Automatic Tube Current Modulation Research Articles

SU‐C‐217BCD‐03: CT QA Revisited in Context of Tube Current Modulation and Iterative Reconstruction

Purpose: Despite their substantial impact on both patient dose and image quality, automatic‐tube‐current‐modulation (ATCM) and iterative reconstruction (IR) are not typically evaluated or monitored in CT quality assurance (CTQA) programs. A new CT phantom and custom software have been incorporated into the CTQA program at our institution. The purpose of this project was to devise a generic testing platform using these newly designed tools, and introduce this platform as part of regular CTQA testing. Methods: The Mercury Phantom comprises three tapered and four uniform regions of polyethylene (16, 23, 30, and 37cm in diameter), each of which includes four inserts of different materials: air, Polystyrene, Acrylic, and Teflon. Images were acquired using ATCM on three Siemens Flash and three GE 750HD scanners, and iteratively reconstructed (SAFIRE and ASIR). A custom MATLAB software package provided mAs and standard deviation per image, HU, MTF, NPS, and detectability indices for each insert. Results were compared within and across systems, to be used as baseline values for comparison with future system performance testing. Results: For each scanner model, the absolute difference in mAs for corresponding images of the baseline scans had a mean value less than 2 mAs, with a standard deviation of 1.3 mAs. For different phantom diameters, HU measurements for all four inserts varied little between identical systems or reconstruction algorithms. For each reconstruction algorithm used, detectability indices based upon MTF and NPS measurements were very reproducible, averaging less than 5% difference. Different CT models varied substantially in terms of their mAs profile. Conclusions: The Mercury Phantom and data analysis technique can be used to benchmark/monitor CT performance, including ATCM and IR, and identify changes over time and across systems. The methodology can be readily implemented across a fleet of CT scanners at a large academic medical center.

Emergency unenhanced multi-slice computed tomography in suspected urinary calculi—with dose reduction method (care dose 4D)

Objectives: To assess urinary calculi and the secondary signs of obstruction, in patients referred from the accident and emergency department, by unenhanced computed tomography examination and to review the radiation dose the patients received with the use of automatic dose modulation technique, care dose 4D. Material and methods Medical records of unenhanced multi-slice computed tomography (MSCT) examinations for 114 patients referred to the emergency department for analysis of suspected urinary calculi were reviewed retrospectively. Their treatment involved automatic tube current modulation, a dose reduction method (care dose 4D), in Hamad General Hospital. The cases were analyzed for the presence of stones, size, site, density and the secondary signs of obstruction, namely hydronephrosis, hydroureter, perinephric fat stranding, peri-ureteric fat stranding and renal enlargement. A search for alternate diagnosis was made if no stone was found. The final diagnosis was noted from the discharge summary in m...

Reducing radiation in CT urography for hematuria: Effect of using 100 kilovoltage protocol

ObjectiveTo compare 128-slice multidetector computed tomography (MDCT) at 100kVp and 120kVp for image quality and radiation dose. Materials and methodsOur study had approval of our institutional review board. We retrospectively selected 25 patients who underwent CT urography for the evaluation of hematuria. The CT scans were taken with 128-slice MDCT, with three phases (precontrast, nephrographic, and excretory), using an automatic tube current modulation with reference tube level of 180mAs and tube voltages of either 100kVp (n=14, mean age 26.71) or 120kVp (n=11, mean age 25.54). The signal-to-noise-ratio (SNR) and contrast-to-noise-ratio (CNR) of the urinary tract, the subjective image quality of the urinary tract evaluated with a five point scale by two radiologists and the effective dose calculated on the basis of dose-length-product (DLP) and volume-CT-dose-index (CTDIvol) were compared between the groups with Student's t test or Mann–Whitney U test. ResultsThere was no significant difference in the SNR (p=0.358), CNR (p=0.303) and the subjective image quality (p=0.486) between the two protocols. The mean CTDIvol, DLP and the effective dose in the 100kVp protocol were significantly lower than the 120kVp protocol (p=0.000, 0.000). ConclusionCT urography using 100kVp protocol resulted in reduction of radiation dose without loss of objective or subjective image quality.

