Overranging at Multisection CT: An Underestimated Source of Excess Radiation Exposure 1

Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, the Netherlands.
Radiographics (Impact Factor: 2.6). 07/2010; 30(4):1057-67. DOI: 10.1148/rg.304095167
Source: PubMed


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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    • "Overscanning (overranging) refers to scanning a body part greater than the one planned for obtaining adequate data for image reconstruction [75]. Its effect is greater in pediatric patients than adults because of the smaller body sizes of children [73] [74] [75] [76] [77] [78]. Generally, the extent of overranging mainly depends on detector collimation and pitch because they affect the dose profile. "
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    ABSTRACT: Introduction: Computed tomography (CT) dose optimization is an important issue in radiography because CT is the largest contributor to medical radiation dose and its use is increasing. However, CT dose optimization for pediatric patients could be more challenging than their adult counterparts. The purpose of this literature review was to identify and discuss the current pediatric CT dose saving techniques. Optimized pediatric protocols were also proposed. Methods: A comprehensive literature search was conducted using the Medline, ProQuest Health and Medical Complete, PubMed, ScienceDirect, Scopus, Springer Link, and Web of Science databases and the keywords CT, pediatric, optimization, protocol, and radiation dose to identify articles focusing on pediatric CT dose optimization strategies published between 2004 and 2014. Results and Summary: Seventy-seven articles were identified in the literature search. Strategies for optimizing a range of scan parameters and technical considerations including tube voltage and current, iterative reconstruction, diagnostic reference levels, bowtie filters, scout view, pitch, scan collimation and time, overscanning, and overbeaming for pediatric patients with different ages and body sizes and compositions were discussed. An example of optimized pediatric protocols specific to age and body size for the 64-slice CT scanners was devised. It is expected that this example could provide medical radiation technologists, radiologists, and medical physicists with ideas to optimize their pediatric protocols.
    Full-text · Article · Jun 2015 · Journal of Medical Imaging and Radiation Sciences
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    • "In contrast, for imaging of the thorax and abdomen, the difference is only 790 mGy cm versus 769 mGy cm. Most differences lie in differences in the extra dose from overranging [12, 13]. In contrast, most of the dose increase is seen in imaging of the thorax and liver CT for lung carcinoma staging, with 557 mGy cm for the lung and liver scanned separately versus 383 mGy cm for the thorax-liver in one run. "
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    ABSTRACT: OBJECTIVES: To assess radiation exposure due to CT in the Netherlands. METHODS: Twenty-one hospitals participated in a dose survey for the 21 most frequently used CT protocols. Hospitals completed a Web survey with detailed parameters for one patient per protocol, including the dose length product (DLP) from the scanner dose report. Only standard-sized patients (1.74 m and 77 kg and BMI 25.4 kg/m(2) ± 15 %) for each protocol and available scanner were considered. Effective dose (E) per protocol was estimated using ICRP-103-based E/DLP coefficients. Dose levels were compared to surveys from other countries and to diagnostic reference levels. RESULTS: Data of 186 patients (247 scan phases) from 14 hospitals and 19 scanners were used for final analysis of DLP and E. Effective doses varied from 0.2 mSv in sinus CT to 19.4 mSv for multiphase liver. The most frequent exams were brain (1.5 mSv), abdomen (8.0 mSv), and thorax-abdomen (11.5 mSv). These results are lower than in Germany and comparable to those in the UK, and are within reference levels. Results between hospitals varied, with per protocol minimum/maximum E ratios ranging from 1.1-5.4. CONCLUSIONS: Compared to surrounding countries, CT in the Netherlands is associated with relatively low radiation doses in standard patients. Important differences remain between hospitals. MAIN MESSAGES : • A national dose survey providing updated, detailed data for patient dose in the most frequently used CT protocols. • CT in the Netherlands is associated with relatively low individual radiation doses in standard patients compared to surrounding European countries. • Considerable differences remain between hospitals for the most frequently used CT protocols, indicating the need for further optimisation.
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