M. Visscher

Evaluation of superparamagnetic iron oxide nanoparticles (Endorem) as a photoacoustic contrast agent for intra-operative nodal staging

Detection of tumor metastases in the lymphatic system is essential for accurate staging of malignancies. Commercially available superparagmagnetic nanoparticles (SPIOs) accumulate in normal lymph tissue after injection at a tumor site, whereas less or no accumulation takes place in metastatic nodes, thus enabling lymphatic staging using MRI. We verify for the first time the potential of SPIOs, such as Endorem(®) as a novel photoacoustic (PA) contrast agent in biological tissue. We injected five Wistar rats subcutaneously with variable amounts of Endorem(®) and scanned the resected lymph nodes using a tomographic PA setup. Findings were compared using histology, vibrating sample magnetometry (VSM) and 14 T MR-imaging. Our PA setup was able to detect the iron oxide accumulations in all the nodes containing the nanoparticles. The distribution inside the nodes corresponded with both MRI and histological findings. VSM revealed that iron quantities inside the nodes varied between 51 ± 4 and 11 ± 1 µg. Nodes without SPIO enhancement did not show up in any of the PA scans. Iron oxide nanoparticles (Endorem(®)) can be used as a PA contrast agent for lymph node analysis and a distinction can be made between nodes with and nodes without the agent. This opens up possibilities for intra-operative nodal staging for patients undergoing nodal resections for metastatic malignancies.

Intra-operative ex vivo photoacoustic nodal staging in a rat model using a clinical superparamagnetic iron oxide nanoparticle dispersion

The ability to accurately detect tumor metastases in lymph nodes is essential for intra-operative staging of various malignancies. Histopathological assessment of nodes has the drawback of a time delay before results are available to the surgeon and a likelihood of missing metastases. Photoacoustic (PA) imaging has been shown to possess the potential to detect melanoma metastases in resected in toto lymph nodes based on intrinsic contrast. To extend application of the method to other malignancies, extrinsic contrast for lymphatic mapping is important. We investigate in a metastatic animal model whether clinically approved superparamagnetic iron oxide (SPIO) nanoparticles, applied for MRI, can help PA imaging for staging in an intra-operative ex vivo setting. Imaging results are compared with 14 Tesla MR images and histology. We observe that irregularities in SPIO distribution in PA images of the nodes and a decrease in contrast correlate with metastatic involvement as seen in MR images and histology. The results show that a PA based imaging technique may be valuable for nodal staging in the field of surgical oncology.

Magnetic properties and paleointensities as function of depth in a Hawaiian lava flow

The outcome of paleointensity experiments largely depends on the rock-magnetic properties of the samples. To assess the relation between volcanic emplacement processes and rock-magnetic properties, we sampled a vertical transect in a ∼6 m thick inflated lava flow at Hawaii, emplaced in ∼588 AD. Its rock-magnetic properties vary as function of distance from the flow top; the observations can be correlated to the typical cooling rate profile for such a flow. The top and to a lesser extent the bottom parts of the flow cooled faster and reveal a composition of ∼TM60 in which the magnetic remanence is carried by fine-grained titanomagnetites, relatively rich in titanium, with associated low Curie and unblocking temperatures. The titanomagnetite in the slower cooled central part of the flow is unmixed into the magnetite and ulvospinel end-members as evidenced by scanning electron microscope observation. The remanence is carried by coarse-grained magnetite lamella (∼TM0) with high Curie and unblocking temperatures. The calibrated pseudo-Thellier results that can be accepted yield an average paleointensity of 44.1 ± 2.4 μT. This is in good agreement with the paleointensity results obtained using the thermal IZZI-Thellier technique (41.6 ± 7.4 μT) and a recently proposed record for Hawaii. We therefore suggest that the chance of obtaining a reliable paleointensity from a particular cooling unit can be increased by sampling lavas at multiple levels at different distances from the top of the flow combined with careful preliminary testing of the rock-magnetic properties.

