共查询到20条相似文献,搜索用时 31 毫秒
1.
Debra E. Horng MS Diego Hernando PhD Catherine D.G. Hines PhD Scott B. Reeder MD PhD 《Journal of magnetic resonance imaging : JMRI》2013,37(2):414-422
Purpose:
To compare the performance of fat fraction quantification using single‐R2* and dual‐R2* correction methods in patients with fatty liver, using MR spectroscopy (MRS) as the reference standard.Materials and Methods:
From a group of 97 patients, 32 patients with hepatic fat fraction greater than 5%, as measured by MRS, were identified. In these patients, chemical shift encoded fat‐water imaging was performed, covering the entire liver in a single breathhold. Fat fraction was measured from the imaging data by postprocessing using 6 different models: single‐ and dual‐R2* correction, each performed with complex fitting, magnitude fitting, and mixed magnitude/complex fitting to compare the effects of phase error correction. Fat fraction measurements were compared with co‐registered spectroscopy measurements using linear regression.Results:
Linear regression demonstrated higher agreement with MRS using single‐R2* correction compared with dual‐R2* correction. Among single‐R2* models, all 3 fittings methods performed similarly well (slope = 1.0 ± 0.06, r2 = 0.89–0.91).Conclusion:
Single‐R2* modeling is more accurate than dual‐R2* modeling for hepatic fat quantification in patients, even in those with high hepatic fat concentrations. J. Magn. Reson. Imaging 2013;37:414–422. © 2012 Wiley Periodicals, Inc. 相似文献2.
Venkata V. Chebrolu Catherine D. G. Hines Huanzhou Yu Angel R. Pineda Ann Shimakawa Charles A. McKenzie Alexey Samsonov Jean H. Brittain Scott B. Reeder 《Magnetic resonance in medicine》2010,63(4):849-857
Noninvasive biomarkers of intracellular accumulation of fat within the liver (hepatic steatosis) are urgently needed for detection and quantitative grading of nonalcoholic fatty liver disease, the most common cause of chronic liver disease in the United States. Accurate quantification of fat with MRI is challenging due the presence of several confounding factors, including T*2 decay. The specific purpose of this work is to quantify the impact of T*2 decay and develop a multiexponential T*2 correction method for improved accuracy of fat quantification, relaxing assumptions made by previous T*2 correction methods. A modified Gauss‐Newton algorithm is used to estimate the T*2 for water and fat independently. Improved quantification of fat is demonstrated, with independent estimation of T*2 for water and fat using phantom experiments. The tradeoffs in algorithm stability and accuracy between multiexponential and single exponential techniques are discussed. Magn Reson Med 63:849–857, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
3.
Shreyas S. Vasanawala Huanzhou Yu Ann Shimakawa Michael Jeng Jean H. Brittain 《Magnetic resonance in medicine》2012,67(1):183-190
MRI imaging of hepatic iron overload can be achieved by estimating T2* values using multiple‐echo sequences. The purpose of this work is to develop and clinically evaluate a weighted least squares algorithm based on T2* Iterative Decomposition of water and fat with Echo Asymmetry and Least‐squares estimation (IDEAL) technique for volumetric estimation of hepatic T2* in the setting of iron overload. The weighted least squares T2* IDEAL technique improves T2* estimation by automatically decreasing the impact of later, noise‐dominated echoes. The technique was evaluated in 37 patients with iron overload. Each patient underwent (i) a standard 2D multiple‐echo gradient echo sequence for T2* assessment with nonlinear exponential fitting, and (ii) a 3D T2* IDEAL technique, with and without a weighted least squares fit. Regression and Bland–Altman analysis demonstrated strong correlation between conventional 2D and T2* IDEAL estimation. In cases of severe iron overload, T2* IDEAL without weighted least squares reconstruction resulted in a relative overestimation of T2* compared with weighted least squares. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc. 相似文献
4.
