Air Gap Technique Research Articles

Dose–image quality study in digital chest radiography using Monte Carlo simulation

One of the main preoccupations of diagnostic radiology is to guarantee a good image-sparing dose to the patient. In the present study, Monte Carlo simulations, with MCNPX code, coupled with an adult voxel female model (FAX) were performed to investigate how image quality and dose in digital chest radiography vary with tube voltage (80–150 kV) using air-gap technique and a computed radiography system. Calculated quantities were normalized to a fixed value of entrance skin exposure (ESE) of 0.0136 R. The results of the present analysis show that the image quality for chest radiography with imaging plate is improved and the dose reduced at lower tube voltage.

News Briefs

IBM researchers have developed a technique that creates vacuum spaces, called airgaps, between the wires in processors using self-assembling technology. These airgaps create a form of insulation that increases the chips' performance up to 35 percent or reduces their power consumption up to 15 percent. The airgap technique lowers the chip insulator's dielectric constant, which describes a material's ability to transmit charge when a voltage is applied. Because the insulator would transmit less charge, the current introduced to other circuits would drop. Vacuums are very good insulators because they don't transmit charge well. Using airgaps between wires can lower a chip insulator's relative dielectric level to a bit below 2.0.

Application of contrast-to-noise ratio in optimizing beam quality for digital chest radiography: comparison of experimental measurements and theoretical simulations

The contrast-to-noise ratio (CNR) has been employed in optimizing beam quality for imaging a chest phantom using digital radiography. The relationship between CNR and tube potential has been studied for regions of different attenuations representing the lung, heart and abdomen, and a figure of merit (FOM) incorporating effective dose has been calculated to enable dose performance to be included. Direct measurements of imaging performance have been compared with simulations based on a model representing object attenuations. The study has shown reasonable agreement between measurements of CNR and calculated values. The CNR values in the lung and heart regions are higher at 60–80 kV, while those for the abdomen are higher at 90–110 kV. Incorporating a 0.2 mm copper filter has minimal effect on image quality and the FOM is higher because of the reduction in dose. For imaging the heart and abdomen, performance was improved through use of a technique to remove scatter, with an air-gap technique giving a higher FOM because of the lower dose. The CNR and FOM can provide useful quantities for evaluating imaging performance to optimize beam quality for different imaging tasks.

Towards optimization in digital chest radiography using Monte Carlo modelling

A Monte Carlo based computer model of the x-ray imaging system was used to investigate how various image quality parameters of interest in chest PA radiography and the effective dose E vary with tube voltage (90–150 kV), additional copper filtration (0–0.5 mm), anti-scatter method (grid ratios 8–16 and air gap lengths 20–40 cm) and patient thickness (20–28 cm) in a computed radiography (CR) system. Calculated quantities were normalized to a fixed value of air kerma (5.0 µGy) at the automatic exposure control chambers. Soft-tissue nodules were positioned at different locations in the anatomy and calcifications in the apical region. The signal-to-noise ratio, SNR, of the nodules and the nodule contrast relative to the contrast of bone (C/CB) as well as relative to the dynamic range in the image (Crel) were used as image quality measures. In all anatomical regions, except in the densest regions in the thickest patients, the air gap technique provides higher SNR and contrast ratios than the grid technique and at a lower effective dose E. Choice of tube voltage depends on whether quantum noise (SNR) or the contrast ratios are most relevant for the diagnostic task. SNR increases with decreasing tube voltage while C/CB increases with increasing tube voltage.

Optimisation of scoliosis examinations in children.

To present results of optimising scoliosis examination by changing from a conventional film/grid (F/G) to air-gap technique using computed radiography (CR), and to evaluate different methods for estimating effective radiation doses. Forty-nine children and adolescents were examined with an F/G technique, and 21 with air-gap and CR techniques. Entrance surface doses (ESD) were determined with lithium fluoride thermoluminescence dosimeters. For all patients, the effective radiation doses were determined using a hermaphrodite PCXMC computer program. For all F/G radiographs, the effective doses were also determined according to the NRPB-R279 report, and for 22 children (>9 years and/or >40 kg) also with the ODS-60 program, which allows separate gender calculations. Accumulated doses for 37 children examined more than once with F/G examinations were assessed. For F/G techniques, the ESDs for both frontal and lateral views varied with age and were significantly correlated to the patients' thickness. The calculated effective doses using the PCXMC program and the NRPB-R279 did not differ significantly for all frontal and lateral radiographs, respectively, but ODS-60 gave significantly higher values in female subjects. With air-gap and CR techniques, the mean effective doses were reduced by a factor over 10. The mean accumulated effective dose for 37 children with a mean of seven F/G examinations was 6.1 mSv, implying a risk of death of about 1:2,000 for boys and at least 1:1,000 for girls. Paediatric scoliosis radiography should be considered a specialised procedure, which has to be optimised using a non-grid technique.

