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.

Screening for breast cancer with Breast-CT in a ProHTA simulation.

AIMS: The potential of dedicated Breast-CT is evaluated by simulating its impact onto the performance of the German breast cancer screening program. Attendance rates, cancer detection and economic implications are quantified. METHODS: Based on a prospective health technology assessment approach, we simulated screening in different scenarios. RESULTS: In the simulation, attendance rates increase from 54 to up to 72% due to reduced pain. Breast cancers will be detected earlier while nodal positives and distant recurrences decrease. Assuming no additional cost, cost savings of up to €55 million in one screening period are computed. CONCLUSION: The simulation indicates that earlier cancer detection, fewer unnecessary biopsies and less pain are potential benefits of Breast-CT resulting in cost savings and higher attendance.

Estimates of effective dose for CT scans of the lower extremities.

PURPOSE: To determine the dose-length product (DLP)-effective dose (ED) (DLP/ED) conversion coefficient (k) tables for the lower extremities that can be used for calculating ED. MATERIALS AND METHODS: Dose calculations were performed on standard phantoms using a validated Monte Carlo calculation tool. Calculations were performed to obtain ED values for tube voltages from 80 kV to 140 kV in steps of 20 kV for the following examinations: hip (femur), knee, ankle, and computed tomographic (CT) angiography of the lower extremities. Values of the DLP were calculated by multiplying measured CT dose index values by the scan length; k values resulted as the quotients of the ED and DLP values. DLP/ED coefficients averaged over the range of voltage values and their standard deviations were determined for the given lower-extremity CT examinations for all age groups and for both sexes. RESULTS: Coefficients depend strongly on the phantom age and size, but little on the kilovolt value. In the case of the newborn, for example, k values were 0.0612, 0.0046, 0.0014, and 0.047 for hip, knee, ankle, and CT angiography, respectively, while in the case of the adult, these respective values were 0.0110, 0.0004, 0.0002, and 0.0062. A substantial difference up to 20% between coefficients in male and female phantoms was observed for CT angiographic examination. CONCLUSION: DLP/ED conversion coefficients are provided for lower extremities and allow estimation of ED for commonly used clinical musculoskeletal CT and CT angiographic protocols.

Assessment of patient dose from CT localizer radiographs.

PURPOSE: With recently introduced technical innovations for CT systems, the dose of CT scan acquisitions has been substantially reduced; even effective dose values below 1 mSv have been reported. Due to this development, dose of the localizer radiograph may contribute substantially to dose of the whole CT examination. Since there are only limited data in the literature regarding patient dose for the different types of localizer radiographs, patient dose values were estimated in our study by measurements and Monte Carlo simulations and compared to dose values of typical CT examinations. METHODS: First, dose distributions were measured in anthropomorphic phantoms for three different body regions (head, thorax, abdomen-pelvic) and three positions of the x-ray tube (AP, PA, and lateral views); measured values were compared to simulated data using Monte Carlo techniques for validation purposes. Second, organ and effective dose values for the various investigated localizer radiograph scenarios were calculated and compared with published dose values for standard CT and low-dose CT examinations. RESULTS: For the anthropomorphic phantom, deviations of the dose values between measured and calculated results were in the range of 15%. Organ and effective dose values showed a strong dependence on the tube position. The largest differences were observed for chest localizer radiographs in the female phantom for the dose to the breast (AP: 1.01 mGy vs PA: 0.24 mGy). Overall effective dose values were in the range of 0.04-0.42 mSv per localizer radiograph acquisition. CONCLUSIONS: In view of the technical dose-reducing innovations in CT, localizer radiographs may substantially contribute to the total dose of the whole CT examination, particularly in the case of dedicated low-dose scans used, e.g., for young patients or screening purposes. Optimization of dose in localizer radiographs should be pursued further in the same way as it was done in CT.

Fast on-site Monte Carlo tool for dose calculations in CT applications.

