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Comparison of lung T₁ mapping using variable flip angle and Look-Locker techniques

Laura Saunders¹, Paul J. C. Hughes¹, James Eaden¹, Andy J Swift¹, Stephen Bianchi², and Jim M Wild¹
¹Infection Immunity and Cardiovascular Disease, University of Sheffield, Sheffield, United Kingdom, ²Sheffield Teaching Hospitals NHS, Sheffield, United Kingdom

Synopsis

Lung T₁ is sensitive to lung pathology, tissue and perfusion, and can be used to the calculation of quantitative DCE perfusion metrics. However, no comparisons have been made of common MRI T₁ mapping sequences in the human lung. The aim of this work was to compare Look-Locker and variable flip angle T₁ mapping sequences in phantoms and in vivo, in the lung, liver and blood. T₁ measured in the phantom, blood and liver was significantly lower when measured using a Look-Locker acquisition than VFA, whereas lung T₁ was significantly higher when measured using Look-Locker acquisition than when measured using VFA.

Introduction

Lung T₁ mapping is sensitive to changes in lung tissue and lung perfusion[1-6] and is used for the calculation of quantitative perfusion MRI[7]. Lung T₁ mapping is usually performed using a Look-Locker inversion recovery sequence with a gradient echo readout or a SPGR sequence acquired at multiple flip angles (variable flip angle, VFA). The Modified Look-Locker Inversion Recovery sequence (MOLLI), which is an interleaved and cardiac gated Look-Locker variant with a SSFP readout has also been used to calculate lung T₁[6, 8].

Comparisons of the accuracy and precision of VFA, Look-Locker and MOLLI sequences have been reported in phantoms and multiple organs. In phantom data, Look-Locker, MOLLI and VFA sequences with B1 correction have excellent agreement with spin-echo inversion recovery[9, 10]. In vivo data has shown less consistent results with significant differences in T₁ measured using different sequences[9, 11]. To the authors knowledge, no comparisons have been made between in vivo human lung T₁ calculated using VFA and Look-Locker sequences, despite both sequences having being performed in multiple studies in the lung[1, 2, 5, 7, 12-15].

In vivo T₁ is sensitive to multiple physiological effects which can alter the measured T₁, including T₂^* and TE dependency[2], T₂ dependency[16], the in-flow of non-inverted blood [16, 17], magnetisation transfer and, for MOLLI sequences, heart-rate[16]. The aim of this work is to compare T1 measured using inversion recovery and VFA sequences in phantoms and in vivo.

Methods

Look-Locker, VFA and MOLLI T₁ mapping was performed on 18 phantoms with known T₁ at 1.5T on a GE HDx scanner.

The 2D Look-Locker inversion recovery sequence[18, 19] acquisition parameters were: TR: 3.2 ms; TE: 0.9 ms; flip angle: 7°; phase × frequency: 128×128; slice thickness: 15 mm; FOV: 440 mm2; overall acquisition time=7s.

The 3D VFA sequence acquisition parameters were: 3 acquisitions with flip angles of 2, 10 and 30°; TE: 0.9ms; phase x frequency = 80x200; slice thickness: 4mm; FOV: 400mm².

The 2D MOLLI sequence acquisition parameters were: image sequence: 3-3-5; TR: 3.2ms ; TE: 1.0 ms; flip angle: 35°; phase x frequency: 192x192; slice thickness: 5.1mm; ASSET: 2; FOV: 400mm²; acquisition time: 17s for 60bpm heart rate.

4 volunteers and 8 patients with IPF underwent Look-Locker and VFA T₁ mapping with the aforementioned parameters. Look-Locker and MOLLI slices were positioned through the descending aorta. A similar slice from the 3D VFA sequence was selected for analysis. Regions of interest were also placed in the liver and descending aorta.

Phantom T₁ measurements were presented using Bland-Altman plots, and mean differences and one-way random intraclass correlation coefficients were calculated. In vivo T₁ was also compared using mean differences and one-way random intraclass correlation coefficients for the lung, liver and descending aorta (blood) separately. Paired-t tests were used to test for difference in T₁ between methods.

VFA and Look-Locker acquisitions in 12 phantoms and 7 patients with IPF and 3 healthy volunteers were re-sized during post-processing to the same size and slice thickness and to assess the effect of image size and slice thickness on resultant T₁.

