Visualization of cerebral and vascular abnormalities by NMR imaging. The effects of imaging parameters on contrast

Nuclear magnetic resonance imaging and spectroscopy in experimental brain edema in a rat model

Many aspects of the use of high-resolution nuclear magnetic resonance (NMR) imaging in the examination of brain edema have not been fully explored. These include the quantitation of edema fluid, the ability to distinguish between various types of edema, and the extent to which tissue changes other than a change in water content can affect NMR relaxation times. The authors have compared NMR relaxation times obtained by both in vivo magnetic resonance imaging (MRI) and in vitro NMR spectroscopy of brain-tissue samples from young adult rats with cold lesions, fluid-percussion injury, hypoxic-ischemic injury, bacterial cerebritis, and cerebral tumor. Changes in relaxation times were compared with changes in brain water content, cerebral blood volume, and the results of histological examination. In general, both in vivo and in vitro longitudinal relaxation times (T1) and transverse relaxation times (T2) were prolonged in the injured hemispheres of all experimental groups. Water content of tissue from the injured hemispheres was increased in all groups. A linear correlation between T2 (but not T1) and water content was found. Changes in the values of T1 and T2 could be used to distinguish tumor from cold-injured tissue. Cerebral blood volume was reduced in the injured hemispheres and correlated inversely with prolongation of T1 and T2. The results of this study suggest that, in a clinical setting, prolongation of T2 is a better indicator of increased water content than prolongation of T1, yet quantitation of cerebral edema based solely upon prolongation of in vivo or in vitro T1 and T2 should be undertaken with caution.

Improved in vivo magnetic resonance imaging of acute myocardial infarction after intravenous paramagnetic contrast agent administration

Gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA), a paramagnetic relaxation agent, has been used in vitro to improve the detection of acute myocardial infarction (MI) by magnetic resonance imaging (MRI). In this study, the ability of Gd-DTPA to improve in vivo magnetic resonance imaging of MI was examined. Ten dogs with MI caused by left anterior descending coronary artery ligation were imaged 1 to 5 days after infarction. Imaging was performed before and for 2 hours after intravenous administration of 0.34 mmol/kg Gd-DTPA. One to 2 days after MI, Gd-DTPA improved visualization of the infarct in 3 of 4 dogs. This effect was more prominent in dogs imaged 4 to 5 days after MI, when 6 of 6 dogs showed substantially improved infarct definition after Gd-DTPA. At both times the intensity ratio, an objective measure of contrast between infarcted and normal tissue that is defined by the ratio of image intensity of infarcted area to that of noninfarcted area, was significantly better after Gd-DTPA administration. The intensity ratio at 24 to 48 hours after infarction was 1.4 +/- 0.2 (mean +/- standard deviation) before Gd-DTPA, and 1.7 +/- 0.6 after Gd-DTPA (p = 0.03). The intensity ratio at 4 to 5 days after infarction was 1.5 +/- 0.3 before Gd-DTPA, and 1.8 +/- 0.5 after Gd-DTPA (p less than 0.001). Thus, Gd-DTPA administration improves in vivo visualization of MI by MRI.

Characterization of experimental ischemic brain edema utilizing proton nuclear magnetic resonance imaging

Correlations between T1 and T2 relaxation times and water and electrolyte content in the normal and ischemic rat and gerbil brains were studied by means of both nuclear magnetic resonance (NMR) spectroscopic and imaging methods. In the spectroscopic experiment on excised rat brains, T1 was linearly dependent on tissue water content and T2 was prolonged in edematous tissue to a greater extent than expected by an increase in water content, showing that T2 possesses a greater sensitivity for edema identification and localization. Changes in Na+ and K+ content of the tissue mattered little in the prolongation of relaxation times. Serial NMR imaging of gerbil brains insulted with permanent hemispheric ischemia offered early lesion detection in T1- and especially T2-weighted images (detection as soon as 30 min after insult). The progressive nature of lesions was also imaged. Calculated T1 and T2 relaxation times in regions of interest correlated excellently with tissue water content (r = 0.892 and 0.744 for T1 and T2, respectively). As a result, detection of cerebral ischemia utilizing NMR imaging was strongly dependent on a change in tissue water content. The different nature of T1 and T2 relaxation times was also observed.

