Fig. 4From: A vibration sensor approach to detect intra-articular needle tip placement in the knee joint: a proof-of-concept studySpectrogram of the vibration signal illustrated frequency (kilohertz or kHz) and time in second (s) when the injection was done at the extra-articular (top row) and intra-articular (bottom row) knee joint for the needle gauge of 18G (A), 21G (B) and 24G (C). The color bars represent the corresponding power spectral density in decibel (dB)Back to article page