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

Advancing Safety: Role of Equipment Design and Configuration Change in Pediatric Fluoroscopy

Diagnostic x-ray exams irradiate the patient to produce an x-ray pattern in space, which is captured and processed into a visible image, followed by clinical interpretation. Diagnostic-quality images at a well-managed radiation dose are required. Improvements to image receptors and image processing algorithms have resulted in improved images at reduced dose levels. However, careful management of x-ray production via design or configuration changes of the imaging device also affect patient dose. This initial dose management step is the focus of this discussion. Imaging equipment vendors, in general, have produced quality images of adults at reasonably managed patient doses. This achievement required teamwork between leading adult hospital staff members and representatives of the imaging equipment vendor within the adult hospital. Most manufacturers have had less opportunity to develop similar optimized configurations for pediatric imaging, the imaging of patients ranging from 2-200 kg between 0-21 y of age. Challenges: The wider dynamic range of patient thicknesses in the pediatric size range compared to the adult range of only 45-140 kg challenges automatic control features. In recent years reduction of patient dose as opposed to proper management has been stressed. The principal objectives of end users and vendors, respectively, are patient care and the bottom line. This too often hampers (if not prevents) a productive working relationship between the vendor and the end user. Too many end users rely too heavily on their equipment vendor to solve imaging/dose concerns. The vendors have an important role to play in this challenge but should not be the sole solution. Qualified medical physicists need an understanding of the design of the imaging device, an understanding that many vendors do not support out of proprietary concerns. The performance of the equipment should be judged based on data acquired with better tools. Solutions: Multiple equipment configurations are needed-each designed to excel at a reduced size of patients. Dose reductions that significantly impact image quality must be rejected. Radiologists and their qualified medical physicists should develop target patient doses (size based) for their unique imaging equipment and preferred level of quantum mottle. Once target doses are established as a function of patient size, vendor application specialists and design engineers should leverage the equipment's strengths and weaknesses to best achieve desired results. The qualified medical physicist should function as an interpreter between the end user and the vendor's design engineers. Are the radiologists' and technologists' expectations of the vendor reasonable and vice versa? While better tools are being developed, vendors may hesitate to make them available or charge excessive dollars for these new features to further slow their adoption. Conclusion: The challenges and solutions require the radiologist, technologist, qualified medical physicist, and vendor representatives to work as a team to manage patient dose and maintain image quality. The installed imaging device will be only as successful as the working relationship between the parties. These challenges and conflicts must continually be overcome to provide the best patient care.

On image quality metrics and the usefulness of grids in digital mammography

Antiscatter grids are used in digital mammography to reduce the scattered radiation from the breast and improve image contrast. They are, however, imperfect and lead to partial absorption of primary radiation, as well as failing to absorb all scattered radiation. Nevertheless, the general consensus has been that antiscatter grids improve image quality for the majority of breast types and sizes. There is, however, inconsistency in the literature, and recent results show that a substantial image quality improvement can be achieved even for thick breasts if the grid is disposed of. The purpose of this study was to investigate if differences in the considered imaging task and experimental setup could explain the different outcomes. We estimated the dose reduction that can be achieved if the grid were to be removed as a function of breast thickness with varying geometries and experimental conditions. Image quality was quantified by the signal-difference-to-noise ratio (SDNR) measured using an aluminum (Al) filter on blocks of poly(methyl methacrylate) (PMMA), and images were acquired with and without grid at a constant exposure. We also used a theoretical model validated with Monte Carlo simulations. Both theoretically and experimentally, the main finding was that when a large [Formula: see text] Al filter was used, the SDNR values for the gridless images were overestimated up to 25% compared to the values for the small [Formula: see text] filter, and gridless imaging was superior for any PMMA thickness. For the small Al filter, gridless imaging was only superior for PMMAs thinner than 4 cm. This discrepancy can be explained by a different sensitivity to and sampling of the angular scatter spread function, depending on the size of the contrast object. The experimental differences were eliminated either by using a smaller region of interest close to the edge of the large filter or by applying a technique of scatter correction by subtracting the estimated scatter image. These results explain the different conclusions reported in the literature and show the importance of the selection of measurement methods. Since the interesting structures in mammography are below the 1-cm scale, we advocate the use of smaller contrast objects for assessment of antiscatter grid performance.

