image quality and mri system: magnet, gradient and rffaculty.neu.edu.cn/bmie/chenshuo/lecture 16...
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Image quality and MRI System: magnet, gradient and RF�
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Image quality: Contrast�For a SE sequence, the signal is proportional to:�
ρ(1− e−TR/T1 )e−TE/T2
� TR TE ����� ρ� >> T1 << T2
T1�� ~ T1 << T2 T2�� >> T1 ~ T2 ��� ~ T1 ~ T2
!
õgn�=® (TR=6000ms, TE=20ms) �
T2=® (TR=4000ms, TE=80ms) �
T1=® (TR=400ms, TE=20 ms) �
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Image quality: Noise�
n ↑ −n ↓≈ ns
γ !B0
2kBT= 3.3×10−6ns ui = γ !mI
M ≈ (!γ )
2nsB04kBT
Net magnetization vector�
The most important noise sources are the thermal noise in the patient and in the receiver part of the MR imaging system. Consequently, the lower the temperature, the less the noise. Cooling the subject would also yield a higher signal. Unfortunately, this cannot be applied to patients. The relation of data acquisition time, signal-to-noise ratio (SNR) and spatial resolution? If only increase number of excitations? If only reduce the slice thickness? If 128�256 -> 256*256?�
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Image quality: Artifacts�1. Technical imperfections: ! The external magnetic field is assumed to be homogenous. ! RF field is inhomogeneous. ! The slice sensitivity profile (¢u���¨Ý) is not rectangular. ! Inappropriate shielding of the magnetic room. ! Tissues are assumed to be stationary. ! Magnetic susceptibility of tissues or foreign particles and implants.�2. Discretization and truncation errors, e.g. inadequate sampling 3. Involuntary phase shift and dephasing�
x,�ky = 1/FOV�M,�ky = 2/FOV � (a)çåR¶j/eÎ'�¶çûË#ÇĒû; (b)��JTE+�¶RçåOÎ'¦��
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Simple Block Diagram View of an MRI scanner
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MR System Overview Scan Room ���
Equipment Room ò`Ć�
Operator Console �9�
Magnet RF Body Coil
Gradient Coil
Patient Handling
Receive Coil ���
Hardware Scanner Control
Display
Input Device
Host Computer
Archival device
Audio Communication
Gradient Amplifier
Data Acquisition
RF Amplifier
RF/Grad Controller
Reconstruction Engine
Multiple, Complex, Highly Integrated Subsystems MRI§��aÌ�ÉcÛáĐ�ĉ�ÌAh�.Ûá�
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Field Strength
LIST PRICE
Open Systems
Cylindrical Systems
Profile 0.2 T Ovation 0.35 T
OpenSpeed 0.7T
HDx 16 3 T
��T
HDe 4/8 1.5 T
HDx 32 3 T
HDx 16 1.5 T HDx 32 1.5 T
MR450 1.5 T
MR750 3.0 T
Current MR Product Line Í:Ð0�#S�
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1. Main magnet�
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" ��Purpose of the main magnetic field (B0) B0ÌÍÌ�
" How is a magnetic field generated ? f*#ÇB0�
" Field strength and SNR Z�RSNR
" ��Magnet Types Ð)Ù]�" �Permanent Magnets ·Ð)�" Superconducting Magnets öpÐ)�
" ��Magnet shielding ÐZvë�" Active shield !>vë�" Passive shield & Room shield ï>vëR�Ćvë�
" ��Image Quality and Magnet shim X.õăRÐ)?Z�– Active shims !>?Z�– Passive shims ï>vë�
Main Magnet and Shim – Overview !Ð)&?Z��
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����� Magnetic field aligns protons
Electron
Proton
Nuclei in random orientations
��
With a magnetic field
Nuclei align to the applied field
Without magnetic field
Magnetic “Spinning Top”
γ = Gyromagnetic ratio
Precession frequency
(Larmor equation)
For protons,γ = 42.58 MHz/T
Bdtd
×= µγµ
Bγω =
B
µ
Why do we need a magnetic field ?
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How to generate a magnetic field ?
