When 30 frames per second is not 30 images per second Catriona M.
1,2,3, Steele Melanie
1 Peladeau-Pigeon
& Ka Lun
3 Tam
1. Toronto Rehabilitation Institute – University Health Network, Toronto, Canada; 2. University of Toronto, Toronto, Canada; 3. Bloorview Research Institute, Holland Bloorview Kids Rehab, Toronto, Canada
Background
Metronome Test Protocol
Current best practice recommendations for Videofluoroscopic Swallowing Studies (VFSS) or Modified Barium Swallows (MBS) are: – Fluoroscopy exposure: • continuous fluoroscopy, or • 30 pulses/s – Image capture : • maximum temporal resolution: • 30 fps (NTSC countries), or • 25 fps (PAL and SECAM countries) As part of establishing a multi-site research study, we inspected image acquisition rate on fluoroscopy systems at 8 different hospitals.
1. Wind the metronome fully before testing by turning the winding key. 2. Slide the beat indicator up along the metronome arm to the upper-most setting of 40 beats per minute. 3. Tap the metronome arm to set it in motion. 4. Position the metronome in the videofluoroscopy system. Zoom out on the image as much as possible while still being able to identify the metronome. 5. Turn on the fluoro for 30 seconds. 6. Zoom in on the metronome, so that it appears larger on the screen. 7. Turn on the fluoro for 30 seconds. 8. Zoom in yet again, while ensuring that the metronome arm is visible during its full range of motion. 9. Turn on the fluoro for 30 seconds. 10. Disassemble the metronome. 11. Repeat the test using 44 beats per minute instead of 40 beats per minute. 12. Review the recording frame by frame to confirm image acquisition rate: • At 40 beats per minute and 30 fps, there should be 45 unique video frames per swing; • At 44 beats per minute and 30 fps, there should be 41 unique video frames per swing; • Frames should progress cleanly with the arm of the metronome remaining straight on all images.
Methods • Fluoroscopic recordings of an analog metronome were collected at 40 and 44 beats/minute. The fluoroscope was turned on for ~ 30 seconds during active pendulum swings at 3 different zoom levels using radiology settings intended to yield 30 unique images/s. • Fluoroscopy recording were obtained on one of two video capture platforms: – one platform was designed for BNC and composite video inputs; – the second was designed for DVI input. • Recordings were analyzed to confirm frame acquisition rate and assess image quality.
Metronome Protocol Images
Fig 1. Wittner mini metronome (model 882051)
Fig 2. Clear fluoroscopic image of metronome at end of its swing
Findings Inspection of the recordings revealed several situations in which a portion of the frames in every second included: • interpolation of adjacent frames, • screen splits, • ghost shadows, and • motion blur.
Conclusions • Although split frames, ghosts and motion blur may be fully expected by imaging specialists, clinicians and researchers need to be aware of these issues and the limitations they bring for videofluoroscopy analysis. • A portion of images in each second may contain visual distortions. • Measurements from these frames are vulnerable to distortion artifacts. The accuracy of pixel based measures of swallowing kinematics and bolus movement may be particularly challenged by these image registration constraints.
References Schueler, B. (2000). The AAPM/RSNA physics tutorial for residents: General overview of fluoroscopic imaging. Radiographics, 20(4), 11151126. Bonilha, H. S., Blair, J., Carnes, B., Huda, W., Humphries, K., McGrattan, K., Martin-Harris, B. (2013). Preliminary investigation of the effect of pulse rate on judgments of swallowing impairment and treatment recommendations. Dysphagia, 28(4).
Acknowledgments • Funded by Nestlé Health Science S.A. (Vevey, Switzerland). • Assistance from Susan Brady, Nancy Swigert, Kayla Brinkman Theimer, Luis Riquelme, Andrea Tobochnik, Susan Langmore, Gintas Kriscinunas, and Liza Blumenfeld with data collection is gratefully acknowledged.
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Fig 3. Split frame and ghost shadows
Fig 4. Motion blur
Discussion with the fluoroscope manufacturers suggests these issues can arise due to recursive filters (frame averaging), transmission rate conversion and dys-synchrony between fluoroscope output and video capture.
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