Off-resonance saturation MRI of superparamagnetic nanoprobes: theoretical models and experimental validations

Bioresponsive Nanomaterials: Recent Advances in Cancer Multimodal Imaging and Imaging-Guided Therapy

Cancer is a serious health problem which increasingly causes morbidity and mortality worldwide. It causes abnormal and uncontrolled cell division. Traditional cancer treatments include surgery, chemotherapy, radiotherapy and so on. These traditional therapies suffer from high toxicity and arouse safety concern in normal area and have difficulty in accurately targeting tumour. Recently, a variety of nanomaterials could be used for cancer diagnosis and therapy. Nanomaterials have several advantages, e.g., high concentration in tumour via targeting design, reduced toxicity in normal area and controlled drug release after various rational designs. They can combine with many types of biomaterials in order to improve biocompatibility. In this review, we outlined the latest research on the use of bioresponsive nanomaterials for various cancer imaging modalities (magnetic resonance imaging, positron emission tomography and phototacoustic imaging) and imaging-guided therapy means (chemotherapy, radiotherapy, photothermal therapy and photodynamic therapy), followed by discussing the challenges and future perspectives of this bioresponsive nanomaterials in biomedicine.

A magnetism/laser-auxiliary cascaded drug delivery to pulmonary carcinoma

Although high-efficiency targeted delivery is investigated for years, the efficiency of tumor targeting seems still a hard core to smash. To overcome this problem, we design a three-step delivery strategy based on streptavidin–biotin interaction with the help of c(RGDfK), magnetic fields and lasers. The ultrasmall superparamagnetic iron oxide nanoparticles (USIONPs) modified with c(RGDfK) and biotin are delivered at step 1, followed by streptavidin and the doxorubicin (Dox) loaded nanosystems conjugated with biotin at steps 2 and 3, respectively. The delivery systems were proved to be efficient on A549 cells. The co-localization of signal for each step revealed the targeting mechanism. The external magnetic field could further amplify the endocytosis of USPIONs based on c(RGDfK), and magnify the uptake distinctions among different test groups. Based on photoacoustic imaging, laser-heating treatment could enhance the permeability of tumor venous blood vessels and change the insufficient blood flow in cancer. Then, it was noticed in vivo that only three-step delivery with laser-heating and magnetic fields realized the highest tumor distribution of nanosystem. Finally, the magnetism/laser-auxiliary cascaded delivery exhibited the best antitumor efficacy. Generally, this study demonstrated the necessity of combining physical, biological and chemical means of targeting.

Theoretical analysis and optimization of ultrashort echo time (UTE) imaging contrast with off-resonance saturation

PURPOSE

Off-resonance saturation (ORS) is a tool which can be used in ultrashort echo time (UTE) magnetic resonance imaging to selectively reduce short T2 signals. When these ORS prepared UTE images are subtracted from a non-suppressed UTE acquisition, the short T2 signals are highlighted. The aim of this paper is to develop a theoretical ORS model and optimize short T2 contrast.

THEORY

Using a theoretical model the sequence parameters such as saturation flip angle and off-resonance frequency were optimized to maximize short T2 contrast. Bloch simulations were performed to demonstrate the accuracy of the theoretical model.

METHODS

Volunteer imaging was performed on the knee using different saturation flip angles and off-resonance frequencies using a Fermi RF pulse with a 3D UTE Cones acquisition.

RESULTS

The off-resonance saturation method showed good long T2 suppression, and highlighted short T2 signals such as the patella tendon. The theoretical signal curves generally agreed with simulated and experimentally measured signals.

CONCLUSION

Off-resonance saturation 3D UTE imaging can be used to effectively suppress long T2 signals and highlight short T2 signals. Theoretical modeling can be used to optimize sequence parameters to maximize long T2 suppression and short T2 contrast. Experimental results confirmed the theoretical predictions.

