Chapter 27- Medical imaging

• X-rays are electromagnetic waves with extremely short wavelenths
• X-ray photons have much more energy than a photon of visible light and are therefore harmful to living cells and can kill them, a property which is utilised in the treatment of cancer
• X-ray photons are produced when fast-moving electrons are decelerated by interaction with atoms of a metal such as tungsten. The kinetic energy of the electrons is transformed into x-ray photons
• X-Rays are produced within an electron tube which consists of an evacuated tube containing two electrodes. An external power supply created a large p.d across the electrodes. The cathode is a heated which produced electrons by thermionic emission. These electrons are accelerated towards the anode which is made of a metal such as tungsten (the target metal) that has a high melting point. photons are produced with the electrons are deaccelerated by hitting the anode. Most of the energy is transferred to thermal energy therefore the anode is rotated to prevent overheating
• An electron accelerated through a potential difference V gains kinetic energy $e$V. One electron released one photon therefor the maximum energy of a photon from an electron tube must equal the kinetic energy of an electron 


  $hf\ =$$\ eV$ and $\frac{hc}{\lambda}=$$\ eV$
• The wavelength from an x-ray tube is inversely proportional to the accelerating potential difference. Increasing the current of the tube will increase the intensity of the x-rays
• Attenuation is the decrease in the intensity of electromagnetic radiation as it passes through matter and or space
• There are four attenuation mechanisms by which x-ray photons interact with atoms;

  • simple scatter
  • photoelectric effect
  • compton scattering
  • pair production
• In the simple scatter attenuation mechanism the x-ray photon is scattered elastically by an electron

  
• In the photoelectric effect attenuation mechanism, the x-ray photon disappears and removes an electron from the atom

  
• In the compton scattering attenuation mechanism, the x-ray photon is scattered by an electron; its energy is reduced and the electron is ejected from the atom

  
• In the pair production attenuation mechanism, the x-ray photon disappears to produce an electron-positron pair

  
• The simple scatter mechanism occurs in the energy range 1-20keV. This is not enough energy to remove an electron so the photon is scattered. Hospital x-ray machines use p.d. greater than 20kev so this mechanism is insignificant in hospital radiography
• The photoelectric effect mechanism occurs for x-ray photons with energy less than 100keV. The x-ray photon is absorbed by an electron and the electron uses the energy to escape from the atom. Hospitals use p.ds of 30-100keV so this attenuation mechanism is dominant when a x-ray image is taken
• The compton scattering mechanism occurs for X-ray photons with energy from 0.5-5MeV. The x-ray interacts with an electron and the electron is ejected while the photon is scattered with the reduced energy
• The pair production mechanism occurs when x-ray photons have energy equal to or greater than 1.02MeV. The photon disappears and the electromagnetic energy is used to create an electron and positron
• The transmitted intensity of X-rays depends on the energy of photons and on the thickness and type of substance. This is an exponential relationship with μμ the attnetuation coefficient and x the thickness of the substance


  $I=$$I_0e^{-\mu x}$
• Soft tissue has a low absorption coefficient whereas bone has a high absorption coefficient.
• To improve the visibility of internal structures on x-rays, contrast medium such as iodine and barium compounds are used.
• Iodine is used as a contrast medium in liquids eg blood flow, it is injected into the blood vesels showing blockages, and structures of organs like the heart
• Barium sulphate is used to image digestive systems and is given as a liquid mixture for the patient to eat. The x-ray will show where the barium accumulates
• X-rays photons in linear accelerators are used to treat cancer. They create high energy photons that use Compton scattering and pair production to kill off cancerous cells
• The types of medical imaging machines that use x-rays are;

  • CAT scans
  • PET scans
  • The gamma camera
• CAT scanners work by having an x-ray tube and detectors rotate around the patient. The x-ray tube produces a fan shaped beam of x-rays 1-10mm thick which irradiates a thin slice of the patient and the x-rays are attenuated different amounts within the body resulting in different intensities that are recorded by the detector. Each time the x-ray tube and detectors complete a full 360 rotation, a complete slice is acquired, the table is moved slightly through the ring, resulting in a spiral path of the scan.
• CAT scan

  
• CAT scans are more expensive and take longer than traditional x-rays however they are able to create a 3D image of the patient. CAT scans can also distinguish between soft tissues with similar attenuation coefficients. CAT scans expose the patient to more ionising radiation as they take longer
• Radioisotopes chosen for medical imaging must have a short half life to ensure high activity from the source so that only a small amount is required to form the image
• Flurine-18 is the radioisotope used in PET scans. It has a short half life and can be produced artificially at the hospital using a particle accelerator.
• Technetium-99m is a versatile isotope used to monitor the function of major organs sucha as the heart, liver, kidneys and brain. The isotope is produced from the natural radioactive decay of molybendum-99. The isotope looses energy by emitting a gamma photo and its half life is about 6 hours
• The m in Tc-99m means metastable and refers to the nucleus remaining stable in a high energy state with more energy than the stable nucleus for a longer period than expected
• In order to ensure radioisotopes reach the correct targeted organ, it has to be chemically combined with elements that will target the desired tissues to make a medical tracer.
• Tc-99m can be combined with sodium and oxygen to create a medical tracer which targets the brain
• A gamma camera detects the gamma photons emitted from a medical tracer within the patient and an image is constructed indicating the concentration of the tracer within the patients body.
• Within the gamma camera, photons travel towards the collimator and any photons arriving at an angle to the tubes are absorbed, so only those that are travelling parallel to the tubes reach the scintillator. A single gamma photon striking the scintillator produces visible light photons which travel through the light guide to the photomultiplier tubes which are arranged in a hexagonal pattern. These convert the visible light photons into a voltage which is recorded and processed into an image.
• The collimator is a honeycomb of thin tubes made of lead.

  the scintillator is a crystalline sheet, often NaI, that produces visible light when hit with gamma
• Gamma camera

  
• Gamma cameras show the function and processes of the organ rather than a snapshot
• Ultrasounds are longitudinal waves with a frequency greater than 20 kHz.
• Ultrasound used for medical imaging has frequencies within the range 1-15 MHz
• An ultrasound transducer is a device that is used to both generate and receive ultrasounds through the piezoelectric effect.