Functional evaluation of emphysema using diffusion-weighted 3Helium-magnetic resonance imaging, high-resolution computed tomography, and lung function tests

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.

Structural and functional changes in COPD: What we have learned from imaging

Chronic obstructive pulmonary disease (COPD) is the third leading cause of mortality worldwide. It is a heterogeneous disease involving different components of the lung to varying extents. Developments in medical imaging and image analysis techniques provide new insights in the assessment of the structural and functional changes of the disease. This article reviews the leading imaging techniques: CT and MRI of the lung in research settings and clinical routine. Both visual and quantitative methods are reviewed, emphasizing their relevance to patient phenotyping and outcome prediction.

Hyperpolarized Helium-3 Diffusion-weighted Magnetic Resonance Imaging Detects Abnormalities of Lung Structure in Children With Bronchopulmonary Dysplasia

PURPOSE

The aim of the study was to determine whether hyperpolarized He diffusion-weighted magnetic resonance imaging detects abnormalities in the lungs in children with bronchopulmonary dysplasia (BPD) as compared with age-matched normal children.

MATERIALS AND METHODS

All experiments were compliant with Health Insurance Portability and Accountability Act (HIPAA) and performed with Food and Drug Administration approval under an IND application. The protocol was approved by our Institutional Review Board, and written informed consent was obtained. Hyperpolarized He diffusion-weighted magnetic resonance imaging was performed in 16 subjects with a history of preterm birth complicated by BPD (age range, 6.8 to 13.5 y; mean, 9.0 y) and in 29 healthy term-birth subjects (age range, 4.5-14.7 y; mean, 9.2 y) using a gradient-echo sequence with bipolar diffusion gradients and with measurements at 2 b values (0 and 1.6 s/cm). Age-related comparison of the whole-lung mean apparent diffusion coefficient (ADC), 90th percentile ADC, and percentage of whole-lung volume with ADC>0.2 cm/s between the 2 groups was examined using ordinary least-squares multiple regression.

RESULTS

The mean ADC was significantly greater in subjects with BPD (0.187 vs. 0.152 cm/s, P<0.001). The 90th percentile ADC and mean percentage lung volume with ADC>0.2 cm/s were also higher in the BPD group (0.258 vs. 0.215 cm/s, 30.3% vs. 11.9%, P<0.001 for both). The body surface area-adjusted ventilated lung volume was similar in the 2 groups (1.93 vs. 1.91 L, P=0.90).

CONCLUSIONS

Children with BPD had higher ADCs and the same lung volumes when compared with age-matched healthy subjects, suggesting that children with BPD have enlarged alveoli that are reduced in number.

Phenotyping emphysema and airways disease: Clinical value of quantitative radiological techniques

The pathophysiology of chronic obstructive pulmonary disease (COPD) and Alpha one antitrypsin deficiency is increasingly recognised as complex such that lung function alone is insufficient for early detection, clinical categorisation and dictating management. Quantitative imaging techniques can detect disease earlier and more accurately, and provide an objective tool to help phenotype patients into predominant airways disease or emphysema. Computed tomography provides detailed information relating to structural and anatomical changes seen in COPD, and magnetic resonance imaging/nuclear imaging gives functional and regional information with regards to ventilation and perfusion. It is likely imaging will become part of routine clinical practice, and an understanding of the implications of the data is essential. This review discusses technical and clinical aspects of quantitative imaging in obstructive airways disease.

Current methods to diagnose small airway disease in patients with COPD

The small airways are characterized by an internal diameter < 2 mm and absence of cartilage. Approximately 10-25% of total airway resistance in healthy lungs is due to the small airways, with their contribution to total airway resistance increasing substantially in chronic obstructive pulmonary disease (COPD). As the small airways are located in the lung periphery, they are not easily evaluable, which can potentially interfere with the diagnosis (especially at early stages), monitoring, detection of responses to clinical interventions, and prognostic evaluation in COPD. Here, we will discuss the currently available methods in clinical practice to evaluate small airway disease in COPD, focusing on the concept, advantages, and disadvantages of each method.

New insights into lung diseases using hyperpolarized gas MRI

Hyperpolarized (HP) gases are a new class of contrast agents that permit to obtain high temporal and spatial resolution magnetic resonance images (MRI) of the lung airspaces. HP gas MRI has become important research tool not only for morphological and functional evaluation of normal pulmonary physiology but also for regional quantification of pathologic changes occurring in several lung diseases. The purpose of this work is to provide an introduction to MRI using HP noble gases, describing both the basic principles of the technique and the new information about lung disease provided by clinical studies with this method. The applications of the technique in normal subjects, smoking related lung disease, asthma, and cystic fibrosis are reviewed.

Advanced imaging in COPD: insights into pulmonary pathophysiology

Chronic obstructive pulmonary disease (COPD) involves a complex interaction of structural and functional abnormalities. The two have long been studied in isolation. However, advanced imaging techniques allow us to simultaneously assess pathological processes and their physiological consequences. This review gives a comprehensive account of the various advanced imaging modalities used to study COPD, including computed tomography (CT), magnetic resonance imaging (MRI), and the nuclear medicine techniques positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Some more recent developments in imaging technology, including micro-CT, synchrotron imaging, optical coherence tomography (OCT) and electrical impedance tomography (EIT), are also described. The authors identify the pathophysiological insights gained from these techniques, and speculate on the future role of advanced imaging in both clinical and research settings.

