Repeat cerebral blood volume assessment with first-pass MR imaging

The feasibility of performing multiple first-pass studies with dynamic, contrast material-enhanced magnetic resonance (MR) imaging was evaluated in a cat model of acute middle cerebral artery (MCA) ischemia. Two dynamic series of SSFP (steady-state free precession) images were acquired in each animal (n = 5) with a conventional 1.5-T imager. The initial first-pass study was acquired at 60 minutes after MCA occlusion, and the second study at 70 minutes, with each performed during an intravenous bolus injection of a 0.5 mmol/kg dose of gadoteridol. In both first-pass studies, differentiation of normal and ischemic gray and white matter was highly statistically significant. At a threshold of P<.01, no statistically significant difference in the peak signal intensity between the first and second studies was noted. A difference between the two studies in the recovery to baseline was seen, presumably due to T1 effects. First-pass MR studies can be repeated within the time frame of a single clinical examination, expanding their utility.     

The Significance of Perfusion Defect at Myocardial Perfusion MR Imaging in a Cat Model of Acute Reperfused Myocardial Infarction

Objective

To determine whether the size of a perfusion defect seen at myocardial perfusion MR imaging represents the extent of irreversibly damaged myocardium in acute reperfused myocardial infarction.

Materials and Methods

In nine cats, reperfused myocardial infarction was induced by occlusion of the left anterior descending coronary artery for 90 minutes and subsequent reperfusion for 90 minutes. At single-slice myocardial perfusion MR imaging at the midventricular level using a turbo-FLASH sequence, 60 short-axis images were sequentially obtained with every heart beat after bolus injection of gadomer-17. The size of the perfusion defect was measured and compared with both the corresponding unstained area seen at triphenyl tetrazolium chloride (TTC) staining and the hyperenhanced area seen at gadophrin-2-enhanced MR imaging performed in the same cat six hours after myocardial perfusion MR imaging.

Results

The sizes of perfusion defects seen at gadomer-17-enhanced perfusion MR imaging, unstained areas at TTC staining, and hyperenhanced areas at gadophrin-2-enhanced MR imaging were 20.4 ± 4.3%, 29.0 ± 9.7%, and 30.7 ± 10.6% of the left ventricular myocardium, respectively. The perfusion defects seen at myocardial perfusion MR imaging were significantly smaller than the unstained areas at TTC staining and hyperenhanced areas at gadophrin-2-enhanced MR imaging (p < .01). The sizes of both the perfusion defect at myocardial perfusion MR imaging and the hyperenhanced area at gadophrin-2-enhanced MR imaging correlated well with the sizes of unstained areas at TTC staining (r = .64, p = .062 and r = .70, p = .035, respectively).

Conclusion

In this cat model, the perfusion defect revealed by myocardial perfusion MR imaging underestimated the true size of acute reperfused myocardial infarction. The defect may represent a more severely damaged area of infarction and probably has prognostic significance.     

MR relative cerebral blood flow mapping of Alzheimer disease: correlation with Tc-99m HMPAO SPECT

RATIONALE AND OBJECTIVES: To evaluate the value of magnetic resonance (MR) perfusion imaging for diagnosis of Alzheimer disease (AD), the authors compared relative cerebral blood flow (CBF) maps obtained with MR perfusion imaging and technetium-99m hexamethyl-propyleneamine oxime (HMPAO) single photon emission computed tomography (SPECT) in patients with AD. MATERIALS AND METHODS: Eight patients with AD were studied with MR perfusion imaging and HMPAO SPECT. The relative CBF maps from the two techniques were spatially coregistered, and relative CBF values in 13 cerebral gray matter regions (total, 26 regions of interest) were compared with regression analysis. To evaluate the degree of deviation of each brain region from the regression line, a P value for the residual was calculated for each region. RESULTS: A significant overall correlation was seen between the relative CBF values produced by the two techniques (r = .68, P < .0001). Smaller P values for the residuals were obtained in the anterior cingulate cortex (P = .05) and posterior cingulate cortex (P < .001), indicating larger deviations in these regions. When data from these two regions were eliminated, the correlation coefficient rose to 0.80 (P < .0001). CONCLUSION: Despite fairly large discrepancies in the anterior and posterior cingulate cortices, the relative CBF map obtained with MR imaging is generally in close agreement with the HMPAO SPECT map, suggesting that MR perfusion imaging can provide clinically useful information regarding CBF abnormalities in patients with AD.     

Perfusion MR Imaging in Gliomas: Comparison with Histologic Tumor Grade

Objective

To determine the usefulness of perfusion MR imaging in assessing the histologic grade of cerebral gliomas.

