AJR 2005; 184:1754-1761
© American Roentgen Ray Society
Optimization of Gadodiamide Concentration for MR Arthrography at 3 T
Jeffrey N. Masi,
David Newitt,
Christian A. Sell,
Heike Daldrup-Link,
Lynne Steinbach,
Sharmila Majumdar and
Thomas M. Link
Department of Radiology, University of California, San Francisco, 400
Parnassus Ave., A 367, Box 0628, San Francisco, CA 94143-0628.
Received June 27, 2004;
accepted after revision August 26, 2004.
Address correspondence to T. M. Link
(tmlink{at}radiology.ucsf.edu).
Abstract
OBJECTIVE. The purpose of our study was to determine the optimal
concentration of a gadolinium-based contrast agent (gadodiamide) for direct MR
arthrography at 3 T compared with 1.5 T in an in vitro study.
CONCLUSION. Optimized concentrations of gadolinium-based contrast
agents for MR arthrography are similar at 3 and 1.5 T, although a slightly
greater dilution may be useful at 3 T. Signal-to-noise ratio peak levels are
significantly reduced by adding an iodinated contrast agent, relatively
significantly more at 3 T than at 1.5 T.
Introduction
The value of using intraarticular gadolinium-based contrast agents
to enhance the depiction of the internal structure of joints has been well
established, and the use of these agents is currently considered routine,
particularly in the shoulder
[1-6].
An accepted procedural approach for direct MR arthrography involves injecting
a fixed concentration of a gadolinium-based contrast agent into the articular
space using fluoroscopic guidance with iodinated contrast injection to confirm
proper needle placement [7]. In
an in vitro study designed to optimize a gadolinium-based contrast agent
protocol for direct MR arthrography
[8], the highest signal
intensity in a series of saline and albumin dilutions imaged at 1.5 T was
found to be in the range of 1.25-2.0 mmol/L. In several other recent studies,
authors have reported using concentrations at or above this range (2.00-5.00
mmol/L) when imaging a variety of joints at 1.5 T
[9-12].
Although MR arthrography at 1.5 T is considered to be the imaging technique
with the highest diagnostic efficacy for assessing internal derangements of
major joints, limitations have also been reported
[5,
13]. MRI at 3 T, with its
inherently higher signal-to-noise ratio (SNR), has the potential to improve
image quality and to provide a higher spatial resolution
[14-16],
which would be particularly favorable in enhancing the diagnostic performance
of MR arthrography. To our knowledge, however, no study has been performed to
optimize direct MR arthrography procedures at 3 T, nor have suitable
gadolinium-based contrast agent concentrations been examined in depth at 3 T.
Optimal gadolinium-based contrast agent concentrations used at 3 T may differ
from those used at 1.5 T because of predictions based on field strength and
associated relaxation times.
Because 3-T scanners will be increasingly used clinically and also to
perform direct arthrographic examinations of joints, the aims of our study
were to determine the optimal concentration of gadolinium-based contrast
agents at 3 T versus 1.5 T and to assess the additional effect of iodinated
contrast agents on imaging at the higher field strength.
Materials and Methods
Experimental Phantom
Studies were performed with a nonionic gadolinium-based contrast agent
(gadodiamide, Omniscan, Amersham Health) that is currently used for MR
arthrography at our institution. Solutions with the following gadodiamide
concentrations were obtained: 0.078 mmol/L (1/6,400 dilution), 0.156 mmol/L
(1/3,200 dilution), 0.313 mmol/L (1/1,600 dilution), 0.625 mmol/L (1/800
dilution), 1.25 mmol/L (1/400 dilution), 2.50 mmol/L (1/200 dilution), 5.0
mmol/L (1/100 dilution), 10.00 mmol/L (1/50 dilution), 20.00 mmol/L (1/25
dilution), and 40.00 mmol/L (1/12.5 dilution). To dilute gadodiamide we used
three solutions: normal saline (0.9% sodium chloride), a 1.7 g/dL
concentration of albumin in normal saline to simulate normal joint fluid (as
previously described by Montgomery et al.
