Tomographic imaging with nuclear magnetic resonance

A technique is described for obtaining tomographic images of hydrogen distribution in animals using nuclear magnetic resonance (NMR). Resonant frequency is proportional to magnetic field strength, so that spatial resolution is achieved by frequency selection and magnetic field shaping. The results of scanning a phantom and two rats are presented.     

Burn injury by nuclear magnetic resonance imaging

Nuclear magnetic resonance imaging has become a standard diagnostic procedure in clinical medicine and is well known to have hazards for patients with pacemaker or metallic foreign bodies. Compared to CT, the frequency of MRI examinations is increasing due to the missing exposure of the patients by X-rays. Furthermore, high-field magnetic resonance tomograph (MRT) with 3 T has entered clinical practice, and 7-T systems are installed in multiple scientific institutions. On the other hand, the possibility of burn injuries has been reported only in very few cases.Based on a clinical finding of a burn injury in a 31-year-old male patient during a routine MRI of the lumbar spine with standard protocol, the MR scanner was checked and the examination was simulated in an animal model.The patient received a third-degree burn injury of the skin of the right hand and pelvis in a small region of skin contact. The subsequent control of the MRI scanner indicated no abnormal values for radiofrequency (RF) and power. In the subsequent animal experiment, comparable injuries could only be obtained by high RF power in a microwave stove.It is concluded that ‘tissue loops’ resulting from a contact between hand and pelvis must be avoided. With regard to forensic aspects, the need to inform patients of such a minimal risk can be avoided if the patients are adequately positioned using an isolating material between the hands and pelvis. These facts must be emphasized more in the future, if high-field MRI with stronger RF gradients is available in routine imaging.     

Urinary tract imaging by nuclear magnetic resonance

Nuclear magnetic resonance scanning has already found application in the urinary tract in man. It is a method of producing diagnostic images from the radiofrequency signals which can be elicited from hydrogen nuclei when the abdomen is placed in a strong magnetic field and subjected to pulses of RF energy. Computer-generated images depict the distribution of signal-emitting nuclei, modified by their relaxation times and macroscopic motion. The images are tomographic and have great potential flexibility in plane orientation. Normal anatomy, space-occupying lesions and lesions characterized by local edema have been visualized to date, and as clinical experience is accumulated, a much greater range of pathology promises to be detectable by this technique.     

Site planning for nuclear magnetic resonance imaging

The unique factors to be considered in selection of a site for a magnetic resonance imaging installation are addressed, with emphasis on the interaction of the magnet with its environment. The effect of ferrous objects on the magnet and the influence of the magnet on its surroundings are discussed. Design issues for magnetic shields are given.     

Urinary tract imaging by nuclear magnetic resonance

Nuclear magnetic resonance scanning has already found application in the urinary tract in man. It is a method of producing diagnostic images from the radiofrequency signals which can be elicited from hydrogen nuclei when the abdomen is placed in a strong magnetic field and subjected to pulses of RF energy. Computer-generated images depict the distribution of signal-emitting nuclei, modified by their relaxation times and macroscopic motion. The images are tomographic and have great potential flexibility in plane orientation. Normal anatomy, space-occupying lesions and lesions characterized by local edema have been visualized to date, and as clinical experience is accumulated, a much greater range of pathology promises to be detectable by this technique.     

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.     

Clinical magnetic resonance imaging with nuclear medicine correlation

The current role of magnetic resonance imaging (MRI) in different organ systems is discussed and compared to nuclear medicine and to other available clinical diagnostic modalities. The value of optimizing radiofrequency pulse sequence selection to provide additional tissue characterization is also described. The results of nuclear medicine and MRI studies in 56 patients are compared to evaluate the clinical diagnostic contribution of each imaging modality for various pathological processes. In addition, the state-of-the-art MRI systems and future development in MRI technology with its potential contribution is defined.     

Clinical nuclear magnetic resonance imaging of the body

NMR promises great advances in diagnosis and has delivered so much already that it is expected that in the future it will replace many applications of the currently used imaging modalities. Although x-ray computed tomography is continuing to advance in speed of scanning and resolving power, NMR will most likely soon eliminate its use in many studies of the central nervous system and also in many other areas of the body. The promise of combining topical spectroscopy with imaging is also exciting and should provide further information about metabolic processes of various organs. Progress in NMR is so rapid and the future is so bright that one of the great problems will be to develop a new breed of radiologists who are versatile in biochemistry, mathematics, and computers, as well as competent in morphologic anatomy and pathologic physiology. As time goes on, advances in NMR will be achieved only by teams of clinical and basic scientists encompassing multiple disciplines.     

