Topic 6 - Optics in Medicine

In the exam you are expected to know about:

• Fibre optics and lasers

• Properties of fibre optics and applications in medical physics including total internal reflection at the core-cladding interface

• Endoscopy: Physical principles of the optical system of a flexible endoscope; the use of coherent and non-coherent fibre bundles; examples of use for internal imaging and related advantages

• Properties of laser radiation

• Absorption by tissue

• Uses of lasers in medicine

• Safety issues

Optical fibres are basically long thin glass rods which are surprisingly flexible.  They conduct light by total internal reflection.

Question 1 What is the condition needed for total internal reflection?  ANSWER

The light is guided down a core surrounded by glass cladding of slightly lower refractive index.  This is to prevent the loss of light if the core were to come into contact with material of a higher refractive index.

Question 2  What would happen if the light ray in the core hit a boundary between the core and a material of higher refractive index? Would it matter?  ANSWER

When dealing with optic fibres we use the Snell's Law Equation:

You may wish to revise this from AS Module 1.

Here is how light is conducted in a glass fibre:

Question 3  An optical fibre has a core refractive index of 1.55, and the cladding refractive index is 1.42.  What is the critical angle?  ANSWER

Question 4  What is the angle of refraction that gives this critical angle?  ANSWER

Question 5  What is the angle of incidence that gives this angle of refraction, as the light crosses from air into the glass?  ANSWER

The maximum angle of incidence in the examples above was about 38^o.  Anything above that angle, the refraction will make the light ray strike the side of the glass at less than the critical angle.  This maximum angle of incidence is called the half angle.

The analysis above works for straight fibres.  If the fibre is bent sharply, then light will escape.  However as long as the radius of the bend is about 20 times the diameter or more, the light loss will be insignificant.

Image formation

Optical fibres are bundled together into a fibre optic light guide and the whole bundle is held together by a sheath.  In the short length that is used in an endoscope, about 1 metre, the light loss is negligible.  Each fibre carries the light that is incident on it independently of the other fibres.  To transmit a picture, the fibres in a bundle have to be kept in the same relative positions with a smooth square surface so that each fibre is contributing to the formation of the image.  This is called a coherent bundle.

You can see how each fibre contributes to the picture.  Each fibre is about 10 mm across.  The more fibres there are, the more detailed picture that is possible.  A good quality endoscope will have about 40 000 fibres packed into a bundle about 3 mm across.

The endoscope has to have a light source for the doctor to see what's going on.  The fibres do not have to be coherent (hence non-coherent), and are somewhat wider, about 30 mm.  This ensures more efficient transfer of light.  Also it is cheaper to produce.

The Endoscope

The first endoscope was invented by a German doctor, Kassmaul, at the turn of the twentieth century, but being non-flexible, was not much use.  A flexible device was made by another German, Rudolf Schindler, but it was still not very flexible unless the patient was contorted to suit the instrument.  Schindler's wife did the manipulation of the patient in the early days.  The endoscope became a much more practical instrument in the 1960's with the invention of the optical fibre.

The modern endoscope has:

• Two light channels to illuminate the area of interest;

• An image channel to enable the doctor to see what's happening;

• An instrument channel through which instruments can be placed

• An air or water channel to wash the area being operated on.

• Control cables to make the business end move.

A range of special instruments can be inserted which enables doctors not only to see inside to make an accurate diagnosis, but also operate on the diseased area.  This might be to:

• cut out diseased tissue;

• take a biopsy;

• seal a site of bleeding with heat (cauterising);

• remove an object that is causing an obstruction.

At the eye piece end, there can also be attached:

• a TV camera

• a still camera

• a movie camera

The endoscope has also enable doctors to carry out keyhole surgery where surgical treatments can be applied without having to make major incisions.  This leads to less complications and more rapid recovery of the patient.

The latest development is to have a video camera at the end of the endoscope.

Question 6 Write down and explain three ways in which a doctor might use an endoscope.  ANSWER

Lasers

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.  If a photon of the right wavelength hits an excited atom of certain materials, it can stimulate the emission of a second photon of exactly the same wavelength and phase as the first.  If enough atoms are excited, the photons can stimulate further emissions of further photons, all travelling in the same direction.  At one end of the material there is a mirror that totally reflects the photons, while at the other is a mirror that partially reflects the photons.

Question 7 If there were no mirror at all, more photons could get out.  Why should there be a partially reflecting mirror instead no mirror at all? ANSWER

The properties of laser light are:

• monochromatic

• coherent (in phase, of the same amplitude).

Lasers can produce continuous light.  Pulsed light is achieved not by turning the laser on and off, but by placing a shutter between the two mirrors.

The table shows some of the continuous lasers that can be used in medicine:

This table shows some of the pulsed lasers:

Skin will absorb laser light:

• far infrared is absorbed to about 0.1 mm;
• near infra red, a couple of cm.

The black pigment melanin increases the absorption.  The pigment haemoglobin in blood absorbs the blue green light of an argon laser.  Tissue damage arises because the water in cells boils and proteins are denatured, just like when they are cooked.

Lasers are often used with optic fibres to guide the laser to the precise location it is needed.

Tissue damage is reduced by using a pulsed laser, but there are difficulties sending high powered laser pulses down glass fibres as they will shatter due to thermal shock.

Lasers are used to cut away diseased material.  The Carbon Dioxide laser can be used with very delicate tissues such as the brain.  However it cannot be used with optical fibres as the far infra red radiation is absorbed by the fibres.  The argon laser can be used to spot weld the retina if it becomes detached.  It also used to remove birthmarks.  Lasers can be used to seal up (cauterise) bleeding wounds and ulcers.

There is work being carried out in which a chemical is injected into a tumour and can be activated by laser light to become toxic to the tumour, but not to the rest of the body.  This is called phototherapy.

Question 8  Which laser would you use for the following procedures?  Give a reason for each one:

(a) removing a tumour from the brain;

(b) spot welding a detached retina.

ANSWER

Safety Issues

Lasers are potentially dangerous.  Hazards arise from:

• the beams causing severe and deep burns

• shining a beam into the eye will cause blindness.

Therefore strict rules have to be enforced so that persons using lasers are fully trained and competent in their use.

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