Decreased gray matter in normal aging: an in vivo magnetic resonance study.

Using in vivo magnetic resonance (MR) images, the percentages of gray matter, white matter, and cerebrospinal fluid (CSF) in the brains of 8 young and 6 elderly normal male community volunteers were quantified. Compared with the young men, the elderly men had a significantly lower percentage of gray matter (p less than .01) and higher percentage of CSF (p less than .01). The percentage of white matter was not significantly different between the two groups. This finding suggests that the age-related decrease in brain tissue is chiefly due to loss of gray matter.     

Atherosclerotic aortic component quantification by noninvasive magnetic resonance imaging: an in vivo study in rabbits

OBJECTIVES: We sought to demonstrate the ability that noninvasive in vivo magnetic resonance imaging (MRI) has to quantify the different components within atherosclerotic plaque. BACKGROUND: Atherosclerotic plaque composition plays a critical role in both lesion stability and subsequent thrombogenicity. Noninvasive MRI is a promising tool for the characterization of plaque composition. METHOD: Thoracic and abdominal aortic atherosclerotic lesions were induced in rabbits (n = 5). Nine months later, MRI was performed in a 1.5T system. Fast spin-echo sequences (proton density-weighted and T2-weighted [T2W] images) were obtained (in-plane resolution: 350 x 350 microns, slice thickness: 3 mm). Magnetic resonance images were correlated with matched histopathological sections (n = 108). RESULTS: A significant correlation (p < 0.001) was observed for mean wall thickness and vessel wall area between MRI and histopathology (r = 0.87 and r = 0.85, respectively). The correlation was also present on subanalysis of the thoracic and upper part of the abdominal aorta, susceptible to respiratory motion artifacts. There was a significant correlation for plaque composition (p < 0.05) between MRI and histopathology for the analysis of lipidic (low signal on T2W, r = 0.81) and fibrous (high signal on T2W, r = 0.86) areas with Oil Red O staining. T2-weighted images showed greater contrast than proton density-weighted between these different components of the plaques as assessed by signal intensity ratio analysis with the mean difference in signal ratios of 0.47 (S.E. 0.012, adjusted for clustering of observations within lesions) being significantly different from 0 (t1 = 39.1, p = 0.016). CONCLUSIONS: In vivo noninvasive high resolution MRI accurately quantifies fibrotic and lipidic components of atherosclerosis in this model. This may permit the serial analysis of therapeutic strategies on atherosclerotic plaque stabilization.     

In vivo heating of pacemaker leads during magnetic resonance imaging.

AIMS: Magnetic resonance imaging (MRI) is well established as an important diagnostic tool in medicine. However, the presence of a cardiac pacemaker is usually regarded as a contraindication for MRI due to safety reasons. In this study, heating effects at the myocardium-pacemaker lead tip interface have been investigated in a chronic animal model during MRI at 1.5 Tesla. METHODS AND RESULTS: Pacemaker leads with additional thermocouple wires as temperature sensors were implanted in nine animals. Temperature increases of up to 20 degrees C were measured during MRI of the heart. Significant impedance and minor stimulation threshold changes could be seen. However, pathology and histology could not clearly demonstrate heat-induced damage. CONCLUSIONS: MRI may produce considerable heating at the lead tip. Changes of pacing parameters due to MRI could be seen in chronic experiments. Potential risk of tissue damage cannot be excluded even though no reproducible alterations at the histological level could be found.     

In vivo magnetic resonance vascular imaging using laser-polarized 3He microbubbles

Laser-polarized gases (³He and ¹²⁹Xe) are currently being used in magnetic resonance imaging as strong signal sources that can be safely introduced into the lung. Recently, researchers have been investigating other tissues using ¹²⁹Xe. These studies use xenon dissolved in a carrier such as lipid vesicles or blood. Since helium is much less soluble than xenon in these materials, ³He has been used exclusively for imaging air spaces. However, considering that the signal of ³He is more than 10 times greater than that of ¹²⁹Xe for presently attainable polarization levels, this work has focused on generating a method to introduce ³He into the vascular system. We addressed the low solubility issue by producing suspensions of ³He microbubbles. Here, we provide the first vascular images obtained with laser-polarized ³He. The potential increase in signal and absence of background should allow this technique to produce high-resolution angiographic images. In addition, quantitative measurements of blood flow velocity and tissue perfusion will be feasible.     