PURPOSE: Monte Carlo (MC) simulation is an established technique for dose calculation in diagnostic radiology. The major drawback is its high computational demand, which limits the possibility of usage in real-time applications. The aim of this study was to develop fast on-site computed tomography (CT) specific MC dose calculations by using a graphics processing unit (GPU) cluster. METHODS: GPUs are powerful systems which are especially suited to problems that can be expressed as data-parallel computations. In MC simulations, each photon track is independent of the others; each launched photon can be mapped to one thread on the GPU, thousands of threads are executed in parallel in order to achieve high performance. For further acceleration, the authors considered multiple GPUs. The total computation was divided into different parts which can be calculated in parallel on multiple devices. The GPU cluster is an MC calculation server which is connected to the CT scanner and computes 3D dose distributions on-site immediately after image reconstruction. To estimate the performance gain, the authors benchmarked dose calculation times on a 2.6 GHz Intel Xeon 5430 Quad core workstation equipped with two NVIDIA GeForce GTX 285 cards. The on-site calculation concept was demonstrated for clinical and preclinical datasets on CT scanners (multislice CT, flat-detector CT, and micro-CT) with varying geometry, spectra, and filtration. To validate the GPU-based MC algorithm, the authors measured dose values on a 64-slice CT system using calibrated ionization chambers and thermoluminesence dosimeters (TLDs) which were placed inside standard cylindrical polymethyl methacrylate (PMMA) phantoms. RESULTS: The dose values and profiles obtained by GPU-based MC simulations were in the expected good agreement with computed tomography dose index (CTDI) measurements and reference TLD profiles with differences being less than 5%. For 10(9) photon histories simulated in a 256 × 256 × 12 voxel thorax dataset with voxel size of 1.36 × 1.36 × 3.00 mm(3), calculation times of about 70 and 24 min were necessary with single-core and multiple-core central processing unit (CPU) solutions, respectively. Using GPUs, the same MC calculations were performed in 1.27 min (single card) and 0.65 min (two cards) without a loss in quality. Simulations were thus speeded up by factors up to 55 and 36 compared to single-core and multiple-core CPU, respectively. The performance scaled nearly linearly with the number of GPUs. Tests confirmed that the proposed GPU-based MC tool can be easily adapted to different types of CT scanners and used as service providers for fast on-site dose calculations. CONCLUSIONS: The Monte Carlo software package provides fast on-site calculation of 3D dose distributions in the CT suite which makes it a practical tool for any type of CT-specific application.

Spectral optimization for micro-CT.

PURPOSE: To optimize micro-CT protocols with respect to x-ray spectra and thereby reduce radiation dose at unimpaired image quality. METHODS: Simulations were performed to assess image contrast, noise, and radiation dose for different imaging tasks. The figure of merit used to determine the optimal spectrum was the dose-weighted contrast-to-noise ratio (CNRD). Both optimal photon energy and tube voltage were considered. Three different types of filtration were investigated for polychromatic x-ray spectra: 0.5 mm Al, 3.0 mm Al, and 0.2 mm Cu. Phantoms consisted of water cylinders of 20, 32, and 50 mm in diameter with a central insert of 9 mm which was filled with different contrast materials: an iodine-based contrast medium (CM) to mimic contrast-enhanced (CE) imaging, hydroxyapatite to mimic bone structures, and water with reduced density to mimic soft tissue contrast. Validation measurements were conducted on a commercially available micro-CT scanner using phantoms consisting of water-equivalent plastics. Measurements on a mouse cadaver were performed to assess potential artifacts like beam hardening and to further validate simulation results. RESULTS: The optimal photon energy for CE imaging was found at 34 keV. For bone imaging, optimal energies were 17, 20, and 23 keV for the 20, 32, and 50 mm phantom, respectively. For density differences, optimal energies varied between 18 and 50 keV for the 20 and 50 mm phantom, respectively. For the 32 mm phantom and density differences, CNRD was found to be constant within 2.5% for the energy range of 21-60 keV. For polychromatic spectra and CMs, optimal settings were 50 kV with 0.2 mm Cu filtration, allowing for a dose reduction of 58% compared to the optimal setting for 0.5 mm Al filtration. For bone imaging, optimal tube voltages were below 35 kV. For soft tissue imaging, optimal tube settings strongly depended on phantom size. For 20 mm, low voltages were preferred. For 32 mm, CNRD was found to be almost independent of tube voltage. For 50 mm, voltages larger than 50 kV were preferred. For all three phantom sizes stronger filtration led to notable dose reduction for soft tissue imaging. Validation measurements were found to match simulations well, with deviations being less than 10%. Mouse measurements confirmed simulation results. CONCLUSIONS: Optimal photon energies and tube settings strongly depend on both phantom size and imaging task at hand. For in vivo CE imaging and density differences, strong filtration and voltages of 50-65 kV showed good overall results. For soft tissue imaging of animals the size of a rat or larger, voltages higher than 65 kV allow to greatly reduce scan times while maintaining dose efficiency. For imaging of bone structures, usage of only minimum filtration and low tube voltages of 40 kV and below allow exploiting the high contrast of bone at very low energies. Therefore, a combination of two filtrations could prove beneficial for micro-CT: a soft filtration allowing for bone imaging at low voltages, and a variable stronger filtration (e.g., 0.2 mm Cu) for soft tissue and contrast-enhanced imaging.

