Hannes Dahnke

Limits of detection of SPIO at 3.0 T using T2* relaxometry

T2* relaxometry for quantitative MR imaging is strongly hampered by large‐scale field inhomogeneities, which lead to signal losses and an overestimation of the relaxation rate R2*. This is of particular importance for the sensitive detection of iron oxide contrast agent distributions. To derive an accurate measurement of T2*, a main field inhomogeneity correction is applied: the main field inhomogeneity is derived from multislice T2* relaxometry data and used as an initial value for an iterative optimization, by which the relaxation signal is corrected for each voxel. These corrected T2* maps show reduced influence of the local field variation and contain information about the local SPIO concentration. The method was tested on phantoms and the limit of detection of SPIO labeled cells using T2* relaxometry was estimated in volunteers to be 120 × 103 cells/mL (2.4 μg Fe/mL) in the brain and 385 × 103 cells/mL (8 μg Fe/mL) in the liver. Magn Reson Med 53:1202–1206, 2005.

Susceptibility gradient mapping (SGM): A new postprocessing method for positive contrast generation applied to superparamagnetic iron oxide particle (SPIO)-labeled cells

Local susceptibility gradients result in a dephasing of the precessing magnetic moments and thus in a fast decay of the NMR signals. In particular, cells labeled with superparamagnetic iron oxide particles (SPIOs) induce hypointensities, making the in vivo detection of labeled cells from such a negative image contrast difficult. In this work, a new method is proposed to selectively turn this negative contrast into a positive contrast. The proposed method calculates the susceptibility gradient and visualizes it in a parametric map directly from a regular gradient‐echo image dataset. The susceptibility gradient map is determined in a postprocessing step, requiring no dedicated pulse sequences or adaptation of the sequence before and during image acquisition. Phantom experiments demonstrated that local susceptibility differences can be quantified. In vivo experiments showed the feasibility of the method for tracking of SPIO‐labeled cells. The method bears the potential also for usage in other applications, including the detection of contrast agents and interventional devices as well as metal implants. Magn Reson Med 60:595–603, 2008.

In vivo magnetic resonance imaging of iron oxide-labeled, arterially-injected mesenchymal stem cells in kidneys of rats with acute ischemic kidney injury: detection and monitoring at 3T.

To evaluate MRI for a qualitative and quantitative in vivo tracking of intraaortal injected iron oxide–labeled mesenchymal stem cells (MSC) into rats with acute kidney injury (AKI).

Ultrashort T2* Relaxometry for Quantitation of Highly Concentrated Superparamagnetic Iron Oxide (SPIO) Nanoparticle Labeled Cells

A new method was developed to measure ultrashort T  2* relaxation in tissues containing a focal area of superparamagnetic iron oxide (SPIO) nanoparticle‐labeled cells in which the T  2* decay is too short to be accurately measured using regular gradient echo T  2* mapping. The proposed method utilizes the relatively long T2 relaxation of SPIO‐labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T  2* decay curve. MRI experiments in phantoms and rats with SPIO‐labeled tumors demonstrated that it can detect ultrashort T  2* down to 1 ms or less. The measured T  2* values were about 10% higher than those from the ultrashort TE (UTE) technique. The shorter the TE, the less the measurements deviated from the UTE T  2* mapping. Combined with the regular T  2* mapping, this technique is expected to provide quantitation of highly concentrated iron‐labeled cells from direct cell transplantation. Magn Reson Med, 2009.

Magnetic Resonance Imaging and Spectroscopy

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are noninvasive techniques that allow the characterization of morphology, physiology and metabolism in vivo. MRI and MRS have become techniques of choice in many pre-clinical and clinical applications. In this chapter, the basic principles and the instrumentation of MRI and MRS are described. Furthermore, the factors that influence the sensitivity are discussed and examples for the limit of contrast agent detection are given.

Tumor Blood Volume Determination by Using Susceptibility-corrected ΔR2* Multiecho MR

