Enabling ultra-high dose rate electron beams at a clinical linear accelerator for isocentric treatments.

BACKGROUND AND PURPOSE: Radiotherapy delivery with ultra-high dose rates (UHDR) has consistently produced normal tissue sparing while maintaining efficacy for tumour control in preclinical studies, known as the FLASH effect. Modified clinical electron linacs have been used for pre-clinical studies at reduced source-surface distance (SSD) and novel intra-operative devices are becoming available. In this context, we modified a clinical linac to deliver 16 MeV UHDR electron beams with an isocentric setup. MATERIALS AND METHODS: The first Varian TrueBeam (SN 1001) was clinically operative between 2009-2022, it was then decommissioned and converted into a research platform. The 18 MeV electron beam was converted into the experimental 16 MeV UHDR. Modifications were performed by Varian and included a software patch, thinner scattering foil and beam tuning. The dose rate, beam characteristics and reproducibility were measured with electron applicators at SSD = 100 cm. RESULTS: The dose per pulse at isocenter was up to 1.28 Gy/pulse, corresponding to average and instantaneous dose rates up to 256 Gy/s and 3⋅105 Gy/s, respectively. Beam characteristics were equivalent between 16 MeV UHDR and conventional for field sizes up to 10x10cm2 and an overall beam reproducibility within ± 2.5% was measured. CONCLUSIONS: We report on the first technical conversion of a Varian TrueBeam to produce 16 MeV UHDR electron beams. This research platform will allow isocenter experiments and deliveries with conventional setups up to field sizes of 10x10 cm2 within a hospital environment, reducing the gap between preclinical and clinical electron FLASH investigations.

Corrigendum to "Robustness of radiomic features in magnetic resonance imaging for patients with glioblastoma: Multi-center study" [Phys. Imaging Radiat. Oncol. 22 (2022) 131-136].

[This corrects the article DOI: 10.1016/j.phro.2022.05.006.].

Robustness of radiomic features in magnetic resonance imaging for patients with glioblastoma: Multi-center study.

Background and purpose: Radiomics offers great potential in improving diagnosis and treatment for patients with glioblastoma multiforme. However, in order to implement radiomics in clinical routine, the features used for prognostic modelling need to be stable. This comprises significant challenge in multi-center studies. The aim of this study was to evaluate the impact of different image normalization methods on MRI features robustness in multi-center study. Methods: Radiomics stability was checked on magnetic resonance images of eleven patients. The images were acquired in two different hospitals using contrast-enhanced T1 sequences. The images were normalized using one of five investigated approaches including grey-level discretization, histogram matching and z-score. Then, radiomic features were extracted and features stability was evaluated using intra-class correlation coefficients. In the second part of the study, improvement in the prognostic performance of features was tested on 60 patients derived from publicly available dataset. Results: Depending on the normalization scheme, the percentage of stable features varied from 3.4% to 8%. The histogram matching based on the tumor region showed the highest amount of the stable features (113/1404); while normalization using fixed bin size resulted in 48 stable features. The histogram matching also led to better prognostic value (median c-index increase of 0.065) comparing to non-normalized images. Conclusions: MRI normalization plays an important role in radiomics. Appropriate normalization helps to select robust features, which can be used for prognostic modelling in multicenter studies. In our study, histogram matching based on tumor region improved both stability of radiomic features and their prognostic value.

Systematic literature review on the benefit of patient protection shielding during medical X-ray imaging: Towards a discontinuation of the current practice.

