Automatic Exposure Control Technique Research Articles

Exploration of the pulse pileup effects in a clinical CdTe-based photon-counting computed tomography.

High tube current generates a high flux of x-rays to photon counting detectors (PCDs) that can potentially result in the piling up of pulses formed by concurrent photons, which can cause count loss and energy resolution degradation. To evaluate the performance of clinical photon-counting CT (PCCT) systems in high flux, potentially influenced by pulse pileup effects, in terms of task-generic image quality metrics. A clinical phantom was scanned on a commercial PCCT scanner (NAEOTOM Alpha, Siemens) at 120kV under fourteen different tube current levels (40-1000mA) with a rotation time of 0.25 s and a pitch of 1. The dose levels corresponded to CTDIvol (32cm phantom) of 0.79-19.8mGy. CT sinograms were reconstructed using QIR-off mode (noniterative reconstruction algorithm), Br44 kernel, and a voxel size of . imQuest, an open-source MATLAB-based software package was used to calculate noise power spectrum (NPS), task transfer function (TTF), contrast-to-noise ratio (CNR), and CT number according to AAPM Task Group 233 metrology. The 50% cut-off frequency of TTF (f50 ) remained mostly constant across all higher tube currents for all inserts, namely polyethylene, bone, air, and acrylic. Using the lowest two data points (40 and 80mA), the expected relationship between noise magnitude and tube current was determined to be noise mA-0.47 . The measured noise magnitude were up to 11.1% higher than the expected value at the highest tube current. The average frequency of NPS (fav ) decreased from 0.32 to 0.29mm-1 as tube current increased from 40 to 1000mA. No considerable effects were observed in CT number measurement of any insert; however, CT numbers for air and bone changed almost monotonically as tube current increased. Absolute CNR increased monotonically for all inserts; however, the difference between measured and expected CNRs were approximately -6% to 12% across all tube currents. Increasing tube currents did not affect the spatial resolution, but slightly affected the CT number and noise measurements of the clinical PCCT system. However, the effects were only considerable at clinically irrelevant tube currents used on a small 20-cm phantom. In general clinical practices, automatic exposure control techniques are used to decrease the variation of flux on the detector, which alleviates the chances of detector saturation due to high count rates. The observed effects could be due to pulse pileup, signal-dependent filtration of the system, or nonlinearities in the reconstruction algorithm. In conclusion, either the deadtime of the detector used in the photon-counting CT system is shorter such that count losses due to pulse pileup are negligible, or pulse pileup has inconsiderable effects on the image quality of clinical photon-counting CT systems in routine clinical practice due to possible corrections applied on the system.

Investigation of the Relationship between Body Parameters and mAs Using Non-Contact Two-Dimensional Thickness Measurement in Chest Digital Radiography.

The current study aimed to investigate the relationship between body parameters and the current-time product (mAs) in chest digital radiography using a non-contact infrared thickness-measurement sensor. An anthropomorphic chest phantom was first used to understand variations in mAs over multiple positionings during chest radiography when using the automatic exposure control (AEC) technique. In a human study, 929 consecutive male subjects who underwent regular chest examinations were enrolled, and their height (H), weight (W), and body mass index (BMI) were recorded. In addition, their chest thickness (T) was measured at exhalation using a non-contact infrared sensor, and chest radiography was then performed using the AEC technique. Finally, the relationship between four body parameters (T, BMI, T*BMI, and W/H) and mAs was investigated by fitting the body parameters to mAs using three curve models. The phantom study showed that the maximum mAs was 1.76 times higher than the lowest mAs during multiple positionings in chest radiography. In the human study, all chest radiographs passed the routine quality control procedure and had an exposure index between 100 and 212. In curve fitting, the comparisons showed that W/H had a closer relationship with mAs than the other body parameters, while the first-order power model with W/H fitted to mAs performed the best and had an R-square of 0.9971. We concluded that the relationship between W/H and mAs in the first-order power model may be helpful in predicting the optimal mAs and reducing the radiation dose for chest radiography when using the AEC technique.

Identification of aspirated radiolucent foreign bodies in the pediatric airway using digital tomosynthesis: a multireader phantom study.

