protocol, including the dose length product (DLP) from the scanner dose report. Only standard-sized patients (1.74 m and 77 kg and BMI 25.4 kg/m(2) ± 15 %) for each protocol and available scanner were considered. Effective dose (E) per protocol was estimated using ICRP-103-based E/DLP coefficients. Dose levels were compared to surveys from other countries and to diagnostic reference levels. RESULTS: Data of 186 patients (247 scan phases) from 14 hospitals and 19 scanners were used for final analysis of DLP and E. Effective doses varied from 0.2 mSv in sinus CT to 19.4 mSv for multiphase liver. The most frequent exams were brain (1.5 mSv), abdomen (8.0 mSv), and thorax-abdomen (11.5 mSv). These results are lower than in Germany and comparable to those in the UK, and are within reference levels. Results between hospitals varied, with per protocol minimum/maximum E ratios ranging from 1.1-5.4. CONCLUSIONS: Compared to surrounding countries, CT in the Netherlands is associated with relatively low radiation doses in standard patients. Important differences remain between hospitals. MAIN MESSAGES: • A national dose survey providing updated, detailed data for patient dose in the most frequently used CT protocols. • CT in the Netherlands is associated with relatively low individual radiation doses in standard patients compared to surrounding European countries. • Considerable differences remain between hospitals for the most frequently used CT protocols, indicating the need for further optimisation.

Radiation exposure and mortality risk from CT and PET imaging of patients with malignant lymphoma.

OBJECTIVE: To quantify radiation exposure and mortality risk from computed tomography (CT) and positron emission tomography (PET) imaging with (18)F-fluorodeoxyglucose ((18)F-FDG) in patients with malignant lymphoma (Hodgkin's disease [HD] or non-Hodgkin's lymphoma [NHL]). METHODS: First, organ doses were assessed for a typical diagnostic work-up in children with HD and adults with NHL. Subsequently, life tables were constructed for assessment of radiation risks, also taking into account the disease-related mortality. RESULTS: In children with HD, cumulative effective dose from medical imaging ranged from 66 mSv (newborn) to 113 mSv (15 years old). In adults with NHL the cumulative effective dose from medical imaging was 97 mSv. Average fractions of radiation-induced deaths for children with HD [without correction for disease-related mortality in brackets] were 0.4% [0.6%] for boys and 0.7% [1.1%] for girls, and for adults with NHL 0.07% [0.28%] for men and 0.09% [0.37%] for women. CONCLUSION: Taking into account the disease-related reduction in life expectancy of patients with malignant lymphoma results in a higher overall mortality but substantial lower incidence of radiation induced deaths. The modest radiation risk that results from imaging with CT and (18)F-FDG PET can be considered as justified, but imaging should be performed with care, especially in children.

Automated assessment of low contrast sensitivity for CT systems using a model observer.

PURPOSE: Low contrast sensitivity of CT scanners is regularly assessed by subjective scoring of low contrast detectability within phantom CT images. Since in these phantoms low contrast objects are arranged in known fixed patterns, subjective rating of low contrast visibility might be biased. The purpose of this study was to develop and validate a software for automated objective low contrast detectability based on a model observer. METHODS: Images of the low contrast module of the Catphan 600 phantom were used for the evaluation of the software. This module contains two subregions: the supraslice region with three groups of low contrast objects (each consisting of nine circular objects with diameter 2-15 mm and contrast 0.3, 0.5, and 1.0%, respectively) and the subslice region with three groups of four circular objects each (diameter 3-9 mm; contrast 1.0%). The software method offered automated determination of low contrast detectability using a NPWE (nonprewhitening matched filter with an eye filter) model observer for the supraslice region. The model observer correlated templates of the low contrast objects with the acquired images of the Catphan phantom and a discrimination index d' was calculated. This index was transformed into a proportion correct (PC) value. In the two-alternative forced choice (2-AFC) experiments used in this study, a PC ≥ 75% was proposed as a threshold to decide whether objects were visible. As a proof of concept, influence of kVp (between 80 and 135 kV), mAs (25-200 mAs range) and reconstruction filter (four filters, two soft and two sharp) on low contrast detectability was investigated. To validate the outcome of the software in a qualitative way, a human observer study was performed. RESULTS: The expected influence of kV, mAs and reconstruction filter on image quality are consistent with the results of the proposed automated model. Higher values for d' (or PC) are found with increasing mAs or kV values and for the soft reconstruction filters. For the highest contrast group (1%), PC values were fairly above 75% for all object diameters >2 mm, for all conditions. For the 0.5% contrast group, the same behavior was observed for object diameters >3 mm for all conditions. For the 0.3% contrast group, PC values were higher than 75% for object diameters >6 mm except for the series acquired at the lowest dose (25 mAs), which gave lower PC values. In the human observer study similar trends were found. CONCLUSIONS: We have developed an automated method to objectively investigate image quality using the NPWE model in combination with images of the Catphan phantom low contrast module. As a first step, low contrast detectability as a function of both acquisition and reconstruction parameter settings was successfully investigated with the software. In future work, this method could play a role in image reconstruction algorithms evaluation, dose reduction strategies or novel CT technologies, and other model observers may be implemented as well.

