Specific Scan Research Articles

Geometric and Colour Data Fusion for Outdoor 3D Models

This paper deals with the generation of accurate, dense and coloured 3D models of outdoor scenarios from scanners. This is a challenging research field in which several problems still remain unsolved. In particular, the process of 3D model creation in outdoor scenes may be inefficient if the scene is digitalized under unsuitable technical (specific scanner on-board camera) and environmental (rain, dampness, changing illumination) conditions. We address our research towards the integration of images and range data to produce photorealistic models. Our proposal is based on decoupling the colour integration and geometry reconstruction stages, making them independent and controlled processes. This issue is approached from two different viewpoints. On the one hand, given a complete model (geometry plus texture), we propose a method to modify the original texture provided by the scanner on-board camera with the colour information extracted from external images taken at given moments and under specific environmental conditions. On the other hand, we propose an algorithm to directly assign external images onto the complete geometric model, thus avoiding tedious on-line calibration processes. We present the work conducted on two large Roman archaeological sites dating from the first century A.D., namely, the Theatre of Segobriga and the Fori Porticus of Emerita Augusta, both in Spain. The results obtained demonstrate that our approach could be useful in the digitalization and 3D modelling fields.

Effective and equivalent doses for CT head examinations calculated using the voxelized phantoms MASH and FASH

The voxelized phantoms MASH and FASH have been used for calculating conversion coefficients (CCs) for a specific CT head examination performed in a specific Toshiba CT scanner. The X-ray spectrum was selected according to the concordance between the calculated C 100,c and measured C 100,c. For 120 kV, the X-raytbc code presented the smaller deviation, equal to −7.11%. Finally, effective doses calculated for the voxelized phantoms MASH and FASH were 6.1 and 4.5, respectively, smaller than the effective dose obtained though the European guidelines on quality criteria for computed tomography.

Radiation safety summit – method to estimate radiation dose to pediatric patients from CT scans

State-of-the-art CT scanners typically display two dose indices, CTDIvol (mGy) and DLP (mGy-cm), along with which two standard CTDI phantoms, 16or 32-cm diameter, are used to estimate the two indices. CTDIvol is based on radiation dose measurements on an individual scanner completed by a medical physicist with a pencil ionization chamber and either a 16or 32-cm diameter CTDI cylindrical plastic phantom. CTDIvol represents the radiation dose delivered to a standard plastic phantom from a specific model CT scanner using specified scan parameters: tube voltage, tube current, rotation time, pitch and selected bowtie filter. CTDIvol provides a standardized method to estimate and compare the radiation output of different CT scanners. CTDIvol is defined to be a dose index of CT scanners, not an indication of patient dose. The inability of CTDIvol to accurately reflect pediatric patient dose is illustrated in Fig. 1. Three cylindrical phantoms with diameters of 10 cm, 16 cm, and 32 cm (representing the smallest pediatric patient to an adult) were scanned with the same scanner using the same scan parameters. While this delivered the same radiation energy to each phantom, the measured CTDIvol in the phantoms (top row of data in figure) ranged from 47 mGy to 18 mGy. This clearly indicates that the dose to various-size patients (represented by the three phantoms) is a function of the energy produced by the scanner (determined by scan techniques) and the size of the patient. The superficial tissues of the large patient attenuate the radiation beam, reducing the dose delivered to the core of the large patient. In the figure, the center of the 10-cm phantom received 47 mGy of dose while the center of the 32-cm phantom received a dose of only 10.8 mGy for the same scan techniques. While the measured dose index in the previous example with an ionization chamber, CTDIvol, is very dependent on patient size when scan parameters for all three phantoms are identical, the displayed CTDIvol on a CT scanner is independent of changes in patient size by definition. The displayed scan index assumes the actual scan parameters but by definition assumes the patient is either a 16-cm or 32-cm diameter cylindrical plastic phantom. If the 16-cm phantom is assumed, the scanner will display 37 mGy for CTDIvol for each of the three represented patient sizes (second row of data in Fig. 1); this overestimates the dose to adult patients and underestimates dose to infants. If the 32-cm phantom is assumed, the scanner displays 18 mGy for all patients (third row of data in figure); this underestimates the dose to all patients smaller than adults. In either case, the displayed CTDIvol does not represent the pediatric patient dose and can mislead the technologist or radiologist. A task group of the American Association of Physicists in Medicine (AAPM), co-chaired by John Boone and Keith Strauss, is currently preparing a report that will provide scaling factors as a function of patient size that can be used by the technologist or radiologist to more accurately estimate the radiation dose to the tissues of the pediatric patient that are directly irradiated by the primary X-ray beam of the CT scanner. These scaling factors will be based on attenuation data collected from numerous CT scanners by multiple investigators. Hopefully, these scaling factors will be available by the time of this presentation. These scaling factors should allow the operator of the CT scanner to estimate the radiation dose to the Disclaimer Dr. Strauss has no financial interests, investigational or off-label uses to disclose.