PURPOSE: Patient shielding during medical X-ray imaging has been increasingly criticized in the last years due to growing evidence that it often provides minimal benefit and may even compromise image quality. In Europe, and as also shown in a short assessment in Switzerland, the use of patient shielding is inhomogeneous. The aim of this study was to systematically review recent literature in order to assess benefits and appraise disadvantages related to the routine use of patient shielding. METHODS: To evaluate benefits and disadvantages related to the application of patient shielding in radiological procedures, a systematic literature review was performed for CT, radiography, mammography and fluoroscopy-guided medical X-ray imaging. In addition, reports from medical physics societies and authorities of different countries were considered in the evaluation. RESULTS: The literature review revealed 479 papers and reports on the topic, from which 87 qualified for closer analysis. The review considered in- and out-of-plane patient shielding as well as shielding for pregnant and pediatric patients. Dose savings and other dose and non-dose related effects of patient shielding were considered in the evaluation. CONCLUSIONS: Although patient shielding has been used in radiological practice for many years, its use is no longer undisputed. The evaluation of the systematic literature review of recent studies and reports shows that dose savings are rather minimal while significant dose- and non-dose-related detrimental effects are present. Consequently, the routine usage of patient protection shielding in medical X-ray imaging can be safely discontinued for all modalities and patient groups.

Radiomic Analysis to Predict Outcome in Recurrent Glioblastoma Based on Multi-Center MR Imaging From the Prospective DIRECTOR Trial.

BACKGROUND: Based on promising results from radiomic approaches to predict O6-methylguanine DNA methyltransferase promoter methylation status (MGMT status) and clinical outcome in patients with newly diagnosed glioblastoma, the current study aimed to evaluate radiomics in recurrent glioblastoma patients. METHODS: Pre-treatment MR-imaging data of 69 patients enrolled into the DIRECTOR trial in recurrent glioblastoma served as a training cohort, and 49 independent patients formed an external validation cohort. Contrast-enhancing tumor and peritumoral volumes were segmented on MR images. 180 radiomic features were extracted after application of two MR intensity normalization techniques: fixed number of bins and linear rescaling. Radiomic feature selection was performed via principal component analysis, and multivariable models were trained to predict MGMT status, progression-free survival from first salvage therapy, referred to herein as PFS2, and overall survival (OS). The prognostic power of models was quantified with concordance index (CI) for survival data and area under receiver operating characteristic curve (AUC) for the MGMT status. RESULTS: We established and validated a radiomic model to predict MGMT status using linear intensity interpolation and considering features extracted from gadolinium-enhanced T1-weighted MRI (training AUC = 0.670, validation AUC = 0.673). Additionally, models predicting PFS2 and OS were found for the training cohort but were not confirmed in our validation cohort. CONCLUSIONS: A radiomic model for prediction of MGMT promoter methylation status from tumor texture features in patients with recurrent glioblastoma was successfully established, providing a non-invasive approach to anticipate patient's response to chemotherapy if biopsy cannot be performed. The radiomic approach to predict PFS2 and OS failed.

Comparison of ultrasound speed-of-sound of the lower extremity and lumbar muscle assessed with computed tomography for muscle loss assessment.

ABSTRACT: To compare the speed of propagation of ultrasound (US) waves (SoS) of the lower leg with the clinical reference standard computed tomography (CT) at the level of lumbar vertebra 3 (L3) for muscle loss assessment. Both calf muscles of 50 patients scheduled for an abdominal CT were prospectively examined with ultrasound. A plexiglas-reflector located on the opposite side of the probe with the calf in between was used as a timing reference for SoS (m/s). CT measurements were performed at the level of L3 and included area (cm2) and attenuation (HU) of the psoas muscle, abdominal muscles, subcutaneous fat, visceral fat and abdominal area. Correlations between SoS, body mass index (BMI) and CT were determined using Pearson's correlation coefficient. Based on reported CT sarcopenia threshold values, receiver operating characteristic (ROC) analysis was performed for SoS. Inter-examiner agreement was assessed with the median difference, inter-quartile range (IQR) and intraclass correlation coefficients. SoS of the calf correlated moderately with abdominal muscle attenuation (r = 0.48; P < .001), psoas muscle attenuation (r = 0.40; P < .01), abdominal area (r = -0.44; P < .01) and weakly with subcutaneous fat area (r = -0.37; P < .01). BMI correlated weakly with psoas attenuation (r = -0.28; P < .05) and non-significantly with abdominal muscle attenuation. Normalization with abdominal area resulted in moderate correlations with abdominal muscle area for SoS (r = 0.43; P < .01) and BMI (r = -0.46; P < .001). Based on sarcopenia threshold values for skeletal muscle attenuation (SMRA), area under curve (AUC) for SoS was 0.724. Median difference between both examiners was -3.4 m/s with IQR = 15.1 m/s and intraclass correlation coefficient = 0.794. SoS measurements of the calf are moderately accurate based on CT sarcopenia threshold values, thus showing potential for muscle loss quantification.

