Overranging at multisection CT: an underestimated source of excess radiation exposure.
ABSTRACT To reconstruct the first and last sections of a helical computed tomographic (CT) scan, the scan length is automatically extended beyond the planned image boundaries, a phenomenon known as overranging. With common 16-section CT scanning protocols, the overrange length is between 3 and 6 cm. For scanners with 64 or more sections, this length will be much greater, since overranging increases as pitch or detector collimation increases. Manufacturers have equipped the latest generation of CT scanners (128 sections or more) with overrange dose-reducing innovations that reduce overranging by typically up to 50%, which in the best cases reduces overranging to that of the previous scanner models (64 sections). To reduce the impact of overranging on radiosensitive organs just outside the planned scan region, it is best to use an axial protocol rather than a helical protocol. If this is not an option, lowering the pitch or the detector collimation will significantly reduce overranging. Finally, CT examinations should be planned in such a way that radiosensitive organs are as far as possible from the imaged volume.
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ABSTRACT: The objective of this study was to determine bony landmarks that can be used on computed tomographic (CT) localizer radiographs to prescribe a reduced scan range for noncontrast CT (NCCT) for suspected urolithiasis. This retrospective review comprised 243 patients who underwent NCCT for suspected urolithiasis. Scout line mode was used to correlate axial images with the z-axis location on CT localizer radiographs. The most inferior vertebral body end plate (superior or inferior) that was above the superiormost aspect of the highest kidney was recorded. The superior or inferior border of the pubic symphysis was determined to be below the inferiormost aspect of the bladder. The CT studies were then viewed to identify any findings that would be missed above or below the defined bony landmarks. Using the top or bottom of T10 included both kidneys in their entirety in 243 (100%) of 243 cases. Using the top or bottom of T11 included both kidneys in 241 (99%) of the 243 and 189 (78%) of the 243 cases, respectively. Using the top or bottom of the pubic symphysis included the entire bladder in 158 (65%) of the 243 and 243 (100%) of the 243 cases, respectively. The mean reduction in scan length was 5.17 cm (11.3%) above the bottom of T10 and 2.89 cm (6.3%) below the pubic symphysis, for a total reduction of 8.06 cm (17.7%). Reducing the scan length is an effective technique to reduce radiation dose in patients undergoing NCCT for suspected urolithiasis. The bottom of T10 and the bottom of the pubic symphysis should be used as landmarks on the CT localizer radiograph.Journal of computer assisted tomography 03/2014; · 1.38 Impact Factor
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ABSTRACT: Overranging or overscanning increases the dose delivered to patients undergoing helical Computed Tomography examinations. In order to reduce it, nowadays most of the multidetector tomographs close the X-ray beam aperture at the scan extremes. This technical innovation, usually referred to as dynamic or adaptive collimation, also influences the overranging assessment methods. In particular, the film free approach proposed in previous studies is not suitable for these modern tomographs. The present study aims to introduce a new method of estimating overranging with real time dosimetry, even suitable for tomographs equipped with adaptive collimation. The approach proposed is very easy to implement and time saving because only a pencil chamber is required. It is also equivalent in precision and in accuracy to the film based one, considered an absolute benchmark.Physica Medica. 06/2014;
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