physics of anode heel effects

1. Anode heel effect, Line focus
principle, Off focus radiation and
its clinical applications
Soni Nagarkoti
B.Sc.MIT 2nd 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.
1. Small focus
electron beam strikes small portion of
target and gives improved resolution and used in
maammo
1. 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 in 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

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

16. 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

17. 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

18. • 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.

20. 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)

22. 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.

24. 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

25. 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

26. 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.

27. • 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

28. • 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

29. • 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%.

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

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

33. 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

34. 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

35. 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

38. 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

40. 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.

42. 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

43. 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

44. •Thank you

45. refrences
• Christensen’s physics of diagnostic radiology
• Radiologic science for technologists by
Stewart Carlyle Bushong
• www.google.com

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