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D R Dance

Publications and source records attributed to D R Dance.

At least 37 records · Page 2Linked to original sources
PubMed1999

A search for improved technique factors in paediatric fluoroscopy.

A Monte Carlo computational model of a fluoroscopic imaging chain was used for deriving optimal technique factors for paediatric fluoroscopy. The optimal technique was defined as the one that minimizes the absorbed dose (or dose rate) in the patient with a constraint of constant image quality. Image quality was assessed for the task of detecting a detail in the image of a patient-simulating phantom, and was expressed in terms of the ideal observer's signal-to-noise ratio (SNR) for static images and in terms of the accumulating rate of the square of SNR for dynamic imaging. The entrance air kerma (or air kerma rate) and the mean absorbed dose (or dose rate) in the phantom quantified radiation detriment. The calculations were made for homogeneous phantoms simulating newborn, 3-, 10- and 15-year-old patients, barium and iodine contrast material details, several x-ray spectra, and for imaging with or without an antiscatter grid. The image receptor was modelled as a CsI x-ray image intensifier (XRII). For the task of detecting low- or moderate-contrast iodine details, the optimal spectrum can be obtained by using an x-ray tube potential near 50 kV and filtering the x-ray beam heavily. The optimal tube potential is near 60 kV for low- or moderate-contrast barium details, and 80-100 kV for high-contrast details. The low-potential spectra above require a high tube load, but this should be acceptable in paediatric fluoroscopy. A reasonable choice of filtration is the use of an additional 0.25 mm Cu, or a suitable K-edge filter. No increase in the optimal tube potential was found as phantom thickness increased. With the constraint of constant low-contrast detail detectability, the mean absorbed doses obtained with the above spectra are approximately 50% lower than those obtained with the reference conditions of 70 kV and 2.7 mm Al filter. For the smallest patient and x-ray field size, not using a grid was slightly more dose-efficient than using a grid, but when the patient size and field size were increased a fibre interspaced grid resulted in lower doses than imaging without a grid. For a 15-year-old patient the mean absorbed doses were up to 40% lower with this grid than without the grid.

Adolescent↗
PubMed1998

Objective assessment of phantom image quality in mammography: a feasibility study.

The need for test objects in mammography quality control programmes to provide an objective measure of image quality pertinent to clinical problems is well documented. However, interobserver variations may be greater than the fluctuations in image quality that the quality control programme is seeking to detect. We have developed a computer algorithm to score a number of features in the Leeds TOR(MAX) mammography phantom. Threshold scoring techniques have been applied in the first instance; scoring schemes which utilize measures such as signal-to-noise ratio and modulation have also been formulated. This fully automatic algorithm has been applied to a set of 10 films which have been digitized at 25 microns resolution using a Joyce-Loebl scanning microdensitometer. The films were chosen retrospectively from quality control test films to demonstrate: (a) a range of optimized imaging systems, and (b) variation from the optimum. The performance of the algorithm has been compared with that of five experienced observers, and has been shown to be as consistent as individual observers, but more consistent than a pool of observers. Problems have been encountered with the detection of small details, indicating that a more sophisticated localization technique is desirable. The computer performs more successfully with the scoring scheme which utilizes the full imaging information available, rather than with the threshold-determined one. However, both the observers and the computer algorithm failed to identify the non-optimum films, suggesting that the sensitivity of the TOR(MAX) test object may not be adequate for modern mammography imaging systems.

Algorithms↗
PubMed1997

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.

Aluminum↗
PubMed1995

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%.

Adult↗
PubMed1995

Segmentation of mammograms using multiple linked self-organizing neural networks.

A possible first stage in the analysis of the mammographic scene is its segmentation into four major components: background (the nonbreast area), pectoral muscle, fibroglandular region (parenchyma), and adipose region. An algorithm has been developed for this task. It is based on the classification of a feature vector constructed from statistical measures of texture calculated at two window sizes. Separate self-organizing neural networks are trained on sample data taken from each of the four regions. The feature vectors from the entire mammogram are then classified with the trained networks linked via a decision logic. To overcome the variability of texture between mammograms the algorithm uses data from a mammogram to classify itself in a staged approach consisting of several binary decisions. The training regions for each successive stage are determined from geometric information produced by the previous stages. The dataset in the study consisted of thirty (fifteen pairs) digitized normal mammograms of variable radiographic appearance. As a measure of performance, the outlines of the parenchyma were compared to those drawn by a radiologist experienced in reading mammograms. Comparison of the areas and perimeters generated by the human and computer observers gives a relationship with correlation coefficients of 0.74 and 0.59 for each measure, respectively. The overlapping areas of the parenchymas segmented by the observers normalized by the combined area was also calculated for each case. The mean and standard deviation of this measure was 0.69 +/- 0.12.

Adipose Tissue↗
PubMed1995

Classification of ductal carcinoma in situ by image analysis of calcifications from digital mammograms.

There is evidence to suggest that non-comedo cases of ductal carcinoma in situ (DCIS) may have a lower risk of progressing to invasive disease than comedo DCIS and may be managed with less radical treatment. Most cases of DCIS present as calcifications on the mammogram and, due to differences in the tumour architecture, comedo calcifications often have a characteristic linear or branching appearance. In this study, computer methods were developed to identify the comedo cases using the imaging features of individual calcifications and of calcification clusters. Classifier performance was measured using the area under the receiver operating characteristic (ROC) curve and was optimized by systematic testing of many different combinations of these features. On a test-set of 42 cases the computer achieved a ROC curve area of 0.91 using six combined cluster features. These results are encouraging given the observed overlap in appearance between the comedo and non-comedo cases.