Effects of patient size on radiation dose reduction and image quality in low-kVp CT pulmonary angiography performed with reduced IV contrast dose

The purpose of the study is to evaluate image quality and radiation exposure as a function of patient size for CT pulmonary angiography (CTPA) performed at reduced tube voltage and reduced intravenous (IV) contrast dose. We reviewed consecutive CTPAs performed between 9/1/2010 and 10/31/2010 on a 128-slice Siemens AS+ scanner using automated tube current modulation with quality reference mAs 200 and IV contrast concentration 370 mg I/ml followed by a saline flush: 99 scans at 120 kVp using 75 ml of contrast at 5 ml/s and 53 scans on patients lighter than 175 lbs at 100 kVp using 50 ml of contrast at 4 ml/s. We measured patient size (mean water-equivalent diameter) using a topogram analysis tool, signal (mean CT density) and noise (standard deviation) in the main pulmonary artery (MPA) on axial images, and calculated local CTDI(vol) from the kVp and mAs. Linear regression models were created for dependent variables ln(CTDI(vol)), signal, noise, and signal to noise ratio (SNR) as a function of independent variables size, age, gender, and kVp. After controlling for other variables, scanning at 100 kVp yielded CTDI(vol) reduction of 33 % (p < 0.0001), signal increase of 96 HU (p < 0.0001), and increased image noise (p < 0.0001), but without significant difference in SNR (p = 0.99). Relative to 120 kVp, 100-kVp CTPA allows simultaneous reduction of radiation exposure by 33 % and IV contrast dose by 33 % while maintaining image quality. Scanning at 100 kVp is recommended in all patients for whom the required mAs does not exceed maximum X-ray tube output.

Variation in Tube Voltage for Adult Neck MDCT: Effect on Radiation Dose and Image Quality

The purpose of this study was to assess the effect of peak kilovoltage on radiation dose and image quality in adult neck MDCT. An anthropomorphic phantom with metal oxide semiconductor field effect transistor detectors was imaged with a 64-MDCT scanner. The reference CT protocol called for 120 kVp, and images obtained with that protocol were compared with CT images obtained with protocols entailing 80, 100, and 140 kVp. All imaging was performed with automatic tube current modulation. Organ dose and effective dose were determined for each protocol and compared with those obtained with the 120-kVp protocol. Image noise was evaluated objectively and subjectively for each protocol. The highest organ doses for all protocols were to the thyroid, ocular lens, skin, and mandible. The greatest reductions in organ dose were for the bone marrow of the cervical spine and mandible: 43% and 35% with the 100-kVp protocol and 63% and 53% with the 80-kVp protocol. Effective dose decreased as much as 9% with the 100-kVp protocol and 12% with the 80-kVp protocol. Use of the 140-kVp protocol was associated with an increase in organ dose as high as 64% for bone marrow in the cervical spine and a 19% increase in effective dose. Image noise increased with lower peak kilovoltage. The measured noise difference was greatest at 80 kVp, absolute increases were less than 2.5 HU. There was no difference in subjective image quality among protocols. Reducing the voltage from 120 to 80 kVp for neck CT can result in greater than 50% reduction in the absorbed organ dose to the bone marrow of the cervical spine and mandible without impairment in subjective image quality.

