Electromagnetic Radiation in Therapy and Diagnosis

Learning Outcomes

• Identify the main components of the electromagnetic spectrum.

• State typical values of frequency or wavelength for each of these.

• Describe the relationship between energy, frequency, and wavelength of photons.

• Recognize the role of each of the main components of the EM spectrum in relation to diagnosis or therapy of patients.

Electromagnetic (EM) Radiation: Diagnosis & Therapy

• EM radiation results from the acceleration of charged particles, creating an interaction between changing Electric and Magnetic fields.

• EM radiation can be described by its energy, wavelength, or frequency.

• An EM spectrum organizes the different types of EM radiation that exist based on wavelength and frequency.

Electromagnetic Radiation Spectrum

• The electromagnetic radiation spectrum includes:-

  • Radio waves:

    • AM/FM radio

    • TV broadcasts

  • Microwaves:

    • Used in microwave ovens and communication systems

  • Infrared (IR):

    • Heat radiation

  • Visible light:

    • The portion of the EM spectrum that the human eye can detect

  • Ultraviolet (UV):

    • UVA, UVB, and UVC

  • X-Rays:

    • Used in medical imaging

  • Gamma Rays:

    • Used in cancer treatment

• Frequency increases from radio waves to gamma rays.

• Wavelength decreases from radio waves to gamma rays.

Nature of EM Radiation

• Various bands of radiation are identical, all consisting of changing electric and magnetic fields.

• The key difference between them is the wavelength or frequency of the radiation.

• All EM radiation travels at the same speed – the speed of light (in a vacuum), c = 3 \times 10^8 m/s.

• All EM radiation obeys the relationship v
  {wave} = c = f \cdot \lambda (i.e., speed = frequency × wavelength).

Relationship Between Energy, Frequency, and Wavelength

• EM radiation consists of photons, which are packets of energy.

• Photon energy (E) is directly proportional to the frequency of the wave (f), i.e., E \propto f.

• Because c = f \cdot \lambda = constant, f and \lambda are inversely proportional to each other.

• E \propto f implies E \propto 1/\lambda, meaning photon energy is inversely proportional to the wavelength.

• Gamma rays are more energetic than radio waves, and blue light contains more energy per photon than red light (f{blue} > f{red} or \lambda{blue} < \lambda{red} \rightarrow E{blue} > E{red}).

Frequency, Wavelength, and Biological Effects

• Radiation with a frequency > ~1 \times 10^{15} Hz can break bonds and free electrons from an atom.

• For biological tissue, this can occur for radiation with wavelengths less than ~300 nm (UV) or photon energies greater than ~4 eV.

• E \propto f \rightarrow E = constant \times f = h f = \frac{hc}{\lambda} (in Joule, J) = \frac{hc}{e\lambda} (in electronvolt, eV)

  • c = speed of light in vacuum = 3 \times 10^8 m/s

  • h = Planck’s constant = 6.63 \times 10^{-34} J s

  • e = charge of an electron = 1.6 \times 10^{-19} C

• E = h \cdot f = \frac{hc}{\lambda}

Response of the Body to Different Types of EM Radiation

• The body responds differently to different frequencies and wavelengths of electromagnetic radiation. Responses can range from heating effects (infrared) to ionization (X-rays and gamma rays).

Uses for EM Radiation in Medicine or Physiotherapy

• EM radiation has various applications in medicine and physiotherapy.

Short Wave Diathermy (SWD)

• Diathermy: "Gentle Heating": non-superficial heat treatment in patients.

• Heat:-

  • Relieves Pain

  • Increases Mobilization

  • Improves Blood Flow

• SWD uses radio wave radiation in the frequency range 10-100 MHz (Typical value is 27.12 MHz).

Mechanism of SWD

• In SWD, radiation is passed to the patient via electrodes (capacitive field diathermy) or coil applicator (inductive field diathermy).

• The varying electrical and magnetic fields cause charged molecules within the tissue to vibrate, converting kinetic energy to heat.

• Tissues with a high number of free ions (muscle tissue, blood, etc.) are good conductors and respond well to SWD.

• Metal and sweat also respond well to SWD (hazards).

• SWD can produce superficial or deep tissue heating (Joule Heating, E = I^2Rt or P = I^2R).

• Capacitive/Electric field diathermy and Inductive/Magnetic field diathermy are two methods.

Observed Effects of SWD

• Increase blood flow.

• Help reduce inflammation.

• Increase the extensibility of deep collagen tissues.

• Decrease joint stiffness.

• Relieve deep muscle pain and spasm.

Microwave Diathermy (MWD)

• Water molecules (polar molecules) in tissue absorb the microwave energy.

• Vibrational energy is converted to heat.

• Frequencies used are ~2450 MHz.

• Provides deep penetration into tissue.

• Caution: Bone reflects microwaves and can cause burns to surrounding tissues.

Precautions on Diathermy

• SWD is contraindicated in areas with metal implants and in patients with pacemakers.

• Avoid areas with excessive fluid accumulation (edematous tissue, moist skin, eyes, fluid-filled cavities, pregnant or menstruating uterus) for both SWD and MWD.

• A rule of “no water and no metal” is generally recommended when using both SWD and MWD.

Infrared (IR) Radiation

• More than half of the energy reaching the earth from the sun is in the form of infrared (IR) radiation – i.e., heat.

• Near IR may penetrate up to 5mm under the skin to reach subcutaneous tissues.