Materials and Methods

In order to determine relative cerebral blood volume (rCBV), 22 patients with pathologically proven gliomas (9 glioblastomas, 9 anaplastic gliomas and 4 low-grade gliomas) underwent dynamic contrast-enhanced T2*-weighted and conventional T1- and T2-weighted imaging. rCBV maps were obtained by fitting a gamma-variate function to the contrast material concentration versus time curve. rCBV ratios between tumor and normal white matter (maximum rCBV of tumor / rCBV of contralateral white matter) were calculated and compared between glioblastomas, anaplastic gliomas and low-grade gliomas.

Results

Mean rCBV ratios were 4.90°±1.01 for glioblastomas, 3.97°±0.56 for anaplastic gliomas and 1.75°±1.51 for low-grade gliomas, and were thus significantly different; p < .05 between glioblastomas and anaplastic gliomas, p < .05 between anaplastic gliomas and low-grade gliomas, p < .01 between glioblastomas and low-grade gliomas. The rCBV ratio cutoff value which permitted discrimination between high-grade (glioblastomas and anaplastic gliomas) and low-grade gliomas was 2.60, and the sensitivity and specificity of this value were 100% and 75%, respectively.

Conclusion

Perfusion MR imaging is a useful and reliable technique for estimating the histologic grade of gliomas.     

MR image contrast at high field strength.

The T1 of soft tissues increases with magnetic field strength. Some tissue contrast may be diminished on high-field-strength magnetic resonance (MR) images when conventional TRs are used, because of altered T1 effects on the MR signals. This necessitates longer TRs in techniques that use long TRs, which prolongs the examination excessively. Behavior of macroscopic magnetization is governed by the Bloch equations. Therefore, T1 contributions to the MR signal can be modulated by means of both timing intervals and radio-frequency pulses. The analytic solution to the Block equations allowed calculation of white matter/gray matter and gray matter/cerebrospinal fluid contrast in both spin-echo and inversion-recovery (IR) imaging. Rabbit brains (normal and tumor-containing) were then imaged in vivo at 1.5 and 4.7 T. In addition, MR images of a human head were obtained at 4.0 T. Experimental results supported the theoretical predictions that brain contrast on long TR spin-echo or IR images increases with field strength. However, varying the excitation flip angle allowed optimization of the T1 contribution to the MR signals, improving image contrast and/or reducing examination time. Thus, the dependence of T1 on field strength determines the optimum choice of imaging techniques and parameters in a predictable fashion.     

脑结节性硬化的MR DWI分析

目的:使用磁共振扩散加权成像(DWI)研究结节性硬化(TS)患者脑灰质和白质内病灶的扩散特点,并观察表观扩散系数(ADC)的变化。方法:对18例临床影像学诊断的TS患儿使用1.5TMR系统行DWI检查,测量大脑皮质和皮质下61个结节的ADC值和对照组15例正常儿童的正常白质的ADC值。结果:TS患儿大脑皮质和白质病灶的ADC值明显高于健康儿童正常脑白质的ADC值,两者存在统计学差异(P<0.001)。其中4例TS伴发的室管膜下巨细胞星形细胞瘤ADC值与正常脑实质的相似,两者无统计学差异(P>0.05)。结论:DWI可提供TS患儿脑皮质和白质病灶的病理信息,TS脑白质内结节灶的高ADC值可区别常规MRI检查T1WI和T2WI信号与TS相似的其他疾病结节状病灶。     

MR imaging signal response to sustained stimulation in human visual cortex

The response of signal intensity to transient (on-off) motor and sensory stimulation has been well studied; however, the dependence of signal response on the duration of stimulus requires further characterization. The objective of this study was to determine the time course of signal response in the human visual cortex to prolonged, sustained stimulation and to examine possible contributory physiologic mechanisms. Nine healthy volunteers underwent magnetic resonance (MR) imaging during sustained visual stimulation with light-proof binocular goggles. With photic stimulation, activation was observed in all subjects as an increase in signal intensity of the visual cortex. With sustained stimulation, a gradual decrease in signal intensity was subsequently observed, with progression toward an apparent steady state. Correlation with positron emission tomographic, MR spectroscopic, and visual evoked-potential data suggests that the initial uncoupling of cerebral blood flow and oxidative metabolism with a neuronal activation burst may represent a transient phenomenon. This quick-response phase may proceed to an equilibrium coupling of flow and oxidative metabolism, with a gradual normalization of venous deoxyhemoglobin levels and signal intensity.     

