catheter may be initially introduced into vasculature of a patient and positioned adjacent a target treatment site. Catheter 12 may then be introduced through an inner lumen of the guide catheter. In some examples, catheter 12 is inserted into vasculature of the patient prior to attachment of a remainder of aspiration system 10, while in other examples, catheter 18 may be inserted into vasculature of the patient with a remainder of aspiration system 10 already attached, such as through aspiration tubing 26. [0081] The technique of FIG. 4 also includes various steps for initiating aspiration to remove a thrombus. For example, a distal end of catheter 12 may be introduced into an intracranial blood vessel and positioned adjacent to and/or proximal of a thrombus. Once catheter 2 is in place, a clinician may flush aspiration system 10 with an aspiration fluid, such as saline. For example, a clinician may control fluid pump 18 to generate a positive pressure to discharge fluid from a port of fluid pump 18. [0082] The technique of FIG. 4 includes generating a negative pressure in catheter 12 (92). For example, once the at least one opening 36 of catheter 12 is positioned at a position corresponding to an estimated location of the thrombus, a clinician operating aspiration system 10 may operate fluid pump 18 to generate the negative pressure on a port coupled to aspiration tubing 30. This negative pressure may be configured to produce a desired flow rate, pressure drop, or other flow condition responsive to a change in pressure. [0083] The technique of FIG. 4 includes initiating flow of fluid from catheter 12 to fluid pump 18 (94). In examples in which aspiration system 10 does not include a flow switch, initiating of flow of fluid may correspond to generating the negative pressure, such that fluid pump 18 may operate to control the negative pressure to control flow of the fluid. In examples in which aspiration system 10 includes flow switch 14, once the negative pressure has been generated, the clinician operating aspiration system 10 may operate flow switch 14 to an “open” position to initiate flow of fluid from catheter 12 through flow switch 14 and fluid flow sensor 16 to fluid pump 18. [0084] The clinician may sense, either directly or indirectly, vibrations generated in response to flow of fluid through fluid flow sensor 16. As fluid flows from catheter 12 to fluid pump 18, fluid flow sensor 16 may oscillate flow of the fluid through fluid flow sensor 16 to generate flow-induced vibrations. In some examples, such as examples in which fluid flow sensor 16 is configured to vibrate above an oscillation threshold at flow rates above a particular fluid flow and not vibrate or vibrate below the oscillation threshold at flow rates below the particular fluid flow, the presence or absence of