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​​What is Focal Spot Size?

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Do you even have any idea of what X-ray technicians mean when they talk about X-ray tubes’ focal spot size? 

X-ray focal spot is often one of the factors causing the loss of resolution for X-ray imaging systems which is usually attributed to the fixed size and shape of the focal spot. Consequently, knowing the limitations that focal spot size imposes on X-ray imaging systems becomes critical especially in the efforts to improve image quality with methods like direct magnification. 

This guide gives you the road map to navigate through all the twists and turns of focal spot size of an X-ray tube. So, if you would like to know more then let’s dive in together. 


I. What is the Focal Spot (Actual Focal Spot)? 

For starters, imaging systems such as X-ray machines and computer tomography rely very much on the quality of the image produced. However, the resolution of the image which is critical to this image quality relies significantly on the focal spot size of the X-ray tube.

The focal spot is the area of the anode in an X-ray tube where the electrons from the filament hit the anode disc from where the X-ray radiation is emitted to the sample under examination. The manufacturer of the X-ray tube determines the size of the cathode and the filament which in turn determines the focal spot size. 


What is the Effective Focal Spot Size


II. What is the Effective Focal Spot Size?

Generally, the effective focal spot size is the area of the focal spot that is projected along the main axis of the major X-ray area. Usually, the actual focal spot size determines the effective spot size, and their relationship is influenced significantly by the angle of the anode. Evaluating the effective focal spot size is important in estimating the efficiency of the X-ray tube as it gives an idea of the quantities of certain parameters used for X-ray operations. These parameters include the amperage, exposure time, and the voltage. It also helps to increase the lifespan of the X-ray tube as you can immediately find out if you are overloading your X-ray tube. 


III. Why is Focal Spot Important?

While the quality of an X-ray imaging system is affected by several important factors, the focal spot size of the X-ray tube is arguably the most important of these factors. One reason is that knowing the geometry and focal spot size helps in careful interpretations of X-ray images. Also, it helps in getting clarity on focal spot system’s limitations.  

The loss of resolution in X-ray imaging systems has been sometimes attributed to the fixed size and shape of the X-ray focal spot. Consequently, focal spot is critical to creating a high-quality image by protecting the imaging system’s resolution. You’ll often find majority X-ray tubes with more than one focal spot size as the focal spot size is influential to the spatial resolution given that smaller focal spot sizes give better spatial resolution for an X-ray imaging system. In any case, smaller focal spot sizes have a higher concentration of heat energy on a smaller area of the anode which requires a longer time for it to cool. 

But while the focal spot needs to be small to generate sharp images it should also be strong enough to sustain the heat load without melting the anode target. However, in practice where spatial resolution is critical the focal spot size used is often small. On the contrary, where short exposure time is the main concern, then a large focal spot size is employed.


IV. What does Focal Spot Size Affect?

The importance of focal spot size cannot be overemphasized seeing that it affects a lot of parameters in an X-ray imaging system, the key one being the spatial resolution that is directly related to the quality of the image. But wait there’s more.

One critical effect of the focal spot size is the exposure time of test samples to X-ray beams. For a small focal spot size, the intensity of the heat is higher since it requires a smaller area of anode to project the X-ray beam. This small area of the anode takes a longer time to cover the entirety of the test sample, but a sharp image is the result. However, using a larger focal spot size ensures that the sample is covered in a shorter period reducing the exposure time of the sample.

The availability of both broad and fine focal spot sizes in most X-ray tubes introduces another side of focal spot effects on X-ray tubes. Apparently, some individuals use more fine focus in their X-ray examinations and this excessive use can lead to a shorter lifespan for the X-ray tube. 

Also, focal spot size affects the accuracy of X-ray inspection since the accuracy of this inspection relies heavily on the quality of the image generated. The focal spot size can significantly influence this image quality as smaller focal spot size have been known to increase the sharpness of images.



In conclusion, if you are in the market for an X-ray machine, it is essential that you critically consider the focal spot size of the X-ray tube that comes with the machine. The inspection effectiveness of your machine is significantly dependent on the manufacturer’s specification of the X-ray tube and filament which greatly impacts the focal spot size. However, while you want to achieve sharp images with high resolution, high-quality images with your X-ray tube, you should also consider the thermal resistivity of the anode. The reason is because, smaller focal spot increases the heat energy generated for a smaller area of the anode, and if the anode is not strong enough to withstand the heat, it might result in melting of the anode.

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