A Monte Carlo based method to estimate radiation dose from multidetector CT (MDCT): cylindrical and anthropomorphic phantoms

The purpose of this work was to extend the verification of Monte Carlo based methods for estimating radiation dose in computed tomography (CT) exams beyond a single CT scanner to a multidetector CT (MDCT) scanner, and from cylindrical CTDI phantom measurements to both cylindrical and physical anthropomorphic phantoms. Both cylindrical and physical anthropomorphic phantoms were scanned on an MDCT under the specified conditions. A pencil ionization chamber was used to record exposure for the cylindrical phantom, while MOSFET (metal oxide semiconductor field effect transistor) detectors were used to record exposure at the surface of the anthropomorphic phantom. Reference measurements were made in air at isocentre using the pencil ionization chamber under the specified conditions. Detailed Monte Carlo models were developed for the MDCT scanner to describe the x-ray source (spectra, bowtie filter, etc) and geometry factors (distance from focal spot to isocentre, source movement due to axial or helical scanning, etc). Models for the cylindrical (CTDI) phantoms were available from the previous work. For the anthropomorphic phantom, CT image data were used to create a detailed voxelized model of the phantom's geometry. Anthropomorphic phantom material compositions were provided by the manufacturer. A simulation of the physical scan was performed using the mathematical models of the scanner, phantom and specified scan parameters. Tallies were recorded at specific voxel locations corresponding to the MOSFET physical measurements. Simulations of air scans were performed to obtain normalization factors to convert results to absolute dose values. For the CTDI body (32 cm) phantom, measurements and simulation results agreed to within 3.5% across all conditions. For the anthropomorphic phantom, measured surface dose values from a contiguous axial scan showed significant variation and ranged from 8 mGy/100 mAs to 16 mGy/100 mAs. Results from helical scans of overlapping pitch (0.9375) and extended pitch (1.375) were also obtained. Comparisons between the MOSFET measurements and the absolute dose value derived from the Monte Carlo simulations demonstrate agreement in terms of absolute dose values as well as the spatially varying characteristics. This work demonstrates the ability to extend models from a single detector scanner using cylindrical phantoms to an MDCT scanner using both cylindrical and anthropomorphic phantoms. Future work will be extended to voxelized patient models of different sizes and to other MDCT scanners.     

A Monte Carlo study of lung counting efficiency for female workers of different breast sizes using deformable phantoms

There are currently no physical phantoms available for calibrating in vivo counting devices that represent women with different breast sizes because such phantoms are difficult, time consuming and expensive to fabricate. In this work, a feasible alternative involving computational phantoms was explored. A series of new female voxel phantoms with different breast sizes were developed and ported into a Monte Carlo radiation transport code for performing virtual lung counting efficiency calibrations. The phantoms are based on the RPI adult female phantom, a boundary representation (BREP) model. They were created with novel deformation techniques and then voxelized for the Monte Carlo simulations. Eight models have been selected with cup sizes ranging from AA to G according to brassiere industry standards. Monte Carlo simulations of a lung counting system were performed with these phantoms to study the effect of breast size on lung counting efficiencies, which are needed to determine the activity of a radionuclide deposited in the lung and hence to estimate the resulting dose to the worker. Contamination scenarios involving three different radionuclides, namely Am-241, Cs-137 and Co-60, were considered. The results show that detector efficiencies considerably decrease with increasing breast size, especially for low energy photon emitting radionuclides. When the counting efficiencies of models with cup size AA were compared to those with cup size G, a difference of up to 50% was observed. The detector efficiencies for each radionuclide can be approximated by curve fitting in the total breast mass (polynomial of second order) or the cup size (power).     