[8], and a solution of
iodinated nonionic contrast agent (iohexol, Omnipaque, Amersham Health),
because iodinated contrast material is used to verify correct needle placement
under fluoroscopic guidance. The concentration of the iohexol injection was
300 mg I/L. The 30 solutions were placed in 1.5-mL test tubes (Fischer
Scientific) with one duplicate of each dilution, resulting in a total of 60
vials. In addition, 12 control tubes were obtained, six of them filled with
normal saline and six filled with corn oil. These vials were placed in a
geometric series of holes punched into a foam block as shown in
Figure 1. The vials with
identical solutions and concentrations were placed in different regions of the
field to account for differences in field homogeneity.
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Fig. 1. —Diagram shows phantom, including individual dilutions of
gadolinium (Gd) in saline, albumin, and iodinated contrast agent (iohexol) as
well as fat and water controls. ROI = region of interest, SNR =
signal-to-noise ratio.
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Fig. 2B. —MR images of phantom. T1-weighted spin-echo image (700/14) at
1.5 T of part of phantom outlined by box in A. Note that highest signal
intensity at 1.5 T was determined in 1.25 mmol/L solution (long arrow
indicates saline, short arrow indicates albumin).
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Fig. 2C. —MR images of phantom. T1-weighted spin-echo image (700/14) at
3 T as outlined by box in A. Note that at 3 T highest signal intensity was
measured in 0.625 mmol/L solution (long arrow indicates saline,
short arrow indicates albumin), but with little change in signal
intensity compared with 1.25 mmol/L solution.
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MRI
This phantom was imaged with 3- and 1.5-T MR scanners (Signa, GE
Healthcare) using a standard head coil (GE Healthcare). Both the 1.5- and 3-T
systems were equipped with 4 G/cm gradients. The following sequences were
applied, all having, except when noted otherwise, a matrix of 256 x 256
pixels, slice thickness of 3 mm, and bandwidth of 15.63 kHz: (1) T1-weighted
spin-echo (TR/TE, 500/15); (2) T1-weighted spin-echo (700/14); (3) proton
density-weighted spin-echo (2,550/30); (4) mildly T2-weighted fast spin-echo
(4,000/45, echo-train length, 4); (5) mildly T2-weighted fast spin-echo
(3,000/45; echo-train length, 4); (6) mildly T2-weighted spin-echo (2,000/45;
echo-train length, 4); (7) gradient-echo (33.3/13; flip angle, 45°) at 3 T
and gradient-echo (34/13; flip angle, 45°) at 1.5 T; (8) single-slice
(4-mm) 4-echo (TE, 20, 40, 60, 80) spin-echo at multiple TRs (2,000; 1,000;
500); and (9) single-slice (4-mm) single-echo (TE, 8) spin-echo at multiple
TRs (800, 600, 400, 200, 100).
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Fig. 3. —Graph shows T1 decay time versus gadolinium concentration for
all examined solutions (gadolinium in normal saline, saline with albumin, and
saline with iodinated contrast agent iohexol) at 1.5 and 3 T. Note that curves
for normal saline and albumin gadolinium solutions are similar at 3 and 1.5 T,
with albumin giving slightly lower T1 for concentrations less than 1 mmol/L.
Iohexol has substantial effect on curves, reducing T1 by approximately factor
of 4, with somewhat greater reduction at 1.5 T than 3 T.
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Fig. 4. —Graph shows T2 decay time versus gadolinium concentration for
all examined solutions at 1.5 and 3 T. Again shown are similarities of saline
and albumin curves, whereas T2 values of iohexol curves are substantially
lower.
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Sequences 1-7 included those used for MR arthrography at 1.5 T in our
standard protocol. Sequence 8 was used to calculate T2 maps using a simple
exponential fit to the pixel intensities for the different TEs, and sequences
8 and 9 were used to calculate T1 maps by fitting the signal intensities to
the standard T1 relaxation equation. Figures
2A,
2B, and
2C shows representative images
of the phantom obtained at 1.5 and 3 T with a T1-weighted spin-echo sequence.
The sample vials were kept sealed throughout all imaging to minimize changes
in the solutions (e.g., changes due to evaporation). The scanners were located
next to each other with air conditioning providing the same room temperatures
and thus the same temperature of the samples during the scanning
procedures.
We measured signal intensities in vials with identical concentrations and
found good homogeneity within the field examined with the head coil at 1.5 T
(differences in signal between the two vials with the same contrast
concentration usually < 5%). However, a higher degree of inhomogeneity was
found in the field using the head coil at 3 T (differences in signal 10% and
more pronounced in the periphery of the coil). The higher degree of
inhomogeneity found at 3 T was most likely due to coil technology, which
currently at 3 T is not as advanced as at 1.5 T.