Microcomputer simulation of nuclear magnetic resonance imaging contrasts

The high information content of magnetic resonance images is due to the multiplicity of its parameters. However, this advantage introduces a difficulty in the interpretation of the contrast: an image is strongly modified according to the visualised parameters. The author proposes a micro-computer simulation program. After recalling the main intrinsic and extrinsic parameters, he shows how the program works and its interest as a pedagogic tool and as an aid for contrast optimisation of images as a function of the suspected pathology.     

A computer simulation of nuclear magnetic resonance imaging

A modular, user-friendly computer simulation of NMR imaging was created for the DEC VAX computer. This simulation is novel in that complex imaging phenomena, such as chemical shifts and flow, may be easily studied. Other features include spin warp and backprojection imaging, movie generation, and simulation of image contrast, composite pulse sequences, motion artifacts, aliasing, selective excitation, field inhomogeneity, noise and signal averaging. The simulation incorporates fast Fourier transform and graphics modules for processing and display of one- and two-dimensional images. Examples which illustrate features of the program are shown. The simulation is a useful tool for both the sophisticated and the beginning user of NMR imaging. It serves as a drawing board for studying MRI phenomena and a teaching program for demonstrating basic techniques.     

Analysis of eddy currents in nuclear magnetic resonance imaging

The eddy currents in nuclear magnetic resonance (NMR) imaging are analyzed from the solutions of Maxwell's equations and their effects are examined over various experimental conditions from whole-body diagnostic imaging to recently developed NMR microscopy. The analysis is focused mainly on the frequency characteristics and intensity variations of the eddy-current-induced field which depends on the overall system size, ratio of the gradient coil size to the magnet bore diameter, and the pulse-sequence-dependent parameters such as input current waveform and repetition time. From the analysis, the frequency response of the eddy-current-induced field is that of a high-pass filter whose cutoff frequency is inversely proportional to the square of the overall system size. The intensity ratio of the generated field to the induced field is not affected by the overall system size, but is sensitively related to the ratio of the gradient coil size to the magnet bore diameter.     

In vivo nuclear magnetic resonance imaging of lithium

Magnetic resonance imaging techniques have previously been applied to 1H, 23Na, 31P, and 19F nuclei. This is the first report of application of these techniques to 7Li. Lithium images of both aqueous phantoms and a rat abdomen are presented. Applications of this technique to humans are discussed.     

Visualization of brain infarction with nuclear magnetic resonance imaging

Summary Nine patients with cerebral or cerebellar hemispheric infarction and two patients with brain stem lesions were studied with NMR imaging in order to examine the lesion with different weightings on the various NMR parameters. The time interval between examination and the acute onset of the stroke varied from one day to six months. The lesions were clearly detected by the T₁ weighted inversion recovery sequence but the tissue contrast was better using the spin echo sequence with a long repetition rate and a long echo delay (SE_2000/100). In two patients the brain stem lesions were invisible with CT, but were easily depicted with NMR. In one of them the further progress of the lesion was also confirmed and visualized with the follow-up NMR study.     

Work in progress: nuclear magnetic resonance imaging of the gallbladder

A preliminary study of the relation between food intake and intensity of gallbladder bile on nuclear magnetic resonance (NMR) images was made. Twelve subjects (seven volunteers, five patients) were imaged following a minimum of 14 hours of fasting. Six of seven volunteers were reimaged one hour after stimulation by either a fatty meal or an alcoholic beverage. An additional seven patients were imaged two hours after a hospital breakfast. It was found that concentrated bile emits a high-intensity spin echo signal (SE), while hepatic bile in the gallbladder produces a low-intensity SE signal. Following ingestion of cholecystogogue, dilute hepatic bile settles on top of the concentrated bile, each emitting SE signals of different intensity. The average T1 value of concentrated bile was 594 msec, while the T1 value of dilute hepatic bile was 2,646 msec. The average T2 values were 104 msec for concentrated bile and 126 msec for dilute bile. The most likely cause for the different SE intensities of bile is the higher water content, and therefore longer T1 and T2 relaxation times, of hepatic bile. It is suggested that NMR imaging has the ability to provide physiological information about the gallbladder and that it may prove to be a simple and safe clinical test of gallbladder function.     

Sodium nuclear magnetic resonance imaging of neuroblastoma in the nude mouse

The average concentration of sodium is known to be elevated in some tumors relative to normal tissues, and necrosis is suspected of being a possible cause. We have performed in vivo sodium-23 magnetic resonance imaging (MRI) of IMR-5 neuroblastoma in the athymic nude mouse on a 1.9-Tesla, small-bore animal imaging system. We compared the sodium images with histologic analysis for necrosis and with proton images, chemical measurements of water and blood content, and sodium and potassium concentrations. We found that the sodium concentrations determined by MRI were proportional to the fraction of the tumor tissue that was necrotic. Correlation coefficients varied from 0.65 to 0.78, depending upon how the data were selected. With further refinement it is possible that the sodium concentration measurements determined noninvasively by MRI may have applications as part of clinical diagnosis and staging of soft tissue tumors.