Holographic applications

This article provides an summary of the numerous medical applications in use and development today. This article has been compiled and reproduced by Integraf with permission of the copyright holder, and contains edits to the original article as needed.

Holographic Techniques

Recent improvements in hologram recording techniques and the availability of tools for the interpretation of holographic interferograms and the success of holographic techniques in imaging through tissues, ophthalmology, dentistry, urology, otology, pathology, and orthopedics shows a strong promise for holography to emerge as a powerful tool for medical applications. Holographic 3D images of eyes and interferometric testing of human teeth and chest motion during respiration were carried out quite early.

• Mostly the holographic interferometric techniques have been used for biomedical applications.
• X-ray holography can be applied for imaging of internal parts of the body and living biological specimens with very high resolution without the need for sample preparation.
• Endoscopic holography has opened up the possibility of noncontact high resolution 3D imaging and nondestructive measurements inside the natural cavities of internal organs.
• Three dimensional images of biological specimens can be synthesised from a series of two dimensional radiological images using the techniques of holographic stereogram, holographic conical stereogram and multiplex hologram.
• Holographic contour generation is useful for measurements for biomedical specimen.

X-Ray Holography

X-ray holography has the potential of examining the samples in aqueous solution with very high resolution without the need for sample preparation that often results in structural alternations. A X-ray hologram with a resolution better than that of a detector can be obtained by Fourier transform holography with a zone plate. The X-ray beam from a selenium X-ray laser (wavelength =20 nm, pulse length 200ps, output power 500 kW) falls on a narrow band X-ray mirror (bandwidth 10% and reflectivity 25% at 20nm wavelength) which reduces the broadband X-ray background produced by the Se laser. The mirror is a substrate with a flatness and roughness of better than 2nm to preserve the coherence of the X-ray laser beam, and is coated with alternate 20 layers each of silicon and molybdenum to provide high X-ray reflectivity. The sample on illumination with the X-ray beam scatters in the forward direction and forms the object beam. The object beam interferes with the portions of the X-ray beam that miss the sample. The recorded hologram can be reconstructed optically.The x-ray resist polymethyl methacrylate has high resolution but it requires development and reconstruction.

If the source size is kept small to ensure spatial coherence and the diffraction pattern is enlarged by shadow projection, a moderate-resolution detector with a high quantum efficiency such as a backilluminated CCD camera can be used for recording the hologram, instead of polymethyl methacrylate. The reconstruction of such a hologram can be performed numerically. The system would permit the observation in real-time, which would be useful for biological samples. A resolution of 1.3 um has been obtained by taking d=1.0 um and N=23 lines/mm, and using a backilluminated CCD camera for hologram recording. The resolution obtained was limited by the resolution of the source size(1 um). These experiments show the promise of real-time observation of holograms of living biological specimens.

Endoscopic Holography

Endoscopic holography has potential of providing a powerful tool for non contact high resolution 3D imaging and nondestructive measurements inside natural cavities of human body or in any difficult to access environment. It combines the features of holography and endoscopy. The ability to record a 3D large focal depth and high resolution image of internal organs and tissues greatly enhances the detection capability. The holographic endoscopy is of two types. In the one form the hologram is recorded inside the endoscope, while the other form uses an external recording device.

Internal Hologram Recording Endoscope

The internal hologram recording endoscope produces full three-dimensionality of the reconstructed image with parallax and a large focal depth. The endoscope requires a miniaturized holographic setup inside the instrument and records a reflection hologram. It mainly consists of three parts; a film cartridge, a diaphragm and a single mode optical fibre (core diameter 4um) cable. The three parts are assembled in three adjusTAbLE stainless steel or brass tubes. The film is placed at 10 to the normal to the endoscope. The holograms are viewed under a powerful microscope allowing for the observation of individual cells. Due to large hologram aperture, the image with a low speckle noise and high lateral resolution is obtained. A lateral resolution of 7 um has been obtained in the reconstructed image that shows that the technique can be used for cellular structure analysis and may even substitute biopsy in tumour diagnosis. Specific dyes can be used to enhance the contrast of the tissue before recording the holograms as has been used extensively in gynecology and gastrointestinal tract.

External Hologram Recording Endoscope

In the external hologram recording endoscope, a conventional endoscope is used. The system records the hologram outside the endoscope using an external reference beam. An endoscope with extremely small outer diameter can be used but this results in a loss of parallax and a small entrance pupil which produces speckles in the reconstructed image. However image plane holograms can be recorded to reconstruct the image without speckles. In order to obtain a high signal-to-noise ratio, the holographic endoscope must use gradient-index (GRIN) rod lenses. The speckle noise is reduced by illuminating and imaging the object by the same GRIN lens. An electro-optic crystal can be used as the photographic storage device in the holographic endoscope to provide in-situ recording, reconstruction, and erasure. These will make a new class of medical instruments for use not only in medical diagnostics but also in industrial testing.

Holographic endoscope can be attached to a salpingoscope for fallopian tube investigations or to otoscope for the inspection of outer and middle ear via an acoustic system to generate vibrations of the tympanic membrane. Holographic endoscope has been used with success for early recognition of cancerous indurations in the wall of urinary bladder.

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