Multitapered x-ray capillary optics for mammography.

X-ray mammography is currently the primary tool used for breast cancer detection. However, studies have shown that 5%-15% of breast cancers are not visualized mammographically. The long term goal of this project is to improve the x-ray mammographic imaging system using capillary optics. A post-patient capillary optic lens has the potential to increase spatial resolution and eliminate the detection of scattered x rays, thereby improving image contrast and the signal-to-noise ratio (SNR). Several individual and two prototype multitapered optics were studied to determine the feasibility of a full-field multitapered optic. Scatter fraction, contrast, transmission, uniformity, and the modulation transfer function (MTF) were measured for a Mo target tube/computed radiography (CR) imaging system when this prototype was applied. The results were compared with standard grid and airgap techniques. The multitapered optic lens removed 85% of the scattered photons as compared to 66% and 39% for the air gap and grid methods, respectively. This resulted in an improvement of contrast by approximately 80% for the optics, 51% for the air gap, and 30% for grid methods. The single optic lens improved the limiting resolution (5% MTF level) of the CR detector by 78% due to magnification with very little focal spot blurring, while the multitapered prototype improved resolution significantly, but not as much as the single optic. These measurements have shown that it is feasible to create a multitapered optic lens that significantly improves system MTF and virtually eliminates scatter. With continued improvements in fabrication techniques, a full-field multitapered lens will be feasible.

Impedance spectroscopy study of hardened Portland cement paste

In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste–electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.

The Influence of Different Technique Factors on Image Quality for Chest Radiographys: Application of the Recent CEC Image Quality Criteria

The aim of the first this work part of the EU-project, Trial I, was to evaluate and possibly improve the CEC image criteria for radiographic chest images. Chest images of healthy volunteers were acquired using different technique factors. The image criteria were used as a tool to discriminate between the different images. The technique factors were chosen so that the image quality would differ slightly. Four different technique parameters, each with two possible settings, used in clinical practice today, were used: tube voltage - 102 and 141 kV; screen/film speed - 160 and 320; maximum optical density in the parenchyma - 1.3 and 1.8; method for scatter reduction - air gap 30/390 and moving grid40/12. The results showed that the image criteria were able to separate between different technique groups. Some conclusions can be drawn from the results Optical density 1.8 was better than 1.3 independent of the other parameters. . Among the six combinations ranked best , four used tube voltage 141 kV and four used air gap technique for scatter reduction. No difference was seen for screen/film speed. No correlation was seen between the ranking of the systems and patient dose. (Less)

Air-gap remeshing technique for rotating machines in 3D finite element modeling

We present a 3D air-gap remeshing technique for rotating machines, which permits independent meshing of rotor and stator to facilitate machine modeling. Comparison of torque and eddy current analysis results for a brushless motor with the results obtained by the conventional lock-step technique validates the air-gap remeshing technique.

Cement paste hardening process studied by impedance spectroscopy

Impedance Spectroscopy measured in the range of 15 MHz to 10 kHz has been used to monitor the hardening process of cement paste with different water over cement (w:c) ratios. The cement used was of commercial grade portland cement containing pozzolans, according to the Spanish standard CEM IV/A 32.5 MR, a Portland cement containing pozzolans. The influence of sample/electrode interfacial phenomena in the impedance spectra has been eliminated by introduction of a non-contact air gap technique. The impedance spectra obtained were interpreted according to the Debye model for dielectric relaxation. The Nyquist plots of the complex permittivity showed symmetrical dispersion (Cole–Cole type) in the first 15 hardening days while asymmetrical dispersion (Cole–Davidson type) was observed later on. The calculated dielectric constant decreases with as w:c ratio increases, which confirms its relation to the variation in solid phase fraction by volume. The evolution of the fitting parameters can be explained in terms of hardening process of cement paste.

Radiation Dose and Image Quality in Neuroangiography: Effects of Increased Tube Voltage, Added X-Ray Filtration and Antiscatter Grid Removal