Computed Tomography: Fundamentals, System Technology, Image Quality, Applications, 3rd revised and enlarged Edition

This book describes the evolution of computed tomography (CT) over four decades, projects into the future concerning new developments, explains the scientific and technological foundations of the imaging methodology, and provides examples of static and dynamic imaging with computed tomography. It describes the introduction and rapid development of CT in the 1970s, a rather static phase in the 80s, the onset of spiral CT in the 90s with the transition from single-slice scanning to volume scanning using multislice and cone-beam approaches, and the introduction of several innovations in the 2000s including newer detector systems, dual-source CT, and dual-energy scanning. Further developments predicted for the next few years are projected to potentially yield submillisievert scanning. The physics and engineering underlying each phase in the evolution of CT are nicely explained in the book. This book is intended as an explanation of the fundamentals of computed tomography for a wide audience, including but not limited to practicing and student medical physicists and biomedical engineers interested in medical imaging. It is a third and updated edition of a book that has encountered a very receptive audience for its previous editions. The book comes with a DVD of images and applications, video clips, and interactive exercises. The authors have provided considerable basic information in the appendices so that individuals with limited physics and engineering backgrounds can appreciate the book. As mentioned earlier, the book is entirely suitable for medical physicists and biomedical engineers, including students. The book should be required reading in medical physics and biomedical engineering graduate programs educating imaging scientists. Radiologists and radiographers specializing in CT will also find the book interesting and helpful in understanding the principles and evolution of computed tomography. The introduction and development of computed tomography has been a major tipping point in the evolution of medical imaging, and persons interested in this evolution will find the book enlightening. This third edition of Computed Tomography describes the evolution of computed tomography to its current state as an essential tool in medical imaging and image-guided interventional therapy. After a brief historical overview, the book provides a brief introduction to the principles of computed tomography (Chap. 1). Chapter 2 describes the technical concepts of CT in greater detail, Chap. 3 is devoted to spiral CT, and Chap. 4 covers the parameters and influences of image quality in computed tomography. Radiation dose from CT is considered in Chap. 5, together with ways that dose can be reduced by methods such as optimization of imaging protocols and use of technical features such as automatic exposure control and tube current modulation. Chapter 6 is devoted to CT image display, visualization, and diagnosis, and Chap. 7 describes special applications of CT, including cardiac and dual-energy imaging. New developments and projections for the future are covered in Chap. 8. For those interested in the mathematical aspects of image reconstruction, Chap. 9 is devoted to that topic. Appendices include a glossary, abbreviations and symbols, references, and an index. As mentioned earlier, a DVD of images and applications, video clips, and interactive exercises is included with the book. The book is printed on high quality paper and has superb and very informative illustrations. The book is easy to read and comprehend. This is a relatively short but superbly done introductory text on computed tomography. The author is a principal developer of multislice computed tomography and has contributed substantially over many years to CT development. The book should be required reading for graduate students in medical imaging and for biomedical engineering students with principal interests in medical imaging. It can be recommended also for radiologists and radiology residents who have a special interest in the science and technology underlying computed tomography. Radiographers responsible for computed tomography applications may find the book interesting even though the mathematics present in the text may extend beyond their reach. The book is written in a lucid and succinct manner, and the illustrations are excellent. The included DVD greatly enhances the educational value of the book. The ability of the author to explain recent advances and current developments in CT, and to expound on their future clinical importance, makes for interesting reading. The book deserves a wide audience, and one can only hope that it will receive substantial attention. Bill Hendee is Distinguished Professor at the Medical College of Wisconsin, Professor at Marquette University, and Adjunct Professor at the University of Wisconsin-Milwaukee, University of New Mexico, University of Colorado, and Mayo Clinic. He is past president of the American Association of Physicists in Medicine and current editor of Medical Physics.

Comparison of image quality and radiation dose between combined automatic tube current modulation and fixed tube current technique in CT of abdomen and pelvis

