Flattening filter removal for improved image quality of megavoltage fluoroscopy. Issue 8 (24th July 2013)

• Record Type: Journal Article
• Authors: Christensen, James D.; Kirichenko, Alexander; Gayou, Olivier
• Is Part Of: Medical physics. Volume 40:Issue 8(2013)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2013-07-24
• Subjects: Digital fluoroscopy -- Biomedical instrumentation and transducers, including micro‐electro‐mechanical systems (MEMS) -- Dosimetry/exposure assessment -- Linear accelerators; biomedical equipment -- diagnostic radiography -- dosimetry -- linear accelerators -- phantoms; flattening filter -- mega‐voltage imaging -- fluoroscopy; Linear accelerators; Medical imaging -- Medical image quality -- Medical image contrast -- Photons -- Fluoroscopy -- Gold -- Medical image noise -- Medical X‐ray imaging -- Cancer -- Spatial resolution; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4812678
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : Removal of the linear accelerator (linac) flattening filter enables a high rate of dose deposition with reduced treatment time. When used for megavoltage imaging, an unflat beam has reduced primary beam scatter resulting in sharper images. In fluoroscopic imaging mode, the unflat beam has higher photon count per image frame yielding higher contrast‐to‐noise ratio. The authors' goal was to quantify the effects of an unflat beam on the image quality of megavoltage portal and fluoroscopic images. Methods: : 6 MV projection images were acquired in fluoroscopic and portal modes using an electronic flat‐panel imager. The effects of the flattening filter on the relative modulation transfer function (MTF) and contrast‐to‐noise ratio were quantified using the QC3 phantom. The impact of FF removal on the contrast‐to‐noise ratio of gold fiducial markers also was studied under various scatter conditions. Results: : The unflat beam had improved contrast resolution, up to 40% increase in MTF contrast at the highest frequency measured (0.75 line pairs/mm). The contrast‐to‐noise ratio was increased as expected from the increased photon flux. The visualization of fiducial markers was markedly better using the unflat beam under all scatter conditions, enabling visualization of thin gold fiducial markers, the thinnest of which was not visible using the unflat beam. Conclusions: : The removal of the flattening filter from a clinical linac leads to quantifiable improvements in the image quality of megavoltage projection images. These gains enable observers to more easily visualize thin fiducial markers and track their motion on fluoroscopic images.