MR velocity imaging by phase display

The ability of the nuclear magnetic resonance signal to encode information about macroscopic motion has been recognized since the works of Hahn and Carr and Purcell. In the medical imaging setting this ability has led to a variety of ingenious magnetic resonance flow imaging schemes that ultimately may become competitive with X-ray angiography in sensitivity and specificity while remaining radically noninvasive. This work demonstrates that conventional spin-echo Fourier transform image acquisitions naturally encode a component of flow velocity that lies within the image plane. By displacing just the real part of the complex image data (phase display), the velocity distribution within the subject is revealed in image form. This method of flow imaging requires neither special pulse sequences nor image reconstruction and format software for its implementation. Further, images that intersect a flow channel longitudinally, demonstrating in-plane flow, yield an unusually large quantity of physiologic information per image. Phantom and in vivo flow images are presented. Also described is a phantom based on a rotating disk that enables calibration of the velocity/phase-shift constant for an untested pulse sequence.     

Diffusion- and perfusion-weighted MR imaging during the hyperacute phase of stroke

The sensitivity of diffusion-weighted MR imaging to detect a lesion within 6 hours of stroke onset was approximately 90%. The false negative results were usually small lesions (1 ml), were seen early, and were usually located in the brain stem. The specificity of this technique was nearly 100% when it was used correctly. The volume and the value of the apparent diffusion coefficient of the detected lesions provided prognostic information. After injection of a contrast agent (perfusion imaging), a time series of volumes were obtained using a T2* sensitive gradient echo EPI sequence. Hemodynamic perturbations of the cerebral parenchyma could be detected as well as the type of perturbation in the lesion. A map representing the mean transit time for each voxel was used to define the maximum volume of the perturbation. A hemodynamic penumbra was defined to be when this volume was larger than the volume detected on the diffusion images. The quantitative measure of cerebral blood flow could predict the irreversibility of the lesions when the value was below 18 ml/min/100g, and the extension of the ischemia in the penumbra zone when the value was below a threshold of 30 ml/min/100g.     

Improved liver lesion conspicuity by increasing the flip angle during hepatocyte phase MR imaging

Objective  

One of the advantages of using hepatobiliary contrast agents in liver MRI is the ability to acquire a delayed, “hepatocyte phase” dataset. This phase highlights many types of lesions as hypointense compared with the liver, because of the accumulation of contrast material in functioning hepatocytes and non-retention within most lesions.     

Role of slice thickness in MR imaging of the internal auditory canal

The relative efficacy of post-Gd-DTPA 5 and 3 mm axial T1-weighted images was compared in the detection of lesions in the internal auditory canal and cerebellopontine angle. One hundred twenty consecutive patients were prospectively evaluated with 5 mm axial T1-weighted slices. If these were negative or questionable. 3 mm axial slices were immediately obtained as the next sequence. Eighteen percent of cases were positive and in none of these was the 5 mm study normal. However, in 22 negative cases and two positive cases, a 3 mm study was necessary for confirmation. Five millimeter axial scanning post Gd-DTPA is recommended as the initial study for detection of masses in the internal auditory canal and cerebellopontine angle. Because this study requires fewer acquisitions than 3 mm sections and can be done satisfactorily on low and midfield systems, there are potential time- and cost-saving benefits to this approach.     

Hippocampal infolding angle changes during brain development assessed by prenatal MR imaging

BACKGROUND AND PURPOSE: Epileptic syndromes or neurodevelopmental delay may be associated with congenital anomalies of the shape or the orientation of the hippocampus. Scarce data are available about quantitative hippocampal developmental changes during fetal life, in particular about the progressive rotational changes of the hippocampal infolding angle (HIA), which can be considered a hallmark of hippocampal development. We hypothesized that prenatal MR imaging could demonstrate the progressive rotation of the hippocampus, providing quantitative data by means of the HIA determination. METHODS: We retrospectively selected 62 fetal MR imaging cases with normal brain at prenatal and postnatal imaging. The gestational age ranged from 20 to 37 weeks. The coronal section encompassing the pons was used to perform the measurement of HIA. HIA was defined as the angle between the line connecting the lateral margin of the cornu ammonis with the medial superior margin of the subiculum and the line passing through the midline structures. RESULTS: A significant positive correlation was found between the HIA value and the gestational age. The HIA was generally below 70 degrees before the gestational week 25 and above 70 degrees after week 30. CONCLUSION: Prenatal MR imaging allowed the progressive rotation of hippocampus to be detected during fetal life, providing normative data about HIA changes. These data could support further investigations to assess how fetal HIA anomalies might affect postnatal neurologic outcome.     

