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Medical Imaging

X-Ray Production
  • X-rays are a form of electromagnetic radiation
  • They are produced when high-speed electrons hit metal targets
  • Production of X-rays:
    • Heated filament undergoes thermionic emission, releasing high-speed electrons
    • P.d. between cathode and anode causes electrons to accelerate
    • Electrons bombard metal target, emitting x-rays which leave through the window
    • Some kinetic energy of electrons transferred into the metal target as thermal energy
    • The metal target is cooled by water or spun around to increase target area
  • Tube Current: is the rate of arrival of electrons at metal target
  • Some ‘soft’ x-rays are always produced which cannot fully pass through the patient and contribute to the total radiation dose of the patient
  • to reduce the radiation dose, hence cut off ‘soft’ x-rays, an aluminum filter should be used to absorb them
X-ray Spectrum
  • X-rays emerge from the tube with a range of energies as represented in the spectra below
  • The spectra is made up of two components:
    • Braking Radiation: X-ray photons released when electrons decelerate as it strikes the anode, they are attracted by the nucleus of an atom in the anode and loses energy
    • Characteristic Radiation: Rearrangement of electrons in the anode when a high speed electron strikes, excites orbital electrons which then de-excite, emitting photons and giving rise to spectrum lines; specific to the material of the anode
X-ray Imaging
  • X-ray radiation blackens photographic plates in the same way as visible light
  • Degree of blackening depends on total x-ray exposure
  • It is mostly used to distinguish bones from tissue because bones have a higher density than the surrounding tissue
  • It can be used to identify organs if the densities of the surrounding tissues are sufficiently different
X-ray Quality
  • Sharpness is the ease with which the edge of a structure can be determined
    • Improving sharpness of x-ray:
      • Reduce the area of target anode
      • Reduce aperture size (window): This reduces the beam width
      • Place lead grid in front of photographic film: This absorbs scattered x-rays and reduces partial images
  • Contrast is the visual difference between the areas of blackening and light
    • Improving contrast of x-ray:
      • Increase the exposure time
      • Use harder x-rays: This increases penetration power
      • Reduce scattering of x-ray beam
      • Use fluorescent ‘contrast medium’
X-ray Intensity
  • In the medium where x-rays are absorbed, intensity of a parallel x-ray bean decreases by a constant fraction in passing through equal small thickness of the medium
  • This gives rise to an exponential decrease in the intensity of the transmitted beam:
    • I = I0e-μx
    • I is the instantaneous intensity of the x-ray beam
    • I0 is the initial intensity of the x-ray beam
    • x is the thickness of the medium passed by x-ray
    • μ is the linear absorption coefficient unique to medium, dependent on photon energy and in
      mm-1
  • This can be represented graphically:
  • Half-value Thickness (x½): is the thickness of the medium required to reduce the transmitted intensity to one half of its initial value
  • It can be related to the linear absorption coefficient by:
    • x½ × μ = ln 2
Computed Tomography (CAT/CT Scan)
  • X-ray imaging only produces a 2-dimensional image with no impression of depth, it cannot tell if a tissue is near to the surface or deep within the body
  • Tomography is a procedure which forms a 3-dimensional plane of the object
  • The diagram below shows the procedure
  • Putting together several planes produces a whole 3-dimensional image which can be rotated
Voxel Development in CT Scans
  • Voxel is a small cube in a three-dimensional image
  • The number in each square is the density that the computer will register for that section of the object
  • As the scanner goes around each part, it has a different density which the computer can model
  • The computer then puts these together to form a 3-dimensional shape
  • Building the image
  • For a well-defined image in a CT scan, we need voxels to be small and to do so:
    • X-ray beams must be well collimated so that it consists of parallel ray — rays must not spread
    • The detector must consist of a regular array of tiny detecting elements — the smaller, the detector the better the image
  • Advantages of a CT Scan:
    • It produces images that show 3-dimensional relationships between different tissues
    • It can distinguish tissues with quite similar densities
Ultrasonic Waves
  • Ultrasound is any sound wave that has a frequency above the upper limit of human hearing, 20 kHz
  • Piezo-electric transducers can be used to record and produce ultrasonic waves
  • A diagram of a Piezo-electric transducer:
  • Quartz crystal is the macromolecule formed by SiO2
  • Generating Ultrasonic Waves:
    • Charged atoms of a transducer in an electric field move closer to oppositely charged plates and the overall crystal either compresses or extends:
    • When an alternating voltage with frequency f is applied to the crystal, it causes it to contract and expand at the same frequency of f
    • This acts as the vibrating source of ultrasound waves
  • Receiving Ultrasonic Waves:
    • Ultrasonic waves change pressure in a medium
    • Charged atoms in the crystal shift position closer to the plates
    • Opposite charges are induced in the silver plates
    • Induced potential difference across the plates
    • Potential difference fluctuates which can be processed
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 partially reflected
  • From the law of conservation of energy:
    • I = IR + IT
  • Specific Acoustic Impedance (Z): is the product of the density of the medium and the speed of sound in the medium
    • Z = ρc
  • Between two media, the difference in acoustic impedance determines the fraction of incident intensity that is reflected
  • Intensity Reflection Coefficient (α): is the ratio of the intensity of the reflected wave and the intensity of the incident wave
    • α = IR/I = (Z2 – Z1/(Z2 + Z1
  • Comparing acoustic impedances (IR/I):
    • A very large fraction is reflected at the air-tissue boundary
    • A large fraction is reflected at the tissue-bone boundary
    • A very little fraction is reflected at the boundary between soft tissues
  • A gel is applied before carrying out scan because when the 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 are very similar
Attenuation of Ultrasonic Waves
  • Similar to x-rays, ultrasonic waves are also absorbed by the medium in which they are passing through
  • They also follow the same decay equation as x-rays:
    • I = I0e-kx
    • Note: The constant is different
    • k is called the absorption coefficient
Ultrasound Imaging Procedure
  • A transducer is placed in contact with the skin and a gel acting as a coupling medium
  • Pulses of ultrasound are directed into the body
  • The wave is reflected at the boundary between tissues
  • The reflected pulse is detected and processed
  • The time for return of echo gives information on the depth
  • The amount of reflection gives information on the structures
  • Two techniques for display:

