BME Introduction to BME. Bioelectrical Engineering Part: Medical Imaging

Transcription

1 BME Introduction to BME Bioelectrical Engineering Part: Medical Imaging Reference Textbook: Principles of Medical Imaging, by Shung, Smith and Tsui Lecturer: Murat EYÜBOĞLU, Ph.D. Dept. of Electrical and Electronics Engineering Middle East Technical University, Ankara - Turkey 1

2 BME Introduction to Biomedical Engineering Bioelectrical Engineering Part: Medical Imaging... 3h (X-ray imaging, Computerized Tomography, Medical Ultrasound Imaging, Nuclear Medicine Imaging, Magnetic Resonance Imaging) (Dr. B. Murat Eyüboğlu) Bioelectric phenomena... 3h (Dr. Yeşim Serinağaoğlu) Medical Instrumentation, mathematical modeling of physiological control systems... 3h (Dr. Nevzat G. Gençer) Lab Practice h 2

3 Outline What is medical imaging History Projection Imaging Computerized Tomography (CT) Nuclear Source Imaging (PET, SPECT) Ultrasonic Imaging Magnetic Resonance Imaging Electrical Impedance Imaging 3

4 What is medical imaging? Medical imaging is a collection of techniques, that are developed to measure and display distribution of a physical property in living subjects, specifically in humans. Why is it useful? Medical imaging, not only provides useful information for diagnosis but also serves to assist in planning and monitoring the treatment of malignant disease. 4

5 Simplified block diagram of a Medical Imaging System 5

6 Which energy types are used for imaging? X-ray Nuclear (radio-isotope) sources, Ultrasonic waves, Magnetic fields, Electrical currents, Mechanical, Optical waves etc. 6

7 Electromagnetic spectrum 7

8 What are the physical properties of interest? X-ray absorption coefficient, Radionuclide concentration, Ultrasonic properties, Spin density and spin relaxation, Electromagnetic properties, Mechanical properties, Optical properties. 8

9 Why are we interested in these physical properties? Certain physical property may vary between different healthy tissue types, with the physiological state of a tissue type, with the pathological condition of a tissue type. 9

10 Why are there so many imaging modalities? All imaging modalities are based on the physics of the interaction of energy and matter. Different imaging modalities are based on physical interaction of different energy types with biological tissues and thus provide images of different physical properties of the tissues. 10

11 History Discovery of X-rays, 1895, Radon transform, 1917, NMR principles, 1946, Nuclear medicine scan, 1948, Ultrasound imaging, 1952, Positron tomography, 1953, Single Photon Emission CT, 1971 Development of X-ray CT, 1972, NMR Imaging, 1976, Impedance Tomography,

12 X-ray Projection Radiography p θ ( t ) β( x, y ) X-ray tube p θ ( t ) + = β( x, y )ds = β( x, y ) δ ( xcosθ + y sinθ t ) dxdy Patient Film t Radon Transform 12

13 Attenuation Coefficients for Biological Tissues at 60 kev Tissue Attenuation coefficient (cm -1 ) Blood Brain matter Water Fat Bone Air

14 Typical Chest X-ray Radiograph 14

15 X-ray tube design Cathode with focusing cup, 2 filaments (different spot sizes) Anode Tungsten, Z w = 74, T melt = 2250 ºC Embedded in copper for heat dissipation Angled (see next slide) Rotating to divert heat 15

16 X-ray tube Working Principle: Accelerated charge causes EM radiation: Cathode filament C is electrically heated (V C = ~10V / I f = ~5 A) to boil off electrons Electrons are accelerated toward the anode target (A) by applied high-voltage (V tube = kv); kinetic electron energy: K e usually rated in peak-kilo voltage kvp Typical: V tube = kvp, I tube = mA Deceleration of electrons on target creates "Bremsstrahlung" V C, I f + - C A kvp, I tube

17 X-ray tube design Tungsten Anode is desirable as: It has high melting point, Little tendency to vaporize, It is strong. 17

18 X-rays characteristics EM radiation at wavelengths kev ( nm). Diagnostic Range X-rays typically have a wavelength from 100nm 0.01nm ~1-100 kev. X-ray radiation is thought to be particles traveling at the speed of light and carrying an energy given by E=hf. (Plank constant h=4.13x10e-18 kev/hz, 1eV=1.6x10E-19Joules) These particles are called QUANTA or PHOTONS. A photon having an energy level greater than a few electron volts is capable of ionizing atoms an molecules. Ionization energy for valence electrons < ~10 ev X-rays is ionizing radiation (harmful) 18

19 Example: UV light bulb Photon energy > a few evolts may result in ionizing radiation. For a UV light bulb: l=100nm. results in f = c/l = 3x10E8 / 1x10E-7 = 3x10E15Hz. E=h f = 12eV is ionizing radiation. 19

