MRI of the lungs using hyperpolarized noble gases

Exercise-induced bronchoconstriction: reproducibility of hyperpolarized 3He MR imaging

PURPOSE

To quantitatively evaluate interday, interreader, and intersite agreement of readers of hyperpolarized helium 3 (HPHe) MR images in patients with exercise-induced bronchoconstriction.

MATERIALS AND METHODS

This HIPAA-compliant, institutional review board approved study included 13 patients with exercise-induced bronchoconstriction. On two separate days, HPHe MR imaging of the lungs was performed at baseline, immediately after a 10-minute exercise challenge (postchallenge), and 45 minutes after exercise (recovery). Patients were imaged at two sites, six at site A and seven at site B. Images were analyzed independently by multiple readers at each site. Lung volume, ventilation defect volume, ventilated volume, and the number of defects were measured quantitatively, and the location of defects was evaluated qualitatively at site A. Interday and interreader agreement were evaluated by using the intraclass correlation coefficient (ICC), and intersite agreement was evaluated by using a modified Bland-Altman analysis.

RESULTS

The ICC between days for ventilation defect volume, ventilated volume, and number of defects was at least 0.74 at both sites. The ICC for lung volume was greater at site B (0.83-0.86) than at site A (0.60-0.65). Defects seen in the same location in the lung on both days included 19.7% of those seen on baseline images and 29.2% and 18.6% of defects on postchallenge and recovery images, respectively. Interreader ICC for each measurement was at least 0.82 for each site. Analysis of intersite agreement showed biases of 612 mL for lung volume, -60.7 mL for ventilation defect volume, 2.91% for ventilated volume, and -6.56 for number of defects.

CONCLUSION

The reported measures of reproducibility of HPHe MR imaging may help in the design and interpretation of single- and multicenter studies of patients with exercise-induced bronchoconstriction.

A 24-ch Phased-Array System for Hyperpolarized Helium Gas Parallel MRI to Evaluate Lung Functions

Hyperpolarized 3He gas MRI has a serious potential for assessing pulmonary functions. Due to the fact that the non-equilibrium of the gas results in a steady depletion of the signal level over the course of the excitations, the signal-tonoise ratio (SNR) can be independent of the number of the data acquisitions under certain circumstances. This provides a unique opportunity for parallel MRI for gaining both temporal and spatial resolution without reducing SNR. We have built a 24-channel receive / 2-channel transmit phased array system for 3He parallel imaging. Our in vivo experimental results proved that the significant temporal and spatial resolution can be gained at no cost to the SNR. With 3D data acquisition, eight fold (2x4) scan time reduction can be achieved without any aliasing in images. Additionally, a rigid analysis using the low impedance preamplifier for decoupling presented evidence of strong coupling.

Ventilation distribution in rats: Part 2--A comparison of electrical impedance tomography and hyperpolarised helium magnetic resonance imaging

BACKGROUND

Hyperpolarised helium MRI (He3 MRI) is a new technique that enables imaging of the air distribution within the lungs. This allows accurate determination of the ventilation distribution in vivo. The technique has the disadvantages of requiring an expensive helium isotope, complex apparatus and moving the patient to a compatible MRI scanner. Electrical impedance tomography (EIT) a non-invasive bedside technique that allows constant monitoring of lung impedance, which is dependent on changes in air space capacity in the lung. We have used He3MRI measurements of ventilation distribution as the gold standard for assessment of EIT.

METHODS

Seven rats were ventilated in supine, prone, left and right lateral position with 70% helium/30% oxygen for EIT measurements and pure helium for He3 MRI. The same ventilator and settings were used for both measurements. Image dimensions, geometric centre and global in homogeneity index were calculated.

RESULTS

EIT images were smaller and of lower resolution and contained less anatomical detail than those from He3 MRI. However, both methods could measure positional induced changes in lung ventilation, as assessed by the geometric centre. The global in homogeneity index were comparable between the techniques.

CONCLUSION

EIT is a suitable technique for monitoring ventilation distribution and inhomgeneity as assessed by comparison with He3 MRI.

Ventilation distribution in rats: Part I--The effect of gas composition as measured with electrical impedance tomography

Abstract

The measurement of ventilation distribution is currently performed using inhaled tracer gases for multiple breath inhalation studies or imaging techniques to quantify spatial gas distribution. Most tracer gases used for these studies have properties different from that of air. The effect of gas density on regional ventilation distribution has not been studied. This study aimed to measure the effect of gas density on regional ventilation distribution.

Methods

Ventilation distribution was measured in seven rats using electrical impedance tomography (EIT) in supine, prone, left and right lateral positions while being mechanically ventilated with either air, heliox (30% oxygen, 70% helium) or sulfur hexafluoride (20% SF₆, 20% oxygen, 60% air). The effect of gas density on regional ventilation distribution was assessed.

Results

Gas density did not impact on regional ventilation distribution. The non-dependent lung was better ventilated in all four body positions. Gas density had no further impact on regional filling characteristics. The filling characteristics followed an anatomical pattern with the anterior and left lung showing a greater impedance change during the initial phase of the inspiration.

Conclusion

It was shown that gas density did not impact on convection dependent ventilation distribution in rats measured with EIT.

Hyperpolarized 129Xe magnetic resonance imaging: tolerability in healthy volunteers and subjects with pulmonary disease

RATIONALE AND OBJECTIVES

The objective of this study was to evaluate the tolerability of hyperpolarized (129)Xe gas inhaled from functional residual capacity and magnetic resonance imaging in healthy subjects and those with pulmonary disease.

MATERIALS AND METHODS

Twelve healthy volunteers (mean age, 59 ± 17 years), seven subjects with asthma (mean age, 47 ± 7 years), 10 subjects with chronic obstructive pulmonary disease (mean age, 74 ± 4 years), three subjects with cystic fibrosis (mean age, 27 ± 10 years), and a single subject with radiation-induced lung injury (age, 66 years) were enrolled and evaluated over 43 visits with 136 anoxic inhalations of 500 mL (129)Xe gas mixed with 500 mL (4)He gas. Oxygen saturation and heart rate were monitored during the breath-hold and imaging; subjects were queried for adverse events (AEs) before and immediately following gas inhalation and for 24 hours after the last dose.

RESULTS

No subjects withdrew from the study or reported serious, hypoxic, or severe AEs. Over the course of 136 dose administrations, two mild AEs (1%) were reported in two different subjects (two of 33 [6%]). One of these AEs (light-headedness) was temporally related and judged as possibly related to (129)Xe administration and resolved without treatment within 2 minutes. Statistically significant but clinically insignificant changes in oxygen saturation and heart rate were observed after inhalation (P < .001), and both resolved 1 minute later, with no difference between subject groups.

