A methodology for direct quantification of over‐ranging length in helical computed tomography with real‐time dosimetry

Abstract

In helical computed tomography (CT), reconstruction information from volumes adjacent to the clinical volume of interest (VOI) is required for proper reconstruction. Previous studies have relied upon either operator console readings or indirect extrapolation of measurements in order to determine the over‐ranging length of a scan.( 1 – 5 ) This paper presents a methodology for the direct quantification of over‐ranging dose contributions using real‐time dosimetry. A Siemens SOMATOM Sensation 16 multislice helical CT scanner is used with a novel real‐time “point” fiber‐optic dosimeter system with 10 ms temporal resolution to measure over‐ranging length, which is also expressed in dose‐length‐product (DLP). (6) Film was used to benchmark the exact length of over‐ranging. Over‐ranging length varied from 4.38 cm at pitch of 0.5 to 6.72 cm at a pitch of 1.5, which corresponds to DLP of 131 to 202 mGy‐cm. The dose‐extrapolation method of Van der Molen et al. yielded results within 3%, while the console reading method of Tzedakis et al.( 2 , 4 ) yielded consistently larger over‐ranging lengths. From film measurements, it was determined that Tzedakis et al.( 2 , 4 ) overestimated over‐ranging lengths by one‐half of beam collimation width. Over‐ranging length measured as a function of reconstruction slice thicknesses produced two linear regions similar to previous publications.( 1 – 4 ) Over‐ranging is quantified with both absolute length and DLP, which contributes about 60 mGy‐cm or about 10% of DLP for a routine abdominal scan. This paper presents a direct physical measurement of over‐ranging length within 10% of previous methodologies.( 1 – 4 ) Current uncertainties are less than 1%, in comparison with 5% in other methodologies. Clinical implantation can be increased by using only one dosimeter if codependence with console readings is acceptable, with an uncertainty of 1.1% This methodology will be applied to different vendors, models, and postprocessing methods – which have been shown to produce over‐ranging lengths differing by 125%. (4) PACS number: 87.57.qp

Real time evaluation of overranging in helical computed tomography

“…As evident in Fig. 4 OV ' uncertainties reported in Table 3 for Tomograph A without AC, reproduce Tien's results [11], confirming the reliability of the proposed method. Moreover all OV ' evaluations of this study were performed even on modern CT scanners equipped with AC, without any loose in precision or in accuracy.…”

“…OV is always directly measurable by films [9]. However van der Molen [10] and Tien [11] improved OV quantification introducing two film free and time saving approaches. In the "dose-slope" method described in Ref.…”

Accuracy of Monte Carlo simulations compared to in‐vivo MDCT dosimetry

“…Scatter from this region can substantially contribute to the total dose measured by the TLD. 42,43 This was the most likely cause for the underestimation of the dose to TLDs in this patient. T II.…”

“…Scatter from this region can substantially contribute to the total dose measured by the TLD. 42,43 This was the most likely cause for the underestimation of the dose to TLDs in this patient. Other possible reasons for discrepancies between measurements and simulated values is the x-y plane shift in the reconstructed images to center the anatomy in the display field of view.…”

Characterization of dynamic collimation mechanisms for helical CT scans with direct measurements

“…Indirect methods have been proposed to measure the integrated radiation dose within a 100 mm ion chamber at different scan lengths. By extrapolating the dose to zero scan length, the dose associated with overranging (or saved with dynamic collimation) can be estimated (Tien et al 2011, Trevisan et al 2014.…”