Determination of in vivo rat muscle Gd-DTPA relaxivity at 6.3 T.

For the in vivo relaxivity of Gd-DTPA at 6.3 T in rat muscle a value of 2.7+/-0.5 (mM s)(-1) was found, and for the in vitro value in water 3.00+/-0.56 (mM s)(-1) at 37 degrees C. The temperature dependence of the in vitro relaxivity was -0.087 (mM s degrees C)(-1). The relation between 1/T1 and the tissue Gd-DTPA concentration is linear for the normally used in vivo Gd-DTPA concentration range.

Influence of the pharmacokinetic model on the quantification of the Gd-DTPA uptake rate in brain tumours using direct T1 measurements.

After a bolus injection of Gd-DTPA, a fast tracer uptake in rat brain tumours was already observed during the tracer bolus passage. For the quantification of the uptake rate constant, pharmacokinetic models are commonly used. For a good quantification, the changes of the plasma tracer concentration directly after the bolus injection must be incorporated into these models as prior knowledge. The aim of this paper is to investigate whether or not it is necessary to include the bolus passage into the description for the plasma tracer concentration. The result of this study indicates that the best quantification of the uptake rate constant is achieved by using only the data points after the bolus passage. Using all the data points and incorporating the bolus passage in the pharmacokinetic models results in a less accurate estimation.

Spatially selective RF pulses and the effects of digitization on their performance.

Spectrometers make use of D/A converters to generate RF and gradient shapes. This paper examines by exact simulations the time and amplitude digitization effects, inherent to the use of D/A converters, on the performance of amplitude modulated (AM) frequency selective RF pulses. By making use of Fourier theory and the small tip angle approximation, an approximate model of these effects on the magnetization slice profiles is derived and verified for several pulse types by computer simulations. This approximate model will be used to derive requirements for D/A converters with respect to spatial localization. The dynamics of the spin system allows pulse width modulation (PWM) as an alternative to AM for pulse envelope encoding. The effects of PWM on the slice profile are examined and compared with conventional AM pulses. It is shown by simulation and measurement that adiabatic PWM pulses can be found. In contrast to AM modulated adiabatic pulses, adiabatic PWM pulses have side bands with the same slice quality as the main slice and might therefore be useful as multislice selective pulses.

Diffusion in red blood cell suspensions: separation of the intracellular and extracellular NMR sodium signal.

It is shown that the signal of intracellular and extracellular sodium of red blood cells can be separated by a difference in diffusion. Comparison with proton diffusion experiments conducted in parallel showed that this difference was caused by restriction to the cell volume. The measured proton and sodium root mean square displacements agreed well with the cell dimensions. However, this experiment is limited to use in vitro by the required gradient strength.

Is the magnetic resonance imaging proton spin-lattice relaxation time a reliable noninvasive parameter of developing liver fibrosis?

During the development of liver fibrosis in rats by an individual dose-titrated CCl4 administration, hepatic proton spin-lattice relaxation time (T1) has been measured in vivo every 2 weeks for 8 weeks. Liver content of collagen, triglycerides and water has been measured biochemically in biopsy material. After 4 weeks of CCl4 treatment, T1 increased significantly and remained at the same level, whereas liver collagen reached its maximum at 8 weeks. It is concluded that, under our experimental conditions, increased hepatic T1 represents drug-induced edema and that hepatic T1 is not a reliable noninvasive parameter for developing liver fibrosis in vivo.

Application of linear prediction and singular value decomposition (LPSVD) to determine NMR frequencies and intensities from the FID.

A parametric spectral analysis based on linear prediction and singular value decomposition has been applied to the 31P FID of a tumor implanted in a mouse. It is found that a number of complications that may arise in in vivo FFT NMR can be circumvented. The new method yields all parameters of the spectral components, i.e., the frequency, amplitude, damping constant, and phase.

N.m.r. relaxation and images of human breast tumours in vitro.

It is found that fat and non-fatty tissue in dissected samples of the mamma differ in their T1/T2 ratios. This opens the possibility of locating tumours by n.m.r. imaging, because they have a lower fat content than their surroundings. By means of a sensitive point method, samples were scanned with a resolution of about 0.4 mm X 0.4 mm. The similarity between the shape of a tumour in an n.m.r. and in an X-ray image of a thin section of mamma tissue is quite convincing.