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

Geometrical model for calculating the effect of surface morphology on total x-ray output of medical x-ray tubes

Purpose

Correlation of characteristic surface appearance and surface roughness with measured air kerma (kinetic energy released in air) reduction of tungsten‐rhenium (WRe) stationary anode surfaces.

Methods

A stationary anode test system was developed and used to alter nine initially ground sample surfaces through thermal cycling at high temperatures. A geometrical model based on high resolution surface data was implemented to correlate the measured reduction of the air kerma rate with the changing surface appearance of the samples. In addition to the nine thermally cycled samples, three samples received synthetic surface structuring to prove the applicability of the model to nonconventional surface alterations. Representative surface data and surface roughness values were acquired by laser scanning confocal microscopy.

Results

After thermal cycling in the stationary anode test system, the samples showed surface features comparable to rotating anodes after long‐time operation. The established model enables the appearance of characteristic surface features like crack networks, pitting, and local melting to be linked to the local x‐ray output at 100 kV tube voltage ,10° anode take off angle and 2 mm of added Al filtration. The results from the conducted air kerma measurements were compared to the predicted total x‐ray output reduction from the geometrical model and show, on average, less than 10 % error within the 12 tested samples. In certain boundaries, the calculated surface roughness R_a showed a linear correlation with the measured air kerma reduction when samples were having comparable damaging characteristics and similar operation parameters. The orientation of the surface features had a strong impact on the measured air kerma rate which was shown by testing synthetically structured surfaces.

Conclusions

The geometrical model used herein considers and describes the effect of individual surface features on the x‐ray output. In close boundaries arithmetic surface roughness R_a was found to be a useful characteristic value on estimating the effect of surface damage on total x‐ray output.

Monte Carlo simulation of the effects of anode surface roughness on x-ray spectra

PURPOSE

Spectral and angular distribution of the x-ray beam generated by medical x-ray tubes as a function of anode surface roughness was analyzed.

METHODS

Different sets of profiles such as ideal flat, regular profiles, and measured profiles adopted from the literature were analyzed by means of MCNPX Monte Carlo simulator. The geometry used was simplified to separate different physical effects. A sphere centered on the origin of the coordinate system was divided into two hemispheres filled with tungsten and a vacuum, respectively. The studied anode surfaces were placed at the center of the plane of the hemisphere. The profiles were realized by means of the general lattice structure of the MCNPX. The energy and angular distributions of the excited photons were recorded with energy and angular resolutions of 0.5 keV and 1 degrees, respectively, by means of point detectors. The range of the studied anode surface roughness was 0-550 micro Ra. The emission angle dependencies of the following quantities were analyzed: Half value layer (HVL) value, intensity, and spectral photon flux.

RESULTS

The analysis of the HVL of the x-ray beam showed that around an emission angle of 5 degrees, the hardness of the beam was practically independent of the surface roughness. The value of this emission angle depends on the filtration. Below this critical angle, the HVL value decreases, while at a higher emission angle, the beam becomes harder with increasing surface roughness. The intensity degradation saturates with increasing roughness. The position of the maximum spectral photon flux shifts to higher emission angles as the anode surface roughness increases. The surface roughness (Ra) was found to be an inadequate quantity to describe the effect of anode surface roughness on x-ray spectra since no definite connection was found between the values of the intensity degradation and surface roughness. At 120 kVp tube voltage and at a 3.84 microm Ra roughness value, the effect of anode surface roughness introduces a 5% and 12% intensity degradation at a 5 degrees and 12 degrees emission angle, respectively. However, it has a higher impact at low tube voltages (<60 keV), e.g., in mammography systems where the intensity degradation could even be 25% at the "newly" polished anode surface.

CONCLUSIONS

The effects of anode surface roughness on x-ray spectra were successfully simulated by a Monte Carlo method. It was proved that the effect of the anode surface roughness could not be modeled by simple filters made from the anode material. The surface roughness (Ra) was found to be an inadequate quantity to describe the effect of anode surface roughness on x-ray spectra.

Estimate of organ radiation absorbed doses in clinical CT using the radiation treatment planning system

Organ absorbed doses in computed tomography (CT) scans can be measured with anatomical phantoms but not inside the human body. In this study, a straightforward method was investigated to estimate organ doses in clinical CT using the radiation treatment planning system (RTPS) and compared them with experimental results of photoluminescence dosemeters (PLD). In a heterogeneous phantom, the average difference between PLD and RTPS values were -5.0% for the body and 7.1% for the lung. Using CT data, organ doses in 30 clinical cases were then calculated. There was a significant inverse correlation between the calculated values of organ doses and body mass index (BMI, correlation coefficients (r) = -0.69 (whole body), -0.80 (right lung), -0.81 (left lung), -0.76 (spinal cord), -0.74 (vertebra bone), -0.74 (heart), and -0.79 (oesophagus), all p < 0.01). An RTPS can be a simple and useful tool for estimating equivalent doses inside the human body, during whole-body CT scans.