A PET cylinder phantom positioned at an oblique angle.

Objectives: A widely-available uniform cylinder phantom positioned at a slightly oblique angle, allows for very fine sampling of the edge spread function with potential for accurate measurement of PET spatial resolution. We show how this technique can be used to measure the effect of positron range and to predict recovery coefficients for extended phantoms with different geometries, contrast ratios and reconstruction parameters.

Methods: A 20 cm diameter uniform ¹⁸F cylinder phantom was centrally positioned at a small angle (~5 degrees) with respect to the z-axis of a Biograph mCT. The oblique angle ensures the phantom edge intersects the image matrix differently in different slices. Combining line profiles from multiple slices allows for very fine sampling of the edge spread function. Spatial resolution was measured as the full width at half maximum (FWHM) by fitting a model to the edge spread functions in both radial and axial directions. The technique was validated by comparison with NEMA point source measurements, repeatability studies and by controlled modulation of image reconstruction parameters. Separate experiments with uniform cylinders containing ¹⁸F, ¹¹C, ¹³N, ⁶⁸Ga and ¹²⁴I allowed for a direct measurement of the effect of positron range on spatial resolution. FWHM measured with an ¹⁸F cylinder was also used to calculate recovery coefficients by equivalent filtering of phantom-specific digital reference objects. Recovery coefficients derived in this way were compared to experimental data for different phantom geometries (NEMA image quality phantom, ACR PET phantom), insert-to-background ratios (2, 4, 6, 8 and 10-to-1), phantom preparations (n = 5 for NEMA, n = 10 for ACR) and reconstruction parameters.

Results: Cylinder estimates of spatial resolution (involving iterative as opposed to analytic reconstruction) were in close agreement with NEMA FWHM values: 5.53 ± 0.19 mm (radial) & 5.78 ± 0.09 mm (axial) for an ¹⁸F cylinder (3D OSEM+TOF, 2i, 21s, all pass) compared to 5.48 ± 0.05 mm (radial) & 5.49 ± 0.12 mm (axial) for an ¹⁸F point source at r = 10 cm (FORE + FBP, ramp filter). Controlled adjustment of a Gaussian post-reconstruction filter was accurately reflected in the measured FWHM values. A linear fit to the data produced FWHM ²_measured = 1.06 FWHM ²_filter + 31.15, implying an intrinsic spatial resolution of √31.15 mm = 5.58 mm for ¹⁸F at r = 10 cm in the radial direction. The effect of increasing positron energy was clearly reflected in the FWHM values measured with different isotopes: 5.53, 5.62, 5.64, 6.55, 6.88 mm for ¹⁸F, ¹¹C, ¹³N, ⁶⁸Ga and ¹²⁴I respectively (radial direction, r = 10 cm, 3D OSEM+TOF, 2i, 21s, all pass). Recovery coefficients derived using the cylinder FWHMs were in very close agreement with recovery coefficients derived from physical phantoms over a range of insert-to-background ratios, phantom geometries and reconstruction protocols (see supporting data).

Conclusion: PET spatial resolution can be conveniently measured by positioning a widely-available uniform cylinder phantom at a slightly oblique angle. Utility of the method was illustrated by measuring the spatial resolution corresponding to various relevant isotopes including ¹⁸F, ¹¹C, ¹³N, ⁶⁸Ga and ¹²⁴I. The method is well-suited to multi-center deployment and may aid standardization of data collection across sites. Research Support: This work was supported by a grant from the National Institutes of Health (HHSN268201500021C).