Sodium-23 MRI of whole spine at 3 Tesla using a 5-channel receive-only phased-array and a whole-body transmit resonator.

Sodium magnetic resonance imaging ((23)Na MRI) is a unique and non-invasive imaging technique which provides important information on cellular level about the tissue of the human body. Several applications for (23)Na MRI were investigated with regard to the examination of the tissue viability and functionality for example in the brain, the heart or the breast. The (23)Na MRI technique can also be integrated as a potential monitoring instrument after radiotherapy or chemotherapy. The main contribution in this work was the adaptation of (23)Na MRI for spine imaging, which can provide essential information on the integrity of the intervertebral disks with respect to the early detection of disk degeneration. In this work, a transmit-only receive-only dual resonator system was designed and developed to cover the whole human spine using (23)Na MRI and increase the receive sensitivity. The resonator system consisted of an already presented (23)Na whole-body resonator and a newly developed 5-channel receive-only phased-array. The resonator system was first validated using bench top and phantom measurements. A threefold SNR improvement at the depth of the spine (∼7cm) over the whole-body resonator was achieved using the spine array. (23)Na MR measurements of the human spine using the transmit-only receive-only resonator system were performed on a healthy volunteer within an acquisition time of 10minutes. A density adapted 3D radial sequence was chosen with 6mm isotropic resolution, 49ms repetition time and a short echo time of 540µs. Furthermore, it was possible to quantify the tissue sodium concentration in the intervertebral discs in the lumbar region (120ms repetition time) using this setup.

Pre-clinical functional Magnetic Resonance Imaging Part II: The heart.

One third of all deaths worldwide in 2008 were caused by cardiovascular diseases (CVD), and the incidence of CVD related deaths rises ever more. Thus, improved imaging techniques and modalities are needed for the evaluation of cardiac morphology and function. Cardiac magnetic resonance imaging (CMRI) is a minimally invasive technique that is increasingly important due to its high spatial and temporal resolution, its high soft tissue contrast and its ability of functional and quantitative imaging. It is widely accepted as the gold standard of cardiac functional analysis. In the short period of small animal MRI, remarkable progress has been achieved concerning new, fast imaging schemes as well as purpose-built equipment. Dedicated small animal scanners allow for tapping the full potential of recently developed animal models of cardiac disease. In this paper, we review state-of-the-art cardiac magnetic resonance imaging techniques and applications in small animals at ultra-high fields (UHF).

Pre-clinical functional Magnetic Resonance Imaging Part I: The kidney.

The prevalence of chronic kidney disease (CKD) is increasing worldwide. In Europe alone, at least 8% of the population currently has some degree of CKD. CKD is associated with serious comorbidity, reduced life expectancy, and high economic costs; hence, the early detection and adequate treatment of kidney disease is important. Pre-clinical research can not only give insights into the mechanisms of the various kidney diseases but it also allows for investigating the outcome of new drugs developed to treat kidney disease. Functional magnetic resonance imaging provides non-invasive access to tissue and organ function in animal models. Advantages over classical animal research approaches are numerous: the same animal might be repeatedly imaged to investigate a progress or a treatment of disease over time. This has also a direct impact on animal welfare and the refinement of classical animal experiments as the number of animals in the studies might be reduced. In this paper, we review current state of the art in functional magnetic resonance imaging with a focus on pre-clinical kidney imaging.

First in vivo potassium-39 (³9K) MRI at 9.4 T using conventional copper radio frequency surface coil cooled to 77 K.

Potassium-39 ((39)K) magnetic resonance imaging (MRI) is a noninvasive technique which could potentially allow for detecting intracellular physiological variations in common human pathologies such as stroke and cancer. However, the low signal-to-noise ratio (SNR) achieved in (39)K-MR images hampered data acquisition with sufficiently high spatial and temporal resolution in animal models so far. Full wave electromagnetic (EM) simulations were performed for a single-loop copper (Cu) radio frequency (RF) surface resonator with a diameter of 30 mm optimized for rat brain imaging at room temperature (RT) and at liquid nitrogen (LN2) with a temperature of 77 K. A novel cryogenic Cu RF surface resonator with home-built LN2 nonmagnetic G10 fiberglass cryostat system for small animal scanner at 9.4 T was designed, built and tested in phantom and in in vivo MR measurements. Aerogel was used for thermal insulation in the developed LN2 cryostat. In this paper, we present the first in vivo (39)K-MR images at 9.4 T for both healthy and stroke-induced rats using the developed cryogenic coil at 77 K. In good agreement with EM-simulations and bench-top measurements, the developed cryogenic coil improved the SNR by factor of 2.7 ± 0.2 in both phantom and in in vivo MR imaging compared with the same coil at RT.

[Bilateral 23Na MR imaging of the breast and quantification of sodium concentration]. / Bilaterale 23Na-MR-Bildgebung der Mamma und Quantifizierung der Natriumkonzentration.

