MR perfusion imaging using encapsulated laser-polarized 3He

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Magnetic resonance elastography of the lungs: A repeatability and reproducibility study

Lung diseases are one of the leading causes of death worldwide, from which four million people die annually. Lung diseases are associated with changes in the mechanical properties of the lungs. Several studies have shown the feasibility of using magnetic resonance elastography (MRE) to quantify the lungs' shear stiffness. The aim of this study is to investigate the reproducibility and repeatability of lung MRE, and its shear stiffness measurements, obtained using a modified spin echo-echo planar imaging (SE-EPI) MRE sequence. In this study, 21 healthy volunteers were scanned twice by repositioning the volunteers to image right lung both at residual volume (RV) and total lung capacity (TLC) to assess the reproducibility of lung shear stiffness measurements. Additionally, 19 out of the 21 volunteers were scanned immediately without moving the volunteers to test the repeatability of the modified SE-EPI MRE sequence. A paired t-test was performed to determine the significant difference between stiffness measurements obtained at RV and TLC. Concordance correlation and Bland-Altman's analysis were performed to determine the reproducibility and repeatability of the SE-EPI MRE-derived shear stiffness measurements. The SE-EPI MRE sequence is highly repeatable with a concordance correlation coefficient (CCC) of 0.95 at RV and 0.96 at TLC. Similarly, the stiffness measurements obtained across all volunteers were highly reproducible with a CCC of 0.95 at RV and 0.92 at TLC. The mean shear stiffness of the lung at RV was 0.93 ± 0.22 kPa and at TLC was 1.41 ± 0.41 kPa. TLC showed a significantly higher mean shear stiffness (P = 0.0004) compared with RV. Lung MRE stiffness measurements obtained using the SE-EPI sequence were reproducible and repeatable, both at RV and TLC. Lung shear stiffness changes across respiratory cycle with significantly higher stiffness at TLC than RV.

Biomedical imaging with hyperpolarized noble gases

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

Agents for polarization enhancement in MRI

The intrinsic low sensitivity of the NMR phenomenon can be overcome thanks to hyperpolarization procedures that break the limits of the Boltzmann equilibrium and may increase the NMR signal by a factor of 10(5). Hyperpolarization procedures have been applied to enhance the signal from noble gases, such as 3He and 129Xe, and small 13C-containing molecules. For the latter class, attention has been focused on the use of methods based on dynamic nuclear polarization (DNP) and para-hydrogen induced polarization (PHIP). After discussion of the basics of the methods, an overview of the main applications with 13C-containing molecules is presented. This includes pre-clinical MR investigations of vascular imaging, perfusion and catheter tracking as well as molecular imaging protocols that allow the development of highly innovative studies in the field of metabolic imaging.

Lung MRI for experimental drug research

Current techniques to evaluate the efficacy of potential treatments for airways diseases in preclinical models are generally invasive and terminal. In the past few years, the flexibility of magnetic resonance imaging (MRI) to obtain anatomical and functional information of the lung has been explored with the scope of developing a non-invasive approach for the routine testing of drugs in models of airways diseases in small rodents. With MRI, the disease progression can be followed in the same animal. Thus, a significant reduction in the number of animals used for experimentation is achieved, as well as minimal interference with their well-being and physiological status. In addition, under certain circumstances the duration of the observation period after disease onset can be shortened since the technique is able to detect changes before these are reflected in parameters of inflammation determined using invasive procedures. The objective of this article is to briefly address MRI techniques that are being used in experimental lung research, with special emphasis on applications. Following an introduction on proton techniques and MRI of hyperpolarized gases, the attention is shifted to the MRI analysis of several aspects of lung disease models, including inflammation, ventilation, emphysema, fibrosis and sensory nerve activation. The next subject concerns the use of MRI in pharmacological studies within the context of experimental lung research. A final discussion points towards advantages and limitations of MRI in this area.

In vivo MR imaging of pulmonary arteries of normal and experimental emboli in small animals

PURPOSE

To demonstrate the feasibility of pulmonary MRA in living rodents.

MATERIALS AND METHODS

A three-dimensional (3D) gradient echo sequence was adapted to perform a time-of-flight (TOF) angiography of rat lung. Angiogram with a spatial resolution of 195 x 228 x 228 microm(3) was acquired in around 33 minutes. The method was then applied in animals before and after pulmonary embolism (PE) induction. Section of the proximal right pulmonary artery was measured and compared between the two populations.

RESULTS

Good quality images were obtained with a contrast-to-noise ratio (CNR) of 9 +/- 3 in the proximal part of the pulmonary artery. Cross-section areas of the right main artery are statistically different before (3.45 +/- 0.69 mm(2)) and after induction of PE (4.3 +/- 0.86 mm(2)).

CONCLUSION

This noninvasive tool permits angiogram acquisition at around 200 microm spatial resolution and objective distinction between healthy and embolized arteries.

