Scan Acceleration with Rapid Gradient-Echo

Transcription

1 Scan Acceleration with Rapid Gradient-Echo Hsiao-Wen Chung ( 鍾孝文 ), Ph.D., Professor Dept. Electrical Engineering, National Taiwan Univ. Dept. Radiology, Tri-Service General Hospital 1 of 214

2 The Need for Faster Scan Patient comfort, motion artifacts, efficiency, more information EPI? You know the difficulty now But there are a lot more ways to accelerate the scanning 2 of 214

3 Back to the Old Formula Scan time for single slice = TR x (# phase encoding) x NEX Reduce phase encoding A little faster, trade in resolution Reduce NEX (1 or 0.5 at most) 3 of 214

4 MRI Scan Time (1990?) Spin-echo : 256x256, 2 NEX PD, T2 : 16 min (TR 2000) T1 : 5 min (TR 600) Note : somewhat exaggerated 4 of 214

5 Short Scan Time? Reduce TR ( msec?) 40 times faster? 256x256, 1 NEX : 13 sec Sounds like an efficient way? 5 of 214

6 Short Scan Time? Reduce TR Increased T1-weighting Reduced SNR 6 of 214

7 Signal Intensity Effects of Reduced TR on T1 Contrast Signal at this TR TR Substantial reduction in TR leads to SNR loss 7 of 214

8 How Low Is The SNR? TR = T1 : ~ 63% of thermal equilibrium TR = 0.1 T1 : ~ 9.5% of thermal equilibrium 8 of 214

9 Compensating SNR? SNR loss due to slow T1 recovery CSF T1 = 0.7 ~ 4.0 sec Can magnetization recover from nonzero (positive) values? Can we retain Mz after RF pulsing? 9 of 214

10 Small Flip Angle RF Excitation B o z' z' B o y' y' x' x' Mxy for data acquisition, some Mz for next excitation 10 of 214

11 The FLASH Technique Reducing TR without sacrificing too much SNR Achievable by lowering the flip angle via B1 amplitude adjustment Fast Low-Angle SHot (FLASH) Haase et al., of 214

12 SNR Comparison TR = T1, a = 90 0 : ~ 63% of Mo TR = 0.1 T1 : a = 90 0 : ~ 9.5% of Mo a = 25 0 : ~ 22% of Mo 12 of 214

13 Example CSF has T1 ~ 700 msec TR lowered to ~ 70 msec Slightly lowered quality Scan time ~ 18 sec allows breath-hold exams 13 of 214

14 Here Comes A Question Spin-echo no longer useable! Imaging has to be done with gradient-echo Bo inhomogeneity is going to affect image quality But can also become another source of diagnostic information 14 of 214

15 Effects of Refocusing Pulse z' z' B o B o x' y' x' y' Mz becomes negative (recovery takes even longer)! 15 of 214

16 Gradient-Echo Properties No refocusing function from pulse Image affected by Bo inhomogeneity T2* decaying (not T2) Instrumentation, air-tissue interface Hemorrhage, hematoma, bone 16 of 214

17 Effects of Bo inhomogeneity Image voxel : Non-uniform Bo in a voxel short T2* 17 of 214

18 Effects of TE TE = 9 msec TE = 18 msec 18 of 214

19 Gradient-echo Is Worse? Image quality for gradient-echo is often harder to control than for spin-echo But that does not mean bad Proper usage gives useful information that cannot be provided by spin-echo 19 of 214

20 Application Examples Hemorrhage (Iron in blood) Brain perfusion (Gd-based agent) Blood oxygenation (deoxy-hb) Brain fmri (BOLD contrast) 20 of 214

21 T2* Signal Loss in Hemorrhage T1 PD T2 GrE 21 of 214

22 T2* Signal Loss & Blood Oxygenation Normal air Pure oxygen 22 of 214

23 Brain Oxygenation & Brain Function 23 of 214

24 That Looks Good Then! Short TR faster scan Small flip angle not much SNR loss Gradient-echo more information Worth some more exploration! 24 of 214

