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.
Index
Holographic
Techniques
X-Ray Holography
Endoscopic
Holography
Internal Hologram Recording
Endoscope
External Hologram recording
Endoscope
Multiplexed Holography
For Medical Tomography
Holographic
Light-in-Flight Recording Method
Holography in Ophthalmology
Diffractive Bifocal Intraocular
Lens
Holography in Dentistry
Holography in Otology
Study of Tympanic Membranes
Holography
in Orthopedics
Summary
Holographic Techniques
<return to Index>
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 <return to Index>
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
<return to Index>
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
<return to Index>
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
<return to Index>
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.
Multiplexed Holography
for Medical Tomography
<return to Index>
Multiplexed holography can be used for complete display of
three-dimensional tomographic medical data. It uses photographically scaled
images of the objects for making the hologram. The technique thus provides a
way to make hologram whose images are of a different size from the original
object. A series of photographic transparencies are made, all of which ate
used to make a multiple-exposure composit hologram. the reconstruction of the
hologram produces the original object with a magnification equal to the scale
factor of the transparencies. Holographic stereograms, known as Cross
holograms uses multiangular views of the object. The method consists of three
steps: data acquisition, image processing, and making of the stereogram.
Since the holographic stereogram retains only the horizontal parallax of the
object, it is limited to those applications in which surfaces are important
such as prostheses and craniofacial surgery. Moreover, most biological data
such as CT or MR scans are generally collected as serial scans rather than
multiangular views, multiplexed holography that reconstructs the image with
all the sections at correct depths (locations) and both horizontal and
vertical parallaxes is more useful.
Multiplexed holograms which retains full parallax and physical depths cues are
known as volumetric multiplexed holograms, or multiplane-multiplexed
holograms.
A volumetric multiplexed hologram is made from a stack of images such as a CT
or MR scan. The first image in the stack is projected onto a screen placed in
front of a holographic film. A hologram of this image is recorded by adding a
reference beam. Next the screen is moved a few millimeters away from the
holographic film and the second image in the stack is projected onto the
screen, and a second exposure is made on the first one. All the images are
similarly recorded, each at slightly greater distance than its predecessor.
The film is then developed. When the hologram is reconstructed, it shows all
the slices distributed in the three-dimensional space at different distances.
Dispersion compensation techniques can be employed for making the multiplexed
hologram so that the image can be viewed by a white light source.
The process of volumetric holographic multiplexing is similar to that of
"inverse tomography" in which the projection screen mimics the tomographic
slicing mechanisms, and the screen's gradual displacements mimics the
patient's's linear translation through the scanner. Since the volumetric
multiplexed hologram uses serial sections at different Z coordinates in space,
it can be termed as 'zeta multiplexed hologram'. in contrast to a holographic
stereogram which is known as 'theta multiplexed hologram' because it involves
images at different angles.
It may be pointed out that the volumetric multiplexed hologram faithfully
reconstructs complete information including physical depth cues and all of the
grey-scale tonality in every slice without geometric or photometric
distortion. the success of the technique, however, depends on the accuracy in
ALIGNment of the serial sections in different exposures.
The complete system for producing and display of clinically useful multiplexed
hologram can be automated.
Holographic Light-in-Flight Recording Method
<return to Index>
The holographic technique of light-in-flight recording can be effectively
adapted for implementing first-arriving light principle for imaging through
human tissue. The holographic method permits the use of both continuos and
pulsed lasers with short coherence length. A short coherence length in
holography is equivalent to a short pulse. Holography permits a gate that is
as short as the pulse itself . A short-pulse laser beam is split into object
beam and reference beam. The object beam passes through the tissue, and the
reference beam is delayed so that it arrives at the recording plate in
coincidence with the first-arriving light. Thus only the first-arriving light
is recorded as a hologram. In practice, the reference beam is incident at a
large angle to the recording plate, so that it arrives at different parts of
the plate at different times. A part of the hologram would record the
first-arriving light and the later-arriving light would be recorded at the
other parts.
Holography in Ophthalmology
<return to Index>
Recording of a three dimensional image of the eye was one of the earliest
applications of holography in the field of ophthalmology. Any retinal
detachment or intraocular foreign body can be detected. Holography can also be
applied for the measurement of corneal topography and crystalline lens changes
and for the study of surface characteristics of both the nerve head and the
cornea. Current methods of determining the shape of the central surface miss
the central part and its periphery. The major advantage of holographic
technique is the ultra high precision (sub-um range) with which such
measurements are possible. The elastic expansion of the cornea can also be
measured by holographic interferometry. This information is vital for corneal
surgery. The expansion of the cornea of fresh enucleated bovine eyes has been
examined as a result of a small increase in intraocular pressure using double
exposure holographic interferometry. Their first investigations have revealed
that each bovine cornea has its own typical expansion. The studies made so far
show that holography has potential to investigate corneal endothermal
morphology, changes on the cornea, crystalline lens changes, and surface
characteristics of both the nerve head and the retina.
Diffractive Bifocal Intraocular Lens
<return to Index>
A very useful application of diffractive optics is in the correction of
refractive errors for old persons who have been operated for cataract by the
use of a bifocal intraocular lens. Such persons have difficulty in changing
the focus of their eyes for near distant and far distant objects. bifocal
lenses are implanted in place of the natural eye lenses. The bifocal lens is a
combination of a conventional refractive lens and a diffractive lens, the
former focussed to infinity and the later for near distance vision. The
efficiency of the diffractive lens is set at 50%, thus both the near and the
far foci are accommodated over the whole visual field. The diffractive lens is
fabricated on the rear of the conventional lens. When the eyes are focussed
for a distant object, a blurred image is superimposed due to the presence of
diffractive lens and vice versa, which obviously reduces the image quality. In
most cases, the blurred image is discarded by the human visual perception and
retinal processing system.
