Detection of acute renal ischemia in swine using blood oxygen level-dependent magnetic resonance imaging

PURPOSE: To determine the feasibility and sensitivity of blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) to detect acute renal ischemia, using a swine model, and to present the causes of variability and assess techniques that minimize variability introduced during data analysis. MATERIALS AND METHODS: BOLD MRI was performed in axial and coronal planes of the kidneys of five swine. Color R2* maps were calculated and mean R2* values and 95% confidence intervals (CIs) for the cortex and medulla were determined for baseline, renal artery occlusion and reperfusion conditions. Paired Student's t-tests were used to determine significance. RESULTS: Mean R2* measurements increased from baseline during renal artery occlusion in the cortex (axial, 13.8-24.6 second(-1); coronal, 14.4-24.7 second(-1)) and medulla (axial, 19.3-32.2 second(-1); coronal, 20.1-30.7 second(-1)). These differences were significant for both the cortex (axial, P < 0.04; coronal, P < 0.005) and medulla (axial, P < 0.02; coronal, P < 0.0005). No significant change was observed in the contralateral kidney. CONCLUSION: R2* values were significantly higher than baseline for medulla and cortex during renal artery occlusion. More variability exists in R2* measurements in the medulla than the cortex and in the axial than the coronal plane.     

Experimental cerebral ischemia studied using nuclear magnetic resonance imaging and spectroscopy

The effects of short-duration forebrain ischemia on cerebral metabolism in the rat have been studied using several nuclear magnetic resonance (NMR) techniques. In vivo phosphorus-31 (31P) NMR spectroscopy showed that the model produces rapid cerebral energy failure and acidosis. Reperfusion was accompanied by recovery of high-energy metabolites in about 30 minutes, with a slower recovery of pH. Proton (1H) NMR spectra of perchloric acid extracts of selected brain regions showed that levels of alanine and gamma-aminobutyric acid (GABA) were elevated and the level of glutamate was depressed immediately after the ischemic insult, returning to normal by 24 hours. The lactate level remained elevated for up to 7 days after ischemia, suggesting ongoing abnormal mitochondrial function. Postischemic cerebral glucose metabolism was monitored using carbon-13 (13C)-labelled glucose as an NMR probe. Glycolysis was impaired immediately after the ischemic insult, resulting in accumulation of glucose in the tissue and reduced formation of amino acids and tricarboxylic acid cycle intermediates. Glycolysis recovered by 1 hour, but underwent a secondary decrease at 24 hours, the time at which neuronal injury became manifest histologically and physiologically. Nuclear magnetic resonance imaging was used to follow the regional development of tissue injury in selectively vulnerable brain regions. Striatal changes were evident by 24 hours after reperfusion, increasing in intensity and accompanied by hippocampal changes by 48 hours, then becoming less pronounced by 72 hours. Histologic analysis of regional neuronal injury correlated well with the imaging results, establishing NMR imaging as a noninvasive method of visualizing the regional development of ischemic tissue injury.     

Evaluation of acute renal failure with magnetic resonance imaging using gradient-echo and Gd-DTPA

Detection of acute renal failure (ARF) using fast-scan magnetic resonance imaging (MRI) with Gd-DTPA was studied in a dog model. ARF was produced in five dogs by infusion of norepinephrine (0.75 micrograms/kg/min) into the renal arteries for 40 minutes. MRI was performed 1 hour later and compared with baseline (pre-ARF) MRI. There was no significant difference in the ratios of signal intensity-vs.-time curves from 0 to 35 seconds after injection of Gd-DTPA. However, a difference between the outer and inner medulla was significant in the time period of 5 to 20 minutes after Gd-DTPA injection. These later signal intensity differences by fast-scan (gradient-echo) technique may be useful in the evaluation of ARF.     