25 Effects of Flip Angle Small flip angle, partial flip angle (< 90) How small should it be? 10 0? 30 0? 70 0? Arbitrary? 25 of 214

26 Flip Angle Contrast Short TR T1WI, long TR T2WI Only true for 90 0 excitation TR already short in gradient-echo No longer use TR to alter T1 contrast 26 of 214

27 How to Vary T1 Contrast? Magnetization vector goes into a steady state after several RF pulsing Image intensity mainly determined by this steady state behavior Steady state: T1 recovery for Mz in one TR = Mz reduction due to RF pulsing 27 of 214

28 Steady State with Many RF Pulses z' B o B o x' y' Assuming no residual Mxy at the end of TR 28 of 214

29 The Formula Is Actually Signal proportional to (1 - e -TR/T1 ) sin a 1 - e -TR/T1 cos a e -TE/T2* a : flip angle 29 of 214

30 Simple Rule for PD or T1 Contrast B o z' recover from 0 z' little room for recovery B o y' y' x' x' T1WI PDWI 30 of 214

31 Control of T1 Contrast Large flip angle (~ 90 0 ) Similar to short-tr images (T1) Small flip angle (20 ~ 40 0 ) Reduced T1 weighting (PD) 31 of 214

32 Flip Angle = 10 0 z' x' y' Proton-density-weighted image 32 of 214

33 Flip Angle = 20 0 z' x' y' 33 of 214

34 Flip Angle = 30 0 z' x' y' 34 of 214

35 Flip Angle = 40 0 z' x' y' 35 of 214

36 Flip Angle = 50 0 z' x' y' T1-weighted image 36 of 214

37 Flip Angle = 60 0 z' x' y' 37 of 214

38 Flip Angle = 70 0 z' x' y' Strong T1-weighted image 38 of 214

39 Comparison of PD & T1 Contrast of 214

40 Control of T2 Contrast Still use TE (< TR) Actually T2* in gradient-echo TE does not have to be too long T2* contrast very similar to T2WI (other than Bo inhomogeneity) 40 of 214

41 T2(*) Weighting z' B o x' y' Decay of transverse magnetization 41 of 214

42 Using TE to Control T2(*) Contrast TE = 10 TE = 30 TE = of 214

43 Myth of Speeding Scan Is the examination time really shortened? 43 of 214

44 Expansion of a Pulse Sequence B1 Gs Gp Gr t t t t TR >> TE : hardware mostly idle 44 of 214

45 Add Different Slices B1 Gs Gp Gr t t t t Making best use of the idle time 45 of 214

46 Add Even More Slices B1 Gs Gp Gr t t t t Multi-slice imaging (scan time not lengthened) 46 of 214

47 Myth of Speeding Scan As TR shortens, the number of slices becomes smaller in a TR Multiple slice coverage repeat the scan several times! Total exam time likely unchanged totally useless?? 47 of 214

48 Pros of Speeding Scan Faster single-slice scan Less motion influences 3D becomes possible! 2D examination time is not necessarily shortened 48 of 214

49 Scan Time Advantages 6.4 seconds 3.8 seconds 2.5 seconds 1.5 seconds 49 of 214

50 Speeding Even Further? TR shortened to ~10 msec : Flip angle reduced to ~ sec scan time!! High-quality MR images for uncooperative patients? 50 of 214

51 RF Pulsing with Very Small Flip Angle B o z' z' B o y' y' x' x' Very small TR and flip angle : PDWI 51 of 214

52 Clinical Restrictions TR ~ 10 msec : PDWI (seldom used clinically) TE < TR Then T2 contrast...? 52 of 214

53 Gradient-echo with Very Short TR B1 TR... t Gs... t Gp... t Gr... t TE TE < TR 53 of 214

54 If You Study Too Hard... CE-FAST, PSIF, SSFP-echo... TE can be larger than TR Complicated principles, questionable applications out of scope! 54 of 214