Grid removal and impact on population dose in full-field digital mammography

The study purpose was to determine the impact of anti-scatter grid removal on patient dose, in full field digital mammography. Dose saving, phantom based, was evaluated with the constraint that images acquired with and without grid would provide the same contrast-to-noise ratio (CNR). The digital equipment employed a flat panel detector with cesium iodide for x-ray to light conversion, 100 microm pixel size; the x-ray source was a dual-track tube with selectable filtration. Poly(methyl-emathocrylate) (PMMA) layers in the range 20-70 mm were used to simulate the absorption of different breast thickness, while two Al foils, 0.1 and 0.2 mm thick were used to provide a certain CNR. Images with grid were acquired with the same beam quality as selected in full automatic exposure mode and the mAs levels as close as possible, and the CNR measured for each thickness between 20 and 70 mm. Phantom images without grid were acquired in manual exposure mode, by selecting the same anode/filter combination and kVp as the image with grid at the same thickness, but varying mAs from 10 to 200. For each thickness, an image without aluminum was acquired for each mAs value, in order to obtain a flat image to be used to subtract the scatter nonuniformity from the phantom images. After scatter subtraction, the CNR was measured on images without grid. The mAs value that should be set to acquire a phantom image without grid so that it has the same CNR as the corresponding grid image was calculated. Therefore, mAs reduction percentage was determined versus phantom thickness. Results showed that dose saving was lower than 30% for PMMA equivalent breast thinner than 40 mm, decreased below 10% for intermediate thickness (45-50 mm), but there was no dose gain for thickness beyond 60 mm. By applying the mAs reduction factors to a clinical population derived from a data base of 4622 breasts, dose benefit was quantified in terms of population dose. On the average, the overall dose reduction was about 8%. It was considered small, not sufficient to justify a clinical implementation, and the anti-scatter grid was maintained.

Optimisation in general radiography

Radiography using film has been an established method for imaging the internal organs of the body for over 100 years. Surveys carried out during the 1980s identified a wide range in patient doses showing that there was scope for dosage reduction in many hospitals. This paper discusses factors that need to be considered in optimising the performance of radiographic equipment. The most important factor is choice of the screen/film combination, and the preparation of automatic exposure control devices to suit its characteristics. Tube potential determines the photon energies in the X-ray beam, with the selection involving a compromise between image contrast and the dose to the patient. Allied to this is the choice of anti-scatter grid, as a high grid ratio effectively removes the larger component of scatter when using higher tube potentials. However, a high grid ratio attenuates the X-ray beam more heavily. Decisions about grids and use of low attenuation components are particularly important for paediatric radiography, which uses lower energy X-ray beams. Another factor which can reduce patient dose is the use of copper filtration to remove more low-energy X-rays. Regular surveys of patient dose and comparisons with diagnostic reference levels that provide a guide representing good practice enable units for which doses are higher to be identified. Causes can then be investigated and changes implemented to address any shortfalls. Application of these methods has led to a gradual reduction in doses in many countries.

Properties of preprocessed sinogram data in x-ray computed tomography

The accurate determination of x-ray signal properties is important to several computed tomography (CT) research and development areas, notably for statistical reconstruction algorithms and dose-reduction simulation. The most commonly used model of CT signal formation, assuming monoenergetic x-ray sources with quantum counting detectors obeying simple Poisson statistics, does not reflect the actual physics of CT acquisition. This paper describes a more accurate model, taking into account the energy-integrating detection process, nonuniform flux profiles, and data-conditioning processes. Methods are developed to experimentally measure and theoretically calculate statistical distributions, as well as techniques to analyze CT signal properties. Results indicate the limitations of current models and suggest improvements for the description of CT signal properties.