Magnetic resonance imaging of cystic fibrosis lung disease

Lung involvement in cystic fibrosis (CF) disease continues to be a major life-limiting factor of this autosomal recessive genetic disorder. Efforts made toward early diagnosis and advances in therapy have led to sustained survival of affected patients, and many are now of adult age. Because imaging provides detailed information on regional distribution of CF lung disease, repetitive imaging is required for severity assessment and therapy monitoring not only in clinical routine but also for interventional trials. Computed tomography has long succeeded chest radiograph because it provides the highest morphologic detail of airway and parenchymal changes. This is inseparably accompanied by an increase in radiation exposure to CF individuals, who are critically susceptible to, and may accumulate, relevant doses during their lifetime. Magnetic resonance imaging (MRI) as an ionizing radiation-free cross-sectional imaging modality is capable of depicting anatomic hallmarks of CF lung disease at lower spatial resolution but with enhanced tissue characterization. Comprehensive functional lung imaging (imaging of respiratory mechanics, ventilation, and lung perfusion) provides valuable additional information that cannot or can hardly be obtained by any other single diagnostic procedure. The present review article strives to present the current state of lung MRI in CF, as well as its future perspectives. Functional MRI of the CF lung is at the threshold of being considered a routine application, which, supporting early diagnosis, may help to further improve the survival of CF patients.

Quantitative pulmonary imaging using computed tomography and magnetic resonance imaging

Measurements of lung function, including spirometry and body plethesmography, are easy to perform and are the current clinical standard for assessing disease severity. However, these lung functional techniques do not adequately explain the observed variability in clinical manifestations of disease and offer little insight into the relationship of lung structure and function. Lung imaging and the image-based assessment of lung disease has matured to the extent that it is common for clinical, epidemiologic and genetic investigation to have a component dedicated to image analysis. There are several exciting imaging modalities currently being used for the non-invasive study of lung anatomy and function. In this review, we will focus on two of them; X-ray computed tomography and magnetic resonance imaging. Following a brief introduction of each method, we detail some of the most recent work being done to characterize smoking-related lung disease and the clinical applications of such knowledge.

Ventilation-based segmentation of the lungs using hyperpolarized (3)He MRI

PURPOSE

To develop an automated segmentation method to differentiate the ventilated lung volume on (3) He magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Computational processing (CP) for each subject consisted of the following three essential steps: 1) inhomogeneity bias correction, 2) whole lung segmentation, and 3) subdivision of the lung segmentation into regions of similar ventilation. Evaluation consisted of two comparative analyses: i) comparison of the number of defects scored by two human readers in 43 subjects, and ii) simultaneous truth and performance level estimation (STAPLE) in 18 subjects in which the ventilation defects were manually segmented by four human readers.

RESULTS

There was excellent correlation between the number of ventilation defects tabulated by CP and reader #1 (intraclass correlation coefficient [ICC] = 0.86), CP and reader #2 (ICC = 0.85), and between the two readers (ICC = 0.97). The STAPLE results from the second analysis yielded the following sensitivity/specificity numbers: CP (0.898/0.905), radiologist #1 (0.743/0.897), radiologist #2 (0.501/0.985), radiologist #3 (0.898/0.848), and the first author (0.600/0.984).

CONCLUSION

We developed and evaluated an automated method for quantifying the ventilated lung volume on (3) He MRI. The findings strongly indicate that our proposed algorithmic processing may be a reliable, automatic method for quantitating ventilation defects.

Can Hyperpolarized Helium MRI add to radiation planning and follow-up in lung cancer?

Locally advanced non‐small‐cell lung cancer (NSCLC) is a common disease with a low overall survival even with aggressive treatments. Standard imaging (CT and PET/CT) provide no information about normal lung function. We therefore, sought to pilot HeMRI in patients with non‐small‐cell lung cancer before and after definitive radiotherapy (RT). Five patients with NSCLC receiving RT were enrolled on a prospective IRB approved study. Patients underwent CT, FDG‐PET and HeMRI before and (within 10 days) following RT. All images (CT, FDG‐PET and HeMRI) were co‐registered. The CT and PET GTVs were contoured, as well as the ventilation defects on HeMRI caused by the tumor. Patients also underwent pulmonary function tests (PFTs). Correlations between the images and PFTs were evaluated by linear regression. CT and FDG‐PET tumor volumes were highly correlated (r2=0.91 before treatment and 0.99 following RT). There was less correlation between HeMRI and CT or PET (r2=0.67 (CT) and 0.38 (PET)) prior to treatment and 0.27 following RT). However, HeMRI volumes correlated very well with FEV1, both prior to and following RT. (r2=0.89 and 0.83, respectively). 3Helium MRI scanning is feasible in NSCLC before and after treatment. HeMRI provides important functional information in addition to CT and CT/PET scanning.

