X-ray Production and X-ray Tubes

X-ray Production

• X-rays arise when charged particles decelerate rapidly, transforming kinetic energy into high-frequency photons of electromagnetic radiation
• X-rays and gamma rays share overlapping frequency spectra, distinguishable only by their origin
• Gamma rays come from radioactive decay or particle collisions with mass defects
• X-rays are produced by Bremsstrahlung, or braking radiation, due to charged particle acceleration
• Soft X-rays, used in medical imaging, generally have lower energies compared to gamma rays

X-ray Tubes

• X-ray tubes accelerate electrons in a high-voltage electric field and then rapidly decelerate them through collisions with a hard metal anode like tungsten
• Electrons are emitted from a heated filament (cathode) into a vacuum tube via thermionic emission, which prevents collisions with air molecules
• An external power supply sets up a potential difference of up to 200kV between the cathode and anode; Therefore, giving electrons kinetic energy of up to 200keV
• About 1% of electrons' kinetic energy is emitted as X-rays when rapidly decelerated
• The rest of the kinetic energy is lost as thermal energy in the anode
• Anode overheating is prevented by either anode rotation or cooling it with a circulating water supply
• X-rays are emitted in every direction, so a straight, parallel, or collimated beam is generated to direct X-rays at specific body parts
• The material encasing the tube is thinner in one area, referred to as the window, to allow X-rays to emerge
• The X-ray beam is directed into a series of parallel metal tubes called a collimator, which absorbs rays not parallel to the tube's axis

X-ray Spectra

• Braking radiation produces a broad range of X-ray wavelengths with a hump-shaped intensity profile
• Characteristic radiation consists of sharp lines not caused by decelerating electrons
• The sharp lines are caused by incident electrons knocking out bound low energy electrons in the anode atoms
• Higher energy electrons transition to the unoccupied shell, emitting excess energy as radiation
• Photons produced via the electron transition have specific wavelengths, increasing the X-ray number and intensity at these energies

Ionising Radiation

• X-rays, gamma rays, and UV rays are high energy and can ionise matter by causing atoms to emit electrons
• X-rays can ionise DNA and other tissues in living cells, potentially leading to harmful mutations
• Tissues should be exposed to low-intensity beams for a short amount of time to minimise damage
• X-rays' ability to destroy cells can be used in the treatment of cancer (radiotherapy)

X-ray Attenuation Mechanisms

• As X-rays pass through matter and ionise it, they lose some or all of their energy to the atoms or molecules they interact with
• This energy loss causes a gradual decrease in intensity

Intensity and Attenuation

• The intensity (power per unit cross-sectional area) of the X-ray beam is known as attenuation
• Different materials attenuate X-rays differently, allowing tissues to be contrasted by measuring the intensity of the attenuated beam
• Bone greatly attenuates X-rays more than soft tissues; Therefore, an X-ray beam passing through bone shows greater attenuation directly behind it
• Photographic film will be blackened less in areas where it is in the path of X-rays that traveled through bone
• Digital detectors are now used for easier image processing, storage, and transfer
• The intensity of a collimated X-ray beam decreases exponentially

Attenuation Equation

• The attenuation of an X-ray beam in a medium is evaluated by: I = I₀e^(-μx)
• I₀ is the initial intensity
• I is the attenuated intensity
• x is the thickness
• μ is the attenuation or absorption coefficient

Absorption Mechanisms

• Simple Scattering: 1-20 keV X-rays reflect off layers of atoms or molecules because they lack the energy for more complex processes
• Photoelectric Effect: X-rays less than 100 keV are absorbed by electrons, releasing a photoelectron
• Compton Effect: 0.5-5 MeV X-rays lose only a fraction of their energy to electrons due to an inelastic interaction
• The scattered X-ray photon has a longer wavelength and less energy than before, and the Compton electron is scattered in a different direction
• Pair Production: X-ray energy greater than 1.02 MeV produces an electron-positron pair in the electric field of an atom, followed by the positron annihilation with another electron to produce photons
  • Because photon energies are not high enough, pair production is not as important as other mechanisms in medical X-rays

X-ray Imaging

• Contrast media are high attenuation coefficient materials with heavy atoms and a large number of electrons
• Contrast media are easily identified on X-ray images as they cause lower detected intensity; Similar to bone
• Common contrast media:
  • Barium (Z = 56)
  • Iodine (Z = 53)
• Barium and iodine absorb X-rays far better than soft tissues, which have low average proton numbers (Z ≈ 7) and so low attenuation coefficients
• The relationship between the absorption coefficient (μ) and the proton number is largely defined by the photoelectric effect: μ ∝ Z³
• Contrast media can be around 500 times better absorbers than surrounding tissue because the attenuation coefficient is proportional to the proton number cubed

