| 1. |
- Sandborg, Michael, et al.
(författare)
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Calculation of contrast and signal-to-noise degradation factors for digital detectors in chest and breast imaging
- 2003
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The Monte Carlo model of an x-ray imaging system, used in the EU 5th framework project by the Linköping and London partner to study chest and breast imaging, was developed jointly by the London and Linköping partners. It incorporates a model of the x-ray imaging system (x-ray tube, filtration, anti-scatter device and image receptor etc.) and the patient by using a voxel phantom of an adult male. Validation and calibration experiments have been performed for both the chest (Ullman et al 2003b) and the breast model. The model allows inclusion of anatomical or pathological details at particular positions in the anatomy and is able to calculate measures of image quality such as contrast and signal-to-noise ratio and measures of radiation risk for example entrance air kerma and effective dose. It allows alteration of imaging system settings such as tube voltage, filtration, beam size and position, choice of anti-scatter device and choice of image detector etc. The model is a useful tool for optimisations since it has been shown that in chest and lumbar spine radiography is able to predict clinical image quality as assessed by a group of radiologists. In the Monte Carlo model (MC-model) the image quality measures are calculated assuming a perfectly sharp imaging system and correction factors need to be applied to the computed data in order to make the image quality measures agree on an absolute scale. The calculation of correction factors for contrast and signal-tonoises are described in this report. A similar report focusing on analogue screen-film chest and lumbar spine radiography was completed some years ago and some of the concepts and methods are similar.
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| 2. |
- Sandborg, Michael, et al.
(författare)
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Report on the study and optimisation of chestand lumbar spine X-ray imaging
- 2000
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The optimisation of radiological equipment and equipment parameters is a key stage in the optimisation of image quality and patient dose in diagnostic radiology. It is essential, however, to underpin such optimisation with theoretical modelling which can provide both the direct quantification of the effect on image quality and dose of changes in system parameters and the opportunity for optimisation of equipment selection and use. Our principal contribution to the joint CEC-project 'Predictivity and Optimisation in Diagnostic Radiology' is in modelling of planar chest and lumbar spine radiographic examinations. The results of this work for the chest PA, lumbar spine AP and lumbar spine lateral examinations are presented in this report. Prior to this, several development stages have been completed which include the calibration and validation of our methods by measurements in the clinical environment on patients and patient images. These important aspects are not dealt with in detail here, but are described in separate reports. This report focuses on three aspects from our results of using our Monte Carlo model of the patient and imaging equipment: (1) Study of the effects on image quality and patient dose when the imaging parameters are varied; Establishment of imaging parameters and systems that result in as least as good image quality as systems with good performance singled out from results of clinical trials (optimisations); Comparison of the results from the model with the results from clinical trials performed by partners in the joint CEC-project. An objective of the report is to present our results at a level of detail not usually possible in the refereed scientific literature. The report should therefore not be read all at once, but preferably used as a ‘reference library’ or documentation of all our efforts. There are many interesting results and findings from this collaborative work and these will be submitted for publication to the appropriate journals.
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| 3. |
- Ullman, Gustav, et al.
(författare)
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Comparison of clinical and physical measures of image quality in chest PA and pelvis AP views at varying tube voltages
- 2004
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Image quality in digital chest PA and pelvis AP was assessed using two different methods; one based on observations of images of an anthropomorphic phantom, one based on computer modelling using an anthropomorphic voxel phantom. The tube voltage was varied within a broad range (50-150 kV), including those values typically used with screen-film radiography. The tube charge was altered so that approximately the same effective dose was achieved in the modelled patient (anthropomorphic phantom). Two x-ray units were employed using a digital image detector (computed radiography, CR, system) with standard tube filtration and anti-scatter device. Clinical image quality was assessed by a group of radiologists using a visual grading analysis (VGA) technique based on the revised CEC image criteria. Physical image quality was derived from the computer model in terms of the signal-to-noise ratio, SNR for fixed effective dose in the voxel phantom. The computer model uses Monte Carlo simulations of the patient and complete imaging system. Both the VGAS (visual grading analysis score) and SNR increase with decreasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages than used today are employed in digital radiology. A positive correlation between clinical and physical measures of image quality was found. The pros and cons of using lower tube voltages with CR digital radiography than typically used in analogue screen-film radiography are discussed as well as the relevance of using VGAS and quantum noise SNR as measures of image quality.
