X-ray Production and X-ray Tubes

X-ray Production

• X-rays are produced when charged particles decelerate rapidly, converting kinetic energy to high-frequency photons.
• X-rays and gamma rays share overlapping frequency spectra.
• Gamma rays originate from radioactive decay or particle collisions, involving a mass defect.
• X-rays result from Bremsstrahlung (braking radiation) due to charged particle acceleration.
• Soft X-rays are X-rays used in medical imaging that typically exhibit lower energies compared to gamma rays.

X-ray Tubes

• X-ray tubes generate X-rays by accelerating electrons in a high-voltage electric field and then rapidly decelerating them through collisions with a hard metal anode, such as tungsten.
• Electrons are emitted from a heated filament (cathode) into a vacuum tube via thermionic emission.
• Thermionic emission describes electrons being emitted from a heated source.
• The vacuum tube prevents collisions between electrons and air molecules
• An external power supply creates a potential difference of up to 200kV between the cathode and anode.
• Electrons gain kinetic energy of up to 200keV.
• Electrons rapidly decelerate upon collision, emitting ~1% of their kinetic energy as X-rays, while the rest converts to thermal energy in the anode
• The anode is either rotated to expose new areas to the electron beam or cooled with circulating water to prevent overheating.
• X-rays are emitted in all directions from the anode
• A collimated beam is more useful and can be directed at specific areas to minimize exposure; a vacuum tube is encased in material with a thin window to direct X-rays
• A collimator further refines the beam by absorbing non-parallel rays.

X-ray Spectra

• Braking radiation produces a broad range of X-ray wavelengths, forming a hump-shaped intensity profile.
• Sharp lines of characteristic radiation result from incident electrons knocking out low-energy-level electrons in anode atoms.

Ionising Radiation

• X-rays, gamma rays, and UV rays can ionise matter by causing electrons to be emitted from atoms, which is a process better known as the photoelectric effect.
• X-rays can ionise DNA.
• Minimizing exposure time and using low-intensity beams helps to limit organism damage and harmful gene mutations
• Radiotherapy utilises X-rays to target and destroy cancerous cells.

X-ray Attenuation Mechanisms

• X-rays lose energy as they ionise matter.
• Attenuation is the gradual decrease in intensity as X-rays pass through matter that relies on measuring the intensity of the attenuated beam of X-rays once it has passed through the patient
• Bone attenuates X-rays more than soft tissues
• Digital detectors are used as the images are easier to process, store, and transfer

Exponential Intensity Decrease

• The intensity of a collimated X-ray beam decreases exponentially. If intensity halves in 1cm of bone, it quarters in 2cm and reduces to one-eighth in 3cm.
• Attenuation Formula: 𝐼 = 𝐼₀ * 𝑒^(-𝜇𝑥), where 𝐼₀ is initial intensity, 𝐼 is attenuated intensity, 𝑥 is thickness, and 𝜇 is the attenuation coefficient.
• The attenuation coefficient describes how well the medium absorbs X-rays

Absorption Mechanisms

• Simple Scattering: X-rays of 1-20 keV reflect off layers of atoms/molecules due to insufficient energy for complex processes.
• Photoelectric Effect: X-rays (<100 keV) are absorbed by electrons with matching ionisation energies, releasing a photoelectron.
• Compton Effect: X-rays of 0.5-5 MeV lose only a fraction of energy to electrons due to inelastic interaction, causing a change in wavelength.
• Pair Production: X-rays (>1.02 MeV) passing through an atom's electric field produce an electron-positron pair via mass-energy conversion. Positron-electron collision leads to annihilation producing photons. This process is not very important in medical X-rays as the photon energies are usually not high enough to produce an electron-positron pair.

X-ray Imaging Contrast Media

• Contrast media are high attenuation coefficient materials that have heavy atoms with a large proton number and a large number of electrons.
• Contrast media causes a lower detected intensity, like bone, and are easily identified
• Barium, with Z=56, or iodine, with Z=53, absorb X-rays better than soft tissue
• Soft tissues have low average proton numbers (Z ≈ 7) and low attenuation coefficients.
• The absorption coefficient u ~ Z^3.

Contrast Media Absorption

• Contrast media can be up to 500 times better absorbers via an attenuation coefficient proportional to the proton number cubed.
• Iodine is a contrast medium used in liquids to observe blood flow.
• Barium sulphate is a contrast medium used in the digestive system.

Computerised Axial Tomography (CAT)

• Conventional X-ray images are cheap and quick, but produce 2D images that cannot distinguish overlapping bones or different soft tissues.
• CAT scans use X-ray imaging to examine internal 3D structures.
• CAT scanners record 2D X-ray images and construct 3D images with software.
• CAT scans have higher resolution and can distinguish differing soft tissues, but expose the patient to a greater dose of ionising radiation.
• CAT scanners use an X-ray tube that generates a fan-shaped beam.
• Electronic detectors detect X-ray beam intensity, which turns into electrical signals and reconstructs tissues.
• The X-ray tube and detectors rotate and move to create a 3D image, displayed on a computer monitor for analysis.

