Nuclear magnetic resonance of the brain

Recent developments in MRI of the brain are reviewed. Over the past year there has been a general improvement in image quality, a paramagnetic contrast agent has been used in clinical practice, and surface coils have been used more generally in imaging. A wider range of disease has been studied, and there has been considerable agreement about the advantages and disadvantages of MRI at a basic level. As the technique moves into routine clinical use, questions about the economics of the technique are becoming more pressing.     

Assessment of severity of experimental pulmonary edema with magnetic resonance imaging. Effect of relaxation enhancement by Gd-DTPA

This study was done to demonstrate the capability of magnetic resonance imaging (MRI), using a spin-echo technique to detect permeability pulmonary edema in vivo, to correlate the signal intensity to the water content of the lung, and to assess the influence of administration of gadolinium-DTPA upon this relationship. Pulmonary edema was induced in 28 rats by intravenous injection of oleic acid (0.05-0.1 cc/kg). This was detected in all animals on images obtained with a repetition time (TR) of 2.0 seconds and echo times (TE) of 28 or 56 msec as increased signal intensity. Compared with control animals, the intensity was increased primarily in the dependent and peripheral portions of the lung. There was a significant linear relationship between the mean signal intensity, obtained from the area of both lungs within one slice, and the water content of both lungs (r = .94). Intravenous administration of gadolinium (Gd)-DTPA, 5 minutes prior to imaging, produced an easily observable increase in signal intensity on images of short TR (0.5 second) in the edematous lung tissue. However, these values did not exceed the intensities obtained without Gd-DTPA, if a long TR (2.0 seconds) was used for imaging. Thus, MRI has the capability for quantitating the severity of oleic acid-induced pulmonary edema. Gd-DTPA distributes rapidly into permeability pulmonary edema. It allows improved sensitivity at shorter pulse sequence repetition times and thus may diminish imaging time.     

Acute and repair stage characteristics of magnetic resonance relaxation times in oxygen-induced pulmonary edema

Proton magnetic relaxation times, T1 and T2, were determined for rat lungs exposed to 80% oxygen for a duration of 2 weeks. The transverse magnetization decay curve of the lung tissue was multiexponential. A linear combination of two decay curves with different T2 values fits the multiexponential decay suggesting that there are at least two different components of tissue water in the lung. Remarkable prolongation of T1 and T2 was demonstrated as lung injuries progressed in the acute stage of pulmonary edema. Both 1/T1 and 1/T2 were significantly correlated with 1/water content of the lung tissue. In the repair stage, T1 and T2 were significantly shortened. Shortening coincided with the spontaneous resolution of pulmonary edema. Relaxation rates showed no significant correlation with 1/water content in this stage. These results indicate that the physical state of water in the tissue is affected not only by the water content but also by the derangement of macromolecules in pulmonary edema. T2 was more sensitive than T1 for detecting pulmonary damage.     

Evaluation of acute brain edema using quantitative magnetic resonance imaging: effects of pretreatment with dexamethasone.

We developed a quantitative magnetic resonance imaging method to permit a rapid assessment of brain water content during osmotic brain edema produced by intraperitoneal (ip) injection of distilled water. Fifteen minutes after water injection, the normalized mean image intensity (MIn) from a spin-echo pulse sequence (TE = 80 ms, TR = 1085 ms) was the same as that measured from control animals not injected with water. Sixty minutes after the water injection, the mean +/- SEM brain image MIn had increased by 10.8 +/- 2.4% compared to 3.4 +/- 0.7% in control animals (P less than 0.05). Blood plasma osmolality decreased by 6-10% during this time interval. A subsequent ip injection of hypertonic NaCl solution (100 gm/liter) caused the blood plasma osmolality and brain image MIn to return toward their initial values. MIn of cerebral gray matter correlated with tissue water content measured in parallel studies. Animals pretreated with 0.25 mg/(kg day) dexamethasone had cerebral gray matter MIn values during osmotic edema which were lower than those of untreated animals.     