Magnetic resonance imaging of extremity masses

Twenty-seven adults with extremity masses were examined by magnetic resonance imaging (MRI). In 26/27 cases, computed tomography (CT) scans were available for comparison. Imaging with multiple pulse sequences is necessary to optimize diagnosis by MRI. Advantages of MRI include direct sagittal and coronal imaging, demonstration of vessels without contrast and superior soft tissue contrast. CT better demonstrates cortical destruction and small calcifications, which may be important in some cases. For noncalcified masses without bone involvement, MRI is equivalent or superior to CT. In those cases with calcification or bone involvement, MRI plays a role complementary to CT.

Instrumentation for rapid MR image synthesis

MR image synthesis has previously been developed as a means of retrospectively optimizing contrast of arbitrary materials in MR images. The first step of this process is to form computed N(H), T1, and T2 images from source images acquired at a variety of echo delay and repetition times. The second step is to take these computed images, along with operator-selected timing parameters, and mathematically generate a synthesized image. Computation is carried out pixel by pixel according to the equation describing the chosen pulse sequence. This paper presents a study of design considerations for a digital image processor capable of rapidly performing the second step, the actual synthesis. In this work the computations inherent to image synthesis are identified, and the feasibility of performing them in high-speed hardware examined. An analysis of the imprecision due to bit-limited calculations shows that an error bound of 0.4% is possible with a 16-bit processor design. A method is described which uses a commercially available image processor by which images can be synthesized according to any of the standard pulse sequences in less than 600 ms.

Magnetic resonance imaging in pediatric hematology/oncology. Part II. Illustrative cases and assessment of technique

Preliminary clinical experience with magnetic resonance imaging (MRI) in 28 pediatric patients with 20 different hematological diseases, benign tumors, or malignant neoplasms is presented. The clinical results are presented in the form of case presentations that are discussed in the context of alternative diagnostic imaging modalities. Also discussed are the known biological effects, or lack thereof, the need for sedation, the effect of motion, the effect of MRI on foreign metallic objects, the financial considerations, and the trends for the future of MRI. This imaging modality has many unique merits. Present difficulties should be overcome by future innovations, making MRI even more efficacious for the diagnosis of blood diseases and cancer in children.

Evaluation of the cardiovascular system using NMR

The current status and suggestions of the future potential for nuclear magnetic resonance imaging of the cardiovascular system are described. With many of the potential applications, there is overlap with existing methods. For example, imaging of the left ventricle can be accomplished with either echocardiography or radionuclide techniques with adequate evaluation of the left ventricular function. At current costs these conventional techniques may provide a more cost-effective approach for morphologic and functional assessment of the cardiovascular system. For this type of imaging, the advantages of NMR include its excellent resolution, the inherent tissue contrast, the sensitivity to blood motion, the 3-dimensional measure, and the lack of ionizing radiation. Because of these, NMR could provide an important adjunct for evaluation of the cardiovascular system. However for NMR to achieve its full promise as a cardiovascular imaging technique, some of its unique potentials need to be developed. These include: the ability to reliably image at least the proximal coronary arteries, the ability to delineate regional myocardial blood flow distribution, the ability to evaluate regional metabolic activity such as high-energy phosphate metabolites, and the ability to characterize myocardial disease using proton T1 and T2 alterations.

Projective imaging of pulsatile flow with magnetic resonance

Noninvasive angiography with magnetic resonance is demonstrated. Signal arising in all structures except vessels that carry pulsatile flow is eliminated by means of velocity-dependent phase contrast, electrocardiographic gating, and image subtraction. Background structures become in effect transparent, enabling the three-dimensional vascular tree to be imaged by projection to a two-dimensional image plane. Image acquisition and processing are accomplished with entirely conventional two-dimensional Fourier transform magnetic resonance imaging techniques. When imaged at 0.6 tesla, vessels 1 to 2 millimeters in diameter are routinely detected in a 50-centimeter field of view with data acquisition times less than 15 minutes. Studies of normal and pathologic anatomy are illustrated in human subjects.

