nqing Li

DANTE-prepared pulse trains: A novel approach to motion-sensitized and motion-suppressed quantitative magnetic resonance imaging

Delay alternating with nutation for tailored excitation (DANTE) pulse trains are well appreciated as frequency‐selective excitation methods in Fourier transform NMR and for spatial tagging in MRI. In this study, nonselective DANTE pulse trains are used in combination with gradient pulses and short repetition times as motion‐sensitive preparation modules. We show that while the longitudinal magnetization of static tissue is mostly preserved, flowing spins are largely (or fully) attenuated as they fail to establish transverse steady state due to a spoiling effect caused by flow along the applied gradient. The attenuation of flowing spins is effectively insensitive to spin velocity (above a low threshold) and can be approximately quantified with a simple T1 longitudinal magnetization decay model. The relevant analytical equations for moving spins and static spins during DANTE module application are derived for both transient and steady state epochs. The equations are validated by comparing analytical solutions and numerical Bloch equation simulations against experimental observations in phantoms and in vivo. Based on this contrast mechanism, the application of the DANTE preparation to black blood vessel imaging is proposed. A simple demonstration of DANTE black blood imaging modules shows that it provides excellent blood signal suppression and static tissue signal preservation. Magn Reson Med, 2012.

Black-Blood Multicontrast Imaging of Carotid Arteries with DANTE-prepared 2D and 3D MR Imaging

PURPOSE To prospectively compare the black-blood ( BB black blood ) imaging efficiency of a delay alternating with nutation for tailored excitation ( DANTE delay alternating with nutation for tailored excitation ) preparation module with conventional double inversion-recovery ( DIR double inversion recovery ) and motion-sensitive driven equilibrium ( MSDE motion-sensitive driven equilibrium ) preparation modules and to introduce a new three-dimensional ( 3D three-dimensional ) T1-weighted magnetic resonance (MR) imaging sequence. MATERIALS AND METHODS Carotid artery wall imaging was performed in 10 healthy volunteers and 15 patients in accordance with an institutional review board-approved protocol. Two-dimensional ( 2D two-dimensional ) turbo spin-echo ( TSE turbo spin echo ) and 3D three-dimensional fast low-angle shot ( FLASH fast low-angle shot ) sequences served as readout modules. DANTE delay alternating with nutation for tailored excitation -prepared T1-, T2-, and proton density-weighted 2D two-dimensional TSE turbo spin echo images, as well as T1-weighted 3D three-dimensional DANTE delay alternating with nutation for tailored excitation -prepared FLASH fast low-angle shot (hereafter, 3D three-dimensional DASH DANTE-prepared FLASH ) images, were acquired in the region of the carotid artery bifurcation. For comparison, 2D two-dimensional DIR double inversion recovery -prepared, 2D two-dimensional MSDE motion-sensitive driven equilibrium -prepared multicontrast TSE turbo spin echo , and 3D three-dimensional MSDE motion-sensitive driven equilibrium -prepared FLASH fast low-angle shot (hereafter, 3D three-dimensional MERGE MSDE-prepared FLASH ) MR images were also acquired. The effective contrast-to-noise ratio ( CNReff effective contrast-to-noise ratio ) per unit time was calculated for all sequences. Paired t tests were performed to test within-group differences in vessel wall CNReff effective contrast-to-noise ratio . RESULTS The CNReff effective contrast-to-noise ratio of DANTE delay alternating with nutation for tailored excitation -prepared T1-, T2-, and proton density-weighted sequences was 27.3, 14.7, and 25.7 mm(-1)min(-1/2), respectively; this represented an improvement of approximately 25%-100% (P < .05) when compared with the CNReff effective contrast-to-noise ratio attained with existing methods. The 3D three-dimensional DASH DANTE-prepared FLASH technique proved to be a fast (<2 seconds per section) and high-spatial-resolution (0.6 mm isotropic) BB black blood technique with higher (75%-100% improvement, P < .001) signal-to-noise ratio efficiency than the 3D three-dimensional MERGE MSDE-prepared FLASH technique. CONCLUSION The DANTE delay alternating with nutation for tailored excitation -prepared multicontrast 2D two-dimensional BB black blood technique is a promising new tool for MR imaging of carotid artery walls. Additionally, the 3D three-dimensional DASH DANTE-prepared FLASH sequence enables 3D three-dimensional high-spatial-resolution fast T1-weighted imaging of carotid artery walls. ©RSNA, 2014 Online supplemental material is available for this article .