In vivo measurement of myocardial mass using nuclear magnetic resonance imaging

To examine the accuracy of nuclear magnetic resonance imaging in measuring left ventricular mass, measurements of left ventricular mass made using this technique were compared with left ventricular weight in 10 mongrel dogs. Left ventricular myocardial volume was measured from five short-axis end-diastolic images that spanned the left ventricle. Left ventricular mass was calculated from left ventricular myocardial volume and compared with the left ventricular weight determined after formalin immersion-fixation. Linear regression analysis yielded the following relation in grams: left ventricular mass determined using nuclear magnetic resonance imaging = (0.94) (left ventricular weight) + 9.1 (r = 0.98, SEE = 6.1 g). The small overestimation of left ventricular weight by nuclear magnetic resonance imaging was judged to be secondary to both difficulty with proper border definition and partial volume effects. Hence, this imaging technique can be used to obtain accurate measurements of left ventricular mass in dogs in vivo.     

In vivo assessment of pituitary gland volume with magnetic resonance imaging: the effect of age

We used sagittal and coronal T1 weighted magnetic resonance images, at 1.5 Tesla, to measure the height, width, length, and cross-sectional area and to generate two estimates of pituitary gland volume in 35 normal volunteers aged 26-79 yr (19 females and 16 males). Subjects over 50 yr of age had significantly smaller pituitary gland height (P = 0.03), area (P = 0.04), and volume (P = 0.04) than those under 50 yr (by two-tailed t test). Overall, age was negatively correlated with pituitary volume (V1: r = -0.51; P = 0.003; V2: r = -0.47; P = 0.008), area (r = -0.43; P = 0.009), and height (r = -0.46; P = 0.005), but not with pituitary length or width. There were no statistically significant differences in pituitary size between men and women (by two-tailed t test). These findings suggest that pituitary gland height provides a good single measure for the assessment of pituitary gland size and that age must be controlled for in studies of pituitary gland size.     

Myocardial material property determination in the in vivo heart using magnetic resonance imaging

Objectives: To determine nonlinear material properties of passive, diastolic myocardium using magnetic resonance imaging (MRI) tissue-tagging, finite element analysis (FEA) and nonlinear optimization.Background: Alterations in the diastolic material properties of myocardium may pre-date the onset of or exist exclusive of systolic ventricular dysfunction in disease states such as hypertrophy and heart failure. Accordingly, significant effort has been expended recently to characterize the material properties of myocardium in diastole. The present study defines a new technique for determining material properties of passive myocardium using finite element (FE) models of the heart, MRI tissue-tagging and nonlinear optimization. This material parameter estimation algorithm is employed to estimate nonlinear material parameters in thein vivo canine heart and provides the necessary framework to study the full complexities of myocardial material behavior in health and disease.Methods and results: Material parameters for a proposed exponential strain energy function were determined by minimizing the least squares difference between FE model-predicted and MRI-measured diastolic strains. Six mongrel dogs underwent MRI imaging with radiofrequency (RF) tissue-tagging. Two-dimensional diastolic strains were measured from the deformations of the MRI tag lines. Finite element models were constructed from early diastolic images and were loaded with the mean early to late left ventricular and right ventricular diastolic change in pressure measured at the time of imaging. A nonlinear optimization algorithm was employed to solve the least squares objective function for the material parameters. Average material parameters for the six dogs wereE=28,722 ± 15,984 dynes/cm² andc=0.00182 ± 0.00232 cm²/dyne.Conclusion: This parameter estimation algorithm provides the necessary framework for estimating the nonlinear, anisotropic and non-homogeneous material properties of passive myocardium in health and disease in thein vivo beating heart.     

