Patrick Omoumi1, Tom Hilbert1,2,3, Marion Roux1, Jean-Baptiste Ledoux1, Ruud B Van Heeswijk1, Reto Meuli1, and Tobias Kober1,2,3
1Radiology, Lausanne University Hospital, Lausanne, Switzerland, 2Advanced Clinical Imaging Technology (HC CMEA SUI DI BM PI), Siemens Healthcare AG, Lausanne, Switzerland, 3LTS5, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Synopsis
A fast
quantitative T2 mapping technique that additionally provides synthetic images
for morphological assessment was validated by two experienced radiologists regarding
(1) the T2 values through a phantom experiment and (2) the image quality
through a quantitative and qualitative assessment for the knee joint in five
healthy volunteers. Introduction
Today,
intermediate-weighted (IW) and T2-weighted (T2w) Turbo Spin Echo (TSE) sequences
with fat suppression are most commonly used to clinically assess morphological
abnormalities of joint structures
1. In addition, quantitative
analysis of the musculoskeletal system using MR relaxometry techniques such as
T2 mapping has gained interest in recent years. This is in particular the case
for imaging of osteoarthritis, since T2 values can be used as a non-invasive biomarker
of early degenerative disease of cartilage and meniscus
2-4. The acquisition of both
morphological and quantitative sequences is however time consuming. We suggest
using a fast quantitative T2 mapping technique that allows the generation of
synthetic images with different TEs, allowing a significant decrease in
acquisition time while preserving image quality in comparison to the consecutive
acquisition of morphological and quantitative sequences.
Methods
A
multi-echo spin-echo (MESE) sequence was modified in order to acquire undersampled
k-spaces. Generalized autocalibrating partially parallel acquisition (GRAPPA)5
and Model-based Accelerated Relaxometry by Iterative Non-linear
Inversion(MARTINI)6 can be subsequently applied to estimate the
transverse relaxation T2 and the equilibrium magnetization M0, a
method termed GRAPPATINI7. Synthetic TSE images with any T2-weighting
can then be generated using the M0/T2 maps in the forward
signal model.
Phantom
experiments were performed to validate the T2 estimation. To that end, the
prototype GRAPPATINI and product MESE sequences were used to estimate the T2
values within tubes with different concentrations of Gadolinium and Agar using
the same acquisition parameters as in the in-vivo experiments. A single-slice
single-spin-echo product sequence was used to achieve reference T2 values using
a standard log-linear fit onto various fully sampled acquisitions with
different TEs=12,24,36,60,100ms.
Subsequently,
the prototype GRAPPATINI sequence was used to estimate T2 and M0
maps of the knee joint at 3T (MAGNETOM Skyra, Siemens Healthcare, Germany)
using a 15-channel knee coil in five healthy volunteers (3 males, age 30.2±3.3
years). Additional synthetic contrasts with TE=34ms and TE=80ms were generated
on the scanner. For comparison, standard IW (TE=34ms) and T2w (TE=80ms) morphological
TSE images were acquired. The detailed acquisition parameters are listed in Table 1.
The synthetic
morphological images were validated quantitatively and qualitatively in
comparison to the conventional TSE images.
ROIs
of at least 15mm2 were placed on fluid, muscle, meniscus and
cartilage and copy-pasted between comparative images. SNR and CNR
(cartilage/fluid and meniscus/fluid) were calculated.
Qualitative
analysis was performed by two radiologists in consensus blinded to the employed
sequence by comparing the synthetic images and the corresponding TSE side-by-side
in random order. A five-grade scale was used for the comparison (-2: first
image significantly worse than second, -1: moderately worse, 0: no difference,
+1: moderately better, +2: significantly better). Each of the following
anatomical structures was assessed: cartilage, menisci, cruciate ligaments,
bone marrow, muscle, joint fluid, quadricipital and patellar tendons.
Furthermore, image contrast, noise, artifacts, and global diagnostic value were
also compared.
Results and Discussion
Fig.
1
shows the T2 values of the phantom experiment. T2 values found by GRAPPATINI
are slightly overestimated in comparison to the reference method, which is most
likely due to stimulated echoes, a typical problem for T2 mapping using MESE sequences.
The fully-sampled MESE sequence experiences a stronger overestimation which can
be explained by the much shorter TR (1.6s versus 4.88s) causing an even
stronger stimulated-echo effect due to increased T1 influences in the signal
decay8.
The quantitative
analysis showed similar SNR and no statistically significant difference between
the synthetic and conventional sequences (average SNR=9.9 for both sequences, p=0.99).
CNR values were not statistically different between the two sequences
(cartilage/fluid: 6.2 vs. 6.6, p=0.62; meniscus/fluid: 11.3 vs. 11.6, p=0.81).
The qualitative
analysis showed no difference in global image quality (cf. Fig 2) or of any of
the anatomical structures that were evaluated (average score of 0, 95%CI=[0;
0.4]). Artefact scores were slightly higher for the synthetic sequences
(average of -0.1, 95%CI=[-0.002;-0.6]), while visual noise and contrast were
slightly better for the synthetic sequences (average score of 0.1,
95%CI=[0.002;0.6]).
Conclusions
The
GRAPPATINI sequence provides accurate T2 values, as well as synthetic sequences
that are quantitatively and qualitatively similar to conventional TSE sequences.
Using this technique, T2 maps, IW and T2w sequences can all be obtained in 6.22min
compared to 12.13min, corresponding to the sum of the acquisition times
obtained with the standard technique.
Acknowledgements
No acknowledgement found.References
1. Huang M, and Schweitzer ME. The role of
radiology in the evolution of the understanding of articular disease. Radiology
2014;273:S1-22.
2.
Kijowski R, Blankenbaker DG, Munoz Del Rio A, Baer GS, and Graf BK. Evaluation
of the articular cartilage of the knee joint: value of adding a T2 mapping
sequence to a routine MR imaging protocol. Radiology 2013;267:503-13.
3.
Zarins ZA, Bolbos RI, Pialat JB, Link TM, Li X, Souza RB, and Majumdar S.
Cartilage and meniscus assessment using T1rho and T2 measurements in healthy
subjects and patients with osteoarthritis. Osteoarthritis Cartilage
2010;18:1408-16.
4.
Rauscher I, Stahl R, Cheng J, Li X, Huber MB, Luke A, et al. Meniscal
measurements of T1rho and T2 at MR imaging in healthy subjects and patients
with osteoarthritis. Radiology 2008;249:591-600.
5.
Griswold, Mark A., et al. Generalized autocalibrating partially parallel
acquisitions (GRAPPA). Magnetic resonance in medicine 47.6 (2002): 1202-1210.
6. Sumpf,
Tilman J., et al. Model-based nonlinear inverse reconstruction for T2 mapping
using highly undersampled spin-echo MRI. Journal of Magnetic Resonance Imaging
34.2 (2011): 420-428.
7.
Hilbert, Tom, et al. MARTINI and GRAPPA-When Speed is Taste. Proc. Intl. Soc.
Mag. Reson. Med.. 22.4077 (2014).
8.
Maier, Cynthia F., Steve G. Tan, Hari Hariharan, and Hollis G. Potter. T2
quantitation of articular cartilage at 1.5 T. Journal of magnetic resonance
imaging 17, no. 3 (2003): 358-364.