Beam Centering and Beam Limiting Devices: A Comprehensive Overview This presentation, titled "Beam Centering and Beam Limiting Devices", is an informative and visual exploration into one of the most essential aspects of diagnostic radiology. Beam centering and beam limiting play a critical role in patient safety, image quality, and radiation protection. The concepts covered in this PowerPoint are particularly valuable for students pursuing degrees in Medical Imaging Technology (such as BSc. MiT), radiographers, and radiologic technologists. This presentation aims to provide learners with a strong foundation in understanding what beam centering and beam limiting devices are, why they are important, the physics behind their function, and how they are practically implemented in clinical radiology settings. The slides are designed to aid both theoretical learning and practical application, making this resource ideal for use in academic institutions, hospital training programs, or self-study modules.

1. Prepared by :
Sajan Bohara
BSc. MIT (1st
Year)
NMCTH

2. Basic Introduction
• Xray- a form of high-energy electromagnetic radiation ( 0.01 to 10
nanometers)which invisible and penetrates materials (like soft tissues),
and is absorbed by denser substances (like bones)
• Beam- A focused stream of particles or waves moving in a specific
direction
• X-rays are the type of radiation, while the beam is how they are delivered
• X-ray Beam- A controlled stream of X-ray photons emitted from an X-ray
tube. Unlike scattered light, the beam is collimated (narrowed) to target
a specific area.

3. X-ray Beam
• Characterized mainly by quality and quantity
• Quality is the overall energy of x-ray beam
• Quantity is the number of photon in x-ray beam
• Beam Quality is the primary factor that effects the image quality in radiography
• Beam Quantity determines the intensity of the image
• Changes in quality and quantity affect the image quality and radiation dose.

4. X-ray emission spectrum
• refers to the energy distribution of the radiation
produced in an x-ray exposure
• In x-ray spectra curve, Bremsstrahlung and
characteristics radiation is represented by
continuous spectrum and line spectrum respectively
• It contain information about quality and quantity of
beam
• The area under the curve represents the quantity,
and the peak of the curve implies the average
energy
• The general shape of an emission spectrum curve is
always the same, but its relative position along the
energy axis can alter
• the farther to the right a spectrum is, the higher the effective enegry or the quality of x ray
• the larger the area under the curve, the higher the x ray quantity
fig:- general form of x-ray emission spectrum

5. Beam Centering
• Beam centering is the process of aligning the central ray of the X-ray beam
with the center of the area of interest on the patient’s body and the image
receptor.
• It ensures the accurate visualization of the area of interest with minimal
distortion and optimal image quality.
• It is important to limit the size of the primary beam and prevent exposure
of body region which are not required in image formation.
• Large X-ray field result in unnecessary exposure, thus scatter radiation is
increased and image contrast is decreased.

6. OFF FOCUS RADIATION
• X-Ray tube are designed so that the
projectile electrons interact with the target
• Some of the electrons bounce off the focal
spot and interact with other areas of target,
causing X-Ray production outside
of the focal spot.

7. SCATTARED RADIATION
• Type of radiation that occurs when a
primary radiation beam interacts with
matter, causing it to scatter in various
directions.
• In diagnostic radiology scatter radiation
leads to Fog in the X-Ray image.
• Two kinds of x-rays are responsible for
the optical density and contrast on
radiograph image
• Those that pass through the patient
without interacting called remnant ray.
• Those that are scattered through
Compton interaction

8. FACTORS AFFECTING SCATTERED RADIATION
Three factors contribute to an increase in scatter radiation:
• KVp
-:Higher kVp → More Compton scatter, less absorption → Lower contrast,
less patient dose
-:Lower kVp → More photoelectric effect→ Higher contrast, more patient dose
• Field Size
-:Larger field size → More scatter radiation → Increased image density
-:Smaller field size→Less scatter radiation→Reduced image density and better
image quality
•Patient thickness
-:More scatter results from imaging thick body parts compared to thin body parts
-:As tissue thickness increases, more of the rays go through multiple scattering

9. The relationship between collimation (field size) and
quantity of scatter radiation
• Collimated beam means
decreasing field size,
and decreasing
collimation means
increasing field size.
• Collimation and
scatter radiation are
Inversely proportional.

10. Beam Restriction:
• The beam restriction and collimation
are refer to decrease in the size and
shape of the projected radiation field.
• Collimation of beam decreases the risk
of radiation and minimizes scattered
radiation with better contrast.

11. BEAM RESTRICTING DEVICES
• Beam restrictors is a type of device attach to the opening in
Xray Tube housing to limit the size of the x-ray field to only
the anatomical structures of interest.
• There are mainly two reasons to restrict any x-ray beam.
Only the tissue being examined should be exposed.
Larger x-ray field results unnecessary patient exposure. So to
reduce patient dose.

12. Importance of beam limiting devices
:
Beam restriction serves for three purposes:
• Limiting patient exposure
• Reducing the amount of scatter radiation
• Focus on the correct anatomy .
• Improve image quality.
Limitation:
Reduce intensity by blocking incoming radiation.

13. BEAM RESTRICTORS
All beam-restricting devices are made of metal or a
combination of metals that readily absorb x-rays.
Classified into 3 categories:
• Aperture Diaphragm
• Cones & Cylinders
• Collimators

14. APERTURE DIAPHRAGM
• Apertures are the simplest of all
restricting devices
• Lead or lead lines metal diaphragm
attached to x-ray tube head
• The opening in the diaphragm is
usually designed to cover just less
then the size of the image receptor
used.
• Thickness 3-5mm

15. Applications of Aperture Diaphragm
• Used in dental radiography with rectangular collimation.
• Also, used in trauma and chest radiography.