A way forward for the development of an exposure computational model to computed tomography dosimetry

A way forward for the development of an exposure computational model to computed tomography dosimetry has been presented. In this way, an exposure computational model (ECM) for computed tomography (CT) dosimetry has been developed and validated through comparison with experimental results. For the development of the ECM, X-ray spectra generator codes have been evaluated and the head bow tie filter has been modelled through a mathematical equation. EGS4 and EGSnrc have been used for simulating the radiation transport by the ECM. Geometrical phantoms, commonly used in CT dosimetry, have been modelled by IDN software. MAX06 has also been used to simulate an adult male patient submitted for CT examinations. The evaluation of the X-ray spectra generator codes in CT dosimetry showed dependence with tube filtration (or HVL value). More generally, with the increment of total filtration (or HVL value) the X-raytbc becomes the best X-ray spectra generator code for CT dosimetry. The EGSnrc/X-raytbc combination has calculated C 100,c in better concordance with C 100,c measured in two different CT scanners. For a Toshiba CT scanner, the average percentage difference between the calculated C 100,c values and measured C 100,c values was 8.2%. Whilst for a GE CT scanner, the average percentage difference was 10.4%. By the measurements of air kerma through a prototype head bow tie filter a third-order exponential decay equation was found. C 100,c and C 100,p values calculated by the ECM are in good agreement with values measured at a specific CT scanner. A maximum percentage difference of 2% has been found in the PMMA CT head phantoms, demonstrating effective modelling of the head bow tie filter by the equation. The absorbed and effective doses calculated by the ECM developed in this work have been compared to those calculated by the ECM of Jones and Shrimpton for an adult male patient. For a head examination the absorbed dose values calculated by the ECM developed by Jones and Shrimpton overestimates up to three times the absorbed dose to brain compared to the ECM developed in this work. The effective dose calculated by the ECM of Jones and Shrimpton was 26% greater than effective dose calculated by the ECM developed in this work.

Attenuation correction without transmission scan for the MAMMI breast PET

Abstract Whole-body Positron Emission Tomography (PET) scanners are required in order to span large Fields of View (FOV). Therefore, reaching the sensitivity and spatial resolution required for early stage breast tumor detection is not straightforward. MAMMI is a dedicated breast PET scanner with a ring geometry designed to provide PET images with a spatial resolution as high as 1.5 mm, being able to detect small breast tumors ( 1 cm ) . The patient lays down in prone position during the scan, thus making possible to image the whole breast, up to regions close to the base of the pectoral without the requirement of breast compression. Attenuation correction (AC) for PET data improves the image quality and the quantitative accuracy of radioactivity distribution determination. In dedicated, high resolution breast cancer scanners, this correction would enhance the proper diagnosis in early disease stages. In whole-body PET scanners, AC is usually taken into account with the use of transmission scans, either by external radioactive rod sources or by Computed Tomography (CT). This considerably increases the radiation dose administered to the patient and time needed for the exploration. In this work we propose a method for breast shape identification by means of PET image segmentation. The breast shape identification will be used for the determination of the AC. For the case of a specific breast PET scanner the procedure we propose should provide AC similar to that obtained by transmission scans as we take advantage of the breast anatomical simplicity. Experimental validation of the proposed approach with a dedicated breast PET prototype is also presented. The main advantage of this method is an important dose reduction since the transmission scan is not required.