Purpose: Foreign body aspiration is difficult to diagnose because many aspirated foreign bodies are low density or radiolucent. Digital radiographs (DR) are poor at detecting radiolucent foreign bodies. Digital tomosynthesis (DTS) has been shown to be ideally suited for applications where DR is insensitive and the increased dose from computed tomography (CT) is not justified. Our objective was to determine if DTS can be a practical alternative to DR and CT in the diagnosis of foreign body aspiration. Approach: A phantom approximating the densities of a pediatric chest was constructed. Radiolucent foreign bodies were placed in the airways. Seven pediatric radiologists assessed DTS and DR images with and without simulated breathing motion. Two rounds were performed with fixed exposure techniques and then automatic exposure control techniques. Interobserver agreement was evaluated using Fleiss' kappa. Results: DTS and DR images using fixed exposure techniques performed very poorly with accuracies of 42% to 60%. DTS with automatic exposure control techniques increased accuracy to 84% for a stationary phantom, but the accuracy dropped to 70% in a phantom with simulated motion. DTS outperformed DR, with DR accuracies of 60% and 63% for stationary simulations and motion, respectively. Interobserver agreement was poor with Fleiss' kappa of 0.476. Conclusion: DTS is superior to DR for radiolucent foreign body detection. However, the overall accuracy and interobserver agreement are likely too low for this modality to be clinically useful.

Estimation of Absorbed and Effective Doses in Organs through Computed Tomography Examinations Using Automatic Exposure Control and Fixed Tube Current Techniques: A Phantom Case Study

Introduction: The study aimed to assess absorbed and effective doses in organs through computed tomography (CT) examinations using automatic exposure control (AEC) and fixed tube current (FTC) techniques. Material and Methods: Scanning parameters were obtained for routine adult CT examinations and used to estimate the organ absorbed and effective doses using CT-Expo software. The estimated effective doses were based on International Commission on Radiological Protection publication 103 recommendations. Results: Regarding the scans performed with AEC, doses to head, chest, abdomen and pelvic organs were within the range of 19.7-41.8, 6.4-17.4, 19.2-20.9, and 10.5-24.9 mGy respectively. Moreover, the effective doses for the mentioned organs were 1.6, 6.1, 6.4 and 5.4 mSv respectively. Considering FTC technique, doses to organs ranged 16.7-75.5, 4.1-52.2, 10.6-33.2 and 5.2-38.7 mGy respectively. Moreover, the mean effective doses of FTC were 2.1, 6.9, 9.4 and 6.1 mSv, respectively. Examinations performed with AEC technique induced a dose reduction of 9% and 34% for head organs, 52, 62 and 25% for chest organs, 16% and 14% for abdomen organs, and 11% and 10% for pelvic organs, compared to the FTC. A dose increase of 3% was observed for testes. The mean effective doses for scans with AEC were 13-46% lower than those obtained by FTC. Conclusion:According to the obtained results of the current study, the estimated doses for scans with AEC technique were in a lower level compared to FTC technique. Accordingly, it is recommended to utilize this technique for CT examinations to ensure optimal dose reduction to radiosensitive organs.

Low-dose computed tomography scans with automatic exposure control for patients of different ages undergoing cardiac PET/CT and SPECT/CT.

This study aimed to evaluate the efficacy of automatic exposure control (AEC) in order to optimize low-dose computed tomography (CT) protocols for patients of different ages undergoing cardiac PET/CT and single-photon emission computed tomography/computed tomography (SPECT/CT). One PET/CT and one SPECT/CT were used to acquire CT images for four anthropomorphic phantoms representative of 1-year-old, 5-year-old and 10-year-old children and an adult. For the hybrid systems investigated in this study, the radiation dose and image quality of cardiac CT scans performed with AEC activated depend mainly on the selection of a predefined image quality index. Multiple linear regression methods were used to analyse image data from anthropomorphic phantom studies to investigate the effects of body size and predefined image quality index on CT radiation dose in cardiac PET/CT and SPECT/CT scans. The regression relationships have a coefficient of determination larger than 0.9, indicating a good fit to the data. According to the regression models, low-dose protocols using the AEC technique were optimized for patients of different ages. In comparison with the standard protocol with AEC activated for adult cardiac examinations used in our clinical routine practice, the optimized paediatric protocols in PET/CT allow 32.2, 63.7 and 79.2% CT dose reductions for anthropomorphic phantoms simulating 10-year-old, 5-year-old and 1-year-old children, respectively. The corresponding results for cardiac SPECT/CT are 8.4, 51.5 and 72.7%. AEC is a practical way to reduce CT radiation dose in cardiac PET/CT and SPECT/CT, but the AEC settings should be determined properly for optimal effect. Our results show that AEC does not eliminate the need for paediatric protocols and CT examinations using the AEC technique should be optimized for paediatric patients to reduce the radiation dose as low as reasonably achievable.

