Hybrid FePt/SiO2/Au nanoparticles as a theranostic tool: in vitro photo-thermal treatment and MRI imaging.

We have produced an innovative, theranostic material based on FePt/SiO2/Au hybrid nanoparticles (NPs) for both, photo-thermal therapy and magnetic resonance imaging (MRI). Furthermore, a new synthesis approach, i.e., Au double seeding, for the preparation of Au nanoshells around the FePt/SiO2 cores, is proposed. The photo-thermal and the MRI response were first demonstrated on an aqueous suspension of hybrid FePt/SiO2/Au NPs. The cytotoxicity together with the internalization mechanism and the intracellular fate of the hybrid NPs were evaluated in vitro on a normal (NPU) and a half-differentiated cancerous cell line (RT4). The control samples as well as the normal cell line incubated with the NPs showed no significant temperature increase during the in vitro photo-thermal treatment (ΔT < 0.8 °C) and thus the cell viability remained high (∼90%). In contrast, due to the high NP uptake by the cancerous RT4 cell line, significant heating of the sample was observed (ΔT = 4 °C) and, consequently, after laser irradiation the cell viability dropped significantly to ∼60%. These results further confirm that the hybrid FePt/SiO2/Au NPs developed in the scope of this work were not only efficient but also highly selective photo-thermal agents. Furthermore, the improvement in the contrast and the easier distinction between the healthy and the cancerous tissues were clearly demonstrated with in vitro MRI experiments, proving that hybrid NPs have an excellent potential to be used as contrast agents.

Magnetic interactions and in vitro study of biocompatible hydrocaffeic acid-stabilized Fe-Pt clusters as MRI contrast agents.

A detailed magnetic study of separated Fe-Pt NPs and Fe-Pt clusters was performed to predict their optimal size and morphology for the maximum saturation magnetization, a factor that is known to influence the performance of a magnetic-resonance-imaging (MRI) contrast agent. Excellent stability and biocompatibility of the nanoparticle suspension was achieved using a novel coating based on hydrocaffeic acid (HCA), which was confirmed with a detailed Fourier-transform infrared spectroscopy (FTIR) study. An in vitro study on a human-bladder papillary urothelial neoplasm RT4 cell line confirmed that HCA-Fe-Pt nanoparticles showed no cytotoxicity, even at a very high concentration (550 µg Fe-Pt per mL), with no delayed cytotoxic effect being detected. This indicates that the HCA coating provides excellent biocompatibility of the nanoparticles, which is a prerequisite for the material to be used as a safe contrast agent for MRI. The cellular uptake and internalization mechanism were studied using ICP-MS and TEM analyses. Furthermore, it was shown that even a very low concentration of Fe-Pt nanoparticles (<10 µg mL-1) in the cells is enough to decrease the T 2 relaxation times by 70%. In terms of the MRI imaging, this means a large improvement in the contrast, even at a low nanoparticle concentration and an easier visualization of the tissues containing nanoparticles, proving that HCA-coated Fe-Pt nanoparticles have the potential to be used as an efficient and safe MRI contrast agent.

Magnetic resonance electrical impedance tomography for measuring electrical conductivity during electroporation.

The electroporation effect on tissue can be assessed by measurement of electrical properties of the tissue undergoing electroporation. The most prominent techniques for measuring electrical properties of electroporated tissues have been voltage-current measurement of applied pulses and electrical impedance tomography (EIT). However, the electrical conductivity of tissue assessed by means of voltage-current measurement was lacking in information on tissue heterogeneity, while EIT requires numerous additional electrodes and produces results with low spatial resolution and high noise. Magnetic resonance EIT (MREIT) is similar to EIT, as it is also used for reconstruction of conductivity images, though voltage and current measurements are not limited to the boundaries in MREIT, hence it yields conductivity images with better spatial resolution. The aim of this study was to investigate and demonstrate the feasibility of the MREIT technique for assessment of conductivity images of tissues undergoing electroporation. Two objects were investigated: agar phantoms and ex vivo liver tissue. As expected, no significant change of electrical conductivity was detected in agar phantoms exposed to pulses of all used amplitudes, while a considerable increase of conductivity was measured in liver tissue exposed to pulses of different amplitudes.

Current density imaging during tissue electroporation.

Delivery of externally applied electric pulses on the target tissue during electroporation increases membrane permeability and induces electric currents in the tissue. To optimize electroporation parameters, the current density and with it associated electric field distributions can be monitored by means of current density imaging (CDI) and magnetic resonance electric impedance tomography (MREIT).

Magnetic resonance microscopy in biomedical research.

Magnetic resonance (MR) microscopy is a special modality of MRI with an emphasis on high spatial resolution. While its main principle is identical to conventional clinical MRI, there are several differences between the two that are mainly associated with a use of stronger magnets and gradients. MR microscopy has numerous interesting applications in material and bio sciences in which high spatial resolution is demanded and long experiment times are allowed.