3D ultrasound reconstruction algorithms from analog and digital data

Freehand 3D ultrasound is increasingly being introduced in the clinic for diagnostics and image-assisted interventions. Various algorithms exist for combining 2D images of regular ultrasound probes to 3D volumes, being either voxel-, pixel- or function-based. Previously, the most commonly used input to 3D ultrasound reconstruction has been digitized analog video. However, recent scanners that offer access to digital image frames exist, either as processed or unprocessed data. To our knowledge, no comparison has been performed to determine which data source gives the best reconstruction quality. In the present study we compared both reconstruction algorithms and data sources using novel comparison methods for detecting potential differences in image quality of the reconstructed volumes. The ultrasound scanner used in this study was the Sonix RP from Ultrasonix Medical Corp (Richmond, Canada), a scanner that allow third party access to unprocessed and processed digital data. The ultrasound probe used was the L14-5/38 linear probe. The assessment is based on a number of image criteria: detectability of wire targets, spatial resolution, detectability of small barely visible structures, subjective tissue image quality, and volume geometry. In addition we have also performed the more “traditional” comparison of reconstructed volumes by removing a percentage of the input data. By using these evaluation methods and data from the specific scanner, the results showed that the processed video performed better than the digital scan-line data, digital video being better than analog video. Furthermore, the results showed that the choice of video source was more important than the choice of tested reconstruction algorithms.

Small lesions detectability with the Biograph 16 Hi-Rez PET/CT scanner and fast imaging protocols: performance evaluation using an anthropomorphic thoracic phantom and ROC analyses

The purpose of this study was to evaluate the impact on lesion detectability of fast imaging protocols using 18F-FDG and a 3-dimensional LSO-based PET/CT scanner. An anthropomorphic thoracic phantom was used simulating the anatomical structures of radioactivity distribution for the upper torso of an underweight patient. Irregularly shaped targets of small dimensions, the zeolites, were located inside the phantom in an unpredictable position for the observers. Target-to background ratios and target dimensions were selected in order to sample the range of detectability. Repeated imaging was performed to acquire PET images with varying emission scan duration (ESD) of 1, 2, 3 and 4 min/bed and background activity concentrations of 10, 5 and 3 kBq/mL in the torso cavity. Three observers ranked the targets and a receiver operating characteristic analysis was performed for each acquisition protocol. Detection performances improved when passing from a short (ESD = 1 min) protocol to longer (ESD C 2 min) protocols. This improvement was established with adequate statistical significance. Short image acquisition times of 1 min/bed using 18F-FDG and the specific scanner model considered in the study lead to reduced lesion detectability and should be avoided also in underweight patients.

Radiation Dose to the Conceptus from Multidetector CT during Early Gestation: A Method That Allows for Variations in Maternal Body Size and Conceptus Position

To develop a method for estimating the radiation dose to the conceptus from multidetector computed tomography (CT) of the abdomen and pelvis in pregnant patients during the first 7 weeks of gestation. This study was approved by the institutional review board and informed consent was obtained. A CT simulation software package was used to (a) develop voxelized models on the basis of image data from 117 nonpregnant patients who underwent abdominal and pelvic multidetector CT and (b) calculate dose at a position of the uterus assumed to be the position of the conceptus in case of pregnancy during the first 7 weeks of gestation. Regression analysis was carried out to establish the relationship among conceptus dose, patient body size, and distance from the conceptus to the anterior skin surface. Normalized conceptus doses calculated by using the software package ranged from 0.335 to 0.785 mGy per absorbed dose to air. Conceptus dose showed a significant correlation with maternal body size and conceptus depth (R² = 0.793, P < .001). A multivariable correlation of conceptus dose normalized to the free-in-air CT dose index (CTDI(F)) with conceptus depth and patient perimeter was produced for estimating conceptus dose from abdominal and pelvic multidetector CT. Conceptus dose data provided for a specific scanner can be applied to other scanners by using correction factors based on ratios between the weighted CT dose index and CTDI(F), resulting in inaccuracies in the estimation of conceptus dose of less than 12%. The radiation dose to the conceptus from abdominal and pelvic multidetector CT can be estimated with a method that allows for variations in maternal body size and conceptus position.

Differentiating Emery-Dreifuss muscular dystrophy and collagen VI-related myopathies using a specific CT scanner pattern

Bethlem myopathy and Ullrich congenital muscular dystrophy are part of the heterogeneous group of collagen VI-related muscle disorders. They are caused by mutations in collagen VI (ColVI) genes ( COL6A1, COL6A2, and COL6A3) while LMNA mutations cause autosomal dominant Emery-Dreifuss muscular dystrophy. A muscular dystrophy pattern and contractures are found in all three conditions, making differential diagnosis difficult especially in young patients when cardiomyopathy is absent. We retrospectively assessed upper and lower limb muscle CT scans in 14 Bethlem/Ullrich patients and 13 Emery-Dreifuss patients with identified mutations. CT was able to differentiate Emery-Dreifuss muscular dystrophy from ColVI-related myopathies in selected thigh muscles and to a lesser extent calves muscles: rectus femoris fatty infiltration was selectively present in Bethlem/Ullrich patients while posterior thigh muscles infiltration was more prominently found in Emery-Dreifuss patients. A more severe fatty infiltration particularly in the leg posterior compartment was found in the Emery-Dreifuss group.

