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Magnetic Particle Imaging (MPI) is an emerging tomographic imaging technology that detects magnetic nanoparticle tracers by exploiting their non-linear magnetization properties. In order to predict the behavior of nanoparticles in an imager, it is possible to use a non-imaging MPI relaxometer or spectrometer to characterize the behavior of nanoparticles in a controlled setting. In this paper we explore the use of ferrohydrodynamic magnetization equations for predicting the response of particles in an MPI relaxometer. These include a magnetization equation developed by Shliomis (Sh) which has a constant relaxation time and a magnetization equation which uses a field-dependent relaxation time developed by Martsenyuk, Raikher and Shliomis (MRSh). We compare the predictions from these models with measurements and with the predictions based on the Langevin function that assumes instantaneous magnetization response of the nanoparticles. The results show good qualitative and quantitative agreement between the ferrohydrodynamic models and the measurements without the use of fitting parameters and provide further evidence of the potential of ferrohydrodynamic modeling in MPI.

Purpose: To rapidly calculate and validate subject-specific field maps based on the three-dimensional shape of the bilateral breast volume. Materials and Methods: Ten healthy female volunteers were scanned at 3 Tesla using a multi-echo sequence that provides water, fat, in-phase, out-of-phase, and field map images. A shape-specific binary mask was automatically generated to calculate a computed field map using a dipole field model. The measured and computed field maps were compared by visualizing the spatial distribution of the difference field map, the mean absolute error, and the 80% distribution widths of frequency histograms. Results: The 10 computed field maps had a mean absolute error of 38 Hz (0.29 ppm) compared with the measured field maps. The average 80% distribution widths for the histograms of all of the computed, measured, and difference field maps are 205 Hz, 233 Hz, and 120 Hz, respectively. Conclusion: The computed field maps had substantial overall agreement with the measured field maps, indicating that breast MRI field maps can be computed based on the air–tissue interfaces. These estimates may provide a predictive model for field variations and thus have the potential to improve applications in breast MRI. J. Magn. Reson. Imaging 2013;37:227–232. © 2012 Wiley Periodicals, Inc.

An integrated Overhauser-enhanced MRI-Prepolarized MRI system was developed to obtain radiobiological information that could be accurately coregistered with diagnostic quality anatomic images. EPR and NMR images were acquired through the double resonance technique and field cycling of the main magnetic field from 5 mT to 0.5 T. Dedicated EPR and NMR coils were devised to minimize radiofrequency power deposition with high signal-to-noise ratio. Trityl and nitroxide radicals were used to characterize oxygen and redox sensitivities of multispin echo Overhauser-enhanced MRI. Oxygen resolution of 3 mmHg was obtained from 2 mM deoxygenated trityl phantoms. Trityl radicals were stable in reducing environments and did not alter the redox-sensitive decaying rate of the nitroxide signals. Nitroxide radicals had a compounding effect for the trityl oximetry. Tumor oxygenation and redox status were acquired with anatomical images by injecting trityl and nitroxide probes subsequently in murine tumors. The Overhauser-enhanced MRI-Prepolarized MRI system is ready for quantitative longitudinal imaging studies of tumor hypoxia and redox status as radiotherapy prognostic factors.

Developing tracers for MPI and related applications requires a thorough understanding of underlying relaxation processes. In X-Space MPI reconstruction, relaxation has a profound impact on the image through a decrease in SNR and direction-dependent blurring. To probe the relaxation of potential MPI tracers, Goodwill et al., have developed an X-Space Relaxometer that measures the MPI point spread function and accurately predicts image quality.[1,2] The present work introduces a new relaxometer with improved eddy current shielding to achieve a significant gain in sensitivity. A simulation of the enhanced shielding has shown to attenuate external interference by more than 40dB in the detection bandwidth.

In this work,a method to achieve higher, more isotropic resolution for x-space MPI through analyzing various FFP trajectories and developing multidimensional transmit/receive hardware was presented. However, many parameters remain to be optimized for this technique, including the various tradeoffs between pFOV coverage, scanning speed, and resolution. Additional hardware challenges include precise control of the FFP through controlling the phase of the transmit fields and compensating for received particle signal based the anisotropic sensitivity profiles of the transmit and receive coils.

