This study investigated the effects of varying the acquisition window length (Tacq) on sodium MRI image quality of articular cartilage. In vivo sodium MRI of knees was performed with Tacq durations of 4-25ms. Increasing Tacq improved signal-to-noise ratio (SNR) by 200% with minimal increases in image blurring. While longer Tacq improves SNR, the short T2 relaxation of cartilage results in some blurring; however, measurements showed this blurring was minimal using their acquisition scheme. Thus, the SNR gains of longer Tacq outweighed the small adverse effects on image quality.

1. Effects of the acquisition window length on articular cartilage sodium MR
image quality
Alireza Akbari1,2, and Michael D. Noseworthy1,2,3
Sodium magnetic resonance imaging (MRI) is
challenging because of the limited signal-to-
noise-ratio (SNR). It also suffers from image
blurring due to its short biexponential
transverse relaxation, T2. SNR is proportional
to the square root of the acquisition window,
Tacq (1), thus lengthening this should be
beneficial. However, SNR gain would be at
the cost of increased image blurring.
One sodium MRI application is the
assessment of articular cartilage. 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 Tacq.
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 split design 18cm
diameter birdcage transmit/receive RF coil
tuned for 23Na (53.311MHz). 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. 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 the
patellar cartilage was measured for each Tacq
and SNR was calculated according to
Madelin et al. (7).
a a a
b
a c
In vivo 23Na MR images of the knee were collected for
the various Tacq durations (Fig. 1). Increasing Tacq
from 4 to 25ms resulted in an increased SNR by 200%
(Fig. 3). FWHM, as a measure of image blurring,
increased by 1±0mm across three different sections of
articular cartilage (Fig.2).
Figure 1. In vivo 23Na images of articular cartilage
obtained at various Tacq = 4-25ms in (a) axial, (b) sagittal,
(c) coronal views. The signal-to-noise ratio (SNR)
improved as Tacq duration was increased.
Figure 2. Blurring Measurements. In vivo FWHM
measurements in one individual’s patellar (a),
posterior femoral condyle (b), and femorotibial
(c) cartilage for Taq=4-25ms.
Figure 3. In vivo 23Na SNR measurements (mean±SE) in
three sections of the articular cartilage. The SNR gain with
respect to Tacq=4 ms is indicated by the number above each
bar.
Our results indicate that SNR benefits significantly
from lengthening Tacq. Image blurring is expected
due to short T2 relaxation (10-30ms) (8); however, the
FWHM measurements indicate minimal image
blurring. This is in agreement with the measured
amount of blurring for a DA-3DPR acquisition
scheme (5). This work demonstrates 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.
1. Haacke EM, et al. John Wiley and Sons; 1999. 2. Lesperance LM, et al. J Orthop Res. 1992;10(30):1-13. 3. Reddy R, et al. Magn
Reson Med. 1998;39(5):697-701. 4. Shapiro EM, et al. Magn Reson Med. 2002; 47(2):284-291. 5. Nagel AM, er al. Magn Reson
Med. 2009;62(6):1565-1573. 6. Fessler J a. J Magn Reson. 2007;188(2):191-195. 7. Madelin G, et al. Magn Reson Med. 2012;68(3):
841-849. 8. Madelin G, et al. Prog Nucl Magn Reson Spectrosc. 2014;79:14-47.
akbaria@mcmaster.ca
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