PACS number: 87.55.D‐

Hyperpolarized 3He magnetic resonance imaging-derived pulmonary pressure-volume curves

We aimed to evaluate the potential for the use of hyperpolarized helium-3 magnetic resonance imaging (MRI) apparent diffusion coefficient (ADC) surrogates of alveolar size, together with literature-based morphological parameters in a theoretical model of lung mechanics to simulate noninvasive transpulmonary pressure-volume curves. Fourteen ex-smokers with chronic obstructive pulmonary disease (COPD) ( n = 8 stage II, n = 6 stage III/IV COPD) and five age-matched never-smokers, provided written, informed consent and were evaluated at baseline and 26 ± 2 mo later ( n = 15 subjects) using plethysmography, spirometry, and 3He MRI at 3.0 T. Total lung capacity, residual volume, and literature-based morphological parameters were used with alveolar volumes derived from 3He ADC to simulate noninvasive pressure-volume curves. The resultant anterior-posterior transpulmonary pressure gradient was significantly decreased for stage II COPD ( P < 0.01) and stage III COPD subjects ( P < 0.001) compared with healthy volunteers. Both COPD subgroups showed increased alveolar radius compared with healthy subjects ( P < 0.01, stage II COPD; P < 0.001, stage III COPD). In addition, surface area and surface tension were significantly increased in stage III COPD compared with healthy volunteers ( P < 0.01). These results suggest that 3He MRI provides a potential noninvasive approach to evaluate lung mechanics regionally and further supports the use of ADC values as a regional noninvasive probe of pulmonary microstructure and compliance.

Chronic obstructive pulmonary disease: longitudinal hyperpolarized (3)He MR imaging

PURPOSE

To quantitatively evaluate a small pilot group of ex-smokers with chronic obstructive pulmonary disease (COPD) and healthy volunteers during approximately 2 years by using hyperpolarized helium 3 ((3)He) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

All subjects provided written informed consent to the study protocol, which was approved by the local research ethics board and Health Canada and was compliant with the Personal Information Protection and Electronic Documents Act and HIPAA. Hyperpolarized (3)He MR imaging, hydrogen 1 MR imaging, spirometry, and plethysmography were performed in 15 ex-smokers with COPD and five healthy volunteers (with the same mean age and age range) at baseline and 26 months +/- 2 (standard deviation) later. Apparent diffusion coefficients (ADCs) derived from (3)He MR imaging were calculated from diffusion-weighted (3)He MR images, and (3)He ventilation defect volume (VDV) and ventilation defect percentage (VDP) were generated after manual segmentation of (3)He MR spin-density images.

RESULTS

For subjects with COPD, significant increases in (3)He MR imaging-derived VDV (P = .03), VDP (P = .006), and ADC (P = .02) were detected, whereas there was no significant change in forced expiratory volume in 1 second (FEV(1)) (P = .97). For healthy never-smokers, there was no significant change in imaging or pulmonary function measurements at follow-up. There was a significant correlation between changes in FEV(1) and changes in VDV (r = -0.70, P = .02) and VDP (r = -0.70, P = .03).

CONCLUSION

For this small pilot group of ex-smokers with COPD, (3)He MR imaging-derived VDV, VDP, and ADC measurements worsened significantly, but there was no significant change in FEV(1), suggesting increased sensitivity of hyperpolarized (3)He MR imaging for depicting COPD changes during short time periods.

Effects of diffusion time on short-range hyperpolarized (3)He diffusivity measurements in emphysema

PURPOSE

To characterize the effect of diffusion time on short-range hyperpolarized (3)He magnetic resonance imaging (MRI) diffusion measurements across a wide range of emphysema severity.

MATERIALS AND METHODS

(3)He diffusion MRI was performed on 19 lungs or lobes resected from 18 subjects with varying degrees of emphysema using three diffusion times (1.6 msec, 5 msec, and 10 msec) at constant b value. Emphysema severity was quantified as the mean apparent diffusion coefficient (ADC) and as the percentage of pixels with ADC higher than multiple thresholds from 0.30-0.55 cm(2)/sec (ADC index). Quantitative histology (mean linear intercept) was obtained in 10 of the lung specimens from 10 of the subjects.

RESULTS

The mean ADCs with diffusion times of 1.6, 5.0, and 10.0 msec were 0.46, 0.40, and 0.37 cm(2)/sec, respectively (P < 0.0001, analysis of variance [ANOVA]). There was no relationship between the ADC magnitude and the effect of diffusion time on ADC values. The mean linear intercept correlated with ADC (r = 0.91-0.94, P < 0.001) and ADC index (r = 0.78-0.92, P < 0.01) at all diffusion times.

CONCLUSION

Decreases in ADC with longer diffusion time were unrelated to emphysema severity. The strong correlations between the ADC at all diffusion times tested and quantitative histology demonstrate that ADC is a robust measure of emphysema.

[Functional 3He-MRI of the lungs]

Pulmonary diseases have a high health-related and economic significance. (3)He-MRI is an alternative imaging method which can detect ventilatory disturbances with a high sensitivity. The application of different pulse sequences allows static and dynamic assessment of ventilation and bronchial gas flow, non-invasive measurement of intrapulmonary oxygen partial pressure and quantification of pulmonary parenchyma destruction and overinflation. Generally, the method is applicable for obstructive and restrictive ventilatory disturbances but initial approaches also exist for vascular pulmonary diseases. Specific clinical applications remain to be determined but (3)He-MRI is an excellent instrument for the assessment of physiologic and pathophysiologic interrelations in the distribution of ventilation.

