Morpho-Functional 1H-MRI of the Lung in COPD: Short-Term Test-Retest Reliability

Advances in COPD imaging using CT and MRI: linkage with lung physiology and clinical outcomes

Recent years have witnessed major advances in lung imaging in patients with COPD. These include significant refinements in images obtained by computed tomography (CT) scans together with the introduction of new techniques and software that aim for obtaining the best image whilst using the lowest possible radiation dose. Magnetic resonance imaging (MRI) has also emerged as a useful radiation-free tool in assessing structural and more importantly functional derangements in patients with well-established COPD and smokers without COPD, even before the existence of overt changes in resting physiological lung function tests. Together, CT and MRI now allow objective quantification and assessment of structural changes within the airways, lung parenchyma and pulmonary vessels. Furthermore, CT and MRI can now provide objective assessments of regional lung ventilation and perfusion, and multinuclear MRI provides further insight into gas exchange; this can help in structured decisions regarding treatment plans. These advances in chest imaging techniques have brought new insights into our understanding of disease pathophysiology and characterising different disease phenotypes. The present review discusses, in detail, the advances in lung imaging in patients with COPD and how structural and functional imaging are linked with common resting physiological tests and important clinical outcomes.

Unsupervised clustering algorithms improve the reproducibility of dynamic contrast-enhanced magnetic resonance imaging pulmonary perfusion quantification in muco-obstructive lung diseases

Background

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) allows the assessment of pulmonary perfusion, which may play a key role in the development of muco-obstructive lung disease. One problem with quantifying pulmonary perfusion is the high variability of metrics. Quantifying the extent of abnormalities using unsupervised clustering algorithms in residue function maps leads to intrinsic normalization and could reduce variability.

Purpose

We investigated the reproducibility of perfusion defects in percent (QDP) in clinically stable patients with cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD).

Methods

15 CF (29.3 ± 9.3y, FEV1%predicted = 66.6 ± 15.8%) and 20 COPD (66.5 ± 8.9y, FEV1%predicted = 42.0 ± 13.3%) patients underwent DCE-MRI twice 1 month apart. QDP, pulmonary blood flow (PBF), and pulmonary blood volume (PBV) were computed from residue function maps using an in-house quantification pipeline. A previously validated MRI perfusion score was visually assessed by an expert reader.

Results

Overall, mean QDP, PBF, and PBV did not change within 1 month, except for QDP in COPD (p < 0.05). We observed smaller limits of agreement (± 1.96 SD) related to the median for QDP (CF: ± 38%, COPD: ± 37%) compared to PBF (CF: ± 89%, COPD: ± 55%) and PBV (CF: ± 55%, COPD: ± 51%). QDP correlated moderately with the MRI perfusion score in CF (r = 0.46, p < 0.05) and COPD (r = 0.66, p < 0.001). PBF and PBV correlated poorly with the MRI perfusion score in CF (r =−0.29, p = 0.132 and r =−0.35, p = 0.067, respectively) and moderately in COPD (r =−0.57 and r =−0.57, p < 0.001, respectively).

Conclusion

In patients with muco-obstructive lung diseases, QDP was more robust and showed a higher correlation with the MRI perfusion score compared to the traditionally used perfusion metrics PBF and PBV.

Echo Time-Dependent Observed Lung T1 in Patients With Chronic Obstructive Pulmonary Disease in Correlation With Quantitative Imaging and Clinical Indices

BACKGROUND

There is a clinical need for imaging-derived biomarkers for the management of chronic obstructive pulmonary disease (COPD). Observed pulmonary T₁ (T₁ (TE)) depends on the echo-time (TE) and reflects regional pulmonary function.

PURPOSE

To investigate the potential diagnostic value of T₁ (TE) for the assessment of lung disease in COPD patients by determining correlations with clinical parameters and quantitative CT.

STUDY TYPE

Prospective non-randomized diagnostic study.

POPULATION

Thirty COPD patients (67.7 ± 6.6 years). Data from a previous study (15 healthy volunteers [26.2 ± 3.9 years) were used as reference.

