动态增强MRI技术评价动脉粥样斑块的实验研究

目的 利用动态增强MRI探讨动脉粥样斑块Gd-DTPA强化的特点和强化机制.方法 12只新西兰大白兔,通过高脂饮食联合内膜剥脱术在腹主动脉建立斑块模型.术后10～16周行动态增强MRI扫描.以71 s为间隔获得35帧TtWI,并在第3次扫描开始的同时静脉注射Gd-DTPA.扫描结束后12 h内取动脉斑块行HE和CD31免疫组织化学染色,计数新生血管.在MRI上测量斑块的时间-信号强度曲线,计算曲线的描述性参数,和新生血管计数进行Pearson相关性分析.以组织病理检查结果为参照,分析斑块强化特点.结果 静脉注射Gd-DTPA后,动脉斑块明显强化,呈现"快进慢出"的特点.斑块内部强化程度不均匀,纤维成分强化较强,脂质和炎细胞聚集灶强化较弱.新生血管计数(117.7±93.3)和时间-信号强度曲线得到的峰值(1.05±0.30)、初始斜率(0.82±0.28)和7 min曲线下面积(4.97±1.67)有显著正相关,相关系数r分别为:0.553(P＜0.01)、0.468(P＜0.05)、0.554(P＜0.01).结论 动脉粥样硬化斑块呈"快进慢出"的强化特点.新生血管生成、内膜通透性增加及细胞外基质增多可能是斑块被Gd-DTPA强化的原因.DEC-MRI定量分析斑块新生血管密度有一定的可行性.     

MRI in characterization of focal liver lesions: Comparison of T2 weighting by conventional spin-echo and turbo spin-echo sequences

Forty-one patients with 61 proved focal liver lesions underwent MRI of the liver at 1.0 T, with the aim of evaluating the usefulness of turbo spin-echo (TSE) sequences in characterizing focal liver lesions, by comparing them with conventional spin-echo (CSE) sequences. Two different TSE protocols were employed, with constant echo time and varying repetition time: TSE-S (3000 msec) and TSE-L (5100 msec). All images were evaluated quantitatively (signal-to-noise ratio ‘SNR’) and qualitatively: because benign lesions were all liquid (12 cysts and 10 hemangiomas), they were well characterized morphologically on the basis of signal intensity. Mean SNR was significantly different between metastases and benign lesions (P < .0001) with all T2 sequences. Among the single T2 sequences tested, logistic regression analysis showed TSE-L to have the best predictive ability of the nature of focal lesions, with a G value of 42.02, compared to 29.87 of TSE-S and 25.55 of CSE second echo (SE II). The combination of TSE-L with TSE-S did not modify these results, whereas the combination of TSE-L with CSE only resulted in slight improvement (G = 46.95). Comparison of the receiver operating characteristic (ROC) curves showed only SE II (area under the ROC curve of .8312) to be significantly inferior to the best single sequence, or TSE-L (area under the ROC curve of .9176; P = .027). All sequences were equivalent in qualitative evaluation, with good reproducibility, sensitivity ranging from .94 to 1.0, and specificity ranging from .86 to .93. This study confirms the value of TSE sequences in characterization of focal liver lesions. Time of acquisition is strongly reduced with these sequences, whereas results are fairly similar to those obtained with CSE. TSE sequences could therefore replace CSE for the study of focal liver lesions.     

Dynamic gadolinium-enhanced rapid acquisition spin-echo MR imaging of the liver

Rapid acquisition spin-echo (RASE) magnetic resonance (MR) imaging allows for coverage of the entire liver with highly T1-weighted SE images during a single 23-second breath-holding period. The RASE sequence was implemented in conjunction with rapid intravenous injection of gadopentetate dimeglumine to enable performance of dynamic contrast material-enhanced MR imaging of the liver. Prospective evaluation of 24 patients with 62 liver lesions 1 cm or greater in diameter was performed. Images obtained with RASE were devoid of respiratory-related ghost artifacts or edge blurring. The dynamic contrast-enhanced RASE technique resulted in contrast-to-noise and contrast-to-artifact values and time efficiency measures significantly greater (P less than .05) than those obtained with use of conventional T1- and T2-weighted pulse sequences, indicating a higher likelihood for lesion detectability. Lesion conspicuity was maximal during or immediately following bolus administration of gadopentetate dimeglumine, with lesions often becoming obscured at delayed postcontrast imaging.     

