A microwave antenna device for treating tissue, the device having: a longitudinal axis, and a plurality of indentations (910a-c; 920a-c) extending into the surface of the device, each of the indentations including: a first surface forming a first plane; a second surface forming a second plane, and a third surface forming a third plane, wherein the first, second and third planes are substantially perpendicular to each other, wherein the indentations are configured to be visible to an ultrasonic imaging system so as to improve visibility of the device by an ultrasonic imaging system; and wherein A) clusters of indentations are provided, A1) each cluster comprising a predetermined pattern of indentations, wherein the orientation of the clusters varies with respect to each other around the circumference and/or along the longitudinal axis, or A2) each cluster includes three indentations in a pattern in which each indention is rotated at 120 degrees with respect to each other; or B) the indentations vary in orientation, position and size. a longitudinal axis, and a plurality of indentations (910a-c; 920a-c) extending into the surface of the device, each of the indentations including: a first surface forming a first plane; a second surface forming a second plane, and a third surface forming a third plane, wherein the first, second and third planes are substantially perpendicular to each other, wherein the indentations are configured to be visible to an ultrasonic imaging system so as to improve visibility of the device by an ultrasonic imaging system; and wherein A) clusters of indentations are provided, A1) each cluster comprising a predetermined pattern of indentations, wherein the orientation of the clusters varies with respect to each other around the circumference and/or along the longitudinal axis, or A2) each cluster includes three indentations in a pattern in which each indention is rotated at 120 degrees with respect to each other; or B) the indentations vary in orientation, position and size. a first surface forming a first plane; a second surface forming a second plane, and a third surface forming a third plane, wherein the first, second and third planes are substantially perpendicular to each other, wherein the indentations are configured to be visible to an ultrasonic imaging system so as to improve visibility of the device by an ultrasonic imaging system; and wherein A) clusters of indentations are provided, A1) each cluster comprising a predetermined pattern of indentations, wherein the orientation of the clusters varies with respect to each other around the circumference and/or along the longitudinal axis, or A2) each cluster includes three indentations in a pattern in which each indention is rotated at 120 degrees with respect to each other; or B) the indentations vary in orientation, position and size. A) clusters of indentations are provided, A1) each cluster comprising a predetermined pattern of indentations, wherein the orientation of the clusters varies with respect to each other around the circumference and/or along the longitudinal axis, or A2) each cluster includes three indentations in a pattern in which each indention is rotated at 120 degrees with respect to each other; or A1) each cluster comprising a predetermined pattern of indentations, wherein the orientation of the clusters varies with respect to each other around the circumference and/or along the longitudinal axis, or A2) each cluster includes three indentations in a pattern in which each indention is rotated at 120 degrees with respect to each other; or B) the indentations vary in orientation, position and size.