Anatomisch adaptierte Variation des Röhrenstroms bei der CT

Cone-beam CT dacryocystography (CBCT-DCG) is a newly developed lacrimal passage imaging technique. This pilot study aimed to determine the effect of reducing tube current on image quality and radiation dose of CBCT-DCG in healthy volunteers. Thirty volunteers were randomly divided into three groups of ten. Each group of subjects underwent CBCT-DCG using a tube current of 13mA, 10mA, and 7mA respectively. The image quality of CBCT-DCG was assessed independently by two observers using three different scoring methods and compared among the groups. The effective dose was estimated and compared among the three different tube current groups. The CBCT-DCG images provided a high spatial and contrast resolution of the bony structures and the contrast medium. No significant differences were found in the image quality between different tube current groups. Compared with the 13mA group (49.44 μSv), the effective dose for 10mA group (38.40 μSv) and 7mA group (27.12 μSv) decreased by 22.33% and 45.15%, respectively. By decreasing the tube current, CBCT-DCG can be performed with a significant reduction of effective dose without loss of image quality in healthy volunteers.

The purpose of this prospective study was to compare the performance of standard-dose MDCT with that of low-dose MDCT with tube current modulation in patients with renal colic. Three hundred patients underwent 6- and 16-MDCT in 150 standard-dose examinations (6-MDCT effective tube current, 95 mAs at 130 kV; 16-MDCT effective tube current, 120 mAs at 120 kV) and 150 low-dose examinations (6-MDCT effective tube current, 51 mAs at 110 kV; 16-MDCT effective tube current, 70 mAs at 120 kV), all performed with 4D tube current modulation. Two experienced radiologists using a clinical workstation and blinded to scan parameters prospectively viewed the images from the 300 examinations. In a second session, one experienced radiologist and two first-year residents using a clinical workstation retrospectively reviewed images from 100 randomly selected standard-dose and 100 randomly selected low-dose examinations. Tube current modulation reduced effective tube current 25-31% in all examinations. Mean effective dose was 1.41-1.58 mSv for low-dose examinations, which reached additional dose reduction of 51.2-64.3% in comparison with standard-dose examinations. Excellent correlation existed between mean tube current and body mass index of the patients. Spearman's correlation coefficient was 0.85-0.88 for all examinations. The sensitivity of low-dose examinations interpreted by two experienced reviewers was 97.3-98.6%; specificity, 93.5%; and accuracy, 95.3%. These findings were comparable with those for standard-dose examinations. Sensitivity, specificity, and accuracy of low-dose examinations of overweight and obese patients reached similar high values: 97-100%, 100%, and 98-100%, respectively. Interobserver agreement for urinary stone detection was excellent between the two reviewers, with kappa values of 0.98 for the low-dose and 0.96 for the standard-dose examinations. An alternative diagnosis was identified in 15% and 16% of cases by two experienced radiologists in the two examinations groups. In the second interpretation session, the residents found an alternative diagnosis in only 10-12% of standard-dose examinations and only 4-5% of low-dose examinations. Low-dose MDCT with tube current modulation can be used as standard procedure in evaluation for urolithiasis, even in overweight and obese patients.

To prospectively assess the effects of lower tube voltage and various effective tube currents on image quality for computed tomographic (CT) angiography of the circle of Willis. Institutional review board approval was obtained. Patients or family provided written informed consent. Signal-to-noise ratios (SNRs) were determined in a head phantom for various effective tube currents with tube voltages of 90, 120, and 140 kVp. Patients were referred for CT angiography because of acute subarachnoid hemorrhage (n = 20) or family history of cerebral aneurysms (n = 20). In each group, 10 patients were scanned with 120 kVp and 200 mAs(eff) and 10 were scanned with 90 kVp and 330 mAs(eff) (CT dose index volumes, 27.2 mGy and 20.6 mGy, respectively). CT numbers were measured in the internal carotid artery at the T junction and compared with a t test. Two radiologists used a five-point scale to subjectively score arterial enhancement, depiction of small arterial detail, image noise, venous contamination, and interference of subarachnoid blood. Mann-Whitney U test was used for statistical analysis. In the phantom, SNR(2) was proportional to effective tube current and CT dose index volume. With an identical effective tube current, SNR(2) was lower at 90 kVp than at 120 or 140 kVp. With identical CT dose index volume, tube voltage of 90 kVp resulted in a 45%-52% increase of SNR(2) compared with SNR(2) at 120 kVp. In patients, mean attenuation in the internal carotid artery T junction was higher with 90 kVp (340 HU) than with 120 kVp (252 HU, P < .001). Although dose at 90 kVp was 30% lower than dose at 120 kVp, scores for arterial enhancement and depiction of small arterial detail were higher at 90 kVp than at 120 kVp (4.0 vs 3.2 and 3.6 vs 3.1, respectively; P < .005). In head phantoms, lower tube voltage improved SNR at equal radiation doses. For CT angiography of the circle of Willis, this translated into superior image quality at 90 kVp.

