PHYSICS A2 SLIDES ON MEDICAL IMAGING

1. MEDICAL IMAGING

2. X-RAY PRODUCTION
 Principles of production of an X-ray beam.
o Electrical current is run through the tungsten
filament, causing it to glow and emit
electrons.
o A large voltage difference (kV) is placed
between the cathode and the anode, causing
the electrons to move at high velocity from
the filament to the anode target.
o The high speed electrons strike the target and
rapidly decelerated on impact, suddenly x-
rays are emitted.

5. CONTINUOUS OF X-RAY
SPECTRA
 Continuous x-rays is electromagnetic radiation produced
by the deceleration of a charged particle when deflected
by another charged particle, typically an electron by an
atomic nucleus.
 The moving particle loses kinetic energy, which is
converted into a photon because energy is conserved.
 This is the Continuous x-rays or Bremsstrahlung rays and
has a continuous spectrum.

7. CHARACTERISTIC
OF X-RAY SPECTRA
 The second type of spectra, called the characteristic
spectra, is produced at high voltage as a result of specific
electronic transitions that take place within individual
atoms of the target material.
 The easiest to see using the simple Bohr model of the
atom.
 In such a model, the nucleus of the atom containing the
protons and neutrons is surrounded by shells of electrons.

10. TUBE CURRENT- rate of arrival of electrons at metal target
The intensity of the X-ray beam is determined by the
rate of arrival of electrons at the metal target, that
is, the tube current.
This tube current is controlled by the heater current
of the cathode.
The greater the heater current, the hotter the
filament and hence the greater the rate of emission
of thermo-electrons.

11. HARDNESS OF X-RAY BEAM
 The hardness of an X-ray beam refers to its penetration power.
 The hardness is controlled by the accelerating voltage between the cathode
and the anode.
 More penetrating X-rays have higher photon energies and thus a larger
accelerating potential is required.
 Referring to the spectrum of X-rays produced, it can be seen that longer
wavelength X-rays (‘softer’ X-rays) are also produced.
 These X-ray photons are of such low energy that they would not be able to
pass through the patient.
 They would contribute to the total radiation dose without any useful purpose.
 Consequently, an aluminium filter is frequently fitted across the window of
the X-ray tube to absorb the ‘soft’ X-ray photons.

12. ATTENUATION- decrease in intensity
 When x-ray is absorbed in medium, intensity of parallel x-
ray beam decreases by constant fraction in passing
through equal small thicknesses of medium

13. Half-value Thickness (HVT)
 The half-value thickness x½ or HVT is the thickness of the medium required
to reduce the transmitted intensity to one half of its initial value.
 It is a constant and is related to the linear absorption coefficient μ by the
expression
 x½ μ = ln2.
 In practice, x½ does not have a precise value as it is constant only when the
beam has photons of one energy only.

14. IMPROVING X-RAY IMAGES
 Three main aims:
 To reduce as much as possible the patient's
exposure to harmful x-rays.
 To improve the sharpness of the images – finer
details can be resolved.
 To improve the contrast of the image – the
different tissues under investigation show up
clearly.

15. REDUCING DOSAGE
 X-rays can damage living cell.
 So, intensifier screens are used.
 These are sheets of a material that contains a
phosphor, a substance that emits visible light
when it absorb x-rays photon.
 The film is sandwiched between two intensifier
screens.
 Each x-rays photon absorbed result in several
thousand light photons catch then blacken the
film.

16. Quality of the Image
 Sharpness is concerned with the ease with which the edges of structures can be
determined. A sharp image implies that the edges of organs are clearly defined.
To Obtain Sharp Images
1.The X-ray tube is designed to generate a beam of X-rays with minimum width.
Factors in the design of the X-ray apparatus that may affect sharpness include:

17.  2.the size of the aperture, produced by overlapping metal plates, through
which the X-ray beam passes after leaving the tube (see Fig. 2.4),

18.  3.the use of a lead grid in front of the photographic film to absorb scattered
X-ray photons, as illustrated in Fig. 2.5.

