The phantom’s application

To acquire an X-ray image, the CDRAD phantom is positioned on the patient table over the automatic exposure control (AEC) and the cassette. Individual experiments are determined by such exposure techniques as manual and automatic exposures, with grid and without grid, focal spot size and tube potential. Moreover, the phantom can provide measurements related to the following [103]:

 The comparison of IQ with different film-screen combinations.

102

 The application of different tube potentials to determine the optimum exposure technique.

 The use of different object thicknesses for IQ comparisons (this is achieved by varying the Perspex amount at a fixed density).

 The impact of filtering by varying the filter thickness added.

To simulate different patient thicknesses, different Perspex thicknesses should be added both over and under the phantom [103].

4.1.2.1

The phantom image’s assessment

The CDRAD phantom must be assessed by at least three experienced observers to provide the best results. To improve the validity, three independent images made during the same sitting can be evaluated using a score from the CDRAD phantom. The evaluation of the image is focused on the area where the holes are just visible, which is indicated by the position at which the non-central holes can be seen. At least three fields, including at least one non- visible choice in each column or row, must be used to conform to the suggested correction scheme [103].

The indicated locations of the eccentric holes should be compared to the true hole positions in the phantom. There are specific rules for evaluating the results (e.g. consider the four nearest neighbours of the field under examination). The assessment of a specific field always refers to the original observations of the nearest neighbours [103].

103

4.1.2.2

Correction scheme

In each observation, there are three options for the observer [103]:

 T: the eccentric hole is indicated at the true position.

 F: the eccentric hole is indicated at the false position.

 N: the eccentric hole is not indicated at all.

There are also two main rules within the correction scheme [103]:

1. A True requires two or more correctly indicated closest neighbours to remain a True. 2. A non-indicated hole or a False can be considered True when it has three or four correctly indicated nearest neighbours.

However, there are two exceptions [103]:

1. A True that has only two nearest neighbours (at the edge of the phantom) requires only one correctly indicated nearest neighbour to remain True.

2. If both nearest neighbours are correctly indicated for a False or for a non-indicated hole that had only two nearest neighbours, it will be regarded as True.

4.1.2.3

The results depictions

There are two methods for presenting the results: 1) the use of formulas and 2) the use of the contrast detail (CD) curve[103].

104

4.1.2.3.1

Formulas

The CD curve is the curve through the threshold fields. The IQ can be illustrated by the ratio calculation of correctly identified hole positions to the total number of squares i.e. , as follows [103]:

(4.1)

The IQF method can also be used to quantify the IQ. This method is defined in formula 4.2, as follows [103]:

∑

(4.2) (similar to equation 1.2)

Where denotes the threshold ( ) diameter in the contrast column . A summation over all contrast columns yields the IQF.

Two extra rules are applied for calculation purposes [103]:

1) For a hole depth between 0.3 and 8 mm, a completely invisible column will result in a

of 10 mm.

2) For a hole depth between 0.3 and 8 mm, a completely visible column will result in a

of 0.3 mm.

The increase in the number of correctly identified hole positions reflects the IQ enhancement. The smaller the appearance of the contrast and the details, the better the system is. In this

105

case, because the values for the depths and diameters of the thresholds are smaller, the IQF will become smaller as well. However, as this could be misleading, an inverse measurement, IQFinv, is adopted [3]. IQFinv values increase as smaller diameter holes are observed. Formula 4.3 presents IQFinv approach for increasing the IQ, as follows [103]:

_∑

(4.3) (similar to equation 2.7)

4.1.2.3.2

Contrast detail (CD) curve

The results can be demonstrated in a graph that depicts the hole depths plotted against the hole diameters. To compare the image performances of different systems, the phantom images should be produced for the same observer in identical conditions and at the same time. The smaller the appearance of the contrast and the details, the better the system is. This generates a shift of the CD curve to the lower-left part of the image. It also makes it possible to compare several performances [103].

4.1.2.4

Analysing of the CDRAD

Analysing the CDRAD involves analysing the images and providing a statistical method to determine whether a specific CD combination is observed or not. This statistical method utilises the standard deviation and the average pixel signal value in relation to the images of the CD combination under evaluation and its pixel background (or variables). It is vital to identify the locations of all 225 different CD combinations of the image phantom, which helps to correctly identify the two variables [103]. First, the programme identifies the location of the CD combinations. This achieved by applying a statistical method that

106

indicates CD combination recognition. Then, the programme depicts the user’s results. The assessment of a single CDRAD image can be broken into seven steps [103]:

1) Deciding the location of the phantom.

2) Deciding the centre of the 225 CD combination of the phantom. 3) Resolving the pattern type of the phantom.

4) Resolving the signal background. 5) Resolving the related CD signal. 6) Allocating True and False.

7) Calculating the contrast detail curve.