Development and validation of a low dose simulator for computed tomography.

PURPOSE: To develop and validate software for facilitating observer studies on the effect of radiation exposure on the diagnostic value of computed tomography (CT). METHODS: A low dose simulator was developed which adds noise to the raw CT data. For validation two phantoms were used: a cylindrical test object and an anthropomorphic phantom. Images of both were acquired at different dose levels by changing the tube current of the acquisition (500 mA to 20 mA in five steps). Additionally, low dose simulations were performed from 500 mA downwards to 20 mA in the same steps. Noise was measured within the cylindrical test object and in the anthropomorphic phantom. Finally, noise power spectra (NPS) were measured in water. RESULTS: The low dose simulator yielded similar image quality compared with actual low dose acquisitions. Mean difference in noise over all comparisons between actual and simulated images was 5.7 +/- 4.6% for the cylindrical test object and 3.3 +/- 2.6% for the anthropomorphic phantom. NPS measurements showed that the general shape and intensity are similar. CONCLUSION: The developed low dose simulator creates images that accurately represent the image quality of acquisitions at lower dose levels and is suitable for application in clinical studies.

Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner.

Computed tomography (CT) dosimetry should be adapted to the rapid developments in CT technology. Recently a 160 mm wide, 320 detector row, cone beam CT scanner that challenges the existing Computed Tomography Dose Index (CTDI) dosimetry paradigm was introduced. The purpose of this study was to assess dosimetric characteristics of this cone beam scanner, to study the appropriateness of existing CT dose metrics and to suggest a pragmatic approach for CT dosimetry for cone beam scanners. Dose measurements with a small Farmer-type ionization chamber and with 100 mm and 300 mm long pencil ionization chambers were performed free in air to characterize the cone beam. According to the most common dose metric in CT, namely CTDI, measurements were also performed in 150 mm and 350 mm long CT head and CT body dose phantoms with 100 mm and 300 mm long pencil ionization chambers, respectively. To explore effects that cannot be measured with ionization chambers, Monte Carlo (MC) simulations of the dose distribution in 150 mm, 350 mm and 700 mm long CT head and CT body phantoms were performed. To overcome inconsistencies in the definition of CTDI100 for the 160 mm wide cone beam CT scanner, doses were also expressed as the average absorbed dose within the pencil chamber (D100). Measurements free in air revealed excellent correspondence between CTDI300air and D100air, while CTDI100air substantially underestimates CTDI300air. Results of measurements in CT dose phantoms and corresponding MC simulations at centre and peripheral positions were weighted and revealed good agreement between CTDI300w, D100w and CTDI600w, while CTDI100w substantially underestimates CTDI300w. D100w provides a pragmatic metric for characterizing the dose of the 160 mm wide cone beam CT scanner. This quantity can be measured with the widely available 100 mm pencil ionization chamber within 150 mm long CT dose phantoms. CTDI300w measured in 350 mm long CT dose phantoms serves as an appropriate standard of reference for characterizing the dose of this CT scanner. A CT dose descriptor that is based on an integration length smaller than the actual beam width is preferably expressed as an (average) dose, such as D100 for the 160 mm wide cone beam CT scanner, and not as CTDI100.

