Higher Tube Voltage Research Articles

이중 엑스선 에너지를 이용한 전자부품 검사

X-ray can be applied to obtain a projection image of an object. It is not easy to obtain an high quality image for the object composed of low and high density materials. For the object with large difference in density, it is possible to realize high contrast image using images of low and high tube voltages and image processing. The plastic and metalic parts of the electronic components can be imaged by the dual energy technique which use low and high tube voltages and by processing pixel-by-pixel using visual C++. The contrast-enhanced image can be used to detect and observe defects within the electronic components.

Radiation hazards in operation theatre: Anaesthesiologist's concerns and preventive strategies.

Sir, The use of X-ray fluoroscopy has led to remarkable advancements in management of various disorders. However, operation theatre staff are repeatedly exposed to radiation, which can cause bone marrow suppression, genetic mutations and carcinogenesis, etc. The aim of this letter is to draw the attention of senior authorities to the fact that guidelines should be prepared and followed strictly to monitor radiation exposure to anaesthesiologists. Knowledge of various physical and biochemical aspects of radiation is important to prevent potential hazardous consequences of its exposure.[1] In a study comparing the effect of the use of single or two C-arms, duplicate thermoluminescent dosimeter (TLD) badges were placed on surgeons’ and scrub technicians’ eyewear, thyroid shields, and waist. It was concluded that the use of lead aprons and thyroid shields reduced radiation exposure by 50%.[2] The National Council on Radiation Protection was established in United States (US) to help set US policy on radiation protection.[3] Various prophylactic measures include increased filtration and use of higher tube voltage or lower tube current. Pulsed fluoroscopic operation should be set at rate of 4 pulses/s and C-arm should be positioned underneath the table. Recommended distance to avoid exposure is approximately 3 feet from the source.[4] The use of biplanar fluoroscopy reduces both radiation exposure and surgical time. Lead aprons should be regularly checked for any breakage by an assigned technician. Mobile full body lead shield can be used, as it can both effectively shield off scattered radiation and do not add extra weight. During repeated and long procedures, patients should also be protected with lead shield. Educational programs should be conducted for awareness of radiation hazards and safety measures among operation theatre personnel. Staff should be taught to avoid close contact with tilted side of fluoroscope as scattered radiation increases when emission tube gets tilted.[5] Certain adjustments in fluoroscopy machine such as avoiding any metal parts within fluoroscopy field, by closing iris and adjusting size of collimator, the amount of radiation generated can be reduced.[5] Lead acrylic glasses can be used to avoid damage to lens. Lead impregnated gloves can be used in high-risk procedures. Both ionising and non-ionising radiations pose potential hazards to anaesthesiologists. If recommended dose limit exceeds 15 mSv/year, cumulative effects of radiation can affect whole body or can result in localised effects such as cataract. Anaesthesiologists posted in OTs where fluoroscopy is used routinely should have two badges including one for cumulative and another for individual exposure. These badges should be made available from the reputed suppliers. Their use establishes the dose received and verifies the adequacies of facilities for radiation protection. As part of safety, according to Atomic Energy (Radiation Protection) Rules 2004 section (3), a licence/registration is necessary for establishing the radiation installation. This procedure involves three steps: The director, directorate of radiation safety (DRS) first approves the site where equipment has to be installed Then quality assurance test is done by the manufacturer or supplier under intimation to DRS After that licence/registration is issued by Atomic Energy Regulatory Board (AERB) through DRS. Recommendations from AERB to improve quality of care include: It is the responsibility of the employer to provide radiation-monitoring devices (TLD badges) to radiation workers X-ray installations should be located as far as possible from areas of high occupancy such as maternity, paediatric and ultrasonography rooms Quality assurance should be conducted once in 2 years on every X-ray equipment and records to be maintained by registrant/licensee. Thus by taking various prophylactic measures, safety of the anaesthesiologists can be enhanced.