Spiral breast computed tomography (CT): signal-to-noise and dose optimization using 3D-printed phantoms.

OBJECTIVES: To investigate the dependence of signal-to-noise ratio (SNR) and calculated average dose per volume of spiral breast-CT (B-CT) on breast size and breast density and to provide a guideline for choosing the optimal tube current for each B-CT examination. MATERIALS AND METHODS: Three representative B-CT datasets (small, medium, large breast size) were chosen to create 3D-printed breast phantoms. The phantoms were filled with four different agarose-oil-emulsions mimicking differences in breast densities. Phantoms were scanned in a B-CT system with systematic variation of the tube current (6, 12.5, 25, 32, 40, 50, 64, 80, 100, 125 mA). Evaluation of SNR and the average dose per volume using Monte Carlo simulations were performed for high (HR) and standard (STD) spatial resolution. RESULTS: SNR and average dose per volume increased with increasing tube current. Artifacts had negligible influence on image evaluation. SNR values ≥ 35 (HR) and ≥ 100 (STD) offer sufficient image quality for clinical evaluation with SNR being more dependent on breast density than on breast size. For an average absorbed dose limit of 6.5 mGy for the medium and large phantoms and 7 mGy for the small phantom, optimal tube currents were either 25 or 32 mA. CONCLUSIONS: B-CT offers the possibility to vary the X-ray tube current, allowing image quality optimization based on individual patient's characteristics such as breast size and density. This study describes the optimal B-CT acquisition parameters, which provide diagnostic image quality for various breast sizes and densities, while keeping the average dose at a level similar to digital mammography. KEY POINTS: • Image quality optimization based on breast size and density varying the tube current using spiral B-CT.

Bone Mineral Density Quantification from Localizer Radiographs: Accuracy and Precision of Energy-integrating Detector CT and Photon-counting Detector CT.

Background Bone mineral density (BMD) could be derived from CT localizer radiographs and could potentially enable opportunistic osteoporosis screening. Purpose To assess the accuracy and precision of BMD measurement using two localizer radiographs obtained with energy-integrating detector CT and a single localizer radiograph obtained with photon-counting detector CT. Materials and Methods A calibration phantom and a porcine phantom with lumbar vertebrae were imaged with a dual-energy x-ray absorptiometry (DXA) scanner, a clinical energy-integrating detector CT scanner, and a prototype photon-counting detector CT scanner. Two localizer radiographs at different combinations of tube voltages were obtained with energy-integrating detector CT, and one localizer radiograph was obtained with photon-counting detector CT using different energy thresholds. BMD was calculated for all three approaches and compared with the known specifications in the calibration phantom. In the animal phantom, BMDs from both CT systems were compared with those from the DXA scanner (the reference standard). Accuracy was defined as the measurement error of BMD (ΔBMD), and precision was defined as the coefficient of variation (in percentage). Radiation doses were estimated. Nonparametric tests were applied. Results In the calibration phantom, ΔBMD was smaller with both CT systems compared with the DXA scanner (both P < .05). ΔBMD ranged from -5% to -1.8% for DXA, from -2.3% to -1.7% for energy-integrating detector CT, and from -1.6% to 1.6% for photon-counting detector CT. Precision (range, 0.3%-2.8%) was high for both CT systems. In the animal phantom, ΔBMD ranged from -0.6% to 0.1% for energy-integrating detector CT and from -0.1% to 0.6% for photon-counting detector CT, with no significant differences between CT systems (P = .65). The dose-area product in the animal phantom was 4.6 cGy ∙ cm2 for DXA, 3.5-11.5 cGy ∙ cm2 for energy-integrating detector CT, and 7.2-11.2 cGy ∙ cm2 for photon-counting detector CT, depending on tube voltage and energy threshold combination. Conclusion Experimental evidence suggests that bone mineral density measurements are accurate and precise using two localizer radiographs at different tube voltages from energy-integrating detector CT and a single localizer radiograph with different energy thresholds from photon-counting detector CT. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Pourmorteza in this issue.