Multiparametric 3T MR approach to the assessment of cerebral gliomas: tumor extent and malignancy

Introduction Contrast-enhanced MR imaging is the method of choice for routine assessment of brain tumors, but it has limited sensitivity and specificity. We verified if the addition of metabolic, diffusion and hemodynamic information improved the definition of glioma extent and grade.Methods Thirty-one patients with cerebral gliomas (21 high- and 10 low-grade) underwent conventional MR imaging, proton MR spectroscopic imaging (¹H-MRSI), diffusion weighted imaging (DWI) and perfusion weighted imaging (PWI) at 3 Tesla, before undergoing surgery and histological confirmation. Normalized metabolite signals, including choline (Cho), N-acetylaspartate (NAA), creatine and lactate/lipids, were obtained by ¹H-MRSI; apparent diffusion coefficient (ADC) by DWI; and relative cerebral blood volume (rCBV) by PWI.Results Perienhancing areas with abnormal MR signal showed 3 multiparametric patterns: “tumor”, with abnormal Cho/NAA ratio, lower ADC and higher rCBV; “edema”, with normal Cho/NAA ratio, higher ADC and lower rCBV; and “tumor/edema”, with abnormal Cho/NAA ratio and intermediate ADC and rCBV. Perienhancing areas with normal MR signal showed 2 multiparametric patterns: “infiltrated”, with high Cho and/or abnormal Cho/NAA ratio; and “normal”, with normal spectra. Stepwise discriminant analysis showed that the better classification accuracy of perienhancing areas was achieved when regarding all MR variables, while ¹H-MRSI variables and rCBV better differentiated high- from low-grade gliomas.Conclusion Multiparametric MR assessment of gliomas, based on ¹H-MRSI, PWI and DWI, discriminates infiltrating tumor from surrounding vasogenic edema or normal tissues, and high- from low-grade gliomas. This approach may provide useful information for guiding stereotactic biopsies, surgical resection and radiation treatment.     

Increased self-diffusion of brain water in normal aging

With magnetic resonance (MR) imaging, brain water self-diffusion was measured in 17 healthy volunteers 22–76 (mean, 44.6) years old. The calculated values for the apparent diffusion coefficients (ADCs) ranged from 0.58 × 10^?9 to 1.23 × 10^?9 m²/sec in cerebral white matter. A significant correlation was found between the ADC in white matter and age (r =.7069, P <.01). The calculated values for ADC in cortical gray matter ranged from 1.06 × 10^?9 to 1.72 × 10^?9 m²/sec no correlation was found between ADCs in gray matter and age. The increased ADC in white matter may be caused by an increase in the extracellular volume due to age-dependent neuronal degeneration or to changes in myelination. These findings have implications for future clinical investigations with diffusion MR imaging techniques in patients with neurologic diseases, and stress the importance of having an agematched group of healthy volunteers for comparison.     

Clinical relevance of diffusion and perfusion magnetic resonance imaging in assessing intra-axial brain tumors

Advanced magnetic resonance (MR) imaging techniques provide physiologic information that complements the anatomic information available from conventional MR imaging. We evaluated the roles of diffusion and perfusion imaging for the assessment of grade and type of histologically proven intraaxial brain tumors. A total of 28 patients with intraaxial brain tumors underwent conventional MR imaging (T2- and T1-weighted sequences after gadobenate dimeglumine injection), diffusion imaging and T2*-weighted echo-planar perfusion imaging. Examinations were performed on 19 patients during initial diagnosis and on nine patients during follow-up therapy. Determinations of relative cerebral blood volume (rCBV) and apparent diffusion coefficient (ADC) were performed in the solid parts of each tumor, peritumoral region and contralateral white matter. For gliomas, rCBV values were greater in high-grade than in low-grade tumors (3.87±1.94 versus 1.30±0.42) at the time of initial diagnosis. rCBV values were increased in all recurrent tumors, except in one patient who presented with a combination of recurrent glioblastoma and massive radionecrosis on histology. Low-grade gliomas had low rCBV even in the presence of contrast medium enhancement. Differentiation between high- and low-grade gliomas was not possible using diffusion-weighted images and ADC values alone. In the peritumoral areas of untreated high-grade gliomas and metastases, the mean rCBV values were higher for high-grade gliomas (1.7±0.37) than for metastases (0.54±0.18) while the mean ADC values were higher for metastases. The rCBV values of four lymphomas were low and the signal intensity–time curves revealed a significant increase in signal intensity after the first pass of gadobenate dimeglumine. Diffusion and perfusion imaging, even with relatively short imaging and data processing times, provide important information for lesion characterization.     

