The area of target where the electrons are absorbed and the x-ray are generated is called focal spot size or target area. If the focal area is very small, the penumbra will be lesser and the picture sharpness will be good but heat removal is difficult. If the focal area is large the penumbra is large the picture sharpness is bad, but heat will be removed quickly. The focal sizes of 0.3 mm to 0.6 mm used in mammography and 1.0 mm to 1.2 mm are commonly used in Radiology. The focal spot has major effect on spatial resolution.
There are two types of focal spot size
a) Actual focal spot :- The actual focal spot size is the area on the anode that is struck by electrons.
b) Effective focal spot size:- The Effective focal spot size is the length and width of the emitted x-ray beam as projected down the central axis of the tube. The Effective focal spot length is smaller than the actual focal spot.
In 1984 the standard specific that method for measuring focal spot size by use of slit camera. A slit camera is also used to measure focal spot blooming and modulation transfer function (MTF). Focal spot orientation and intensity distribution are still measured with a pinhole camera. The resolution of focal spot is measured by star pattern.
What does focal spot size effect.
The focal spot size in an x-ray tube affects the resolution of the image that can be produced and penumbra size also effect Blurring due to this factor is determined by the relative sizes of the penumbra and the radiographic image . A smaller focal spot size allows for higher resolution images because it produces a more finely focused x-ray beam. However, using a smaller focal spot size also requires more precise alignment of the x-ray tube and the detector, and it can lead to increased patient dose if not used properly.
What determines the size of focal spot
The size of the focal spot in an x-ray tube is determined by the shape and size of the target on the x-ray tube, as well as the current and voltage of the x-ray tube.
The target is typically made of tungsten or another high-Z material, and it is shaped and positioned so that the x-ray beam is focused onto a small spot on its surface. The size of this spot is known as the focal spot size. The shape and size of the target can be adjusted to produce different focal spot sizes.
The current and voltage of the x-ray tube also play a role in determining the focal spot size. Increasing the current or the voltage can result in a smaller focal spot size, but it can also lead to increased patient dose. The optimal current and voltage settings will depend on the specific x-ray tube and the desired focal spot size.
How do you choose focal spot size.
In general, the focal spot size should be chosen to match the size of the object being imaged and the resolution required for the particular application. For example, a larger focal spot size may be necessary when imaging a large object or when a lower resolution image is sufficient, while a smaller focal spot size may be necessary when imaging a small object or when a higher resolution image is required. It is important to balance the need for higher resolution with the potential for increased patient dose when selecting the focal spot size.