Experimental application of thermosensitive paramagnetic liposomes for monitoring magnetic resonance imaging guided thermal ablation

Spatiotemporal modeling of nano-delivered chemotherapeutics for synergistic microwave ablation cancer therapy

BACKGROUND AND OBJECTIVE

The effectiveness of current microwave ablation (MWA) therapies is limited. Administration of thermosensitive liposomes (TSLs) which release drugs in response to heat has presented a significant potential for enhancing the efficacy of thermal ablation treatment, and the benefits of targeted drug delivery. However, a complete knowledge of the mechanobiological processes underlying the drug release process, especially the intravascular drug release mechanism and its distribution in response to MWA needs to be improved. Multiscale computational-based modeling frameworks, integrating different biophysical phenomena, have recently emerged as promising tools to decipher the mechanobiological events in combo therapies. The present study aims to develop a novel multiscale computational model of TSLs delivery following MWA implantation.

METHODS

Due to the complex interplay between the heating procedure and the drug concentration maps, a computational model is developed to determine the intravascular release of doxorubicin from TSL, its transvascular transport into the interstitium, transport in the interstitium, and cell uptake. Computational models can estimate the interplays among liposome and drug properties, tumor perfusion, and heating regimen to examine the impact of essential parameters and to optimize a targeted drug delivery platform.

RESULTS

Results indicated that the synergy of TSLs with MWA allows more localized drug delivery with lower side effects. The drug release rate and tumor permeability play crucial roles in the efficacy of TSLs during MWA treatment. The computational model predicted an unencapsulated drug lime around the ablated zone, which can destroy more cancer cells compared to MWA alone by 40%. Administration of TSLs with a high release rate capacity can improve the percentage of killed cancer cells by 24%. Since the heating duration in MWA is less than 15 min, the presented combination therapy showed better performance for highly permeable tumors.

CONCLUSION

This study highlights the potential of the proposed computational framework to address complex and realistic scenarios in cancer treatment, which can serve as the future research foundation, including advancements in nanomedicine and optimizing the pair of TSL and MWA for both preclinical and clinical studies. The present model could be as a valuable tool for patient-specific calibration of essential parameters.

Imaging-based internal body temperature measurements: The journal Temperature toolbox

Noninvasive imaging methods of internal body temperature are in high demand in both clinical medicine and physiological research. Thermography and thermometry can be used to assess tissue temperature during thermal therapies: ablative and hyperthermia treatments to ensure adequate temperature rise in target tissues but also to avoid collateral damage by heating healthy tissues. In research use, measurement of internal body temperature enables us the production of thermal maps on muscles, internal organs, and other tissues of interest. The most used methods for noninvasive imaging of internal body temperature are based on different parameters acquired with magnetic resonance imaging, ultrasound, computed tomography, microwave radiometry, photoacoustic imaging, and near-infrared spectroscopy. In the current review, we examine the aforementioned imaging methods, their use in estimating internal body temperature in vivo with their advantages and disadvantages, and the physical phenomena the thermography or thermometry modalities are based on.

Theranostics in Interventional Oncology: Versatile Carriers for Diagnosis and Targeted Image-Guided Minimally Invasive Procedures

We are continuously progressing in our understanding of cancer and other diseases and learned how they can be heterogeneous among patients. Therefore, there is an increasing need for accurate characterization of diseases at the molecular level. In parallel, medical imaging and image-guided therapies are rapidly developing fields with new interventions and procedures entering constantly in clinical practice. Theranostics, a relatively new branch of medicine, refers to procedures combining diagnosis and treatment, often based on patient and disease-specific features or molecular markers. Interventional oncology which is at the convergence point of diagnosis and treatment employs several methods related to theranostics to provide minimally invasive procedures tailored to the patient characteristics. The aim is to develop more personalized procedures able to identify cancer cells, selectively reach and treat them, and to assess drug delivery and uptake in real-time in order to perform adjustments in the treatment being delivered based on obtained procedure feedback and ultimately predict response. Here, we review several interventional oncology procedures referring to the field of theranostics, and describe innovative methods that are under development as well as future directions in the field.