MR imaging of hypervascular liver tumors: timing optimization during the arterial phase

To analyze the optimal timing strategy for the detection of hypervascular liver tumors during the arterial phase of magnetic resonance (MR) imaging, a test examination after injection of 2 mL of gadopentetate dimeglumine was performed in 47 patients. The time course of the tumor-to-liver contrast-to-noise ratio (CNR) for all studies together was determined relative to the start of injection, the time of peak aortic enhancement, and the time of peak enhancement in the tumor. All studies were grouped together and the highest CNR was transiently observed at the time of peak tumor enhancement. This CNR was significantly higher than those observed at fixed delays after peak aortic enhancement. However, the CNRs at peak tumor enhancement+/-1.5 seconds did not differ significantly from those obtained after peak aortic enhancement. Finally, the CNRs obtained at fixed delays after the start of injection remained significantly lower. In hypervascular liver tumors, a higher CNR can be obtained during the arterial phase when the MR imaging delay is determined relative to the time of peak enhancement in the tumor or the aorta rather than being fixed after the start of contrast material injection. Timing based on the enhancement profile in the tumor rather than in the aorta should be performed only if rapid MR imaging is available with a time resolution of about 1.5 seconds to image the whole liver.     

Lobar intensity differences of the liver on MR imaging

Differences in signal intensity involving lobes of the liver were noted in seven cases of liver tumor. The clinical significance and possible cause of these differences in lobar intensity are discussed.     

Evaluation of normal brain development by prenatal MR imaging

PURPOSE: The aim of this study was to describe the normal pattern of development and maturation of the foetal brain with respect to gestational age as assessed with magnetic resonance imaging (MRI) and to provide an overview of the possibilities of the technique. MATERIALS AND METHODS: Foetal cerebral MRI was performed on 56 pregnant women between 19 and 37 weeks of gestation. Half-Fourier single-shot turbo spin-echo (HASTE), true fast imaging with steady precession (FISP), T1-weighted fast low angle shot (FLASH) two-dimensional (2D) and diffusion-weighted (DW) sequences with apparent diffusion coefficient (ADC) were obtained. Biometric parameters and developmental areas of the cerebral cortex were correlated to gestational age by using the Spearman rank correlation test. RESULT: We found a negative correlation between the germinal matrix/biparietal diameter ratio and gestational age and a positive correlation between the germinal and cortical matrix when expressed as external intraocular diameter ratio (R=0.452, p=0.02). The cortical mantle was correlated with biometric parameters, such as the biparietal diameter and the frontooccipital diameter, and with gestational age. The interhemispheric fissure, the parietooccipital fissure and the sylvian fissure were detectable by the 22nd week. In the grey matter, the mean ADC values varied from 1.76 x 10(-3) mm(2)/s (at week 19) to 0.89 x 10(-3) mm(2)/s (at week 37), whereas in the white matter, the values varied from 2.03 x 10(-3) mm(2)/s (at week 19) to 1.25 x 10(-3) mm(2)/s (at week 37). CONCLUSIONS: MRI provides a reliable valuation of brain maturation during pregnancy.     

Quantitative flow measurement in phase contrast MR angiography

The quantitative nature of phase contrast magnetic resonance angiography is explored, and a technique of blood flow measurement that is independent of system and patient parameters is presented. Phantom and patient studies demonstrate that quantitative flow measurements by phase contrast angiography can be routinely obtained with good accuracy despite nonuniform sensitivity profiles, blood flow pulsatility, and patient-to-patient changes in system gain. The calibration method requires the acquisition of only two flow images and thus can be performed as part of an angiographic session. Flow calibration of any flow profile can be accomplished with this technique. The solutions for plug flow and parabolic flow are presented.     

