CNRs of CPC to dentin in ZTE images of goat teeth. Across three consecutive slices
over three goat teeth at TP1 and three at TP2, mean SNRs of CPC and dentin as well as mean CNRs of CPC to dentin. Error bars
indicate ± 1 SD.
A BE model was used for T2* estimations in the CPC and dentin regions for all three goat
teeth at TP2. A robust fit was achieved with a high goodness of fit (mean R2: 0.99; Figure 7). Two components with distinctly different T2* and fraction values were found for CPC and
dentin by using the BE model. The mean T2* values for both components and the mean
fraction values of the long T2* components were calculated for CPC and dentin in each tooth
across two (for CPC) and three (for dentin) consecutive slices (Figures. 8A, B, C). The mean T2* values across all three teeth for CPC were 1110±280 µs and 94±38 µs (with CVs 25.2%
and 40.4%), and for dentin these were 2211±180 µs and 91±2 µs (with CVs 8.1% and 1.7%). The mean fraction percentage for the long T2* component for CPC was 55.5±5.4% (CV:
9.8%), and for dentin this was 66.9±1.6% (CV: 2.5%).
The mean T1 values in each goat tooth at TP2 were also estimated for CPC and dentin
across two (for CPC) and three (for dentin) consecutive slices (Figure 8D). The average T1
values and the corresponding CV values over all three teeth were: 742±103 ms and 13.9% for CPC, and 930±57 ms and 5.0% for dentin. Compared to freshly applied CPC in goat teeth at TP1, much lower T1 values were found in the CPC region at TP2 (742 ms vs 1008 ms).
3.3. Water content
Pore and bound water volume percentages of all five CPC cylinders were calculated. The mean value for pore water over all CPCs was 24.8±2.7%, and for bound water this was 10.4±0.8%.
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Figure 7. MR images of goat teeth obtained with UTE at TE=
18 µs. A) coronal view at TP2. ROIs (pink) with identical size of
3 by 2 voxels were selected in CPC and dentin regions, respectively. A bi-exponential (BE) model fit the multi-TE UTE
data (*) using a semi-log plot for the ROIs in CPC (B) and dentin
(C) regions, which (blue) achieved high goodness of fit: R2
= 0.99 (CPC: T2.1*= 1310 µs
(fraction: 53.2%), T2.2*= 114 µs;
dentin: T2.1*= 2208 µs (fraction:
68.5%), T2.2*= 88 µs).
Figure 8. Mean levels of the T2* values in the long T2* component (A) with its fraction (B), the T2* values in the
short T2* component (C) and T1 values (D) of the CPC and dentin across two (for CPC) or three (for dentin)
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4. Discussion
In this paper we present high field proton MR properties of a new CPC material as applied in the restoration of teeth. This includes T1 and T2* relaxation times of this material implanted in
teeth and of the enamel and dentin tissues in these teeth. Because of the very short T2*
relaxation times of CPC and dental tissues, we used a ZTE sequence to obtain high SNR images of CPC restored teeth. In these images, CPC is hyperintense and has a strong image contrast with respect to dentin and enamel. In addition, we demonstrate that seven weeks after implantation in goat teeth CPC exhibits a less homogeneous signal distribution, a decreased T1, and a change in T2* values, implying a certain CPC degradation over time.
Previously, the T2* value of a comparable CPC was reported, although the ME model used
to analyze the multi-TE data in that study did not result in a satisfactory fit [36]. Because non- exponential behavior was also observed for the T2* relaxation of the CPC investigated in this
study, we applied the GAME model to fit our gradient-echo data [28]. This model assumes that the intra-voxel frequency distribution responsible for the reversible relaxation component is Gaussian- rather than Lorentzian- shaped, which is particularly relevant at higher field when this component becomes more prominent in T2* relaxation. It has been successfully
used for T2* determinations from gradient-echo measurements of human brain and prostate
and gynecologic cancers at 3T [28,32,33]. In this study it was applied for the first time to estimate the T2* values of teeth and engineered tissue.
Unlike the ME model that gives a single T2* value from fitting the signals of a multiple TE
gradient-echo measurement, the GAME model provides both the irreversible relaxation time (T2) and reversible rela ation time (1/ ) from such a measurement. As the reversible
relaxation reflects local inhomogeneities, the relatively high CV (about 10%) that we obtained for the 1/ times of CPC across three human tooth samples indicates differences in microstructures such as small (air) cavities in the material and at the interface of CPC with tooth tissues, which may cause complex susceptibility gradients. Fitting with the GAME model also appeared valuable for T2* estimations of dentin and enamel. While T2 relaxation
dominates their T2* rela ation times, the effect of 1/ on T2* of these tissues is not negligible
in some teeth (Figures 1 and 2).
Previously, relaxation times for enamel and dentin have been investigated at 1T [7,37]. These studies determined for a semi-liquid component in enamel a T2* of about 240 μs, a T2
of 10 ms and a T1 in the order of 300 ms. For dentin a T2* of below 1000 μs and T2 of 38 ms
were reported. A study performed at 1.5T reported T2 (T2*) values for dentin and enamel in
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and in vivo using a UTE sequence with multiple TEs [31,39]. Fitting by a standard ME model, the T1 and T2* relaxation times of dentin tissue ex vivo were 545 ms and 478 μs and the T2*
time of dentin in vivo was 324 μs, which are closer to the results we have obtained in the present study, than to those in any of the other previous studies. The discrepancies between the reported tooth relaxation properties of these studies might be because of the use of different field strengths, different MR sequences and different analysis and fitting methods, as well as of variations between the conditions of the tooth samples used in each examination.
Because of their very short T2* relaxation times and limited water content, ZTE is a favorite
sequence to record high SNR images of teeth [8,15]. As CPC, dentin and enamel appeared to have comparable T1 values, a FA (3°) for near-optimum steady state magnetization at a TR of
2 ms could be applied in the ZTE sequence for tooth imaging, which essentially produces proton density weighted images. The observed hyperintense signal in CPC and strong image contrast to surrounding tissues can therefore only be explained by its higher water content relative to dentin and enamel. To confirm this assumption, a gravimetric analysis was performed to quantify water content in CPC. Two water components, namely, pore water and bound water, were found with a much higher water volume percentage (about 25 and 10% respectively) than that reported for enamel and dentin (4 and 10% respectively) [8]. As CPC thus has at least two water compartments, a bi-exponential T2* decay may have been expected
that would not have been possible to fit by the GAME model. A plausible reason that this is not observed is that the bound water probably has an ultra-short T2*. A T2* value of 100 μs has
been reported for collagen-bound water in cortical bone at 3 T [29]. Together with the substantial contribution from local field inhomogeneities, which follows from the GAME fit, the T2* of bound water in CPC matrices is expected to be significantly shorter at 11.7 T. Also,
considering the lower volume percentage of this bound water it is expected that in the UTE examinations for T2* fitting, bound water contributed little to the recorded signal.
To investigate if CPC degradation in teeth can be monitored over time, we separately implanted CPC into three goat teeth ex vivo and three in vivo. While the fresh CPC implanted in teeth ex vivo was scanned immediately (TP1), the CPC injected in teeth in vivo was measured at seven weeks post-surgery (TP2). With respect to the CPC at TP1, we observed a less homogeneous signal distribution in ZTE images for the CPC component at TP2. We also compared their relaxation properties and found a decreased T1 time and T2* separated into two
components (Figure. 9). Based on these findings, we infer that the CPC at TP2 had undergone compositional changes. The observed less homogeneous structure might indicate that a certain degree of CPC dissolution had occurred at TP2, giving rise to a different phase cement
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