Medical Physicist ’s Testing Procedures for MRI Equipment ... · AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation Slide 1 Medical Physicist

AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation

Slide 1

Medical Physicist’s Testing Procedures for MRI

Equipment Evaluation

Geoffrey D. Clarke, Ph.D.

[email protected] Annual Meeting

May, 2010

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2 Overview

• Phantoms for ACR MRI Accreditation

• Geometric Distortion, RF Receiver Bandwidth and Magnetic Field Homogeneity

• Resolution, Signal-to-Noise Ratio & Signal Intensity Uniformity

• Slice Thickness, RF Transmission & Ghosting Artifacts

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3

Phantoms for ACR MRI Accreditation

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4 ACRMagnetic

ResonanceImaging QualityControl Manual

(rev. 2004)

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5 MRI Accreditation

Standard Phantom Design Goals

• Easy to Use– Multiple inserts

– Not too bulky

– Applicable to all MRI systems

• Moderately Priced– $1050

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6 ACR Standard MRI PhantomsACR MRI Phantoms Produced

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J-97

J-97

F-98

A-98

M-99

S-99

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D-01

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J-03

J-03

F-04

S-04

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M-07

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J-09

Month-Year

Nu

mb

er o

f Ph

anto

ms

Pro

du

ced

~1000/yr~1000/yr

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7 MRAP Standard Phantom Models

S/N 2256 S/N 2857

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8 ACR MRAP Small Phantom

Small version of standard ACR MRAP phantom – designed for use is clinics that have dedicated extremity imagers that allow

application for ACR Accreditation of knee module only.

Small version of standard ACR MRAP phantom – designed for use is clinics that have dedicated extremity imagers that allow

application for ACR Accreditation of knee module only.

Phantom Test Guidance for Use of the Small MRI Phantom for the ACR MRAPPhantom Test Guidance for Use of the Small MRI Phantom for the ACR MRAP

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9 MRAP Small Phantom Specs• inside length 100 mm

• inside diam. 100 mm

• filled with a solution of NiCl

& NaCl

– 10 mM NiCl2 and 0.45% by

weight aqueous NaCl.

• A separate vial is filled with

20 mM NiCl2 but no

aqueous NaCl.

Saggital image show positions for seven slices acquired with small ACR MRAP phantom

Saggital image show positions for seven slices acquired with small ACR MRAP phantom

Phantom Test Guidance for Use of the Small MRI Phantom for the ACR MRAPPhantom Test Guidance for Use of the Small MRI Phantom for the ACR MRAP

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10 MRAP Small Phantom Specs

• Slices are nominally 5mm with 3 mm gaps

• 12 cm FOV; matrix size is nominally 192 (fe) × 152 (pe)

• High contrast resolution arrays are 0.9mm, 0.8mm & 0.7mm

• Crossed wedges have 45o slopes - the bar length difference is twice the actual slice displacement error

• Slices are nominally 5mm with 3 mm gaps

• 12 cm FOV; matrix size is nominally 192 (fe) × 152 (pe)

• High contrast resolution arrays are 0.9mm, 0.8mm & 0.7mm

• Crossed wedges have 45o slopes - the bar length difference is twice the actual slice displacement error

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11 MRI Phantoms: General Features

– Nonsignal-producing container

– Proton density similar to water

– Shorten T1: NiCl & CuSO4

– Mimic Conductivity of tissues: NaCl

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12

AgaroseAgaroseGel Gel

Phantom Phantom with Insertswith Inserts

Uniform Uniform Spherical Spherical PhantomsPhantoms

Phantoms Developed by UsersPhantoms Developed by Users

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13

Geometric Distortion, RF Receiver Bandwidth and

Magnetic Field Homogeneity

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14 Geometric Accuracy

• Measure distance along main axes of phantom

• Compare with known values

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15 Air Bubble

• When air bubble is in phantom, geometric distortion measurement may have to be taken along diagonal.