Dosimetry concepts for scanner quality assurance and tissue dose assessment in micro-CT.

PURPOSE: At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissue dose assessment for micro-CT. METHODS: The computed tomography dose index (CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissue dose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDI measurements were compared to Monte Carlo (MC) dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLD measurements was further investigated in two mice cadavers. RESULTS: CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDI measured free in air showed larger systematic deviations after the 6 month interval compared to the CTDI measured in PMMA phantoms. MC calculations were found to match CTDI measurements within 3% when using x-ray spectra measured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLD measurements in mice was found to be better than 10% with a mean deviation of 4.5%. CONCLUSIONS: Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissue dose assessment, MC calculations offer an accurate and fast alternative to TLD measurements allowing for dose calculations specific to any geometry and scan protocol.

Radiation dose considerations by intra-individual Monte Carlo simulations in dual source spiral coronary computed tomography angiography with electrocardiogram-triggered tube current modulation and adaptive pitch.

OBJECTIVES: To evaluate radiation dose levels in patients undergoing spiral coronary computed tomography angiography (CTA) on a dual-source system in clinical routine. METHODS: Coronary CTA was performed for 56 patients with electrocardiogram-triggered tube current modulation (TCM) and heart-rate (HR) dependent pitch adaptation. Individual Monte Carlo (MC) simulations were performed for dose assessment. Retrospective simulations with constant tube current (CTC) served as reference. Lung tissue was segmented and used for organ and effective dose (ED) calculation. RESULTS: Estimates for mean relative ED was 7.1 ± 2.1 mSv/100 mAs for TCM and 12.5 ± 5.3 mSv/100 mAs for CTC (P < 0.001). Relative dose reduction at low HR (≤60 bpm) was highest (49 ± 5%) compared to intermediate (60-70 bpm, 33 ± 12%) and high HR (>70 bpm, 29 ± 12%). However lowest ED is achieved at high HR (5.2 ± 1.5 mSv/100 mAs), compared with intermediate (6.7 ± 1.6 mSv/100 mAs) and low (8.3 ± 2.1 mSv/100 mAs) HR when automated pitch adaptation is applied. CONCLUSIONS: Radiation dose savings up to 52% are achievable by TCM at low and regular HR. However lowest ED is attained at high HR by pitch adaptation despite inferior radiation dose reduction by TCM. KEY POINTS: • Monte Carlo simulations allow for individual radiation dose calculations. • ECG-triggered tube current modulation (TCM) can effectively reduce radiation dose. • Slow and regular heart rates allow for highest dose reductions by TCM. • Adaptive pitch accounts for lowest radiation dose at high heart rates. • Women receive higher effective dose than men undergoing spiral coronary CT-angiography.

Dynamic contrast-enhanced micro-CT on mice with mammary carcinoma for the assessment of antiangiogenic therapy response.

