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Medical Physics: 2. X-rays, CAT scans and endoscopes

Syllabus reference (October 2002 version)
2. The physical properties of electromagnetic radiation can be used as diagnostic tools	Students learn to: describe how X-rays are currently produced compare the differences between ‘soft’ and ‘hard’ X-rays explain how a computed axial tomography (CAT) scan is produced describe circumstances where a CAT scan would be a superior diagnostic tool compared to either X-rays or ultrasound explain how an endoscope works in relation to total internal reflection discuss differences between the role of coherent and incoherent bundles of fibres in an endoscope explain how an endoscope is used in: observing internal organs obtaining tissue samples of internal organs for further testing	Students: gather information to observe at least one image of a fracture on an X-ray film and X-ray images of other body parts gather secondary information to observe a CAT scan image and compare the information provided by CAT scans to that provided by an X-ray image for the same body part perform a first-hand investigation to demonstrate the transfer of light by optical fibres gather secondary information to observe internal organs from images produced by an endoscope

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 19 Aug 08]

gather information to observe at least one image of a fracture on an X-ray film and X-ray images of other body parts

To gather an X-ray film image of a fracture may require a number of strategies. You may know someone who has sustained a fracture and has kept their X-ray films. Your doctor or a radiologist may have X-ray film of fractures that you can inspect. Other images of body parts may be obtained from print and on-line searches.
Your observations should include image clarity, contrast and brightness; the soft tissues and their appearance, bone appearance, the appearance of any metal and any apparent pattern between tissue density and image intensity.

describe how X-rays are currently produced

X-radiation is high frequency electromagnetic radiation. Frequencies of 10¹⁷ to 10²⁰ Hz mean that the wavelengths are on the atomic scale. X-rays can be thought of as high-energy photons or quanta.
X-rays for medical use are produced by a diagnostic X-ray tube consisting of an evacuated glass envelope containing a cathode and an anode. Electrons released from the cathode by thermionic emission are focussed into a beam and accelerated towards the anode by a very high voltage electric field. X-rays produced when the electrons strike the anode are directed by the angle of the anode towards a small hole in a dense metal shield around the tube.
An X-ray tube produces X-rays when the electrons in the beam interact with the anode atoms. An incident electron will be slowed down by interacting with the nucleus of a target atom. The electron’s kinetic energy is converted to a photon of X-radiation. These X-rays have a continuous spectrum of energy as the incident electrons lose differing amounts of energy.
At the same time X rays can be produced by another process. Some incident electrons knock inner atomic shell electrons from their orbitals. Other target atom electrons change orbitals to fill the gap and in doing so release an X-ray photon. These latter X-rays, unlike the former, have just a few energy values (ie just a few frequencies).
The process of producing X-rays is very inefficient, typically less than 1%. Most of the electrons’ energy goes into heating the anode. Hence the need to have external cooling surrounded by a jacket of cooling oil, air or water circulated, a high melting point anode and to rotate the anode to distribute target area heating. A high atomic mass target increases the efficiency of X-ray production. The target is usually tungsten, chosen for its high melting point and atomic weight.

compare the differences between ‘soft’ and ‘hard’ X-rays

Your comparison should indicate that ‘soft’ and ‘hard’ are descriptive terms devised to indicate the relative penetrating power of a beam of X-radiation. Hard and soft X-rays are similar in that they are X-rays. When the X-radiation is produced it has a range of energies, corresponding to a range of frequencies. Passing the beam through a filter of aluminium means that the lower energy X-rays are attenuated (weakened, dissipated). The emerging beam thus has a higher percentage of high energy X-rays. These ‘hard’ X-rays have greater penetration into tissue and produce a sharper image with less radiation needed. This is safer for the patient. X-ray hardness can also be increased by increasing the X-ray tube voltage.

