The physics of medical imaging US, CT, MRI. Prof. Peter Bogner

Transcription

1 The physics of medical imaging US, CT, MRI Prof. Peter Bogner

2 Clinical radiology curriculum blocks of lectures and clinical practice (7x2) Physics of medical imaging Neuroradiology Head and neck I. Head and neck II. Thoracic radiology Gastrointestinal radiology Urogenital and interventional radiology Final exam: test + oral

3 Medical imaging Energy matter is required or not Imaging needs energy-matter interaction Part of the energy is deposited

4 Sound waves mechanical disturbance, a pressure wave moves along longitudinal wave compression rarefaction zones c = nl, (c: velocity, n: frequency, l: wavelength diagnostic range: MHz

5 Ultrasound velocity Relaitvely low in gases higher in solids Velocity changes with the wavelength, frequency remains approx. constant c = E/ (E ~ elasticity, ~ density) Variation in velocity artifacts

6 60-90% US interactions in tissues

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8 Reflection the basis of US imaging it occurs on the interface of two media, that have different acoustic impedances (Z) acoustic impedance ~ density + elasticity (similarly to velocity) impedance (Z) = ( )(v) (rayl = kgm 2 sec -1 )

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11 Transducer

12 Adjustable focus

13 A(mplitude) B(rightness) M(otion) doppler Presentation modes

14 Presentation modes M doppler

15 Resolution

16 Resolution

17 CT - computed tomography Godfrey N. Hounsfield Allan M. Cormack The Nobel Prize in Physiology or Medicine 1979

18 MRI - magnetic resonance imaging Sir Peter Mansfield Paul C. Lauterbur The Nobel Prize in Medicine 2003

19 Superimposition

20 Limitations of radiography superimposition slice thickness in direction! estimation of x-ray attenuation tissue contrast sensitivity: ~ 10%

21 computer tomography minimal superimposition improves tissue contrast tissue characterization (CT number)

22 computed tomography

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24 CT principles attenuation coefficient reconstruction algorithm CT number Hounsfield value

25 element of CT image: voxel FOV field of view image matrix slice thickness

26 Spatial encoding detectors number of slices : 16,32,64,128,256,384

27 Detector configuration

28 CT contrast factor physical (electron) density 1 HU is 0.1% change in attenuation 4 HU can be differentiated 0,008 g/cm 3 between white matter and white matter edema: 2.6 HU 1 % difference in water content (% wet weight) cytotoxic edema can produce a 3% increase in water within 1 hour (hypoxia)

29 relationship between electron and physical density x-ray attenuation

30 windowing

31 windowing

32 window settings 1 HU is 0.1% change in attenuation 4 HU can be differentiated experienced reader can differentiate 10-15% change of contrast in radiography

33 CT contrast media Oral contains iodine or barium Intravenous iodinated, non-ionic Injector volume, timing (phases), flow (ml/s) Adverse effects, contraindications, nephrotoxicity (ESUR guidelines)

34 Pre- and post-contrast (iv.)

35 CT methods morphologic/structural imaging preand postcontrast (multiple phases) CT angiography, perfusion CT neuro, orbits, sinuses, cervical soft tissue, thorax, abdomen, pelvis, traumaemergency, staging, radiation oncology planning, surgical planning, cardiac, virtual colonoscopy, etc.

36 isotropic voxel reconstruction

37 cardiac CT

38 virtual endoscopy

39 CT angiography

40 characteristics of modern CT imaging isotropic voxels powerful reformatting possibilities high contrast good temporal resolution (e.g. cardiac imaging, CT-fluoroscopy) volume scanning: good spatial resolution lower patient dose

41 Damadian R Tumor detection by nuclear magnetic resonance. Science 1971, 171:

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43 Hydrogen without magnetic field Normally, spins are randomly oriented No net magnetization (M=0)

44 Hydrogen in magnetic field Two possible orientations Paralell: lower-energy state Anti-paralell: higher-energy state Natural systems tend to have minimal energy Thermal agitation : only small imbalance

45 Hydrogen in magnetic field At 1.5 T only 1 spin in a contribute to the net magnetization (M) M

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47 Larmor equation w=gxb 0

48 Excitation

49 Signal detection

50 Signal detection

51 Relaxation in z direction: T1

52 Relaxation in the x-y plane: T2

53 spin echo (SE) pulse sequence

54 sequence parameters TR 90º TE TR: time to repeat TE: time to echo

55 Basis of imaging : spatial localization

56 Gradient magnetic field Magnetic field gradients are used to change the strength of the main magnetic field (B0) Different spatial locations become associated with different precession frequencies

57 Spatial encoding It is done using magnetic gradients Slice selection - z Frequency encoding - x Phase encoding - y

58 Slice selection Based on Larmour equation: ω 0 =γb 0 The central frequency of RF pulse determines the particular location excited Different slice positions are achieved by changing the central frequency Hydrogen nuclei located outside the slice plane are not excited, they will not emit a signal

59 Frequency encoding It causes range of Larmor frequencies to exist in the direction in which it is applied

60 Frequency encoding Fourier transform to separate the different frequencies out after the signal detection

61 Phase encoding Protons located at different positions in the phase encoding direction experience different amounts of phase shift. Repeating the signal detection multiple times with different amplitude of phase-encoding gradient.

62 Review: Image Formation Fourier transform k y k x k-space image space Data gathered in k-space (Fourier domain of image) Gradients change position in k-space during data acquisition (location in k-space is integral of gradients) Image is Fourier transform of acquired data

63 The timing of spin echo sequence

64 Magnet types in MRI In Permanent magnets the magnetic field is always there and always at full strength (<0,5T). Resistive magnets are made from many coils of wire wrapped around a cylinder through which an electric current is passed. This generates a magnetic field. When the electricity is shut off, the magnetic field dies. Superconducting magnets are somewhat similar to resistive magnets - coils of wire with a passing electrical current create the magnetic field. The important difference is that in a superconducting magnet the wire is continually bathed in liquid helium. Always on at full strength.

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67 MR safety!

68 MR safety!

69 Coils

70 MR contrast, methods T1W pre- and post-contrast T2W T1/T2W mixed Fat or water sturation (pl. FLAIR, STIR, Dixon ) Diffusion weighted (DWI, DTI) Susceptibility weighted T2* (SWI, fmri) Flow-sensitive MR angiography (TOF, PC, 3D CE) in vivo spectroscopy

71 molecular basis of MRI tissue contrast 1. water content Relaxation rate is directly proportional to water content 2. restricted water movement Originates from the interaction of water and macromolecules. This phenomenon is common in pathologic tissues. 3. macromolecular motion It also influences water motion. Other parameters might also be important, like ph, ion concentration, polimerisation of macromolecules, etc. 4. lipid content Hidrophobic lipids membranes 5. paramagnetic ions Primarily paramagnetic iron; contrast agents..

72 T1 weighted

73 T2 weighted

74 FLAIR & T2 weighted

75 Myelinisation T1 weighted

76 T2 weighted submilimeter slice thickness

77 Pre- and postcontrast T1 weighted Note the metallic artifact in the mouth!

78 T2 weighted and DWI

79 Inversion Recovery TR = 2000 ms TI (ms)

80 1.5T vs. 3T

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82 fat suppression

83 Late enhancement method myocardial infarction

84 MR-guided neurosurgery

85 Fetal imaging