Source can be a current or a permanent magnet
B0B0
Magnet)Coils
Magnet'Coils
B0
Shield'Coils
Magnetic field is generated by a circulating current
Ä�ʺ��ʺR@��²´ Field strength linearly proportional
to current and number of loops OR
Magnetic field generated by distributed magnetic material 6{ÌÐ�¯¡�
Field strength proportional to mass and material properties �õăRÁ��²´�
B ~ m•(BH)m
I
LNIB 0µ≈
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����� SNR Linearly proportional to magnetic field strength
�cm More field -> larger difference in population of states -> more signal
Low Field
High Field
7T
1.5T 3T
GroundState
ExcitedState
Energy
Magnetic FieldStrength
GroundState
ExcitedState
Energy
Magnetic FieldStrength
Field Strength and SNR
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����� Magnets exist in many shapes and strengths
Magnet Types
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����� Permanent magnets for field range from
0.2 to 0.45 T
" Magnetic field generated by two poles of permanent magnet material (typically NdFeB) ąĄÏ�
" Field shaping with laminated iron pole-faces ÐZ�ÂÉuKĄ\�9�
" �ãÊ�" à°ÖD�ā%( " ÐZI�t�oQWÄ_�Ts
" Large forces; heavy structure ;�c�Ă
" Z�(�ÐZ[?�z " =wÚ�ð¸Đ�
Permanent Magnets ·Ð)�
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Iron Yoke Iron
Post
Tile 瓦片Assembly PolePiece
Permanent Material Block
" Field adjustment done mechanically or through tight temperature control ÐZóêĀý¬±ó �½��9lÅ�
" Very temperature sensitive ½�п�
-5
0
5
10
15
20
-15 -12 -9 -6 -3 0
H, kOe
B, k
Gs
20C30C40C
Permanent Magnet Design ·Ð)òñ��
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����� Superconducting magnets for field ranges from 0.5 to >9.4 T
Coil made of superconducting wire
Typically used for higher fields (>0.5 T)
Cryogenic Cooling to keep wire superconducting (½93â�öp�" Liquid Helium ¼µ�" Mechanical Cryocooler ¬±93¬�
Extremely Stable - not connected to power supply during operation ċ|Ók�þíýÒ��Ċðb�æ¾�
Cross-section of a 7T superconducting magnet
3T magnet in testing
Superconducting Magnets öpÐ)�� !ðÈ"ĐZ�2�
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# Homogeneity typically expressed in parts per million (PPM)
# Requirements are < ~1-10 ppm for MRI imaging (much smaller for spectroscopy) �éð¸<1-10 ppm
�
# Shimming (fine field adjustment) used to reach desired ppm levels ü8ppmÜ7�?Z��ßZó �§�ðÌ�
IQ is strongly correlated to magnet shim ���������� ���������
PPM = ΔB
Bavg
×106
Image Quality and Magnet Shim X.õăR?Z�
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����� Magnets are unshielded, passively or actively shielded
Radial
Axial
Without shielding, magnetic field reaches far from the magnet ¹ªÐvë�ÐZL$ü8�ÿ�
Typical requirements are <5G or <0.5 mT at the scan-room wall/ceiling �éð¸<5Đ£�
Shielding done either actively or passively vëL$§!>�ï>Ì�" Active shield (bucking) coils " Steel shield on magnet
Magnet shielding Ðvë�
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����� Passive shield uses heavy steel plates on magnet or room walls
# A passively shielded magnet uses steel cylinders, plates or blocks to contain (return) the magnetic flux ��-1���2)/3��%,0��� # Just enough steel is used to prevent saturation within the shield " ��2)��&(* # Mechanical structure inside the cryostat is less complex Iron shielded magnets are typically very heavy #�.�'��,!�4�-12)�� For 7T and beyond, passive shields in the wall are currently the only feasible option ��7T�0�4 ���+���-1
B-H Curve
0
5000
10000
15000
20000
25000
30000
0 1000 2000 3000 4000 5000H(Oe)
B(G
s)
B 0 B 0
Passive shield & Room Shield ��-1$�-1
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����� Active shield uses cancellation field �� ��������
Main Coils
Bucking Coils �����
# An actively shielded magnet uses pairs of coils with equal but opposite current ( I ) (bucking coils) to the main coils !>vë+È�o�»ÝY�ʺÎÕ&¤PÎH��
# Bucking coils typically reduce the stray field to about 2.5 m (radial) by 3.5 m (axial) from iso-center �»ÝY�ér�Z4sè�P2.5Ø�øP3.5Ø�
# Bucking coils counter the main field, thus more wire needed �»ÝY�!ÐZ¤PÎH�V³Ċð©cöpÝ�
# Mechanical structure inside the cryostat becomes more complex (½�½U1¬±à°J�B6a�
Total system is light-weight compared to passive shield Ûá´úùy�
I I
Active shield !>vë�
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World’s highest field whole-body magnet – 9.4T
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World’s highest field whole-body magnet – 9.4T
700 ton shield
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Magnetic field aligns protons for imaging Resistive or permanent magnets for fields up to 0.45 T
" Heavy " Temperature sensitive
Superconducting magnets for fields from 0.5 T to 9.4 T " Stable " Need cryogens/cooling
Shielding & shims can be active or passive
