Li greater than about 0.5 centimeters proximal to inner catheter distal opening 22 may correspond to a distance at which the distal-most sidewall opening is unlikely to be plugged (e.g., covered or otherwise blocked) by a thrombus drawn into inner catheter lumen 20. As another example, a length Li less than about 10 centimeters proximal to inner catheter distal opening 22 may correspond to a distance at which fluid flowing through sidewall openings 24 may have a flow rate or position adequate (e.g., maximum residence time, minimum Reynolds number, minimum flow rate, etc.) for providing suction, circulation, and/or turbulence to a volume within inner catheter lumen 20 near inner catheter distal opening 22. [0074] In some examples, the plurality of sidewall openings may be distributed axially along inner catheter 14 (i.e., along a longitudinal axis 28 of catheter 14). For example, as shown in FIG. IB, distal-most sidewall opening 24B is distal to proximal- most sidewall opening 24A. While only two axially-distributed sidewall openings are shown, inner catheter 24 may include more than two axially-distributed sidewall openings, such as three, four, five or more axially distributed sidewall openings. In some examples, a size of sidewall openings may vary according to an axial distribution. For example, a size of sidewall openings may increase or decrease along a distal direction. [0075] In some examples, the plurality of sidewall openings may be distributed around the outer perimeter of inner catheter 14, e.g., circumferentially distributed in the case of inner catheter 14 having a circular cross-section (the cross-section being take in a direction orthogonal to the longitudinal axis of inner catheter 14). FIG. 1C is a schematic axial cross-sectional diagram illustrating example aspiration catheter system 10 of FIG. 1A that includes outer catheter 12 and inner catheter 14. As shown in FIG. 1C, sidewall opening 24C is at a different radial position than sidewall opening 24D. In some examples, a position and/or size of the sidewall openings may be relatively even or constant according to a radial distribution around inner catheter 14. For example, consistency in position and/or size at different radial positions may provide a more even flow of fluid into inner catheter lumen 20. In some examples, a position and/or size of sidewall openings may vary according to a radial distribution around inner catheter 14. For example, variation in sidewall opening sizes at different radial positions may encourage more turbulent flow, as the flow may be less uniform. [0076] Referring back to FIG. IB, aspiration catheter system 10 can include an alignment element 26. Alignment element 26 can be configured to indicate a predetermined position Pi of inner catheter 14 relative to outer catheter 12 when inner