In-situ calibration of clinical built-in KAP meters with traceability to a primary standard using a reference KAP meter

The air kerma-area product (KAP) is used for settings of diagnostic reference levels. The International Atomic Energy Agency (IAEA) recommends that doses in diagnostic radiology (including the KAP values) be estimated with an accuracy of at least ±7% (k = 2). Industry standards defined by the International Electrotechnical Commission (IEC) specify that the uncertainty of KAP meter measurements should be less than (k = 2). Medical physicists willing to comply with the IAEA’s recommendation need to apply correction factors to KAP values reported by x-ray units. The aim of this work is to present and evaluate a calibration method for built-in KAP meters on clinical x-ray units. The method is based on (i) a tandem calibration method, which uses a reference KAP meter calibrated to measure the incident radiation, (ii) measurements using an energy-independent ionization chamber to correct for the energy dependence of the reference KAP meter, and (iii) Monte Carlo simulations of the beam quality correction factors that correct for differences between beam qualities at a standard laboratory and the clinic. The method was applied to the KAP meter in a Siemens Aristos FX plus unit. It was found that values reported by the built-in KAP meter differed from the more accurate values measured by the reference KAP meter by more than 25% for high tube voltages (more than 140 kV) and heavily filtered beams (0.3 mm Cu). Associated uncertainties were too high to claim that the IEC’s limit of 25% was exceeded. Nevertheless the differences were high enough to justify the need for a more accurate calibration of built-in KAP meters.

Evaluation of entrance surface air kerma in pediatric chest radiography

Abstract The objective of this study was to evaluate the entrance surface air kerma in pediatric chest radiography. An evaluation of 301 radiographical examinations in anterior–posterior (AP) and posterior–anterior (PA) (166 examinations) and lateral (LAT) (135 examinations) projections was performed. The analyses were performed on patients grouped by age; the groups included ages 0–1 y, 1–5 y, 5–10 y, and 10–15 y. The entrance surface air kerma was determined with DoseCal software (Radiological Protection Center of Saint George׳s Hospital, London) and thermoluminescent dosimeters. Two different exposure techniques were compared. The doses received by patients who had undergone LAT examinations were 40% higher, on average, those in AP/PA examinations because of the difference in tube voltage. A large high-dose “tail” was observed for children up to 5 y old. An increase in tube potential and corresponding decrease in current lead to a significant dose reduction. The difference between the average dose values for different age ranges was not practically observed, implying that the exposure techniques are still not optimal. Exposure doses received using the higher tube voltage and lower current-time product correspond to the international diagnostic reference levels.

Study of dose profiles using automatic tube voltage selection in an anthropomorphic phantom

Background: Rapid and user-friendly software tools for individual estimates of organ doses in CT are of the utmost importance to support the professionals in the choice of the appropriate imaging modalities. Most CT-dosimetry tools, like CT Expo [1], CT Dosimetry [2], as well as a new dosimetry tool [3], rely on the assumption that organ doses estimated for one CT scanner can be converted to other CT scanner using the ratio of the Computed Tomography Dose Index (CTDI) between two CT scanners. We tested this assumption for paediatric patients using physical phantom measurements for various protocols and CT scanners. Measurements were compared with dosimetry tools. Materials and methods: Dose to 22 radiosensitive organs were measured using thermoluminescence dosimeters inserted within an anthropomorphic 5-year old phantom. Dose measurements were performed for wholebody scans on 4 scanner models from the 4 major manufacturers (Siemens, GE, Philips and Toshiba); both axial and helical modes were tested with small collimations (below 12 mm). Measured doses were compared with the results of Monte-Carlo simulations [3]. Results: Whereas the measured organ doses showed important variability across the different scanner models, doses normalised to CTDI showed significantly less variation. Higher tube voltage also showed less variation. The simulated organ doses reasonably compared with the measurements. For example, at 120 kV in axial mode with 10 mm collimation, the mean coefficient of variation between the scanner models was below 10%; the relative difference between measured and simulated organ doses was within ±15% for most organs. Discussion: For the tested collimations (below 12 mm), both in axial and helical modes, the CTDI fairly accounts for the difference in CT design across different models. Further measurements are being carried out to test the algorithm against protocols more representative of the clinical practice. This study proved that, for small collimations and paediatric patients, organ doses simulated from one CT scanner can be converted to other CT scanners using CTDI-ratio. Those results are of interest for fast dose assessment and retrospective dosimetry studies. Acknowledgments: These measurements were performed within the framework of the EPI-CT study, supported by the European Community's Seventh Framework Programme (FP7/2007e2013) under grant agreement number 269912-EPI-CT: Epidemiological study to quantify risks for paediatric computerized tomography and to optimise doses.