Perfusion and diffusion MR imaging of thromboembolic stroke

A carotid embolic stroke model in rats was studied with a combination of diffusion- and perfusion-sensitive magnetic resonance (MR) imaging at 4.7 T. Capillary blood deoxygenation changes were monitored during formation of focal ischemia by acquiring multisection magnetic susceptibility-weighted echo-planar images. A signal intensity decrease of 7% ± 3 in ischemic brain (1% ± 2 in normal brain) was attributable to a T2* decrease due to increased blood deoxygenation, which correlated well with subsequently measured decreases in the apparent diffusion coefficient. The same multisection methods were used to track the first-pass transit of a bolus of dysprosium-DTPA-BMA [diethylenetriaminepentaacetic acid-bis(methylam-ide)] to assess relative tissue perfusion before and after stroke and after treatment with a thrombolytic agent. Analysis of contrast agent transit profiles suggested a total perfusion deficit in ischemic tissue and essentially unchanged perfusion in normal brain tissue after stroke.     

急性期放射性脑损伤磁共振扩散张量成像的定量研究

目的 利用磁共振扩散张量成像(DTI)定量分析,探讨正常脑组织不同部位急性期放射损伤的敏感性。方法 44例欲行全颅放疗的颅内肿瘤患者,在放疗前及放疗后3周行磁共振常规扫描、增强扫描及扩散张量成像,测量非肿瘤侧大脑半球接受总放射剂量为27 Gy时的等剂量区域内脑回灰质、脑回白质、深部灰质、深部白质的表观扩散系数(ADC)、部分各向异性(FA)、相对各向异性(RA)、容积比率(VR)等指标,并进行对比分析。 结果 所有患者常规及增强磁共振扫描非肿瘤侧大脑半球均未发现异常信号,而放疗后脑回灰质ADC值升高(t=-3.819,P<0.05),脑深部灰质核团ADC、容积比率值升高(t=-3.31、-2.810,P<0.05),脑深部灰质核团FA、RA值降低(t=2.906、2.349,P<0.05),其余部位放疗前后DTI各指标差异无统计学意义。结论 在急性期脑灰质较白质对放疗损伤敏感,DTI能从组织细胞功能水平对放射性脑损伤急性反应进行评价。     

Echo-planar MR imaging of human brain oxygenation changes

Echo-planar magnetic resonance (MR) imaging was used to observe signal intensity changes in the human brain during hypoxia. Increasing arterial blood levels of deoxyhemoglobin (0%–42%) during prolonged apnea were monitored with a pulse oximeter and correlated with gray matter and white matter signal attenuation of 13% and 20%, respectively. The results suggest the possibility of using deoxyhemoglobin boluses as a physiologic, intravascular susceptibility contrast agent for assessment of local cerebral oxygen utilization.     

Diffusion tensor imaging and magnetic resonance spectroscopy of the brain in a patient with Sturge-Weber syndrome

Quantitative brain MR spectroscopy (MRS) and diffusion tensor imaging (DTI) were used to characterize one patient with Sturge-Weber syndrome. Choline increases and N-acetylaspartate decreases were observed in pathologic frontal gray matter tissue compared to contralateral unaffected brain tissue without any change in the diffusion tensor imaging parameters (fractional anisotropy, apparent diffusion coefficient). The N-acetylaspartate decreases and/or choline increases observed here and in eight previously described Sturge-Weber patients probably reflect neuronal loss or dysfunction and demyelination as a result of recurrent seizures.     

Echo-planar imaging of intravoxel incoherent motion

The recently established single-shot technique of echo-planar imaging of intravoxel incoherent motion (IVIM) for determining and imaging the variations of microscopic motions of water has been applied to studies of water perfusion in phantoms and to in vivo studies of diffusion and perfusion in cat and human brains. The phantom results demonstrate that perfusion levels comparable with those found in vivo have easily observable and reproducible effects on signal amplitude that are consistent with previous IVIM theory. Reliable measurements of the diffusion coefficient in various types of brain tissue have been obtained. The results for white matter are consistent with the existence of anisotropic diffusion in oriented bundles of myelinated nerve fibers. The results for gray matter can be fitted to the IVIM theory and suggest a value of up to 14% for the fraction of the signal contributed by randomly perfusing fluid in normal cerebral cortex.     

