We have studied the impact of using rectangular-aperture pinholes in a multi-pinhole, stationary system for cardiac imaging of mice. The system will utilize the standard NaI(Tl) detectors on a clinical SPECT system. There are at least two advantages to using rectangular apertures: (1) the fields of view (FOV) in the axial and transverse directions become separable since they are determined by the walls of the aperture insert, which can have independent acceptance angles; and (2) the tiling of projections onto the detector is more efficient for the same solid angle (i.e., allows more pinholes) than traditional, circular apertures since the circular-aperture projections have a packing fraction of less than π=4. The system design places these rectangular apertures, which will be cast using a Pt/Ir alloy that is denser and harder than gold, into a tungsten-polymer tube that will be cast from a mold that is 3D printed in plastic. This fabrication technique will provide flexibility in the placement and orientation of the pinhole inserts yet will be less expensive to manufacture than traditional machining. The system sensitivity depends on the resolution and FOV; we plan to limit the FOV to a region that accommodates a mouse heart with some margin. We have tested the reconstruction of simulated data, with both point phantoms and the MOBY™phantom, using an MLEM algorithm and found both good resolution and fidelity throughout the targeted FOV, with good angular sampling. The system can achieve an estimated best system resolution of 0.4 mm with high magnification - before resolution recovery by system modeling during reconstruction - but this operating point would yield low sensitivity. Comparisons with commercial and research systems indicate improved sensitivity at the same resolution.