Enzymatic activity monitoring through dynamic nuclear polarization in Earth magnetic field.

Cost-effective and portable MRI systems operating at Earth-field would be helpful in poorly accessible areas or in developing nations. Furthermore Earth-field MRI can provide new contrasts opening the way to the observation of pathologies at the biochemical level. However low-field MRI suffers from a dramatic lack in detection sensitivity even worsened for molecular imaging purposes where biochemical specificity requires detection of dilute compounds. In a preliminary spectroscopic approach, it is proposed here to detect protease-driven hydrolysis of a nitroxide probe thanks to electron-nucleus Overhauser enhancement in a home-made double resonance system in Earth-field. The combination of the Overhauser effect and the specific enzymatic modification of the probe provides a smart contrast reporting the enzymatic activity. The nitroxide probe is a six-line nitroxide which lines are shifted according to its substrate/product state, which requires quantum mechanical calculations to predict EPR line frequencies and Overhauser enhancements at Earth field. The NMR system is equipped with a 13-mT prepolarization coil, a 153-MHz EPR coil and a 2-kHz NMR coil. Either prepolarized NMR or DNP-NMR without prepolarization provide NMR spectra within 3 min. The frequency dependence of Overhauser enhancement was in agreement with theoretical calculations. Protease-mediated catalysis of the nitroxide probe could only be measured through the Overhauser effect with 5 min time resolution. Future developments shall open the way for the design of new low-field DNP-MRI systems.

Microglia used as vehicles for both inducible thymidine kinase gene therapy and MRI contrast agents for glioma therapy.

Microglia are phagocytic cells that are chemoattracted by brain tumors and can represent up to 70% of the tumor cell population. To get insight into gene therapy against glioma, we decided to take advantage of those microglia properties and to use those cells as vehicles to transport simultaneously a suicide gene (under the control of a heat-sensitive promoter) and contrast agents to localize them by magnetic resonance imaging before applying any therapeutic treatment. Thymidine kinase (TK) expression and its functionality after gancyclovir administration were investigated. After the heat shock (44 degrees C and 20 min), TK was expressed in 50% of the cells. However, after gancyclovir treatment, 90% of the cells died by apoptosis, showing an important bystander effect. Then, the cells were incubated with new lanthanide contrast agents to check both their potential toxicity and their MR properties. Results indicate that the nanoparticles did not induce any cell toxicity and yield a hypersignal on MR images at 4.7 T. These in vitro experiments indicate that microglia are good candidates as vectors in gene therapy against brain tumors. Finally, microglia containing gadolinium-grafted nanoparticles were injected in the close vicinity of C6 tumor, in a mouse. The hyperintensive signal obtained on in vivo images as well as its retention time show the potential of the novel contrast agents for cellular imaging.

A standardised method for measuring magnetisation transfer ratio on MR imagers from different manufacturers--the EuroMT sequence.

Magnetisation transfer ratio (MTR) is increasingly used to evaluate neurological disorders, especially those involving demyelination. It shows promise as a surrogate marker of disease progression in treatment trials in multiple sclerosis (MS) but the value measured is highly dependent on pulse sequence parameters, making it hard to include the technique in large multi-centre clinical trials. The variations can be reduced by a normalisation procedure based on the flip angle and timing of the presaturation pulse, but correction for parameters such as saturation pulse shape, amplitude, duration and offset frequency remains problematic. We have defined a standard pulse sequence, to include a standard presaturation pulse and set of parameters, which can be implemented on scanners from both General Electric and Siemens, and has also been used on Phillips scanners. To validate the sequence and parameters, six European centres measured MTR in the frontal white matter of normal volunteers. It was possible to measure MTR values in controls which were consistent to within approximately +/-2.5 percentage units across sites. This degree of precision may be adequate in many situations. The remaining differences between sites and manufacturers are probably caused by B1 errors.

Gadolinium-enhanced small-animal TOF magnetic resonance angiography.

So far, magnetic resonance angiography (MRA) of rodents has only been performed by using time-of-flight (TOF) MRI techniques. This is because applications of first-passage contrast agents as in humans are hampered by pronounced physiologic differences (blood volume and heart beat rate). Here we describe the use of low-dose Gd-DOTA to enhance the performance of TOF MRA in rat brain. While no improvement in contrast was achieved, the measuring time could be reduced by almost a factor of three. This decrease in total acquisition time has been used to study the impact of a model of ligatured common carotid on the upper part of the blood system of the rat.

Single-coil surface imaging using a radiofrequency field gradient

A method for in-plane imaging of large objects as compared to the RF coil is proposed based on the use of a single specially designed surface coil, without using B(0) gradients. A constant B(1) gradient was generated along the main axis of a ladder-shaped coil, and RF-encoding along the direction of the gradient made it possible to obtain spin-density profiles. Successive acquisitions of profiles obtained by translation of the NMR coil resulted in distorted images-due to the presence of non-zero gradients perpendicular to the constant gradient-that were successfully processed using a mathematical treatment based on linear combinations of calculated altered images from single-pixel objects. Copyright 2000 Academic Press.