FIELD STRENGTH/SEQUENCE

Study participants were examined at 1.5 T using dynamic contrast-enhanced three-dimensional gradient echo keyhole perfusion sequence and a multi-echo inversion recovery two-dimensional UTE (ultra-short TE) sequence for T₁ (TE) mapping at TE_1-5  = 70 μsec, 500 μsec, 1200 μsec, 1650 μsec, and 2300 μsec.

ASSESSMENT

Perfusion images were scored by three radiologists. T₁ (TE) was automatically quantified. Computed tomography (CT) images were quantified in software (qCT). Clinical parameters including pulmonary function testing were also acquired.

STATISTICAL TESTS

Spearman rank correlation coefficients (ρ) were calculated between T₁ (TE) and perfusion scores, clinical parameters and qCT. A P-value <0.05 was considered statistically significant.

RESULTS

Median values were T₁ (TE_1-5 ) = 644 ± 78 msec, 835 ± 92 msec, 835 ± 87 msec, 831 ± 131 msec, 893 ± 220 msec, all significantly shorter than previously reported in healthy subjects. A significant increase of T₁ was observed from TE₁ to TE₂ , with no changes from TE₂ to TE₃ (P = 0.48), TE₃ to TE₄ (P = 0.94) or TE₄ to TE₅ (P = 0.02) which demonstrates an increase at shorter TEs than in healthy subjects. Moderate to strong Spearman's correlations between T₁ and parameters including the predicted diffusing capacity for carbon monoxide (DLCO, ρ < 0.70), mean lung density (MLD, ρ < 0.72) and the perfusion score (ρ > -0.69) were found. Overall, correlations were strongest at TE₂ , weaker at TE₁ and rarely significant at TE₄ -TE₅ .

DATA CONCLUSION

In COPD patients, the increase of T₁ (TE) with TE occurred at shorter TEs than previously found in healthy subjects. Together with the lack of correlation between T₁ and clinical parameters of disease at longer TEs, this suggests that T₁ (TE) quantification in COPD patients requires shorter TEs. The TE-dependence of correlations implies that T₁ (TE) mapping might be developed further to provide diagnostic information beyond T₁ at a single TE.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 1.

[Magnetic resonance imaging of the lungs in cystic fibrosis]

CLINICAL ISSUE

Disease severity and mortality in patients with cystic fibrosis (CF) is mainly determined by (progressive) pulmonary lung disease. Early diagnosis and therapy are important and of prognostic value to conserve lung function.

STANDARD RADIOLOGICAL METHODS

Primary imaging techniques for lung imaging are x‑ray and computed tomography (CT) to monitor disease severity and regional distribution.

METHODICAL INNOVATIONS

Radiation-free imaging techniques such as magnetic resonance imaging (MRI) have gained interest over the last decade in order to prevent radiation damage.

PERFORMANCE

The main findings of CF lung disease are airway wall thickening, bronchiectasis, and mucus plugging, which are found in up to 60% of preschool age children. Pleural abnormalities and consolidations are often associated with pulmonary exacerbation. Young CF patients often show a mosaic pattern as functional changes and also perfusion defects can be seen from birth in 50% of CF patients by contrast-enhanced perfusion imaging, and in up to 90% of adult patients, with varying degrees of severity. Dilated bronchial arteries indicate an increased risk for hemoptysis.

ACHIEVEMENTS

Proton MRI is the sole imaging technique that can show structural and functional lung changes in one examination. Structured assessment using a scoring system helps to systematically grade the extent and severity of all CF-associated changes.

CONCLUSIONS

Lung MRI for cystic fibrosis has been recently established as a clinical standard examination and is routinely performed at experienced centers. More recently, it has also been used as an endpoint within the framework of clinical studies.

Echo Time-Dependence of Observed Lung T1 in Patients With Cystic Fibrosis and Correlation With Clinical Metrics

BACKGROUND

Noninvasive monitoring of early abnormalities and therapeutic intervention in cystic fibrosis (CF) lung disease using MRI is important. Lung T₁ mapping has shown potential for local functional imaging without contrast material. Recently, it was discovered that observed lung T₁ depends on the measurement echo time (TE).

