 Today, I will be discussing about an important and interesting phenomenon to be kept in mind by positioning a patient for radiography. Do check it out. Hi, I am Dr. Indra Neel and I will be discussing about the anode heal effect. This effect stems from the fact that x-ray photons are produced not only at the surface of the anode, but also at deeper levels within the substance of the anode. X-rays are isotropic in nature. That means they are emitted equally in all directions. Now imagine a group of people say around 10 people had to run a 100 meters dash on a road with less traffic and obstacles, then all of them would cross the finish line. But if the same were to run a 5K long distance marathon on a busy road filled with traffic, probably only a few of them would make it to the finish line. Applying the same analogy, the number of x-ray photons that emerge nearer to the target surface which is facing towards the cathode side will be high as they travel a shorter path compared to the x-ray photons that travel a longer path through the substance or the heel of the anode. And during this process some of the x-ray photons get absorbed. As you can see, some of the x-ray photons are absorbed while traveling the longer distance through the anode atoms. As a result of this, there is a variation of intensity across the projected x-ray beam and this is called the anode heel effect. As you can see when the relative intensities are measured, we have a central ray as a standard 100% intensity and those towards the anode side have lesser intensities and those towards the cathode side have higher intensities. Now we all together cannot eliminate the anode heel effect completely, but we can make use of this during patient positioning such that the thicker body part is placed towards the cathode side and the thinner less denser part is placed towards the anode side which consists of less intense beams. Thereby we achieve a uniform exposure. For example, when we image the abdomen, the less dense upper abdomen can be placed towards the anode side and the more denser pelvic girdle can be placed towards the cathode side. Similarly in mammography, the more dense chest wall end is placed towards the cathode side and the less dense nipple end can be placed towards the anode side. Also, we can reduce the severity of this effect and make it less pronounced in three ways. The first is with the help of polymeters. These are like our window curtain blinds that restrict the size of the primary beam projected towards the patient. Lesser field size means the extreme values or the ranges of the X-ray beams on ether ends are cut out and the beam is made less diverse and the central field is allowed to be focused on the patient. Again, when the field size increases, the beam diversity that reaches the patient increases and anode yield effect becomes more pronounced. Second is by altering the SID. SID is the source image receptor distance. We can reduce or increase the anode yield effect by altering this SID, also called as focal film distance. Notice when the film is close to the source, it is exposed to beams of both extreme ends, a wider range of beams and thereby the anode yield effect increases, but when the film distance increases, the diversity of exposure is less. Finally, the anode angle also plays an important role in increasing or decreasing the yield effect. When the anode angle decreases, more X-rays are made to traverse through the substance of the anode, through the heel of the anode, thereby increasing the effect and increasing the anode angle The path away from the main substance of the anode reduces the heel effect. Summarizing the anode yield effect, the beam intensity is high towards the cathode side and less intense towards the anode side. We can take advantage of anode yield effect and position the patient accordingly such that the more denser parts are placed towards the cathode side and the less denser parts are placed towards the anode side. The anode yield effect is inversely proportional to the anode angle and the source image receptor distance, also called as focal film distance and anode yield effect is directly proportional to the field size. That is, if field size increases, the anode yield effect increases. Thank you.