Chris V. Bowen

Application of the static dephasing regime theory to superparamagnetic iron-oxide loaded cells.

The relaxation rates of iron‐oxide nanoparticles compartmentalized within cells were studied and found to satisfy predictions of the static dephasing (SD) regime theory. THP‐1 cells in cell culture were loaded using two different iron‐oxide nanoparticles (superparamagnetic iron‐oxide (SPIO) and ultrasmall SPIO (USPIO)) with four different iron concentrations (0.05, 0.1, 0.2, and 0.3 mg/ml) and for five different incubation times (6, 12, 24, 36, and 48 hr). Cellular iron‐oxide uptake was assessed using a newly developed imaging version of MR susceptometry, and was found to be linear with both dose and incubation time. R  2* sensitivity to iron‐oxide loaded cells was found to be 70 times greater than for R2, and 3100 times greater than for R1. This differs greatly from uniformly distributed nanoparticles and is consistent with a cellular bulk magnetic susceptibility (BMS) relaxation mechanism. The cellular magnetic moment was large enough that R2′ relaxivity agreed closely with SD regime theory predictions for all cell samples tested where the local magnetic dose (LMD) is the sample magnetization due to the presence of iron‐oxide particles). Uniform suspensions of SPIO and USPIO produced R2′ relaxivities that were a factor of 3 and 8 less, respectively, than SD regime theory predictions. These results are consistent with theoretical estimates of the required mass of iron per compartment needed to guarantee SD‐regime‐dominant relaxivity. For cellular samples, R2 was shown to be dependent on both the concentration and distribution of iron‐oxide particles, while R2′ was sensitive to iron‐oxide concentration alone. This work is an important first step in quantifying cellular iron content and ultimately mapping the density of a targeted cell population. Magn Reson Med 48:52–61, 2002.

Detection Threshold of Single SPIO-Labeled Cells With FIESTA

MRI of superparamagnetic iron oxide (SPIO)‐labeled cells has become a valuable tool for studying the in vivo trafficking of transplanted cells. Cellular detection with MRI is generally considered to be orders of magnitude less sensitive than other techniques, such as positron emission tomography (PET), single photon emission‐computed tomography (SPECT), or optical fluorescence microscopy. However, an analytic description of the detection threshold for single SPIO‐labeled cells and the parameters that govern detection has not been adequately provided. In the present work, the detection threshold for single SPIO‐labeled cells and the effect of resolution and SNR were studied for a balanced steady‐state free precession (SSFP) sequence (3D‐FIESTA). Based on the results from both theoretical and experimental analyses, an expression that predicts the minimum detectable mass of SPIO (mc) required to detect a single cell against a uniform signal background was derived: mc = 5v/(Kfsl · SNR), where v is the voxel volume, SNR is the image signal‐to‐noise ratio, and Kfsl is an empirical constant measured to be 6.2 ± 0.5 × 10−5 μl/pgFe. Using this expression, it was shown that the sensitivity of MRI is not very different from that of PET, requiring femtomole quantities of SPIO iron for detection under typical micro‐imaging conditions (100 μm isotropic resolution, SNR = 60). The results of this work will aid in the design of cellular imaging experiments by defining the lower limit of SPIO labeling required for single cell detection at any given resolution and SNR. Magn Reson Med 53:312–320, 2005.

Confirming white matter fMRI activation in the corpus callosum: co-localization with DTI tractography.

Recently, functional magnetic resonance imaging (fMRI) activation has been detected in white matter, despite the widely-held belief that fMRI activation is restricted to gray matter. The objective of the current study was to determine whether the regions of white matter fMRI activation were structurally connected to the functional network in gray matter. To do this, we used fMRI-guided tractography to evaluate whether tracts connecting regions of gray matter fMRI activation were co-localized with white matter fMRI activation. An established interhemispheric transfer task was employed to elicit activation in the corpus callosum. Diffusion tensor imaging (DTI) tractography was used to determine the existence of tracts that connected regions of gray matter fMRI activation to regions of activation in the corpus callosum. Corpus callosum activation was detected in the majority of participants. While there was individual variability in the location of corpus callosum activation, activation was commonly observed in the callosal mid-body, isthmus/splenium, or both. Despite the variability, gray matter fMRI-guided tractography identified tracts that were co-localized with corpus callosum fMRI activation in all instances. In addition, callosal activation had tracts to bilateral gray matter fMRI activation for 7/8 participants. The results confirmed that the activated regions of the corpus callosum were structurally connected to the functional network of gray matter regions involved in the task. These findings are an important step towards establishing the functional significance of white matter fMRI, and provide the foundation for future work combining white matter fMRI and DTI tractography to study brain connectivity.