CONCLUSIONS

Inhalation of hyperpolarized (129)Xe gas and subsequent magnetic resonance imaging were well tolerated in healthy subjects and ambulatory subjects with obstructive and restrictive pulmonary disease.

NMR-based diffusion pore imaging

Nuclear magnetic resonance (NMR) diffusion experiments offer a unique opportunity to study boundaries restricting the diffusion process. In a recent Letter [Phys. Rev. Lett. 107, 048102 (2011)], we introduced the idea and concept that such diffusion experiments can be interpreted as NMR imaging experiments. Consequently, images of closed pores, in which the spins diffuse, can be acquired. In the work presented here, an in-depth description of the diffusion pore imaging technique is provided. Image artifacts due to gradient profiles of finite duration, field inhomogeneities, and surface relaxation are considered. Gradients of finite duration lead to image blurring and edge enhancement artifacts. Field inhomogeneities have benign effects on diffusion pore images, and surface relaxation can lead to a shrinkage and shift of the pore image. The relation between boundary structure and the imaginary part of the diffusion weighted signal is analyzed, and it is shown that information on pore coherence can be obtained without the need to measure the phase of the diffusion weighted signal. Moreover, it is shown that quite arbitrary gradient profiles can be used for diffusion pore imaging. The matrices required for numerical calculations are stated and provided as supplemental material.

Imaging the paediatric lung: what does nanotechnology have to offer?

This review will provide an overview of current research into lung imaging with nanoparticles, with a focus on the use of nanoparticles as molecular imaging agents to observe pathological processes and to monitor the effectiveness of nanoparticulate drug delivery systems. Various imaging modalities together with their advantages and limitations for lung imaging will be discussed. We will also explore the range of nanoparticles used, as well as active or passive targeting of nanoparticles.

Effects of corticosteroid treatment on airway inflammation, mechanics, and hyperpolarized ³He magnetic resonance imaging in an allergic mouse model

The purpose of this study was to assess the effects of corticosteroid therapy on a murine model of allergic asthma using hyperpolarized (3)He magnetic resonance imaging (MRI) and respiratory mechanics measurements before, during, and after methacholine (MCh) challenge. Three groups of mice were prepared, consisting of ovalbumin sensitized/ovalbumin challenged (Ova/Ova, n = 5), Ova/Ova challenged but treated with the corticosteroid dexamethasone (Ova/Ova+Dex, n = 3), and ovalbumin-sensitized/saline-challenged (Ova/PBS, n = 4) control animals. All mice underwent baseline 3D (3)He MRI, then received a MCh challenge while 10 2D (3)He MR images were acquired for 2 min, followed by post-MCh 3D (3)He MRI. Identically treated groups underwent respiratory mechanics evaluation (n = 4/group) and inflammatory cell counts (n = 4/group). Ova/Ova animals exhibited predominantly large whole lobar defects at baseline, with significantly higher ventilation defect percentage (VDP = 19 ± 4%) than Ova/PBS (+2 ± 1%, P = 0.01) animals. Such baseline defects were suppressed by dexamethasone (0%, P = 0.009). In the Ova/Ova group, MCh challenge increased VDP on both 2D (+30 ± 8%) and 3D MRI scans (+14 ± 2%). MCh-induced VDP changes were diminished in Ova/Ova+Dex animals on both 2D (+21 ± 9%, P = 0.63) and 3D scans (+7 ± 2%, P = 0.11) and also in Ova/PBS animals on 2D (+6 ± 3%, P = 0.07) and 3D (+4 ± 1%, P = 0.01) scans. Because MCh challenge caused near complete cessation of ventilation in four of five Ova/Ova animals, even as large airways remained patent, this implies that small airway (<188 μm) obstruction predominates in this model. This corresponds with respiratory mechanics observations that MCh challenge significantly increases elastance and tissue damping but only modestly affects Newtonian airway resistance.

Diagnostic imaging

Physical techniques have always had a key role in medicine, and the second half of the 20th century in particular saw a revolution in medical diagnostic techniques with the development of key imaging instruments: x-ray imaging and emission tomography (nuclear imaging and PET), MRI, and ultrasound. These techniques use the full width of the electromagnetic spectrum, from gamma rays to radio waves, and sound. In most cases, the development of a medical imaging device was opportunistic; many scientists in physics laboratories were experimenting with simple x-ray images within the first year of the discovery of such rays, the development of the cyclotron and later nuclear reactors created the opportunity for nuclear medicine, and one of the co-inventors of MRI was initially attempting to develop an alternative to x-ray diffraction for the analysis of crystal structures. What all these techniques have in common is the brilliant insight of a few pioneering physical scientists and engineers who had the tenacity to develop their inventions, followed by a series of technical innovations that enabled the full diagnostic potential of these instruments to be realised. In this report, we focus on the key part played by these scientists and engineers and the new imaging instruments and diagnostic procedures that they developed. By bringing the key developments and applications together we hope to show the true legacy of physics and engineering in diagnostic medicine.

Diagnostic evaluation and management of chronic thromboembolic pulmonary hypertension: a clinical practice guideline

BACKGROUND

Pulmonary embolism is a common condition. Some patients subsequently develop chronic thromboembolic pulmonary hypertension (CTEPH). Many care gaps exist in the diagnosis and management of CTEPH patients including lack of awareness, incomplete diagnostic assessment, and inconsistent use of surgical and medical therapies.

METHODS

A representative interdisciplinary panel of medical experts undertook a formal clinical practice guideline development process. A total of 20 key clinical issues were defined according to the patient population, intervention, comparator, outcome (PICO) approach. The panel performed an evidence-based, systematic, literature review, assessed and graded the relevant evidence, and made 26 recommendations.

RESULTS

Asymptomatic patients postpulmonary embolism should not be screened for CTEPH. In patients with pulmonary hypertension, the possibility of CTEPH should be routinely evaluated with initial ventilation/perfusion lung scanning, not computed tomography angiography. Pulmonary endarterectomy surgery is the treatment of choice in patients with surgically accessible CTEPH, and may also be effective in CTEPH patients with disease in more 'distal' pulmonary arteries. The anatomical extent of CTEPH for surgical pulmonary endarterectomy is best assessed by contrast pulmonary angiography, although positive computed tomography angiography may be acceptable. Novel medications indicated for the treatment of pulmonary hypertension may be effective for selected CTEPH patients.

