Image quality and MRI System: magnet, gradient and RFfaculty.neu.edu.cn/bmie/chenshuo/Lecture 16...

Image quality and MRI System: magnet, gradient and RF�

� / Medical Imaging Technolog�

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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'¦��

/ Medical Imaging Technolog�

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Simple Block Diagram View of an MRI scanner


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MR System Overview Scan Room ��Ć�

Equipment Room ò`Ć�

Operator Console �9Ć�

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��

�� / Medical Imaging Technolog�

�� 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 ?


��

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�Êº��ÊºR@��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≈


�� 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


�� Magnets exist in many shapes and strengths

Magnet Types


�� 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�Ð¿�

-5

0

5

10

15

20

-15 -12 -9 -6 -3 0

H, kOe

B, k

Gs

20C30C40C

Permanent Magnet Design ·Ð)òñ��


�� 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

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# 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�


�� 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�


�� 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


�� 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�ÊºÎÕ&¤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


��

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


��

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


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


��

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)


��

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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Y - axis

Z

Y

X

Current

Current

Current

Current

Usable Gradient Volume


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Y - axis

Current

Current

Current

Current

Z

Y

X


��

Sample Transverse Gradient Coil

Fingerprint Gradient Coils


��

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


GradientSwitch

GradientFilter

GradientCoil

Amplifiers

Controller

MainDisconnect

PI

HospitalPower

Distribution

ControlSignals


GradientSwitch

GradientFilter

GradientCoil

Amplifiers

Controller

MainDisconnect

PI

HospitalPower

Distribution


��

Generating Current Waveforms �]�B�x�

" ��Current waveforms are arbitrary due to: – Scan protocol design XHw�\2�

DW EPI FIESTA Spiral EPI

X

Y

Z


��

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


��

" �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 −= ∫


��

"  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


��

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��


��

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:)�


��

3. RF subsystem�


��

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


��

qĎÝYÌ^«FÆ§ô�Ê÷�ÉÊmRÊ�$�ä�}äÌ¤�Þ��äô��Ê�ÊERÊmÊEĐ�È"��ÝY�r��Ð0�-N�d��}äô��Ê�RÊmÊºd�È"IqÝY��ôóÝYÌĂð�æG�§SõV��iÕ"��ÝY��ĎÃĈ$T~¨Ý�C<ÃÀĆÌĎÃz��


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Î�ćÝY��

5ÑqĎÝY�x�§D@ÊºÄîČÝY�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. �

Image quality and MRI System: magnet, gradient and RFfaculty.neu.edu.cn/bmie/chenshuo/Lecture 16...

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Transcript of Image quality and MRI System: magnet, gradient and RFfaculty.neu.edu.cn/bmie/chenshuo/Lecture 16...