Background

Lung T₁ mapping has been shown to be sensitive to pulmonary perfusion and lung T₁ reflects a combination of tissue composition and density, blood volume and blood oxygenation [1]. Pulmonary hypertension is a heterogeneous condition defined by increased blood pressure in the pulmonary artery, which may have a variety of causes including stiffening or thickening of the small pulmonary artery branches (pulmonary arterial hypertension, PAH) or blood clots in the lungs (chronic thromboembolic pulmonary hypertension, CTEPH). Patient treatment and outcome depends upon the cause (and therefore subgroup) of pulmonary hypertension and therefore differentiating patients is key to patient management and treatment. Lung T₁ mapping may be a contrast-free, radiation-free method of differentiating subgroups in pulmonary hypertension however patients may struggle with a breath hold due to dyspnoea.

Purpose

To explore lung T₁ mapping as a contrast-free, radiation-free method of differentiating subgroups in pulmonary hypertension, measuring during both free breathing and inspiration breath hold acquisitions.

Methods

65 patients with suspected pulmonary hypertension and 9 healthy volunteers underwent lung T₁ mapping on a 1.5T GE scanner using a Look-Locker inversion recovery sequence. T₁ mapping images were acquired during inspiration breath hold and also during an identical acquisition during free breathing with post-processing image registration for motion correction. The Look-Locker sequence comprises a global 180 degree inversion pulse followed by 16 gradient echo readout images [2]. Imaging parameters were as follows: inversion time (TI): 229ms; TR=3.2ms; flip angle: 7; phase x frequency: 128x128; slice thickness: 15mm; pixel bandwidth: 244.14; normal field of view 400-440mm. Breathing instructions for the free breathing acquisition were to breathe normally.

For analysis, patients were grouped into the 5 subcategories of pulmonary hypertension (PAH: n=29; pulmonary hypertension with left heart disease (PH LHD): n=9; pulmonary hypertension due to lung disease (PH-lung): n=7; CTEPH: n=8; other pulmonary hypertension (MISC): n=3) and patients without pulmonary hypertension (no-PH: n=9). Of the patients with PAH, 8 had a diagnosis of idiopathic PAH (IPAH), 10 had a diagnosis of PAH due to connective tissue disease (PAH-CTD) and 8 had a diagnosis of PAH due to congenital heart disease (PAH-CHD), 1 had PAH due to drugs and 1 had PAH associated with portal hypertension.

For free breathing acquisitions, post-processing image registration was used to spatially align acquired images based creating spatially aligned synthetic images. Automatically selected ROIs from the acquired images are input into a combined respiratory and inversion recovery model to determine images which are in the similar respiratory states. Spatially aligned images are then input into a simplified inversion recovery model to create synthetic images. Acquired images are then co-registered to synthetic images, see Figure 1. Image alignment before and after image registration was evaluated using Dice Similarity Coefficient (DSC).

Results

Lung image alignment was significantly improved after image registration for free breathing images (DSC before registration: 0.93±0.02; DSC after registration: 0.95±0.02; p<0.001) and post registration DSC was not significantly different from breath hold DSC (p=0.651).

T₁ maps were created for all patients, see Figure 2. Mean lung T₁ during inspiration was significantly lower in CTEPH (T₁=0.97±0.2s) than both healthy volunteers (T₁=1.10±0.12s, p=0.038) and PAH (T₁=1.14±0.11, p=0.016), see Figure 3. Mean free breathing T₁ was significantly lower in patients with PH-Lung (T1=0.93±0.33s) when compared to healthy volunteers (T₁=1.22±0.3s, p=0.049). Within the group of patients with PAH, during inspiration and free breathing acquisition, mean lung T1 was significantly lower in patients with IPAH than patients (T₁=1.03±0.09s) with PAH-CTD (T₁=1.22±0.06s, inspiration: p<0.001; free breathing: p=0.019) or PAH-CHD (T₁=1.16±0.07s, inspiration: p=0.005; free breathing: p=0.004), see Figure 4. There was no significant difference in mean lung T₁ between patients with PAH-CHD and PAH-CTD.

T₁ values from inspiration and breath hold significantly correlated with one another (r=0.534, p<0.001) and did not differ from one another significantly in this patient cohort (p=0.461).

Discussion and conclusion

Inspiration lung T₁ mapping is significantly different between patients with PAH and CTEPH, as well as between patients with IPAH and PAH due to CTD and CHD. Free breathing lung T₁ mapping was significantly lower in patients with PH-Lung when compared to healthy volunteers, as well as being significantly different between patients with IPAH and PAH due to CTD and CHD. Free breathing T₁ mapping acquisition may be particularly useful in patients who struggle to maintain breath hold due to dyspnoea.

Acknowledgements

No acknowledgement found.

References

[1] Jobst et a., (2015). Functional Lung MRI in Chronic Obstructive Pulmonary Disease: Comparison of T1 Mapping, Oxygen-enhanced T1 Mapping and Dynamic Contrast Enhanced Perfusion.