55 Clinical Restrictions TR ~ 10 msec : PDWI (seldom used clinically) TE < TR Then T2 contrast...? 55 of 214

56 Back to Imaging Principle RF with very small flip angle T2 & T1 relaxation Next RF pulsing Repeat many times 56 of 214

57 RF Pulsing with Very Small Flip Angle B o z' z' B o y' y' x' x' Magnetization almost the same before/after RF 57 of 214

58 When TR Is Very Short RF with very small flip angle T2 & T1 relaxation not obvious Next RF pulsing Repeat many times Contrast determined by initial Mo 58 of 214

59 No Obvious Relaxation? Can be useful! Use even smaller flip angle and TR Relaxation plays a minor role Manipulate the initial magnetization vector to change contrast! 59 of 214

60 How to Manipulate M? RF pulsing, of course! (-90 0 ) : T2-weighted magnetization! 60 of 214

61 (-90 0 ) B1 z' x' y' Spin echo principle 61 of 214

62 (-90 0 ) B1 z' Short T2 x' y' Long T2 Mz becomes T2-dependent! 62 of 214

63 Hints for the Processes Very small flip angle + very short TR T1/T2 do not affect signal intensity Contrast determined by initial Mz (-90 0 ) : T2-weighted Mz! 63 of 214

64 Combine These Two B TR... t Gs... t Gp... t Gr... t Preparation Fast acquisition 64 of 214

65 Magnetization Preparation B TR... t TE ~ 200 msec TR x 256 ~ 2 sec Total scan time ~ 2-3 sec Preparation Fast acquisition 65 of 214

66 Magnetization Preparation Images (T2) PD (no prep) T2 (with prep) 66 of 214

67 Names for The Technique Magnetization preparation Turbo-FLASH, MP-RAGE (Siemens) Driven-equilibrium fast SPGR (GE) of 214

68 Properties TR often very short (< 20 msec) Flip angle often very small (5~20 0 ) SNR often low (system-dependent) Contrast determined by the magnetization preparation part 68 of 214

69 The Reason for Low SNR B o z' z' B o y' y' x' x' Not much Mxy available for sampling 69 of 214

70 Expand the Applications! Different preparation modules Different acquisition modules To form many combinations 70 of 214

71 Preparation Modules STIR/FLAIR (inversion recovery) Fat-Sat (off-reson pulse) Diffusion (RF + gradient) MTC (bipolar pulses) of 214

72 Inversion Recovery Preparation TI B1 z' z' short T1 x' y' x' y' long T1 T1-related preparation or suppression 72 of 214

73 STIR/FLAIR Turbo-FLASH TI ~ 130 msec B1 fat relaxes to TI ~ 2000 msec B1 CSF relaxes to 0 73 of 214

74 Fat-Sat Preparation 90 0 fat only B1 Gs Gp Gr strong gradient for spoiling 74 of 214

75 Other Preparation Schemes B1 diffusion Gs B1 a 0 -a 0 a 0 -a 0 a 0 -a 0 a 0 -a 0 magnetization transfer Some will be mentioned in the future 75 of 214

76 Acquisition Modules FLASH, GRASS, SPGR,... EPI (echo-planar imaging) FSE (TurboSE) Conventional spin-echo! 76 of 214

77 FLASH (short TR) B1 TR... t Gs... t Gp... t Gr... t Continual RF excitation 77 of 214

78 Echo Planar Imaging RF t Gs t Gp t Gr t 78 of 214

79 Fast Spin-Echo (Turbo Spin-Echo) RF t Gs Gp t t Gr t 79 of 214

80 Preparation + Acquisition B1 Gs TI ~ 2000 Gp Gr FLAIR Fat-Sat EPI 80 of 214

81 FLAIR Fat-sat EPI Picker Vista msec TE = 120 msec 256x160 From Picker (Marconi Philips) brochure 81 of 214

82 Note (1) Short TR gradient-echo actually has very complicated contrast behavior Greatly simplified in this course Complex parts saved for the future 82 of 214