Spatial and Temperature Resolutions of Magnetic Nanoparticle Temperature Imaging with a Scanning Magnetic Particle Spectrometer

This paper quantitatively investigates the spatial and temperature resolutions of magnetic nanoparticle (MNP) temperature imaging with a multiline phantom filled with MNPs. The multiline phantom in total consists of seven lines with different distances between two adjacent lines. A scanning magnetic particle spectrometer is used to measure the spatial distributions of the MNP harmonics for MNP concentration and temperature imaging, whereas an iterative deconvolution method is used to improve the spatial resolution. A modulation transfer function calculated from the MNP concentration image is used to quantitatively present the spatial resolution, whereas the standard deviation of the measured temperatures is used to quantitatively present the temperature resolution. The spatial resolution is about 4 mm while the temperature resolution is about 1.0 K without deconvolution. With increasing the number of the iterative loops in the deconvolution, the spatial resolution is improved to 2 mm while the temperature resolution is worsened to about 9.6 K due to deconvolution-based oscillation.

Ferromagnetic particles as magnetic resonance imaging temperature sensors

Magnetic resonance imaging is an important technique for identifying different types of tissues in a body or spatial information about composite materials. Because temperature is a fundamental parameter reflecting the biological status of the body and individual tissues, it would be helpful to have temperature maps superimposed on spatial maps. Here we show that small ferromagnetic particles with a strong temperature-dependent magnetization, can be used to produce temperature-dependent images in magnetic resonance imaging with an accuracy of about 1 °C. This technique, when further developed, could be used to identify inflammation or tumours, or to obtain spatial maps of temperature in various medical interventional procedures such as hyperthermia and thermal ablation. This method could also be used to determine temperature profiles inside nonmetallic composite materials.

Magnetic resonance imaging can distinguish between different tissue types by measuring the proton distribution in a living sample. Here, the authors demonstrate how this technique can be extended to provide temperature information by using ferromagnetic particles with temperature-dependent magnetization.

The release of Doxorubicin from liposomes monitored by MRI and triggered by a combination of US stimuli led to a complete tumor regression in a breast cancer mouse model

The work aimed at developing a novel MRI-based theranostic protocol for improving the anticancer efficacy of a Doxil-like liposomal formulation. The goal was achieved stimulating the intratumor release of the drug from the nanocarrier and favoring its diffusion in the lesion by the sequential application of low-intensity pulsed ultrasound. The protocol was tested on mice bearing a syngeneic breast cancer model. The combination of acoustic waves with different characteristics allowed for: i) the release of the drug and the co-encapsulated MRI agent (Gadoteridol) from the liposomes in the vessels of the tumor region, and ii) the extravasation of the released material, as well as intact liposomes, in the tumor stroma. The MR-T1 contrast enhancement measured in the tumor reported on the delivery and US-triggered release of Doxorubicin. The developed protocol resulted in a marked increase in the intratumor drug concentration that, in turn, led to the complete regression of the lesion. The protocol has a good clinical translatability because all the components of the theranostic agent (Doxorubicin, liposomes, Gadoteridol) are approved for human use.

Six-coordinate Iron(II) and Cobalt(II) paraSHIFT Agents for Measuring Temperature by Magnetic Resonance Spectroscopy