Amnestic syndrome and vertical gaze palsy: early detection of bilateral thalamic infarction by CT and NMR

A 27 year old woman with mitral valve prolapse presented with somnolence, bilateral Babinski signs, and grasp reflexes. As somnolence cleared, vertical gaze palsy and Korsakoffian memory deficit were apparent. Initial CT scan was normal, but NMR scan 24 hours after the onset of symptoms revealed prolonged T2 relaxation in medial thalami bilaterally, facilitating diagnosis of bithalamic infarction. Subsequent CT scans delineated infarction in the vascular territory of the paramedian thalamic arteries. Previous clinical reports and the neuro- and vascular anatomy underlying this syndrome are reviewed, including cases that suggest a relationship to the syndrome of transient global amnesia.

Nuclear magnetic resonance imaging of the region of the sella turcica

Nuclear magnetic resonance (NMR) imaging is a versatile technique which has proved more sensitive in the detection of perisellar lesions than CT. A multiplanar facility, the demonstration of blood flow, and the lack of bone artefacts are its main advantages in this respect. Limitations are seen in its inability to identify small calcifications and to distinguish clearly perifocal oedema and tumour especially in malignant lesions. Relatively thick slices of 5-10 mm make the detection of microadenomas very difficult, mainly because of partial volume effects. T1 and T2 values alone are not sufficient to provide specific diagnosis. No reliable criteria have been determined yet, which would indicate the quality of pathological involvement of peritumorous structures by tumours of the sella region. At present NMR still plays a more complementary role to CT in the evaluation of this complex anatomical area. We are confident that with future technical refinements NMR will be able to replace ionizing radiation for the diagnostic imaging of the sella.

Variations in slice shape and absorption as artifacts in the determination of tissue parameters in NMR imaging

Existing models for the contrast between one tissue and another in NMR imaging have shown that there are regions of ambiguity particularly in spin-echo sequences. This paper indicates that there are further problems due to variations in slice formation with tissue parameters, and that it is necessary to be very conservative in assessing the magnitude of T1 in any imaging system. The change in shape affects the appearance of images due, in essence, to adjustments in partial volume effects, with some of the contributions to the signal from lesions with long T1 coming from different regions to those for normal tissue. A simple model for a slice is used because the actual form in a real imaging situation is the result of a number of factors which are usually not accurately enough known.

Correlations between proton nuclear magnetic resonance imaging and retrospective histochemical images in experimental cerebral infarction

Evaluation of ischemic brain injury in experimental cerebral infarction in gerbils and rats was performed by means of both proton nuclear magnetic resonance imaging ([1H]NMR-CT) and various histochemical analyses. In vivo nuclear magnetic resonance (NMR) imaging was carried out employing saturation recovery, inversion recovery, and spin echo pulse sequences. Spatial resolution of the images was excellent. The ischemic lesions were detected with a remarkable contrast in inversion recovery and spin echo images within a few hours after insult. Those changes in NMR images consistently corresponded with the various retrospective histochemical observations, especially with methods related to brain edema (K+ staining) rather than structural (enzymatic) studies. Calculated T1 and T2 relaxation times indicated the evolution of the edema state in the brain in situ. They correlated excellently with the retrospective water content measurement. As a result, detailed characterization of the edema state induced by cerebral ischemia was possible in vivo using [1H]NMR imaging.

Nuclear magnetic resonance in oncology

The application of nuclear magnetic resonance (NMR) techniques to the diagnosis of cancer was first explored by Damadian, who proposed that benign and malignant tissues could be differentiated on the basis of characteristic differences in spin-lattice and spin-spin relaxation times (T1 and T2) as determined in vitro with NMR spectrometers. Damadian's thesis was very controversial and never gained widespread acceptance. Not all investigators were able to confirm his findings. Moreover, it was improbable that NMR would ever play an important role in the diagnosis of malignancy as long as biopsy was necessary to obtain material for analysis. However, the potential usefulness of NMR in oncology was enhanced considerably by the work of Lauterbur, who showed that NMR signals could be spatially encoded to produce images of the examined object. NMR imaging has made feasible the measurement of the T1 and T2 of lesions without biopsy. Unfortunately, initial efforts at characterizing tissues by in vivo determination of proton relaxation times have yielded disappointing results. Nonetheless, NMR imaging will be a powerful tool for evaluating patients with malignant disease because of the unique anatomic information it can provide without exposure of the patient to ionizing radiation. In vivo NMR spectroscopy of 31P and other sensitive nuclei may add a new dimension to clinical and experimental oncology.

