ISO/TS 24560-1:2022

TECHNICAL ISO/TS
SPECIFICATION 24560-1
First edition
2022-07
Tissue-engineered medical
products — MRI evaluation of
cartilage —
Part 1:
Clinical evaluation of regenerative
knee articular cartilage using delayed
gadolimium-enhanced MRI of
cartilage (dGEMRIC) and T2 mapping
Produits médicaux issus de l'ingénierie tissulaire — Évaluation du
cartilage par IRM —
Partie 1: Évaluation clinique de la régénération du cartilage
articulaire du genou par séquences IRM tardives après injection de
gadolinium (dGEMRIC) et cartographie T2
Reference number
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 3
5 T2 mapping evaluation in human knee articular cartilage . 4
5.1 Characterization parameters and methods . 4
5.2 T2 value measurement process . 6
5.2.1 Post-processing of imaging . 6
5.2.2 Measurement method . 6
5.2.3 ROIs of regenerative cartilage . 7
5.2.4 ROIs of normal control cartilage . 8
5.3 T2 value evaluation . 8
5.3.1 Purpose of evaluation . 8
5.3.2 In vivo evaluation of regenerative cartilage with T2 value . 8
6 dGEMRIC evaluation in human knee articular cartilage . 9
6.1 Characterization parameters and methods . 9
6.2 T1 value measurement process . 11
6.2.1 Post-processing of imaging . 11
6.2.2 Measurement method . 11
6.2.3 ROIs of regenerative cartilage .12
6.2.4 ROIs of normal control cartilage .12
6.3 ΔR1 value calculation . 12
6.4 ΔR1 value evaluation .12
6.4.1 Purpose of evaluation .12
6.4.2 In vivo evaluation of regenerative cartilage with ΔR1 values .13
7 Acceptable standard for MR evaluation .14
7.1 Requirements for MR equipment . 14
7.2 Requirements for MR parameters. 14
7.3 Requirements for the MR longitudinally evaluation . 14
7.4 Exclusion criteria . 14
8 Limitation .15
Annex A (informative) Example of measurement results .16
Annex B (informative) Introduction of T1ρ MR Imaging technology .26
Bibliography .28
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 150, Implants for surgery, Subcommittee
SC 7, Tissue-engineered medical products.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
Tissue-engineered cartilage has shown desirable results for the repair of cartilage defects, and histologic
findings indicate that the repaired tissue has a hyaline-like cartilage structure. Kang H.J. et al., Zheng
M.H. et al. and Behrens P. et al. reported that the histologic change after matrix-associated autologous
[1-3]
chondrocyte implantation/transplantation (MACI/MACT) was a hyaline-like cartilage. The knee
articular cartilage can also be repaired or regenerated via other tissue engineering approaches using
other seed cells such as mesenchymal stem cells or even by tissue regeneration free of external seed
[4-6]
cells . MACI and other approaches lead to a maturation of the cartilage matrix over time with the
development of an organized collagen architecture. For long-term follow-up of regenerative cartilage,
clinical scores and morphological evaluations are commonly used. Furthermore, histological evaluation
from arthroscopic biopsies provides a gold standard for morphological and biochemical assessments
of regenerative cartilage tissue. However, this process is invasive and unacceptable for patients after
cartilage repair surgery. Magnetic resonance (MR) is a noninvasive technique that can be used for the
evaluation of a cartilage microstructure. Xu X and other researchers reported that MR-based biochemical
imaging techniques, such as delayed gadolinium-enhanced MRI of the cartilage (dGEMRIC) and T2
[7-12]
mapping, show the capability of evaluating the biochemical character of articular cartilage . The
T2 relaxation time is sensitive to the content of effective hydrogen atoms, and thus to the concentration
[13]
of collagen, the main component of cartilage extracellular matrix . Besides, the orientation changes
in the collagen network of articular cartilage produce the depthwise T2 anisotropy through the magic
[14]
angle effect . The dGEMRIC technique enables an indirect estimation of the fixed charge density (FCD)
[15]
of cartilage, which mainly arises from the aggregated proteoglycan biomacromolecules . Since both
collagen and proteoglycan components are important for determining the functional characteristics
of cartilage, a combination of T2 mapping and dGEMRIC techniques provides a better evaluation of
articular regenerative cartilage. Therefore, standardization of T2 mapping and dGEMRIC techniques is
needed for the evaluation of regenerative articular cartilage.