19. To Obtain Good Contrast
 Use a ‘contrast medium’.
 For example, the stomach may be examined by giving the patient a drink
containing barium sulphate.
 Similarly, to outline blood vessels, a contrast medium that absorbs strongly
the X-radiation would be injected into the bloodstream.
 The contrast of the image produced on the photographic film is affected by
 exposure time,
 X-ray penetration and
 scattering of the X-ray beam within the patient’s body.
 Contrast may be improved by backing the photographic film with a
fluorescent material.

20. Computed Tomography (CAT/CT Scan)
 Purpose:
●X-ray imaging only produce 2-Dimensional image with no impression of depth, cannot
tell if tissue is near to the surface or deep within the body
●Tomography is a procedure which forms 3-D image of the object
The word ‘tomo’ means crossectional or slice

21. Principles of CT Scan
 X-rays is directed from different angles
 X ray taken of slice/plane/section are repeated at different angles
 These images are combined and processed
 Combined images give 2-D image of slice which is formed on the computer
screen
 The procedure is repeated for successive slice to build up 3-D image
 Image can be viewed from different angles/rotated

22. Voxel Development in CT Scan
 The section (or slice) through body is divided up into a small units called voxels.
 The image of each voxel would have a particular intensity, called pixels.
 The pixels is the density that the computer will register for that section of the
object.
 As the scanner goes around, each part has
different density which the computer can
model

23. Building the Image

29.  For a well-defined image in a CT Scan, we need voxels to be small.
 How?
● X-ray beams must well be collimated so that it consists of parallel ray
- rays must not be spread
●Detector must consists of regular array of tiny detecting element
-the smaller the detector the better the image

30. Advantages of CT Scan
 Produces images that show 3-Dimensional relationships between different
tissues
 Can distinguish tissues with quite similar densities (attenuation coefficients)

48.  Generating Ultrasonic Waves: (electrical to ultrasound)
 A quartz crystal with the two sides coated with silver is used to act as electrodes
 When a p.d. is applied across it, it expands to generate sound wave
 Charged atoms of a transducer in an electric field move closer to oppositely charged plates (positive
silicon ions to cathode and negative oxygen ions to anode)
 When a constant alternating voltage is applied to the crystal, it causes the crystal to contract and
expand, making it vibrate at the same frequency with maximum amplitude (resonance)
 This acts as the vibrating source of ultrasound waves
 Receiving Ultrasonic Waves: (ultrasound to electrical)
 Ultrasonic waves change pressure in medium. When the crystal contracts, a p.d. is generated.
 Charged atoms in crystal move towards plates
 Opposite charges induced in the silver plates
 Induced potential difference across the plates
 Potential difference fluctuates which can be amplified and processed

51. Reflection of Ultrasonic Waves
 Ultrasound requires ultrasonic waves to pass from one medium to another
 When a beam of ultrasound wave reaches a boundary between two different media,
the beam is partially refracted and reflected.
 Reflected waves are used to construct an image of the body
 Refracted waves allow transmission of ultrasound from transducer to medium and
vice versa
 I = IR + IT

53. Intensity Reflection Coefficient
Definition: Ratio of intensity of reflected wave and intensity of incident wave
-Images are clearer if there is a strong reflection (large difference in acoustic
impedance at the reflected boundary)
Comparing acoustic impedances:
• Very large fraction reflected at air-tissue boundary
• Large fraction reflected at boundary between soft tissues
• Very little reflected at boundary between soft tissues (fat and muscle) because most has been
absorbed
A gel is applied before carrying out scan because when wave travels in or out of the body there is :
• Very little transmission at an air-skin boundary
• Almost complete transmission at a gel-skin boundary because acoustic impedance of gel and skin
very similar, this allows transmission of wave from medium back to transducer

68. EXAMPLE 1: ULTRASOUND IMAGING PROCEDURE
 Explain the main principles behind the use of ultrasound to obtain diagnostic
information about internal body structures
 Transducer is placed in contact with skin and a gel acting as a coupling
medium
 Pulses of ultrasound are directed into the body
 The wave is reflected at boundary between tissues
 The reflected pulse is detected and processed
 The time for return of echo gives information on depth
 Amount of reflection gives information on structures