Efficient methodologies for system matrix modelling in iterative image reconstruction for rotating high-resolution PET

A fully 3D iterative image reconstruction algorithm has been developed for high-resolution PET cameras composed of pixelated scintillator crystal arrays and rotating planar detectors, based on the ordered subsets approach. The associated system matrix is precalculated with Monte Carlo methods that incorporate physical effects not included in analytical models, such as positron range effects and interaction of the incident gammas with the scintillator material. Custom Monte Carlo methodologies have been developed and optimized for modelling of system matrices for fast iterative image reconstruction adapted to specific scanner geometries, without redundant calculations. According to the methodology proposed here, only one-eighth of the voxels within two central transaxial slices need to be modelled in detail. The rest of the system matrix elements can be obtained with the aid of axial symmetries and redundancies, as well as in-plane symmetries within transaxial slices. Sparse matrix techniques for the non-zero system matrix elements are employed, allowing for fast execution of the image reconstruction process. This 3D image reconstruction scheme has been compared in terms of image quality to a 2D fast implementation of the OSEM algorithm combined with Fourier rebinning approaches. This work confirms the superiority of fully 3D OSEM in terms of spatial resolution, contrast recovery and noise reduction as compared to conventional 2D approaches based on rebinning schemes. At the same time it demonstrates that fully 3D methodologies can be efficiently applied to the image reconstruction problem for high-resolution rotational PET cameras by applying accurate pre-calculated system models and taking advantage of the system's symmetries.

Reverse Engineering: Statistical Threshold for New Selective Sampling Morphological Descriptor

During the digitization process of a physical object, the operator has to choose an acquisition pitch. Currently, 3D scanners employ constant pitches. For this reason the grid dimension choice normally represents a compromise between the scanner performances and specific applications, and the resolution and accuracy of the specific application. This is a critical problem because, normally, the object shape is assumed as a combination of different geometries with different morphological complexities. As a consequence of this, while some basic geometries (i.e., planes, cylinders, and cones) require only few points to describe their behavior, others need much more information. Normally, this problem is solved with a significant operator involvement. Starting from the object morphology and from the 3D scanner performances, the author finds the optimal acquisition strategy with an iterative and refining process made of many attempts. This approach does not guarantee an efficient acquisition of the object, because it depends strongly on the subjective ability of the operator involved in the acquisition. Many approaches propose points cloud management methodologies that introduce or erase punctual information, working with statistical hypothesis after the acquisition phase. This research work proposes an operative strategy, which starts from, first, a raw point acquisition, then it partitions the object surface, identifying different morphological zone boundaries (shape changes). As a consequence, some of the identified regions will be redigitized with deeper scansions in order to reach a more precise morphological information. The proposed partitioning methodology has been developed to directly interact with the 3D scanner. It integrates the use of a global morphological descriptor (Gaussian curvature), managed in order to be applicable in a discrete context (points cloud), with the concept of the 3D scanner measuring uncertainty. This integration has been proposed in order to provide an automatic procedure and a “curvature variation threshold,” able to identify real significant shape changes. The proposed methodology will neglect those regions where the shape changes are only correlated with the uncontrolled noise introduced by the specific 3D scanner performances.

Real Time Coincidence Detection Engine for High Count Rate Timestamp Based PET

Coincidence engines follow two main implementation flows: timestamp based systems and AND-gate based systems. The latter have been more widespread in recent years because of its lower cost and high efficiency. However, they are highly dependent on the selected electronic components, they have limited flexibility once assembled and they are customized to fit a specific scanner's geometry. Timestamp based systems are gathering more attention lately, especially with high channel count fully digital systems. These new systems must however cope with important singles count rates. One option is to record every detected event and postpone coincidence detection offline. For daily use systems, a real time engine is preferable because it dramatically reduces data volume and hence image preprocessing time and raw data management. This paper presents the timestamp based coincidence engine for the LabPET?, a small animal PET scanner with up to 4608 individual readout avalanche photodiode channels. The engine can handle up to 100 million single events per second and has extensive flexibility because it resides in programmable logic devices. It can be adapted for any detector geometry or channel count, can be ported to newer, faster programmable devices and can have extra modules added to take advantage of scanner-specific features. Finally, the user can select between full processing mode for imaging protocols and minimum processing mode to study different approaches for coincidence detection with offline software.