Tumor imaging using hyperpolarized 13C magnetic resonance spectroscopy

Dynamic nuclear polarization is an emerging technique for increasing the sensitivity of magnetic resonance imaging and spectroscopy, particularly for low‐γ nuclei. The technique has been applied recently to a number of ¹³C‐labeled cell metabolites in biological systems: the increase in signal‐to‐noise allows the spatial distribution of an injected molecule to be imaged as well as its metabolic product or products. This review highlights the most significant molecules investigated to date in preclinical cancer models, either in terms of their demonstrated metabolism in vivo or the biological processes that they can probe. In particular, label exchange between hyperpolarized ¹³C‐labeled pyruvate and lactate, catalyzed by lactate dehydrogenase, has been shown to have a number of potential applications. Finally, techniques to image these molecules are also discussed as well as methods that may extend the lifetime of the hyperpolarized signal. Hyperpolarized magnetic resonance imaging and magnetic resonance spectroscopic imaging have shown great promise for the imaging of cancer in preclinical work, both for diagnosis and for monitoring therapy response. If the challenges in translating this technique to human imaging can be overcome, then it has the potential to significantly alter the management of cancer patients. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Advances in magnetic resonance imaging and spectroscopy

Over the past decade, MR imaging and MR spectroscopy have provided a classic example of the rapid progress that can be accomplished by closely coupled technological advancement and clinical application. This article reviews some of the technical and clinical advances in MR imaging and MR spectroscopy that have occurred over the past year.     

Demonstration of acute retinal ischemia on diffusion weighted magnetic resonance imaging

In this case, we demonstrate the ability of diffusion weighted magnetic resonance imaging to localize retinal ischemia in a patient with acute vision loss. MRI is widely used for the evaluation of acute stroke, but there has only been one other case report showing the ability of DWI to rapidly localize retinal pathology, including central retinal artery occlusion [1]. Central retinal artery occlusion is an ocular emergency presenting with acute, painless loss of vision as a result of retinal ischemia. This case demonstrates the role of diffusion weighted imaging as an adjunctive test in the prompt diagnosis and management of central retinal artery occlusion. As the technology improves, we anticipate diffusion weighted imaging will become more sensitive for the identification of acute retinal ischemia and other retinal pathologies.     

Phenylketonuria: diffusion magnetic resonance imaging and proton magnetic resonance spectroscopy

Two patients with phenylketonuria are reported with white matter lesions. Diffusion magnetic resonance (MR) imaging revealed restricted diffusion patterns (high signal) on b = 1000 s/mm2 images associated with low apparent diffusion coefficient values ranging between 0.44 x 10-3 mm2/s and 0.56 x 10-3 mm2/s. On proton MR spectroscopy obtained in 1 of the patients, a prominent peak resonating at approximately 3.80 ppm was consistently present attributable to the peak rising from the alpha-proton of the phenylalanine molecule.     

Simultaneous multinuclear magnetic resonance imaging and spectroscopy

A technique has been developed to perform simultaneous multinuclear magnetic resonance imaging and spatially localized spectroscopy. It is inherently superior in terms of time efficiency over current approaches which use sequential or interleaved methods. The pulse sequence uses a parallel excitation and acquisition scheme to acquire multislice proton images concurrently with phosphorus-31 spectroscopic images. Because the phosphorus signal is necessarily collected in the presence of a gradient, an essential element of the technique is an algorithm to extract pure chemical-shift information.     

Gadolinium-DTPA-enhanced magnetic resonance imaging of the isolated rat heart after ischemia and reperfusion.

The objective of this study was to assess the potential of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) to identify myocardial ischemia and reperfusion in the isolated rat heart model. Ischemia was induced by reducing the perfusion pressure from 80 to 30 mm Hg for 2 hours. Hearts were not reperfused, or were reperfused for 20 minutes or for 2 hours. Perfusion was performed with Evans blue dye and/or Gd-DTPA for 3 minutes. Twenty isolated rat hearts were perfused according to the Langendorff method, and divided into five groups according to the perfusion status and the use of Gd-DTPA and/or Evans blue as perfusion markers. The Evans blue distribution in the hearts was assessed by point-counting volumetry. The Gd-DTPA distribution was assessed by magnetic resonance microimaging at 6.3 T field strength. Evans blue staining clearly identified areas with "no flow" or "no reflow." Perfusion with Gd-DTPA enhanced signal intensity significantly, both in ischemic and reperfused myocardium. Signal intensity in hearts reperfused for 2 hours was increased significantly compared to nonreperfused ischemic hearts, but not to ischemic hearts reperfused for 20 minutes. Magnetic resonance imaging with the aid of Gd-DTPA can identify ischemia and reperfusion in the isolated rat heart, dependent on residual perfusion.     

