MR Application Essentials Part 2
MR Orthopedics:
MapIt
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Agenda
• Biochemical imaging of cartilage
• T1 mapping of cartilage
• T2 mapping of cartilage
• T2* mapping of cartilage
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Biochemical imaging
of cartilage
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Biochemical imaging of cartilage
CELL
GAG
glycosaminoglycan
COLLAGEN
Water / Mobile ions
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Biochemical imaging of cartilage
• Several biochemical imaging techniques that map biochemical changes within the
cartilage have been trialed within the scientific community
o T1 mapping: used in dGEMRIC studies to measure the proteoglycan content of the cartilage
o T2 and T2* mapping: provides information on the collagen orientation and the water content of the
cartilage
o DWI: provides information on the anisotropy and collagen orientation
o Magnetization transfer: collagen orientation
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Biochemical imaging of cartilage
• Several biochemical imaging techniques that map biochemical changes within the
cartilage have been trialed within the scientific community
o T1 mapping: used in dGEMRIC studies to measure the proteoglycan content of the cartilage
o T2 and T2* mapping: provides information on the collagen orientation and the water content of the
cartilage
o DWI: provides information on the anisotropy and collagen orientation
o Magnetization transfer: collagen orientation
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T1 mapping
of cartilage
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T1 mapping of cartilage
dGEMRIC: Delayed Gd-Enhanced MRI of Cartilage
• It is an imaging technique to estimate joint
cartilage glycosaminoglycan content by T1-
relaxation time measurements after
penetration of the Gd-DTPA2-
• The resulting T1 maps are used in cartilage to
track proteoglycans
o Example: it is demonstrated that human knee
cartilage adapts to exercise by increasing the
glycosaminoglycan content à T1-relaxation
time becomes longer
• With dGEMRIC: Pre-operative staging, Prediction
of therapy outcome, Therapy follow up
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T1 mapping of cartilage
dGEMRIC: Delayed Gd-Enhanced MRI of Cartilage
Why?
• Glycosaminoglycan is negatively charged
• If there is lack of glycosaminoglycan, there is a
lack of negative charges
• After the injection of a ionic contrast agent, this
lack of negative charges is substituted by Gd-
DTPA2-
• The accumulation of Gd-DTPA2- in the cartilage
provokes a T1 shortening
• This T1 shortening can be evaluated in a T1 map
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T1 mapping of cartilage
dGEMRIC: Delayed Gd-Enhanced MRI of Cartilage
The examination
• Outside the magnet
o Intravenous injection of Gd-DTPA2- (ionic contrast agent)
o Patient performs exercise during some minutes (~ 15-30 min)
• MRI Examination: T1 maps to estimate joint cartilage glycosaminoglycan
o If low glycosaminoglycan content, the T1 value is shortened
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T1 mapping of cartilage
syngo.MapIt
T1 mapping
• Gold standard for T1 mapping:
o was multiple IR spin echo measurement
o very long acquisition time of up to 30 minutes à makes it clinically difficult
• syngo MapIt T1 mapping:
o uses a multi-angle VIBE measurement
o the acquisition time is significantly reduced to 3 minutes
o T1 map is produced by a pixel-by-pixel analysis (results in ms)
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T1 mapping of cartilage
syngo.MapIt
T1 mapping – Ernst angle
• The signal for spoiled GRE is
1 - exp(- TR T1 )
GRE signal = M 0 sin (a ) ×
1 - cos(a ) × exp(- TR T1 )
71% Ernst
signal
• This can be transformed into the linear version
GRE signal GRE signal
= exp(- TR T1 ) × + M 0 × (1 - exp(- TR T1 ))
sin (a ) tan (a )
• For a given T1 there is an optimum set of angles: syngo MapIt will
calculate this if the user requires (“Auto angle calculation” must be
selected). The optimum angles correspond to 71% of the Ernst signal
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T1 mapping of cartilage
syngo.MapIt
Auto angle calculation ON
• For a given T1 there is an optimum set of
angles: syngo.MapIt will calculate this if
the user requires
• The optimum angles correspond to 71%
of the Ernst signal
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T1 mapping of cartilage
syngo.MapIt
Auto angle calculation OFF
• The user can also set up the flip angles
independently
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T1 mapping of cartilage
syngo.MapIt
Measurements
• The user can define the number of different
flip angles that will be scanned
o Minimum: 2 different flip angles
o Maximum: 10 different flip angles
Noise threshold
• Signals less than this value will be ignored in
the fit
o If in doubt, leave at default value
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T1 mapping of cartilage
syngo.MapIt: protocols @1.5T
“T1Map” or “T1Map_p2”
• 3D measurements with different flip angles for T1
measurement and calculation
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T1 mapping of cartilage
syngo.MapIt: protocols @3T
B1 correction: “B1mapForT1mapping”
• Extends mapIt T1 mapping functionality by doing
correction of the B1 inhomogeneity influence
o First step in the T1 mapping with B1 correction is to
measure B1 inhomogeneity
o B1 mapping measurement, based on the tfl_b1Map
sequence, is applied to determine the actual flip angle
for each voxel
o Resulting flip angle maps from this measurement are
stored in the memory (in PARC data store (PDS) on
MARS) and used later by T1 mapping protocols
“T1Map_anatomical” or “T1Map_p2_anatomical”
• 3D measurements with different flip angles for T1
measurement and calculation
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T1 mapping of cartilage
