Triple contrast technique for black blood imaging with double inversion preparation.

This work reports on the development of a pulse sequence to simultaneously acquire proton density, T(1), and T(2) weighted images in a single magnetization prepared fast spin echo acquisition. The technique is based upon the application of a magnetization preparation consisting of a global inversion followed by slice-selective 180 degrees and 90 degrees pulses to prepare the signal of specific slices. Slices are acquired in an interleaved manner with time delays appropriate for the desired image contrasts. Data acquisition is repeated for all combinations of slice interleaving covering the region of interest until images from all slice locations have been acquired with all desired image contrasts. The multiple image contrasts obtained with this technique should be useful in applications where discrimination between different types of tissue components is desired, such as in the analysis of plaque in cervical carotid artery disease.     

Inversion-recovery-prepared SSFP for cardiac-phase-resolved delayed-enhancement MRI.

Delayed-enhancement magnetic resonance imaging (DE-MRI) can be used to visualize myocardial infarction (MI). DE-MRI is conventionally acquired with an inversion-recovery gradient-echo (IR-GRE) pulse sequence that yields a single bright-blood image. IR-GRE imaging requires an accurate estimate of the inversion time (TI) to null the signal from the myocardium, and a separate cine acquisition is required to visualize myocardial wall motion. Simulations were performed to examine the effects of a steady-state free precession (SSFP) readout after an inversion pulse in the setting of DE-MRI. Using these simulations, a segmented IR-SSFP sequence was optimized for infarct visualization. This sequence yields both viability and wall motion images over the cardiac cycle in a single breath-hold. Viability images at multiple effective TIs are produced, providing a range of image contrasts. In a study of 11 patients, IR-SSFP yielded infarct sizes and left ventricular ejection fractions (LVEFs) similar to those obtained by IR-GRE and standard SSFP, respectively. IR-SSFP images yielded improved visualization of the infarct-blood border because of the simultaneous nulling of healthy myocardium and blood. T(1) (*) recovery curves were extracted from IR-SSFP images and showed excellent qualitative agreement with theoretical simulations.     

Contrast and accuracy of relaxation time measurements in acquired and synthesized multislice magnetic resonance images

The effects of interslice spacing, the number of data points and other factors on the accuracy of relaxation time measurements and contrast have been investigated for both acquired and synthesized multislice MR images using experiments and computer simulations.The cross-excitation between adjacent slices in multislice imaging affects both contrast and derived relaxation times. Such measurements also are affected by the T1 and T2 of the materials imaged, the pulse sequence timing parameters, and the number of data points used to estimate the relaxation times. Errors in T1 and T2 may be severe, particularly for slice spacings less than 0.5 slice thickness and for long T1 and T2 materials. Consequently, the difference in signal intensities between two materials with different relaxation times also varies with slice spacing and between acquired and synthetic images, particularly for strongly T1-weighed images.     

In vivo quantification of T1rho using a multislice spin-lock pulse sequence.

A multislice spin-lock (MS-SL) pulse sequence is implemented on a clinical scanner to acquire multiple images with spin-lock-generated contrast of the knee joints of six healthy human subjects. The MS-SL sequence produces images with T1rho contrast with an additional factor of intrinsic T2rho weighting, which hinders direct measurement of T1rho. A method is presented to compensate the MS-SL-generated data with regard to T2rho in an effort to accurately calculate multislice T1rho maps in a feasible experimental time. The T2rho-compensated multislice T1rho maps produced errors in the measurement of T1rho in healthy patellar cartilage of approximately 5% compared to the gold standard measurement of T1rho acquired with single-slice spin-lock pulse sequence. The MS-SL sequence has potential as an important clinical tool for the acquisition of multislice T1rho-weighted images and/or quantitative multislice T1rho maps.     

