Progression of emphysema evaluated by MRI using hyperpolarized (3)He (HP (3)He) measurements in patients with alpha-1-antitrypsin (A1AT) deficiency compared with CT and lung function tests

Undersampled Diffusion-Weighted 129Xe MRI Morphometry of Airspace Enlargement: Feasibility in Chronic Obstructive Pulmonary Disease

Multi-b diffusion-weighted hyperpolarized gas MRI measures pulmonary airspace enlargement using apparent diffusion coefficients (ADC) and mean linear intercepts (L_m). Rapid single-breath acquisitions may facilitate clinical translation, and, hence, we aimed to develop single-breath three-dimensional multi-b diffusion-weighted ¹²⁹Xe MRI using k-space undersampling. We evaluated multi-b (0, 12, 20, 30 s/cm²) diffusion-weighted ¹²⁹Xe ADC/morphometry estimates using a fully sampled and retrospectively undersampled k-space with two acceleration-factors (AF = 2 and 3) in never-smokers and ex-smokers with chronic obstructive pulmonary disease (COPD) or alpha-one anti-trypsin deficiency (AATD). For the three sampling cases, mean ADC/L_m values were not significantly different (all p > 0.5); ADC/L_m values were significantly different for the COPD subgroup (0.08 cm²s^-1/580 µm, AF = 3; all p < 0.001) as compared to never-smokers (0.05 cm²s^-1/300 µm, AF = 3). For never-smokers, mean differences of 7%/7% and 10%/7% were observed between fully sampled and retrospectively undersampled (AF = 2/AF = 3) ADC and L_m values, respectively. For the COPD subgroup, mean differences of 3%/4% and 11%/10% were observed between fully sampled and retrospectively undersampled (AF = 2/AF = 3) ADC and L_m, respectively. There was no relationship between acceleration factor with ADC or L_m (p = 0.9); voxel-wise ADC/L_m measured using AF = 2 and AF = 3 were significantly and strongly related to fully-sampled values (all p < 0.0001). Multi-b diffusion-weighted ¹²⁹Xe MRI is feasible using two different acceleration methods to measure pulmonary airspace enlargement using L_m and ADC in COPD participants and never-smokers.

The history of magnetic resonance imaging and its reflections in Acta Radiologica

The first reports in Acta Radiologica on magnetic resonance imaging (MRI) were published in 1984, four years after the first commercial MR scanners became available. For the first two years, all MR papers originated from the USA. Nordic contributions started in 1986, and until 2020, authors from 44 different countries have published MR papers in Acta Radiologica. Papers on MRI have constituted, on average, 30%-40% of all published original articles in Acta Radiologica, with a high of 49% in 2019. The MR papers published since 1984 document tremendous progress in several areas such as magnet and coil design, motion compensation techniques, faster image acquisitions, new image contrast, contrast-enhanced MRI, functional MRI, and image analysis. In this historical review, all of these aspects of MRI are discussed and related to Acta Radiologica papers.

Hyperpolarized 3 helium MRI measured apparent diffusion coefficient and its correlations with pulmonary functions tests in patients with chronic obstructive pulmonary disease: A meta-analysis

BACKGROUND

This study evaluates role of hyperpolarized ³ helium (³ He) MRI measured apparent diffusion coefficient (ADC) in examining pulmonary function of chronic obstructive pulmonary disease (COPD) patients.

METHODS

After literature search in electronic databases, studies were selected by following precise eligibility criteria. Meta-analyses were performed to estimate mean difference in ADC between COPD patients and healthy individuals and to seek correlations between lung ADC and pulmonary function. Metaregression analyses were performed to seek relationships between ADC and age, gender, BMI, cigarette pack-years, and pulmonary function tests.