Gradient-induced acoustic and magnetic field fluctuations in a 4T whole-body MR imager

Both the acoustic and magnetic fluctuation frequency response functions for a Siemens AS25 body gradient coil inside a 4 Tesla whole‐body MR system were measured and analyzed in this study. In an attempt to correlate the acoustic noise inside the gradient coil with magnetic field oscillations, triangular and trapezoidal gradient impulses of varying amplitudes and widths were used to excite the gradient coil. The acoustic and magnetic responses to these inputs were measured. The results show the existence of discrete resonances in both acoustic and uniform magnetic field fluctuation spectra, while gradient magnetic field fluctuation spectra show no such resonances. In addition, the dominant amplitude peaks in spectra fluctuate similarly with respect to trapezoidal gradient impulse flat‐top widths. This implies that these phenomena are correlated, and that the trapezoidal impulse flat‐top width may be used as a way to suppress both acoustic noise and uniform magnetic field oscillations. Magn Reson Med 44:532–536, 2000.

Compressed sensing reconstruction improves sensitivity of variable density spiral fMRI

Functional MRI (fMRI) techniques that can provide excellent blood oxygen level dependent contrast, rapid whole brain imaging, and minimal spatial distortion are in demand. This study explored whether fMRI sensitivity can be improved through the use of compressed sensing (CS) reconstruction of variable density spiral fMRI.

In Vivo Detection of Human TRPV6-Rich Tumors with Anti-Cancer Peptides Derived from Soricidin

Soricidin is a 54-amino acid peptide found in the paralytic venom of the northern short-tailed shrew (Blarina brevicauda) and has been found to inhibit the transient receptor potential of vallinoid type 6 (TRPV6) calcium channels. We report that two shorter peptides, SOR-C13 and SOR-C27, derived from the C-terminus of soricidin, are high-affinity antagonists of human TRPV6 channels that are up-regulated in a number of cancers. Herein, we report molecular imaging methods that demonstrate the in vivo diagnostic potential of SOR-C13 and SOR-C27 to target tumor sites in mice bearing ovarian or prostate tumors. Our results suggest that these novel peptides may provide an avenue to deliver diagnostic and therapeutic reagents directly to TRPV6-rich tumors and, as such, have potential applications for a range of carcinomas including ovarian, breast, thyroid, prostate and colon, as well as certain leukemias and lymphomas.

Relaxometry model of strong dipolar perturbers for balanced-SSFP: Application to quantification of SPIO loaded cells

Magnetic resonance microscopy using magnetically labeled cells is an emerging discipline offering the potential for non‐destructive studies targeting numerous cellular events in medical research. The present work develops a technique to quantify superparamagnetic iron‐oxide (SPIO) loaded cells using fully balanced steady state free precession (b‐SSFP) imaging. An analytic model based on phase cancellation was derived for a single particle and extended to predict mono‐exponential decay versus echo time in the presence of multiple randomly distributed particles. Numerical models verified phase incoherence as the dominant contrast mechanism and evaluated the model using a full range of tissue decay rates, repetition times, and flip angles. Numerical simulations indicated a relaxation rate enhancement (ΔR2b=0.412γ · LMD) proportional to LMD, the local magnetic dose (the additional sample magnetization due to the SPIO particles), a quantity related to the concentration of contrast agent. A phantom model of SPIO loaded cells showed excellent agreement with simulations, demonstrated comparable sensitivity to gradient echo ΔR*2 enhancements, and 14 times the sensitivity of spin echo ΔR2 measurements. We believe this model can be used to facilitate the generation of quantitative maps of targeted cell populations. Magn Reson Med, 2006.

Asymmetric spin-echo (ASE) spiral improves BOLD fMRI in inhomogeneous regions.

