Use of 19F magnetic resonance imaging to measure local cerebral blood volume

Non-invasive evaluation of the location, the functional integrity and the oxygen supply of implants: 19F nuclear magnetic resonance imaging of perfluorocarbon-loaded Ba2+-alginate beads

19F nuclear magnetic resonance imaging (MRI) can be used as a non-invasive tool to simultaneously determine the location, the integrity and the oxygen supply of Ba2+-alginate implants. This requires that the beads (implants) are pre-loaded with the perfluorocarbon compound F-44E. Implantation of solid 19F-labelled beads into the peritoneum, below the kidney capsule or into the muscle of Wistar WU rats demonstrated that these beads could be detected by 19F-MRI for up to 18 months after implantation. This indicated that F-44E is not considerably released from the beads during implantation. The signal to noise ratio of liquid-core beads was higher by a factor of 4 than the signal to noise ratio of solid beads, but liquid-core beads were more fragile and also too large for implantation under the kidney capsule and into the intramuscular tissue. Quantitative 2-dimensional 19F-T1 maps (resolution 0.5 x 0.5 mm) could be deduced from 19F-MRI measurements. These T1-maps correlated to the local pO2-values. The partial oxygen pressure estimated in F-44E-loaded Ba2+-alginate beads showed that the oxygen supply inside the beads was very poor when they were implanted below the kidney capsule or into the peritoneal cavity. These low pO2-values obtained for the renal subcapsular site and the peritoneum may explain the failure of previous immunoisolated islet transplantation studies using these locations.

Comparison of gadopentetate dimeglumine and albumin-(Gd-DTPA)30 for microvessel characterization in an intracranial glioma model

To compare the performance of macromolecular albumin gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)30 and low molecular weight gadopentetate dimeglumine for microvessel characterization, we examined an intracranial 9L glioma model in which increased angiogenesis, hypervascularity, and hyperpermeability mimic characteristics of clinical malignant brain tumors. Dynamic MRI data were analyzed using a bidirectional, two-compartment kinetic model to extract quantitative estimates for fractional blood volume (fBV) and permeability surface area product (PS). Three criteria were used for comparison of contrast agent performance: (a) tumor conspicuity, defined as the contrast-to-noise ratio (CNR); (b) dynamic range of differential permeability estimates between tumor and normal brain; (c) reasonableness of blood volume estimates. Gadopentetate was superior to macromolecular albumin-(Gd-DTPA)30 for detection of 9L brain gliomas and for measurements of hyperpermeability.

Accuracy of gamma-variate fits to concentration-time curves from dynamic susceptibility-contrast enhanced MRI: influence of time resolution, maximal signal drop and signal-to-noise

Concentration-time curves derived from dynamic susceptibility-contrast enhanced magnetic resonance imaging are widely used to calculate cerebrovascular parameters. To exclude effects of recirculation, a non-linear regression method is used to fit a gamma-variate function to the concentration-time course. In previous studies the errors arising from the fitting procedure have not been quantified. In a computer simulation we investigate the uncertainties of parameters calculated from the fitted gamma-variate function, exploring the dependencies on signal-to-noise (SNR), time resolution (delta t), and maximal signal drop (MSD). Our study was performed to give a framework on how to design MR-sequences and choose contrast media and their application in order to yield concentration-time curves which allow a reliable performance of the gamma-variate fitting procedure. We recorded 396 concentration-time curves from regions of interest of 40 patients. The gamma-variate fitting procedure was applied to these curves resulting in 396 parameter sets. Ideal concentration-time curves as gamma-variate functions were generated from these sets with a given delta t, MSD, and SNR. Recirculation effect was simulated. Then the gamma-variate fitting was performed again. From ideal and simulated gamma-variate function the area and the normalized first moment were calculated. The uncertainties of the values calculated from the simulated curve relating to the values of the original one were determined. Increase of SNR decreases the involved errors. With SNR values of 100 and more there is only minor influence of delta t and MSD and the fitted curve approximates the original data very well. Smaller values of SNR lead to a stronger influence of delta t and MSD and a higher number of fitting failures. With increasing delta t the uncertainties also increase. Intermediate values of MSD (30% to 70%) yield the smallest errors while increasing or decreasing MSD yields an increase of uncertainty. To achieve low uncertainties in the calculation of cerebrovascular parameters from gamma-variate fits, delta t of the imaging sequence and MSD must be considered. This is more important the lower SNR is. The shown dependencies should be taken into account when choosing MR sequence parameters and application of contrast media.

