ASPIRATION CATHETER SYSTEM TECHNICAL FIELD BACKGROUND SUMMARY [0005] In some examples, an aspiration catheter system includes an outer catheter and an inner catheter configured to be positioned within the outer catheter lumen, and an alignment element. The outer catheter defines an outer catheter lumen and an outer catheter distal opening. The inner catheter defines an inner catheter lumen, an inner catheter distal opening, and a plurality of sidewall openings proximal to the inner catheter [0004] In some aspects, this disclosure describes example aspiration catheter systems configured to provide suction from and continuous flow of fluid through an aspiration catheter even when a distal opening of the aspiration catheter is blocked by a tissue. Aspiration catheter systems include an inner catheter with sidewall openings that allow fluid to flow into a lumen of the inner catheter when a distal opening of the inner catheter is blocked by the tissue. An outer catheter surrounding the inner catheter provides a flow path for aspiration fluid to a site of the tissue and through the sidewall openings of the inner catheter, ensuring continuous flow into the lumen of the inner catheter for removing any portions of the tissue that may come loose. A clinician can adjust the position of the inner catheter relative to the outer catheter to achieve a desired flow characteristic. In this way, a clinician may more effectively remove the tissue from vasculature of the patient when compared to aspiration catheters that provide reduced or no flow during engagement of a tissue. [0003] A medical catheter defining at least one lumen has been proposed for use with various medical procedures. For example, in some cases, a medical catheter may be used to access and treat defects in blood vessels, such as, but not limited to, lesions or occlusions in blood vessels. This treatment may involve drawing fluid through the medical catheter. [0002] This disclosure relates to medical aspiration. [0001] This application claims priority to U.S. Application No. 16/38 1,642, entitled, ‘ASPIRATION CATHETER SYSTEM,”and filed April 11, 2019.