New and current clinical imaging techniques to study chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a heterogeneous disease characterized by both small airway and parenchymal abnormalities. There is increasing evidence to suggest that these two morphologic phenotypes, although related, may have different clinical presentations, prognosis, and therapeutic responses to medications. With the advent of novel imaging modalities, it is now possible to evaluate these two morphologic phenotypes in large clinical studies using noninvasive or minimally invasive methods such as computed tomography (CT), magnetic resonance imaging (MRI), and optical coherence tomography (OCT). In this article, we provide an overview of these imaging modalities in the context of COPD and discuss their strengths as well as their limitations for providing quantitative COPD phenotypes.

Relationship between structural changes and hyperpolarized gas magnetic resonance imaging in chronic obstructive pulmonary disease using computational simulations with realistic alveolar geometry

Both the development of accurate models of lung function and their quantitative validation can be significantly enhanced by the use of functional imaging techniques. The advent of hyperpolarized noble gas magnetic resonance imaging (MRI) technology has increased the amount of local, functional information we can obtain from the lung. In particular, application of (3)He to measure apparent diffusion coefficients has enabled some measure of lung microstructure and airspace size within the lung. Models mimicking image acquisition in hyperpolarized gas MRI can improve understanding of the relationship between image findings and lung structure, and can be used to improve the definition of imaging protocols. In this paper, we review the state of the art in hyperpolarized gas MRI modelling. We also present our own results, obtained using a Monte Carlo approach and a realistic alveolar sac geometry, which has previously been applied in functional lung studies. In this way, we demonstrate the potential for models combining lung function and image acquisition, which could provide valuable tools in both basic studies and clinical practice.

Free breathing hyperpolarized 3He lung ventilation spiral MR imaging

OBJECTIVES

Current clinical hyperpolarized He lung ventilation MR imaging protocols rely on the patient's ability to control inhalation and exhalation and hold their breath on demand. This is impractical for intensive care unit patients under ventilation or for pediatric populations under the age of 3 to 4 years. To address this problem, we propose a free-breathing protocol for hyperpolarized He lung ventilation spiral imaging. This approach was evaluated in vitro and on rabbits.

MATERIALS AND METHODS

The protocol was implemented on a clinical 1.5-T magnetic resonance imaging scanner. Ventilation images were acquired using a spiral sequence, in vitro on a lung phantom and in vivo on rabbits, the animal breathing freely from a gas reservoir. Dynamic spiral ventilation images were reconstructed using retrospective Cine synchronization. Magnetic resonance (MR) signal dynamics was modeled taking account of gas inflow and outflow, radiofrequency depolarization and oxygen-induced relaxation.

RESULTS

Cine ventilation images acquired in spontaneously breathing rabbits were reconstructed with a temporal resolution of 50 milliseconds. Gas volume variations and time-to-maximum maps were obtained. The numerical model was validated in vitro and in vivo with various gas mixtures. Ventilation parameters (functional residual capacity, tidal volume, and alveolar pO2) were extracted from the MR signal dynamics.

CONCLUSIONS

Ventilation imaging can be performed at tidal volume using a simple experimental protocol, without any ventilation device or breath-hold period. Acquisition time, SNR and pO2 decay can be optimized using the developed numerical model. Free-breathing ventilation images can be obtained without artifacts related to motion or gas flow. Lastly, parametric maps can be derived from the time-resolved ventilation images and physiological parameters extracted from the global signal dynamics.

Assessment of the lung microstructure in patients with asthma using hyperpolarized 3He diffusion MRI at two time scales: comparison with healthy subjects and patients with COPD

PURPOSE

To investigate short- and long-time-scale (3)He diffusion in asthma.

MATERIALS AND METHODS

A hybrid MRI sequence was developed to obtain co-registered short- and long-time-scale apparent diffusion coefficient (ADC) maps during a single breath-hold. The study groups were: asthma (n = 14); healthy (n = 14); chronic obstructive pulmonary disease (COPD) (n = 9). Correlations were made between mean-ADC and %ADC-abn (abnormal) (%pixels with ADC > mean +2 SD of healthy) at both time scales and spirometry. Sensitivities were determined using receiver operating characteristic (ROC) analysis.

RESULTS

For asthmatics, the short- and long-time-scale group-mean ADCs were 0.254 +/- 0.032 cm(2)/s and 0.0237 +/- 0.0055 cm(2)/s, respectively, representing a 9% and 27% (P = 0.038 and P = 0.005) increase compared to the healthy group. The group-mean %ADC-abn were 6.4% +/- 3.7% and 17.5% +/- 14.2%, representing a 107% and 272% (P = 0.004 and P = 0.006) increase. For COPD much greater elevations were observed. %ADC-abn provided better discrimination than mean-ADC between asthmatic and healthy subjects. In asthmatics ADC did not correlate with spirometry.

CONCLUSION

With long-time scale (3)He diffusion magnetic resonance imaging (MRI) changes in lung microstructure were detected in asthma that more conspicuous regionally than at the short time scale. The hybrid diffusion method is a novel means of identifying small airway disease.

