1. Ray Traces and Pinhole Images of the Xoft Axxent™ X-ray Source
Steve Axelrod, Thomas W. Rusch, Zirao Zheng – Xoft, Inc Tom Case, Brian Wilfley – Triple Ring Technologies

2. The Xoft Axxent™ X-ray Tube
2.2 mm diameter
Runs at 50 kV, 300 µA
Breast brachytherapy
X-Ray Tube
HV Cable

3. The Xoft Axxent X-ray Treatment System
Controller
Source and balloon applicator

4. Motivation For the Study
To develop and manufacture a miniature x-ray tube, one needs to understand the detailed physics
Electron beam transport critical performance parameter
(Ideal) geometry can be modeled
Beam distribution at anode determined with X-ray pinhole images
Output compared to actual behavior
A vetted computer model is a powerful tool in design and manufacturing

5. Ray Traces of Electron Trajectories
Electrons emitted from hot cathode accelerated by the 50 kV potential, impact the anode, create x-rays
Right: very fine mesh defined near filament
Modeling program is OmniTrak, by Field Precision LLC
Below: less fine mesh transports electrons  anode

6. Electron Beam Intensity Distributions
Filament launch points weighted by a thermal model of the filament temperature profile
Followed by a Richardson-Dushman thermionic emission/extraction model
Left: side views
Below: end view

7. Modeled Effects of Filament Misalignment
Offset filament a few 0.001” crosswise and lengthwise
Determine manufacturing tolerances
Misaligned crosswise
Misaligned lengthwise
Above: aligned just right

8. Pinhole Images - Principle
A very good way to see things upside down
Key fact: The last Sunday in April is “Worldwide Pinhole Photography Day” - Wikipedia

9. Pinhole Images – Apparatus Description
Source held inside a brass enclosure, with a 30 μm pinhole
Can look at either the end or the side of the anode
ShadoCam image plane* used to capture x-ray images
X-ray sensitivity 10 – 50 kV
The x-ray launch points closely match electron distribution
Little scatter in the anode
Ran sources at various kV and μA operating points
Typically integrated for 10 seconds
* Shad-o-Snap 1024, Rad-icon Imaging Corp

10. Sample Images – Side View
Model prediction 
Actual images

11. Sample Images – “Typical” End View
Central diamond and surrounding oval are clearly visible

12. Effect of Pinhole Aperture Diameter
400, 100 and 30 μm pinholes
Source was an experimental design

13. Out-takes
Not all sources built pass the stringent criteria for human use
Polar, azimuthal, depth dose, output level, output stability
Not all have pretty pictures…

14. Rogues’ Gallery of Images
Not Human Use Tubes!

15. Conclusion
Beam optics modeling provides important insight into design parameters
And allowable ranges of variation
Major features of the electron beam distribution predicted by computer modeling are observable in pinhole x-ray images of the sources
Variations seen among pinhole images of different sources do not correlate with spatial distributions significantly until the variations are extreme

16. The End
Thanks for sticking around!