Image Analysis
The images were transferred to SUN/SPARC workstations (Sun Microsystems) on
which all analyses were performed with in-house software written in IDL
(Research Systems). Regions of interest (ROIs) were determined at the center
of each vial, and signal intensities were measured. In addition, five
background signal intensity measurements were obtained and SNRs at each
gadodiamide concentration were determined using the equation:
 | (1) |
Because two vials were available for each concentration, signal intensity
measurements were obtained for both and averaged to take into account any
field heterogeneity. SNRs at each concentration were plotted on a graph and
peaks were determined. Because the plots of the normal saline series and
saline with albumin series were similar, the albumin plot was excluded from
some of the graphs for clarity in peak comparison.
The multiple TEs (20, 40, 60, and 80 msec) from sequence 8 were used to
calculate T2 maps using a simple exponential fit to the pixel intensities for
the different TEs. Images from sequences 8 and 9 were used to calculate T1
maps by fitting the signal intensities at different TRs to the standard T1
relaxation equation. Sequence 8 was used for solutions with T1 greater than
approximately 300 msec, corresponding to concentrations smaller than 1 mmol/L
for saline and saline-plus-albumin solutions and concentrations smaller than
0.2 mmol/L for the iodinated contrast material. Sequence 9 was used for all
other solutions. For both T1 and T2 maps, any pixels with intensities less
than a threshold of approximately 10 times the background noise level were
excluded from calculations. Circular ROIs of 5 mm in diameter were defined for
each vial on the T1 and T2 maps, and mean and SD were calculated for each
region. The predicted signal variation versus contrast concentration plots for
basic spin-echo sequences with a given TE and TR were calculated using the
equation:
 | (2) |
Results
The T1 and T2 decay curves are shown in Figures
3 and
4. Mean and SDs of the
percentage changes in T1 and T2 from 1.5 to 3 T were calculated for contrast
concentrations from 0.078 to 5 mmol/L. There are no substantial differences in
the T1 curves between 1.5 and 3 T for either saline or saline- and-albumin
gadodiamide solutions (0.5% ±; 2.8% and -0.4% ±; 6.6%). For
gadodiamide concentrations less than 1 mmol/L, the saline solutions showed a
10-15% increase in T1 over albumin solutions at the same concentration. The T2
values for these solutions were generally greater at 3 T than 1.5 T, with the
difference increasing at lower concentrations of contrast material. For
iodinated contrast material, we found a consistent increase in T1 (21%
±; 3.8%) and decrease in T2 (-20% ±; 4.8%) at 3 T compared with
1.5 T. Calculating NSIs versus gadodiamide concentrations from these data for
T1-weighted sequences resulted in a small shift of the curves for saline and
saline-plus-albumin to lower gadodiamide concentrations at 3 T compared with
1.5 T, and a significantly larger signal decrease due to iodinated contrast
material at 3 T (Figs. 5A, and
5B). Calculated T2- and proton
density-weighted sequences showed similar increased signal losses for
iodinated contrast material at 3 T.
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Fig. 5A. —Graphs show calculated normalized signal intensity variation
due to measured T1 and T2 versus gadolinium concentration for T1-weighted
spin-echo sequence. Calculated graph for T1-weighted spin-echo sequence with
TR/TE of 500/15. Note that at 3 T, curve is shifted slightly to lower
concentrations for albumin solutions. Curves for normal saline solutions were
similar and are not shown. Substantially lower signal intensities are found
for solutions with gadodiamide and iohexol, with significantly greater
decrease at 3 T than at 1.5 T.
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Fig. 5B. —Graphs show calculated normalized signal intensity variation
due to measured T1 and T2 versus gadolinium concentration for T1-weighted
spin-echo sequence. Calculated graph for more heavily T1-weighted spin-echo
sequence with TR/TE of 500/8. Note that for gadodiamide concentrations below
about 1 mmol/L, the addition of iodinated contrast material should increase
signal-to-noise ratio (SNR) for this sequence, although peak SNR achievable
with iodinated contrast material is still less than that achievable with
gadodiamide alone.