Comparison of Radiation Dose and Image Quality with Various Computed-Tomography Scout Views: The Angular Modulation Technique Based on Information Calculated from Scout Views

Background: While using the automatic exposure control technique in computed tomography (CT), the scout view determines the radiation dose for the CT scan by taking anatomical morphology into consideration. Objectives: The goal of this study was to estimate radiation doses during scout scanning and to evaluate the hypothesis that the radiation dose during CT depends on the location and order of the X-ray tubes. Materials and Methods: A CT scanner (GE VCT 64-channel) with the X-ray tubes placed in the anteroposterior (AP), lateral, and posteroanterior (PA) positions was used to acquire the scout views. The effective doses were assessed using an anthropomorphic chest phantom (RSD Inc., Long Beach, CA), an AAPM CT performance phantom (CIRS model 610), and CT dose indices (CTDI) head phantom. Both single and two scout views were tested with the three views combined according to the order of scanning. The CTDIs obtained from the CT unit dose report were used to compare the chest CT radiation doses, while the image quality was evaluated based on the root-mean-square error values calculated using the Image program. Results: The results of this study indicate that single AP (0°) scout views obtained with the z-axis modulation technique (AutomA) during chest CT scanning resulted in CTDIs that were lower than those generated by using lateral (90°) or PA (180°) scout views. The analysis of two-scout views using AutomA alone revealed the lowest CTDI in the 90° - 0° scout views. However, based on the CT image noise analysis, a scout angle order of 0°-90° resulted in the optimal scout view combination with the minimal dose. Conclusion: Scout views may be useful in determining dose and image quality in CT scanning; however, it is necessary to first determine the most appropriate scout view for CT scans.

SU‐F‐I‐31: Reproducibility of An Automatic Exposure Control Technique in the Low‐Dose CT Scan of Cardiac PET/CT Exams

Purpose:Two separate low‐dose CT scans are usually performed for attenuation correction of rest and stress N‐13 ammonia PET/CT myocardial perfusion imaging (PET/CT). We utilize an automatic exposure control (AEC) technique to reduce CT radiation dose while maintaining perfusion image quality. Our goal is to assess the reproducibility of displayed CT dose index (CTDI) on same‐day repeat CT scans (CT1 and CT2).Methods:Retrospectively, we reviewed CT images of PET/CT studies performed on the same day. Low‐dose CT utilized AEC technique based on tube current modulation called Smart‐mA. The scan parameters were 64 × 0.625mm collimation, 5mm slice thickness, 0.984 pitch, 1‐sec rotation time, 120 kVp, and noise index 50 with a range of 10–200 mA. The scan length matched with PET field of view (FOV) with the heart near the middle of axial FOV. We identified the reference slice number (RS) for an anatomical landmark (carina) and used it to estimate axial shift between two CTs. For patient size, we measured an effective diameter on the reference slice. The effect of patient positioning to CTDI was evaluated using the table height. We calculated the absolute percent difference of the CTDI (%diff) for estimation of the reproducibility.Results:The study included 168 adults with an average body‐mass index of 31.72 ± 9.10 (kg/m2) and effective diameter was 32.72 ± 4.60 cm. The average CTDI was 1.95 ± 1.40 mGy for CT1 and 1.97 ± 1.42mGy for CT2. The mean %diff was 7.8 ± 6.8%. Linear regression analysis showed a significant correlation between the table height and %diff CTDI. (r=0.82, p<0.001)Conclusion:We have shown for the first time in human subjects, using two same‐day CT images, that the AEC technique in low‐dose CT is reproducible within 10% and significantly depends on the patient centering.