ObjectiveTo compare the effect of arm positioning on radiation dose, scan length, and image noise in computed tomography (CT) scans of the medial clavicular epiphysis for forensic age estimation performed with the arms alongside the body (arms-down) versus elevated above the head (arms-up).MethodsTwenty consecutive CT scans were analysed, ten performed with arms-down and ten with arms-up. The scans were conducted at 120 kVp and 37 mAs reference tube current. Scan length extended from 10 mm above to 10 mm below the medial clavicular epiphysis. Dose-relevant parameters (effective CT tube current, volume CT dose index (CTDIvol), CT dose length product (DLP), and effective dose) as well as scan length and image noise were compared between arms-up and arms-down CT scans.ResultsPopulation characteristics: 19 males, 1 female; mean weight 65.8 ± 9.2 kg; height 174.6 ± 7.8 cm; and body mass index (BMI) 21.6 ± 2.5 kg/m². Dose-relevant parameters were significantly lower with arms-up compared to arms-down (effective tube current: 80.9 ± 21.7 mAs vs. 146.0 ± 47.5 mAs, p = 0.001; CTDIvol: 5.5 ± 1.5 mGy vs. 9.9 ± 3.2 mGy, p = 0.001; DLP: 40.2 ± 13.7 mGy*cm vs. 63.8 ± 21.9 mGy*cm, p = 0.010; effective dose: 0.6 ± 0.2 mSv vs. 0.9 ± 0.3 mSv, p = 0.010). No significant differences were found in scan length, image noise, or population characteristics.ConclusionsRemoving the arms from the CT beam path reduced radiation dose by 33% without affecting scan length or image noise. Given the importance of dose optimisation in non-medical examinations of potentially minor individuals, CT scans of the medial clavicular epiphysis should be performed with arms elevated above the head.

Radiation dose reduction in chest CT—Review of available options

To evaluate effects of varying tube current and voltage on radiation dose, image noise, and image contrast with different phantom sizes and shapes. Four round lucite phantoms with 8-32-cm diameters were scanned with multi-detector row computed tomography (CT) and 80-120 kVp. Radiation dose was based on CT dose index, image noise, and iodine contrast and measured with constant and variable tube currents that were age appropriate for each tube voltage. Radiation dose and image noise and contrast were compared in round and oval 24-cm phantoms. For various combinations of technical factors and phantom sizes and shapes, percentage differences were calculated for radiation dose and image noise and contrast. Associations between tube voltage and radiation dose, image noise, and image contrast in round and oval phantoms were determined by fitting second-degree polynomials to data. Differences in radiation dose and image noise and contrast, which were attributable to differences in tube voltage, were tested with paired t tests. With 165-mAs tube current, radiation doses with 140- and 80-kVp tube voltages were 103% ([41.9 mGy - 20.6 mGy]/20.6 mGy) and 58% ([10.2 mGy - 4.2 mGy]/10.1 mGy) higher in the 8-cm phantom than in the 32-cm phantom. When tube current was adapted for phantom size, radiation dose at 80 kVp in the 8-cm phantom was reduced by 82% ([10.1 mGy - 1.8 mGy]/10.1 mGy). In the 8-cm phantom, tube voltage was decreased from 120 to 80 kVp and tube current remained at 165 mAs, resulting in a 68% noise increase ([3.1 HU - 1.8 HU]/1.8 HU). With variable tube current, 80-kVp tube voltage in the 8-cm phantom led to a 138% noise increase ([7.3 HU - 3.1 HU]/3.1 HU). With reduced tube voltage, image contrast increased. In the 8-cm phantom, with a constant 165-mAs tube current and a decrease in tube voltage from 120 to 80 kVp, there was a 35% ([333 HU - 217 HU]/333 HU) increase in contrast. No difference was noted in radiation dose or noise between round and oval phantoms (P = .604 and P = .06, respectively), but a small statistically significant difference (1%) in contrast attenuation was demonstrated (P = .025). Reduced tube voltage for pediatric contrast material-enhanced CT reduces radiation dose and maintains image contrast. Image noise increases, but the effect is minimal in smaller phantoms. An additional reduction in tube current further reduces radiation dose.

