Controlling Radiographic Quality
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 go undetected.
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.