RESULTS

Twenty-five studies (622 COPD patients and 469 healthy controls) were included. Lung ADC was 0.402 (95% confidence interval [CI]: 0.374, 0.429) in COPD patients and 0.228 (95% CI: 0.205, 0.252) in healthy individuals (mean difference 0.160 [95% CI: 0.127, 0.193]; p < 0.001). In metaregression, age (coefficient: 0.006; p = 0.004), pack-years (coefficient: 0.005; p = 0.018), forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) ratio (coefficient: -1.815; p = 0.007), percent predicted diffusion capacity of carbon monoxide (DLCO) (coefficient: -0.004; p = 0.008), and percent predicted inspiratory capacity (coefficient: -0.004; p = 0.012) were significantly associated with ADC in COPD patients. In meta-analysis of correlation coefficients, ADC was significantly correlated with FEV1 (r = -0.62; p < 0.00001), FEV1/FVC (r = -0.80; p < 0.00001), DLCO (r = -0.85; p < 0.00001), functional residual capacity (r = 0.71; p < 0.00001), reserve volume (r = 0.53; p = 0.0001), and emphysema index (r = 0.89; p < 0.00001).

CONCLUSION

Hyperpolarized ³ He MRI measured ADC was higher in COPD patients than in healthy individuals and was inversely associated with FEV1, FEV1/FVC, DLCO, and inspiratory capacity.

Imaging in alpha-1 antitrypsin deficiency: a window into the disease

Imaging modalities such as plain chest radiograph and computed tomography (CT) are important tools in the assessment of patients with chronic obstructive pulmonary disease (COPD) of any etiology. These methods facilitate differential diagnoses and the assessment of individual lung pathologies, such as the presence of emphysema, bullae, or fibrosis. However, as emphysema is the core pathological consequence in the lungs of patients with alpha-1 antitrypsin deficiency (AATD), and because AATD is associated with the development of other lung pathologies such as bronchiectasis, there is a greater need for patients with AATD than those with non-AATD-related COPD to undergo more detailed assessment using CT. In the field of AATD, CT provides essential information regarding the presence, distribution, and morphology of emphysema. In addition, it offers the option to quantify the extent of emphysema. These data have implications for treatment decisions such as initiation of alpha-1 antitrypsin (AAT) therapy, or suitability for surgical or endoscopic interventions for reducing lung volume. Furthermore, CT has provided vital insight regarding the natural history of emphysema progression in AATD, and CT densitometry has underpinned research into the efficacy of AAT therapy. Moving forward, hyperpolarized xenon gas (129Xe) lung magnetic resonance imaging (MRI) is emerging as a promising complement to CT by adding comprehensive measures of regional lung function. It also avoids the main disadvantage of CT: the associated radiation. This chapter provides an overview of technological aspects of imaging in AATD, as well as its role in the management of patients and clinical research. In addition, perspectives on the future potential role of lung MRI in AATD are outlined.

Accelerated 129 Xe MRI morphometry of terminal airspace enlargement: Feasibility in volunteers and those with alpha-1 antitrypsin deficiency

PURPOSE

Multi-b diffusion-weighted hyperpolarized inhaled-gas MRI provides imaging biomarkers of terminal airspace enlargement including ADC and mean linear intercept (L_m ), but clinical translation has been limited because image acquisition requires relatively long or multiple breath-holds that are not well-tolerated by patients. Therefore, we aimed to accelerate single breath-hold 3D multi-b diffusion-weighted ¹²⁹ Xe MRI, using k-space undersampling in imaging direction using a different undersampling pattern for different b-values combined with the stretched exponential model to generate maps of ventilation, apparent transverse relaxation time constant ( T 2 ∗ ), ADC, and L_m values in a single, short breath-hold; accelerated and non-accelerated measurements were directly compared.

METHODS

We evaluated multi-b (0, 12, 20, 30, and 45.5 s/cm² ) diffusion-weighted ¹²⁹ Xe T 2 ∗ /ADC/morphometry estimates using acceleration factor (AF = 1 and 7) and multi-breath sampling in 3 volunteers (HV), and 6 participants with alpha-1 antitrypsin deficiency (AATD).