Functional magnetic resonance imaging study of human olfaction and normal aging

BACKGROUND: The function of human olfaction declines with advancing age. An important question centers on whether functional alterations to olfactory brain structures accompany age-related behavioral changes. In the present study, we tested the hypothesis that aged adults have intact though reduced activity in the central olfactory system using functional magnetic resonance imaging (fMRI). METHODS: University of Pennsylvania Smell Identification Test (UPSIT) was used to test the smell function of 11 young (23.9 +/- 1.6 years) and 8 aged (66.4 +/- 4.4 years) healthy participants. Then, the participants received fMRI at 3.0 T with lavender and spearmint as stimulants. After fMRI, the participants provided ratings for the odorants' intensity and pleasantness. RESULTS: The average UPSIT score of the aged adults was 34.1 +/- 1.5, which was significantly lower than that of the young adults (37.3 +/- 1.1) (p =.0004). Both age groups showed significant activation in major olfactory brain structures, including the primary olfactory cortex, entorhinal cortex, hippocampus and parahippocampal cortex, thalamus, hypothalamus, orbitofrontal cortex, and insular cortex and its extension into the inferior lateral frontal region. The aged adults showed less brain activity in olfactory structures (p =.022), consistent with lower ratings of odor intensity and UPSIT scores. Activation intensity in bilateral primary olfactory cortex areas and right insular cortex was also comparatively weaker (p <.019). CONCLUSION: Results demonstrate that significant activation in aged adults can be observed in all the olfactory brain structures that are activated in young adults, but with lower activation volume and intensity. This finding provides a necessary baseline for further investigations in olfaction and aging.     

Cerebral metabolism in streptozotocin-diabetic rats: an in vivo magnetic resonance spectroscopy study

Aims/hypothesis. It is increasingly evident that the brain is another site of diabetic end-organ damage. The pathogenesis has not been fully explained, but seems to involve an interplay between aberrant glucose metabolism and vascular changes. Vascular changes, such as deficits in cerebral blood flow, could compromise cerebral energy metabolism. We therefore examined cerebral metabolism in streptozotocin-diabetic rats in vivo by means of localised ³¹P and ¹H magnetic resonance spectroscopy. Methods. Rats were examined 2 weeks and 4 and 8 months after diabetes induction. A non-diabetic group was examined at baseline and after 8 months. Results. In ³¹P spectra the phosphocreatine:ATP, phosphocreatine:inorganic phosphate and ATP:inorganic phosphate ratios and intracellular pH in diabetic rats were similar to controls at all time points. In ¹H spectra a lactate resonance was detected as frequently in controls as in diabetic rats. Compared with baseline and 8-month controls ¹H spectra did, however, show a statistically significant decrease in N-acetylaspartate:total creatine (–14 % and –23 %) and N-acetylaspartate:choline (–21 % and –17 %) ratios after 2 weeks and 8 months of diabetes, respectively. Conclusion/interpretation. No statistically significant alterations in cerebral energy metabolism were observed after up to 8 months of streptozotocin-diabetes. These findings indicate that cerebral blood flow disturbances in diabetic rats do not compromise the energy status of the brain to a level detectable by magnetic resonance spectroscopy. Reductions in N-acetylaspartate levels in the brain of STZ-diabetic rats were shown by ¹H spectroscopy, which could present a marker for early metabolic or functional abnormalities in cerebral neurones in diabetes. [Diabetologia (2001) 44: 346–353] Received: 3 August 2000 and in revised form: 17 October 2000     

Measurement of left ventricular mass in vivo using gated nuclear magnetic resonance imaging

