Biomedical subjects

David R Dance

Publications and source records attributed to David R Dance.

11 recordsLinked to original sources

PubMed2006

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.

Humans↗

PubMed2006

How do lesion size and random noise affect detection performance in digital mammography?

RATIONALE AND OBJECTIVES: We investigated the effect of random noise and lesion size on detection performance in mammography. MATERIALS AND METHODS: Digital mammograms were obtained of an anthropomorphic breast phantom with and without simulated mass lesions. Digital versions of the mass lesions, ranging in size from 0.8 to 12 mm, were added back to the breast phantom image. Four alternate forced choice experiments were performed to determine the lesion contrast required to achieve a 92% correct lesion detection rate, denoted I92. Experiments were performed using identical phantom images and different versions of phantom images obtained using the same techniques but with different random noise patterns. RESULTS: For lesions larger than 1 mm, the slope of the contrast detail curves was always positive. This behavior contrasts with conventional contrast-detail curves in uniform backgrounds in which the slope is approximately -0.5. There was no difference between twinned experiments and those obtained using different patterns of random noise for lesions greater than 1 mm. When the lesion size was reduced below 1 mm, the detection threshold increased indicating a deterioration of lesion detectability, and detection performance was significantly lower when random noise patterns were used. CONCLUSION: Our results suggest that lesion detection is dominated by anatomical structure for lesions with a size >1 mm, but by random noise for submillimeter sized lesions.

Artificial Intelligence↗

PubMed2006

Evaluation of two methods of predicting MLC leaf positions using EPID measurements.

In intensity modulated radiation treatments (IMRT), the position of the field edges and the modulation within the beam are often achieved with a multileaf collimator (MLC). During the MLC calibration process, due to the finite accuracy of leaf position measurements, a systematic error may be introduced to leaf positions. Thereafter leaf positions of the MLC depend on the systematic error introduced on each leaf during MLC calibration and on the accuracy of the leaf position control system (random errors). This study presents and evaluates two methods to predict the systematic errors on the leaf positions introduced during the MLC calibration. The two presented methods are based on a series of electronic portal imaging device (EPID) measurements. A comparison with film measurements showed that the EPID could be used to measure leaf positions without introducing any bias. The first method, referred to as the "central leaf method," is based on the method currently used at this center for MLC leaf calibration. It mimics the manner in which leaf calibration parameters are specified in the MLC control system and consequently is also used by other centers. The second method, a new method proposed by the authors and referred to as the "individual leaf method," involves the measurement of two positions for each leaf (-5 and +15 cm) and the interpolation and extrapolation from these two points to any other given position. The central leaf method and the individual leaf method predicted leaf positions at prescribed positions of -11, 0, 5, and 10 cm within 2.3 and 1.0 mm, respectively, with a standard deviation (SD) of 0.3 and 0.2 mm, respectively. The individual leaf method provided a better prediction of the leaf positions than the central leaf method. Reproducibility tests for leaf positions of -5 and +15 cm were performed. The reproducibility was within 0.4 mm on the same day and 0.4 mm six weeks later (1 SD). Measurements at gantry angles of 0 degrees, 90 degrees, and 270 degrees for leaf positions of -5 and +15 cm showed no significant effect of gravity. The individual leaf method could be used in various applications to improve the accuracy of radiotherapy treatment from planning to delivery. Three cases are discussed: IMRT beam verification, MLC calibration and dose calculation.

Calibration↗

PubMed2006

Comparison of clinical and physical measures of image quality in chest and pelvis computed radiography at different tube voltages.

The aim of this work was to study the dependence of image quality in digital chest and pelvis radiography on tube voltage, and to explore correlations between clinical and physical measures of image quality. The effect on image quality of tube voltage in these two examinations was assessed using two methods. The first method relies on radiologists' observations of images of an anthropomorphic phantom, and the second method was based on computer modeling of the imaging system 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 the same effective dose was achieved for each projection. Two x-ray units were employed using a computed radiography (CR) image detector with standard tube filtration and antiscatter 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 a Monte Carlo computer model in terms of the signal-to-noise ratio, SNR, of anatomical structures corresponding to the image criteria. Both the VGAS (visual grading analysis score) and SNR decrease with increasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages are employed. Hence, 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 analog screen-film radiography are discussed, as well as the relevance of using VGAS and quantum-noise SNR as measures of image quality in pelvis and chest radiography.

Algorithms↗

PubMed2005

A search for optimal x-ray spectra in iodine contrast media mammography.