Improvement of the Off-Resonance Saturation, an MRI sequence for positive contrast with SPM particles: Theoretical and experimental study

The SuperParaMagnetic particles (SPM particles) are used as contrast agents in MRI and produce negative contrast with conventional T2 or T2(∗)-weighted sequences. Unfortunately, the SPM particle detection on images acquired with such sequences is sometimes difficult because negative contrast can be created by artifacts such as air bubbles or calcification. To overcome this problem, new sequences as Off-Resonance Saturation (ORS) were developed to produce positive contrast with SPM particles. This work explores a new way to optimize the contrast generated by the ORS sequence by increasing the number of saturation pulses applied before the imaging sequence. This modified sequence is studied with numerical simulations and experiments on agarose gel phantoms. A theoretical model able to predict the contrast for different values of the sequence parameters is also developed. The results show that the contrast increases with the saturation pulses number with an optimal value of three saturation pulses in order to avoid artifacts and limit the Specific Absorption Rate (SAR) effect. The dependence of the contrast on the SPM particle concentration and sequence parameters is comparable to what was observed for the ORS sequence.

Recent Advances in 19Fluorine Magnetic Resonance Imaging with Perfluorocarbon Emulsions

The research roots of ¹⁹fluorine (¹⁹F) magnetic resonance imaging (MRI) date back over 35 years. Over that time span, ¹H imaging flourished and was adopted worldwide with an endless array of applications and imaging approaches, making magnetic resonance an indispensable pillar of biomedical diagnostic imaging. For many years during this timeframe, ¹⁹F imaging research continued at a slow pace as the various attributes of the technique were explored. However, over the last decade and particularly the last several years, the pace and clinical relevance of ¹⁹F imaging has exploded. In part, this is due to advances in MRI instrumentation, ¹⁹F/¹H coil designs, and ultrafast pulse sequence development for both preclinical and clinical scanners. These achievements, coupled with interest in the molecular imaging of anatomy and physiology, and combined with a cadre of innovative agents, have brought the concept of ¹⁹F into early clinical evaluation. In this review, we attempt to provide a slice of this rich history of research and development, with a particular focus on liquid perfluorocarbon compound-based agents.

Bottom-up study of the MRI positive contrast created by the Off-Resonance Saturation sequence

Superparamagnetic iron oxide nanoparticles (SPM particles) are used in MRI to highlight regions such as tumors through negative contrast. Unfortunately, sources as air bubbles or tissues interfaces also lead to negative contrast, which complicates the image interpretation. New MRI sequences creating positive contrast in the particle surrounding, such as the Off-Resonance Saturation sequence (ORS), have thus been developed. However, a theoretical study of the ORS sequence is still lacking, which hampers the optimization of this sequence. For this reason, this work provides a self-consistent analytical expression able to predict the dependence of the contrast on the sequence parameters and the SPM particles properties. This expression was validated by numerical simulations and experiments on agarose gel phantoms on a 11.7 T scanner system. It provides a fundamental understanding of the mechanisms leading to positive contrast, which could allow the improvement of the sequence for future in vivo applications. The influence of the SPM particle relaxivities, the SPM particle concentration, the echo time and the saturation pulse parameters on the contrast were investigated. The best contrast was achieved with SPM particles possessing the smallest transverse relaxivity, an optimal particle concentration and for low echo times.

Nanostructured magnetic nanocomposites as MRI contrast agents

Magnetic resonance imaging (MRI) has become an integral part of modern clinical imaging due to its non-invasiveness and versatility in providing tissue and organ images with high spatial resolution. With the current MRI advancement, MRI imaging probes with suitable biocompatibility, good colloidal stability, enhanced relaxometric properties and advanced functionalities are highly demanded. As such, MRI contrast agents (CAs) have been an extensive research and development area. In the recent years, different inorganic-based nanoprobes comprising inorganic magnetic nanoparticles (MNPs) with an organic functional coating have been engineered to obtain a suitable contrast enhancement effect. For biomedical applications, the organic functional coating is critical to improve colloidal stability and biocompatibility. Simultaneously, it also provides a building block for generating a higher dimensional secondary structure. In this review, the combinatorial design approach by a self-assembling pre-formed hydrophobic inorganic MNPs core (from non-polar thermolysis synthesis) into various functional organic coatings (e.g. ligands, amphiphilic polymers and graphene oxide) to form water soluble nanocomposites will be discussed. The resultant magnetic ensembles were classified based on their dimensionality, namely, 0-D, 1-D, 2-D and 3-D structures. This classification provides further insight into their subsequent potential use as MRI CAs. Special attention will be dedicated towards the correlation between the spatial distribution and the associated MRI applications, which include (i) coating optimization-induced MR relaxivity enhancement, (ii) aggregation-induced MR relaxivity enhancement, (iii) off-resonance saturation imaging (ORS), (iv) magnetically-induced off-resonance imaging (ORI), (v) dual-modalities MR imaging and (vi) multifunctional nanoprobes.