CONCLUSIONS

The present guideline requires formal dissemination to relevant target user groups, the development of tools for implementation into routine clinical practice and formal evaluation of the impact of the guideline on the quality of care of CTEPH patients. Moreover, the guideline will be updated periodically to reflect new evidence or clinical approaches.

Measurement of 129Xe gas apparent diffusion coefficient anisotropy in an elastase-instilled rat model of emphysema

Hyperpolarized noble gas ((3)He and (129)Xe) apparent diffusion coefficient (ADC) measurements have shown remarkable sensitivity to microstructural (i.e., alveolar) changes in the lung, particularly emphysema. The ADC of hyperpolarized noble gases depends strongly on the diffusion time (Δ), and (3)He ADC has been shown to be anisotropic for Δ ranging from a few milliseconds down to a few hundred microseconds. In this study, the anisotropic nature of (129)Xe diffusion and its dependence on Δ were investigated both numerically, in a budded cylinder model, and in vivo, in an elastase-instilled rat model of emphysema. Whole lung longitudinal ADC (D(L)) and transverse ADC (D(T)) were measured for Δ = 6, 50, and 100 ms at 73.5 mT, and correlated with measurements of the mean linear intercept (L(m)) obtained from lung histology. A significant increase (P = 0.0021) in D(T) was measured for Δ = 6 ms between the sham (0.0021 ± 0.0005 cm(2)/s) and elastase-instilled (0.005 ± 0.001 cm(2)/s) cohorts, and a strong correlation was measured between D(T) (Δ = 6 ms) and L(m), with a Pearson's correlation coefficient of 0.90. This study confirms that (129)Xe D(T) increases correlate with alveolar space enlargement due to elastase instillation in rats.

In vivo MR imaging of pulmonary perfusion and gas exchange in rats via continuous extracorporeal infusion of hyperpolarized 129Xe

BACKGROUND

Hyperpolarized (HP) (129)Xe magnetic resonance imaging (MRI) permits high resolution, regional visualization of pulmonary ventilation. Additionally, its reasonably high solubility (>10%) and large chemical shift range (>200 ppm) in tissues allow HP (129)Xe to serve as a regional probe of pulmonary perfusion and gas transport, when introduced directly into the vasculature. In earlier work, vascular delivery was accomplished in rats by first dissolving HP (129)Xe in a biologically compatible carrier solution, injecting the solution into the vasculature, and then detecting HP (129)Xe as it emerged into the alveolar airspaces. Although easily implemented, this approach was constrained by the tolerable injection volume and the duration of the HP (129)Xe signal.

METHODS AND PRINCIPAL FINDINGS

Here, we overcome the volume and temporal constraints imposed by injection, by using hydrophobic, microporous, gas-exchange membranes to directly and continuously infuse (129)Xe into the arterial blood of live rats with an extracorporeal (EC) circuit. The resulting gas-phase (129)Xe signal is sufficient to generate diffusive gas exchange- and pulmonary perfusion-dependent, 3D MR images with a nominal resolution of 2×2×2 mm(3). We also show that the (129)Xe signal dynamics during EC infusion are well described by an analytical model that incorporates both mass transport into the blood and longitudinal relaxation.

CONCLUSIONS

Extracorporeal infusion of HP (129)Xe enables rapid, 3D MR imaging of rat lungs and, when combined with ventilation imaging, will permit spatially resolved studies of the ventilation-perfusion ratio in small animals. Moreover, EC infusion should allow (129)Xe to be delivered elsewhere in the body and make possible functional and molecular imaging approaches that are currently not feasible using inhaled HP (129)Xe.

Evaluating bronchodilator effects in chronic obstructive pulmonary disease using diffusion-weighted hyperpolarized helium-3 magnetic resonance imaging

The objective of this study was to evaluate the regional effects of bronchodilator administration in chronic obstructive pulmonary disease (COPD) using hyperpolarized helium-3 (3He) MRI apparent diffusion coefficient (ADC). Ten COPD ex-smokers provided written, informed consent and underwent diffusion-weighted, hyperpolarized 3He MRI, spirometry, and plethysmography before and 25 ± 2 min after bronchodilator administration. Pre- and postsalbutamol whole-lung (WL) ADC maps were generated and registered together to identify the lung regions containing the 3He signal at both time points, and mean ADC within those regions of interest (ROI) was determined for a measurement of previously ventilated ROI ADC (ADCP). Lung ROI with 3He signal at both time points was used as a binary mask on postsalbutamol WL ADC maps to obtain an ADC measurement for newly ventilated ROI (ADCN). Postsalbutamol, no significant differences were detected in WL ADC ( P = 0.516). There were no significant differences between ADCN and ADCP postsalbutamol ( P = 1.00), suggesting that the ADCN lung regions were not more emphysematous than the lung ROI participating in ventilation before bronchodilator administration. Postsalbutamol, a statistically significant decrease in ADCP ( P = 0.01) was detected, and there were significant differences between ADCP in the most anterior and most posterior image slices ( P = 0.02), suggesting a reduction in regional gas trapping following bronchodilator administration. Regional evaluation of tissue microstructure using hyperpolarized 3He MRI ADC provides insights into lung alterations that accompany improvements in regional 3He gas distribution after bronchodilator administration.

Hyperpolarized 3He magnetic resonance functional imaging semiautomated segmentation

RATIONALE AND OBJECTIVES

To improve intra- and interobserver variability and enable the use of functional magnetic resonance imaging (MRI) for multicenter, multiobserver studies, we generated a semiautomated segmentation method for hyperpolarized helium-3 ((3)He) MRI. Therefore the objective of this study was to compare the reproducibility and spatial agreement of manual and semiautomated segmentation of (3)He MRI ventilation defect volume (VDV) and ventilation volume (VV) in subjects with asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF).

MATERIALS AND METHODS

The multistep semiautomated segmentation method we developed employed hierarchical K-means clustering to classify (3)He MRI pixel intensity values into five user-determined clusters ranging from signal void to hyperintense. A seeded region-growing algorithm was also used to segment the (1)H MRI thoracic cavity for coregistration to the (3)He cluster-map, generating VDV and VV.