Parkinson's Disease (PD) is a degenerative disorder that causes the malfunction and death of neurons in the substantia nigra and afflicts hundreds of thousands of Americans. There is no known cure for PD. However, stem cell transplant therapy holds promise for reversing the effects of PD. Preclinically, the mouse model of stem cell therapy for PD is used to assess stem cell viability and analyze neuron regeneration in the substantia nigra. However, tracking stem cell viability in these experiments remains difficult at best because conventional imaging methods may inaccurately quantify stem cell number in vivo due to penetration depth or other effects. Here, we propose a new magnetic nanoparticle cytometer for small animals to quantify cells implanted in vivo labeled with superparamagnetic iron oxide (SPIO) nanoparticles. This cytometer, which uses MPI principles, is well-suited for determining stem cell viability as it provides linear signals for accurate quantification of cell number in vivo without suffering from depth or tissue attenuation or relying on ionizing radiation or radioactive materials with short half-lives.

The point spread function (PSF) in MPI has been found to be the derivative of the Langevin function. This PSF has noticeably longer “tails” than a single Gaussian. These tails of the PSF manifest as baseline blurring, which significantly decreases contrast and “glare” in MPI. Here, we develop a robust post-processing method to equalize (similar to deconvolution) the PSF, to essentially remove these tails. Fortunately we find this equalization method does not incur the enormous loss of SNR observed in conventional deconvolution.

Summary form only given. Magnetic Particle Imaging (MPI) commonly utilizes a Field Free Point (FFP) magnetic field gradient to localize magnetic nanoparticles [12]. With the benefits of two orders of magnitude reduced acquisition time or one order of magnitude signal-to-noise ratio (SNR) improvement, a gradient called a Field Free Line (FFL), which localizes particles to a line instead of a point, has been theoretically developed [3-6], and experimentally demonstrated [4,6]. In this work, we use a FFL with sample rotation and projection reconstruction to demonstrate experimental images with a 20 fold improvement in acquisition time compared to the first projection reconstruction (PR) MPI results [6]. To gain this 20 fold speed up, we implement a z direction focus field coil configuration instead of the previously utilized translation stage. Our imaging system included a 2.3 T/m permanent magnet FFL, a solenoidal drive coil, two focus field x and z direction electromagnet pairs with Helmholtz configurations, a solenoidal receive coil with a gradiometer configuration, and a motor driven rotary table (see Figure 1). The system drive coil was excited to create a 22.9 kHz drive field with a 1.3 cm z partial field of view (FOV). The x slow shift (focus) field operated with a 3.3 Hz triangle wave, which produced a 5 cm x FOV. A linear ramp z focus field traversed 6 cm in 3 s, once per projection image. The drive and z shift fields summed, producing a 7.3 cm z FOV. There were 20 x axis traversals (10 cycles) per projection image. With this sequence, we acquired 40 images at linearly spaced angles over 180 degrees. The image acquisition time was 2.1 min. We collected all the necessary projection data to produce a MPI tomographic 3D volume using the above parameters. Images were reconstructed using x-space reconstruction with filtered backprojection (FBP) [5-6]. The final imaging volume was limited by bore size and the z slow shift magnets to a 4.8 cm by 4.8 cm by 7.3 cm 3D volume. The 3D volume was exported in DICOM file format and subsequently imported to Osirix (Pixmeo, Switzerland) where maximum intensity projection (MIP) images were rendered. To test our imaging sytem, we have designed a phantom with a 3D distribution of magnetic nanoparticles (see Figure 1). Polyurethane tubing with inner diameter 1.6 mm (outer diameter 3.2) filled with 43 mM Fe Micromod Nanomag-D-spio was wrapped around a cylindrical piece of acrylic with a 3.4 cm outer diameter. Resulting MIP images illustrate the ability of the FFL imager to accurately resolve nanoparticle distributions in 3D. The MIP image can be rotated to any orientation, and two such views are shown in Figure 1. In previous work, an image with a similar FOV would have taken 39 min using a translation stage [6]. The two minute acquisiton time using electromagnetic z shift demonstrates an approximately 20 fold improvement in acquisition time.