This document is intended to guide the clinical biochemical evaluation of regenerative articular
cartilage with MR. dGEMRIC and T2 mapping are recommended for the clinical evaluation of
regenerative cartilage. These techniques have been used for patients who received tissue-engineered
cartilage implantation or transplantation (MACI/MACT). The validation data from different hospitals
are provided Annex A.
This document provides general principles for imaging and the measurement method of T2 mapping
and dGEMRIC of knee cartilage using 1,5 T or 3,0 T MRI equipment. These techniques are also applicable
for other articular cartilage, such as the ankle joint, hip joint, and shoulder joint, but the imaging
parameters should be adjusted and modified for better image quality.
v
TECHNICAL SPECIFICATION ISO/TS 24560-1:2022(E)
Tissue-engineered medical products — MRI evaluation of
cartilage —
Part 1:
Clinical evaluation of regenerative knee articular cartilage
using delayed gadolimium-enhanced MRI of cartilage
(dGEMRIC) and T2 mapping
1 Scope
This document provides a principle to determine the parameter settings and operating methods for the
evaluation of the composition and structure of articular cartilage by dGEMRIC and T2-mapping MRI
in humans with a typical example of the methods; each are distinct MRI technologies that allow for
noninvasive observation of soft tissue characteristics.
The methods provided in this document are intended for application in the evaluation of the clinical
effects of tissue-engineered cartilage or other cartilage regeneration products used in the knee joint,
and are also applicable for the evaluation of regenerative cartilage in other joints, although some
modification of parameters is needed.
This document describes a longitudinal evaluation of the water content, the glycosaminoglycan (GAG)
concentration, and the concentration and orientation of collagen fibres in regenerative cartilage when
using dGEMRIC and T2-mapping techniques in 1,5 T or 3,0 T magnetic resonance imaging equipment.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
pulse sequences
train of programmed radio frequency pulses and gradient pulses
Note 1 to entry: In MRI, it is a time protocol for encoding images to obtain k-space data.
3.2
number of averages
NA
number of repeated acquired identical MR signals from the same programmed pulse sequence
3.3
voxel
three-dimensional cuboid representing the minimum unit comprising a three-dimensional image
3.4
pixel
two-dimensional cuboid representing the minimum unit comprising an image
3.5
field of view
FOV
width and height of an imaged region
Note 1 to entry: It is expressed in cm by cm or mm by mm.
3.6
matrix
array of scalars arranged in frequency encoding direction and phase encoding direction in a two-
dimensional MR image
Note 1 to entry: It is typically expressed in number of pixels in frequency encoding direction by number of pixels
in phase encoding direction.
Note 2 to entry: In MRI, the scalars in the array are called pixel of the matrix.
3.7
slice thickness
thickness of the imaging plane
Note 1 to entry: It is expressed in cm or mm.
3.8
signal-to-noise ratio
SNR
single number obtained by dividing the image signal by the image noise
3.9
region of interest
ROI
user-defined area on an image in which parameter of interested is calculated
3.10
echo time
TE
time from the centre of the 90-degree excitation RF-pulse to the centre of the echo
Note 1 to entry: It is expressed in ms.
3.11
repetition time
TR
time interval for repetition of the basic unit of magnetic resonance pulse sequences
Note 1 to entry: It is expressed in ms.
3.12
proton density-weighted image
PDWI
magnetic resonance image reflecting the concentration of protons in tissue
3.13
matrix-associated autologous chondrocyte implantation/transplantation
MACI/MACT
procedure involving expansion of autologous chondrocytes and seeding the cells onto a three-
dimensional biomaterial scaffold
3.14
scaffold
support or structural component or delivery vehicle, or matrix, consisting of synthetic and/or naturally-
derived material(s), for modulating the biological properties or transport of administered and/or
endogenous cells and/or binding/transport of bioactive agents
Note 1 to entry: Biological properties include (but are not limited to) adhesion, migration, proliferation, and
differentiation.