Fat-suppressed T2-weighted MRI of the liver: comparison of respiratory-triggered fast spin-echo, breath-hold single-shot fast spin-echo, and breath-hold fast-recovery fast spin-echo sequences

The purpose of our study was to compare the value of respiratory-triggered fast spin-echo, breath-hold single-shot fast spin-echo, and breath-hold fast-recovery fast spin-echo sequences in detecting hepatic lesions. Fat-suppressed T2-weighted magnetic resonance (MR) images obtained with the three sequences in 36 patients with 138 lesions and nine patients without lesions were prospectively analyzed. Quantitative and qualitative analyses, including receiver operating characteristic (ROC) analyses, were performed. The mean lesion-to-liver contrast-to-noise ratio (CNR) for hepatic lesions was highest with the respiratory-triggered fast spin-echo sequence. On the basis of receiver-operating characteristic analyses, tumor detection rates were higher with the breath-hold fast-recovery fast spin-echo sequence (Az = 0.94) than with the respiratory-triggered fast spin-echo sequence (AZ = 0.80, P < 0.0001) or the single-shot fast spin-echo sequence (Az = 0.77, P < 0.0001). The image quality with the breath-hold fast-recovery fast spin-echo sequence was acceptable in all patients. The breath-hold fast-recovery fast spin-echo sequence provided the highest tumor detection in a short imaging time, although the mean lesion-to-liver CNRs were inferior to those of the respiratory-triggered fast spin-echo and the breath-hold single-shot fast spin-echo sequences.     

Characterization of musculoskeletal masses using dynamic Gd-DTPA enhanced spin-echo MRI.

Early studies evaluating the utility of Gd-diethylenetriamine pentaacetic acid (DTPA) enhanced MR imaging for characterization of musculoskeletal masses have demonstrated inconsistent and often conflicting results. In this study a new method, dynamic Gd-DTPA enhanced rapid acquisition spin echo MR imaging, was implemented in the evaluation of 18 musculoskeletal lesions and the enhancement features of these lesions were analyzed. Lesions were evaluated before, during, and sequentially following bolus Gd-DTPA injection. Analysis of intensity, volume, timing of onset, progression, uniformity, and pattern of enhancement did not demonstrate significant differences between benign (n = 8) and malignant (n = 10) masses. Significant variations in enhancement were noted in different regions within these masses, which limits the utility of previous dynamic contrast enhanced methods that provide only a single imaging slice for analysis, and are therefore subject to sampling error. This pilot study indicates no advantage for using dynamic Gd-DTPA enhanced imaging for qualitative lesion characterization.     

Lesion discrimination in optic neuritis using high-resolution fat-suppressed fast spin-echo MRI

Fast spin-echo (FSE) is a new sequence with acquisition times currently down to one-sixteenth of those obtained with conventional spin-echo sequences, which allows high-resolution (512×512 matrix) images to be acquired in an acceptable time. We compared the higher resolution of FSE with the medium resolution of a short inversion-time inversion-recovery (STIR) sequence in depicting the optic nerves of healthy controls and patients with optic neuritis. Optic nerve MRI examinations were performed in 18 patients with optic neuritis and 10 normal controls. Two sequences were obtained coronally: fat-suppressed FSE (FSE TR 3250 ms/TEef 68 ms, echo-train length 16, 4 excitations, 24 cm rectangular field of view, 3 mm interleaved contiguous slices, in-plane resolution 0.5×0.5 mm) and STIR (TR 2000 ms/TE 50 ms/TI 175 ms, inplane resolution 0.8×0.8 mm, slice thickness 5 mm). FSE demonstrated much more anatomical detail than STIR, e. g. distinction of optic nerve and sheath. Lesions were seen in 20 of 21 symptomatic nerves using FSE and in 18 of 21 using STIR. Nerve swelling or partial cross-sectional lesions of the optic nerve were each seen only on FSE in 3 cases. Fatsuppressed FSE imaging of the optic nerve improves anatomical definition and increases lesion detection in optic neuritis.     

