450 kV MesoFocus CT

35 µm resolution at first trial with 450 kV MesoFocus.

By Dr. Daniel Stickler / October 26, 2021

Isn't the unsharpness of a mini-focus system the real limitation of what you can achieve with it, especially at 
450 kV?
 
"We shall see what we shall see." (W. Röntgen 1896)
 
In Spring of 2021, the long-awaited 450 kV MesoFocus tube came to Yxlon, and the blur or let's say the fog disappeared from the images. A Computed Tomography (CT) system with the reliable 450 kV MesoFocus now enables up to 35 µm resolution with a level of details that was previously known from micro-focus systems only with normally much less energy.
 
In this blog, we would like to give you first insights into the very convincing laboratory results and show you an idea of future possibilities with closed 450 kV sources.

MesoFocus

In X-ray technology, “Meso”, the Greek prefix for “middle”, describes radiation sources with focal spot sizes that lie between mini- and micro-focus. With focal spot sizes of down to 50 µm, the MesoFocus sources support applications for which only micro-focus tubes could previously be used.

Comet X-Ray's MesoFocus technology attracted a lot of attention in 2021. The 225 kV variant very reliably supports a large number of applications with requirements for high resolution and high stability, as well as maintenance-free at-line and in-line NDT.
 
The MesoFocus X-ray tube combines:
 
  • High resolution to the point 
  • Simple operation, 'on/off' set parameters, and nothing else 
  • Fast operational readiness 
  • Extremely high reliability and availability 
  • Minimal maintenance

Is it already over at 225 kV?

For objects of heavy metals or higher dense material-thickness, it is necessary to switch to a higher kV class (320, 450, or 600 kV). With closed tubes, there is currently no solution with high resolution which is at the same time reliable, simple, and with low-maintenance effort. [1]
 
For years, many industries such as aerospace and additive manufacturing, from the laboratory to manufacturing, have been demanding a high-resolution and robust 450 kV solution for DR and CT applications.  In addition to high resolution and detail recognizability, there are always other requirements such as quick readiness, high voltage stability, maintenance-free, ease of use, and use without special tube know-how.
 
With the 450 kV MesoFocus, the solution is not only visible on the horizon or within reach, but it is already in use in our laboratory. As with the 225 kV MesoFocus, Yxlon was also the first address for the new 450 kV tube in order to evaluate the X-ray source in DR and CT in detail and to compare it with other solutions. Different from the 225, the 450 kV MesoFocus for the present offers 5 instead of 3 focal spots (with 1W/µm): 450, 350, 250, 100, and 60 µm (or W).

First example
 
A 450 kV HP11 mini-focus X-ray source has a focal spot of about 400 µm. Large detectors in the range of 150 µm pixel size, which offer a good compromise between resolution, efficiency, and data volume, allow a spatial resolution of about 100 - 120 µm or 4.5 - 5 lp/mm. To get this resolution in the CT system, a magnification of about 2 is chosen. Smaller objects made of or surrounded by heavy metals are thus only imaged with a little amount of detail.
 
Instead of choosing a special application, we want to go first with an easily recognizable object with fine structures to show how the resolution will change from today to tomorrow, even at 450 kV.
 
Fine Swiss mechanics with blocked vision
 
To be in line with the Swiss technological innovation we have chosen a fine-mechanical Swiss wristwatch in the first example. The case and bracelet made of 14 K (585) gold make the CT much more demanding so that 450 kV is a good choice for the multi-material CT.
 

Fig. 1: Photo of the wristwatch, scanned with high resolution
 
As of today, choosing the small focal spot of a 450-HP11, only parts of the coarser wristwatch mechanics can be seen. Due to a lack of details, it is difficult or even impossible to recognize the mechanical functions, interactions, or the connection between individual elements.


Fig. 2: When opening the watch, it’s no problem to see the wristwatch
echanics in much higher resolution than is possible with a mini-focus CT of Fig. 3.
The case and the mechanics of the watch are a mix of different materials.
 
The wristwatch is a perfect example to see that a CT scan with a mini-focus tube doesn’t achieve the resolution we are used to with our own eyes. For example, the leaf spring can almost only get recognized as a ring, and here and on other layers, gear wheels merge into each another. (Fig. 3 - on the left)

That is now changing in the 450 kV range. With a 60 µm focal spot, it’s no longer absurd to CT-scan such mechanics or small gold soldering porosities with a closed tube. (Fig. 4 - on the right)

Please click on the pictures to see them in original size.
In this example, the blurring has significantly reduced such that the material surfaces are sharp and in evidence. The threads of tiny screws are just as clearly visible as the leaf spring, only to describe a few details. The higher sharpness in the CT also reduces the possibility that details get reconstructed into other layers as disturbing ghosting artifacts.

Fig. 7: 450 kV MesoFocus projection at a magnification of 5.65

Key scan parameters for this example:
  • MesoFocus, 450 kV 2mm Sn
  • 60 Watt, 60 µm focal spot
  • 4343HE, 0.5 Hz, 60 min. scan time FDD/FDD (mm): 1000/177
 
When working with the MesoFocus tube, the most striking feature is its quick readiness for use. As with any closed X-ray tube, a new warm-up is only necessary after a pause of more than 12 hours, and it runs through within a few minutes since a breakdown, which often occurs with an open micro-focus tube, is an extremely rare interruption here. Not one of the continuous scans was interrupted by breakdowns or other tube problems. There were no filament changes or target position changes, and no brightness fluctuations occurred.
  
[1] At higher kV classes such as 450 kV, the detector could provide better resolution in the 2D range. Here, requirements better than 100 µm (D10 duplex wire IQI) could be met.


In the next blog, we will look at porosities in a golden solder joint in the same scan and observe how detail detectability changes with the choice of the focal spot.
 
Later in this series, we will show an example from additive manufacturing with a 100 mm thick, disk-like copper object and the ability to analyze internal roughness.