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Image quality parameters

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Similar to other medical imaging systems quality control measurements can be performed to evaluate the image quality of the cone-beam CT systems. These image quality parameters are very similar to those are measured and calculated on a third generation human CT scanner. Therefore the methods are basically the same, although they are modified in some cases due to the different structure of the cone-beam CT.

The spatial resolution of the system can be measured using phantoms with special high contrast pattern or it can be calculated based on the modulation transfer function (MTF). Since the spatial resolution of the cone-beam CT systems are usually very good and fall in the range of 10 micron or even better, in practice it is easier to calculate the MTF than to produce phantoms with this precision. The transfer function can be calculated either from the point spread function (PSF) or from the edge spread function (ESF) and both can be determined based on measurements with relatively simple phantoms. For the third generation human CT systems a very thin (thinner than the expected resolution) tungsten wire is used to measure the PSF, but in cone-beam tomography it is easier to measure the ESF with a so called slanted edge phantom and derive the MTF. Based on the MTF, the spatial resolution is defined where the value of the function reaches 2%, but usually the 10% and 50% values are also calculated.

The homogeneity is measured with a cylindrical phantom filled with water. Four peripheral and one central region of interests are defined in 3, 6, 9 and 12 o’clock position and in the centre and the average density of the water is measured in all positions. All the five values should be within a certain tolerance.

The image noise is defined as the standard deviation of the pixels within a homogeneous region of the image. It is measured on the same image used to evaluate the homogeneity. The analysis should be performed for all tube voltage settings at various tube current and exposure time values, because the product of the last two parameters (exposure or also known as milliampere-seconds, mAs) is inversely proportional to the image noise. Artefacts, changes in the environment (e.g. temperature) or in the processing change (e.g. electrical noise) can increase the image noise.
Checking the stability of the Hounsfield scale should be done on all tube voltage settings by measuring the air and water values on a phantom measurement. If the Hounsfield values calculated for the water and air ROIs are differ more than specified from the theoretical numbers (-1000 HU for air and 0 HU for water) the Hounsfield calibration should be repeated (see previous sections).

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