Quantification of synovitis in the cranio-cervical region: dynamic contrast enhanced and diffusion weighted magnetic resonance imaging in early rheumatoid arthritis--a feasibility follow up study.

OBJECTIVE: To test the feasibility of dynamic contrast enhanced (DCEI) and diffusion weighted (DWI) magnetic resonance imaging (MRI) for quantifying synovitis of the cranio-cervical (C-C) region in patients with early rheumatoid arthritis (RA) and neck pain at the beginning and at a six month follow up. METHODS: 27 patients with duration of RA of less than 24 months and neck pain were studied with standard qualitative MRI evaluation and two quantitative MRI methods (DCEI and DWI) at the level of atlantoaxial joints. Rate of early enhancement (REE), enhancement gradient (Genh) and apparent diffusion coefficient (ADC) were extracted from DCEI and DWI data. MRI was coupled with clinical assessment and radiographic imaging. RESULTS: Using standard qualitative MRI evaluation, unequivocal active synovitis (grade 2 or 3 contrast enhancement) was proved in 16 (59%) patients at baseline and 14 (54%) at follow up. DCEI and DWI measurements confirmed active synovitis in 25 (93%) patients at baseline and 24 (92%) at follow up. Average REE, Genh and ADC values decreased during follow up, however the difference was not statistically significant (p>0.05). Both qualitative and quantitative MRI methods confirmed active inflammatory disease in the C-C region following therapy although all clinical criteria showed signs of improvement of the peripheral disease. CONCLUSIONS: The study proved the feasibility of DCEI and DWI MRI for quantifying synovitis of the C-C region in patients with early RA and neck pain. Both techniques can be used as additional method for evaluation of synovitis of the C-C region in RA.

Magnetic resonance electrical impedance tomography for monitoring electric field distribution during tissue electroporation.

Electroporation is a phenomenon caused by externally applied electric field of an adequate strength and duration to cells that results in the increase of cell membrane permeability to various molecules, which otherwise are deprived of transport mechanism. As accurate coverage of the tissue with a sufficiently large electric field presents one of the most important conditions for successful electroporation, applications based on electroporation would greatly benefit with a method of monitoring the electric field, especially if it could be done during the treatment. As the membrane electroporation is a consequence of an induced transmembrane potential which is directly proportional to the local electric field, we propose current density imaging (CDI) and magnetic resonance electrical impedance tomography (MREIT) techniques to measure the electric field distribution during electroporation. The experimental part of the study employs CDI with short high-voltage pulses, while the theoretical part of the study is based on numerical simulations of MREIT. A good agreement between experimental and numerical results was obtained, suggesting that CDI and MREIT can be used to determine the electric field during electric pulse delivery and that both of the methods can be of significant help in planning and monitoring of future electroporation based clinical applications.

T1 relaxation time and magnetic resonance imaging of inflamed gingival tissue.

OBJECTIVES: The aim of this study was to evaluate the use of MRI as a non-invasive method for the characterization of the inflammation and healing processes in periodontal tissues. METHODS: For the in vitro study, 99 gingival samples were collected during periodontal surgical treatment and T1 relaxation time measurements were performed and correlated to the probing depth measurements recorded at the collection sites. For the in vivo study, a group of eight patients with moderate to advanced periodontal disease was examined with pre-contrast and Gd-DTPA contrast-enhanced T1 weighted MRI both before and 3 months after non-surgical periodontal therapy. On the MR images of the 8 patients, 53 regions of interest (ROIs) were selected. For each ROI, the ratio between post- and pre-contrast signal intensity (RSI) was calculated and used as a measure for the degree of inflammation. RESULTS: The in vitro T1 relaxation times measurements of gingival samples showed an increase in relaxation times with the increase of probing depth at the sites of tissue removal. The in vivo studies demonstrated that the reduction of inflammation and probing depth in gingival tissues after non-surgical periodontal therapy correlates with a decrease of RSI in T1 weighted MR images. The non-invasively obtained data provide the characteristic ratio U, which shows that two distinct types of inflammation occurred in the examined group of patients. CONCLUSIONS: The results of MRI provide a new possibility to characterize the type and healing process of periodontal inflammation.

A mathematical model for the dissolution of non-occlusive blood clots in fast tangential blood flow.