83 Note (2) TR > T2? TR < T2? Steady-state and non-steady-state imaging families Approaching steady state Destroying steady state (spoiler) 83 of 214

84 The Fast Spin-echo Imaging Sequence Hsiao-Wen Chung ( 鍾孝文 ), Ph.D., Professor Dept. Electrical Engineering, National Taiwan Univ. Dept. Radiology, Tri-Service General Hospital 84 of 214

85 Fast (Turbo) Spin-echo Sequence RF t Gs t Gp t Gr echo 1 echo 2 echo 3... Every echo forms one k-space line t 85 of 214

86 Review: Accelerate Scan? Example : EPI Fill in the entire k-space after one single RF excitation 86 of 214

87 Echo Planar Imaging (EPI) RF t k y G z t k x G y t G x t 87 of 214

88 From EPI to FSE EPI : series of gradient echoes with proper encoding gradient FSE : series of spin echoes with proper encodign gradient 88 of 214

89 Echo Planar Imaging (EPI) RF t k y G z t k x G y t G x t 89 of 214

90 Fast Spin-Echo (FSE) RF t k y G z t k x G y G x t t TR TR 90 of 214

91 Also Similar to Spin-Echo Spin-echo has the multi-echo option echo echo Multi-echo : forms many images FSE : All echoes used in one image 91 of 214

92 Multi-echo Sequence RF Gs Gp t t t Gr image 1 image 2 image 3... Every echo belongs to a unique image t 92 of 214

93 Fast Spin-echo Sequence RF t Gs t Gp t Gr echo 1 echo 2 echo 3... All echoes belong to the same image t 93 of 214

94 What You Can Infer Fast spin-echo sequence can be easily modified from multi-echo FSE image behavior should be similar to traditional spin-echo 94 of 214

95 Comparison between FSE T2 & SE T2 SE (TE = 100) FSE (TE = 100) 95 of 214

96 Also Similar to Spin-Echo Multiple k-space lines obtained with every single RF excitation Just with several pulses Single-slice scan must be much faster than spin-echo 96 of 214

97 20-sec Scan for the Eye GE 1.5 Tesla Fast Spin-echo ETL = 12 TR = 2000 Scan time = 20 sec No motion artifacts visible 97 of 214

98 Let s Name It Then Turbo spin-echo (Siemens) Fast spin-echo (GE & others) RARE (Bruker) Rapid acquisition with relaxation enhancement 98 of 214

99 Why Is FSE Important? Spin-echo : traditional MRI standard FSE similar to SE Much faster scan TR = 2000 : 7 min to 1 min 99 of 214

100 FSE Similar to SE (256x128) SE (6 min) FSE (48 sec) 100 of 214

101 Acceleration Achieved 8 echoes (e.g.) with each x256 : 32xTR only 8 times faster than spin-echo Echo train length (ETL) = of 214

102 Multi-shot FSE Scan time = TR x (phase#) / ETL The larger ETL, the faster singleslice scan 102 of 214

103 How about Contrast? Echoes have different TE! What determines T2 contrast? Effective TE 103 of 214

104 Multi-shot FSE Sequence RF t k y G z t G y t k x G x Signal t t The k-space lines have different TE?? TR TR 104 of 214

105 Don t Forget about k-space k y boundary contrast k x boundary Contrast mainly determined by central k-space 105 of 214

106 A 256x256 Image Is Composed of Central k: contrast Outer k : boundary 106 of 214

107 TE in FSE Central k-space determines the image contrast Data passing central k-space dominate the contrast despite of different TEs 107 of 214

108 TE & Phase Encoding Data location in k-space controlled by phase encoding Phase encoding order determines TE eff 108 of 214

109 k-space Filling Pattern in FSE RF t k y G z t G y t k x G x t Signal t Early echo placed at central k-space: PDWI 109 of 214