Functional MRI (fMRI) is of limited use in areas such as the orbitofrontal and inferior temporal lobes due to the presence of local susceptibility‐induced field gradients (SFGs), which result in severe image artifacts. Several techniques have been developed to reduce these artifacts, the most common being the dual‐echo spiral sequences (spiral‐in/out and spiral‐in/in). In this study, a new multiple spiral acquisition technique was developed, in which the later spiral acquisitions are acquired asymmetrically with the peak of a spin‐echo causing increased R2‐weighting but matched R2′‐weighting. This sequence, called asymmetric spin‐echo (ASE) spiral, has demonstrated significant improvements in minimizing the signal loss and increasing the image quality as well as optimal blood‐oxygen‐level‐dependent (BOLD)‐weighting. The ASE spiral is compared to conventional spiral‐out using both signal‐to‐noise ratio (SNR) and whole brain fMRI activation volumes from a breath‐hold task acquired at 4 Tesla. The ASE dual spiral has exhibited SNR increases of up to 300% in areas where strong SFGs are present. As a result, the ASE spiral is highly efficient for recovering lost activation in areas of SFGs, as demonstrated by a 16% increase in the total number of activated voxels over the whole brain. Post spin‐echo ASE spiral images have decreasing SNR due to R2 signal losses, however the increase in R2‐weighting leads to a higher percentage of signal changes producing ASE spiral images with equivalent contrast‐to‐noise ratio (CNR) for each echo. The use of this sequence allows for recovery of BOLD activation in areas of SFG without sacrificing the CNR over the whole brain. Copyright

Clearance of depot vaccine SPIO-labeled antigen and substrate visualized using MRI

Immunotherapies, including peptide-based vaccines, are a growing area of cancer research, and understanding their mechanism of action is crucial for their continued development and clinical application. Exploring the biodistribution of vaccine components may be key to understanding this action. This work used magnetic resonance imaging (MRI) to characterize the in vivo biodistribution of the antigen and oil substrate of the vaccine delivery system known as DepoVax(TM). DepoVax uses a novel adjuvanted lipid-in-oil based formulation to solubilise antigens and promote a depot effect. In this study, antigen or oil were tagged with superparamagnetic iron oxide (SPIO), making them visible on MR images. This enables tracking of individual vaccine components to determine changes in biodistribution. Mice were injected with SPIO-labeled antigen or SPIO-labeled oil, and imaged to examine clearance of labeled components from the vaccine site. The SPIO-antigen was steadily cleared, with nearly half cleared within two months post-vaccination. In contrast, the SPIO-oil remained relatively unchanged. The biodistribution of the SPIO-antigen component within the vaccine site was heterogeneous, indicating the presence of active clearance mechanisms, rather than passive diffusion or drainage. Mice injected with SPIO-antigen also showed MRI contrast for several weeks post-vaccination in the draining inguinal lymph node. These results indicate that MRI can visualize the in vivo longitudinal biodistribution of vaccine components. The sustained clearance is consistent with antigen up-take and trafficking by immune cells, leading to accumulation in the draining lymph node, which corresponds to the sustained immune responses and reduced tumor burden observed in vaccinated mice.

Using MRI to evaluate and predict therapeutic success from depot-based cancer vaccines.

In the preclinical development of immunotherapy candidates, understanding the mechanism of action and determining biomarkers that accurately characterize the induced host immune responses is critical to improving their clinical interpretation. Magnetic resonance imaging (MRI) was used to evaluate in vivo changes in lymph node size in response to a peptide-based cancer vaccine therapy, formulated using DepoVax (DPX). DPX is a novel adjuvant lipid-in-oil–based formulation that facilitates enhanced immune responses by retaining antigens at the injection site for extended latencies, promoting increased potentiation of immune cells. C57BL/6 mice were implanted with C3 (HPV) tumor cells and received either DPX or control treatments, 5 days post-implantation. Complete tumor eradication occurred in DPX-vaccinated animals and large volumetric increases were observed in the vaccine-draining right inguinal lymph node (VRILN) in DPX mice, likely corresponding to increased localized immune response to the vaccine. Upon evaluating the relative measure of vaccine-potentiated immune activation to tumor-induced immune response (VRILN/VLILN), receiver-operating characteristic (ROC) curves revealed an area under the curve (AUC) of 0.90 (±0.07), indicating high specificity and sensitivity as a predictive biomarker of vaccine efficacy. We have determined that for this tumor model, early MRI lymph node volumetric changes are predictive of depot immunotherapeutic success.