Use of X-rays in medicine

Question 1

What are the two types of x-rays? How are x-rays produced?

Answer 1

The source of x-rays is an evacuated x-ray tube containing a heated cathode and cooled anode. A high voltage exists between them.

-electrons escaping from the heated cathode are accelerated by an accelerating voltage U (unit is the volt) and have energy E =eU

X-rays are of 2 types: * At high accelerating voltages, both types of x-rays and their respective spectra are produced in an x-ray tube and superimposition of the two spectra occurs 1) Bremsstrahlung – results in a continuous spectrum.

Bremsstrahlung x-rays are produced by electromagnetic interaction of the accelerated e- with the fields of atomic nuclei -They are created when the kinetic energy of the accelerated electrons is transformed into the energy of EM waves in the field of atomic nuclei -The highest possible photon energy = eU = hfmax c=fλ then λ = c/f -the maximum energy and frequency therefore correspond to the shortest wavelength -Because the interaction with the fields of atomic nuclei occurs at various distances from the nucleus, the photon energies will vary as will their corresponding wavelengths and a continuous spectrum will be created. -The peak of the spectrum is shifted to the left (shorter wavelengths, higher energy) with an increased accelerating voltage -The emitted power of Bremstrahlung is proportional to the accelerating voltage, intensity of the e- current, and atomic # Z of the anode target P=kV²IZ *from this relationship, power can be increased with a higher accelerating voltage, intensity of electron current, and atomic # of anode Efficiency: Only 1% of the energy transported by e- to the anode is transformed into electromagnetic waves, 99% is converted into thermal energy. Therefore, a method of cooling the anode is needed and the anode must have a high melting point. The efficiency n of a x-ray tube is given by N= kVZ 2)

Characteristic x-rays produce a line spectrum. -Characteristic x-rays are emitted from the electron shells of anode atoms by ionization due to the energy of the incoming accelerated electron beam. -If the energy of the accelerated electrons is higher than the binding energy of the electrons in the inner electron shells, ionization of these electrons will occur. -The subsequent de-exitation results in emission of photons -A line spectrum is formed since electron transitions have well defined energies associated with them -Higher atomic #s of the target anode will shift the line spectra to the left (shorter wavelengths, higher energies) X-rays have several effects: -Primary effects are excitation and ionization of target nuclei -Other effects occur as well and they are the result of primary effects of x-rays: Luminescence: Ionizing effect: the electrical conductivity of some materials is increased (due to free flowing electrons that were ionized and now free to roam) Photographic effect: darkening of a photographic plate Chemical effect: production of hydrogen peroxide in water Biological effect: structural and functional changes in cells

Question 2

Control of the energy and intensity of X-rays

Answer 2

• The penetrating ability of Bremsstrahlung can be controlled by the accelerating voltage of in the x-ray tube, the higher the voltage the higher the x ray energy and the greater it’s penetrating ability E = qV
• The intensity can be controlled by changing the intensity of the anode current. This can be done by varying the degree of heating of the cathode filament. It is a useful technique when is desired to change the intensity of without varying the energy of radiation.

P=kV²IZ

Question 3

X-ray apparatus

Answer 3

An x-ray machine is a machine used to produce x-rays via x-ray tubes.

• X-ray machines generating a beam of x-rays from a source (x-ray tube).
• The beam is projected through the body. Some of the X-rays will pass through the body and others will be attenuated in the tissues. The transmitted x-rays then fall onto a photocathode which contains luminescent screen or onto a photographic film. Semiconductor plates or image intensifiers.
• Images taken with such devices are known as x-ray photographs or radiographs.

X-ray lamp

If the energy of an x-ray photon that strikes the anode in the x ray tube is greater than the binding energy of e- in the e- shell of nucleus, the e- excitation are e- transition results in production of the line x ñ ray spectrum since electron energy levels are well defined. Increasing atomic number Z of the target material results in the shift of these line spectra to shorter wavelengths.

Question 4

X-ray absorption and Attenuation of X-rays

Answer 4

Attenuation = absorption

The intensity of a monchromatic x-ray beam propagating in some medium decreases according to

Two processes cause the attenuation of X-rays:

1) Photoelectric effect

a photon transfers its whole energy to an electron, therby ionising the atom.

After leaving the atom, the electron induces ionization and excitation until its excess energy is lost

• Therefore, x-rays do not cause ionization and excitation directly but rather through the high energy electrons that are released as a result of their collisions with atoms
• This process occurs during Characteristic x-ray emission and is accompanied by a line spectrum
• Attenuation in bone due to the photoelectric effect is higher in bone than in soft tissue because the effective atomic number of bone is higher
• tissues that attenuate x-rays well appear white on a radiograph while those that do not are black
• Bone is seen as the unexposed region on the x-ray film (white).
  2) Compton scattering
• During compton scattering a photon transfers only a fraction of its energy to an electron causing a scattering effect.
• The scattered photon moves in a changed direction with a lower energy hf. (longer wavelength)

This type of interaction does not depend on the atomic number of the absorber and the probability of its occurence depends on incident photon energy.

-compton scattering plays an important role in x-ray contrast

*The total linear mass attenuation coefficient is equal to the sum of the attenuation coefficients for the photoelectric effect and compton scattering

Question 5

X-ray contrast

Answer 5

The intensity of an x-ray beam passing through a patient decreases due to attenuation by the photoelectric effect and compton scattering.