MR for the evaluation of obstructive pulmonary disease

Obstructive lung diseases include emphysema, chronic bronchitis, chronic obstructive pulmonary disease, asthma, and cystic fibrosis. These diseases are a heterogeneous group of pulmonary disorders that share in common obstruction of air flow and deranged gas exchange. Traditionally these diseases are evaluated with clinical testing, such as pulmonary function tests, but such tests provide only global measures of respiratory function. MR techniques designed for obstructive lung disease have the capability of directly imaging the anatomic and pathophysiologic derangements and may prove useful for monitoring response to therapy.

Extending the range of diffusion times for regional measurement of the 3He ADC in human lungs

A stimulated-echo-based technique was developed to measure the regional apparent diffusion coefficient (ADC) of hyperpolarized 3He during a single breathhold for diffusion times of 25 ms or greater. Compared to previous methods, a substantially shorter minimum diffusion time was achieved by decoupling diffusion sensitization from image acquisition. A hyperpolarized-gas phantom was used to validate the method, which was then tested in four healthy subjects in whom regional ADC maps were acquired with diffusion times of 50, 200, and 1500 ms and a tag wavelength of 5 or 10 mm. ADC values from healthy subjects were in good agreement with reported literature values and decreased with increasing diffusion time. Mean ADC values were approximately 0.07, 0.03, and 0.015 cm2/s for diffusion times of 50, 200, and 1500 ms, respectively. ADC maps were generally homogeneous, with similar mean values when measured with the same parameters in different subjects.

Validity of apparent diffusion coefficient hyperpolarized 3He-MRI using MSCT and pulmonary function tests as references

PURPOSE

To compare apparent diffusion coefficient (ADC) measurements from hyperpolarized (HP) helium ((3)He)-magnetic resonance imaging (MRI) with quantitative data from multislice Computed Tomography (CT) (MSCT) of the whole lungs and pulmonary function tests (PFT).

MATERIALS AND METHODS

Twenty-seven subjects, 22 with established emphysema and 5 with preclinical emphysema defined by PFT criteria, were examined with HP (3)He-MRI and MSCT. Mean age was 55 (+/-12) years, 18 female and 9 male. Mean ADC from (3)He-MRI was compared with emphysema index (EI), 15th percentile and mean lung density (MLD) values from MSCT. Both mean ADC and MSCT data were compared to PFT, especially percent of predicted diffusing capacity of carbon monoxide (%predicted DLCO), using Pearson's correlation test.

RESULTS

Mean ADC and standard deviation values were 0.392+/-0.119 cm(2)/s for the established emphysema group and 0.216+/-0.046 for the pre-clinical emphysema group. MSCT values for the established emphysema group and pre-clinical emphysema group were: EI (%) 11+/-12 and 0.4+/-0.6, respectively; 15th percentile (Hounsfield Units (HU)), -956+/-25 and -933+/-13, respectively and MLD (HU) -877+/-20 and -863+/-15, respectively. Correlations between mean ADC and EI and 15th percentile were both r=0.90 and for MLD r=0.59. There was higher correlation between mean ADC and %predicted DLCO (r=0.90) than between EI and %predicted DLCO (r=0.76).

CONCLUSION

HP (3)He-MRI correlates well with density measurements from MSCT and agrees better than MSCT with %predicted DLCO which is the PFT most related to emphysema.

Hyperpolarized 3He magnetic resonance imaging of ventilation defects in healthy elderly volunteers: initial findings at 3.0 Tesla

RATIONALE AND OBJECTIVES

Hyperpolarized (3)He magnetic resonance imaging ventilation defects have been observed in subjects with respiratory disorders. We quantified (3)He ventilation defects in elderly and middle-aged subjects who had no history of smoking, respiratory, or cardiovascular disorders.

MATERIALS AND METHODS

Hyperpolarized (3)He magnetic resonance imaging ventilation defect volume (VDV) and ventilation defect score (VDS) were assessed in eight elderly healthy volunteers (mean 67+/-6 years) scanned twice within 7+/-2 minutes and again 7+/-2 days later. A younger cohort of 24 subjects (mean 44+/-10 years) was also scanned for direct comparison. Four observers blinded to scan timepoint and subject identity scored VDS and manually segmented VDV in all center coronal slices.

RESULTS

Center coronal slice ventilation defects were observed in six of eight elderly subjects (ages 63-74 years, 5 males) in all scans acquired and in no middle-aged subjects. At the scan timepoint, mean VDS was 2.7 (mean VDV 52+/-34 cm(3)), whereas for same-day rescan, mean VDS was 2.5 (mean VDV 53+/-35 cm(3)) and at 7-day rescan, mean VDS was 3.6 (mean VDV 48+/-39 cm(3)). Interscan coefficients of variation (COV) for mean VDV was 1.8% (same-day rescan) and 5.3% (7-day rescan) and interobserver COV ranged from 10-12%.

CONCLUSION

Elderly subjects have ventilation defects that are reproducible in same-day scanning and 7-day scanning visits. The observation of reproducible pulmonary ventilation defects in otherwise healthy elderly volunteers suggests caution must be used in interpreting results from (3)He studies of elderly subjects.