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Fig. 6A. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, and saline with iohexol (iodinated contrast
material), for four sample sequences. SNR curves for other T1- and T2-weighted
sequences were similar in form to those shown here. T1-weighted spin-echo
sequence, TR/TE, 500/15 (1.5 and 3 T) (A); proton density-weighted
spin-echo sequence, 2,550/30 (1.5 and 3 T) (B); T2-weighted spin-echo
sequence, 4,000/45 (1.5 and 3 T) (C); and gradient-echo sequences,
33.3/13 (3 T) and 34/13 (1.5 T) (D). In A and B, observe
some shift to lower concentrations for saline solutions at 3 T relative to 1.5
T. Note increase in SNR at 3 T for all sequences except for T2-weighted
sequence with iodinated contrast material. Also note decrease in SNR with
iodinated contrast material except for T1-weighted sequence at low gadolinium
concentrations.
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Fig. 6B. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, and saline with iohexol (iodinated contrast
material), for four sample sequences. SNR curves for other T1- and T2-weighted
sequences were similar in form to those shown here. T1-weighted spin-echo
sequence, TR/TE, 500/15 (1.5 and 3 T) (A); proton density-weighted
spin-echo sequence, 2,550/30 (1.5 and 3 T) (B); T2-weighted spin-echo
sequence, 4,000/45 (1.5 and 3 T) (C); and gradient-echo sequences,
33.3/13 (3 T) and 34/13 (1.5 T) (D). In A and B, observe
some shift to lower concentrations for saline solutions at 3 T relative to 1.5
T. Note increase in SNR at 3 T for all sequences except for T2-weighted
sequence with iodinated contrast material. Also note decrease in SNR with
iodinated contrast material except for T1-weighted sequence at low gadolinium
concentrations.
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The signal intensity versus gadodiamide concentration curves that resulted
from directly measuring the signal intensities in the vials showed a similar
shape at 3 and 1.5 T, as shown in Figures
6A,
6B,
6C, and
6D—for example,
T1-weighted, proton density-weighted, T2-weighted, and gradient-echo sequences
for normal saline and iodinated contrast material. The curves for saline plus
albumin were similar to the normal saline curves and are omitted for clarity.
For T1-weighted sequences, the shift of the signal intensity curve at 3 T to
lower gadodiamide concentrations relative to the curve obtained at 1.5 T was
more pronounced than that seen in the calculated NSI curves, as can be seen in
Figure 7A, where the curves
have been plotted normalized to the peak signal intensity at each field
strength. For the T1-weighted images at a TR/TE of 700/14, the peak shifted
from a concentration of 1.25 to 0.625 mmol/L
(Figure 7A); whereas for the
TR/TE of 500/15, the peak shifted from 2.50 mm/L to 1.25 mmol/L
(Figure 6A). However, the peaks
of the intensity versus concentration curves were relatively broad. For
example, for the T1-weighted 700/14 sequence, the intensity for 3 T at 1.25
mmol/L was similarly high to that at the peak concentration of 0.625
mmol/L.
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Fig. 6C. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, and saline with iohexol (iodinated contrast
material), for four sample sequences. SNR curves for other T1- and T2-weighted
sequences were similar in form to those shown here. T2-weighted spin-echo
sequences, TR/TE, 4,000/45 (1.5 and 3 T).
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Fig. 6D. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, and saline with iohexol (iodinated contrast
material), for four sample sequences. SNR curves for other T1- and T2-weighted
sequences were similar in form to those shown here. Gradient-echo sequences,
34/13 (1.5 T) and 33.3/13 (3 T).
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Fig. 7A. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, with curves scaled to equal peak heights for 1.5
and 3 T. T1-weighted spin-echo sequence, TR/TE of 700/14 (A) and
T2-weighted spin-echo sequence, 4,000/45 (B). Note shift of peak to
lower concentrations at 3 T for T1-weighted sequence.
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Fig. 7B. —Measured signal-to-noise ratio (SNR) versus concentration of
gadodiamide in normal saline, with curves scaled to equal peak heights for 1.5
and 3 T. T1-weighted spin-echo sequence, TR/TE of 700/14 (A) and
T2-weighted spin-echo sequence, 4,000/45 (B). Note shift of peak to
lower concentrations at 3 T for T1-weighted sequence.
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For the T1-weighted sequences, the signal intensity versus gadodiamide
concentration curves for the iodinated contrast solutions were similar at the
two field strengths, each having a relatively flat peak region, between 0.156
and 0.625 mmol/L (Figure 6A).