Our aim was to study the effect of an axially directed blood plasma flow on the dissolution rate of cylindrical non-occlusive blood clots in an in vitro flow system and to derive a mathematical model for the process. The model was based on the hypothesis that clot dissolution dynamics is proportional not only to the biochemical proteolysis of fibrin but also to the power of the flowing blood plasma dissipated along the clot. The predicted rate of thrombolysis is then proportional to the square of the average blood plasma velocity for laminar flow and to the third power of the average velocity for turbulent flow. To verify the model, the time dependence of the clot cross-sectional area was measured by dynamic magnetic resonance microscopy during fast (turbulent) and slow (laminar) flow of plasma through an axially directed channel along the clot. The flowing plasma contained a magnetic resonance imaging contrast agent (Gd-DTPA) and a thrombolytic agent (recombinant tissue-type plasminogen activator). The experimental data fitted well to the model, and confirmed the predicted increase in the dissolution rate when blood flow changed from a laminar to a turbulent flow regime.

Modelling the effect of laminar axially directed blood flow on the dissolution of non-occlusive blood clots.

Axially directed blood plasma flow can significantly accelerate thrombolysis of non-occlusive blood clots. Viscous forces caused by shearing of blood play an essential role in this process, in addition to biochemical fibrinolytic reactions. An analytical mathematical model based on the hypothesis that clot dissolution dynamics is proportional to the power of the flowing blood plasma dissipated along the clot is presented. The model assumes cylindrical non-occlusive blood clots with the flow channel in the centre, in which the flow is assumed to be laminar and flow rate constant at all times during dissolution. Effects of sudden constriction on the flow and its impact on the dissolution rate are also considered. The model was verified experimentally by dynamic magnetic resonance (MR) microscopy of artificial blood clots dissolving in an in vitro circulation system, containing plasma with a magnetic resonance imaging contrast agent and recombinant tissue-type plasminogen activator (rt-PA). Sequences of dynamically acquired 3D low resolution MR images of entire clots and 2D high resolution MR images of clots in the axial cross-section were used to evaluate the dissolution model by fitting it to the experimental data. The experimental data fitted well to the model and confirmed our hypothesis.

Lysability of arterial thrombi assessed by magnetic resonance imaging.

BACKGROUND: Intravascular thrombi change in time due to retraction and organization, which is reflected in the appearance of magnetic resonance images of clots. We have hypothesized that MRI has the potential to improve patient selection for thrombolytic treatment. The aim of our study was to analyze occlusive arterial thrombi with MRI, and to correlate the MRI parameters with the therapeutic outcome in patients with occlusive atherothrombotic disease of the superficial femoral artery who were treated with catheter-directed thrombolysis by streptokinase. PATIENTS AND METHODS: We included 13 patients with subacute (2 weeks to 3 months old) occlusive arterial thrombi and 4 patients with chronic (more than 6 months old) arterial occlusions. We measured the MRI signal intensity on gradient echo images of 98 axial slices of the subacute occlusive thrombi and in 45 slices of 4 chronic thrombi. Following MRI, the patients with subacute history were treated with catheter-directed thrombolysis. RESULTS: Thrombolysis was successful in 11/13 patients. The normalized MRI signal intensity was significantly higher in the unsuccessfully treated thrombi than in the successfully treated thrombi (1.10 +/- 0.08 vs. 0.72 +/- 0.17, p < 0.003), but the subacute and chronic thrombi did not differ in signal intensity. CONCLUSIONS: High signal intensity of arterial thrombi on gradient echo MRI might predict resistance to thrombolytic therapy.

Three-dimensional in vivo magnetic resonance microscopy of beech (Fagus sylvatica L.) wood.

Spatial structure and water distribution in branch tissues after mechanical injury were investigated in vivo by three-dimensional (3D) magnetic resonance (MR) microscopy. On a beech tree (Fagus sylvatica L.), transplanted in a portable pot, a branch was topped and then MR imaged. High-resolution 3D MR images revealed structures which could not be identified by conventional MR images or by light microscopy. MR measurements confirmed our assumption that moisture content is decreasing towards the wounded part of the branch. This indicates that quick moisture loss from mechanically wounded tissues represents the initial passive response of compromised tissue.

Magnetic resonance imaging of alternating electric currents.

Electric Current Density Imaging (CDI) is a new modality of magnetic resonance imaging that enables electric current distribution imaging in conductive samples containing water. So far, two CDI techniques have been in use: DC-CDI operating at zero frequency and RF-CDI operating at the RF Larmor frequency. In this paper we present a new CDI technique, which extends the CDI frequency range to alternating electric currents (AC-CDI). First, a theoretical model for the electric current response to the alternating voltage is presented. Later, this model is used for the frequency analysis of the AC-CDI sequence. Additionally, the effect of off-resonance spins and imperfect refocusing RF pulses on the stability of the AC-CDI sequence and the echo formation is studied. The new theory is verified by experiments on a model system and compared to the other two methods: DC-CDI and RF-CDI. Finally, an application of the AC-CDI sequence to biological systems is demonstrated by an experiment on a wood twig in which an increase of approximately 30% was obtained at AC as compared to DC electric current.