110 k-space Filling Pattern in FSE RF t k y G z t G y t k x G x t Signal t Late echo placed at central k-space: T2WI 110 of 214

111 Expand: Dual-Contrast FSE is an expanded version of multiecho spin-echo Dual echo naturally feasible in FSE T2 weighting also determined by TE eff 111 of 214

112 Dual-contrast FSE Sequence RF t Gs t Gp t Gr echo 1 echo 2 echo 3... t image 1 image of 214

113 Or Even... k y Contrast Early echo : PD Late echo : T2 Data sharing boundary (shared) k x boundary (shared) 113 of 214

114 Data Sharing FSE Sequence RF t Gs t Gp t Gr echo 1 echo 2 echo 3... image 1 image 2 t 114 of 214

115 Data Sharing in Dual-Echo FSE TE eff = 17 msec TE eff = 85 msec 115 of 214

116 Data Sharing Only central k-space acquired multiple times with different TE For different T2 weightings Outer k-space acquired only once Dual contrast with < 2 time penalty 116 of 214

117 Move Further: Single-shot Entire acquisition + wasted time < 1~2 T2 (100 ms to sec range) 256x256 : an echo every 4 msec Echo spacing (ESP) 117 of 214

118 Multi-shot FSE Sequence ETL = 3 TR RF t G z t G y t G x ESP Scan time = TR x (phase #) / ETL t 118 of 214

119 Single-shot FSE Sequence ETL = # of phase encoding RF t G z t G y t G x ESP No TR (or TR is infinite) t 119 of 214

120 Certainly Possible But ESP has ~4 ms lower limit ETL ~ 256 to yield 1-2 sec scan Most signals decay due to T2 relaxation 120 of 214

121 Single-shot FSE Sequence RF t k y G z t G y t k x G x t Signal t Very late echoes show no signals at all 121 of 214

122 ESP Can t Be To Short! Specific Absorption Rate (SAR) RF power proportional to (flip angle) power: 4x of 90 0, 36x of 30 0! RF power deposition causes an increase of local body temperature 122 of 214

123 Single-shot FSE Sequence ETL = # of phase encoding RF t G z t G y t G x ESP So many high-power RF pulses! t 123 of 214

124 Single-shot FSE Usage You want only long T2 tissues Myelogram, MRCP Motion so severe that scan time becomes the dominant factor Fetal imaging, GI imaging 124 of 214

125 Only Long-T2 Tissues Have Signals CSF in spinal cord : long T2 tissue 125 of 214

126 Myelogram (Strongly T2-weighted FSE) Original slices (heavy T2 images) MIP 126 of 214

127 FSE MRCP (Same Principle) Original slices MIP MRCP 127 of 214

128 1-sec Fetal Scan Siemens 1.5 Tesla HASTE ETL = x240 Scan time = 1 sec 22 weeks gestation No artifacts from fetal motion 128 of 214

129 Including My Own Son 5-month photo Future look? 28-week gestation 35-week gestation Courtesy Cheng-Yu Chen, M.D., Tri-Service General Hospital 129 of 214

130 One Variation : HASTE Half-Fourier acquisition single-shot TurboSE (Siemens) Single-shot fast spin-echo (GE) Half Fourier + TSE = ~1s scan Reduce to ~130 0 for SAR 130 of 214

131 Multi-shot FSE Usage Almost the new standard for T2 Much faster than traditional SE HASTE best in GI Motion and susceptibility artifacts 131 of 214

132 FSE Advantages FSE similar to traditional SE Spin-echo already a standard FSE widely accepted as well No gradient-echo artifacts 132 of 214

133 Comparison between FSE T2 & SE T2 SE (TE = 100) TSE (TE = 100) 133 of 214

134 Speed Advantages Overcome motion artifacts Multiple signal averages for SNR in reasonable scan time Trade SNR for spatial resolution Long TR for proton density weighting 134 of 214