A Monte Carlo-based method to estimate radiation dose from spiral CT: from phantom testing to patient-specific models

The purpose of this work is to develop and test a method to estimate the relative and absolute absorbed radiation dose from axial and spiral CT scans using a Monte Carlo approach. Initial testing was done in phantoms and preliminary results were obtained from a standard mathematical anthropomorphic model (MIRD V) and voxelized patient data. To accomplish this we have modified a general purpose Monte Carlo transport code (MCNP4B) to simulate the CT x-ray source and movement, and then to calculate absorbed radiation dose in desired objects. The movement of the source in either axial or spiral modes was modelled explicitly while the CT system components were modelled using published information about x-ray spectra as well as information provided by the manufacturer. Simulations were performed for single axial scans using the head and body computed tomography dose index (CTDI) polymethylmethacrylate phantoms at both central and peripheral positions for all available beam energies and slice thicknesses. For comparison, corresponding physical measurements of CTDI in phantom were made with an ion chamber. To obtain absolute dose values, simulations and measurements were performed in air at the scanner isocentre for each beam energy. To extend the verification, the CT scanner model was applied to the MIRD V model and compared with published results using similar technical factors. After verification of the model, the generalized source was simulated and applied to voxelized models of patient anatomy. The simulated and measured absolute dose data in phantom agreed to within 2% for the head phantom and within 4% for the body phantom at 120 and 140 kVp; this extends to 8% for the head and 9% for the body phantom across all available beam energies and positions. For the head phantom, the simulated and measured absolute dose data agree to within 2% across all slice thicknesses at 120 kVp. Our results in the MIRD phantom agree within 11% of all the different organ dose values published by the UK's ImPACT group for a scan using an equivalent scanner, kVp, collimation, pitch and mAs. The CT source model was shown to calculate both a relative and absolute radiation dose distribution throughout the entire volume in a patient-specific matrix geometry. Results of initial testing are promising and application to patient models was shown to be feasible.     

Digital mammography image simulation using Monte Carlo

Monte Carlo simulations of digital images of the contrast detail phantom and the ACR phantom are presented for two different x-ray digital mammography modalities: a synchrotron mammography system and a next-generation scanning slot clinical system. A combination of variance reduction methods made it possible to simulate accurate images using real pixel dimensions within reasonable computation times. The complete method of image simulation, including a simple detector response model, a simple noise model, and the incorporation of system effects (MTF), is presented. The simulated images of the phantoms show good agreement with images measured on the two systems.     

Reconstruction of 2D PET data with Monte Carlo generated system matrix for generalized natural pixels

In discrete detector PET, natural pixels are image basis functions calculated from the response of detector pairs. By using reconstruction with natural pixel basis functions, the discretization of the object into a predefined grid can be avoided. Here, we propose to use generalized natural pixel reconstruction. Using this approach, the basis functions are not the detector sensitivity functions as in the natural pixel case but uniform parallel strips. The backprojection of the strip coefficients results in the reconstructed image. This paper proposes an easy and efficient way to generate the matrix M directly by Monte Carlo simulation. Elements of the generalized natural pixel system matrix are formed by calculating the intersection of a parallel strip with the detector sensitivity function. These generalized natural pixels are easier to use than conventional natural pixels because the final step from solution to a square pixel representation is done by simple backprojection. Due to rotational symmetry in the PET scanner, the matrix M is block circulant and only the first blockrow needs to be stored. Data were generated using a fast Monte Carlo simulator using ray tracing. The proposed method was compared to a listmode MLEM algorithm, which used ray tracing for doing forward and backprojection. Comparison of the algorithms with different phantoms showed that an improved resolution can be obtained using generalized natural pixel reconstruction with accurate system modelling. In addition, it was noted that for the same resolution a lower noise level is present in this reconstruction. A numerical observer study showed the proposed method exhibited increased performance as compared to a standard listmode EM algorithm. In another study, more realistic data were generated using the GATE Monte Carlo simulator. For these data, a more uniform contrast recovery and a better contrast-to-noise performance were observed. It was observed that major improvements in contrast recovery were obtained with MLEM when the correct system matrix was used instead of simple ray tracing. The correct modelling was the major cause of improved contrast for the same background noise. Less important factors were the choice of the algorithm (MLEM performed better than ART) and the basis functions (generalized natural pixels gave better results than pixels).     