70. MRI
Magnetic resonance imagining

71. LEARNING OUTCOMES

72. INTRO (HUMAN BODY)
 Made up of water molecules
 Consist only H & O₂ atoms
 Fat also contains H atoms
 We are made up of 60% of H atoms
 Nucleus of H atom is a proton – very sensitive to the magnetic field
 Nucleus of a H behaves like a tiny magnet

74. PRINCIPLES
 Hydrogen nucleus (proton) behave as a tiny magnet
 When a large / strong uniform magnetic field applied, protons will line up in the field with
most line up with their N facing S (stable low energy state) and If N facing N OR S facing S
(unstable higher energy state)
 Pulses are applied at RF waves which cause the protons to resonate
 External magnetic field applied causes nuclei to precess
 H atoms give off RF waves
 RF detected and processed
 To give positions of H atoms
 Non uniform field enables
 Positions of resonating atoms to be defined

75. PRECESSION
 Protons are not static when they align with the field
 Magnetic axis rotates around the direction of external field
 gyration action
 Depends on individual nucleus & magnetic flux density,B₀
 Stronger the external field, the faster the proton precess about it
 Angular frequency of precession is called the Larmor frequency, ω₀

76. FREQUENCY
 Frequency of radio waves applied (Larmor frequency)
 Causes the nuclei to resonate and flip into higher energy state
 Depends on the strength of the magnetic field where each nucleus is
 Scanner can workout the location of each nucleus

77. RELAXATION TIMES
 Time taken for a nucleus to fall back to a lower energy state
 When RF waves are switched off & protons gradually relax into their lower energy
state
 The protons in higher energy state are unstable so must ‘relax’ and come back to
lower energy state
 The excess energy is transmitted back as radio waves which can be detected
 The time taken for the waves to be detected determine the relaxation times
 Depends on : water and watery tissues
fatty tissues
cancerous tissues
Different tissues can be distinguished by different rates at which they release energy
after they have been forced to resonate

79. Formula
Larmor frequency
Gyromagnetic ratio
(2.68 𝑥 108
𝑟𝑎𝑑 𝑠−1
𝑇−1
)
Magnetic flux density

80. MRI SCANNER
 Main features :
1. A large superconducting magnet
2. An RF coil that transmits RF pulses to the body
3. An RF that detects the signal emitted by the relaxing protons
4. A set of gradient coils
5. A computer

81. PROCEDURE
 Before :
 Eat, drink & take your medicine as usual ( except magnetic resonance
cholangiopancreatography, MRCP)
 Remove any mtal objects from your body – wear a hospital gown
 Injection (a special dye – as contrast agent )

82.  During :
 A friend/ family member maybe allowed to stay in the room with you (follow
the same clothing guideline)
 Lasts between 15-90 minutes
 Scanner will make a loud sound – caused by the magnet – given earplug
 Keep your body still to avoid images being blurred

83.  After :
 Can resume their normal activities as usual immediately
 Someone should stay with you if you decide to sedated
 Drink a lot of water for the following 24hours

84. ADVANTAGES
 Do not involve exposure to ionizing radiation
 Showing soft tissues structures
 Provide info about how the blood moves through certain organs & blood
vessels
 Painless
 No moving mechanisms
 No short/ long term effects demonstrated

85. DISADVANTAGES
 Very expensive
 Long scan times
 Audible noise (65-115dB)
 Claustrophobic
 Can be affected by movement
 Metallic object in the patient can be heated
 Heart pacemaker can be affected

87. EASY RIGHT?
SO NOW LET’S MOVE ON FOR A
SHORT EXERCISE

89.  Large/ strong uniform magnetic field applied
 Pulses applied at RF waves
 Causes the protons/ H atoms to resonate
 H atoms give off RF waves
 RF detected and processed
 To give positions of H atoms
 Non uniform field enables
 Positions of resonating atoms to be defined
SOLUTION

91.  For nuclei spin/ precess
 Spin/ precess about the direction of magnetic field
 Frequency of precession depends on magnetic field strength
 Large field means frequency in radio frequency range
 Means frequency of precession are different in different regions of subject
 Enables location of precessing nuclei to be determined
 Enables thickness of slice to be varied / location of slice to be changed