Radiation Dose to the Fetus From Computed Tomography of Pregnant Patients-Development and Validation of a Web-Based Tool.

OBJECTIVE: Estimations of radiation dose absorbed by the fetus from computed tomography (CT) in pregnant patients is mandatory, but currently available methods are not feasible in clinical routine. The aims of this study were to develop and validate a tool for assessment of fetal dose from CT of pregnant patients and to develop a user-friendly web interface for fast fetal dose calculations. METHODS: In the first study part, 750 Monte Carlo (MC) simulations were performed on phantoms representing pregnant patients at various gestational stages. The MC code simulating vendor-independent dose distributions was validated against CT dose index (CTDI) measurements performed on CT scanners of 2 vendors. The volume CTDI-normalized fetal dose values from MC simulations were used for developing the computational algorithm enabling fetal dose assessments from CT of various body regions at different exposure settings. In the institutional review board-approved second part, the algorithm was validated against patient-specific MC simulations performed on CT data of 29 pregnant patients (gestational ages 8-35 weeks) who underwent CT. Furthermore, the tool was compared with a commercially available software. A user-friendly web-based interface for fetal dose calculations was created. RESULTS: Weighted CTDI values obtained from MC simulations were in excellent agreement with measurements performed on the 2 CT systems (average error, 4%). The median fetal dose from abdominal CT in pregnant patients was 2.7 mGy, showing moderate correlation with maternal perimeter (r = 0.69). The algorithm provided accurate estimates of fetal doses (average error, 11%), being more accurate than the commercially available tool. The web-based interface (www.fetaldose.org) enabling vendor-independent calculations of fetal doses from CT requires the input of gestational age, volume CTDI, tube voltage, and scan region. CONCLUSIONS: A tool for fetal dose assessments from CT of pregnant patients was developed and validated being freely available on a user-friendly web interface.

Lesion Detectability and Radiation Dose in Spiral Breast CT With Photon-Counting Detector Technology: A Phantom Study.

OBJECTIVES: The aim of the article was to evaluate the lesion detectability, image quality, and radiation dose of a dedicated clinical spiral breast computed tomography (CT) system equipped with a photon-counting detector, and to propose optimal scan parameter settings to achieve low patient dose levels and optimal image quality. METHODS: A breast phantom containing inserts mimicking microcalcifications (diameters 196, 290, and 400 µm) and masses (diameters 1.8, 3.18, 4.76, and 6.32 mm) was examined in a spiral breast CT system with systematic variations of x-ray tube currents between 5 and 125 mA, using 2 slabs of 100 and 160 mm. Signal-to-noise ratio and contrast-to-noise ratio measurements were performed by region of interest analysis. Two experienced radiologists assessed the detectability of the inserts. The average absorbed dose was calculated in Monte Carlo simulations. RESULTS: Microcalcifications in diameters of 290 and 400 µm and masses in diameters of 3.18, 4.76, and 6.32 mm were visible for all tube currents between 5 and 125 mA. Soft tissue masses in a diameter of 1.8 mm were visible at tube currents of 25 mA and higher. Microcalcifications with a diameter of 196 µm were detectable at a tube current of 25 mA and higher in the small, and at a tube current of 40 mA and higher in the large slab. For the small and large breast, at a tube current of 25 and 40 mA, an average dose value of 4.30 ± 0.01 and 5.70 ± 0.02 mGy was calculated, respectively. CONCLUSIONS: Optimizing tube current of spiral breast CT according to the breast size enables the visualization of microcalcifications as small as 196 µm while keeping dose values in the range of conventional mammography.

Vertical Off-Centering in Reduced Dose Chest-CT: Impact on Effective Dose and Image Noise Values.

