Focus 2 - The physical properties of electromagnetic radiation can be used as diagnostic tools.
X-Rays
X-rays are high frequency electromagnetic waves that sit between gamma and ultraviolet rays of the electromagnetic spectrum. As they are high frequency waves, they have short wavelengths (between 0.001 nm and 10nm) which make x-rays useful for visualising points that are close together. X-rays are also high energy (E=hf) which makes them able to penetrate through objects.
Note that wavelength is measured in Angstroms. 1A = 10^-10 m or 0.1 nm
X-rays can be classified as either "hard" or "soft", and this depends on their wavelength/frequency which is closely linked to their penetrative power.
- Hard x-rays are higher frequency and, therefore, higher resolution due to their shorter wavelength (approx 0.01nm). Hard x-rays are higher energy and have more penetrating power, making them useful for medical imaging as they can penetrate deep into the body and reach a film on the other side.
- Soft x-rays are lower frequency and, therefore, lower resolution due to their longer wavelength (approx 1 nm). Soft x-rays are lower energy waves and have less penetrating power. They are unable to penetrate the body (most absorbed by the skin!) and reach the film, making them useless for medical imaging. Soft x-rays are used in soft x-ray microscopy.
How are x-rays produced?
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X-rays are produced in vacuum tubes by thermionic emission.. yes.. THAT again!
While x-rays were initially discovered by Wilhelm Roentgen in 1895 using Crooke's tubes, a man named William Coollidge improved this set up in 1913 with what is now called a Coollidge tube. |
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X-rays are produced in an x-ray tube (below). A voltage is applied across the cathode and anode. Electrons at the cathode are freed from the surface by thermionic emission. The electrons accelerate towards the anode and decelerate as the collide with the atoms of the metal anode. This collision converts the kinetic energy of the electrons into a lot of heat and... x-rays!!
Now that we've produced x-rays, we can narrow the beam through a lead collimator to produce a good quality image. The useless soft x-rays are then filtered out by 3mm thin aluminium sheet (which it cannot penetrate). Note that, while we've established that soft x-rays are useless, they are still x-rays and reducing exposure can prevent skin burning. Hard x-rays are able to pass through the body but are attenuated (reduced in intensity) differently by different tissues. Denser tissue like bone will attenuate x-rays more than muscle/fat and much more than air. The x-rays travel through the body and "expose" the film, so more attenuation of x-rays results in less exposure (kind of like casting a shadow!) The result is a pattern on the x-ray film as different tissues vary in their attenuation of x-rays. Today, x-ray film is not used frequently. X-ray machines can be connected to a computer that will interpret the data collected as an image on the screen. This is far more convenient, saves storage as there are no hard copies, and has the ability to adjust contrast. |
As mentioned above, A LOT of heat is produced as we convert the kinetic energy of high velocity electrons into x-rays. This makes the energy conversion very inefficient. It is for this reason that the vacuum tube, and the anode is particular, is able to withstand the heat and dissipate it quickly and efficiently.
- High melting point metals (tungsten or molybdenum) are used for the anode as they can withstand high heat.
- Anode is placed at an angle to the beam of electrons to increase the surface area exposed to electrons and distribute the heat over a larger area (less heat per square unit prevents melting of the metal).
- A motor rotates the anode at 3600 revs/min so that different parts of the metal are exposed at different times and heat is evenly distributed over the metal.
- The tungsten or molybdenum is mounted on a piece of copper to conduct heat away from the exposed metal.
- A coolant (e.g. oil) circulates through the anode to carry away excess heat.
Types of X-ray Radiation
There are two types of x-ray radiation produced using the process above - bremsstrahlung (braking) radiation and characteristic radiation.
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Bremsstrahlung (braking) radiation is produced as the electrons decelerate when the hit the atoms of the anode (technically the electrons bend around the nucleus of the anode atoms, but this is still deceleration). Electrons in the x-ray tube have a very high accelerating potential (voltage) of 25000-250000 volts! so when they collide with the anode, the collision produces 98% heat and 2% x-rays. These x-rays are the bremsstrahlung (braking radiation).
The frequency of the x-rays relate to the to the energy of the x-rays and the intensity is proportional to the number of electrons that strike the anode. The number of electrons striking the anode can be changed by changing the alternating current passing through the filament of the electron gun. The electrons can possess variations of kinetic energy or have different proportions of kinetic energy converted to x-rays. This results in a spectrum of frequencies. |
The area under the curve is the total energy of the x-rays at that voltage. Increasing the voltage increases the energy of the x-rays.