Assessment of the increase in variability when combining volumetric data from different scanners

In multicenter MRI studies, pooling of volumetric data requires a prior evaluation of compatibility between the different machines used. We tested the compatibility of five different scanners (2 General Electric Signa, 2 Siemens Symphony, and a Philips Gyroscan) at five different sites by repeating the scans of five volunteers at each of the sites. Using a semiautomatic method based on the Talairach atlas, and SPM algorithms for tissue segmentation (multimodal T1 and T2, or T1-only), we obtained volume measurements of the main brain lobes (frontal, parietal, occipital, temporal) and for each tissue type. Our results suggest that pooling of multisite data adds small error for whole brain measurements, intersite coefficient of variation (CV) ranging from 1.8 to 5.2%, respectively, for GM and CSF. However, in the occipital lobe, intersite CV can be as high as 11.7% for WM and 17.3% for CSF. Compared with the intersite, intrasite CV values were always much lower. Whenever possible, T1 and T2 tissue segmentation methods should be used because they yield more consistent volume measurements between sites than T1-only, especially when some of the scans were obtained with different sequence parameters and pixel size from those of the other sites. Our study shows that highest compatibility among scanners would be obtained using equipments of the same manufacturer and also image acquisition parameters as similar as possible. After validation, data from a specific ROI or scanner showing values markedly different from the other sites might be excluded from the analysis.

Safety of localizing epilepsy monitoring intracranial electroencephalograph electrodes using MRI: Radiofrequency‐induced heating

PurposeTo investigate heating during postimplantation localization of intracranial electroencephalograph (EEG) electrodes by MRI.Materials and MethodsA phantom patient with a realistic arrangement of electrodes was used to simulate tissue heating during MRI. Measurements were performed using 1.5 Tesla (T) and 3T MRI scanners, using head- and body-transmit RF-coils. Two electrode-lead configurations were assessed: a “standard” condition with external electrode-leads physically separated and a “fault” condition with all lead terminations electrically shorted.ResultsUsing a head-transmit–receive coil and a 2.4 W/kg head-average specific absorption rate (SAR) sequence, at 1.5T the maximum temperature change remained within safe limits (<1°C). Under “standard” conditions, we observed greater heating (≤2.0°C) at 3T on one system and similar heating (<1°C) on a second, compared with the 1.5T system. In all cases these temperature maxima occurred at the grid electrode. In the “fault” condition, larger temperature increases were observed at both field strengths, particularly for the depth electrodes. Conversely, with a body-transmit coil at 3T significant heating (+6.4°C) was observed (same sequence, 1.2/0.5 W/kg head/body-average) at the grid electrode under “standard” conditions, substantially exceeding safe limits. These temperature increases neglect perfusion, a major source of heat dissipation in vivo.ConclusionMRI for intracranial electrode localization can be performed safely at both 1.5T and 3T provided a head-transmit coil is used, electrode leads are separated, and scanner-reported SARs are limited as determined in advance for specific scanner models, RF coils and implant arrangements. Neglecting these restrictions may result in tissue injury. J. Magn. Reson. Imaging 2008;28:1233–1244. © 2008 Wiley-Liss, Inc.

G.P.6.03 Specific CT scanner muscle pattern helps to differentiate contractural lamin A/C and collagen VI related myopathies

G.P.6.03 Specific CT scanner muscle pattern helps to differentiate contractural lamin A/C and collagen VI related myopathies