• The degree of attenuation varies for different tissues.
• The contrast Cr of traditional x-ray image (shadow image) resulting from different x ray absorption coefficients in various tissues (of various density and effect atomic number) is defined by

where I₂,I₁ are different x ray intensities

-The contrast of the resulting photographic plate is evaluated by brightness

*Since the linear attenuation coef. of bone or soft tissue decreases with increasing incident photon energy, the contrast between bone and soft tissue decreases with increasing accelerating voltage installed in the x ray tube.

• In oder to differentiate between tissues with similar absorbing properties, contrast materials are added.
• Positive contrast:* stronger absorbers than their surroundings
• Negative contrast:* weaker absorbers than their surroundings

The following conclusions can be drawn according to the above equation:

A: the contrast is negative: more absorbing material will lower x-ray beam intensity on the photographic film

B: contrast doesn‘t depend on the thickness of the irradiated object

C: for empty spaces the contrast is positive

Question 6

Use of X-rays for diagnostic purposes

Answer 6

• X-rays can be used for diagnostic purposes due to the fact that different tissues in the body absorb x-rays to various extents
• X-rays emitted by the x-ray tube pass through a certain body part and are then viewed on a screen do to the luminescent properties of x-rays (summation image) or on photographic film due to the photographic properties of x-rays.
• the image produced is a summation image
• depth can only be assessed by moving the patient
• -Electronic image transfer* can be used to increase the brightness. This is acheived by acceleration of the e- produced by the photoelectric effect as they pass through the body

Because x-rays are damaging, least dose is desirable.

Scintigraphy: technique used to lower the dose of radiation for patients during examination

This technique utilizes a photographic film that contains a photographic emulsion on both sides which increases the sensitivity

The density of blackening is equal to

D=log/0/I

The difference between the densities of two neighboring areas is called the radiographic contrast

*photographic plates provide better resolution than due luminescent screens

Radiography : X-rays are highly penetrating, and x-ray machines are used in radiology to take pictures of bones and teeth. They can also be used to diagnose fractured bones. Imaging of the digestive tract can be done with the help of barium sulphate as a contrast medium.

Question 7

X-ray therapy

Answer 7

• The use of x-rays in therapy is based on the principle that certain types of cells are more susceptible to x-ray damage than others (i.e. young and actively dividing cells such as are present in cancerous growths)
• Used in destruction of malignant tumours.

Process:

• Low energy photons must be filtered out from the x-ray beam because they cause damage to superficial tissues. (can use Zn, Cu, Al filters)
• This results in a more homogeneous x-ray beam that can be targeted to a particular tissue.
• The more narrow the the energy range of photons, the greater the quality of the beam.
• Quality is assessed by the HVL (half value layer) which reduces initial intensity by 50%. The quality factor is : HVL1/HVL2 and should be about 1.5.

HVL1 absorbs x-rays with longer wavelength and HVL2 absorbs higher wavelength

Absorbed dose of radiation or exposure: the energy absorbed per unit mass. =E/m

(unit is Grey Gy)

I Gy = J/kg

There are several types

• Air dose – dose measured in the air at a certain distance from the focus of X-ray tube*
• Surface dose – measured at the surface of the patients body, higher because radiation is scattered back from the patients body*
• Depth dose – measures the absorbed dose at a certain depth below the surface*
• To avoid superficial damage, high energy photons are positioned sufficiently distant from the patient;
• In the treatment of superficial lesions, deep tissue damage is avoided by using low energy photons near to the skin.
• Physicians and technicians must be protected by shielding. The radiation hazard has to be checked by film dosemeters. (1mGy/week is max)
• Gamma ray therapy has become increasingly more common and often replaces x-ray therapy. It utilizes high energy photons emitted by radioisotopes

Question 8

Depth dose

Answer 8

This is one of the various types of dose (exposure).

• Depth dose* is the amount of radiation absorbed at a certain depth below the surface.
• -*Since intensity of radiation from a point source of radiation decreases proportionally to the squared distance from the source, the depth dose (Dd) observed at a depth d is related to the surface dose (Ds) as:

where FS = distance between the focus of the X ray tube and the body surface

The implication of this relationship is that objects closer to the source are more exposed to x-rays than objects further away.

• Geometrical considerations play an important role in x-ray therapy
• To avoid superficial damage, high energy photons must be positioned sufficiently distant from the patient;
• In the treatment of superficial lesions, deep tissue damage is avoided by using low energy (low accelerating voltage) photons near to the skin.
• Physicians and technicians must be protected by shielding. The radiation hazard has to be checked by film dose meters.

Question 9

Principle of computed tomography, Roentgen methods

Answer 9

• Computed tomography is the most advanced x-ray imaging technique and has a high resolving power and produces a higher contrast
• produces an image of a single layer (crossection) of the body
• The body cross-section to be examined is divided into small pixels of size less than 1 mm
• A narrow x-ray beam is passes through the body and it’s intensity is measured by a photomultiplier.
• The value of attenuation coefficients of each individual pixel is calculated.
• This allows for the calculation of the emerging intensity I and subsequent image production by a computer.
• produces a detailed image with good contrast, results are stored electronically, radiation load for patient is the same as traditional x-ray

How it works:

• X-ray slices data is generated by using a X-ray source which rotates around the body
• On the opposite side of the X-ray source are X-ray sensors (photomultipliers)
• Through very thin x-ray beams a large quantetiy of data can be collected
• This data is reconstructed to a range of crosssections through tomographic reconstruction
• Tomographic reconstruction uses algrorithms based on the Radon transformation