An assessment of image distortion and CT number accuracy within a wide-bore CT extended field of view

Abstract

Although wide bore computed tomography (CT) scanners provide increased space for patients, the scan field of view (sFOV) remains considerably smaller than the bore size. Consequently, patient anatomy which spans beyond the sFOV is truncated and the information is lost. As a solution, some manufacturers provide the capacity to reconstruct CT images from a partial dataset at an extended field of view (eFOV). To assess spatial distortion within this eFOV three phantoms were considered a 30 x 30 x 20 cm 3 slab of solid water, the Gammex electron density CT phantom and a female anthropomorphic phantom. For each phantom, scans were taken centrally within the sFOV as a reference image and with the phantom edge extended at 1 cm intervals from 0 to 5 cm beyond the sFOV into the eFOV. To assess CT number accuracy various tissue equivalent materials were scanned in the eFOV and resulting CT numbers were compared to inserts scanned within the sFOV. For all phantom geometries, objects within the eFOV were geometrically overestimated with elongation of phantom shapes into the eFOV. The percentage increase in size ranged from 0.22 to 15.94 % over all phantoms considered. The difference between eFOV and sFOV CT numbers was dependent upon insert density. The eFOV underestimated CT numbers in the range of −127 to −230 HU for soft tissue densities and −278 to −640 for bone densities. This trend reversed for low tissue densities with the CT numbers in the eFOV being overestimated by 100-130 HU for lung equivalent inserts. Initial correlation between eFOV and sFOV CT numbers was seen and a correction function was successfully applied to better estimate the CT number representative of that seen within the sFOV. Abstract Although wide bore computed tomography (CT) scanners provide increased space for patients, the scan field of view (sFOV) remains considerably smaller than the bore size. Consequently, patient anatomy which spans beyond the sFOV is truncated and the information is lost. As a solution, some manufacturers provide the capacity to reconstruct CT images from a partial dataset at an extended field of view (eFOV). To assess spatial distortion within this eFOV three phantoms were considered a 30 9 30 9 20 cm 3 slab of solid water, the Gammex electron density CT phantom and a female anthropomorphic phantom. For each phantom, scans were taken centrally within the sFOV as a reference image and with the phantom edge extended at 1 cm intervals from 0 to 5 cm beyond the sFOV into the eFOV. To assess CT number accuracy various tissue equivalent materials were scanned in the eFOV and resulting CT numbers were compared to inserts scanned within the sFOV. For all phantom geometries, objects within the eFOV were geometrically overestimated with elongation of phantom shapes into the eFOV. The percentage increase in size ranged from 0.22 to 15.94 % over all phantoms considered. The difference between eFOV and sFOV CT numbers was dependent upon insert density. The eFOV underestimated CT numbers in the range of -127 to -230 HU for so...

Evaluating the impact of extended field‐of‐view CT reconstructions on CT values and dosimetric accuracy for radiation therapy

“…In cases where patient anatomy extends past the scan field of view (FOV), it is necessary to account for the missing tissue in the CT image. Extended field‐of‐view (eFOV) reconstruction algorithms attempt to recreate the original patient geometry that extends past the original scan window . However, these reconstruction techniques may result in artifacts that can distort the image intensities and patient external contours which can produce inaccurate calculation of the treatment beam attenuation.…”

“…Extended field-of-view (eFOV) reconstruction algorithms attempt to recreate the original patient geometry that extends past the original scan window. [10][11][12][13][14][15][16][17] However, these reconstruction techniques may result in artifacts that can distort the image intensities and patient external contours which can produce inaccurate calculation of the treatment beam attenuation. The purpose of this study is to determine the accuracy of the CT HU numbers and external body contour and to quantify the resulting change in radiation dose calculations for three different eFOV reconstructions algorithms implemented on Siemens CT scanners.…”

Evaluation of the high definition field of view option of a large-bore computed tomography scanner for radiation therapy simulation

“…Results from the current study are consistent with these findings. Another group used anthropomorphic phantoms to study image distortion and CT number accuracy of a wide-bore CT scanner, and found similar results [8]. As suggested by a previous study [19], the dosimetric accuracy of treatment plans can reasonably be assumed to be within ±4% of target dose for all treatment sites in a worst-case scenario if the CT number variations are within ±100 HU of its standard value.…”

“…Significant variation in both CT number accuracy and image distortion have been reported as a function of phantom displacement outside the sFOV [3,7,8], which could affect the dosimetric accuracy of the treatment plan created with these images. The extent and impact of dose discrepancies vary among systems, and the uncertainties involved have made some users reluctant to use HD FOV or eFOV.…”

Impact of computed tomography (CT) reconstruction kernels on radiotherapy dose calculation

“…The more traditionally customizable CT scan parameters such as kilovoltage, current, resolution, slice thickness, field-of-view (FOV) and reconstruction algorithm, have been more heavily studied in terms of the induced HU changes and subsequent impact on dose calculation. [10][11][12][13][14][15][16][17][18][19] These options can be varied through the selection of various anatomic scan protocols pre-installed onto the scanner. In addition, there is also a large variety of reconstruction kernel options available from the manufacturer to choose from.…”