Dose reduction techniques
The system for patient radiation protection is based upon two principles: (a) justification and (b) optimisation. It is essential that all X-ray exposures used for estimation of bone status are clinically justified. Examinations that do not influence patient care must be avoided.
Patient preparation for bone densitometry is important for reducing the radiation dose. Careful checking for the presence of items on patient’s clothing such as jewellery and coins that cause artefacts will optimise the quality of the imaging and avoid the imaging having to be repeated with additional radiation exposure. In paediatric examinations, proper interaction with the children and parents is essential. All actions should be taken to avoid movement of the child during imaging and defer imaging if artefacts (e.g. plaster of Paris on fractures) are present at the imaging site.
The length of the DXA or CT should be minimal and should take into account patient’s body size. If possible, radiosensitive organs should not be exposed to the primary radiation beam. Modern MDCT employ multiple rows of detector arrays allowing rapid imaging. As a consequence, the length of imaging is sometimes increased to include multiple body sites. This increases the DLP and the patient effective dose.
After the introduction of multi-detector CT, significant attention was given to doses and associated risks from CT. In spiral CT, reconstruction algorithms require a number of additional rotations to reconstruct the first and last slice of imaged volume. Z-overscanning refers to the extent of the tissue beyond the boundaries of the volume to be imaged that is exposed to X-rays. The increase in patient effective dose due to z-overscanning may reach 36% for a 16-slice CT system [47]. In hip MDQCT and HR MDCT imaging the influence of z-overscanning on the patient effective dose should always be taken into account. Careful selection of beam collimation, reconstructed image slice width and pitch is needed to limit the extent of z-overscanning [48]. A recent study showed that adaptive section collimation allows considerable dose reduction of unnecessary exposure due to z-overscanning [49].
Variation in the tube voltage causes a substantial change in patient CT dose and image quality [50]. The use of 80 kVp for spinal single-slice QCT reduces the dose substantially in comparison with acquisition at 120 kVp. The possibility of tube voltage reduction in MDQCT should be investigated, especially for patients with small body size. Reduction of tube load may reduce patient dose considerably. Automatic exposure control (AEC) tools adjust the tube current in the x–y plane and along z axis simultaneously, based on the size and attenuation of the anatomical area being examined to achieve an operator-defined level of image quality with the lowest possible dose. The use of AEC provides a substantial potential for dose reduction compared with the fixed mA technique. In AEC-activated examinations, the mAs product can be reduced by typically between 15% and 60% [51, 52].