The aim of this work was to search for the optimal x-ray tube voltage and anode-filter combination in digital iodine contrast media mammography. In the optimization, two entities were of interest: the average glandular dose, AGD, and the signal-to-noise ratio, SNR, for detection of diluted iodine contrast medium. The optimum is defined as the technique maximizing the figure of merit, SNR2/AGD. A Monte Carlo computer program was used which simulates the transport of photons from the x-ray tube through the compression plate, breast, breast support plate, anti-scatter grid and image detector. It computes the AGD and the SNR of an iodine detail inside the compressed breast. The breast thickness was varied between 2 and 8 cm with 10-90% glandularity. The tube voltage was varied between 20 and 55 kV for each anode material (Rh, Mo and W) in combination with either 25 microm Rh or 0.05-0.5 mm Cu added filtration. The x-ray spectra were calculated with MCNP4C (Monte Carlo N-Particle Transport Code System, version 4C). A CsI scintillator was used as the image detector. The results for Rh/0.3 mmCu, Mo/0.3 mmCu and W/0.3 mmCu were similar. For all breast thicknesses, a maximum in the figure of merit was found at approximately 45 kV for the Rh/Cu, Mo/Cu and W/Cu combinations. The corresponding results for the Rh/Rh combination gave a figure of merit that was typically lower and more slowly varying with tube voltage. For a 4 cm breast at 45 kV, the SNR2/AGD was 3.5 times higher for the Rh/0.3 mmCu combination compared with the Rh/Rh combination. The difference is even larger for thicker breasts. The SNR2/AGD increases slowly with increasing Cu-filter thickness. We conclude that tube voltages between 41 and 55 kV and added Cu-filtration will result in significant dose advantage in digital iodine contrast media mammography compared to using the Rh/Rh anode/filter combination at 25-32 kV.

Algorithms↗

PubMed2005

Evaluation of a novel method of noise reduction using computer-simulated mammograms.

A novel method of noise reduction has been tested for mammography using computer-simulated images for which the truth is known exactly. This method is based on comparing two images. The images are compared at different scales, using a cross-correlation function as a measure of similarity to define the image modifications in the wavelet domain. The computer-simulated images were calculated for noise-free primary radiation using a quasi-realistic voxel phantom. Two images corresponding to slightly different geometry were produced. Gaussian noise was added with certain properties to simulate quantum noise. The added noise could be reduced by >70% using the proposed method without any noticeable corruption of the structures. It is possible to save 50% dose in mammography by producing two images (each 25% of the dose for a standard mammogram). Additionally, a reduction of the anatomical noise and, therefore, better detection rates of breast cancer in mammography are possible.

Artifacts↗

PubMed2005

Distributions of scatter-to-primary and signal-to-noise ratios per pixel in digital chest imaging.

The aim of this work was to calculate distributions of scatter-to-primary ratios (epsilon(s)/epsilon(p)) and signal-to-noise ratios per pixel (SNRp) in chest images. Such distributions may provide useful information on how physical image quality (contrast, SNR) is distributed over the posterior/anterior (PA) chest image. A Monte Carlo computer program was used for the calculations, including a model of both the patient (voxel phantom) and the imaging system (X-ray tube, anti-scatter grid and image detector). The calculations were performed for three PA thicknesses 20, 24 and 28 cm. For a 24 cm patient, the epsilon(s)/epsilon(p) varies between 0.5 in the lung to 2.5 behind the spine and heart. The corresponding variation of the SNRp is a factor of 3, with the highest values in the lung. Increasing the patient thickness from 20 to 28 cm increases the epsilon(s)/epsilon(p) by a factor of 2.2 behind the spine and heart.

Computers↗

PubMed2005

The influence of patient thickness and imaging system on patient dose and physical image quality in digital chest imaging.

The aim of this work was to study the influence of patient thickness, tube voltage and image detector on patient dose, contrast and ideal observer signal-to-noise ratio (SNR(I)), for pathological details positioned at different regions in the image in posterior-anterior (PA) chest radiology. A Monte Carlo computational model was used to compute measures of physical image quality (contrast, SNR(I)) and patient effective dose, E. Two metastasis-like details positioned in the central right lung and right lung near the spine, respectively, were studied. The tube voltage was varied between 100 and 150 kV and the patient thickness between 20 and 28 cm. Both, a computed radiography (CR) system and a direct radiography (DR) system, were investigated. The DR system provides both lower doses and better image quality compared with the CR system. The SNR(I)2/E is approximately 2.9 times higher for the DR system compared with the CR system.

Artifacts↗

PubMed2005

Patient radiation doses for electron beam CT.