PURPOSE

To examine TE-dependence of observed T₁ in patients with CF and its correlation with clinical metrics.

STUDY TYPE

Prospective.

POPULATION

In all, 75 pediatric patients with CF (8.6 ± 6.1 years, range 0.1-23 years), with 32 reexamined after 1 year.

FIELD STRENGTH/SEQUENCE

Patients were examined at 1.5T using an established MRI protocol and a multiecho inversion recovery 2D ultrashort echo time (UTE) sequence for T₁ (TE) mapping at five TEs including TE₁ = 70 μs.

ASSESSMENT

Morphological and perfusion MRI were assessed by a radiologist (M.W.) with 11 years of experience using an established CF-MRI scoring system. T₁ (TE) was quantified automatically. Clinical data including spirometry (FEV1pred%) and lung clearance index (LCI) were collected.

STATISTICAL TESTS

T₁ (TE) was correlated with the CF-MRI score, clinical data, and LCI.

RESULTS

T₁ (TE) showed a different curvature in CF than in healthy adults: T₁ at TE₁ was shorter in CF (1157 ms ± 73 ms vs. 1047 ms ± 70 ms, P < 0.001), but longer at TE₃ (1214 ms ± 72 ms vs. 1314 ms ± 68 ms, P < 0.001) and later TEs. The correlations of T₁ (TE) with patient age (ρ_TE1-TE5 = -0.55, -0.44, -0.24, -0.30, -0.22), and LCI (ρ_TE1-TE5 = -0.43, -0.42, -0.33, 0.27, -0.22) were moderate at ultra-short to short TE (P < 0.001) but decreased for longer TE. Moderate but similar correlations at all TE were found with MRI perfusion score (ρ_TE1-TE5 = -0.43, -0.51, -0.47, -0.46, -0.44) and FEV1pred% (ρ_TE1-TE5 = +0.44, +0.44, +0.43, +0.40, +0.39) (P < 0.05).

DATA CONCLUSION

TE should be considered when measuring lung T₁ , since observed differences between CF and healthy subjects strongly depend on TE. The different variation of correlation coefficients with TE for structural vs. functional metrics implies that TE-dependence holds additional information which may help to discern effects of tissue structural abnormalities and abnormal perfusion.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1 J. MAGN. RESON. IMAGING 2020;52:1645-1654.

Towards quantitative perfusion MRI of the lung in COPD: The problem of short-term repeatability

Purpose

4D perfusion magnetic resonance imaging (MRI) with intravenous injection of contrast agent allows for a radiation-free assessment of regional lung function. It is therefore a valuable method to monitor response to treatment in patients with chronic obstructive pulmonary disease (COPD). This study was designed to evaluate its potential for monitoring short-term response to hyperoxia in COPD patients.

Materials and methods

19 prospectively enrolled COPD patients (median age 66y) underwent paired dynamic contrast-enhanced 4D perfusion MRI within 35min, first breathing 100% oxygen (injection 1, O₂) and then room air (injection 2, RA), which was repeated on two consecutive days (day 1 and 2). Post-processing software was employed to calculate mean transit time (MTT), pulmonary blood volume (PBV) and pulmonary blood flow (PBF), based on the indicator dilution theory, for the automatically segmented whole lung and 12 regions of equal volume.

Results

Comparing O₂ with RA conditions, PBF and PBV were found to be significantly lower at O₂, consistently on both days (p<10–8). Comparing day 2 to day 1, MTT was shorter by 0.59±0.63 s (p<10–8), PBF was higher by 22±80 ml/min/100ml (p<3·10–4), and PBV tended to be lower by 0.2±7.2 ml/100ml (p = 0.159) at both, RA and O_2, conditions.

Conclusion

The second injection (RA) yielded higher PBF and PBV, which apparently contradicts the established hypothesis that hyperoxia increases lung perfusion. Quantification of 4D perfusion MRI by current software approaches may thus be limited by residual circulating contrast agent in the short-term and even the next day.