Magnetic resonance imaging in a patient with soft tissue sarcoma of the leg

A patient with a high grade malignant fibrous histiocytoma of the leg is presented. Staging studies included a 99mTc diphosphonate bone scan, an arteriogram, a computed tomogram (CT) and a proton (1H) magnetic resonance imaging (MRI) study. By manipulating imaging parameters to enhance contrast between normal and neoplastic tissues, the latter technique more accurately delineated the extent of the soft tissue sarcoma than the other imaging modalities. Although spatial resolution of MRI is inferior to that of CT, it is likely that MRI will provide important information relative to surgical staging and preoperative planning that is not available from current imaging techniques.

Magnetic resonance imaging of primary intracranial tumors: a review

The experience in magnetic resonance (MR) imaging of primary intracranial neoplasia at University of California, San Francisco is reviewed. Seventy patients have been evaluated by MR and computerized tomography (CT). MR scans were performed using a multi-slice spin echo technique with a long pulse repetition time (TR = 2000 msec), and long echo sampling delay (TE = 56 msec). This method was most sensitive in differentiating normal gray and white matter and in detecting both cerebral edema and abnormal tissue with prolonged T2 characteristics. More sensitive to slight alterations in normal tissue, MR may detect a focal lesion in cases in which CT shows only mass effect. Moreover, MR may demonstrate more thoroughly the extent of tumor infiltration and broaden the characterization of abnormal tissue. Posterior fossa and brainstem anatomy are invariably better depicted by MR. The major limitations of MR include its inability to detect foci of tumor calcification, demonstrate the severity of bone destruction, or reliably distinguish tumor nidus from surrounding edema.

Magnetic resonance imaging of the neoplasms involving the head

Magnetic resonance (MR) imaging is a new method for producing high quality tomographic images with improved soft-tissue definition. MR imaging is conceptually quite different from conventional imaging modalities. A number of variations in MR technique may be chosen to provide better resolution or improved specificity. Some of the current and future roles of MR in the evaluation of both cranial and extracranial neoplasms involving the head are discussed.

Improvements in the clinical utility of calculated T2 images of the human brain

Magnetic Resonance Imaging (MRI) affords a considerable improvement in image contrast over other methods by virtue of the intrinsic NMR parameters spin density, T1, and T2. However, the clinical utility of routine quantification of these parameters is currently unknown. Calculated T2 images might afford additional disease specific information provided the calculation algorithm generates accurate T2 values. In this study, calculated T2 images of a MnCl2 phantom (spanning a T2 range of interest of 45.7 ms to 346.6 ms at 6 MHz) were generated utilizing a variety of calculation algorithms based upon a data set of 32 sequential spin-echo (SE) images. In general, when utilizing only the earliest sequential SE after the 90 degree pulse for the T2 calculation, the greater the number of SE used in the calculation algorithm, regardless of how they were averaged, the more accurate and less noisy was the calculated image. When only limited numbers of SE were used in the calculation algorithm, accuracy and noise varied with the choice of TE suggesting that there may be optimal timings for TE for a particular T2 range of interest. Forty-two calculated T2 head images of normal subjects, based upon data sets of 16 sequential SE, were evaluated for the T2 values of normal brain. These were compared to T2 images calculated via 7 different algorithms based upon 16 SE data sets from two patients with CNS pathology. An optimal algorithm was identified in which 16 SE Carr-Purcell-Meiboom-Gill (CPMG) were averaged into two images for the T2 calculation. With this algorithm, calculated images could be generated efficiently which were accurate and relatively noise free. The availability of such images maximized whatever disease specificity, and thus clinical utility, T2 information affords.

