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CT and PET/CT technology examins that  it's critical to keep a balance between the potential diagnostic benefits and the possible risks brought by radiation exposure. Protocols can be adapted to each patient by selecting parameters according to their size, age, gender and the clinical question. Reducing scanning length and minimizing the number of scans in an imaging study can help in dose optimization.
Another factor that can contribute significantly to overall patient exposure is "over-scanning", the additional rotation of the X-ray tube at each end of the scan length to allow the first and last slices to be reconstructed. Typically, over-scanning doses are not included in calculations of effective dose, which are usually based on the planned scan volume and not the actual volume exposed.
There are three parameters:

• Beam collimation
• Pitch
• Reconstruction slice width - to limit the extent of over-scanning.

PET/CT now plays an integral role in oncology, replacing standalone PET for many applications. Here, the CT protocol and exposure varies greatly with application. Often, the CT scan is only needed to give a rough anatomical framework to the PET images, for instance when assessing therapy response in lymphoma patients. But while PET/CT scans have led to increased diagnostic accuracy, this benefit frequently comes at the expense of higher radiation exposure. Thomas Beyer, a teaching professor at the University Hospital Essen, Germany, and CEO of Swiss cross-modality imaging company cmi-experts, told ECR delegates that it's crucial for radiologists and nuclear-medicine physicians to work together to define appropriate imaging parameters for PET/CT of different clinical indications.

Minimizing radiation dose without compromising diagnostic quality is obviously key. This fast scan speed, along with the submillimeter resolution of MDCT, has led to its use in more and more clinical indications, including CT angiography, polytrauma, virtual endoscopy, cardiac imaging and screening. It should be noted that patients who receive regular CT scans are very ill, and the likely reduction in life expectancy from their medical condition is much greater than that associated with the CT exam. The justification of CT use is more critical when it comes to outpatients and participants in screening programmes.

Source: ECR 2009