Tube current is an important determinant of radiation dose and image quality in X-ray-based examination. The combined automatic tube current modulation technique (ATCM) enables automatic adjustment of the tube current in various planes (x-y and z) based on the size and attenuation of the body area scanned. To compare image quality and radiation dose of the ATCM with those of a fixed tube current technique (FTC) in CT of the abdomen and pelvis performed with a 16-slice multidetector row CT. We reviewed 100 patients in whom initial and follow-up CT of the abdomen and pelvis were performed with FTC and ATCM. All acquisition parameters were identical in both techniques except for tube current. We recorded objective image noise in liver parenchyma, subjective image noise and diagnostic acceptability by using a five-point scale, radiation dose, and body mass index (BMI, kg/m(2)). Data were analyzed with parametric and non-parametric statistical tests. There was no significant difference in image noise and diagnostic acceptability between two techniques. All subjects had acceptable subjective image noise in both techniques. The significant reduction in radiation dose (45.25% reduction) was noted with combined ATCM (P < 0.001). There was a significant linear statistical correlation between BMI and dose reduction (r = -0.78, P < 0.05). The ATCM for CT of the abdomen and pelvis substantially reduced radiation dose while maintaining diagnostic image quality. Patients with lower BMI showed more reduction in radiation dose.

Correlation of Cross-Sectional Diameter With Image Quality and Radiation Exposure in MDCT Examinations of the Neck

The purpose of this study was to identify an optimal cross-sectional neck diameter that correlates with image quality and radiation exposure in MDCT examinations of the neck performed with automatic tube current modulation. Ninety-six adults underwent 64-MDCT of the neck with automatic tube current modulation at the same noise setting. On frontal and lateral scout images, maximal body diameters were measured in the transverse and anteroposterior planes at two levels: just below the mandible (upper neck) and at the lung apex (lower neck). Neck diameters were correlated with image quality on a subjective 4-point scale and with radiation exposure (volume CT dose index). As continuous variables, both anteroposterior and transverse diameters in the lower neck were associated with image quality (p ≤ 0.0012). Diameters in the upper neck were not associated with image quality. When diameters in the lower neck were categorized into small, medium, and large, image quality grades were higher for smaller patients (p < 0.001). Images of 81% of small patients (lower neck transverse diameter < 40 cm) had a high image quality grade, compared with images of 7-20% of large patients (diameter > 48 cm). Transverse diameter in the lower neck correlated best with radiation dose measured as volume CT dose index (r = 0.78). When transverse diameter in the lower neck was used to categorize patients' size, the mean volume CT dose index for small patients was 34.1 mSv and that for large patients was 63.5 mSv. Lower neck transverse diameter on the CT scout image best correlates with image quality and radiation exposure for neck MDCT examinations performed with automatic tube current modulation. Images of patients with a lower neck transverse diameter less than 40 cm are of higher quality than those of larger patients. Individualized dose reduction techniques therefore may be appropriate for smaller patients.

Organ dose evaluation for multi-slice spiral CT scans based on China Sichuan chest anthropomorphic phantom measurements

The authors measured organ radiation doses during multi-slice computed tomography (MSCT) chest scans using a China Sichuan anthropomorphic phantom (CDP-1C). Chest CT images from live volunteers based on automatic tube current modulation (ATCM) techniques were similar to those obtained using the CDP-1C phantom, indicating that the phantom accurately modelled the anatomic structure and X-ray absorbance of the human torso. Indeed, attenuation values differed by <5%. Organ radiation doses were measured using thermoluminescence dosemeters in the CDP-1C. With increased noise index, the CT dose index, the dose-length product and the average organ dose all decreased. Thus, the CDP-1C phantom can also assess dose levels during CT examinations in Chinese patients. The noise index (based on ATCM techniques) should be set to 8.5 or higher to reduce X-ray exposure while maintaining appropriate resolution for diagnosis.

