2/19/2009

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Clinical MRI Education Lecture 3 will start @ 7am Tue. 2/10

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Lecture 3MRI System and Hardware

Components

Chen Lin, PhD

Indiana University School of Medicine & Clarian Health Partners

Components of MR System • Magnet / Shimming / Shielding

• RF system (Transmitting and receiving, RF coils, RF shielding)

• Gradients

• Signal processing and image construction.

• Patient handling and communication.

• Physiological monitoring, triggering and gating.

• Image display, post-processing, analysis, database and networking.

• Other accessories (contrast administration, fMRI paradigm delivery, anesthesia machine, respiratory support, etc.)

Chen Lin, PhD, 2/09

B0

Whole Body MRI Magnet

• Cylindrical Horizontal Field

– Higher field strength (0.5T - 3.0T)

– Better image quality

• Open-sided vertical field

– Lower field strength (0.2-1.0T)

– Less claustrophobic

Chen Lin, PhD, 2/09

B0

Modern Superconducting Magnet

• Cryogen: Liquid helium + nitrogen

• Windings are kept at a 4.2K i.e. immersed in liquid helium.

• Liquid helium compartment is surrounded by liquid nitrogen at 77.4K

• Isolated with vacuum space and thermal radiation shield.

• Active (superconductive) and passive shimming

• Active and passive shielding

Chen Lin, PhD, 2/09

Magnet Related Specifications

• Field Strength

• Field Homogeneity

• Max. FOV: i.e. 50cm

• Max. Ramp

• Fringe Field

• Eddy currents

• Bore diameter, length, aperture style

• Field Stability: < 1.5ppm per ACR

• Weight, size, 4He boil off rate

Chen Lin, PhD, 2/09

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Field Inhomogeneity Solutions

• Create a buffer zone or use self-shield magnet

• Scan at the iso-center and apply high order shim.

• Increase rBW -> steeper frequency encoding gradient.

• Use short TE or SE sequence instead of GRE.

With garbage truck Without garbage truck

Chen Lin, PhD, 2/09

Shimming of Magnetic Field

At the time of installation:• Super conducting shim set

Additional windings in the magnet.• Passive shim

Positioning of magnetic material.

Prior to each Exam or Scan:• Localized to scan volume (can be override)• Linear shim (gradient shim) {X, Y, Z}• Room temperature high order shim (HOS) set. {Z2,

ZX, ZY, XY, X2-Y2}• Can be performed manually.

Chen Lin, PhD, 2/09

High Order Shim Coils

Z2 X2 – Y2XY, YZ, XZChen Lin, PhD, 2/09

Siemens High Order Shim Interface

Chen Lin, PhD, 2/09

4,6 m

0.90m

4,7

m

Vacuum chamber

77K screen

Compensation

Main coils

Courtesy NeuroSpin project CEA, Paris, France

Wide bore 11.74T MRI Magnet

Chen Lin, PhD, 2/09

Radio Frequency (RF) System

Synthesizer (63MHz)

RF Amplifier

Waveform Generator

RF Coils

T/R SwitchCoil Detune

Pre-Amp(s)

Filters& ADCs

Tx Path

Rx PathRF Shield

Chen Lin, PhD, 2/09

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RF System Specifications

• Tx Power Amplifier

– Output power and bandwidth

– Stability and linearity (Amplitude & Phase)

• Rx Pre-amplifiers and Receivers

– # of channels and connections (to match the Rx coils)

– Bandwidth/speed and dynamic range

– Sensitivity, stability and noise characteristics

• RF Coil

– B1 uniformity (For Tx coil)

– # of elements and geometry (For Rx coils)

– SNR

Chen Lin, PhD, 2/09

Type of RF Coil

• Linear Polarized (LP) versus Circularly Polarized (CP)

Waveform Simulation: http://www.school-for-champions.com/science/experiments/simwaveform.htm

CP or Quadrature:

– 2+ LP coils with appropriate (i.e. 900) phase offset.

– 50% Tx power & 140% Rx SNR

• Transmit & Receive (Tx/Rx) versus Receive (Rx) Only

• Volume Coil versus Surface Coil versus Phased Array

• Maximize filling factor / Reduce distance.

• Proper positioning is very important for good image quality.

Chen Lin, PhD, 2/09

Solenoid Coil

• Used for vertical field magnet

Chen Lin, PhD, 2/09

“Bird Cage” RF Coil

• Circular polarized.

• Transmit and receive.

• Volume coil i.e. uniform RF field, B1

• Often used as body RF coil.