Oxygen-sensitive 3He-MRI in bronchiolitis obliterans after lung transplantation

Oxygen-sensitive 3He-MRI was studied for the detection of differences in intrapulmonary oxygen partial pressure (pO2) between patients with normal lung transplants and those with bronchiolitis obliterans syndrome (BOS). Using software developed in-house, oxygen-sensitive 3He-MRI datasets from patients with normal lung grafts (n = 8) and with BOS (n = 6) were evaluated quantitatively. Datasets were acqiured on a 1.5-T system using a spoiled gradient echo pulse sequence. Underlying diseases were pulmonary emphysema (n = 10 datasets) and fibrosis (n = 4). BOS status was verified by pulmonary function tests. Additionally, 3He-MRI was assessed blindedly for ventilation defects. Median intrapulmonary pO2 in patients with normal lung grafts was 146 mbar compared with 108 mbar in patients with BOS. Homogeneity of pO2 distribution was greater in normal grafts (standard deviation pO2 34 versus 43 mbar). Median oxygen decrease rate during breath hold was higher in unaffected patients (-1.75 mbar/s versus -0.38 mbar/s). Normal grafts showed fewer ventilation defects (5% versus 28%, medians). Oxygen-sensitive 3He-MRI appears capable of demonstrating differences of intrapulmonary pO2 between normal lung grafts and grafts affected by BOS. Oxygen-sensitive 3He-MRI may add helpful regional information to other diagnostic techniques for the assessment and follow-up of lung transplant recipients.

Clinical aspects of the apparent diffusion coefficient in 3He MRI: results in healthy volunteers and patients after lung transplantation

PURPOSE

To measure the apparent diffusion coefficient (ADC) after inhalation of hyperpolarized (3)He in healthy volunteers and lung transplant recipients, and demonstrate the gravity dependence of ADC values.

MATERIALS AND METHODS

Six healthy volunteers, 10 patients after single-lung transplantation, and six patients after double-lung transplantation were examined at 1.5T during inspiration and expiration. The inhalation of 300 mL of hyperpolarized (3)He was performed with a computer-controlled delivery device. A two-dimensional fast low-angle shot (FLASH) sequence measured the (3)He diffusive gas movement. From these data the ADC was calculated.

RESULTS

The mean ADC was 0.143 cm(2)/second in healthy individuals, 0.162 cm(2)/second in transplanted healthy lungs, and 0.173 cm(2)/second in rejected transplanted lungs, whereas it was 0.216 cm(2)/second in native fibrotic lungs and 0.239 cm(2)/second in emphysematous lungs. The difference in mean ADC values among healthy lungs, healthy transplanted lungs, and native diseased lungs was significant (P < 0.001). In inspiration the healthy volunteers showed higher ADC values in the anterior than in the posterior parts of the lungs. In expiration this gradient doubled.

CONCLUSION

An anterior-posterior (A/P) gradient was found in inspiration and expiration in healthy lungs. Healthy, transplanted, and native diseased lungs had significantly different mean ADC values. From our preliminary results, (3)He MRI appears to be sensitive for detecting areas of abnormal ventilation in transplanted lungs.

Hyperpolarized 3He Ventilation Defects and Apparent Diffusion Coefficients in Chronic Obstructive Pulmonary Disease

OBJECTIVE

Hyperpolarized 3He magnetic resonance imaging (3He MRI) at 3.0 Tesla of healthy volunteers and chronic obstructive pulmonary disease (COPD) patients was performed for quantitative evaluation of ventilation defects and apparent diffusion coefficients (ADC) and for comparison to published results acquired at 1.5 Tesla. The reproducibility of 3He ADC and ventilation defects was also assessed in subjects scanned 3 times, twice within 10 minutes, and again within 7 +/- 2 days of the first MRI visit.

MATERIALS AND METHODS

Hyperpolarized 3He MRI was performed in 6 subjects. Two interleaved images with and without additional diffusion sensitization were acquired with the first image serving as a ventilation image from which defect score and volume were measured and the combination of the 2 images used to compute ADC maps and ADC histograms.

RESULTS

He MRI at 3.0 Tesla showed increased mean ADC and ADC standard deviation for subjects with COPD compared with healthy volunteers (ADC healthy volunteer (0.24 +/- 0.12 cm2/s), mild-moderate COPD (0.34 +/- 0.14 cm2/s), and severe COPD (0.47 +/- 0.21 cm2/s), and these values were similar to previously reported results acquired at 1.5 Tesla. Reproducibility of mean ADC was high (coefficient of variation 2% in severe COPD, 3% in mild-moderate COPD, 4% in healthy volunteers) across all 3 scans. Higher same-day scan reproducibility was observed for ventilation defect volume compared with 1-week scan reproducibility in this small group of subjects.

CONCLUSIONS

ADC values for emphysematous lungs were significantly increased compared with healthy lungs in age-matched subjects, and all values were comparable to those reported previously at 1.5 Tesla. Ventilation defect score and ventilation defect volume results were also comparable to results previously reported in COPD subjects Reproducibility of ADC for same-day scan-rescan and 7-day rescan was high and similar to previously reported results.