Quantitative Analysis of Superparamagnetic Contrast Agent in Sentinel Lymph Nodes Using Ex Vivo Vibrating Sample Magnetometry

As the first step in developing a new clinical technique for the magnetic detection of colorectal sentinel lymph nodes (SLNs), a method is developed to measure the magnetic content in intact, formalin fixated lymph nodes using a vibrating sample magnetometer (VSM). A suspension of superparamagnetic nanoparticles is injected ex vivo around the tumor in the resected colon segments. A selection of three lymph nodes is excised from the region around the tumor and is separately measured in the VSM. The iron content in the lymph nodes is quantified from the magnetic moment curve using the Langevin model for superparamagnetism and a bimodal particle size distribution. Adverse, parasitic movements of the sample were successfully reduced by tight fixation of the soft tissue and using a small vibration amplitude. Iron content in the lymph nodes is detected with 0.5 μg accuracy and ranged from 1 to 51 μg. Histological staining confirmed iron presence. The current method of measuring intact biological tissue in a VSM is suitable to show the feasibility and merit of magnetic detection of SLNs in colorectal cancer. For clinical validation of magnetic SLN selection in colorectal cancer, a new magnetometer with high specificity for superparamagnetic nanoparticles is required.

Magnetic Detection of the Sentinel Lymph Node in Ex Vivo Tissue with Colorectal Cancer

A new method for sentinel lymph node detection was investigated by ex-vivo experiments in colorectal cancer. A dispersion of superparamagnetic iron oxide (SPIO) nanoparticles was injected in the resected tissue around the tumor. A magnetic probe with a 2 nd order gradiometer configuration was used to localize the sentinel lymph node with accumulated SPIO nanoparticles. Additional analysis of SPIO content was performed on the resected lymph nodes. High-field MRI and conventional microscopy revealed the presence and distribution of SPIO inside the lymph nodes. Magnetometry confirmed SPIO content in lymph nodes and could give an estimate of the amount of SPIO uptake.

A handheld SPIO-based sentinel lymph node mapping device using differential magnetometry

Sentinel lymph node biopsy has become a staple tool in the diagnosis of breast cancer. By replacing the morbidity-plagued axillary node clearance with removing only those nodes most likely to contain metastases, it has greatly improved the quality of life of many breast cancer patients. However, due to the use of ionizing radiation emitted by the technetium-based tracer material, the current sentinel lymph node biopsy has serious drawbacks. Most urgently, the reliance on radioisotopes limits the application of this procedure to small parts of the developed world, and it imposes restrictions on patient planning and hospital logistics. Magnetic alternatives have been tested in recent years, but all have their own drawbacks, mostly related to interference from metallic instruments and electromagnetic noise coming from the human body. In this paper, we demonstrate an alternative approach that utilizes the unique nonlinear magnetic properties of superparamagnetic iron oxide nanoparticles to eliminate the drawbacks of both the traditional gamma-radiation centered approach and the novel magnetic techniques pioneered by others. Contrary to many other nonlinear magnetic approaches however, field amplitudes are limited to 5 mT, which enables handheld operation without additional cooling. We show that excellent mass sensitivity can be obtained without the need for external re-balancing of the probe to negate any influences from the human body. Additionally, we show how this approach can be used to suppress artefacts resulting from the presence of metallic instruments, which are a significant dealbreaker when using conventional magnetometry-based approaches.

A Magnetometer Cooled with Liquid Nitrogen for the Characterization and Quantification of Magnetic Nanoparticles in Biological Samples at Room Temperature

For several medical applications of magnetic nanoparticles (MNP) it is desired to know the quantity and characteristics of the particles in the tissue of interest. That can either be necessary to determine how successful a procedure was of how it will be. Therefore a system is built, that is suitable to analyze small intact biological samples at room temperature. The magnetometer is used for the analysis and selection of sentinel lymph nodes in colorectal cancer. In this clinical procedure MNPs are administered in the resected part of the colon to determine the sentinel lymph node. The magnetometer is based on copper wound coils and comprises of two detection coils in series opposition, enclosed by two separately driven excitation coils.