Effect of surface coating on water migration into resin-modified glass ionomer cements: a magnetic resonance micro-imaging study.

Magnetic resonance micro-imaging was applied to study water diffusion into resin-modified glass ionomer cement restoration and to evaluate the effect of surface coating over restoration. Two cavities were prepared on the labial surface of extracted teeth and restored with resin-modified glass ionomer cement; one was protected with surface coating and the other was not. Immediately after restoration, the teeth were immersed in water. Progress of water diffusion into restorations was monitored by T(1) weighted spin-echo MRI at one-day intervals after the start of immersion. To quantify the water diffusion, a model was developed and compared with imaging data. Best fit yielded an effective water diffusion coefficient D = (2.3 +/- 0.4) 10(-12) m(2)/sec. Experimental results demonstrated that surface coating protects the dental cement against water intrusion from the surface of the restoration which faces the oral cavity. Such coating, however, does not prevent water penetration from the dentine side.

Volume selective detection by weighted averaging of constant tip angle scans.

We propose a method to improve the sensitivity in volume selective detection based on the CARVE excitation sequence (I. Sersa and S. Macura, J. Magn. Reson. 135, 466-477 (1998)) which consists of signal acquisition with constant tip angle excitation and a short phase-encoding gradient pulse. Volume selectivity is achieved using the weighted average of a number of scans whose weights and gradient steps are determined by the shape of the excitation profile. The method is particularly useful for broadband volume selective detection of insensitive spins where the volume selection can be merged with the standard signal averaging process, without compromising the excitation bandwidth or sensitivity.

MRI edge enhancement as a diffusive discord of spin phase structure.

The enhancement of magnetic resonance image intensity near impermeable boundaries can be nicely described by a new approach where the diffusional spin echo attenuation is linked to the correlation function of molecular motion. In this method the spin phase structure created by the applied gradient is considered to be a composition of plane waves with the wave vectors representing feasible momentum states of a particle in confinement. The enhancement of edges on the magnetic resonance images (MRI) comes out as a discord of plane waves due to particle motion. It results from the average of the wave phase by using the cumulant expansion in the Gaussian approximation. The acquired analytical expression describes the MRI signal space distribution where the enhancement of edges depends on the intensity and the duration of gradient sequence as well as on the length of the mean squared particle displacement in restricted geometry. This new method works well with gradients of general waveform and is, therefore, suitable for imaging sequences where finite or even modulated gradients are usually used.

Excitation of complicated shapes in three dimensions.

We experimentally verified a recently proposed technique for the excitation of a complicated three-dimensional profile (CARVE, completely arbitrary regional volume excitation). CARVE is based on a generalized DANTE RF pulse sequence and a synchronous string of gradient steps. Provided there is no limitation in the number of pulses, CARVE can generate an excitation profile of any shape with any resolution. However, hardware limitations and sample properties restrict the number of RF pulses and gradient steps and, thus, limit attainable resolution of the excitation profile. We theoretically and experimentally showed that spatial resolution can be increased by distributing a long sequence among several CARVE experiments and summing up their signals. This is particularly important for three-dimensional excitation profiles where an n-fold increase in resolution requires an n3-fold increase of the number of events in the sequence. The potential use of three-dimensional CARVE might be in spectroscopic imaging where the excitation profile can be tailored to match the shape of a selected organ or body part.

Excitation of arbitrary shapes by gradient optimized random walk in discrete k-space.

A new technique for the excitation of arbitrary shapes is proposed. It is based on a parallel sequence of small tip angle RF pulses and gradient pulses. The small tip angle rotations co-add yielding a 90 degrees excitation pulse within the selected excitation profile while outside the profile, the rotations cancel each other. A full theory of the completely arbitrary regional volume excitation (CARVE) method is presented and experimentally verified. In CARVE, k-space is discrete because the RF is applied in pulses. The discrete character of k-space permits an arbitrary trajectory for the k-space walk. The optimal random trajectory is found by minimizing the gradient load using simulated annealing. It is shown, both theoretically and experimentally, that such a trajectory is much better than any other systematic or random trajectory in k-space.

Electric current density imaging of mice tumors.

The use of electric current density imaging (CDI) to map spatial distribution of electric currents through tumors is presented. Specifically, a method previously tested on phantoms was implemented in vivo and in vitro for mapping electric current pulses of the same order of magnitude (j approximately 2500 A/m2) as in electrochemotherapy through T50/80 mammary carcinomas, B-16 melanomas and SA-1 sarcomas. A technically simplified method of electric current density imaging is discussed as well. Three geometries of electrodes (flat-flat, point-point, point-flat) indicate altered electric current distribution for the same tumor. This indicates that the method can be used for monitoring the effects of electrochemotherapy as a function of electrode geometry.