Vascular Enhancement and Image Quality of CT Venography: Comparison of Standard and Low Kilovoltage Settings

The objective of our study was to investigate the vascular enhancement and image quality of CT venography (CTV) with a lower peak kilovoltage (kVp) setting than the standard setting. In this retrospective study, the clinical records of 100 consecutive patients with suspected pulmonary embolism were analyzed. All patients underwent pulmonary CT angiography and CTV of the abdomen, pelvis, and lower extremities using 64-MDCT with automatic tube current modulation: 50 patients underwent CT at 120 kVp, the standard kVp setting, and 50 patients were scanned at 100 kVp; we refer to these groups as the "standard-kVp group" and the "low-kVp group," respectively. Vessel enhancement and image noise were assessed in the inferior vena cava (IVC), femoral vein, and popliteal vein. Two radiologists who were blinded to the kVp setting placed the regions of interest on vessels by consensus and assessed image quality using a 5-point visual scale. Effective dose was estimated using the dose-length product. The Wilcoxon rank test was used to evaluate differences between the two groups using statistics software (JMP, version 5.1). A p value of less than 0.05 was considered to indicate statistical significance. Mean vascular enhancement was significantly higher in the low-kVp group than in the standard-kVp group: IVC, 138.4 ± 12.2 (SD) HU versus 164.5 ± 17.4 HU, respectively; femoral vein, 130.2 ± 18.0 HU versus 152.0 ± 24.5 HU; and popliteal vein, 136.7 ± 17.5 HU versus 158.3 ± 26.0 HU. Although the images of the low-kVp group had significantly higher image noise, there were no significant differences in image quality in the IVC and popliteal vein. The mean effective dose for the low-kVp protocol was significantly lower than that for the standard-kVp protocol. Lowering the kVp setting for CTV examinations improved vascular enhancement while providing sufficient image quality.

Real-time estimation of dose reduction for pediatric CT using bismuth shielding

The purpose of this study is to estimate the dose reduction of bismuth shielding for pediatric CT using a real-time CT dose probe. Routine CT examinations of head, chest and abdomen were performed on a 10-year pediatric anthropomorphic phantom, with and without bismuth shield. A solid-state dosimeter was used to obtain real-time dose measurements and to estimate the dose reduction. The maximum dose reduction of eyes using bismuth shield for in-plane CT scanning can achieve 56% during spiral head scanning interval. The average dose reduction is around 25% for in-plane CT scanning, and reaches 62% combining the technique of automatic tube current modulation. The solid-state probe can provide fast and convenient measurements for dose estimation of superficial organs. It is suggested to use Bismuth shielding in routine CT examinations to protect the radiosensitive tissues.

Image Noise and Liver Lesion Detection With MDCT: A Phantom Study

The purpose of this study was to determine the upper limit of noise for detection of small low-contrast lesions in a liver phantom. A CT liver phantom containing 21 low-contrast, low-attenuation, circular simulated lesions ranging in size from 2.4 to 10 mm was scanned 23 times at different tube current ranges (varying noise index) on a 64-MDCT scanner with automatic tube current modulation. The attenuation of the simulated lesions was 20 HU less than that of the liver-equivalent background. Three radiologists independently reviewed the resultant CT images, which contained either a low-contrast lesion or no lesion and scored certainty of lesion detection using a 4-point Likert scale. Overall performance was evaluated by sensitivity analysis with receiver operator curve and area under the curve (A(z)) computation for ranges of noise index. The reviewers achieved 100% sensitivity with a noise index of 15 or less for lesions measuring 6.3-10.0 mm (A(z) = 0.96). Increasing noise index to the 17-21 range resulted in a minor decrease in sensitivity and overall performance (sensitivity, 92.3%; A(z) = 0.93). A further increase in noise index to the 23-27 range resulted in a moderate decrease in sensitivity (sensitivity, 81.4%; A(z) = 0.77). Beyond the noise index 23-27 range, sensitivity dropped markedly from 81.4% to 39%. Agreement between the three readers in assessing the image sets was moderate. For detection of small low-contrast lesions in the liver phantom model used in this study, the upper limit of noise index may be in the 15-21 range for sensitivity greater than 90%.

