The effect of x-ray spectra from molybdenum and tungsten target tubes on image quality in mammography

Quantifying the effect of anode surface roughness on diagnostic x-ray spectra using Monte Carlo simulation

PURPOSE

The accurate prediction of x-ray spectra under typical conditions encountered in clinical x-ray examination procedures and the assessment of factors influencing them has been a longstanding goal of the diagnostic radiology and medical physics communities. In this work, the influence of anode surface roughness on diagnostic x-ray spectra is evaluated using MCNP4C-based Monte Carlo simulations.

METHODS

An image-based modeling method was used to create realistic models from surface-cracked anodes. An in-house computer program was written to model the geometric pattern of cracks and irregularities from digital images of focal track surface in order to define the modeled anodes into MCNP input file. To consider average roughness and mean crack depth into the models, the surface of anodes was characterized by scanning electron microscopy and surface profilometry. It was found that the average roughness (Ra) in the most aged tube studied is about 50 pm. The correctness of MCNP4C in simulating diagnostic x-ray spectra was thoroughly verified by calling its Gaussian energy broadening card and comparing the simulated spectra with experimentally measured ones. The assessment of anode roughness involved the comparison of simulated spectra in deteriorated anodes with those simulated in perfectly plain anodes considered as reference. From these comparisons, the variations in output intensity, half value layer (HVL), heel effect, and patient dose were studied.

RESULTS

An intensity loss of 4.5% and 16.8% was predicted for anodes aged by 5 and 50 microm deep cracks (50 kVp, 6 degrees target angle, and 2.5 mm A1 total filtration). The variations in HVL were not significant as the spectra were not hardened by more than 2.5%; however, the trend for this variation was to increase with roughness. By deploying several point detector tallies along the anode-cathode direction and averaging exposure over them, it was found that for a 6 degrees anode, roughened by 50 microm deep cracks, the reduction in exposure is 14.9% and 13.1% for 70 and 120 kVp tube voltages, respectively. For the evaluation of patient dose, entrance skin radiation dose was calculated for typical chest x-ray examinations. It was shown that as anode roughness increases, patient entrance skin dose decreases averagely by a factor of 15%.

CONCLUSIONS

It was concluded that the anode surface roughness can have a non-negligible effect on output spectra in aged x-ray imaging tubes and its impact should be carefully considered in diagnostic x-ray imaging modalities.

A dual-energy subtraction technique for microcalcification imaging in digital mammography--a signal-to-noise analysis

Breast cancer may manifest as microcalcifications (microCs) in x-ray mammography. However, the detection and visualization of microCs are often obscured by the overlapping tissue structures. The dual-energy subtraction imaging technique offers an alternative approach for imaging and visualizing microCs. With this technique, separate high- and low-energy images are acquired and their differences are used to "cancel" out the background tissue structures. However, the subtraction process could increase the statistical noise level relative to the calcification contrast. Therefore, a key issue with the dual-energy subtraction imaging technique is to weigh the benefit of removing the cluttered background tissue structure over the drawback of reduced signal-to-noise ratio in the subtracted microC images. In this report, a theoretical framework for calculating the (quantum) noise in the subtraction images is developed and the numerical computations are described. We estimate the noise levels in the dual-energy subtraction signals under various imaging conditions, including the x-ray spectra, microC size, tissue composition, and breast thickness. The selection of imaging parameters is optimized to evaluate the feasibility of using a dual-energy subtraction technique for the improved detection and visualization of microCs. We present the results and discuss its dependence on imaging parameters.

Screen-film and digital mammography. Image quality and radiation dose considerations

Factors affecting image quality and patient dose in screen-film and digital mammography have been discussed. Some proposed parameters for judging image quality and breast exposure measurements and dose calculations relating to changes in image quality factors have been reviewed. It is important to remember that the goal in making a mammogram is to obtain as much diagnostic information as possible at the lowest dose compatible with that information. As noted previously, this necessitates compromises (i.e., an optimization of factors that affect image quality). These include beam quality, compression, imaging geometry, grids, receptor characteristics, processing of the film or digital image, and display and viewing conditions. If this is done correctly, a high-quality mammogram can be obtained at a reasonably low dose to the patient. The goal is not simply to use as low a dose as possible, because if this is done there is a large risk of degrading the performance of mammography in detecting or accurately characterizing small, node-negative cancers.

The sensitivity to technological innovations of the doses delivered by optimized mammography systems

Multiparameter optimizations have been carried out to study the effects of hypothetical technological advances on the patient doses required to maintain a given image quality in mammography. The assumed advances include: improvement in the power loading limits of the tube focal spot, increased absorption efficiency for a given detector resolution, increases in detector system gain, and changes in the exposure time limitations that result from patient motion. The optimization permits system geometry, kVp of the examination, filtration, detector resolution, focal spot size, and grid characteristics to vary simultaneously and self-consistently subject to image quality as well as technological constraints. The effects of technological innovations were measured by systematically varying the technological constraints to reflect hypothetical improvements and comparing the resulting minimized doses, required to maintain constant image quality, to a baseline optimized system derived from a set of baseline technological capabilities.

Effect of tungsten absorption edge filter on diagnostic x-ray spectra, image quality and absorbed dose to the patient

The X-ray spectra from a tungsten-target diagnostic tube were measured with a lithium-drifted silicon detector. Four characteristic X-ray peaks were clearly observed. When a 0.05-0.3 mm thick tungsten absorption edge filter was added to the tube, the number of photons in the spectra above the K-absorption edge decreased dramatically. The effect of the absorption edge filter on image quality and on the absorbed dose were investigated by both measurement and Monte Carlo calculation. The absorbed dose to the patient is reduced without image quality, being degraded.