Comparison of Measured and Estimated CT Organ Doses for Modulated and Fixed Tube Current:: A Human Cadaver Study

The aim of this study was to compare the directly measured and the estimated computed tomography (CT) organ doses obtained from commercial radiation dose-tracking (RDT) software for CT performed with modulated tube current or automatic exposure control (AEC) technique and fixed tube current (mAs). With the institutional review board (IRB) approval, the ionization chambers were surgically implanted in a human cadaver (88 years old, male, 68 kg) in six locations such as liver, stomach, colon, left kidney, small intestine, and urinary bladder. The cadaver was scanned with routine abdomen pelvis protocol on a 128-slice, dual-source multidetector computed tomography (MDCT) scanner using both AEC and fixed mAs. The effective and quality reference mAs of 100, 200, and 300 were used for AEC and fixed mAs, respectively. Scanning was repeated three times for each setting, and measured and estimated organ doses (from RDT software) were recorded (N = 3*3*2 = 18). Mean CTDIvol for AEC and fixed mAs were 4, 8, 13 mGy and 7, 14, 21 mGy, respectively. The most estimated organ doses were significantly greater (P < 0.01) than the measured organ doses for both AEC and fixed mAs. At AEC, the mean estimated organ doses (for six organs) were 14.7 mGy compared to mean measured organ doses of 12.3 mGy. Similarly, at fixed mAs, the mean estimated organ doses (for six organs) were 24 mGy compared to measured organ doses of 22.3 mGy. The differences among the measured and estimated organ doses were higher for AEC technique compared to the fixed mAs for most organs (P < 0.01). The most CT organ doses estimated from RDT software are greater compared to directly measured organ doses, particularly when AEC technique is used for CT scanning.

Determination of the optimal dose reduction level via iterative reconstruction using 640-slice volume chest CT in a pig model.

AimTo determine the optimal dose reduction level of iterative reconstruction technique for paediatric chest CT in pig models.Materials and Methods27 infant pigs underwent 640-slice volume chest CT with 80kVp and different mAs. Automatic exposure control technique was used, and the index of noise was set to SD10 (Group A, routine dose), SD12.5, SD15, SD17.5, SD20 (Groups from B to E) to reduce dose respectively. Group A was reconstructed with filtered back projection (FBP), and Groups from B to E were reconstructed using iterative reconstruction (IR). Objective and subjective image quality (IQ) among groups were compared to determine an optimal radiation reduction level.ResultsThe noise and signal-to-noise ratio (SNR) in Group D had no significant statistical difference from that in Group A (P = 1.0). The scores of subjective IQ in Group A were not significantly different from those in Group D (P>0.05). There were no obvious statistical differences in the objective and subjective index values among the subgroups (small, medium and large subgroups) of Group D. The effective dose (ED) of Group D was 58.9% lower than that of Group A (0.20±0.05mSv vs 0.48±0.10mSv, p <0.001).ConclusionsIn infant pig chest CT, using iterative reconstruction can provide diagnostic image quality; furthermore, it can reduce the dosage by 58.9%.

Effects of Iterative Reconstruction Algorithm, Automatic Exposure Control on Image Quality, and Radiation Dose: Phantom Experiments with Coronary CT Angiography Protocols

†† ‡ § ‡ § In this study, we investigated the effects of an iterative reconstruction algorithm and an automatic exposure control (AEC) technique on image quality and radiation dose through phantom experiments with coronary computed tomography (CT) angiography protocols. We scanned the AAPM CT performance phantom using 320 multi-detector-row CT. At the tube voltages of 80, 100, and 120 kVp, the scanning was repeated with two settings of the AEC technique, i.e., with the target standard deviations (SD) values of 33 (the higher tube current) and 44 (the lower tube current). The scanned projection data were reconstructed also in two ways, with the filtered back projection (FBP) and with the iterative reconstruction technique (AIDR-3D). The image quality was evaluated quantitatively with the noise standard deviation, modulation tr ansfer function, and the contrast to noise ratio (CNR). More specifically, we analyzed the influences of selection of a tube voltage and a reconstruction algorithm on tube current modulation and consequently on radiation dose. Reduction of image noise by the iterative reconstruction algorithm compared with the FBP was revealed eminently, especially with the lower tube current protocols, i.e., it was decreased by 46% and 38%, when the AEC was established with the lower dose (the target SD=44) and the higher dose (the target SD=33), respectively. As a side effect of iterative reconstruction, the spatial resolution was decreased by a degree that could not mar the remarkable gains in terms of noise reduction. Consequently, if coronary CT angiogprahy is scanned and reconstructed using both the automatic exposure control and iterative reconstruction techniques, it is anticipated that, in comparison with a conventional acquisition method, image noise can be reduced significantly with slight decrease in spatial resolution, implying clinical advantages of radiation dose reduction, still being faithful to the ALARA principle.

