Image Quality Indicators

The purpose of an IQI is to indicate the overall sensitivity of the technique and as a measure of how well the radiograph will reveal discontinuities. As the name implies, an IQI is an indicator of the quality of the radiographic image. An IQI is a device made from the same material as the test specimen. It is placed on the test specimen in a position where its image will be recorded on the radiograph.

The quality of a radiographic image can be assessed in terms of three factors:

1. Image sharpness. Usually, in radiography, the inverse of sharpness - unsharpness or blurring - is used.
2. Image contrast. The density change on a film for a given thickness change in the specimen. If a small image detail shows only faintly, this is a low contrast image; if the detail is easily seen, this is a higher contrast image.
3. Image Noise. For radiography-on-film, this is effectively graininess. In radioscopy, there are additional features affecting image noise.

A good design of IQI should be able to show changes in all these three factors.

There are four broad patterns of IQI in use today - the wire type, the step/hole type, the hole-in-plaque type and the duplex wire type.

Probably the most widely used IQI in radiography-on-film is the wire type. Various patterns of wire type IQIs are described in German, Scandinavian, British, Japanese, Chinese, American CEN and ISO Standards. These consist of a series of straight wires of the same or similar material to the specimen, the wires being of different diameters taken from the series 0.10, 0.125, 0.16, 0.20, 0.25, 0.32, 0.40, 0.50, 0.63, 0.80, 1.00. The wires may be 10, 20, 30, 50mm according to the various Standards and are held parallel to one another in a low density plastic mount, with appropriate identification symbols.

Placement of IQIs
Standard practice is to place the IQI on the side of the specimen facing the radiation source, on the grounds that this is the region of the image where geometric unsharpness and therefore image blurring, will be the largest. Most tables of IQI values assume that the IQI is in this position.

In the case of double-wall/single-image pipe weld radiography, where there is no access to the inside of the pipe, the only practical place for the IQI is on the film side of the weld, that is, effectively on the film cassette. If the IQI is used in this position, a lead identification letter (F or FS) should be placed on the IQI. In this position, the IQI image is completely insensitive to geometric unsharpness factors such as source-to-film distance or source diameter and is an indication of contrast only. Normally, the IQI readings are 1 - 2 wires or holes better than with the IQI on the source side of the same specimen. Some Standards ask for a trial exposure on a sample piece of pipe, with IQIs on both source and film sides of the weld, for comparison. Some experts regard the IQI reading from the film side position as misleading and of little practical value, but tables of IQI sensitivities for the IQI on the film side are included in EN 1453:1997 and EN 462-3:1997.

The smallest element readily visible in the area under inspection is used to determine IQI sensitivity

For the wire type IQI:

sensitivity % equals diamter of smallest wire divided by thickness of metal times 100

For the step-hole IQI:

sensitivity % equals diamter of smallest hole divided by thickness of metal times 100

Selecting the correct IQI is important

Typical radiographic sensitivities range from around 1% to 4%, depending on:

the thickness of the item being radiographed
the type (energy) of the radiation
the type of film
other factors discussed in the previous task.

In some cases, a code, specification or customer order will state the required sensitivity.

Selecting the correct IQI involves:

Selecting the correct metal (steel, aluminium, copper).
Selecting the correct type, again normally specified in the code or specification.
Selecting the correct model, i.e. the model with an element that will correspond with the required sensitivity. Hence, if radiographing 20 mm steel for a 2% sensitivity, with the wire type IQI, the model with a wire closest to 0.4 mm (2% of 20 mm) is required. This is wire number 10, so the 6 to 12 wire type IQI is required. If the step hole type is specified, then the model with a 0.4 mm step is required. This is the 1 to 6 model.

An example of IQI selection as per client requirement in radiography (Qatar Petroleum requirement)

Image Quality Indicators

Image quality indicators conforming to ASTM E747 shall be employed to verify the radiographic sensitivity achieved. Subject to clients approval prior to commencement of work. Selection of indicator shall be by the sensitivity requirement for the material thickness under examination and shall be compatible with material involved e.g. Steel for steel etc.

The image quality indicator shall be selected for single wall thickness plus maximum permissible reinforcement next to the film without regard to backing strips or rings. The image quality indicator shall be located on the source side of weld under examination however where not practical then indicator shall be placed on film side with identifying lead marker "F" placed adjacent to the indicator. Dimensions of marker shall be similar to those of identification markers. Image quality indicator shall be placed at 90 deg. to the weld axis at the furthest point of radiographic interest on radiograph except where panoramic exposure technique is utilized; a minimum of three shall be placed equidistant around the circumference of the weld.

Some Excercises on IQI

1. What model wire type IQI would you select in the following cases?
25 mm thick plate where the weld has 3 mm reinforcement and a 2% sensitivity is specified.
40 mm thick casting where a 2.5% sensitivity is specified.

2. Determine the IQI sensitivity in the following cases:

in a weld in a 20 mm thick plate, number 10 is visible in the parent metal, and wire number 8 is visible in the weld. The total weld thickness is 24 mm.

in a 40 mm thick casting, wire number 5 is visible in the area being examined.

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