[SOURCE: ASTM F2312 -11: 2020, Clause 4]
3.15
gradient recalled echo
GRE
MR sequence that generates gradient echoes as a consequence of echo refocusing
3.16
delayedgadolinium enhanced MRI of the cartilage
dGEMRIC
pre-contrast and post-contrast T1 mapping of cartilage
3.17
longitudinal relaxation time
T1
time taking for the longitudinal magnetization to recover approximately 63 % of its initial value after
being flipped into the magnetic transverse plane by a 90° radiofrequency pulse
Note 1 to entry: It is expressed in ms.
3.18
transverse relaxation time
T2
time taking for the magnetic resonance signal to irreversibly decay to 37 % of its initial value after
being flipped into the magnetic transverse plane by a 90° radiofrequency pulse
Note 1 to entry: It is expressed in ms.
3.19
T1 mapping
two-dimensional spatial distributions of T1 value of tissue
3.20
T2 mapping
two-dimensional spatial distributions of T2 value of tissue
3.21
R1
longitudinal relaxation rate calculated as 1/T1
4 Principles
Articular cartilage is a type of hyaline cartilage that is characterized by an extracellular matrix that
[16]
contains a fine network of collagen and proteoglycan . In regenerative articular cartilage, it is
important to evaluate whether the implanted tissues regenerate to hyaline or hyaline-like cartilage
with time. MRI is a noninvasive technique that can provide an indirect method for assessing the
composition and microstructure of articular regenerative cartilage, including content and organization
[12],[17]
of the collagen network and the proteoglycan, as the main component in the extracellular matrix .
Delayed gadolinium enhanced MRI of the cartilage (dGEMRIC) is a technique pertinent to the T1
relaxation-time measurement that uses the negative ionic charge of gadopentetate dimeglumine (Gd-
2- 2-
DTPA ) to map the fixed charge density of the cartilage GAG. Gd-DTPA is repelled by negatively
charged GAGs and is therefore negatively related to the local proteoglycan concentration. Consequently,
2-
Gd-DTPA accumulates in areas of low GAG content, and a cartilage will have a shorter T1 relaxation
time in these regions. The ability to measure spatial variations in the cartilage GAG concentration in
vitro with dGEMRIC has been validated biochemically and histologically using both bovine and human
cartilage. The feasibility of using dGEMRIC in vivo has also been demonstrated, and the interpretation
[18-21]
of MR images as representing a GAG distribution is supported by literature evidence . The GAG is a
component of normal hyaline cartilage that is critical to its mechanical strength. Thus, as a noninvasive
method of indirectly monitoring the GAG concentration in cartilage, dGEMRIC is a potentially useful
method for assessing regenerative cartilage.
[22]
T2 mapping usually involves imaging at several echo times along the T2 decay curve and T2
relaxation time of different tissues can be calculated after data processing. In cartilage, changes in the
T2-relaxation times are dependent upon the quantity of water and the integrity of the proteoglycan–
collagen matrix. T2 relaxation time mapping provides an indirect assessment of the collagen structure
and orientation as it relates to the free water content. The presence of unbound water molecules slows
the loss of transverse magnetization following an RF pulse, such that regions of cartilage with more free
water have higher T2 relaxation times. In healthy cartilage, the collagen matrix traps and immobilizes
water molecules. When this structured matrix breaks down, the extra space is filled with free, unbound
water, and leads to elevated T2 relaxation times. The correlation between T2 relaxation time mapping
[23],[24]
and the collagen content has been validated, both in vitro and in vivo . The T2 value of cartilage is
a dipolar interaction due to the slow anisotropic motion of water molecules in the collagen matrix and
[14],[25]
varies as a function of the collagen arrangement in the static magnetic field , the strength of this
interaction is orientation-dependent and reaches its minimum at an angle of 54,7 (between the static
field and the axis of interacting protons, the so-called “magic angle”. Consequently, T2 changes along
cartilage thickness are reported to follow the orientational changes in the collagen fibril network.
