A method of cannulating a coronary sinus within a heart chamber includes deploying, from a catheter, an imaging hood to a deployed configuration by extending the imaging hood from a distal end of the catheter and radially expanding the imaging hood to define a constant deployed volume within an open area of the imaging hood. The method further includes positioning a contact edge of the imaging hood and the open area of the imaging hood in the deployed configuration over or upon an ostium of the coronary sinus, displacing an opaque fluid with a transparent fluid from the open area defined by the imaging hood and tissue surrounding the ostium, visualizing the ostium through the transparent fluid by viewing the ostium via an imaging element attached to an inner surface of the imaging hood, and introducing a guidewire through the imaging hood and into the ostium while under visual guidance.