135 Speed Advantages in terms of Motion SE (ECG gating) FSE (no gating) 135 of 214

136 Speed Advantage in GI Imaging R-L frequency encoding 4:30 min scan, 512 matrix (readout) 136 of 214

137 Resolution Advantage with SNR 256x256, 57 sec 512x512, 2:45 min High-resolution in reasonable scan time 137 of 214

138 Long TR Advantage in Nerve Roots Siemens 1.5 Tesla Turbo Spin-echo 512 matrix 3 mm slice Scan time = 7 min Strong CSF & high resolution for nerve roots 138 of 214

139 FSE Properties Compared with SE at same TE Stronger magnetization transfer contrast Weaker diffusion weighting Bright fat at long ETL No time to explain in this semester 139 of 214

140 FSE Unique Artifacts Point-spread function blurring Will be briefly mentioned Pseudo edge enhancement Ghosts from data discontinuity No time to explain either 140 of 214

141 k-space Filling Pattern in FSE RF t k y G z t G y t k x G x t Signal t Early echo placed at central k-space : blurring 141 of 214

142 Blurring in FSE with Long ETL HASTE (176x256) HASTE (128x256) 142 of 214

143 ETL Comparison in Chest Imaging ETL 15 (ECG, BH, 14 sec) ETL 85 (0.4 sec) 143 of 214

144 Parallel MRI with RF Phased Array Coils Hsiao-Wen Chung ( 鍾孝文 ), Ph.D., Professor Dept. Electrical Engineering, National Taiwan Univ. Dept. Radiology, Tri-Service General Hospital 144 of 214

145 Review : Phased Array Surface coil: high SNR with limited coverage Phased array: multi coils with geometric arrangement to cancel mutual inductance Achieve high SNR and wide coverage simultaneously 145 of 214

146 Phased Array Coil 146 of 214

147 Spine Phased Array 147 of 214

148 Phased Array Image Formation Computer (reconstruction) Receiver Receiver Receiver Receiver Signals received and processed separately 148 of 214

149 Combine to Form Phased Array Image Wide FOV for larger coverage 149 of 214

150 Phased Array Imaging Coil elements receive signals separately Send to individual receiver channel No other difference at all RF pulsing, phase encoding, etc. 150 of 214

151 What Is Parallel Imaging? Signals in different coils must be different If data in different coils show little redundancy, can some steps be omitted? SMASH (1997),SENSE (1999) 151 of 214

152 Method 1 Produce various spatial frequency waveforms in k-space using the coil profiles Multiple k-space lines in one phase encoding 152 of 214

153 Review : k-space & MRI Each point in the k-space coordinate (kx,ky) coordinate : specific waveform Signal intensity : relative weighting of that waveform All MRIs are formed by these waveforms 153 of 214

154 A k-space point represents a waveform ky kx 154 of 214

155 Many waveforms summed to an MRI Waveforms weighed by signal intensity 155 of 214

156 Phased Array Helps in Signals received at various locations Adjust weights of signals according to the coil locations to form different waveforms from one single acquisition 156 of 214

157 Waveform Formation from Coil Profiles Coil arrangement Equal weights Form cosine Form sine High-freq cosine High-freq sine 8 elements arranged linearly 157 of 214

158 Many Waveforms from One Acquisition k y phase encoding k x freq encoding Many k-space lines from one phase encoding 158 of 214

159 Parallel Imaging Multiple k-space lines with one phase encoding due to separate signal receiving with phased array coils N waveforms N times acceleration 159 of 214

160 Let s Name It Many harmonics formed at once SiMultaneous Acquisition of Spatial Harmonics (SMASH) Sodickson of 214

161 Acceleration Factor Theoretically, N coil elements could form at most N harmonics Nothing is perfect in practice acceleration factor < N 161 of 214