Pediatric interventional radiography equipment: safety considerations

This paper discusses pediatric image quality and radiation dose considerations in state-of-the-art fluoroscopic imaging equipment. Although most fluoroscopes are capable of automatically providing good image quality on infants, toddlers, and small children, excessive radiation dose levels can result from design deficiencies of the imaging device or inappropriate configuration of the equipment's capabilities when imaging small body parts. Important design features and setup choices at installation and during the clinical use of the imaging device can improve image quality and reduce radiation exposure levels in pediatric patients. Pediatric radiologists and cardiologists, with the help of medical physicists, need to understand the issues involved in creating good image quality at reasonable pediatric patient doses. The control of radiographic technique factors by the generator of the imaging device must provide a large dynamic range of mAs values per exposure pulse during both fluoroscopy and image recording as a function of patient girth, which is the thickness of the patient in the posterior-anterior projection at the umbilicus (less than 10 cm to greater than 30 cm). The range of pulse widths must be limited to less than 10 ms in children to properly freeze patient motion. Variable rate pulsed fluoroscopy can be leveraged to reduce radiation dose to the patient and improve image quality. Three focal spots with nominal sizes of 0.3 mm to 1 mm are necessary on the pediatric unit. A second, lateral imaging plane might be necessary because of the child's limited tolerance of contrast medium. Spectral and spatial beam shaping can improve image quality while reducing the radiation dose. Finally, the level of entrance exposure to the image receptor of the fluoroscope as a function of operator choices, of added filter thickness, of selected pulse rate, of the selected field-of-view and of the patient girth all must be addressed at installation.

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 microGy) 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/C(B)) as well as relative to the dynamic range in the image (C(rel)) 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/C(B) increases with increasing tube voltage.

The influence of antiscatter grids on soft-tissue detectability in cone-beam computed tomography with flat-panel detectors

The influence of antiscatter x-ray grids on image quality in cone-beam computed tomography (CT) is evaluated through broad experimental investigation for various anatomical sites (head and body), scatter conditions (scatter-to-primary ratio (SPR) ranging from approximately 10% to 150%), patient dose, and spatial resolution in three-dimensional reconstructions. Studies involved linear grids in combination with a flat-panel imager on a system for kilovoltage cone-beam CT imaging and guidance of radiation therapy. Grids were found to be effective in reducing x-ray scatter "cupping" artifacts, with heavier grids providing increased image uniformity. The system was highly robust against ring artifacts that might arise in CT reconstructions as a result of gridline shadows in the projection data. The influence of grids on soft-tissue detectability was evaluated quantitatively in terms of absolute contrast, voxel noise, and contrast-to-noise ratio (CNR) in cone-beam CT reconstructions of 16 cm "head" and 32 cm "body" cylindrical phantoms. Imaging performance was investigated qualitatively in observer preference tests based on patient images (pelvis, abdomen, and head-and-neck sites) acquired with and without antiscatter grids. The results suggest that although grids reduce scatter artifacts and improve subject contrast, there is little strong motivation for the use of grids in cone-beam CT in terms of CNR and overall image quality under most circumstances. The results highlight the tradeoffs in contrast and noise imparted by grids, showing improved image quality with grids only under specific conditions of high x-ray scatter (SPR> 100%), high imaging dose (Dcenter> 2 cGy), and low spatial resolution (voxel size > or = 1 mm).

Technical note: a comparison of antiscatter grids for digital radiography

The use of digital radiography (DR) systems offers a number of advantages over film-screen detectors. One potential disadvantage, however, is that some fixed DR systems do not allow the user to change the antiscatter grid to suit the imaging task. Instead, the user must choose the grid at the time of purchase. Six grids, which are offered as installation options for one commercial fixed-room DR system, were experimentally evaluated, using a range of scatter conditions and tube voltages. In addition, three grids, which are available with a portable DR system in which the user can change the grid to suit the imaging task, were also evaluated. The grids were compared using the primary transmission, scatter fraction, and calculated signal-to-noise improvement factor (SIF). It was found that the grids with low atomic number interspace and cover material had an SIF up to 15% higher than did the grids with aluminium interspace and cover material; the grid with a grid ratio of 12:1 had the highest SIF for all tube voltages and scatter conditions tested here. This 12:1 grid probably represents a good general-purpose non-removable grid in DR.

Using simulated annealing for 3D reconstruction of orthopedic fracture

Three-dimensional (3D) reconstruction from two orthogonal images has been realized, and a Monte Carlo program MCNP4B has been applied to simulate the x-ray images. These two approaches can be applied to reconstruction of orthopedic fractures, using computed radiography images. The purpose of this paper is to utilize the information from pairs of orthogonal images at different stages of healing to generate a 3D reconstruction of the callus which builds up during the treatment, thus facilitating understanding and enabling assessment of the process of fracture healing. The reconstruction from Monte Carlo simulated images and x-ray images shows that this approach may be feasible in clinical practice.