Medical Tracers

• Radioactive isotopes for medicine combined with specific elements form compounds that collect in particular locations in the body.
• These compounds are known as medical tracers and Radiopharmaceuticals and are used in both diagnosis and therapy
• Must be placed inside the patient's body during a diagnosis.
• Gamma-emitters are the most useful as they are the least ionising and most penetrative; beta and alpha emitters are more ionising and would cause significant damage.
• Medicine radioisotopes have high activity and short half-lives to achieve imaging quickly, minimize harmful radiation, and require only small amounts
• Many of these radioisotopes are produced artificially on-site because they must be utilised almost immediately or their activity will decay below measurable levels.

Medical Tracer Examples

• Flourine-18 undergoes plus beta decay, releasing a positron and forming a neutron; half-life is approximately 110 minutes.
• Positron then interacts with an electron and creates a pair of gamma photons.
• Sodium fluoride is a tracer of F-18 and helps with bone imaging.
• Technetium-99m is versatile and the m stands for metastable
• 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

Gamma Cameras

• Gamma cameras identify gamma photons emitted from the tracers in the body.
• A collimator limits direction because the rays travel in every direction.
• The collimator has to be made of a high-density metal

Scintillation Crystal

• Barium iodide is a material that will emit many photons when a high-energy photon is incident upon it.
• Approximately 1/10th of the gamma photons are absorbed in the scintillator, but each photon produces many visible photons
• The visible photons are directed towards a photocathode, which produces an electron for each visible photon detected.
• These electrons pass through a photomultiplier tube where dynodes generate cascades of electrons and amplify the signal.
• The impact in the scintillator is used to locate the emission and detected by a computer and displayed on a screen.

Positron Emission Tomography (PET)

• Similar to X-rays and CAT scans, a gamma camera produces a 2D low resolution compared with PET scanner.
• PET scanners utilize a ring of gamma cameras to generate an accurate 3D image.
• Positron-emitters such as F-18 produce paired gamma photons from electron-positron annihilation in opposite directions, obeying momentum conservation.
• Detectors identify where to record arrival times to identify the exact location as the speed of photons are known.
• Fluorodeoxyglucose (glucose with F-18) is used in PET scans to locate regions with high respiration rates, which may be cancerous tumors or active parts of the brains

Non-invasive PET and Limitations

• PET is a non-invasive technique that demonstrates organ function and the effects of different medications.
• PET imaging is both very expensive and requires tracers to be made on-site.

Ultrasound

• Ultrasound uses longitudinal sound waves with a frequency>20 kHz (5 MHz in medical diagnosis).
• Ultrasound waves can be refracted, reflected, doppler shifted and diffracted.
• The images use diffraction to identify apertures or features of a few millimeters in size and the properties of different media.
• This is a non-ionising, non-invasive, quick and affordable technique.
• A transducer creates signals that are studied and used to generate images
• The boundary can be found in media.

Piezoelectric Effect

• When certain materials expands and contracts they generate a voltage
• Certain crystals vibrate when exposed to an alternating voltage.
• Piezoelectric crystals are quartz, polymeric, or ceramic.
• Ultrasound transducers use an alternating potential difference to compress and stretch crystals, generating ultrasound.
• The ultrasound is created here and the potential difference is turned off to create signal

Ultrasound Scans

• A-scans use a single transducer to determine distances from an ultrasound device to a point of reflection by measuring time delay.
• B-scans use B scans, which are more complex and can produce a 2D B scans are a combination of B scans.
• Ultrasound waves are pulsed to ensure reflected waves have time to return
• Smaller wavelengths allow for more detailed images that can see finer points and details.

Acoustic Impedance

• Defined as Z=pc kgm-2s-1 and its measurements depend on a medium's properties, or p and c.
• The media's acoustic impedance causes a wave of energy to be transmitted and reflected when an ultrasound hits the wall between the media. The media's acoustic impedances determine the wave of energy.

Accounting for Intensity

• Ir is a fraction of initial wave intensity and accounts for any reflections: r = (Z2 – Z1)2 / (Z2 + Z1)2
• High reflection: Z1 and Z2 are very different.
• To maximise level of detail and reflected intensities: impedance matching gel similar to skin's is used to maximise transmitted media.

Doppler Effect Usage

• The shift in wave frequency caused by production/reflection is known as the Doppler effect.
• Doppler imaging is used to measure blood flow without using invasive techniques.
• Iron that reflects waves is sent throughout a blood vessel by ultrasound waves
• Frequency shifts up or down, depending on the direction and speed of flow: DELTA f = 2fvcos(THETA)/c
• Shift can be used to demonstrate blood clots or narrow volume.