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:
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:
- Specific Acoustic Impedance (Z): is the product of the density of the medium and the speed of sound in the medium
- 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)²/(Z2 + Z1)²
- 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