Hard X-rays	Soft X-rays
Higher Frequency, Smaller Wavelength ~0.01nm	Lower Frequency, Larger Wavelength ~1nm
Higher penetration (photons have more energy)	Lower penetration (photons have less energy)
Produces higher “quality” X-rays	Produces very low “quality” X-rays
Higher voltages are applied to produce	Lower voltages are applied to produce

Since soft X-rays are absorbed easily, not a lot of X-rays get through so a very poor image is formed. Also because it is absorbed more, it is also very dangerous to the body as it can cause genetic mutations and cancers.

This is why X-ray machines have an aluminium filter to try to reduce exposure to these soft X-rays that are dangerous and don’t have much use.

X-ray machines also have a collimator that is designed to control the size and shape of X-ray beams
Also, beams that don’t get absorbed and don’t pass through get scattered making the image fuzzy. To stop this a grid of lead is used before the beam hits the screen so only the transmitted X-rays are used for image.
Screen is used to convert the X-ray to visible light which exposes the film. This is done since a smaller exposure time needed so patient isn’t subject to too much X-rays.
Soft X-rays:
- Longer wavelength
- High frequency
- Generated though lower velocity electrons (low voltages)
- Not very penetrative
Hard X-rays:
- Short wavelength
- Low frequency
- Generated though higher velocity electrons (high voltages)
- Highly penetrative

gather secondary information to observe a CAT scan image and compare the information provided by CAT scans to that provided by an X-ray image for the same body part

CAT scan images are readily available from a range of sources. You will find CAT scan images in medical physics books, on the Internet and through your doctor or medical imaging centre. Your observations of a CAT scan should include general X-ray image observations of brightness related to tissue type and image clarity. CAT scan images typically include many images and you should observe how these are related to each other and organised and presented to the viewer.
Your comparison should include both similarities and differences between CAT scan and traditional X-ray images. CAT scans are produced when information from multiple X-rays is manipulated by computer to form a composite image. CAT scan images can be enhanced to highlight structures. Structures can also be ‘digitally removed’ by the computer eg the rib cage). CAT scans can be presented as a 3-dimensional image on a computer screen. This image can be digitally rotated and sectioned by the viewer. CAT scans can be printed like a traditional X-ray.

explain how a computed axial tomography (CAT) scan is produced

CAT scans are based on a series of X-rays.
In one type of machine, a patient is placed in a fixed position (eg lying down). The X-ray tube and detectors are attached to a movable C-shaped structure. This structure follows a pre-determined path around the patient, recording X-ray images to a computer.
In another version the patient’s bed moves through the X-ray equipment which is located in a fixed cylindrical structure. (Like a large donut). The X-ray tube (and detector) rotate around the donut as the patient goes through the central hole. This produces a spiral of images.
In each case, the X-ray tube directs a narrow beam through the patient. The intensity of the emerging beam is recorded by the detector.
This information, along with the position and angle of the beam, is used by a sophisticated computing program to develop a 3 dimensional intensity map of the structure in question. This can be presented as a set of two-dimensional slices or as a movable image on a computer screen.

describe circumstances where a CAT scan would be a superior diagnostic tool compared to either X-rays or ultrasound

Your description should indicate the diagnostic advantages of CAT scans over X-rays. Your answer could include the wealth of data a CAT scan provides, the ability to manipulate the computed image to obtain different views, the ability to selectively enhance or remove structures from the image and safety aspects for the patient.
Your description should indicate the diagnostic advantages of CAT scans over ultrasound. Your answer could include the ability to image structures that ultrasound is unable to penetrate, such as gas or bone.

perform a first-hand investigation to demonstrate the transfer of light by optical fibres