Magnetic resonance of brain tumors: considerations of imaging contrast on the basis of relaxation measurements

Proton spin-lattice and spin-spin relaxation times have been measured in surgically-removed normal CNS tissues and a variety of tumors of the brain. All measurements were made at 20 MHz and 37 degrees C. Between grey and white matter from autopsy human or canine specimens significant differences in T1 or T2 were observed, with greater differences seen in T1. Such discrimination was reduced in samples obtained from live brain-tumor patients due to lengthening in T1 and T2 of white matter near tumorous lesions. Edematous white matter showed T1 and T2 values higher than those of autopsy disease-free white matter. Compared to normal CNS tissues, most brain tumors examined in this study demonstrated elevated T1 and T2 values. Exceptions, however, did exist. No definitive correlation was indicated on a T1 or T2 basis which allowed a distinction to be made between benign and malignant states. Furthermore, considerable variation in relaxation times occurred from tumor to tumor of the same type, suggesting that within a tumor type there are important differences in physiology, biology, and/or pathologic state. Such variation caused partial overlap in relaxation times among certain tumor types and hence may limit the capability of magnetic resonance imaging (MR) alone for the diagnosis of specific disease. Nonetheless, this study predicts that on the basis of T1 or T2 differences most brain tumors are readily detectable by MR via saturation recovery or inversion recovery with appropriate selections of pulse-spacing parameters. In general, tumors can be discriminated against white matter better than grey matter and contrast between glioma and grey matter is usually superior to that between meningioma and grey matter. This work did not consider tissue-associated proton density which should be addressed together with T1 and T2 for a complete treatment of MR contrast.

Resistive NMR of intracranial hematomas

Comparison between computed tomography and nuclear magnetic resonance imaging in 17 patients with intracranial hematomas indicates a distinct role for NMR in evaluating the stable patient with hematoma. NMR is useful for delineating the extent of the hematoma, the relationship of the hematoma to brain anatomy, and the presence of hematoma at a time when the hematoma is isodense on CT.

Magnetic resonance imaging and characterization of normal and abnormal intracranial cerebrospinal fluid (CSF) spaces

A retrospective review of twenty-five normal MRI brain studies performed with the spin-echo technique focused special attention on the ventricular and extraventricular cerebrospinal fluid (CSF) and revealed unique signal intensity characteristics in the two locations. In addition, MRI studies of ten patients with abnormal extraaxial fluid collections either missed with CT or indistinguishable from CSF on CT images were also analyzed. MRI is more sensitive when compared to CT in evaluating the composition of CSF. Unique signal intensity characterizes the two major CSF compartments and presumably reflects their known but subtle difference in protein concentration (10-15 mg%). Normal variant or abnormal developmental fluid collections can be better characterized with MRI than with CT. These preliminary observations are offered in view of their implications for patient management and suggest further investigation.

Magnetic resonance imaging (MRI): method and early clinical experiences in diseases of the central nervous system

Magnetic resonance imaging (MRI) has undergone a rapid development which is still continuing. In this article a survey is given of the present status of this new diagnostic tool in the evaluation of diseases of the central nervous system. When atoms with uneven numbers of protons or neutrons in a homogeneous magnetic field are tilted against the main vector of this field by a radiofrequency pulse, nuclear magnetic resonance can be observed. During the relaxation of the little dipoles back to the direction of the underlying magnetic field, a resonance signal is generated. The superposition of variable field gradients enables the scanning of sectional images in the axial, frontal and sagittal plane. The variables of H+-magnetic resonance which can be utilized for imaging are: the proton density, the relaxation times T1 (spin-lattice) and T2 (spin-spin) and flow effects. While the proton density in organic tissue fluctuates only by some 10%, the relaxation times may vary by several hundred per cent. Tissue contrast, therefore, is mainly based on relaxation times differences. The image character can also be influenced by variations of imaging parameters (i.e. repetition rate, interpulse delay, read out or echo delay) in different imaging sequences, such as the spin-echo and the inversion recovery technique. Depending on these imaging parameters T1 and T2 will contribute to the signal to a varying degree. This fact is most important for the diagnostic information of MRI. In initial clinical experiences in the diagnosis of diseases of the central nervous system, MRI has demonstrated high sensitivity in the detection of lesions (such as oedema, neoplasms, demyelinating disease), but less significance in lesion discrimination. In spinal disease the direct sagittal imaging of MRI enables MRI-myelography without contrast medium, superior to conventional myelography in many cases. For detailed evaluation of disc disease, however, the spatial resolution still has to be improved. Promising results have been obtained from flow effects. Depending on the flow velocity of blood, vessels appear white with intensive signals (slow flow) or black due to low signal intensities (rapid flow). MRI-angiography including measurement of blood flow seems possible. MRI-contrast media are not yet available for routine clinical use. Promising results have been reported on the basis of rare-earth elements, such as gadolinium Gd3+. These substances decrease T1 and T2 with subsequent increase in signal intensity. Concerning harmful side-effects of MRI, three possible sources have to be considered: the static magnetic field, the changing magnetic field, and radiofrequency heating.(ABSTRACT TRUNCATED AT 400 WORDS)

