Anode heel effect, line focus principle,

physics of anode heel effects

1. Anode heel effect, Line focus principle, Off focus radiation and its clinical applications Soni N agarkoti B.Sc.MIT 2 nd Year NAMS, Bir Hospital

2. Overview Review of xray tube Production of xrays Effective and actual focal spot Line focus principle Anode heel effect Off focus radiation Clinical applications

3. Xray tube

4. Xray tube An X-ray tube is a vacuum tube. Contains a pair of electrodes i.e. a Cathode and an Anode. Cathode is a filament that releases electrons when high voltage is applied. Anode is made up of tungsten, which attracts the electrons. When the electrons released from the cathode come in contact with the tungsten , they release energy in the form of x-ray photons.

5. Xray tube components Tube housing Glass envelope Cathode component Anode component

6. Cathode component Negative electrode of xray tube Made up of filament, connecting wires and focusing cup Filament is made up of tungsten that is the source of electron Connecting wires is used to supply voltage and electron to filament Focusing cup focuses electron to the anode

7. Nowadays two filaments are used in xray tube that gives rise to the two focal spots i.e. Small focus electron beam strikes small portion of target and gives improved resolution and used in maammo Large anode electron beam strikes larger portion of the target and used in general radiography

8. Anode component Positive electron of the xray tube Made up of target (focal spot) and cylindrical cu block or tungsten Rhenium disk. Made up of a small plate of tungsten 2 or 3mm thick that is embedded i n a large mass of cu . anode is generally angled at 15-20 deg. Could be stationary or rotating type

9. Anode

10. Production of xrays

11. Focal spot It is the area actually bombarded by the electron stream on the target. It can be larger or smaller in size. A small focal spot is required for producing good radiographic detail but it may also lead to overheating of target. Whereas large focal spot allows greater heat loading but doesn't produce sharp image. This problem was solved in 1918 by the development of line focus principle .

12. Actual focal spot the area of the target material being bombarded by electrons from the filament Depends on the filament size and dimension of the focusing cup

13. Effective focal spot the imaginary geometric line that can be drawn based on the actual focal spot size vs. the angle of the anode i.e. is the beam projected onto the patient Depends on anode angle and actual focal spot

14. Apparent (effective) focal spot is determined by the sine of the angle of the anode surface Apparent focal spot = real focal spot * sin Ѳ (where Ѳ varies from 6 to 20 degrees depending on variety of tube)

15. Line focus principle   explains the relationship between the anode surface and the effective focal spot size It states that as the anode angle is made small, the apparent focal spot also becomes small but with increased heat loading . Acccording to this, by angling the target, effective area of the target is made much smaller that the actual area of electron interaction

16. Basic concept of line focus principle During xray production, heat is dissipiated uniformly across the focal spot and anode surface So large focal spot is useful to protect tungsten from melting as heat is accumulated and dissipated within area of focal spot However, a small focal spot is required to achieve good radiographic image So line focus principle is important which states that angulation of anode surface results in apparent decrease in focal spot size

17. for a given apparent focal spot size, the real area covered by the electron beam is larger for smaller target angles which, as stated above allows a greater area over which to dissipate the heat. for a smaller target angle, the area covered by the x-ray beam will be smaller so it is not possible to cover large areas at smaller FFDs therefore it can be appreciated that choice of target angle is a compromise between tube loading, geometric unsharpness and desired area to be covered by the useful beam For eg , at 40" FFD the anode angle should be no smaller than 15 degrees.

18. Relationship between apparent and real focal spot -DIRECT relationship  -the SMALLER the actual focal spot, the SMALLER the effective focal spot  -the LARGER the actual focal spot, the LARGER the effective focal spot size  -large actual focal spot will have less heat on the anode, than a small focal spot (same quantity of photons over a larger area vs a smaller area)

19. Relationship between effective focal spot and tube angle As anode angle increases effective focal spot size also increases for the same actual focal spot Larger tube angle will have large effective focal spot so the tube angle should be chosen properly so that there wont be compromising in the resolution of the image.

20. Advantages of line focus principle This slope allows x-rays produced at focal spot to leave the tube sideways In such a way that the x-ray beam emerges at right angle to the long axis of the x-ray tube . It permits large area for electron bombardment and a small x-ray source. more heat loading with good radiographic detail. Sin 6⁰= 0.104 Sin 21⁰=0.358

21. Disadvantages of line focus principle Anode heel effect Area covered by the beam reduces with the target angle i.e. To cover 17” the angle must be 12 degree To cover 36” the angle must be 14 degree

22. Anode heel effect The intensity of the x ray beam as it leaves the x ray tube is not uniform throughout all portions of the beam. The intensity of the beam depends on the angle at which the x rays are emitted from the focal spot . The intensity of the film exposure on the anode side of the x ray tube is less than that on the cathode side of the tube.

23. is one unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side Electrons interact with target atoms at various depths into the target. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target, and therefore increased absorption. This is the heel effect

24. The x-rays that constitute the useful beam emitted toward the anode side must deal with a greater thickness of target material than the x-rays emitted toward the cathode direction The decreased intensity of the x ray beam that is emitted more nearly parallel to the surface of the angled target is caused by the absorption of some of the x ray photons by the target itself

25. The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as 45%. The central ray of the useful beam is the imaginary line generated by the centermost x-ray in the beam. If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%.

26. Factors affecting anode heel effect Anode angle Focus to film distance Film size

27. As the angle of the anode decreases the anode heel effect increases The distance from the anode to the image receptor greatly influence the apparent magnitude of the anode heel effect. This effect is less noticeable at large FFD Larger the field size more prominent the heel effect Smaller the field size results in less pronounced heel effect

28. Curved anode VS flat anode Curved anode in comparison to flat anode do not produce objectionable heel effect and also anode curvature might seem to offer a potential improvement in heat capacity

29. Applications of anode heel effect Anode heel effect is important during the radiography of non uniform anatomical structures The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image

30. Contd …… For eg , radiography of foot radiography of lumbar spine radiography of abdomen Also, it is important in context of the radiation protection for the patients For eg , head of the female patient is placed at the cathode end of the x-ray tube to achieve a significant dose reduction to the ovaries and hence lower effective dose in lumber spine radiography

31. Anode heel effect in the mammography Anode angulation = 6 degree Tube angle = 23 to 25 degree Focal spot = 0.1 to 0.3 mm As a result we get perpendicular beam towards cathode where the chest wall is positioned that cause less scattering of radiation and increases resolution as well

32. Off focus radiation X-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot. However , some of the electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot). These x-rays are called off-focus radiation The main source of off focus radiation is scattered electrons at the target.

33. Control of off focus radiation A diaphragm is placed between the tube and the collimator to reduce off focus rays. Metal enclosure decreases off focus radiation by attracting off- focus electrons to the grounded metal wall of the x-ray tube

34. Summary Line focus principle is the angulation of the anode to achive smaller effective focal spot on larger actual focal spot for greater heat dissipation and the improvement in the image quality. Anode heel effect is the decrease in the intensity of radiation at anode side than in cathode side due to the absorption and attenuation of the radiation by the heel of the anode Off focal radiation is the radiation which is produced by the bombardment if the electrons on other areas of the target outside the focal track

35. Thank you

36. refrences Christensen’s physics of diagnostic radiology Radiologic science for technologists by S tewart Carlyle Bushong www.google.com

37. Questions What is the variation in the beam intensity that appears due to anode heel effect? What is difference between apparent and actual focal spot? Describe the relationship between apparent and actual focal spot and tube angle Importance of anode heel effect with examples Correlate the importance of angulation of anode surface with line focus principle