One of the methods of controlling the quality of
a radiograph is through the use of image quality indicators (IQIs).
IQIs, which are also referred to as penetrameters, provide a means of visually informing the film interpreter
of the contrast sensitivity and definition of the radiograph.
The IQI indicates that a specified amount of change in material thickness will be detectable in the radiograph, and that the radiograph
has a certain level of definition so that the density changes
are not lost due to unsharpness. Without such a reference point,
consistency and quality could not be maintained and defects could
Image quality indicators take many shapes and forms due to the
various codes or standards that invoke their use. In the United
States, two IQI styles are prevalent: the placard, or hole-type
and the wire IQI. IQIs comes in a variety of material types so
that one with radiation absorption characteristics similar to
the material being radiographed can be used.
ASTM Standard E1025 gives detailed requirements for the design
and material group classification of hole-type image quality indicators.
E1025 designates eight groups of shims based on their radiation
absorption characteristics. A notching system is incorporated
into the requirements, which allows the radiographer to easily determine
if the IQI is the correct material type for the product.
The notches in the IQI to the right indicate that it is made of aluminum. The thickness in thousands of an inch is noted on each pentameter
by one or more lead number. The IQI to the right is 0.005 inch thick. IQIs may also be manufactured to a military or other industry specification and the material type
and thickness may be indicated differently. For example, the IQI on the left in the image
above uses lead letters to indicate the material. The numbers on this same IQI indicate the sample thickness that the IQI would typically be placed on when attempting to achieve two percent contrast sensitivity.
Image quality levels are typically designated using a two part
expression such as 2-2T. The first term refers to the IQI thickness
expressed as a percentage of the region of interest of the part
being inspected. The second term in the expression refers to the
diameter of the hole that must be revealed and it is expressed
as a multiple of the IQI thickness. Therefore, a 2-2T call-out
would mean that the shim thickness should be two percent of the material
thickness and that a hole that is twice the IQI thickness must
be detectable on the radiograph. This presentation of a 2-2T IQI
in the radiograph verifies that the radiographic technique is
capable of showing a material loss of 2% in the area of interest.
It should be noted that even if 2-2T sensitivity
is indicated on a radiograph, a defect of the same diameter and
material loss may not be visible. The holes in the IQI
represent sharp boundaries, and a small thickness change. Discontinues
within the part may contain gradual changes and are often less
visible. The IQI is used to indicate the quality of the radiographic
technique and not intended to be used as a measure of the size of
a cavity that can be located on the radiograph.
ASTM Standard E747 covers the radiographic examination
of materials using wire IQIs to control image
quality. Wire IQIs consist of a set of six wires arranged in order
of increasing diameter and encapsulated between two sheets of
clear plastic. E747 specifies four wire IQI sets, which control
the wire diameters. The set letter (A, B, C or D) is shown in
the lower right corner of the IQI. The number in the lower left
corner indicates the material group. The same image quality levels
and expressions (i.e. 2-2T) used for hole-type IQIs are typically
also used for wire IQIs. The wire sizes that correspond to various
hole-type quality levels can be found in a table in E747 or can
be calculated using the following formula.
F = 0.79 (constant form factor for wire)
d = wire diameter (mm or inch)
l = 7.6 mm or 0.3 inch (effective length of wire)
T = Hole-type IQI thickness (mm or inch)
H = Hole-type IQI hole diameter (mm or inch)
Placement of IQIs
IQIs should be placed on the source side of the
part over a section with a material thickness equivalent to the
region of interest. If this is not possible, the IQI may be placed
on a block of similar material and thickness to the region of
interest. When a block is used, the IQI should be the same distance
from the film as it would be if placed directly on the part in
the region of interest. The IQI should also be placed slightly
away from the edge of the part so that at least three of its edges
are visible in the radiograph.