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Assessment of Flying-Focal Spot in MPR Images of High-Frequency Objects at Various Distances for the Isocenter

R Al-Senan*, K Brown, S King, Penn State Milton S. Hershey Med Ctr., Hershey, PA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To quantitatively evaluate the spatial resolution of CT MPR images with the flying focal spot using a simple test object.

Methods: A star pattern phantom of 2? spoke angle was used as the test object. Temporal bone (zUHR) protocol was used on our 128-slice scanner. For coronal MPR assessment, the phantom was laid flat, and for sagittal MPR, the phantom was standing 90? with its edge aligned with Z-plane. Scans were performed at isocenter and at different distances (4cm, 8cm, and 12cm) from isocenter in both vertical and lateral directions. Images were reconstructed with a sharp kernel (Ur77u/3). A plugin in ImageJ software was used to define the limiting resolution, which corresponded to where the phase reversal (of black and white) occurs. By measuring the distance from the center to the phase-reversal point, the line-pairs per mm (lp/mm) was then calculated as 90/2pr, for both directions in phantom images. Also, an anthropomorphic phantom’s temporal bone was scanned at the same vertical intervals.

Results: When the test object was at isocenter, the limiting resolution in all three planes was about 1.9 lp/mm. Z-plane’s spatial resolution showed only a slight loss (between 1.7 and 1.8 lp/mm) with lateral and vertical shifts of 8 and 12-cm from isocenter. Both X and Y planes’ resolution showed significant loss at distances > 4cm in the lateral and vertical directions (from 1.9 lp/mm down to 1.0 lp/mm at 12cm from isocenter). Coronal and sagittal images of the anthropomorphic phantom demonstrated similar effect.

Conclusions: 1) The use of the star phantom in evaluating the spatial resolution of MPR images can provide a reliable objective assessment. 2) The flying focal spot provides almost shift-invariant high-resolution in the z-plane. However, in both x and y planes, the resolution suffers significant loss at distances > 4cm from isocenter.

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