20 Tomographic Imaging cut image 3-dimensional subject 2-dimensional slice 20

21 X-ray CT Source Patient Detector array 21

22 CT Scan First scan Second scan 22

23 CT Scan First scan Second scan Third scan 23

24 CT Scan First scan Second scan Third scan Fourth scan 24

25 Image Reconstruction - Backprojection β b, θ + ( x, y ) = p ( t ) δ ( xcosθ + y sinθ t ) dt θ 25

26 Image Reconstruction - Backprojection β ( x, y ) = π + p xcosθ + y sinθ t b BME-501 θ ( t ) δ ( )dtdθ 26

27 Backprojection Example 1: True distribution 27

28 Example 1: Backprojection

29 Backprojection 5/5 5/5 5/5 5/5 5/5 11/5 11/5 11/5 11/5 11/5 7/5 7/5 7/5 7/5 7/5 7/5 7/5 7/5 7/5 7/5 5/5 5/5 5/5 5/5 5/

30 Backprojection 5/5 +5/5 5 11/5 +5/5 7/5 +5/5 7/5 +5/5 5/5 +5/5 5/5 +7/5 11/5 +7/5 7/5+ +7/5 7/5 +7/5 5/5 +7/5 5/5 +11/ 11/ / 5 7/5 +11/ 5 7/5 +11/ 5 5/5 +11/ /5 +7/5 11/5 +7/5 7/5 +7/5 7/5 +7/5 5/5 +7/5 7 5/5 +5/5 11/5 +5/5 7/5 +5/5 7/5 +5/5 5/5 +5/

31 Backprojection 10/5 12/5 16/5 12/5 10/5 16/5 18/5 22/5 18/5 16/5 12/5 14/5 18/5 14/5 12/5 12/5 14/5 18/5 14/5 12/5 10/5 12/5 16/5 12/5 10/

32 Backprojection 10/5 12/5 16/5 12/5 10/5 16/5 18/5 22/5 18/5 16/5 12/5 14/5 18/5 14/5 12/ /5 14/5 18/5 14/5 12/5 10/5 12/5 16/5 12/5 10/

33 Backprojection 10/5 +9/5 12/5 +6/4 16/5 +5/3 12/5 10/5 16/5 +6/4 18/5 +9/5 22/5 +6/4 18/5 +5/3 16/5 12/5 +3/3 14/5 +6/4 18/5 +9/5 14/5 +6/4 12/5 +5/3 12/5 14/5 +3/3 18/5 +6/4 10/5 12/5 16/5 +3/3 14/5 +9/5 12/5 +6/4 12/5 +6/4 10/5 +9/

34 34 Backprojection 10/5 +9/5 12/5 +6/4 16/5 +5/3 +5/3 12/5 +6/4 10/5 +9/5 16/5 +6/4 18/5 +9/5 +5/3 22/5 +6/4 +6/4 18/5 +5/3 +9/5 16/5 +6/4 12/5 +3/3 +5/3 14/5 +6/4 +6/4 18/5 +9/5 +9/5 14/5 +6/4 +6/4 12/5 +5/3 +3/3 12/5 +6/4 14/5 +3/3 +9/5 18/5 +6/4 +6/4 14/5 +9/5 +3/3 12/5 +6/4 10/5 +9/5 12/5 +6/4 16/5 +3/3 +3/3 12/5 +6/4 10/5 +9/

35 Backprojection

36 Backprojection 36

37 Two basic strategies for producing an image that doesn t have the blurring seen in the preceding example: Backproject, and then perform a second, repair operation on the image to correct the blur (Backprojection Filtering algorithms), Modify the projection data in an appropriate manner, so they will produce an unblurred image, before backprojecting (Filtered backprojection algorithms). 37

38 Filtered Backprojection Backprojected image represents a blurred version of the original distribution: 1 βb x, y ) = β( x,y )** F2 r This blurring effect can be removed as, { βb( x, y )} = F { β( x,y )} ρ ( 2 β bf 1 = { ρ F { β ( x, y )}} ( x, y ) F2 2 b Filtering can be applied to projections prior to backprojection which is computationally more effective: F { p ( t )} { } 1 F ρ = p ( t )** { ρ } θ θ F1 1 38

39 Filtered Backprojection Measure projections from all possible view angles Convolve all projections with the filtering function h(t) Backproject the filtered projections 39

40 Performance of CT Spatial resolution of 1 mm. (minimal distance between two pixels which can be discriminated is 1 mm.) Contrast resolution of 1 % (i.e, pixel density which is 1% different than the background density can be discriminated.) Soft tissue contrast is low. Invasive : X-rays are harmful for living organisms i.e. contains ionizing radiation. 40

41 Nuclear Source Imaging Planar Scintigraphy : Radioisotopes (radionuclides) are injected to the body, They emit radiation which can be detected by photon detectors and the position of the isotopes can be determined, Two-dimensional representations of the projections of three-dimensional activity distributions are reconstructed. 41

42 Nuclear Source Imaging Emission Computed Tomography: is a technique to obtain cross sectional images of activity, SPECT: Single gamma ray is emitted per nuclear disintegration. PET: Two gamma rays are emitted when a positron from a nuclear disintegration annihilates in tissue. 42

