Medical Physics

Medical Physics

1. X - rays

   • Production
     • Electrons are produced from a heated filament (cathode) and accelerated through a large p.d towards an anode.
       • Thermionic emission
       • In a vacuum
       • When the electrons collide with the target, the fast-moving electrons are decelerated by interaction with atoms of the target.
         • A small fraction of the electron’s kinetic energy is transformed into very high frequency radiation in the X-ray region of the EM spectrum.
           • The kinetic energy of the electrons is mainly transferred to thermal energy in the target.
     • Cathode
       • Cup shape to focus the beam of electrons onto the target metall
     • Anode
       • The anode contains a heavy metal target, usually tungsten. That has a high melting point.
     • avoiding overheating
       • The tungsten anode is rotated at about 3000 rpm - to spread the heat over a large surface area.
       • It is mounted on copper which conducts the heat away effectively
       • Oil is circulated to cool the anode.
     • X-rays are also produced when beam electrons knock out other electrons from the inner shells of the tungsten atoms.
       • Electrons in the outer shells move into the vacancies in the lower energy levels, and release energy in the form of x-ray photons.
   • X - ray tube
     • An x-ray tube consists of an evacuated tube containing two electrodes.
     • The tube is evacuated so that electrons pass through the tube without interacting with gas atoms.
     • An external power supply is used to create a large p.d between these electrodes.
     • The x-ray tube is lined with lead to shield the radiographer from any x-rays emitted in other directions.
   • Energy
     • An electron accelerated through a potential difference V gains kinetic energy eV.
     • One electron releases one X-ray photon - (one-to-one interaction)
       • MAX energy of X-ray photon = MAX kinetic energy of a single electron.
         • hf=eV
   • increasing the intensity of an X-ray beam
     • Increase the tube voltage - This gives the electrons more kinetic energy.
       • Higher energy electrons ‘knock out’ electrons from shells deeper within the tungsten atoms.
     • Increase the current supplied to the filament. - As current increases, the filament temperature rises.
       • This frees more electrons per second (with the same final energy per electron as before.) which then produce more X-ray photons per second.

2. Interaction of X-rays with matter

   • X-ray photons interact with the atoms of the material they pass through - The photons can be scattered or absorbed by the atoms
     • This reduces the intensity of the x-rays
   • Attenuation
     • The decrease in intensity of EM radiation as it passes through matter and space
     • Attenuation coefficients
       • For a given substance and energy of photons - intensity falls exponentially with thickness of substance
         • The transmitted intensity I is given by:
       • A measure of the absorption of X-ray photons by a substance - which depends on the photon energy
         • unit: m^-1 / mm^-1
       • Contrast medium
         • Soft tissues have low absorption coefficients
           • X -ray images of soft tissue can be improved by including a contrast medium
         • Iodine and barium are used - which are relatively harmless to humans
           • They are both elements with large atomic numbers and absorb X-rays strongly
           • Iodine is used as a contrast medium in liquids - e.g to view blood flow
           • Barium is often used to image digestive systems
   • Attenuation mechanisms
     • Simple scattering
       • X -ray photon interacts with an electron in the atom
       • It has less energy than the energy required to remove the electron - so the x-ray is scattered
       • The photon energy is unchanged in the process
     • Photoelectric effect
       • When the x-ray photon energy is < 0.1 Mev
       • The x-ray photon may make an inelastic collision with an electron in the atom
       • The electron uses this energy to escape the atom
       • Photoelectric effect is dominant when an x-ray image is taken - hospitals use 30-100kV supplies.
     • Compton scattering
       • Significant for x-ray photons with energy in the range 0.5 - 5 MeV
       • The high energy X-ray photon interacts with an electron within the atom
       • The electron is ejected from the atom and the the photon is scattered.
       • The energy of the scattered photon is less than the energy of the incident photon, as energy is transferred to the electron.
       • In the interaction both energy and momentum are conserved.
     • Pair production
       • Pair production only occurs when X-ray photons have energy to or greater than 1.02MeV
       • An X-ray photon interact with the nucleus of the atom
         • The photon disapears and the electromagnetic energy of the photon is used to create an electron and a positron.
       • Pair production dominates at photon energies above 5 MeV
   • X -rays are also used for therapy rather than imaging
     • specialised X-ray machines (LInear ACcelerators) are used to kill off cancerous cells
       • They do so by compton scattering and pair production