Nuclear magnetic resonance imaging in evaluation of clinical stroke syndromes

Proton nuclear magnetic resonance (NMR) imaging was performed on normal volunteers and patients with various types of clinical strokes. True three-dimensional volumetric data were obtained for subsequent reconstruction of images at various orientations, including transverse, coronal, and sagittal, and for specific matching to x-ray computed tomographic planes. A variety of radiofrequency pulse sequences was used to generate images weighted by the NMR parameters spin density (p) and spin-lattice (T1) and spin-spin (T2) relaxation times. Quantitative T1 data and qualitative T2 information were obtained from appropriate sequences. Proton NMR images embodying T1 information provided adequate delineation of normal intracranial anatomical structure and a high level of gray matter-white matter contrast; high lesion detection sensitivity in various clinical forms of stroke was seen in images providing both T1 and T2 information. T1 and T2 relaxation times were prolonged in embolic, thrombotic, and watershed infarctions as well as in lacunae; shortening of T1 and T2 was found in hematomas. Serpiginous vessels in an arteriovenous malformation were seen that had particular pulse sequences.

Nuclear magnetic resonance technology for medical studies

Nuclear magnetic resonance proton imaging provides anatomical definition of normal and abnormal tissues with a contrast and detection sensitivity superior to those of x-ray computed tomography in the human head and pelvis and parts of the cardiovascular and musculoskeletal systems. Recent improvements in technology should lead to advances in diagnostic imaging of the breast and regions of the abdomen. Selected-region nuclear magnetic resonance spectroscopy of protons, carbon-13, and phosphorus-31 has developed into a basic science tool for in vivo studies on man and a unique tool for clinical diagnoses of metabolic disorders. At present, nuclear magnetic resonance is considered safe if access to the magnet environment is controlled. Technological advances employing field strengths over 2 teslas will require biophysical studies of heating and static field effects.

Magnetic resonance imaging: skeletal applications

It has been assumed that magnetic resonance imaging (MRI) would have limited utility in visualizing bones, due to the lack of hydrogen atoms in cortical bone. Nonetheless, that same lack makes it possible to clearly distinguish cortical bone from soft tissue and from marrow. Thus, roles are described for magnetic resonance imaging in depicting bone and soft tissue tumors, the spine (including the spinal cord and vertebral discs), and appendicular soft tissue, as well as the detection of avascular necrosis in the hip.

Nuclear magnetic resonance imaging of the brain in children

Nuclear magnetic resonance imaging of the hydrogen nucleus provides a unique noninvasive display of proton dynamics in biologic tissues and fluids as well as internal anatomy in a sectional imaging format. No ionizing radiation is utilized. Our experience with NMR imaging of the brain in 14 pediatric patients is presented and compared with computed tomography. The major advantages of NMR over CT include its greater sensitivity to blood flow, edema, hemorrhage, and myelinization and its lack of beam-hardening artifacts. In addition, the potential for tissue characterization exists by determination of T1 and T2 relaxation times and of mobile proton density. Disadvantages of NMR over CT include its failure to demonstrate calcification and bone detail and longer data acquisition times.

