use of filters and screens

1. M.KARTHIKEYAN
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING
AAA COLLEGE OF ENGINEERING & TECHNOLOGY, SIVAKASI
karthikeyan@aaacet.ac.in
ME8097 NON DESTRUCTIVE
TESTING AND EVALUATION

2. UNIT V RADIOGRAPHY (RT)
1. Principle, interaction of X-Ray with matter,
2. imaging, film and film less techniques,
3. types and use of filters and screens,
4. geometric factors, Inverse square, law,
5. characteristics of films - graininess, density, speed, contrast,
6. characteristic curves, Penetrameters,
7. Exposure charts, Radiographic equivalence.
8. Fluoroscopy- Xero-Radiography,
9. Computed Radiography, Computed Tomography

3. FILTERS IN RADIOGRAPHY
 Filters are metal sheets placed in the x-ray beam between the
window and the patient that are used to attenuate the low-
energy (soft) x-ray photons from the spectrum.
 Filtering is the removal of these low energy x-rays from the
beam spectrum which would otherwise not contribute to image
quality but would add to patient dose and scatter.
 If unfiltered these low-energy x-ray photons are generally
absorbed by superficial structures of the body and contribute to
the entrance surface dose (ESD).
 As they are absorbed by the superficial structures they
contribute minimally to image formation. Using a filter reduces
the ESD and to a lesser extent effective dose for the patient.
 The units of filtration are expressed in mm of aluminum

4. There are two types of filtration:
 inherent filtration from components in the x-ray tube, i.e.
window, housing, cooling oil (equivalent to 0.5-1.0 mm Al)
 added filtration from interchangeable metal sheets (Al, Cu, etc.)
 Total filtration is the combined effect of inherent and added
filtration, with US guidelines stating a minimum total filtration
of 2.5 mm of aluminum is required for x-ray tubes operating
above 70 kVp.
 The added filtration component is customized (filter thickness,
type of metal) for individual examinations and procedures (e.g
fluoroscopy) and takes advantage of specific metals filtration
characteristics (e.g. absorption edges) to improve image quality
and contrast.

5.  Beryllium is commonly used in mammography (which use low-
energy photons) as it provides very little filtration.
 Other types of x-ray generally use aluminum, copper or tin.
 Filtration reduces x-ray intensity (quantity) and quality (shape
of beam spectrum), but not the maximum energy of the x-ray
beam spectrum.
 The change in the shape of beam spectrum with filtration is
referred to as beam hardening.
 This is due to the loss of lower energy photons from a
polychromatic beam.
 The average x-ray energy is increased and becomes more
penetrating.

6. FILTERS IN RADIOGRAPHY
 At x-ray energies, filters consist of material placed in the useful
beam to absorb, preferentially, radiation based on energy level
or to modify the spatial distribution of the beam.
 Filtration is required to absorb the lower-energy x-ray photons
emitted by the tube before they reach the target.
 The use of filters produce a cleaner image by absorbing the
lower energy x-ray photons that tend to scatter more.
 The total filtration of the beam includes the inherent filtration
(composed of part of the x-ray tube and tube housing) and the
added filtration (thin sheets of a metal inserted in the x-ray
beam).
 Filters are typically placed at or near the x-ray port in the direct
path of the x-ray beam.

7.  Placing a thin sheet of copper between the part and the film
cassette has also proven an effective method of filtration.
 For industrial radiography, the filters added to the x-ray beam
are most often constructed of high atomic number materials
such as lead, copper, or brass.
 Filters for medical radiography are usually made of aluminum
(Al).
 The amount of both the inherent and the added filtration are
stated in mm of Al or mm of Al equivalent.
 The amount of filtration of the x-ray beam is specified by and
based on the voltage potential (keV) used to produce the beam.

8.  The thickness of filter materials is dependent on atomic
numbers, kilovoltage settings, and the desired filtration factor.
 Gamma radiography produces relatively high energy levels at
essentially monochromatic radiation, therefore filtration is not
a useful technique and is seldom used.