Andreas K. Bitz

RF excitation using time interleaved acquisition of modes (TIAMO) to address B1 inhomogeneity in high‐field MRI

As the field strength and, therefore, the operational frequency in MRI is increased, the wavelength approaches the size of the human head/body, resulting in wave effects, which cause signal decreases and dropouts. Several multichannel approaches have been proposed to try to tackle these problems, including RF shimming, where each element in an array is driven by its own amplifier and modulated with a certain (constant) amplitude and phase relative to the other elements, and Transmit SENSE, where spatially tailored RF pulses are used. In this article, a relatively inexpensive and easy to use imaging scheme for 7 Tesla imaging is proposed to mitigate signal voids due to B  1+ field inhomogeneity. Two time‐interleaved images are acquired using a different excitation mode for each. By forming virtual receive elements, both images are reconstructed together using GRAPPA to achieve a more homogeneous image, with only small SNR and SAR penalty in head and body imaging at 7 Tesla. Magn Reson Med, 2010.

An eight-channel phased array RF coil for spine MR imaging at 7 T

Objective:To develop a transmit/receive radiofrequency (RF) array for MR imaging of the human spine at 7 T. The prototype is characterized in simulations and bench measurements, and the feasibility of high-resolution spinal cord imaging at 7 T is demonstrated in in vivo images of volunteers. Materials and Methods:The RF phased array consists of 8 overlapping surface loop coils with a dimension of 12 cm × 12 cm each. Bench measurements were obtained with a phantom made of body-simulating liquid and assessed with a network analyzer. For safety validation, numerical computations of the RF field distribution and the corresponding specific absorption rate were performed on the basis of 3 different human body models. In vivo images of 3 volunteers (2 with a documented scoliosis) were acquired using a 3D-FLASH sequence with a high spatial resolution of 0.57 mm isotropic. Results:The 7 T transmit/receive RF coil could be easily integrated into the patient table for examinations of the cervicothoracic or thoracolumbosacral spine. Comparable results were found for all 3 numerical calculations using different human body models. Measurements of the g-factor indicated good image quality for parallel imaging acceleration factors up to 2.7 along the head-feet direction, which could be validated in the in vivo images. The in vivo images demonstrated very fine anatomic features such as the longitudinal ligaments or the venous drainage through the vertebral bodies. A largely homogeneous excitation over an extensive field-of-view of 40 cm could be obtained. Conclusions:These early results indicate that a multichannel transmit/receive phased array RF coil can be used for in vivo spine imaging at 7 T, thereby rendering high-resolution spine imaging a promising new application in 7 T clinical research.

Proton spectroscopic imaging of the human prostate at 7 T.

The sensitivity of proton MR Spectroscopic Imaging (1H‐MRSI) of the prostate can be optimized by using the high magnetic field strength of 7 T in combination with an endorectal coil. In the work described in this paper we introduce an endorectal transceiver at 7 T, validate its safety for in vivo use and apply a pulse sequence, optimized for three‐dimensional (3D) 1H‐MRSI of the human prostate at 7 T. A transmit/receive endorectal RF coil was adapted from a commercially available 3 T endorectal receive‐only coil and validated to remain within safety guidelines for radiofrequency (RF) power deposition using numerical models, MR thermometry of phantoms, and in vivo temperature measurements. The 1H‐MRSI pulse sequence used adiabatic slice selective refocusing pulses and frequency‐selective water and lipid suppression to selectively obtain the relevant metabolite signals from the prostate. Quantum mechanical simulations were used to adjust the inter‐pulse timing for optimal detection of the strongly coupled spin system of citrate resulting in an echo time of 56 ms. Using this endorectal transceiver and pulse sequence with slice selective adiabatic refocusing pulses, 3D 1H‐MRSI of the human prostate is feasible at 7 T with a repetition time of 2 s. The optimized inter‐pulse timing enables the absorptive detection of resonances of spins from spermine and citrate in phase with creatine and choline. These potential tumor markers may improve the in vivo detection, localization, and assessment of prostate cancer. Copyright

Lymphoma development in mice chronically exposed to UMTS-modulated radiofrequency electromagnetic fields.

