The listed image criteria refer to characteristic features of imaged anatomical structures that are defined in the region of examination with a specific degree of visibility. At the present time there are no internationally accepted definitions. For the purpose of these guidelines the degree of visibility is defined as follows:

Visualization - The organs and structures are detectable in the volume of investigation.

Critical reproduction - The structures particular to the specific indication are discriminated to a level essential for diagnosis. This will include the terms:

reproduction - details of anatomical structures are visible but not necessarily clearly defined.

visually sharp reproduction - anatomical details are clearly defined.

Diagnostic reference dose values are indicated for two dose descriptors weighted CT DI (CTDI_W) and dose-length product (DLP) on the basis of absorbed dose to air, in relation to technique for a standard-sized patient.

CTDI_w is the approximation of average dose over a single slice in the standard head or body CT dosimetry phantom, expressed in terms of absorbed dose to air (mGy).

DLP characterises exposure for a complete examination in relation to linear integration of the dose to the standard head or body CT dosimetry phantom on the basis of absorbed dose to air (mGy cm).

Comparison of CTDI_w or DLP values for a particular type of procedure provides a useful indication of relative performance. However, data for examinations on different regions of the body can not be compared directly in order to assess relative patient risk.

Appendix 1 to Chapter 1 gives further information concerning the definition of these quantities and methods to check compliance with the dose criteria. The derivation of the diagnostic reference dose values and additional background information is given in Chapter 2.

Parameters are listed that contribute to the fulfilment of the Diagnostic Requirements and the Criteria for Radiation Dose to the Patient.

Patient position

Volume of investigation - anatomical landmarks for beginning and end of the scan.

Nominal slice thickness for serial or collimation for helical CT - in mm.

Inter-slice distance/pitch - in mm/factor.

Field of View (FOV) - maximum diameter (in cm) of the reconstructed image.

Gantry tilt - angle (°) between vertical plane and plane containing the x-ray tube, the x-ray scan beam and the detector array.

X-ray tube voltage - in kV. This should, if possible, be selected so as to achieve the required image quality at lowest practicable dose.

Tube current and exposure time product - in mAs. Selection of tube current (mA) and exposure time (s) to determine radiographic exposure (mAs) is of critical importance. Absolute values of mAs cannot be recommended in view of significant differences in operating characteristics between types of scanner. Operators should be aware of the characteristics particular to their scanner and understand the range of settings that are consistent with meeting required image quality and reference dose values.

Reconstruction algorithm - broad type of mathematical filter for the reconstruction of the CT image.

Window width - in HU. Range of CT numbers converted into grey levels and displayed on the image monitor.

Window level - in HU. Central value of the window used for the display of the reconstructed CT image.

Protective shielding - additional protection devices to reduce exposure of sensitive organs and tissues.

Motion - patient or organ movements.

Problems and pitfalls - mostly site specific clinical or technical problems which impede image quality.

Modification of technique - in order to provide clinically relevant examination in case of technical or diagnostic problems.