We evaluated the capability of a 3 mm thick CsI:Tl FC scintillator to compensate parallax errors by comparing the FWHM of a projected source to the FWHMs for a 3 mm thick continuous CsI:Tl scintillator and for a 1.5 mm thick CsI:Tl SC scintillator. The thinner SC scintillator is used as the gold standard with which to compare the DOI compensation effect of the FC scintillator. The FC scintillator was laser-cut to create 350 µm pixels. The angular range of the cuts was between −20°and 20° and was limited by the angular tilt range of the goniometer used to set the cut angles in the etching process. The SC scintillator was laser-cut to create 325 µm pixels. The FC scintillator was fabricated through excimer laser ablation (KrF (λ = 248 nm)) from a continuous CsI:Tl scintillator at Resonetics Inc. The CsI:Tl crystals of thickness of 3 mm, were cut to produce structured columns, oriented towards the pinhole collimator aperture. The cut width averaged 30 µm, which is much thinner than the 325 µm pixel size and thinner than what is achievable by a wire saw. More details of the fabrication procedure are discussed elsewhere [10
]. All three scintillators have an active area of 10 × 10 mm2
. The experimental setup is shown in . The FC and SC scintillators are mounted on a fiber-optic substrate, which is then pressure-coupled to the fiber-optic plug of the EMCCD camera. Index-matching fluid is placed between the substrate and plug for each scintillator. The continuous scintillator is directly pressure-coupled to the fiber-optic plug, also with a layer of index-matching fluid between it and the plug.
We imaged a ~100 µCi Tc-99m (140 keV) point source, a 1.12 mm diameter resin ion exchange bead previously immersed in a NaTcO4 solution, as it was translated across the FOV of the scintillators. A tungsten collimator with a 1 mm diameter knife-edge pinhole and a thickness of 6 mm was used. The photons emitted from a scintillator were transmitted through a 1 mm thick 1:1 fiber optic plug and collected by a cooled electron-multiplying charged-coupled device (EMCCD) camera, a back-illuminated Andor iXon DV887. The EMCCD has an 8 × 8 mm2 detector, a 16 µm pixel width, and 512 × 512 binning. The detector area is less than that of the scintillator area (64 mm2 vs. 100 mm2, respectively) and is thus not able to capture all of the emitted scintillation light.
The values for the geometrical parameters of the experimental setup shown in were calculated as follows. A primary constraint is that the collimated rays are properly aligned with the angled pixels of the FC scintillator. We aligned the rays by setting f
so that the maximum ray angle at the edge of the scintillator, θmax
, is equal to 20°, the maximum pixel cut angle. We calculated the necessary values of f
in order to achieve θ = θmax
at the edge of scintillator (x
= 13.737 mm. We took into account the mean DOI
for each scintillator when determining the necessary spacings between the physical components in our experimental setup in order to keep the focal length fixed at the same value for all scintillators. However, we were limited by the precision of our mechanical setup and thus set f
to within 13.8 ± 2 mm. The pinhole-to-object distance, b
, was set to 31.6 mm, large enough so that the 1.12 mm diameter resin bead would best approximate a point source given the 2.42 mm theoretical knife-edge pinhole collimator resolution while maintaining an acceptable level of sensitivity (3.38 × 10−5
) for imaging. The resulting magnification was 0.437, yielding a FOV of 18.3 mm at a distance b
from the pinhole. We adjusted the spacing between the object, collimator, and detector based on each scintillator’s dimensions so that f
were maintained approximately constant for each experimental setup.
It is important to note that the expression for the magnification of an FC-scintillator-based pinhole camera is different from that of a conventional pinhole camera (M
). In the case of the FC scintillator, there is additional magnification through the thickness of the scintillator, which is illustrated in . The resulting magnification of the FC scintillator setup, Mf
, is thus
and is equal to 0.530 in this setup.
We acquired the images in “integration mode”, exposing the EMCCD for 30 s per image and measuring the number of counts accumulated in each pixel during the exposure-time. We then performed background-subtraction on each image and median filtering in order to remove dark current noise.
We measured the FWHM of the projected image along its axis of the source displacement (x
in ) using a 1D Gaussian-fitting algorithm. The FWHM of the projected spot as ϕ is varied for each scintillator is shown in from 0° to 17°, corresponding to a change in θ of 1° to 18°. Three projections were obtained for each angle in order to obtain mean and standard deviation values for each measurement. In addition, the theoretical collimator resolution for the knife-edge pinhole collimator used in experiment, 2.42 mm, [13
] is plotted in for sake of reference. The FWHM values were actually scaled from the measured profiles to object space, i.e., they were divided by the camera magnification M
in order to provide a common reference plane for comparison [13
]. The continuous and SC FWHM values were thus divided by M
= 0.437. The “uncorrected” FWHM values were divided by with M
= 0.437 while the “corrected” FWHM values were divided by the corrected magnification, Mf
Fig. 4 (a) FWHM, including the magnification corrections, of the projected image vs. central ray angle, ϕ, for the continuous, straight-cut, and focused-cut scintillators (with and without magnification correction) along with the theoretical collimator (more ...)