SU‐E‐I‐68: Practical Considerations On Implementation of the Image Gently Pediatric CT Protocols

Purpose:One limitation associated with the Image Gently pediatric CT protocols is practical implementation of the recommended manual techniques. Inconsistency as a result of different practice is a possibility among technologist. An additional concern is the added risk of data error that would result in over or underexposure. The Automatic Exposure Control (AEC) features automatically reduce radiation for children. However, they do not work efficiently for the patients of very small size and relative large size. This study aims to implement the Image Gently pediatric CT protocols in the practical setting while maintaining the use of AEC features for pediatric patients of varying size.Methods:Anthropomorphological abdomen phantoms were scanned in a CT scanner using the Image Gently pediatric protocols, the AEC technique with a fixed adult baseline, and automatic protocols with various baselines. The baselines were adjusted corresponding to patient age, weight and posterioranterior thickness to match the Image Gently pediatric CT manual techniques. CTDIvol was recorded for each examination. Image noise was measured and recorded for image quality comparison. Clinical images were evaluated by pediatric radiologists.Results:By adjusting vendor default baselines used in the automatic techniques, radiation dose and image quality can match those of the Image Gently manual techniques. In practice, this can be achieved by dividing pediatric patients into three major groups for technologist reference: infant, small child, and large child. Further division can be done but will increase the number of CT protocols. For each group, AEC can efficiently adjust acquisition techniques for children. This implementation significantly overcomes the limitation of the Image Gently manual techniques.Conclusion:Considering the effectiveness in clinical practice, Image Gently Pediatric CT protocols can be implemented in accordance with AEC techniques, with adjusted baselines, to achieve the goal of providing the most appropriate radiation dose for pediatric patients of varying sizes.

Computed tomography automatic exposure control techniques in 18F-FDG oncology PET-CT scanning

Computed tomography (CT) automatic exposure control (AEC) systems are now used in all modern PET-CT scanners. A collaborative study was undertaken to compare AEC techniques of the three major PET-CT manufacturers for fluorine-18 fluorodeoxyglucose half-body oncology imaging. An audit of 70 patients was performed for half-body CT scans taken on a GE Discovery 690, Philips Gemini TF and Siemens Biograph mCT (all 64-slice CT). Patient demographic and dose information was recorded and image noise was calculated as the SD of Hounsfield units in the liver. A direct comparison of the AEC systems was made by scanning a Rando phantom on all three systems for a range of AEC settings. The variation in dose and image quality with patient weight was significantly different for all three systems, with the GE system showing the largest variation in dose with weight and Philips the least. Image noise varied with patient weight in Philips and Siemens systems but was constant for all weights in GE. The z-axis mA profiles from the Rando phantom demonstrate that these differences are caused by the nature of the tube current modulation techniques applied. The mA profiles varied considerably according to the AEC settings used. CT AEC techniques from the three manufacturers yield significantly different tube current modulation patterns and hence deliver different doses and levels of image quality across a range of patient weights. Users should be aware of how their system works and of steps that could be taken to optimize imaging protocols.

SU‐E‐I‐43: Automatic Exposure Control Or Fixed Tube Current? : Evaluating Dose Effectiveness of AEC in Pediatric Chest CT Examinations with CTDIvol, SSDE, and Water‐Equivalent SSDE