T1rho MR imaging of the human wrist in vivo

RATIONALE AND OBJECTIVES: The purpose of this study was (a) to demonstrate the feasibility of computing T1rho maps of, and T1rho dispersion in, human wrist cartilage at MR imaging in vivo and (b) to compare T1rho and T2 weighting in terms of magnitude of relaxation times and signal intensity contrast. MATERIALS AND METHODS: T2 and T1rho magnetic resonance images of wrist joints in healthy volunteers (n = 5) were obtained with a spin-echo sequence and a fast spin-echo sequence pre-encoded with a spin-lock pulse cluster. A 1.5-T clinical imager was used (Signa; GE Medical Systems, Milwaukee, Wis) with a 9.5-cm-diameter transmit-receive quadrature birdcage coil tuned to 63.75 MHz. RESULTS: T1rho relaxation times at a spin-lock frequency of 500 Hz vary from 40.5 msec +/- 0.85 to 56.6 msec +/- 4.83, and T2 relaxation times vary from 28.1 msec +/- 1.88 to 34.5 msec +/- 2.63 (mean +/- standard error of the mean, n = 5, P < .016) in various regions of the wrist. T1rho dispersion was observed in the range of spin-lock frequencies studied. T1rho-weighted images not only have higher signal-to-noise ratios but also show better fluid and fat signal suppression than T2-weighted images. CONCLUSION: It was possible to perform T2- and T1rho-weighted MR imaging of human wrist cartilage in vivo with standard clinical imagers. The higher signal-to-noise ratio and improved contrast between cartilage and surrounding fat achieved with T1rho imaging may provide better definition of lesions and accurate quantitation of small changes in cartilage degeneration.     

k-space weighted image contrast (KWIC) for contrast manipulation in projection reconstruction MRI.

A novel technique for manipulating contrast in projection reconstruction MRI is described. The method takes advantage of the fact that the central region of k-space is oversampled, allowing one to choose different filters to enhance or reduce the amount that each view contributes to the central region, which dominates image contrast. The technique is implemented into a fast spin-echo (FSE) sequence, and it is shown that multiple T(2)-weighted images can be reconstructed from a single image data set. These images are shown to be nearly identical to those acquired with the Cartesian-sampled FSE sequence at different effective echo times. Further, it is demonstrated that T(2) maps can be generated from a single image data set. This technique also has the potential to be useful in dynamic contrast enhancement studies, capable of yielding a series of images at a significantly higher effective temporal resolution than what is currently possible with other methods, without sacrificing spatial resolution.     

Intracranial meningeal disease: comparison of contrast-enhanced MR imaging with fluid-attenuated inversion recovery and fat-suppressed T1-weighted sequences

BACKGROUND AND PURPOSE: Contrast-enhanced fluid-attenuated inversion recovery (FLAIR) imaging has been reported to have higher sensitivity for detecting leptomeningeal disease compared with contrast-enhanced T1-weighted MR imaging. The purpose of this study was to compare contrast-enhanced T1-weighted MR images with fat suppression to contrast-enhanced FLAIR images to determine which sequence was superior for depicting meningeal disease. METHODS: We reviewed MR images of 24 patients (35 studies) with a variety of meningeal diseases. The MR imaging protocol included contrast-enhanced T1-weighted MR images with fat suppression (FS) and contrast-enhanced fluid-attenuated inversion recovery (FLAIR) images that were reviewed by three neuroradiologists and were assigned a rating of positive, equivocal, or negative for abnormal meningeal enhancement. The two sequences were compared side by side to determine which better depicted meningeal disease. RESULTS: Abnormal meningeal enhancement was positive in 35 contrast-enhanced T1-weighted MR images with FS and in 33 contrast-enhanced FLAIR studies. In the first group, which had the T1-weighted sequence acquired first (21 of 33 studies), contrast-enhanced T1-weighted images with FS showed superior contrast enhancement in 11 studies (52%), inferior contrast enhancement in six studies (29%), and equal contrast enhancement in four studies (19%) compared with the contrast-enhanced FLAIR images. In the second group, which had the FLAIR sequence acquired first (12 of 33), contrast-enhanced T1-weighted images with FS showed superior contrast enhancement in seven studies (58%), inferior contrast enhancement in two studies (17%), and equal contrast enhancement in three studies (25%). CONCLUSION: Contrast-enhanced T1-weighted MR imaging with FS is superior to contrast-enhanced FLAIR imaging in most cases for depicting intracranial meningeal diseases.     