Yongxian Qian Ashley A. Williams Constance R. Chu Fernando E. Boada 《Magnetic resonance in medicine》2010,64(5):1426-1431
Disorganization of collagen fibers is a sign of early‐stage cartilage degeneration in osteoarthritic knees. Water molecules trapped within well‐organized collagen fibrils would be sensitive to collagen alterations. Multicomponent effective transverse relaxation (T2*) mapping with ultrashort echo time acquisitions is here proposed to probe short T2 relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home‐developed ultrashort echo time sequence with echo times optimized via Monte Carlo simulations. Time constants and component intensities of T2* decays were calculated at individual pixels, using the nonnegative least squares algorithm. Four T2*‐decay types were found: 99% of cartilage pixels having mono‐, bi‐, or nonexponential decay, and 1% showing triexponential decay. Short T2* was mainly in 1‐6 ms, while long T2* was ~22 ms. A map of decay types presented spatial distribution of these T2* decays. These results showed the technical feasibility of multicomponent T2* mapping on human knee cartilage explants. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
5.
Catherine D.G. Hines MS Huanzhou Yu PhD Ann Shimakawa MSE Charles A. McKenzie PhD Jean H. Brittain PhD Scott B. Reeder MD PhD 《Journal of magnetic resonance imaging : JMRI》2009,30(5):1215-1222
Purpose:
To validate a T1‐independent, T2*‐corrected fat quantification technique that uses accurate spectral modeling of fat using a homogeneous fat‐water‐SPIO phantom over physiologically expected ranges of fat percentage and T2* decay in the presence of iron overload.Materials and Methods:
A homogeneous gel phantom consisting of vials with known fat‐fractions and iron concentrations is described. Fat‐fraction imaging was performed using a multiecho chemical shift‐based fat‐water separation method (IDEAL), and various reconstructions were performed to determine the impact of T2* correction and accurate spectral modeling. Conventional two‐point Dixon (in‐phase/out‐of‐phase) imaging and MR spectroscopy were performed for comparison with known fat‐fractions.Results:
The best agreement with known fat‐fractions over the full range of iron concentrations was found when T2* correction and accurate spectral modeling were used. Conventional two‐point Dixon imaging grossly underestimated fat‐fraction for all T2* values, but particularly at higher iron concentrations.Conclusion:
This work demonstrates the necessity of T2* correction and accurate spectral modeling of fat to accurately quantify fat using MRI. J. Magn. Reson. Imaging 2009;30:1215–1222. © 2009 Wiley‐Liss, Inc. 相似文献6.
Ning Jin Zhuoli Zhang Longjiang Zhang Guangming Lu Andrew C. Larson 《Magnetic resonance in medicine》2011,66(1):207-212
Abdominal effective transverse relaxation rate (R2*) mapping is critical for a wide range of applications. However, respiratory motion can lead to significant image quality deterioration and R2* overestimation. For this work, we explored the feasibility of combining respiratory self‐gating techniques with a multiple gradient‐recalled echo sequence for free‐breathing abdominal R2* measurements. In a series of eight normal volunteers, respiratory self‐gated–multiple gradient‐recalled echo methods effectively avoided motion artifacts to produce quantitative R2* measurements in liver, spleen, and kidneys that were comparable to R2* measurements produced while breath‐holding. Respiratory self‐gated–multiple gradient‐recalled echo methods demonstrated the potential to avoid the need for breath‐holding during abdominal R2* mapping. For clinical application, respiratory self‐gated–multiple gradient‐recalled echo approaches could be particularly useful for R2* measurements in those patients unable or unwilling to sustain sufficiently long breath‐holds to avoid motion artifacts. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc. 相似文献
7.