Magnet shim is very important for image quality
Summary
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2. Gradient system�
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Gradient Subsystem Block Diagram�
Gradient driver applies high fidelity current waveforms to the gradient Coil. i�R�M'i�tTj(%����B�x�
Gradient Driver
Scan Protocol
Gradient Coil
ControlSignals
PowerDistribution Power Supply
GradientSwitch
GradientFilter
GradientCoil
Amplifiers
Controller
MainDisconnect
PI
HospitalPower
Distribution
ControlSignals
PowerDistribution Power Supply
GradientSwitch
GradientFilter
GradientCoil
Amplifiers
Controller
MainDisconnect
PI
HospitalPower
Distribution
Gradient coil converts the current waveform into a gradient field required by the given Pulse Sequence
i�tT7�B�x��oF�|A{}�i���
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Gradient CTQs )6�c�• Magnetic Field Strength • Slew Rate • Rise Time • Magnetic Field Linearity
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Gradient Linearity i���tz��• Two Types of Linearity CTQs • Integral Linearity �ktz��• Expected field value from a perfectly
linear gradient versus true value (voxel location error)
• Bz expected versus Bz actual • Differential Linearity n!tz��• Expected (ideal) strength versus field
gradient (voxel size error) • dBz / dx expected versus dBz / dx actual
FOVx
xz
rGxGB −
x
xz
G
GxB −∂∂
FOV Spec: < ±50% differential & ± 15% integral
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Sample Gradient Fields -0.2925
-0.2574
-0.2223
-0.1872
-0.1521
-0.117
-0.0819
-0.0468
-0.0117
0.0234
0.0585
0.0936
0.1287
0.1638
0.1989
0.234
0.2691
-0.2925-0.2691-0.2457-0.2223-0.1989-0.1755-0.1521-0.1287-0.1053-0.0819-0.0585-0.0351-0.01170.01170.03510.05850.08190.10530.12870.15210.17550.19890.22230.24570.26910.2925
Y [m]
Z [m]
0.02-0.0250.015-0.020.01-0.0150.005-0.010-0.005-0.005-0-0.01--0.005-0.015--0.01-0.02--0.015-0.025--0.02
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
y [m]
Bz
[T]
0.000
0.010
0.020
0.030
0.040
0.050
0.060
d B
z / d
y [T
/m]
BzSeries3Series4d Bz / dy
Integral Linearity Limits
Y-Z Plane (x=0)
-0.2925
-0.2574
-0.2223
-0.1872
-0.1521
-0.117
-0.0819
-0.0468
-0.0117
0.0234
0.0585
0.0936
0.1287
0.1638
0.1989
0.234
0.2691
-0.2925-0.2691-0.2457-0.2223-0.1989-0.1755-0.1521-0.1287-0.1053-0.0819-0.0585-0.0351-0.01170.01170.03510.05850.08190.10530.12870.15210.17550.19890.22230.24570.26910.2925
y [m]
x [m]
0.02-0.0250.015-0.020.01-0.0150.005-0.010-0.005-0.005-0-0.01--0.005-0.015--0.01-0.02--0.015-0.025--0.02
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0.02
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
y [m]
Bz
[T]
0
0.01
0.02
0.03
0.04
0.05
0.06
d B
z / d
y [T
/m]
Bzd Bz / dy
Y-X Plane (z=0)
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Typical Gradient Coil Designs
Magnetic Field
Current
��
Current
Z
Y
X Z - axis
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Transverse Gradient Design . Gi�\2�
• Simplest transverse gradient design is four saddle coils �5��e&E��tT�
• Real gradients are nested saddle coils (for linearity) _3[bOg�E��tT�o@tz��
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Typical Gradient Coil Designs
Y - axis
Z
Y
X
Current
Current
Current
Current
Usable Gradient Volume
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Typical Gradient Coil Designs
Y - axis
Current
Current
Current
Current
Z
Y
X
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Sample Transverse Gradient Coil
Fingerprint Gradient Coils
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Gradient Driver Overview i�R�M�" �Gradient driver applies current waveforms to the gradient coil to alter the magnetic field, which affects the precessional frequency of hydrogen protons in the body. " Driver must generate arbitrary current waveforms with extremely high fidelity (minimal error between command waveform and actual waveform). j(V~9�%��hz��B�x�
Gradient Driver
Scan Protocol
Gradient Coil
ControlSignals
PowerDistribution Power Supply
GradientSwitch
GradientFilter
GradientCoil
Amplifiers
Controller
MainDisconnect
PI
HospitalPower
Distribution
ControlSignals
PowerDistribution Power Supply
GradientSwitch
GradientFilter
GradientCoil
Amplifiers
Controller
MainDisconnect
PI
HospitalPower
Distribution
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Generating Current Waveforms �]�B�x�
" ��Current waveforms are arbitrary due to: – Scan protocol design XHw�\2�
DW EPI FIESTA Spiral EPI
X
Y
Z
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Fundamental Gradient Driver Circuit i�R�M�C1��• H-bridge circuit topology to generate current waveforms in the gradient coil (inductor) H-�P�i�tT��$I�]�B�x�
Curr
ent
Curr
ent
Curr
ent
Ideal Switch
+/- Voltage applied to an inductor generates +/- current.