Purpose: To investigate the effectiveness of automatic exposure control (AEC) in pediatric CT examinations in terms of volume CT dose index (CTDIvol), size‐specific dose estimates (SSDE), and water‐equivalent size‐specific dose estimates (SSDEw). Methods: We selected 179 pediatric chest CT examinations of patient ages 1 ∼ 15 taken with or without AEC (AEC: 88, FIXED: 91). A 16 slice MDCT(Sensation 16, Siemens) was used with tube potentials of 80kVp. The reference level for AEC was different for each patient. We assigned CT exams into one of four age groups: under 2, 25, 6&amp;–10, and under 15. We segmented body regions using a seeded region growing method. The effective diameter (Deff) and the water‐equivalent diameter (DW) were calculated at each slice. CTDIvol was extracted from the conventional DICOM dose report image using optical character recognition (OCR) technique, and then SSDE and SSDEw were derived according to AAPM TG 204 based on Deff and DW. The patient dose was compared for with and without AEC exams using those 3 CT dose metrics. Results: Patient dose of exams with AEC was lower in all age groups regardless of the 3 CT dose metrics. CTDIvol, SSDE, and SSDEw with AEC were 1.61±1.13, 2.85±1.29, and 3.23±1.53 mGy and those without AEC were 2.86±1.34, 5.42±2.61, and 6.21±1.93 mGy, which represent dose reduction of AEC exams by 56.3%, 52.6%, and 52.0%, respectively. Conclusions: Our study revealed that patient dose of exams with AEC was substantially lower than that without AEC by 52 ∼ 56.3 % in pediatric chest CT examinations. Our study results suggest that AEC technique is recommended in pediatric CT exams. The research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001498)

How Much Dose Can Be Saved in Three-Phase CT Urography? A Combination of Normal-Dose Corticomedullary Phase With Low-Dose Unenhanced and Excretory Phases

The purpose of this study was to investigate the degree to which the total radiation dose for CT urography can be lowered by selective reduction of the dose in the unenhanced and excretory phases when images in these phases are systematically evaluated alongside normal-dose corticomedullary phase images. Twenty-seven patients (mean age, 74±9 years) underwent single-bolus CT urography with acquisition in the unenhanced, corticomedullary, and 5-minute excretory phases. The scanning parameters for normal-dose CT urography were as follows: 16×0.75 mm, 120 kV, and automatic exposure control technique reference tube loads of 100, 120, and 100 effective mAs (mAseff). The patients also underwent low-dose unenhanced and excretory phase scanning, in which the dose was escalated stepwise from a volume CT dose index (CTDIvol) of 1.7 to 6.6 mGy (reference 20-40-60-80 mAseff). Images were analyzed for quality and diagnostic confidence. If low-dose scans of three patients were inadequate, the study continued to the next dose level. When 20 patients were successfully included in the unenhanced and excretory phase groups, the study ended. Doses were calculated with a CT patient dosimetry calculator. Combined with the normal dose for corticomedullary phase scanning, doses of CTDIvol 1.5 mGy for the unenhanced phase and CTDIvol 2.7 mGy for the excretory phase were sufficient. The effective dose for three-phase CT urography was lowered from 16.2 to 9.4 mSv, a decrease of 42%. Diagnostic confidence in low-dose images was equal to that in normal-dose images when low-dose unenhanced and excretory phase images were read along-side normal-dose corticomedullary phase images. With a three-phase CT urographic protocol, significant dose reductions in the unenhanced and excretory phases can be achieved when these phases are combined with a normal-dose corticomedullary phase.

Automatic exposure control at MDCT based on the contrast-to-noise ratio: Theoretical background and phantom study

PurposeTo develop a new automatic exposure control (AEC) technique based on the contrast-to-noise ratio (CNR) and provide constant lesion detectability. MethodsLesion detectability is affected by factors such as image noise, lesion contrast, and lesion size. We performed ROC analysis to assess the relationship between the optimum CNR and the lesion diameter at various levels of lesion contrast. We then developed a CNR-based AEC algorithm based on lesion detectability. Using CNR- based AEC algorithm, we performed visual evaluation of low-contrast detectability by 5 radiologists on a low-contrast module of the Catphan phantom, a contrast-difference level of 1.0% (difference in the CT number = 10 HU), and objects 3.0–9.0 mm in diameter. ResultsOn step-and-shoot scans the mean detection fraction with CNR-based AEC remained almost constant from 88 to 99 % regardless of the lesion size. We observed the same trend on helical scans, the mean detection fraction with CNR-based AEC exhibited a high score from 91 to 100%. Although CNR-based AEC maintains higher CNR for smaller size or lower contrast lesion, radiation dose on 3 mm lesion resulted in about 13 times larger than that of 9 mm lesion size. CTDIvol for the CNR-based AEC technique changed dramatically with the SDZ from 7.5 to 100.0 mGy for step-and-shoot scans and from 9.1 to 121.5 mGy for helical scans. ConclusionsFrom the viewpoint of ROC analysis-based CNR for lesion detection, CNR-based AEC potentially provide image quality advantages for clinical implementation.