Paramagnetic Fe(II) and Co(II) complexes are utilized as the first transition metal examples of (1)H NMR shift agents (paraSHIFT) for thermometry applications using Magnetic Resonance Spectroscopy (MRS). The coordinating ligands consist of TACN (1,4,7-triazacyclononane) and CYCLEN (1,4,7,10-tetraazacyclododecane) azamacrocycles appended with 6-methyl-2-picolyl groups, denoted as MPT and TMPC, respectively. (1)H NMR spectra of the MPT- and TMPC-based Fe(II) and Co(II) complexes demonstrate narrow and highly shifted resonances that are dispersed as broadly as 440 ppm. The six-coordinate complex cations, [M(MPT)](2+) and [M(TMPC)](2+), vary from distorted octahedral to distorted trigonal prismatic geometries, respectively, and also demonstrate that 6-methyl-2-picolyl pendents control the rigidity of these complexes. Analyses of the (1)H NMR chemical shifts, integrated intensities, line widths, the distances obtained from X-ray diffraction measurements, and longitudinal relaxation time (T1) values allow for the partial assignment of proton resonances of the [M(MPT)](2+) complexes. Nine and six equivalent methyl protons of [M(MPT)](2+) and [M(TMPC)](2+), respectively, produce 3-fold higher (1)H NMR intensities compared to other paramagnetically shifted proton resonances. Among all four complexes, the methyl proton resonances of [Fe(TMPC)](2+) and [Co(TMPC)](2+) at -49.3 ppm and -113.7 ppm (37 °C) demonstrate the greatest temperature dependent coefficients (CT) of 0.23 ppm/°C and 0.52 ppm/°C, respectively. The methyl groups of these two complexes both produce normalized values of |CT|/fwhm = 0.30 °C(-1), where fwhm is full width at half-maximum (Hz) of proton resonances. The T1 values of the highly shifted methyl protons are in the range of 0.37-2.4 ms, allowing rapid acquisition of spectroscopic data. These complexes are kinetically inert over a wide range of pH values (5.6-8.6), as well as in the presence of serum albumin and biologically relevant cations and anions. The combination of large hyperfine shifts, large temperature sensitivity, increased signal-to-noise ratio, and short T1 values suggests that these complexes, in particular the TMPC-based complexes, show promise as paraSHIFT agents for thermometry.

Microgel-Based Thermosensitive MRI Contrast Agent

Monitoring subtle temperature changes noninvasively remains a challenge for magnetic resonance imaging (MRI). A temperature-sensitive contrast agent based on thermosensitive microgel is proposed and synthesized using a manganese tetra(3-vinylphenyl) porphyrin core reacting with N-isopropylacrylamide (NIPAM) or N-isopropylmethacrylamide (NIPMAM) monomers and N,N'-methylenebis(acrylamide) (MBA) cross-linkers. The volume of the NIPAM-incorporated microgel (M-1) decreased sharply around its lower critical solution temperature (LCST, 29-33 °C), whereas the volume of the NIPMAM-incorporated microgel (M-2) decreased gradually. MR longitudinal relaxivity (r₁) enhancement (44%) was obtained for M-1, while the corresponding change for M-2 was much smaller. M-1 was further optimized in synthesis without an MBA cross-linker to obtain M-3 which showed a 67% increase in r₁ around its LCST. Our results suggested that the longitudinal relaxivity is strongly modulated by microgel volume change around the LCST, leading to a significant increase in r₁. This novel thermally sensitive microgel could potentially be applied to monitor small temperature changes using MRI methods.

Magnetic resonance guided high-intensity focused ultrasound for image-guided temperature-induced drug delivery

Magnetic resonance guided high-intensity focused ultrasound (MR-HIFU) is a versatile technology platform for noninvasive thermal therapies in oncology. Since MR-HIFU allows heating of deep-seated tissue to well-defined temperatures under MR image guidance, this novel technology has great potential for local heat-mediated drug delivery from temperature-sensitive liposomes (TSLs). In particular, MR provides the ability for image guidance of the drug delivery when an MRI contrast agent is co-encapsulated with the drug in the aqueous lumen of the liposomes. Monitoring of the tumor drug coverage offers possibilities for a personalized thermal treatment in oncology. This review focuses on MR-HIFU as a noninvasive technology platform, temperature-sensitive liposomal formulations for drug delivery and image-guided drug delivery, and the effect of HIFU-induced hyperthermia on the TSL and drug distribution. Finally, the opportunities and challenges of localized MR-HIFU-mediated drug delivery from temperature-sensitive liposomes in oncology are discussed.

Computed tomography and magnetic resonance imaging

Imaging in Oncology is rapidly moving from the detection and size measurement of a lesion to the quantitative assessment of metabolic processes and cellular and molecular interactions. Increasing insights into cancer as a complex disease with involvement of the tumor stroma in tumor pathobiological processes have made it clear that for successful control of cancer, treatment strategies should not only be directed at the tumor cells but also targeted at the tumor microenvironment. This requires understanding of the complex molecular and cellular interactions in cancer tissue. Recent developments in imaging technology have increased the possibility to image various pathobiological processes in cancer development and response to treatment. For computed tomography (CT) and magnetic resonance imaging (MRI) various improvements in hardware, software, and imaging probes have lifted these modalities from classical anatomical imaging techniques to techniques suitable to image and quantify various physiological processes and molecular and cellular interactions. Next to a more general overview of possible imaging targets in oncology this chapter provides an overview of the various developments in CT and MRI technology and some specific applications.