Using appropriate arrangement of the articular surface with respect to the B0 field the resulting
laminated appearance in T2 maps approximately corresponds to the histological collagenous zones: the
superficial zone (orientation of collagen fibrils parallel to the articular surface), the transitional zone
(random fibril orientation) and the deep or radial zone (fibrils perpendicular to the articular surface
and perpendicular to the bone), which reveals the spatial collagen architecture in articular cartilage.
This spatial variation is a marker for hyaline-like matrix organization after cartilage repair.
MACI/MACT uses biomaterial scaffolds (natural or synthetic materials) as a carrier and seeds cells of
autologous chondrocytes. The repaired tissue can develop an organized collagen network, which is the
[1-3],[26],[27]
basis for histological characterization of normal hyaline articular cartilage over time . It is
possible to longitudinally evaluate the water content, the GAG concentration, and the concentration and
orientation of collagen fibres in regenerative cartilage after MACI/MACT by using the dGEMRIC and T2
mapping techniques.
In this document, T2 mapping and dGEMRIC data obtained from subjects who received MACI using
different MRI equipment are included Annex A.
5 T2 mapping evaluation in human knee articular cartilage
5.1 Characterization parameters and methods
The 1,5 T or 3,0 T magnetic resonance imaging equipment and multichannel phased-array knee coil
are recommended for T2 mapping examination of knee cartilage. It is recommended to use the same
field strength equipment for longitudinal evaluation to avoid the influence of static magnetic field B0
on the relaxation time of the tissue. Before MRI examinations, the subject should rest for more than
30 min to avoid mechanical loading by exercise, which can influence the T2 value of knee cartilage. B0
and B1 shimming is highly recommended before scanning the T2-mapping sequence for every patient.
Sagittal proton density-weighted images with fat saturation (FS-PDWI) and three-dimensional gradient
recalled echo (3D-GRE) pulse sequences are recommended for morphological evaluation of cartilage.
3D-GRE pulse sequences with spoiled gradient (such as SPGR, FLASH, and VIBE) or steady-state free
precession (such as DESS) can be chosen in different MR manufactures. The pixel size in plane of the
3D-GRE pulse sequence should be consistent with pixel size in plane of the T2 mapping sequence, which
can ensure the accuracy of the image fusion registration.
A regularly repeated phantom test is recommended to ensure the status and stability of the MR system.
Phantom-based quality control is required after any change in the MR system hardware and software.
The protocol of T2 mapping consists of a sagittal, multi-echo spin echo pulse sequence for T2
measurement. Table 1 lists the recommended imaging parameters of T2 mapping in 1,5 T and 3,0 T MR
equipment, as a reference.
Table 1 — Recommended Magnetic resonance parameters of T2 mapping evaluation
T2 mapping
Parameters
1,5 T 3,0 T
FOV (mm x mm) 160 × 160 160 × 160
TR (ms) range 1 200 to 2 000 range 1 200 to 2 000
multiple TE (no less than 4 echo times), more multiple TE (no less than 4 echo times), more
echo times corresponds to more accurate echo times corresponds to more accurate
TE (ms)
T2 calculation, and the maximum echo time T2 calculation, and the maximum echo time
should be shorter than 80 ms should be shorter than 80 ms
Parallel the acceleration factor should be no larger the acceleration factor should be no larger
acquisition than 2 than 2
Matrix no less than 256 × 256 no less than 320 × 320
Pixel size in plane
no larger than 0,6 × 0,6 no larger than 0,5 × 0,5
(mm )
Number of
1 or 2 1 or 2
averages (NA)
Slice thickness
3 is recommended (ranging 3,0 to 4,0) 3 is recommended (ranging 3,0 to 4,0)
(mm)
Image plane sagittal plane sagittal plane
Number of slices no more than 30 slices no more than 30 slices
NOTE The parameters were suggested to be adjusted with different MR equipment and different signal-receiving coil.
