A Comparison of Broadband Holographic and Tomographic Imaging Concepts
Originally, holography is a monofrequency technique which also was applied to acoustical waves after its introduction into the optical range1. While acoustical holography was not very successful in medical imaging because of the inhomogenity of biological tissue it became a powerful tool for non-destructive testing7. Unfortunately, its excellent lateral resolution obtained by the synthetic aperture concept of scanning holographic techniques is accompanied by a poor axial resolution due to its zero bandwidth and large acoustical wavelengths. Axial resolution is considerably better in the case of conventional pulse-echo imaging, here axial positions of scatterers are related to the echo travelling time while axial resolution corresponds to the pulse length. Large signal bandwidth, for example representated by short pulses, leads to good axial resolution. Lateral resolution of conventional pulse-echo imaging is determined by the transducer beam shape and is generally 2 to 3 times lower than axial resolution. Consequently, high resolution with respect to both the axial and the lateral direction can be obtained by a combination of synthetic aperture and bandwidth. The paper deals with an investigation of various holographic imaging concepts including diffraction tomography. Broadband modifications will be presented and significant differences will be pointed out.
KeywordsMicrowave lItHIA Radar Acoustics
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