Combination radiofrequency (RF) ablation and IV liposomal heat shock protein suppression: reduced tumor growth and increased animal endpoint survival in a small animal tumor model

BACKGROUND

To investigate the effect of IV liposomal quercetin (a known down-regulator of heat shock proteins) alone and with liposomal doxorubicin on tumor growth and end-point survival when combined with radiofrequency (RF) tumor ablation in a rat tumor model.

METHODS

Solitary subcutaneous R3230 mammary adenocarcinoma tumors (1.3-1.5 cm) were implanted in 48 female Fischer rats. Initially, 32 tumors (n=8, each group) were randomized into four experimental groups: (a) conventional monopolar RF alone (70°C for 5 min), (b) IV liposomal quercetin alone (1 mg/kg), (c) IV liposomal quercetin followed 24hr later with RF, and (d) no treatment. Next, 16 additional tumors were randomized into two groups (n=8, each) that received a combined RF and liposomal doxorubicin (15 min post-RF, 8 mg/kg) either with or without liposomal quercetin. Kaplan-Meier survival analysis was performed using a tumor diameter of 3.0 cm as the defined survival endpoint.

RESULTS

Differences in endpoint survival and tumor doubling time among the groups were highly significant (P<0.001). Endpoint survivals were 12.5±2.2 days for the control group, 16.6±2.9 days for tumors treated with RF alone, 15.5±2.1 days for tumors treated with liposomal quercetin alone, and 22.0±3.9 days with combined RF and quercetin. Additionally, combination quercetin/RF/doxorubicin therapy resulted in the longest survival (48.3±20.4 days), followed by RF/doxorubicin (29.9±3.8 days).

CONCLUSIONS

IV liposomal quercetin in combination with RF ablation reduces tumor growth rates and improves animal endpoint survival. Further increases in endpoint survival can be seen by adding an additional anti-tumor adjuvant agent liposomal doxorubicin. This suggests that targeting several post-ablation processes with multi-drug nanotherapies can increase overall ablation efficacy.

Radiofrequency ablation combined with liposomal quercetin to increase tumour destruction by modulation of heat shock protein production in a small animal model

PURPOSE

To investigate the effect of heat shock protein (HSP) modulation on tumour coagulation by combining radiofrequency (RF) ablation with adjuvant liposomal quercetin and/or doxorubicin in a rat tumour model.

METHODS

Sixty R3230 breast adenocarcinoma tumours/animals were used in this IACUC-approved study. Initially, 60 tumours (n=6, each subgroup) were randomised into five groups: (1) RF alone, (2) intravenous (IV) liposomal quercetin alone (1 mg/kg), (3) IV liposomal quercetin followed 24 h later with RF, (4) RF followed 15 min later by IV liposomal doxorubicin (8 mg/kg), (5) IV liposomal quercetin 24 h before RF followed by IV liposomal doxorubicin 15 min post-ablation. Animals were sacrificed 4 or 24 h post-treatment and gross coagulation diameters were compared. Next, immunohistochemistry staining was performed for Hsp70 and cleaved caspase-3 expression. Comparisons were performed by using Student t-tests or ANOVA.

RESULTS

Combination RF-quercetin significantly increased coagulation size compared with either RF or liposomal quercetin alone (13.1±0.7 mm vs. 8.8±1.2 mm or 2.3±1.3 mm, respectively, P<0.001 for all comparisons). Triple therapy (quercetin-RF-doxorubicin) showed larger coagulation diameter (14.5±1.0 mm) at 24 h than quercetin-RF (P=0.016) or RF-doxorubicin (13.2±1.3 mm, P=0.042). Combination quercetin-RF decreased Hsp70 expression compared with RF alone at both 4 h (percentage of stained cells/hpf 22.4±13.9% vs. 38.8±16.1%, P<0.03) and 24 h (45.2±10.5% vs. 81.1±3.6%, P<0.001). Quercetin-RF increased cleaved caspase-3 expression at both 4 h (percentage of stained cells/hpf 50.7±13.4% vs. 41.9±15.1%, P<0.03) and 24 h (37.4±7.8% vs. 33.2±6.5%, P=0.045); with, triple therapy (quercetin-RF-doxorubicin) resulting in the highest levels of apoptosis (45.1±10.7%) at 24 h. Similar trends were observed for rim thickness.