Study of the right ventricle using magnetic resonance imaging

Magnetic resonance (MR) imaging has proven efficacy in the study of the heart. Its clinical applications are directed primarily at the study of the left ventricle, and the right ventricle is relegated to the background. This article reviews the anatomy and physiology of the right ventricle, as well as the manifestations of most common diseases affecting this chamber of the heart: infarction, cardiomyopathy, masses, and right heart failure. Knowing the distinctive features of the right ventricle with respect to the left and the particularities of the MR imaging protocol results in better technical performance in cases in which the reason for the examination or imaging findings point to the right ventricle. The importance of the right ventricle in the management of cardiopulmonary disease is growing and MR imaging can provide clinicians with the support they need.     

31P magnetic resonance spectroscopy of mesenteric ischemia

31P magnetic resonance spectroscopy (MRS) was performed in vivo on normal and ischemic rat intestine. Within 3 min after induction of ischemia, there is a dramatic fall in adenosine triphosphate (ATP) and rise in inorganic phosphate (Pi). Our results suggest that MRS may prove useful in the early detection of mesenteric ischemia.     

大鼠C6胶质瘤模型的磁共振成像及1H波谱研究

目的 :评价磁共振成像及其波谱技术在大鼠胶质瘤模型研究中的应用价值。方法 :大鼠C6脑胶质瘤细胞体外培养 ,应用立体定向架将肿瘤细胞注入大鼠脑尾状核区域 ,接种后 12～ 18d分别行磁共振成像及其波谱检查 ,观察大鼠脑C6胶质瘤的磁共振成像及其波谱表现 ,并与病理学变化进行对照分析。结果 :2 4只接种大鼠脑内有 16只胶质瘤生长良好 ,磁共振成像表现为长T1、长T2 信号 ,增强后肿瘤强化明显 ,呈均匀强化或环状强化 ;磁共振波谱表现为NAA峰明显下降甚至消失 ,Cho升高为第一高峰 ,NAA/Cr比值明显下降、Cho/Cr比值明显上升 ,部分肿瘤出现乳酸峰 (Lac)或脂质峰(Lip)。病理学显示肿瘤呈浸润性生长 ,边界不清。随着肿瘤增大 ,肿瘤内部坏死明显 ,乳酸峰 (Lac)或脂质峰 (Lip)出现率增加。结论 :大鼠脑C6胶质瘤模型是一种生长稳定、较为理想的脑胶质瘤模型 ;磁共振成像 (MRI)可以准确提供胶质瘤模型的病理形态学信息 ,磁共振波谱 (MRS)则可以提供胶质瘤的代谢及生化信息。     

Evaluation of tissue perfusion in a rat model of hind-limb muscle ischemia using dynamic contrast-enhanced magnetic resonance imaging

PURPOSE: To evaluate the feasibility of using dynamic contrast-enhanced magnetic resonance imaging (MRI) for assessment of muscle perfusion in a rat model of hind-limb ischemia. MATERIALS AND METHODS: The acute alteration and chronic recovery in muscle perfusion and perfusion reserve after femoral artery ligation were quantified using the maximum Gd-DTPA uptake rate obtained by a T(1)-weighted gradient-recalled echo sequence. Radionuclide-labeled microsphere blood flow measurements were performed for comparison with the MR perfusion measurement on a separate set of animals. RESULTS: After femoral artery ligation, a significant reduction in resting muscle perfusion was only observed at 1 hour post-ligation during the 28-day follow-up period. Muscle perfusion reserve was severely diminished following the ligation. Despite significant recovery over time, perfusion reserve to the ligated limb reached only 63% of the perfusion capacity in the unaffected limb by 42 days post ligation. A strong correlation (r = 0.86) between MR perfusion and microsphere blood flow measurements was observed for evaluation of relative changes in muscle perfusion. CONCLUSION: Dynamic contrast-enhanced MRI with Gd-DTPA is useful to assess time-dependent changes in muscle perfusion and perfusion reserve in this hind-limb ischemia model.     