Rising numbers of computed tomography (CT) examinations worldwide have led to a focus on dose reduction in the latest developments in CT technology. Iterative reconstruction (IR) models bear the potential to effectively reduce dose while maintaining adequate image quality. To assess the impact of adaptive statistical iterative reconstruction (ASIR) technique on dose reduction and image quality in a dedicated whole body CT (WBCT) protocol for trauma patients. A total of 122 subjects with multiple trauma was prospectively included in our study. Subjects who had to undergo a WBCT following a severe trauma were randomly assigned to two different groups: Group A was examined with an ASIR protocol for the body series (n = 64), group B (n = 58) was examined using a standard filtered back projection (FBP) protocol. Image quality was assessed both quantitatively by calculating signal-to-noise ratio (SNR) and contrast-to-noise ratios (CNRs) and qualitatively by two observers who evaluated image quality using a 5-point scale system. Applied dose was analyzed as CTDIvol (mGy), total DLP (mGyxcm), and effective dose (mSv). Applied dose for the body series in group A was about 23% lower than in group B (P < 0.05). SNR and CNRs for different tissues were not significantly different. Subjective image quality ratings were excellent and showed no significant difference, with a high inter-reader agreement. ASIR contributes to a relevant dose reduction without any loss of image quality in a dedicated WBCT protocol for patients with multiple trauma.

Purpose:The quality of a computed tomography (CT) image and the dose delivered during its acquisition depend upon the acquisition parameters used. Tube current, tube voltage, and pitch are acquisition parameters that potentially affect image quality and dose. This study investigated physicians' abilities to characterize small, solid nodules in low‐dose CT images for combinations of current, voltage and pitch, for three CT scanner models.Methods:Lung CT images was acquired of a Data Spectrum anthropomorphic torso phantom with various combinations of pitch, tube current, and tube voltage; this phantom was used because acrylic beads of various sizes could be placed within the lung compartments to simulate nodules. The phantom was imaged on two 16‐slice scanners and a 64‐slice scanner. The acquisition parameters spanned a range of estimated CTDI levels; the CTDI estimates from the acquisition software were verified by measurement. Several experienced radiologists viewed the phantom lung CT images and noted nodule location, size and shape, as well as the acceptability of overall image quality.Results:Image quality for assessment of nodules was deemed unsatisfactory for all scanners at 80 kV (any tube current) and at 35 mA (any tube voltage). Tube current of 50 mA or more at 120 kV resulted in similar assessments from all three scanners. Physician‐measured sphere diameters were closer to actual diameters for larger spheres, higher tube current, and higher kV. Pitch influenced size measurements less for larger spheres than for smaller spheres. CTDI was typically overestimated by the scanner software compared to measurement.Conclusion:Based on this survey of acquisition parameters, a low‐dose CT protocol of 120 kV, 50 mA, and pitch of 1.4 is recommended to balance patient dose and acceptable image quality. For three models of scanners, this protocol resulted in estimated CTDIs from 2.9–3.6 mGy.

Purpose: To compare the dose reduction and image quality achieved using bismuth shielding or global decreasing of the tube current in head and thoracic CT. Methods: An anthropomorphic head phantom and four sizes of thorax phantoms were used for dose measurement and image quality assessment. Three scans were performed on each phantom with a 128‐slice clinical CT scanner: 1) reference (relevant clinical protocol); 2) reference plus bismuth shield; 3) reference with a global reduction in tube current chosen to match the dose reduction from bismuth shielding. Dose to the eye was measured with OSL dosimeters. Dose to the breast was measured with an ion chamber on the surface of the phantom. Image quality was evaluated by measuring the mean and standard deviation of CT numbers within the brain region for head CT and the lung and heart regions for thoracic CT. Results: Compared to reference scan, dose to the eye was reduced by 26.4% with bismuth shielding; dose to the breast region was decreased by 21% for the 15‐cm phantom with a pediatric (2‐ply) shield and by 37% for the 30, 35 and 40‐cm phantoms with adult (4‐ply) shields. The dose reduction achieved with bismuth shielding was limited to the anterior surface. Globally decreasing tube current reduced dose by similar amounts, but for all projection angles. Image noise was increased in the brain, lung and heart regions by similar amounts with both techniques. However, streak artifacts and an increase in CT numbers were observed for using bismuth shields, but not for global tube current reduction. Conclusions: Globally decreasing tube current can produce the same dose reduction, but for all projections, as bismuth shielding reduces dose to the anterior surface (eye and breast), with a similar noise increase, yet without the streak artifacts and CT number errors caused by bismuth shields.

