the vacuum force, which may cause an initial reduction or cessation of flow through the catheter lumen followed by a continuous or increased flow through the catheter lumen as the thrombus is sucked piecemeal from the vasculature. In either case, engagement of the thrombus may cause a change in flow characteristics, such as flow rate, of fluid through the catheter lumen. [0043] The clinician performing the medical aspiration may use the change in flow characteristics of the suctioned fluid through an inner lumen of the aspiration catheter as an indication that the aspiration catheter is engaged with the thrombus and/or has removed the thrombus. For example, the clinician may visually confirm that suction has reduced as the thrombus is engaged by observing a decrease in a flow of fluid into the canister, visually confirm that suction has increased as the thrombus is removed by observing an increase in a flow of fluid into the canister, reposition the aspiration catheter, e.g., using the medical image, and continue to visually confirm the flow characteristics and reposition the aspiration catheter as needed until the flow of fluid is constant or the thrombus is removed. Due to this alternating visual confirmation of flow characteristics, the clinician may divert her gaze and attention from the medical image display, the patient, or other devices to an apparatus containing the aspirated fluid, such as a canister or tubing. Additionally, visual confirmation of an opaque or semi-opaque liquid by the clinician, such as blood, may be relatively difficult in some circumstances, such as a medical procedure room that is relatively dimly lit to enable better viewing of the medical image by the clinician. [0044] In examples described in this disclosure, an aspiration system may include a fluid flow sensor configured to provide a tactile indication of fluid flow through an inner lumen of an aspiration catheter that is directly produced by the fluid flow. The fluid flow sensor may include a flow oscillator configured to oscillate flow of the fluid through the fluid flow sensor to generate flow-induced vibrations. These flow-induced vibrations provide haptic feedback to a clinician, e.g., may be sensed by a clinician holding the fluid flow sensor as an indication of a presence, absence, or change in fluid flow through the aspiration catheter. In this way, the clinician may detect fluid flow through the aspiration catheter without diverting her gaze to the aspiration tubing or fluid canister. The fluid flow sensor may be relatively small, reliable and/or relatively inexpensive, as the fluid flow sensor may have a reduced number or an absence of electronics or moving parts. [0045] FIG. 1 is a schematic diagram illustrating an example aspiration system 10 that includes a fluid flow sensor 16 configured to generate flow-induced vibrations in