Similar findings were obtained in the calculated NSI versus concentration
curves shown in Figures 5A, and
5B. Note that the calculated
NSI takes into account only changes due to T1 and T2, whereas the experimental
SNR curves of Figures 6A,
6B,
6C, and
6D are also reflective of
other factors such as coil sensitivity and overall increase in signal strength
at 3 T.
Similarly to the T1-weighted spin-echo sequences, the saline SNR curve for
the proton density-weighted spin-echo sequence at TR/TE of 2,550/30 shifted to
the left at 3 T, with the peak shifting from a concentration of 0.625 to 0.313
mmol/L (Figure 6B). Corresponding iodinated contrast signal intensities at 3 T appear to approach
a maximum value as gadolinium concentrations decreased to the lowest
concentration measured (0.078 mmol/L), whereas at 1.5 T the SNR reaches a flat
peak at concentrations of 0.156 mmol/L and below.
Peak SNR for saline for the more heavily T2-weighted fast spin-echo TR/TE
of 4,000/45 (Figure 6C), fast
spin-echo TR of 3,000/45, and fast spin-echo TR of 2,000/45 sequences at both
3 and 1.5 T all occurred at a gadodiamide concentration of 0.313 mmol/L.
However, at the optimal concentrations obtained from T1-weighted
images—0.6-2.5 mmol/L—no major decrease in SNR was noted on these
moderately T2-weighted scans because the peaks were quite broad. Iodinated
contrast SNRs were similar at the two field strengths, increasing as
gadodiamide concentration decreased. Any peak of SNR for these solutions
appears to be at or below the 0.078 mmol/L gadodiamide concentration.
SNR for the gradient-echo sequences (33.3/13 at 3 T and 34/13 at 1.5 T,
Figure 6D) peaked at an
identical gadolinium concentration of 2.50 mmol/L for the saline and
saline-with-albumin solutions. The iodinated contrast SNR curve also showed no
shift between 1.5 and 3 T for these sequences, with the peak occurring at a
concentration of 1.25 mmol/L.
Among the normal saline dilution series and the saline-with-albumin
dilution series, no substantial differences in signal intensities were
observed at either field strength for any sequence investigated. Adding iodine
to gadodiamide solutions caused a decline in SNR for almost all gadodiamide
concentrations for all investigated pulse sequences at both 1.5 and 3 T
because of the large susceptibility-induced decrease in T2. Because this
effect is offset to some extent by the iodine-induced increase in T1,
T1-weighted sequences show the least SNR decrease. For low concentrations of
gadodiamide (< 0.5 mmol/L) and heavy T1 weighting, an increase in SNR with
the addition of iodine may even be observed, as shown in Figures
5A,
5B, and
6A.
Figure 5B shows the predicted
normalized signal variations using our measured T1 and T2 values for a more
heavily T1-weighted sequence (500/8) than was performed in this study, where
for any gadodiamide concentration below approximately 1 mmol/L we would expect
an SNR increase with the addition of iodine.
On all sequences, the relative decline in signal intensity caused by adding
iodine was more pronounced at 3 T than at 1.5 T, and the SNR at the optimum
gadodiamide concentration was always reduced with iodination. These decreases
at optimum gadolinium concentrations for saline and iodine gadolinium
solutions on T1-weighted images (500/15) were 36% at 3 T and 24% at 1.5 T. For
proton density-weighted TR/TE of 2,550/30, decreases were 52% at 3 T and 40%
at 1.5 T; and for T2-weighted TR/TE of 4,000/45, they were 61% and 49%. From
the predicted intensity curves for the most heavily T1-weighted image
investigated (500/8), we find decreases of 27% at 3 T and 14% at 1.5 T.
Using identical sequence parameters and comparing peak SNR in the saline
solutions, an increase from 1.5 to 3 T was found by factors of 1.50 for TR/TE
of 500/15, of 1.51 for 700/14, of 1.62 for 2,550/30, of 1.38 for 4,000/45, of
1.35 for 3,000/45, of 1.31 for 2,000/45, and of 1.18 for the gradient echo
sequences, respectively.