Alterations of left ventricular mass occur in a variety of congenital and acquired heart diseases. In vivo determination of left ventricular mass, using several different techniques, has been previously reported. Problems inherent in some previous methods include the use of ionizing radiation, complicated geometric assumptions and invasive techniques. We tested the ability of gated nuclear magnetic resonance imaging to determine in vivo left ventricular mass in animals. By studying both dogs (n = 9) and cats (n = 2) of various sizes, a broad range of left ventricular mass (7 to 133 g) was examined. With a 0.5 tesla superconducting nuclear magnetic resonance imaging system the left ventricle was imaged in the transaxial plane and multiple adjacent 10 mm thick slices were obtained. Endocardial and epicardial edges were manually traced in each computer-displayed image. The wall area of each image was determined by computer and the areas were summed and multiplied by the slice thickness and the specific gravity of muscle, providing calculated left ventricular mass. Calculated left ventricular mass was compared with actual postmortem left ventricular mass using linear regression analysis. An excellent relation between calculated and actual mass was found (r = 0.95; SEE = 13.1 g; regression equation: magnetic resonance mass = 0.95 X actual mass + 14.8 g). Intraobserver and interobserver reproducibility were also excellent (r = 0.99). Thus, gated nuclear magnetic resonance imaging can accurately determine in vivo left ventricular mass in anesthetized animals.     

Comparison of three methods for the estimation of the pituitary gland volume using magnetic resonance imaging: a stereological study

Stereological techniques using point counting and planimetry have been used to estimate pituitary gland volume. However, many studies have estimated pituitary gland volume by the mathematical approach the elliptic formula. The objective of the current study was to determine pituitary gland volume using stereological methods and elliptic formula on magnetic resonance imaging (MRI). In this study, pituitary gland volumes were estimated in a total of 28 subjects (22 females, 6 males,) who were free of any pituitary or neurological symptoms and signs. The mean ± SD pituitary gland volumes for the point counting, planimetry and elliptic formulae groups were 582.14 ± 140.16 mm³, 610.08 ± 133.17 mm³, and 432.82 ± 147.38 mm³, respectively. The mean CE for the pituitary gland volume estimates derived from the point counting technique was 8.07%. No significant difference was found between point counting and planimetric methods for the pituitary gland volume (P > 0.05). In addition, there was a 26.14 and 29.71% underestimation of pituitary volume as measured by the elliptic formula compared to the point counting and planimetric techniques, respectively. From these results, it can be concluded that stereological techniques are unbiased, efficient and reliable methods and are ideally suitable for in vivo examination of MRI data for pituitary gland volume estimation. Hence, we suggest that estimating pituitary gland volume using MRI study and stereology may be clinically relevant for pituitary surgeons for the investigation of the structure and function of the pituitary gland.     

High prevalence of cardiac hypertophy without detectable signs of fibrosis in patients with untreated active acromegaly: an in vivo study using magnetic resonance imaging

Objective Left ventricular (LV) hypertrophy and myocardial fibrosis are considered the main pathological features of acromegalic cardiomyopathy. The aim of the study was to evaluate the proportion of LV hypertrophy and the presence of fibrosis in acromegalic cardiomyopathy in vivo using cardiac magnetic resonance (CMR). Design and patients Fourteen consecutive patients (eight women, mean age 46 ± 10 years) with untreated active acromegaly were submitted to two‐dimensional (2D) colour Doppler and integrated backscatter (IBS) echocardiography and CMR. Measurements LV volume, mass and wall thickness and myocardial tissue characterization (IBS and CMR). Results On echocardiography: mean LV mass (LVM) and LVM index (LVMi) were 209 ± 48 g and 110 ± 24 g/m², respectively; hypertrophy was revealed in five patients (36%); abnormal diastolic function [evaluated by isovolumic relaxation time (IVRT) or early (E) to late or atrial (A) peak velocities (E/A ratio)] was found in four patients (29%). Systolic function evaluated by measuring LV ejection fraction (LVEF) was normal (mean 72 ± 12%) in all patients. Six patients (43%) had increased IBS (mean 57·4 ± 6·2%). On CMR: mean LVM and LVMi were 151 ± 17 g and 76 ± 9 g/m², respectively; 10 patients (72%) had LV hypertrophy. Contrastographic delayed hyperenhancement was absent in all patients; on the contrary, mild enhancement was revealed in one patient. Systolic function was normal in all patients (LVEF 67 ± 11%). LVMi was not related to serum IGF‐1 concentrations or the estimated duration of disease. Conclusions CMR is considered to be the gold standard for evaluating cardiac hypertrophy, fibrosis and systolic function. Using CMR, 72% patients with untreated active acromegaly had LV hypertrophy, which was only detected in 36% patients by echocardiography. However, cardiac fibrosis was absent in all patients irrespective of the estimated duration of disease. Although a very small increase in collagen content (as suggested by increased cardiac reflectivity at IBS), not detectable by CMR, could not be ruled out, it is unlikely that it would significantly affect cardiac function.     