The time-varying drive field in MPI is subject to magnetostimulation and specific absorption rate (SAR) limits. For frequencies up to 50 kHz, magnetostimulation is the dominant safety concern [1-2]. In this work, we investigate both the frequency and duration dependence of magnetostimulation limits for the drive field in MPI.

This prototype system has already produced high quality images with spatial resolutions very similar to the theoretically predicted resolution across mouse and rat sized FOVs. We have imaged both plastic and biological imaging phantoms, as well as ex vivo mice (not shown). Some early images are shown below in Fig. 1b-c. Imaging times are comparable to micro-MRI: full mouse and rat images (up to 12 cm × 5 cm × 5 cm) require between 2-5 minutes per image.

For angiography, stem cell imaging, and cancer imaging, magnetic particle imaging (MPI) can replace conventional techniques due to its safety in vivo, exquisite image contrast, and high detection sensitivity. However, compared to the theoretical physical sensitivity limit of a MPI scanner with 1 mm3 resolution using image 17 nm iron oxide nanoparticles, which lies between 100 nM and 1 μM1, the detection sensitivity for current MPI systems is worse by over 4 orders of magnitude. Factors contributing to this lack of sensitivity include non-optimal noise matching and feedthrough interference, of which the latter is a more dominant effect due to spectral overlap between the interfering signals and particle signals. Gleich et al., Schmale et al. and the authors of this study have previously attempted to decrease the interference through high-power filters and reducing capacitor distortion. In this work, we attempt to actively cancel interfering magnetic fields through the use of a feedforward transformer coupling circuit.

MPI is intrinsically a linear and shift-invariant (LSI) system as described in x-space theory [3,4] assuming perfect signal acquisition and that the imaging equation can be written as a convolution in realspace. However, the received MPI signal is corrupted by a direct feedthrough signal from the excitation source at the drive frequency, which is 106 times larger than the nanoparticle signal. Hardware filtering is required to suppress the feedthroughsignalbutalso inevitably removes this frequency component from the particle signal. This signal loss in the received particle signalbreaks the LSI propertiesof x-space MPI. This study will investigate the impact of losing the fundamental frequency component in the image domain and propose an algorithm to recover the lost information. Finally, experimental results demonstrate that LSI propertiescan be restored after applying a simple and robust baseline recovery algorithm.

Magnetic resonance imaging (MRI) pulse sequence consoles typically employ closed proprietary hardware, software, and interfaces, making difficult any adaptation for innovative experimental technology. Yet MRI systems research is trending to higher channel count receivers, transmitters, gradient/shims, and unique interfaces for interventional applications. Customized console designs are now feasible for researchers with modern electronic components, but high data rates, synchronization, scalability, and cost present important challenges. Implementing large multichannel MR systems with efficiency and flexibility requires a scalable modular architecture. With Medusa, we propose an open system architecture using the universal serial bus (USB) for scalability, combined with distributed processing and buffering to address the high data rates and strict synchronization required by multichannel MRI. Medusa uses a modular design concept based on digital synthesizer, receiver, and gradient blocks, in conjunction with fast programmable logic for sampling and synchronization. Medusa is a form of synthetic instrument, being reconfigurable for a variety of medical/scientific instrumentation needs. The Medusa distributed architecture, scalability, and data bandwidth limits are presented, and its flexibility is demonstrated in a variety of novel MRI applications.

Prepolarized magnetic resonance imaging (PMRI) uses two pulsed electromagnets to achieve high-field image quality with the benefits of low-field data acquisition. The principal challenge with all resistive MRI systems is the implementation of a highly precise magnet current supply. The noise current through the magnet is fundamentally limited by the current transducer used to provide feedback and the voltage reference used to generate the demand signal. Field instability in the main field magnet can both corrupt the received data and degrade the robustness of Carr¿Purcell¿Meiboom¿Gill (CPMG) echo trains, which are paramount to efficient imaging in PMRI. In this work, we present the magnet control system that achieved sufficient field stability for PMRI at $0.5/0.13$ T, identify the dominant sources of noise in the control system, examine the imaging artifacts that can occur if the field stability is insufficient, and identify how the design can be improved for better field stability, should it be required for future implementations of PMRI.