During endovascular treatment the patient may be subject to fluoroscopy for long periods as well as multiple x-ray exposures. The radiation dose to the patient can be considerable, and cause local deterministic effects such as alopecia or even skin burn. The potential carcinogenic effects should also be noted, being especially important in the paediatric population. We measured radiation doses to patients and personnel during neuroendovascular procedures and diagnostic neuroangiography. We also tried to reduce the radiation dose to the patient utilising increased tube voltage, additional primary X-ray filtration and by removing the antiscatter grid in front of the image intensifier, employing air gap technique. We investigated radiation doses to patient and personnel during neuroangiographic procedures and optimized the examination technique with regard to radiation dose with maintained image quality. Radiation exposure to patients and personnel was measured with thermoluminescent dosimeters and permanently mounted KermaDoseArea-Product meters in front of the X-ray tubes during 13 cerebral angiographies and six neuroendovascular procedures. We performed experiments with radiation dose measurements and evaluation of image quality with 80 and 90 kV tube voltage during image acquisition and 75 and 85 kV during fluoroscopy, as well as with different primary X-ray filtration. Images from patient studies acquired with the original grid in front of the image intensifier were compared with images from patient studies acquired with the grid removed and air gap technique (30 cm). Images from patient studies acquired with the original examination technique were compared to images from patient studies acquired with increased x-ray tube voltage, increased x-ray filtering and with the antiscatter grid removed using an airgap as scatter reduction method. Radiation exposure to personnel was very low using standard protective devices. Measurable doses were recorded only on the hands and forehead of the neuroradiologist. Maximum entrance skin dose was about 1 Gy on the side of the patientspatient's forehead during an endovascular procedure. Increasing the tube voltage from 75 to 85 - 85 and 90 kV, exchanging the original 0.5 mm aluminium primary filtration for 0.2 mm copper and removing the antiscatter grid allowed us to reduce entrance skin dose to the patient by 70% with unchanged or slightly improved image quality.

Reduction of inrush current in single-phase transformer using virtual air gap technique

In this paper the authors present a technique based on the use of a virtual air gap in which equivalent thickness varies as a function of controllable parameters adapted to the configuration of the magnetic circuit and the associated control system. The principle is presented using a magnetic circuit similar to the transformer one. Among the numerous applications, this technique can be applied to reduce inrush current in single-phase transformers.

Trends in the use of pelvimetry techniques

Assessment of maternal pelvic dimensions is usually considered necessary where vaginal delivery is contemplated in a breech presentation or if reduced pelvic dimensions are suspected in a current or previous pregnancy. Pelvimetry techniques include computed tomography (CT), conventional radiography, digital fluorography and magnetic resonance imaging (MRI). The first three techniques result in a radiation dose to mother and fetus which, depending on how the technique is performed, can vary by up to 40-fold. Of the techniques using X-rays, CT pelvimetry with a lateral scanogram generally gives the lowest radiation dose and conventional radiography using an air gap technique with a single lateral view is a relatively low-dose alternative where CT is not available. A questionnaire was sent to 227 hospitals during 1993 and 1996 to assess whether there was a move towards lower dose techniques of pelvimetry. The results show a trend away from conventional pelvimetry (48.4% in 1993 to 28% in 1996) with a small proportion of centres using MRI (4%) in 1996. Of the centres still using conventional pelvimetry, relatively few were using a low-dose air-gap technique (2.1% in 1993 to 10.9% in 1996). An increasing majority of centres were using one-view CT (69.3% in 1993 and 80.4% in 1996) but a significant proportion were still performing more than one view. This study shows that there was a move towards lower dose techniques of pelvimetry but that there were still many hospitals that had not implemented a policy of reducing radiation exposure in these patients.

The effect of tetraethylammonium and 4-aminopyridine on mechano- and electrosensitive channels of the Pacinian corpuscle

The action of tetraethylammonium (TEA) (20 mM) and 4-aminopyridine (4-AP) (5 mM) on mechano- and electrically excitable membranes of the Pacinian corpuscles was studied by means of the air gap technique in constantly perfused preparations. Extracellular recordings of receptor potential have shown that application of TEA causes 1.5-fold prolongation of the receptor potential and a decrease of its amplitude by 40%. 4-AP has no effect on the mechanosensitive membrane. The difference in the blockers effect testifies that repolarization of the receptor potential is not regulated by the voltage-dependent potassium channels. The blockers considered are likely effect on the calcium-dependent potassium currents activated by Ca 2+ ions passing through the non-selective channels opening in response to mechanical stimulus. TEA and 4-AP induce a 2- to 3-fold increase of the action potential duration in the electrically excitable membrane of the Ranvier node. Computations based on the Dodge model help reveal that inhibition of the voltage-dependent potassium channels accounts for this increase.

X-ray pelvimetry — Which is the best technique?