A Monte Carlo based computer model has been developed for electron beam computed tomography (EBCT) to calculate organ and effective doses in a humanoid hermaphrodite phantom. The program has been validated by comparison with experimental measurements of the CT dose index in standard head and body CT dose phantoms; agreement to better than 8% has been found. The robustness of the model has been established by varying the input parameters. The amount of energy deposited at the 12:00 position of the standard body CT dose phantom is most susceptible to rotation angle, whereas that in the central region is strongly influenced by the beam quality. The program has been used to investigate the changes in organ absorbed doses arising from partial and full rotation about supine and prone subjects. Superficial organs experience the largest changes in absorbed dose with a change in subject orientation and for partial rotation. Effective doses for typical clinical scan protocols have been calculated and compared with values obtained using existing dosimetry techniques based on full rotation. Calculations which make use of Monte Carlo conversion factors for the scanner that best matches the EBCT dosimetric characteristics consistently overestimate the effective dose in supine subjects by typically 20%, and underestimate the effective dose in prone subjects by typically 13%. These factors can therefore be used to correct values obtained in this way. Empirical dosimetric techniques based on the dose-length product yield errors as great as 77%. This is due to the sensitivity of the dose length product to individual scan lengths. The magnitude of these errors is reduced if empirical dosimetric techniques based on the average absorbed dose in the irradiated volume (CTDIvol) are used. Therefore conversion factors specific to EBCT have been calculated to convert the CTDIvol to an effective dose.

Body Burden↗

PubMed2003

Experimental investigation of the dose and image quality characteristics of a digital mammography imaging system.

Our purpose in this study was to investigate the image quality and absorbed dose characteristics of a digital mammography imaging system with a CsI scintillator, and to identify an optimal x-ray tube voltage for imaging simulated masses in an average size breast with 50% glandularity. Images were taken of an ACR accreditation phantom using a LORAD digital mammography system with a Mo target and a Mo filter. In one experiment, exposures were performed at 80 mAs with x-ray tube voltages varying between 24 and 34 kVp. In a second experiment, the x-ray tube voltage was kept constant at 28 kVp and the technique factor was varied between 5 and 500 mAs. The average glandular dose at each x-ray tube voltage was determined from measurements of entrance skin exposure and x-ray beam half-value layer. Image contrast was measured as the fractional digital signal intensity difference for the image of a 4 mm thick acrylic disk. Image noise was obtained from the standard deviation in a uniformly exposed region of interest expressed as a fraction of the background intensity. The measured digital signal intensity was proportional to the mAs and to the kVp5.8. Image contrast was independent of mAs, and dropped by 21% when the x-ray tube voltage increased from 24 to 34 kVp. At a constant x-ray tube voltage, image noise was shown to be approximately proportional to (mAs)(-05), which permits the image contrast to noise ratio (CNR) to be modified by changing the mAs. At 80 mAs, increasing the x-ray tube voltage from 24 to 34 kVp increased the CNR by 78%, and increased the average glandular dose by 285%. At a constant lesion CNR, the lowest average glandular dose value occurred at 27.3 kVp. Increasing or decreasing the x-ray tube voltage by 2.3 kVp from the optimum kVp increased the average glandular dose values by 5%. These results show that imaging simulated masses in a 4.2 cm compressed breast at approximately 27 kVp with a Mo/Mo target/filter results in the lowest average glandular dose.

Equipment Failure Analysis↗

PubMed2002

How good is the ACR accreditation phantom for assessing image quality in digital mammography?

RATIONALE AND OBJECTIVES: The purpose of this study was to evaluate the American College of Radiology (ACR) accreditation phantom for assessing image quality in digital mammography. MATERIALS AND METHODS: Digital images were obtained of an ACR accreditation phantom at varying mAs (constant kVp) and varying kVp (constant mAs). The average glandular dose for a breast with 50% glandularity was determined for each technique factor. Images were displayed on a 5 mega-pixel monitor, with the window width and level settings individually optimized for viewing the fibers, specks, and masses in the ACR phantom. Digital images of the ACR phantom were presented in a random manner to eight observers, each of whom indicated the number of objects visible in each image. RESULTS: Intraobserver variability was greater than interobserver variability for the detection of fibers and specks, but the reverse was true for the detection of masses. As the mAs increased, the number of fibers visible increased from less than one at 5 mAs to all six being visible at 80 mAs. The corresponding number of visible specks increased from 12 to 24, and the number of visible masses increased from 1.25 to about four. Above 26 kVp, object visibility was constant with increasing x-ray tube voltage. Reducing the x-ray tube voltage to 24 kVp, however, reduced the number of visible fibers from six to five, the number of visible specks from 24 to 21.1, and the number of visible masses from four to 3.1. Observer performance was approximately constant for average glandular doses greater than 1.6 mGy, so that the range of lesion detectability in the ACR phantom occurs at doses lower than those normally encountered in clinical practice. CONCLUSION: The current design of the ACR phantom is unsatisfactory for assessing image quality in digital mammography.

Accreditation↗