Nontriggered magnetic resonance velocity measurement of the time-average of pulsatile velocity

The feasibility of the determination of the time-average of pulsatile velocity obtained via a nontriggered magnetic resonance (MR) acquisition is studied. The advantage of this method, in comparison with a triggered acquisition, is a considerable reduction (≈15×) in acquisition time. However, pul-satility causes image artifacts, known as ghosts, and the Fourier transform technique required for the imaging procedure accomplishes time-averaging of the complex MR signal. Both effects can result in errors in the velocity determined. Calculations show that these errors depend on the velocity time function and the acquisition parameters. In vivo comparison of triggered and nontriggered MR velocity measurements in the femoral artery of volunteers (n = 7) shows larger statistical and systematic errors in the latter, which depend on the excitation angle. Therefore, this nontriggered average velocity measurement is only useful as a fast and rough estimation of the time-averaged velocity.     

In vivo imaging of a stable paramagnetic probe by pulsed-radiofrequency electron paramagnetic resonance spectroscopy

Imaging of free radicals by electron paramagnetic resonance (EPR) spectroscopy using time domain acquisition as in nuclear magnetic resonance (NMR) has not been attempted because of the short spin-spin relaxation times, typically under 1 μs, of most biologically relevant paramagnetic species. Recent advances in radiofrequency (RF) electronics have enabled the generation of pulses of the order of 10–50 ns. Such short pulses provide adequate spectral coverage for EPR studies at 300 MHz resonant frequency. Acquisition of free induction decays (FID) of paramagnetic species possessing inhomogenously broadened narrow lines after pulsed excitation is feasible with an appropriate digitizer/averager. This report describes the use of time-domain RF EPR spectrometry and imaging for in vivo applications. FID responses were collected from a water-soluble, narrow line width spin probe within phantom samples in solution and also when infused intravenously in an anesthetized mouse. Using static magnetic field gradients and back-projection methods of image reconstruction, two-dimensional images of the spin-probe distribution were obtained in phantom samples as well as in a mouse. The resolution in the images was better than 0.7 mm and devoid of motional artifacts in the in vivo study. Results from this study suggest a potential use for pulsed RF EPR imaging (EPRI) for three-dimensional spatial and spectral-spatial imaging applications. In particular, pulsed EPRI may find use in in vivo studies to minimize motional artifacts from cardiac and lung motion that cause significant problems in frequency-domain spectral acquisition, such as in continuous wave (cw) EPR techniques.     

Blood attenuation with SSFP-compatible saturation (BASS)

PURPOSE: To investigate a rapid flow-suppression method for improving the contrast-to-noise ratio (CNR) between the vessel wall and the lumen for cardiovascular imaging applications. MATERIALS AND METHODS: In this study a new dark-blood steady-state free precession (SSFP) sequence utilizing two excitation pulses per TR was developed. The first pulse is applied immediately adjacent to the slice of interest, while the second is a conventional slice-selective pulse designed to excite an SSFP signal for the static spins in the slice of interest. The slice-selective pulse is followed by fully refocused gradients along all three imaging axes over each TR. The signal amplitude (SA) from the moving spins excited by the "saturation" pulse is attenuated since they are not fully refocused at the TE. RESULTS: This work provides confirmation, by both simulation and experiments, that modest adaptations of the basic True-FISP structure can limit unwanted "bright blood" signal within the vessels while simultaneously preserving the contrast and speed advantages of this well-established rapid imaging method. CONCLUSION: Animal imaging trials confirm that dark-blood contrast is achieved with the BASS sequence, which substantially reverses the lumen-to-muscle CNR of a conventional True-FISP "bright blood" acquisition from 14.77 (bright blood) to -13.96 (dark blood) with a modest increase (24.2% of regular TR of SSFP for this implementation) in acquisition time to accommodate the additional slab-selective excitation pulse and gradient pulses.     