Iodine

• Iodine: a contrast medium in liquids to observe blood flow
• An organic iodine compound is injected and the patient is exposed to X-rays
• Areas with low attenuated intensities identify healthy blood flow, while areas with little attenuation locate poor blood flow

Barium Sulphate

• Barium sulphate: used as a contrast medium in the digestive system
• The patient swallows a white liquid mixture (a 'barium meal')
• Areas with lower measured intensities outline the intestines and can be used to locate blockages

Computerised Axial Tomography (CAT)

• CAT is an effective way of examining the internal three-dimensional structure of a patient
• Conventional X-ray images are cheap and quick but only provide a two-dimensional image, unable to distinguish overlapping bones or different soft tissues
• CAT scanners record multiple 2D X-ray images and create a 3D image with computer software
• Yields higher resolution images and distinguishes differing soft tissues
• CAT scans take significantly longer and expose the patient to a far greater dose of ionising radiation
• X-ray tube generates a fan-shaped beam directed onto the patient, a ring of electronic detectors opposite detects the X-ray beam intensity

CAT Scan Process

• Information about X-ray intensity is converted into electrical signals
• The signals are processed to reconstruct the tissues the beam has passed through
• The X-ray tube and detectors rotate about the patient and move up and down their length
• A full 3D image of the patient’s body is created when all images of each slice are stitched together
  • The image can then be displayed on a computer monitor and analysed

Medical Tracers

• Medical tracers combine radioactive isotopes with specific elements to form compounds that collect in particular body locations
• Also known as Radiopharmaceuticals
• Used in both diagnosis and therapy

Non-Invasive Diagnosis

• Sources have to be placed inside the patient’s body and their emissions detected from the outside
• Gamma-emitters are the most useful due to least ionising and more penetrative properties
• Beta and alpha emitters are more ionising and cause significant damage
• Radioisotopes used in medicine tend to have high activities and short half-lives minimize imaging time and radiation dosage to patient
• Many short half-life radioisotopes are produced artificially on-site so they can be used almost immediately

Flourine-18

• Flourine-18 source used in Positron Emission Tomography (PET)
• It undergoes beta plus decay, forming a positron from a proton, releasing a neutron in the nucleus
• (ref. Nuclear and Particle Physics 6.4).
• The positron annihilates with an electron in the patient’s body, forming a pair of gamma photons that are detected to locate the F-18 source
• F-18 has a half-life of approximately 110 minutes and is produced through the nuclear transformation of oxygen-18
• i.e. O-18 is bombarded with protons in a particle accelerator
• Tracers of F-18: sodium fluoride is used for skeletal imaging, displaying rapid bone uptake

Technetium-99m

• Technetium-99m is a versatile radioisotope used to monitor many major organs
• The "m" stands for metastable (remains in a high energy state for prolonged periods of time)
• When it decays via gamma emission (half-life ~ 6 hours) it then forms Tc-99 which is stable with a half-life of 210,000 years
• Ensures the patient receives limited ionising radiation
• The photon is released with an exact energy of 140 keV
• Tc-99m can be combined to form NaTcO₄, when injected, targets brain cells to image the brain

Gamma Cameras

• Gamma cameras detect gamma photons emitted from medical tracers within the body
• Since gamma rays travel in every direction, a collimator is needed for accurate location tracing
• A collimator is made of parallel honeycomb-shaped tubes, absorbing photons not aligned with the tube's axis
• The collimator is made of a high-density metal to ensure gamma absorption

Gamma Camera Process

• Collimated photons hit a scintillation crystal (e.g. sodium iodide)
• Each photon produces thousands of visible photons
• Visible photons are directed onto a photocathode
• A photocathode produces an electron for each visible photon detected
• The electron enters a photomultiplier tube
• Dynodes amplify the signal
• The position of impact in the scintillator locates the emission site
• A computer detects and displays the signal

Positron Emission Tomography (PET)