Low voltage CTPA for patients with suspected pulmonary embolism

To test a low dose, low voltage protocol for the diagnosis of pulmonary embolism. 50 Patients with clinically suspected pulmonary embolism underwent CTPA with 80kVp and 295mAs (test group) on a 64-row MDCT scanner. These patients were compared to a similar group of 50 patients scanned on the same scanner with the 120kVp protocol with automatic tube current modulation (control group). All patients received 100-110ml of a 370mgI/ml solution at 4.5cm(3)/s. Scans were compared for quantitative imaging parameters (attenuation and standard deviation in the main, right and left pulmonary arteries, in a lobar and segmental pulmonary artery and in the aorta) and for dose parameters (CTDI and DLP), using an unpaired t-test. Phantom measurements were also performed for image quality parameters and radiation dose. Mean attenuation was significantly higher in the test group than in the control group in the main pulmonary trunk, in the right pulmonary artery, in the left pulmonary artery, in a lobar and segmental pulmonary artery and in the ascending aorta (all: p≤0.0001). Noise was significantly higher in the test group than in the control group, but contrast to noise ratio was not significantly different between the two protocols. Radiation dose was significantly lower in the test group than in the control group, as measured by CTDI, DLP and effective dose to organs (all: p<0.0001). The use of 80kVp for pulmonary CTA significantly reduces patient radiation exposure, and significantly increases contrast medium attenuation in the pulmonary arteries, with no effect on the diagnostic quality of the exams.

Estimation and comparison of effective dose (E) in standard chest CT by organ dose measurements and dose-length-product methods and assessment of the influence of CT tube potential (energy dependency) on effective dose in a dual-source CT

To determine effective dose (E) during standard chest CT using an organ dose-based and a dose-length-product-based (DLP) approach for four different scan protocols including high-pitch and dual-energy in a dual-source CT scanner of the second generation. Organ doses were measured with thermo luminescence dosimeters (TLD) in an anthropomorphic male adult phantom. Further, DLP-based dose estimates were performed by using the standard 0.014mSv/mGycm conversion coefficient k. Examinations were performed on a dual-source CT system (Somatom Definition Flash, Siemens). Four scan protocols were investigated: (1) single-source 120kV, (2) single-source 100kV, (3) high-pitch 120kV, and (4) dual-energy with 100/Sn140kV with equivalent CTDIvol and no automated tube current modulation. E was then determined following recommendations of ICRP publication 103 and 60 and specific k values were derived. DLP-based estimates differed by 4.5-16.56% and 5.2-15.8% relatively to ICRP 60 and 103, respectively. The derived k factors calculated from TLD measurements were 0.0148, 0.015, 0.0166, and 0.0148 for protocol 1, 2, 3 and 4, respectively. Effective dose estimations by ICRP 103 and 60 for single-energy and dual-energy protocols show a difference of less than 0.04mSv. Estimates of E based on DLP work equally well for single-energy, high-pitch and dual-energy CT examinations. The tube potential definitely affects effective dose in a substantial way. Effective dose estimations by ICRP 103 and 60 for both single-energy and dual-energy examinations differ not more than 0.04mSv.

The measurement of organic radiation dose of multi-slice CT scanning by using the Chinese anthropomorphic chest phantom

Objective Using the Chinese anthropomorphic chest phantom to measure the absorbed dose of various tissues and organs under different noise index, and to assess the radiation dose of MSCT chest scanning with the effective dose(ED). Methods The equivalence of the Chinese anthropomorphic chest phantom(CDP-1C) and the adult chest on CT sectional anatomy and X-ray attenuation was demonstrated. The absorbed doses of various tissues and organs under different noise index were measured by laying thermoluminescent dosimeters(TLD) inside the phantom, and the corresponding dose-length products(DLP) were recorded. Both of them were later converted into ED and comparison was conducted to analyze the dose levels of chest CT scanning with automatic tube current modulation (ATCM) under different noise index. Student t-test was applied using SPSS 12.0 statistical software. Results The Phantom was similar to the human body on CT sectional anatomy. The average CT value of phantom are -788.04 HU in lung,45.64 HU in heart,65.84 HU in liver,254.32 HU in spine and the deviations are 0.10%,3.04%, 4.49% and 4.36% respectively compared to humans. The difference of average CT value of liver was statistically significant(t=-8.705,P＜0.05),while the differences of average CT values of lung, heart and spine were not significant(t value were -0.752,-1.219,-1.138,respectively and P＞0.05).As the noise index increased from 8.5 to 22.5, the DLP decreased from 393.57 mGy·cm to 78.75 mGy·cm and the organs dose declined. For example, the average absorbed dose decreased from 22.38 mGy to 3.66 mGy in lung. Compared to ED calculating by absorbed dose, the ED calculating by DLP was lower. The ED values of the two methods were 6.69 mSv and 8.77 mSv when the noise index was set at 8.5. Conclusions Application of the Chinese anthropomorphic chest phantom to carry out CT dose assessment is more accurate. The noise index should be set more than 8.5 during the chest CT scanning based on ATCM technique. Key words: Phantoms, imaging; Densitometry,X-ray; Radiation dosage