syngo.MapIt: B1 corrected T1 maps
Without B1 correction With B1 correction
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T1 mapping of cartilage
syngo.MapIt
Shows a sagittal 3D GRE image in a patient Precontrast T1-map shows no difference in T1 Postcontrast T1-map shows a significant drop
after microfracturing at 3T. In the area of relaxation times in the repair tissue in T1 values in the repair tissue, which
microfracture only slight thinning of the corresponds to a low glycosaminoglycan
cartilage layer can be seen content
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T2 mapping
of cartilage
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T2 mapping of cartilage
Clinical use
Information on the collagen orientation and the water content of the cartilage
• Cartilage repair therapies (microfracture – MFX – therapy vs. Matrix-associated Autologous Chondrocyte
Transplantation – MACT – therapy)
• Cartilage repair therapies follow up (MACT therapy follow up)
• Early Osteoarthritis – OA – detection (femoral acetabular impingement)
T2 has been used to study cartilage repair therapies and to provide information on cartilage softening
• T2 provides information on structural changes in the collagen within the cartilage
• T2 provides information on changes in the water content of the cartilage
o Compressed areas show less water content à lower T2
o Less compressed areas show greater water content à higher T2
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T2 mapping of cartilage
syngo.MapIt
T2 is measured using a multi-echo spin echo
• Contrast: up to 32 echoes
o More echoes provides a better fit
o Each echo produces an image
• TE: set the echo time for each echo
o Keep maximum TE to less than or similar to expected
T2 values
• TR: use values higher than 1000 ms
• Noise threshold: Signals less than this value will be
ignored in the fit
o If in doubt, leave at default value
T2 map is produced by a pixel-by-pixel
analysis (results in ms)
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T2 mapping of cartilage
Example: volunteer
Axial T2-map of the cartilage layer of the patella at 7 Tesla
It nicely demonstrates the zonal variation in the collagen fiber network in
cartilage in this volunteer
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T2 mapping of cartilage
Example: MFX therapy
Microfracture therapy (MFX)
• It creates microfractures in the bone
• Cartilage regeneration is promoted by
the released blood and marrow
• Reduced T2 in the region of MFX
therapy
Courtesy of S. Trattnig, Dept. Radiology, University Vienna
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T2 mapping of cartilage
Example: MACT therapy
Matrix-associated autologous
chondrocyte transplantation
therapy (MACT)
• It is an operative procedure using a
cell seeded collagen matrix
• It is used for the treatment of
localized full thickness cartilage
defects
• Reduced T2 in the region of MACT
therapy
Courtesy of S. Trattnig, Dept. Radiology, University Vienna
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T2 mapping of cartilage
syngo.MapIt: R2 mapping
R2 map is also possible
• R2 = 1/T2 (relaxivity values)
• Instead of the T2 values in ms (relaxation
times), MapIt provides the R2 values in 1/s
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T2* mapping
of cartilage
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T2* mapping of cartilage
Clinical use
It can be used as a substitute for T2 mapping
• T2* is the apparent transverse relaxation rate
• 1/T2* = 1/T2+1/T2’
• T2* has a T2 component: 1/T2
• T2* also has a component which depends on the field change within the voxel: 1/T2′ = γ ΔBinhom
resulting from:
o Macroscopic field changes, i.e. main field homogeneity, large susceptibility fields from implants
o Macroscopic field changes due to susceptibility variations, i.e. bone-cartilage interface
o Microscopic field changes from the microstructure within the voxel
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T2* mapping of cartilage
syngo.MapIt
T2* is measured using a multi-echo gre sequence
• Contrast: up to 12 echoes
o More echoes provides a better fit
o Each echo produces an image
• TE: set the echo time for each echo
o Keep maximum TE to less than or similar to expected
T2* values
• Noise threshold: Signals less than this value will be
ignored in the fit
o If in doubt, leave at default value
• Bandwidth: use the same bandwidth for all the echoes
T2* map is produced by a pixel-by-pixel
analysis (results in ms)
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T2* mapping of cartilage
Example: volunteer
Axial T2*-map of the cartilage layer of the patella at 7 Tesla
It nicely demonstrates the zonal variation in the collagen fiber network in
cartilage in this volunteer
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T2* mapping of cartilage
Example: volunteer
Axial T2*-map of the cartilage Axial T2-map of the cartilage
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T2* mapping of cartilage
Example: volunteer
T2* map should be more efficient than T2 map
• SNR of GRE is higher than SE
• Inter echo spacing is lower in GRE than in SE
• A small patient study suggests that variability is better with T2* values than with T2 values
Axial T2*-map of the cartilage Axial T2-map of the cartilage
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T2* mapping of cartilage
Example: MFX therapy
Sagittal T2*-map of the cartilage
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T2* mapping of cartilage
syngo.MapIt: R2* mapping
R2* map is also possible
• R2* = 1/T2* (relaxivity values)
• Instead of the T2* values in ms (relaxation
times), MapIt provides the R2* values in 1/s
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MR Application Essentials Part 2
MR Orthopedics: MapIt
Laura Nieto Charques
Allee am Roethelheimpark 3a
91052 Erlangen
E-Mail: laura.nieto@siemens.com
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