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Improved infarct scar imaging and scar classification using native T1ρ-mapping

Maximilian Gram^1,2, Daniel Gensler^1,3, Michael Seethaler^2,3, Anton Xu^2,3, Peter Michael Jakob², and Peter Nordbeck^1,3

¹Department of Internal Medicine I, University Hospital Würzburg, Würzburg, Germany, ²Experimental Physics V, University of Würzburg, Würzburg, Germany, ³Comprehensive Heart Failure Center (CHFC), University Hospital Würzburg, Würzburg, Germany

Synopsis

Myocardial mapping techniques are known to provide new diagnostic possibilities for morphological and functional information including accurate tissue classification for several diseases. However, without the use of contrast agents, the differentiation between healthy and diseased tissue is hardly possible. In the present study, a scar/infarct model has been employed to investigate the contrast ratio performance of different native MRI mapping techniques (T1, T2, and T1ρ) under controlled conditions. Here, T1ρ provides the best results with an up to 12-times increased contrast ratio. Hence, T1ρ-mapping might be a very promising technique for scar imaging without the use of contrast agents.

Introduction

Cardiac quantitative MR imaging, especially myocardial T1-mapping, has become an increasingly important imaging technique over the last years, which establishes new non-invasive diagnostic possibilities. Multiple recent studies have shown that cardiac T1 measurements can be used to acquire diverse morphological and functional information [1,2] including precise tissue classification for several diseases. However, without the use of contrast agents, the differentiation between healthy and diseased tissue is hardly possible due to an insufficient contrast ratio performance of common spin-lattice and spin-spin relaxation techniques [3]. Therefore, T1ρ-mapping is a promising alternative that has distinct advantages over traditional T1 and T2 quantification methods. The relaxation mechanism under the spin-lock condition shows a high sensitivity to low frequency processes at the molecular and cellular level [4]. Another important advantage of T1ρ-based imaging is the ability to modulate the contrast ratio by regulating the spin-lock amplitude. This is known as T1ρ-dispersion imaging [5].

In the present work, a precisely adjustable scar/infarct model has been employed to investigate the contrast ratio performance of different MRI relaxation quantification techniques (T1, T2, and T1ρ) under controlled and reproducible conditions. Due to the remarkable contrast ratio performance and tissue differentiation ability, T1ρ-mapping might be a very promising quantification method for scar imaging and scar classification without the use of contrast agents.

Methods

All measurements were performed on a 7.0T small animal imaging system Bruker BioSpec 70/30 (Bruker BioSpin MRI GmbH, Ettlingen, Germany). The adjustable scar/infarct model contained porcine myocardial tissue pieces (ex-vivo) with defined ablation scars. For this purpose, the tissue was locally heated by a thin stainless-steel wire (diameter=0.5mm) with supplied current. Subsequently, a detailed quantitative analysis of T1, T2 and T1ρ relaxation times of the scar core, scar border, and remote myocardial tissue was performed. The contrast ratio performance of the different methods was calculated by fitting a suitable multi-gaussian function. For all quantification techniques a Turbo-Spin-Echo (TSE) sequence was used as basis. T2 quantification was done by varying the echo time TE. T1-mapping was performed by an inversion recovery prepared TSE with various inversion times TI. For T1ρ-mapping a novel spin-lock pulse-sequence with effective compensation mechanisms against B0 and B1 field inhomogeneities was applied. The quantitative measurement of T1ρ was performed by variation of the spin-lock time TSL. For the T1ρ-dispersion measurements the spin-lock amplitude was varied. The parameter maps were acquired and compared under equal conditions in the same slices. In order to obtain significant results, the measurements were repeated in different slices.

Further sequence parameters were: TR=10.000ms, fov=32x32mm², slice=3.5mm, resolution=250x250µm², TI=64-8304ms, TE=8-173ms, TSL=2-259ms, FSL=256-2048Hz

Results

Figure 1 shows some exemplary relaxation maps. The myocardial tissue was provided with three scars. The relaxation times of scar cores saw an increase across all parameters in every map. In the scar border the relaxation times decreased in comparison to the remote tissue. As seen in the contrast profile through scar I (Figure 2), the T1ρ maps reveal the highest contrast of the scar core. In addition, T1ρ also shows an excellent contrast of the scar border. The contrast ratio increases by increasing the spin-lock amplitude. Hence, the relaxation behavior shows a distinct dispersion as expected. In Table 1 (Figure 3) the relaxation times and calculated contrasts of the T1, T2 and T1ρ measurements are depicted. The highest contrast (+298%) of the scar core can be observed for a spin-lock amplitude of FSL=2048Hz. In comparison, T1 only shows a contrast of +24.7% and T2 a value of +105%. In the border area, the highest contrast was achieved with T1 (-14.7%) and T1ρ at FSL=2048Hz (-14.7%). T2 indicates the lowest contrast (-7.64%).

Discussion

Modeling and examining ablation scars provide valuable insights into the processes that occur in damaged tissue. The comparative study under controlled conditions with porcine myocardial tissue shows that T1ρ based imaging provides the best results with 3- to 12-times increased contrast. In addition, T1ρ provides both high positive and negative contrast mechanisms, compared to the other techniques that only show good contrast for positive (T2) or negative (T1) relaxation time changes. Hence, T1ρ-mapping might be a promising technique for scar imaging without the use of contrast agents.

The current study is only concerned with ex-vivo measurements of ablation scars and not with true infarct scars. However, at present, measurements are carried out on various types of true myocardial infarctions (acute/chronic) in the ex-vivo animal model. Furthermore, in-vivo measurements with progression studies are in preparation. Here, our new T1ρ-mapping sequence will enable improved imaging as well as quantitative data quality without spin-lock artifacts as first measurements have already shown.

Acknowledgements

This work was supported by the Federal Ministry for Education and Research of the Federal Republic of Germany (BMBF 01EO1504, MO6).

References

1. Gensler, et al. Radiology. 2015 Mar;274(3):879-87.

2. Messroghli, et al. Magn Reson Med. 2004 Jul;52(1):141-6.

3. Nezafat. JACC: Cardiovascular Imaging. 2015 8(9): 1031-1033.

4. van Oorschot, et al. J Magn Reson Imaging. 2017 Jan;45(1):132-138.

5. Wáng, et al. Quant Imaging Med Surg. 2015 Dec;5(6):858-85.

Figures

Fig. 1) Exemplary parameter maps (T1, T2, T1ρ (1024Hz), T1ρ (2048Hz)) of the porcine myocardial tissue. The three scars were created with an ablation time of 140s. Relaxation times increase in the core of the scar and decrease in the border area. In Figure 2, the contrast profiles of a vertical trajectory through scar I are considered.

Fig. 2) Contrast profiles of scar I in a vertical trajectory. The profiles indicate the relative contrast ratio to the intact remote tissue. Besides the peak (scar core), two drops (scar border) are clearly visible. The relaxation maps based on T1ρ show a significantly higher contrast of the scar core and a good contrast of the border area. The contrast increases with the spin lock amplitude FSL.

Fig. 3 / Tab. 1) Measured relaxation times T1, T2 and T1ρ in the area of diseased (scar core) tissue, scar border and intact remote tissue. The T1ρ-maps were acquired with the spin lock amplitudes FSL=256Hz, 512Hz, 1024Hz, and 2048Hz. The relaxation times given are averages of scar I and II from multiple slices. The errors represent the corresponding standard deviations. The relaxation times were used to calculate the percentage contrast ratio between the scar core and scar border to intact remote tissue.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

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