Perfusion magnetic resonance imaging and magnetic resonance spectroscopy of cerebral gliomas showing imperceptible contrast enhancement on conventional magnetic resonance imaging

The purpose of the present paper was to evaluate the utility of perfusion MRI in cerebral gliomas showing imperceptible contrast enhancement on conventional MRI, and to evaluate the relationships of perfusion MRI and magnetic resonance (MR) spectroscopic results in these tumours. Twenty-two patients with histopathologically proven cerebral gliomas and showing insignificant contrast enhancement on conventional MR were included in the present study. All patients underwent perfusion MRI and MR spectroscopy on a 1.5-T MR system. Significant differences of the relative cerebral blood volume (rCBV) values and the choline : creatine ratios were noted between low-grade and anaplastic gliomas (P < 0.01). Good correlation was found between the rCBV values and the choline : creatine values (y = 0. 532x + 1.5643; r = 0.67). Perfusion MRI can be a useful tool in assessing the histopathological grade of non-contrast-enhancing cerebral gliomas. Along with MR spectroscopic imaging it can serve as an important technique for preoperative characterization of such gliomas, so that accurate targeting by stereotactic biopsies is possible.     

大脑胶质瘤病磁共振成像结合磁共振波谱成像的诊断价值

目的：探讨磁共振成像（MRI）结合磁共振波谱成像（MRS）对大脑胶质瘤病的诊断价值。方法对15例经活体组织检查或手术病理证实的大脑胶质瘤病患者的临床表现及MRI平扫、增强,MRS影像学资料进行回顾性分析。MRI常规行T1WI、T2WI及FLAIR序列,采用时间飞跃法（TOF）的磁共振血管成像（MRA）,T1WI增强扫描。氢质子MRS采用单体素STEAM序列,并分析N-乙酰天门冬氨酸（NAA）、肌酸（Cr）、胆碱复合物（Cho）等物质峰值改变。结果所有病例均侵犯2个或2个以上脑叶,以颞叶、枕叶、胼胝体、基底节和丘脑等部位侵犯受累常见。病变区T1WI呈低或等低信号、T2WI呈高或混杂高信号、FLAIR上为高信号,未见明显坏死、钙化,受累区域脑组织肿胀,占位效应轻。注射钆喷酸葡胺增强扫描示10例无明显强化、3例斑片状强化、1例结节状强化、1例线状轻度强化。病变区域MRS表现为不同程度NAA降低,NAA/Cr比值降低;Cho上升,Cho/Cr和Cho/NAA的比值上升。结论 MRI结合MRS对大脑胶质瘤病的诊断及鉴别诊断具有临床价值,是目前诊断大脑胶质瘤病的首选影像学方法。     

DWI、MRA在急性脑缺血中的联合诊断价值

目的探讨磁共振弥散加权成像（DWI）与磁共振血管成像（MRA）在急性期脑缺血中的联合诊断价值。方法收集经临床和影像学确诊为急性脑梗死的患者40例,所有患者行常规MRI、FLIAR、DWI及MRA检查。对MRA原始图像进行最大密度投影（MIP）重建,选取病灶中心和健侧相应部位8×8像素大小区域,测定各自的表观扩散系数（ADC）,并计算rADC值。结果 40例患者的DWI图像均显示与临床症状及体征相符的病灶,尤其是在超急性期,病灶具有较高的信号强度和清晰度。MRA检查均获得清晰血管图像,结果显示动脉硬化3例,动脉狭窄或闭塞28例;40例中,24例大面积病灶的血管病变阳性率为100%。DWI显示病灶解剖定位均与MRA上血管异常解剖定位有较好的一致性。结论 DWI对脑梗死超急性期、急性期均有很高的敏感性,MRA对颅内大血管及其部分分支的狭窄或闭塞情况均能很好地显示。通过DWI和MRA联合扫描进行比较,可以对应显示同一区域脑实质和脑血管情况,进一步判定责任血管。     

Diffusion-weighted magnetic resonance imaging and magnetic resonance spectroscopy in the evaluation of focal cerebral tubercular lesions

Purpose: To review magnetic resonance (MR) diffusion-weighted imaging (DWI) and spectroscopy findings in patients with focal cerebral tuberculosis and to assess whether these techniques can adequately characterize focal cerebral tubercular lesions.