MR examination of PDWI and T1-weighted 3D-GRE pulse sequences should achieve the following
standards:
a) the field of view (FOV) should be no larger than 160 mm × 160 mm and no smaller than 140 mm ×
140 mm;
b) the pixel size in plane of the PDWI pulse sequence should not be larger than 0,5 mm × 0,5 mm in 3,0
Tesla MRI equipment and should not be larger than 0,6 mm x 0,6 mm in 1,5 Tesla MRI equipment;
c) a 3,0-4,0 mm slice thickness is suggested in the PDWI pulse sequence;
d) for image matching, some parameters, such as FOV, the scanning centre and slice thickness, are
suggested to be kept the same for both PDWI and T2 mapping;
e) the voxel size of the 3D-GRE pulse sequence should be isotropic and not larger than 0,5 mm ×
0,5 mm × 0,5 mm in 3,0 Tesla MRI equipment and should not be larger than 0,6 mm × 0,6 mm ×
0,6 mm in 1,5 Tesla MRI equipment;
f) the fat-saturation technique is suggested in PDWI and 3D-GRE pulse sequences, such as water-
excitation or fat water separation methods;
g) imaging with high resolution can require multiple signal averages in 1,5 Tesla MR equipment for a
higher signal-to-noise ratio (SNR);
h) if images are acquired with fat suppression, lowering the imaging bandwidth improves the overall
SNR.
5.2 T2 value measurement process
5.2.1 Post-processing of imaging
Post-processing of the multiple images generated by the T2 mapping sequences can be performed
online on the scanner or offline using algorithms written in separate programs, such as MATLAB (the
MathworksInc, Natick, MA). Automated processing on the scanner typically generates a pixel-by-pixel
map of T2 relaxation times, and the T2 maps can be overlain on anatomical images through image
registration. Generally, sagittal PDW images and 3D-GRE images are recommended for morphological
evaluation of regenerative cartilage and native cartilage. PDW images are sensitive to the signal
abnormality of regenerative tissue, and 3D-GRE pulse sequence is used to obtain anatomical images for
its high resolution. T2 map images can be registered to 3D GRE images for verification of regenerative
cartilage and native cartilage (see Figure 1).
5.2.2 Measurement method
T2 relaxation time is obtained by pixel-wise mono-exponential fitting of signal decay at different echo
times, and discarding the first echo for curve fitting is recommended in post-processing to minimize
[28]
the error in T2 . If the regenerative cartilage showed longer T2 component not covered by the entire
ETL, bi-exponential curves including the offset as an additional parameter should be applied and the
corresponding model can be manually selected in the MATLAB software for imaging processing.
The SE pulse sequence signal intensity (S) shall be calculated by Formula (1).
SM=×()11−−expT()RT//×−expT()ET2 (1)
where
S is the SE pulse sequence signal intensity;
M is equilibrium longitudinal magnetization;
TR is the repetition time;
T1 is the longitudinal relaxation time;
TE is the echo time;
T2 is the transverse relaxation time.
When TR>>T1, (1-exp(-TR/T1)) approaches 1. When TR is not much longer than T1(mostly in multi
echo spin echo T2 mapping sequence), TR is fixed, and the T1 value of the tissue is also relatively fixed
according to TR, thus, 1-exp(-TR/T1) is relatively constant with multiple TEs. The M ⨯(1-exp(-TR/T1))
can be calculated as constant S . Therefore, the above formula can be simplified as Formula (2):
SS=×expT()− ET/ 2 (2)
where
S is the SE sequence signal intensity;
S is the steady longitudinal magnetization during TR recovery after the RF pulse;
T2 is the transverse relaxation time;
TE is the echo time.
And the T2 value is calculated by curve fitting.
Key
A high-resolution FS-PDW image of cartilage in a patient at 6 months after MACI of the patella
B T2 map matched anatomical image (3D-GRE) of cartilage in a patient at 6 months after MACI of the
patella
white arrow the location of regenerative cartilage
white triangle the location of control cartilage
NOTE 1 ROIs are placed in T2 map matched anatomical image to locate regenerative cartilage and control
cartilage and to measure the T2 values.
NOTE 2 The scale bar of the T2 maps shows the T2 values ranging from 0 ms to 100 ms. The white represents
a high T2 value, and the black represents a low T2 value. Besides, the coloured scale bar is also recommended.
