Point spread function for concave transducer in near field
The calculation of point spread functions is done by placing a point in front of the transducer and then sweeping the beam over the point. This gives the point spread function of the imaging system. This example calculates the point spread function for a concave, round transducer with a radius of 8 mm, a geometric focus at 80 mm and a center frequency of 3 MHz.
The scanning with such a transducer is done by moving the transducer relative to the tissue. Field II does not have the capability of moving the aperture, so the point is moved instead. The point is moved from -10 mm laterally in steps of 0.2 mm til +10 mm at a distance of 30 mm to the aperture.
The envelope of the responses is then found and it is log compressed to a dynamic range of 60 dB. A mesh plot as the one shown below is then produced.
The m-file for calculating the images can be found at:
The routine field.m initializes the field system, and should be modified to point to the directory holding the Field II code and m-files. The routine concave_logo.m is then called to calculate the point spread function and make the plot.