Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering <s> B. Resolution <s> We present first results of near-field imaging with ultrashort, broadband far-infrared pulses. By focusing the radiation into a tapered metal tip with a small exit aperture and scanning a sample in the near field of this aperture, sub-wavelength spatial resolution better than λ/4 is demonstrated. <s> BIB001 </s> Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering <s> B. Resolution <s> We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems. <s> BIB002
The resolution of any conventional imaging system is diffraction limited by the wavelength of the radiation. In the case of THz imaging this corresponds to features in the range 1µm to 3mm, not detailed enough for many medical applications. There are however ways to overcome this and the foremost among them is near-field imaging. Near-field imaging is based on the principle that the distribution of radiation directly behind a small aperture is a function of the diameter of the aperture rather than a function of wavelength or diffraction. When the object to be imaged is placed close to the aperture (less than a wavelength separation), a spatial resolution of a fraction of a wavelength (λ) can be achieved. Resolutions of better than λ/4 have been demonstrated BIB001 . The drawback to this technique is the proximity in which the sample must be placed to the aperture. A distance of less than 1mm is required for frequencies of 300 GHz and above, which makes the technique impractical for in vivo reflection mode use. The technique is however still valuable for in vitro imaging on prepared samples. Other researchers have developed a technique that combines THz images taken over different frequency intervals to display details in histological samples that are not visible under normal microscopic inspection BIB002 . The technique does however require some image processing to mitigate the artifacts introduced by combining images of different resolutions (the resolution being related to the wavelength used).