distal opening. The alignment element is configured to indicate a predetermined position of the inner catheter relative to the outer catheter when the inner catheter is received within the outer catheter distal opening. When the inner catheter is at the predetermined position, at least one sidewall opening of the plurality of sidewall openings remains positioned within the outer catheter lumen and at least one other sidewall opening of the plurality of sidewall openings is positioned distal to the outer catheter distal opening. [0006] Clause 1: In one example, an aspiration catheter system comprises an outer catheter defining an outer catheter lumen and an outer catheter distal opening; an inner catheter configured to be positioned within the outer catheter lumen, the inner catheter defining an inner catheter lumen, an inner catheter distal opening, and a plurality of sidewall openings proximal to the inner catheter distal opening; and an alignment element configured to indicate a predetermined position of the inner catheter relative to the outer catheter when the inner catheter is received within the outer catheter distal opening, w’hercinwhen the inner catheter is at the predetermined position, at least one sidewall opening of the plurality of sidewall openings remains positioned within tire outer catheter lumen and at least one other sidewall opening of the plurality of sidewall openings is positioned distal to the outer catheter distal opening. [0007] Clause 2: In some examples of the aspiration catheter system of clause 1, tire alignment element comprises a visible marker on the inner catheter. [0008] Clause 3: In some examples of the aspiration catheter system of clause 1 or clause 2, the alignment element comprises an alignment structure extending from the inner catheter and is configured to engage with a proximal end of the outer catheter. [0009] Clause 4: In some examples of the aspiration catheter system any of clauses 1- 3, the alignment element comprises a plurality of markers, each marker corresponding to a predetermined position of at least one sidewall opening of the plurality of sidewall openings of the inner catheter relative to the outer catheter distal opening. [0010] Clause 5: In some examples of the aspiration catheter system any of clauses 1- 4, an inner circumference of the outer catheter is between about 50 microns and about 200 microns greater than an outer circumference of the inner catheter. [0011] Clause 6: In some examples of the aspiration catheter system any of clauses 1- 5, at least one sidewall opening of the plurality of sidewall openings comprises a one-way valve configured to permit flow of fluid into the inner catheter lumen. [0012] Clause 7: In some examples of the aspiration catheter system of clause 6, the one-way valve is configured to open in response to a differential pressure between the