强直性脊柱炎骶髂关节影像学分析(附24例骶髂关节平片、CT和MR影像对比观察)

目的：旨在评估强直性脊柱炎（ＡＳ）患者骶髂关节炎的ＭＲ影像特征，并比较Ｘ线平片、ＣＴ和ＭＲ影像在诊断骶髂关节炎中的作用。材料与方法：搜集２４例ＡＳ患者，分别行Ｘ线平片、ＣＴ和ＭＲＩ检查。增强前ＭＲ扫描序列包括ＳＥＴ１ＷＩ、ＦＳＥＴ２ＷＩ和梯度回波的准Ｔ２ＷＩ（ＧＲＴ２＊ＷＩ）。增强后ＭＲ扫描序列参数与增强前ＳＥＴ１ＷＩ相同。另选９例志愿者，行ＭＲ平扫检查。结果：８例志愿者１６个骶髂关节的Ｔ１ＷＩ和Ｔ２ＷＩ可直接显示正常骶髂关节软骨，表现为线形或点样的中等信号影。９例志愿者１１个骶髂关节的骨髓内可见局灶性脂肪沉积。２４例ＡＳ患者的４２个骶髂关节可见软骨异常，表现为Ｔ１ＷＩ和Ｔ２ＷＩ上正常线形中等信号的软骨影像消失，代之以不规则增粗或扭曲样中等信号。分析表明：在骶髂关节炎的诊断方面，ＭＲ和ＣＴ明显优于Ｘ线平片（Ｐ＜０．００１）。结论：同Ｘ线片比较，虽ＣＴ和ＭＲＩ均有助于ＡＳ骶髂关节炎的诊断，但ＭＲ影像可显示ＣＴ和Ｘ线所不能显示的软骨异常和骨髓内水肿改变；骨髓内脂肪沉积可属正常变异；扭曲样中等强度信号应视为软骨的异常征象。     

Assessment and compensation of susceptibility artifacts in gradient echo MRI of hyperpolarized 3He gas.

The effects of macroscopic background field gradients upon 2D gradient echo images of inhaled (3)He in the human lung were investigated at 1.5 T. Effective compensation of in-slice signal loss in (3)He gradient echo images was then demonstrated using a multiple acquisition interleaved single gradient echo sequence. This method restores signal dephasing through a combination of separate images acquired with different slice refocusing gradients. In vivo imaging of volunteers with the sequence shows substantial restoration of signal at the lung periphery and close to blood vessels. The technique presented may be useful when using (3)He MRI for volumetric measurements of lung ventilation and in studies using (3)He combined with intravenous contrast as a means of assessing lung ventilation/perfusion (V/Q).     

Radial GRASE: implementation and applications.

RAD-GRASE is an MRI sequence that combines radial (RAD) k-space scanning with the gradient and spin-echo (GRASE) technique. RAD-GRASE has the advantages of all radial data acquisition methods in that it can reduce motion sensitivity and correct motion-induced data errors, which can be exploited to achieve high-resolution diffusion-weighted imaging (DWI). One can obtain different types of image contrast, including DWI, T(1), T(2), and T(2)*, in RAD-GRASE by controlling the magnetization preparation and sequence timing. Moreover, because there is oversampling of the low spatial frequencies inherent to radial sequences, partial data reconstruction can be used to achieve multiple forms of image contrast from a single acquired data set, and to generate parametric image maps of equilibrium magnetization, T(2), and T(2) (dagger). The RAD-GRASE technique can also be used to achieve fat-suppressed and/or separated fat and water images by choosing the appropriate timing parameters.     

Single-dose gadolinium with magnetization transfer versus triple-dose gadolinium in the MR detection of multiple sclerosis lesions.

PURPOSETo compare the efficacy of single-dose gadolinium with magnetization transfer contrast (MTC) with that of triple-dose gadolinium in detecting enhancing multiple sclerosis lesions.METHODSTwenty-one patients with multiple sclerosis were examined with MR imaging first with 0.1 mmol/kg gadolinium (single dose) and then, after 24 to 72 hours, with 0.3 mmol/kg gadolinium (triple dose). T2-weighted fast spin-echo and T1-weighted spin-echo MR images with and without MTC were obtained before contrast administration followed by either T1-weighted spin-echo images with MTC (single dose) or conventional T1-weighted spin-echo images (triple dose), starting 5, 17, and 29 minutes after contrast administration. All images were evaluated in a blinded fashion and scored in random order by two readers. Outcome parameters included number of enhancing lesions, number of active MR examinations (those containing at least one enhancing lesion), contrast ratio (signal intensity of enhancing lesion divided by signal intensity of normal-appearing white matter), and size of enhancing lesions.RESULTSEighty-one percent more enhancing lesions and 49% more active MR examinations were detected when a triple dose of gadolinium was used as compared with a single dose. The level of agreement between readers as to the number of enhancing lesions was significantly higher for triple-dose than for single-dose gadolinium. With triple-dose gadolinium, contrast ratios and areas of enhancement increased by 10% and 33%, respectively. Delayed imaging increased the size of the lesion by 11% on single-dose MTC images and by 18% on triple-dose images.CONCLUSIONTriple-dose gadolinium is more effective (higher sensitivity and interobserver agreement) than single-dose gadolinium in combination with MTC in detecting enhancing multiple sclerosis lesions.     

