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Lecture #6

Lecture #6. INFO 590: Fundamentals of Clinical Care for Health Informaticians. Medical Imaging. kharrazi@iupui.edu http://www.info590.com. Medical Imaging Ultrasound X-ray(Contrast Enhancement) CT Scan MRI Nuclear Medicine Radiology Information System (RIS) PACS and DICOM

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Lecture #6

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  1. Lecture #6 INFO 590: Fundamentals of Clinical Care for Health Informaticians Medical Imaging kharrazi@iupui.edu http://www.info590.com

  2. Medical Imaging Ultrasound X-ray(Contrast Enhancement) CT Scan MRI Nuclear Medicine Radiology Information System (RIS) PACS and DICOM Advances in Medical Imaging Acquisition and Sampling, Colors, Restoration, Segmentation Measurement, Compression, Registration, Visualization Pattern Recogniation Syntactic Statistical Lecture in a Nutshell

  3. Medical (Diagnostic) Imaging

  4. Introduction Ultrasound Radiology (Contrast Enhancement) CT Scan MRI Nuclear Medicine Section Topics

  5. Medical Imaging • Introduction • Medical imaging is to a large extent impossible without the use of computers. Computers are applied in medical imaging to: • Construct an image from measurements • Obtain an image reconstructed for optimal extraction of a particular feature from an image • Present images • Improve image quality by image processing • Store and retrieve images • In medical imaging, images of organisms, organs, or parts of organs, are generated by means of radiation. This radiation is often of an electromagnetic (EM) nature.

  6. Medical Imaging cont. Overview of the use of electromagnetic radiation for medical applications.

  7. Medical Imaging cont. • Ultrasound • Ultrasound is produced by piezoelectric crystals that transform electrical energy into acoustic energy and vice versa. These crystals can vibrate with frequencies of about 2 to 10 MHz which is much higher than audible sound (max 20kHz). • The principles of ultrasound are similar to those of radar and sonar. Pulsed sound waves are emitted and the energies and arrival times of the received echoes caused by the reflections are measured. The main interest lies in determining the distance between the ultrasound source and possible reflectors to show the anatomical images. • The distance between the source and the reflector can be computed by multiplying the time interval between emission of the sound-wave pulse and the detection of the echo coming from a reflector and then dividing the result by two.

  8. Medical Imaging cont. Ultrasound principles

  9. Medical Imaging cont. Ultrasound sound beam and beam return

  10. Medical Imaging cont. • The ability to discern two reflectors that are positioned behind each other is called the axial resolution. • Ultrasound waves are either absorbed, scattered or reflected in the patient. Reflections occur at interfaces between media that are different with respect to density and the velocity of sound (sound is reflected at interfaces with different acoustic impedance). • Soft tissue and water have about equal densities and sound has about equal velocities in the two media. Therefore most of the sound waves are not reflected at their interface. At an interface between soft tissue on one side and bone or air on the other side, a strong reflection is observed. • Scattering takes place if the dimension of the object is small.

  11. Medical Imaging cont. Ultrasound problems with detecting returned beam: (Left) Reflection (Right) Scattering

  12. Medical Imaging cont. • The resolution of an echo scan is the degree with which details located close together can still be distinguished. Resolution is determined by both the wavelength of the sound waves and the duration of the emitted pulse. The smaller the wavelength the better the resolution; therefore the resolution is proportional to the frequency  resolution ~ frequency • The attenuation coefficient (because of scatter and absorption) is also proportional to frequency and therefore the depth of penetration of the sound waves is inversely proportional to the frequency depth ~ 1/frequency • Resolution and penetration depth pose contradictory requirements. Therefore, as deeper structures can only be visualized with relatively low frequencies, their resolution will be lower. Air and bone are strong absorbers whereas muscle and water hardly attenuate the beam.

  13. Medical Imaging cont. • Ultrasound Modes • A-Mode: In the amplitude mode (A-mode) the energy of each echo is displayed as the function of the time interval between the pulse and the echo. Therefore in the image the horizontal axis is time while amplitude will be the vertical axis. This mode provides one-dimensional information about the location of the reflecting boundaries. • B-Mode and M-Mode: The motion mode (M-mode) is similar to A-mode (one dimensional) however the amplitude of each echo belonging to an individual pulse is represented as the brightness (B-mode) of the point located along the time axis. (Top) A-Mode (Bottom) B-Mode

  14. Medical Imaging cont. • C-Scan: In a compound scan (C-scan) the crystal can be moved so that the direction of the beam is changed. The crystal is connected to a flexible arm that is linked to a fixed point of reference. • Sector Scan: In parallel scanning the crystal is moved in such as way that the mean is continuously shifted in a certain direction. Therefore each successive beam makes a small angle with the previous one. Two dimensional images are obtained in which the intensity of the image represents the amplitude of the echo (B-mode). From one-dimensional to two-dimensional