Biomedical nanomaterials for imaging-guided cancer therapy

To date, even though various kinds of nanomaterials have been evaluated over the years in order to develop effective cancer therapy, there is still significant challenges in the improvement of the capabilities of nano-carriers. Developing a new theranostic nanomedicine platform for imaging-guided, visualized cancer therapy is currently a promising way to enhance therapeutic efficiency and reduce side effects. Firstly, conventional imaging technologies are reviewed with their advantages and disadvantages, respectively. Then, advanced biomedical materials for multimodal imaging are illustrated in detail, including representative examples for various dual-modalities and triple-modalities. Besides conventional cancer treatment (chemotherapy, radiotherapy), current biomaterials are also summarized for novel cancer therapy based on hyperthermia, photothermal, photodynamic effects, and clinical imaging-guided surgery. In conclusion, biomedical materials for imaging-guided therapy are becoming one of the mainstream treatments for cancer in the future. It is hoped that this review might provide new impetus to understand nanotechnology and nanomaterials employed for imaging-guided cancer therapy.

Principles and emerging applications of nanomagnetic materials in medicine

The development of nanometer-scale magnetic materials for biomedical applications spans the interface between the physical sciences and biology. Applications of these materials are rapidly becoming important in medicine and enable targeted therapies and diagnostics. At the same time, specific applications add focus to the development of novel magnetic materials and facilitate a deeper understanding of the physical mechanisms behind their function. This review presents a broad, nontechnical overview of the basis of magnetism in materials at the nanometer scale and describes how these materials are created, characterized, and used. Specific emerging applications in medical diagnostics and therapies are discussed, including cancer cell targeting for thermal ablation, tissue engineering, and three-dimensional noninvasive molecular imaging. Challenges in these fields are discussed, including the toxicity and delivery of magnetic nanomaterials and the sensitivity of imaging and therapeutic techniques. The development of novel nanomagnetic nanomaterials should continue to accelerate as new applications are identified and researchers uncover new mechanisms to increase and modulate magnetism at the nanometer scale.

Improving the magnetic resonance imaging contrast and detection methods with engineered magnetic nanoparticles

Engineering and functionalizing magnetic nanoparticles have been an area of the extensive research and development in the biomedical and nanomedicine fields. Because their biocompatibility and toxicity are well investigated and better understood, magnetic nanoparticles, especially iron oxide nanoparticles, are better suited materials as contrast agents for magnetic resonance imaging (MRI) and for image-directed delivery of therapeutics. Given tunable magnetic properties and various surface chemistries from the coating materials, most applications of engineered magnetic nanoparticles take advantages of their superb MRI contrast enhancing capability as well as surface functionalities. It has been found that MRI contrast enhancement by magnetic nanoparticles is highly dependent on the composition, size and surface properties as well as the degree of aggregation of the nanoparticles. Therefore, understanding the relationships between these intrinsic parameters and the relaxivities that contribute to MRI contrast can lead to establishing essential guidance that may direct the design of engineered magnetic nanoparticles for theranostics applications. On the other hand, new contrast mechanism and imaging strategy can be developed based on the novel properties of engineered magnetic nanoparticles. This review will focus on discussing the recent findings on some chemical and physical properties of engineered magnetic nanoparticles affecting the relaxivities as well as the impact on MRI contrast. Furthermore, MRI methods for imaging magnetic nanoparticles including several newly developed MRI approaches aiming at improving the detection and quantification of the engineered magnetic nanoparticles are described.