Chia‐Ying Liu Alban Redheuil Ronald Ouwerkerk Joao A. C. Lima David A. Bluemke 《Magnetic resonance in medicine》2010,63(4):892-901
Proton MR spectroscopy (1H‐MRS) has been used for in vivo quantification of intracellular triglycerides within the sarcolemma. The purpose of this study was to assess whether breath‐hold dual‐echo in‐ and out‐of‐phase MRI at 3.0 T can quantify the fat content of the myocardium. Biases, including T1, T*2, and noise, that confound the calculation of the fat fraction were carefully corrected. Thirty‐four of 46 participants had both MRI and MRS data. The fat fractions from MRI showed a strong correlation with fat fractions from MRS (r = 0.78; P < 0.05). The mean myocardial fat fraction for all 34 subjects was 0.7 ± 0.5% (range: 0.11–3%) assessed with MRS and 1.04 ± 0.4% (range: 0.32–2.44%) assessed with in‐ and out‐of‐phase MRI (P < 0.05). Scanning times were less than 15 sec for Dixon imaging, plus an additional minute for the acquisition used for T*2 calculation, and 15‐20 min for MRS. The average postprocessing time for MRS was 3 min and 5 min for MRI including T*2 measurement. We conclude that the dual echo method provides a rapid means to detect and quantifying myocardial fat content in vivo. Correction/adjustment for field inhomogeneity using three or more echoes seems crucial for the dual echo approach. Magn Reson Med 63:892–901, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
8.
Taigang He Peter D. Gatehouse Paul Kirk Raad H. Mohiaddin Dudley J. Pennell David N. Firmin 《Magnetic resonance in medicine》2008,60(2):350-356
Myocardial T measurement has been increasingly used for iron quantification to assess the risk of cardiac complications in thalassemia patients. In this study the noise effects were evaluated along with different curve‐fitting models on an iron overloaded ex vivo heart in order to determine the optimal method of T measurement and to help understand issues affecting reproducibility and accuracy. Gradient multiecho short axis images were acquired with differing numbers of excitations to generate varying signal‐to‐noise ratio (SNR) images. A noise correction method was implemented; linear and nonlinear curve‐fitting algorithms were compared and different curve‐fitting models (monoexponential, truncation, baseline subtraction, and offset) were evaluated. This study suggests that the T decay curve in an ex vivo heart can be fitted by a monoexponential model and accurate T measurements can be obtained with proper noise correction. With MRI noise, T is generally overestimated by including late low SNR data points, but underestimated by the offset or baseline subtraction models, which are in fact equivalent. In this situation the truncation model proves to be reproducible and more accurate than the other models. The study also shows that the nonlinear algorithm is preferred in T curve fitting. Magn Reson Med 60:350–356, 2008. © 2008 Wiley‐Liss, Inc. 相似文献
9.
Jens H. Jensen Haiying Tang Christina L. Tosti Srirama V. Swaminathan Alvaro Nunez Kristi Hultman Kamila U. Szulc Ed X. Wu Daniel Kim Sujit Sheth Truman R. Brown Gary M. Brittenham 《Magnetic resonance in medicine》2010,63(5):1201-1209
A new MRI method is proposed for separately quantifying the two principal forms of tissue storage (nonheme) iron: ferritin iron, a dispersed, soluble fraction that can be rapidly mobilized, and hemosiderin iron, an aggregated, insoluble fraction that serves as a long‐term reserve. The method utilizes multiple spin echo sequences, exploiting the fact that aggregated iron can induce nonmonoexponential signal decay for multiple spin echo sequences. The method is validated in vitro for agarose phantoms, simulating dispersed iron with manganese chloride, and aggregated iron with iron oxide microspheres. To demonstrate feasibility for human studies, preliminary in vivo data from two healthy controls and six patients with transfusional iron overload are presented. For both phantoms and human subjects, conventional R2 and R2* relaxation rates are also measured in order to contrast the proposed method with established MRI iron quantification techniques. Quantification of dispersed (ferritin‐like) iron may provide a new means of monitoring the risk of iron‐induced toxicity in patients with iron overload and, together with quantification of aggregated (hemosiderin‐like) iron, improve the accuracy of estimates for total storage iron. Magn Reson Med 63:1201–1209, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
10.