Detection of magnetic nanoparticles for clinical interventions

In this thesis the possibilities of detection of magnetic nanoparticles for clinical interventions are investigated. Detection of these particles in, for example, the sentinel lymph node procedure, is a good alternative for the radionuclide based methods currently in use. The thesis shows that the magnetic method can be only successful if the magnetic properties of the human tissue and the particles are effectively discriminated. Because of the dominating diamagnetic response of the surrounding tissue volume, a tiny amount of magnetic nanoparticles deeply located in the body cannot be detected using only a simple sinusoid excitation field. For an experimental exploration of the ex vivo sentinel lymph node procedure in colorectal cancer using magnetic nanoparticles, a method has been developed using vibrating sample magnetometry, to quantify the amount of magnetic nanoparticles in small tissue samples. In a small group of ten patients the magnetic tracer is tested ex vivo in combination with blue dye, directly after resection of the tumor area. Based on the selection of the three nodes closest to the tumor, the magnetic nanoparticles are detected in at least one of the three nodes in nine patients. This demonstrates the feasibility of the magnetic tracer for ex vivo sentinel lymph node mapping. In four patients occult metastases were found by the sentinel lymph node procedure. For clinical detection of magnetic nanoparticles during interventions, the patented DiffMag concept has been developed, which specifically exploits the nonlinear magnetic properties of the nanoparticles and eliminates the diamagnetic contribution of tissue. A sinusoidal excitation field (B ~2 mT, f ~ 5 kHz) is combined with a sequence of offsets that alternates between zero and a constant value, to modulate the nonlinear response of the particles, while the diamagnetic response remains constant. The modulation in the measured signal is the DiffMag response, which is uniquely attributed to the particles and depends on the particle core size distribution. In lymph node tissue this response is reduced by environmental effects on Brownian relaxation. The effect of environmental factors is confirmed using samples with different viscosities, particle volume change by biomolecule adhesion and particle uptake in macrophages.

Analyzing magnetic nanoparticle content in biological samples: Acsusceptometry using offset fields

The clinical application of magnetic nanoparticles is a developing field with promising perspectives in treatment and diagnosis [1]. After the first applications as a contrast agent in MRI, other magnetic methods have been developed for excitation and detection of magnetic nanoparticles. For magnetic detection, the nonlinear behavior of superparamagnetic iron oxides provide excellent contrast in the linear magnetic human body. To exploit these properties, the design of magnetic nanoparticles as well as detection systems has to be optimized for clinical practice. The particles have to provide optimal sensitivity in contrast to tissue, whereas the signal-to-noise ratio and applicability of a measurement system are important for successful clinical implementation. In this contribution a setup is presented that is able to assess these both elements for sentinel lymph node mapping. Small intact biological samples, such as lymph nodes, can be measured at room temperature to characterize the magnetic nanoparticle content by differential magnetometry. Furthermore, the system can be used as a tool to analyze the magnetic properties of nanoparticles, providing insight in the quality for nonlinear particle detection.

Finding the Sentinel Lymph Node with a handheld differential magnetometer

In cancer staging, the Sentinel Lymph Node (SLN) procedure is a common method to assess the stage to which a cancer has progressed[1]. Currently, the SLN procedure is performed by injecting both a blue dye and a radionuclide tracer near or into the tumor area, and the first lymph node(s) draining the tumor area are located by means of visual inspection and a gamma probe which detects the radiation emitted by the radionuclide tracer. This combined procedure has proven to be very reliable and is used frequently, but suffers from some serious drawbacks that limit its applicability in general practice. These problems are mostly caused by the usage of ionizing radiation, which poses occupational hazards to medical staff and requires extensive logistics, which not all hospitals can offer.