RESULTS

We compared manual segmentation performed by an expert observer and semiautomated measurements of (3)He MRI VDV and observed strong significant correlations between the volumes generated using each method (asthma, n = 5, r = 0.89, P < .0001; COPD, n = 5, r = 0.84, P < .0001; CF, n = 5, r = 0.89, P < .0001). Semiautomated VDV had high interobserver reproducibility (coefficient of variation [CV] = 7%, intraclass correlation coefficient [ICC] = 0.96); intraobserver reproducibility was significantly higher for semiautomated (CV = 5%, ICC = 1.00) compared to manual VDV (CV = 12%, ICC = 0.98). Spatial agreement for VV determined using the Dice coefficient (D) was also high for all disease states (asthma, D = 0.95; COPD, D = 0.88; CF, D = 0.90).

CONCLUSIONS

Semiautomated segmentation (3)He MRI provides excellent inter- and intraobserver precision with high spatial and quantitative agreement with manual measurements enabling its use in longitudinal studies.

Magnetic resonance imaging of the chest: current and new applications, with an emphasis on pulmonology

The objective of the present review study was to present the principal applications of magnetic resonance imaging (MRI) of the chest, including the description of new techniques. Over the past decade, this method has evolved considerably because of the development of new equipment, including the simultaneous interconnection of phased-array multiple radiofrequency receiver coils and remote control of the table movement, in addition to faster techniques of image acquisition, such as parallel imaging and partial Fourier acquisitions, as well as the introduction of new contrast agents. All of these advances have allowed MRI to gain ground in the study of various pathologies of the chest, including lung diseases. Currently, MRI is considered the modality of choice for the evaluation of lesions in the mediastinum and in the chest wall, as well as of superior sulcus tumors. However, it can also facilitate the diagnosis of lung, pleural, and cardiac diseases, as well as of those related to the pulmonary vasculature. Pulmonary MRI angiography can be used in order to evaluate various pulmonary vascular diseases, and it has played an ever greater role in the study of thromboembolism. Because cardiac MRI allows morphological and functional assessment in the same test, it has also become part of the clinical routine in the evaluation of various cardiac diseases. Finally, the role of MRI has been extended to the identification and characterization of pulmonary nodules, the evaluation of airway diseases, and the characterization of pleural effusion.

Dynamic ventilation 3He MRI for the quantification of disease in the rat lung

Pulmonary diseases are known to be largely inhomogeneous. To evaluate such inhomogeneities, we are testing an image-based method to measure gas flow in the lung regionally. Dynamic, spin-density-weighted hyperpolarized (3)He MR images performed during slow inhalation of this gas were analyzed to quantify regional inflation rate. This parameter was measured in regions of interest (ROIs) that were defined by a rectangular grid that covered the entire rat lung and grew dynamically with it during its inflation. We used regional inflation rate to quantify elastase-induced emphysema and to differentiate healthy (n = 8) from elastase-treated (n = 9) rat lungs as well as healthy from elastase-treated areas of one rat unilaterally treated with elastase in the left lung. Emphysema was also assessed by gold standard morphological and well-established hyperpolarized (3)He MRI diffusion measurements. Mean values of regional inflation rates were significantly different for healthy and elastase-treated animals and correlated well with the apparent diffusion coefficient of (3)He and morphological measurements. The image-based biomarker inflation rate may be useful for the assessment of regional lung ventilation.

Hyperpolarized 129Xe lung MRI in spontaneously breathing mice with respiratory gated fast imaging and its application to pulmonary functional imaging

In the present study, a balanced steady-state free precession pulse sequence combined with compressed sensing was applied to hyperpolarized (129) Xe lung imaging in spontaneously breathing mice. With the aid of fast imaging techniques, the temporal resolution was markedly improved in the resulting images. Using these protocols and respiratory gating, (129) Xe lung images in end-inspiratory and end-expiratory phases were obtained successfully. The application of these techniques for pulmonary functional imaging made it possible to simultaneously evaluate regional ventilation and gas exchange in the same animal. A comparative study between healthy and elastase-induced mouse models of emphysema showed abnormal ventilation as well as gas exchange in elastase-treated mice.

Chronic obstructive pulmonary disease: safety and tolerability of hyperpolarized 129Xe MR imaging in healthy volunteers and patients

PURPOSE

To evaluate the safety and tolerability of inhaling multiple 1-L volumes of undiluted hyperpolarized xenon 129 ((129)Xe) followed by up to a 16-second breath hold and magnetic resonance (MR) imaging.

MATERIALS AND METHODS

This study was approved by the institutional review board and was HIPAA compliant. Written informed consent was obtained. Forty-four subjects (19 men, 25 women; mean age, 46.1 years ± 18.8 [standard deviation]) were enrolled, consisting of 24 healthy volunteers, 10 patients with chronic obstructive pulmonary disease (COPD), and 10 age-matched control subjects. All subjects received three or four 1-L volumes of undiluted hyperpolarized (129)Xe, followed by breath-hold MR imaging. Oxygen saturation, heart rate and rhythm, and blood pressure were continuously monitored. These parameters, along with respiratory rate and subjective symptoms, were assessed after each dose. Subjects' serum biochemistry and hematology were recorded at screening and at 24-hour follow-up. A 12-lead electrocardiogram (ECG) was obtained at these times and also within 2 hours prior to and 1 hour after (129)Xe MR imaging. Xenon-related symptoms were evaluated for relationship to subject group by using a χ(2) test and to subject age by using logistic regression. Changes in vital signs were tested for significance across subject group and time by using a repeated-measures multivariate analysis of variance test.

RESULTS

The 44 subjects tolerated all xenon inhalations, no subjects withdrew, and no serious adverse events occurred. No significant changes in vital signs (P > .27) were observed, and no subjects exhibited changes in laboratory test or ECG results at follow-up that were deemed clinically important or required intervention. Most subjects (91%) did experience transient xenon-related symptoms, most commonly dizziness (59%), paresthesia (34%), euphoria (30%), and hypoesthesia (30%). All symptoms resolved without clinical intervention in 1.6 minutes ± 0.9.

CONCLUSION

Inhalation of hyperpolarized (129)Xe is well tolerated in healthy subjects and in those with mild or moderate COPD. Subjects do experience mild, transient, xenon-related symptoms, consistent with its known anesthetic properties.

A shift from cell cultures to creatures: in vivo imaging of small animals in experimental regenerative medicine

Although the use of small animals for in vivo experimentation has been widespread, only recently has there been easy availability of techniques that allow noninvasive in vivo imaging of small animals. Because these techniques allow the same individual subject to be followed longitudinally throughout the duration of an experiment, their use is rapidly changing the way small animals are employed in the laboratory. In this review, we focus on six imaging modalities that are increasingly employed for small animal in vivo imaging: optical imaging (OI), magnetic resonance imaging (MRI), computed tomography (CT), single-photon emission tomography (SPECT), ultrasound (US), and positron-emission tomography (PET). Each modality allows for the noninvasive tracking of cells and cell products in vivo. In addition, multimodality imaging, combining two or more of these techniques, has also been increasingly employed to overcome the limitations of each independent technique. After reviewing these available imaging modalities, we detail their experimental application, exemplified by the emerging field of regenerative medicine, referring to publications whose conclusions would otherwise be difficult to support without the availability of in vivo imaging.