Methods for reconstructing phase sensitive slice profiles in magnetic resonance imaging

The experimental determination of slice profiles excited by applying radiofrequency pulses in the presence of a gradient generally results in magnitude profiles. The conditions necessary to obtain a phase-sensitive picture of the profile of a slice are discussed. A distinction is made between the “excitation profile” (distribution of the transverse magnetization immediately after the RF pulse) and the “slice profile” (distribution after refocusing by gradient reversal and/or imperfect gradient switching). Methods are presented that allow one to obtain either the excitation profile or the slice profile. It is shown that phase encoding along the direction of the slice selection gradient provides a convenient protocol for obtaining the distribution of both the real and imaginary parts of the slice profile. The phase sensitive excitation profile can be obtained by frequency encoding. These methods were used to evaluate the performance of various shaped pulses.     

Magnetic susceptibility-weighted MR phase imaging of the human brain

BACKGROUND AND PURPOSE: MR gradient echo imaging is sensitive to the magnetic susceptibility of different tissue types. The purpose of this study was to investigate the diagnostic potential of MR phase imaging of the human brain. METHODS: High-spatial-resolution, T2*-weighted, single-echo images were acquired in five volunteers and one patient with a brain tumor on a 1.5T system by applying a 3D, first-order, velocity-compensated gradient echo sequence by using a quadrature transmit-receive head coil. Phase images were reconstructed from the raw data and unwrapped by using a region-growing phase-unwrapping algorithm. Low-spatial-frequency components originating from static background susceptibility effects were removed by high-pass filtering. RESULTS: Phase images showed excellent image contrast and revealed anatomic structures that were not visible on the corresponding magnitude images. CONCLUSION: Improved processing of susceptibility-weighted MR phase images offers a new means of contrast for neuroimaging applications.     

血管性痴呆的MRI定量研究

目的 寻找可预测血管性痴呆（ｖａｓｃｕｌａｒ ｄｅｍｅｎｔｉａ,Ｖａｌ）的神经影像学指标。方法 对３０例ＶａＤ和３０名中风后非痴呆（ｓｔｒｏｋｅ ｗｉｔｈｏｕｔ ｄｅｍｅｎｔｉａ,ＳＷＤ）患者进行ＭＲＩ测量研究,测量指标包括：大脑白质病变面积、脑梗死面积、脑室－脑比率和正中矢状面胼胝体面积与幕上颅腔面积比率,并分析脑梗死所在部位。用判别分析法寻找区分ＶａＤ与ＳＷＤ的测量指标。结果 小血管病和多发脑     

Improved wedge method for the measurement of sub-millimeter slice thicknesses in magnetic resonance imaging

The standard method for measuring the slice thickness of magnetic resonance images uses the inclined surface of a wedge (wedge method); it is sensitive to small increases in noise because of the differentiation of the edge response function (ERF) required. The purpose of this study was to improve the wedge method by fitting a curve to the ERF. The curve-fit function was obtained by convolving an ideal ERF (a ramp function) with a Gaussian function to represent ERF blurring. Measurements of 5- and 3-mm slice thicknesses were performed on a 3T scanner using the conventional wedge method, the improved wedge method, and another standard method using an inclined slab (slab method). Subsequently, 0.5- and 0.25-mm slice thicknesses from multiple slices acquired using a three-dimensional sequence were measured using the improved wedge method. When measuring 5-mm slices, the differences in measurements obtained using the improved wedge method and the conventional slab and wedge methods were very small: <0.6% of the 5-mm slice thickness. The difference was ≤1.7% for 3-mm slices. For 0.5- and 0.25-mm slices, the mean values obtained using the improved wedge method were 0.543 ± 0.007 mm and 0.247 ± 0.015 mm, with a 1.2 and 5.9% coefficient of variation across slices, respectively. The improved wedge method is valid and potentially applicable to the measurement of sub-millimeter slice thicknesses.     