162 SMASH Phantom Image (1997 MRM) Usual scan (10 sec) 3 coils (5 sec) 162 of 214

163 SMASH Body Image (1997 MRM) Usual scan (22 sec) 4 coils (11 sec) 163 of 214

164 SMASH CE-MRA (2000 Radiology) Philips ACS NT 1.5T FLASH, 7.0/1.5/30 0 3D (128px256x20) 6 coils, R = 3 [Gd] = 0.13 mm/kg 8 sec per 3D frame Shorten breath-hold time or high temporal resolution 164 of 214

165 SNR in SMASH Accelerate from reduced phase encoding SNR lowers according to the square root relationship Half scan time SNR lowered to 70% Used when reducing motion effects outweighs SNR loss 165 of 214

166 SMASH Pitfalls Coil size, shape, arrangement relatively restricted in order to form perfect sinusoids Direction of multi coils often not used for phase encoding 166 of 214

167 Waveform Formation from Coil Profiles Coil arrangement Equal weights Form cosine Form sine High-freq cosine High-freq sine 8 elements arranged linearly 167 of 214

168 Combine to Form Phased Array Image Head-foot direction is often freq encoding 168 of 214

169 Even Arrangement OK Sinusoids formed by coil profiles often non-perfect Imperfect reconstruction results in residual aliasing 169 of 214

170 Aliasing in SMASH with R = 2 Usual scan (10 sec) 3 coils (5 sec) 170 of 214

171 Aliasing in SMASH with R = 2 Usual scan (22 sec) 4 coils (11 sec) 171 of 214

172 SMASH Extensions Auto-SMASH VD Auto-SMASH GRAPPA (Siemens) Details omitted 172 of 214

173 GRAPPA Lung Images HASTE 128x256 GRAPPA 256x256 Usual scan 207 ms 150 ms (8-coil array) 173 of 214

174 GRAPPA Liver Images HASTE 128x256 GRAPPA 256x256 Usual scan 252 ms 252 ms (8-coil array) 174 of 214

175 Method 2 Coils have different sensitivity profiles, all relatively small Reduce FOV for less phase encoding Accelerated, aliasing occurs Compute image according to the different aliasing patterns 175 of 214

176 3-coil Example Coil sensitivity profiles roughly occupy 1/3 FOV Prescribe a small FOV (~1/3) Resolution unchanged reduced matrix size 176 of 214

177 Example Using 3 Coils Phantom & coil locations 3 Aliased images 177 of 214

178 No Panic with Aliasing Aliased image = signals within FOV + outside FOV Signals stronger within coil profile, weaker outside weighted sum 178 of 214

179 Aliased Image from Coil 1 FOV aliasing 1 aliasing Phantom & coil location 179 of 214

180 Aliased Image from Coil 1 FOV aliasing (weaker) 1 Coil #1 local intensity aliasing (even weaker) 180 of 214

181 Aliased Image from Coil 1 aliasing (weaker) 1 Phantom & coil aliasing (even weaker) image 181 of 214

182 Aliased Image = Weighted Sum An aliased images (D1) = weighted sum of 3 sub-fov images In the form of D 1 = A 1 x + B 1 y + C 1 z 182 of 214

183 Aliased Image from Coil 2 aliasing (weaker) 2 Coil #2 local intensity aliasing (weaker) image 183 of 214

184 Aliased Image from Coil 3 aliasing (weaker) Coil #3 local intensity 3 aliasing (even weaker) image 184 of 214

185 3 Aliased Images from the 3 Coils Phantom & coil location 3 Aliased images 185 of 214

186 3 Aliased Images (D) D 1 = A 1 x + B 1 y + C 1 z D 2 = A 2 x + B 2 y + C 2 z D 3 = A 3 x + B 3 y + C 3 z Solving the equations (matrix inversion) gives (x, y, z) 186 of 214

187 Algebraic Problem Now 3 aliased images (D) in D = Ax + By + Cz form A, B, C: known from coil profiles Matrix inversion to get (x, y, z) 187 of 214

188 Don t Forget Each aliased pixel has one unique set of D = Ax + By + Cz equations Matrix inversion performed 256x256/3 times (for R = 3) 188 of 214