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16 Watch Out for Distortion Correction

• Distortion correction can warp the FOV

GeometricDistortioncorrection

RF FilterRoll-off

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17 Gradient Correction

• If gradients are inherently non-linear gradient correction may be applied before images are displayed

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18 Magnetic Field HomogeneityMagnetic Field Homogeneity

ωoωo ωoωo ωoωo

Fourier transform of signal produces a Lorentzian peak in

well-shimmed magnet

Fourier transform of Fourier transform of signal produces a signal produces a LorentzianLorentzian peak in peak in

wellwell--shimmed shimmed magnetmagnet

Magnet firldhomogeneity can be characterized using FWHM of resonance

peak

Magnet Magnet firldfirldhomogeneity can be homogeneity can be characterized using characterized using FWHM of resonance FWHM of resonance

peakpeak

Denotes a totally uniform magnetic

field.All signal is at

resonant frequency, ωo.

Denotes a totally Denotes a totally uniform magnetic uniform magnetic

field.field.All signal is at All signal is at

resonant resonant

frequency, frequency, ωωo.o.

Ideal HomogeneityIdeal Homogeneity Good HomogeneityGood Homogeneity Poor HomogeneityPoor Homogeneity

FWHMFWHM

FWHMFWHM

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19 ACR Phantom – Phase Maps

Phase and Unwrapped

Phase Images

AxialAxial

SagittalSagittal

Best homogeneity is in center, edges of phantom degrade uniformity of B-field in phantom

Best homogeneity is in center, edges of phantom degrade uniformity of B-field in phantom180o phase variation180180oo phase variationphase variation

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20 Magnetic Field Homogeneity

spectral line widths phase-difference map

Data from MRI System Manufacturer’s Phantom

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• Overall, the phase mapping technique provides the best mechanism for evaluating field homogeneity.

• Phase-maps in several planes can be obtained to determine the spherical harmonic coefficients and allows a means of “shimming” the magnet.

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22 EstimatingEstimatingReceiver BandwidthReceiver Bandwidth

( )(mm)shift chemical

(Hz) frequency proton 10 3.5 (mm) FOV

(Hz) Bandwidth

6- ×××

=

Distance between Fat & Water represents the amount of the field of view which spans 3.5 ppm of the resonant frequency:

Distance between Fat & Water represents the amount of the field of view which spans 3.5 ppm of the resonant frequency:

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23 Image with Small Bandwidth

• Large chemical shift indicates small BW

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24 Small Bandwidth = Large Distortion

• Smaller BW on T2W image leads to increased image distortion

T1W T2W

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25 Spherical Homogeneity Phantom

saggital

axial

coronal

Photo of homogeneity phantom

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26 Bandwidth-Difference Method

• The MFH is measured from the change in distance between landmarks in the phantoms between the two bandwidths.

• FOV = field of view in m

• γ = γ/2π= 42,567 Hz/mT

)(

)(),(

12

2121

BWBWFOV

ddBWBWyxMFH

−⋅−⋅= ⋅γ

d

Chen HH et al. Medical Physics, 2006, 33(11): 4299-4306.Chen HH et al. Medical Physics, 2006, 33(11): 4299-4306.

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27 Optimization of Parameters

Minimum ppm Measured

0

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0 5000 10000 15000

BW1 (Hz)

min

imum

ppm

BW2 =10000 Hz

BW2 =25000 Hz

BW2 =50000 Hz

FOV = 330 mm

d1-d2 = 1 mm

FOV = 330 mm

d1-d2 = 1 mm

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• For some systems, service personnel may provide use of phase-mapping acquisition and analysis tools.

• Filmed copy of vendor’s final homogeneity map and shim coefficients is useful for documenting initial conditions and establishing a baseline.

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29

Resolution, Signal-to-Noise Ratio

& Signal Intensity Uniformity

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30 Spatial Resolution Matrix: Registration with PhantomSpatial Resolution Matrix: Spatial Resolution Matrix: Registration with PhantomRegistration with Phantom

ImageMatrixImageMatrix

ResolutionHolesResolutionResolutionHolesHoles

PARTIAL VOLUME ARTIFACT

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31 Total Point Spread Function

∆∆yy∆∆xx

Instrumental PSFInstrumental PSF Total PSFTotal PSF

Due mainly to T2*Due mainly to T2*

Due mainlyto matrix size and FOV

Due mainlyto matrix size and FOV

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32 Excessive Truncation Artifacts

If receiver bandwidth is

set too low, images

become prone to

major truncation artifacts.