Quantification of Lipid-Rich Core in Carotid Atherosclerosis Using Magnetic Resonance T2 Mapping: Relation to Clinical Presentation.

Objectives The aim of this study was to: 1) provide tissue validation of quantitative T2 mapping to measure plaque lipid content; and 2) investigate whether this technique could discern differences in plaque characteristics between symptom-related and non–symptom-related carotid plaques. Background Noninvasive plaque lipid quantification is appealing both for stratification in treatment selection and as a possible predictor of future plaque rupture. However, current cardiovascular magnetic resonance (CMR) methods are insensitive, require a coalesced mass of lipid core, and rely on multicontrast acquisition with contrast media and extensive post-processing. Methods Patients scheduled for carotid endarterectomy were recruited for 3-T carotid CMR before surgery. Lipid area was derived from segmented T2 maps and compared directly to plaque lipid defined by histology. Results Lipid area (%) on T2 mapping and histology showed excellent correlation, both by individual slices (R = 0.85, p < 0.001) and plaque average (R = 0.83, p < 0.001). Lipid area (%) on T2 maps was significantly higher in symptomatic compared with asymptomatic plaques (31.5 ± 3.7% vs. 15.8 ± 3.1%; p = 0.005) despite similar degrees of carotid stenosis and only modest difference in plaque volume (128.0 ± 6.0 mm3 symptomatic vs. 105.6 ± 9.4 mm3 asymptomatic; p = 0.04). Receiver-operating characteristic analysis showed that T2 mapping has a good ability to discriminate between symptomatic and asymptomatic plaques with 67% sensitivity and 91% specificity (area under the curve: 0.79; p = 0.012). Conclusions CMR T2 mapping distinguishes different plaque components and accurately quantifies plaque lipid content noninvasively. Compared with asymptomatic plaques, greater lipid content was found in symptomatic plaques despite similar degree of luminal stenosis and only modest difference in plaque volumes. This new technique may find a role in determining optimum treatment (e.g., providing an indication for intensive lipid lowering or by informing decisions of stents vs. surgery).

T2-Weighted intracranial vessel wall imaging at 7 Tesla using a DANTE-prepared variable flip angle turbo spin echo readout (DANTE-SPACE).

To optimize intracranial vessel wall imaging (VWI) at 7T for sharp wall depiction and high boundary contrast.

Quantification of carotid plaque lipid content with magnetic resonance T2 mapping in patients undergoing carotid endarterectomy

Background and purpose Techniques to stratify subgroups of patients with asymptomatic carotid artery disease are urgently needed to guide decisions on optimal treatment. Reliance on estimates of % luminal stenosis has not been effective, perhaps because that approach entirely disregards potentially important information on the pathological process in the wall of the artery. Methods Since plaque lipid is a key determinant of plaque behaviour we used a newly validated, high-sensitivity T2-mapping MR technique for a systematic survey of the quantity and distribution of plaque lipid in patients undergoing endarterectomy. Lipid percentage was quantified in 50 carotid endarterectomy patients. Lipid distribution was tested, using two imaging indices (contribution of the largest lipid deposit towards total lipid (LLD %) and a newly-developed LAI ‘lipid aggregation index’). Results The bifurcation contained maximal lipid volume. Lipid percentage was higher in symptomatic vs. asymptomatic patients with degree of stenosis (DS ≥ 50%) and in the total cohort (P = 0.013 and P = 0.005, respectively). Both LLD % and LAI was higher in symptomatic patients (P = 0.028 and P = 0.018, respectively), suggesting that for a given plaque lipid volume, coalesced deposits were more likely to be associated with symptomatic events. There was no correlation between plaque volume or lipid content and degree of luminal stenosis measured on ultrasound duplex (r = -0.09, P = 0.53 and r = -0.05, P = 0.75), respectively. However, there was a strong correlation in lipid between left and right carotid arteries (r = 0.5, P <0.0001, respectively). Conclusions Plaque lipid content and distribution is associated with symptomatic status of the carotid plaque. Importantly, plaque lipid content was not related to the degree of luminal stenosis assessed by ultrasound. Determination of plaque lipid content may prove useful for stratification of asymptomatic patients, including selection of optimal invasive treatments.