A Monte Carlo comparison of the response of the PTW-diamond and the TL-diamond detectors in megavoltage photon beams

A detailed Monte Carlo study of the PTW-diamond solid state detector response in megavoltage photon beams (60Co gamma rays to 25 MV x rays) has been performed with the EGS4 Monte Carlo Code. The sensitive volume of the diamond detector is a disk of diameter 4.4 mm and thickness 0.40 mm. The phantom material was water and the irradiation depth was usually 3 cm but additional simulations were performed at six other depths for the 10 and 25 MV x rays. Results show that the PTW-diamond detector response per unit of absorbed dose is constant within 1% for photon beam energies ranging from 60Co gamma rays to 25 MV x rays. Accurate depth dose curves for 10 and 25 MV x-ray beams may be measured with the diamond detector since the response per unit of absorbed dose at different depths in a water phantom is also constant to within 1% for depths ranging from 3 to 25 cm and field sizes ranging from 2.5 cm by 2.5 cm to 10 cm by 10 cm. An examination of the difference between the PTW-diamond detector and the wall-less form of the detector (e.g., TLDs) revealed that there is no significant difference in their response in megavoltage photon beams. This implies that the encapsulation of the diamond dosimeter causes less than a 1.3% change in its response for these megavoltage photon beams. Analysis of the total dose deposited in the sensitive volume of the detector shows that the PTW-diamond detector behaves as an intermediate-sized cavity, not a simple Bragg-Gray cavity, since the dose contribution from photon interactions within the cavity (alpha(c)) to the total cavity dose is 8% for 25 MV x rays and increases to 42% for 60Co gamma rays.     

Photon counting spectral CT versus conventional CT: comparative evaluation for breast imaging application

Spectral CT systems with photon counting detectors have more advantages compared to conventional CT systems. However, clinical applications have been hampered for a long time due to the high demands of clinical systems and limitations of spectroscopic x-ray detectors. Photon counting detector technology has gained considerable improvements in the past decade, and spectral CT has become a hot topic. Several experimental spectral CT systems are under investigation. The purpose of this work was to perform the first direct, side-by-side comparison of existing spectral CT technology with a mature clinical CT system based on a conventional energy integrating detector. We have built an experimental spectral CT system whose main parameters are similar to the parameters of a clinical CT system. The system uses a spectroscopic cadmium zinc telluride (CZT) detector. The detector includes two rows of CZT pixels with 256 pixels in each row. The pixel size is 1 × 1 mm(2), and the maximum count rate is 2 Mcounts/pixel/s. The spectral CT system has a magnification factor of 1.62 and the source to detector and source to image distances of 85 and 53 cm, respectively. The above parameters are similar to those of the clinical CT system, Siemens Sensation 16, used for comparison. The two systems were compared by imaging spatial resolution and contrast resolution phantoms made from acrylic cylinders with 14 cm diameters. The resolution phantom included Al wires with 0.3, 0.6, and 1 mm diameters, and 0.25 g cc(-1) CaCO(3) contrast. The contrast phantom included contrast elements with 1.7, 5, and 15 mg cc(-1) iodine, and 1.1, 3.3, and 10 mg cc(-1) gadolinium. The phantoms were imaged with the two systems using 120 kVp tube voltage and 470 mR total skin exposure. The spectral CT showed CT numbers, image noise, and spatial and contrast resolutions to be similar within 10% compared to the Siemens 16 system, and provided an average of 10% higher CNR. However, the spectral CT system had a major advantage in that the iodine, gadolinium, and CaCO(3) contrasts were decomposed by dual-energy and K-edge subtraction methods using energy selective CT data acquired in a single CT scan and fixed tube voltage. It is concluded that photon counting spectral CT technology is close to feasibility for routine clinical applications. Furthermore, it is ready for some clinical applications such as dedicated breast CT which has relatively lower demands on photon counting detectors.     