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Remember that hard x-rays have greater penetrating power? The hardness/penetrating power of x-rays depends of the energy of the electrons hitting the anode. These electrons have greater energy if the accelerating potential (voltage) is greater.
This means the intensity and the hardness of the x-rays can be controlled independently.
This means the intensity and the hardness of the x-rays can be controlled independently.
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Characteristic radiation is produced when electrons reach the anode and knock the inner electrons of the atom out of its usual position. As electrons from outer shells of the atom fill the vacancies in the lower shells, energy is emitted as x-rays (if the atoms are large).
This type of radiation is called 'characteristic radiation' because the x-rays produced depend on the type of metal used for the anode. The x-rays are seen as spikes in a graph. 
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Uses of X-rays for Medical Imaging
X-rays have many uses in medical imaging. They can be used to view bones for fractures, breakages, spurs, abnormalities and cartilage degeneration. They can also be used to image teeth.
X-rays are also useful for searching for masses and blockages in organs that normally don't attenuate x-rays well. For example, the outline of the lungs can be seen in a normal chest x-ray, but any blockages/masses will attenuate x-rays better than the air in the lungs and be clearly visible on the film.
Objects can be radiopaque (can be seen) or radiolucent (cannot be seen). The organs of the digestive system are radiolucent - they don't attenuate x-rays well and organs are not distinguishable. To image the digestive system, the patient can do a barium meal test. Barium sulfate is taken orally as a radiocontrast medium to make the bowel radiopaque.
To collect information about blood flow through an artery (e.g. searching for blockages), an iodine based contrast medium can be used.
X-rays are also useful for searching for masses and blockages in organs that normally don't attenuate x-rays well. For example, the outline of the lungs can be seen in a normal chest x-ray, but any blockages/masses will attenuate x-rays better than the air in the lungs and be clearly visible on the film.
Objects can be radiopaque (can be seen) or radiolucent (cannot be seen). The organs of the digestive system are radiolucent - they don't attenuate x-rays well and organs are not distinguishable. To image the digestive system, the patient can do a barium meal test. Barium sulfate is taken orally as a radiocontrast medium to make the bowel radiopaque.
To collect information about blood flow through an artery (e.g. searching for blockages), an iodine based contrast medium can be used.
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X-rays are a useful tool for medical imaging, but it isn't perfect and can't be used for everything. There are advantages and disadvantages to its use.
Advantages
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Disadvantages
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A-Level Physics Video Lessons - understand higher level content to make HSC level easier.
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Questions 2.2.1,2) Complete the summary, practice and past questions on X-rays.
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Task 2.3.1) Observe x-ray images of fractures and other body parts. Consider the following points.
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Computed Axial Tomography (CAT) Scans
Endoscopes
In this module, we've been looking how the properties of light are used in medical imaging technologies.
Let's look back at the refraction of light. Refraction is the change in the speed of light as it travels from one medium into another. This change in speed results in a change in direction if the incident ray hits the boundary outside the perpendicular normal. Remember that travelling into a more dense medium slows down the light resulting in a smaller angle of refraction, and travelling into a less dense medium causes the light to speed up resulting in a larger angle or refraction.
When travelling from a less dense into a more dense medium, increasing the angle of incidence also increases the angle of refraction. This can continue until the angle of refraction is 90 degrees and travelling along the boundary. The angle of incidence needed to produce this effect is called the critical angle.
Increasing the angle of incidence beyond this interface/boundary between the two media causes the light to reflect back into the first medium. This is called total internal reflection. This is an interesting phenomena because this reflection is off a non-reflective surface, is caused by refraction, but still follows the law of reflection - angle of incidence and angle of reflection are equal and on the same plane!
Let's look back at the refraction of light. Refraction is the change in the speed of light as it travels from one medium into another. This change in speed results in a change in direction if the incident ray hits the boundary outside the perpendicular normal. Remember that travelling into a more dense medium slows down the light resulting in a smaller angle of refraction, and travelling into a less dense medium causes the light to speed up resulting in a larger angle or refraction.
When travelling from a less dense into a more dense medium, increasing the angle of incidence also increases the angle of refraction. This can continue until the angle of refraction is 90 degrees and travelling along the boundary. The angle of incidence needed to produce this effect is called the critical angle.
Increasing the angle of incidence beyond this interface/boundary between the two media causes the light to reflect back into the first medium. This is called total internal reflection. This is an interesting phenomena because this reflection is off a non-reflective surface, is caused by refraction, but still follows the law of reflection - angle of incidence and angle of reflection are equal and on the same plane!