RESULTS

For the HV subgroup, mean differences of 5%, 2%, and 8% were observed between fully sampled and undersampled k-space for ADC, L_m , and T 2 ∗ values, respectively. For the AATD subgroup, mean differences were 9%, 6%, and 12% between fully sampled and undersampled k-space for ADC, L_m and T 2 ∗ values, respectively. Although mean differences of 1% and 4.5% were observed between accelerated and multi-breath sampled ADC and L_m values, respectively, mean ADC/L_m estimates were not significantly different from corresponding mean ADC^M /L_m ^M or mean ADC^A /L_m ^A estimates (all P > 0.60 , ^A = undersampled and ^M = multi-breath sampled).

CONCLUSIONS

Accelerated multi-b diffusion-weighted ¹²⁹ Xe MRI is feasible at AF = 7 for generating pulmonary ADC and L_m in AATD and normal lung.

Pulmonary 3He Magnetic Resonance Imaging Biomarkers of Regional Airspace Enlargement in Alpha-1 Antitrypsin Deficiency

RATIONALE AND OBJECTIVES

Thoracic x-ray computed tomography (CT) and hyperpolarized ³He magnetic resonance imaging (MRI) provide quantitative measurements of airspace enlargement in patients with emphysema. For patients with panlobular emphysema due to alpha-1 antitrypsin deficiency (AATD), sensitive biomarkers of disease progression and response to therapy have been difficult to develop and exploit, especially those biomarkers that correlate with outcomes like quality of life. Here, our objective was to generate and compare CT and diffusion-weighted inhaled-gas MRI measurements of emphysema including apparent diffusion coefficient (ADC) and MRI-derived mean linear intercept (L_m) in patients with AATD, chronic obstructive pulmonary disease (COPD) ex-smokers, and elderly never-smokers.

MATERIALS AND METHODS

We enrolled patients with AATD (n = 8; 57 ± 7 years), ex-smokers with COPD (n = 8; 77 ± 6 years), and a control group of never-smokers (n = 5; 64 ± 2 years) who underwent thoracic CT, MRI, spirometry, plethysmography, the St. George's Respiratory Questionnaire, and the 6-minute walk test during a single 2-hour visit. MRI-derived ADC, L_m, surface-to-volume ratio, and ventilation defect percent were generated for the apical, basal, and whole lung as was CT lung area ≤-950 Hounsfield units (RA₉₅₀), low attenuating clusters, and airway count.

RESULTS

In patients with AATD, there was a significantly different MRI-derived ADC (P = .03), L_m (P < .0001), and surface-to-volume ratio (P < .0001), but not diffusing capacity of carbon monoxide, residual volume or total lung capacity, or CT RA₉₅₀ (P > .05) compared to COPD ex-smokers with a significantly different St. George's Respiratory Questionnaire.

CONCLUSIONS

In this proof-of-concept demonstration, we evaluated CT and MRI lung emphysema measurements and observed significantly worse MRI biomarkers of emphysema in patients with AATD compared to patients with COPD, although CT RA₉₅₀ and diffusing capacity of carbon monoxide were not significantly different, underscoring the sensitivity of MRI measurements of AATD emphysema.

Pulmonary MRI morphometry modeling of airspace enlargement in chronic obstructive pulmonary disease and alpha-1 antitrypsin deficiency

PURPOSE

We generated lung morphometry measurements using single-breath diffusion-weighted MRI and three different acinar duct models in healthy participants and patients with emphysema stemming from chronic obstructive lung disease (COPD) and alpha-1 antitrypsin deficiency (AATD).

METHODS

Single-breath-inhaled ³ He MRI with five diffusion sensitizations (b-value = 0, 1.6, 3.2, 4.8, and 6.4 s/cm² ) was used, and signal intensities were fit using a cylindrical and single-compartment acinar-duct model to estimate MRI-derived mean linear intercept (L_m ) and surface-to-volume ratio (S/V). A stretched exponential model was also developed to estimate the mean airway length and L_m .

RESULTS

We evaluated 42 participants, including 15 elderly never-smokers (69 ± 5 years), 12 ex-smokers without COPD (67 ± 11 years), 9 COPD ex-smokers (80 ± 6 years), and 6 AATD patients (59 ± 6 years). In the never- and ex-smokers, the diffusing capacity of the lung for carbon monoxide (DL_CO ) and computed tomography relative area of less than -950 Hounsfield units (RA₉₅₀ ) were normal, but these were abnormal in the COPD and AATD patients, which is reflective of emphysema. Although cylindrical and stretched-exponential-model estimates of L_m and S/V were not significantly different, the single-compartment-model estimates were significantly different (P < 0.05) for the never- and ex-smoker subgroups. All models estimated significantly worse L_m and S/V in the AATD and COPD subgroups compared with the never- and ex-smokers without emphysema.