Characterization and correction of temporal artifacts in CT

Image artifacts, caused by a temporally delayed response of a computed tomography (CT) detector, were investigated. To study its consequences, a computer model for a standard third generation CT scanner was devised and simulations were carried out. Resulting image artifacts were studied for various voxel-based and mathematical phantoms using three time constants, which characterize the delayed signal responses of different detector configurations. Furthermore, a theory was developed to understand temporal artifacts in reconstructed images. A filter function was derived, which compensates for temporal artifacts. For a given phantom, simulations and theory demonstrate that artifacts scale with the time constant, characteristic for the detector, and the angular speed of the scanner.     

Development and validation of MCNP4C-based Monte Carlo simulator for fan- and cone-beam x-ray CT

An x-ray computed tomography (CT) simulator based on the Monte Carlo N-particle radiation transport computer code (MCNP4C) was developed for simulation of both fan- and cone-beam CT scanners. A user-friendly interface running under Matlab 6.5.1 creates the scanner geometry at different views as MCNP4C's input file. The full simulation of x-ray tube, phantom and detectors with single-slice, multi-slice and flat detector configurations was considered. The simulator was validated through comparison with experimental measurements of different nonuniform phantoms with varying sizes on both a clinical and a small-animal CT scanner. There is good agreement between the simulated and measured projections and reconstructed images. Thereafter, the effects of bow-tie filter, phantom size and septa length on scatter distribution in fan-beam CT were studied in detail. The relative difference between detected total, primary and scatter photons for septa length varying between 0 and 95 mm is 11.2%, 1.9% and 84.1%, respectively, whereas the scatter-to-primary ratio decreases by 83.8%. The developed simulator is a powerful tool for evaluating the effect of physical, geometrical and other design parameters on scanner performance and image quality in addition to offering a versatile tool for investigating potential artefacts and correction schemes when using CT-based attenuation correction on dual-modality PET/CT units.     

Computer simulation of image intensifier-based computed tomography detector: vascular application

This present study reports the results of a computer simulation whose aim was to predict the low-contrast imaging performance of which a conventional x-ray image intensifier with charge coupled device (CCD) camera would be capable if incorporated into a computed tomography (CT) volume imager. A vascular imaging task was modeled in our simulation. The effects of detector noise, x-ray exposure levels, analog-to-digital conversion (ADC) precision and residual levels of detected x-ray scatter were considered. The results of this simulation indicate that the low-contrast imaging performance of an image intensifier-based CT system was most limited by the CCD detector readout noise. Given this limitation the detection of greater than about 100,000 detected photons/pixel/projection gave marginal improvement in low-contrast resolution. At these exposures 12 bit ADC precision resulted in little additional image noise. The effects of detecting scattered x rays are twofold; decreasing the signal-to-noise ratio associated with our modeled artery and introducing a cupping artifact. Based on the results from the simulation, it appears that an image intensifier-based CT system is a feasible concept from a noise viewpoint, if the anticipated imaging task is intravenous angiography.     

用两种体模作CT性能检测的对比研究

本文介绍使用两种不同的标准体模(AAPM体模和RMI461A体模),对3台新安装的CT设备作性能检测,把两组实验数据进行双边对比研究.结果:大多数性能项目,包括高对比度分辨率、影像均匀性和噪声、层厚偏差和CT值线性等的测量数据有较好的一致性.发现低对比度分辨率的测量值存在明显差异,可能是体模(插件)长期受X线照射,化学效应引起材料CT值漂移及本底对比度升高所致,需要用对比度-细节反比关系校正.本文还就当前CT性能检测的若干技术问题,包括检测方法与评价标准的改进等进行探讨.     

基于蒙特卡洛方法和数字体模的质子治疗设施屏蔽优化设计

目的:评估采用蒙特卡洛（MC）模拟方法和中国科学技术大学数字人体模型（USTC体模）在质子治疗设施中的辐射屏蔽优化设计。方法:使用MC模拟方法和USTC体模计算数字体模处于不同位置时在不同部位的当量剂量率（EDR），对安徽省合肥市离子医学中心（HIMC）的新型质子治疗设施的屏蔽设计进行评估，并将其与采用经验公式计算得出的EDR进行比较。结果:使用铁靶时，经验公式计算得出的EDR值比MC模拟方法得到的结果偏高27.6倍；使用水靶时，经验公式计算结果较MC模拟结果高36.6倍，说明使用经验公式进行屏蔽计算将使得剂量被高估，从而导致成本增加，不符合辐射防护最优化原则。结论:利用USTC体模对质子治疗设施进行基于MC模拟的屏蔽计算可以得到更加准确和优化的结果。     