Dynamics of magnetization in hyperpolarized gas MRI of the lung

The magnetization in hyperpolarized gas (HP) MRI is generated by laser polarization that is independent of the magnet and imaging process. As a consequence, there is no equilibrium magnetization during the image acquisition. The competing processes of gas inflow and depolarization of the spins lead to large changes in signal as one samples k-space. A model is developed of dynamic changes in polarization of hyperpolarized ³He during infusion and in vivo imaging of the lung and verified experimentally in a live guinea pig. Projection encoding is used to measure the view-to-view variation with temporal resolution <4 ms. Large excitation angles effectively sample the magnetization in the early stages of inflow, highlighting larger airways, while smaller excitation angles produce images of the more distal spaces. The work provides a basis for pulse sequences designed to effectively exploit HP MRI in the lung.     

Chemical shift encoding‐based water–fat separation methods

The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so‐called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water–fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed. J. Magn. Reson. Imaging 2014;40:251–268 . © 2014 Wiley Periodicals, Inc .     

Improved 2D time-of-flight angiography using a radial-line k-space acquisition

For flow imaging applications, radial-line k-space acquisition methods offer advantages over conventional 2DFT methods. Specifically, radial-line acquisition methods mitigate artifacts resulting from pulsatile flow while offering a potential reduction in scan times. In this paper, radial-line and 2DFT acquisitions are compared in a two-dimensional time-of-flight angiography sequence. The twisting radial-line (TwiRL) trajectory, a variant of 2D projection reconstruction, is used to represent the family of radial-line trajectories. In both phantom and in vivo studies, the TwiRL images demonstrate improved vessel depiction including a more uniform signal intensity and better delineation of the vasculature in comparison with images obtained via the 2DFT method.     

Per‐subject characterization of bolus width in pulsed arterial spin labeling using bolus turbo sampling

Quantification of cerebral blood flow using QUIPSSII pulsed arterial spin labeling requires that the QUIPSS saturation delay TI₁ is shorter than the natural temporal bolus width. Yet the duration of the bolus of tagged spins entering the region of interest varies during vasoactive stimuli such as gaseous challenges or across subjects due to differences in blood velocity or vessel geometry. A new technique, bolus turbo sampling, to rapidly measure the duration of the inflowing bolus is presented. It allows to optimize the arterial spin labeling acquisition to ensure reliable quantification of perfusion while maximizing the arterial spin labeling signal by avoiding the use of unnecessarily short label durations. The average bolus width measured in the right and left middle cerebral artery territories using the bolus turbo sampling technique has a repeatability coefficient of 75 ms and correlates significantly with the TI_1,max determined from a novel multi‐TI₁ protocol (R = 0.65, P < 0.05). The possibility to measure the bolus width under hypercapnia is demonstrated. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

MRA studies of arterial stenosis: Improvements by diastolic acquisition

Cardiac-phase-specific data acquisition is used to reduce signal loss in MR Angiography resulting from disturbed flow. RF pulses are delivered continuously throughout the cardiac cycle, but incrementation of phase-encoding gradients and data storage are enabled only during the chosen part of the cycle. Studies in a stenotic pulsatile flow phantom demonstrate that poststenotic signal loss is primarily determined by the mean flow velocity, and is not appreciably affected by acceleration or deceleration of the mean flow rate. The signal loss is least in diastole. In vivo studies in patients with carotid artery disease show that data acquisition in diastole reduces the apparent degree and extent of carotid bifurcation stenosis and provides a crisper definition of the vascular lumen. The additional time required for cardiac-phase-specific acquisition can be reduced by gating only the lower-order phase-encoding lines while retaining acceptable image quality.     

A comparison of RIGR and SVD dynamic imaging methods

Several constrained imaging methods have recently been proposed for dynamic imaging applications. This paper compares two of these methods: the Reduced-encoding Imaging by Generalized-series Reconstruction (RIGR) and Singular Value Decomposition (SVD) methods. RIGR utilizes a priori data for optimal image reconstruction whereas the SVD method seeks to optimize data acquisition. However, this study shows that the existing SVD encoding method tends to bias the data acquisition scheme toward reproducing the known features in the reference image. This characteristic of the SVD encoding method reduces its capability to capture new image features and makes it less suitable than RIGR for dynamic imaging applications.     

