X-Rays and Nuclear Medicine

X-Rays: Structure of X-Ray Tube

• 
• X-ray tube = cathode and anode in vacuum
• Cathode: focuses the emitted electrons and conducts current
• (Think cathode - current!)
• Filament = tightly wound wire coil
• Thermoionic emission = when the filament heats up so much that electrons are emitted from the filament
• Electrons are accelerated by high voltage
• X-rays produced when electrons hit metal target.

X-Rays: Bremstrahlung

• Bremstrahlung means braking
• Occur when high energy electrons hit metal target then rapidly DECELERATE on impact.
• (Think braking = slowing down! Bremstrahlung - decelerate!)
• Bremstrahlung Spectra:
• Distribution is dependent on kV & independent of anode material
• Sharp cut off at low wavelengths (because all energy is converted to X-ray photons!)
• Peak on a spectrum is the most common wavelength!
• Cut off wavelength = hc/E

Characteristic X-Rays

• The opposite of Bremstrahlung!
• Electrons from the cathode have enough energy that rather than slow down on collision with the metal target, they knock an inner electron out of it!
• This leaves a hole.
• Electrons inside the metal plate move to fill the hole.
• X-rays are emitted with precise energies! These are characteristic X-rays!
• (Precise energy = unique = character!)
• The wavelength is dependent on the target material.

Producing X-rays: Summary

• X-ray Tube: Anode and cathode in vacuum with a filament. (Thermoionic emission)
• Bremstrahlung
• High energy electrons hit metal target and SLOW DOWN rapidly. X-ray photons released.
• Characteristic X-rays
• High energy electrons hit target.
• Inner e- knocked out.
• Electrons move to fill hole.
• X-ray photons released.

Attenuation

• When X-ray photon enters tissue (eg. breast) where molecules are tightly packed, 3 things can happen:
• 1. No interaction
• 2. Absorption
• 3. Scattering
• Because of this, fewer photons come out of the tissue than went in. This is ATTENUATION
• Different materials attenuate different amounts: bone and metal (dense) attenuate more so fewer photons pass through.

Applications of X-rays

• Mammography - X-ray imaging of breast tissue
• Photons are detected using film, photons cause chemical changes in the film emulsion.
• Digital imaging increasingly used instead!
• Identifying Fractures

Nuclear Medicine

• Radiotherapy - using radiation to kill tumours
• Imaging - X-rays, radiograms, tomography
• Gamma rays - Gamma camera, PET (Positron Emission Tomography)

Radiotherapy

• Tumour is irradiated with ionising radiation.
• Tumour cells cannot repair the damage done to DNA.
• External Radiotherapy:
• Targeted beam of X-rays or gamma rays. (Produced by linear accelerator!)
• Internal Radiotherapy:
• Radioisotope inserted directly into tumour tissue: usually alpha source with short range.
• Advantage - radiation is targeted directly at the tissue
• Disadvantage - usually requires surgery
• Brachytherapy (Internal radiotherapy for prostate cancer etc.)
• Radioactive source placed inside or near tumour.
• Advantage: delivers cell killing dose directly into tumour whilst sparing surrounding healthy tissue!

Gamma Camera: 4 main parts!

• 
• Collimator
• Scintillator
• Photomultiplier
• Camera

Collimator

• Collimator: Defines gamma location! (where in the patient the gamma is being emitted from: eg. where the tumour is!)
• Made up of parallel lead tubes.
• Lead absorbs gamma rays.
• Collimator only allows gamma rays from certain directions to reach the scintillator.
• Without the collimator, you would not be able to tell where the gamma rays were coming from.

Scintillator

• Scintillator: Gamma rays -------> Visible Photon
• Incoming gamma photons are absorbed and re-emitted as visible photons.
• (More than one visible photon is produced per gamma photon absorbed.)

Photomultiplier & Computer

• Visible photon -----> Electronic signal
• Photomultiplier amplifies the electronic signal.
• Made up of photomultiplier tubes with cathode and anode. Increasing positive voltage gradient in tube (potential).
• Electron is knocked out of photocathode.
• Electron proceeds to knock out multiple electrons inside the tube: produces cascade of electrons towards anode.
• Computer: Electronic signal ------> Image
• Pulse height analysis: turns signal into image!

Gamma Camera: Summary

• Collimator - defines gamma location
• Scintillator - Gamma to visible photon
• Photomultiplier - Visible photon to electronic signal
• Computer - Electronic signal to image

PET (Positron Emission Tomography)

• 1. Radiotracer injected into patient.
• 2. Radiotracer accumulates in body tissues with a high energy demand such as tumour cells.
• 3. Radiotracer decays internally emitting POSITRONS.
• 4. Poistrions interact with nearby electrons in ANNIHILATION reaction: produces pair of gamma rays.
• 5. Gamma rays emitted can then be detected by the PET scanner.
• Uses of PET: identifying cancers, heart problems, systemic diseases. Can be used to inspect blood flow, O2 intake and tissue/organ metabolism.

Gallium Imaging

• Gallium forms a complex with lactoferrin at sites of infection.
• Lactoferrin is found in neutrophils (white blood cells) which increase in number during infection.
• Lactoferrin is released during inflammation.
• Example: acute interstital nephritis - spaces between kidney tubules become inflamed.
• Intensive renal gallium uptake seen in images.