Breast Diseases↗
PubMed1995

Assessment of organ radiation doses and associated risk for digital bifemoral arteriography.

An assessment has been made of the absorbed dose associated with femoral arteriography using a digital imaging system. A bilateral femoral arteriogram was performed on 17 patients, using a filmless 1024 matrix digital image acquisition system with a discrete stepping tube-stand and 40 cm image intensifier. A standardized protocol of manual patient/tube-stand positioning under fluoroscopic control and automatic stepping digital acquisition was followed. Skin entry doses were measured with a dose-area product meter for each stage of the procedure, and the total gonad dose was assessed with thermoluminescent dosimeters (TLDs). Published Monte Carlo simulations were supplemented with further calculations to evaluate organ doses from the dose-area products measured. Comparison with the TLD measurements indicated that this technique over-estimated organ doses by about 30%. A mean effective dose of 3.1 +/- 1.8 mSv was calculated for the procedure, with the greatest dose burden being imposed by fluoroscopy during catheter manipulation. The related radiation detriment is 0.018%, which is insignificant when compared with the overall mortality from peripheral vascular disease.

Aged↗
PubMed1994

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.

Computer Simulation↗
PubMed1994

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.

Algorithms↗
PubMed1994

Estimation of tube potential in mammography from transmission measurements.

A computer program has been used to study the performance of a method for estimating the potential of mammographic x-ray tubes. The method uses measurements of the transmission of the x-ray beam through a series of filters. The transmissions are fitted to values calculated using tables of mammographic x-ray spectra, thus providing an estimate of tube potential. The performance of the method was studied using a Monte Carlo simulation of experimental errors. The number of measurements and the selection of filter material were both varied, and the precision and accuracy were used as performance indicators. It was found that, for a single filter material, the precision was improved by increasing the number of transmission data, but the accuracy was unaffected. The poorest accuracy and precision were obtained when K-edge filters, or filters with very high or very low atomic number were used. Using combinations of two filter materials, a high transmission (25%) K-edge filter in combination with a low transmission (3%) non-K-edge filter produced results comparable to the best results obtained from seven transmission measurements using a single filter material. By reducing the non-K-edge filter transmission to 1%, an accuracy of +/- 0.4% and a precision of +/- 1.4% were achieved at both 25 kV and 30 kV.

Algorithms↗
PubMed1994

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.

Adult↗
PubMed1993

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.

Adult↗
PubMed1993

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.

Adult↗
PubMed1992

Calculation of dose and contrast for two mammographic grids.

To aid in selecting optimal conditions for screening mammography practice in Sweden, the performance of two mammographic grids (one moving and one stationary) has been investigated. Monte Carlo techniques were used to simulate photon transport in the breast. Transport through the breast support, grid covers, grid and image receptor (33.9 mg cm-2 Gd2O2S) was treated analytically. The contrast of a 100 microns calcification has been evaluated for three tissue compositions (adipose, glandular, 50:50 fractions by weight of adipose and glandular tissue) as a function of breast thickness (2-8 cm) and potential difference (25-30 kV, Mo anode). Contrast for a 5 cm 'average' breast at 28 kV was improved by 40% using the moving grid and by 30% using the stationary one; the corresponding increases in breast absorbed dose, keeping the energy imparted to the image receptor constant, were 90% and 150%, respectively. The superior properties of the moving grid were due to (i) equal scatter rejection ability and higher transmission of primary photons yielding lower scatter-to-primary ratios behind the grid, and (ii) less attenuation and filtering of the primary photons in the interspace material yielding lower degradation of primary contrast.

Breast Neoplasms↗
PubMed1991

Measurement of small-angle photon scattering for some breast tissues and tissue substitute materials.

For photon energies encountered in diagnostic radiology the shape of the scattering distributions for low-atomic-number media exhibits peaks in intensity close to the forward direction that are not predicted by conventional theoretical models. The positions and shapes of the peaks depend upon the interatomic and intermolecular configurations of the scatterers. The phenomenon is of particular interest because of its relevance to the understanding and modelling of x-ray imaging processes and the possibility that the peaking may be characteristic of tissue type. In the present study, peaks in the forward scattering distributions have been demonstrated for 19 samples of breast tissue and three tissue substitute materials using a position-sensitive photon detector and a 60 kVp x-ray source. Prominent features were observed for all samples investigated. Large differences were found in the shapes of the distributions between adipose and fibroglandular tissues and only small differences were found between carcinomas and fibroglandular tissues.

Adenofibroma↗
PubMed1990

Automatic computer detection of clustered calcifications in digital mammograms.

The automatic detection of clusters of calcifications in digital mammograms has been investigated using image analysis techniques. The calcifications were segmented from the background of normal breast structure in the mammogram using a local area thresholding process. This procedure also identified other breast structures and the digital image properties of all segmented objects were analysed to extract clusters of calcifications. Seventy five clinical mammograms were digitised. These were divided into training and test sets of 25 and 50 films respectively. The results for the test set of 50 complete clinical mammograms show that the computer system achieves a 25/25 true positive film classification (i.e. those containing clusters of calcifications) with false positive clusters detected in 4/50 films. There were no false negative film classifications.

Algorithms↗