189 Summary 3 RF coils to receive signals 1/3 FOV prescribed with same resolution Scan accelerated 3 times from a reduction in matrix size (phase encoding) Full FOV image can be computed 189 of 214

190 3 Times Acceleration, Solve Equations Full FOV image obtained from aliased images 190 of 214

191 Let s Name It It is about the usage of coil sensitivity profiles SENSitivity Encoding (SENSE) Pruessmann 1999 Rumor has it that it s Philips patent 191 of 214

192 Acceleration Factor Theoretically, N coil elements provide at most N aliased images Smallest prescribed FOV is FOV/N Like SMASH, < N in reality 192 of 214

193 SENSE Brain Image (1999 MRM) Usual scan (170 sec) 2 coils (85 sec) 193 of 214

194 SENSE Heart Image (Short Axis) Usual scan (15 beats) 5 coils (5 beats) 194 of 214

195 SNR in SENSE Acceleration thru reduced phase encoding SNR lowers according to square root relationship Like SMASH, Used when reducing motion effects outweighs SNR loss 195 of 214

196 SENSE Heart Image (Axial) Usual scan (128x128) 6 coils (R = 3) 196 of 214

197 SENSE Coil Requirement D 1 = A 1 x + B 1 y + C 1 z D 2 = A 2 x + B 2 y + C 2 z D 3 = A 3 x + B 3 y + C 3 z Solvable as long as equations are linearly independent 197 of 214

198 SENSE RF Coil Arrangement Phase RF coil element Phase Phase direction can be either one 198 of 214

199 SENSE Coil Requirement Linearly independent equations No need to form perfect sinusoid Easier than SMASH No many variations like GRAPPA 199 of 214

200 SENSE Pitfalls Incompatible with restricted FOV Example : cardiac imaging Full FOV contains some aliasing Can t distinguish after mixture with 1/3 FOV aliasing 200 of 214

201 Theoretical Comparison SMASH : Fast computation (Fourier transform) Artifact performance better than SENSE at high acceleration factors 201 of 214

202 Theoretical Comparison SENSE : Coil arrangement flexible Flexible slice orientation as in MRI General artifacts less than SMASH 202 of 214

203 Practical Comparison SENSE & SMASH performance highly depends on human resources devoted to R&D No major difference when commercialized 203 of 214

204 Parallel Imaging Families Philips : SENSE Siemens : ipat GRAPPA + msense General Electric : ASSET 204 of 214

205 Parallel MRI Advantages Phased array coils have long been commercialized ( 94) Matrix inversion software simple Acceleration is basically pulsesequence independent 205 of 214

206 SENSE Coronary Angiogram Usual scan (3.0 T) Similar quality at 3x 206 of 214

207 Speed Advantage of SENSE Abdominal CE-MRA 512x512 T2W FSE 207 of 214

208 More Advantages Shorten EPI acquisition time Less EPI geometric distortion Reduce ETL in FSE Less blurring in HASTE 208 of 214

209 SMASH EPI of Brain (Sagittal) Usual scan Four coils 209 of 214

210 SENSE DW-EPI of Brain (Axial) 8 coils (R = 4) to reduce distortions (3.0T) 210 of 214

211 SMASH HASTE of Chest (192x256) Usual scan Four coils 211 of 214

212 Trade Scan Time for Resolution (R = 2) Usual scan 192x256, 450 ms 4 coils 192x256, 225 ms 4 coils 384x256, 450 ms 212 of 214

213 Parallel MRI Advantages Major medical centers will have it soon after its first introduction Taiwan will have it after 3-5 years at the latest time (a matter of $$) 213 of 214

214 The Fast Imaging Techniques Hsiao-Wen Chung ( 鍾孝文 ), Ph.D., Professor Dept. Electrical Engineering, National Taiwan Univ. Dept. Radiology, Tri-Service General Hospital 214 of 214