Development and assessment of a quality assurance device for radiation field-light field congruence testing in diagnostic radiology

Abstract

Purpose: Existing methods for checking the light field-radiation field congruence on x-ray equipment either do not fully meet the conditions of various quality control standards regarding inherent uncertainty requirements or contain subjective steps, further increasing the uncertainty of the end result. The aim of this work was to develop a method to check the light field-radiation field congruence on all x-ray equipment. The result should have a low uncertainty which is accomplished by eliminating most subjective user steps in the method. A secondary aim was to maintain the same level of usability as of comparable methods but still able to store the result. Approach: A new device has been developed where the light field and corresponding radiation field are monitored through measurements of the field edge locations (in total: $2\times 4\text{edges}$ ). The maximum field size location deviation between light field and radiation field in the new method is constrained by the physical limitations of the sensors used in various versions of the prototype: linear image sensors (LISs) of 25 to 29 mm active sensor length. The LISs were sensitized to x-rays by applying a phosphor strip of ${\mathrm{Gd}}_2{O}_{2}S:\mathrm{Tb}$ covering the light sensor input area. Later prototypes of the completed LIS device also have the option of a Bluetooth (100-m range standard) connection, thus increasing the mobility. Results: The developed device has a special feature of localization a field edge without any prior, subjective, alignment procedure of the user, i.e., the signals produced were processed by software storing the associated field edge profiles, localizing the edges in them, and finally displaying the calculated deviation. The uncertainty in field edge location difference was estimated to be $<0.1\mathrm{mm}$ ( $k=2$ ). The calculated uncertainty is lower than for other, commercially available, methods for light field-radiation field congruence also presented in this work. Conclusions: A method to check the light field-radiation field congruence of x-ray systems was developed to improve the limitations found in existing methods, such as device detector resolution, subjective operator steps, or the lack of storing results for later analysis. The development work overcame several challenges including mathematically describing real-life edges of light and radiation fields, noise reduction of radiation edges, and mapping/quantification of the rarely observed phenomenon of focal spot wandering. The assessment of the method showed that the listed limitations were overcome, and the aims were accomplished. It is therefore believed that the device can improve the work in quality controls of x-ray systems.

Keywords: congruence; field edge; focal spot wandering; linear image sensor; quality control; radioluminescence.

Fig. 1

One of the prototype versions of the LIS device in this work. (a) Assembled. (b) LIS connected to main board comprising a USB connection for further profile analysis on a PC. Phosphor strip on sensor is clearly seen in this picture.

Fig. 2

Acquisition of the respective field edge profiles using an early LIS prototype. (a) The device in place with the slit perpendicular to the edge of one of the four sides of the light field and the profile appears in the GUI. A 1D image of the profile is stored. Field size $20\times 20\mathrm{cm}$. (b) Without moving the device, an x-ray exposure is made and the 1D image of the radiation field edge profile is stored for one side at a time. The x-ray field is illustrated with the superposed square, not normally visible. Note the noise in x-ray field edge profile (lower right).

Fig. 3

Illustration of noise suppression process. Radiation field edge profile: (a) before and (b) after applying a positive noise suppression filter. An eight-pixel window slides across the profile replacing the maximum pixel value within the window equal to the calculated mean pixel value in the very same window. The spikes are effectively removed without altering the baseline of the profile. The remaining noise is assumed random and a Gaussian filter has been applied on the intermediate profile to remove the random noise before a 5PL-function (see Appendix) is finally fitted to the bottom noise processed edge profile.

Fig. 4

The 25% edges of each profile, indicated by circular dots (light field and radiation field), are compared and the difference in localization is converted to millimeters, producing the alignment. In this example, there is a deviation of $+1.8\mathrm{mm}$ between the light field and radiation field edges.

Fig. 5

Compared methods/devices.

Fig. 6

Consecutive edge location results for varying settings. (a) X-ray edge location mammographic equipment. $1\text{pixel}=8\mu \mathrm{m}$. Apparent focal spot wandering is shown (see text for explanation). X-ray tube is cold at starting point (#1). Thermal balance is achieved following $\sim 15$ exposures, i.e., wandering effect ends. Settings: 1 to 5: $28\mathrm{kVp}/50\mathrm{mAs}$, 6 to 21: 25 to $34\mathrm{kVp}/50\mathrm{mAs}$, Rh-filter, and 22 to 31: $28\mathrm{kVp}/40$ to 200 mAs. (b) Edge location results for conventional system. Apparent focal spot wandering is observed. X-ray tube cold at starting point (#1). Allowed to cool between exposure #5 and #6. Settings: 1 to 5: $80\mathrm{kVp}/50\mathrm{mAs}$, 6 to 9: 60 to $100\mathrm{kVp}/50\mathrm{mAs}$, 10 to 15: $80\mathrm{kVp}/20$ to 100 mAs.

Fig. 7

A five-parameter logistics curve. Parameters $a$ and $b$ are the asymptotic value when $x\to -\infty$ and $x\to +\infty$, respectively, parameter $c$ shifts the curve in the $x$-direction. Parameters $d$ and $E$ dictate the near-asymptotic behavior of the curve. The 25% intensity value is indicated, i.e., the edge of the curve. Based on a figure from Ref. .

Fig. 8

Light output change in arbitrary units with increasing phosphor layer thickness shown for two particle sizes; 7 and $25\mu \mathrm{m}$. The applied phosphor strip in the prototype is indicated in the diagram ($25\mu \mathrm{m}$ particle size, $100\mu \mathrm{m}$ thickness). Arrow and encircled value indicate currently chosen thickness in the prototype.