OBJECTIVES: To assess the effect of vertical off-centering in tube current modulation (TCM) on effective-dose and image-noise in reduced-dose (RD) chest-CT. METHODS: One-hundred consecutive patients (36 female; mean age 56 years) were scanned on a 192-slice CT scanner with a standard-dose (ND) and a RD chest-CT protocol using tube current modulation. Image-noise was evaluated by placing circular regions of interest in the apical, middle, and lower lung regions. Two independent readers evaluated image quality. Study population was stratified according to patient position in the gantry: positioned in the gantry isocenter (i), higher than the gantry isocenter (ii), and lower than the gantry isocenter, (iii). Pearson correlation was used to determine the correlation between effective radiation dose and vertical off-centering. Student's t test was used to evaluate for differences in image-noise between groups (i-iii). RESULTS: Mean vertical off-centering was of 10.6 mm below the gantry-isocenter (range -45.0-27.9 mm). Effective radiation dose varied in a linear trend, with the highest doses noted below gantry isocenter, and the lowest doses noted above gantry isocenter (ND: r = -0.296; p = 0.003 - RD: r = -0.258; p = 0.010). Lowest image-noise was observed where patients were positioned below the gantry isocenter, and highest in patients positioned above (ND: 79.35 HU vs. 94.86 HU - RD: 143.44 HU vs. 160.13 HU). Subjective image quality was not significantly affected by patient-position (p > 0.05). Overall, there was no over-proportional noise-increase from the ND to the RD protocol in patients which were positioned off-center. CONCLUSION: Vertical off-centering influences effective radiation dose and image-noise on ND and RD protocols. ADVANCES IN KNOWLEDGE: There is no over-proportional noise increase in RD compared to ND protocols when patients are positioned off-center.

How patient off-centering impacts organ dose and image noise in pediatric head and thoracoabdominal CT.

OBJECTIVES: To assess the impact of patient off-centering on organ dose and image noise for head and thoracoabdominal CT in a pediatric phantom. METHODS: An anthropomorphic phantom simulating a 5-year-old child was used. Semiconductor dosimeters were placed in various cranial and thoracoabdominal organs. Head and thoracoabdominal CT were performed using automatic tube current modulation (ATCM) and default bowtie filters. The phantom was imaged repeatedly at vertical table positions ranging from - 6 to + 6 cm from the 0-position. Tube current time products (TCTP), organ doses, and image noise were recorded. Scatter radiation was measured in the thyroid for head CT. The effect of ATCM and bowtie filters was assessed. RESULTS: Depending on patient position, organ doses differed up to 22% for the supratentorial brain, 34% for the infratentorial brain, 19% for the eyes, 28% for the lungs, 25% for the stomach, and 22% for the liver compared with those in the 0-position. The relation between position and dose was linear and mainly affected by the bowtie filter in head CT, while it was quadratic and affected by ATCM and bowtie filter in thoracoabdominal CT. It further depended on the relative position of each organ to the isocenter. An inverse relation was found between position and image noise. Scatter radiation was not significantly related to patient positioning (p = 0.21). CONCLUSIONS: In pediatric CT, vertical patient positioning had a substantial impact on radiation dose with differences of up to 34%, depending on the body region and location of each individual organ. KEY POINTS: • Patient off-centering has a substantial impact on organ radiation dose and image noise in pediatric CT. • Impact of patient off-centering on radiation dose and noise differs between head and thoracoabdominal CT. • Differences are caused by both ATCM and bowtie filter in thoracoabdominal CT, but mainly by bowtie filter in head CT.

Dedicated Breast Computed Tomography With a Photon-Counting Detector: Initial Results of Clinical In Vivo Imaging.