CONCLUSIONS

Suppression of HSP production using adjuvant liposomal quercetin can increase apoptosis and improve RF ablation-induced tumour destruction. Further increases in tumour coagulation can be seen including an additional anti-tumour adjuvant agent such as liposomal doxorubicin.

Formulation and characterisation of magnetic resonance imageable thermally sensitive liposomes for use with magnetic resonance-guided high intensity focused ultrasound

PURPOSE

Objectives of this study were to: 1) develop iLTSL, a low temperature sensitive liposome co-loaded with an MRI contrast agent (ProHance® Gd-HP-DO3A) and doxorubicin, 2) characterise doxorubicin and Gd-HP-DO3A release from iLTSL and 3) investigate the ability of magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) to induce and monitor iLTSL content release in phantoms and in vivo.

METHODS

iLTSL was passively loaded with Gd-HP-DO3A and actively loaded with doxorubicin. Doxorubicin and Gd-HP-DO3A release was quantified by fluorescence and spectroscopic techniques, respectively. Release with MR-HIFU was examined in tissue-mimicking phantoms containing iLTSL and in a VX2 rabbit tumour model.

RESULTS

iLTSL demonstrated consistent size and doxorubicin release kinetics after storage at 4°C for 7 days. Release of doxorubicin and Gd-HP-DO3A from iLTSL was minimal at 37°C but fast when heated to 41.3°C. The magnitude of release was not significantly different between doxorubicin and Gd-HP-DO3A over 10 min in HEPES buffer and plasma at 37°, 40° and 41.3°C (p > 0.05). Relaxivity of iLTSL increased significantly (p < 0.0001) from 1.95 ± 0.05 to 4.01 ± 0.1 mMs⁻¹ when heated above the transition temperature. Signal increase corresponded spatially and temporally to MR-HIFU-heated locations in phantoms. Signal increase was also observed in vivo after iLTSL injection and after each 10-min heating (41°C), with greatest increase in the heated tumour region.

CONCLUSION

An MR imageable liposome formulation co-loaded with doxorubicin and an MR contrast agent was developed. Stability, imageability, and MR-HIFU monitoring and control of content release suggest that MR-HIFU combined with iLTSL may enable real-time monitoring and spatial control of content release.

Multi-functional liposomes having temperature-triggered release and magnetic resonance imaging for tumor-specific chemotherapy

For development of tumor-specific chemotherapy, we designed liposomes with temperature-triggered drug release and magnetic resonance imaging (MRI) functions. We prepared multi-functional liposomes by incorporating thermosensitive poly(2-ethoxy(ethoxyethyl)vinyl ether) chains with a lower critical solution temperatures around 40 °C and polyamidoamine G3 dendron-based lipids having Gd(3+) chelate residues into pegylated liposomes. These stable doxorubicin (DOX)-loaded liposomes retained DOX in their interior below physiological temperature but released DOX immediately at temperatures greater than 40 °C. They exhibited excellent ability to shorten the longitudinal proton relaxation time. When administered intravenously into colon 26 tumor-bearing mice, accumulated liposomes in tumors increased with time, reaching a constant level 8 h after administration by following T(1)-weighted MRI signal intensity in tumors. Liposome size affected the liposome accumulation efficiency in tumors: liposomes of about 100 nm diameter were accumulated more efficiently than those with about 50 nm diameter. Tumor size also affected accumulation: more efficient accumulation occurred in larger tumors. Tumor growth was strongly suppressed when liposomes loaded with DOX were administered intravenously into tumor-bearing mice and the tumor was heated mildly at 44 °C for 10 min at 8 h after administration. Multi-functional liposomes having temperature-triggered drug release and MRI functions might engender personalized chemotherapy, providing efficient patient-optimized chemotherapy.

