System (100, 400) for determining the position of an ultrasound detector (109, 509) that is attached to a medical device (108, 508), the system (100, 500) comprising: at least three ultrasound emitters (107a, b, c, 507a, b, c); and a position triangulation unit (106, 506); wherein the at least three ultrasound emitters (107a, b, c, 507a, b, c) are arranged in a predetermined configuration on a frame (110, 410, 510) that is adapted for attachment to an ultrasound imaging probe (101, 401, 501) having an imaging field of view; wherein the position triangulation unit (106, 506) is configured to communicate with the at least three ultrasound emitters (107a, b, c, 507a, b, c) and to cause each ultrasound emitter to emit ultrasound signals; wherein the position triangulation unit (106, 506) is adapted for receiving, from the ultrasound detector (109, 509), signals indicative of ultrasound signals detected by the ultrasound detector; and is further configured to determine, by triangulation, a spatial position of the ultrasound detector (109, 509) relative to the at least three ultrasound emitters (107a, b, c, 507a, b, c) based on a first set of time delays (ΔT 1, ΔT 2, ΔT 3) between the emission of an ultrasound signal by each of the at least three ultrasound emitters (107a, b, c, 507a, b, c) and its detection by the ultrasound detector (109, 509); characterised in that the frame (110, 410, 510) comprises a detachable reference volume comprising a background volume and at least one inclusion or void; and in that the at least one inclusion or void has an ultrasound acoustic impedance that differs from the ultrasound acoustic impedance of the background volume such that when the detachable reference volume is attached to the frame (110, 410, 510) and the frame is attached to the ultrasound imaging probe (101, 401, 501), the at least one inclusion or void is configured to provide a corresponding at least one image feature within the field of view of the ultrasound imaging probe (101, 401, 501). at least three ultrasound emitters (107a, b, c, 507a, b, c); and a position triangulation unit (106, 506); wherein the at least three ultrasound emitters (107a, b, c, 507a, b, c) are arranged in a predetermined configuration on a frame (110, 410, 510) that is adapted for attachment to an ultrasound imaging probe (101, 401, 501) having an imaging field of view; wherein the position triangulation unit (106, 506) is configured to communicate with the at least three ultrasound emitters (107a, b, c, 507a, b, c) and to cause each ultrasound emitter to emit ultrasound signals; wherein the position triangulation unit (106, 506) is adapted for receiving, from the ultrasound detector (109, 509), signals indicative of ultrasound signals detected by the ultrasound detector; and is further configured to determine, by triangulation, a spatial position of the ultrasound detector (109, 509) relative to the at least three ultrasound emitters (107a, b, c, 507a, b, c) based on a first set of time delays (ΔT 1, ΔT 2, ΔT 3) between the emission of an ultrasound signal by each of the at least three ultrasound emitters (107a, b, c, 507a, b, c) and its detection by the ultrasound detector (109, 509); characterised in that the frame (110, 410, 510) comprises a detachable reference volume comprising a background volume and at least one inclusion or void; and in that the at least one inclusion or void has an ultrasound acoustic impedance that differs from the ultrasound acoustic impedance of the background volume such that when the detachable reference volume is attached to the frame (110, 410, 510) and the frame is attached to the ultrasound imaging probe (101, 401, 501), the at least one inclusion or void is configured to provide a corresponding at least one image feature within the field of view of the ultrasound imaging probe (101, 401, 501).