OBJECTIVES: The purpose of this work is to present the data obtained from the first clinical in vivo application of a new dedicated spiral breast computed tomography (B-CT) equipped with a photon-counting detector. MATERIALS AND METHODS: The institutional review board approved this retrospective study. Twelve women referred for breast cancer screening were included and underwent bilateral spiral B-CT acquired in prone position. Additional sonography was performed in case of dense breast tissue or any B-CT findings. In 3 women, previous mammography was available for comparison. Soft tissue (ST) and high-resolution (HR) images were reconstructed. Two independent radiologists performed separately the readout for subjective image quality and for imaging findings detection. Objective image quality evaluation was performed in consensus and included spatial resolution, contrast resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio. All women were asked to report about positioning comfort and overall comfort during data acquisition. RESULTS: The major pectoral muscle was included in 15 breast CT scans (62.5%); glandular component was partially missing in 2 (8.3%) of the 24 scanned breasts. A thin "ring artifact" was present in all scans but had no influence on image interpretations; no other artifacts were present. Subjective image quality assessment showed excellent agreement between the 2 readers (κ = 1). Three masses were depicted in B-CT and were confirmed as simple cysts in sonography. Additional 5 simple cysts and 2 solid benign lesions were identified only in sonography. A total of 12 calcifications were depicted with a median size of 1.1 mm (interquartile range, 0.7-1.7 mm) on HR and 1.4 mm (interquartile range, 1.1-1.8 mm) on ST images. Median SNRgl, SNRfat, and contrast-to-noise ratio were significantly higher in ST than in HR reconstructions (each, P < 0.001). A mild discomfort due to positioning of the rib cage on the table was reported by 2 women (16.7%); otherwise, no discomfort was reported. CONCLUSIONS: The new dedicated B-CT equipped with a photon-counting detector provides high-quality images with potential for screening of breast cancer along with minor patient discomfort.

Technical Note: Radiation dose reduction from computed tomography localizer radiographs using a tin spectral shaping filter.

PURPOSE: To evaluate the possibility of lowering radiation dose from a localizer radiograph (LR) using a tin spectral shaping filter and to investigate the effect of this adaptation on the radiation dose and image quality of subsequent computed tomography (CT) examination. METHODS: The study utilized a set of semianthropomorphic abdomen phantoms, representing small, medium, and large patients. The LR scans were performed with and without a tin spectral shaping filter using various kVp/mA settings. The tube current values of spiral CT examinations following the LR were assessed to evaluate the effect of LR settings on automatic exposure control (AEC). The image quality of CT examinations with various LRs was evaluated by measuring image noise in several regions-of-interest. Organ dose values from LR scans were derived from Monte Carlo simulations performed on a set of virtual anthropomorphic phantoms and the effective dose (ED) values were calculated. RESULTS: The radiation dose from the LR can be strongly reduced by using a tin spectral shaping filter (P < 0.001). The optimal settings of the LR scan depend on the size of the scanned subject: for small and medium size subjects, the combination of a tin spectral shaping filter with 100 kVp and 20 mA resulted in the lowest possible radiation dose (ED = 0.007 mGy) without compromising the AEC and image quality of subsequent CT. In contrast, the LR settings of 100 kVp with a tin spectral shaping filter and the tube current values of 20 and 35 mA in large subject (47.4 cm in diameter) resulted in significant variation of the TCM values (11.1% and 8.4%, respectively) and the corresponding increase of noise by >5% in subsequent CT examination. For all investigated phantom sizes, the combination of 100 Sn kV with a tin spectral shaping filter and tube current values of 75 mA results in the lowest possible radiation dose, while still keeping the AEC function unchanged. CONCLUSION: The study indicated that tin spectral shaping filtration can be applied to LRs for radiation dose reduction, but such adaptation needs to take patient size into account.

Precise and Automatic Patient Positioning in Computed Tomography: Avatar Modeling of the Patient Surface Using a 3-Dimensional Camera.