Optimal beam quality for chest digital radiography

To investigate the optimal beam quality for chest computed radiography (CR), we measured the radiographic contrast and evaluated the image quality of chest CR using various X-ray tube voltages. The contrast between lung and rib or heart increased on CR images obtained by lowering the tube voltage from 140 to 60 kV, but the degree of increase was less. Scattered radiation was reduced on CR images with a lower tube voltage. The Wiener spectrum of CR images with a low tube voltage showed a low value under identical conditions of amount of light stimulated emission. The quality of chest CR images obtained using a lower tube voltage (80 kV and 100 kV) was evaluated as being superior to those obtained with a higher tube voltage (120 kV and 140 kV). Considering the problem of tube loading and exposure in clinical applications, a tube voltage of 90 to 100 kV (0.1 mm copper filter backed by 0.5 mm aluminum) is recommended for chest CR.

Validation of a calculation algorithm for organ doses in the existing CT dosimetry tools using paediatric anthropomorphic phantom measurements

Background: Rapid and user-friendly software tools for individual estimates of organ doses in CT are of the utmost importance to support the professionals in the choice of the appropriate imaging modalities. Most CT-dosimetry tools, like CT Expo [1], CT Dosimetry [2], as well as a new dosimetry tool [3], rely on the assumption that organ doses estimated for one CT scanner can be converted to other CT scanner using the ratio of the Computed Tomography Dose Index (CTDI) between two CT scanners. We tested this assumption for paediatric patients using physical phantom measurements for various protocols and CT scanners. Measurements were compared with dosimetry tools. Materials and methods: Dose to 22 radiosensitive organs were measured using thermoluminescence dosimeters inserted within an anthropomorphic 5-year old phantom. Dose measurements were performed for wholebody scans on 4 scanner models from the 4major manufacturers (Siemens, GE, Philips and Toshiba); both axial and helical modes were tested with small collimations (below 12 mm). Measured doses were compared with the results of Monte-Carlo simulations [3]. Results:Whereas the measured organ doses showed important variability across the different scanner models, doses normalised to CTDI showed significantly less variation. Higher tube voltage also showed less variation. The simulated organ doses reasonably compared with the measurements. For example, at 120 kV in axial mode with 10 mm collimation, the mean coefficient of variation between the scanner models was below 10%; the relative difference between measured and simulated organ doses was within ±15% for most organs. Discussion: For the tested collimations (below 12 mm), both in axial and helical modes, the CTDI fairly accounts for the difference in CT design across different models. Further measurements are being carried out to test the algorithm against protocols more representative of the clinical practice. This study proved that, for small collimations and paediatric patients, organ doses simulated from one CT scanner can be converted to other CT scanners using CTDI-ratio. Those results are of interest for fast dose assessment and retrospective dosimetry studies. Acknowledgments: These measurements were performed within the framework of the EPI-CT study, supported by the European Community's Seventh Framework Programme (FP7/2007e2013) under grant agreement number 269912-EPI-CT: Epidemiological study to quantify risks for paediatric computerized tomography and to optimise doses. [4].

Fabrication of improved multi-slit equipment to obtain the input-output characteristics of computed radiography systems: correction of the heel effect, and application to high tube-voltage experiments

Multi-slit equipment is a new experimental apparatus that can measure the input-output characteristics of a CR (computed radiography) system with limited influence of the fading effect. Kimoto et al. recently proposed a new type of multi-slit apparatus in which the multi-slit setup, the insertion region of the phosphor plate, and plate shielding are integrated to create a single handy-type item (an all-in-one type multi-slit apparatus). However, some problems remained unsolved. The aims of this study were to devise a setup for application to high tube voltage conditions, and to improve the all-in-one type multi-slit equipment so as to correct the heel effect. We examined the capabilities of our improved multi-slit equipment using diagnostic X-ray apparatus and found that it can obtain input-output characteristics with 5% accuracy for tube voltages of 40-140 kV and SID (source to image receptor distances) of 50-200 cm.

