Proton NMR Imaging Research Articles

Identification and characterization of tissues by T2-selective whole-body proton NMR imaging.

T2-selective proton imaging at 10 MHz is based on a Carr-Purcell-Meiboom-Gill pulse sequence with at least 36 echoes and 2 tau greater than or equal to 6 ms employing the projection reconstruction technique. The transverse magnetization decay of the volume elements is multiexponential composed of up to three monoexponential T2 components. The T2 values are arranged in T2 histograms which are slice specific. T2 windows are defined in the T2 histogram and used for imaging and for selecting T2 values which are assigned to proton classes and tissues. Tissue characterization is based on the visualization of differences in T2 behavior. The T2 values are transformed into 256 gray levels or, by use of a red-green-blue lookup table, into a rainbow code. Two display techniques have been developed and implemented in the RWTH Aachen Magnetic Resonance Software System (RAMSES): (i) for nonselective T2 imaging with secondary T2 selection and encoding of molecular classes on the basis of T2 values; (ii) for T2-selective imaging by employing combined primary and secondary T2 selection and encoding particular tissues and variations therein. The tissues can be identified by selecting different classes of molecules, like water, lipids, and proteins, when shifting the primary T2 window on the T2 axis of the T2 histogram.

Studies of factors affecting the design of NMR contrast agents: manganese in blood as a model system.

Some factors affecting the performance of paramagnetic ions as contrast agents for proton NMR imaging have been studied. It is demonstrated that the relaxation rate of an aqueous solution of the ion is not reliably predicted by its magnetic moment, but that significant relaxation enhancement may result when the ion is complexed with large molecules, which increases the dominant correlation time. This enhancement in turn can be altered by factors such as pH and competition for binding. Chelation of the paramagnetic ion, which may be implemented to lower its toxicity, can considerably reduce its efficacy by not only limiting its access to water but also by preventing the enhancement from associations and macromolecules. For manganese the ratio T1/T2 is a useful parameter which is sensitive to the degree of metal binding. These features of paramagnetic relaxation enhancement in tissue are demonstrated in a series of experiments on systems consisting of blood components and manganese.

Paramagnetic contrast agents in nuclear magnetic resonance medical imaging

Relaxation time differences are the sources of most of the contrast observed in proton NMR images, not only among normal organs and tissues but between lesions and the adjacent tissue. Although these differences are often large, there are low-contrast situations in which it would be desirable to increase the visibility of an organ or region. The study of time-dependent phenomena would also be aided by the ability to change selected relaxation times deliberately. One way to achieve these goals is to administer substances that change proton relaxation times in tissues without causing significant toxic effects or other physiologic changes. Paramagnetic ions and molecules, those with unpaired electrons, may be useful for this purpose because the very large magnetic effects associated with such electrons can drastically decrease water proton relaxation times at concentrations of the order of 100 to 1000 μM , which may be reached in certain organs after doses of 10 to 100 μM /kg. The general characteristics of such paramagnetic substances are described, and specific animal experiments with manganous ion and its complexes, and with stable nitroxide free radicals and molecular oxygen, are reviewed. The paramagnetic contrast agents already studied are effective, and many more are potentially possible, but the most important questions to be answered are whether acute and chronic toxicity are low enough to permit research and diagnosis on humans.

Differentiating senile dementia of Alzheimer type and multi-infarct dementia by proton NMR imaging

Differentiating senile dementia of Alzheimer type and multi-infarct dementia by proton NMR imaging

Potential hazards of NMR imaging. No evidence of the possible effects of static and changing magnetic fields on cardiac function of the rat and guinea pig

Clinical proton NMR imaging uses magnetic field strengths in the range 0.1 to 0.5 T. In addition to the large static magnetic field, patients are exposed to magnetic field gradients during imaging and under extreme conditions, such as power failure or quenching, the field may collapse precipitously. A potential source of hazard to patients under these conditions is the induction of thoracic currents which may trigger ventricular fibrillation. In the present experiments, a 0.16 T resistive magnet with a time constant of 60 ms, powered by a programmable power supply, was used to examine any possible effects of static and changing magnetic field on the ECG and arterial blood pressure of anesthetized rats and guinea pigs. Animals were exposed to the following field conditions: static fields of 0.16 T; sine, triangular, and square wave modulated fields from 0.1 to 2 Hz; rapid field switches in excess of 2.0 T/s for 25 ms timed to occur at different stages of the cardiac cycle, including the vulnerable period during ventricular repolarization; and AC fields of 50 Hz. No change was observed in the blood pressure, heart rate, or ECG under any of the field conditions examined.

The physics of proton NMR

The physics of pulse NMR which is pertinent to an understanding of proton NMRimaging has been condensed and directed toward the medical physicist. The basic physical principles of spin manipulations using rf pulses are presented, and the relation between the quantum mechanical and the classical descriptions is covered in a rigorous fashion. The physics of relaxation is described and the relaxation timesT 1 and T 2 are explained in some detail. Application of these spin manipulation techniques is illustrated by showing how they may be used in creating an image.

Paramagnetic contrast agents in nuclear magnetic resonance medical imaging

Relaxation time differences are the sources of most of the contrast observed in proton NMR images, not only among normal organs and tissues but between lesions and the adjacent tissue. Although these differences are often large, there are low-contrast situations in which it would be desirable to increase the visibility of an organ or region. The study of time-dependent phenomena would also be aided by the ability to change selected relaxation times deliberately. One way to achieve these goals is to administer substances that change proton relaxation times in tissues without causing significant toxic effects or other physiologic changes. Paramagnetic ions and molecules, those with unpaired electrons, may be useful for this purpose because the very large magnetic effects associated with such electrons can drastically decrease water proton relaxation times at concentrations of the order of 100 to 1000 microM, which may be reached in certain organs after doses of 10 to 100 microM/kg. The general characteristics of such paramagnetic substances are described, and specific animal experiments with manganous ion and its complexes, and with stable nitroxide free radicals and molecular oxygen, are reviewed. The paramagnetic contrast agents already studied are effective, and many more are potentially possible, but the most important questions to be answered are whether acute and chronic toxicity are low enough to permit research and diagnosis on humans.

Proton NMR imaging: a new approach for the evaluation of cardiac structure and function

Proton NMR imaging: a new approach for the evaluation of cardiac structure and function

The physics of proton NMR.

The physics of pulse NMR which is pertinent to an understanding of proton NMR imaging has been condensed and directed toward the medical physicist. The basic physical principals of spin manipulations using rf pulses are presented, and the relation between the quantum mechanical and the classical descriptions is covered in a rigorous fashion. The physics of relaxation is described and the relaxation times T1 and T2 are explained in some detail. Application of these spin manipulation techniques is illustrated by showing how they may be used in creating an image.

Design of a radiofrequency coil suitable for NMR imaging of heads

A whole-body proton NMR imaging machine operating at 1.7 MHz has been built in Aberdeen (Hutchison et al. 1980, Johnson et al. 1982). In order to improve the signal-to-noise ratio of head images, a radiofrequency coil of approximately half the whole-body coil diameter has been built. The head coil slides easily into its working position centrally within the whole-body coil, which is then short circuited.

A comparative evaluation of proton NMR imaging results

A comparative evaluation of proton NMR imaging results

A demonstration of the resolution of NMR imaging in biological systems.

A proton NMR imaging study of several fruit specimens demonstrates the integrity and resolution of this new imaging method.