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Minna Tuominen 12.3.2002

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Minna Tuominen 12.3.2002

  1. 1. Minna Tuominen 12.3.2002
  2. 2. • Introduction • Radionuclide imaging • Ultrasonic imaging • Magnetic resonance imaging • CT imaging • Electrical Impedance Tomography (EIT)
  3. 3. • Medical imaging has been used since the discovery of x-rays (1895) • Other methods than x-rays (NM, US, CT, MR) have become available during last 50 years • Digital images on a monitor have replaced films • New methods allow images (digital) to be manipulated in variety of ways
  4. 4. • Measurements that use radiopharmaceuticals (radioactive form of drugs) or another radioactive tracer • Radiopharmaceutical has two components, a carrier and a radionuclide. The carrier component (e.g. glucose) concentrates in parts of the body with increased metabolic activity. Therefore, over time the radioactivity accumulates in the area with increased metabolic activity. • Imaging doesn’t only show the anatomy (structure) of an organ or body part, but the function of the organ as well. • This "functional information" allows to diagnose certain diseases and various medical conditions much sooner than other medical imaging examinations.
  5. 5. • The radionuclides are absorbed by or taken up at varying rates (or in different concentrations) by different tissue types. E.g. the thyroid gland (kilpirauhanen) takes up more radioactive iodine than other parts of the body. • The amount of radiation that is taken up and then emitted by a specific body part is linked to the metabolic activity of the organ or tissue. For example, cells which are dividing rapidly may be seen as "hot spots" of metabolic activity on a nuclear medicine image, since they absorb more of the radionuclide. • Radionuclide emits gamma rays (energetic photons of electromagnetic radiation emitted in nuclear-energy-state transition) • Spatial distribution of radionulides is determined by γ-detector • Current radionuclide imaging system uses a gamma camera
  6. 6. • A nucleus with measurable half-life (T½) of radioactive decay • Nuclei with unfavorable neutron/proton ratio will decay into stable nuclei by spontaneous emission of nuclear particles • 99mTc (mostly used), 131I, 67Ga (soft-tissue tumors), 133Xe (lung ventilation), 101Tl (ischemia) ,eNN(t) t o λ = λ/693.0T½ =
  7. 7. • Require the oral or intravenous introduction of very low-level radioactive chemicals (radionuclides) into the body. • The radionuclides are taken up by the organs in the body • Radionuclides emit faint gamma ray signals which are measured by a gamma camera. • The gamma camera has a large crystal detector, which detects the emitted radiation signal and converts that signal into faint light. The light is then converted to an electric signal, which is digitized and reconstructed into an image by a computer. • The resulting image is viewed on the system monitor and can be manipulated (post-processed) and filmed.
  8. 8. • Consist of collimator, scintillation detector, PM-tubes and position logistic circuits
  9. 9. • Collimator – is a pattern of holes through gamma ray absorbing material, that allows the projection of the gamma ray image onto the detector crystal. – allows only gamma rays traveling along certain directions to reach the detector. • Scintillation detector – A gamma ray photon interacts with the iodide ions of the crystal by means of the Photoelectric Effect or Compton Scattering. This interaction causes the release of electrons which in turn interact with the crystal lattice (kidehila) to produce light. (scintillation)
  10. 10. • Photomultiplier tube (PMT) – instrument that detects and amplifies the electrons that are produced by the photocathode – At the face of a photomultiplier tube (PMT) is a photocathode which, when stimulated by light photons, ejects electrons. This electron is focused on a dynode which absorbs this electron and re-emits many more electrons (usually 6 to 10). These new electrons are focused on the next dynode and the process is repeated over and over in an array of dynodes. At the base of the photomultiplier tube is an anode which attracts the final large cluster of electrons and converts them into an electrical pulse. – Each gamma camera has several photomultiplier tubes arranged in a geometrical array.
  11. 11. Forte (Philips)
  12. 12. • During nuclear medicine imaging body emits gamma rays, which are similar to x-rays but have a shorter wavelength. • In x-ray imaging the radiation comes from an external radiation source and then passes through the patient's body before being detected. • Nuclear medicine uses the opposite approach: a radioactive material is introduced into the patient, and is then detected by a machine called a gamma camera.