SU‐E‐I‐14: Image Quality and Patient Dose Dependence on CT Scan Parameters for ATCM (CareDose4D)

Purpose: To develop pediatric CT protocols by characterizing effects of CT parameters on pediatric CT dose using automatic tube‐current modulation (ATCM). Methods: Sections of varying AP thicknesses (∼80, 117, 149, 214mm) from the adult, 10‐year‐old, 5‐year‐old, and 1‐year‐old ATOM phantoms (CIRS, Norfolk, VA) were placed contiguously. They were scanned on a 16‐channel CT (Emotion 16, Siemens) using ATCM (CareDose4D) to characterize the relationships between image quality and patient‐dose under different scan conditions: scan‐kVp, adult/pediatric patient selection, and patient size. Effective‐mAs/slice was extracted from the DICOM header of each CT image to calculate the DLP/slice. Noise was estimated as the standard deviation in ROIs. Two figures of merit (FOM) were defined: FOM1 as (noisê2/DLP)̂^−1; FOM2 as the (CNR̂2/DLP). Each FOM was calculated for each image of each series acquired. Results: for scan acquired as pediatric patients, the effective‐mAs/slice was lower than the reference‐mAs value for patient AP thicknesses below 120 mm but greater above AP thicknesses of 149mm. These differences increased with patient thickness and at lower kVp. For scans acquired as adult patients, the effective‐mAs/slice was always lower than the reference‐mAs. Under ATCM, the percent difference between the FOM1 for 110 and 80kVp relative to 130kVp ranged from [7,−12]% and [13,−58]% for pediatric, and [3, −16]% and [5,−92]% for adult patients. Under ATCM, the percent difference between the FOM2 for 110 (80)kVp and 130kVp for increasing thickness ranged from 54 to 36 (130 to 78)% for pediatric‐entered patients, and 51 to 32 (125 to 43)% for adult entered patients. Conclusion: CT dose with ATCM decreased for adult‐patient and increased for pediatric‐patient selection. The AP thickness at which ATCM will use effective‐mAs/slice &gt; reference‐mAs value for pediatric‐patients selection is 120–149mm. Adult ‐ patient selection maintains effective‐mAs/slice &lt; reference‐mAs value.