Aim: To provide definitive data on the radiation doses given to both mother and fetus using different pelvimetry techniques at a number of centres. This data was combined with an assessment of image quality, to determine the optimum technique. Patients and Methods: Multiple thermoluminescent dosimeters (TLDs) were used to measure patient and fetal doses inside an anthropomorphic phantom of a pregnant women at full term. Measurements were made at twenty hospitals that routinely perform pelvimetry using techniques including computed tomography (CT), conventional radiography, conventional radiography using an air-gap technique, and digital fluorography. To assess image quality the images obtained by the different techniques were reviewed by a panel of two radiologists and a senior radiographer. Results: A wide range of absorbed doses and relative risks to mother and fetus was observed. The largest and smallest doses encountered in the study differed by a factor of approximately forty. CT pelvimetry was seen to give an average of 25% of the dose of conventional pelvimetry, and at some centres dose savings of over 90% would be possible by performing lateral CT scanograms in preference to plain film pelvimetry. However, a surprisingly wide variation in doses was observed between centres performing CT pelvimetry, and in some cases the potential dose saving would be small. An air-gap technique practised by one of the centres in the survey was found to give absorbed doses and relative risks comparable to average CT scanograms. A digital fluorography technique also gave a very low dose at one centre. However, a similar technique performed at another of the centres conferred little benefit over conventional pelvimetry. There was no observable correlation between the dose administered to the patient and image quality for any of the techniques, and CT and conventional pelvimetry both gave similar image quality. Conclusion: While CT deserves its reputation as a low dose technique for pelvimetry, in some cases there may be little dose saving over conventional techniques. The total mAs from the CT examination should be kept as low as possible. Low absorbed doses with good image quality may also be achieved without specialized equipment using an air gap technique. This method is highly recommended as an alternative to CT, for example if no CT scanner is available.

Scatter rejection by air gaps in diagnostic radiology. Calculations using a Monte Carlo collision density method and consideration of molecular interference in coherent scattering

The air gap technique is an old method for scatter rejection. It is still used in lung examinations and may be reconsidered for use in digital radiography. Using magnification techniques, for example in mammography, the air gap thereby introduced simultaneously yields scatter rejection. A Monte Carlo collision density method is exploited to investigate the physical parameters relevant to this technique. Radiation quantities of scattered photons at points behind a water slab both on and laterally displaced from the central axis are calculated and their dependence on field area, slab thickness, air gap length and detector type is derived. Values of `scatter-to-primary' ratios of the plane energy fluence (the energy imparted to a totally absorbing detector) are given for perpendicularly incident 30, 70 and 130 kV energy spectra, slab thicknesses of 0.05 and 0.2 m (30 kV: 0.05 m), air gaps of length 0.002 - 1.0 m and field areas from to . Contrast degradation factors are derived for both totally absorbing and thin detectors. The influence on the scatter-to-primary ratios of using divergent instead of parallel beams and of neglecting molecular interference in coherent scattering is analysed.

Computed Radiography in Scoliosis

The diagnostic information and radiation dose in scoliosis examinations performed with air-gap technique using stimulable phosphor imaging plates were determined in a prospective study. Overlapping p.a. images of the thoracic and lumbar spine in 9 patients were obtained with 4 different exposure settings according to patient size. Equal exposure settings were used for the 2 images. Two images of 18 were judged inferior in depicting the landmarks of scoliosis measurement, requiring re-exposure. Sixteen images were judged of adequate or good quality. The mean entrance doses in the central beam for the 4 patient groups were in the interval of 0.05 to 0.12 mGy for both images. The skin doses on the breasts were in the range of 0.00 to 0.03 mGy. The presented technique thus results in a low radiation dose with sufficient diagnostic information in radiography of scoliosis.

217.CRシステムにおけるAir Gap法の有用性 : 胸部遠距離撮像法における検討

217.CRシステムにおけるAir Gap法の有用性 : 胸部遠距離撮像法における検討

376. Air gap technique を併用した CR システムの応用(CR 被曝低減)

376. Air gap technique を併用した CR システムの応用(CR 被曝低減)

Monte Carlo study of grid performance in diagnostic radiology: factors which affect the selection of tube potential and grid ratio

A Monte Carlo computational model has been developed for the study of the performance of anti-scatter grids in diagnostic radiology. It is used here to estimate the scatter in the image plane from soft tissue phantoms (representing the patient) and to calculate image contrast and the mean absorbed dose in the phantom. Different scattering conditions, representative of various examinations, have been investigated: adult lumbar spine; small field radiography and fluoroscopy; adult chest and paediatric pelvis and chest. For each scattering condition, the combinations of tube potential and grid ratio have been found which, for a well designed grid, result in the lowest mean absorbed dose in the phantom for a fixed contrast level. In examinations which generate large amounts of scatter, the use of high grid ratios in combination with high tube potentials is favourable with regard to both mean absorbed dose in the phantom and tube charge. When less scatter is generated, either the grid ratio or the tube potential can be varied to achieve the desired contrast level. High grid ratios require shorter exposure times, but need careful alignment in the beam to prevent primary radiation cut-off. It is shown that the air gap technique can be used to reduce patient dose in examinations with small amounts of scatter, but in combinations with a lower tube potential than when a grid is used.