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Medical imaging X-ray scan Define the terms attenuation coefficient and half-value thickness. The intensity I is the amount of energy per unit area in the beam. X is the distance travelled through the material. The attenuation coefficient describes the extent to which the intensity of an energy beam is reduced as it passes through a specific material. When used in the context of X-rays or Gamma-rays, it is represented using the symbol μ, and measured in cm-1. Lead has a high attenuation coefficient. The half-value thickness is the thickness of the material that reduces the intensity of the X-rays by 50% Derive the relation between attenuation coefficient and half-value thickness. Staring with As the intensity drops to 50% Taking logs Solve problems using the equation I = I0 e− μx . Describe X-ray detection, recording and display techniques. Students should be aware of photographic film, enhancement, electronic detection and display. Photographic plates are sensitive (the efficiency of X-Ray film to absorb x-ray photons is only » 1%) to X-rays, they provide a means of recording the image, but require a lot of exposure (to the patient), so intensifying screens were devised (scintilator and photomultiplier tube). They allow a lower dose to the patient, because the screens take the X-ray information and intensify it so that it can be recorded on film positioned next to the intensifying screen. Scintillators Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier tube. These detectors are called “scintilators", filmscreens or “scintilation counters". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays. Scintillators Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier tube. These detectors are called “scintilators", filmscreens or “scintilation counters". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays. Photostimulable phosphors (PSPs) (Electronic detection) An common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. After the plate is x-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography (also referred to as digital radiography). The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. Image plates have the following advantages. 1. dynamic range larger than 5 orders of magnitude in x-ray dose, 2. lower limit of useful dose compared to the x-ray film, 3. reusability, 4. no wet chemical processing, 5. images are available in digital form, 6. image processing and pattern recognition possible, and 7. simple data storage on optical or digital media. Disadvantages of image plates compared to the conventional x-ray film are 1. a poorer spatial resolution due to light scattering at the storage phosphor grains during the readout process. How X Rays Work X Rays (continued) X-rays: x-radiography How do we obtain a clear x-ray image? X rays are scattered or absorbed as they pass through the body. If the energy (tube voltage) of the x rays is too high, then all the x – rays get through (penetration) and there is no contrast on the image. If the x-ray energy is too low, then no x-rays reach the x-ray plate. The choice of tube voltage (energy) depends upon the type of tissue and the thickness of the tissue. X ray contrast • Structures in the body like bones are very dense and contain elements such as calcium that have a high atomic number. This makes bone absorb a high proportion of the x-rays. Soft tissues like fat and muscle allow more x-rays to pass though. The body casts an x-ray shadow onto the film. Where the x-rays have passed though bone, the film is less exposed so it looks white; where they have not passed though anything the film is exposed and turns black; and where the x-rays have passed through soft tissues the film has different levels of grey. • In order to make some parts of the body show up better, contrast media with a high atomic number can be used. This can be a 'barium meal', where the patient drinks a liquid containing barium (atomic number 56) which makes the digestive tract show up clearly on x-rays, or the patient can have an injection of iodine (atomic number 53) which makes the blood vessels stand out (this is called angiography). Explain standard X-ray imaging techniques used in medicine. Students should appreciate the causes of loss of sharpness and of contrast in X-ray imaging. They should be familiar with techniques for improving sharpness and contrast. The general relationship between scattering angle and size of the object is, Angle = (object size)-1 This will cause the edge of objects to lose sharpness X-ray Sharpness A quantitative measure of the loss of edge detail which is due to geometric properties of the object and imaging system and not due to image noise or X-ray scatter. It is usually expressed as the width of the band of changing density or brightness arising from a sudden change in the intensity of the radiation incident on the film or fluorescent screen. From this definition it can be understood that unsharpness and resolution are different concepts. It is possible for an edge to be "spread" by one of many factors, and at the same time for two such edges to be resolved in the image. The factors which contribute to the total image unsharpness include geometric unsharpness, movement unsharpness, absorption unsharpness, image receptor unsharpness, and parallax unsharpness. The various unsharpness factors all contribute to the observed unsharpness of structures in an image. However, the quantitative manner in which the factors combine is in general complicated and is not completely understood. It is known from observation that the total unsharpness is not the direct sum of the contributing factors. In general, it appears that the total image unsharpness is dominated by the unsharpness of the largest individual factor. X-ray Sharpness • Unsharpness is introduced by 1.A wide source (parallax) 2.A moving patient IB Questions Questions 7,11, 19b,c from review pack. (b) An X-ray beam that consists of photons of the same energy is used to image a possible bone fracture in the leg of a patient. At this photon energy attenuation coefficient of bone = 0.62 cm–1 attenuation coefficient of tissue = 0.12 cm–1. In passing through the leg, the X-rays effectively encounter a thickness of tissue equal to 14 cm and thickness of bone equal to 8.0 cm. Use the above data to explain why X-rays of this energy are suitable for imaging a possible leg fracture. ............................................................... ....... ....................................................... ............... ............................................... ....................... ....................................... ............................... ............................... ............................................................