ESTABLISHMENT OF REFERENCE DOSE VALUES SELECTION OF REFERENCE DOSE QUANTITIES

Reference doses are intended to allow comparison of performance. In order to achieve this objective for CT, reference doses had to be expressed in terms of quantities which fulfil the following criteria:

(a) provide a meaningful indication of patient exposure, taking into account the details of scanning technique used by individual centres for particular examinations;
(b) well-defined and simple to measure or easy to determine in order to encourage widespread use at CT centres of all sizes and levels of sophistication;
(c) applicable to all current and new types of scanner and to all common techniques, including helical scanning;
(d) consistency of approach with other reference doses and dose descriptors already in widespread use.

There are a number of dosimetric quantities that are employed routinely under various circumstances to characterise exposure from CT scanners. One of the most practical measurements concerns the computed tomography dose index (CTDI) (23). This quantity is simple and can easily be determined free-in-air on the axis of rotation of the scanner for a single scan (CTDIair). This approach has formed the basis for national surveys in several countries. By itself, CTDIair is only a coarse indicator of patient exposure for an examination, for example, the relationship between CTDIair and effective dose for a standard examination varies by up to a factor three between models of scanner as a result of inherent differences in design, and in particular the use of shaped beam filtration (24). CTDIair therefore is not well-suited for use as a reference dose quantity since the setting of a single level for a given procedure would not equitably dictate practice for all types of scanner. CTDIair can, however, still be an important element in the implementation of patient dosimetry.

Effective dose (25) is certainly a useful indicator of patient exposure, although it is also not particularly suitable as a reference dose quantity since it can not be measured directly and its definition may be subject to further changes.

Measurements with phantoms offer the advantage of taking into account differences in dose distribution arising from scanner design, particularly if measurements are not confined to the phantom surface. However, any such dosimetric approach should utilise well-defined and commonly available phantoms in order to gain wide acceptance. For a series of multiple scans with constant separation, the multiple scan average dose (MSAD) (23) is an indication of the magnitude of the dose along the length of the scanned volume at a particular radial depth in a phantom . This quantity has been recommended by the American Association of Physicists in Medicine (AAPM) in relation to the specification and acceptance testing of CT scanners (26) and has been reported in surveys of CT practice in the USA (27). MSAD is equal to CTDI when the distance between scans is equal to the slice thickness (23).

Another quantity in wide-spread use is the particular definition of CTDI given by the Food and Drug Administration (FDA), i.e. CTDIFDA (28), in relation to measurements in a phantom for the purposes of compliance testing of CT systems in the USA. It involves the integration of D(z) over a distance of 14 times the slice thickness, where D(z) is the dose at a point z on any line parallel to the z (rotational) axis for a single slice of nominal thickness T.

Under requirements of FDA in the USA, manufacturers of CT scanners are obliged to report values of CTDIFDA for all modes of operation. Values of such measurements in standard CT dosimetry phantoms are quoted in terms of absorbed dose in PMMA.

For a given type of scanner and CT dosimetry phantom (head or body), values of CTDIFDA measured simultaneously at the surface and the centre of the phantom may vary by up to a factor of three. Variations in this ratio between scanners reflect differences in equipment design, and in particular the shape of the beam filtration.

Although measurements of CTDIFDA represent an established body of data, this quantity is not ideal, however, from the point of view of practical dosimetry. Not only is it expressed in terms of dose to PMMA, which requires the introduction of an additional calibration factor together with its associated uncertainty, but also the length of integration (14 slice thicknesses) varies in absolute terms between settings and is difficult to realise experimentally.

In practice, it is more convenient to measure CTDI over a fixed length of integration using a pencil ionisation chamber with an active length of 100 mm. This provides a measurement of CTDI100, expressed in terms of absorbed dose to air (mGy). When measured in phantoms, such values are larger than corresponding values of CTDIFDA under similar conditions of exposure, with this difference being most significant at small slice thicknesses (Table 1, Appendix I to Chapter 1). Although the ratio of absorbed doses in air and PMMA is approximately 1.1 for the radiation qualities commonly found in CT, this difference is lower (29) for measurements with slice thicknesses in excess of 7 mm by the relatively shorter lengths of integration for CTDI100 in comparison with CTDIFDA; conversely, the difference is exacerbated at smaller slice thicknesses by the relatively longer lengths of integration for CTDI100. The definition of CTDI100 in this guidelines is consistent with the IEC standard on computed tomography (30). Comparing the properties of the various dose quantities, it has been decided to take the CTDI100 at the surface and centre of the head or body CT dosimetry phantoms as an adequate basis for specifying reference doses for CT. From these measurements a weighted CTDI (CTDIw), representing the average dose to a single slice, and an associated dose-length product (DLP) for a complete examination, can be derived. More details about the definition of these quantities are given in Appendix I to Chapter 1.

DERIVATION OF REFERENCE DOSES VALUES

In concept, reference dose values for diagnostic medical exposures are essentially investigation levels which relate to typical practice rather than to individual patients. Such doses are not intended to inhibit the development of sound clinical practice. Reference dose values should be examination-specific and be set to provide an indication of potentially unacceptable practice. They may, for example, be based on the results of large-scale surveys which take into account the variation in performance between centres (31). This approach has been successfully applied to common conventional x-ray examinations in the UK, whereby examination-specific reference dose values were set pragmatically at the third quartile values of the distributions of mean doses observed for representative samples of patients at each centre in a national survey (32). Accordingly, the top dose quartiles have been taken to represent the bounds of potentially unacceptable practice; centres with doses above this level of the distributions are encouraged to carry out urgent investigations with a view to correct action or provide a thorough clinical justification for the use of exceptionally high doses.

Levels of dose from CT examinations depend on the general technique and equipment in use, and also the clinical and physical characteristics of the patient. Wide-scale survey data relating to CT practice may also provide a convenient means for deriving initial values of reference dose quantities for CT. Dose data for some routine examinations (head, chest, abdomen and pelvis) are available from a national survey in the UK at the beginning of the 1990's (8). Distributions are shown in Figures 1 and 2 illustrating the variations in typical values of CTDIw per single slice and DLP per complete examination, respectively, observed between CT centres for routine head examinations (8). For completeness, the values of CTDIair underlying these data are shown in Figure 3 in order to demonstrate that this quantity is more dependent on scanner design and hence shows greater variation than CTDIw. This is why a single value of CTDIair is impractical as a universal reference dose quantity, as discussed above. More recent information concerning some specific examinations (face and sinuses, vertebral trauma, HRCT of the lung, liver and spleen, and osseous pelvis) have been provided by a pilot study of the quality criteria (33). More detailed analyses of the survey data described above are given in Tables 1 and 2, including quartile values for the distributions of CTDIw and DLP, respectively. Accordingly, initial reference dose values for CT, proposed on the basis of the third quartile values from these distributions, are given in Table 3. Effective dose can be calculated from the operational dose values (CTDIair or DLP) thus enabling the different examinations to be compared meaningfully taking into account the relative radiosensitivities of the body regions involved.

The suitability of the initial reference dose values proposed in Table 3 should be checked by means of a wide-scale trial. Consequently the setting and review of reference dose values should be seen as a continuing process in order to promote continous improvement over time.