Magnetic resonance image of index finger: psoriatic arthritis (mutilans form). Shown is a T2 weighted fat suppressed sagittal image of the index finger in a patient with PsA (mutilans form). Focal increased signal (probable erosion) is seen at the base of the middle phalanx (long thin arrow). There is synovitis at the proximal interphalangeal joint (long thick arrow) plus increased signal in the overlying soft tissues indicating oedema (short thick arrow). There is also diffuse bone oedema (short thin arrows) involving the head of the proximal phalanx and extending distally down the shaft.