Functional MRI of liver using BOLD MRI: effect of glucose

Purpose

To demonstrate feasibility of functional MRI of liver using glucose as a stimulus to monitor metabolic changes using blood oxygenation level dependent (BOLD) contrast. We hypothesized that during hyperglycemia, liver stores the glucose and consequently there is a reduction in oxygen consumption, which can be detected using BOLD MRI.

Materials and Methods

In four mini pigs, measurements were made before and after 54 g of glucose administered intravenously. In six healthy young human subjects, measurements were made before and after oral ingestion of 75 g of glucose. T₂* weighted images of the liver were obtained on a Siemens 3 Tesla Verio MRI scanner using multiple gradient recalled echo (mGRE) sequence.

Results

A statistically significant decrease (P < 0.05) in R2* (1/T₂*) was observed postglucose both in swine (110.41 ± 14.1 s^?1 to 72.22 ± 5.7 s^?1) and human (55.84 ±3.8 s^?1 to 50.6 ±0.5 s^?1), suggesting improved liver oxygenation during hyperglycemia.

Conclusion

Our preliminary data presented here demonstrate the feasibility of obtaining functional liver images that illustrate the changes in oxygen consumption. Further studies are necessary to fully validate the technique. J. Magn. Reson. Imaging 2010;32:988–991. © 2010 Wiley‐Liss, Inc.

     

Comparison of fast spin-echo versus conventional spin-echo MRI for evaluating meniscal tears

OBJECTIVE: We compare sensitivities of conventional spin-echo and fast spin-echo sequences in the evaluation of meniscal tears. Furthermore, we reevaluate the results from prior studies comparing these two sequences to understand why there are differing conclusions regarding the efficacy of fast spin-echo sequences as a commensurate replacement for conventional spin-echo sequences. MATERIALS AND METHODS: We used records from a control group of 64 patients (128 menisci) who had undergone arthroscopy to determine the sensitivity of conventional spin-echo sequences for detecting meniscal tears. Two hundred sixteen consecutive patients (432 menisci) were then imaged using conventional spin-echo and fast spin-echo sequences to evaluate for meniscal tears. Both sequences were proton density-weighted with fat suppression. RESULTS: Of the 432 menisci, 170 tears were detected on conventional spin-echo imaging. Only 128 tears were detected on the fast spin-echo sequence. The sensitivities of conventional spin-echo and fast spin-echo imaging were found to be 93% and 80%, respectively. In addition, findings from conventional spin-echo and fast spin-echo sequences were discordant for 72 menisci (17%, p < 0.01). CONCLUSION: The sensitivities of conventional spin-echo and fast spin-echo imaging for detecting meniscal tears have been shown to be greater than 90% and approximately 80%, respectively. However, some authors advocate substituting conventional spin-echo imaging with fast spin-echo imaging. We urge the abandonment of fast spin-echo imaging for evaluating meniscal tears because a loss of greater than 10% in sensitivity is unacceptable.     