Dose comparison between conventional and quasi-monochromatic systems for diagnostic radiology

Several techniques have been introduced in the last year to reduce the dose to the patient by minimizing the risk of tumour induced by radiation. In this work the radiological potential of dose reduction in quasi-monochromatic spectra produced via mosaic crystal Bragg diffraction has been evaluated, and a comparison with conventional spectra has been performed for four standard examinations: head, chest, abdomen and lumbar sacral spine. We have simulated quasi-monochromatic x-rays with the Shadow code, and conventional spectra with the Spectrum Processor. By means of the PCXMC software, we have simulated four examinations according to parameters established by the European Guidelines, and calculated absorbed dose for principal organs and the effective dose. Simulations of quasi-monochromatic laminar beams have been performed without anti-scatter grid, because of their inherent scatter geometry, and compared with simulations with conventional beams with anti-scatter grids. Results have shown that the dose reduction due to the introduction of quasi-monochromatic x-rays depends on different parameters related to the quality of the beam, the organ composition and the anti-scatter grid. With parameters chosen in this study a significant dose reduction can be achieved for two out of four kinds of examination.

A phantom approach to find the optimal technical parameters for plain chest radiography

A simple approach based on phantom measurements is proposed in this study to find the filtration, tube potential and antiscatter device that are optimal in respect of patient dose and image quality, at constant film-screen combination, film processing and viewing conditions. An original quasi-anthropomorphic chest phantom was exposed with 18 different beam qualities and three antiscatter devices. The entrance surface dose, organ doses and effective dose were estimated for each radiograph. The image quality was compared using two objective quality indexes, a contrast index and a scatter fraction, as well as two subjective indexes, a low contrast visualization index and a high contrast visualization index. It was found that for this X-ray unit, routinely using a 7:1 antiscatter grid, the optimal imaging technique is added filtration of 0.1 mm Cu+1 mm Al at a tube potential 100 kVp. Using a 25 cm air gap instead of the grid allows the tube potential to be increased to the upper limit of 120 kVp for this unit. The entrance surface dose of 0.075 mGy at 120 kVp with an air gap is less than half the value of the same quantity with a grid at 100 kVp and is significantly below the European reference level of 0.3 mGy. This phantom method, comprising both objective measurements and subjective estimation, is suitable for dose-image quality optimization in a clinical environment.

Optimization of x-ray imaging geometry (with specific application to flat-panel cone-beam computed tomography)

A theoretical method is presented that allows identification of optimal x-ray imaging geometry, considering the effects of x-ray source distribution, imaging task, x-ray scatter, and imager detective quantum efficiency (DQE). Each of these factors is incorporated into the ICRU-recommended figure of merit for image quality, the detectability index, which is maximized to determine the optimal system configuration. Cascaded systems analysis of flat-panel imagers (FPIs) is extended to incorporate the effects of x-ray scatter directly in the DQE, showing that x-ray scatter degrades DQE as an additive noise source. Optimal magnification is computed for FPI configurations appropriate to (but not limited to) cone-beam computed tomography (CBCT). The sensitivity of the results is examined as a function of focal spot size, imaging task (e.g., ideal observer detection or discrimination tasks), x-ray scatter fraction, detector resolution, and additive noise. Nominal conditions for FPI-CBCT result in optimal magnification of approximately 1.4-1.6, depending primarily on the magnitude of the x-ray scatter fraction. The methodology is sufficiently general that examination of optimal geometry for other FPI applications (e.g., chest radiography, fluoroscopy, and mammography) is possible. The degree to which increased exposure can be used to compensate for x-ray scatter degradation is quantified.

Balancing patient dose and image quality

The formation of images in diagnostic radiology involves a complex interdependence of many factors. The ideal balance is to obtain an image which is adequate for the clinical purpose with the minimum radiation dose. Factors which affect radiation dose and image quality can be grouped under three headings; radiation quality, photon fluence and removal of scattered radiation. If optimal performance is to be achieved, it is necessary to understand how these factors influence image formation and affect radiation dose, and apply methodology for image quality and dose analysis at each stage in the development and use of X-ray equipment.

