WHITE PAPER - Small Focal Spot

By David Campos / January 02, 2017
Reduced geometrical un-sharpness
Geometric unsharpness (Ug) is the lack of sharpness in an image that results from variation in the geometry of an X-ray system set-up. Most heavily influenced by the physical size of the focal spot, Ug has a major effect on image resolution and the ability to detect defects and resolve small details, also referred to as detail detectability.

Radiation originates from the entire surface of the focal spot, not just from a single point. This causes variation in the path of individual X-rays from the source to any one point in an object. A tighter focal spot results in a sharper image, whereas a larger focal spot increases the blurring of details in the X-ray projection, (Figure 1).  

This is clearly illustrated when radiation is projected through an object and onto a detector. The paths of the X-rays vary from their origin in the focal spot to the edge of the sample. This penumbra, or area of variation, is defined as the Ug, (Figure 2).

Geometric unsharpness is characterized by three parameters; the size of the focal spot (f), the distance between the test object and the source (SOD), and the distance between the test object and the detector (ODD). This relationship is represented by the equation:

Ug = f*(ODD/SOD)

Since Ug and focal spot size are directly related, a smaller focal spot leads to a smaller Ug. It follows that as Ug decreases, detail detectability increases and smaller defects and details can be detected, and can be distinguished in closer proximity to one another.

In the case of the small focal spot for PXS EVO, the focal spot is reduced from 3.0 mm to 1.0 mm and the geometric unsharpness is reduced by a factor of three. This extends the limits for flaw detection of 225 kV and 300 kV portable directional X-ray systems.

Reduced exposure time
Lengthy exposure times can reduce workflow and limit the number of applications where X-ray technology can be used. Advances in X-ray tube technology can minimize the effect of long exposure times on work place efficiency.

Focal spot size (f) and source-to-detector distance (SDD), central parameters of X-ray imaging, are directly related and can be reduced without increasing geometric unsharpness (Ug) and degrading image quality. When the source-to-detector distance is decreased, the dose rate (I) increases, according to the inverse square law, by a power of two.

I = 1 / SDD2

In the case of the PXS EVO, when the focal spot size is reduced from 3.0 mm to 1.0 mm, the source-to-detector distance can be reduced up to a third without a significant decrease in image quality. As a result, the dose rate will increase by a factor of up to nine times the original dose rate. This results in a potential reduction in exposure time of up to 90% as well.

Not only can the work flow be significantly increased with the reduction of focal spot size, but the range of applications where 225 kV and 300 kV directional X-ray equipment can be used may be extended.

COMET offers the PXS EVO with a small focal spot. High quality portable X-ray systems featuring a combination of 160 to 300 kV, a 1.0 mm focal spot, and constant potential X-ray power for high penetration, shorter exposure times, and high resolution results.

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