6/22/2011. RT 4912 Review. Rex T. Christensen MHA RT (R) (MR) (CT)

Transcription

1 RT 4912 Review Rex T. Christensen MHA RT (R) (MR) (CT) 1

2 Questions? ARRT Content Specifications: Content-Specification.pdf Tests Can take twice Review Books Websites:

3 MR Active Nuclei Nuclei that aligns its axis to a magnetic field. They do this because of electromagnetic induction: Angular momentum or spin. Posses an electrical charge (+) If two of these characteristics are present it induces the third - magnetism Called a magnetic moment 3

4 MR Active Nuclei (cont.) Odd atomic number. (even proton, odd neutron or vice versa). Spinning mass (magnetic field-example earth). Spinning mass with a charge. 4

5 Precession Precession is the wobble of a top Precessional Frequency is the speed it wobbles. (Mhz) 5

6 Larmor Frequency: The Larmor frequency is also called the Precessional frequency. Precession is a wobble of the hydrogen atom. 6

7 Larmor Equation: MEMORIZE THIS! 7

8 Spin Echo vs. Gradient Echo 8

9 T1 - Fat vs. Water 9

10 Longitudinal Relaxation Time T1 Mo Mz 63% T1 t Longitudinale Relaxation = Energy transfer between excited spins and Tissue (Spin-Lattice-Relaxation) Reestablishing of longitudinal magnetization with time constant T1 10

11 T2 Fat vs. Water 11

12 Transverse Relaxation Time T2 Mxy 37% T2 t Transverse Relaxation = Decay of magnetization by interaction between nuclei (Spin-Spin-Relaxation) 12

13 13

14 Relaxation Times are Tissue Specific M z S Tissue 1 Tissue 1 Tissue 2 Tissue 2 TR Short TE Medium TE Long TE Longitudinal Relaxation Transverse Relaxation 14

15 15

16 Relaxation Times: T1 Relaxation Time: 63 % relaxation of the tissue along the longitudinal (z) axis. T2 Relaxation Time: 63% decay of the tissue along the transverse (xy) axis. 16

17 Free Induction Decay (FID): The induction in reduced signal is called Free Induction Decay (FID) 17

18 Spin Echo Contrasts TR PD T1 T TE 18

19 K-space 19

20 K-space 20

21 K-space 21

22 K-Space 22

23 K-Space 23

24 K-Space 24

25 K-Space (Filling) TR and K-space filling 25

26 K-Space (Filling) Outer Lines = Low signal amplitude (resolution) Inner Lines = High signal amplitude (contrast) 26

27 K-Space (Filling) Low-high High-low Linear Reverse Linear Elliptical concentric 27

28 K-Space 28

29 SNR/Volume size 29

30 TE vs. SNR 30

31 SNR/NSA 31

32 Factors that effect SNR/Spatial Res./Scan Time 32

33 Maximizing SNR/Spatial Res./Scan Time 33

34 Benefits and Limitations 34

35 Types of flow 35

36 Time of Flight (TOF) 36

37 TOF vs. TE 37

38 Co and Counter-Current flow 38

39 TOF pro s and con s 39

40 Cardiac Imaging 40

41 Cardiac Imaging 41

42 Pulse Sequence Diagrams These diagrams show the relation of the 3 gradients (slice,freq, and phase) to the signal formation. 42

43 Pulse Sequence Diagrams 43

44 Reading a Pulse Diagram: Spinecho revisited The timing diagram for a spinecho imaging sequence has entries for the RF pulses, the gradients in the magnetic field, and the signal. A slice selective 90 o RF pulse is applied in conjunction with a slice selection gradient. A period of time equal to TE/2 elapses and a 180 o slice selective 180 o pulse is applied in conjunction with the slice selection gradient.. ignored 44

45 Reading a Pulse Diagram: Spinecho revisited A phase encoding gradient is applied between the 90 o and 180 o pulses. As in the previous imaging sequences, the phase encoding gradient is varied in 128 or 256 steps between G m and -G m. The phase encoding gradient could be applied after the 180 o pulse, however if we want to minimize the TE period the pulse is applied between the 90 o and 180 o RF pulses. ignored 45