  13. 13. • Visualization of organs and regions within organs that can not be seen on conventional x-ray images • Space occupying lesions are seen as areas of reduced radioactivity ("coldspot"); however, in some instances, like bone scanning, areas of increased activity ("hotspot") represent disease or injury. • Therapeutic uses are: treatment of hyperthyroidism (kilpirauhasen liikatoiminta), thyroid cancer, blood imbalances and pain relief from certain types of bone cancers.
  14. 14. • Bone scanning: – It can reveal if the cancer has spread beyond its primary site and developed secondary cancer growths in the bones – Metabolic changes caused by fine fractures, small tumors, or degenerative diseases such as arthritis (niveltulehdus) can be seen
  15. 15. • Heart Disease: – cardiac angiography yields images of the beating heart and the blood vessels (coronary arteries) that supply the heart muscle (myocardium) with blood. – A stress thallium nuclear medicine study shows the function of the myocardium.
  16. 16. Single-photon emission tomography (SPET) • To produce 3D-images data are collected by rotating the gamma camera around the patient • Profile is taken from each image of the sequence • Image of the radioactivity distribution through the patient is done with filtered back-projection reconstruction algorithm • SPET is used for cardiac imaging and imaging of brain perfusion
  17. 17. • Uses positron-emitting isotopes (11C, 15O, 13N) • A positron emitted by a nucleus annihilates with an electron to form two, in opposite direction moving γ-rays. • Involves cross-sectional data acquisition and reconstruction much like CT scanning.
  18. 18. • Positron Emission Tomography, (PET) allows physicians to view biological functions. Areas with increased metabolic activity show up as "hot areas" in a PET image. • Physicians use metabolic information from PET in conjunction with other diagnostic tests to assess certain cancers, cardiac diseases and brain disorders. Abnormal increased metabolic activity may indicate a malignant cancerous tumor. Abnormal metabolic activity may also indicate heart disease or brain disorders. • Lung cancer imaging is a new, emerging application of PET and involves inhalation of the radionuclide.
  19. 19. • Study of epilepsy (nervous system disorders that cause convulsive seizures) • Evaluation of stroke (blood clot or bleeding in the brain) • Study of dementia (for example in patients with Alzheimer's or Parkinson's disease) • Imaging and evaluation of brain tumors • Evaluation of coronary artery disease and detection of transient ischemia (poor blood flow) • Differentiation between recurrent, active tumor growth and necrotic (dead) soft tissue masses in cancer treatment patients
  20. 20. • Ultrasound is sound at frequency above 20kHz • Diagnostic imaging uses frequencies 1-15MHz • Based on reflection of sound waves at an interface between two media of different acoustic impedance • Relatively inexpensive, fast and radiation-free imaging modality • Pulse-echo technique produces images of soft tissues and fluid filled spaces (e.g. heart, pelvis, kidneys) • Doppler imaging provides a means of measuring blood flow in arteries and veins
  21. 21. • No ionising radiation • Soft tissues can be imaged directly without the injection of any sort of radio-opaque substances or isotopes • Entire abdomen and pelvis can be rapidly scanned while the patient is lying on the table and images can be made of the area in question • Real-time images
  22. 22. • Organs filled with air (e.g. lungs, stomach and intestines) and hard tissues (bone) are opaque to sound • Sound is not able to travel through certain organs → the interiors of these organs and those lying directly beneath them cannot be imaged • Formerly it was almost impossible to view the cervix (kohdunkaula) and lower uterus (kohtu) because they lay under the air-filled intestines. If patient drinks fluid one hour before an ultrasound exam, the distended bladder will push the air- filled intestines out of the way and permit sound to reach the reproductive organs. • The air layer between the patient’s skin and the transducer is a barrier for the sound. To overcome the reflections, some lubricant with similar acoustic impedance to tissue is applied on the patient’s skin.