Myocardial perfusion imaging using dual-energy computed tomography: a clinical case

Evaluation of myocardial perfusion imaging (MPI) by single-energy computed tomography is limited by beam hardening (BH). Recently, dual-energy CT (DECT) has been introduced for a more reproducible MPI evaluation through the reduction of BH-artefacts by using monochromatic image reconstruction obtained with rapid switching between low and high tube voltage. We present …

CT attenuation measurements are valuable to discriminate pledgets used in prosthetic heart valve implantation from paravalvular leakage

Sutures with polytetrafluorethylene (PTFE) felt pledgets are commonly used in prosthetic heart valve (PHV) implantation. Paravalvular leakage can be difficult to distinguish from PTFE felt pledgets on multislice CT because both present as hyperdense structures. We assessed whether pledgets can be discriminated from contrast-enhanced solutions (blood/saline) on CT images based on attenuation difference in an ex vivo experiment and under in vivo conditions. PTFE felt pledgets were sutured to the suture ring of a mechanical PHV and porcine aortic annulus, and immersed and scanned in four different contrast-enhanced (Ultravist®; 300 mg jopromide ml(-1)) saline concentrations (10.0, 12.0, 13.6 and 15.0 mg ml(-1)). Scanning was performed on a 256-slice scanner with eight different scan protocols with various tube voltage (100 kV, 120 kV) and tube current (400 mAs, 600 mAs, 800 mAs, 1000 mAs) settings. Attenuation of the pledgets and surrounding contrast-enhanced saline were measured. Additionally, the attenuation of pledgets and contrast-enhanced blood was measured on electrocardiography (ECG)-gated CTA scans of 19 patients with 22 PHVs. Ex vivo CT attenuation differences between the pledgets and contrast-enhanced solutions were larger by using higher tube voltages. CT attenuation values of the pledgets were higher than contrast-enhanced blood in patients: 420±26 Hounsfield units (mean±SD, range 383-494) and 288±41 Hounsfield units (range 202-367), respectively. PTFE felt pledgets have consistently higher attenuation than surrounding contrast-enhanced blood. CT attenuation measurements therefore may help to differentiate pledgets from paravalvular leakage, and detect paravalvular leakage in patients with suspected PHV dysfunction.

Reducing the effects of metal artefact using high keV monoenergetic reconstruction of dual energy CT (DECT) in hip replacements

The aim of this study was to determine whether high keV monoenergetic reconstruction of dual energy computed tomography (DECT) could be used to overcome the effects of beam hardening artefact that arise from preferential deflection of low energy photons. Two phantoms were used: a Charnley total hip replacement set in gelatine and a Catphan 500. DECT datasets were acquired at 100, 200 and 400 mA (Siemens Definition Flash, 100 and 140 kVp) and reconstructed using a standard combined algorithm (1:1) and then as monoenergetic reconstructions at 10 keV intervals from 40 to 190 keV. Semi-automated segmentation with threshold inpainting was used to obtain the attenuation values and standard deviation (SD) of the streak artefact. High contrast line pair resolution and background noise were assessed using the Catphan 500. Streak artefact is progressively reduced with increasing keV monoenergetic reconstructions. Reconstruction of a 400 mA acquisition at 150 keV results in reduction in the volume of streak artefact from 65 cm(3) to 17 cm(3) (74 %). There was a decrease in the contrast to noise ratio (CNR) at higher tube voltages, with the peak CNR seen at 70-80 keV. High contrast spatial resolution was maintained at high keV values. Monoenergetic reconstruction of dual energy CT at increasing theoretical kilovoltages reduces the streak artefact produced by beam hardening from orthopaedic prostheses, accompanied by a modest increase in heterogeneity of background image attenuation, and decrease in contrast to noise ratio, but no deterioration in high contrast line pair resolution.

Single-Energy Material Decomposition Using X-Ray Path Length Estimation

Clinical computed tomographies (CTs) can typically use only a single energy at a time. The main purpose of the present paper was to study whether the calculated x-ray path lengths can help replace one of the 2 dual-energy measurements by 2-material decomposition. The proposed single-energy material decomposition method (SEMD) is based on the evaluation of a single CT scan. The SEMD combines postreconstruction and prereconstruction algorithms for the determination of x-ray path length and material decomposition, respectively. The simulation results of the proposed and dual-energy methods were compared using pregenerated look-up tables. The results show that SEMD is more sensitive to CT signal errors at higher tube voltages. The dual-energy method is generally less sensitive to CT signal bias but more sensitive to the noise. In the case of inferior signal errors, the proposed method gives the same results as the dual-energy variant. Although the x-ray path length estimation method with SEMD is more complex, the dose is considerably lower.

