Coronary sinus cannulation apparatus and methods relating to tissue visualization catheters that can locate and/or cannulate a morphological feature within a body lumen, such as the coronary sinus, are described. Such devices can also perform a variety of therapeutic tissue treatments under direct in vivo visualization thereafter. Moreover, such apparatus and methods may also be utilized with guidewires to facilitate delivery into the coronary sinus to eliminate repositioning of the device and/or repeating an entire sequence of operations, consequently improving procedure effectiveness and reducing procedure time.