Results

In phantoms, Look-Locker and MOLLI sequences systematically underestimate T₁ (Figure 1), with an increasing underestimation at longer T₁. VFA calculated a small overestimation of T₁ with a wider standard deviation in T₁ difference. MOLLI and LL T₁ showed excellent agreement.

Lung T₁ values were calculated for healthy volunteers and patients with IPF (Figure 2). Lung T₁ was significantly different between patients with IPF and healthy volunteers when measured using the Look-Locker sequence (volunteers: T₁=1180±30ms; patients: T₁=1040±90ms; p=0.010) but not when measured using the VFA sequence (volunteers: T₁=1040±90ms; patients: T₁=880±190ms; p=0.231).

In phantom data, VFA calculated a significantly higher T₁ than Look-Locker (p<0.001). For in vivo data, VFA calculated a significantly higher T₁ than Look-Locker in the liver and blood (p=0.030 and p=0.003, respectively). However, in the lung VFA calculated a significantly lower T₁ than Look-Locker (p=0.007) (Table 1).
Decreasing slice thickness (4mm to 15mm) during post-processing, decreased VFA T₁ significantly in the blood and lung. Downsizing Look-Locker images in the phase direction from 128 to 80mm significantly increased lung T₁. No other significant differences in T₁ due to resizing were found (Figure 4).

Discussion

In phantoms, there were significant differences between VFA and inversion recovery methods, which are consistent with literature[11, 16].

In vivo data showed significant differences between T₁ measured using VFA and Look-Locker, which varied depending on the tissue. Blood and liver T₁ was lower when measured using Look-Locker sequences than VFA, however T₁ measured using VFA was lower in the lung than T₁ measured using Look-Locker.

Significant differences between lung T₁ in healthy volunteers and patients were not found in VFA T₁, which is likely due to the large standard deviation of T₁ in these results.

Decreasing VFA slice thickness during post-processing decreased lung T₁, suggesting it can not be responsible for the differences in T₁ between the sequences.

Conclusion

In vivo, there are significant differences in T₁ measured with inversion recovery and VFA sequences, which differ in different tissues. Lung T₁ calculated using a VFA sequence is significantly lower than T₁ calculated using a Look-locker sequence.

Acknowledgements

This work was supported by MRC grant MR/M008894/1 and Wellcome Trust grant: 205188/Z/16/Z. The views expressed in this work are those of the author(s) and not necessarily those of the NHS, or the Department of Health.

References

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12. Jobst, B.J., et al., Functional Lung MRI in Chronic Obstructive Pulmonary Disease: Comparison of T1 Mapping, Oxygen-Enhanced T1 Mapping and Dynamic Contrast Enhanced Perfusion. Plos One, 2015. 10(3).

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Figures

Figure 1: Bland Altman plots showing agreement of T₁ measured using Look-Locker, VFA and MOLLI sequences in phantoms. Look-Locker and MOLLI sequences show a consistent underestimation of T₁ that increases at longer T₁. VFA had the largest 95% confidence interval and MOLLI had the smallest confidence interval. Look-Locker mean difference: 56±33ms, p<0.001; VFA mean difference: -69±58ms, 0.001; MOLLI mean difference: 107±26ms, p<0.001.

Figure 2: Example T₁ maps in a healthy volunteer and patient with IPF using Look-Locker and VFA acquisitions. A similar-slice UTE image is also shown for the patient, to visualise regions of increases lung density.

Figure 3: Scatter plot showing the correlation in lung T₁ measured using the VFA acquisition and Look-Locker acquisition, for both phantom and in vivo data. Regions of interest were drawn in the lung, liver and descending aorta (blood) for each participant. 4 participants were healthy volunteers, 7 were patients with IPF.

Table 1: Comparison of T₁ calculated with Look-Locker, VFA and MOLLI sequences with one another for phantom and in vivo data.

Figure 4: The mean difference in VFA T₁ when slice thickness was decreased (4mm vs 15mm) was: phantoms: -10±10ms, p=0.055; lung: 77±70ms, p=0.007; liver: -10±46ms, p=0.606; blood: 77±70ms, p=0.016, with a larger increase seen in the lung and aorta. The mean difference in Look-Locker T₁ when image size in the phase direction was decreased (128 to 800) was: phantoms: -0.3±5.5ms, p=0.855; lung: -33±26ms, p=0.004; liver: 8±14ms, p=0.104; blood: -8±23ms, p=0.314.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)

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