Reducing Abdominal CT Radiation Dose With Adaptive Statistical Iterative Reconstruction Technique

To assess radiation dose reduction for abdominal computed tomography (CT) examinations with adaptive statistical iterative reconstruction (ASIR) technique. With institutional review board approval, retrospective review of weight adapted abdominal CT exams were performed in 156 consecutive patients with ASIR and in 66 patients with filtered back projection (FBP) on a 64-slice MDCT. Patients were categorized into 3 groups of <60 kg (n = 42), 61 to 90 kg (n = 100), and >or=91 kg (n = 80) for weight-based adjustment of automatic exposure control technique. Remaining scan parameters were held constant at 1.375:1 pitch, 120 kVp, 55 mm table feed per rotation, 5 mm section thickness. Two radiologists reviewed all CT examinations for image noise and diagnostic acceptability. CT dose index volume, and dose length product were recorded. Image noise and transverse abdominal diameter were measured in all patients. Data were analyzed using analysis of variance. ASIR allowed for an overall average decrease of 25.1% in CT dose index volume compared with the FBP technique (ASIR, 11.9 +/- 3.6 mGy; FBP, 15.9 +/- 4.3 mGy) (P < 0.0001). In each of the 3 weight categories, CT examinations reconstructed with ASIR technique were associated with significantly lower radiation dose compared with FBP technique (P < 0.0001). There was also significantly less objective image noise with ASIR (6.9 +/- 2.2) than with FBP (9.5 +/- 2.0) (P < 0.0001). For the subjective analysis, all ASIR and FBP reconstructed abdominal CTs had optimal or less noise. However, 9% of FBP and 3.8% of ASIR reconstructed CT examinations were diagnostically unacceptable because of the presence of artifacts. Use of ASIR reconstruction kernel results in a blotchy pixilated appearance in 39% of CT sans which however, was mild and did not affect the diagnostic acceptability of images. The critical reproduction of visually sharp anatomic structures was preserved in all but one ASIR 40% reconstructed CT examination. ASIR technique allows radiation dose reduction for abdominal CT examinations whereas improving image noise compared with the FBP technique.

흉부 및 복부에서 AEC 적용에 따른 MDCT의 선량 감소 효과

CT검사에 따른 방사선 선량의 유해성에 대한 관심이 고조되고 있어, 고정관전류기법과 AEC(automatic exposure control)를 적용하여 선량의 감소 효과를 비교하였다. 64MDCT를 이용하여 인체조직등가물질 팬텀으로 흉부 및 복부 부위를 고정관전류기법과 AEC를 적용하여 CTDIvol과 DLP를 비교하였고, 영상의 평가는 관심영역으로 CT감약계수와 노이즈를 측정하여 비교하였다. 흉부에서 고정관전류기법과 AEC적용에 따른 선량감소 효과는 CTDIvol 35.2%, DLP 49.3%, 복부에서는 CTDIvol 5.9%, DLP 3.2% 감소 효과가 있었다. CT검사에서 자동노출장치인 AEC를 이용하여 선량의 감소효과가 있다. CT 검사에 따른 환자의 피폭선량을 줄이기 위해서는 AEC를 이용하여야 한다. The purpose of the current study was to compare radiation dose of 64MDCT performed with automatic exposure control (AEC) with manual selection fixed tube current. We evaluated the CT scans of phantom of the chest and abdomen using the fixed tube current and AEC technique. Objective image noise shown as the standard deviation of CT value in Hounsfield units was measured on the obtained images. Compared with fixed tube current, AEC resulted in reduction of the chest and abdomen in the CTDIvol (35.2%, 5.9%) and DLP (49.3%, 3.2%). Compared with manually selected fixed tube current, AEC resulted in reduced radiation dose at MDCT study of chest and abdomen.