Predicting and Suppressing Oversensing of a Pacemaker in Plain X-Ray Photography

We conducted experiments of oversensing generation of pacemaker (PM) and X-irradiation direction dependency of PM, and examined the oversensing suppression method, using 8 different types of PMs. It was found out from this experiment that oversensing would occur when some conditions (X-irradiation direction, X-irradiation intensity) are met. Oversensing occurred with the most low irradiation conditions (kV × mA) when PM was irradiated at 90° (vertically to C-MOS; Complementary Metal Oxide Semiconductor). The acuter the angle of irradiation is (α > 90° < α), the higher the irradiation conditions (kV × mA) at which oversensing start to occur. In plain X-ray photography, oversensing was confirmed under the irradiation conditions of (cervical spine, thoracic spine, lateral thoracic spine, rib, shoulder joint, collarbone, humerus, and chest).Once the irradiation angle and irradiation conditions (kV × mA) are available, oversensing is predictable to some extent. Our findings will help to predict oversensing generation of plain X-ray photography and suppress oversensing. Oversensing can be suppressed in most of the radiography by lowering tube current to 100 mA, but a 1.0 mm High-Density Tungsten Sheet must be put on PM in high tube voltage radiography.

Investigation of the effect of tube voltage and imaging geometry on phase contrast imaging for a micro-CT system

Based upon a bench-top micro-CT system, propagation-based phase-contrast imaging has been investigated using insects and a thin plastic sheet. The system mainly includes a micro-focus source with focal spot size of 13–20 μm and a cooled X-ray CCD detector with pixel size of 24 μm. The edge-enhancement effect can be found clearly in the acquired images. With a 0.5 mm thickness plastic edge phantom, the effects of X-ray tube voltage and imaging geometry on the phase-contrast imaging were investigated, and quantitative index, edge-enhancement index (EEI), were also calculated. In our study, an interesting phenomenon was observed that the phase-contrast effect becomes more pronounced as the tube voltage increases from 20 kVp to 90 kVp. Further investigation indicates that smaller focal spot size resulting from the reduction of tube current at higher tube voltage, has caused the unexpected phenomenon. Inferred from our results, phase-contrast effect is insensitive to the tube voltage in the range of 20–90 kVp (widely used in medical diagnosis); however, it is sensitive to the focal spot size. In addition, for the investigation of the effect of imaging geometry, an optimal geometric magnification range of 2.5–4.5 is suggested to get a good phase-contrast imaging for a micro-CT system with source-to-detector distance of 720 mm.

Potential of high‐Z contrast agents in clinical contrast‐enhanced computed tomography

Currently, only iodine- and barium-based contrast media (CM) are used in clinical contrast-enhanced computed tomography (CE-CT). High-Z metals would produce a higher contrast at equal mass density for the x-ray spectra used in clinical CT. Using such materials might allow for significant dose reductions in CE-CT. The purpose of this study was to quantify the potential for dose reduction when using CM based on heavy metals. The contrast-to-noise ratio weighted by dose (CNRD) was determined as a function of scan protocol by means of measurements and simulations on a clinical CT scanner. For simulations, water cylinders with diameters 160, 320, 480, and 640 mm were used to cover a broad range of patient sizes. Measurements were conducted with 160 and 320 mm water-equivalent plastic cylinders. A central bore of 13 mm diameter was present in all phantoms. The tube voltage was varied from 80 to 140 kV for measurements and from 60 to 180 kV for simulations. Additional tin filtration of thicknesses 0.4, 0.8, and 1.2 mm was applied in the simulation to evaluate a range of spectral hardness. The bore was filled with a mixture of water and 10 mg/ml of pure iodine, holmium, gadolinium, ytterbium, osmium, tungsten, gold, and bismuth for the simulations and with aqueous solutions of ytterbium, tungsten, gold, and bismuth salts as well as Iopromid containing 10 mg/ml of the pure materials for the measurements. CNRDs were compared to iodine at phantom size-dependent reference voltages for all high-Z materials and the resulting dose reduction was calculated for equal contrast-to-noise ratio. Dose reduction potentials strongly depended on phantom size, spectral hardness, and tube voltage. Depending on the added filtration, a dose reduction of 19%-60% could be reached at 80 kV with gadolinium for the 160 mm phantom, 52%-69% at 100 kV with holmium for the 320 mm phantom, 62%-78% with 120 kV for hafnium and the 480 mm phantom and 74%-86% with 140 kV for gold and the 640 mm phantom. While gadolinium might be considered at 160 mm diameter, hafnium showed the best overall performance for phantom sizes of 320 mm and above. The measurements conducted on the clinical CT scanner showed very good agreement with simulations with deviations in the order of 5 to 10%. The results of this study encourage the development and use of CM based on high-Z materials, especially for adipose patients, where high tube voltages are necessary to reach sufficiently short scan times. Hafnium proved to be the best compromise for average-size and for adipose patients. Even higher-Z materials such as gold and bismuth showed a good overall performance in conjunction with high tube voltage, large patients or strong added filtration and may be recommended for scans under these conditions.