  23. 23. • An ultrasonic pulse emitted from a transmitter moves through the medium and some of its energy reflects back from the objects within the body • Interfaces between different structures in the body produce echoes at different times • If the velocity of the pulse is known, the time taken for the echo to return can be translated into distance from transmitter. • When the position and the orientation of the transducer and receiver are known, image of the stuctures within the body can be obtained t c d 2 =
  24. 24. • The skin is covered with mineral oil • Transducer is connected to a console with a television screen and placed against the skin near the region of interest. • Transducer emits sound and receives sound. • Transducer produces a stream of inaudible, high frequency sound waves which penetrate into the body and bounce off the organs inside. • Transducer detects sound waves as they bounce off or echo back from the internal structures and contours of the organs. • Different tissues reflect these sound waves differently • Received waves converted to electrical signals and turned into live pictures with computers.
  25. 25. • Piezoelectric crystal is used to excite and detect ultrasound waves. • Applying AC voltage across the crystal faces causes a changing mechanical strain → pressure wave at certain frequency • Conversely, ultrasound waves received at the transducer causes the crystal to vibrate → alternating electric voltage
  26. 26. • Detailed images of the fetus and uterus. • Ultrasound is very operator-dependent. • Transabdominal prenatal ultrasound is used to check on the development of the fetus and look for abnormalities e.g.. – Determining the age of the pregnancy – Examining the baby's physical development and functions – Imaging the limbs and spinal column to check for proper formation and growth – Imaging the development of the brain and other major organs – Determining whether the pregnancy is ectopic or whether there is a multiple pregnancy – Guiding other prenatal tests (e.g.. Amniocentesis) – Guiding prenatal surgery and the safe delivery of medications to the fetus the uterus)
  27. 27. • Used to determine velocities. • Based on the Doppler frequency shift (fd), which is related to the velocity of the object (v) • Can be used to measure blood flow c d v v f θ =
  28. 28. • An application of nuclear magnetic resonance phenomenon (NMR) • Can be used to image nuclei with magnetic moment • Produces images with high contrast between different tissues • Nuclear density and recovery times vary from a tissue to another • Safe procedure (non-radioactive) • Can measure properties which reflect the chemical environment of the nucleus • Can also visualize the flow in the blood vessels
  29. 29. • Measures the density of protons (1H) in the body as a function of the position • In the human body there is a lot of hydrogen (mainly in water and fat) • MRI is used to image soft tissues with high proton concentration (e.g. brains)
  30. 30. • A fundamental property of nature • Comes in multiples of ½ and can be + or - • Almost every nucleus has an isotope with nuclear spin • Nuclear spins having the opposite sign can pair up to eliminate the observable manifestation • Outside magnetic field nucleus are randomly oriented • In magnetic field (B0) nuclei align themselves parallel or antiparallel to the magnetic field – There are little more nuclei in lower energy state – Net magnetic moment (sum of individual magnetic moments) parallel to B0
  31. 31. • In magnetic field protons’ magnetic moment vector precess around the magnetic field at Larmor frequency • At equilibrium the net magnetization is parallel to B0 (z-axis) Y Z X
  32. 32. • When placed in a magnetic field, a particle with a net spin can absorb RF-pulse’s photon at Larmor frequency → net magnetization turns to tranverse plane Y Z X • After RF-pulse the longitudial magnetization recovers and net magnetization returns to the original equilibrium • Precession of transverse magnetization induces a electrical signal in coil
  33. 33. • Depends on nucleus’ gyromagnetic ratio γ and the strength of the magnetic field B0 • Similar nuclei can be differentiated from each other by changing the magnetic field strength BL γω = B0
  34. 34. • Net magnetic vector M consists of two components Mz and Mxy • Two relaxation processes: longitudial T1 and transverse T2 Y Z X MXY MZ M(t) B0