Structural imaging of the cervical spinal cord with suppressed CSF signal using DANTE pulse trains

We propose DANTE (Delays Alternating with Nutation for Tailored Excitation) moving fluid attenuation preparation pulse trains, in conjunction with T1, T2, and proton‐density‐weighted fast spin‐echo (T1w‐TSE, T2w‐TSE and PDw‐TSE) imaging readout, and three‐dimensional fast low flip angle shots (3D‐FLASH) T1‐weighted imaging readout to achieve CSF‐suppressed high‐spatial resolution multicontrast cervical spinal cord images.

Histological validation of carotid plaque characterization by in-vivo T2 mapping in patients with recent cerebrovascular events: preliminary results

Background Multicontrast CMR is the conventional method for invivo characterization of carotid plaques. However, its non-quantitative nature and the need for extensive postacquisition interpretation limit its clinical application. We have recently proposed using in-vivo quantitative T2 mapping for atherosclerotic plaque characterization [Biasiolli et al. JCMR 2013, 15:69]. T2 maps have the advantage of providing an absolute physical measure of plaque components that can be standardized among different MR systems and widely adopted in multi-centre studies. This pilot study investigates the agreement between T2 mapping and histology using AHA plaque type classification.

Fast three-dimensional black-blood MR imaging for carotid artery intra-plaque haemorrhage using DANTE-prepared FLASH (3D-DASH)

Background DANTE (Delays Alternating with Nutation for Tailored Excitation) pulse trains are a rapid series of low flip angle RF pulses interspersed with gradients. We have previously demonstrated that when using DANTE pulse trains as a preparation module prior to imaging readout, the longitudinal magnetization of flowing spins is substantially attenuated, whereas the longitudinal magnetization of static tissue/fluid is mostly preserved [1]. In this study we introduce a new DANTE-prepared 3D FLASH T1 weighted (T1w) sequence (denoted ‘3DDASH’) [2] that is able to generate 0.6 mm isotropic resolution images with an average imaging speed better than 2 sec/slice.

A novel approach to probing in vivo metabolite relaxation: Linear quantification of spatially modulated magnetization

Conventional sequences for metabolite transverse relaxation quantification all generally measure signal changes at different echo times (TEs). However, quantification results obtained via these conventional methods can be very different and are highly dependent on the type of sequence being applied. TE‐dependent effects such as diffusion, macromolecule baseline, and J‐coupling modulation contribute significantly to these differences. Here, we propose a novel technique—multiple flip angle pulse‐driven ratio of longitudinal steady states (MARzss)—for preparing magnetization with T2/T1 weighting. Using premeasured T1 values, T2 values for metabolites can thereby be determined. The measurement procedure does not require varying TE and is TE independent; T2, diffusion, and J‐coupling effects induced by the readout sequence are cancelled.

In-vivo carotid T2 mapping can accurately quantify plaque lipid content to discriminate between symptomatic and asymptomatic patients: histological validation, scan-rescan reproducibility and clinical study

Background In-vivo carotid CMR is able to identify features of plaque vulnerability such as lipid core size. However, the current standard (multicontrast CMR) requires contrast media, extensive post-processing and subjective interpretation. Recently we proposed to use quantitative T2 mapping to distinguish plaque lipid from surrounding fibrous tissue and measure lipid core size. This study aimed to (1) validate plaque lipid quantification by T2 mapping against histology and (2) investigate if it could discriminate between symptomatic and asymptomatic patients.