Time dependence of 3He diffusion in the human lung: measurement in the long-time regime using stimulated echoes

A stimulated-echo-based technique was developed to measure the long-time-scale apparent diffusion coefficient (ADC) of hyperpolarized 3He during a single breath-hold acquisition. Computer simulations were used to evaluate the performance of the technique and guide the selection of appropriate parameter values for obtaining accurate ADC values. The technique was used in 10 healthy subjects and two subjects with chronic obstructive pulmonary disease (COPD) to measure the global ADC for diffusion times between a few tenths of a second and several seconds, and to acquire spatial maps of the ADC for a diffusion time of 1.5 s. The reproducibility of the technique and its sensitivity to the direction of diffusion sensitization were also investigated. In healthy subjects, global ADC values decreased by severalfold over the range of diffusion times measured (mean values = 0.039 and 0.023 cm2/s at diffusion times of 0.61 and 1.54 s, respectively). ADC maps were generally uniform, with mean values similar to the corresponding global values. For the two COPD subjects, global ADC values were substantially greater than those of every healthy subject at all diffusion times measured. In addition, regional elevations of ADC values were far more conspicuous on long-time-scale ADC maps than on short-time-scale ADC maps.

[Microstructure of the lung: diffusion measurement of hyperpolarized 3Helium]

Imaging methods to study the lung are traditionally based on x-ray or on radioactive contrast agents. Conventional magnetic resonance imaging (MRI) has only limited applications for lung imaging because of the low tissue density of protons concentration of hydrogen atoms, which are usually the basis for the imaging. The introduction of hyperpolarized noble gases as a contrast agent in MRI has opened new possibilities for lung diagnosis. The present paper describes this new technique. Diffusion-weighted MRI for assessment of the lung microstructure is presented here as an example of the new possibilities of functional imaging. Studies to determine the sensitivity of the diffusion measurement and regarding the correlation with traditionally established methods are also presented, along with results of the measurement of the reproducibility determined in a clinical pilot study on healthy volunteers and patients. Furthermore, a pilot measurement of the 3He diffusion tensor in the lung is presented.

Monitoring of Lung Motion in Patients With Malignant Pleural Mesothelioma Using Two-Dimensional and Three-Dimensional Dynamic Magnetic Resonance Imaging

PURPOSE

To monitor lung motion in patients with malignant pleural mesothelioma (MPM) before and after chemotherapy (CHT) using 2-dimensional (2D) and 3-dimensional (3D) dynamic MRI (dMRI) in comparison with spirometry.

METHODS AND MATERIALS

Twenty-two patients with MPM were examined before CHT, as well as after 3 and 6 CHT cycles (3 months and 6 months) using 2D dMRI (trueFISP; 3 images/s) and 3D dMRI (FLASH 3D, 1 slab (52 slices)/s) using parallel imaging in combination with view-sharing technique. Maximum craniocaudal lung dimensions (2D) and lung volumes (3D) were monitored, separated into the tumor-bearing and nontumor-bearing hemithorax. Vital capacity (VC) was measured for comparison using spirometry.

RESULTS

Using 2D technique, there was a significant difference between the tumor-bearing and the nontumor-bearing hemithorax before CHT (P < 0.01) and after 3 CHT cycles (P < 0.05), whereas difference was not significant in the second control. In the tumor-bearing hemithorax, mobility increased significantly from the status before versus after 3 CHT cycles (4.1 +/- 1.1 cm vs. 4.8 +/- 1.4 cm, P < 0.05). Using 3D technique, at maximum inspiration, the volume of the tumor-bearing hemithorax was 0.6 +/- 0.4 L and of the nontumor-bearing hemithorax 1.25 +/- 0.4 L before CHT. In the follow-up exams, these volumes changed to 1.05 +/- 0.4 L (P < 0.05) and 1.4 +/- 0.5 L, respectively. Using spirometry, there was no significant change in VC (1.9 +/- 0.4 L vs. 2.2 +/- 0.7 L vs. 2.2 +/- 0.9 L).

CONCLUSION

dMRI is capable of monitoring changes in lung motion and volumetry in patients with MPM not detected by global spirometry. Thus, dMRI is proposed for use as a further measure of therapy response.

Assessment of lung function in children by cross-sectional imaging: techniques and clinical applications

Imaging techniques are indispensable for diagnosis and follow-up of paediatric pulmonary diseases. In the past, interest was focused on morphological aspects of pulmonary tissue. With the development of novel CT and MRI techniques, functional pulmonary imaging became available. In this review, the new techniques of cross-sectional functional imaging of the lung are presented and the value of these methods for investigating paediatric pulmonary diseases and their potential clinical applications are discussed.

Evaluation of lung volumetry using dynamic three-dimensional magnetic resonance imaging

RATIONALE AND OBJECTIVES

We sought to investigate lung volume and surface measurements during the breathing cycle using dynamic three-dimensional magnetic resonance imaging (3D MRI).

MATERIALS AND METHODS

Breathing cycles of 20 healthy volunteers were examined using a 2D trueFISP sequence (3 images/second) in combination with a model and segmented 3D FLASH sequence (1 image/second) MR images using view sharing. Segmentation was performed semiautomatically using an interactive region growing technique. Vital capacity (VC) was calculated from MRI using the model (2D) and counting the voxels (3D) and was compared with spirometry.

RESULTS

VC from spirometry was 4.9+/-0.9 L, 4.4+/-1.2 L from 2D MRI measurement, and 4.7+/-0.9 L for 3D MRI. Using the 3D technique, correlation to spirometry was higher than using the 2D technique (r>0.95 vs. r>0.83). Using the 3D technique, split lung volumes and lung surface could be calculated. There was a significant difference between the left and right lung volume in expiration (P<0.05).

