M. E. Bellemann

Magnetic Nanoparticles for Biomedical Heating Applications

Summary In the past, magnetic nanoparticles have found increasing interest in different biomedical applications, e.g. the magnetic hyperthermia of tumor cells or the remote controlled drug delivery to the gut. These applications are based on a magnetically induced heating effect caused by different magnetic loss mechanisms in the nanoparticles. To advance the present state of the art of these methods, it is important to use particles with a higher specific heating power (SHP) at lower magnetic field amplitudes. To this aim, several iron oxide nanoparticle powders, consisting of particles in the diameter range from 10 nm up to 100 nm, were prepared by two different chemical methods and magnetically as well as morphologically characterized. The magnetic characterization was done by using a vibrating sample magnetometer and the calorimetrical determination of SHP. The dependence of the magnetic losses on the morphological properties was investigated. Magnetic characterization showed that several suitable iron oxide absorbers can be utilized. With decreasing particle size, hysteresis loss underestimates SHP at higher frequencies as measured calorimetrically. The effect of measurement frequency on the hysteresis losses is shown experimentally. Experimental results are discussed in the frame of known theoretical models of nanoparticle magnetism.

Evaluation of inverse algorithms in the analysis of magnetic flux leakage data

We evaluate the use of linear and nonlinear inverse algorithms (maximum entropy method, low resolution electromagnetic tomography, L/sub 1/ and L/sub 2/ norm methods) in the analysis of magnetic flux leakage (MFL) measurements commonly used for the detection of flaws and irregularities in gas and oil pipelines. We employed MFL data from a pipe with well-defined artificial surface breaking flaws at the internal and external wall. Except for the low-resolution electromagnetic tomography, all algorithms show, on average, similar accuracy in the flaw extent estimation. Maximum entropy and the L/sub 1/ norm have a tendency to yield better results for smaller flaws, while the L/sub 2/ norm performs slightly better for larger flaws. The errors of the flaw location estimation are comparable for the maximum entropy and the L/sub 2/ norm algorithm. The L/sub 1/ norm performs worse for those flaws situated on the internal pipe wall. Linear methods (L/sub 2/ norm) are easier to implement and require less computation time than nonlinear methods (maximum entropy method, L/sub 1/ norm). In conclusion, inverse algorithms potentially provide a powerful means for the detection and characterization of flaws in MFL data.

Passive vortex currents in magneto- and electrocardiography: comparison of magnetic and electric signal strengths

Vortex currents may be of importance in the early diagnosis of myocardial infarction caused by an occlusion of a coronary artery. We investigated the influence of a passive vortex current distribution, modelled by different conductivities in a hollow cylinder, on the localization error and on the signal strength in both the magnetocardiogram and the electrocardiogram. The hollow cylinder was mounted in a realistically shaped physical torso phantom. A platinum dipole was inserted into the cylinder. The compartment boundaries were modelled with two special ionic exchange membranes. The conductivity ratio of the cylinder compartment to the torso compartment was varied from 0.25 to 100. We compared the simultaneously measured magnetic and electric signal strengths as a function of this conductivity ratio. We found that an increasing conductivity ratio causes only a slight increase (about 19%) of the magnetic signal strength but a strong decrease (about 81%) of the electric signal strength. Using a homogeneous torso model, the dipole localization errors were, depending on the conductivity ratio, up to 16 mm. In conclusion, passive vortex currents might partially explain the differences between magnetocardiographic and electrocardiographic recordings observed both experimentally and clinically.