Material and Methods: Sixteen patients with single or multiple lesions were evaluated on a 1.5T MR system. DWI was performed with three 'b' values of 50, 500, and 1000 s/mm² and the apparent diffusion coefficient maps were calculated. MR spectroscopy was performed using the point-resolved single-voxel technique with 2 echo time values of 135 ms and 270 ms. The signal intensities of the tubercular lesions on diffusion images and the apparent diffusion coefficients (ADCs) of their centers, along with MR spectroscopy findings, were analyzed in relation to their T2-weighted MR appearances.

Results: DWI identified 17 of the 20 lesions evaluated. Increased signal intensity was seen in 9 of the 17 lesions. The ADCs of the lesions ranged from 0.406 to 2.64×10^-3 mm²/s (mean±SD: 1.038±0.609 mm²/s). Most of the lesions with hyperintense centers on T2-weighted images were of increased intensity on diffusion images, while those with hypointense centers on T2-weighted images were of decreased signal intensity on diffusion images. However, no statistical difference in the ADCs was found between lesions with increased and those with decreased signal intensity centers on T2-weighted images. MR spectroscopy revealed a lipid peak at 0.9-1.3 ppm in all of the 14 lesions evaluated. An increase in normalized choline:creatine ratio was found in all the lesions in which the spectra were obtained with the voxel, including a variable portion of the lesion wall.

Conclusion: DWI and MR spectroscopy help in determining the nature of cerebral tubercular lesions; however, since the findings are varied, they do not help in specific characterization.     

Diffusion-weighted magnetic resonance imaging detects early neuropathology following four vessel occlusion ischemia in the rat

Early neuropathology following a prolonged duration of four-vessel occlusion (4 VO) ischemia in the rat was charted using magnetic resonance imaging (MRI). Animals received either 30 minutes of 4 VO (N = 6) or sham operation (N = 6) prior to in vivo assessment. Proton density and T(2) and combined T(2)/diffusion-weighted (T(2)/DW) MRI were performed at 6, 24, and 72 hours postocclusion. T(2)/DW imaging was the most effective sequence for delineating between injured and intact tissues, indicating neuropathology in the dorsolateral striatum at 24 hours and in the CA1/CA2 subfields of the hippocampus at 72 hours following ischemia. Apparent diffusion coefficient values were significantly reduced in the striatum (P = 0.03) and hippocampus (P = 0.005) at 24 and 72 hours, respectively. This is the first report, to our knowledge, of T(2)/DW imaging detecting lesions following 4 VO in accord with the known temporal evolution of ischemic brain damage.     

Sturge-Weber syndrome: diffusion magnetic resonance imaging and proton magnetic resonance spectroscopy findings

We report on the diffusion magnetic resonance imaging (MRI) and proton MR spectroscopy findings of a 26-year-old female patient with Sturge-Weber syndrome. Echo-planar trace diffusion MRI revealed mildly high signal intensity changes at parieto-occipital lobes on b = 1000 s/mm² images, suggesting restricted diffusion. On corresponding apparent diffusion coefficient maps, those areas had moderately high signal intensity and high apparent diffusion coefficient values (around 0.9×10(-3) mm²/s) compared with the contralateral symmetrical normal side of the brain (0.776×10(-3) mm²/s). This finding was consistent with increased motion of water molecules (disintegration of the neural tissue) in these regions. Proton MR spectroscopy revealed decreased N-acetyl aspartate and increased choline peaks, indicating disintegration of neural tissue associated with neuronal loss as well.