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [4] (b) intensity after passing through bone = I0e-0.62x8.0 = 7x10-3I0 intensity after passing through tissue =0.19I0 reduction by bone much greater than by tissue so good contrast between bone and tissue; Outline the principles of computed tomography (CT). A CT scan (sometimes called computed axial tomography, or a CAT scan) uses x-rays. In a CT scan the patient lies on a table and is moved though a doughnut-shaped machine. It creates images that are slices through the patient. It does this by moving the x-ray tube and detector in a circle taking xray images of the slice from all angles around the body. A computer then processes these images to produce a cross sectional image (a picture of a slice through the body). CT scans are useful as they can show a range of very different tissue types clearly: lung tissue, bone, soft tissue and blood vessels. CT Scan X-rays: Computed tomography image (CT scan) Second metatarsal bone (the bone that David Beckham and Wayne Rooney broke!) X-rays: Computed tomography image (CT scan) Ultrasound Ultrasound scan Ultrasound Ultrasound imaging: What does it look like? Ultrasound imaging: development of a pregnancy 24 weeks 8 weeks gestation (out of a 40 week pregnancy) 18 weeks Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals. When the ultrasound beam is generated, an electrical signal is sent to the crystal, and converted to mechanical vibrations. Naturally-occurring crystals Cane sugar, quartz, topaz, bone, enamel, dentine and tendons. When the reflected signal is received, the mechanical vibrations are converted back to electrical signals and sent to the computer for processing into an image. Define acoustic impedance (Z) as the product of the density (ρ) of a substance and the speed of sound (c) in that substance. Z = ρc The acoustic impedance of the eardrum, for instance, corresponds well with that of the auditory canal, guaranteeing maximum efficiency of energy transfer, but it does not correspond well with air. The pinna may be described as an impedance matching device between the air and the auditory canal. Likewise, the flared end of a trumpet results in less energy being lost by being reflected back down the tube of the instrument. Solve problems involving acoustic impedance. Students should understand the use of a gel on the surface of the skin. Z = ρc Z1 Z2 Gel is generally necessary because the acoustic impedance mismatch between air and the body is large. Without gel, nearly all of the energy is reflected and very little is transmitted into the body. Outline the differences between A-scans and B-scans. B-scan A-scan An A-scan, is routine type of diagnostic test used in ophthalmology. The A-scan provides data on the length of the eye. (i.e. is 1 dimensional) A B-scan, is a diagnostic test used in ophthalmology to produce a two-dimensional, cross-sectional view of the eye. Ultrasound imaging: A-scan How does it work? • An ultrasound element acts like a bat. • Emit ultrasound and detect echoes • Map out boundary of object Ultrasound imaging: B-scan How does it work? • Now put many elements together to make a probe and create an image How do the scans work? A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe. Identify factors that affect the choice of diagnostic frequency. Students should appreciate that attenuation and resolution are dependent on frequency. High frequency gives you high resolution but high attenuation i.e. limited depth penetration. Low frequencies are less absorbed (attenuated), therefore can penetrate deeply. But have lower resolution. Tsokos Page 710 Q’s 1-8. IB Q 2 from the review pack. NMR and lasers Hyperlink to video MRI Scan Hyperlink Outline the basic principles of nuclear magnetic resonance (NMR) imaging. Students need only give a simple qualitative description of the principle, including the use of a non-uniform magnetic field in conjunction with the large uniform field. Magnetic Field When a person is lying in the magnetic field of the MRI scanner the nuclei of the hydrogen atoms in their body line up, like compass needles in the Earth's magnetic field, either pointing in the direction of the field or opposite to it. The hydrogen nuclei (protons) don’t stay still though, but move like a spinning top around the direction of the magnetic field. How MRI works 1.The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field. 2. A second radio frequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy. When this field is turned off the protons release this energy at a radio frequency which can be detected by the scanner. 3. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan. Proton nuclear magnetic resonance (NMR) detects the presence of hydrogen nuclei (protons) by subjecting them to a large magnetic field to align the nuclear spins, then exciting the spins with properly tuned radio frequency (RF) radiation, and then detecting weak radio frequency radiation from them as they “relax" from this magnetic interaction. The frequency of this proton "signal" is proportional to the magnetic field to which they are subjected during this relaxation process. In the medical application known as Magnetic Resonance Imaging (MRI), an image of a cross-section of tissue can be made by producing a well-calibrated magnetic field gradient across the tissue so that a certain value of magnetic field can be associated with a given location in the tissue. Since the proton signal frequency is proportional to that magnetic field, a given proton signal frequency can be assigned to a location in the tissue. This provides the information to map the tissue in terms of the protons present there. Since the proton density varies with the type of tissue, a certain amount of contrast is achieved to image the organs and other tissue variations in the subject tissue. Since the MRI uses proton NMR, it images the concentration of protons. Many of those protons are the protons in water, so MRI is particularly well suited for the imaging of soft tissue, It is estimated that about 80% of the body's atoms are hydrogen atoms, so most parts of the body have an abundance of sources for the hydrogen NMR signals which make up the magnetic resonance image. Describe examples of the use of lasers in clinical diagnosis and therapy. Applications such as the use in pulse oximetry and in endoscopes should be discussed. Students should be familiar with the use of a laser as a scalpel and as a coagulator. Infrared: Pulse oximetry Heart rate:81 bpm Blood oxygenation: 99% Pulse oximetry Pulse oximetry is a simple non-invasive method of monitoring the percentage of haemoglobin (Hb) which is saturated with oxygen. The pulse oximeter consists of a probe attached to the patient's finger or ear lobe which is linked to a computerised unit. The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated haemoglobin. Oxygenated haemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) haemoglobin absorbs more red light and allows more infrared light to pass through. Visible: Endoscopy Visible: Endoscopy Visible: Endoscopy This is the endoscope coming out of the oesophagus Parasitic Worm! The stomach wall has relapsed back into the oesophagus. This is a hernia.