Relaxation of hyperpolarized 129Xe in a deflating polymer bag

In magnetic resonance imaging with hyperpolarized (HP) noble gases, data is often acquired during prolonged gas delivery from a storage reservoir. However, little is known about the extent to which relaxation within the reservoir will limit the useful acquisition time. For quantitative characterization, 129Xe relaxation was studied in a bag made of polyvinyl fluoride (Tedlar). Particular emphasis was on wall relaxation, as this mechanism is expected to dominate. The HP 129Xe magnetization dynamics in the deflating bag were accurately described by a model assuming dissolution of Xe in the polymer matrix and dipolar relaxation with neighboring nuclear spins. In particular, the wall relaxation rate changed linearly with the surface-to-volume ratio and exhibited a relaxivity of κ=0.392±0.008 cm/h, which is in reasonable agreement with κ=0.331±0.051 cm/h measured in a static Tedlar bag. Estimates for the bulk gas-phase 129Xe relaxation yielded T1bulk=2.55±0.22 h, which is dominated by intrinsic Xe-Xe relaxation, with small additional contributions from magnetic field inhomogeneities and oxygen-induced relaxation. Calculations based on these findings indicate that relaxation may limit HP 129Xe experiments when slow gas delivery rates are employed as, for example, in mouse imaging or vascular infusion experiments.

Optical and magnetic resonance imaging as complementary modalities in drug discovery

Imaging has the ability to study various biological and chemical processes noninvasively in living subjects in a longitudinal way. For this reason, imaging technologies have become an integral part of the drug-discovery and development program and are commonly used in following disease processes and drug action in both preclinical and clinical stages. As the domain of imaging sciences transitions from anatomical/functional to molecular applications, the development of molecular probes becomes crucial for the advancement of the field. This review summarizes the role of two complementary techniques, magnetic resonance and fluorescence optical imaging, in drug discovery. While the first approach exploits intrinsic tissue characteristics as the source of image contrast, the second necessitates the use of appropriate probes for signal generation. The anatomical, functional, metabolic and molecular information that becomes accessible through imaging can provide invaluable insights into disease mechanisms and mechanisms of drug action.

Effects of pulmonary inhalation on hyperpolarized krypton-83 magnetic resonance T1 relaxation

The (83)Kr magnetic resonance (MR) relaxation time T(1) of krypton gas in contact with model surfaces was previously found to be highly sensitive to surface composition, surface-to-volume ratio, and surface temperature. The work presented here explored aspects of pulmonary (83)Kr T(1) relaxation measurements in excised lungs from healthy rats using hyperpolarized (hp) (83)Kr with approximately 4.4% spin polarization. MR spectroscopy without spatial resolution was applied to the ex vivo lungs that actively inhale hp (83)Kr through a custom designed ventilation system. Various inhalation schemes were devised to study the influence of anatomical dead space upon the measured (83)Kr T(1) relaxation times. The longitudinal (83)Kr relaxation times in the distal airways and the respiratory zones were independent of the lung inhalation volume, with T(1) = 1.3 s and T(1) = 1.0 s, depending only on the applied inhalation scheme. The obtained data were highly reproducible between different specimens. Further, the (83)Kr T(1) relaxation times in excised lungs were unaffected by the presence of up to 40% oxygen in the hp gas mixture. The results support the possible importance of (83)Kr as a biomarker for evaluating lung function.

A flexible 32-channel receive array combined with a homogeneous transmit coil for human lung imaging with hyperpolarized 3He at 1.5 T

Parallel imaging presents a promising approach for MRI of hyperpolarized nuclei, as the penalty in signal-to-noise ratio typically encountered with (1)H MRI due to a reduction in acquisition time can be offset by an increase in flip angle. The signal-to-noise ratio of hyperpolarized MRI generally exhibits a strong dependence on flip angle, which makes a homogeneous B(1)(+) transmit field desirable. This paper presents a flexible 32-channel receive array for (3) He human lung imaging at 1.5T designed for insertion into an asymmetric birdcage transmit coil. While the 32-channel array allows parallel imaging at high acceleration factors, the birdcage transmit coil provides a homogeneous B(1)(+) field. Decoupling between array elements is achieved by using a concentric shielding approach together with preamplifier decoupling. Coupling between transmit coil and array elements is low by virtue of a low geometric coupling coefficient, which is reduced further by the concentric shields in the array. The combination of the 32-channel array and birdcage transmit coil provides (3)He ventilation images of excellent quality with similar signal-to-noise ratio at acceleration factors R = 2 and R = 4, while maintaining a homogeneous B(1)(+).

Signal-to-noise ratio for hyperpolarized ³He MR imaging of human lungs: a 1.5 T and 3 T comparison

The signal-to-noise ratio in hyperpolarized noble gas MR imaging is expected to be independent of field strength at frequencies typical of clinical systems (e.g., 1.5 T), where body noise dominates over coil noise. Furthermore, at higher fields (e.g., 3 T), the SNR of lung images may decline due to decreases in T(2) originating from increases in susceptibility-induced field gradients at the air-tissue interface. In this work, the SNR of hyperpolarized (3) He lung imaging at two commonly used clinical field strengths (1.5 T and 3 T) were compared in the same volunteers. Thermally polarized and hyperpolarized (3) He phantoms were used to account for differences in MR imaging system and (3) He polarizer performance, respectively, at the two field strengths. After correcting for T(2) values measured at 1.5 T (16 ± 2 ms) and 3 T (7 ± 1 ms), no significant difference in image SNR between the two field strengths was observed, consistent with theory.