3. CAT scans

   • The CAT scanner has an X-ray tube housed in a ring.
     • It sends a thin fan-shaped beam of x-rays (typically only 1-10mm thick) through the patient's body towards detectors on the opposite side of the ring.
       • The X-ray tube and detectors rotate around the patient, recording many images from different angles.
         • The intensity of the transmitted X-rays is recorded by the detectors.
           • These are assembled into a 3D image by software to create cross-sections through the body that can be viewed on screen.
             • The radiographer can view each two-dimensional slice through the patient.
       • The X-rays are attenuated by different amounts by different tissues.
   • Each time the X-ray tube makes a 360o rotation - a 2D image or ‘slice’ is acquired.
     • By the time the X-ray tube has made one complete revolution - The table has moved about 1cm through the ring.
       • The X-ray beam follows a spiral path during a 10-30 minute scan.
   • Advantages
     • Used to create a 3D image - Helps doctors to assess the shape, size and position of disorders
     • Distinguish between soft tissues of similar attenuation coefficients.
   • Disadvantages
     • Some CAT scans can be quite prolonged - Patients are exposed to high radiation dose.
     • Patients have to remain very still during scanning process - any movement blurs the slice.

4. The Gamma Camera

   • Gamma-emitting sources are ideal for medical imaging
     • Gamma photons are the least ionising
       • Alpha and beta cause more ionisation
     • Can penetrate through the patient to be detected externally.
   • Radioisotopes
     • Used for medical imaging - Placed in the bloodstream of the patient
     • Must have a short half-life to ensure high activity from the source
       • So only a small amount is required to from the image
       • So the patient is not subject to a high dosage of radiation that continues long after the procedure.
     • Technetium-99m
       • Loses energy by emitting a gamma photon
         • with energy of exactly 0.14Mev and a wavelength about the same as X -rays
       • Half life of 6 hours
         • Long enough for scanning procedures
         • Short enough to keep radiation exposure low for patient
       • Versatile isotope - it can be used to monitor function of organs and also blood flow
       • The daughter nuclei in the decay of molybdenum-99
         • Mo-99 decays by beta minus emission
         • half-life of 67 hours
       • Metastable
         • A nucleus that stays in a high energy state, with more energy than the stable nucleus for a longer period than expected
     • Fluorine-18
       • Decays by beta plus decay
       • Half-life of 110 minutes
       • Used in PET scans
   • Medical tracers (radiotracers)
     • A radiopharmaceutical, that is, a compound labelled with a radioisotope that can be that can be traced inside the body using a gamma camera
     • The radioisotope has to be chemically combined with elements that will target the desired tissues.
   • Use of the Gamma Camera
     • A gamma camera detects the gamma photons emitted from the medical tracer injected into the patient
       • an image is constructed indicating the concentration of the tracer within the patient's body.
     • The gamma photons travel towards the collimator - a honeycomb of long, thin tubes made from lead.
       • Any photons arriving at an angle to the axis of the tubes - are absorbed by the tubes. So only those travelling along the axis of the tubes reach the scintillator.
         • The scintillator absorbs gamma photons and emits visible light.
           • A single gamma photon striking the scintillator produces thousands of photons of visible light.
             • The photons of visible light travel through the light guide into the photomultiplier tubes.
               • A photomultiplier tube consists of an earthed cathode that emits electrons when photons are incident upon it via the photoelectric effect.
                 • The photoelectrons are accelerated towards an anode (dynode), where each electron releases at least 2 electrons.
                 • These 2 electrons are then accelerated to a second anode where the same process occurs.
                   • Hence each anode increase the number of electrons
                 • A single photon of light entering a photomultiplier tube is converted into an electrical pulse (voltage).
                   • The outputs of all the photomultiplier tubes are connected to a computer.
                     • The electrical signals can be processed to locate the impacts of the gamma photons on the scintillator.
                       • These impact positions are used to construct a high-quality image that shows the concentrations of the medical tracer within the patient's body.
   • Advantages
     • A gamma camera produces an image that shows the function and processes of the body, rather than its anatomy.
     • Cheaper than a PET scanner
     • Very low radiation risk - because only a small amount of radioisotope is injected - so the procedure results in relatively low radiation exposure to the patient.
   • Disadvantages
     • Use ionising radiation - which can damage or kill living cells
     • Gamma Cameras are fairly expensive