Tie‐Qiang Li Bing Yao Peter van Gelderen Hellmut Merkle Stephen Dodd Lalith Talagala Alan P. Koretsky Jeff Duyn 《Magnetic resonance in medicine》2009,62(6):1652-1657
Recent in vivo MRI studies at 7.0 T have demonstrated extensive heterogeneity of T2* relaxation in white matter of the human brain. In order to study the origin of this heterogeneity, we performed T2* measurements at 1.5, 3.0, and 7.0 T in normal volunteers. Formalin‐fixed brain tissue specimens were also studied using T2*‐weighted MRI, histologic staining, chemical analysis, and electron microscopy. We found that T2* relaxation rate (R2* = 1/T2*) in white matter in living human brain is linearly dependent on the main magnetic field strength, and the T2* heterogeneity in white matter observed at 7.0 T can also be detected, albeit more weakly, at 1.5 and 3.0 T. The T2* heterogeneity exists also in white matter of the formalin‐fixed brain tissue specimens, with prominent differences between the major fiber bundles such as the cingulum (CG) and the superior corona radiata. The white matter specimen with substantial difference in T2* has no significant difference in the total iron content, as determined by chemical analysis. On the other hand, evidence from histologic staining and electron microscopy demonstrates these tissue specimens have apparent difference in myelin content and microstructure. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc. 相似文献
11.
Simon Walker‐Samuel Matthew Orton Lesley D. McPhail Jessica K. R. Boult Gary Box Suzanne A. Eccles Simon P. Robinson 《Magnetic resonance in medicine》2010,64(3):914-921
Although the biasing of R*2 estimates by assuming magnitude MR data to be normally distributed has been described, the effect on changes in R*2 (ΔR*2), such as induced by a paramagnetic contrast agent, has not been reported. In this study, two versions of a novel Bayesian maximum a posteriori approach for estimating ΔR*2 are described and evaluated: one that assumes normally distributed data and the other, Rice‐distributed data. The approach enables the robust, voxelwise determination of the uncertainty in ΔR*2 estimates and provides a useful statistical framework for quantifying the probability that a pixel has been significantly enhanced. This technique was evaluated in vivo, using ultrasmall superparamagnetic iron oxide particles in orthotopic murine prostate tumors. It is shown that assuming magnitude data to be normally distributed causes ΔR*2 to be underestimated when signal‐to‐noise ratio is modest. However, the biasing effect is less than is found in R*2 estimates, implying that the simplifying assumption of normally distributed noise is more justifiable when evaluating ΔR*2 compared with when evaluating precontrast R*2 values. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
12.
Yamamura J Grosse R Graessner J Janka GE Adam G Fischer R 《Journal of magnetic resonance imaging : JMRI》2010,32(5):1104-1109
Purpose
To assess regional iron distribution by magnetic resonance imaging (MRI)‐R2* within the heart of patients with β‐thalassemia major (TM) and other iron overload diseases.Materials and Methods
Breathhold electrocardiogram (ECG)‐gated MRI (1.5 T) of the heart was used for the measurement of transverse relaxation rates R2* in 32 patients (11–79 years). In a mid‐papillary short‐axis slice divided into septal, anterior, lateral, and posterior quadrants, R2* was analyzed from region of interest (ROI)‐based signal intensities from 12 echo times (TE = 1.3–26 msec). Typical boundary effects were evaluated in detail.Results
The segmentation of the cardiac wall resulted in highly significant correlations of R2* between septal and all other quadrants. In the patient group with R2* < 50 s?1 (normal), all quadrants show higher normalized median rates (126%–174%) than the septum (P < 10?4), while this was relatively smaller in the group with septal R2* > 50 s?1. Typical boundary effects on segmental R2* from blood, lung tissue, epicardial fat, and hepatic iron could not be easily separated from segmental iron distribution.Conclusion
The measurement of MRI‐R2* in the interventricular septum is the least affected method by boundary effects to detect patients with iron overload at risk of developing heart failure. J. Magn. Reson. Imaging 2010;32:1104–1109. © 2010 Wiley‐Liss, Inc.13.