WE-B-351-01: Patient Dose Estimates From CT Examinations

The assessment of radiation dose from computed tomography has become an important issue due to the increased utilization of computed tomography in a large number of clinical applications, from CT urography to cardiac CT. The traditional metrics used for CT dose assessment (CTDI-100) have come under assault from a number of investigators, because of its limitations in describing the radiation dose in realistic CT examinations. While there remains no real consensus in the field, a number of groups are working on the development of CT dose metrics which convey a better understanding of the radiation dose received by individual patients for specific CT examinations. In this presentation, the perspective of a number of groups will be presented. Primarily, the work of a committee of the International Commission of Radiological Units (ICRU) commissioned in 2005 will be discussed. The ICRU committee has preliminarily defined a multi-tier system for the assessment of radiation dose metrics in computed tomography. At the first level, machine-dependent performance factors are described and measured, and these include the traditional CTDI-100 metric for the 16 cm and 32 cm diameter polymethylmethacrylate dosimetry phantoms, at the center and at the periphery. There is a consensus amongst the ICRU community that radiation dose to patients undergoing CT examinations is best established using the known geometry of the CT examination, coupled with measured output characteristics of the specific CT scanner (including bow-tie characteristics), combined with Monte Carlo computations. In the rare instance in which the CT dose to a specific patient needs to be computed, image-based methodologies will be presented which enable these computations with a high degree of accuracy. It is emphasized that the effective dose (measured in milliseiverts) is not an appropriate measure for individual patient doses, as this metric includes population-based radiation epidemiological data which may not be applicable to a specific individual. Therefore, the ICRU efforts towards radiation dosimetry of the individual patient focus on dosimetric units which are physical in nature, and describe the individual organ doses and the overall average dose to the patient, depending upon the specific CT examination and the patient's physical characteristics. Educational Objectives: 1. Convey some of the issues in accurate dose assessment in CT. 2. Describe some of the ongoing efforts of the ICRU committee on CT towards dose assessment.

On the use of Monte Carlo‐derived dosimetric data in the estimation of patient dose from CT examinations

The purpose of this work was to investigate the applicability and appropriateness of Monte Carlo-derived normalized data to provide accurate estimations of patient dose from computed tomography (CT) exposures. Monte Carlo methodology and mathematical anthropomorphic phantoms were used to simulate standard patient CT examinations of the head, thorax, abdomen, and trunk performed on a multislice CT scanner. Phantoms were generated to simulate the average adult individual and two individuals with different body sizes. Normalized dose values for all radiosensitive organs and normalized effective dose values were calculated for standard axial and spiral CT examinations. Discrepancies in CT dosimetry using Monte Carlo-derived coefficients originating from the use of: (a) Conversion coefficients derived for axial CT exposures, (b) a mathematical anthropomorphic phantom of standard body size to derive conversion coefficients, and (c) data derived for a specific CT scanner to estimate patient dose from CT examinations performed on a different scanner, were separately evaluated. The percentage differences between the normalized organ dose values derived for contiguous axial scans and the corresponding values derived for spiral scans with pitch = 1 and the same total scanning length were up to 10%, while the corresponding percentage differences in normalized effective dose values were less than 0.7% for all standard CT examinations. The normalized organ dose values for standard spiral CT examinations with pitch 0.5-1.5 were found to differ from the corresponding values derived for contiguous axial scans divided by the pitch, by less than 14% while the corresponding percentage differences in normalized effective dose values were less than 1% for all standard CT examinations. Normalized effective dose values for the standard contiguous axial CT examinations derived by Monte Carlo simulation were found to considerably decrease with increasing body size of the mathematical phantom used. When the body-mass index was increased from 23.0 to 32.7 kg/m2 discrepancies in patient effective dose were up to 34%. The error in estimating effective dose from a CT exposure performed on a specific CT scanner using Monte Carlo data derived for a different CT scanner was estimated to be up to 25%. A simple method was proposed and validated for the determination of scanner-specific normalized dosimetric data from data derived from Monte Carlo simulation of a specific scanner. In conclusion, computed tomography dose index (CTDI) to effective dose conversion coefficients derived by Monte Carlo simulation of axial CT scans may provide a good approximation of corresponding coefficients applicable in helical scans. However, the use of Monte Carlo conversion coefficients for the estimation of patient dose from a CT examination involves a remarkable inaccuracy when the body size of the mathematical anthropomorphic phantom used in Monte Carlo simulation differs from the body of the patient. Therefore, separate sets of Monte Carlo dosimetric CT data shall be generated for different patient body sizes. Besides calculation of different sets of Monte Carlo data for each commercially available scanner is not necessary, since scanner specific data may be derived with acceptable accuracy from the Monte Carlo data calculated for a specific scanner appropriately modified for the different CTDI(W)/CTDI(air) ratio.

