Synopsis
We
assessed articular cartilage sodium MR image SNR and blurring as a function of
acquisition window length. Increasing
the acquisition window dramatically improved the SNR with minimal reduction in
image quality as shown by minimal blurring.Introduction
Sodium
magnetic resonance imaging (MRI) is challenging because it is a
signal-to-noise-ration (SNR)-limited technique. It also suffers from image
blurring due to its short biexponential transverse relaxation, T
2. One way to improve SNR,
would be lengthening the acquisition window while keeping the overall
acquisition time the same. SNR is proportional to the square root of the
acquisition window, T
acq (1). Therefore, if for example, T
acq were doubled then SNR
would theoretically increase by $$$\sqrt[]{2}$$$. However, the SNR gain
would be at the cost of increased image blurring. One sodium MRI application is in assessment of articular
cartilage as studies have shown direct correlation between osteoarthritis (OA)
severity and articular cartilage sodium content
(2-4). Enhancing articular cartilage sodium MR image quality would
lead to more accurate sodium quantification and a better assessment of OA
severity. The goal of this
work was to investigate the balance between sodium image SNR gain and
deleterious image blurring when lengthening T
acq.
Methods
A
density-adapted 3-dimentional projection reconstruction (DA-3DPR) sequence
(5) was implemented on a GE MR750 3T (General
Electric Healthcare, Milwaukee WI) and home-built 12-rung 23Na split
design 18cm diameter birdcage transmit/receive RF coil was used for
23Na
image acquisition. Sodium DA-3DPR datasets
corresponding to Tacq = 4,
8, 12, 16, 20, and 25ms were acquired with the following imaging parameters
: TE/TR
= 0.25/100ms, 11310 projections, isotropic resolution/FOV = 3mm/18cm, and averaging
= 2. All images were reconstructed into 60 slices of 540x540 (i.e. 0.3mm in-plane
resolution x 3mm thick) using a non-uniform fast Fourier transform (NUFFT)
(6). To quantify
the effect of readout window length on blurring, the full-width-at-half-maximum
(FWHM) of the slice profile across patellar cartilage was measured for each T
acq. The SNR was calculated
in the patellar cartilage according to Madelin et al.
(7).
Results
In
vivo axial views of the knee with various T
acq
durations are shown in figure 1. The background noise was noted to markedly
decrease as T
acq was increased,
with only a slight increase in image blurring. The SNR measurement for patellar
cartilage is shown in figure 2 for each Tacq.
The SNR increased considerably with respect to the T
acq=4ms data as indicated by the numbers above each
bar. The blurring caused by T
acq
lengthening in the patellar cartilage is shown in Figure 3. The FWHM as a
measure of blurring was observed to slightly broaden by a maximum of 1mm in the articular cartilage as
T
acq was increased from 4
to 25ms.
Discussion
Our
results indicate that SNR is doubled when T
acq
is increased from 4 to 25 ms. This is a significant improvement in SNR.
However, one would expect this increase to be greater, i.e. $$$\sqrt[]{25 ms/4 ms} = 2.5$$$,
or 250% increase. One
reason SNR imrovement was less than theoretical may be because articular
cartilage has short T
2
relaxation (10-30ms)
(8), that leads
to signal loss with longer acquisition
window lengths. In fact, SNR is proportional to the square root of the acquisition
window in the absence of T
2
effects. Image blurring is expected due to biexponential T
2 relaxation; however, the FWHM measurements indicate
small image blurring. This is in agreement with the measured amount of blurring
for a DA-3DPR acquisition scheme
(5)
even when the acquisition window is extended beyond the T
2s.
Conclusion
This
work demonstrates that the benefits of increasing Tacq in terms of SNR gain outweighs the minimal adverse
effects of blurring on articular cartilage sodium MR image quality using
DA-3DPR.
Acknowledgements
No acknowledgement found.References
1.
Haacke EM, Brown RW, Thompson MR, Venkatesan R. Magnetic Resonance Imaging:
Physical Principles and Sequence Design. New York: A John Wiley and Sons; 1999.
2.
Lesperance LM, Gray ML, Burstein D. Determination of fixed charge density in
cartilage using nuclear magnetic resonance. J Orthop Res. 1992;10(30):1-13.
3.
Reddy R, Insko EK, Noyszewski E a, Dandora R, Kneeland JB, Leigh JS. Sodium MRI
of human articular cartilage in vivo. Magn Reson Med. 1998;39(5):697-701.
4.
Shapiro EM, Borthakur A, Gougoutas A, Reddy R. 23Na MRI accurately measures
fixed charge density in articular cartilage. Magn Reson Med. 2002;47(2):284-291.
5.
Nagel AM, Laun FB, Weber MA, Matthies C, Semmler W, Schad LR. Sodium MRI using
a density-adapted 3D radial acquisition technique. Magn Reson Med.
2009;62(6):1565-1573.
6.
Fessler J a. On NUFFT-based gridding for non-Cartesian MRI. J Magn Reson.
2007;188(2):191-195.
7.
Madelin G, Babb JS, Xia D, Chang G, Jerschow A, Regatte RR. Reproducibility and
repeatability of quantitative sodium magnetic resonance imaging in vivo in
articular cartilage at 3 T and 7 T. Magn Reson Med. 2012;68(3):841-849.
8.
Madelin G, Lee J, Regatte RR, Jerschow A. Sodium MRI: methods and applications.
Prog Nucl Magn Reson Spectrosc. 2014;79:14-47.