Figure 1 — A typical T2-mapping MR image of the knee joint in a patient to demonstrate the
measurement of T2 values
After pixel-by-pixel map of T2 relaxation times with either a colour or grayscale map are generated, T2
values of regenerative cartilage and healthy cartilage are measured on T2 maps. The regions of interest
(ROIs) are placed in T2 map matched anatomical image to locate regenerative cartilage and control
cartilage and to measure the T2 values (see Figure 1).
5.2.3 ROIs of regenerative cartilage
ROIs of regenerative cartilage are drawn manually by an experienced senior musculoskeletal
radiologist. The location and extent of regenerative cartilage are identified by at least two radiologists
to ensure the accuracy of the ROI placement; in addition, it is recommended that the same radiologists
perform a longitudinal evaluation to ensure consistency in the placement of ROIs. For imaging analysis,
the ROI of regenerative cartilage should cover the full thickness of the cartilage. In the slice of implanted
plugs, the ROI is placed between the edges of each plug (see Figure 1).
5.2.4 ROIs of normal control cartilage
To compare the T2 values between regenerative cartilage and healthy cartilage, a region of
morphologically normal-appearing cartilage within the same anatomical region should be selected as
a reference (control) cartilage, which is defined as a normal signal on the PDW images if the cartilage
[29]
thickness is preserved, the surface is intact, and no intrachondral signal alterations are visible .
5.3 T2 value evaluation
5.3.1 Purpose of evaluation
The T2 relaxation time is sensitive to the content of effective hydrogen atoms (mostly in water), and
thus to the concentration and orientation of collagen, the main component of cartilage extracellular
matrix.
To longitudinally evaluate the water content and collagen fibre orientation of cartilage with T2 mapping,
a 1-year follow-up MR examination of T2 mapping is suggested, and the time points for follow up at 3, 6,
and 12 months after MACI are recommended. T2 values of both regenerative cartilage and its neighbour
normal cartilage (control) should be measured, including 3, 6, and 12 months after MACI. To avoid the
influence of static magnetic field B0 on the relaxation time of the tissue, this document suggests the
longitudinal evaluation of the articular cartilage in the same equipment or different equipment with
the same field strength.
5.3.2 In vivo evaluation of regenerative cartilage with T2 value
For longitudinal evaluation the biochemical change of regenerative knee articular cartilage, the
variation tendency of the T2 values of regenerative cartilage should be assessed, and statistical analysis
should be performed 3, 6 and 12 months after MACI when the sample size is sufficient (multiple patients)
to prove the clinical efficacy of the tissue-engineered product. In addition, to compare the biochemical
microstructure of the regenerative cartilage and the native cartilage, a horizontal comparison is
recommended between the T2 values of the regenerative cartilage and those of the control cartilage at
3, 6, and 12 months after MACI, and the statistical analysis should be performed when the sample size
(multiple patients) is sufficient for a clinical effect evaluation of cartilage regenerative products or for a
clinical trial.
When the regenerative tissues undergo a gradually hyaline-like repair, the T2 values of the repair
tissue should show a downward trend between 3, 6 and 12 months after MACI. The T2 values of the
repaired tissue should be significantly higher than those of the control cartilage 6 months after MACI,
which reflect greater hydration and an unorganized collagen network in the regenerative cartilage. If
the T2 values show no significant difference between the plugs and the control cartilage 1 year after
MACI, it can indicate the maturation of the collagen network and water content of the repaired tissue.
This result can indirectly demonstrate that repaired tissues can develop a hyaline-like structure (see
Figure 2).
Key
A the location of regenerative cartilage (white arrow)
B, C and D T2 map matched anatomical image (3D-GRE) of cartilage in a patient at 3, 6 and 12 months after
MACI
white arrow the location of regenerative cartilage
a 3 months
b 6 months
c 12 months
NOTE The scale bar of the T2 maps shows the T2 values ranging from 0 ms to 100 ms. The white represents
a high T2 value, and the black represents a low T2 value. Besides, the coloured scale bar is also recommended.