Evaluation of a model-based treatment planning system for dose computations in the kilovoltage energy range

The ability to determine dose distribution and calculate organ doses from diagnostic x rays has become increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedures. In an attempt to determine the dose from both diagnostic and orthovoltage x rays, we have used a commercial treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kilovoltage x rays. The planning system's capabilities for dose computation have been extended to lower energies by the addition of energy deposition kernels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp beams using the system. We compared the dose calculated by the system with that measured using thermoluminescent dosimeters (TLDs) placed in various positions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogeneities, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment plan was generated using CT scans of each of these phantoms and point doses at the positions of TLD chips were calculated. Comparisons of measured and computed values show an average difference of less than 2% within materials of atomic number less than and equal to that of water. The algorithm, however, does not produce accurate results in and around bone inhomogeneities and underestimates attenuation of x rays by bone by an average of 145%. A modification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyond bone.     

Comparison of whole-body phantom designs to estimate organ equivalent neutron doses for secondary cancer risk assessment in proton therapy

Secondary neutron fluence created during proton therapy can be a significant source of radiation exposure in organs distant from the treatment site, especially in pediatric patients. Various published studies have used computational phantoms to estimate neutron equivalent doses in proton therapy. In these simulations, whole-body patient representations were applied considering either generic whole-body phantoms or generic age- and gender-dependent phantoms. No studies to date have reported using patient-specific geometry information. The purpose of this study was to estimate the effects of patient–phantom matching when using computational pediatric phantoms. To achieve this goal, three sets of phantoms, including different ages and genders, were compared to the patients' whole-body CT. These sets consisted of pediatric age specific reference, age-adjusted reference and anatomically sculpted phantoms. The neutron equivalent dose for a subset of out-of-field organs was calculated using the GEANT4 Monte Carlo toolkit, where proton fields were used to irradiate the cranium and the spine of all phantoms and the CT-segmented patient models. The maximum neutron equivalent dose per treatment absorbed dose was calculated and found to be on the order of 0 to 5 mSv Gy(-1). The relative dose difference between each phantom and their respective CT-segmented patient model for most organs showed a dependence on how close the phantom and patient heights were matched. The weight matching was found to have much smaller impact on the dose accuracy except for very heavy patients. Analysis of relative dose difference with respect to height difference suggested that phantom sculpting has a positive effect in terms of dose accuracy as long as the patient is close to the 50th percentile height and weight. Otherwise, the benefit of sculpting was masked by inherent uncertainties, i.e. variations in organ shapes, sizes and locations.Other sources of uncertainty included errors associated with beam positioning, neutron weighting factor definition and organ segmentation. This work demonstrated the importance of hybrid phantom height matching for more accurate organ dose calculation in proton therapy and the potential limitations of reference phantoms released by regulatory bodies for radiation therapy applications.     

Intensity distribution and impact of scatter for dual-source CT

Apart from forward scatter, which is given for all CT scanners, dual-source CT (DSCT) is also affected by cross-scatter photons from the second tube-detector system arranged at 90 degrees . We investigated the magnitude and distribution of scatter for DSCT and its impact on image quality. Simulations and measurements of homogeneous and anthropomorphic phantoms were conducted for a DSCT scanner (SOMATOM Definition, Siemens Medical Solutions, Forchheim, Germany) at tube voltages of 80 and 120 kV. The simulations of forward scatter were carried out using combined analytical and Monte Carlo simulation methods for a collimation of 19.2 mm for both tube-detector systems. Measurements of cross scatter were performed by switching one tube off, still reading out the corresponding detector. The relative scatter fractions and the distribution of cross scatter were registered for various imaging conditions. Additionally, a detailed noise analysis with respect to the correction of cross-scatter artifacts is provided to evaluate the performance of correction algorithms. The forward-scatter fraction increased with increasing phantom diameter from 0.02 up to 0.11 for PMMA phantoms of 80 to 400 mm diameter. For cross scatter, the mean intensity was equivalent to forward scatter for small phantoms but was larger for increased phantom size and resulted in severe artifacts in the reconstructed images. The outer dimensions and shape of the object are decisive for the cross-scatter intensity distribution whereas the influence of the degree of inhomogeneity of the respective phantom appears to be negligible. Scatter correction suppressed cross-scatter artifacts but increased noise as a function of the cross-scatter fraction. The magnitude of scatter is not negligible for DSCT systems and dedicated corrections are necessary for the assurance of unimpaired image quality.     