Jake S. Burrell Simon Walker‐Samuel Lauren C. J. Baker Jessica K. R. Boult Anderson J. Ryan John C. Waterton Simon P. Robinson 《Magnetic resonance in medicine》2011,66(1):227-234
A combined carbogen ultrasmall superparamagnetic iron oxide (USPIO) imaging protocol was developed and applied in vivo in two murine colorectal tumor xenograft models, HCT116 and SW1222, with established disparate vascular morphology, to investigate whether additional information could be extracted from the combination of two susceptibility MRI biomarkers. Tumors were imaged before and during carbogen breathing and subsequently following intravenous administration of USPIO particles. A novel segmentation method was applied to the image data, from which six categories of R2* response were identified, and compared with histological analysis of the vasculature. In particular, a strong association between a negative ΔR2*carbogen followed by positive ΔR2*USPIO with the uptake of the perfusion marker Hoechst 33342 was determined. Regions of tumor tissue where there was a significant ΔR2*carbogen but no significant ΔR2*USPIO were also identified, suggesting these regions became temporally isolated from the vascular supply during the experimental timecourse. These areas correlated with regions of tumor tissue where there was CD31 staining but no Hoechst 33342 uptake. Significantly, different combined carbogen USPIO responses were determined between the two tumor models. Combining ΔR2*carbogen and ΔR2*USPIO with a novel segmentation scheme can facilitate the interpretation of susceptibility contrast MRI data and enable a deeper interrogation of tumor vascular function and architecture. Magn Reson Med 66:227–234, 2011. © 2011 Wiley‐Liss, Inc. 相似文献
14.
Lee SS Lee Y Kim N Kim SW Byun JH Park SH Lee MG Ha HK 《Journal of magnetic resonance imaging : JMRI》2011,33(6):1390-1398
Purpose:
To compare the accuracy of four chemical shift magnetic resonance imaging (MRI) (CS‐MRI) analysis methods and MR spectroscopy (MRS) with and without T2‐correction in fat quantification in the presence of excess iron.Materials and Methods:
CS‐MRI with six opposed‐ and in‐phase acquisitions and MRS with five‐echo acquisitions (TEs of 20, 30, 40, 50, 60 msec) were performed at 1.5 T on phantoms containing various fat fractions (FFs), on phantoms containing various iron concentrations, and in 18 patients with chronic liver disease. For CS‐MRI, FFs were estimated with the dual‐echo method, with two T2*‐correction methods (triple‐ and multiecho), and with multiinterference methods that corrected for both T2* and spectral interference effects. For MRS, FF was estimated without T2‐correction (single‐echo MRS) and with T2‐correction (multiecho MRS).Results:
In the phantoms, T2*‐ or T2‐correction methods for CS‐MRI and MRS provided unbiased estimations of FFs (mean bias, ?1.1% to 0.5%) regardless of iron concentration, whereas the dual‐echo method (?5.5% to ?8.4%) and single‐echo MRS (12.1% to 37.3%) resulted in large biases in FFs. In patients, the FFs estimated with triple‐echo (R = 0.98), multiecho (R = 0.99), and multiinterference (R = 0.99) methods had stronger correlations with multiecho MRS FFs than with the dual‐echo method (R = 0.86; P ≤ 0.011). The FFs estimated with multiinterference method showed the closest agreement with multiecho MRS FFs (the 95% limit‐of‐agreement, ?0.2 ± 1.1).Conclusion:
T2*‐ or T2‐correction methods are effective in correcting the confounding effects of iron, enabling an accurate fat quantification throughout a wide range of iron concentrations. Spectral modeling of fat may further improve the accuracy of CS‐MRI in fat quantification. J. Magn. Reson. Imaging 2011;33:1390–1398. © 2011 Wiley‐Liss, Inc.15.