Abstract Sommer, A. M., Bitz, A. K., Streckert, J., Hansen, V. W. and Lerchl, A. Lymphoma Development in Mice Chronically Exposed to UMTS-Modulated Radiofrequency Electromagnetic Fields. Radiat. Res. 168, 72–80 (2007). There are public concerns regarding possible carcinogenic or cancer-promoting effects of electromagnetic fields (EMFs) from mobile phones and base stations. The objective of the present study was to investigate whether chronic exposure to EMFs of the UMTS (Universal Mobile Telecommunication System) influences the development of lymphoma in a lymphoma animal model, the AKR/J mouse. Unrestrained mice were chronically sham-exposed (n = 160) or exposed (n = 160) in identical exposure systems (radial waveguides) to a generic UMTS test signal (24 h per day, 7 days per week, 0.4 W/kg SAR). Additionally, 30 animals were kept as cage controls. Animals were checked visually each day and were weighed and palpated weekly to detect swollen lymph nodes. Starting at the age of 6 months, blood samples were taken from the tail every 2 weeks to perform differential leukocyte counts and to measure the hematocrit. Visibly diseased animals or those older than 43 weeks were killed humanely, and tissue slices were examined for metastatic infiltrations and lymphoma type. The study was performed in a blinded way. Cage control animals had a significantly lower growth rate than those kept in the radial waveguides. The number of ill animals, the mean survival time, and the severity code of the disease did not differ between the experimental groups. Therefore, the data show no negative effects from exposure and corroborate earlier findings in AKR/J mice exposed to GSM EMF (Sommer et al., BMC Cancer 4, 77–90, 2004).

Effects of universal mobile telecommunications system (UMTS) electromagnetic fields on the blood-brain barrier in vitro

Abstract Franke, H., Streckert, J., Bitz, A., Goeke, J., Hansen, V., Ringelstein, E. B., Nattkämper, H., Galla, H-J. and Stögbauer, F. Effects of Universal Mobile Telecommunications System (UMTS) Electromagnetic Fields on the Blood-Brain Barrier In Vitro. Radiat. Res. 164, 258–269 (2005). The extensive use of mobile phone communication has raised public concerns about adverse health effects of radiofrequency (RF) electromagnetic fields (EMFs) in recent years. A central issue in this discussion is the question whether EMFs enhance the permeability of the blood-brain barrier (BBB). Here we report an investigation on the influence of a generic UMTS (Universal Mobile Telecommunications System) signal on barrier tightness, transport processes and the morphology of porcine brain microvascular endothelial cell cultures (PBEC) serving as an in vitro model of the BBB. An exposure device with integrated online monitoring system was developed for simultaneous exposure and measuring of transendothelial electrical resistance (TEER) to determine the tightness of the BBB. PBEC were exposed continuously for up to 84 h at an average electric-field strength of 3.4–34 V/m (maximum 1.8 W/kg) ensuring athermal conditions. We did not find any evidence of RF-field-induced disturbance of the function of the BBB. After and during exposure, the tightness of the BBB quantified by 14C-sucrose and serum albumin permeation as well as by TEER remained unchanged compared to sham-exposed cultures. Permeation of transporter substrates at the BBB as well as the localization and integrity of the tight-junction proteins occludin and ZO1 were not affected either.