Chen Lin, PhD, 2/09

Body Coil

Head Coil

CP Volume Coil Examples

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Elliptical Polarization

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Conventional

TrueForm

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“Loop” RF Coil• Linear polarized

• Receive only

• Surface coil

• Often used as elements of phased array

Chen Lin, PhD, 2/09

CTL Spine Coil for GE

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Siemens Matrix Coils

Chen Lin, PhD, 2/09

Siemens Matrix Coil Modes

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Sensitivity Profile and Correction

Re

l. S

NR

6

5

4

3

2

1

05 1510

a

b

c

Body coil

Volume head coil

(a) 8 cm dia. Surface coil(b) 10 cm dia. Surface coil(c) 14 cm dia. Surface coil

Depth (cm)

• GE– Surface coil

intensity correction (SCIC)

– Phased array uniformity enhancement (PURE)

• Siemens– Normalization

– Pre-scan Normalization

Chen Lin, PhD, 2/09

Phased Array

• Multiple Rx channels instead of simple multiplexing.

• Approximate vector combination with matched filters by sum-of-square for high SNR condition.

• Receiver path calibration (FFT Scale).Chen Lin, PhD, 2/09

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SNR at location (130,110) = 121

(18,7

9)

Combined SNR (SoS) SL:1 File: meas_MID34_se_15b130_trans_32ch_signal_FID3683.mat Variable=SNR

50 100 150 200 250

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100

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2500

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SNR at location (131,113) = 101

(4,8

0)

Combined SNR (SoS) SL:1 File: meas_MID42_se_15b130_trans_12ch_signal_Produktspule_FID3691.mat Variable=SNR

50 100 150 200 250

50

100

150

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2500

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32 versus 12 Channel Head Coil

Yang Ming University, TaiwanChen Lin, PhD, 2/09

Number of Coil Elements & SNR

Average over entire the brain

Center of the brain

Chen Lin, PhD, 2/09

Tx w. CP Coil & RX w. PA Coils

• CP Coil: Large CP volume coil provides uniform B1 and efficient excitation, but low sensitivity.

• Surface coils: Smaller receive coil provides high local signal, but has limited coverage and poor uniformity.

• CP Tx + PA Rx (4 – 128 elements)– Multiple receive coils in a phased array to improve both signal and

coverage and enables parallel imaging.

– Requires multiple receive channels ($).

Chen Lin, PhD, 2/09

Siemens Tx/Rx Adjustment Results

Chen Lin, PhD, 2/09

Siemens Tx/Rx Adjustment Results

Chen Lin, PhD, 2/09

96 Channel Parallel Receive

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8-channel Parallel TransmissionTx/RxMod

Mod

Mod

Mod

Mod

Mod

Mod

Mod

Modulator RFPA Tx/Rx switch RF coil

Tx/Rx

Tx/Rx

Tx/Rx

Tx/Rx

Tx/Rx

Tx/Rx

Tx/Rx

Phase (top) and magnitude (bottom) of array coil maps. Coil locations are indicated (white).

-180º

0º

180º

Courtesy of MGH, Larry Wald, Vijay Alagappan- MIT,

Elfar Adalsteinsson, Kawin Setsompopp and SiemensChen Lin, PhD, 2/09

Transmission Array Applications

• B1 shimming

• Selective excitation of arbitrary shape (Single organ imaging)

• TX-SENSE (SAR reduction)

Chen Lin, PhD, 2/09Courtesy of MGH, Larry Wald, Vijay Alagappan- MIT,

Elfar Adalsteinsson, Kawin Setsompopp and Siemens.

3D Volume Excitation

RF “Feed-through” Artifact

• A line of alternating intensity (Zipper)

• Along the phase encoding direction at the location of zero frequency encoding.

• Caused by excitation RF pulse not completely turn off during the data acquisition.

• Alternate phase of excitation RF pulses by 180o

on successive acquisitions (Phase cycling) to cancel out the RF leakage.

Chen Lin, PhD, 2/09

RF Noise/Interference Artifact

• Improve/repair RF shielding

• Remove/turn off/shield other devices in the scanner room

Chen Lin, PhD, 2/09

Gradient Coil Design

Chen Lin, PhD, 2/09

Gradient Assembly

Three gradient coils (X, Y, Z) are

encased within the magnet bore

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Body RF CoilGradient Coils

Gradient Coil Example

Chen Lin, PhD, 2/09

Gradient System Specifications

• Maximum amplitude and slew rate

i.e. 45mT/m & 200T/m/s

• FOV

• Nonlinearity -> Distortion

• Stability -> Ghosting

• Duty Cycle

• Fidelity

– Eddy current compensation with pre-emphasis

– Maxwell (cross-term) calibration

Trapezoid Gradient Pulse

t

A

Chen Lin, PhD, 2/09

Other Gradient Related Artifacts

Star Artifact “Annefact”

Chen Lin, PhD, 2/09

Aliasing artifact caused by phase errors at both sides of the magnet. Image shows a moiré artifact produced by the

addition and cancellation of signals.

Zhuo, J. et al. Radiographics 2006;26:275-297Chen Lin, PhD, 2/09

Adopted from Joseph P. Hornak, Ph.D., The Basics of MRI

Summary

Chen Lin, PhD, 2/09

Thank you !

Please send your additional questions and comments to:

clin1@iupui.edu