BOSTES HSC Physics 2015 Question 32

Question: 
This question is asked in the context of medical physics. Students are expected to explain the impact of advances in an understanding of waves on the development of imaging technologies. Three examples should be provided in the answer.

Marking Guidelines:

Criteria	Marks
Relate three imaging technologies to an understanding of waves. Assess the impact of advances on the development of imaging technologies.	6

(Source: https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-physics.pdf)

Possible answer:

There are more than dozens of specific applications of imaging technologies such as magnetic resonance imaging and positron emission tomography. Importantly, the answers should be related to an understanding of waves in different frequencies (or wavelengths). Below are three examples of imaging technologies that are closely related to medical physics:

1. Thermal (infrared) imaging: The thermal sensors essentially record the emitted infrared radiations from the skin surface of a patient. In some applications, thermal imaging provides an objective measurement of temperature changes that are clinically significant. Currently, the medical applications include not only fever screening but also inflammatory diseases and complex regional pain syndrome.

2. Ultrasound imaging: The depth of penetration of ultrasound waves is dependent on human tissues and the ultrasonic frequency. The principle of ultrasound is based on the reflection and refraction of ultrasound waves while propagating the tissues which have different densities. In Doppler-based modes, ultrasound waves help to determine the velocity of a moving tissue and the blood circulation in a baby during pregnancy.

3. X-ray imaging: Different organs and tissues have different sensitivities and absorptions of X-ray waves. Essentially, denser tissues such as bone can absorb more X-ray radiations as compared to other organs and tissues, and there is a greater attenuation of the X-ray waves. In X-ray imaging, the image of the human body has a higher resolution due to the shorter wavelengths of X-ray waves.

Feynman’s insights or goofs?:

Physics teachers may use Feynman’s lectures to explain the concept of waves in imaging technologies as shown below.

1. Thermal (infrared) imaging: Feynman says that “there are also infrared waves traveling from the warm foreheads to the cold blackboard (Feynman et al., 1964, section 20–2 Three-dimensional waves).” However, this does not mean that infrared waves only propagate from the warm bodies. Of course, infrared waves are emitted from the warm bodies as well as cold bodies and even dead bodies. Physics teachers should clarify that oscillations of molecules in bodies near room temperature can emit infrared waves. On the other hand, objects at a lower temperature can emit microwaves.

More importantly, Feynman adds that “[w]e are blind when we measure the infrared reflection coefficient of sodium chloride, or when we talk about the frequency of the waves that are coming from some galaxy that we can’t see — we make a diagram, we make a plot (Feynman et al., 1964, section 20–3 Scientific imagination).” That is, we can make a plot of different frequencies of the infrared waves and arbitrarily assign different colors to temperatures of bodies. It does help to visualize or distinguish the different temperatures of warm bodies.

Interestingly, Feynman suggests that “one day the physical review of the blind men might publish a technical article with the title ‘The Intensity of Radiation as a Function of Angle under Certain Conditions of the Weather’ (Feynman et al., 1964, section 20–3 Scientific imagination).” In a sense, we are also the blind men when the atmospheric radiations include infrared waves. However, we are not completely blind because we can visualize the intensity of atmospheric radiations as a function of wavelength and angle, under certain meteorological conditions.

2. Ultrasound imaging: According to Feynman, “waves like sound waves start out from such a source very much longer in wavelength than one usually considers in sound waves, but still they are sound waves, and they travel around in the earth. The earth is not homogeneous, however, and the properties, of pressure, density, compressibility, and so on, change with depth, and therefore the speed varies with depth. Then the waves do not travel in straight lines — there is a kind of index of refraction and they go in curves (Feynman et al., 1963, section 51–3 Waves in solids).” Similarly, ultrasonic waves travel in curves because of the changes in density of human tissues. Importantly, there are also reflections of waves and attenuations of amplitude due to density changes.

In addition, Feynman’s explanation of Doppler effect of moving atoms is a good analogy for moving tissues. In Feynman’s own words, “[s]uppose that the atoms were emitting, instead of sine waves, a series of pulses, pip, pip, pip, pip, at a certain frequency ω₁. At what frequency would they be received by us? The first one that arrives has a certain delay, but the next one is delayed less because in the meantime the atom moves closer to the receiver. Therefore, the time between the “pips” is decreased by the motion. If we analyze the geometry of the situation, we find that the frequency of the pips is increased by the factor 1/(1−v/c) (Feynman et al., 1963, section 34–6 The Doppler effect).” That is, we can detect an increase in the frequency of reflected ultrasound waves when the tissues are moving toward the ultrasonic receiver.

3. X-ray imaging: Feynman clarifies that “[x]-rays are nothing but very high-frequency light. If we go still higher, we get gamma rays. These two terms, x-rays and gamma rays, are used almost synonymously. Usually, electromagnetic rays coming from nuclei are called gamma rays, while those of high energy from atoms are called x-rays, but at the same frequency they are indistinguishable physically, no matter what their source (Feynman et al., 1963, section 2–2 Physics before 1920).” Thus, physics teachers may explain that x-rays are basically higher frequency electromagnetic waves. Importantly, Feynman also distinguishes gamma rays from nuclei, cosmic rays, and artificial sources.

In defining x-rays, Feynman explains that “[w]here the ultraviolet stops, the x-rays begin, but we cannot define precisely where this is; it is roughly at 10⁻⁸ m, or 10⁻² μm. These are ‘soft’ x-rays; then there are ordinary x-rays and very hard x-rays; then γ-rays, and so on, for smaller and smaller values of this dimension called the wavelength (Feynman et al., 1963, section 26–1 Light).” In short, the hard x-rays have shorter wavelengths as compared to soft x-rays. This is because electromagnetic radiations that have shorter wavelengths behave more like particles. Simply put, x-rays appears to be soft particles or hard particles depending on their wavelengths.

References: 

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley. 

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1964). The Feynman Lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley.