MR梯度回波T2^＊WI检出脑微出血

目的：MR梯度回波T2^＊WI可检出无症状性脑微出血,本研究旨在探讨脑微出血的发生率、部位以及与高血压、脑卒中、脑室周围白质高信号的相关性。方法：在1年时间内共有375例患者在常规MRI／序列的基础上加扫T2^＊加权序列,排除了存在脑血管畸形出血、颅内占位、外伤、手术史等病例,共有324例进行分析处理。结果：脑微出血37例,总的发生率为11．4％,尤其位于基底节／放射冠（n=22）、丘脑（n=15）、小脑（n=14）和脑叶区域（n=15）,脑微出血主要见于存在脑中风史病例,与年龄、高血压、脑白质疏松症（脑室周围白质高信号）存在相关性（P〈0．001）。在2年多的影像学随访过程中,共发现2例（5．4％）出现新鲜脑出血。结论：MR T2^＊加权像可用于检出脑微出血,其发生提示存在脑微小血管病变,存在继发出血的可能性,建议对于60岁以上的老年人尤其是存在脑中风的病例,MR T2^＊加权应作为常规扫描序列。     

Brain microhemorrhages detected on T2*-weighted gradient-echo MR images

BACKGROUND AND PURPOSE: Multifocal microhemorrhages have been reported to be commonly found in the brain of patients with systemic hypertension and spontaneous brain hemorrhage. The factors associated with these lesions detected on T2*-weighted gradient-echo images were examined to determine whether these lesions serve to indicate different types of microangiopathy and to predict a patient's risk for symptomatic hemorrhage. METHODS: The study population consisted of 2164 patients who underwent 2416 consecutive brain MR imaging studies performed during 3 years. The patients with intracerebral hemorrhages due to vascular malformations, neoplasms, trauma, or intracranial surgery and those with incomplete medical records were excluded; 2019 cases were analyzed. RESULTS: The overall incidence of microhemorrhages was 9.8%, predominantly in the lentiform nucleus (n = 96), thalamus (n = 88), and cortical-subcortical region (n = 93). Presence of microhemorrhages had the highest significant correlation with history of hemorrhagic stroke (P <.0001); advancing age, hypertension, and prominent white matter hyperintensity on T2-weighted images had the next highest significant correlation. Cortical-subcortical microhemorrhages were more frequently observed in patients who had previous lobar hemorrhagic stroke (P <.005). Among 139 patients with microhemorrhages who could be clinically followed up for more than 1 month, four (2.9%) had new hemorrhagic stroke. CONCLUSION: The presence of microhemorrhages may be not only a direct marker of bleeding-prone small-vessel diseases but also an indicator of different types of microangiopathy and a predictor of further hemorrhagic stroke.     

磁共振钆类对比剂对扩散加权成像的影响

目前 MR 检查中的常用序列包括增强扫描和扩散加权成像(diffusion-weighted imaging,DWI),在增强扫描中临床最常用的 MR 对比剂为钆类对比剂,其常规用于 MR 普通增强扫描、灌注加权成像(perfusion weighted imaging,PWI)、对比增强 MR 血管成像(contrast enhanced MRA,CE-MRA)等。DWI 近些年来在临床扫描中应用广泛[1],而表观扩散系数(apparent diffusion coefficient,ADC)值是 DWI 中的重要量化参数,DWI 通常在增强扫描前完成,但作为常规序列的补充序列,临床工作中有时会遇到需要在增强扫描完成后进行 DWI 扫描的情况[2],例如：在增强完成后才发现病变而希望增加 DWI 序列以提供更多鉴别诊断信息,一些序列设计中为获得足够的延迟扫描时间需要将增强扫描前置,如此也可以节省整体扫描时间等。因此,要确保临床扫描的序列设计方案,以及依赖 ADC 值进行计算得出的研究结果是否可信,必须首先明确钆类对比剂是否影响 DWI,特别是对DWI 中极为重要的量化指标 ADC 存在何种程度的影响。本文仅针对较常应用在临床中的钆类对比剂钆喷酸葡胺(Gd-DTPA)和钆塞酸二钠(Gd-EOB-DTPA)做一综述。     

Spleen: dynamic enhancement patterns on gradient-echo MR images enhanced with gadopentetate dimeglumine.

To examine the pattern of immediate enhancement with gadopentetate dimeglumine on dynamic magnetic resonance (MR) images of the spleen, this study was divided into two parts: In the first part, the authors retrospectively reviewed the dynamic MR images obtained with a fast low-angle shot (FLASH) sequence in the abdomen immediately after injection of gadopentetate dimeglumine in 137 patients. In the second part, dynamic gadolinium-enhanced FLASH images were prospectively compared with contrast material-enhanced computed tomographic (CT) scans in 17 patients with focal splenic lesions discovered on CT scans. In the first part, 108 patients (79%) had an arciform pattern of contrast enhancement; 22 patients (16%), a uniform pattern of high signal intensity; and seven patients (5%), a uniform pattern of low signal intensity. Most patients had arciform enhancement of the spleen; uniform enhancement occurred in some patients with underlying malignant or inflammatory disease. In the second part, all focal lesions seen on CT scans were seen on dynamic MR images (75 lesions), significantly more than were seen on FLASH images (15 lesions) (P < .001).     

Bright pleural effusion and ascites on gradient-echo MR images: a potential source of confusion in vascular MR studies

Motion of fluids other than blood can cause flow-related signal enhancement on MR images, including MR angiograms. In order to study this problem, the appearance of ascites (20 patients) and pleural effusions (five patients) was assessed on MR images made during suspended respiration with flow-compensated gradient-echo sequences as well as T1- and T2-weighted sequences. Signal intensities of vessels, fluid collections, and muscle were measured and vessel/muscle and vessel/fluid contrast were calculated. Fluid motion was measured with a bolus tracking technique that tags a selected volume of fluid with an RF presaturation. Fluid collections had a bright signal in four of five patients with pleural effusion and in 15 of 20 patients with ascites. The average contrast ratio between bright components of the fluid collections and vessels was only 0.03 +/- 0.09. Bright fluid collections were seen on MR angiograms and could obscure blood vessels. Bolus tracking measurements of ascites revealed multidirectional flow, suggesting that its bright signal is related to motion that continues during suspended respiration. Fluid collections appeared dark on T1-weighted images in all patients, indicating that a short T1 relaxation time was not a cause of the high signal intensity. The results indicate that, despite breath-holding, ascites and pleural effusions can show bright signal intensity on gradient-echo images. Awareness of this phenomenon will avoid confusion between moving fluid collections and flowing blood and identify a source of image degradation on both gradient-echo and T2-weighted spin-echo MR acquisitions.     

An alternative to GRASE: Toward spin-echo-like contrast with independent reconstruction of gradient-echo images

Multiple gradient echoes are generated for each RF echo of a Carr-Purcell-Meiboom-Gill (CPMG) train. Independently, phase-encoded fast spin-echo images are obtained from the different gradient echoes. Presently, three images are formed from three gradient-echoes from each of four RF echoes. The two peripheral gradient echo images are encoded for a late effective TE, then summed after reconstruction: this image has decreased fat intensity and increased susceptibility contrast compared with fast spin echo. The central gradient echoes yielded another image of intermediate contrast useful for neuroimaging. Raw data from the variously timed gradient echoes are not combined as they are in GRASE.     

Magnetization transfer contrast of hepatic lesions in breath-hold gradient-echo images of different T1 weighting

Seventeen patients with hepatic lesions [six metastases from colon, breast, and gallbladder carcinoma; one gallbladder carcinoma; five hepatocellular carcinoma; three focal nodular hyperplasia (FNH); one adenoma; and one cyst] were examined by MR breath-hold two-dimensional gradient-echo imaging to assess the potential of magnetization transfer contrast (MTC) for improved conspicuity and classification. Imaging sequences were applied with and without irradiation of off-resonant radiofrequency (RF) prepulses, but other parameters were unchanged. Therefore, quantitative assessment of MTC could be performed. In contrast to former examinations of other researchers, no significant difference of MTC was found between malignant liver lesions and benign lesions as FNH or adenoma. MTC might provide differentiation between hemangioma and cysts versus solid tumors, but MTC is not capable of distinguishing benign and malignant types of solid liver tumors. Effects of unchanged MTC prepulses on signal intensity of normal liver tissue and most lesions were more pronounced for nearly proton density-weighted fast low-angle shot (FLASH) images than for T1-weighted FLASH images, obtained by using higher excitation flip angles. Liver-to-lesion contrast could not be improved clearly by MTC prepulses. The contrast between liver and lesions in the gradient-echo breath-hold images was compared with standard T1- and T2-weighted spin-echo images. Liver-to-lesion contrast in the breath-hold images was found to be inferior to T2-weighted spin-echo images in 14 of 17 cases. Lesion conspicuity in regions near the diaphragm was better in breath-hold images, because problems with marked breathing motion (as in standard imaging) could be avoided.     

Magnetization transfer contrast in fat-suppressed steady-state three-dimensional MR images.

We demonstrate that magnetization transfer contrast can be used to improve the diagnostic utility of fat-suppressed steady-state three-dimensional gradient-recalled images. Fat suppression is achieved using a "jump-return" pair of contiguous shaped pulses. No time interval exists between the pulses, and no RF echo is generated. The sequence normally produces images with "density" weighting. Preparation of the spin magnetization with off-resonance frequency-selective excitation creates magnetization transfer contrast which attenuates signal intensity in proportion to the exchange rate of magnetization from free water with magnetization from water bound to macromolecules or protons that have restricted mobility. The resulting images have excellent fat suppression with low sensitivity to motion since no subtraction is used. In addition, the mechanism of signal attenuation is independent of paramagnetic effects, and addition of Gd-DTPA produces signal enhancement from vascularized regions of tissue. Examples are presented for the knee and breast, where the observation of pathology with signal enhancement from Gd-DTPA is improved over conventional 3D fat-suppressed images.     

Ascorbate-induced cancellation of nitroxide contrast media enhancement of MR images

Ascorbate (Vitamin C), a naturally occurring reducing substance, was tested as an in vivo chemical agent to cancel magnetic resonance imaging (MRI) tissue contrast enhancement induced by a nitroxide spin label contrast agent. Paramagnetic nitroxide compounds can be reduced in vitro by ascorbate to nonparamagnetic hydroxylamine derivatives. A nitroxide agent, TES, was injected intravenously, 2 mmol/kg, in 11 anesthetized rats. Renal cortical and hepatic intensities were monitored by serial T1-weighted images (TR/TE 310/15) acquired precontrast and postcontrast. Fourteen minutes after TES administration, ascorbate (1 mmol/kg) was injected in 6 rats, and saline in 5 control rats. At twenty-nine minutes postcontrast, a second TES-injection was given to all rats. The initial TES-injection resulted in a marked enhancement of kidney cortex and liver. Ascorbate administration immediately cancelled this enhancement. Contrast enhancement could be successfully reinduced by a repeat administration of TES. Results indicate that in vivo administration of reducing agents can be used to immediately cancel enhancement induced by nitroxide contrast media, thus nonenhanced images could be obtained after enhanced images without lengthy delays for contrast media elimination.     

MR imaging of acute intracranial hemorrhage: findings on sequential spin-echo and gradient-echo images in a dog model

Seven intraparenchymal hematomas (four venous and three arterial) were placed in the brains of six dogs in order to study the MR appearance of acute hemorrhage and to evaluate the effects of several variables on the signal intensity of the hematoma. MR imaging at 0.6 and 1.5 T was performed by using standard short and long TR spin-echo and low-flip-angle gradient-echo sequences. Sequential examinations were performed during the first week following hematoma creation. MR findings were compared with CT and postmortem examinations. Three patterns of signal intensity were observed, which varied according to the size (small vs large) and location (parenchymal vs intraventricular) of the hematomas. The small parenchymal hematomas did not undergo evolutionary changes. On short TR scans they were isointense at both field strengths, and therefore not detectable; on long TR scans these hematomas were of variable intensity at 1.5 T and were hyperintense at 0.6 T. On gradient-echo scans, they were hypointense at all times at both field strengths. The large parenchymal hematomas underwent evolutionary changes typical of those seen in clinical imaging. On short TR scans they were initially isointense and became hyperintense 1-3 days later. Long TR scans demonstrated initial hyperintensity, followed by the development of hypointensity within 12 hr in the venous hematomas and within 60 hr in the arterial hematoma. The intensity changes on long TR scans were seen at both 0.6 and 1.5 T, but occurred sooner and to a greater degree at 1.5 T. Gradient-echo imaging of these large lesions demonstrated hypointensity at all times at both field strengths. The intraventricular hemorrhages demonstrated more rapid development of hyperintensity on short TR scans and slower and less pronounced development of hypointensity on long TR scans compared with the parenchymal clots in the same animal. Gradient-echo imaging of the intraventricular hemorrhages demonstrated hypointensity at all times at both field strengths. A multifactorial hypothesis is proposed to explain the differences in intensity between venous, arterial, and intraventricular blood. Gradient-echo sequences should prove to be highly useful in detecting and delineating hemorrhages and are recommended for the MR protocol of patients with acute neurologic ictus and suspected hemorrhage.     

Perilesional hyperintense rim of malignant hepatic tumors on ferumoxide-enhanced T1-weighted gradient-echo MR images: correlation between MR imaging and histopathologic findings

PURPOSE: To correlate the perilesional hyperintense rim of malignant hepatic tumors seen on ferumoxide-enhanced T1-weighted gradient-echo (GE) MR images with histopathologic findings. MATERIALS AND METHODS: In 13 tumors in 12 patients, T1-weighted GE images (TE of 1.4 msec, flip angle of 90 degrees) obtained after IV administration of ferumoxide were evaluated. MR imaging was initiated within one hour of the completion of ferumoxide administration. Surgical resection for tumors was performed within an interval of two weeks of the MR imaging. Resected specimens were histopathologically examined for peritumoral sinusoidal congestion, desmoplastic reaction, compressed hepatic parenchyma, lymphocytic infiltration, and vascular proliferation. RESULTS: In twelve tumors (92%), prominently (N = 2), moderately (N = 5), and mildly to minimally (N = 5), a perilesional hyperintense rim was observed. Among histopathologic findings, the degree of peritumoral sinusoidal congestion correlated (R =.75, P <.04) with the degree of perilesional hyperintense rim. The thickness of the perilesional hyperintense rim showed a moderate positive correlation (R =.65, P <.02) with the thickness of peritumoral area with sinusoidal congestion. CONCLUSION: Perilesional hyperintense rim of malignant hepatic tumors on ferumoxide-enhanced T1-weighted GE images may correlate with sinusoidal congestion surrounding malignant hepatic tumors.     

Magnetization transfer effects on T1-weighted three-dimensional gradient-echo MR images of a phantom simulating enhancing brain lesions.

PURPOSETo develop a simple tissue phantom to study the effects of various imaging parameters and gadolinium concentrations on magnetization transfer (MT) and lesion-to-background ratios.METHODSA commercial egg product was doped with gadolinium in concentrations of 0.0 to 1.0 mmol/L and cooked. The T1 and T2 values were determined for the phantom materials and for the white and gray matter of a healthy volunteer subject. The gadolinium-doped egg phantom and human brain were studied using a short-repetition-time three-dimensional gradient-echo MT sequence with various effective MT powers, frequency offsets, and section-select flip angles. The normalized signal intensities, MT ratios (MTRs), and simulated lesion-to-background normal white matter contrast ratios were determined for a variety of experimental conditions.RESULTSThe MTR and lesion-to-background contrast ratios for all materials were greatest at the highest effective MT power (270 Hz, root-mean-square of amplitude) and the narrowest MT pulse frequency offset (1000 Hz). There was an inverse relationship between gadolinium concentration and MTR, and a positive relationship between the gadolinium concentration and lesion-to-background contrast. MTR was greatest at low flip angles, where there was little T1 weighting. The simulated lesion-to-background contrast showed a complex, gadolinium concentration-dependent relationship with section excitation flip angle.CONCLUSIONSThe tissue phantom has relaxation properties and MT behavior close to that expected for enhancing brain lesions, allowing a rigorous analysis of simulated lesion-to-background contrast for high MT power, short-repetition-time, three-dimensional gradient-echo sequences.     

Para-cavernous sinus venous structures: anatomic variations and pathologic conditions evaluated on fat-suppressed 3D fast gradient-echo MR images

BACKGROUND: The cavernous sinus communicates with several para-cavernous sinus venous structures, receiving blood flow from the superficial middle cerebral vein (SMCV), the sphenoparietal sinus (SPS), and the superior ophthalmic vein, and draining into the superior and inferior petrosal sinuses and pterygoid and basilar plexuses. Anatomic variations of these veins have been previously reported; however, some details, such as the relationship between the SPS and the SMCV, are incompletely characterized. The anatomic variations of para-cavernous sinus veins, especially drainage patterns of the SMCV, were evaluated on MR imaging. MATERIALS AND METHODS: Thirty-seven patients, including those without any lesions affecting the cavernous sinus or para-cavernous veins and patients with carotid cavernous fistulas, were examined by using fat-suppressed contrast-enhanced 3D fast gradient-echo MR imaging. Two neuroradiologists evaluated the images on a viewer, regarding the normal anatomy and the pathologic findings of the para-cavernous sinus veins. RESULTS: The fat-suppressed 3D fast gradient-echo MR images clearly depicted the para-cavernous sinus venous structures in all patients. SMCVs had 4 variations in the drainage patterns. The most frequent pattern was drainage into the SPS (39%), and other types were draining into cavernous sinus, pterygoid plexus, and tentorial sinus. The SPS had 3 variations. The most frequent pattern was drainage into cavernous sinus (72%), and others were the hypoplastic type or those draining into pterygoid plexus. CONCLUSION: The fat-suppressed 3D fast gradient-echo MR image is useful for evaluating the venous structures in the skull base. Knowledge of the variations is important for diagnosis and endovascular treatment of the cavernous sinus lesions.     

Delayed contrast enhancement on MR images of myocardium: past, present, future

Differential enhancement of myocardial infarction was first recognized on computed tomographic (CT) images obtained with iodinated contrast material in the late 1970s. Gadolinium enhancement of myocardial infarction was initially reported for T1-weighted magnetic resonance (MR) imaging in 1984. The introduction of an inversion-recovery gradient-echo MR sequence for accentuation of the contrast between normal and necrotic myocardium was the impetus for widespread clinical use for demonstrating the extent of myocardial infarction. This sequence has been called delayed-enhancement MR and MR viability imaging. The physiologic basis for differential enhancement of myocardial necrosis is the greater distribution volume of injured myocardium compared with that of normal myocardium. It is now recognized that delayed enhancement occurs in both acute and chronic (scar) infarctions and in an array of other myocardial processes that cause myocardial necrosis, infiltration, or fibrosis. These include myocarditis, hypertrophic cardiomyopathy, amyloidosis, sarcoidosis, and other myocardial conditions. In several of these diseases, the presence and extent of delayed enhancement has prognostic implications. Future applications of delayed enhancement with development of MR imaging and CT techniques will be discussed.