Sonochemically produced fluorocarbon microspheres: a new class of magnetic resonance imaging agent

With the intent of increasing the signal-to-noise ratio (SNR) of fluorine magnetic resonance imaging and enabling new applications, we have developed a novel class of agents based on protein encapsulation of fluorocarbons. Microspheres formed by high-intensity ultrasound have a gaussian size distribution with an average diameter of 2.5 microns. As with conventional emulsions, these microspheres target the reticuloendothelial system. However, our sonochemically produced microspheres, because of a high encapsulation efficiency, show increases in the SNR of up to 300% compared to commercially available emulsions. We also demonstrate an increase in the circulation lifetime of the microspheres with the bloodstream by more than 30-fold with a chemical modification of the outer surface of the microsphere. Finally, by encapsulating mixtures of fluorocarbons that undergo solid/liquid phase transitions, we can map temperature in the reticuloendothelial system, with signal changes of approximately 20-fold over a 5 degrees C range.

In vivo measurement of partial oxygen pressure in large vessels and in the reticuloendothelial system using fast 19F-MRI

Quantitative in vivo 19F-MRI was performed in a rat model to monitor partial oxygen pressure (pO2) using a perfluorocarbon (PFC) emulsion as contrast agent. On Days 1, 4, and 8 postinjection of the PFC emulsion, transaxial T1 and pO2 maps were acquired of the abdomen of rats that were consecutively ventilated with pure oxygen, air, and a mixture of 10% oxygen and 90% nitrogen. The images had a resolution of 0.75 mm x 0.75 mm x 2 mm and a total acquisition time of 24 min. In these images it was possible to distinguish between different vessels and hepatic and splenic tissue in the selected imaging plane. Serial 19F-MRI measurements on the different days postinjection of the PFC allowed to determine separately the pO2 of arterial and venous blood and the intracellular pO2 in macrophages of the liver and spleen.

A matched filter echo summation technique for MRI

A technique is presented to increase the signal-to-noise ratio (SNR) in two-dimensional (2D), phase-encoded imaging at low SNR. The essence of this technique is to combine multiple echoes in the time domain. As analyzed in the paper, phase discrepancies exist among different echoes and may deteriorate the combined echo. In particular, extraneous phase shifts can be created if unshielded gradient coils are used. To overcome these phase discrepancies, a matched filter was derived from the k = 0 component of image. This matched filter has the same phase discrepancies among its echoes as the imaging signal and its magnitude decays with an average T2. In the echo summation with the matched filter, the phase of the matched filter was subtracted from the imaging signal and the magnitude of the matched filter was used as the weighting function. We have shown that this matched filter echo summation technique has better SNR than the case of 2D, phase-encoded imaging in both simulation and experiment. The SNR improvement is up to 60% in a phantom experiment. This technique is mostly useful in low SNR imaging that requires long imaging time, such as spectroscopic imaging and 19F imaging.

Non-invasive physiology: 19F NMR of perfluorocarbons

Ever since it was shown that the 19F NMR spin-lattice relaxation rates (R1) of perfluorocarbon (PFC) emulsions are highly sensitive to oxygen tension (pO2), there has been a developing interest in the use of PFCs to probe tissue physiology. Oxygen is required for efficient function by most tissues and hypoxia leads to rapid cellular dysfunction and damage. In addition, hypoxic tumor cells are refractory to radiotherapy. Thus, the opportunity to measure tissue oxygen tension non-invasively may be significant in understanding mechanisms of tissue function and in clinical prognosis. PFC NMR parameters are also sensitive to temperature, facilitating NMR thermometry with potential applications in hyperthermia studies. I will review the development of experimental techniques, applications to specific tissues and discuss the challenges and opportunities presented by 19F NMR of perfluorocarbons.

Effect of sample numbers on the kinetic data analysis of MR contrast agents

The Monte Carlo simulation method is used to determine the uncertainties of the estimated are and mean transit time (MTT) of a regional cerebral concentration-time curve of a bolus injected paramagnetic contrast agent. Many factors contribute to these uncertainties. This study concentrates on the error propagation from the sample noise at different sample time points along the concentration-time curve. The uncertainties of area and MTT were determined as a function of the sample noise and the number of sample time points. For example, the percentage standard deviations (uncertainties) of both area and MTT are less than 12% with five samples whose signal-to-noise ratios are greater than 50.