Indirect detection of lung perfusion using susceptibility-based hyperpolarized gas imaging

PURPOSE

To address the problem of inadequate signal-to-noise ratio (SNR) encountered in lung perfusion magnetic resonance imaging (MRI) by developing an indirect detection based on the strong hyperpolarized (HP) gas signal.

MATERIALS AND METHODS

Our model is based on detecting the effects of gadolinium (Gd) flowing through lung capillaries by recording the phase of the nearby alveolar HP gas. In a HP gas 3He phantom we imaged gas phases before and after removing tubes containing paramagnetic solution away from the phantom. We also imaged HP gas phases in pig lungs before and after injection of Gd. Finally, parenchymal spin phase in excised lungs was measured as a function of Gd concentration.

RESULTS

In the phantom, the differential phase map displayed a pattern characteristic of a susceptibility-induced dipole field, showing the possibility of an indirect detection. In vivo, the differential phase map showed homogeneous appearance, as expected for uniform perfusion in healthy lungs. Ex vivo, the parenchymal spin phases were shown to depend linearly on Gd concentration.

CONCLUSION

Our method should allow indirect perfusion (Q) and direct ventilation (V) to be assessed simultaneously, thus allowing for diagnosis of V/Q mismatches. The linear dependency of parenchymal spin phase vs. Gd concentration may allow for quantification of lung perfusion.

Perfusion assessment with bolus differentiation: A technique applicable to hyperpolarized tracers

A new technique for assessing tissue blood flow using hyperpolarized tracers, based on the fact that the magnetization of a hyperpolarized substance can be destroyed permanently, is described. Assessments of blood flow with this technique are inherently insensitive to arterial delay and dispersion, and allow for quantification of the transit time and dispersion in the arteries that supply the investigated tissue. Renal cortical blood flow was studied in six rabbits using a 13C-labeled compound (2-hydroxyethylacrylate) that was polarized by the parahydrogen-induced polarization (PHIP) technique. The renal cortical blood flow was estimated to be 5.7/5.4 +/- 1.6/1.3 ml/min per milliliter of tissue (mean +/- SD, right/left kidney), and the mean transit time and dispersion in the renal arteries were determined to be 1.47/1.42 +/- 0.07/0.07 s and 1.78/1.93 +/- 0.40/0.42 s2, respectively.

Hyperpolarized xenon in NMR and MRI

Hyperpolarized gases have found a steadily increasing range of applications in nuclear magnetic resonance (NMR) and NMR imaging (MRI). They can be regarded as a new class of MR contrast agent or as a way of greatly enhancing the temporal resolution of the measurement of processes relevant to areas as diverse as materials science and biomedicine. We concentrate on the properties and applications of hyperpolarized xenon. This review discusses the physics of producing hyperpolarization, the NMR-relevant properties of 129Xe, specific MRI methods for hyperpolarized gases, applications of xenon to biology and medicine, polarization transfer to other nuclear species and low-field imaging.

Cerebral perfusion assessment by bolus tracking using hyperpolarized13C

Cerebral perfusion was assessed with 13C MRI in a rat model after intravenous injections of the 13C-labeled compound bis-1,1-(hydroxymethyl)-1-13C-cyclopropane-D8 in aqueous solutions hyperpolarized by dynamic nuclear polarization (DNP). Since the tracer acted as a direct signal source, several of the problems associated with techniques based on traditional dynamic susceptibility contrast (DSC) MRI contrast agents were avoided. Maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) were calculated. The MTT was determined to be 2.8 +/- 0.8 sec. However, arterial partial-volume effects in the animal model prevented accurate absolute quantification of CBF and CBV. It was demonstrated that depolarization of the hyperpolarized 13C tracer via relaxation and the imaging sequence had little influence on CBF assessment when the time resolution of the imaging sequence was short compared to the MTT. However, CBV and MTT were increasingly underestimated as MTT or the depolarization rate increased if depolarization was not taken into account. With a modified bolus-tracking theory depolarization could be compensated for, assuming that the depolarization rate was known. Three separate compensation methods were investigated experimentally and by numerical simulations.

Hyperpolarized 13C MR angiography using trueFISP

A (13)C-enriched water-soluble compound (bis-1,1-(hydroxymethyl)-1-(13)C-cyclopropane-D(8)), with a (13)C-concentration of approximately 200 mM, was hyperpolarized to approximately 15% using dynamic nuclear polarization, and then used as a contrast medium (CM) for contrast-enhanced magnetic resonance angiography (CE-MRA). The long relaxation times (in vitro: T(1) approximately 82 s, T(2) approximately 18 s; in vivo: T(1) approximately 38 s, T(2) approximately 1.3 s) are ideal for steady-state free precession (SSFP) imaging with a true fast imaging and steady precession (trueFISP) pulse sequence. It was shown both theoretically and experimentally that the optimal flip angle was 180 degrees. CE-MRA was performed in four anesthetized live rats after intravenous injection of 3 ml CM. The angiograms covered the thoracic/abdominal region in two of the animals, and the head-neck region in the other two. Fifteen consecutive images were acquired in each experiment, with a flip-back pulse at the end of each image acquisition. In the angiograms, the vena cava (SNR approximately 240), aorta, renal arteries, carotid arteries (SNR approximately 75), jugular veins, and several other vessels were visible. The SNR in the cardiac region was 500. Magnetization was preserved from one image acquisition to the next using the flip-back technique (SNR(cardiac) approximately 10 in the 15th image).