Objective To investigate the radiation dose to phantoms of thick dense breasts (compression thickness > 65 mm) and the influence of Molybdenum-Rhodium (Mo-Rh) and Wolfram-Rhodium (W-Rh)) target-filter combination on image quality in full field digital mammography (FFDM). Methods An aluminium plate was placed on polymethyl methacrylate (PMMA) plates to simulate dense breasts. Two of 68 mm in-depth phantoms were used to simulate photography condition of thick dense breasts with 0.1 and 0.2 mm aluminium plates respectively. The two phantoms were exposed automatically, half-automatically and manually. The parameters were recorded for each exposure including the target-filter(T-F), tube voltage (kV), tube current (mAs), average glandular dose (AGD), entrance surface dose (ESD) and the image quality figure inverse (IQFInv). Pearson correlation and regression was used to analyze the influence of AGD on IQFInv. Paired sample t-test was used to analyze the influence of different target-filters on IQFInv and AGD. Results AGD had a positive correlation with IQFInv (r=0.806-0.953, P 0.05). In addition, when tube current was 100 mAs, IQFInv had statistically significant difference for two phantoms in W-Rh(t=2.345, P<0.05). Conclusions W-Rh target-filter combination can achieve lower radiation dose under the guarantee of the image quality compared with Mo-Rh target-filter combination. At the same tube current, W-Rh target-filter combination can provide a better image quality under a relatively high tube voltage. Key words: Radiation dose; Mammography; Dense breast; Phantom; Image quality figure inverse

The aim of this study was to evaluate the image quality and radiation dose in adenosine-stress dynamic myocardial CT perfusion (CTP) imaging using different tube voltages, tube current settings, and contrast materials containing different iodine concentrations in subjects with normal body mass indices (BMI). We included 92 patients (BMI range, 18.5-24.8) who underwent dynamic CTP for the evaluation of coronary artery disease using a 128-slice dual-source computed tomography. The protocols employed the following dynamic scan parameters: protocol I with 100 kV, fixed tube current (FTC), and medium-concentration contrast material (MC, 350 mg iodine/mL); protocol II with 100 kV, automatic tube current modulation (ATCM), and MC; protocol III with 100 kV, ATCM, and high-concentration contrast material (HC, 400 mg iodine/mL); protocol IV with 80 kV, adopted FTC, and HC. Subjective image quality on a 1-3 point scale and objective image quality with respect to the contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) were assessed. Protocol IV showed higher CNR and SNR than the other protocols (P < 0.01), while the CNR and SNR values did not significantly differ among the other three protocols. There was no significant difference in subjective image quality among the protocols. The radiation dose in protocol IV was the lowest among the protocols (P < 0.01), while protocol IV resulted in a 54% overall reduction in mean effective radiation dose compared with protocol I. Dynamic myocardial CTP performed at 80 kV with adapted FTC provided high CNR and SNR while preserving subjective image quality and reducing radiation exposure.

