alignment structures 352, 354 are shown as interlocking structures, in other examples, the alignments structures may include other mechanisms that limit travel of inner catheter 348 within outer catheter 346, such as a single structure at a proximal portion of inner catheter 346 that interacts with an edge of an outer catheter proximal opening 356 of outer catheter 346. [0089] FIG. 5 is a flow diagram of an example method of using an aspiration catheter system that includes an outer catheter and an inner catheter configured to maintain suction and continuous flow of fluid on an engaged thrombus. The techniques of FIG. 5 are described with reference to the various aspects of aspiration system 2 of FIGS. lA-lC for illustrative purposes; however, such descriptions are not intended to be limiting, and the techniques of FIG. 5 may be used with other examples of the aspiration systems disclosed herein, or otherwise. The technique of FIG. 5 includes introducing aspiration catheter system 10 into vasculature of a patient (400), positioning inner catheter 14 within outer catheter 12 at a predetermined position (410), and delivering a fluid through outer catheter lumen 16 into outer catheter distal opening 18 and/or a sidewall opening 24 of inner catheter 14 (420), thereby maintaining suction and continuous flow of fluid on an engaged thrombus. In some examples, the techniques described herein include removing aspiration catheter system 10 from the vasculature of the patient once the procedure is complete. [0090] In some examples, introducing at least a portion of outer catheter 12 and inner catheter 14 into vasculature of a patient (400) may be aided by initially introducing a guidewire, guide catheter or another guide member into the vasculature of the patient to a target treatment site. In some examples, outer catheter 12 is a guide catheter that is initially introduced into the vasculature of the patient, e.g., over a guidewire and advanced to the target treatment site. Inner catheter 14 may then be introduced through an inner lumen of the guide catheter. In other examples, outer catheter 12 and inner catheter 14 may be navigated to a target treatment site inside of a separate guide catheter. [0091] In some examples, aspiration catheter system 10 is inserted into vasculature of the patient prior to attachment of a remainder of aspiration system 2, while in other examples, aspiration catheter system 10 may be inserted into vasculature of the patient with a remainder of aspiration system 2 already attached, such as through aspiration tubing 8. [0092] The technique of FIG. 5 may include various steps for initiating aspiration to remove a thrombus. For example, a distal end of aspiration catheter system 10 may be