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| 4. |
- Ullman, Gustav, et al.
(författare)
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Distributions of scatter-to-primary ratios in chest PA radiography using grid or air gap for scatter-rejection
- 2004
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The aim for this work was to calculate distributions of scatter-to-primary ratios (εs/εp) and signal-to-noise ratios per pixel (SNRp) for a large set of imaging systems with either grid or air gap for scatter rejection. Grids with ratio 8-16 and air gap length 20 and 40 cm were used. The tube voltage was varied between 90-150 kV and three patient thickness between 20-28 cm were tested in order to compare scatter-rejections techniques for different conditions. Distributions of this sort may provide useful information on how physical image quality (contrast, SNR) is distributed over the chest PA image. A Monte Carlo computer program was used for the calculations, including a model of both the patient (voxel phantom) and the imaging system. The mean value of the εs/εp is 0.39 in the hilar region and 1.72 in the lower mediastinal region. For a 28 cm patient, the corresponding values are 0.42 in the hilar region and 2.58 in the lower mediastinal region. The grid with ratio 16 is the most efficient scatter rejection technique in all regions except the hilar region. In the hilar region, the most efficient technique is the 40 cm air gap.
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| 5. |
- Ullman, Gustav, et al.
(författare)
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Implementation of pathologies in the Monte Carlo model in chest and breast imaging
- 2003
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The Monte Carlo simulation model of the chest and breast imaging systems including a voxelised model of the patient are used to compute measures of image quality and patient absorbed dose. It is important that the model computes measures of image quality of pathological details that are similar in size, composition and position as real pathological details in typical chest and breast images. Moreover, the other partners of the co-coordinated research project will produce hybrid images with pathological details and have these images assessed by a group of radiologist. The model will then be used to study to what extent variations in clinical image quality can be explained by variations in physical image quality, for example signal-to-noise ratio. This report summarizes the selection of pathological details to include in the model of chest and breast imaging systems.
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| 6. |
- Ullman, Gustav, et al.
(författare)
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On the extent of quantum noise limitation in digital chest radiography
- 2004
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The aim for this work was to study to what extent the detection of nodules is quantum noise limited, based on the combined results from a nodule-detection clinical trial and a Monte Carlo computational model of a digital chest imaging system. The Monte Carlo computer program computes measures of physical image quality such as image contrast, C and signalto-noise ratio, SNR for nodules of any size. A computed radiography (CR) imaging system used simulated. The patient anterior-posterior thickness was 25 cm and nodules with diameters between 1-40 mm were included. The image contrast and SNR was calculated for 1600 (40x40) positions in the chest image and averaged over five anatomical regions of interest (lateral pulmonary, retrocardial, hilar, lower- and upper mediastinal regions). Threshold contrasts for each region, Cth, corresponding to Az=0.80 for detecting a 10 mm nodule, were deduced from the clinical trial. A threshold is also used for the quantum noise signal-to-noise ratio, SNRth. The model computes the diameter of a disk-shaped object that is required to comply with the two criteria: SNR≥SNRth and C≥Cth. A system is said to be quantum noise limited when the nodule size required to fulfil both criteria is not limited by the contrast but by the SNR. The required nodule diameter is largest in the hilar region (25 mm) and smallest in the lateral pulmonary region (11 mm). When the threshold SNRth=25 is used, the lower mediastinal region is quantum noise limited already at low speed classes (S>100). The hilar region is never quantum noise limited at realistic speed classes (S<1000). The accuracy of this model will be tested in the future by more sophisticated modelling of anatomical background and noise in the SNR-expression.
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| 7. |
- Ullman, Gustav, et al.
(författare)
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Optimisation of chest radiology by computer modelling of image quality measures and patient effective dose
- 2004
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A set of modelled computed radiography (CR) systems are compared with a reference system. Calculations are performed, which compares the effective dose and a set of figures of merit corresponding to the image quality of both the modelled systems and the reference system. For a nodule with soft tissue corresponding, the signal-to-noise ratio, SNR, is found to decrease with increasing tube voltage. On the other hand, the ratio of the contrast of the nodule compared to the contrast of a rib (nodule-to-rib contrast-ratio) is found to increase with increasing tube voltage.
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