16. DISADVANTAGE
•Size and shape is fixed
•Produce large penumbra
•Reduces sharpness and resolution

17. CONES AND CYLINDERS
• Cones and cylinders are modifications to the
aperture
• Heavy metal base like lead
• Copper lined barrel
• Cones and cylinders are shaped differently.
• Both have extended metal structures (10-
20inch) that restrict the useful circular beam
to require size.
• Positioning and size of the distal end
determines the field size.

18. Application:
• Designed to operate most effectively at a
designed SID
• Sinuses
• Dental x ray and treatment

19. ADVANTAGES
• Better restriction to x-ray field
• Produce less penumbra than
aperture diaphragm
• Round image on a rectangular film
• Reduce scatter radiation

20. • If the x ray source, film and cone are not aligned properly, then one
side of the film may not be exposed, which is called cone cutting.
• These systems provide only limited number of field sizes.
Disadvantages

21. Collimators
• The most useful, and accepted beam-restricting device is the
collimator.
• The terms collimation and beam restriction are used
interchangeably.
It has two advantages over the other types:
 it provides an infinite variety of rectangular x-ray fields
 a light beam shows the center and exact configuration of the x-ray field.

22. Structure of the collimator:
• 2 sets of shutters control the dimensions.
• Each shutter contains 4 or more lead plates of 3mm thickness, which
move in independent pairs.
• When the shutters are closed, they meet at the center.
• Collimator also has a light and mirror arrangement to create radiation
and optical coincidence.

23. • The light beam is deflected by a mirror mounted in the path of x ray
beam at an angle of 45 degree.
• The target of x ray tube and the light bulb should be the exactly
same distance from the center of the mirror.
• The light field and radiation field should match exactly with each
other.

26. VARIABLE APERTURE COLLIMATOR
•Proper collimation of the x-ray beam has the primary
effect of reducing patient dose by restricting the
volume of tissue irradiated.
•Proper collimation also reduces scatter radiation that
improves contrast.

27. LIGHT LOCALIZING COLLIMATOR
• The light localizing variable aperture
collimator is the most common beam
restricting device in diagnostic
radiography

28. COLLIMATOR
Two stages of shutter to control the beam
• First stage shutters protrude into the tube
housing to control the off-focus radiation.
• Adjustable second stage shutter pairs are
used to restrict the beam.

29. • Light localization is accomplished by a small projector
lamp and mirror to project the setting of the shutters on
the patient
• The light field and x-ray beam should match to avoid
collimator cut-off
• A scale on the collimator is used to match the beam to the
film size at fixed target film distance.

30. Automatic Collimator or Positive Beam
Limiting Devices
• A sensor in the Bucky and the motor used to automatically
collimate the image to film size called a positive-beam limiting
(PBL) device
• When film loaded cassette is inserted in the Bucky tray and
clamped into place sensing devices in the tray identify the
size and alignment of the cassette.
• An electric signal is transmitted to the collimator housing, and
accurate the synchronous motors that drive the collimator
leave to a pre-calibrated position

31. COLLIMATION RULES
• Requires 4 borders of collimation to be seen on the film.
• Collimation must be slightly less than film size or to the area
of clinical interest, whichever is smaller.
• Any exposure beyond the film is unnecessary patient
exposure
• Collimate the area within the patient not beyond the body

32. Quality assurance :
• Do collimator –beam alignment check at least every six months
• Do check of alignment of center of x ray beam at least every six
months.

33. FILTERS
•When the X ray beam is produced many energies of
photons exit (polychromatic). Many are of such low
energies that they were offered nothing to production
of that radiograph. So, removal of low energy x rays
from beam spectrum is called filtration. And the device
for that filtration is called Filters.
• It is placed in the path of x-ray beam.

34. WHY WE USE FILTERS IN THE DIAGNOSTIC
RADIOLOGY?
In diagnostic radiology, if the filters are not used,
• The patient acts as a filter, which will result in absorption of X rays
by the tissues.
• Unnecessary dosage of radiation to the patient.
• Also, to Increase the tube age

35. Preferred metal:
Most commonly preferred in diagnostic radiology
• Copper (Z= 29)
• Aluminum (Z=13)
low atomic number therefore excellent material for absorbing soft
rays.
low energy X ray photons low in weight therefore make the extra
tube lighter in weight and easily handling

36. Types of Filtration
Mainly 3 types:
a. Inherent
b. Added
c .Compensating

37. INHERENT FILTRATION
• A type of filtration that results from the
composition(aluminum of about 0.5-1 mm thickness) of
the tube and housing

38. ADDED FILTRATION
• Any filtration that occurs outside the tube and housing and before the
image receptor. Added to the port of the x-ray tube to absorb as many
low-energy photons as possible.
• Thickness 1- 2mm
Copper : pediatric applications
Aluminum : general radiography x ray tubes
Rhodium : mammography with Rh anode
Molybdenum : Mammography
Silver : mammography with tungsten anode

39. COMPENSATING FILTRATION
• They are filters designed to solve a problem involving
unequal subject densities
• Wedge Filter-Most common kind of compensating filter
which is used to radiograph body parts that vary
considerably, such as the foot. (1-3mm)
• Trough Filter-Compensating filter used mainly for chest
radiography. (Around 3mm)

40. CONE FILTERS
• Concave or convex shaped, used in applications in digital
fluoroscopy, where image-intensifier tubes and receptors
are round

41. 1. Chesneys' Equipment for Student Radiographers, 4th Edition
2. The Physics of Radiology and imaging by K.Thayalan
3. Christensen’s Physics of Diagnostic Radiology
4. Handbook of medical radiography C Ramamohan
REFERENCE