It is possible to place an ultra-thin (5 to 10 um) diffractive lens directly
on the eye's cornea, where natural tissue growth over it would secure it,
creating a semitransparent contact lens that would provide full visual
ability.
Holography in Dentistry
<return to Index>
Both continuous wave and pulse laser holography have been used for
applications of holography in dentistry.
- The holograms offer a convenient way of
storing toothprints, i.e. dental records of the casts of upper and lower
centitions for legal and forensic purpose.
- Holograms can be used for storing
orthodontic study models that can be retrieved by a laser beam or a white
light source for accurate 3D measurements. This saves a lot of storage
space.
- The holographic images are clinically
reliable and random errors are not clinically significant. by studying the
old and new records, orthodontists can watch the progress of their patients.
- Holograms can be employed as training aids
in the disciplines of dental anatomy and operative dentistry.
- Holographic interferometry has been used
for the contactless measurement of in-vivo tooth mobility and its movement
in three dimensions, and measurement of dimensional changes of the tissue
bearing surfaces of maxillary full dentures due to deformation of dentures
material by oral fluids.
- Holographic contouring technique can
reveal the topography of teeth.
Holography in Otology
<return to Index>
The human ear is embedded in temporal bone that forms part of the skull base.
The ear may be broadly divided into three parts: the outer ear, the middle ear
and the inner ear. The outer ear consists of pinna (not shown in the figure),
the external auditory canal and the tympanic membrane. The tympanic membrane
is the inlet of the middle ear, which transmits the sound coming from the
outer ear to the inner ear through auditory ossicles viz. malleus, incus and
stapes. The middle ear is located within the tympanic cavity and contains a
chain of auditory ossicles. The inner ear is the cochlea that has a coil like
shape. The cochlea is separated into two parallel canals by basilar membrane.
The receptor cells for hearing sense are spatially distributed on the basilar
membrane.
- Double exposure and time-average
holographic interferometric techniques are powerful for studying different
parts of the human peripheral hearing organ.
- The vibration behaviour of models of the
inner ear parts such as unrolled cochlea and coiled basilar membrane have
been studied by time-average holography.
- Time-average holographic interferometry
has also been used for the study of vibration analysis of incudo-mallar
joint with forces applied to the middle ear muscles has demonstrated that
the incus and malleus move like a lever around a frequency dependent axis.
Study of Tympanic Membrane
<return to Index>
The tympanic membrane is important in coupling the acoustic sound pressure in
the outer ear canal to the motion of the middle ear ossicles. Holographic
contouring techniques may be applied for precise measurement of the shape of
the tympanic membrane. When sound waves fall on the tympanic membrane in the
outer ear, it vibrates. These vibrations are transmitted by the leverage
action of the auditory ossicles to the stapes footplate attached to the
annular ligament and finally to the fluid system of the inner ear. The
vibrations propagate through the lymphatic fluid in the cochlea and cause
basilar membrane to vibrate which in turn cause stimulation of the receptor
cells on the cochlea. In order to use holography as a clinical tool for living
persons, special care is required with regard to the optical system due to the
difficult optical access through the narrow and curved outer ear canal. The
device is made flexible by using a endoscope fibre bundle for the object
illumination and using a thermoplastic film for recording the hologram. The
device produces the results in 10 seconds, therefore is useful for routine
clinical applications for quick study of the vibration pattern of tympanic
membrane.
The holographic time-average interferometry is particularly suitable in
studying the modes of vibration of the eardrum of living human as the modes of
vibration are observed simultaneously over the whole area.
Many infants and old persons develop severe otitis media with effusion causing
the malfunction of the eustachian tube leading to medial recess of the
tympanic membrane under the compression of atmospheric pressure. This results
in hearing loss, ear obstruction sensation, autophony, etc.
The time-average holography of a fresh human temporal bone sample has revealed
a small tilt movement with a piston like oscillation of stapes . The vibration
patterns of macerated human skull using time-average holographic
interferometry has thrown light on the mechanism of sound transmission of
sound transmission by bone conduction. The sample was excited by bone
conduction vibrator.
Hearing impairments occur when micro-fractures of petrous pyramids in the
human skull base are created by accidents. Double-exposure holographic
interferometry has been applied for the study of deformation of the human
skull under different load conditions to investigate petrous pyramids.
Holography in Orthopedics
<return to Index>
Holography offers an excellent tool for the contactless study of
orthopedic structures, specifically external fixtures to reveal and measure
strains on fixation pins and rods. Such studies are important in
osteosynthesis with external fixture used for long bone fractures, to prevent
dislocations of both fractured ends that are mainly caused by decrease in
strength of the fixation pins. Dry bonein cantilever bending mode has also
been studied by heterodyne holographic interferometry to determine the
piezoelectric coefficients of bone.
Summary <return to
Index>
Holographic interferometric techniques have been widely applied with success
for the study of different parts of human body including cornea, tooth
mobility, tympanic membrane, basilar membrane, cochlea, temporal bone,
incudo-mallar joint, chest, stull, and bones. Endoscopic holography is a
powerful tool for non contact high resolution imaging and nondestructive
measurements inside the natural cavities of human internal organs. X-ray
holography has shown the promise of real-time observation of living biological
specimens.
Copyright © 2005 Prakash Mehta,
www.hololight.net. All Rights Reserved