CONCLUSIONS

Differences in airspace enlargement may be estimated using L_m and S/V, generated using MRI and a stretched-exponential or cylindrical model of the acinar ducts. Magn Reson Med 79:439-448, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Biomedical imaging with hyperpolarized noble gases

Hyperpolarized noble gases (HNGs), polarized to approximately 50% or higher, have led to major advances in magnetic resonance (MR) imaging of porous structures and air-filled cavities in human subjects, particularly the lung. By boosting the available signal to a level about 100 000 times higher than that at thermal equilibrium, air spaces that would otherwise appear as signal voids in an MR image can be revealed for structural and functional assessments. This review discusses how HNG MR imaging differs from conventional proton MR imaging, how MR pulse sequence design is affected and how the properties of gas imaging can be exploited to obtain hitherto inaccessible information in humans and animals. Current and possible future imaging techniques, and their application in the assessment of normal lung function as well as certain lung diseases, are described.

Effect of lung inflation level on hyperpolarized 3He apparent diffusion coefficient measurements in never-smokers

PURPOSE

To evaluate the effects of lung volume differences on apparent diffusion coefficient (ADC) measurements on a regional basis, with breath holds at volumes adjusted for differences in lung size across individuals according to the subject's vital capacity (VC).

MATERIALS AND METHODS

This study was approved by the local institutional review board and was compliant with HIPAA. Informed consent was obtained from all subjects. Imaging was performed under a physician's Investigational New Drug application from the Food and Drug Administration. ADC changes as a function of inflation levels were evaluated in 24 healthy never-smokers across three lung volumes (20%, 60%, and 100% VC) on the basis of the spirometric data collected from each subject. Response variables based on lung volume and anatomic position were assessed with multifactorial analysis of variance followed by posthoc pair-wise testing. Imaging was performed with a 1.5-T magnetic resonance (MR) unit with use of a two-dimensional gradient-echo fast low-angle shot sequence.

RESULTS

Significant differences in ADCs between lung volumes were observed for all inflation levels (20%, 60%, and 100% VC; P < .001), along with significant dependent-nondependent vertical gradients at 20% VC (P < .0001) and 60% VC (P < .0001, left lung only). In addition, significant differences between mean values in the left and right lungs with respect to those in the whole lung were observed at the lower lung inflation levels (20% and 60% VC, P < .01), reaching more uniform expansion at 100% VC.

CONCLUSION

The results confirm known anatomic differences in patterns of regional inflation and ventilation with corresponding lung volume changes, emphasizing the need for tight control over lung volume when performing hyperpolarized helium 3 ((3)He) lung studies if (3)He MR imaging is to be used to follow up small longitudinal changes in lung abnormalities.

Hyperpolarized (3)He magnetic resonance imaging: comparison with four-dimensional x-ray computed tomography imaging in lung cancer

RATIONALE AND OBJECTIVES

Pulmonary functional imaging using four-dimensional x-ray computed tomographic (4DCT) imaging and hyperpolarized (3)He magnetic resonance imaging (MRI) provides regional lung function estimates in patients with lung cancer in whom pulmonary function measurements are typically dominated by tumor burden. The aim of this study was to evaluate the quantitative spatial relationship between 4DCT and hyperpolarized (3)He MRI ventilation maps.

MATERIALS AND METHODS

Eleven patients with lung cancer provided written informed consent to 4DCT imaging and MRI performed within 11 ± 14 days. Hyperpolarized (3)He MRI was acquired in breath-hold after inhalation from functional residual capacity of 1 L hyperpolarized (3)He, whereas 4DCT imaging was acquired over a single tidal breath of room air. For hyperpolarized (3)He MRI, the percentage ventilated volume was generated using semiautomated segmentation; for 4DCT imaging, pulmonary function maps were generated using the correspondence between identical tissue elements at inspiratory and expiratory phases to generate percentage ventilated volume.

