The study is carried out at a university hospital, where two GE Revolution EVO CT devices were recently installed. The protocols in use were initially proposed by GE and adapted by the radiologists to their needs and likes.
In order to accomplish some optimizations in CT protocols, a multidisciplinary dose optimization committee has been created, with radiographers, radiologists and medical physicists.
EUCLID Project has selected 10 clinical CT indications to establish European DRLs. Protocols corresponding to these indications have been searched for and identified in our devices by the radiologists. Table 1 shows the selected indications, organized by anatomical location, and the corresponding protocols.
DRLs in EUCLID are established for CTDIvol (mGy) and DLP (mGy.cm). These quantities are indicated by the CT units and annually validated within the quality assurance programme. The maximum allowed difference according to Spanish protocol between measured and indicated value is 10%.
The dose online management system available at our hospital, General Electric DoseWatch, was used to extract CTDIvol from each series and DLP for each study. Then, for each clinical indication, median values of both quantities have been calculated and compared to the EUCLID DRLS. Outlier values have been searched for and extracted to each sample, as follows:
Superior Outlier: Q3 + 3*(Q3 –Q1)
Where Q1 and Q3 are the first and third quartile of the sample. Values over this outlier where extracted and then median values calculated.
To optimize doses in a Revolution EVO CT, the process involves changing the “noise index”, which is a parameter related to the accepted noise in the images and therefore the radiation dose delivered to the patient (Figure 1).
For each protocol, GE recommends a noise index level, but it can easily be changed. Depending on the slice thickness selected and anatomical thickness of the area to be irradiated, the noise index should be different. For example, noise index recommended for head protocols with 2.5 mm slice thickness is 3.80, while the noise index for this thickness in an abdomen protocol is 15.8. According to GE, when noise index is increased by a 5%, mA are decreased by the automatic exposure control by 10%. So, to optimize our protocols, noise index has to be changed according to necessities. Also pitch or slice thickness can be reviewed.