Micro-Focus X-ray Tube
Micro focus tubes have been used to refine image quality by providing the smallest focal spot size and enabling the magnification factor to increase thus increasing visible resolution. Micro-focus and Micro CT systems provide reliable resolution as low as 2 microns. As you know from the inverse square law and focal spot size discussions earlier, the smaller the focal spot to less area surface is available to boil off electrons. The result is that your tube current is limited because your filament current is limited. If you have ever experienced or experimented with an adjustable power supply connected to an incandescent light bulb, you know that the light can be driven brighter and brighter as current increase until the point that the light bulbs filament will burn up. The same concept applies to the X-ray tube filament. So there are physical limitations to developing a filament that is sized in the micron range.
Development of a smaller high powered focal spots is a key engineering task that has been researched for almost as long as the x-ray tube has been in existence. Indeed, even high energy requirements and the development of linear accelerator tubes main requirement is a stable small focal spot. Both of these technologies require that electrons flow to the target area in a stabile electron beam. The control of the electrons that are boiled off by the X-ray tube filament can also be achieved with a grid bias. To achieve the precise sizing and shaping of the electron beam, magnetic field are generated to steer and shape the electron beam.
Grid Bias X-ray Tube
The grid tube was patented in the early 1960's. The technology was developed to control the X-ray beam on and off time. In early and some modern day x-ray systems, high tension cables are used to carry the high voltage from the X-ray transformer to the X-ray tube. Because the cables are conductors separated by an insulator, they have a capacitive value and will store a charge. If we were to think of the tube as a large variable resistor, the circuit would be a simple R/C network and the cable charge will drain depending on the tube conduction where as the higher the conduction the higher the tube current and the faster the cable is discharged. In early X-ray systems the HV capacitance or lag was particularly undesirable because the x-ray would continue for a short while after the exposure was commanded to stop. The use of a grid tube will allow the operator to run at higher frame rates and shut the X-rays off by essentially shorting the cathode electron beam. Early cardiology examinations used a 35mm movie camera and an image intensifier (we will discuss imaging later) to record the flow of the blood mixed with a radio-opaque substance through the heart muscle and shut the beam off between the frames so that the patient was not over radiated and so that the exposure provided a quick blur free image on each frame. The subsequent control of the discharge to shut off the X-rays caused a large amount of heat. However, this technology led to grid developments and improvements that allowed the control of the beam size and shape.
Micro-Focus Transmission Tube
The shape of the filament, the grid shape and bias, the electromagnetic control of the beam width and the target thickness and materials will determine the focal spot size and beam quality in a modern micro-focus X-ray tube. That together with the ability to keep the system cool will provide the operator with a stabile focal spot of optimal size and energy. In the transmission tube picture you can see the sections of a micro-focus tube that is capable of a focal spot size of 2 microns and almost unlimited exposure time. This type of X-ray tube is used in the diondo micro focus systems to provide the operator with a a high resolution CT image quickly and accurately.
The selection of X-ray tube is very important and the target material and thickness are created to match several applications. "This layer has a thickness of 1 - 10 microns, depending on the application of the X-ray tube. For special tasks different materials can be used for this layer. Which type of transmission target and which materials are best suited for your application?" Maximum heat dissipation for the target is crucial because if you think about it, you can compare this 300,000 electron volt beam to an electron beam welder such as used in metal deposition additive manufacturing. The result of a tube target that is not cooled properly can cause disfiguration due to melting. Contact us to get more information about X-ray tube selection for CT and digital radiographic systems.