Reduction of phase error ghosting artifacts in thin slice fast spin-echo imaging

Fast spin-echo (FSE) imaging techniques are very sensitive to the relative phase between the 90° (excitation) RF pulse and the 180° (refocusing) RF pulses. In this paper, it is demonstrated that a phase shift can be created between the excitation and refocusing pulses in such a manner that the received signal is divided into two components of distinctly different phase shifts. The nature of these two components is reviewed. It is demonstrated that ghosting artifacts will occur when images are reconstructed from this received signal. The ghosting is shown to be object dependent. A correction technique is presented which calculates the phase errors among different echoes based on measurements from a single echo train acquired without phase encoding gradients. The results in both phantom and human studies show that this method is capable of reducing the ghosting artifact in thin slice FSE images.     

Normal postnatal development of the corpus callosum as demonstrated by MR imaging

Sixty-three patients, 3 days to 12 months old, were examined by MR imaging to evaluate the normal development of the corpus callosum in the first year of life. During the first month of life the corpus callosum is uniformly thin and of the same signal intensity as white matter throughout the brain. During the second month, a variable spurt of growth occurs in the genu, followed by a similar period of rapid growth in the splenium between 4-6 months of age. High signal intensity on T1-weighted images related to the myelination process begins to appear in the splenium by about 4 months and in the genu by about 6 months. The corpus callosum has an adult appearance on sagittal scans by about 8 months of age.     

Diffusion-weighted MR imaging in the acute phase of transient ischemic attacks

BACKGROUND AND PURPOSE: Radiologic assessment of acute transient ischemic attacks (TIAs) has been handicapped by the low sensitivity of CT and conventional MR imaging for acute small-vessel infarction and the difficulty in differentiating between acute and chronic lesions by use of these methods. Our purpose was to evaluate the incidence of TIA-related infarction by using diffusion-weighted MR imaging to determine whether the presence of a diffusion imaging abnormality correlates with the duration of symptoms or cause of TIA. METHODS: We prospectively studied 58 consecutive patients with acute TIA by use of diffusion-weighted imaging. All MR imaging was performed with a 1.5-T whole-body system with 24-mT/m gradient strength and an echo-planar-capable receiver. All patients were imaged within 10 days of stroke onset. RESULTS: Thirty-nine patients (67%) manifested a diffusion imaging abnormality consistent with acute ischemia. Cortical lesions were identified in 54% of these patients; most of them associated with other acute ischemic lesions. Subcortical lesions were identified in 46%; most of them were isolated from other lesions. The mean duration of symptoms in patients with no TIA-related diffusion imaging abnormalities was 0.96 hours (median, 0.33 hours) compared with a mean of 6.85 hours (median, 1.53 hours) in patients with diffusion imaging abnormalities (P =.025, Mann-Whitney U test). This significant correlation between the duration of TIA symptoms and the presence of TIA-related abnormalities was lost when we excluded from the analysis patients whose symptoms lasted longer than 6 hours (P =.513, Mann-Whitney U test). No significant correlation was observed between the size of TIA-related lesions and the duration of symptoms or cause of TIA. CONCLUSION: Two thirds of our TIA patients showed focal abnormalities indicative of acute ischemic lesions on diffusion-weighted images. This incidence is higher than that previously reported in the literature. The presence of such abnormalities increased with increasing total symptom duration, but this relation was not observed when only patients whose symptoms lasted less than 6 hours were considered. No significant correlation was observed between the cause and presence of TIA-related lesions on diffusion-weighted MR images. These TIA-related lesions are probably irreversible and may lead to subsequent infarct.     

宫内期暴露于MR成像听觉噪声的新生儿耳蜗功能测试

目的确定新生儿耳蜗损伤和后继的听力丧失是否与胎儿期暴露于1.5TMR操作噪声有关。材料与方法此项研究经伦理委员会批准,双亲签署知情同意书。收集在     

Cardiac-gated phase MR imaging of aqueductal CSF flow

The direction of CSF flow within the cerebral aqueduct was studied by cardiac-gated magnetic resonance (MR) phase images in five healthy volunteers and 10 patients with presumably normal cerebral CSF circulation. Caudal CSF flow was observed during systole and cranial flow during diastole. Using phantom based calibrations of the imager, aqueductal CSF velocities of 3-5 mm/s were calculated. Cardiac-gated phase MR is a potentially major tool for the investigation of the CSF circulation.