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For a multi-pinhole SPECT system, it is important to choose an optimal scanning orbit to fulfill the sampling completeness requirement. In this study, we propose a novel four-degree-of-freedom (4-DOF) orbit which combines a conventional helical scanning path and a superimposed periodic reciprocating 3×3-pioint shift movement in transaxial plane. An Orlov-sphere-based numerical evaluation method was used and a sampling completeness percentage (SCP) index was introduced to access the sampling completeness for the merits of scanning orbits over entire field of view (FOV) in terms of sampling completeness. By comparing distribution of sampling completeness over the FOV of conventional circular orbit, helical orbits, and 4-DOF orbit applied to an experimental small animal SPECT system, it is shown that the novel 4-DOF orbit has better sampling performance than conventional orbits. The evaluation results of different orbits and the mouse whole-body imaging experiment demonstrate that this method is feasible and applicable for guiding the design of scanning orbits and determining the merits of the orbits.