A portal ultrasound imaging apparatus having an adjustable hinge assembly, which includes a key having a narrower end and an end wider in the circumferential direction of the hinge assembly. The key is movable in a slot in an axial direction of the hinge assembly to a locked position, where the side surfaces of the key presses against the walls of the slot to eliminate circumferential gaps between the key and the slot. When at the locked position in the slot, the key can be pushed towards the opposite of the axial direction to an unlocked position where an angular lock of the hinge assembly is released, allowing the hinge assembly to be adjusted to a desired angular coupling. The key can then be pushed in the axial direction back to the locked position in the slot to prevent change in the coupling angle of the hinge assembly.