In vivo 31P MR spectroscopic imaging of the human prostate at 7 T: Safety and feasibility

31P MR spectroscopic imaging of the human prostate provides information about phosphorylated metabolites that could be used for prostate cancer characterization. The sensitivity of a magnetic field strength of 7 T might enable 3D 31P MR spectroscopic imaging with relevant spatial resolution in a clinically acceptable measurement time. To this end, a 31P endorectal coil was developed and combined with an eight‐channel 1H body‐array coil to relate metabolic information to anatomical location. An extensive safety validation was performed to evaluate the specific absorption rate, the radiofrequency field distribution, and the temperature distribution of both coils. This validation consisted of detailed Finite Integration Technique simulations, confirmed by MR thermometry and B  1+ measurements in a phantom and in vivo temperature measurements. The safety studies demonstrated that the presence of the 31P endorectal coil had no influence on the specific absorption rate levels and temperature distribution of the external eight‐channel 1H array coil. To stay within a 10 g averaged local specific absorption rate of 10 W/kg, a maximum time‐averaged input power of 33 W for the 1H array coil was allowed. For transmitting with the 31P endorectal coil, our safety limit of less than 1°C temperature increase in vivo during a 15‐min MR spectroscopic imaging experiment was reached at a time‐averaged input power of 1.9 W. With this power setting, a second in vivo measurement was performed on a healthy volunteer. Using adiabatic excitation, 3D 31P MR spectroscopic imaging produced spectra from the entire prostate in 18 min with a spatial resolution of 4 cm3. The spectral resolution enabled the separate detection of phosphocholine, phosphoethanolamine, inorganic phosphate, and other metabolites that could play an important role in the characterization of prostate cancer. Magn Reson Med, 2012.

Exposure to high‐field MRI does not affect cognitive function

To assess potential cognitive deficits under the influence of static magnetic fields at various field strengths some studies already exist. These studies were not focused on attention as the most vulnerable cognitive function. Additionally, mostly no magnetic resonance imaging (MRI) sequences were performed.

RF exposure of biological systems in radial waveguides

Two radial waveguide exposure systems for multiple biological targets are described that were conceived, dimensioned, implemented, measured and dosimetrically analyzed. In the first case, a number of 120 hamsters was exposed with mobile communication signals at 383 and 900 MHz. The variation of the dissipated energy within the animals could be reduced to less than 30% though the objects were not restrained. Whole body specific absorption rates of 80 mW/kg were applied. The other system was used for exposing hamsters pineal glands in culture. The small size of the samples yields an even smaller variation of the exposure of 2.4%. This is a consequence of the excellent uniformity of the electromagnetic field distribution across the exposure region, which was achieved by a good rotational symmetry of the waveguide and a careful design of the feed antenna and the absorber/shortcut combination. The paper reports on the technical aspects of the RF exposure.

Feasibility of T2 -weighted turbo spin echo imaging of the human prostate at 7 tesla.

To demonstrate that high quality T2‐weighted (T2w) turbo spin‐echo (TSE) imaging of the complete prostate can be achieved routinely and within safety limits at 7 T, using an external transceive body array coil only.

Renal imaging at 7 Tesla: preliminary results

ObjectiveTo investigate the feasibility of 7T MR imaging of the kidneys utilising a custom-built 8-channel transmit/receive radiofrequency body coil.MethodsIn vivo unenhanced MR was performed in 8 healthy volunteers on a 7T whole-body MR system. After B0 shimming the following sequences were obtained: 1) 2D and 3D spoiled gradient-echo sequences (FLASH, VIBE), 2) T1-weighted 2D in and opposed phase 3) True-FISP imaging and 4) a T2-weighted turbo spin echo (TSE) sequence. Visual evaluation of the overall image quality was performed by two radiologists.ResultsRenal MRI at 7T was feasible in all eight subjects. Best image quality was found using T1-weighted gradient echo MRI, providing high anatomical details and excellent conspicuity of the non-enhanced vasculature. With successful shimming, B1 signal voids could be effectively reduced and/or shifted out of the region of interest in most sequence types. However, T2-weighted TSE imaging remained challenging and strongly impaired because of signal heterogeneities in three volunteers.ConclusionThe results demonstrate the feasibility and diagnostic potential of dedicated 7T renal imaging. Further optimisation of imaging sequences and dedicated RF coil concepts are expected to improve the acquisition quality and ultimately provide high clinical diagnostic value.