The V3 loop of the ENV glycoprotein exerts a dominant influence on the interaction of gp120 with coreceptors. Primary env genes cloned from sequential isolates from two seroconverters revealed Pro--Ala conversion in the conserved GPG motif of the V3 crown in seven of 17 R5 ENV. ENV containing the GPG motif in the V3 crown had fusogenic activity with chimeric receptors containing either the N terminus or loops of CCR5, whereas those with the GAG variant utilized only the former. Site-directed mutagenesis of multiple primary and prototypic R5 env genes demonstrated that the GPG motif was necessary for dual utilization of the N terminus and body of CCR5 in both gain and loss-of-function experiments. All ENV containing the GPG V3 crown showed CCR5 binding in the presence of soluble CD4, whereas it was not detected with the GAG variants. Molecular dynamic simulations of a V3 peptide predicts that the Pro--Ala substitution results in a conformational change with loss of the crown structure. These studies demonstrate that sequences in the third hypervariable region determine the specificity of coreceptor utilization for fusion, and that a conserved motif in the crown directly influences the molecular anatomy of the interaction between gp120 and CCR5.

Purpose: To develop a multi‐parametric molecular imager to assess tumor hypoxia and redox status by integrating Overhauser‐enhanced MRI (OMRI) with prepolarized MRI (PMRI). Method and Materials: PMRI system acquires anatomic MR images comparable to conventional MRI. With a moderate hardware augmentation of the PMRI, we implemented field‐cycled OMRI and obtained electron paramagnetic resonance(EPR)images of nitroxide spin probes. Phantoms were made of 2‐cc tubes filled with pure water

The high incidence of osteoarthritis and the recent advent of several new surgical and non-surgical treatment approaches have motivated the development of quantitative techniques to assess cartilage loss. Although magnetic resonance (MR) imaging is the most accurate non-invasive diagnostic modality for evaluating articular cartilage, improvements in spatial resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) would be valuable. Cartilage presents an imaging challenge due to its short T(2) relaxation time and its low water content compared with surrounding materials. Current methods sacrifice cartilage signal brightness for contrast between cartilage and surrounding tissue such as bone, bone marrow, and joint fluid. A new technique for imaging articular cartilage uses driven equilibrium Fourier transform (DEFT), a method of enhancing signal strength without waiting for full T(1) recovery. Compared with other methods, DEFT imaging provides a good combination of bright cartilage and high contrast between cartilage and surrounding tissue. Both theoretical predictions and images show that DEFT is a valuable method for imaging articular cartilage when compared with spoiled gradient-recalled acquisition in the steady state (SPGR) or fast spin echo (FSE). The cartilage SNR for DEFT is as high as that of either FSE or SPGR, while the cartilage-synovial fluid CNR of DEFT is as much as four times greater than that of FSE or SPGR. Implemented as a three-dimensional sequence, DEFT can achieve coverage comparable to that of other sequences in a similar scan time. Magn Reson Med 42:695-703, 1999.

A new technique for designing resistive homogeneous multicoil magnets for magnetic resonance imaging (MRI) is presented. A linearly independent subset of coils is chosen from a user-defined feasible set using an efficient numerical algorithm. The coil currents are calculated using a linear least squares algorithm to minimize the deviation of the actual magnetic field from the target field. The solutions are converted to practical coils by rounding the currents to integer ratios, selecting the wire gauge, and optimizing the coil cross-sections. To illustrate the technique, a new design of a short, homogeneous MRI magnet suitable for low-field human torso imaging is presented. Magnets that satisfy other constraints on access and field uniformity can also be designed. Compared with conventional techniques that employ harmonic expansions, this technique is flexible, simple to implement, and numerically efficient.