Susceptibility gradient quantization by MRI signal response mapping (SIRMA) to dephaser

Normalized averaged range (nAR), a robust quantification method for MPIO-content

Micron-sized paramagnetic iron oxide particles (MPIO) are commonly used as contrast agents in magnetic resonance imaging (MRI) that produce negative contrast enhancement, i.e. darkening, on T2^*-weighted images. However, estimation and quantification of MPIO in vivo is still challenging. This limitation mainly arises from smearing and displacement of the negative contrast of the MPIO, so-called blooming, potentially leading to false-positive detection. Further, the bias field induced by the MR coils also hinders visualization and quantification of the MPIO. To mitigate these drawbacks, a positive contrast image can be generated, for example by using a frequency offset technique, which can significantly improve the accuracy of quantification methods. In this research, we introduce the normalized average range (nAR) as a new way to quantify the relative MPIO content within a study. The method compares the average value of test ROIs to that of a control ROI in range filtered images. The nAR can be used on both positive and negative contrast images. The nAR was tested on agar phantoms containing various MPIO concentrations, and on a rostral migration model for MPIO labeled stem cells in mice. The amount of MPIO was quantified for biased and unbiased data, and both for positive and negative contrast images. In addition, the presence of MPIOs in the olfactory bulb was verified by histology. The results show the nAR can indicate the presence and relative content of MPIO for both negative and positive images. However, the nAR showed slightly higher sensitivity in optimized positive contrast images compared to negative contrast images. In all cases, the bias field played a minor role in the quantification, making debiasing less of a concern. The dependency of the nAR values on the MPIO content in the ROI was further validated histologically. Thus, the nAR provides a robust and reliable tool for quantification of MPIO in mice.

Perfluorocarbon-Loaded Lipid Nanocapsules to Assess the Dependence of U87-Human Glioblastoma Tumor pO2 on In Vitro Expansion Conditions

Growing tumor cell lines, such as U87-MG glioma cells, under mild hypoxia (3% O2) leads to a ca. 40% reduction in growth rate once implanted in the brain of nude mice, as compared to normoxia (21% O2) grown cells, wherein the former over-express HIF-1 and VEGF-A. Despite developing differently, the tumors have similar: blood perfusion, oxygen consumption, and vascular surface area parameters, whereas the number of blood vessels is nearly doubled in the tumor arising from normoxia cultured cells. Interestingly, tumor oxygen tension, measured using 19F-oximetry, showed that the normoxia grown cells led to tumors characterized by mild hypoxic environment (approximately 4%) conditions, whilst the hypoxia grown cells led to tumors characterized by physioxic environment (approximately 6%) conditions. This reversal in oxygen concentration may be responsible for the apparent paradoxical growth profiles.

Using MRI to detect and differentiate calcium oxalate and calcium hydroxyapatite crystals in air-bubble-free phantom

Calcium oxalate (CaOX) crystals and calcium hydroxyapatite (CaHA) crystals were commonly associated with breast benign and malignant lesions, respectively. In this research, CaOX (n = 6) and CaHA (n = 6) crystals in air-bubble-free agarose phantom were studied and characterized by using MRI at 9.4 T scanner. Calcium micro-crystals, with sizes that ranged from 200 to 500 µm, were made with either 99% pure CaOX or CaHA powder and embedded in agar to mimic the dimensions and calcium content of breast microcalcifications in vivo. MRI data were acquired with high spatial resolution T2-weighted (T2W) images and gradient echo images with five different echo times (TEs). The crystal areas were determined by setting the threshold relative to agarose signal. The ratio of crystal areas was calculated by the measurements from gradient echo images divided by T2W images. Then the ratios as a function of TE were fitted with the radical function. The results showed that the blooming artifacts due to magnetic susceptibility between agar and CaHA crystals were more than twice as large as the susceptibility in CaOX crystals (p < 0.05). In addition, larger bright rings were observed on gradient echo images around CaHA crystals compared to CaOX crystals. Our results suggest that MRI may provide useful information regarding breast microcalcifications by evaluating the apparent area of crystal ratios obtained between gradient echo and T2W images.

Positive contrast of SPIO-labeled cells by off-resonant reconstruction of 3D radial half-echo bSSFP

This article describes a new acquisition and reconstruction concept for positive contrast imaging of cells labeled with superparamagnetic iron oxides (SPIOs). Overcoming the limitations of a negative contrast representation as gained with gradient echo and fully balanced steady state (bSSFP), the proposed method delivers a spatially localized contrast with high cellular sensitivity not accomplished by other positive contrast methods. Employing a 3D radial bSSFP pulse sequence with half-echo sampling, positive cellular contrast is gained by adding artificial global frequency offsets to each half-echo before image reconstruction. The new contrast regime is highlighted with numerical intravoxel simulations including the point-spread function for 3D half-echo acquisitions. Furthermore, the new method is validated on the basis of in vitro cell phantom measurements on a clinical MRI platform, where the measured contrast-to-noise ratio (CNR) of the new approach exceeds even the negative contrast of bSSFP. Finally, an in vivo proof of principle study based on a mouse model with a clear depiction of labeled cells within a subcutaneous cell islet containing a cell density as low as 7 cells/mm(3) is presented. The resultant isotropic images show robustness to motion and a high CNR, in addition to an enhanced specificity due to the positive contrast of SPIO-labeled cells.

