Congress:
EuroSafe Imaging 2019
Keywords:
Action 2 - Clinical diagnostic reference levels (DRLs), Action 3 - Image quality assessment based on clinical indications, Head and neck, CT, Technical aspects, Safety, Quality assurance, Dosimetric comparison
Authors:
T. D. Do, K. Graf, R. Mueller, T. Mokry, T. F. Weber, W. Stiller, H. U. Kauczor
DOI:
10.26044/esi2019/ESI-0085
Description of activity and work performed
Head support shells
Studies in anthropomorphic phantoms have shown that a gantry tilt approximately reduces the lens dose for 75% (3).Since a gantry tilt is not possible with every CT scanner and an alignment of the scan plane along the skull base head support shells were introduced to achieve similar effects in dose reduction.
On the one hand the angulated head positioning shortens the scan length that is necessary to image the whole brain.
On the other the direct radiation of the orbits with the primary beam can be avoided.
However,
the angulation of the head or the cervical spine is not possible in all cases e.g.
patients suspicious of cervical spine trauma.
Another limitation to adequately angulate the cervical spine is a feet-first position,
as a mobile head support shell placed on the scanner table is needed.
Lens protection devices
The International commission on Radiological Protection (ICRP) has reduced the equivalent dose thresholds for the eye lens effects (cataract and opacities) caused by ionizing radiation to 0.5 Gy for patients (4).
Irrespective of CT scanner type with or without gantry tilt eye shields are provided for patients during the scan at our institution.
The eye shield is placed after the acquisition of the scout to prevent tube current modulation and dose increase compensating for the elevated x-ray attenuation and resulting in an increased radiation dose.
Thus ideally the eye shield should not be visible on the scout.
It has been reported that the eye shield does not provide a marked dose reduction to the eye lens when an angled gantry is used (5).
However when the orbits are exposed to the primary beam an eye shield should be used.
Moreover,
the application of the eye shield increases noise in the anterior and central parts of the brain whereas posterior parts were not significantly affected (6).
By implication no eye shield is used for facial or orbit trauma.
Image quality improvement with iterative reconstructions and kernel
Iterative reconstruction techniques have shown to reduce the radiation dose while maintaining the diagnostic image quality through lowering image noise and increasing signal-to-noise ratio (7-9).
Different iterative reconstruction algorithms are offered,
which are vendor-specific and based on sinogram impainting techniques.
Several strength levels are available and a trade-off between reduced noise and conspicuity/sharpness of the structures has to be made (Figure 4).
The newly generated images may appear altered and show smoother texture (10).
Iterative reconstruction has become a clinical standard in head CT.
To ensure a good demarcation brain structures and delineation of pathologies a sharp soft tissue kernel is used (Figure 5).