A Monte Carlo study of grid performance in diagnostic radiology: task-dependent optimization for digital imaging

A Monte Carlo computational model has been used to optimize grid design in digital radiography. The optimization strategy involved finding grid designs that, for a constant signal-to-noise ratio, resulted in the lowest mean absorbed dose in the patient. Different examinations were simulated to explore the dependence of the optimal scatter-rejection technique on the imaging situation. A large range of grid designs was studied, including grids with both aluminium and fibre interspaces and covers, and compared to a 20 cm air gap. The results show that the optimal tube potential in each examination does not depend strongly on the scatter-rejection technique. There is a significant dose reduction associated with the use of fibre-interspaced grids, particularly in paediatric radiography. The optimal grid ratio and strip width increase with increasing scattering volume. With increasing strip density, the optimal strip width decreases, and the optimal grid ratio increases. Optimal grid ratios are higher than those used today, particularly for grids with large strip density. It is, however, possible to identify grids of good performance for a range of strip densities and grid ratios provided the strip width is selected accordingly. The computational method has been validated by comparison with measurements with a caesium iodide image receptor.

Use of the concept of energy imparted in diagnostic radiology

The concept of energy imparted by ionizing radiation to the matter in a volume is analyzed and methods to determine the energy imparted epsilon to the patient are reviewed, in particular, determinations based on measurements of the air kerma integrated over beam area [the kerma-area-product (KAP)] and calculations needed to derive conversion factors epsilon/KAP. The energy imparted to the image receptor, epsilon rec, including the statistical aspects of the concept, and the effect of epsilon rec on image quality and patient dose are also analysed. Finally, use of the energy imparted to the patient as a risk indicator is discussed.

Monte Carlo generated mammograms: development and validation

We have developed a model using Monte Carlo methods to simulate x-ray mammography. All possible physical processes of interaction of x-rays with matter have been taken into account. A simplified geometry of the mammographic apparatus has been considered along with a software phantom of compressed breast. The phantom may contain inhomogeneities of various compositions and sizes. We have used this model to produce Monte Carlo mammograms under realistic conditions. The validation of the simulation includes both the modelling of physical processes and the production of Monte Carlo mammograms. The first part is accomplished by the demonstration of the coincidence between Monte Carlo and theoretical data, whereas the second is accomplished by the comparison of real mammograms, taken from irradiation of a simplified breast phantom that we have constructed, and Monte Carlo mammograms taken from simulation of the above phantom under the corresponding exposure conditions. The limitations of the model as well as the future use of Monte Carlo mammograms are discussed.

The use of carbon fibre material in radiographic cassettes: estimation of the dose and contrast advantages

A Monte Carlo simulation has been used to estimate the dose and contrast advantages of replacing radiographic cassette fronts fabricated from aluminium with cassette fronts fabricated from low atomic number material (carbon fibre). The simulation used a realistic imaging geometry and calculations were made both with and without an anti-scatter grid. Account was taken of the scatter generated in the cassette front and the effect of beam hardening on primary contrast. Dose and contrast were evaluated for a range of cassette front thicknesses and tube potentials (60-150 kV) as well as for four examinations representative of situations with varying amounts of scatter. The results with an anti-scatter grid show a clear dose and contrast advantage in all cases when an aluminium cassette front is replaced with a low attenuation cassette front. The contrast advantage is dependent upon the examination and is generally greater for imaging bony structures than for imaging soft tissue. If a 1.74 mm aluminium cassette front is compared with a 1.1 mm carbon fibre cassette front, then the dose advantages are 16%, 9%, 8% and 6% and the contrast advantages are 10%, 7%, 4% and 5% for the AP paediatric pelvis examination at 60 kV, the anteroposterior (AP) lumbar spine examination at 80 kV, the lateral lumbar spine examination at 100 kV and the posteroanterior (PA) chest examination at 150 kV, respectively. The results without an anti-scatter grid show an increased dose advantage when a low attenuation cassette front is used, but the contrast advantage is small and in some situations negative.

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 8 x 10(-5) to 0.3 m2. 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.