Diffusion-weighted MR imaging of anisotropic water diffusion in cat central nervous system

The diffusion behavior of intracranial water in the cat brain and spine was examined with the use of diffusion-weighted magnetic resonance (MR) imaging, in which the direction of the diffusion-sensitizing gradient was varied between the x, y, and z axes of the magnet. At very high diffusion-sensitizing gradient strengths, no clear evidence of anisotropic water diffusion was found in either cortical or subcortical (basal ganglia) gray matter. Signal intensities clearly dependent on orientation were observed in the cortical and deep white matter of the brain and in the white matter of the spinal cord. Greater signal attenuation (faster diffusion) was observed when the relative orientation of white matter tracts to the diffusion-sensitizing gradient was parallel as compared to that obtained with a perpendicular alignment. These effects were seen on both premortem and immediate postmortem images obtained in all axial, sagittal, and coronal views. Potential applications of this MR imaging technique included the stereospecific evaluation of white matter in the brain and spinal cord and in the characterization of demyelinating and dysmyelinating diseases.     

Diffusion-Weighted Mr and Apparent Diffusion Coefficient in the Evaluation of Severe Brain Injury

Purpose: To study apparent diffusion coefficient (ADC) maps in severely brain-injured patients.Material and Methods: Four deeply comatose patients with severe brain injury were investigated with single-shot, diffusion-weighted, spin-echo echoplanar imaging. The tetrahedral diffusion gradient configuration and four iterations of a set of b-values (one time of 0 mm²/s, and four times of 1000 mm²/s) were used to create isotropic ADC maps with high signal-to-noise ratio. ADC values of gray and white matter were compared among patients and 4 reference subjects.Results: One patient was diagnosed as clinically brain dead after the MR examination. The patient's ADC values of gray and white matter were significantly lower than those of 3 other brain-injured patients. In addition the ADC value of white matter was significantly lower than that of gray matter.Conclusion: The patient with fatal outcome shortly after MR examination differed significantly from other patients with severe brain injury but non-fatal outcome, with regard to ADC values in gray and white matter. This might indicate a prognostic value of ADC maps in the evaluation of traumatic brain injury.     

Intracranial solitary fibrous tumor: Imaging findings

Objective

To study the neuroimaging features of intracranial solitary fibrous tumors (ISFTs).

Materials and methods

Retrospective study of neuroimaging features of 9 consecutive histopathologically proven ISFT cases. Location, size, shape, density, signal intensity and gadolinium uptake were studied at CT and MRI. Data collected from diffusion-weighted imaging (DWI) (3 patients), perfusion imaging and MR spectroscopy (2 patients), and DSA (4 patients) were also analyzed.

Results

The tumors most frequently arose from the intracranial meninges (7/9), while the other lesions were intraventricular. Tumor size ranged from 2.5 to 10 cm (mean = 6.6 cm). They presented multilobular shape in 6/9 patients. Most ISFTs were heterogeneous (7/9) with areas of low T2 signal intensity that strongly enhanced after gadolinium administration (6/8). Erosion of the skull was present in about half of the cases (4/9). Components with decreased apparent diffusion coefficient were seen in 2/3 ISFTs on DWI. Spectroscopy revealed elevated peaks of choline and myo-inositol. MR perfusion showed features of hyperperfusion.

Conclusion

ISFT should be considered in cases of extra-axial, supratentorial, heterogeneous, hypervascular tumor. Areas of low T2 signal intensity that strongly enhance after gadolinium injection are suggestive of this diagnosis. Restricted diffusion and elevated peak of myo-inositol may be additional valuable features.     

Wallenberg's lateral medullary syndrome: diffusion-weighted imaging findings

Purpose: To investigate the efficacy of diffusion-weighted imaging in patients with Wallenberg's lateral medullary syndrome.

Material and Methods: Thirteen patients with Wallenberg's lateral medullary syndrome were examined with conventional and echoplanar diffusion-weighted magnetic resonance (MR) imaging in a 1.5 T magnetic resonance unit. MR examinations were obtained in the acute or subacute stage of clinical syndrome, and diffusion-weighted imaging (DWI) was considered to be positive for infarction when an increase in signal was seen on b=1000 s/mm² images in the posterolateral medullary localization.

Results: DWIs were positive in 12 patients in the acute or subacute stages of this clinical syndrome. A false-negative result was obtained in only one patient examined within the first day, 10 h after onset of the symptoms. In the visual evaluation of the DWI, the contrast between normal and infarcted brainstem area was better in the high b-value images than in the apparent diffusion coefficient map images.

Conclusion: DWI is a valuable technique for examining patients presenting with the signs and symptoms of Wallenberg's syndrome and high b-value images can provide complementary data to T2-weighted images. However, because most of our case group were in either the acute or subacute stage, true sensitivity of the method in the hyperacute stage of the syndrome remains unclear.