A PET imaging system, with parallel detector rings sharing a common axis (e.g., rings with one or more detector elements in the axial direction and two or more detector elements in the transaxial direction), may have an adaptive axial and/or transaxial FOV by employing a sparse detector configuration and adapting the size of axial gaps between rings and/or the size of transaxial gaps between detector elements in each ring. The axial FOV may be dynamic, enabling PET data acquisition in multiple modes (e.g., “retracted” with detector rings in a compact configuration, and “extended” with detector rings extended for longer axial FOV). The transaxial FOV may be dynamic, enabling an adaptive detector ring diameter for different body part contours. The sparse detector ring configurations may be used to extend the scanner axial and/or transaxial FOV, or retain the current system's FOV with half the number of (or otherwise fewer) detector elements.