MRI of the neonatal brain: optimization of spin-echo parameters

OBJECTIVE: Our aim was to measure the relaxation times of the neonatal brain and to use these to derive pulse sequence parameters that enhance the signal-to-noise ratio (SNR) and contrast of MRI scans of the neonatal brain. SUBJECTS AND METHODS: The transverse (T2) and longitudinal (T1) relaxation times were measured for 10 healthy neonates, and the average relaxation times were calculated for both gray and white matter. Simulations using these values were then performed to estimate the optimal pulse sequence parameters. Images were obtained in three neonates using both the optimized and conventional sequence parameters. RESULTS: The measured (mean +/- SD) relaxation times of the neonatal brain at 1.5 T were T1 equals 1712 +/- 235 msec and T2 equals 394 +/- 52 msec in white matter and T1 equals 1144 +/- 245 msec and T2 equals 206 +/- 26 msec in gray matter. The optimized T1-weighted imaging used a turbo spin-echo sequence with an echo-train length of 3 and TR/TE of 850/11 msec and showed increases in both the contrast and the SNR. The optimized T2-weighted sequence used a TE of 270 msec and markedly increased the contrast but at the expense of a reduction in the SNR. CONCLUSION: Parameters of MRI turbo spin-echo sequences for scanning neonates are different from those required for adult studies, and appropriate protocols should be used.     

Hemodynamic analysis of bladder tumors using T1-dynamic contrast-enhanced fast spin-echo MRI

Objectives

To evaluate the hemodynamics of bladder tumors, we developed a method to calculate change in R₁ value (ΔR₁) from T₁-dynamic contrast-enhanced fast spin-echo magnetic resonance imaging (T₁DCE-FSE-MRI).

Materials and methods

On a 1.5-T MR system, T₁DCE-FSE-MRI was performed. This study was applied to 12 patients with urinary bladder tumor, i.e. urothelial carcinoma. We compared ΔR₁–time and ΔSI–time between a peak in the ΔR₁–time and ΔSI–time curve occurred during the first pass within 60 s. Next, we assessed the slope of increase for 180 s after CA injection (Slope_0–180).

Results

The mean slope of the first pass was significantly higher for bladder tumors on both the ΔR₁–time and the ΔSI–time curve compared with normal bladder walls. Moreover, a significant difference was apparent between bladder tumors and normal bladder walls on the mean Slope_0–180 in the ΔR₁-time curve. However, no significant difference in the mean Slope_0–180 was observed on the ΔSI-time curve between bladder tumors and normal bladder walls.

Conclusion

T₁DCE-FSE-MRI offers three advantages: quantitative analysis; high-quality (i.e., artifact-free) images; and high temporal resolution even for SE images. Use of ΔR₁ analysis with T₁DCE-FSE-MRI allows more detailed information on the hemodynamics of bladder tumors to be obtained and assists in differentiation between bladder tumors and the normal bladder wall.     

Dynamic spin-echo imaging: Theoretical assessment and implementation

A spin-echo method for obtaining dynamic magnetic resonance (MR) images is described. The method combines the RARE (rapid acquisition with relaxation enhancement) pulse sequence with a data acquisition scheme in which only a centric fraction of the raw data is sampled to increase the time resolution of the dynamic images. The missing high-resolution data are supplied from reference images. By these means, an effective time resolution of about 10 seconds per image is achieved, which is suitable for diagnostic assessment of contrast enhancement procedures. A promising clinical application is contrast-enhanced MR imaging of the pituitary. Although the resolution of small objects in this size range is potentially degraded, this shortcoming is compensated for with use of variable refocusing flip angles. In the context of pituitary imaging, the centric 30%–40% of the raw data is shown to be the optimal fraction to acquire for the low-resolution dynamic images. Ten patients with previous history of pituitary disease have been imaged with dynamic and conventional spin-echo techniques. In six of these patients, an equivalent diagnosis was reached with dynamic and conventional images, while in two, only the dynamic images showed the lesion; in the final two patients, only the conventional images showed the lesion.     