43 Nuclear Medicine - Brain 43

44 SPECT and PET CT SPECT DUAL PET perfusion scan of heart SPECT Neuroblastoma p θ + ( t ) = A( x, y ) δ ( xcosθ + y sinθ t )e dxdy s β ( s )ds 44

45 Advantages and Disadvantages of Nuclear Source Imaging Functional images can be obtained, Spatial resolution is poor, Good tissue specific contrast, Involves ionizing radiation. 45

46 Ultrasonic Imaging Body is probed by Ultrasonic waves, Ultrasound wave propagates through the body, Fraction of the ultrasound waves are reflected at various tissue interfaces along the wave path, producing echoes, The reflected echo signals are measured and used to reconstruct the reflection coefficient distribution along the path. 46

47 Reflectivity of normally incident waves Materials at interface Reflectivity Brain-skull bone 0.66 Fat-bone 0.69 Fat-blood 0.08 Muscle-blood 0.03 Muscle-liver 0.01 Soft tissue-water 0.89 Soft tissue-air

48 Ultrasound Imaging x x Burst of US wave is transmitted + p r ( t ) = p t ( t Reflected wave is measured x 2 c ) f ( x )dx f(x): total reflectivity from a line at x 48

49 Ultrasound imager 49

50 Ultrasound Imaging Ultrasound scanner US image of a fetus hand 50

51 Ultrasound Doppler 51

52 B-Scan ultrasound 52

53 3D ultrasound 53

54 What is your infant upto? 54

55 Advantages and Disadvantages of Ultrasound Functional images can be obtained, Involves no ionizing radiation, Portable. 55

56 Magnetic Resonance Imaging MR imaging system 56

57 Magnetic Resonance Imaging GRADIENT COILS MAGNET RF COIL 57

58 58

59 Magnetic Resonance Imaging 59

60 Use of gradient fields in MRI S( t ) = K [( γg x )t ( γg y )t ] M( x, y )exp{ j }dxdy x + y y The emitted magnetization signal is measured which is the 2-dimensional Fourier Transform of the spin density (proton density) distribution. 60

61 First in-vivo MRI experiment in 1977, by Damadian, Minkoff and Goldsmith 61

62 MR Images of human head Coronal Slice of Head Axial Slice of Head 62

63 Advantages and Disadvantages of MRI Superior spatial resolution, Good soft tissue contrast, Functional imaging is possible, Involves no ionizing radiation, Relatively expensive. 63

64 Electrical Impedance Tomography EIT : cross-sectional imaging of electrical impedance injected EIT induced EIT 64

65 Electrical Impedance Tomography 65

66 ACEIT ventilation scan Left lung ANTERIOR Right lung Mediastenum 4th intercostal space level dynamic ventilation scan 66

67 Cardiac Gated EIT Images 67

68 Advantages and Disadvantages of EIT Functional images can be obtained, Good soft tissue contrast, Involves no ionizing radiation, Poor and position dependent spatial resolution, Low sensitivity to inner regions. 68

69 X-Ray Imaging - 1: History and Physics background Modified from SUNY Downstate Medical Center BMI Lecture Notes Reference Textbook: Principles of Medical Imaging, by Shung, Smith and Tsui 69

70 Discovery of x-rays Wilhelm Konrad Röntgen ( ) 1923) (photographed in 1896) X-ray history on the web: Physical Institute, University of Würzburg, Germany. 70

71 First x-ray images The famous radiograph made by Roentgen on 22 December 1895, and sent to physicist Franz Exner in Vienna. This is traditionally known as "the first X-ray picture" and "the radiograph of Mrs. Roentgen's hand. " Radiograph of the hand of Albert von Kolliker, made at the conclusion of Roentgen's lecture and demonstration at the Würzburg Physical-Medical Society on 23 January

72 Early x-ray setup 72

73 Physical foundations 73

74 Complex Atoms Number of protons Z: Atomic number (determines element) Number of neutrons N: Neutron number Number of protons + neutrons A m = Z + N: Mass number 22 Na 11 Valence electron K-shell (n=1, strongly bound) L-shell (n=2) M-shell (n=3, weakly bound)... 74

75 Atom and electronic transitions Electrons (-) are organized in shells around nucleus (+) Higher shell (greater shell radius) = higher electronic energy Electronic transitions between shells require or release energy Emission E = hν = E 2 -E 1 Absorption E = hν = E 3 -E 2 Excitation Relaxation + n = 1 n = 2 n = 3 75

76 Energy scheme Binding energy (BE): energy binding electron to atom Ionization energy I K,L, = - BE: amount of energy needed to remove electron from atom BE counted in negative units of electron volts (ev) At infinity, BE = 0. N M Continuum Zero L K E Binding energy for 53 I: kev (K), -4.3 kev (L), -0.6 kev (M) BE for valence electrons: ~ -10 ev (H: ev) 76

77 Energy units SI unit: 1 Joule [J] = 1 Nm = 1 kg m 2 s -2 Electron volt [ev]: The potential energy of one elementary charge gained/lost (e = C) when crossing a potential difference of 1V: - 1V + 1 ev = C 1 V = [A s V] = J 100 kev = J = J = 16 fj ( per photon) 77