Magnetic resonance imaging and histologic findings of experimental cerebral fat embolism

RATIONALE AND OBJECTIVES: The purpose of this study was to determine whether cerebral fat embolism demonstrated reversible or irreversible findings in magnetic resonance (MR) imaging over time and to compare the features in MR images with histologic findings in a cat model. MATERIALS AND METHODS: MR images were obtained serially at 2 hours, 1 and 4 days, and 1, 2, and 3 weeks after embolization with 0.05 mL of triolein into the internal carotid artery in 19 cats. Any abnormal signal intensity and change in the signal intensity were evaluated on T2-weighted images, T1-weighted images, diffusion-weighted images (DWIs; including apparent diffusion coefficient [ADC] maps), and gadolinium-enhanced T1-weighted images (Gd-T1WI) over time. After MR imaging at 3 weeks, brain tissue was obtained and evaluated for light microscopic (LM) examination using hematoxylin-eosin and Luxol fast blue staining. For electron microscopic examination, the specimens were obtained at the cortex. The histologic and MR findings were compared. RESULTS: The embolization lesions showed hyperintensity on T2-weighted images, hyperintensity, or isointensity on DWIs, hypointensity, or isointensity on ADC maps and contrast enhancement on Gd-T1WIs at 2 hours. The T2-weighted hyperintensity extended to the white matter at day 1 and decreased thereafter. Contrast enhancement decreased continuously from day 1, and hyperintensity on DWI decreased after day 4. Hypointensity on ADC maps became less prominent after day 4. By week 3, most lesions had reverted to a normal appearance on MR images and were correlated with LM findings. However, small focal lesions remained in the gray matter of 8 cats and in the white matter of 3 cats on MR images, and this correlated with the cystic changes on LM findings. Electron microscopic examination of the cortical lesions that reverted to normal at week 3 in MR images showed that most of these lesions appeared normal but showed sporadic intracapillary fat vacuoles and disruption of the endothelial walls. CONCLUSIONS: The embolized lesions of the hyperacute stage were of 2 types: type 1 lesions, showing hyperintensity on DWIs and hypointensity on ADC maps, have irreversible sequelae, such as cystic changes; whereas type 2 lesions, showing isointensity or mild hyperintensity on DWIs and ADC maps, reverted to a normal appearance in the subacute stage.     

A new steady-state imaging sequence for simultaneous acquisition of two MR images with clearly different contrasts

We present a new steady-state imaging sequence, which simultaneously allows in a single acquisition the formation of two MR images with clearly different contrasts. The contrast of the first image is FISP-like, whereas the second image is strongly T2-weighted. In principle the T2 values in the image can be calculated from the combination of the first and second images. We also show calculated T2 images.     

Relationship between contrast enhancement on fluid-attenuated inversion recovery MR sequences and signal intensity on T2-weighted MR images: visual evaluation of brain tumors