Preliminary observations on magnetic resonance imaging in refractory epilepsy

We are studying the use of magnetic resonance (MR) imaging in localization of epileptogenic foci in patients with medically refractory partial epilepsy. Imaging is performed using a prototype resistive unit operating at 0.15 T. All studies include 7 mm axial sections obtained with a partial saturation sequence (TR = 200 msec) in which signals are recovered with a spin echo (TE = 11 msec) and images reconstructed using a modified 2D Fourier transform technique. Since the temporal lobe and limbic system are the commonest site of seizure foci in this group of patients, examinations were performed with the plane of section parallel to the temporal horns of the lateral ventricles. Consensus interpretations by a radiologist, neurologist and neurosurgeon have recognized findings considered possibly abnormal in six of 11 epilepsy patients and none of six normal volunteers. These preliminary results indicate that further study is warranted in this group of patients. Critical evaluation of such findings must be carried out in a larger group including normals and patients with a variety of neurologic disorders.

Contrast manipulation in NMR imaging

The past few years have shown rapid growth of NMR imaging in both image quality and diagnostic usefulness. It has become apparent, as the images have been published, that both inter- and intra-group imaging of the same underlying pathology produces images which can have vastly differing appearance. This effect is mainly due to imaging techniques which use different pulse sequence types and timings thus varying the relative contribution of the protpn density, T1, and T2 properties of the tissues. In this paper we investigate the contrast manipulation effects and methods for SNR optimization for the saturation recovery, inversion recovery, spin echo, and inversion recovery spin echo pulse sequences when applied to three clinically relevant imaging tasks.

Optimizing tissue contrast in magnetic resonance imaging

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.

Nuclear magnetic resonance imaging in central nervous system disease

From the preliminary work of many investigators, it appears that proton nuclear magnetic resonance (NMR) imaging will have wide application in the diagnostic assessment (and potential management) of patients with vascular, neoplastic, and demyelinating diseases of the central nervous system (CNS). Findings in isolated cases and small series suggest that NMR imaging may play a role in the evaluation of patients with other CNS conditions including hydrocephalus, malformations, infections, developmental and metabolic disorders, and degenerative processes. Because of the dynamic nature of disease processes involving the CNS, the precise meaning of NMR image parameters (rho, T1, and T2) remains unclear. A comprehensive study correlating NMR images in neurologic disease with precise neuropathologic examination is required. In the future, with accurate quantitative measurements of these NMR parameters, in vivo imaging may provide insight into the dynamic nature of neurologic disease.

Nuclear magnetic resonance imaging of the heart

Diagnostic imaging of the heart has, with the exception of echocardiography, employed techniques utilizing ionizing radiation. NMR imaging, a new technique, not only can generate images with spatial resolution closely approaching that of x-ray computed tomography without ionizing radiation, but can potentially detect the presence of intrinsic tissue damage. Although the exact role that NMR techniques will play in the assessment of cardiac disease is not currently defined, there are many instances where NMR may provide both unique and complementary information when compared with existing modalities.

Nuclear magnetic resonance imaging in multiple sclerosis

Ten patients with definite multiple sclerosis underwent hydrogen nuclear magnetic resonance imaging with a 3.5 kilogauss superconducting magnet, using the inversion recovery and spin-echo techniques of signal acquisition. Results were compared with high-resolution x-ray computed tomography. Spin-echo images demonstrated abnormal regions as areas of variably increased signal intensity. The contrast between abnormal and normal white matter improved as the intervals between sequential radiofrequency pulses and between pulse administration and signal sampling were increased. Inversion recovery images demonstrated abnormal areas as regions of decreased signal intensity but did not visualize lesions as well as spin-echo imaging. Spin-echo and inversion recovery imaging each demonstrated more extensive abnormalities than did computed tomography.

Nuclear magnetic resonance imaging of the orbit

Images

Nuclear magnetic resonance imaging

Nuclear magnetic resonance imaging has reached the point at which it is clear that such images will have a definite role in clinical practice. This article reviews the basic physical principles of nuclear magnetic resonance imaging, its current uses in disorders of the central nervous system, and its potential future applications in this field. The technique is also compared with computed tomography and positron emission tomography. Because nuclear magnetic resonance imaging is still in its infancy and its potential is great, definitive statements on present clinical use are difficult. Continual change and expansion of the role of nuclear magnetic resonance imaging in clinical practice in the next few years should be the rule.