Comparison of anti-scatter grids for digital imaging with use of a direct-conversion flat-panel detector

Our purpose in this study was to establish a selection standard for anti-scatter grids for a direct conversion flat-panel detector (FPD) system. As indices for grid evaluation, we calculated the selectivity, Bucky factor, and the signal-to-noise ratio improvement factor (SIF) by measuring rates of scatter transmission, primary transmission, and total transmission (based on the digitally displayed measurement values of the FPD system), using 4 acrylic phantoms of different thicknesses. The results showed that the SIF was less than 1.0 when the phantom thickness was 5 cm. When the phantom thickness was 25 cm and the grid ratio was 16:1, the SIF was 1.505 and 1.518 (maximum value) at 90 and 120 kV, respectively. Compared with the grid ratio of 12:1, the SIF at the grid ratio 16:1 was improved by 6.1% at 90 kV, and by 7.0% at 120 kV. In a direct-conversion FPD system, the grid ratio of 16:1 is considered adequate for eliminating the scattered-radiation effect when much scattered radiation is present, such as with a thick imaged object or a high X-ray tube voltage.

Feasibility of bone density evaluation using plain digital radiography

For the radiographic evaluation of subchondral bone changes (sclerosis) in osteoarthritis (OA), bone density (BD) is commonly subjectively assessed. BD evaluation using plain digital radiography might be influenced by acquisition and post-processing (PP) settings. Objective of this study was to evaluate the effects of these settings on the measurement of BD using digital radiographs. A bone density standard (BDS) of hydroxyapatite (HA) mimicked a BD range of 1.0-5.75 g/cm(2). Digital radiographs were acquired with variation in acquisition settings, and with clinical and minimal PP. An aluminum step wedge served as an internal reference to express the gray values of the BDS in mm aluminum equivalents (mmAl). The relation (R(2)) between actual BD and BD normalized to the reference wedge was evaluated with linear regression analyses for radiographs with variations in PP and acquisition settings. Precision of BD measurement of the BDS was evaluated for application in clinical practice. The correlation between actual BD and BD normalized to the reference was improved by changing PP from clinical (R(2)=0.96) to minimal (R(2)=0.98). Higher tube voltage [kilovolt (kV)] improved the correlation further. Even for clinical PP, average standard deviation (SD) was 0.97 mmAl, much smaller than the change of 2.51 mmAl clinically observed in early OA, which implies the feasibility of BD measurements on digital radiographs. Changing PP and acquisition settings in clinical practice can have profound effect on outcome. If done with care, accurate BD measurement is feasible using plain digital radiography.

Quantitative analysis of metallic artifacts caused by dental metals: comparison of cone-beam and multi-detector row CT scanners

To quantitatively compare the streak artifacts produced by dental metals in a cone-beam computed tomography (CBCT) device and a multi-detector row computed tomography (MDCT) scanner in relation to metal types and imaging parameters. Cubes of aluminum, titanium, cobalt–chromium alloy, and type IV gold alloy were scanned with CBCT and MDCT scanners at tube voltages of 80 and 100 peak kV (kVp), and currents of 100 and 170 mAs by MDCT, and 102 and 170 mAs by CBCT. Artifact areas were quantified using ImageJ software. Artifact areas for the same metals and imaging parameters were smaller with CBCT than with MDCT under most conditions. Type IV gold alloy caused the largest artifact areas, followed by cobalt–chromium alloy, titanium, and aluminum, respectively. Higher tube voltage was associated with smaller artifact areas under most conditions, whereas increasing tube current had no consistent effect on artifact area using either CT device. CBCT was associated with smaller artifact areas than MDCT for the same parameters. Type IV gold alloy produced the largest artifact areas among the tested metals, but metallic artifacts could be reduced by increasing the tube voltage.