OBJECTIVE: To evaluate the potential of in vivo dynamic contrast-enhanced micro-computed tomography (DCE micro-CT) for the assessment of antiangiogenic drug therapy response of mice with mammary carcinoma. METHODS: 20 female mice with implanted MCF7 tumours were split into control group and therapy group treated with a known effective antiangiogenic drug. All mice underwent DCE micro-CT for the 3D analysis of functional parameters (relative blood volume [rBV], vascular permeability [K], area under the time-enhancement curve [AUC]) and morphology. All parameters were determined for total, peripheral and central tumour volumes of interest (VOIs). Immunohistochemistry was performed to characterise tumour vascularisation. 3D dose distributions were determined. RESULTS: The mean AUCs were significantly lower in therapy with P values of 0.012, 0.007 and 0.023 for total, peripheral and central tumour VOIs. K and rBV showed significant differences for the peripheral (P(per)(K) = 0.032, P(per) (rBV) = 0.029), but not for the total and central tumour VOIs (P(total)(K) = 0.108, P(central)(K) = 0.246, P(total) (rBV) = 0.093, P(central) (rBV) = 0.136). Mean tumour volume was significantly smaller in therapy (P (in vivo) = 0.001, P (ex vivo) = 0.005). Histology revealed greater vascularisation in the controls and central tumour necrosis. Doses ranged from 150 to 300 mGy. CONCLUSIONS: This study indicates the great potential of DCE micro-CT for early in vivo assessment of antiangiogenic drug therapy response. KEY POINTS: Dynamic contrast enhanced micro-CT (computed tomography) is a new experimental laboratory technique. DCE micro-CT allows early in vivo assessment of antiangiogenic drug therapy response. Pharmaceutical drugs can be tested before translation to clinical practice. Both morphological and functional parameters can be obtained using DCE micro-CT. Antiangiogenic effects can be visualised with DCE micro-CT.

Quantitative whole heart stress perfusion CT imaging as noninvasive assessment of hemodynamics in coronary artery stenosis: preliminary animal experience.

PURPOSE: To quantify differences in regional myocardial perfusion in coronary artery stenosis by the use of dual source computed tomography (DSCT) in an animal model. MATERIAL AND METHODS: In 5 pigs, an 80% stenosis of the left anterior descending artery was successfully induced by partial balloon occlusion (ischemia group). Five animals served as control group. All animals underwent contrast enhanced whole heart DSCT (Definition Flash, Siemens, Germany) perfusion imaging using a prototype electrocardiogram -triggered dynamic scan mode. Imaging was performed at rest as well as under stress conditions during continuous infusion of adenosine (240 mg/kg/min). For contrast enhancement 60 mL Iopromide 300 (Ultravist 300, Bayer-Schering Pharma, Berlin, Germany) were injected at a rate of 6 mL/s. Myocardial blood flow (MBF), first pass distribution volume, and intravascular blood volume were volumetrically quantified. RESULTS: In the control group MBF increased significantly from 98.2 mL/100 mL/min to 134.0 mL/100 mL/min if adenosine was administered (P = 0.0153). There were no significant differences in the perfusion parameters comparing the control and ischemia group at rest. In the ischemia group MBF under stress was 74.0 +/- 21.9 mL/100 mL/min in the poststenotic myocardium and 117.4 +/- 18.6 mL/100 mL/min in the remaining normal myocardium (P = 0.0024). CONCLUSION: DSCT permits quantitative whole heart perfusion imaging. As this technique is able to show the hemodynamic effect of high grade coronary artery stenosis, it exceeds the present key limitation of cardiac computed tomography, which currently only allows a morphologic assessment of coronary artery stenosis.

Exercise effects on bone mineral density, falls, coronary risk factors, and health care costs in older women: the randomized controlled senior fitness and prevention (SEFIP) study.