PURPOSE To evaluate a susceptibility-corrected multiecho magnetic resonance (MR) relaxometry technique for an accurate and robust determination of DeltaR2* as a noninvasive surrogate parameter of the perfused tumor blood volume. MATERIALS AND METHODS All experiments were approved by the institutional animal care committee. In a glass tube phantom with different superparamagnetic iron oxide (SPIO) particle concentrations and at tumor mice xenografts with DU-4475, HT-1080, and MDA-MB-435 tumors (n = 15 total, n = 5 per model) with different degrees of neovascularization after injection of different ultrasmall SPIO (USPIO) doses changes of the transverse relaxation rate (DeltaR2*) were determined by using a fixed echo time (TE) of 22 msec and a susceptibility-corrected multigradient-echo technique. The mean DeltaR2* value and the vascular volume fraction (VVF) of each tumor was determined and compared with independent in vivo fluorescent tumor perfusion measurements and histologic analysis helped determine microvessel density (MVD). Statistical differences were tested by using analysis of variance and linear correlations. RESULTS For the phantom study, DeltaR2* maps calculated with a fixed TE of 22 msec showed a higher standard deviation of the noise index compared with the susceptibility-corrected multiecho technique. For the xenograft model, mean tumor DeltaR2* values (+/- standard error of the mean) showed significant differences between the various tumors (eg, DU-4475: 12.3 sec(-1) +/- 2.67, HT-1080: 36.47 sec(-1) +/- 5.84, and MDA-MB-435: 64.01 sec(-1) +/- 8.87 at 80 mumol of iron per kilogram; P < .05). DeltaR2* values increased dose dependently and in a linear fashion, resulting in reproducibly stable VVF measurements. Fluorescent tumor perfusion measurements and MVD counts corroborated the MR results. CONCLUSION Susceptibility-corrected multiecho MR relaxometry allows a highly accurate and robust determination of DeltaR2* and VVF with an excellent dynamic range for tumor characterization at clinically relevant doses of USPIO.

Iterative Off-Resonance and Signal Decay Estimation and Correction for Multi-Echo MRI

Signal dephasing due to field inhomogeneity and signal decay due to transverse relaxation lead to perturbations of the Fourier encoding commonly applied in magnetic resonance imaging. Hence, images acquired with long readouts suffer from artifacts such as blurring, distortion, and intensity variation. These artifacts can be removed in reconstruction, usually based on separately collected information in form of field and relaxation maps. In this work, a recently proposed gridding-based algorithm for off-resonance correction is extended to also address signal decay. It is integrated into a new fixed-point iteration, which permits the joint estimation of an image and field and relaxation maps from multi-echo acquisitions. This approach is then applied in simulations and in vivo experiments and demonstrated to improve both images and maps. The rapid convergence of the fixed-point iteration in combination with the efficient gridding-based correction promises to render the running time of such a joint estimation acceptable.

Positive visualization of implanted devices with susceptibility gradient mapping using the original resolution

In magnetic resonance imaging, implantable devices are usually visualized with a negative contrast. Recently, positive contrast techniques have been proposed, such as susceptibility gradient mapping (SGM). However, SGM reduces the spatial resolution making positive visualization of small structures difficult. Here, a development of SGM using the original resolution (SUMO) is presented. For this, a filter is applied in k‐space and the signal amplitude is analyzed in the image domain to determine quantitatively the susceptibility gradient for each pixel. It is shown in simulations and experiments that SUMO results in a better visualization of small structures in comparison to SGM. SUMO is applied to patient datasets for visualization of stent and prostate brachytherapy seeds. In addition, SUMO also provides quantitative information about the number of prostate brachytherapy seeds. The method might be extended to application for visualization of other interventional devices, and, like SGM, it might also be used to visualize magnetically labelled cells. Magn Reson Med, 2011.

Quantification of the magnetic resonance signal response to dynamic (C)O2‐enhanced imaging in the brain at 3 T: R*2 BOLD vs. balanced SSFP

To compare two magnetic resonance (MR) contrast mechanisms, R*2 BOLD and balanced SSFP, for the dynamic monitoring of the cerebral response to (C)O2 respiratory challenges.

Fast T2 relaxometry with an accelerated multi-echo spin-echo sequence

A new method has been developed to reduce the number of phase‐encoding steps in a multi‐echo spin‐echo imaging sequence allowing fast T2 mapping without loss of spatial resolution. In the proposed approach, the k‐space data at each echo time were undersampled and a reconstruction algorithm that exploited the temporal correlation of the MR signal in k‐space was used to reconstruct alias‐free images. A specific application of this algorithm with multiple‐receiver acquisition, offering an alternative to existing parallel imaging methods, has also been introduced. The fast T2 mapping method has been validated in human brain T2 measurements in a group of nine volunteers with acceleration factors up to 3.4. The results demonstrated that the proposed method exhibited excellent linear correlation with the regular T2 mapping with full sampling and achieved better image reconstruction and T2 mapping with respect to SNR and reconstruction artifacts than the selected reference acceleration techniques. The new method has also been applied for quantitative tracking of injected magnetically labeled breast cancer cells in the rat brain with acceleration factors of 1.8 and 3.0. The proposed technique can provide an effective approach for accelerated T2 quantification, especially for experiments with single‐channel coil when parallel imaging is not applicable. Copyright