Evaluation of 4 multisection CT systems in postoperative imaging of a cochlear implant: a human cadaver and phantom study.

BACKGROUND AND PURPOSE: Postoperative imaging of cochlear implants (CIs) needs to provide detailed information on localization of the electrode array. We evaluated visualization of a HiFocus1J array and accuracy of measurements of electrode positions for acquisitions with 64-section CT scanners of 4 major CT systems (Toshiba Aquilion-64, Philips Brilliance-64, GE LightSpeed-64, and Siemens Sensation-64). MATERIALS AND METHODS: An implanted human cadaver temporal bone, a polymethylmethacrylate (PMMA) phantom containing a CI, and a point spread function (PSF) phantom were scanned. In the human cadaver temporal bone, the visibility of cochlear structures and electrode array were assessed by using a visual analog scale (VAS). Statistical analysis was performed with a paired 2-tailed Student t test with significant level set to .008 after Bonferroni correction. Distinction of individual electrode contacts was quantitatively evaluated. Quantitative assessment of electrode contact positions was achieved with the PMMA phantom by measurement of the displacement. In addition, PSF was measured to evaluate spatial resolution performance of the CT scanners. RESULTS: VAS scores were significantly lower for Brilliance-64 and LightSpeed-64 compared with Aquilion-64 and Sensation-64. Displacement of electrode contacts ranged from 0.05 to 0.14 mm on Aquilion-64, 0.07 to 0.16 mm on Brilliance-64, 0.07 to 0.61 mm on LightSpeed-64, and 0.03 to 0.13 mm on Sensation-64. PSF measurements show an in-plane and longitudinal resolution varying from 0.48 to 0.68 mm and 0.70 to 0.98 mm, respectively, over the 4 scanners. CONCLUSION: According to PSF results, electrode contacts of the studied CI can be visualized separately on all of the studied scanners unless curvature causes intercontact spacing narrowing. Assessment of visibility of CI and electrode contact positions, however, varies between scanners.

Effects of heart rate, filling and slice thickness on the accuracy of left ventricular volume measurements in a dynamic cardiac phantom using ECG-gated MDCT.

The objective of the study was to assess the effect of heart rate, filling condition and slice thickness on the accuracy of volumetric analysis based on multidetector-row computed tomography (MDCT) of a cardiac phantom. Retrospective electrocardiogram-gated MDCT of a pulsating phantom was performed under different conditions. End-diastolic volume (EDV) and end-systolic volume (ESV) for different heart rates (60-75 beats per minute), filling volumes and reconstructed slice thicknesses (2 mm and 5 mm) were obtained by three observers. Results were analysed by a linear mixed-effects model. Significant effects on the accuracy were found for heart rate (F-value, 7.3-39.2; p<0.004) and filling condition (F-value, 7.4-55.6; p<0.004), but not for slice thickness. Small relative differences in the assessment of EDV were found (range, -3% to 3%), but there was a trend for overestimation of the ESV (range, -1% to 18%). Underestimation of stroke volume and ejection fraction (range, -1% to -11%) became smaller under conditions of improved temporal resolution and larger EDV. Good interobserver agreement was found (SD <1.8 ml and <0.5%). In conclusion, MDCT allows sufficient and reliable measurements of ventricular volumes and calculation of left ventricle function for clinical applications. Heart rate and filling conditions significantly affect the accuracy of volumetrics, as demonstrated in this cardiac phantom. Thicker slices provide similar accuracy to thin slices.

Dose reduction in digital chest radiography and perceived image quality.

The aim of the study was to investigate whether radiologists can rank the image quality of digital radiographs with different doses; a preliminary study investigated whether reduced dose images provide sufficient diagnostic quality. Raw data of 40 chest radiographs (posteroanterior (PA) and lateral) obtained with a full-field slot-scan charge-coupled device system in 20 patients with chest pathology were used. Noise was added to simulate reduced dose levels to 50%, 25% and 12%. Four observers ranked the quality of the corresponding images and judged the diagnostic quality. Linear regression analysis was performed. Differences were found in image quality at the different dose levels for both PA (p

Artifacts in ECG-synchronized MDCT coronary angiography.

