at a distal portion 322 of aspiration catheter system 320, and an alignment element at a proximal portion 324 of aspiration catheter system 320 that includes three inner catheter visible marker 332A, 332B, 332C on inner catheter 328. Each marker 332A, 332B, 332C corresponds to a respective predetermined position Pi, Pz, Pz of at least one sidewall opening of the plurality of sidewall openings of the inner catheter relative to the outer catheter distal opening when the respective marker 332 is at a respective marker position Mi, Mz, Mz aligned with an outer catheter proximal opening 334 (or, alternatively, a visual marker on outer catheter 326). When outer catheter proximal opening 334 is aligned with a respective inner catheter visible marker 332A, 332B, 332C, aspiration catheter system 320 may be at the respective predetermined position Pi, Pz, Pz. For example, each predetermined position may have certain flow characteristics due to the proportion of sidewall openings 330 that are distal to the distal opening of outer catheter 326 (compared to the total number of sidewall openings 330 (including those within the outer catheter lumen 316)). In this way, a clinician operating aspiration catheter system 300 may adjust aspiration catheter system 300 using an easy to view visual indication to a variety of different predetermined positions. [0087] FIG. 4C is a schematic longitudinal cross-sectional diagram illustrating example aspiration catheter system 340 that includes an alignment element including alignment structures. Aspiration catheter system 340 includes an outer catheter 346, an inner catheter 348 within outer catheter 346, a plurality of sidewall openings 350 at a distal portion 342 of aspiration catheter system 340, and an alignment element at a proximal portion 344 of aspiration catheter system 340, the alignment element including an inner catheter alignment structure 354 extending from an outer surface of inner catheter 348 and an outer catheter alignment structure 352 extending from an inner surface of outer catheter 346. Inner catheter alignment structure 354 may configured to engage with outer catheter alignment structure 352. [0088] When inner catheter alignment structure 354 and outer catheter alignment structure 352 are engaged, inner catheter 348 may be at the predetermined position Pi relative to outer catheter 346. In this way, a clinician operating aspiration catheter system 300 may adjust aspiration catheter system 300 and determine sidewall openings 350 are positioned at the desired location relative to the outer catheter distal opening without having to divert her gaze to observe a relative position of inner catheter 348 and outer catheter 346. While alignment structures 352, 354 are shown at proximal end 344, in some examples, alignment structures 353, 354 may be located at other locations. While