An application of total internal reflection is in optical fibres - useful in telecommunications and medical imaging. In medical imaging, this device is called an ENDOSCOPE.
Revision 2.2.5) Complete the revision activity to test your prior understanding of refraction and total internal reflection.
| reviewing_refraction_-_endoscopes.docx |
What is an Endoscope?
An endoscope is a medical imaging device for examining the interior organs of the body. The endoscope uses a large number of optical fibres (up to 40,000) as a camera and is inserted into the body through a natural orifice or surgically created opening. They are flexible enough to allow the endoscopist to see around corners for a direct view of the interior organs without cutting the organs open. Key-hole surgeries can also be done at the same time, making endoscopy both diagnostic and therapeutic.
An endoscope is a medical imaging device for examining the interior organs of the body. The endoscope uses a large number of optical fibres (up to 40,000) as a camera and is inserted into the body through a natural orifice or surgically created opening. They are flexible enough to allow the endoscopist to see around corners for a direct view of the interior organs without cutting the organs open. Key-hole surgeries can also be done at the same time, making endoscopy both diagnostic and therapeutic.
The optical fibres consist of the core, cladding and sheath in concentric circles (share same centre). It is engineered so that refractive index is greater in the core than the cladding and the critical angle is very small, so that light entering the core will always undergo total internal reflection. This allows light to reflect inside the core and propagate from one end of the fibre to the other.
The sheath acts to reduce the amount of light entering from the external environment, which would otherwise cause interference. Optical fibres can transmit information as flashes of light, translating into 1s and 0s that code for digital data. In an endoscope, optical fibres use continuous light that is transmits visual images from one end to be viewed/displayed at the other end.
Components of an Endoscope
Endoscopes come in many different designs depending on their function. The typical example to use is the colonoscope, used in colonoscopies.
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These components are enclosed in a plastic shaft with a metal frame, making it both strong and flexible as well as resistant to bodily secretions. They are normally 1-2 metres long with a controller on the proximal end to manipulate the shaft through the colon. The distal end is inserted into the anus.
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Coherent and Incoherent Bundles
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An incoherent bundle is a bundle of individual fibres randomly placed alongside each other so that they aren't in the same relative position at the other end. The light patterns that enter are, therefore, distorted at the other end. The illuminating fibre bundle is an incoherent bundle - it transmits light from an exterior source into the organ. As it simply carries light to the interior, the distortion does not affect the outcome. Hundreds of thicker fibres can be used to transmit the light, making them cheaper to manufacture.
A coherent bundle is a bundle of individual fibres kept parallel throughout their length so that they are in the same relative position on the other end. This is important for producing an image without distortion on the other end. The objective optical fibre bundle is a coherent bundle - it carries visual images of the interior of the organ. As it is required for viewing/displaying the images, distortion does affect the outcome. Lenses on both ends of the bundle produce a focussed image, and thousands of thin fibres improve the resolution of the image. This is technically difficult to manufacture and is, therefore, more expensive to produce. |
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Uses of Endoscopes
Endoscopes can be used to:
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Previously, open surgery was needed to examine and treat internal organs. Using endoscopes is a less invasive alternative that carries less risk, allowing for faster recovery without always requiring admission to hospital. This advancement in technology has significantly reduced the cost of health care.
Below is a table of the uses of endoscopes as well as some videos of each procedure for you to watch.
Below is a table of the uses of endoscopes as well as some videos of each procedure for you to watch.
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Activities 2.2.5, 2.2.6, 2.2.7) Complete the activities below to explain the principle behind endoscopes, their components and uses.
- Review the content on refraction and optical fibres from the prelim course.
- Produce a table summarising the function of each of the components in an endoscope (Check your answers here)
- Create a table to compare coherent and incoherent bundles in endoscopes.
- Use your table to write a discussion of the differences between their role in an endoscope.
- Explain how endoscopes are used in medicine and list some examples of their use in both these categories.
- Download and answer the summary questions in the document below. Check your answers.
- Complete the endoscope questions in the Jacaranda chapter review and check your answers.
| endoscope_summary_questions_and_answers.docx |
Practical 2.3.3,4) Complete the practical activities to demonstrate the transfer of light in an optical fibre.
- Look through the slides and experiment with the set up of each demonstration.
- Summarise the method you followed for each practical.
- Explain how this demonstrates light transfer in optical fibres.
- Use the resources above and your notes to compare the information obtained with an endoscope to ultrasounds, x-rays and CT scans.
- Read through page 360 in your textbook and evaluate the use of endoscopes in medicine.
| optic_fibre_demonstrations.pptx |