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33

0.90 mm0.90 mm1.0 mm1.0 mm1.1 mm1.1 mm

ChemicalShift (mm)ChemicalChemicalShift (mm)Shift (mm)

FatFatFat

H2OHH22OO

Slice ThicknessSlice ThicknessSlice Thickness

Resolution InsertResolution InsertResolution Insert

ACR MRI PhantomACR MRI PhantomChemical Shift InsertChemical Shift Insert

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34 Truncation (Gibbs) Artifact

• Appears as lines of alternating darkness and brightness

• Occurs in both read-out and phase-encoding directions

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35

Fourier Convolution Theorem:

Fourier Truncation Artifact

( ) ( )

( ) )(or

)()(2/

2/

xhx

dxhtdxh

DFT

L

L

DFT

x

x

∗=

−=−= ∫∫∞

∞−−

ρρ

ττρτττρρ

The convolution kernel, h(x), is oscillatory andmerges closely spaced features togethergives rise to spurious ringing

This effect is most pronounced where the image exhibits a step discontinuity of signal intensity

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36 Truncation ErrorsTruncation Errors (Gibbs Artifact)(Gibbs Artifact)

True imagefunction, ρ(x)

Convolutionkernel, h(x)

Fourier reconstruction,

ρDFT(x)

Truncation Artifact is prominent in ACR slice

thickness insert

Truncation Artifact is Truncation Artifact is prominent in ACR slice prominent in ACR slice

thickness insertthickness insert

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37 Gibbs Artifacts

• Use of edge enhancement filtering enhances truncation artifacts

“Scalloping”

LCD perception

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38 Methods for Reducing

Truncation (Gibbs) Artifacts

• Use smoothing filter

– Will cause high contrast spatial resolution to be degraded

• Use large matrix size

• Don’t have regions with abrupt signal intensity transitions in the phantom

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39

Slice #8Slice #8 Slice #9Slice #9

Low Contrast DetectabilityLow Contrast Detectability

1.4% Contrast Detectability1.4% Contrast Detectability1.4% Contrast Detectability 2.4% Contrast Detectability2.4% Contrast Detectability2.4% Contrast Detectability

Four sets of plastic membranes with holes 1.5 mm to 7 mm in diameter

Four sets of plastic membranes with holes 1.5 mm to 7 mm in diameter

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40

Slice #10Slice #10 Slice #11Slice #11

Low Contrast DetectabilityLow Contrast Detectability

5.1% Contrast Detectability5.1% Contrast Detectability5.1% Contrast Detectability3.7% Contrast Detectability3.7% Contrast Detectability3.7% Contrast Detectability

Partial volume effect used to obtain contrast from membranes of variable slice thicknessPartial volume effect used to obtain contrast from membranes of variable slice thickness

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41 LCD and Signal-to-Noise

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al N

umbe

r of

Spo

kes

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r of

Spo

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Signal-to-Noise RatioSignal-to-Noise Ratio0 50 100 150 200 250 300 3500 50 100 150 200 250 300 350

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5

0

40

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5

0

++SDSD

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42 Truncation Errors

True imagefunction, ρρρρ(x)

Convolutionkernel, h(x)

Fourier reconstruction, ρρρρDFT(x)

• Truncation Artifact is prominent in ACR slice thickness insert

• Occurs in both read-out and phase-encoding directions

• Truncation Artifact is prominent in ACR slice thickness insert

•• Occurs in both readOccurs in both read--out and out and phasephase--encoding directionsencoding directions

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43 Radio Frequency Coil Checks

• Volume coils

– Signal-to-noise ratio

– Percent integral uniformity

– Percent signal ghosting

• Surface Coils only Maximum SNR Tests

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44 Image Uniformity Reflects RF

Field Distribution

Slotted ResonatorBirdcage with Eight Straight Elements

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45 Uniformity Pattern

Birdcage CoilHigh Field


Solenoid CoilLow Field


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46

ACR phantom - Slice #7

Image UniformityImage Uniformity

Max SignalMax SignalMax Signal

Min SignalMin SignalMin Signal

Big ROI = 195 cm2

(19,500 mm2)