Reproducibility and comparison of oxygen-enhanced T1 quantification in COPD and asthma patients

T₁ maps have been shown to yield useful diagnostic information on lung function in patients with chronic obstructive pulmonary disease (COPD) and asthma, both for native T₁ and ΔT₁, the relative reduction while breathing pure oxygen. As parameter quantification is particularly interesting for longitudinal studies, the purpose of this work was both to examine the reproducibility of lung T₁ mapping and to compare T₁ found in COPD and asthma patients using IRSnapShotFLASH embedded in a full MRI protocol. 12 asthma and 12 COPD patients (site 1) and further 15 COPD patients (site 2) were examined on two consecutive days. In each patient, T₁ maps were acquired in 8 single breath-hold slices, breathing first room air, then pure oxygen. Maps were partitioned into 12 regions each to calculate average values. In asthma patients, the average T_1,RA = 1206ms (room air) was reduced to T_1,O2 = 1141ms under oxygen conditions (ΔT₁ = 5.3%, p < 5⋅10⁻⁴), while in COPD patients both native T_1,RA = 1125ms was significantly shorter (p < 10⁻³) and the relative reduction to T_1,O2 = 1081ms on average ΔT₁ = 4.2%(p < 10⁻⁵). On the second day, with T_1,RA = 1186ms in asthma and T_1,RA = 1097ms in COPD, observed values were slightly shorter on average in all patient groups. ΔT₁ reduction was the least repeatable parameter and varied from day to day by up to 23% in individual asthma and 30% in COPD patients. While for both patient groups T₁ was below the values reported for healthy subjects, the T₁ and ΔT₁ found in asthmatics lies between that of the COPD group and reported values for healthy subjects, suggesting a higher blood volume fraction and better ventilation. However, it could be demonstrated that lung T₁ quantification is subject to notable inter-examination variability, which here can be attributed both to remaining contrast agent from the previous day and the increased dependency of lung T₁ on perfusion and thus current lung state.

Assessing the Relationship between Lung Density and Function with Oxygen-Enhanced Magnetic Resonance Imaging in a Mouse Model of Emphysema

Purpose

A magnetic resonance imaging method is presented that allows for the simultaneous assessment of oxygen delivery, oxygen uptake, and parenchymal density. The technique is applied to a mouse model of porcine pancreatic elastase (PPE) induced lung emphysema in order to investigate how structural changes affect lung function.

Method

Nine-week-old female C57BL6 mice were instilled with saline or PPE at days 0 and 7. At day 19, oxygen delivery, oxygen uptake, and lung density were quantified from T1 and proton-density measurements obtained via oxygen-enhanced magnetic resonance imaging (OE-MRI) using an ultrashort echo-time imaging sequence. Subsequently, the lungs were sectioned for histological observation. Blood-gas analyses and pulmonary functional tests via FlexiVent were performed in separate cohorts.

Principal Findings

PPE-challenged mice had reduced density when assessed via MRI, consistent with the parenchyma loss observed in the histology sections, and an increased lung compliance was detected via FlexiVent. The oxygenation levels, as assessed via the blood-gas analysis, showed no difference between PPE-challenged animals and control. This finding was mirrored in the global MRI assessments of oxygen delivery and uptake, where the changes in relaxation time indices were matched between the groups. The heterogeneity of the same parameters however, were increased in PPE-challenged animals. When the oxygenation status was investigated in regions of varying density, a reduced oxygen-uptake was found in low-density regions of PPE-challenged mice. In high-density regions the uptake was higher than that of regions of corresponding density in control animals. The oxygen delivery was proportional to the oxygen uptake in both groups.

Conclusions

The proposed method allowed for the regional assessment of the relationship between lung density and two aspects of lung function, the oxygen delivery and uptake. When compared to global indices of lung function, an increased sensitivity for detecting heterogeneous lung disorders was found. This indicated that the technique has potential for early detection of lung dysfunction–before global changes occur.