  15. Medical Imaging cont. Three-Dimensional (3D) Ultrasound: usually used in Obstetrics

  16. Medical Imaging cont. • Doppler Effect: It is used for the measurement of flow velocities. A continues stream of sound is generated by a transmitter and another transducer is necessary to detected the reflected sound. In new Doppler the flow map can be superimposed on the echo image to present the colored flow information and anatomical information simultaneously. Doppler Ultrasound: (Left) Doppler mechanism for umbilical cord (Right) Aorta

  17. Medical Imaging cont. • Ultrasound AdvantageIt is harmless and without side effects • Ultrasound DisadvantageLow resolution or complicated views may affect clinical judgment • Ultrasound Applications • Determination of heart functions (echocardiography + doppler) • Examination of the brain • Obstetric examinations (remember that it is harmless) • Eye examinations • Measuring the perfusion of tissues (doppler) • Detection of tumors and cysts • Guided surgery

  18. Medical Imaging cont. • Radiography • X-ray was discovered by Roentgen (1895). Röntgen's original paper, "On A New Kind Of Rays" (Über eine neue Art von Strahlen), was published on December 28, 1895. • On January 5, 1896, an Austrian newspaper reported Röntgen's discovery of a new type of radiation. Röntgen was awarded an honorary Doctor of Medicine degree from the University of Würzburg after his discovery. • He published a total of 3 papers on X-rays between 1895 and 1897. Today, Röntgen is considered the father of diagnostic radiology, the medical specialty which uses imaging to diagnose disease. • He won the first noble prize in physics in 1901.

  19. Medical Imaging cont. An x-ray picture (radiograph) taken by Röntgen of Albert von Kölliker's hand at a public lecture on 23 January 1896

  20. Medical Imaging cont. • X-Ray Imaging • A beam of X rays is generated by an X-ray tube. Behind the patient the transmitted X rays are detected by a fluoroscopic screen which finally exposes a film. • The attenuation of the X-ray beam depends on the type of tissue and the amount of tissue traversed. Lung tissue (air) attenuates the X-ray much less than bone. • Images can be generated on a storage phosphor plate instead of a screen-film. As this image can be scanned by laser, digital X-ray images can be obtained (computed radiology). • The main problem with X-ray imaging is the contrast which is usually low and makes the images not good enough to visualize.

  21. Medical Imaging cont. Principle of an X-ray system with image intensifier. X rays impinging on the image intensifier are transformed into a distribution of electrons, which produces an amplified light image on a smaller fluorescent screen after acceleration. The image is observed by a television camera and a film camera and can be viewed on a computer screen and stored on a CD-ROM or a PACS.

  22. Medical Imaging cont. Chest X-Ray

  23. Medical Imaging cont. • Contrast Enhancement • Blood vessels often absorb as much radiation as the surrounding tissues and therefore cannot be discerned. By injecting contrast agents into blood vessels (iodine which absorbs X-ray) they can be visualized. • A common contrast enhancement procedure is DSA (Digital Subtraction Angiography) where the subtraction of a simple X-ray image from a contrast enhanced image will further increase the visibility of the blood vessels. • Contrast enhancement can be produced in the gastrointestinal system by swallowing a contrast medium (Barium).

  24. Medical Imaging cont. DSA of the bifurcation of the aorta. (a) (b) (c) Principle of contrast enhancement: (a) intensity distribution along a line of an image; (b) same after injection of the contrast medium; (c) intensity distribution after subtraction; (d) intensity distribution after contrast enhancement. (d)

  25. Medical Imaging cont. Barium x-ray of the GI tract (various timings)

  26. Medical Imaging cont. • X-Ray AdvantageEasy to perform, cheap, fast results, bed side • X-Ray DisadvantageLow resolution may affect clinical judgment + does not show the soft tissueRadiation limits its application in pregnant women • X-Ray Applications • Fractures • CxR • Enhanced contrast (Vessels and GI tract) • and many more…

  27. Medical Imaging cont. • Computer Tomography (CT Scan) • Conventional X-ray images are two dimensional and the shadows of organs hides the real geometric distribution of organs. • Hounsfield introduced CT in 1971. He was awarded the Nobel Prize for this invention in 1979. • Now CT scan has passed four generations (latest ones are called helical CT scans…) Hounsfiled’s lab (Original CT-Scan)

  28. Medical Imaging cont. Principle of computed tomography. The combination of X-ray tube and detector is translated across the patient, producing a density profile p(k,f). By rotating the X-ray tube-detector combination, a number of profiles will be obtained. From these profiles the attenuation coefficients of each pixel can be determined.

  29. Medical Imaging cont. The intensity of the transmitted beam as a function of the attenuation coefficient of the pixels traversed. Upper part, the intensity after crossing one volume element; middle part, after traversing n volume elements; lower part, the analog case.