Parametric display of myocardial function

Quantitative assessment of regional heart motion has significant potential to provide more specific diagnosis of cardiac disease and cardiac malfunction than currently possible. Local heart motion may be captured from various medical imaging scanners. In this study, 3-D reconstructions of pre-infarct and post-infarct hearts were obtained from the Dynamic Spatial Reconstructor (DSR)[Ritman EL, Robb RA, Harris LD. Imaging physiological functions: experience with DSR. Philadelphia: Praeger, 1985; Robb RA, Lent AH, Gilbert BK, Chu A. The dynamic spatial reconstructor: a computed tomography system for high-speed simultaneous scanning of multiple cross sections of the heart. J Med Syst 1980;4(2):253-88; Jorgensen SM, Whitlock SV, Thomas PJ, Roessler RW, Ritman EL. The dynamic spatial reconstructor: a high speed, stop action, 3-D, digital radiographic imager of moving internal organs and blood. Proceedings of SPIE, Ultrahigh- and High-speed Photography, Videography, Photonics, and Velocimetry 1990;1346:180-91.] (DSR). Using functional parametric mapping of disturbances in regional contractility and relaxation, regional myocardial motion during a cardiac cycle is color mapped onto a deformable heart model to facilitate appreciation of the structure-to-function relationships in the myocardium, such as occurs in regional patterns of akinesis or dyskinesis associated with myocardial ischemia or infarction resulting from coronary artery occlusion.

ROC analysis for assessment of lesion detection performance in 3D PET: influence of reconstruction algorithms.

Image quality in positron emission tomography (PET) can be assessed with physical parameters, as spatial resolution and signal-to-noise ratio, or using psychophysical approaches, which include the observer performance and the considered task (ROC analysis). For PET in oncology, such a task is the detection of hot lesions. The aim of the present study was to assess the lesion detection performance due to adequate modeling of the scanner and the measurement process in the image reconstruction process. We compared the standard OSEM software of the manufacturer with a sophisticated fully 3D iterative reconstruction technique (USC MAP). A rectangular phantom with 6 oblique line sources in a homogeneous background (2.6 kBq/ml 18F) was imaged dynamically with an ECAT EXACT HR+ scanner in 3D mode. Reconstructed activity contrasts varied between 15 and 0, as the line sources were filled with 11C (3.2 MBq/ml). Measured attenuation and standard randoms, dead time, and scatter corrections of the manufacturer were employed. For the ROC analysis, a software tool presented a cut-out of the phantom (15 x 15 pixels) to two observers. These cut-outs were rated (5 classes) and the area Az under the ROC curve was determined as a measure of detection performance. The improvement for Az with USC MAP compared to the OSEM reconstructions ranged between 0.02 and 0.23 for signal-to-noise ratios of the background between 2.8 and 3.1 and lesion contrast between 2.1 and 4.2. This study demonstrates that adequate modeling of the measurement process in the reconstruction algorithm improves the detection of small hot lesions markedly.

DTI measurements of isotropic and anisotropic media.

The ability to measure different rates of diffusion along different directions is one of the features that distinguish DTI from other imaging methods. It allows to extract and visualize information on tissue microstructure and microdynamics. However, to correctly determine the full diffusion tensor, the so-called b-matrix has to be calculated by taking into account the non-negligible influences of image gradients and cross-terms between imaging and diffusion gradients. In this work validation of this b-matrix correction was investigated by determining self-diffusion coefficients of several isotropic media on a 1.5 T clinical whole-body scanner. To investigate these influences on the measurements of anisotropic media the same experiments were performed with a carrot.

Magnetresonanztomographie von Stents: Quantitative MR-Untersuchungen in vitro bei 3 Tesla

PURPOSE The aim of this study was to qualitatively and quantitatively study MR artifacts of various stents on the basis of in vitro experiments. We were particularly interested whether sequence type and orientation of the stent with respect to the static magnetic field influences the artifact. MATERIAL AND METHODS We examined 18 stents of different material (nitinol, stainless steel, cobalt alloy), different design of the stent meshes (AccuLink, OmniLink, DynaLink, Xact, Protoge, Wallstent Monorail), different diameter (5-10mm) and different length (18-58 mm) with a turbo spin echo (TSE), a 2D-fast low angle shot (FLASH) and a 3D-FLASH sequence. The MR images were examined qualitatively with respect to possible artifacts. Furthermore we examined the MR data quantitatively: The contrast-noise-ratio (CNR) was determined both within the stent and outside (within the tube); based on these values we calculated the transparency factor P, furthermore we calculated the apparent vascular lumen within the tube and within the stent. RESULTS The stents made of stainless steel and cobalt alloy displayed severe susceptibility artifacts. Therefore the vessel lumen within the stent could not be assessed. The nitinol stents showed different artifact patterns: The AccuLink and DynaLink stents showed less artifacts compared to the Xact and Protoge stents. Besides the susceptibility artifacts we found artifacts due to RF shielding by the stent mesh, particularly in TSE sequences. CONCLUSION A MR control of patients after stenting is possible and may yield diagnostic information when using the AccuLink or DynaLink stents. However, it is important to make sure that the stent is MR safe for the field strength used for the examination.