Focused ultrasound as a local therapy for liver cancer

Conventional surgical treatments of liver cancer are invasive (including minimally invasive) with a high incidence of new metastasis and poor success, even after multiple resections or ablations. These limitations motivated research into new, less invasive solutions for liver cancer treatment.Focused ultrasound surgery (FUS), or high-intensity focused ultrasound, has been recognized as a noninvasive technology for benign and malignant tumor treatment. Previously, FUS was guided with ultrasound that has limited target definition and monitoring capability of the ablation process. Combining magnetic resonance imaging (MRI) with multiple-element phased-array transducers to create MRI-guided focused ultrasound thermal therapy provides more accurate targeting and real-time temperature monitoring. This treatment is hindered by the ribcage that limits the acoustic windows to the liver and the respiratory motion of the liver. New advances in MRI and transducer design will likely resolve these limitations and make MRI-guided FUS a powerful tool in local liver cancer therapy. This article reviews this technology and advances that can expand its use for cancer treatment in general and liver cancer in particular.

Target molecular therapies: methods to enhance and monitor tumor drug delivery

Noninvasive monitoring/quantification of drug delivery to tumors is an ideal goal of chemotherapy treatment. However, the ability to overcome the barriers to the developing of targeted therapies-along with the physiological barriers that the tumor presents-is still needed. Recent advances demonstrate that targeted therapies can be used for diagnostic and therapeutic applications. The most mature of these technologies are liposomes that encapsulate a therapeutic drug in conjunction with a contrast agent. Through selective manipulation of the liposome composition, modification of the tumor microphysiology, and temporal sequencing of liposome administration with tumor microphysiology modification improvement in efficacy can be achieved. The future application of these targeted therapies will allow the radiologist to become a more central member of the cancer treatment team, further expanding the field and the radiologist's unique skills.

Non‐invasive thermometry for monitoring hepatic radiofrequency ablation

This paper reviews a selection of methods for non-invasive thermometry with special attention to limitations of possible relevance for hepatic radiofrequency ablation.

A new MRI method, tested in vitro for the assessment of thermal coagulation and demonstrated in vivo on focused ultrasound ablation

A new MRI method is described. It is implemented for the assessment of thermal coagulation after thermal treatment. The method does not require injection of a contrast agent. The image is based on the amount of magnetization exchange between macromolecules and water in the tissue. Coagulated tissue has a faster magnetization exchange rate, forming the new contrast. In this study, the new imaging method is shown to have good contrast between coagulated and untreated excised tissues of porcine liver and muscle. The method is also successfully demonstrated in vivo in rat thigh muscle thermally treated with focused ultrasound.

Magnetic resonance guidance for radiofrequency ablation of liver tumors

Image-guided thermal ablation of liver tumors is a minimally invasive treatment option. Techniques used for thermal ablation are radiofrequency (RF) ablation, laser interstitial thermotherapy (LITT), microwave (MW) ablation, high-intensity focused ultrasound (HIFU), and cryoablation. Among these techniques RF ablation attained widespread consideration. Image guidance should ensure a precise ablation therapy leading to a complete coagulation of tumor tissue without injury to critical structures. Therefore, the modality of image guidance has an important impact on the safety and efficacy of percutaneous RF ablation. The current literature regarding percutaneous RF ablation mainly describes the use of computed tomography (CT) and ultrasonography (US) guidance. In addition, interventional MR systems offer the possibility to utilize the advantages of MR imaging such as excellent soft-tissue contrast, multiplanar and interactive capabilities, and sensitivity to thermal effects during the entire RF ablation procedure. Monitoring of thermally induced coagulation by MR imaging is supportive to control the ablation procedure. MR imaging can be advantageously used to guide overlapping ablation if necessary as well as to define the endpoint of RF ablation after complete coverage of the target tissue is verified. Furthermore, monitoring of thermal effects is essential in order to prevent unintended thermal damage from critical structures surrounding the target region. Therefore, MR-guided RF ablation offers the possibility for a safe and effective therapy option in the treatment of primary and secondary hepatic malignancies. The article summarizes the role of MR guidance for RF ablation of liver tumors.

