Medical Physicist ’s Testing Procedures for MRI Equipment ... · AbstractID: 14612 Title: Medical...
Transcript of Medical Physicist ’s Testing Procedures for MRI Equipment ... · AbstractID: 14612 Title: Medical...
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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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
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M-99
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~1000/yr~1000/yr
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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21 Magnetic Field Homogeneity
• 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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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
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BW1 (Hz)
min
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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28 Magnetic Field Homogeneity
• 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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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
Tot
al N
umbe
r of
Spo
kes
Tot
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umbe
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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++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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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
Birdcage CoilHigh Field
Solenoid CoilLow Field
Solenoid CoilLow Field
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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
Birdcage CoilHigh Field
Birdcage CoilHigh Field
Solenoid CoilLow Field
Solenoid CoilLow Field
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment Evaluation
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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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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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