  30. Medical Imaging cont. Chest CT Scan / Multiple cuts

  31. Medical Imaging cont. A modern (2006) CT scanner with the cover removed, demonstrating the principle of operation. The X-ray tube and the detectors are mounted on a ring shaped gantry. The patient lies in the center of the gantry while the gantry rotates around them.

  32. Medical Imaging cont. Example of cross-sections through several parts of the body: skull, thorax, and abdomen, obtained by computed tomography.

  33. Medical Imaging cont. Attenuation coefficients of several tissues expressed in Hounsfield units.

  34. Medical Imaging cont. Three-Dimensional (3D) CT-Scan

  35. Medical Imaging cont. • CT-Scan AdvantageHigher resolution that x-ray. Non-invasive. • CT-Scan DisadvantageExpensive. Radiation. Soft tissue problem. • CT-Scan Applications • Fractures • Chest • Vertebral Column • Abdomen (Vessels and GI tract) (with contrast media) • and many more…

  36. Medical Imaging cont. • Magnetic Resonance Imaging (MRI) • Certain atomic nuclei spin! They behave like small magnets. The hydrogen nuclei is present in all body tissues. Under normal circumstances the body is not magnetic because hydrogen magnets point to randomly different directions and neutralize each other. • Magnetic nuclei tend to align themselves with external magnetic fields (like a compass), but the room temperature decreases this effect. • In an external magnetic field (0.1 tesla) only 1 in 106 nuclei will be realigned. A millimeter of water contains 3x1022 molecules which makes it almost 1017 aligning to the magnetic field.

  37. Medical Imaging cont. • While the nuclei are under the magnetic field, pulses of electro-magnetic (EM) radiation are beamed into the tissue. The magnetic component of the EM radiation exerts a force on the magnetic nuclei. • If the EM radiation has a certain frequency called Larmor frequency and if they become perpendicular to the main magnetic field, it creates the 90 degree RF excitation pulse which causes the nuclei to resonance. • Certain factors may influence MRI such as flow phenomena in blood or cerebrospinal fluid. Flow can either enhance or void the process. Magnetic resonance angiography (MRA) is based on signal enhancement.

  38. Medical Imaging cont. The phenomenon of spins aligning themselves to an external magnetic field. At 0 K all spins are aligned when an external magnetic field is present:(a) when no external magnetic field is present the spins will point in all directions (b) at room temperature only a small part of the nuclei will align themselves: 1/million (c) at a field strength of 0.1 tesla and 5 per million (d) at a magnetic field strength of 0.5 tesla

  39. Medical Imaging cont. Precession of magnetization under the influence of an external magnetic field with strength Bo and an oscillating field B1 (due to electromagnetic radiation) during a 90 RF pulse as seen from the observer (A) and as seen from the standpoint of the rotating field (B)

  40. Medical Imaging cont. Brain MRI

  41. Medical Imaging cont. (Left) Knee MRI (Right) Chest MRI

  42. Medical Imaging cont. Three-Dimensional (3D) MRI

  43. Medical Imaging cont. • MRI AdvantageHigher resolution. Non-invasive. No radiation. Shows the soft tissue. • MRI DisadvantageExpensive. Bone tissue problem. Magnetic field harmful for metal implants. • MRI Applications • Soft Tissue • Brain and Spine • Liver, pancreas, and … • Chest • and many more…

  44. Medical Imaging cont. • Nuclear Medicine/Imaging • During nuclear medicine examination, radioactive labeled material (radiopharmaceutical) is injected depending on the type of the exam. For example iodine is taken up by thyroid. • The radiation of the radioactive material can be detected by a gamma-camera which is sends the information to a computer for further processing. • For medical examinations it is better to use an isotope with a relatively short half-life. • Different types of nuclear medicine are: Histogram Mode, List Mode, Synchronized Recordings and Three Dimensional Reconstruction (SPECT and PET).

  45. Medical Imaging cont. Principle of the gamma camera.

  46. Medical Imaging cont. • Histogram Mode: In this mode the viewing area of the gamma camera is divided into a matrix of picture elements (pixels). The pixels become darker (or more red) by adding more pictures of the same place by time. This method can be used to examine the activity distribution in the target organ. Histogram mode is based on a predefined time interval. • List Mode: In this mode no predefined time interval is used. The system tries to detect as much sections it can. • Synchronized Recordings: Many physiological phenomena occur very fast. This type of recording is suitable for periodic or quasi-periodic phenomena such as the R-wave from ECG. By dividing the RR interval into a sufficient number of intervals it becomes possible to obtain scintigraphic images of the beating heart.

  47. Medical Imaging cont. Histogram Mode: Example of a scintigram above the lungs, obtained with a gamma camera. Different intensities have been coded with different colors.

  48. Medical Imaging cont. Synchronized Recordings: Principle of ECG-gated scintigraphy.

  49. Medical Imaging cont. A whole body PET/CT Fusion image

  50. Medical Imaging cont. Nuclear Medicine myocardial perfusion scan with Thallium-201

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