６４列ＭＤＣＴを用いたＣＴ−ＡＥＣにおける設定最大管電流の影響：線量低減率と画像ノイズの変動との関係

To investigate the effect of maximum tube current threshold for dose modulation at MDCT on the reduction rate of radiation dose and variability of image noise. A chest CT phantom was scanned with a 64-detector row CT scanner using an automatic exposure control (AEC) technique. The technique consisted of X-Y-Z modulation and Z modulation. We set the desired image noise at 10 HU. Maximum tube current threshold was set at 200 and 300 mA and minimum tube current threshold was fixed at 10 mA. For comparison, we also performed scanning with a fixed tube current at 300 mA. Parameters for helical scanning were: tube voltage, 120 kV; rotation time, 0.5 sec/rot; collimation, 64x0.5 mm; beam pitch, 0.825; reconstructed section thickness, 5.0 mm. After scanning, we recorded the actual mA value of each image. Image noise was also measured with and without the dose modulation technique. Using X-Y-Z modulation, when maximum tube current threshold was 300 and 200 mA, the reduction rates of radiation dose with the dose modulation technique were 29.9% and 45.0%, respectively, compared with the fixed tube current technique. Actual mean image noise was 9.6 and 11.4 HU corresponding to 300 and 200 mA. The standard deviation of image noise at 200 mA (2.47 HU) was larger than that at 300 mA (1.96 HU). A lower maximum tube current setting (200 mA) could achieve larger dose reduction. However, it induced larger variability of the level of image noise in the AEC technique at MDCT.

Automatic Patient Centering for MDCT: Effect on Radiation Dose

The purpose of this study was to determine with phantom and patient imaging the effect of an automatic patient-centering technique on the radiation dose associated with MDCT. A 32-cm CT dose index (CTDI) phantom was scanned with 64-MDCT in three positions: gantry isocenter and 30 and 60 mm below the isocenter of the scanner gantry. In each position, surface, peripheral, and volume CTDIs were estimated with a standard 10-cm pencil ionization chamber. The institutional review board approved the study with 63 patients (36 men, 27 women; mean age, 51 years; age range, 22-83 years) undergoing chest (n = 18) or abdominal (n = 45) CT using the z-axis automatic exposure control technique. Each patient was positioned according to the region being scanned and then was centered in the gantry. Before scanning of a patient, automatic centering software was used to estimate patient off-centering and percentage of dose reduction with optimum recentering. Data were analyzed with linear correlation and the Student's t test. Peripheral and surface CTDIs increased approximately 12-18% with 30-mm off-center distance and 41-49% with 60-mm off-center distance. Approximately 95% (60/63) of patients were not positioned accurately in the gantry isocenter. The mean radiation dose saving with automatic centering of all patients was 13.0% +/- 0.9% (range, 2.6-29.9%). There was strong correlation between off-center distance and percentage of surface CTDI reduction with recentering of patients in the gantry isocenter (r2 = 0.85, p < 0.0001). Surfaces doses can be reduced if radiologic technologists can better center patients within the CT gantry. Automatic centering technique can help in optimum patient centering and result in as much as 30% reduction in surface dose.

SU‐FF‐I‐27: CT Urography Radiation Dose Using Automatic Exposure Control

Purpose: To measure and compare patient radiation doses during CT urography with automatic exposure control (both angular and z‐axis tube current modulation) to patient radiation doses without automatic exposure control (tube current modulation alone), and to correlate these doses with corresponding image noise measurements. Method and Materials: Skin doses were measured by exposing thermoluminescent dosimeters placed on the abdomen (AP) and side (lateral position) of 18 patients examined with CT urography consisting of automatic exposure control (CARE Dose 4D group), and 20 different patients examined with angular tube current modulation only (CARE Dose group). Mean and standard deviation of patient skin doses were calculated. The CT urography protocol included three volumetric acquisitions of the abdomen and pelvis. Effective doses were calculated and used to compare radiation risk between the two patient groups. The variation in effective dose with patient size was also evaluated. Image noise was evaluated by calculating the standard deviation of pixel values from a region of interest in patient liver images. Results: The mean skin dose for CARE Dose 4D patients (63.4 ± 16.4 mGy) was 14.2% higher than that of CARE Dose patients (54.4 ± 7.39 mGy). The mean effective dose for CARE Dose 4D patients (10.6 ± 2.3 mSv) was 17.1% higher than that of CARE Dose patients (8.8 ± 1.8 mSv). Image noise increased with increasing patient size, however the increase was less for patients exposed with automatic exposure control. Conclusion: Patient effective dose, and thus radiation risk was 17.1% higher for CT urography patients examined with automatic exposure control compared to those examined with tube current modulation alone. Using automatic exposure control techniques in CT may be useful in reducing dose in small patients, however, for large patients the dose may actually increase to compensate for the increase in image noise.