1. An image signal processing unit for use in a pulse echo ultrasonic imaging system utilizing consecutive, partly overlapping, line-wise scans to produce digital image signals corresponding to the received echoes, the scan lines comprising a predetermined number of scanning dots, and wherein the system includes an ultrasonic scanner; a transmitter connected thereto; a receiver connected to the ultrasonic scanner; a television monitor connected to the receiver and the television monitor; said receiver including a detector circuit and said image processing unit for producing an analog output signal representing the received echo; said image signal processing unit being characterized in that: (a) it is connected to be responsive to the output of the detector circuit; (b) it comprises: an analog-to-digital converter connected to be responsive to the output of the detector circuit; a digital image memory connected to be responsive to the output of the converter; a digital-to-analog converter connected to be responsive to the output of the image memory; and a non-linear digital filter having the structure of a transverse filter; (c) the filter is interposed between the analog-to-digital converter and the digital-to-analog converter and comprises at least one delay element for delaying the filter input signal for a period equal to the predetermined time interval between corresponding scanning dots of consecutive scans; and (d) the filter includes a coefficient B1 for filter signals not delayed in the filter and a coefficient B2 for filter input signals delayed in the filter, said coefficients B1 and B2 being defined by the following functions of the amplitude N of the input signal of the filter: (1) for relatively very low values of N in a range from N4 to N5: B1=B2=K3; (2) for relatively low and medium values of N in range between N5 and N7: B1 increases from B1=K3 with increasing N to reach a maximum B1=K4 when N=N6, thereafter decreasing with increasing N to equal K5 when N=N7; B2 decreases from B2=K3 with increasing N until reaching a minimum value B2=K6 when N=N6, B2=K6 being less than zero, then increasing with increasing N to equal K11 when N=N7, K11 being equal to or less than zero; (3) for relatively high values of N equal to or greater than N7: B1=K5 and B2=K11; (4) N4, N5, N6 and N7 are predetermined values of the amplitude N of the input signal and K3, K4, K5, K6 and K11 are fixed values.