Intraoperative CT (iCT) angiography of the brain with stereotactic frames is an integral part of navigated neurosurgery. Validated data regarding radiation dose and image quality in these special examinations are not available. We therefore investigated two iCT protocols in this IRB-approved study. Retrospective analysis of patients, who received a cerebral stereotactic iCT angiography on a 128 slice CT scanner between February 2016 and December 2017. In group A, automated tube current modulation (ATCM; reference value 410mAs) and automated tube voltage selection (reference value 120kV) were enabled, and only examinations with a selected voltage of 120kV were included. In group B, fixed parameters were applied (300mAs, 120kV). Radiation dose was measured by assessing the volumetric CT dose index (CTDIvol), dose length product (DLP) and effective dose (ED). Signal-to-noise ratio (SNR) and image noise were assessed for objective image quality, visibility of arteries and grey-white differentiation for subjective image quality. Two hundred patients (n = 100 in each group) were included. In group A, median selected tube current was 643mAs (group B, 300mAs; p < 0.001). Median values of CTDIvol, DLP and ED were 91.54mGy, 1561mGycm and 2.97mSv in group A, and 43.15mGy, 769mGycm and 1.46mSv in group B (p < 0.001). Image quality did not significantly differ between groups (p > 0.05). ATCM yielded disproportionally high radiation dose due to substantial tube current increase at the frame level, while image quality did not improve. Thus, ATCM should preferentially be disabled. • Automated tube current modulation (ATCM) yields disproportionally high radiation dose in intraoperative CT angiography of the brain with stereotactic head frames. • ATCM does not improve overall image quality in these special examinations. • ATCM is not yet optimised for CT angiography of the brain with major extracorporeal foreign materials within the scan range.

Compressed sensing for reduced hardware footprint in medical ultrasound

In previous works, we have presented concepts for dynamic beam attenuators (DBAs) allowing for substantial dose reductions. So far, we have used a tube current modulation (TCM) scheme where the tube current is proportional to the square root of the maximum object attenuation in each projection. As DBAs show particularly beneficial behavior in region-of-interest (ROI) imaging, the question arose whether the employed TCM method would still be the most meaningful in this case. We present a simulation framework calculating i) the dose distribution in the object and ii) the image quality in the ROI of the reconstructed images for a given primary fluence: The dose to every voxel was calculated from individual Monte- Carlo simulations for every beamlet. From this we can approximate the effective dose, that incorporates tissue-specific weighting factors describing the stochastic health risk, for any fluence distribution. Using the same fluence distribution, the image variance according to the propagated fluence can be calculated for a given ROI. We employed a homogeneous phantom with a centered ROI and a female thorax phantom where the ROI is described by either the heart or the spine. Eventually, we optimized the tube current according to the product of mean image variance in the ROI and patient dose. Furthermore, the tube current obtained was compared with a heuristic square root TCM (hsqTCM) method. In result, the optimized TCM matches the hsqTCM well for the idealized case of a central ROI in an elliptical, homogeneous phantom. For a more complex case with an off-center ROI, the optimized TCM differs substantially from the hsqTCM rule, offering an additional dose reduction of up to 30%.

BackgroundRotational beam delivery enables concurrent acquisition of cone-beam CT (CBCT), thereby facilitating further geometric verification of patient setup during radiation treatment. However, it is challenging to acquire CBCT during stereotactic body radiation therapy (SBRT) using flattening-filter free X-ray beams, in which a high radiation dose is delivered. This study presents quantitative evaluation results of the image quality in four-dimensional (4D) in-treatment CBCT acquired during SBRT delivery.MethodsThe impact of megavoltage (MV) scatter and acquisition parameters on the image quality was evaluated using Catphan 503 and XSight lung tracking phantoms. The in-treatment CBCT images of the phantoms were acquired while delivering 16 SBRT plans. The uniformity, contrast, and contrast-to-noise ratio (CNR) of the in-treatment CBCT images were calculated and compared to those of CBCT images acquired without SBRT delivery. Furthermore, the localizing accuracy of the moving target in the XSight lung phantom was evaluated for 10 respiratory phases.ResultsThe CNR of the 3D-reconstucted Catphan CBCT images was reduced from 6.3 to 2.6 due to the effect of MV treatment scatter. Both for the Catphan and XSight phantoms, the CBCT image quality was affected by the tube current and monitor units (MUs) of the treatment plan. The lung target in the XSight tracking phantom was most visible for extreme phases; the mean CNRs of the lung target in the in-treatment CBCT images (with 40 mA tube current) across the SBRT plans were 3.2 for the end-of-exhalation phase and 3.0 for the end-of-inhalation phase. The lung target was localized with sub-millimeter accuracy for the extreme respiratory phases.ConclusionsFull-arc acquisition with an increased tube current (e.g. 40 mA) is recommended to compensate for degradation in the CBCT image quality due to unflattened MV beam scatter. Acquiring in-treatment CBCT with a high-MU treatment beam is also suggested to improve the resulting CBCT image quality.