Roberto Alonzi MD N. Jane Taylor PhD J. James Stirling MSc James A. d'Arcy MPhys David J. Collins BA Michele I. Saunders MD Peter J. Hoskin MD Anwar R. Padhani MB BS 《Journal of magnetic resonance imaging : JMRI》2010,32(1):155-164
Purpose:
To assess the reproducibility of relaxivity‐ and susceptibility‐based dynamic contrast‐enhanced magnetic resonance imaging (MRI) in the benign and malignant prostate gland and to correlate the kinetic parameters obtained.Materials and Methods:
Twenty patients with prostate cancer underwent paired scans before and after androgen deprivation therapy. Quantitative parametric maps for T1‐ and T2*‐weighted parameters were calculated (Ktrans, kep,ve, IAUC60, rBV, rBF, and R2*). The reproducibility of and correlation between each parameter were determined using standard methods at both timepoints.Results:
T1‐derived parameters are more reproducible than T2*‐weighted measures, both becoming more variable following androgen deprivation (variance coefficients for prostate Ktrans and rBF increased from 13.9%–15.8% and 42.5%–90.8%, respectively). Tumor R2* reproducibility improved after androgen ablation (23.3%–11.8%). IAUC60 correlated strongly with Ktrans, ve, and kep (all P < 0.001). R2* did not correlate with other parameters.Conclusion:
This study is the first to document the variability and repeatability of T1‐ and T2*‐weighted dynamic MRI and intrinsic susceptibility‐weighted MRI for the various regions of the human prostate gland before and after androgen deprivation. These data provide a valuable source of reference for groups that plan to use dynamic contrast‐enhanced MRI or intrinsic susceptibility‐weighted MRI for the assessment of treatment response in the benign or malignant prostate. J. Magn. Reson. Imaging 2010;32:155–164. © 2010 Wiley‐Liss, Inc. 相似文献16.
Stefanie Remmele Alois M. Sprinkart Andreas Müller Frank Träber Marec von Lehe Jürgen Gieseke Sebastian Flacke Winfried A. Willinek Hans H. Schild Julien Sénégas Jochen Keupp Petra Mürtz 《Magnetic resonance in medicine》2013,70(1):136-146
This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady‐state spoiled multigradient‐echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid‐rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen‐enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc. 相似文献
17.
Heiko Schmiedeskamp Rexford D. Newbould Laura J. Pisani Stefan Skare Gary H. Glover Klaas P. Pruessmann Roland Bammer 《Magnetic resonance in medicine》2010,63(4):959-969
Multiecho echo‐planar imaging (EPI) was implemented for blood‐oxygenation‐level‐dependent functional MRI at 1.5 T and compared to single‐echo EPI with and without parallel imaging acceleration. A time‐normalized breath‐hold task using a block design functional MRI protocol was carried out in combination with up to four echo trains per excitation and parallel imaging acceleration factors R = 1–3. Experiments were conducted in five human subjects, each scanned in three sessions. Across all reduction factors, both signal‐to‐fluctuation‐noise ratio and the total number of activated voxels were significantly lower using a single‐echo EPI pulse sequence compared with the multiecho approach. Signal‐to‐fluctuation‐noise ratio and total number of activated voxels were also considerably reduced for nonaccelerated conventional single‐echo EPI when compared to three‐echo measurements with R = 2. Parallel imaging accelerated multiecho EPI reduced geometric distortions and signal dropout, while it increased blood‐oxygenation‐level‐dependent signal sensitivity all over the brain, particularly in regions with short underlying T*2. Thus, the presented method showed multiple advantages over conventional single‐echo EPI for standard blood‐oxygenation‐level‐dependent functional MRI experiments. Magn Reson Med 63:959–969, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
18.