Commercially available toys, and lamps that use optical fibres for effects could form the basis of this investigation.
Your investigation involves the use of a light source and an optical fibre. A number of factors will determine the success of this demonstration:
- A darkened room.
- The light source should be of sufficient intensity that an appreciable amount of light will be transmitted for the fibre used. An intense beam from a ray box or torch may give good results. A laser will give good results. Students are cautioned that lasers are a hazard and that using them may result in personal harm. There are guidelines for the safe use of lasers which must be followed. No student should attempt to use a laser of any kind.
- The connection between optical fibre and light source. This should be such that light does not escape. Any escaped light will brighten the room. Laser beams are very narrow and the optical fibre needs to be directly in front of the beam. Shielding should be in place before the beam is turned on. Adjustments should only be made with the beam turned off. A smooth end to the optical fibre will aid light entry and exit to a screen.
- The characteristics of the optical fibre. Optical fibres can be glass or plastic. The latter will give good results over short distances. Glass fibres are very fine and can present an injury hazard.

explain how an endoscope works in relation to total internal reflection

Prior knowledge: Physics syllabus 8.2.4
Your explanation should include the concept of total internal reflection, the role of the cladding on optical fibres, what an endoscope is thus how the endoscope works in relation to total internal reflection.
Total internal reflection may happen when a wave attempts to pass from one medium to another. If the refractive index of the second medium is greater than the first there will be an angle, the critical angle, at which the wave will be refracted along the boundary. At greater angles than the critical angle the wave will be unable to cross the boundary. Instead it will reflect. This is called total internal reflection.
Optical fibres have a thin coating of plastic or other high refractive index substance. This acts to provide a boundary across which the light cannot pass if it strike at an angle beyond the critical angle.
An endoscope contains a flexible, narrow bundle of optical fibres. It is used to image internal structures. Light is directed into the body through the endoscope and the image is transmitted back to the operator. In each case, total internal reflection allows the light to reflect (bounce) its way along the flexible optical fibre. In this way the light can ‘travel around corners’.

discuss differences between the role of coherent and incoherent bundles of fibres in an endoscope

Your discussion should include the meaning of coherent and incoherent, the need for a bundle of fibres in an endoscope and the uses of each type of bundle. Issues include suitability for purpose and cost.
Optical fibres are very narrow. This allows them to be flexible and to access narrow structures. To obtain an image a number of fibres need to be placed side by side, much like lengths of spaghetti in the packet. If the relative position of the fibres at each end is the same, the bundle will give a coherent image. Each image element will be in the correct place at each end and so the picture as a whole will be complete. If the relative position is not the same this will not happen and the bundle will give an incoherent image where the picture elements bear no relationship to each other.
Endoscopes need bundles of fibres to provide the lighting for viewing and to transmit the image back to the operator.
A coherent bundle is best used to transmit the image. An incoherent bundle can be used for introducing the light.
Making a coherent bundle is more expensive than making an incoherent bundle. Each fibre needs to be carefully positioned. The endoscope may undergo tight turns and twists during use and so the construction needs to maintain these positions during use.

gather secondary information to observe internal organs from images produced by an endoscope

Endoscope images can be gathered from a wide variety of sources including texts, the Internet and medical practitioners. Endoscope images can be videotaped. Organs such as the stomach can be viewed in real time and tests can be carried out and the results observed.

explain how an endoscope is used in:

observing internal organs

obtaining tissue samples of internal organs for further testing

Your explanation should include the procedure of using an endoscope and the observations that can be made with one.

A practical endoscope has an optical fibre bundle that directs light into the patient, another bundle for collecting the visible image and a guidance system. The endoscope has a smooth and flexible exterior that also provides some rigidity. This allows the endoscope to be inserted into the patient and guided on its journey. The endoscope can be used to investigate the alimentary canal, lower abdomen and as part of keyhole (small incision) surgery. A camera may be attached to obtain still and video images.
Your explanation should include the modifications to an endoscope that allows for tissue sampling.

A tissue sample (biopsy) can be readily taken by a modified endoscope. At the appropriate position the operator can project small curved ‘cups’ from the endoscope. These open to nip a small tissue sample and then retract. Another modification uses a small wire ‘basket’ to collect tissue.