MR thermometry

Minimally invasive thermal therapy as local treatment of benign and malignant diseases has received increasing interest in recent years. Safety and efficacy of the treatment require accurate temperature measurement throughout the thermal procedure. Noninvasive temperature monitoring is feasible with magnetic resonance (MR) imaging based on temperature-sensitive MR parameters such as the proton resonance frequency (PRF), the diffusion coefficient (D), T1 and T2 relaxation times, magnetization transfer, the proton density, as well as temperature-sensitive contrast agents. In this article the principles of temperature measurements with these methods are reviewed and their usefulness for monitoring in vivo procedures is discussed. Whereas most measurements give a temperature change relative to a baseline condition, temperature-sensitive contrast agents and spectroscopic imaging can provide absolute temperature measurements. The excellent linearity and temperature dependence of the PRF and its near independence of tissue type have made PRF-based phase mapping methods the preferred choice for many in vivo applications. Accelerated MRI imaging techniques for real-time monitoring with the PRF method are discussed. Special attention is paid to acquisition and reconstruction methods for reducing temperature measurement artifacts introduced by tissue motion, which is often unavoidable during in vivo applications.

Polymer platforms for drug delivery and biomedical imaging

Biocompatible synthetic polymers have demonstrated advantageous pharmacokinetic properties as compared to small molecular agents. Incorporation of low molecular weight therapeutics and imaging agents into biocompatible polymers can optimize their pharmacokinetic properties with improved efficacy of therapy and diagnostic imaging, respectively. We have applied the concept of drug delivery to design safe and effective contrast agents for magnetic resonance imaging (MRI) and used biomedical imaging in non-invasive evaluation of drug delivery and image-guided therapy. We summarize here the recent progress in our research on biodegradable macromolecular MRI contrast agents, non-invasive visualization of in vivo drug delivery of polymeric conjugates with contrast enhanced MRI, and contrast enhanced MRI guided photodynamic therapy. The preliminary results have shown that biocompatible polymers can be used as an effective platform for drug delivery and biomedical imaging. Safe and effective imaging agents can be designed by using the concept of polymeric drug delivery. Biomedical imaging can be used as a non-invasive method for the evaluation of in vivo drug delivery of polymeric drug delivery systems. The combination of drug delivery and biomedical imaging can result in image-guided therapies, which include tumor detection, therapy and non-invasive evaluation of therapeutic responses.

Magnetic resonance temperature imaging for focused ultrasound surgery: a review

Magnetic resonance temperature imaging (MRTI) is an enabling technology that has recently demonstrated the potential to bring the emerging minimally invasive image-guided thermal therapy procedures, such as radiofrequency, microwave, laser, ultrasound, and cryosurgery, into the clinical setting with a level of safety and efficacy not previously possible. By coupling the wealth of soft tissue contrast mechanisms available with magnetic resonance imaging with its intrinsic temperature sensitivity, magnetic resonance imaging is in a unique position to provide image-guided treatment planning and verification and quantitative or qualitative feedback during treatment delivery, heightening of the control the physician has over the method, and enhancement of the ability to deliver conformal treatments. The basic principles behind MRTI technology and its application to minimally invasive thermal therapy during ultrasound thermal therapy delivery are reviewed in this study.

Theoretical modeling for radiofrequency ablation: state-of-the-art and challenges for the future

Radiofrequency ablation is an interventional technique that in recent years has come to be employed in very different medical fields, such as the elimination of cardiac arrhythmias or the destruction of tumors in different locations. In order to investigate and develop new techniques, and also to improve those currently employed, theoretical models and computer simulations are a powerful tool since they provide vital information on the electrical and thermal behavior of ablation rapidly and at low cost. In the future they could even help to plan individual treatment for each patient. This review analyzes the state-of-the-art in theoretical modeling as applied to the study of radiofrequency ablation techniques. Firstly, it describes the most important issues involved in this methodology, including the experimental validation. Secondly, it points out the present limitations, especially those related to the lack of an accurate characterization of the biological tissues. After analyzing the current and future benefits of this technique it finally suggests future lines and trends in the research of this area.

