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Medical Imaging PowerPoint Presentation

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Description : Check out this medical presentation on Radiology and Imaging, which is titled "Medical Imaging", to learn about Medical Imaging and its types.

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Published on : Jul 28, 2021 | Views : 2712 | Downloads : 15

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MEDICAL IMAGING
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X-RAY PRODUCTION  Principles of production of an X-ray beam. Electrical current is run through the tungsten filament, causing it to glow and emit electrons. A large voltage difference (kV) is placed between the cathode and the anode, causing the electrons to move at high velocity from the filament to the anode target. The high speed electrons strike the target and rapidly decelerated on impact, suddenly x- rays are emitted.
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CONTINUOUS OF X-RAY SPECTRA  Continuous x-rays is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus.  The moving particle loses kinetic energy, which is converted into a photon because energy is conserved.  This is the Continuous x-rays or Bremsstrahlung rays and has a continuous spectrum.
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CHARACTERISTIC OF X-RAY SPECTRA  The second type of spectra, called the characteristic spectra, is produced at high voltage as a result of specific electronic transitions that take place within individual atoms of the target material.  The easiest to see using the simple Bohr model of the atom.  In such a model, the nucleus of the atom containing the protons and neutrons is surrounded by shells of electrons.
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TUBE CURRENT- rate of arrival of electrons at metal target The intensity of the X-ray beam is determined by the rate of arrival of electrons at the metal target, that is, the tube current. This tube current is controlled by the heater current of the cathode. The greater the heater current, the hotter the filament and hence the greater the rate of emission of thermo-electrons.
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HARDNESS OF X-RAY BEAM  The hardness of an X-ray beam refers to its penetration power.  The hardness is controlled by the accelerating voltage between the cathode and the anode.  More penetrating X-rays have higher photon energies and thus a larger accelerating potential is required.  Referring to the spectrum of X-rays produced, it can be seen that longer wavelength X-rays (‘softer’ X-rays) are also produced.  These X-ray photons are of such low energy that they would not be able to pass through the patient.  They would contribute to the total radiation dose without any useful purpose.  Consequently, an aluminium filter is frequently fitted across the window of the X-ray tube to absorb the ‘soft’ X-ray photons.
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ATTENUATION- decrease in intensity  When x-ray is absorbed in medium, intensity of parallel x- ray beam decreases by constant fraction in passing through equal small thicknesses of medium
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Half-value Thickness (HVT)  The half-value thickness x½ or HVT is the thickness of the medium required to reduce the transmitted intensity to one half of its initial value.  It is a constant and is related to the linear absorption coefficient μ by the expression  x½ μ = ln2.  In practice, x½ does not have a precise value as it is constant only when the beam has photons of one energy only.
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IMPROVING X-RAY IMAGES  Three main aims:  To reduce as much as possible the patient's exposure to harmful x-rays.  To improve the sharpness of the images – finer details can be resolved.  To improve the contrast of the image – the different tissues under investigation show up clearly.
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REDUCING DOSAGE  X-rays can damage living cell.  So, intensifier screens are used.  These are sheets of a material that contains a phosphor, a substance that emits visible light when it absorb x-rays photon.  The film is sandwiched between two intensifier screens.  Each x-rays photon absorbed result in several thousand light photons catch then blacken the film.
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Quality of the Image  Sharpness is concerned with the ease with which the edges of structures can be determined. A sharp image implies that the edges of organs are clearly defined. To Obtain Sharp Images 1.The X-ray tube is designed to generate a beam of X-rays with minimum width. Factors in the design of the X-ray apparatus that may affect sharpness include:
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 2.the size of the aperture, produced by overlapping metal plates, through which the X-ray beam passes after leaving the tube (see Fig. 2.4),
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 3.the use of a lead grid in front of the photographic film to absorb scattered X-ray photons, as illustrated in Fig. 2.5.
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To Obtain Good Contrast  Use a ‘contrast medium’.  For example, the stomach may be examined by giving the patient a drink containing barium sulphate.  Similarly, to outline blood vessels, a contrast medium that absorbs strongly the X-radiation would be injected into the bloodstream.  The contrast of the image produced on the photographic film is affected by  exposure time,  X-ray penetration and  scattering of the X-ray beam within the patient’s body.  Contrast may be improved by backing the photographic film with a fluorescent material.
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Computed Tomography (CAT/CT Scan)  Purpose: X-ray imaging only produce 2-Dimensional image with no impression of depth, cannot tell if tissue is near to the surface or deep within the body Tomography is a procedure which forms 3-D image of the object The word ‘tomo’ means crossectional or slice
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Principles of CT Scan  X-rays is directed from different angles  X ray taken of slice/plane/section are repeated at different angles  These images are combined and processed  Combined images give 2-D image of slice which is formed on the computer screen  The procedure is repeated for successive slice to build up 3-D image  Image can be viewed from different angles/rotated
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Voxel Development in CT Scan  The section (or slice) through body is divided up into a small units called voxels.  The image of each voxel would have a particular intensity, called pixels.  The pixels is the density that the computer will register for that section of the object.  As the scanner goes around, each part has different density which the computer can model
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Building the Image
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 For a well-defined image in a CT Scan, we need voxels to be small.  How? X-ray beams must well be collimated so that it consists of parallel ray - rays must not be spread Detector must consists of regular array of tiny detecting element -the smaller the detector the better the image
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Advantages of CT Scan  Produces images that show 3-Dimensional relationships between different tissues  Can distinguish tissues with quite similar densities (attenuation coefficients)