BACKGROUND: Physical exercise affects many risk factors and diseases and therefore can play a vital role in general disease prevention and treatment of elderly individuals and may reduce costs. We sought to determine whether a single exercise program affects fracture risk (bone mineral density [BMD] and falls), coronary heart disease (CHD) risk factors, and health care costs in community-dwelling elderly women. METHODS: We conducted a randomized, single-blinded, controlled trial from May 1, 2005, through July 31, 2008, recruiting women 65 years or older who were living independently in the area of Erlangen-Nuremberg, Germany. In all, 246 women were randomly assigned to an 18-month exercise program (exercise group) or a wellness program (control group). The exercise group (n = 123) performed a multipurpose exercise program with special emphasis on exercise intensity; the controls (n = 123) focused on well-being with a low-intensity, low-frequency program. The main outcome measures were BMD, the number of falls, the Framingham-based 10-year CHD risk, and direct health care costs. RESULTS: For the 227 women who completed the 18-month study, significant exercise effects were observed for BMD of the lumbar spine (mean [95% confidence interval (CI)] percentage of change in BMD [baseline to follow-up] for the exercise group: 1.77% [1.26% to 2.28%] vs controls: 0.33% [-0.24% to 0.91%]; P < .001), femoral neck (exercise group: 1.01% [0.37% to 1.65%] vs controls: -1.05% [-1.70% to -0.40%]; P < .001), and fall rate per person during 18 months (exercise group: 1.00 [0.76 to 1.24] vs controls: 1.66 [1.33 to 1.99]; P = .002). The 10-year CHD risk was significantly affected in both subgroups (absolute change for the exercise group: -1.96% [95% CI, -2.69% to -1.23%] vs controls: -1.15% [-1.69% to -0.62%]; P = .22), with no significant difference between the groups. The direct health care costs per participant during the 18-month intervention showed nonsignificant differences between the groups (exercise group: 2255 euros[95% CI, 1791 euros-2718 euros] vs controls: 2780 euros [2187 euros-3372 euros]; P = .20). CONCLUSION: Compared with a general wellness program, our 18-month exercise program significantly improved BMD and fall risk, but not predicted CHD risk, in elderly women. This benefit occurred at no increase in direct costs. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00267839.

A fast and pragmatic approach for scatter correction in flat-detector CT using elliptic modeling and iterative optimization.

Scattered radiation is a major source of artifacts in flat detector computed tomography (FDCT) due to the increased irradiated volumes. We propose a fast projection-based algorithm for correction of scatter artifacts. The presented algorithm combines a convolution method to determine the spatial distribution of the scatter intensity distribution with an object-size-dependent scaling of the scatter intensity distributions using a priori information generated by Monte Carlo simulations. A projection-based (PBSE) and an image-based (IBSE) strategy for size estimation of the scanned object are presented. Both strategies provide good correction and comparable results; the faster PBSE strategy is recommended. Even with such a fast and simple algorithm that in the PBSE variant does not rely on reconstructed volumes or scatter measurements, it is possible to provide a reasonable scatter correction even for truncated scans. For both simulations and measurements, scatter artifacts were significantly reduced and the algorithm showed stable behavior in the z-direction. For simulated voxelized head, hip and thorax phantoms, a figure of merit Q of 0.82, 0.76 and 0.77 was reached, respectively (Q = 0 for uncorrected, Q = 1 for ideal). For a water phantom with 15 cm diameter, for example, a cupping reduction from 10.8% down to 2.1% was achieved. The performance of the correction method has limitations in the case of measurements using non-ideal detectors, intensity calibration, etc. An iterative approach to overcome most of these limitations was proposed. This approach is based on root finding of a cupping metric and may be useful for other scatter correction methods as well. By this optimization, cupping of the measured water phantom was further reduced down to 0.9%. The algorithm was evaluated on a commercial system including truncated and non-homogeneous clinically relevant objects.

The effect of angular and longitudinal tube current modulations on the estimation of organ and effective doses in x-ray computed tomography.

