Abstracts Posters SICOT-SOF meeting Gothenburg 2010 _2_

Poster
Topic: Arthroplasty - Hip
Abstract number: 26473
ASSESSMENT OF LOW RESOLUTION DIGITAL DETECTORS FOR
RADIOSTEREOMETRY
Martin DOWNING 1 , George ASHCROFT 2
1 Grampian University Hospitals NHS Trust, Aberdeen (UNITED KINGDOM),
2 Aberdeen University, Aberdeen (UNITED KINGDOM)
Radiostereometry (RSA) is an important method for assessing migration and
inducible movement of orthopaedic implants. Direct digital imaging offers potential
enhancements; however suitable detector choice remains problematic due primarily
to either large size or low resolution. A compact detector of similar proportions to hip
film cassettes is available; however its resolution is only 160 dpi, which is below that
recommended for RSA. We investigated whether with improved marker analysis
methods such detectors could measure markers with sufficient precision and
accuracy.A phantom was constructed with a reference grid of 1400 spherical 2mm
markers arranged in a 10mm spaced grid format. Additionally 25 tantalum markers
each of diameter 1.0, 0.8 and 0.5 mm were added within a 40mm square. The
phantom was imaged repeatedly with a Cannon CXDI50C detector with small
movements between each scan. Marker positions were measured using model
based fitting, and registered to the reference grid identifying measurement
error.Additionally, RSA image pairs from a hip study were reanalysed at half
resolution (150dpi) and compared to the originals. With the phantom, median (max)
radial error was 4 (22), 5 (29), and 4 microns for marker diameters 1.0, 0.8 and
0.5mm respectively. For the 56 RSA image pairs, median (95%ile) marker deviation
was 2 (9) microns, segment movement 5 (12) microns and 0.01 (0.03) degrees, and
rigid body fitting mean error 8 microns.We conclude the application of 160 dpi
detectors will not significantly degrade precision when applied to RSA.
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