OBJECTIVE: In MDCT coronary angiography, image artifacts are the major cause of false-positive and false-negative interpretations regarding the presence of coronary artery stenoses. Hence, it is important that observers reporting these investigations are aware of the potential presence of image artifacts and that these artifacts are recognized. CONCLUSION: The article explores the technical causes for various artifacts in MDCT coronary angiography imaging and clinical examples are given.

16-slice CT: achievable effective doses of common protocols in comparison with recent CT dose surveys.

The aim of the study was to investigate achievable dose levels in 16-slice CT by evaluating CT dose indices (CTDI) and effective doses of dose-optimized protocols compared with 4-slice dose surveys. Normalized CTDI free in air and in 16 cm and 32 cm diameter phantoms were measured on four different 16-slice CT scanners in the Netherlands. All collimation and tube potential settings were analysed. Volume CTDI was calculated for adult protocols for brain, chest, pulmonary angiography (CTPA), abdomen and biphasic liver CT. Effective doses were calculated first using volume CTDI with conversion factors and second from CTDIair values using the ImPACT dose calculator. Average results of the 16-slice scanners were correlated to results of dose surveys with predominantly 4-slice scanners. Statistical analysis was done with Student t-tests with a Bonferroni correction; therefore p < 0.017 was significant. The results of CTDIair and weighted CTDI were documented for all scanners. Effective doses averaged over four scanners for brain, chest, CTPA, abdomen and biphasic liver protocols were 1.9+/-0.4, 3.8+/-0.4, 3.0+/-0.2, 7.2+/-0.9 and 10.2+/-1.3 mSv, respectively. Compared with dose surveys achievable effective doses were equal (p = 0.069) to significantly lower (p < 0.017) for chest and abdomen protocols. For 16-slice spiral brain CT there was a trend of equal doses compared with sequential brain CT in the dose surveys. Thus, with dose-optimized protocols 16-slice CT can achieve equal to lower effective doses in examinations of the chest and abdomen compared with 4-slice CT, while doses can remain stable in the brain.

An inter-hospital comparison of patient dose based on clinical indications.

Patient dose is usually estimated for a single radiographic projection or computed tomography (CT) series. In this study, patient dose was calculated for predefined clinical indications (24 radiography, 11 CT). Members of the radiology staff of each of 11 hospitals were trained in dose measurement and calculation techniques. Based on clinical indications participants decided on imaging protocols and calculated cumulative effective dose for a complete examination. Effective dose ranged from <1 microSv to 0.6 mSv for examinations with radiographs and from 0.2 to 12 mSv for CT scans. Differences in the imaging protocols contributedd to a substantial variation in patient dose. For mammography, average glandular dose (AGD) was estimated for 32-, 53- and 90-mm compressed breast thicknesses, with a median value of 0.74, 1.74 and 3.40 mGy, respectively. The results presented here demonstrate that a pragmatic choice of dosimetry methods enables local staff to estimate effective dose. The inclusion of imaging protocols in the dose surveys provided a broader view on the variations in patient dose between hospitals.

Quantitative assessment of selective in-plane shielding of tissues in computed tomography through evaluation of absorbed dose and image quality.

This study aimed at assessment of efficacy of selective in-plane shielding in adults by quantitative evaluation of the achieved dose reduction and image quality. Commercially available accessories for in-plane shielding of the eye lens, thyroid and breast, and an anthropomorphic phantom were used for the evaluation of absorbed dose and image quality. Organ dose and total energy imparted were assessed by means of a Monte Carlo technique taking into account tube voltage, tube current, and scanner type. Image quality was quantified as noise in soft tissue. Application of the lens shield reduced dose to the lens by 27% and to the brain by 1%. The thyroid shield reduced thyroid dose by 26%; the breast shield reduced dose to the breasts by 30% and to the lungs by 15%. Total energy imparted (unshielded/shielded) was 88/86 mJ for computed tomography (CT) brain, 64/60 mJ for CT cervical spine, and 289/260 mJ for CT chest scanning. An increase in image noise could be observed in the ranges were bismuth shielding was applied. The observed reduction of organ dose and total energy imparted could be achieved more efficiently by a reduction of tube current. The application of in-plane selective shielding is therefore discouraged.