PURPOSE: Tube current modulation (TCM) is one of the recent developments in multislice CT that has proven to reduce the patient radiation dose without affecting the image quality. Presently established methods and published coefficients for estimating organ doses from the dose measured free in air on the axis of rotation or in the CT dose index (CTDI) dosimetry phantoms do not take into account this relatively new development in CT scanner design and technology. Based on these organ dose coefficients effective dose estimates can be made. The estimates are not strictly valid for CT scanning protocols utilizing TCM. In this study, the authors investigated the need to take TCM into account when estimating organ and effective dose values. METHODS: A whole-body adult anthropomorphic phantom (Alderson Rando) was scanned with a multislice CT scanner (Somatom Definition, Siemens, Forchheim, Germany) utilizing TCM (CareDose4D). Tube voltage was 120 kV, beam collimation 19.2 mm, and pitch 1. A voxelized patient model was used to define the tissues and organs in the phantom. Tube current values as a function of tube angle were obtained from the raw data for each individual tube rotation of the scan. These values were used together with the Monte Carlo dosimetry tool IMPACTMC (VAMP GmbH, Erlangen, Germany) to calculate organ dose values both with and without account of TCM. Angular and longitudinal modulations were investigated separately. Finally, corresponding effective dose conversion coefficients were determined for both cases according to the updated 2007 recommendations of the ICRP. RESULTS: TCM amplitude was greatest in the shoulder and pelvic regions. Consequently, dose distributions and organ dose values for particular cross sections changed considerably when taking angular modulation into account. The effective dose conversion coefficients were up to 11% lower for a single rotation in the shoulder region and 17% lower in the pelvis when taking angular TCM into account. In the head, neck, thorax, and upper abdominal regions, conversion coefficients changed similarly by only 5% or less. Conversion coefficients for estimating effective doses for scans of complete regions, e.g., chest or abdomen, were approximately 8% lower when taking angular and longitudinal TCMs into account. CONCLUSIONS: The authors conclude that for accurate organ and effective dose estimates in individual cross sections in the shoulder or pelvic regions, the angular tube current modulation should be taken into account. In general, using the average of the modulated tube current causes an overestimation of the effective dose.

Application- and patient size-dependent optimization of x-ray spectra for CT.

Although x-ray computed tomography (CT) has been in clinical use for over 3 decades, spectral optimization has not been a topic of great concern; high voltages around 120 kV have been in use since the beginning of CT. It is the purpose of this study to analyze, in a rigorous manner, the energies at which the patient dose necessary to provide a given contrast-to-noise ratio (CNR) for various diagnostic tasks can be minimized. The authors used cylindrical water phantoms and quasianthropomorphic phantoms of the thorax and the abdomen with inserts of 13 mm diameter mimicking soft tissue, bone, and iodine for simulations and measurements. To provide clearly defined contrasts, these inserts were made of solid water with a 1% difference in density (DD) to represent an energy-independent soft-tissue contrast of 10 Hounsfield units (HU), calcium hydroxyapatite (Ca) representing bone, and iodine (I) representing the typical contrast medium. To evaluate CT of the thorax, an adult thorax phantom (300 x 200 mm2) plus extension rings up to a size of 460 x 300 mm2 to mimic different patient cross sections were used. For CT of the abdomen, we used a phantom of 360 x 200 mm2 and an extension ring of 460 x 300 mm2. The CT scanner that the authors used was a SOMATOM Definition (Siemens Healthcare, Forchheim, Germany) at 80, 100, 120, and 140 kV. Further voltage settings of 60, 75, 90, and 105 kV were available in an experimental mode. The authors determined contrast for the density difference, calcium, and iodine, and noise and 3D dose distributions for the available voltages by measurements. Additional voltage values and monoenergetic sources were evaluated by simulations. The dose-weighted contrast-to-noise ratio (CNRD) was used as the parameter for optimization. Simulations and measurements were in good agreement with respect to absolute values and trends regarding the dependence on energy for the parameters investigated. For soft-tissue imaging, the standard settings of 120-140 kV were found as adequate choices with optimal values increasing for larger cross sections, e.g., for large abdomens voltages higher than 140 kV may be indicated. For bone and iodine imaging the optimum values were generally found at significantly lower voltages of typically below 80 kV. This offers a potential for dose reduction of up to 50%, but demands significantly higher power values in most cases. The authors concluded that voltage settings in CT should be varied more often than is common in practice today and should be chosen not only according to patient size but also according to the substance imaged in order to minimize dose while not compromising image quality. A reduction from 120 to 80 kV, for example, would yield a reduction in patient dose by more than half for coronary CT angiography. The use of lower voltages has to be recommended for contrast medium studies in cardiac and pediatric CT.

Modulation transfer function-based assessment of temporal resolution: validation for single- and dual-source CT.

The purpose of this study was to determine a manufacturer-independent quality assurance measurement for temporal resolution with a three-dimensional cardiac motion robot; validation was with single-source (SS) and dual-source (DS) computed tomography (CT). Image acquisition was performed by using standard cardiac protocols. Image contrast-based modulation transfer function (MTF) was assessed as function of time. For motion frequency of 60 beats per minute, MTF slightly decreased by 14% and 6% for SS CT and DS CT, respectively. For higher frequencies, a stronger decrease of MTF (eg, by 50% [SS CT] and 18% [DS CT] at 120 beats per minute) was detected. Effect of manufacturer's adaptive bisegment algorithm for SS CT and corresponding resonance effects of rotation time and heart rate were quantified. The robot-based approach is a reproducible, objective way to assess temporal resolution; it allows practical measurement of temporal resolution and comparison of CT scanners and protocols.