Demonstration of the heterogeneous distribution of asthma in the lungs using CT and hyperpolarized helium-3 MRI

Asthma is a chronic inflammatory disease that affects both the large and small airways and results in bronchoconstriction, mucous hypersecretion, smooth muscle hypertrophy, and subepithelial fibrosis. To gain insight into the pathophysiology of asthma, chest computed tomography (CT) has been investigated as a noninvasive method to evaluate airway wall thickness of medium and large airways. Hyperpolarized gas MRI can assess the functional alterations of airflow within the lung resulting from the structural changes in the airways. In this article, we review the application of CT-based techniques and hyperpolarized gas MRI to study structural and functional changes in asthma. From the result of studies with CT and hyperpolarized gas MRI, it is becoming apparent that asthma has a regional distribution within the lung, that is, some areas of the lung are more affected than others. Furthermore, there appears to be some persistence to this distribution which may explain the observed patterns of airway remodeling and provide targets for localized therapies such as local application of anti-inflammatory agents or bronchial thermoplasty. Thus, cross sectional imaging in asthma is providing new insights into the pathophysiology of the disease and has the potential to become essential in the guidance of localized treatments.

Imaging of lung function using hyperpolarized helium-3 magnetic resonance imaging: Review of current and emerging translational methods and applications

During the past several years there has been extensive development and application of hyperpolarized helium-3 (HP (3)He) magnetic resonance imaging (MRI) in clinical respiratory indications such as asthma, chronic obstructive pulmonary disease, cystic fibrosis, radiation-induced lung injury, and transplantation. This review focuses on the state-of-the-art of HP (3)He MRI and its application to clinical pulmonary research. This is not an overview of the physics of the method, as this topic has been covered previously. We focus here on the potential of this imaging method and its challenges in demonstrating new types of information that has the potential to influence clinical research and decision making in pulmonary medicine. Particular attention is given to functional imaging approaches related to ventilation and diffusion-weighted imaging with applications in chronic obstructive pulmonary disease, cystic fibrosis, asthma, and radiation-induced lung injury. The strengths and challenges of the application of (3)He MRI in these indications are discussed along with a comparison to established and emerging imaging techniques.

Measurement of alveolar oxygen partial pressure in the rat lung using Carr-Purcell-Meiboom-Gill spin-spin relaxation times of hyperpolarized 3He and 129Xe at 74 mT

Regional measurement of alveolar oxygen partial pressure can be obtained from the relaxation rates of hyperpolarized noble gases, (3) He and (129) Xe, in the lungs. Recently, it has been demonstrated that measurements of alveolar oxygen partial pressure can be obtained using the spin-spin relaxation rate (R(2) ) of (3) He at low magnetic field strengths (<0.1 T) in vivo. R(2) measurements can be achieved efficiently using the Carr-Purcell-Meiboom-Gill pulse sequence. In this work, alveolar oxygen partial pressure measurements based on Carr-Purcell-Meiboom-Gill R(2) values of hyperpolarized (3) He and (129) Xe in vitro and in vivo in the rat lung at low magnetic field strength (74 mT) are presented. In vitro spin-spin relaxivity constants for (3) He and (129) Xe were determined to be (5.2 ± 0.6) × 10(-6) Pa(-1) sec(-1) and (7.3 ± 0.4) × 10(-6) Pa(-1) s(-1) compared with spin-lattice relaxivity constants of (4.0 ± 0.4) × 10(-6) Pa(-1) s(-1) and (4.3 ± 1.3) × 10(-6) Pa(-1) s(-1), respectively. In vivo experimental measurements of alveolar oxygen partial pressure using (3) He in whole rat lung show good agreement (r(2) = 0.973) with predictions based on lung volumes and ventilation parameters. For (129) Xe, multicomponent relaxation was observed with one component exhibiting an increase in R(2) with decreasing alveolar oxygen partial pressure.

Helical tomotherapy planning for lung cancer based on ventilation magnetic resonance imaging

To investigate the feasibility of lung ventilation-based treatment planning, computed tomography and hyperpolarized (HP) helium-3 (He-3) magnetic resonance imaging (MRI) ventilation images of 6 subjects were coregistered for intensity-modulated radiation therapy planning in Tomotherapy. Highly-functional lungs (HFL) and less-functional lungs (LFL) were contoured based on their ventilation image intensities, and a cylindrical planning-target-volume was simulated at locations adjacent to both HFL and LFL. Annals of an anatomy-based plan (Plan 1) and a ventilation-based plan (Plan 2) were generated. The following dosimetric parameters were determined and compared between the 2 plans: percentage of total/HFL volume receiving ≥20 Gy, 15 Gy, 10 Gy, and 5 Gy (TLV(20), HFLV(20), TLV(15), HFLV(15), TLV(10), HFLV(10), TLV(5), HFLV(5)), mean total/HFL dose (MTLD/HFLD), maximum doses to all organs at risk (OARs), and target dose conformality. Compared with Plan 1, Plan 2 reduced mean HFLD (mean reduction, 0.8 Gy), MTLD (mean reduction, 0.6 Gy), HFLV(20) (mean reduction, 1.9%), TLV(20) (mean reduction, 1.5%), TLV(15) (mean reduction, 1.7%), and TLV(10) (mean reduction, 2.1%). P-values of the above comparisons are less than 0.05 using the Wilcoxon signed rank test. For HFLV(15), HFLV(10), TLV(5), and HTLV(5), Plan 2 resulted in lower values than plan 1 but the differences are not significant (P-value range, 0.063-0.219). Plan 2 did not significantly change maximum doses to OARs (P-value range, 0.063-0.563) and target conformality (P = 1.000). HP He-3 MRI of patients with lung disease shows a highly heterogeneous ventilation capacity that can be utilized for functional treatment planning. Moderate but statistically significant improvements in sparing functional lungs were achieved using helical tomotherapy plans.

Noninvasive detection of pulmonary tissue destruction in a mouse model of emphysema using hyperpolarized 129Xe MRS under spontaneous respiration

In the present study, a chemical shift saturation recovery method in hyperpolarized (129)Xe MR spectroscopy measurements was applied to two groups of spontaneously breathing mice, an elastase-induced emphysema model and a control group. Parameters detected were those related to lung structures and functions, such as alveolar septal thickness, h, the ratio of the alveolar septal volume relative to gas space volume, V(s)/V(a), and the transit time of blood through the gas exchange region, τ. To investigate the potential of these parameters as biomarkers, an attempt was made to detect physiologic changes in the lungs of elastase-treated mice. Our results showed that V(s)/V(a) was significantly reduced in elastase-treated mice, reflecting emphysema-like destruction of the alveolar wall. Compared with histologic results, this degree of reduction was shown to reflect the severity of wall destruction. On the other hand, significant changes in other parameters, h and τ, were not shown. This study is the first application of hyperpolarized (129)Xe MR spectroscopy to a mouse model of emphysema and shows that the V(s)/V(a) volume ratio is an effective biomarker for emphysema that could become useful in drug research and development through noninvasive detection of pathologic changes in small rodents.

Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.

Hyperpolarized noble gases as contrast agents

Hyperpolarized noble gases ((3)He and (129)Xe) can provide NMR signal enhancements of 10,000 to 100,000 times that of thermally polarized gases and have shown great potential for applications in lung magnetic resonance imaging (MRI) by greatly enhancing the sensitivity and contrast. These gases obtain a highly polarized state by employing a spin exchange optical pumping technique. In this chapter, the underlying physics of spin exchange optical pumping for production of hyperpolarized noble gases is explained and the basic components and procedures for building a polarizer are described. The storage and delivery strategies of hyperpolarized gases for in vivo imaging are discussed. Many of the problems that are likely to be encountered in practical experiments and the corresponding detailed approaches to overcome them are also discussed.

³He lung morphometry technique: accuracy analysis and pulse sequence optimization

The (3)He lung morphometry technique (Yablonskiy et al., JAP, 2009), based on MRI measurements of hyperpolarized gas diffusion in lung airspaces, provides unique information on the lung microstructure at the alveolar level. 3D tomographic images of standard morphological parameters (mean airspace chord length, lung parenchyma surface-to-volume ratio, and the number of alveoli per unit lung volume) can be created from a rather short (several seconds) MRI scan. These parameters are most commonly used to characterize lung morphometry but were not previously available from in vivo studies. A background of the (3)He lung morphometry technique is based on a previously proposed model of lung acinar airways, treated as cylindrical passages of external radius R covered by alveolar sleeves of depth h, and on a theory of gas diffusion in these airways. The initial works approximated the acinar airways as very long cylinders, all with the same R and h. The present work aims at analyzing effects of realistic acinar airway structures, incorporating airway branching, physiological airway lengths, a physiological ratio of airway ducts and sacs, and distributions of R and h. By means of Monte-Carlo computer simulations, we demonstrate that our technique allows rather accurate measurements of geometrical and morphological parameters of acinar airways. In particular, the accuracy of determining one of the most important physiological parameter of lung parenchyma - surface-to-volume ratio - does not exceed several percent. Second, we analyze the effect of the susceptibility induced inhomogeneous magnetic field on the parameter estimate and demonstrate that this effect is rather negligible at B(0) ≤ 3T and becomes substantial only at higher B(0) Third, we theoretically derive an optimal choice of MR pulse sequence parameters, which should be used to acquire a series of diffusion-attenuated MR signals, allowing a substantial decrease in the acquisition time and improvement in accuracy of the results. It is demonstrated that the optimal choice represents three not equidistant b-values: b(1)=0, b(2)∼2 s/cm(2), b(3)∼8 s/cm(2).

Simultaneous magnetic resonance imaging of ventilation distribution and gas uptake in the human lung using hyperpolarized xenon-129

Despite a myriad of technical advances in medical imaging, as well as the growing need to address the global impact of pulmonary diseases, such as asthma and chronic obstructive pulmonary disease, on health and quality of life, it remains challenging to obtain in vivo regional depiction and quantification of the most basic physiological functions of the lung-gas delivery to the airspaces and gas uptake by the lung parenchyma and blood-in a manner suitable for routine application in humans. We report a method based on MRI of hyperpolarized xenon-129 that permits simultaneous observation of the 3D distributions of ventilation (gas delivery) and gas uptake, as well as quantification of regional gas uptake based on the associated ventilation. Subjects with lung disease showed variations in gas uptake that differed from those in ventilation in many regions, suggesting that gas uptake as measured by this technique reflects such features as underlying pathological alterations of lung tissue or of local blood flow. Furthermore, the ratio of the signal associated with gas uptake to that associated with ventilation was substantially altered in subjects with lung disease compared with healthy subjects. This MRI-based method provides a way to quantify relationships among gas delivery, exchange, and transport, and appears to have significant potential to provide more insight into lung disease.

Feature analysis of hyperpolarized helium-3 pulmonary MRI: a study of asthmatics versus nonasthmatics

A computational framework is described that was developed for quantitative analysis of hyperpolarized helium-3 MR lung ventilation image data. This computational framework was applied to a study consisting of 55 subjects (47 asthmatic and eight normal). Each subject was imaged before and after respiratory challenge and also underwent spirometry. Approximately 1600 image features were calculated from the lungs in each image. Both the image and 27 spirometric features were ranked based on their ability to characterize clinical diagnosis using a mutual information-based feature subset selection algorithm. It was found that the top image features perform much better compared with the current clinical gold-standard spirometric values when considered individually. Interestingly, it was also found that spirometric values are relatively orthogonal to these image feature values in terms of informational content.

Pulsed-gradient spin-echo monitoring of restricted diffusion in multilayered structures

A general mathematical basis is developed for computation of the pulsed-gradient spin-echo signal attenuated due to restricted diffusion in multilayered structures (e.g., multiple slabs, cylindrical or spherical shells). Individual layers are characterized by (different) diffusion coefficients and relaxation times, while boundaries between adjacent layers are characterized by (different) permeabilities. Arbitrary temporal profile of the applied magnetic field can be incorporated. The signal is represented in a compact matrix form and the explicit analytical formulas for the elements of the underlying matrices are derived. The implemented algorithm is faster and much more accurate than classical techniques such as Monte Carlo simulations or numerical resolutions of the Bloch-Torrey equation. The algorithm can be applied for studying restricted diffusion in biological systems which exhibit a multilayered structure such as composite tissues, axons and living cells.

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.

Ventilation defects observed with hyperpolarized 3He magnetic resonance imaging in a mouse model of acute lung injury

Regions of diminished ventilation are often evident during functional pulmonary imaging studies, including hyperpolarized gas magnetic resonance imaging (MRI), positron emission tomography, and computed tomography (CT). The objective of this study was to characterize the hypointense regions observed via (3)He MRI in a murine model of acute lung injury. LPS at doses ranging from 15-50 μg was intratracheally administered to C57BL/6 mice under anesthesia. Four hours after exposure to either LPS or saline vehicle, mice were imaged via hyperpolarized (3)He MRI. All images were evaluated to identify regions of hypointense signals. Lungs were then characterized by conventional histology, or used to obtain tissue samples from regions of normal and hypointense (3)He signals and analyzed for cytokine content. The characterization of (3)He MRI images identified three distinct types of hypointense patterns: persistent defects, atelectatic defects, and dorsal lucencies. Persistent defects were associated with the administration of LPS. The number of persistent defects depended on the dose of LPS, with a significant increase in mean number of defects in 30-50-μg LPS-dosed mice versus saline-treated control mice. Atelectatic defects predominated in LPS-dosed mice under conditions of low-volume ventilation, and could be reversed with deep inspiration. Dorsal lucencies were present in nearly all mice studied, regardless of the experimental conditions, including control animals that did not receive LPS. A comparison of (3)He MRI with histopathology did not identify tissue abnormalities in regions of low (3)He signal, with the exception of a single region of atelectasis in one mouse. Furthermore, no statistically significant differences were evident in concentrations of IL-1β, IL-6, macrophage inflammatory protein (MIP)-1α, MIP-2, chemokine (C-X-C motif) ligand 1 (KC), TNFα, and monocyte chemotactic protein (MCP)-1 between hypointense and normally ventilated lung regions in LPS-dosed mice. Thus, this study defines the anatomic, functional, and biochemical characteristics of ventilation defects associated with the administration of LPS in a murine model of acute lung injury.