Small ROI’s = 100 mm2

Big ROI = 195 cm2

(19,500 mm2)

Small ROI’s = 100 mm2

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47 Volume Coil Data

Ghost SignalMean SignalBackground Signal

Percent Signal Ghosting

Mean SignalSD of Background Signal

Signal-to-Noise

Max SignalMin Signal

% Image Uniformity

Surface Coil DataMaximum signalSD of Background Signal

Maximum Signal-to-Noise

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48

ACR PhantomSlice #7

Volume RF Coil MeasurementsVolume RF Coil Measurements

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49 Image Intensity Uniformity

• Performance criteria: PIU ≥ 87.5%except 3T (82%)

• Measurement Considerations:• Display may not show signal values

• Display may not allow user to set signal display level

• There may not be a well-defined high/low intensity level

percent integral uniformity = 100 × −−+

1

( )

( )

high low

high low

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50 Uniformity Patterns





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51 Uniformity at 3 Tesla

• B1 field maps in a conductive saline phantom (18 cm diameter)

RL Greenman et al. JMRI 2003, 17(6): 648-655RL Greenman et al. JMRI 2003, 17(6): 648-655

1.5T1.5T

3 T3 T Requirement for percent integral uniformity (PIU) for a 3T MRI system is equal to or greater

than 82%.

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52 Surface RF Coil Measurements

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53 Ghosting is Nonspecific

• Instability in MRI signal from pulse to pulse

• Phantom motion

• Loose connections or bad cable

• Partial failure of radio frequency coils

• Pulse sequence calibration error

– Eddy currents in Fast Spin Echo series

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54 Ghosting• Small amounts

of ghosting may not be above ACR T1W GR limits

• May still cause failure on other tests

• Ghosting on T2W images should be noted & corrected

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55 Ghosting & LCD

• Ghosting that doesn’t cause a failure of itself may obscure otherwise visible LCD spokes

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56

Slice Thickness, & RF Transmission

Artifacts

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57 RF Noise/Leaks/ Spikes

Single frequency

artifact shows up as

liner in image.

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58 DC-Offset Artifacts• MR Signal rides

on top of DC offset

• Leads to zero frequency artifacts

• Not important if off to side

• Can be controlled

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59 Zipper Artifact• Due to

transverse magnetization created by 180o refocusing pulse.

• Can be eliminated with crusher gradients.

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60 Zipper in Sagittal Image

• May be due to large slice thickness

• Inadequate suppression of stimulated echo artifact

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61 RF Clipping / Shine Through

• Due to mis-calibration of RF receiver gain or attenuation

• Clipping of RF by ADC introduces spurious high frequencies

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62 Susceptibility Artifacts

Small inclusions

in LCD insert can hamper

test

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63 Susceptibility Artifacts

Flaw is in Flaw is in phantom, phantom, however however

measurement measurement can still be can still be completed completed

without without compromising compromising

accuracyaccuracy

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64 Air in Slice Thickness Insert

Flaw is in phantom, however

measurement can still be completed

without compromising

accuracy

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65 Slice Thickness Profiles

• Hitachi Aris slice profiles may look strange on certain models and/or versions of software

Hitachi Slice Profile #1

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Hitachi Slice Profile #2

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66 SummarySummary

• The ACR phantom is not adequate for all QC tests – the QMP/MRS must use other phantoms & methods

• All radiofrequency coils must be tested in every mode of operation used clinically.

• It is advisable to use coil manufacturer’s phantoms and tests where available.

•• The ACR phantom is not adequate for The ACR phantom is not adequate for all QC tests all QC tests –– the QMP/MRS must use the QMP/MRS must use other phantoms & methodsother phantoms & methods

•• All radiofrequency coils must be tested All radiofrequency coils must be tested in every mode of operation used in every mode of operation used clinically.clinically.

•• It is advisable to use coil It is advisable to use coil manufacturermanufacturer’’s phantoms and tests s phantoms and tests where available.where available.

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Documents

Medical Physicist ’s Testing Procedures for MRI Equipment ... · AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation Slide 1 Medical Physicist