46 Reading a Pulse Diagram: Spinecho revisited The frequency encoding gradient is applied after the 180 o pulse during the time that echo is collected. The recorded signal is the echo. The FID, which is found after every 90 o pulse, is not used. One additional gradient is applied between the 90 o and 180 o pulses. This gradient is along the same direction as the frequency encoding gradient. It dephases the spins so that they will rephase by the center of the echo. This gradient in effect prepares the signal to be at the edge of k-space by the start of the acquisition of the echo. ignored 46

47 Multi Echo Sequence 47

48 Turbo Spin Echo 48

49 Inversion Recovery 49

50 Pulse Sequence Pro s & Con s 50

51 Spatial Encoding 51

52 Spatial Localization (image encoding) Magnetic Isocenter. Steepness of gradients. Slice selecting gradient. Frequency encoding (long axis of body part). Phase encoding (short axis of body part). 52

53 Spatial Localization (image encoding) Magnetic Isocenter. 53

54 Spatial Localization (image encoding) Slice selecting gradients. Axial Sagittal Coronal 54

55 Spatial Localization (image encoding) Slice selecting gradient (slice thickness). Steep slope = narrow transmit bandwidth (thin slice). Shallow slope = broad transmit bandwidth (thick slice). 55

56 Slice thickness and location 56

57 Changing slice thickness and location with the gradient. 57

58 Changing slice thickness and location with the pulse 58

59 Spatial Localization (image encoding) Slice selecting gradient (cont.) A slice can be selectively excited by transmitting RF with a and of frequencies coinciding wih the Larmor frequencies of spins in a particular slice defined by the slice select gradient. 59

60 Spatial Localization (image encoding) Frequency encoding gradient (long axis of body part). The frequency encoding gradient is switched on when the signal is received and is often called the readout gradient. The echo is centered in the middle of the frequency encoding gradient. The steepness of the slope of the frequency encoding gradient determines the size of the anatomy covered along the frequency encoding axis during the scan. This is called the field of view (FOV). 60

61 Spatial Localization (image encoding) Phase encoding gradient (short axis of body part). Small phase shift = shallow gradient. Large phase shift = steep gradient. 61

62 Gradient Amplitude vs. Rise Time 62

63 Components of an MRI system 63

64 Magnet Safety As you approach the magnet, the fringe magnetic field gets STRONGER 64 64

65 Fringe Diagram of Active Shield1.5 T* 5 Gauss Siemens Symphony 65 65

66 Room Safety MR Safe MR Conditional MR Unsafe 66

67 SAR and FDA Guidelines -Specific Absorption Rate (SAR) Tissue heating and the body s ability to dissipate excess heat. -Measured in Watts/Kg FDA: -Updated July 2004: 0.4 W/kg whole body 3.2 W/kg head 8.0 W/kg small volume 67

68 Time-Varying Magnetic Field (TVMF) Safety Consider electrical conductivity (Faraday s law of induction) and not just ferromagnetic properties. Scalp burn from a cervical halo 68

69 External Magnet Field Strength Remember safety considerations are different depending on field strength. ALWAYS double and triple check for safety! 69

70 Gd-DTPA (Gad + Chelate) Gadolinium + DiethyleneTriaminePentaAcetic acid-chelate The gadolinium chelates are 100% renally excreted, with the exception of two agents with combined renal and hepatobiliary excretion(multihance and Primovist) 70

71 Relaxivity The ability of magnetic compounds to increase the relaxation rates of the surrounding water proton spins. 71

72 Relaxivity How does it work? T1-relaxation is dependent on the molecular tumbling rate. (T1 weighted exams) The gadolinium molecule is a big tumbling magnet The slower a molecule tumbles, the shorter the T1-relaxation time of the hydrogen molecule (the greater the relaxation rate). 72

73 Relaxivity How does it work? When the gadolinium molecule gets next to the water molecule, it effects the rate at which the water molecule tumbles, slowing it down. 73

74 Efficacy The gadolinium chelates currently available for clinical use can be differentiated on the basis of: charge (ionic or nonionic) structure (linear or cyclic) stability 74

75 Charge (Ionicity) Given that the gadolinium ion carries a 3 charge, if the ligand, for example, is HPDO3A (that for ProHance, with a charge of 3), the metal chelate itself will carry a net charge of zero, and thus be nonionic. In the U.S. market, considering only the gadolinium chelates with 100% renal excretion, there are three nonionic agents (ProHance, Omniscan, and Optimark) and one ionic agent (Magnevist). 75