The effect of pulmonary edema on proton nuclear magnetic resonance relaxation times

This study was done to determine the effect of permeability pulmonary edema on proton nuclear magnetic resonance (NMR) relaxation times. Permeability edema was induced in rats by the intravenous injection of alloxan in saline. Control animals received only saline. The rats were ventilated through a tracheostomy; and after a time sufficient for the edema to become uniform, they were sacrificed. T1, and T2 and extravascular lung water were measured on lung samples. A linear relationship was found between the relaxation times and the extravascular lung water. Any diffuse alveolar process including pulmonary edema can increase proton density as well. The T1 and T2 relaxation times may be used to distinguish among different causes of increased proton density in the lung.     

Nuclear magnetic relaxation studies of the compartmentalized water in crosslinked polymer gels

Nuclear magnetic relaxation measurements of water in gels made of crosslinked dextran or polyacrylamide (water content, 30-60 wt%) were carried out using broad-line proton-pulsed NMR. Both T1 and T2 values showed inflection against pore size, i.e., the size of the compartment made by crosslinks of the polymer gels. T2 (or T1) values obtained for gels with compartments smaller than the critical size remained low within the range of the water content used in this experiment. Those values of gels with larger compartments, on the other hand, became higher with increasing size. When the size of the compartment was larger than the critical size, T1 and T2 values also became higher with increasing water content. The inflection point can be considered to correspond to the critical compartment size below which the motion of compartmentalized water in gels is more or less restricted. When the compartment is small, however, not only the effect of the molecular motion of water but also that of the proton exchange between compartmentalized water and gel matrix or hydration water should be taken into consideration for the interpretation of the short relaxation times observed, especially by T2. The results obtained in this investigation might provide useful information in the explanation and evaluation of the relaxation values in tissue.     

Tissue distribution and magnetic resonance spin lattice relaxation effects of gadolinium-DTPA

Gadolinium-DTPA complex (Gd-DTPA) is a potential clinical magnetic resonance (MR) contrast agent that enhances images primarily by decreasing spin-lattice relaxation time (T1) in tissues in which it localizes. This study was designed to determine the immediate tissue distribution of intravenously administered Gd-DTPA in selected organs of interest as a function of administered dose and tissue Gd-DTPA concentration. An intravenous bolus of Gd-DTPA with a tracer quantity of Gd-153 was administered to three groups of rabbits at the following doses: 0.01 mM/kg (n = 6); 0.05 mM/kg (n = 6); 0.10 mM/kg (n = 6). A control group received sham injections. Five minutes after Gd-DTPA was administered, all animals were killed; samples of serum, lung, heart, kidney, liver, and spleen were analyzed in a 0.25 T MR spectrometer to measure T1, and then in a gamma well counter to determine tissue concentration of Gd-DTPA. Tissue distribution (per cent dose/tissue weight in g) at five minutes after injection was proportionally constant over the range of doses given. Tissue concentration varied linearly with injected dose (r greater than 0.98 for all tissues). Relaxation rate (1/T1) varied linearly with injected dose and with tissue Gd-DTPA concentration (r greater than 0.97 for all tissues). The order of tissue relaxation rate response to a given dose was: kidney greater than serum greater than lung greater than heart greater than liver greater than spleen. We conclude that because of its extracellular distribution and linear relaxation rate versus concentration relationship, Gd-DTPA enhancement in MR images may be a good marker of relative organ perfusion.     

A magnetic resonance imaging study of experimental cerebral edema and its response to dexamethasone

Vasogenic edema following cortical freezing, and its response to dexamethasone were studied in cats. The time course of the edema shown by MRI was the same as that observed by invasive techniques. Dexamethasone retards early edema formation and may reduce its total volume, but does not accelerate its disappearance.     

