Slit-Slat and Multi-Slit-Slat Collimator Design and Experimentally Acquired Phantom Images From a Rotating Prototype

We have previously found and validated expressions for slit-slat (SS) geometric efficiency and resolution. These expressions have suggested that SS may be a good choice for imaging midsize objects or objects that are long axially since i) the geometric efficiency increases near the slit as
h

^-1
[instead of
h

^-2
for pinhole (PIN) and either decreases near the collimator for fan-beam (FB) or remains constant for parallel-beam (PB)], where
h
is the distance from a point to the slit plane; (ii) the transverse resolution is comparable to that of PIN, which is better than that of FB and PB for small objects; iii) the axial resolution is worse than that of PIN since there is no axial magnification; iv) there is a large axial field of view, unlike PIN, which is likely to be useful when imaging midsize or long objects; and v) there is no need for three-dimensional orbits (e.g., helical) since each slice is complete (like PB and FB). We have developed a rotating prototype SS collimator that is capable of single-slit or multi-slit acquisition of data. The focal length (FL) is shorter than that of a typical PIN since increasing the FL requires taller slats to maintain resolution; taller slats reduce geometric efficiency. A lead rectangular box was used to provide support and shielding around the slit-slat collimator. Lead slats, spaced with Rohacell foam, were mounted in an assembly with 3 mm pitch. We have performed preliminary characterization with point sources and acquired micro hot- and cold-rod phantoms and a deluxe Jaszczak phantom. The projections have been reconstructed using a maximum likelihood expectation maximization algorithm and show good resolution. Comparisons indicate that SS is more sensitive than PB and FB for the same resolution for objects with smaller diameter. The advantage of SS over PB and FB increases as the desired resolution improves. SS can also be used in configurations that yield projections that have nonisotropic resolution; it is possible for SS to achieve transverse resolutions that are unreachable by PB, since PB does not magnify, and by FB, since its magnification factor for small objects is much smaller than that of SS. Experimental results show that the resolution of the reconstructed phantoms is consistent with theoretical expectations.