Figure 2 — Example of in vivo evaluation of regenerative cartilage with T2 values
6 dGEMRIC evaluation in human knee articular cartilage
6.1 Characterization parameters and methods
The dGEMRIC technique is based on the measurement of the T1 relaxation time enhanced by delayed
2–
administration of Gd-DTPA and is currently the most widely used method for analysing GAG depletion
[20],[30-32]
in articular cartilage, which have provided valuable results in vitro and in vivo . It contains
both pre-contrast T1 mapping and post-contrast T1 mapping. There are many different methods
for creating T1 relaxation time maps based on progressive inversion or saturation of longitudinal
[33-37]
magnetization . The classical method of T1 mapping uses an inversion-recovery prepared fast
spin-echo (IR-FSE) T1-weighted pulse sequence with multiple TI (inversion times) to measure the
[38]
T1 relaxation time ; however, the acquisition times required for T1 maps are usually long and are
often limited to a small number of slice locations. Recently, the 3D GRE T1-weighted pulse sequence
[39-41]
is gradually used in T1 mapping for greater image coverage and a faster acquisition time . This
document recommends a 3D GRE T1-weighted pulse sequence with variable flip angles in the dGEMRIC
technique. This method allowed the acquisition of a slab covering a whole compartment of the knee
[42]
joint with a relatively high resolution within clinically acceptable scan times . As shown in a phantom
study, central positioning of the 3D GRE slab is critical for achieving the best results of T1 mapping for
eliminating partial volume effects and increasing the SNR. When this positioning is performed, a good
correlation between the variable flip angle technique and the standard inversion recovery technique
[41],[42]
for T1 mapping has been established in phantoms and in vivo .
The 1,5 T or 3,0 T magnetic resonance equipment and multichannel phased-array knee coil are
recommended for T1-mapping examination of the knee joint. B0 and B1 shimming are highly
recommended before scanning the T1-mapping sequence for every patient. Sagittal PDW images and
3D-GRE images are recommended for morphological evaluation of regenerative cartilage and native
cartilage. PDW images are sensitive to the signal abnormality of regenerative tissue, and 3D-GRE pulse
sequence is used to obtain anatomical images for its high resolution. T1 mapping (3D GRE T1-weighted
pulse sequence with variable flip angle) is performed both before and after slow manual intravenous
2- -
injection of Gd-DTPA (0,2 mM/kg body weight, Magnevist, Schering, Germany), and the Gd-DOTA
2- [43,44] 2-
can be an alternative when Gd-DTPA cannot be used . To optimize the penetration of Gd-DTPA
into knee cartilage, patients are asked to flex and extend the knee joint (walk or other motion) for
approximately 10-15 min. Post-contrast T1 mapping is then assessed 90-120 min after the injection
until complete diffusion of the contrast agent into the cartilage.
A regularly repeated phantom test is recommended to ensure the status and stability of the MR system.
Phantom-based quality control is required after any change of the MR system hardware and software.
The protocol of T1 mapping consists of a sagittal, multi-flip angle 3D spoiled GRE T1-weighted pulse
sequence. Table 2 lists the imaging parameters of T1 mapping in 1,5 T and 3,0 T MR equipments, just as
a reference.
Table 2 — Recommended Magnetic resonance parameters of T1 mapping evaluation
T1 mapping
Parameters
1,5 T 3,0 T
FOV (mm x mm) 160 × 160 160 × 160
TR (ms) ranging 6 to 50 (depend on MR equipment) ranging 7 to 50 (depend on MR equipment)
TE (ms) minimum echo time minimum echo time
at least 2 flip angles, and 3 or 4 flip angles at least 2 flip angles, and 3 or 4 flip angles
Flip angles
are recommended are recommended
Parallel the acceleration factor should be no larger the acceleration factor should be no larger
acquisition than 2 than 2
Matrix no less than 256 × 256 no less than 320 × 320
Pixel size in plane
no larger than 0,6 × 0,6 no larger than 0,5 × 0,5
(mm )
Number of
1 or 2 1 or 2
averages
Slice thickness
3 is recommended (ranging 3,0 to 4,0) 3 is recommended (ranging 3,0 to 4,0)
(mm)
Image plane sagittal plane sagittal plane
Number of slices no more than 30 slices no more than 30 slices
NOTE 1 The parameters are suggested to be adjusted with different MR equipments and different signal-receiving coils.