The physical performance of different x-ray contrast agents: calculations using a Monte Carlo model of the imaging chain

A Monte Carlo computational model of the imaging chain has been used to investigate the performance of x-ray contrast agents with atomic number, Z, 53 < or = Z < or = 90 with respect to physical image quality descriptors (contrast and signal to noise ratio, SNR) and patient mean absorbed dose. Contrast agents of equal molar concentrations were used within a water slab (simulating the patient). The imaging conditions were chosen to represent adult and paediatric examinations. For all tube potentials studied (40-140 kV), the contrast agents with the highest atomic numbers (bismuth and thorium) gave the highest contrast. In analogue screen-film imaging, several other contrast agents could produce a higher image contrast than iodine in a limited range of tube potentials. This advantage could alternatively be effected as a reduced amount of administered contrast agent, or as a reduced mean absorbed dose in the patient. In digital imaging, a lower mean absorbed dose for a constant SNR than that with iodine can be achieved for ranges of tube potentials and contrast agents. Bismuth and thorium yield a lower dose than iodine at all studied tube potentials. Gadolinium and erbium could alternatively be used at a broad range of tube potentials above 90 kV with a dose penalty of only 5-20%.

A Monte Carlo program for the calculation of contrast, noise and absorbed dose in diagnostic radiology

A Monte Carlo computer program has been developed for the simulation of X-ray photon transport in diagnostic X-ray examinations. The simulation takes account of the incident photon energy spectrum and includes a phantom (representing the patient), an anti-scatter grid and an image receptor. The primary objective for developing the program was to study and optimise the design of anti-scatter grids. The program estimates image quality in terms of contrast and signal-to-noise ratio, and radiation risk in terms of mean absorbed dose in the patient. It therefore serves as a tool for the optimisation of the radiographic procedure. A description is given of the program and the variance-reduction techniques used. The computational method was validated by comparison with measurements and other Monte Carlo simulations.

Monte Carlo study of grid performance in diagnostic radiology: task dependent optimization for screen-film imaging

An optimization of anti-scatter grid design using Monte Carlo techniques in diagnostic radiology is presented. The criterion for optimization was to find the combinations of the grid parameters (lead strip width, grid ratio and strip density) and tube potential which result in the lowest mean absorbed dose in the patient at fixed image contrast. The optimization was performed in three irradiation geometries, representing different scattering conditions (paediatric examinations, and two adult lumbar spine examinations) and was restricted to grids using fibre materials in covers and interspaces. Grid designs currently available were studied, as were designs which use thinner strips (< 30 microns) and higher grid ratios (> 18). It was found that grids with widely different strip densities (strips cm-1) and grid ratios can have good performance provided that they are used with appropriate strip width and tube potential. With increasing amounts of scatter, the optimal grid requires thicker strips and higher grid ratios. Increasing the strip density and using thinner strips and higher grid ratios are generally required. Grids with low strip density (25 strips cm-1) were found to be less sensitive to alterations in strip width. Optimal grids for paediatric radiology require thinner strips (10-20 microns) than those in currently available grids. Grids on the market are best suited for examinations of the adult body in anteroposterior (AP) view. In the adult lateral view, representing the largest scattering volume, higher grid ratios (> 18) than those in existing grids would be optimal. Examples of good grid designs are given for each examination.

Selection of anti-scatter grids for different imaging tasks: the advantage of low atomic number cover and interspace materials

A Monte Carlo computer program has been developed for the study of anti-scatter grids used in diagnostic radiology. The program estimates the scatter from soft tissue phantoms representative of either adult or paediatric examinations and uses dose increase, signal-to-noise ratio improvement and contrast improvement factors to study grid performance. It has been used to quantify the advantage of replacing grids with aluminium covers and interspaces by grids using materials of low atomic number for these components. Two approaches are used. First, the aluminium and low atomic number alternatives are compared for five grid ratios at fixed strip density and width and for tube potentials of 50, 70, 100 and 150 kV. Second, 44 commercially available grids are compared for three different imaging situations (lumbar spine, chest and paediatric). The results demonstrate that grids made with carbon fibre cover and cotton fibre interspace result in greater improvements in contrast and signal-to-noise ratio, and lower dose increase factors, than do grids made with aluminium. The dose reduction varies with irradiation conditions and is generally larger at lower tube potentials, higher grid ratios and lower strip densities. A typical reduction in mean absorbed dose in the patient is 30% in an adult lumbar spine (AP view) at 70 kV with a grid with 36 strips per centimetre and ratio 12.