RESULTS

After accounting for differences in image acquisition lung volumes ((3)He MRI: 1.9 ± 0.5 L ipsilateral, 2.3 ± 0.7 L contralateral; 4DCT imaging: 1.2 ± 0.3 L ipsilateral, 1.3 ± 0.4 L contralateral), there was no significant difference in percentage ventilated volume between hyperpolarized (3)He MRI (72 ± 11% ipsilateral, 79 ± 12% contralateral) and 4DCT imaging (74 ± 3% ipsilateral, 75 ± 4% contralateral). Spatial correspondence between 4DCT and (3)He MRI ventilation was evaluated using the Dice similarity coefficient index (ipsilateral, 86 ± 12%; contralateral, 88 ± 12%).

CONCLUSIONS

Despite rather large differences in image acquisition breathing maneuvers, good spatial and significant quantitative agreement was observed for ventilation maps on hyperpolarized (3)He MRI and 4DCT imaging, suggesting that pulmonary regions with good lung function are similar between modalities in this small group of patients with lung cancer.

Role of computed tomography in quantitative assessment of emphysema

Pulmonary emphysema, together with chronic bronchitis is a part of chronic obstructive pulmonary disease (COPD), which is one of the leading causes of death in the United States and worldwide. There are many methods to diagnose emphysema. Unfortunately many of them, for example pulmonary function tests (PFTs), clinical signs and conventional radiology are able to detect emphysema usually in its late stages when a great portion of lung parenchyma has been already destroyed by the disease. Computed tomography (CT) allows for early detection of emphysema. CT also makes it possible to quantify the total amount of emphysema in the lungs which is important in order to precisely estimate the severity of the disease. Those abilities of CT are important in monitoring the course of the disease and in attempts to prevent its further progression. In this review we discuss currently available methods for imaging emphysema with emphasis on the quantitative assessment of emphysema. To date, quantitative methods have not been widely used clinically, however, the initial results of several research studies regarding this subject are very encouraging.

Diffusion-weighted hyperpolarized 129Xe MRI in healthy volunteers and subjects with chronic obstructive pulmonary disease

Given its greater availability and lower cost, (129) Xe apparent diffusion coefficient (ADC) MRI offers an alternative to (3) He ADC MRI. To demonstrate the feasibility of hyperpolarized (129) Xe ADC MRI, we present results from healthy volunteers (HV), chronic obstructive pulmonary disease (COPD) subjects, and age-matched healthy controls (AMC). The mean parenchymal ADC was 0.036 ± 0.003 cm(2) sec(-1) for HV, 0.043 ± 0.006 cm(2) sec(-1) for AMC, and 0.056 ± 0.008 cm(2) sec(-1) for COPD subjects with emphysema. In healthy individuals, but not the COPD group, ADC decreased significantly in the anterior-posterior direction by ∼ 22% (P = 0.006, AMC; 0.0059, HV), likely because of gravity-induced tissue compression. The COPD group exhibited a significantly larger superior-inferior ADC reduction (∼ 28%) than the healthy groups (∼ 24%) (P = 0.00018, HV; P = 3.45 × 10(-5) , AMC), consistent with smoking-related tissue destruction in the superior lung. Superior-inferior gradients in healthy subjects may result from regional differences in xenon concentration. ADC was significantly correlated with pulmonary function tests (forced expiratory volume in 1 sec, r = -0.77, P = 0.0002; forced expiratory volume in 1 sec/forced vital capacity, r = -0.77, P = 0.0002; diffusing capacity of carbon monoxide in the lung/alveolar volume (V(A) ), r = -0.77, P = 0.0002). In healthy groups, ADC increased with age by 0.0002 cm(2) sec(-1) year(-1) (r = 0.56, P = 0.02). This study shows that (129) Xe ADC MRI is clinically feasible, sufficiently sensitive to distinguish HV from subjects with emphysema, and detects age- and posture-dependent changes.