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Phantom study on magnetic resonance imaging (MRI) T₁ and T₂ relaxation times measurements standardization

Davide Cicolari¹, Domenico Lizio², Patrizia Pedrotti³, Monica Teresa Moioli², Alessandro Lascialfari¹, Manuel Mariani¹, and Alberto Torresin^2,4
¹Department of Physics, Università degli Studi di Pavia, Pavia, Italy, ²Department of Medical Physics, ASST Grande Ospedale Metropolitano Niguarda, Milan, Italy, ³Department of Cardiology, ASST Grande Ospedale Metropolitano Niguarda, Milan, Italy, ⁴Department of Physics, Università degli Studi di Milano, Milan, Italy

Synopsis

Relaxation times measurement standardization, a great issue in clinical inter-centre and inter-scanner applications, is studied by comparing relaxation times maps of a MnCl₂ phantom, scanned with two different MRI imagers, with reference values measured with an NMR laboratory spectrometer. For this study standard sequences were used: NMR reference T₁ and T₂ values were obtained from IR (Inversion-Recovery) and CPMG (Carr-Purcell-Meiboom-Gill) sequences respectively; MRI maps were generated from clinical IR and SE (Spin-Echo) sequences. The MRI and NMR results agreement within the experimental error limits (5%) suggests that the estimation of the relaxation times is independent from the spectrometer/scanner utilized.

Introduction

Performing clinical diagnosis by means of MRI pure relaxation times maps ¹ is a procedure that requires a high level of standardization between methods of images acquisition and post-processing ^2,3 and, most of all, among different scanners ⁴: qualitative and quantitative information on tissues physical properties should be obtained through the appropriate image acquisition and analysis.
User-verified validation of MRI scanners standardization in measuring relaxation times is necessary especially using MRI scanner-independent methods.
This study has the aim to assess the standardization in measuring relaxation times values for ¹H nuclei from post-processing of phantom images acquired by means of different vendor clinical MRI scanners taking as a reference T₁ and T₂ values estimated with an NMR laboratory spectrometer.

Methods

For this study an MRI phantom was realised: the phantom is composed by twelve vials (30 ml) made of borosilicate glass with Teflon caps filled with different concentrations of MnCl₂ aqueous solutions. ⁵
Reference T₁ and T₂ values at 1.5 T of the solutions were measured with an NMR spectrometer (Techmag Apollo spectrometer, Bruker electromagnet) scanning twelve smaller vials (2 ml) made of the same materials and filled with the same solutions of the bigger ones (see Figure 1). The temperature dependence of relaxation time values was measured as well: the vials were analysed inside a cryostat in which the temperature was set by a flux of liquid nitrogen and a heating resistance; the temperature was measured with a thermocouple nearby the vial with a sensibility of ±0.2 K.
The NMR sequences used to obtain the reference values were standard sequences: SR (Saturation Recovery) and IR (Inversion Recovery) for T₁ measurements; SE (Spin-Echo) and CPMG (Carr-Purcell-Meiboom-Gill) for T₂ measurements. ⁶
MRI measurements of relaxation times were performed with two different scanners: a Siemens Magnetom Aera (1.5 T) and a General Electric Signa (1.5 T) MRI scanners, using clinical SE and IR sequences. The temperature of the phantom was monitored by means of an optical fibres sensor TempSense (Opsense Inc., Quebec, Canada, Figure 2) with a sensibility of ±0.2 K. Images were post-processed by the vendor independent open source software Segment ⁷ (http://www.medviso.com) in order to obtain the relaxation times maps: measurements of relaxation times were performed with Segment itself selecting ROIs (Regions Of Interest) on the generated maps.