  35. 35. 1 • Describes the recovery of the longitudinal magnetization (Mz) to its equilibrium state • Nuclei returns by dissipating their excess energy • Depends on the surrounding having at Larmor frequency fluctuating magnetic field 1/ 0 Tt z eMM − −=
  36. 36. 2 • Net field B0 around the protons differ from proton to proton → protons don’t precess at the same angular velocity • Transverse magnetization (Mxy) decays because its magnetic moments get out of phase • Loss of coherence is irreversible and results in Mxy falling to zero • T2 is always less than or equal to T1 • Large molecules, which move slowly, promote T2 relaxation 2/ )( Tt xyeMtM − =
  37. 37. PÄÄMAGNEETTI LÄHETYSKELA GRADIENTTIKELAT (X,Y,Z) PÄÄMAGNEETTI LÄHETYSKELA GRADIENTTIKELAT (X,Y,Z) ESIVAHVISTIN MIXERI ADC AMP LIT UDI- MODULAATTORI TEHOVAHVIST IN SYNT ETISAAT T ORI GRADIENT TI- VAHVISTIN TIETOKONE VASTAANOTTOKELA Structure of MR-scanner • Main parts are: magnet, gradients and coils
  38. 38. • A magnetic field that increases in strength along a particular direction • There are x, y and z gradients • Gradients are resistive electromagnets consisting of metallic coils driven by power amplifier • The strength of the gradient refers to the rate at which its magnetic field changes with distance
  39. 39. • If a sample is in uniform magnetic field, the Larmor frequency will be the same of all parts of the sample. • The size of the returning signal reflect the total number of protons in the sample • To localize the signal to a particular point in space, the applied field is made non-uniform. • The localization is done with slice selection, phase and frequency encoding
  40. 40. • Z-gradient generates an extra field Gz in z-direction • Larmor-frequency is now a function of the position in z- direction ( ) ωωγω ∆+=+= 00 GzB LeikkeenvalintaGz VaihekoodausGy TaajuuskoodausGx 9.8 MHz Lukugradientti 9.7 MHz 9.9 MHz9.8 MHz
  41. 41. • Phase encoding in slice – Protons in higher field has a higher angular velocity of precession than protons in lower field – Initially all components are in phase, but gradient field Gy causes the components become out of phase – After a while gradient field is removed and all the components precess again at the same frequency but in different phase • With frequency encoding the protons are localized in x-direction LeikkeenvalintaGz VaihekoodausGy TaajuuskoodausGx 9.8 MHz Lukugradientti 9.7 MHz 9.9 MHz9.8 MHz
  42. 42. • Slice selection with a gradient Gz • Phase encoding with Gy and frequency encoding with Gx • Echo signal from the nuclides received bu a receiving coil • Received signal is amplified and digitized for the image formation • After time TR the process is repeated with different Gy • This is continued until all the data are collected • An image is done from the data with help of Fourier transform
  43. 43. Panorama 0.23T (Philips) Intera (Philips)
  44. 44. Philips
  45. 45. Philips
  46. 46. • Computed Tomography (CT) imaging combines the use of a digital computer together with a rotating x- ray device • Creates detailed cross sectional images or "slices" of the different organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis, extremities, brain, spine, and blood vessels. • CT has the ability to image a combination of soft tissue, bone, and blood vessels. • CT imaging provides both good soft tissue resolution (contrast) as well as high spatial resolution.
  47. 47. • A rotating frame has an x-ray tube mounted on one side and a serie of detectors on the opposite side. • A fan beam of x-ray is created as the rotating frame spins the x-ray tube and detector around the patient. • As x-rays pass through the body they are absorbed or attenuated (weakened) at differing levels creating a profile of x-ray beams of different strength. • The amount of attenuation of the beam depends on the density of the structures the beam passes through at different angles. The densest structures in the human body (like bones and teeth), which attenuate x-rays most, appear most brightly in CT images.
  48. 48. • A rotating frame has an x-ray tube mounted on one side and a detector mounted on the opposite side. • A fan beam of x-ray is created as the rotating frame spins the x-ray tube and detector around the patient. • As x-rays pass through the body they are absorbed or attenuated (weakened) at differing levels creating a profile of x-ray beams of different strength.