A novel setup for (23)Na MRI, which allows bilateral imaging of the breast, is presented. For this purpose a figure-eight receive-only (23)Na surface coil was developed. For our experiments on three samples with NaCl solutions of different sodium concentrations and two female subjects we used an asymmetric birdcage coil in transmit mode and the developed surface coil for receiving the signal at 3T. Imaging of the samples showed the applicability of the employed normalization method for measuring the distribution of sodium concentration. In a sample of concentration [Na(+)]=51mM we achieved SNR=70 at a nominal isotropic resolution of 2,5mm (TR=66ms, TE=0,6ms, TA=20min). Furthermore we showed that by means of this setup it is possible to quantify the sodium concentration in breast tissue (TSC) of a female subject with an accuracy of 23% (TR=150ms, TE=0,5ms, TA=45min).

Bilateral kidney sodium-MRI: Enabling accurate quantification of renal sodium concentration through a two-element phased array system.

PURPOSE: To develop a sodium-MRI ((23) Na-MRI) method for bilateral renal sodium concentration (RSC) measurements in rat kidneys at 9.4 Tesla (T). MATERIALS AND METHODS: To simultaneously achieve high B1 -field homogeneity and high receive sensitivity a dual resonator system composed of a double-tuned linearly polarized (1) H/(23) Na volume resonator and a newly developed two-element (23) Na receive array was used. In conjunction with three-dimensional (3D) ultra-short Time-to-Echo sequence a quantification accuracy of ± 10% was achieved for a nominal spatial resolution of (1 × 1 × 4) mm(3) in 10 min acquisition time. The technique was applied to study the RSC in six kidneys before and after furosemide-induced diuresis. RESULTS: The loop diuretic agent induced an increase of cortical RSC by 22% from 86 ± 16 mM to 105 ± 18 mM (P = 0.02), whereas the RSC in the inner medulla decreased by 38% from 213 ± 24 mM to 132 ± 25 mM (P = 0.8×10(-4) ). The RSC changes measured in this study agreed well with the qualitative sodium signal intensity variations reported elsewhere. CONCLUSION: Furosemide-induced diuresis has been investigated accurately with herein presented quantitative (23) Na-MRI technique. In the future, RSC quantification could allow for defining pathological and nonpathological RSC ranges to assess sodium concentration changes, e.g., induced by drugs.

Inner-volume imaging in vivo using three-dimensional parallel spatially selective excitation.

This work describes the first experimental realization of three-dimensional spatially selective excitation using parallel transmission in vivo. For the design of three-dimensional parallel excitation pulses with short durations and high excitation accuracy, the choice of a suitable transmit k-space trajectory is crucial. For this reason, the characteristics of a stack-of-spirals trajectory and of a concentric-shells trajectory were examined in an initial simulation study. It showed that, especially when undersampling the trajectories in combination with parallel transmission, experimental parameters such as transmit-coil geometry and off-resonance conditions have an essential impact on the suitability of the selected trajectory and undersampling scheme. Both trajectories were applied in MR inner-volume imaging experiments which demonstrate that acceptably short and robust three-dimensional selective pulses can be achieved if the trajectory is temporally optimized and its actual path is measured and considered during pulse calculation. Pulse durations as short as 3.2 ms were realized and such pulses were appropriate to accurately excite arbitrarily shaped volumes in a corn cob and in a rat in vivo. Reduced field-of-view imaging of these selectively excited targets allowed high spatial resolution and significantly reduced measurement times and furthermore demonstrates the feasibility of three-dimensional parallel excitation in realistic MRI applications in vivo.

Whole body sodium MRI at 3T using an asymmetric birdcage resonator and short echo time sequence: first images of a male volunteer.

Sodium magnetic resonance imaging (²³Na MRI) is a non-invasive technique which allows spatial resolution of the tissue sodium concentration (TSC) in the human body. TSC measurements could potentially serve to monitor early treatment success of chemotherapy on patients who suffer from whole body metastases. Yet, the acquisition of whole body sodium (²³Na) images has been hampered so far by the lack of large resonators and the extremely low signal-to-noise ratio (SNR) achieved with existing resonator systems. In this study, a ²³Na resonator was constructed for whole body ²³Na MRI at 3T comprising of a 16-leg, asymmetrical birdcage structure with 34 cm height, 47.5 cm width and 50 cm length. The resonator was driven in quadrature mode and could be used either as a transceiver resonator or, since active decoupling was included, as a transmit-only resonator in conjunction with a receive-only (RO) surface resonator. The relative B1-field profile was simulated and measured on phantoms, and 3D whole body ²³Na MRI data of a healthy male volunteer were acquired in five segments with a nominal isotropic resolution of (6 × 6 × 6) mm³ and a 10 min acquisition time per scan. The measured SNR values in the ²³Na-MR images varied from 9 ± 2 in calf muscle, 15 ± 2 in brain tissue, 23 ± 2 in the prostate and up to 42 ± 5 in the vertebral discs. Arms, legs, knees and hands could also be resolved with applied resonator and short time-to-echo (TE) (0.5 ms) radial sequence. Up to fivefold SNR improvement was achieved through combining the birdcage with local RO surface coil. In conclusion, ²³Na MRI of the entire human body provides sub-cm spatial resolution, which allows resolution of all major human body parts with a scan time of less than 60 min.