Pituitary magnetic resonance imaging findings do not influence surgical outcome in adrenocorticotropin-secreting microadenomas

The pituitary origin of ACTH secretion in ACTH-dependent hypercortisolism can be difficult to assess, as magnetic resonance imaging (MRI) frequently fails to identify ACTH-secreting microadenomas or, on the contrary, may give false positive images of microadenomas. The choice of therapeutic option for patients with such normal MRI findings is controversial. Some groups propose routinely pituitary surgery, whereas others consider that neurosurgical exploration may be less successful and more harmful, and therefore prefer other types of management. The aim of this study was to compare surgical outcomes between patients with Cushing's disease (CD) and normal vs. positive pituitary MRI findings. Fifty-four patients (44 women and 10 men) with CD, operated on after 1996 in two centers (Kremlin-Bicêtre and Bordeaux) and followed postoperatively during a mean period of 19.9 +/- 22.7 months (range, 1-89 months), were enrolled in this retrospective study. Twenty-eight patients had normal pituitary MRI findings, and the pituitary origin of ACTH was established by bilateral petrosal sinus sampling in all of these cases. Twenty-six patients had positive MRI findings clearly showing a microadenoma. The two groups were not significantly different in terms of the sex ratio, age, frequency of hypertension, or diabetes, basal 24-h urinary free cortisol levels and follow-up. All of the patients were operated on by two experienced neurosurgeons using the same surgical protocol. Selective adenomectomy was performed when a tumor was identified, and subtotal hypophysectomy was performed when the lesion was uncertain or when no tumor was found during surgical exploration. Respectively, 50% and 84% of patients with normal and positive MRI results underwent adenomectomy (P < 0.05). A pituitary adenoma (confirmed by pathological examination) was found at surgery in 53% and 88% of patients in the normal and positive MRI groups, respectively (P < 0.05). The early surgical success rate (combining patients with corticotropic deficiency and patients with eucortisolism) was similar in the normal and positive MRI groups (78% and 88%, respectively; P = 0.85). The recurrence rate was lower in the normal MRI group, but the difference did not reach statistical significance (9% vs. 30%; P = 0.07). The final remission rate at the last visit was similar in the normal and positive MRI groups (72% and 61%, respectively; P = 0.29). Postoperative complications were also similar: 10 patients (36%) with normal MRI and five patients (20%) with positive MRI had at least one postoperative complication (surgical and/or pituitary deficiency; P = 0.12). Thus, the outcome of pituitary surgery in CD appears to be similar regardless of whether pituitary MRI shows a microadenoma. We recommend neurosurgical pituitary exploration as the first-line treatment of CD, provided that the pituitary origin of ACTH secretion is confirmed by bilateral petrosal sinus sampling in patients with normal pituitary MRI findings.     

重度颅内动脉狭窄活体高分辨磁共振成像初探

目的 观察症状性重度颅内动脉狭窄(SSIS)活体高分辨磁共振成像(HRMRI)的初探结果.方法 用3.0T MR扫描仪对5例SSIS患者进行HRMRI检查,盲法分析影像资料.结果 3例管壁呈典型的粥样硬化改变,呈偏心性增厚、大的脂质坏死核心、纤维帽破裂、不均匀强化.2例管壁为非粥样硬化可能性大,1例管腔内可见斜形隔膜与轻度增厚的管壁相连,增强后呈均匀强化;另1例管壁向心性明显增厚,增强后无强化.结论 SSIS的活体HRMRI成像是可行的,能够提供狭窄处的管壁信息,但仍需进一步研究其应用价值.