Apply Positive Voltage Cycle Positive/Negative Voltage
Apply Negative Voltage
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" �MR Physicists Want This:
" They get this:
" Fidelity Specifications:
" Reproducibility: < 5 µAsec " Symmetry: < 10 µAsec " Max. Error: < 30 µAsec
Example: Edge ghost
What is Fidelity? Cu
rren
t Cu
rren
t
Perfect corners are impossible
dtIIFidelity command
TR
0coil −= ∫
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" Heat loads are exponentially increasing with current. �c���" Limits of air-cooling with current product (HFD)…must move to
water in DVMR. �;N=S�ud=�
XGD660A
ACGD 320A
SGD 200A
GRAM1 200A
HFD 320A
Water
Air
Current and Heat Load Trends �B-U#,Qa�
Performance HFD XGD
Current 320A 660A
Voltage 1600V 1800V
Control Analog Digital
ICV $33k $53k
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Protocol Selected
Calculate Coil Heat
Calculate Driver
Temperatures
Add dead time tdriver
Add dead time tcoil
Max(tdriver, tcoil)
> Limit? > Limit? Add dead
time to Protocol
Scan
0
0
Dead time
" �Protocols are adjusted so that driver temperatures or coil heat removal capability are not exceeded. ��|A`�R�M-tTp���*LWUJ>"�j%r�^4��
Adding dead time reduces number of
slices / second (a.k.a. rms current, duty cycle, heat load)
N
Y
N
Y
Gradient Sub-System Limits i��qms��
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Gradient Driver IGBT Challenges l��" Bandwidth/Fidelity requires Gradient Driver IGBTs to be
switched at 31.25 kHz {}i�R�M�������KHz�KD8y: �
" High switching frequency $ high heat generation %U+�" Need to minimize switching losses J?fZ�v/�" Few vendors make switching-optimized IGBTs
0
50
100
150
200
250
300
350
1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
Vcesat @ 300A [V]
Fall
Tim
e [n
s]
Mitsubishi Data
Curve Fit
IGBT Performance Trade-Off
High conduction loss, low switching loss
Low conduction loss, high switching loss
MR
Current Waveform (blue) and IGBT switching (yellow) <Y�x-0Y:)�
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3. RF subsystem�
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RF Subsystem Overview
Analog to Digital
Converter
Computer for Recon, Storage,
Display
Amplifiers
Demodulator
RF Coil
Computer Pulse sequence
Generator Digital to Analog
Modulator
RF Coil
Power Amplifier
Turbo FLASH Flip Angle 20o
Transmit Chain Receive Chain
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5ÑqĎÝY�x�§D@ʺÄîČÝY�M�§ì¥×�ÝY�x�§ďčÂÝY�M�§đÔ�ÝY�
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Questions?�
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Image quality: Resolution in the Fourier space�
To avoid aliasing in the image space, the Nyquist criterion must be satisfied.
Δkx ≤1
2xmaxxmax =
FOVx
2
Δkx ≤1
FOVx
Δkx =γ2π
GxΔt
GxΔt ≤2π
γ FOVx
GyTph ≤2π
γ FOVy
In practice, Δt is fixed and Gx
is scaled to the field of view. �In practice, Tph is fixed and Gy
is scaled to the field of view. �