sidewall opening is positioned distal to the outer catheter distal opening; and delivering a fluid through the outer catheter lumen and through at least one of: the outer catheter distal opening into the vasculature of the patient, or the at least one sidewall opening into the inner catheter lumen. [0031] The examples described herein may be combined in any permutation or combination. [0032] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims. [0033] FIG. 1A is a conceptual block diagram illustrating an example aspiration system that includes an aspiration catheter system configured to help maintain suction and continuous flow of fluid on an engaged thrombus. [0034] FIG. IB is a schematic longitudinal cross-sectional diagram illustrating the example aspiration catheter system of FIG. 1A. [0035] FIG. 1C is a schematic axial cross-sectional diagram illustrating the example aspiration catheter system of FIG. 1A. [0036] FIG. 2A is a schematic longitudinal cross-sectional diagram illustrating an example aspiration catheter system that includes an outer catheter and an inner catheter positioned near a thrombus prior to engagement of the thrombus. [0037] FIG. 2B is a schematic longitudinal cross-sectional diagram illustrating the example aspiration catheter system of FIG. 2A after engagement of the thrombus. [0038] FIG. 3A is a schematic diagram illustrating an example duckbill valve. [0039] FIG. 3B is a schematic diagram illustrating an example slit valve. [0040] FIG. 3C is a schematic diagram illustrating an example flap valve. [0041] FIG. 4A is a schematic longitudinal cross-sectional diagram illustrating an example aspiration catheter system that includes an alignment element comprising one or more visible markers. [0042] FIG. 4B is a schematic longitudinal cross-sectional diagram illustrating an example aspiration catheter system that includes an alignment element including a plurality of visible markers on an inner catheter. BRIEF DESCRIPTION OF THE DRAWINGS