PURPOSE: To investigate the relationship between the degree of contrast enhancement in fluid-attenuated inversion recovery (FLAIR) sequences and tumor signal intensity on T2-weighted images. MATERIALS AND METHODS: A total of 96 patients suspected of having brain tumors were examined by MR imaging, and whenever a brain tumor with an enhancing part larger than the slice thickness was demonstrated on postcontrast T1-weighted images, postcontrast FLAIR images were additionally acquired. The tumor signal intensity on the T2-weighted images was visually classified as follows: equal or lower compared with normal cerebral cortex (group 1), higher than normal cortex (group 2), and as high as cerebrospinal fluid (CSF) (group 3). When a lesion contained several parts with different signal intensities on T2-weighted images, we assessed each part separately. In each group, we visually compared pre- and postcontrast FLAIR images and assessed whether tumor contrast enhancement was present. When contrast enhancement was present on FLAIR sequence, the degree of contrast enhancement in T1-weighted and FLAIR sequences was visually compared. RESULTS: Postcontrast T1-weighted images showed 46 enhancing lesions, including 48 parts, in 31 MR examinations. FLAIR images of the lesion-parts in group 1 (N=18) did not show significant contrast enhancement. In group 2 (N=12), all the parts were enhanced in FLAIR sequences, and three parts were enhanced more clearly in the FLAIR sequences than in the T1-weighted sequences. In group 3 (N=18), all the parts were enhanced equally or more clearly in the FLAIR sequences than in the T1-weighted sequences. CONCLUSION: The signal intensity in FLAIR sequences is largely influenced by both T1 and T2 relaxation time; there is a close relationship between the signal intensity of brain tumors on T2-weighted images and the degree of contrast enhancement on FLAIR sequences. When tumors have higher signal intensity than normal cortex on T2-weighted images, additional postcontrast FLAIR imaging may improve their depiction.     

Multiple echo frequency-domain image contrast: improved signal-to-noise ratio and T2 (T2*) weighting.

Conventional T2- and T2*-weighted image contrasts are produced by waiting a TE period for the transverse magnetic resonance (MR) signals to decay to differentiate tissue types with distinct relaxation rates. Significant image signal-to-noise ratio (SNR) is compromised by this contrast-producing process. In this report, a multiple echo frequency-domain image contrast (MEFIC) method is presented. During the conventional TE period, a multiple echo train modulated by T2 or T2* decay is acquired. A third Fourier transform along the echo direction produces an image set with pixel signal intensity modulated by the spectrum of the decay curve. This method simultaneously enhances image contrast with a large increase in SNR. Experimental studies of cerebral vasogenic edema in immature rats and functional MR imaging studies of the human motor cortex have demonstrated that the MEFIC method produces superior image quality over conventional methods for generating T2- and T2* weighted images.     

Radial fast spin-echo method for T2-weighted imaging and T2 mapping of the liver

PURPOSE: To evaluate a multishot radial fast-spin echo (RAD-FSE) method developed to improve the quality of abdominal T2-weighted imaging as well as the characterization of focal liver lesions. MATERIALS AND METHODS: The RAD-FSE sequence used in this work consisted of a preparatory period followed by a short echo train (ETL = 16). A novel radial k-space trajectory was used to minimize streaking artifacts due to T2 variations and motion. Small diffusion gradients (b = 1.2 mm/s(2)) were used to improve flow suppression. The quality of images obtained with RAD-FSE was compared to multishot 2DFT fast spin-echo (2DFT-FSE) and half-Fourier acquisition single-shot turbo-spin-echo (HASTE) images using data from 16 patients. A postprocessing algorithm was used to generate multiple high-resolution images (at different effective TE values) as well as a T2 map from a single RAD-FSE data set. The T2 maps were used to differentiate malignant from benign lesions for a set of 33 lesions ranging from 0.8-194 cm(3). RESULTS: RAD-FSE produces high-resolution images of the liver in a breath-hold without the motion artifacts of 2DFT-FSE methods, and without the blurriness and loss of small lesion detectability of HASTE. The inclusion of diffusion weighting in RAD-FSE decreases the signal from blood in hepatic vessels, which improves lesion visualization. The T2 values obtained by postprocessing a single RAD-FSE data set can differentiate malignant from benign lesions. The mean T2 values obtained for malignancies, hemangiomas, and cysts are 108 +/- 30 msec, 240 +/- 14 msec, and 572 +/- 334 msec, respectively. CONCLUSION: These results indicate that RAD-FSE produces abdominal images of higher quality than 2DFT-FSE and HASTE. In addition, lesions can be characterized using T2 maps generated from a single RAD-FSE data set.     

