Study objective: The three-dimensional shape of the ultrasound beam produces a thicker scan plane than most users assume. Viewed longitudinally, a needle placed lateral to a vessel just outside the central scanning plane can be displayed incorrectly in the ultrasound image as if placed intravascularly. This phenomenon is called the beam width artefact, also known as the elevation or slice thickness artefact. The goal of this study was to demonstrate the potential negative effect of the beam width artefact on the performance of in-plane ultrasound-guided vascular access procedures, and to provide a solution. Design: Randomized, double-blinded study Setting: Department of anaesthesiology and intensive care of a teaching hospital Participants: 31 experienced (anesthesiologists and intensivists) and 36 inexperienced (anesthetic nurses) ultrasound users Interventions: We developed an acoustic lens that narrows the scan plane to reduce the beam width artefact. The lens was tested in a simulated vascular access study. Measurements: The primary endpoint was first pass success. Secondary endpoints were the number of punctures and needle withdrawals, procedure time, needle visibility and operator satisfaction. Main results: First pass success was highly enhanced using the acoustic lens, with a success rate of 92.5% versus 68.7% without the lens (difference 23.8, 95% confidence interval 11.0–35.3, p < 0.001). The total number of punctures needed to obtain intravenous access was also reduced using the lens (1.10 versus 1.38, difference 0.27, 95% CI 0.11–0.43, p = 0.002). Procedure time, needle withdrawals, needle visibility and satisfaction were similar. Both inexperienced and experienced users benefited from the acoustic lens. Conclusions: The beam width artefact has a significant effect on the performance of ultrasound-guided needle-based procedures. The efficacy of in-plane superficial vascular access procedures can be enhanced by narrowing the imaging plane using an acoustic lens.