NOTE 2 Two flip angles can be used in T1 mapping with the benefit of saving scan time but show the following limitation:
1) it is possible that two flip angles are not enough to get accurate T1 value for a large range of T1 relaxation time; 2) it is
susceptible to patient motions, and that is a significant downside for longitudinal analysis.
MR examination of PDWI should achieve the following standards:
a) the FOV should be no larger than 160 × 160 mm and no smaller than 140 mm × 140 mm;
b) the pixel size in plane of the PDWI pulse sequence should not be larger than 0,5 mm × 0,5 mm
in 3,0 Tesla MRI equipment, and should not be larger than 0,6 mm x 0,6 mm in the 1,5 Tesla MRI
equipment;
c) a slice thickness from 3,0 mm to 4,0 mm is suggested for the PDWI pulse sequence;
d) for image matching, some of the parameters, such as FOV, the scanning centre and slice thickness,
are suggested to be kept the same for both PDWI and T1 mapping;
e) a fat-saturation technique is suggested for PDWI, such as water-excitation or fat water separation
methods;
f) imaging with high resolution can require multiple signal averages in 1,5 Tesla MR equipment for a
higher signal-to-noise ratio (SNR);
g) if imaging is done with fat suppression, lowering the imaging bandwidth improves the overall SNR.
6.2 T1 value measurement process
6.2.1 Post-processing of imaging
Post-processing of the multiple images generated by the T1 mapping sequences can be performed
online on the scanner or offline using algorithms written in separate programs, such as MATLAB (the
Mathworks Inc, Natick, MA), and the formulas of calculating T1 values are indicated in 6.2.2. Automated
processing on the scanner typically generates a pixel-by-pixel map of T1 relaxation times, and the T1
maps can be overlain on anatomical images through image registration. Generally, sagittal PDW images
and 3D-GRE images are recommended for morphological evaluation of regenerative cartilage and native
cartilage. PDW images are sensitive to the signal abnormality of regenerative tissue, and 3D-GRE pulse
sequence is used to obtain anatomical images for its high resolution. T1 map images can be registered
to 3D GRE images for verification of regenerative cartilage and native cartilage.
6.2.2 Measurement method
3D-GRE pulse sequences with spoiled gradient (such as SPGR, FLASH, and VIBE) can be used as T1
mapping sequences. The spoiled GRE pulse sequence with multiple flip angles is used to calculate the
T1 relaxation times. The spoiled GRE signal intensity shall be calculated using Formula (3).
*
ME12−−expTET/sinα
()()
S = (3)
spoiledG RE
1−E cosα
where
M is equilibrium longitudinal magnetization;
E is obtained by the formula exp(-TR/T1);
TR is the repetition time;
T1 is the longitudinal relaxation time;
TE is the echo time;
T2 is the transverse relaxation time;
α is the flip angle.
Here, E = exp(-TR/T1), and minimum TE which is much smaller than T2* is used in the spoiled GRE
pulse sequence, Formula (3) can be represented in the linear form, Y = mX+b, shown as Formula (4).
S S
spoiledG RE spoiledGRE
=+E ME()1− (4)
10 1
sintααan
From which the slope m is E , and the Y-intercept b is M (1-E ), can be estimated by regression, allowing
1 0 1
T1 and M to be extracted:
−TR
T1= (5)
lnm
b
M = (6)
()1−m
So, the T1 mapping can be calculated by two or more flip angles. Using two flip angles, the system
calculates the slope m and Y-intercept by those two points. The linear fitting with the least-square
[45,46]
algorithm is done when the available number of flip angles is more than two .
After a series of pixel-by-pixel map of T1 relaxation times are generated, T1 values of regenerative
cartilage and healthy cartilage are measured on T1 maps. Regions of interest (ROIs) are placed in the
native and regenerative cartilage to measure the T1 values.
6.2.3 ROIs of regenerative cartilage
The ROIs of regenerative cartilage are drawn manually by an experienced senior musculoskeletal
radiologist. The location and extent of regenerative cartilage are identi
...