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 Generating Ultrasonic Waves: (electrical to ultrasound)  A quartz crystal with the two sides coated with silver is used to act as electrodes  When a p.d. is applied across it, it expands to generate sound wave  Charged atoms of a transducer in an electric field move closer to oppositely charged plates (positive silicon ions to cathode and negative oxygen ions to anode)  When a constant alternating voltage is applied to the crystal, it causes the crystal to contract and expand, making it vibrate at the same frequency with maximum amplitude (resonance)  This acts as the vibrating source of ultrasound waves  Receiving Ultrasonic Waves: (ultrasound to electrical)  Ultrasonic waves change pressure in medium. When the crystal contracts, a p.d. is generated.  Charged atoms in crystal move towards plates  Opposite charges induced in the silver plates  Induced potential difference across the plates  Potential difference fluctuates which can be amplified and processed
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Reflection of Ultrasonic Waves  Ultrasound requires ultrasonic waves to pass from one medium to another  When a beam of ultrasound wave reaches a boundary between two different media, the beam is partially refracted and reflected.  Reflected waves are used to construct an image of the body  Refracted waves allow transmission of ultrasound from transducer to medium and vice versa  I = IR + IT
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Intensity Reflection Coefficient Definition: Ratio of intensity of reflected wave and intensity of incident wave -Images are clearer if there is a strong reflection (large difference in acoustic impedance at the reflected boundary) Comparing acoustic impedances: Very large fraction reflected at air-tissue boundary Large fraction reflected at boundary between soft tissues Very little reflected at boundary between soft tissues (fat and muscle) because most has been absorbed A gel is applied before carrying out scan because when wave travels in or out of the body there is : Very little transmission at an air-skin boundary Almost complete transmission at a gel-skin boundary because acoustic impedance of gel and skin very similar, this allows transmission of wave from medium back to transducer
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EXAMPLE 1: ULTRASOUND IMAGING PROCEDURE  Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures  Transducer is placed in contact with skin and a gel acting as a coupling medium  Pulses of ultrasound are directed into the body  The wave is reflected at boundary between tissues  The reflected pulse is detected and processed  The time for return of echo gives information on depth  Amount of reflection gives information on structures
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MRI Magnetic resonance imagining
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LEARNING OUTCOMES
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INTRO (HUMAN BODY)  Made up of water molecules  Consist only H & O₂ atoms  Fat also contains H atoms  We are made up of 60% of H atoms  Nucleus of H atom is a proton – very sensitive to the magnetic field  Nucleus of a H behaves like a tiny magnet
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PRINCIPLES  Hydrogen nucleus (proton) behave as a tiny magnet  When a large / strong uniform magnetic field applied, protons will line up in the field with most line up with their N facing S (stable low energy state) and If N facing N OR S facing S (unstable higher energy state)  Pulses are applied at RF waves which cause the protons to resonate  External magnetic field applied causes nuclei to precess  H atoms give off RF waves  RF detected and processed  To give positions of H atoms  Non uniform field enables  Positions of resonating atoms to be defined
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PRECESSION  Protons are not static when they align with the field  Magnetic axis rotates around the direction of external field  gyration action  Depends on individual nucleus & magnetic flux density,B₀  Stronger the external field, the faster the proton precess about it  Angular frequency of precession is called the Larmor frequency, ω₀
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FREQUENCY  Frequency of radio waves applied (Larmor frequency)  Causes the nuclei to resonate and flip into higher energy state  Depends on the strength of the magnetic field where each nucleus is  Scanner can workout the location of each nucleus
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RELAXATION TIMES  Time taken for a nucleus to fall back to a lower energy state  When RF waves are switched off & protons gradually relax into their lower energy state  The protons in higher energy state are unstable so must ‘relax’ and come back to lower energy state  The excess energy is transmitted back as radio waves which can be detected  The time taken for the waves to be detected determine the relaxation times  Depends on : water and watery tissues fatty tissues cancerous tissues Different tissues can be distinguished by different rates at which they release energy after they have been forced to resonate
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Formula Larmor frequency Gyromagnetic ratio (2.68 𝑥 108 𝑟𝑎𝑑 𝑠−1 𝑇−1 ) Magnetic flux density
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MRI SCANNER  Main features : A large superconducting magnet An RF coil that transmits RF pulses to the body An RF that detects the signal emitted by the relaxing protons A set of gradient coils A computer
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PROCEDURE  Before : Eat, drink & take your medicine as usual ( except magnetic resonance cholangiopancreatography, MRCP) Remove any mtal objects from your body – wear a hospital gown Injection (a special dye – as contrast agent )
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 During : A friend/ family member maybe allowed to stay in the room with you (follow the same clothing guideline) Lasts between 15-90 minutes Scanner will make a loud sound – caused by the magnet – given earplug Keep your body still to avoid images being blurred
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 After : Can resume their normal activities as usual immediately Someone should stay with you if you decide to sedated Drink a lot of water for the following 24hours
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ADVANTAGES  Do not involve exposure to ionizing radiation  Showing soft tissues structures  Provide info about how the blood moves through certain organs & blood vessels  Painless  No moving mechanisms  No short/ long term effects demonstrated
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DISADVANTAGES  Very expensive  Long scan times  Audible noise (65-115dB)  Claustrophobic  Can be affected by movement  Metallic object in the patient can be heated  Heart pacemaker can be affected
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EASY RIGHT? SO NOW LET’S MOVE ON FOR A SHORT EXERCISE
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SOLUTION  Large/ strong uniform magnetic field applied  Pulses applied at RF waves  Causes the protons/ H atoms to resonate  H atoms give off RF waves  RF detected and processed  To give positions of H atoms  Non uniform field enables  Positions of resonating atoms to be defined
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 For nuclei spin/ precess  Spin/ precess about the direction of magnetic field  Frequency of precession depends on magnetic field strength  Large field means frequency in radio frequency range Means frequency of precession are different in different regions of subject Enables location of precessing nuclei to be determined Enables thickness of slice to be varied / location of slice to be changed