Aims and objectives
Optimization of image quality and reducing patient radiation dose is achieved in part by positioning the patient at the CT scanner isocenter.The isocenter of a CT scanner is defined as the axis of rotation of the x-ray source and detector. In the absence of automatic exposure control (AEC),
if the patient is not aligned with the isocenter of the CT scanner the resultant image will demonstrate increased noise1,
which will negatively impact on the diagnostic acceptability of the images (Fig.1).
If AEC is employed,
isocenter...
Methods and materials
222 consecutive outpatient adult CT-colonography (CTC) studies performed between January and December 2016 were retrospectively analyzed.
One hundred CT-KUB and one hundred CT-AP over the same time period were used as comparison studies.
Automated dose-tracking softwarewas used to calculate patient anatomical position and distance from the isocenter along both the x-axis (horizontal) and y-axis (vertical).
Results
Isocenter misalignment in the y-axis was significantly greater than in the x-axis during CTC (p=0.002).
Patients were significantly more likely to be off-center during prone CTC compared to CT-KUB (p=0.008) and during supine CTC compared to CT-AP (p=0.008).
IM was shown to be slightly increased in studies performed by more experienced radiographers (p=0.04).
Patient size or age were not associated with IM (p=0.57 and p=0.91 respectively).
Conclusion
Isocenter misalignment,
which results in increased patient radiation dose and decreased image quality1-3,
is greater during CT colonography compared with CT-KUB or CT-AP; this is likely multifactorial and due in part to patient discomfort and change in body contour due to colonic distension.
Isocenter misalignment during CTC was found to be marginally worse during CTC performed by more experienced radiographers.Our resultshighlight the need to consider implementation of strategies for improving patient positioning such as radiographer education and automated patient positioning solutions.
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