  49. 49. • As the x-ray tube and the detector make a 360° rotation, the detector takes numerous profiles of the attenuated x-ray beam. • ”Slice" is collimated (focused) to a thickness between 1 mm and 10 mm using lead shutters in front of the x- ray tube and x-ray detector. • Each profile is subdivided spatially by the detectors and fed into channels. • Each profile is then backwards reconstructed by a dedicated computer into a 2D-image of the “slice”.
  50. 50. (Siemens) CT detector X-ray tube
  51. 51. • Enables direct imaging and differentiation of soft tissue structures (liver, lung tissue, and fat). • Useful in searching for large space occupying lesions, tumors and metastasis (etäpesäke) and examining of their size, spatial location and extent of tumors.
  52. 52. Virtual reality 3-D image of the lungs. The bronchial trees are colored in green and the heart, aorta and vertebrae are colored in red Siemens
  53. 53. • Scanner collects data for a single slice, so the position of the appropriate slice needs to be known. • The x-ray power was transferred to the x-ray tube using high voltage cables. • The rotating frame would spin 360° in one direction and make an image (or a slice), and then spin 360° back in the other direction to make a second slice. • In between each slice, the gantry would come to a complete stop and then reverse directions while the patient table would be moved forward by an increment equal to the slice thickness.
  54. 54. • In the mid 1980's, an innovation called the power slip ring allows electric power to be transferred from a stationary power source onto the continuously rotating gantry. • CT scanners with slip rings can rotate continuously. • Patient is moved continuosly through the detector ring while the source traces a spiral around the patient. • It acquires a volume of data with the patient anatomy all in one position. • This volume data set can be computer-reconstructed to provide 3D-pictures of complex blood vessels like the renal arteries or aorta.
  55. 55. • The primary digital technique for imaging the chest, lungs, abdomen and bones due to its ability to combine fast data acquisition and high resolution in the same study. • It can provide detailed information of nearly every organ in the upper abdomen and pelvis in one quick examination. • New "multi-slice" spiral CT scanners can collect up to four slices of data during spiral CT mode and some rotate at speeds up to 120 rpm (previously 60rpm).
  56. 56. • Measures the distribution of impedance in a cross- section of the body. • Does not use ionising radiation • Safe and often pleasant method for the patient. • Electrical resistivities of different body tissues varies widely from 0.65 ohm m for cerebrospinal fluid to 150 ohm m for bone.
  57. 57. • Measure of how electricity travels though a given material. • Every tissue has different electrical impedance determined by its molecular composition. • Eg. cancerous breast tissue has a much lower electrical impedance than normal tissue and non- cancerous tumors.
  58. 58. • Cancerous tissue causes alterations in intracellular and extracellular fluid compartments, cell membrane surface area, macromolecules, ionic permeability, and membrane associated water layers. • These changes cause measurable changes in tissue electrical impedance. • When a small alternating current is placed across the area of interest, the focal increase in electrical conductance and capacitance of the cancer tissue distorts the electric field. • The impedance map shows the cancer as a focal brightness on the gray scale image of conductivity and capacitance measured by an array of signal sensors on the skin surface.
  59. 59. • A series of electrodes are attached to a subject in a transverse plane. • Electrodes are linked to a data acquisition unit which outputs data to a PC. • By applying a series of small currents to the body potential difference measurements can be made from non-current carrying pairs of electrodes. • Since electric currents take the paths of least impedance, the currents flow depends on the subject's conductivity distribution.
  60. 60. •AC voltage is connected to the wand held by the patient •Electrical current flows through the body from the wand to the scanning probe. •The scanning probe is moved over the breast and its many sensors measures the current signal at the skin level. • The computer reconstructs the information and shows images immediately on the monitor. •Impedance objects are defined as spots or regions that are brighter (or darker) than their surrounding.
  61. 61. Electrical Impedance Tomography (EIT) of the human thorax. The images on the left are of six normal subjects. On the right are six images of patients with lung water associated with cardiac failure. The fluid in the lungs is clearly visible.
  62. 62. • Medical Physics and Biomedical Engineering (Chapter 12) • Imaginis (http://www.imaginis.com/) • Philips Medical Systems (http://www.medical.philips.com)

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