 Hi, I am Dr. Indanil, Associate Professor in Radiology at GSL Medical College, Rajmandri, I am the Pradesh. Teaching is my passion, my areas of interest are physics, conventional, ultrasound and CT. Hi, I am Dr. Indanil and I will be discussing about the line focus principle. Now before we discuss what this principle is, we need to know a few concepts and terms. The first term is the incident electron beam. It is the electron beam that emanates from the filament from the cathode side and bombards the target anode. And the area on the anode where this incident electron beam strikes is called the actual focal spot. And the focal spot projected towards the patient side is called the effective focal spot. It is also called as the apparent focal spot because it appears to be of a different width than the actual focal spot, which is achieved by the angle of the anode and we will be discussing about this shortly. Now for a crisper and sharper radiographic images, what do we need? We need a thinner focal spot. The thinner the beam in the sense the narrower the beam, the sharper will be the image. And the wider the beam, the more blood will be the image due to a number effect. How do we achieve the thinner and thicker beams? That we can get by manipulating the width of this electron beam. The width of this electron beam depends on three factors. Number one, the size of the filament, the shorter the filament, the narrower will be the beam. The longer the filament, the wider will be the beam. The second one being the construction of the focusing cup in which the filament is placed. And the third one, the position of the filament within the cup. So these are the three factors that control the size of the electron beam. As this electron beam strikes the anode, so we can achieve a narrow electron beam by having a shorter filament. But as it strikes the anode, more amount of heat will be concentrated on a smaller area of the anode. As we know, X-ray production releases tremendous amount of heat energy. In fact, 99% of the energy is lost as heat and only 1% is fruitful in the production of X-ray. So this high energy beam will be converging only on a smaller surface area of the anode and this results in poor heat dissipation. And which in turn leads to reduced life of the X-ray tube. So the width of the beam should be longer so that the surface area increases and there will be more heat dissipation. But as I told you, if we have a wider beam, the image will be blurred. So we need a sharper image and we need a higher heat dissipation. So both are contrary to each other. So what do we do? We need a trade-off between the two. How do we achieve that? That is achieved by this simple line focus principle. Here we tilt the anode so that it has a certain angle with respect to the incident electron beam. And due to the angulation of the anode, the beam that is released, the X-ray beam that is projected towards the patient side, appears to be shortened. So we are achieving both purposes here. We are maintaining the width of the beam and we are maintaining a smaller size of the effective focal spot thereby achieving a sharper X-ray beam. Let me show you with a simple analogy, a simple mechanism what I mean by this effective focal spot shortening when related to the anode. Here you can see two blocks which are very close to each other and there is a very small gap through which the light can pass. A thin strip of light is passing through. But actually in reality, these two blocks that I will rotate and show you are placed wide apart. You can see the gap between the two is very wide because they are not placed on the same line with each other. But they are, one is in front and one is behind. They are angulated actually. They are placed in an angulation. So as you can see, if you see from this side, the gap is very less. It seems to be very close situated. But while in reality, this is the actual gap between the two blocks. So this is a simple 3D model that shows you the perception when viewed from a different angle how it appears. So when the beam strikes the anode and this is the projected beam that is called the effective focal spot, you can see that these two beams are making a right angle triangle here. You can observe there is a right angle triangle here and this is the anode angle which is constituting one angle of this right angle triangle. Now in your basic high school trigonometry, you know that sin theta is equal to opposite by hypotenuse. What is the opposite side of this triangle here? It is the effective focal spot. This line is the effective width of the effective focal spot. What is the hypotenuse here? This is the actual focal spot. So we substitute this and we get sin theta is effective focal spot divided by the actual focal spot. And from this, we get effective focal spot is equal to sin theta into actual focal spot. So if we know the anode angle and the width of the actual focal spot, we can calculate the size of the effective focal spot. In diagnostic radiology, the anode angles are usually range from 10 to 13 degrees and the size of the effective focal spot ranges from 0.6 to 1.3 millimeter. For process of like sineanography, the anode angle is further reduced around 7 to 9 degrees. So this is the principle behind line focus principle. I hope you understand the mechanism that lies behind the instant electron beam, the actual focal spot and the effective focal spot. Thank you.