子宫肌瘤的MR成像研究以--DSA为参照

目的 以DSA为参照评价子宫肌瘤的MRI表现。资料与方法 对DSA上有明显血供的子宫肌瘤行MRI检查,所有病例采用SE T1、turbo—SE T2WI和SET1WI增强扫描,对比剂为Gd—DTPA,剂量0．1mmol／kg体重,对不同成像序列的MRI信号特点和相关征象进行分析。结果 37个肌瘤T1WI均表现为等、低信号T2WI多为等、高信号(28个肌瘤),增强扫描肌瘤均出现明显强化,信号多数高于或等于子宫肌层(35个肌瘤)。多数肌瘤内部为均匀结节状或团块状改变,其间可见分隔结构,肌瘤包膜在T1WI、T1WI和增强扫描时能明确显示。结论 DSA表现明显血供的子宫肌瘤MRI有一定的特征性表现,增强扫描能够较为准确地反映肌瘤血供。     

Lateral geniculate nucleus: anatomic and functional identification by use of MR imaging.

BACKGROUND AND PURPOSE: MR imaging has the potential capacity for noninvasively depicting the anatomy and function of thalamic nuclei. The purpose of this study was to identify the lateral geniculate nucleus (LGN), which is the thalamic relay nucleus for vision, with anatomic and functional MR imaging at 1.5 T. METHODS: Three-millimeter-thick axial images were obtained from eight volunteers by using a double-echo turbo spin-echo sequence for proton density- and T2-weighted contrast and a spin-echo 3D gradient-echo sequence for T1-weighted contrast. Each participant underwent a visual activation experiment using gradient-echo echo-planar imaging at the same location as that of the anatomic study. RESULTS: In all cases, the LGN was recognized on proton density-weighted images as a small wedge-shaped area of high signal intensity relative to that of the surrounding white matter tracts. However, it was difficult to identify the LGN on T1- and T2-weighted images because of the smaller contrast-to-noise ratios between the LGN and the adjacent white matter tracts, compared with those of proton density-weighted images (P <.001). Bilateral thalamic activation and activation in the occipital cortex were shown in all participants. Each region of thalamic activation (23 +/- 3 mm2) was localized to the anatomically identified LGN. CONCLUSION: The excellent correspondence between the anatomically and functionally identified LGN confirms that MR imaging is an indispensable method for visualizing functional neuroanatomy in thalamic nuclei.     

T1 maps by K-space reduced snapshot-FLASH MRI.

The T1 maps evaluated from k-space reduced Snapshot fast low angle shot (FLASH) images provide high contrast parameter images for tissue characterization in vivo of any body region. An algorithm for computing T1 values that allows a fast and reliable evaluation of T1 maps and yields reproducible values of tissue parameters in MR imaging is presented. The algorithm combined with the Snapshot FLASH inversion recovery imaging sequence permits a precise determination of T1 values, even for T1 times as low as 50 ms. Comparison with a spectroscopical inversion recovery method on identical phantoms demonstrates the accuracy of this technique. With its total acquisition time of approximately 2 s, IR Snapshot FLASH is fast enough to be used in monitoring fast T1 dynamics.     

Evaluation of the RF field uniformity of a double-tuned 31P/1H birdcage RF coil for spin-echo MRI/MRS of the diabetic foot

PURPOSE: To evaluate the B1 field uniformity of a double-tuned birdcage coil designed for (31)P/(1)H MRI/MRS spin-echo (SE) imaging of the metatarsal head region of the foot in neuropathic diabetic patients. MATERIALS AND METHODS: A low-pass double-tuned (31)P/(1)H RF birdcage coil was constructed to fit over the adult forefoot. Flip angle (FA) maps were created from B1 data acquired at the 3T (31)P (four normal subjects) and (1)H (five normal subjects) frequencies. T2-weighted (T2-W) (1)H images, (31)P rapid acquisition with relaxation enhancement (RARE) images, and composite SE pulse CSI data were acquired to demonstrate the uniformity of the resulting images and data. RESULTS: The means and standard deviations (SDs) of the range of FAs across the feet of the volunteer subjects indicated good uniformity (the maximum coefficients of variation (CVs) for all of the (31)P and (1)H FA maps were 7.6% and 7.3%, respectively). The FA values across the metatarsal head region indicated a maximum signal intensity variation of +/-3% in a RARE image acquired using an echo train length of 32. CONCLUSION: A (31)P/(1)H birdcage coil constructed for MRI/MRS studies of the human forefoot provided sufficient signal uniformity of SE data to facilitate accurate (31)P concentration measurements in muscle.