Concepts for dose determination in flat-detector CT.

Flat-detector computed tomography (FD-CT) scanners provide large irradiation fields of typically 200 mm in the cranio-caudal direction. In consequence, dose assessment according to the current definition of the computed tomography dose index CTDI(L=100 mm), where L is the integration length, would demand larger ionization chambers and phantoms which do not appear practical. We investigated the usefulness of the CTDI concept and practical dosimetry approaches for FD-CT by measurements and Monte Carlo (MC) simulations. An MC simulation tool (ImpactMC, VAMP GmbH, Erlangen, Germany) was used to assess the dose characteristics and was calibrated with measurements of air kerma. For validation purposes measurements were performed on an Axiom Artis C-arm system (Siemens Medical Solutions, Forchheim, Germany) equipped with a flat detector of 40 cm x 30 cm. The dose was assessed for 70 kV and 125 kV in cylindrical PMMA phantoms of 160 mm and 320 mm diameter with a varying phantom length from 150 to 900 mm. MC simulation results were compared to the values obtained with a calibrated ionization chambers of 100 mm and 250 mm length and to thermoluminesence (TLD) dose profiles. The MCs simulations were used to calculate the efficiency of the CTDI(L) determination with respect to the desired CTDI(infinity). Both the MC simulation results and the dose distributions obtained by MC simulation were in very good agreement with the CTDI measurements and with the reference TLD profiles, respectively, to within 5%. Standard CTDI phantoms which have a z-extent of 150 mm underestimate the dose at the center by up to 55%, whereas a z-extent of 600 mm appears to be sufficient for FD-CT; the baseline value of the respective profile was within 1% to the reference baseline. As expected, the measurements with ionization chambers of 100 mm and 250 mm offer a limited accuracy, whereas an increased integration length of 600 mm appeared to be necessary to approximate CTDI(infinity) in within 1%. MC simulations appear to offer a practical and accurate way of assessing conversion factors for arbitrary dosimetry setups using a standard pencil chamber to provide estimates of CTDI(infinity). This would eliminate the need for extra-long phantoms and ionization chambers or excessive amounts of TLDs.

Validation of a Monte Carlo tool for patient-specific dose simulations in multi-slice computed tomography.

Estimating the dose delivered to the patient in X-ray computed tomography (CT) examinations is not a trivial task. Monte Carlo (MC) methods appear to be the method of choice to assess the 3D dose distribution. The purpose of this work was to extend an existing MC-based tool to account for arbitrary scanners and scan protocols such as multi-slice CT (MSCT) scanners and to validate the tool in homogeneous and heterogeneous phantoms. The tool was validated by measurements on MSCT scanners for different scan protocols under known conditions. Quantitative CT Dose Index (CTDI) measurements were performed in cylindrical CTDI phantoms and in anthropomorphic thorax phantoms of various sizes; dose profiles were measured with thermoluminescent dosimeters (TLD) in the CTDI phantoms and compared with the computed dose profiles. The in-plane dose distributions were simulated and compared with TLD measurements in an Alderson-Rando phantom. The calculated dose values were generally within 10% of measurements for all phantoms and all investigated conditions. Three-dimensional dose distributions can be accurately calculated with the MC tool for arbitrary scanners and protocols including tube current modulation schemes. The use of the tool has meanwhile also been extended to further scanners and to flat-detector CT.

Combining deterministic and Monte Carlo calculations for fast estimation of scatter intensities in CT.

