Qi Peng1, Can Wu2,3, Jee Hun Kim4,5, and Xiaojuan Li4,5,6
1Department of Radiology, Albert Einstein College of Medicine, Bronx, NY, United States, 2Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY, United States, 3Philips Healthcare, Andover, MA, United States, 4Program of Advanced Musculoskeletal Imaging (PAMI), Cleveland Clinic, Cleveland, OH, United States, 5Department of Biomedical Engineering, Cleveland Clinic, Cleveland, OH, United States, 6Department of Diagnostic Radiology, Cleveland Clinic, Cleveland, OH, United States
Synopsis
Magnetization-prepared
gradient echo (MP-GRE)
sequences have been commonly used for quantitative MRI in the literature to
improve imaging speed. Paired acquisitions with RF phase-cycling could eliminate
image blurring due to longitudinal relaxation and loss of MP contrast along the GRE
readouts, with doubled scan time. This study
introduces a novel unpaired phase cycling strategy to eliminate the time
penalty with reduced sensitivity to B0 field inhomogeneities for
high resolution quantitative mapping in MP-GRE sequences. The feasibility and
efficacy of this strategy were demonstrated in both phantom and human studies.
INTRODUCTION
Magnetization-prepared
gradient echo (MP-GRE)
sequences have been commonly used for quantitative MRI in the literature to
improve imaging speed in 3D quantitative imaging, where MR signals are acquired
in a GRE readout train after each MP. Although multiple k-space
lines can be acquired after each preparation, quantitative accuracy and spatial
fidelity are both compromised by the contaminating T1 relaxation and
loss of MP
contrast along the GRE readouts. The combination of PC and flip angle
sweep in the 3D MP-GRE sequence led to the development of the magnetization-prepared angle-modulated partitioned k-space spoiled
gradient-echo snapshots (MAPSS) sequence, which has been applied for high
resolution T1ρ mapping for knee cartilage evaluation.1 The MAPSS sequence, however,
doubles the already long scan time in 3D quantitative mapping, which may
undermine its reliability and reproducibility due to potential motion during
the lengthened scan. We propose here an
efficient PC strategy to eliminate the confounding term in 3D-GRE quantitative
mapping. THEORY AND METHODS
In
a train of uniformly spaced RF pulses, the signal after the nth
RF excitation based on the Block equation can be expressed as Mxy(n)=A(n)Mprep+B(n).2 In MAPSS, two acquisition with opposite phase cycles are
obtained before subtraction to eliminate the contaminating B(n) term. We propose
to use S, where paired PC
is no longer mandated before curve-fitting. All experiments were performed using a 3T (Philips Ingenia) using a phantom with three pairs of tubes (2, 3, and 4 % agarose). The 3D T1ρ MAPSS scan had 16 acquisitions with different TSL and
PC, and the T1ρ maps using the six TSL PC
schemes were generated retrospectively (Table 1). The quantitative results from
six different tubes within the center plane, as well as 4 additional off-center
slices with 1.5cm gap were compared. T1ρref was measured from the ROI on the T1ρ map of the center slice using
TSL_set1. The T1ρ
measurements from all 5 ROIs at different slices of each tube were averaged to
get the overall T1ρmean
and T1ρstdev
of each tube. T1ρ quantification performance for each PC scheme was evaluated using the Mean
Percentage Error (MPE) and Coefficient of Variation (CV). A volunteer knee scan
was also performed to confirm its validity on human. RESULTS
Representative
phantom results are shown in Figure 1. No significant boundary signal
oscillation/blurring can be observed from these maps, indicating that the
unpaired PC strategy effectively eliminated the signal contaminations from T1
recovery as expected. This observation is further confirmed when comparing the y
line profiles against the x line profiles, indicating similar boundary
response for all methods tested. Based on visual observation, TSL_set3
outperforms the rest including TSL_set1 with the least T1ρ
non-uniformity within each phantom, as well as the best consistency between
phantom pairs. Quantitatively, the average MPE from the center slices were all
less than 0.5% (Figure 2A), regardless of which TSL schemes used. Five-slice
averaged results however had much different T1ρ than T1ρref. Among
them all, TSL_set3 had minimum average MPE and CV of 3.51% and 5.79%, compared
to these of 5.76% and 8.33% using TSL_set1 (Figure 2B).
Overall, of the four newly proposed unpaired PC schemes, TSL_set3 compared
favorably with the traditional MAPSS with paired PC (TSL_set1 and TSL_set2),
TSL_set5 had slightly better or similar performance, and TSL_sets 4, 6 had
worse performance when compared to TSL_set1. Figure 3
shows the representative results from the human study. All T1ρ maps had high spatial fidelity along both directions with little
difference between them. In the zoom-in figures of Figure 3B, all maps had nice T1ρ variations along cartilage layers at the interface of the patellar femoral joint, and without adjacent synovial
fluids contaminations due to minimal spatial blurring. The high spatial
fidelity of the T1ρ maps from all four unpaired TSL PC schemes further
confirmed the effectiveness of the unpaired PC strategy, consistent with the
phantom results. DISCUSSION
The originally
proposed MAPSS sequence acquires paired RF PC datasets and eliminates T1
recovery signal contamination along a long readout train with paired
subtraction.1 In this work, we
demonstrated the feasibility and the additional advantages of using an unpaired
PC strategy for more efficient quantitative MR mapping. Our
proposed PC framework alleviated the time penalty to achieve high spatial
fidelity in quantitative imaging required by the original MAPSS sequence.
Unlike other recent fast imaging such as PI or CS, this 50% reduction of scan
time comes without loss of SNR efficiency, compromised spatial fidelity, or undesirable
artifacts. It
can be combined with all existing fast imaging techniques including PI and CS. This
approach allows additional flexibility in protocol optimization to ultimately
achieve high spatial resolution and high accuracy quantitative mapping. Carefully
chosen schemes with this strategy potentially allow more accurate quantitative
mapping with halved scan time needed, without reducing number of sampling
points, to obtain more accurate quantitative mapping within clinically
affordable scan duration in a variety of applications. CONCLUSION
Unpaired
PC strategy potentially allows more accurate quantitative mapping with halved
scan time compared to the paired PC approach. It therefore offers additional
flexibility in SNR optimization, spatial resolution improvement, and choice of
sampling points to obtain more accurate quantitative mapping. Acknowledgements
No acknowledgement found.References
1. Li X, Han ET, Busse RF, Majumdar S. In
vivo T(1rho) mapping in cartilage using 3D magnetization-prepared
angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS).
Magn Reson Med 2008;59(2):298-307.
2. Williams CF, Redpath
TW. Sources of artifact and systematic error in quantitative snapshot of FLASH
imaging and methods for their elimination. Magn Reson Med 1999;41(1):63-71.