    • A-scan: Measures the distance of different boundaries from the transducer, held in one position graphically
    • B-scan: Repeats A-scan from multiple angles to form a 2-dimensional image
Nuclear Magnetism
  • Atomic nuclei with an odd number of protons behave as tiny magnets when introduce to a magnetic field
  • Hydrogen nucleus (proton) is used most because it is abundant in all organic tissues
  • When there is no magnetic field;
    • All protons are aligned randomly
  • When a magnetic field is introduced:
    • Most protons align themselves with ‘north’ facing ‘south’ — stable low energy state
    • Some protons align themselves in the opposite way with ‘south’ facing ‘south’ — unstable high energy state
Nuclear Magnetic Resonance
  • Aligned protons are not stationary; the spin on their axis of rotation
  • Atomic Spin: is a fundamental property (like charge or mass) of a subatomic particle that defines how the particle rotates on its axis
  • Precession: The movement of the axis of rotation of a spinning object (proton) around another external axis
  • Larmor Frequency (ω0): The angular frequency of the circular path of precession of the object (proton)
  • MRI Scanners use a very strong external magnetic field causing ω0 to be in the radio frequency range
Relaxation Times
  • The protons in high energy state are unstable so they must ‘relax’ and come back to their lower energy state
  • The excess energy is transmitted back as radio waves which can be detected
  • The time taken for these radio waves to be detected determines the relaxation time
  • Relaxation Time: is the time taken for a nucleus to fall back to a lower energy state
  • Relaxation times depend on the environment of the protons:
    • Water and watery tissues: Several seconds — high
    • Fatty tissues: Hundreds of milliseconds — low
    • Cancerous tissues: intermediate
MRI Scanning
  • A large uniform magnetic field causes all protons in the body to have the same Larmor frequency
  • A non-uniform magnetic field is applied to locate a particular position of a proton within a person
  • Procedure of an MRI:
    • A strong, constant magnetic field is applied along the body
    • Hydrogen nuclei precess about the direction of the field
    • A radio frequency (r.f.) pulse is applied
    • The pulse is at the Larmor frequency which causes resonance in the hydrogen nuclei
    • On relaxation, the nuclei de-excite and emit a pulse of r.f.
    • R.f. pulses are detected, processed and displayed
    • A calibrated non-uniform field enables position of nuclei to be located and for location of detection to be changed
Comparing Medical Imaging Methods

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