Performance simulation of an x-ray detector for spectral CT with combined Si and Cd[Zn]Te detection layers

The most obvious problem in obtaining spectral information with energy-resolving photon counting detectors in clinical computed tomography (CT) is the huge x-ray flux present in conventional CT systems. At high tube voltages (e.g. 140 kVp), despite the beam shaper, this flux can be close to 109 Mcps mm−2 in the direct beam or in regions behind the object, which are close to the direct beam. Without accepting the drawbacks of truncated reconstruction, i.e. estimating missing direct-beam projection data, a photon-counting energy-resolving detector has to be able to deal with such high count rates. Sub-structuring pixels into sub-pixels is not enough to reduce the count rate per pixel to values that today's direct converting Cd[Zn]Te material can cope with (⩽10 Mcps in an optimistic view). Below 300 µm pixel pitch, x-ray cross-talk (Compton scatter and K-escape) and the effect of charge diffusion between pixels are problematic. By organising the detector in several different layers, the count rate can be further reduced. However this alone does not limit the count rates to the required level, since the high stopping power of the material becomes a disadvantage in the layered approach: a simple absorption calculation for 300 µm pixel pitch shows that the required layer thickness of below 10 Mcps/pixel for the top layers in the direct beam is significantly below 100 µm. In a horizontal multi-layer detector, such thin layers are very difficult to manufacture due to the brittleness of Cd[Zn]Te. In a vertical configuration (also called edge-on illumination (Ludqvist et al 2001 IEEE Trans. Nucl. Sci. 48 1530–6, Roessl et al 2008 IEEE NSS-MIC-RTSD 2008, Conf. Rec. Talk NM2-3)), bonding of the readout electronics (with pixel pitches below 100 µm) is not straightforward although it has already been done successfully (Pellegrini et al 2004 IEEE NSS MIC 2004 pp 2104–9). Obviously, for the top detector layers, materials with lower stopping power would be advantageous. The possible choices are, however, quite limited, since only ‘mature’ materials, which operate at room temperature and can be manufactured reliably should reasonably be considered. Since GaAs is still known to cause reliability problems, the simplest choice is Si, however with the drawback of strong Compton scatter which can cause considerable inter-pixel cross-talk. To investigate the potential and the problems of Si in a multi-layer detector, in this paper the combination of top detector layers made of Si with lower layers made of Cd[Zn]Te is studied by using Monte Carlo simulated detector responses. It is found that the inter-pixel cross-talk due to Compton scatter is indeed very high; however, with an appropriate cross-talk correction scheme, which is also described, the negative effects of cross-talk are shown to be removed to a very large extent.

Evaluation of Scattering in Cone-Beam Breast Computed Tomography: A Monte Carlo and Experimental Phantom Study

In this paper Monte Carlo simulations were performed for X-ray irradiations of breast phantoms of various sizes such as PMMA cylinders of different diameters and a hemi-ellipsoidal PMMA phantom. The aim was the evaluation of the 2D distribution of primary and scattered photons and Scatter-to-Primary Ratio (SPR) in projection images in cone-beam breast Computed Tomography (CT). Irradiation geometry and technique factors reproduce the experimental conditions used for validation measurements with a prototype CT breast scanner. Simulations were performed with GEANT4 software. We varied the phantom diameter and shape, the X-ray tube voltage and added filtration. Magnification was 1.31. SPR increases from 0.4 (at 8 cm cylinder diameter) up to 1.5 (14 cm cylinder diameter) at the centre of the phantom. In the same phantom, SPR has lower values toward the bases of the cylinder than at its centre. The scatter component increases by adopting a 50 kVp or higher tube voltages, up to 80 kVp, and by increasing the added filtration. Simulated and measured lateral profiles across a 14 cm cylinder diameter in projection images show a relative deviation of 4%. Simulations show a different distribution of scatter and SPR in a 14 cm diameter cylinder and 14 cm hemi-ellipsoidal phantom, so questioning the use of simple cylindrical geometries when simulating the attenuation of the pendant breast for scatter correction procedures. The strength and the non-uniformity of the SPR inside the cylindrical phantom decrease as the size of the air gap between object and detector increases.