Lipid-based nanoparticles for contrast-enhanced MRI and molecular imaging

In the field of MR imaging and especially in the emerging field of cellular and molecular MR imaging, flexible strategies to synthesize contrast agents that can be manipulated in terms of size and composition and that can be easily conjugated with targeting ligands are required. Furthermore, the relaxivity of the contrast agents, especially for molecular imaging applications, should be very high to deal with the low sensitivity of MRI. Lipid-based nanoparticles, such as liposomes or micelles, have been used extensively in recent decades as drug carrier vehicles. A relatively new and promising application of lipidic nanoparticles is their use as multimodal MR contrast agents. Lipids are amphiphilic molecules with both a hydrophobic and a hydrophilic part, which spontaneously assemble into aggregates in an aqueous environment. In these aggregates, the amphiphiles are arranged such that the hydrophobic parts cluster together and the hydrophilic parts face the water. In the low concentration regime, a wide variety of structures can be formed, ranging from spherical micelles to disks or liposomes. Furthermore, a monolayer of lipids can serve as a shell to enclose a hydrophobic core. Hydrophobic iron oxide particles, quantum dots or perfluorocarbon emulsions can be solubilized using this approach. MR-detectable and fluorescent amphiphilic molecules can easily be incorporated in lipidic nanoparticles. Furthermore, targeting ligands can be conjugated to lipidic particles by incorporating lipids with a functional moiety to allow a specific interaction with molecular markers and to achieve accumulation of the particles at disease sites. In this review, an overview of different lipidic nanoparticles for use in MRI is given, with the main emphasis on Gd-based contrast agents. The mechanisms of particle formation, conjugation strategies and applications in the field of contrast-enhanced, cellular and molecular MRI are discussed.

Contrast enhanced MRI-guided photodynamic therapy for site-specific cancer treatment

Photodynamic therapy (PDT) is a minimally invasive and effective approach for cancer treatment. It is potentially useful for treating tumors that are not accessible to surgery, radiation, or destructive ablations, and are resistant to chemotherapy. Efficacious treatment of interstitial tumors with PDT requires efficient delivery of photosensitizers and accurate location of tumor tissues for effective light irradiations. In this study we performed contrast-enhanced (CE) MRI-guided PDT with a bifunctional polymer conjugate containing both a magnetic resonance imaging (MRI) contrast agent and a photosensitizer, poly(L-glutamic acid) (PGA)-(Gd-DO3A)-mesochlorin e(6) (Mce(6)). The efficacy of the bifunctional conjugate in cancer CE-MRI and cancer treatment was evaluated in athymic nude mice bearing MDA-MB-231 human breast carcinoma xenografts, with PGA-(Gd-DO3A) used as a control. The polymer conjugates preferentially accumulated in the solid tumor due to the hyperpermeability of the tumor vasculature, resulting in significant tumor enhancement for accurate tumor detection and localization by MRI. Significant therapeutic response was observed for PDT with the bifunctional conjugate as compared to the control. CE-MRI-guided PDT with the bifunctional conjugate is effective for tumor detection and minimally invasive cancer treatment.

Local delivery of magnetic resonance (MR) contrast agent in kidney using thermosensitive liposomes and MR imaging-guided local hyperthermia: A feasibility study in vivo

PURPOSE

To investigate the feasibility of local delivery of a magnetic resonance (MR) contrast agent in vivo using paramagnetic thermosensitive liposomes and infrared (IR) laser-induced local hyperthermia under real-time MR thermometry on rabbit kidney.

MATERIALS AND METHODS

Respiratory gated, radio frequency (RF)-spoiled gradient-echo sequences were used for precise MR temperature mapping (SD = 1 degrees C). In vivo heating experiments confirmed local release of MR contrast agent from liposomes.

RESULTS

T1 decreased from 800 msec to about 500 msec, as measured after tissue cooling, in those locations where the renal parenchyma was heated above the phase transition temperature of the liposome membrane.

CONCLUSION

The release of MR contrast agent has been demonstrated in rabbit kidney in vivo. This may be used as a reporter for simultaneous release of therapeutic agents.