A side effect of increased volume coverage by using multi-row and flat-panel detectors in computed tomography (CT) is the concurrently growing contribution of scattered radiation to the measured signal. In order to investigate the effect of scatter on x-ray projections used for CT imaging, our study aimed at the development of a simulation tool for fast calculation of primary and scatter intensities. We developed a deterministic method to assess the contribution of single-scatter events to the measured signal. The investigation of multiple scatter by Monte Carlo simulations showed that it results in a smooth signal as compared to single scatter. A hybrid method is proposed in order to optimize the performance of the scatter simulation: a fast and exact analytical calculation of the single-scatter intensity combined with a coarse Monte Carlo (MC) estimate of multiple scatter to reduce overall computational expenses, while assuring an acceptable signal quality. The results of the hybrid simulation of total scatter were in excellent agreement with the corresponding MC only simulations, thereby allowing us to reduce computational time by orders of magnitude. Estimates of two-dimensional scatter distributions for flat-panel CT imaging took about 30-40 s (per projection). The hybrid method provides a realistic simulation of x-ray scatter and offers a basis for scatter correction approaches.

[Advanced method for calculating the scatter signal contribution in CT detectors by the Monte Carlo method]. / Beschleunigte Methode zur Berechnung des Streusignals am CT-Detektor mittels Monte-Carlo-Methode.

Multislice spiral CT scanners allow to acquire multiple slices simultaneously. With increasing numbers of slices, not only the total extent of slice collimation increases, but also the contribution of scatter radiation to the detector signal. A fast method for calculating the scatter signal would offer the possibility to correct the measured detector signal. Monte Carlo methods allow to simulate the paths of photons through a 3D volume, both in a patient- and scanner-specific fashion. If a scatter photon leaves the volume, its path can be followed and its interaction with an element of the detector be checked. This conventional way of calculating the scatter signal is time-consuming. In order to reduce the calculation time, a more efficient method was developed (Method of Weights). Every time an interaction occurs inside of the 3D volume, the probability of a detector hit due to photon scattering is calculated for each detector channel. The respective value is added to the scatter signal per detector with the corresponding weight. Simulated values of scatter-to-primary-signal ratios were confirmed by data available in the literature. Both the conventional and fast methods for the calculation of scatter signals yielded identical values within the range of statistical accuracy. Assuming the same computing time, the standard deviation for the conventional method was 5 times higher than for the fast one. The presented method allows to significantly reduce the computation time. It may therefore provide a basis for "real time" methods to correct for the scatter signal, especially in case of increasing numbers of slices.

Assessment of calcium scoring performance in cardiac computed tomography.

Electron beam tomography (EBT) has been used for cardiac diagnosis and the quantitative assessment of coronary calcium since the late 1980s. The introduction of mechanical multi-slice spiral CT (MSCT) scanners with shorter rotation times opened new possibilities of cardiac imaging with conventional CT scanners. The purpose of this work was to qualitatively and quantitatively evaluate the performance for EBT and MSCT for the task of coronary artery calcium imaging as a function of acquisition protocol, heart rate, spiral reconstruction algorithm (where applicable) and calcium scoring method. A cardiac CT semi-anthropomorphic phantom was designed and manufactured for the investigation of all relevant image quality parameters in cardiac CT. This phantom includes various test objects, some of which can be moved within the anthropomorphic phantom in a manner that mimics realistic heart motion. These tools were used to qualitatively and quantitatively demonstrate the accuracy of coronary calcium imaging using typical protocols for an electron beam (Evolution C-150XP, Imatron, South San Francisco, Calif.) and a 0.5-s four-slice spiral CT scanner (Sensation 4, Siemens, Erlangen, Germany). A special focus was put on the method of quantifying coronary calcium, and three scoring systems were evaluated (Agatston, volume, and mass scoring). Good reproducibility in coronary calcium scoring is always the result of a combination of high temporal and spatial resolution; consequently, thin-slice protocols in combination with retrospective gating on MSCT scanners yielded the best results. The Agatston score was found to be the least reproducible scoring method. The hydroxyapatite mass, being better reproducible and comparable on different scanners and being a physical quantitative measure, appears to be the method of choice for future clinical studies. The hydroxyapatite mass is highly correlated to the Agatston score. The introduced phantoms can be used to quantitatively assess the performance characteristics of, for example, different scanners, reconstruction algorithms, and quantification methods in cardiac CT. This is especially important for quantitative tasks, such as the determination of the amount of calcium in the coronary arteries, to achieve high and constant quality in this field.