Gadolinium-DTPA as a contrast agent in MRI: initial clinical experience in 20 patients

Magnetic resonance imaging (MRI) was performed in 20 patients before and after intravenous administration of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) in a dose of 0.1 mmol/kg. Twelve of the patients had clinical and histologic diagnoses of cerebral tumor, six had hepatic tumors, one had hepatic cysts, and one had transitional cell carcinoma of the bladder. Contrast enhancement was seen with all tumors, but not with the hepatic cysts. The degree of enhancement was greater than that seen with computed tomography (CT) in 13 cases, equal to it in six, and less in one. Contrast enhancement was detectable as long as 18 hr after injection of Gd-DTPA in one case of cerebral tumor. The margin between cerebral tumor and peritumoral edema could be delineated with contrast-enhanced MRI to the degree possible with contrast-enhanced CT. In the liver isointense enhancement was seen with saturation-recovery (SR), inversion-recovery (IR), and spin-echo (SE) sequences although not with all three sequences simultaneously. In general IR sequences were most sensitive for display of the contrast agent, but the enhancement often decreased the difference between abnormal and normal tissue. No short-term side effects were encountered and no significant change was seen in urea, creatinine, electrolytes, liver function tests, blood coagulation, or urine testing after injection of Gd-DTPA. Although much more work will be required to evaluate this contrast agent, these initial experiences are very promising.     

Model-based nonlinear inverse reconstruction for T2 mapping using highly undersampled spin-echo MRI

Purpose:

To develop a model‐based reconstruction technique for T2 mapping based on multi‐echo spin‐echo MRI sequences with highly undersampled Cartesian data encoding.

Materials and Methods:

The proposed technique relies on a nonlinear inverse reconstruction algorithm which directly estimates a T2 and spin‐density map from a train of undersampled spin echoes. The method is applicable to acquisitions with single receiver coils but benefits from multi‐element coil arrays. The algorithm is validated for trains of 16 spin echoes with a spacing of 10 to 12 ms using numerical simulations as well as human brain MRI at 3 Tesla (T).

Results:

When compared with a standard T2 fitting procedure using fully sampled T2‐weighted images, and depending on the available signal‐to‐noise ratio and number of coil elements, model‐based nonlinear inverse reconstructions for both simulated and in vivo MRI data yield accurate T2 estimates for undersampling factors of 5 to 10.

Conclusion:

This work describes a promising strategy for T2‐weighted MRI that simultaneously offers accurate T2 relaxation times and properly T2‐weighted images at arbitrary echo times. For a standard spin‐echo MRI sequence with Cartesian encoding, the method allows for a much higher degree of undersampling than obtainable by conventional parallel imaging. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Correction of inhomogeneous RF field using multiple SPGR signals for high-field spin-echo MRI.

PURPOSE: To propose a simple and useful method for correcting nonuniformity of high-field (3 Tesla) T(1)-weighted spin-echo (SE) images based on a B1 field map estimated from gradient recalled echo (GRE) signals. METHODS: To estimate B1 inhomogeneity, spoiled gradient recalled echo (SPGR) images were collected using a fixed repetition time of 70 ms, flip angles of 45 and 90 degrees, and echo times of 4.8 and 10.4 ms. Selection of flip angles was based on the observation that the relative intensity changes in SPGR signals were very similar among different tissues at larger flip angles than the Ernst angle. Accordingly, spatial irregularity that was observed on a signal ratio map of the SPGR images acquired with these 2 flip angles was ascribed to inhomogeneity of the B1 field. Dual echo time was used to eliminate T(2)(*) effects. The ratio map that was acquired was scaled to provide an intensity correction map for SE images. Both phantom and volunteer studies were performed using a 3T magnetic resonance scanner to validate the method. RESULTS: In the phantom study, the uniformity of the T(1)-weighted SE image improved by 23%. Images of human heads also showed practically sufficient improvement in the image uniformity. CONCLUSION: The present method improves the image uniformity of high-field T(1)-weighted SE images.     