Compton scatter axial tomography with x-rays: SCAT-CAT

A method of extracting information from the backscattered field produced in parallel beam x-ray computed tomography (CT) is presented. A calculational model to predict the backscattered field based on Compton scattering is described, and the model is verified by measurements of simple phantoms. The phantoms tested--cylinders of polymethylmethacrylate (PMM) with air gaps and aluminium rods placed internally--are irradiated on a scanning assembly, built to simulate a first generation CT scanner with a transmission and a scatter detector (the SCAT-CAT). Data from the transmission detector are reconstructed by traditional CT methods to provide a transmission image; it is the data from the backscatter detector which are analysed in this study. After verification of the model for the scattered field calculations, a method of extracting information from the scattered field is developed, based on ratios of scatter signals from non-uniform to uniform phantoms. This method is demonstrated for predicted data of a simulated phantom and for measured data of the same and two additional phantoms. The method is very sensitive to air gaps in the phantoms because of the relative electron density of air with respect to PMM; it is not as sensitive to aluminium rods for the same reason. Various methods of applying the scattered field information to produce an image representing a simulated phantom are considered, and a preferred method is chosen to reconstruct scattered field data into an image for the three phantoms studied.     

Monte carlo simulations of dose from microCT imaging procedures in a realistic mouse phantom

The purpose of this work was to calculate radiation dose and its organ distribution in a realistic mouse phantom from micro-computed tomography (microCT) imaging protocols. CT dose was calculated using GATE and a voxelized, realistic phantom. The x-ray photon energy spectra used in simulations were precalculated with GATE and validated against previously published data. The number of photons required per simulated experiments was determined by direct exposure measurements. Simulated experiments were performed for three types of beams and two types of mouse beds. Dose-volume histograms and dose percentiles were calculated for each organ. For a typical microCT screening examination with a reconstruction voxel size of 200 microm, the average whole body dose varied from 80 mGy (at 80 kVp) to 160 mGy (at 50 kVp), showing a strong dependence on beam hardness. The average dose to the bone marrow is close to the soft tissue average. However, due to dose nonuniformity and higher radiation sensitivity, 5% of the marrow would receive an effective dose about four times higher than the average. If CT is performed longitudinally, a significant radiation dose can be given. The total absorbed radiation dose is a function of milliamperes-second, beam hardness, and desired image quality (resolution, noise and contrast). To reduce dose, it would be advisable to use the hardest beam possible while maintaining an acceptable contrast in the image.     

Organ and effective doses in pediatric patients undergoing helical multislice computed tomography examination