Renal Cyst Pseudoenhancement: Influence of Multidetector CT Reconstruction Algorithm and Scanner Type in Phantom Model

To prospectively determine the dependence of renal cyst pseudoenhancement on multidetector computed tomographic (CT) scanner type and convolution kernel in a phantom model. A customized anthropomorphic phantom was created to accept interchangeable 40-, 140-, and 240-HU renal inserts that contained stacked 0- and 50-HU cylindric cysts measuring 7, 10, and 15 mm in diameter. Each phantom and insert was scanned with five different multidetector CT scanners on five separate occasions by using 120 kVp, low and high tube current settings, 3.00-3.75-mm collimation, and standard and high-spatial-resolution kernels. A total of 2340 CT attenuation measurements were obtained by using standardized regions of interest. The effect of multidetector CT imaging regimen, tube current, cyst diameter, and renal attenuation on pseudoenhancement incidence was assessed by using generalized estimating equations based on a binary logistic regression model. Within this framework, a Bonferroni multiple comparison correction was used to assess pseudoenhancement frequency differences among imaging regimens. Pseudoenhancement occurred in both 0- and 50-HU cysts; was significantly correlated with multidetector CT imaging regimen (P<.0001), cyst diameter (P<.0001), and renal attenuation (P<or=.032); and was independent of tube current (P>.3). When convolution kernels on specific scanners were compared, significant differences (P<.04) between kernels were identified with all five scanners in terms of observed pseudoenhancement incidence. Generational differences in equipment were noted, with pseudoenhancement incidence ranging from 1.7% to 8.3%, 1.7% to 16.7%, and 18.3% to 56.7% across relevant kernels for three scanners from one manufacturer. Pseudoenhancement is strongly dependent on multidetector CT convolution kernel. Varying this parameter may mitigate this phenomenon, which is independent of volume-averaging effects.

TU‐SAMS‐L100J‐01: Determination of Dose From CT Examinations

The assessment of radiation dose from computed tomography has become an important issue due to the increased utilization of computed tomography in a large number of clinical applications, from CT urography to cardiac CT. The traditional metrics used for CT dose assessment (CTDI‐100) have come under assault from a number of investigators, because of its limitations in describing the radiation dose in realistic CT examinations. While there remains no real consensus in the field, a number of groups are working on the development of CT dose metrics which convey a better understanding of the radiation dose received by individual patients for specific CT examinations. In this presentation, the perspective of a number of groups will be presented. Primarily, the work of a committee of the International Commission of Radiological Units (ICRU) commissioned in 2005 will be discussed.The ICRU committee has preliminarily defined a multi‐tier system for the assessment of radiation dose metrics in computed tomography. At the first level, machine‐dependent performance factors are described and measured, and these include the traditional CTDI‐100 metric for the 16 cm and 32 cm diameter polymethylmethacrylate dosimetry phantoms, at the center and at the periphery. There is a consensus amongst the ICRU community that radiation dose to patients undergoing CT examinations is best established using the known geometry of the CT examination, coupled with measured output characteristics of the specific CT scanner (including bow‐tie characteristics), combined with Monte Carlo computations. In the rare instance in which the CT dose to a specific patient needs to be computed, image‐based methodologies will be presented which enable these computations with a high degree of accuracy. It is emphasized that the effective dose (measured in milliseiverts) is not an appropriate measure for individual patient doses, as this metric includes population‐based radiation epidemiological data which may not be applicable to a specific individual. Therefore, the ICRU efforts towards radiation dosimetry of the individual patient focus on dosimetric units which are physical in nature, and describe the individual organ doses and the overall average dose to the patient, depending upon the specific CT examination and the patient's physical characteristics.Educational Objectives:1. Convey some of the issues in accurate dose assessment in CT.2. Describe some of the ongoing efforts of the ICRU committee on CT towards dose assessment.

Study of ambient light influence for three-dimensional scanners based on structured light

Ambient light in a scene can introduce errors into range data from most commercial three-dimensional range scanners, particularly scanners that are based on projected patterns and structured lighting. We study the effects of ambient light on a specific commercial scanner. We further present a method for characterizing the range accuracy as a function of ambient light distortions. After a brief review of related research, we first describe the capabilities of the scanner we used and define the experimental setup for our study. Then we present the results of the range characterization relative to ambient light. In these results, we note a systematic error source that appears to be an artifact due to a structured light pattern. We conclude with a discussion of this error and the physical meaning of the results overall.