The effects of mannitol and furosemide on brain water changes in normal and MCA-occluded rats: a magnetic resonance study

Mannitol and furosemide treatment of ischaemic brain oedema caused by middle cerebral artery occlusion (MCAO) was studied by MRI in 87 rats. MRI was performed in all rats before and 30–360 min after drug infusion. The examinations were performed in the presence of an intact blood-brain barrier (BBB) 6 h after MCAO, and 3 days after MCAO at the time of maximal disruption of the BBB. Spin echo (SE) sequences were used for imaging and for determination of the relaxation times T1 and T2. Subtraction images were constructed. Furosemide dehydrated healthy and ischaemic brain. Mannitol had no dehydrating effect on healthy brain tissue. However, when the BBB was disrupted in severe oedema mannitol produced a decrease in water content, a shortening of T1 and T2, and a decrease in intracranial pressure (ICP), while in less severe oedema mannitol could increase brain water content, thus aggravating ICP. The subtraction technique allowed visualisation of the transient change in bulk in water animals with disruption of the BBB after mannitol treatment. Correspondence to: K.-Å. Thuomas     

Myocardial collagen concentration and nuclear magnetic resonance relaxation times in the spontaneously hypertensive rat.

In order to test the hypothesis that the increased myocardial collagen concentration in the older, spontaneously hypertensive (SH) rat is associated with altered T2 and T1, we performed in vitro studies of 70 left ventricles from 8-, 22-, and 33-week-old SH and Wistar-Kyoto (WKY) rats. We also measured the left ventricle/body weight (LV/BW) ratio (as a measure of hypertrophy), left ventricular water and fat content, and hydroxyproline concentration (as a measure of collagen). The LV/BW ration was not significantly different between 8-week-old SH rats and WKY rats but was significantly greater in SH rats than in WKY rats at 22 and 33 weeks of age. Comparing SH rats with WKY rats at 22 weeks of age, no significant difference existed in T1, T2, water content, or hydroxyproline concentration. However, at 33 weeks of age in SH rats compared with WKY rats, hydroxyproline concentration was significantly greater (4.3 +/- 0.6 mg/g, respectively; P less than .0005), water content was significantly greater (77.1% +/- 0.3% vs. 76.2% +/- 0.3%, respectively; P less than .0001), and T2 and T1 were significantly longer (T2: 52.6 +/- 2.1 msec vs. 48.6 +/- 2.2 msec, respectively; P less than .0001; T1: 656 +/- 14 msec vs. 619 +/- 12 msec, respectively; P less than .0001). In all SH rats combined, T2 and hydroxyproline concentration were significantly correlated (r = .63; P less than .0001). Thus, in SH rats, myocardial hypertrophy precedes increased collagen deposition. These data suggest that estimation of magnetic resonance relaxation times may permit noninvasive identification of increased myocardial collagen deposition independent from changes in myocardial hypertrophy.     

In vivo visualization of myelin water in brain by magnetic resonance

We exploit the intrinsic difference in magnetic resonance spin-spin relaxation time, T₂, between water associated with myelin sheaths and water in other central nervous system tissue in order to measure myelin water content within any region of an image or to generate indirectly a myelin map of the brain. In normal volunteers, myelin water maps give the expected myelin distribution. In multiple sclerosis patients, lesions exhibit different myelin water contents providing insight into the de-myelination process unavailable from conventional magnetic resonance images. In vivo myelin measurement has important applications in the clinical management of multiple sclerosis and other white matter diseases.     

Nuclear magnetic resonance of the liver,spleen, and pancreas

This review includes the initial experience with NMR imaging of the liver, spleen, and pancreas at the University of California, San Francisco, using a prototype 0.35 Tesla system. This experience shows great promise for detection of hepatic metastases using T₁-weighted pulse sequences. T₂-weighted pulse sequences appear sensitive for detecting cavernous hemangioma of the liver and may allow tissue specific discrimination of the benign lesion from cancer. NMR is also suitable for evaluating diffuse metabolic alterations and is sensitive and specific for the diagnosis of iron overload. Detection of fatty liver requires use of chemical shift techniques as conventional NMR imaging pulse sequences are relatively insensitive. Motion artifacts and lack of an effective bowel contrast agent limits imaging of the pancreas and retroperitoneum, where CT remains the procedure of choice. The normal spleen has longer T1 and T2 relaxation times than liver or pancreas and NMR has not been successful in diagnosing splenic metastases or lymphoma on a routine basis. We conclude that NMR imaging will be valuable in the diagnosis of focal liver disorders; until fast scan techniques and effective magnetic contrast agents are available for oral and/or intravenous use, other abdominal applications will remain limited.     

