POBLACIÓN OBJETIVO

For radiography and fluoroscopy, the primary dose indicators (K_i and P_KA) are based on measures of tube output in air, whereas for CT the indicators (C_VOL and P_KL,CT) are based on both tube output and absorption in standard sized phantoms. These CT dose indicators quantify the amount of radiation generated by the CT scanner during a clinical examination. Since the standardized phantoms used for these measurements are not good models of patient attenuation for

patients of varying sizes, from an infant to large adult, C_VOL and P_KL,CT should never be used as an estimate of patient dose during a CT examination.

4.4.1. C_VOL

C_VOL was defined in Ref. [27] to indicate the radiation output of a CT scanner measured in a specifically defined phantom. It is suitable for comparing radiation production of different scanners. As such, C_VOL is affected by many scan parameters: tube current, rotation time, pitch, tube voltage, location of the phantom relative to the scanner detector, X ray tube focal spot, and the shape, thickness and material of the bowtie filter inserted into the X ray beam. C_VOL may be considered a dose index of CT scanners [28].

Radiation dose in a clinical CT examination is determined by the amount of energy delivered to locations within the patient’s body by the CT scanner. Thus, an estimation of patient dose can be determined by adjusting C_VOL (radiation output of the scanner) using a conversion factor that accounts for the attenuation of the patient’s body, which is, in turn, a function of body size, in comparison with the attenuation properties of the standard CTDI phantom. Assuming that the scanner’s displayed CTDI values for an abdominal scan are based on the 32 cm CTDI phantom, the estimated patient dose for an adult is similar to the displayed C_VOL. The small infant’s estimated abdominal dose will be approximately three times greater than the C_VOL, while the abdominal dose to the largest adult abdomen will be approximately 70% of C_VOL [29]. In the event that the scanner displays C_VOL based on the 16 cm CTDI phantom, the estimated abdominal dose to the infant will only be 1.5 times greater than C_VOL. It is imperative that the operator know which of the two standard CTDI phantoms their scanner uses for each type of clinical examination when displaying C_VOL.

A set of conversion factors have been derived to convert C_VOL to a size specific dose estimate (SSDE) for children, small adults and large adults receiving CT scans [29]. These were derived using four independent methodologies to study the attenuation properties of patients. When the data from the four methodologies were combined, a logarithmic relationship between the effective diameter of the patient and the normalized conversion factor was found, giving a single set of conversion factors for all high voltage scan parameters between 80 and 140 kV [29]. The correction factors are currently limited to variations in patient size for the trunk of the body. The accuracy of the SSDE estimate as a function of patient size is believed to be within 20% [29]. The SSDE provides a patient dose estimate that can assist in assigning risks that result from CT paediatric scans as discussed in Section 5. This allows the radiologist and radiographer to use the SSDE to better manage the radiation dose delivered to paediatric patients, especially in institutions that scan paediatric patients a minority of

the time. Calculating SSDE is relatively simple with the use of look-up tables based on patient thickness. For example, if the electronic measurement of the scan projection image of the patient gives a lateral dimension of 12.3 cm, and the scanner displayed C_VOL is 5.4 mGy based on the 32 cm CTDI phantom, the correct look-up table [29] gives a correction factor of 2.5 for a 12 cm patient. The size specific dose estimated for the patient is given by Eq. (33):

SSDE = 5.4 [mGy] × 2.5 = 13 [mGy] (33)

It is important to note that the SSDE should not be used to compute a modified DLP nor to compute effective dose using currently available conversion factors (see Ref. [29]).

4.4.2. Dose length product

The concept of DLP incorporates both the mean C_VOL and the total distance scanned. It has often been considered to be the more useful of the two commonly used CT dose metrics, as it can be considered to be related more closely to risk.

however, for paediatric dosimetry, in particular, it is important to understand both the drawbacks and the advantages of this quantity through a consideration of the factors affecting the total scan length:

— The size of the patient will have a large effect on the scan length for a single phase and the higher DLP resulting from scanning a taller patient will not necessarily correspond to higher organ doses.

— The choice of anatomical start and stop positions should normally be dictated by the clinical requirements of the examination but, for small children in particular, may have a critical effect on the dose to organs at the edge of the area of interest.

— The degree of over-ranging in helical scans, i.e. the additional scan volume either side of the region of interest, depends on the scanner type, nominal beam width setting and the pitch, as a specific type of scanner will usually use a set number of tube rotations before and after the selected range, and the distance this equates to in the z direction will vary with beam width and pitch. Over-ranging will result in additional dose to the patient, which is likely to be a greater percentage of the total dose as the patient size decreases.

— The number of phases of the examination will affect the total dose to organs within the scan volume, e.g. a two-phase scan could double the dose to scanned organs.

— One might be tempted to estimate the DLP of paediatric patients by substituting SSDE for C_VOL. While this might be helpful in some cases to

estimate risk, this cannot be done under any circumstance if the calculated paediatric DLP is going to be used with published effective dose conversion factors to estimate effective dose.

— Regarding CTDI values, it is important to know to which size phantom the displayed DLP values relate. This may be the head phantom for all paediatric protocols or may be the 32 cm phantom for body protocols. It may also depend on the FOV selected, rather than on head or body mode, e.g. selecting a small FOV on an abdomen protocol may use the head phantom value.