• Using conventional X-rays and CAT scans, a gamma camera produces only a 2D low resolution
• A PET scanner is a ring of gamma cameras placed around the patient: an accurate 3D image is generated based on the emission site of the gamma photons
• F-18 emission results in a pair of gamma photons emitted in opposite directions to obey momentum conservation
• The photons are detected by diametrically opposed detectors in the ring, and their arrival times are recorded
• The exact location of the annihilation event is calculated based on the detectors' arrival times (since speed of light is known)
• The site of the tracer is estimated (annihilation occurs soon after beta emission)
• This repeats until a 3D model of the tracer locations is produced
• The tracer density can be determined from the photon rate and tracer uptake
• Uses fluorodeoxyglucose (F-18 substituted glucose) to locate cancerous tumours and active parts of the brain
• A non-invasive technique that accurately demonstrates organ function, used to observe medication effects
• PET is expensive and requires on-site synthesis of tracers

Ultrasound

• Ultrasound is a longitudinal sound wave with a frequency greater than human hearing range (above 20 kHz, typically 5MHz in medical diagnosis)
• Can be refracted, reflected, Doppler shifted, and diffracted
• Properties of the inspected media can be calculated from these characteristics
• Diffraction can be used to identify apertures or features of a few millimetres in size
• Quick and affordable, non-ionising and non-invasive
• Useful for finding the boundary between two media
• A transducer in the ultrasound device produces electrical signals from the soundwaves

Ultrasound Image Processing

• Electrical signals are analyzed by computer software
• An image is generated and displayed

Piezoelectric Effect

• Piezoelectric Material generates a voltage when it is contracted or expanded or will contract and expand if a voltage is applied
• Applying a voltage to a piezoelectric crystal generates ultrasound vibrations
• A piezoelectric crystal absorbing ultrasound will produce an alternating voltage
• Piezoelectric crystals: made from Quartz, polymeric or ceramic Material
• Ultrasound transducer:
  • Has an alternating potential difference (causes repetitive compression and stretching)
  • Using resonant Crystal increases the Intensity
  • Once created, the ultrasound's potential differences is turned off

Ultrasound Scans

• A scan (simplest type): a single transducer emits and receives the reflected signal to determine the distance to a boundary between two media
• the time delay between the signal and its reception is measured to estimate the distance
• the approximate speed of sound is used
• B scan (more complex): a more complex scan creates a 2D image by moving the transducer over the patient's skin
• each position measures a time from signal production to reception (distance to the reflection point)
• B scan is a series of A scans stitched together
• Ultrasound waves are pulsed to allow time for a wave being received
• Smaller wavelengths give more detailed images
• sound waves are allowed to diffract around finer points of detail on the object under inspection

Acoustic Properties of Ultrasound

• Acoustic impedance, Z: product of the density (ρ) and speed of sound (c) in a medium
• Measured in units of kgm⁻²s⁻¹
  • Z = ρc
• At a boundary between two media, the 𝐼r (wave intensity) is reflected, the rest is transmitted
• The fraction reflected depends on acoustic impedances of the media
• A sound wave travelling through a medium of impedance (Z₁) incident on in a medium if a second medium has impedance Z₂, the fraction of the original wave intensity (I₀) is reflected
• Ir/ I₀ =(Z₂ − Z₁)²/ (Z₂ + Z₁)²

Reflection/Transmission

• When Z₁ is similar to Z₂, the the fraction of the wave intensity that is reflected 𝐼r is small so most energy is transmitted
• When Z₁ is very different from Z₂, then most of the fraction of the wave intensity that is transmitted 𝐼r is small so most energy is reflected
• If operating in air next to skin: most sound is reflected due to different impedances (99.9% reflection)
• To maximize the transmission into the patient and maximize the detail in reflection an impedance matching gel that is familiar to skin

Doppler Effect

• The Doppler Effect is the change in wave frequency due to a moving source
• Doppler imaging: non-invasive, measures the speed of blood flow
  • Ultrasound waves are sent into a blood vessel
  • Iron in blood, reflects the wave back to the transducer
  • Depending on the speed and direction: the ultrasound wave is shifted up or down Δf = 2fv cos θ/c
  • Δf : is the OBSERVED frequency shift
  • f : is the ORIGINAL frequency,
  • v : is the speed of FLOW in a known direction
  • c : is the speed of wave
  • θ : is the angle between the bean and the direction of BLOOD FLOW
• A typical shift from a 5-15 MHz ultrasound is around 3 kHz, which can be easily detected
• This can be used to demonstrate blood flow through veins, arteries, reveal clots, narrowing and calculate the volume of blood flow