Dynamic MRI of bladder cancer: evaluation of staging accuracy

OBJECTIVE: The purpose of this study was to evaluate the accuracy of gadoliniumenhanced MRI in staging bladder cancer in a series of patients with surgically proven bladder cancer. MATERIALS AND METHODS: Seventy-one patients with biopsy-proven bladder cancer underwent MRI on a 1.5-T scanner with a phased-array pelvic coil. Conventional T1-weighted spin-echo, T2-weighted spin-echo, and unenhanced and enhanced (0.1 mmol/kg gadolinium) fast spoiled gradient-echo images with fat suppression were obtained. Two blinded reviewers evaluated the MR images and assigned a stage that was compared with the pathologic stage (n = 67) or with clinical follow-up for at least 2 years after MRI (n = 4). RESULTS: Agreement among the reviewers was good in assigning a radiologic stage for bladder cancer (kappa = 0.80). On a stage-by-stage basis, MRI accuracy was 62%, and overstaging was the most common error (32%). Staging accuracy improved to 85% and 82% in differentiating superficial from invasive tumors and organ-confined from non-organ-confined tumors, respectively. The time interval between MRI and transurethral resection (/=61 days) was not a statistically significant factor in differentiating superficial from invasive and organ-confined from non-organ-confined tumors (p > 0.05). MRI accuracy in staging transitional cell carcinoma was not significantly different from that obtained in staging non-transitional cell carcinoma (p > 0.05). CONCLUSION: MRI shows good reproducibility between reviewers for staging bladder cancer. Although overall staging accuracy was only moderate, the accuracy for differentiating superficial versus invasive disease and organ-confined versus non-organ-confined disease was high.     

MRI of the knee: value of short echo time fast spin-echo using high performance gradients versus conventional spin-echo imaging for the detection of meniscal tears

Objective. Fast spin-echo (FSE) sequences reduce imaging time compared with conventional spin-echo (CSE) sequences, but may result in blurring. High-performance gradients permit shorter interecho spacing and use of the second echo as the effective TE (20 ms); both improvements reduce blurring. This randomized observer study compared a short TE, second-echo FSE sequence obtained using high-performance gradients and a CSE sequence with similar TR/TE for the detection of meniscal tears in the knee. Design and patients. One hundred consecutive MR examinations of the knee using FSE and CSE sequences at 1.5 T were evaluated. The FSE sequence used an effective TE of 20 ms (centered on the second echo at 2 times minimal interecho spacing) and an echo train length of 4. FSE and CSE parameters were otherwise similar. Four independent, masked readers reviewed randomized sagittal FSE and CSE sequences. Results. Cases were assessed for the presence or absence of meniscal tears and, if present, whether tears were medial or lateral and anterior or posterior. Sequence concordance was 93.5% (1496 of 1600 meniscal segments); the intermethod kappa value was 0.78. Sequence quality was graded from 1 to 5. Average quality of CSE images was slightly but statistically significantly preferred by three of the four readers. Conclusion. There was no statistically significant difference between CSE imaging and FSE imaging centered on the second echo (20 ms) using high-performance gradients for the detection of meniscal tears in the knee. There was a small preference for the quality of CSE images. Received: 22 July 1999 Revision requested: 27 October 1999 Revision received: 7 February 2000 Accepted: 21 March 2000     

Gadolinium-DTPA enhancement of symptomatic nerve roots in MRI of the lumbar spine

Disc prolapse presenting with sciatica may be associated with enhancement of the symptomatic nerve root following magnetic resonance imaging (MRI) with intravenous gadolinium (Gd)-DTPA. Previous studies have shown, however, that this does not occur in all cases. The aim of this study was to assess the incidence of nerve root enhancement in patients with sciatica and disc prolapse and to try to identify any specific features that might be associated with the phenomenon. A total of 227 patients presenting with low back pain and/or sciatica underwent a MRI study of the lumbar spine with intravenous contrast enhancement. Nineteen of 81 (23.5 %) patients with disc prolapse demonstrated nerve root enhancement. Nerve root enhancement had a highly significant association with sequestrated disc lesions (13/19, 68 %; P < 0.0005), and was primarily seen in the symptomatic ipsilateral nerve root (16/19, 84 %). The sensitivity of nerve root enhancement associated with disc prolapse was 23.5 % with a specificity of 95.9 %, a positive predictive value of 76 % and a negative predictive value of 69.3 %. Nerve root enhancement may be indicative of the symptomatic level but its poor sensitivity negates the routine use of Gd-DTPA in MRI for sciatica. Received 7 October 1996; Accepted 2 May 1997