OBJECTIVES: The aim of this study was to evaluate the accuracy of a 3-dimensional (3D) camera algorithm for automatic and individualized patient positioning based on body surface detection and to compare the results of the 3D camera with manual positioning performed by technologists in routinely obtained chest and abdomen computed tomography (CT) examinations. MATERIALS AND METHODS: This study included data of 120 patients undergoing clinically indicated chest (n = 68) and abdomen (n = 52) CT. Fifty-two of the patients were scanned with CT using a table height manually selected by technologists; 68 patients were automatically positioned with the 3D camera, which is based on patient-specific body surface and contour detection. The ground truth table height (TGT) was defined as the table height that aligns the axial center of the patient's body region in the CT scanner isocenter. Off-centering was defined as the difference between the ground truth table height (TGT) and the actual table position used in all CT examinations. The t test was performed to determine significant differences in the vertical offset between automatic and manual positioning. The χ test was used to check whether there was a relationship between patient size and the magnitude of off-centering. RESULTS: We found a significant improvement in patient centering (offset 5 ± 3 mm) when using the automatic positioning algorithm with the 3D camera compared with manual positioning (offset 19 ± 10 mm) performed by technologists (P < 0.005). Automatic patient positioning based on the 3D camera reduced the average offset in vertical table position from 19 mm to 7 mm for chest and from 18 mm to 4 mm for abdomen CT. The absolute maximal offset was 39 mm and 43 mm for chest and abdomen CT, respectively, when patients were positioned manually, whereas with automatic positioning using the 3D camera the offset never exceeded 15 mm. In chest CT performed with manual patient positioning, we found a significant correlation between vertical offset greater than 20 mm and patient size (body mass index, >26 kg/m, P < 0.001). In contrast, no such relationship was found for abdomen CT (P = 0.38). CONCLUSIONS: Automatic individualized patient positioning using a 3D camera allows for accurate patient centering as compared with manual positioning, which improves radiation dose utilization.

Vertical off-centering affects organ dose in chest CT: Evidence from Monte Carlo simulations in anthropomorphic phantoms.

PURPOSE: The objective of our study was to assess the effect of patient vertical off-centering on organ dose in chest CT with tube current modulation. METHODS: For this purpose, anthropomorphic phantoms representing adult male, female, and overweight male were scanned on 192-slice CT scanner at 11 different vertical positions (maximal off-centering ± 5 cm). Monte Carlo simulations were performed for each of the investigated setup, using tube current values extracted from the raw data, in order to obtain 3D dose distributions. Organ doses were calculated as a function of vertical off-centering and compared with the reference values, calculated for the phantoms positioned in the gantry isocenter. Image noise was also calculated as a function of phantoms vertical position using few circular regions of interest. Pearson statistical analysis was used to determine the correlation coefficient between image noise and organ dose values with vertical off-centering. RESULTS: Results of our study showed a significant difference in tube currents applied by the CT scanner when the phantom was scanned in off-centered vertical positions compared to those obtained when the phantom was positioned in the gantry isocenter (P < 0.005). For all investigated phantom configurations the vertical off-centering below 20 mm in both directions resulted in relative organ dose differences below 7%, while the off-centering above 40 mm was associated with higher organ dose changes of about 20%. The highest relative dose difference of 38% was observed for the thyroid gland at the lowest table positions. A significant correlation between organ doses for breasts, heart, lungs, thyroid, and liver, and vertical off-centering (R2 = 0.909-0.998, P < 0.005) was found. The relative dose increase associated with lower table position was more pronounced in peripheral organs: breast and thyroid gland. Image noise behaved opposite to the tube current and organ doses and increased at higher table positions. CONCLUSION: Strong vertical off-centering in chest CT with tube current modulation results in misoperation of the TCM function affecting both radiation dose and image noise. Therefore, special attention must be paid to a correct patient positioning in order to optimize organ doses and image quality of the respective CT examination.

Dose Optimization for Computed Tomography Localizer Radiographs for Low-Dose Lung Computed Tomography Examinations.