As multidetector computed tomography (CT) serves as an increasingly frequent diagnostic modality, radiation risks to patients became a greater concern, especially for children due to their inherently higher radiosensitivity to stochastic radiation damage. Current dose evaluation protocols include the computed tomography dose index (CTDI) or point detector measurements using anthropomorphic phantoms that do not sufficiently provide accurate information of the organ-averaged absorbed dose and corresponding effective dose to pediatric patients. In this study, organ and effective doses to pediatric patients under helical multislice computed tomography (MSCT) examinations were evaluated using an extensive series of anthropomorphic computational phantoms and Monte Carlo radiation transport simulations. Ten pediatric phantoms, five stylized (equation-based) ORNL phantoms (newborn, 1-year, 5-year, 10-year, and 15-year) and five tomographic (voxel-based) UF phantoms (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) were implemented into MCNPX for simulation, where a source subroutine was written to explicitly simulate the helical motion of the CT x-ray source and the fan beam angle and collimator width. Ionization chamber measurements were performed and used to normalize the Monte Carlo simulation results. On average, for the same tube current setting, a tube potential of 100 kVp resulted in effective doses that were 105% higher than seen at 80 kVp, and 210% higher at 120 kVp regardless of phantom type. Overall, the ORNL phantom series was shown to yield values of effective dose that were reasonably consistent with those of the gender-specific UF phantom series for CT examinations of the head, pelvis, and torso. However, the ORNL phantoms consistently overestimated values of the effective dose as seen in the UF phantom for MSCT scans of the chest, and underestimated values of the effective dose for abdominal CT scans. These discrepancies increased with increasing kVp. Finally, absorbed doses to select radiation sensitive organs such as the gonads, red bone marrow, colon, and thyroid were evaluated and compared between phantom types. Specific anatomical problems identified in the stylized phantoms included excessive pelvic shielding of the ovaries in the female phantoms, enhanced red bone marrow dose to the arms and rib cage for chest exams, an unrealistic and constant torso thickness resulting in excessive x-ray attenuation in the regions of the abdominal organs, and incorrect positioning of the thyroid within the stylized phantom neck resulting in insufficient shielding by clavicles and scapulae for lateral beam angles. To ensure more accurate estimates of organ absorbed dose in multislice CT, it is recommended that voxel-based phantoms, potentially tailored to individual body morphometry, be utilized in any future prospective epidemiological studies of medically exposed children.     

Experimental validation of a rapid Monte Carlo based micro-CT simulator

We describe a newly developed, accelerated Monte Carlo simulator of a small animal micro-CT scanner. Transmission measurements using aluminium slabs are employed to estimate the spectrum of the x-ray source. The simulator incorporating this spectrum is validated with micro-CT scans of physical water phantoms of various diameters, some containing stainless steel and Teflon rods. Good agreement is found between simulated and real data: normalized error of simulated projections, as compared to the real ones, is typically smaller than 0.05. Also the reconstructions obtained from simulated and real data are found to be similar. Thereafter, effects of scatter are studied using a voxelized software phantom representing a rat body. It is shown that the scatter fraction can reach tens of per cents in specific areas of the body and therefore scatter can significantly affect quantitative accuracy in small animal CT imaging.     

Adaptation and applications of a realistic digital phantom based on patient lung tumor trajectories

Digital phantoms continue to play a significant role in modeling and characterizing medical imaging. The currently available XCAT phantom incorporates both the flexibility of mathematical phantoms and the realistic nature of voxelized phantoms. This phantom generates images based on a regular breathing pattern and can include arbitrary lung tumor trajectories. In this work, we present an algorithm that modifies the current XCAT phantom to generate 4D imaging data based on irregular breathing. First, a parameter is added to the existing XCAT phantom to include any arbitrary tumor motion. This modification introduces the desired tumor motion but, comes at the cost of decoupled diaphragm, chest wall and lung motion. To remedy this problem diaphragm and chest wall motion is first modified based on initial tumor location and then input to the XCAT phantom. This generates a phantom with synchronized respiratory motion. Mapping of tumor motion trajectories to diaphragm and chest wall motion is done by adaptively calculating a scale factor based on tumor to lung contour distance. The distance is calculated by projecting the initial tumor location to lung edge contours characterized by quadratic polynomials. Data from ten patients were used to evaluate the accuracy between actual independent tumor location and the location obtained from the modified XCAT phantom. The RMSE and standard deviations for ten patients in x, y, and z directions are: (0.29 ± 0.04, 0.54 ± 0.17, and0.39 ± 0.06) mm. To demonstrate the utility of the phantom, we use the new phantom to simulate a 4DCT acquisition as well as a recently published method for phase sorting. The modified XCAT phantom can be used to generate more realistic imaging data for enhanced testing of algorithms for CT reconstruction, tumor tracking, and dose reconstruction.