Contrast-enhanced magnetic resonance angiography: development and optimization of techniques for paramagnetic and hyperpolarized contrast media

Contrast-enhanced magnetic resonance angiography (CE-MRA) is a diagnostic method for imaging of vascular structures based on nuclear magnetic resonance. Vascular enhancement is achieved by injection of a contrast medium (CM). Studies were performed using two different types of CM: conventional paramagnetic CM, and a new type of CM based on hyperpolarized (HP) nuclei. The effects of varying CM concentration with time during image acquisition were studied by means of computer simulations using two different models. It was shown that a rapid concentration variation during encoding of the central parts of k-space could result in signal loss and severe image artifacts. The results were confirmed qualitatively with phantom experiments. A postprocessing method was developed to address problems with simultaneous enhancement of arteries and veins in CE-MRA of the lower extremities. The method was based on the difference in flow-induced phase in the two vessel types. Evaluation of the method was performed with flow phantom measurements and with CE-MRA in two volunteers using standard pulse sequences. The flow-induced phase in the vessels of interest was sufficient to distinguish arteries from veins in the superior-inferior direction. Using this method, the venous enhancement could be extinguished. The possibility of using HP nuclei as CM for CE-MRA was evaluated. Signal expressions for a flow of HP CM imaged with a gradient echo sequence were derived. These signal expressions were confirmed in phantom experiments using HP 129Xe dissolved in ethanol. Studies were also performed with a new CM based on HP 13C. The CM had very long relaxation times (T1, in vivo/T2, in vivo approximately 38/1.3 s). The long relaxation times were utilized in imaging with a fully balanced steady-state free precession pulse sequence (trueFISP), where the optimal flip angle was found to be 180 degrees. CE-MRA with the 13C-based CM in rats resulted in images with high vascular SNR (approximately 500). CE-MRA is a useful clinical tool for diagnosing vascular disease. With the development of new contrast media, based on hyperpolarized nuclei for example, there is a potential for further improvement in the signal levels that can be achieved, enabling a standard of imaging of vessels that is not possible today.

Recent advances in magnetic resonance perfusion imaging of the lung

Magnetic resonance imaging has been relatively underused for clinical application in the lung; however, developments in magnetic resonance perfusion imaging using contrast agents and spin labeling techniques have shown significant potential for clinical application in lung perfusion. This article reviews the recent publications on magnetic resonance pulmonary perfusion.

Gradient echo imaging of flowing hyperpolarized nuclei: theory and phantom studies on 129Xe dissolved in ethanol

The influence of flip angle and flow velocity on the signal intensity achieved when imaging a hyperpolarized substance with a spoiled gradient echo sequence was investigated. The study was performed both theoretically and experimentally using hyperpolarized xenon dissolved in ethanol. Analytical expressions regarding the optimal flip angle with respect to signal and the corresponding signal level are presented and comparisons with thermally polarized substances are made. Both experimentally and theoretically, the optimal flip angle was found to increase with increasing flow velocity. Numerical calculations showed that the velocity dependence of the signal differs between the cases of hyperpolarized and thermally polarized substances.

MRI of the lungs using hyperpolarized noble gases

The nuclear spin polarization of the noble gas isotopes (3)He and (129)Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments.

Vascular and perfusion imaging using encapsulated laser-polarized helium

In this work, the use of hyperpolarized (HP) 3He for in vivo intravascular imaging on animal is reported. To overcome the problem of the low solubility of helium in blood, we propose an approach based on helium encapsulation in lipid-based carrier agents. The mean diameter of the 3He microbubbles, measured equal to 3.0+/-0.2 microm, makes it possible to conduct in vivo studies. In vitro spectroscopy yielded a longitudinal relaxation time T(1) equal to 90 s and an apparent transverse relaxation time T(2)(*) of 4.5 ms. Angiographic imaging (venous and cardiac cavity visualization), as well as lung perfusion imaging, were demonstrated in rats using intravenous injections of microbubble suspensions. Suitable signal and spatial resolution were achieved. The potential of this technique for lung perfusion assessment was assessed using an experimental animal embolism model. Lung perfusion defects and recovery towards a normal perfusion state were visualized. This study was completed with the demonstration of a new ventilation-perfusion lung exploration method based entirely on HP 3He.