INTRODUCTION: Recent studies have shown a substantial reduction of radiation dose from computed tomography (CT) scans down to 0.1 mSv for lung cancer screening and cardiac examinations, when applying optimization techniques. Hence, CT localizer radiographs (LRs) might now be considered a significant contributor to the total dose of the CT examination. We investigated in our study the potential for reducing dose of the LRs by adapting the patient-specific acquisition parameters of the LR. MATERIALS AND METHODS: Localizer radiographs covering the lungs were acquired on 2 clinical scanners (64 slices, conventional detector [CD]; 96 slices, fully integrated detector [ID]) for 3 semianthropomorphic phantoms, representing a slim, a normal, and an obese adult. Starting at 120-kV tube voltage and 250-mA current were reduced until the image quality of the LR, and thereby the accuracy of the automatic exposure control was compromised; this was defined as a deviation of measured attenuation values in the center of the LR of more than 5% from the reference values measured at the highest tube voltage and current. Subsequent Monte Carlo calculations on anthropomorphic phantoms were performed to calculate organ and effective dose values for the respective optimal settings. In addition, effective dose values normalized to CTDIvol for tube voltages ranging from 60 to 160 kV were determined for the different combinations of phantom sizes, sexes, and LR views to evaluate dose efficiency. RESULTS: For the CD scanner, the optimal LR settings depended strongly on phantom size. Higher tube voltage and current were necessary for the larger phantoms. The ID scanner showed uncompromised LR quality for all phantoms using the lowest possible tube voltage-tube current combination of 80 kV and 20 mA. Depending on patient size and LR direction, effective dose values for the optimal settings ranged from 6 to 53 µSv and 3 to 11 µSv for the CD and ID scanner, respectively. For the example of an anterior-posterior LR on a normal patient, using the optimal settings instead of the standard settings on the ID scanner reduced LR dose from 53 µSv (120 kV, 30 mA) to 10 µSv (80 kV, 20 mA). The simulations for the different tube voltages show that effective dose and CTDIvol behave similarly for different views and patient sizes. However, the tube voltage level itself impacts the relationship between CTDIvol and effective dose, by up to a factor of 2. DISCUSSION: Dose from LRs may contribute significantly to the total effective dose of low-dose CT examinations such as lung cancer screening. Optimal LR settings can reduce LR dose substantially, but adaptations have to consider scanner characteristics, detector technology, and patient size. Thus, for low-dose CT examinations, such as cardiac examinations and lung cancer screening, LR optimization may result in a significant dose reduction and thereby in a substantial reduction of total dose.

Effect of Localizer Radiography Projection on Organ Dose at Chest CT with Automatic Tube Current Modulation.

Purpose To calculate the effect of localizer radiography projections to the total radiation dose, including both the dose from localizer radiography and that from subsequent chest computed tomography (CT) with tube current modulation (TCM). Materials and Methods An anthropomorphic phantom was scanned with 192-section CT without and with differently sized breast attachments. Chest CT with TCM was performed after one localizer radiographic examination with anteroposterior (AP) or posteroanterior (PA) projections. Dose distributions were obtained by means of Monte Carlo simulations based on acquired CT data. For Monte Carlo simulations of localizer radiography, the tube position was fixed at 0° and 180°; for chest CT, a spiral trajectory with TCM was used. The effect of tube start angles on dose distribution was investigated with Monte Carlo simulations by using TCM curves with fixed start angles (0°, 90°, and 180°). Total doses for lungs, heart, and breast were calculated as the sum of the dose from localizer radiography and CT. Image noise was defined as the standard deviation of attenuation measured in 14 circular regions of interest. The Wilcoxon signed rank test, paired t test, and Friedman analysis of variance were conducted to evaluate differences in noise, TCM curves, and organ doses, respectively. Results Organ doses from localizer radiography were lower when using a PA instead of an AP projection (P = .005). The use of a PA projection resulted in higher TCM values for chest CT (P < .001) owing to the higher attenuation (P < .001) and thus resulted in higher total organ doses for all investigated phantoms and protocols (P < .001). Noise in CT images was lower with PA localizer radiography than with AP localizer radiography (P = .03). The use of an AP projection allowed for total dose reductions of 16%, 15%, and 12% for lungs, breast, and heart, respectively. Differences in organ doses were not related to tube start angles (P = .17). Conclusion The total organ doses are higher when using PA projection localizer radiography owing to higher TCM values, whereas the organ doses from PA localizer radiography alone are lower. Thus, PA localizer radiography should be used in combination with reduced reference tube current at subsequent chest CT. © RSNA, 2016 Online supplemental material is available for this article.