76 Structure (Linear or Cyclic) The structure of the chelate can be linear or macrocyclic (ring-like), with the cyclic chelates demonstrating higher in vivo stability and thus an improved safety margin. ProHance is the only macrocyclic chelate available in the United States. Internationally, two other extracellular gadolinium chelates are in widespread use, both macrocyclic: Dotarem (ionic) and Gadovist (nonionic). 76

77 MRI Contrast Agents 77

78 Stability Stability is related to structure of the chelate. Gadolinium Chelates differ in their thermodynamic stability constants and their kinetic stability. Macrocyclic chelates are more stable than linear molecules. Even among linear agents there are differences in stabilty. Fundam. Clin. Pharmacol Dec: 20 (6):

79 Osmolality Osmotic (Ion) concentration. An indicator of fluid balance in the bodies tissues. The term "osmolal" describes an ion concentration of a solution in moles per kilogram of solvent (mol/kg), while "osmolar" describes an ion concentration in moles per liter (mol/l). mosm = milliosmole 79

80 Contrast Agents 80

81 Low osmolar contrast media (LOCM) are less nephrotoxic than high osomolar contrast media (HOCM). 81

82 Nephrogenic Systemic Fibrosis (NFS) Development of the disease is due to gadolinium chelate dissociation, with deposition of the free metal, and is thus related to chelate stability, dose, and cumulative (lifetime) dose. 82

83 Pregnancy 83

84 Artifacts Motion (types) Susceptibility Chemical Shift Wrap Around Partial Volume Gibbs Zebra Zipper Cross-Excitation 84

85 Artifacts 85

86 Chemical Shift This is due to the precessional differences in fat and water (fat has a lower precessional frequency than water). This difference is 220 Hz in a 1.5T magnet. It is worse in higher field strength magnets. It creates a dark band at the interfaces of water and fat. It occurs in the frequency direction. 86

87 87

88 Partial Volume Averaging 88

89 Magnetic susceptibility artifact The ability of a substance to become magnetized. (metal and hemosiderin) The differences in these substances with the surrounding tissues results in different precessional frequencies. It occurs in the frequency direction. More obvious in gradient echo sequences. 89

90 Magnetic Susceptibility Artifacts (Understanding MRI) 90

91 Image Artifacts: Magnetic Susceptibility 91

92 Image Artifacts: Aliasing: Anatomy that is bigger than the FOV will wrap back into itself if anti-aliasing techniques are not used. 92

93 Phase Wrap Artifact 93

94 Phase Mismapping Artifact 94

95 Image Artifacts: Zipper: RF leak in the room. Caused by doors (sticky fingers), holes in the wall (nails, screws), door open while scanning, and pipes from air gases or water. 95

96 Image Artifacts: Field Distortion: Scanning off of Isocenter in the z- axis direction with a large FOV. 96

97 Slice Gap 97

98 Motion Artifacts 98

99 Zebra Artifact 99

100 Bandwidth 100

101 6 Key Imaging Goals Optimize Contrast Optimize Coverage and Slice Thickness Optimize Resolution Minimize Artifacts Maximize Patient Throughput Optimize Signal to Noise (SNR) 101

102 Scenario 1 Radiologist asks for a MRCP to visualize the biliary system. 102

103 Scenario 2 Radiologist wants a sagittal T2 FSE fatsat knee with better signal and better resolution. 103

104 Scenario 3 A coronal T1 wrist is too grainy 104

105 Scenario 4 A FSE T2 fatsat brachial plexus does not get good fat saturation. 105

106 Scenario 5 FSE T2 IACs have low spatial resolution 106

107 Build a Protocol/Optimize It! Brain w/wo contrast Routine knee Routine L-spine 107

108 Advantages/Disadvantages Changing FOV Changing Slice Thickness Changing ETL Changing NSA/NEX Changing TE Changing Bandwidth Changing TR 108

109 If this..then this.. P. 137 and P.138 MRI in Practice 109

110 I have a question? 110

111 Website MRI/MRI-Sequences/Sequences-acronyms 111

112 References: Westbrook, Catherine and Kaut, Carolyn (1998). MRI In Practice, 3rd Edition, Malden, MA: Blackwell Science, Inc. 112