Classification of breast lesions pre-contrast injection using water resonance lineshape analysis

Inhomogeneously broadened, non-Lorentzian water resonances have been observed in small image voxels of breast tissue. The non-Lorentzian components of the water resonance are probably produced by bulk magnetic susceptibility shifts caused by dense, deoxygenated tumor blood vessels (the 'blood oxygenation level-dependent' effect), but can also be produced by other characteristics of local anatomy and physiology, including calcifications and interfaces between different types of tissue. Here, we tested the hypothesis that the detection of non-Lorentzian components of the water resonance with high spectral and spatial resolution (HiSS) MRI allows the classification of breast lesions without the need to inject contrast agent. Eighteen malignant lesions and nine benign lesions were imaged with HiSS MRI at 1.5 T. A new algorithm was developed to detect non-Lorentzian (or off-peak) components of the water resonance. After a Lorentzian fit had been subtracted from the data, the largest peak in the residual spectrum in each voxel was identified as the major off-peak component of the water resonance. The difference in frequency between these off-peak components and the main water peaks, and their amplitudes, were measured in malignant lesions, benign lesions and breast fibroglandular tissue. Off-peak component frequencies were significantly different between malignant and benign lesions (p < 0.001). Receiver operating characteristic (ROC) analysis was used to assess the diagnostic performance of HiSS off-peak component analysis compared with dynamic contrast-enhanced (DCE) MRI parameters. The areas under the ROC curves for the 'DCE rapid uptake fraction', 'DCE washout fraction', 'off-peak component amplitude' and 'off-peak component frequency' were 0.75, 0.83, 0.50 and 0.86, respectively. These results suggest that water resonance lineshape analysis performs well in the classification of breast lesions without contrast injection and could improve the diagnostic accuracy of clinical breast MR examinations. In addition, this approach may provide an alternative to DCE MRI in women who are at risk for adverse reactions to contrast media.

Perfluorocarbon-loaded lipid nanocapsules as oxygen sensors for tumor tissue pO₂ assessment

The assessment of tumor oxygenation is a crucial factor in cancer therapy and may be carried out using fluorine MRI once fluorine probes have been distributed within the tumor. However, the deposit of those highly fluorinated compounds often jeopardizes anatomical image quality and requires emulsification of the probes. Due to the high density and the high lipophilicity of perfluorocarbons, nanoemulsion of these molecules usually requires high-energy processes. In the present work, we discuss the synthesis and the physico-chemical characterization of perfluorocarbon nanocapsules using a low-energy phase-inversion process. The nanocapsules were tested on a mouse tumor brain model to assess oxygenation.

Selective depiction of susceptibility transitions using Laplace-filtered phase maps

In this work, we aim to demonstrate the ability of Laplace-filtered three-dimensional (3D) phase maps to selectively depict the susceptibility transitions in an object. To realize this goal, it is first shown that both the Laplace derivative of the z component of the static magnetic field in an object and the Laplacian of the corresponding phase distribution may be expected to be zero in regions of constant or linearly varying susceptibility and to be nonzero when there is an abrupt change in susceptibility, for instance, at a single point, a ridge, an interface, an edge or a boundary. Next, a method is presented by which the Laplace derivative of a 3D phase map can be directly extracted from the complex data, without the need for phase unwrapping or subtraction of a reference image. The validity of this approach and of the theory behind it is subsequently demonstrated by simulations and phantom experiments with exactly known susceptibility distributions. Finally, the potential of the Laplace derivative analysis is illustrated by simulations with a Shepp-Logan digital brain phantom and experiments with a gel phantom containing positive and negative focal susceptibility deviations.

Utilizing echo-shifts in k-space for generation of positive contrast in areas with marked susceptibility alterations

A technique for generation of positive contrast near susceptibility alterations utilizing echo-shifts in k-space is introduced, based on altered Larmor-frequencies and resulting phase-shifts accumulating during the echo-time at the site of local magnetic field gradients. 3D gradient-echo raw-data is acquired and weighted with an inverse Hanning filter. The filter partly suppresses central raw-data points, while maintaining outer areas. Reconstruction of the filtered raw-data results in images where pixels with apparent magnetic field gradients are highlighted against homogeneous pixels. Further processing steps are introduced to remove remaining intensities in the homogeneous parts of the filtered image. Feasibility is shown by an agar phantom containing magnetically labeled cells, with concentrations of 25, 50, 100, and 250 cells/μL, and by images of the human head. The technique allows detection of echo-shifted pixels with automatic suppression of magnetically homogeneous parts while keeping post-processing time short. Fewer than four labeled cells per pixel were clearly displayed with positive contrast. Application to the human head shows bright veins and complete suppression of homogeneous regions. The presented technique has high potential for specific detection of low concentrations of labeled cells or susceptibility altered regions in vivo with positive contrast, whereas areas with low spin density are not highlighted.