Discussion
In this phantom study comparing signal intensities of gadolinium-based
contrast medium dilutions at 3 and 1.5 T, we found that the signal intensity
peaks tended to shift to lower gadolinium concentrations for the T1- and
proton density-weighted spin-echo sequences at 3 T. These changes, however,
were not pronounced and were even less well appreciated when signal
intensity/gadodiamide concentration curves were calculated from T1 and T2
data. Considering these results, a substantial change in previously
[5,
6,
17] proposed optimal
concentration of gadolinium-based contrast agents at 1.5 T (2 mmol Gd/L) may
not be required at 3 T for MR arthrography.
A mild decrease in concentration, down to 1-1.25 mmol Gd/L, may be
considered to optimize SNR in T1-weighted sequences. In addition, the
administration of additional iodinated contrast agents should be minimized,
because they decrease signal intensity at 3 T more substantially than at 1.5
T. These effects are greatest for T2- and proton density-weighted images and
for higher concentrations of gadolinium. It is possible for the addition of
iodine to give a net SNR increase if combined with low concentrations of
gadolinium and heavily T1-weighted imaging. Compared with the T1-weighted
imaging sequences, proton density- and T2-weighted sequences had a
significantly lower optimum SNR contrast concentration (< 0.5 mmol/L), with
little or no shift in the intensity versus concentration curves between 1.5
and 3 T. For iodinated solutions, no peak SNR was observed for proton density-
and T2-weighted sequences, with the signal increasing monotonically to the
lowest gadolinium concentration measured.
Previously there was some discussion whether to use diluted
gadolinium-based contrast agents or just to apply normal saline for MR
arthrography, and studies examined whether a difference existed in image
quality with saline or gadolinium-based contrast agents
[18,
19]. Although Zanetti and
Hodler [19] found better
results for gadolinium-based contrast agents, Yao et al.
[18] found a similar
performance of saline and gadolinium-based contrast agents. Currently,
however, most centers apply diluted gadolinium solutions, and a recent
meta-analysis of 112 published studies found it to be a safe and efficient
technique for diagnosing internal derangement of joints
[5].
Although our study found the best SNR for a 2 mmol/L solution of
gadolinium-based contrast agents, other investigators have suggested the use
of higher gadolinium concentrations
[20-24].
Kopka et al. [23] examined
2.5, 10, and 45 mmol/L solutions of gadopentetate dimeglumine and found the
highest contrast-to-noise ratio for the 10 mmol/L solution. Those authors
added 50% iotrolan to the 2.5 mmol/L solution, although they added less than
5% of iotrolan to the 10 mmol/L solution, which may explain these discordant
findings compared to those of our study. Jacobson et al.
[22] and Loew et al.
[24] described the use of a
1:200 dilution, which corresponds to a concentration of 2.5 mmol/L. Pfirrman
et al. [20] and Czerny et al.
[21] used even higher
concentrations of 4 mmol/L. If a large joint effusion is present, such as in
the postoperative knee, or if a long delay (> 1 hr) between the injection
and the MR examination is expected, the application of a higher gadolinium
concentration may be justified to compensate for a subsequent dilution of the
applied contrast agent in joint effusion or excretion from the joint with
time.
Similar to the findings of Montgomery et al.
[8], we observed that iodinated
contrast material, although helpful in directing needle placement, may
compromise the increase in signal intensity derived from gadolinium. This
decline in signal intensity seen in our test solutions on all pulse sequences
by adding iodine is most likely due to the well-known magnetic susceptibility
of iodine (mass magnetic susceptibility, -4.40 x 10-9
m3kg-1. This susceptibility effect was more pronounced
at 3 T than at 1.5 T and was also stronger on the proton density- and
T2-weighted imaging sequences than on the T1-weighted sequences. To minimize
the confounding susceptibility effect of additive iodine, the amount of
iodinated contrast material injected into the joint should be minimized
[23]. Nevertheless, several
investigators described the use of high concentrations of iodinated contrast
agents, which were mixed with the gadolinium solution and injected into the
joint. Jacobson et al. [22]
recommended the use of a solution of 10 mL saline, 10 mL iohexol, and 0.1 mL
gadopentetate dimeglumine. Brown et al.
[25] showed that these
mixtures are safe and that no significant dissociation of the gadolinium ion
is found. Similar mixtures of saline and iodinated contrast agents with
gadolinium-based agents were also used in other studies
[26,
27]. The advantages of using a
single iodine-and-gadolinium solution are constant contrast agent
concentrations and easier handling (without the required change of syringe and
potential air bubble deposition), but the caveat is that a lower signal
intensity of the applied contrast agent solution visualized on the MR images.