5. PET scans

   • A radioactive isotope that decays by emitting positrons is introduced into the patient - such as fluorine-18
     • When a positron meets an electron inside the patient - annihilation produces two gamma rays of equal energy which travel in opposite directions.
       • The PET scanner detects these gamma rays to produce a high-resolution image of organs and tissues in the body and show how they are working.
     • Fluorine-18 can be incorporated into a radioactive substance similar to glucose.
       • When injected into the patient it accumulates in tissues with a high rate of respiration.
         • The activity from the radioactive substance in the body is monitored using gamma detectors.
   • Another medical tracer used in PET scanning is carbon monoxide
     • This isotope emits a positron and has a half-life of about 20 mins.
     • Carbon monoxide can cling onto red blood cells So it can be transported through the body and the concentrations of carbon monoxide can be monitored in a PET scan.
   • The patient lies on a horizontal table and is surrounded by a ring of gamma detectors.
     • Each detector consists of a photomultiplier tube and a scintillator.
       • Which produces a voltage pulse for every gamma photon incident at its scintillator.
       • The patient is injected with the medical tracer.
         • The PET scanner detects the gamma photons emitted - when the positrons from decaying fluorine-18 nuclei annihilate with electrons inside the patient.
           • Note that the gamma photons detected for the PET can come from the annihilation of the positrons - not the gamma photons emitted by the decaying fluorine-18 nuclei.
           • On average a positron travels 1mm from its emission point before it annihilates an electron.
           • The annihilation of a positron and an electron produces two gamma photons travelling in opposite directions
             • So momentum is conserved.
             • The computer can determine the point of annihilation from:
               • The difference in the arrival times of the photons at the diametrically opposite detectors
               • The speed of photons c
               • The voltage signals from all the detectors are fed into the computer
                 • This creates an image in which different concentrations of the tracer show up as areas of different colours and brightness.
   • Advantages
     • The patient is not subjected to the risks of surgery
     • PET scans are used to help diagnose abnormal activity within cells - It can lead to diagnosis of conditions such as Alzheimer’s disease.
   • Disadvantages
     • The technique is very expensive - because of the facilities required to produce the medical tracers.
     • Only patients with complex health problems are recommended for PET scans.

6. Ultrasound

   • longitudinal sound waves with frequency greater than 20 KHz.
   • Ultrasound can be refracted, reflected and diffracted
     • The wavelength of ultrasound in the human body is less than 1mm, so ultrasound can be used to identify features as small as a few millimeters.
   • ultrasound transducer
     • A device used both to generate and to receive ultrasound. It changes electrical energy into sound - and sound into electrical energy, by the piezoelectric effect.
     • To generate ultrasound - a high frequency (e.g 5Mhz) alternating p.d is applied across opposite faces of a crystal.
       • The frequency is chosen to be the natural frequency of oscillation of the crystal.
         • The crystal resonates and produces an intense ultrasonic signal.
       • This repeatedly compresses and expands the crystal.
       • An ultrasound transducer emits pulses of ultrasound (typically 5000 pulses every second)
       • The same transducer is used to detect ultrasound.
         • Any ultrasound incident on the crystal will make it vibrate
           • The crystal is compressed and expanded by tiny amount.
             • The vibration generates an alternating e.m.f across the ends of the crystal, which can be detected by electronic circuits.
   • piezoelectric effect
     • The production of an e.m.f by some crystals, such as quartz, when they are compressed, stretched, twisted or distorted.
       • This piezoelectric effect is reversible process.
     • When an external p.d is applied across opposite faces of the crystal - The electric field can either compress or stretch the crystal.
   • A scan
     • The simplest type of ultrasound scan is called an A - scan.
       • A stands for Amplitude
       • A - scans are only used to measure distance/thickness.
     • A single transducer is used to record along a straight line through the patient.
     • The transducer emits an ultrasound pulse - The time taken for the pulse to reflect from an object and return is used to determine the distance.
       • Each pulse of ultrasound will be partly reflected and partly transmitted at the boundary between any two different tissues.
         • The reflected pulse will be received at the transducer. It will have less energy than the original pulse Because of energy losses within the body and also because some of the energy of the original pulse is transmitted through the boundary.
           • The pulsed voltage at the ultrasound transducer is displayed on an oscilloscope screen or computer screen as a voltage against time plot.
   • B scan
     • B - scans can be used to make 2D images.
     • The transducer produces multiple pulses and, the transducer is swept over the patient's skin.
     • For each position of the transducer - the computer produces a row of dots on the screen - Each dot corresponds to the boundary between two tissues.
     • The brightness of the dot is proportional to the intensity of the reflected ultrasound pulse.
       • The collection of dots produced correspond to the different positions of the transducer over the patient.
         • Making a 2D image of a section through the patient.

Imágenes y archivos del mapa