Tyrosinemia: computed tomography, magnetic resonance imaging, diffusion magnetic resonance imaging, and proton spectroscopy findings in the brain

In a 5-month-old boy with tyrosinemia, computed tomography revealed diffuse hypodensity in the centrum semiovale. Magnetic resonance (MR) imaging revealed partial signal differences in the white matter and perirolandic regions, and the posterior limbs of the internal capsules revealed higher signal compared with the remainder of the white matter. There were high-signal changes (restricted water diffusion) in the corresponding regions on images of diffusion-weighted MR imaging (b = 1000 s/mm). This likely represented the presence of intramyelinic edema attributable to status spongiosus. Proton MR spectroscopy (repetition time = 1500 milliseconds, echo time = 40 milliseconds) from the lesion sites revealed 2 separate prominent peaks spread between 3.4 and 3.9 ppm. These peaks could represent the CH and CH2 aliphatic protons of the tyrosine molecule.     

Nuclear magnetic resonance in adrenoleukodystrophy: report of a case

The nuclear magnetic resonance images obtained in a case of adrenoleukodystrophy are presented. The areas of demyelination are clearly seen. The T1 values of the affected areas are not specific and coincide with those previously recorded in the lesions of multiple sclerosis or cerebrovascular disease. A diagnosis of adrenoleukodystrophy should be considered in younger male patients presenting with behavioural disturbances, especially when these are associated with abnormalities of vision or gait. There may be no clinical evidence of the associated adrenocortical insufficiency.     

Nuclear magnetic resonance: principles of blood flow imaging

Nuclear magnetic resonance (NMR) imaging with spin-echo techniques defines vascular structures with superb anatomic detail. Contrast agents are not necessary as there is intrinsic contrast between flowing blood and the vascular wall. The signal intensity from blood within the vessel lumen varies with the sequence of gradient and radiofrequency pulses used to generate the image as well as with the velocity of blood flow. Appropriate imaging techniques can optimize anatomic detail, distinguish slow from rapidly flowing blood, and serve to identify marked impairment or complete obstruction of flow in an artery or vein. Some examples of these principles in the intracranial circulation are illustrated.     

Flow effects in multislice, spin-echo magnetic resonance imaging. Model, experimental verification, and clinical examples

Flowing blood is responsible for a number of complex effects on clinical magnetic resonance (MR) images. To help elucidate these effects, a computer model of a conventional multislice spin-echo pulse sequence was developed. Using TR, TE, and direction of slice acquisition, the model calculates and plots a profile of MR signal intensity vs. z-axis velocity. The model predicts complex profiles with multiple segments of MR signal loss depending on TR, TE, direction of flow, sequence and timing of slice excitation, and slice location relative to adjacent slices. Model predictions were verified by imaging a bulk-flow phantom, consisting of a rotating cylinder filled with a manganese chloride solution with T1 = 840 msec and characterized by a velocity-gradient resolution of 0.23 cm/sec/pixel. In conventional spin-echo MRI of medium and large vessels using body coils, in which the velocity gradients exceed 2-5 cm/sec/pixel, most of the flow artifacts are averaged and are difficult to appreciate. However, bright crescents or rings of MR signal occasionally are seen in the inferior vena cava and portal vein, which the model is invoked to explain. The bulk-flow phantom will find use as a tool for calibrating flow-sensitive pulse sequences when these become widely available.