A catheter insertion device has a generally hollow housing having a closed base end opposite an open distal end. Input pulses to the arrays are generally non-linear, frequency-modulated pulses. The apparatus then presents the detected difference, together with display of the contents of the interpretation report information, while changing the display form in accordance with the determined importance. An input signal is Fourier-expanded, and a frequency-domain output spectrum is obtained from the transfer function and the Fourier-expanded input signal.