Another potential advantage of a mixture of gadolinium and iodinated contrast
material would be the production of an arthrogram in case the MR study is
aborted or the patient becomes claustrophobic during the MR examination. The
differences in signal intensity when using different concentrations of
iodinated contrast material in gadolinium solutions should therefore be
further investigated, and a concentration may have to be found that will
provide a high signal at 3 T but that will also allow good depiction of the
joint cavity at fluoroscopy.
Although gadodiamide concentrations that maximized signal intensity were
determined in this in vitro study, an in vivo correlation is warranted.
Factors such as further dilution of the injected concentration because of
synovial fluid or large joint effusions, motion effects, and timing between
injection and imaging, need to be evaluated at 3 T. We tried to simulate the
effect of proteins found in synovial fluid by diluting the gadodiamide in an
albumin solution as previously described
[8] but found no substantial
effect on signal intensity peaks at 1.5 and 3 T with the different sequences.
Similar results were reported in the previous study performed at 1.5 and 0.2 T
[8]. On the other hand, to
eliminate the confounding effect of a large joint effusion, the radiologist
could remove the native effusion before injecting the gadolinium mixture into
the joint.
An interesting finding in that previous study performed at 1.5 and 0.2 T
[8] is that the same contrast
dilutions for MR arthrography (i.e., 1.25-2.0 mmol/L) were recommended for
imaging at 1.5 and 0.2 T. Other investigators also favored the use of a single
concentration of gadolinium-based contrast agents for imaging at 1.5 and 0.2 T
[24,
28].
We determined SNR data as a quantitative measure for the observed signal
effect of our test solutions on the MR images. These SNR data are the result
of intrinsic relaxivities of the test solutions, relaxivity effects by the
added contrast agents, and background noise. SNR data should not be confused
with changes in R1 relaxivities due to the contrast agents. The generally
higher SNR at 3 T compared with 1.5 T is due to the balance of a number of
factors. These include the inherent increase in MR intensity with increased
field strength, differences in receiver coil sensitivity between the two
systems, and differences in the T1 and T2 relaxation times with or without
added contrast agent. Not only our injected contrast agent solution, but also
the surrounding joint tissues, will appear brighter at 3 T than at 1.5 T.
Therefore, the resultant contrast between injected solution and surrounding
tissues at different field strengths needs to be further investigated in in
vivo studies.
Our results are limited to the use of gadodiamide, which is a nonionic
gadolinium-based contrast agent, and must be proven for other small-molecular
gadolinium chelates. We also did not study the effect of different dilutions
of the iodinated contrast agent or different iodinated contrast agents.
However, our findings at 1.5 T are consistent with these previously published
in a contrast optimization study
[8] and with clinical findings
concerning contrast dilutions in MR arthrography of the shoulder at 1.5 T
[5,
6,
17]. We therefore think that
they should also be transferable to clinical examinations at 3 T.
Another potential limitation of this study is that we did not include any
fat-containing media immediately adjacent to the contrast-media-filled vessels
and thus did not investigate any fat-saturated sequences, which are routinely
used in MR arthrography. We would not expect good fat-saturation pulses to
significantly affect the relative SNR of different concentrations of contrast
agents because these are set primarily by the T1 and T2 changes induced by the
contrast media. However, associated effects, such as magnetization transfer or
partial saturation of the water signal due to imperfect fat-saturation pulses,
could affect the results in a true clinical setting. These effects may also
affect the 1.5- to 3-T SNR comparisons for these sequences as a result of the
differences in fat-saturation capabilities of the different MR systems.
Because of the complex interactions involved in saturation pulses, only an in
vivo study will adequately address this issue.
In conclusion, the results of this in vitro study suggest that similar
concentrations of gadolinium-based contrast agents for MR arthrography may be
used at 3 and 1.5 T, although a mildly increased dilution at 3 T should be
considered to further optimize contrast. With a focus on T1-weighted
sequences, concentrations of 2-1.25 mmol/L may be recommended. In addition,
because SNR peak levels for iodinated contrast dilutions are relatively lower
at 3 T than at 1.5 T, administration of iodinated contrast agents should be
minimized during MR arthrography at 3 T.
Acknowledgments
We are grateful to Niles Bruce for assisting with the imaging procedures
and to Ruth Calderon for editing the manuscript.
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Abstract
Introduction