• Therapeutic heat lamps produce a high percentage of high-intensity near-IR radiation (\lambda ~ 1000-2000 nm).-

  • Can be used to deep-heat tissues.

  • Increased metabolism results in a relaxation of the capillary system (vasodilation).

  • Increased blood flow in the region of treatment.

  • Excellent treatment for muscular and soft tissue injuries.

Hazards and Applications of IR Radiation

• IR radiation is invisible and can penetrate through the lens.

• Accidental exposure to IR laser can be hazardous in the form of a retinal burn.

• Thermography is another application.

• All objects with a temperature greater than absolute zero emit radiation with different intensity over a spectrum of wavelengths.

• The peak wavelength of the radiation spectrum is inversely proportional to the object's temperature.

• Wien’s Law: \lambda
  {peak} = \frac{B}{T}, where B is Wien’s constant (2.9 \times 10^{-3} m K) and T is temperature in Kelvin.

Wien's Law and Stefan's Law

• Graph of intensity vs. wavelength of radiation emitted from an object at temperature T (in K).

• Stefan’s Law: The power of radiation emitted by the body (W) is determined primarily by the temperature of the body, i.e., W = eA\sigmaT^4-

  • Where:-

    • e is the emissivity

    • A is the area of the body

    • \sigma is the Stefan-Boltzmann constant (5.67 \times 10^{-8} W m^{-2} K^{-4}).

Thermography

• Since the power of radiation is proportional to T^4, mapping radiation power as a function of position gives a good map of surface temperature.

• Humans emit EM radiation in the infrared region, allowing infrared cameras to accurately map the surface temperature of the body via Wien’s Law or Stefan’s Law.

• Thermography can give a good indication of surface blood distribution.

• It has been used as a ‘first-indicator’ of tumors in breast cancer patients and identifying areas of reduced blood flow in patients with diabetes or vascular problems.

Visible Radiation

• Eye – simply look at your patient.

• Fibre-optic Endoscopes.

• Ophthalmoscope.

• Otoscope.

• Much of the white light used in endoscopy contains Infrared (IR) radiation, so IR filters are desirable to absorb this and minimize unwanted heating of healthy tissue (cold light endoscopy).

Endoscopy

• Regions which can be investigated using endoscopes:-

  • Colon

  • Vocal cords

  • Hematoma

  • Colon - Polyp

  • Trapped meat

Transillumination

• Transillumination refers to the transmission of light (usually visible) into various parts of the body for diagnosis.

• A red glow is often associated with Transillumination.

• Red light penetrates further into tissue and undergoes scattering, while blue light is more easily absorbed at the tissue surface.

Uses of Transillumination

• Can be used effectively to detect hydrocephalus in infants (skull is not fully calcified).

• Can also be used to diagnose pneumothorax and study problems with the gums, sinus cavities, and breasts.

Visible Lasers – Photodynamic Therapy (PDT)

• A photosensitive drug is administered to the patient and selectively taken up by cancerous cells.

• When the cancerous area is exposed to light of a certain wavelength, singlet oxygen is produced, which is extremely toxic to the cancer cells.

Fluorescence in PDT

• Once the photosensitive drug has been taken up by the cancer cells, the extent of the lesion may be observed using fluorescence.

• Fluorescence image shows clear boundaries of a skin lesion that cannot be seen with the naked eye.

Clinical Examples of PDT

• Recurrent cancer of the lower lip treated with PDT.

• Recurrent tumors on the chest wall following mastectomy treated with PDT.

Lasers in Medicine

• Note: Lasers also have many other uses in medicine and physiotherapy – see later.

• Most clinically useful lasers operate in the visible part of the spectrum.

Blue Light

• Infants suffering from jaundice (excess bilirubin excretion from the liver) respond very well to exposure from Blue light – phototherapy.

• Broadband light (430nm – 490nm, centered on ~450 nm) seems to produce the best effect when the infant is exposed to the lamp source for a period of 12-24 hours.

UV Radiation

• UV light has a higher frequency than visible or red light and, therefore, has greater energy per photon.

• UV light with \lambda < 300 nm is germicidal.

• UV light can convert molecular products in the skin to produce vitamin D, which can improve skin conditions such as psoriasis.

• UV radiation also affects melanin in the skin to cause tanning.

• Excessive exposure may burn and can be avoided by sunblock, etc.

Dangers of UV Radiation

• These wavelengths are also well absorbed by DNA cells in the skin, and prolonged exposure can lead to the formation of skin cancers such as Basal Cell Carcinoma (BCC) and Squamous Cell Carcinoma (SCC).

• Be aware of ‘scattered’ UV light.

Why We Can't See UV Light

• UV light is absorbed by the cornea & the lens.

• UV light is very easily absorbed in the surface tissues and never gets through the lens at the front of the eye.

• Excess absorption of UV light by the lens can result in cataracts.

• People who have had the lens removed because of excess cataract formation can often see into the UV part of the spectrum.

X-rays

• High energy photons produced due to decelerating electrons.-

  • They can penetrate soft tissues but are absorbed by high-density tissues – can be used to differentiate tissue types, i.e., medical imaging.

  • They can cause ionization – can be used to kill cells and destroy tumors, i.e., treatment of cancer.

Gamma-rays

• High energy photons that are originated from deexcitation of nuclei.-

  • They can penetrate all tissue types – can be used for medical imaging.

  • They can cause ionization – can be used to kill cells and destroy tumors, i.e., treatment of cancer.