Results

NMR results concerning the dependence of relaxation times from MnCl₂ concentration and from temperature are shown in Figure 3. An experimental error of ±5% was applied to all values (from previous studies on the systematic error of the NMR experimental set up). SR results are not shown because statistically coincident with the IR ones (within the experimental error of 5%, as shown in the Bland-Altman analysis ¹⁰ in Figure 4). SE results were affected by a high diffusion that induce a severe underestimation of T₂ values: the suppression of the signal due to diffusion is related also to the static field inhomogeneity, that in our experimental system is estimated to be approximately of 90 ppm. ⁶
A conservative experimental error of ±5% was associated to clinical MRI imagers results too.
The comparisons between MRI results and NMR results (these last ones taken as reference) are illustrated in Figure 5. To obtain the reference values for the MRI results, which were acquired at different temperatures with respect to the NMR measurements, a linear regression of NMR data vs. temperature was used: with the coefficients obtained from the linear regressions we could determine the relaxation times reference values of each vials at any temperature. Again, an error of ±5% is associated to the NMR reference values calculated in this way. Compatibility tests between MRI values and NMR reference values were performed by means of the two-sample t-test, assuming the experimental errors equal to the standard deviations, thus obtaining for all the comparisons significance level (p-values) greater than 0.1.

Discussion

In our study we assessed T₁ and T₂ values obtained from NMR measurements and set them as reference values for inter-scanner T₁ and T₂ evaluation. A good agreement was found for both relaxation times of each vials in the limits of experimental errors (±5% for both T₁ and T₂) using different and independent MR scanners (an NMR spectrometer and two MRI scanners).
An error of ±5% was applied to MRI results because Segment provides the mean value and the error (standard deviation) related to the pixel intensities within a selected ROI (the pixel intensity in a relaxation time map corresponds to the relaxation time value): the error calculated in this way does not provide an accurate estimation of the error related to the several processes that occur in the mapping process (image acquisition and generation, pixel per pixel fitting procedure). Analysis methods as those proposed by Kellman et al. in 2013 ¹¹ could lead to a better estimation of experimental errors.

Conclusion

Results suggest that standard sequences are scanner-independent methods to establish pure relaxation times values within the experimental error limits (5%): the sequence-dependence and scanner-dependence, well known in literature especially for fast mapping sequences ^3,4,12, could be overcome by an harmonization of acquisition parameters in order to obtain results as close to standard sequences ones as possible.

Acknowledgements

No acknowledgement found.

References

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Figures

Figure 1 The twelve vials composing the MRIMnCl₂ phantom (bigger ones, 30 ml) and the twelve vials for the NMR measurements (smaller ones, 2 ml). Both 30 ml and 2 ml vials were provided by Fisher Scientific (https://fishersci.com). MnCl₂ was provided by Sigma-Aldrich (www.sigmaaldrich.com).

Figure 2 The twelve vials in a support (plexiglass filled with a solution of copper sulphate) for the MRI measurements and the optical fibres sensor TempSense (Opsense Inc.) for the temperature monitoring.

Figure 3 NMR results of relaxation rates R₁ and R₂ (the inverse of relaxation times) as a function of MnCl₂ concentration for IR (left) and CPMG (right) sequences at five different temperatures. The linear dependence of relaxation rates from MnCl₂ concentration expected from Solomon-Bloembergen-Morgan theory for paramagnetic dilute solutions ^8,9 was well observed for all temperatures.

Figure 4 Bland-Altman plot of NMR longitudinal relaxation times T₁ measured at different temperatures with SR and IR sequences. Mean +0.26 % (solid line), confidence intervals for the mean from -0.08% to +0.60% (grey area), agreement limits from -2.30% to +2.82% (dotted lines).

Figure 5 Comparison of MRI results (black dots) of relaxation rates R₁ and R₂ (the inverse of relaxation times) with NMR reference values (grey): GE Signa IR results (top left), GE Signa SE results (bottom left), Siemens Aera IR results (top right) and Siemens Aera SE results (bottom right). NMR reference values were obtained from linear regression of relaxation times data as a function of temperature. MRI values and NMR references are always statistically compatible (two sample t-test, p > 0.1).

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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