Experimental investigation and numerical simulation of 3He gas diffusion in simple geometries: implications for analytical models of 3He MR lung morphometry

Models of lung acinar geometry have been proposed to analytically describe the diffusion of (3)He in the lung (as measured with pulsed gradient spin echo (PGSE) methods) as a possible means of characterizing lung microstructure from measurement of the (3)He ADC. In this work, major limitations in these analytical models are highlighted in simple diffusion weighted experiments with (3)He in cylindrical models of known geometry. The findings are substantiated with numerical simulations based on the same geometry using finite difference representation of the Bloch-Torrey equation. The validity of the existing "cylinder model" is discussed in terms of the physical diffusion regimes experienced and the basic reliance of the cylinder model and other ADC-based approaches on a Gaussian diffusion behaviour is highlighted. The results presented here demonstrate that physical assumptions of the cylinder model are not valid for large diffusion gradient strengths (above approximately 15 mT/m), which are commonly used for (3)He ADC measurements in human lungs.

4DCT-based measurement of changes in pulmonary function following a course of radiation therapy

PURPOSE

Radiation therapy (RT) for lung cancer is commonly limited to subtherapeutic doses due to unintended toxicity to normal lung tissue. Reducing the frequency of occurrence and magnitude of normal lung function loss may benefit from treatment plans that incorporate the regional lung and radiation dose information. In this article, the authors propose a method that quantitatively measures the regional changes in lung tissue function following a course of radiation therapy by using 4DCT and image registration techniques.

METHODS

4DCT data sets before and after RT from two subjects are used in this study. Nonlinear 3D image registration is applied to register an image acquired near end inspiration to an image acquired near end expiration to estimate the pulmonary function. The Jacobian of the image registration transformation, indicating local lung expansion or contraction, serves as an index of regional pulmonary function. Approximately 120 annotated vascular bifurcation points are used as landmarks to evaluate registration accuracy. The authors compare regional pulmonary function before and after RT to the planned radiation dose at different locations of the lung.

RESULTS

In all registration pairs, the average landmark distances after registration are on the order of 1 mm. The pulmonary function change as indicated by the Jacobian change ranges from -0.15 to 0.1 in the contralateral lung and -0.22 to 0.23 in the ipsilateral lung for subject A, and ranges from -0.4 to 0.39 in the contralateral lung and -0.25 to 0.5 in the ipsilateral lung for subject B. Both of the subjects show larger range of the increase in the pulmonary function in the ipsilateral lung than the contralateral lung. For lung tissue regions receiving a radiation dose larger than 24 Gy, a decrease in pulmonary function was observed. For regions receiving a radiation dose smaller than 24 Gy, either an increase or a decrease in pulmonary function was observed. The relationship between the pulmonary function change and the radiation dose varies at different locations.

CONCLUSIONS

With the use of 4DCT and image registration techniques, the pulmonary function prior to and following a course of radiation therapy can be measured. In the preliminary application of this approach for two subjects, changes in pulmonary function were observed with a weak correlation between the dose and pulmonary function change. In certain sections of the lung, detected locally compromised pulmonary function may have resulted from radiation injury.

Design and evaluation of a 32-channel phased-array coil for lung imaging with hyperpolarized 3-helium

Imaging with hyperpolarized 3-helium is becoming an increasingly important technique for MRI diagnostics of the lung but is hampered by long breath holds (>20 sec), which are not always applicable in patients with severe lung disease like chronic obstructive pulmonary disease (COPD) or alpha-1-anti-trypsin deficiency. Additionally, oxygen-induced depolarization decay during the long breath holds complicates interpretation of functional data such as apparent diffusion coefficients. To address these issues, we describe and validate a 1.5-T, 32-channel array coil for accelerated (3)He lung imaging and demonstrate its ability to speed up imaging (3)He. A signal-to-noise ratio increase of up to a factor of 17 was observed compared to a conventional double-resonant birdcage for unaccelerated imaging, potentially allowing increased image resolution or decreased gas production requirements. Accelerated imaging of the whole lung with one-dimensional and two-dimensional acceleration factors of 4 and 4 x 2, respectively, was achieved while still retaining excellent image quality. Finally, the potential of highly parallel detection in lung imaging is demonstrated with high-resolution morphologic and functional images.

Imaging of the distal airways

Imaging techniques of the lung continue to advance, with improving ability to image the more distal airways. Two imaging techniques are reviewed: computed tomography and magnetic resonance with hyperpolarized helium-3.

A mathematical model of pulmonary gas exchange under inflammatory stress

During a severe local or systemic inflammatory response, immune mediators target lung tissue. This process may lead to acute lung injury and impaired diffusion of gas molecules. Although several mathematical models of gas exchange have been described, none simulate acute lung injury following inflammatory stress. In view of recent laboratory and clinical progress in the understanding of the pathophysiology of acute lung injury, such a mathematical model would be useful. We first derived a partial differential equations model of gas exchange on a small physiological unit of the lung ( approximately 25 alveoli), which we refer to as a respiratory unit (RU). We next developed a simple model of the acute inflammatory response and implemented its effects within a RU, creating a single RU model. Linking multiple RUs with various ventilation/perfusion ratios and taking into account pulmonary venous blood remixing yielded our lung-scale model. Using the lung-scale model, we explored the predicted effects of inflammation on ventilation/perfusion distribution and the resulting pulmonary venous partial pressure oxygen level during systemic inflammatory stresses. This model represents a first step towards the development of anatomically faithful models of gas exchange and ventilation under a broad range of local and systemic inflammatory stimuli resulting in acute lung injury, such as infection and mechanical strain of lung tissue.