SU‐E‐I‐77: Reduction of Fetal Dose in Computed Tomography Using Anterior Shields

Purpose: The potential application of “in beam” anterior shields were examined from fetal dose reduction and image quality perspectives.Methods: Dose reduction afforded by commercially available bismuth impregnated synthetic rubber fabricated as 12.5 and 25 cm wide anterior shields was measured. Thermoluminescent dosimeters (TLDs) irradiated in a Rando phantom were used to estimate fetal dose. Average fetal position at 0 and 18 weeks post conception were determined based on ultrasound data of fetal position by gestational age. The latter stage of pregnancy (18 weeks) was simulated by adding tissue equivalent bolus material over the anterior surface of the phantom. To assess the impact of anterior shielding on image quality, an RMI 461A body phantom with low contrast resolution phantom insert was employed. The minimum size of low contrast resolution target visualize was recorded with no shielding, 12.5 cm wide shielding and 25 cm wide shielding.Results: At 120kVp, the 12.5 cm wide bismuth impregnated synthetic rubber shield was shown to reduce fetal dose 14% and 17% at 0 and 18 weeks post conception while the 25 cm shield reduced fetal dose 22% and 38% at 0 weeks and 18 weeks post conception. These shields produce only minor artifacts and do not adversely impact low contrast resolution. It should be noted that the use of these shields in automatic tube current modulation mode may not reduce fetal dose. Conclusions: These shields may prove to be useful for clinical fetal dose reduction early in pregnancy. It should be noted that similar results may also be achieved by tube current modulation to reduce the anterior dose contribution.

Implementation of Automated Tube Current Modulation in PET/CT: Prospective Selection of a Noise Index and Retrospective Patient Analysis to Ensure Image Quality

Automated tube current modulation (ATCM) has been shown to be a useful tool for reducing CT dose. However, its implementation can be complicated, because the correlation between noise index (NI) settings and noise production can change as parameters are manipulated. The goal was to create a methodology to prospectively select ATCM parameters and retrospectively ensure consistent image quality. An anthropomorphic phantom was scanned at various NIs to determine a baseline NI versus image noise. The noise was measured in SDs of the CT number reported in Hounsfield units. A physician then reviewed 45 studies performed with the same fixed-tube-current protocol to obtain a preferred noise level. The noise level was compared with our phantom baseline scans to find a suitable NI value. This value was implemented in clinical operation. Then, the next 50 patient examinations were retrospectively reviewed to ensure that image quality was maintained to our physician's cutoff noise levels. Radiation dose reductions through tube current reduction were measured for all CT slices of each patient study. In the phantom study, tube current modulation was observed at an NI of 15. The preferred noise level established in the physician's review correlated with an NI of 20. In our postimplementation analysis, we found that our noise level was 10.75 SDs in Hounsfield units. CT dose reductions of up to 52% were seen. We were able to prospectively select an NI for ATCM CT by correlating phantom scans to a physician's preferred noise level while maintaining consistent image quality.

Low-dose Unenhanced CT for IV Contrast Bolus Timing:: Is It Reliable to Assess Hepatic Steatosis?

To determine whether an unenhanced low-dose image acquired during automated contrast bolus timing can be used to assess hepatic steatosis. Fifty subjects (29 male, 21 female; 26-92 years; mean body mass index (BMI; 26.9) with abdominal multiphasic computed tomography were included. Abdominal diameters and circumferences were derived from anteroposterior and lateral scout radiographs. Hepatic attenuation (HA) was measured on unenhanced low-dose images (120 kV; 40 mA; 0.5 seconds' rotation time) and corresponding unenhanced standard-dose images (120 kV, z-axis automatic tube current modulation, noise index 11.5). Noise estimates were measured in surrounding air. Pearson correlation was calculated between abdominal circumference and BMI. Mean HA assessed on low-dose images and standard-dose images was compared using a paired Student's t-test and Bland Altman plots. Abdominal circumference (mean, 142.8cm) correlated well with BMI (r = 0.83). No significant difference was found for HA on low-dose images (mean +57.7 HU) compared to HA on standard-dose images (+56.0 HU) (P = .077). Image noise (+11.5 HU) was significantly higher on low-dose images compared to image noise (+8.1 HU) on standard-dose images (P < .05). For HA mean difference comparing low- and standard-dose images was -1.7 HU (limits of agreement: -14.6, 11.2). In all subjects, hepatic attenuation can be correctly assessed on unenhanced low-dose images.

CT Dose Index and Patient Dose: They AreNotthe Same Thing

Estimates of individual patient risk, and epidemiologic studies assessing potential late effects, must use patient size–specific dose estimates—they cannot use only scanner output (volume CT dose index or dose-length product).