CONCLUSIONS

Dynamic 3D MRI is a noninvasive tool to evaluate split lung volumes and lung surfaces during the breathing cycle with a high correlation to spirometry.

Assessment of lung microstructure with magnetic resonance imaging of hyperpolarized Helium-3

Magnetic resonance imaging of the apparent diffusion coefficient (ADC) of hyperpolarized Helium-3 is a new technique for probing pulmonary microstructure in vivo. The aim of this study was the assessment of potential sources of systematic errors of the ADC measurement. The influence of macroscopic motion was determined by measurements at two different delays after initiating the breath-hold, and before and after cardiac arrest. An intercentre comparison was performed in two age- and lung function-matched groups of lung-healthy volunteers at two research sites. Moreover, measurements of diffusion anisotropy were performed. We found no dependency of the ADC as a function of the delay after stop of inspiration. The influence of cardiac motion was less than 10%. In the intercentre comparison study, an excellent agreement between the two sites was found. First measurements of the diffusion tensor of intrapulmonary Helium-3 are shown.

Diffusion-weighted MRI of the lung with hyperpolarized helium-3: a study of reproducibility

PURPOSE

To determine the reproducibility of several parameters of the ADC measurement by calculating the scan-to-scan intrasubject variability.

MATERIALS AND METHODS

Measurements were performed using a gradient-echo sequence with a bipolar gradient for diffusion weighting (b=3.89 sec/cm2). Five patients with pulmonary emphysema, and six healthy-lung volunteers were included in the study. Images were acquired after inspiration of 3He during a single inspiratory breath-hold. To assess the reproducibility, the measurement was performed twice (time between measurements=20 minutes) without repositioning the subjects. Analysis was performed on the basis of region-of-interest (ROI) analysis and global lung ADC histograms.

RESULTS

The mean ADC of a ROI varied by 5.1% between two measurements for volunteers and by 6.1% for patients. In the global evaluation, the 75th percentile demonstrated the best reproducibility (2%), while other parameters showed variations up to 12%. Only the variation of the standard deviation (SD) and the measure of homogeneity of the ADC map showed a significant difference between patients and volunteers.

CONCLUSION

Diffusion-weighted imaging (DWI) is a well-reproducible method for assessing the lung microstructure.

Two-dimensional and three-dimensional oxygen mapping by 3He-MRI validation in a lung phantom

The aim of this study was to validate oxygen-sensitive 3He-MRI in noninvasive determination of the regional, two- and three-dimensional distribution of oxygen partial pressure. In a gas-filled elastic silicon ventilation bag used as a lung phantom, oxygen sensitive two- and three-dimensional 3He-MRI measurements were performed at different oxygen concentrations which had been equilibrated in a range of normal and pathologic values. The oxygen partial pressure distribution was determined from 3He-MRI using newly developed software allowing for mapping of oxygen partial pressure. The reference bulk oxygen partial pressure inside the phantom was measured by conventional respiratory gas analysis. In two-dimensional measurements, image-based and gas-analysis results correlated with r=0.98; in three-dimensional measurements the between-methods correlation coefficient was r=0.89. The signal-to-noise ratio of three-dimensional measurements was about half of that of two-dimensional measurements and became critical (below 3) in some data sets. Oxygen-sensitive 3He-MRI allows for noninvasive determination of the two- and three-dimensional distribution of oxygen partial pressure in gas-filled airspaces.

Feasibility of Pulmonary Ventilation Visualization With Aerosolized Magnetic Resonance Contrast Media

OBJECTIVE

The objective of this study was to evaluate a new approach to noninvasive magnetic resonance assessment of human pulmonary ventilation with aerosolized Gd-DTPA.

MATERIALS AND METHODS

Fifteen experimental procedures were carried out in 15 healthy volunteers on a 1.5-T imager. For a timespan of 10 minutes, the subjects spontaneously inhaled a commercially available Gd-DTPA magnetic resonance contrast agent in aerosolized form through an inflatable facemask. Gd-DTPA was aerosolized by means of a small-particle generator with integrated heater to increase aerosol production. Respiratory gated dynamic T1-weighted turbo spin echo images were obtained before and after contrast agent aerosol administration. After nebulization, homogeneity of aerosol distribution was graded by 2 experienced readers and pulmonary signal intensity (SI) changes were measured in corresponding regions of both lungs.

RESULTS

Pulmonary ventilation visualization, and hence contrast agent delivery, was rated homogeneously distributed by both readers in 14 of 15 cases (93.3%) and slightly inhomogeneous in 1 case (6.7%). Pulmonary SI increased by an average of +37% +/- 8.5 (range, 10-48%). Allergic responses were not noted.

CONCLUSIONS

Human ventilation imaging with aerosolized gadolinium-chelates is viable. The presented modality might evolve as an alternative to current nuclear medicine and magnetic resonance image ventilation imaging procedures, avoiding radiation exposure while offering functional ventilation assessment with an acceptable temporal and spatial resolution. Nevertheless, further evaluation is required to define the potential of gadolinium-based ventilation magnetic resonance imaging in illustrating lung disease.