Julien Vautier Melanie Heilmann Christine Walczak Joël Mispelter Andreas Volk 《Magnetic resonance in medicine》2010,64(1):313-318
Dynamic contrast‐enhanced MRI is extensively studied to define and evaluate biomarkers for early assessment of vasculature‐targeting therapies. In this study, two‐dimensional and three‐dimensional radial multi‐gradient‐echo techniques for dynamic R*2‐corrected R1 mapping based on the spoiled gradient recalled signal equation were implemented and validated at 4.7 T. The techniques were evaluated on phantoms and on a respiratory motion animated tumor model. R1 measurements were validated with respect to a standard inversion‐recovery spin‐echo sequence in a four‐compartment phantom covering a range of relaxation rates typically found in tumor tissue. In the range of [0.4, 3] sec?1, R1 differences were less than 10% for both two‐dimensional and three‐dimensional experiments. A dynamic contrast‐enhanced MRI pilot study was performed on a colorectal tumor model subcutaneously implanted in mice at the abdominal level. Low motion sensitivity of radial acquisition allowed image recording without respiratory triggering. Three‐dimensional Ktrans maps and significantly different mean Ktrans values were obtained for two contrast agents with different molecular weights. The radial multi‐gradient‐echo approach should be most useful for preclinical experimental conditions where the tissue of interest experiences physiologic motion, like spontaneous extracerebral tumors developed by transgenic mice, and where dynamic contrast‐enhanced MRI is performed with high‐relaxivity contrast agents. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc. 相似文献
19.
Martin H. Deppe Juan Parra‐Robles Helen Marshall Titus Lanz Jim M. Wild 《Magnetic resonance in medicine》2011,66(6):1788-1797
Parallel imaging presents a promising approach for MRI of hyperpolarized nuclei, as the penalty in signal‐to‐noise ratio typically encountered with 1H MRI due to a reduction in acquisition time can be offset by an increase in flip angle. The signal‐to‐noise ratio of hyperpolarized MRI generally exhibits a strong dependence on flip angle, which makes a homogeneous B1+ transmit field desirable. This paper presents a flexible 32‐channel receive array for 3He human lung imaging at 1.5T designed for insertion into an asymmetric birdcage transmit coil. While the 32‐channel array allows parallel imaging at high acceleration factors, the birdcage transmit coil provides a homogeneous B1+ field. Decoupling between array elements is achieved by using a concentric shielding approach together with preamplifier decoupling. Coupling between transmit coil and array elements is low by virtue of a low geometric coupling coefficient, which is reduced further by the concentric shields in the array. The combination of the 32‐channel array and birdcage transmit coil provides 3He ventilation images of excellent quality with similar signal‐to‐noise ratio at acceleration factors R = 2 and R = 4, while maintaining a homogeneous B1+. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc. 相似文献
20.
Yongxian Qian Ashley A. Williams Constance R. Chu Fernando E. Boada 《Magnetic resonance in medicine》2013,69(6):1564-1571
Repeatability of in vivo measurement of multicomponent T2* relaxation in articular cartialges in human knee is important to clinical use. This study evaluated the repeatability of two‐component T2* relaxation on seven healthy human subjects. The left knee was scanned once a day in three consecutive days, on a clinical 3T MRI scanner with eight‐channel knee coil and ultrashort echo time pulse sequence at 11 echo times = 0.6–40 ms. The intrasubject and intersubject repeatability was evaluated via coefficient of variation (CV = standard deviation/mean) in four typical cartilage regions: patellar, anterior articular, femoral, and tibial regions. It was found that the intrasubject repeatability was good, with CV < 10% for the short‐ and long‐T2* relaxation time in the layered regions in the four cartilages (with one exception) and CV < 13% for the component intensity fraction (with two exceptions). The intersubject repeatability was also good, with CV ~8% (range 1–15%) for the short‐ and long‐T2* relaxation time and CV ~10% (range 2–20%) for the component intensity fraction. The long‐T2* component showed significantly better repeatability (CV ~8%) than the short‐T2* component (CV~12%) (P < 0.005). These CV values suggest that in vivo measurement of two‐component T2* relaxation in the knee cartilages is repeatable on clinical scanner at 3 T, with a signal‐to‐noise ratio of 90. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc. 相似文献