Computed tomography (CT) constitutes a mainstay of present day radiological diagnostics.
Through a number of technical innovations,
the range of clinical applications of CT has been constantly extended,
and new radiodiagnostic indications have become accessible.
At the same time,
this has also lead to an ever increasing contribution of CT examinations to the collective exposure of patients to ionizing radiation:
Fig. 1: Contribution of various diagnostic X-ray procedures to total examination frequency in the years (a) 1994 and (c) 2012 as well as to collective effective dose in the years (b) 1994 and (d) 2012 for the example of Germany [1, 2]. Only a relatively small fraction of all examinations are computed tomography (CT) procedures, however, these have the highest contribution to collective effective dose. Note that despite numerous developments and improvements in CT technology over the course of 18 years, the doubling in CT examination frequency results in an almost doubled contribution to collective dose.
References: Bundesamt für Strahlenschutz [1, 2]
In the year 2012,
for example,
9 % of all diagnostic procedures requiring the use of ionizing radiation performed in Germany were CT examinations,
contributing to 62 % of collective effective dose [2],
cf.
Fig. 1.
Over the course of 18 years,
the doubling in CT examination frequency has thus resulted in an almost doubled contribution to collective dose,
despite numerous developments and improvements in CT technology (Fig. 1) [1,
2].
The increase in the number of CT examinations worldwide raises concerns about the associated radiation risks,
and there is great interest in approaches for its reduction [1-6,
11,
12].
In this regard,
a key problem in CT imaging is that subjective image quality perception of a CT examination is strongly dominated by image noise,
i.e.
less noisy CT image data is perceived as being of higher diagnostic quality (Fig. 2),
regardless of potentially not impacting diagnostic confidence for a given clinical indication.
Fig. 2: Example of cross-sectional abdominal CT image slices: (a) Higher radiation exposure results in higher subjective image quality characterized by lowered noise and clearly defined borders of anatomical structures (sharpness) leading to good contrast-detail discernibility , whereas (b) lower radiation exposure implies increased noise eventually blurring out borders of anatomical structures, thereby impairing contrast-detail discernibility and resulting in low subjective image quality. Note that contrary to conventional radiography (cf. Fig. 3), in CT an increase of radiation exposure is always associated with a decrease in image noise that is subjectively perceived as an increase in image quality.
References: Unknown source
Therefore,
an increase in radiation exposure associated is rewarded by the acquisition of CT image data of higher image quality,
i.e.
exhibiting less image noise.
This is contrary to conventional radiography,
where an increase of radiation exposure beyond an optimum level is associated with a decrease in image quality,
cf.
Fig. 3.
Fig. 3: Example of conventional radiography of a finger: (a) Overexposure results in lowered image quality characterized by an overall darkening along with decreased contrast detail, whereas (b) the use of correct exposure parameters yields appropriate image quality, especially in view of contrast detail. Note that in conventional radiography an increase of radiation exposure beyond an optimum level is associated with a decrease in image quality.
References: Unknown source
In clinical routine,
different imaging tasks require different levels of quality: Images of the highest quality are not essential for all diagnostic tasks.
Instead,
the level of image quality (e.g.
low noise,
medium or low radiation exposure) is dependent on the clinical task,
i.e.
it should be based on the indication as well as on the individual patient characteristics [6].
Correct selection of the image quality “appropriate” for each specific clinical task will thus result in an optimum use of radiation and in its exposure reduction for most patients [6].