Magnetic Marker Monitoring Using a Permanent Magnetic Sphere Oriented by a Rotating Magnetic Field

Magnetic marker monitoring (MMM) is a diagnostic technique known since about 1990 and mainly applied for motility assessment in the digestive tract. A particularly favorable MMM method uses a rotating marker, which can be aligned along an externally applied magnetic field HP. This novel method of rotating magnetic marker monitoring (RMMM) provides the starting point for the construction of small portable monitoring equipment. Friction effects and background fields, however, may cause deviations of the magnetically determined marker position from the actual location. In order to determine the magnitude of possible deviations, we measured the effect of friction acting on the marker in its bearing case by means of a realistic measuring principle and found a torque of static friction ranging from 10−7 N·m (air as bearing liquid) to about 10−6 N·m (special silicone oil). The torque linearly increases with increasing rotational frequency of the marker sphere. Furthermore, we estimated the influence of a background field HB by applying the method of look-up table and found that the operating distance D for monitoring mainly depends on the ratio HP/HB. Our preliminary set-up is working with pulsed HP fields and provides D ≈ 20 cm. However, D can be enlarged with increasing amplitude of HP. Taking into account practical demands of RMMM applications as well as technical restrictions, we propose means to enlarge the operating distance. It should be pointed out that the rotating marker can be used for remote controlled drug release as well.

Assessment of image reconstruction parameters in PET using physical and statistical figures of merit

The aim of this study was to analyze the recommended OSEM image reconstruction parameters in positron emission tomography (PET). Spatial resolution, signal-to-noise ratio, and contrast were used as physical figures of merit (FOM). For statistical FOMs, the t-value and the area under the receiver operating characteristic (ROC) were employed. The spatial resolution was measured with 21 point sources. The signal-to-noise ratio, the contrast, and the t-value were investigated with a whole-body phantom with hollow spheres inserted. A phantom containing line sources was used for ROC analysis. As result, the reconstruction parameters recommended for visual evaluation lead to images with an adequate lesion detectability. The quantitative reconstruction, however, needs improvement.

Quantitative analysis and parametric display of regional myocardial mechanics

Quantitative assessment of regional heart motion has significant potential for more accurate diagnosis of heart disease and/or cardiac irregularities. Local heart motion may be studied from medical imaging sequences. Using functional parametric mapping, regional myocardial motion during a cardiac cycle can be color mapped onto a deformable heart model to obtain better understanding of the structure- to-function relationships in the myocardium, including regional patterns of akinesis or diskinesis associated with ischemia or infarction. In this study, 3D reconstructions were obtained from the Dynamic Spatial Reconstructor at 15 time points throughout one cardiac cycle of pre-infarct and post-infarct hearts. Deformable models were created from the 3D images for each time point of the cardiac cycles. Form these polygonal models, regional excursions and velocities of each vertex representing a unit of myocardium were calculated for successive time-intervals. The calculated results were visualized through model animations and/or specially formatted static images. The time point of regional maximum velocity and excursion of myocardium through the cardiac cycle was displayed using color mapping. The absolute value of regional maximum velocity and maximum excursion were displayed in a similar manner. Using animations, the local myocardial velocity changes were visualized as color changes on the cardiac surface during the cardiac cycle. Moreover, the magnitude and direction of motion for individual segments of myocardium could be displayed. Comparison of these dynamic parametric displays suggest that the ability to encode quantitative functional information on dynamic cardiac anatomy enhances the diagnostic value of 4D images of the heart. Myocardial mechanics quantified this way adds a new dimension to the analysis of cardiac functional disease, including regional patterns of akinesis and diskinesis associated with ischemia and infarction. Similarly, disturbances in regional contractility and filling may be detected and evaluated using such measurements and displays.