Practical signal-to-noise ratio quantification for sensitivity encoding: application to coronary MR angiography

Purpose:

To develop and evaluate a practical method for the quantification of signal‐to‐noise ratio (SNR) on coronary MR angiograms (MRA) acquired with parallel imaging.

Materials and Methods:

To quantify the spatially varying noise due to parallel imaging reconstruction, a new method has been implemented incorporating image data acquisition followed by a fast noise scan during which radiofrequency pulses, cardiac triggering and navigator gating are disabled. The performance of this method was evaluated in a phantom study where SNR measurements were compared with those of a reference standard (multiple repetitions). Subsequently, SNR of myocardium and posterior skeletal muscle was determined on in vivo human coronary MRA.

Results:

In a phantom, the SNR measured using the proposed method deviated less than 10.1% from the reference method for small geometry factors (≤2). In vivo, the noise scan for a 10 min coronary MRA acquisition was acquired in 30 s. Higher signal and lower SNR, due to spatially varying noise, were found in myocardium compared with posterior skeletal muscle.

Conclusion:

SNR quantification based on a fast noise scan is a validated and easy‐to‐use method when applied to three‐dimensional coronary MRA obtained with parallel imaging as long as the geometry factor remains low. J. Magn. Reson. Imaging 2011;33:1330–1340. © 2011 Wiley‐Liss, Inc.     

Multi-gradient echo with susceptibility inhomogeneity compensation (MGESIC): Demonstration of fMRI in the olfactory cortex at 3.0 T

Short image acquisition times and sensitivity to magnetic susceptibility favor the use of gradient echo imaging methods in functional MRI (fMRI). However, magnetic susceptibility effects attributed to air-tissue interfaces also lead to severe signal loss in images of the large inferior frontal and lateral temporal cortices of the human brain, which renders these regions inaccessible to fMRI. The signal loss is caused by the local field gradients in the slice selection direction. A multigradient echo with magnetic susceptibility inhomogeneity compensation method (MGESIC) is proposed to overcome this problem. The MGESIC method effectively corrects the susceptibility artifacts and maintains the advantages of gradient echo methods to both BOLD sensitivity and fast image acquisition. The effectiveness of the MGESIC method is demonstrated by fMRI experimental results within the olfactory cortex.     

Quantitative auto-fluorescence quenching of free and bound NADH in HeLa cell line model with Carbonyl cyanide-p-Trifluoromethoxy phenylhydrazone (FCCP) as quenching agent

The autofluorescence of endogenous biomolecules (Nicotinamide adenine dinucleotide (NAD, its reduced form NADH and the phosphorylated form NAD(P)H take part in cellular metabolic pathways and has vital importance for in vivo and ex vivo photo diagnostic applications of biological tissues. We present a detailed quenching analysis of Carbonyl cyanide-p-Trifluoromethoxy phenylhydrazone (FCCP) 50−1000 µM and analyzed the fluorescence signal from NADH/ NAD(P)H in vitro (in solution) and in vivo (HeLa cell suspension).The in vitro samples of pure NADH/ NAD(P)H were excited at λ=340±1 nm while the fluorescence signal was collected in the range of 400−550 nm. The quenching process was characterized using excitation emission matrix (EEM) fluorescence spectroscopy and Stern- Volmer plots. The experimental results illustrated maximum fluorescence emission for the control NADH samples (i.e., no FCCP), while the fluorescence signal from the solution progressively decreased with the increasing concentration of the FCCP, until it reaches the base line (i.e., no fluorescence signal) at 1000 µM of FCCP. In vitro study shows that the fluorescence quenching of free NADH was found to be lower than the bound NAD(P)H with similar diminishing trend. The quenching of bound NAD(P)H in cells is attenuated compared to solution quenching possibly due to a contribution from the metabolic/antioxidant response in cells and fluorescence exponential decay curve lies between plated and suspended HeLa cells. A two-fold increase in the fluorescence intensity of NAD(P)H was observed after the bond formation with L-Malate Dehydrogenase (L-MDH, Sigma Aldrich #10127248001) protein This work has applications for sharp tumor demarcation during sensitive surgical procedures as well as to enhance fluorescence based diagnosis of biological tissues.     

双光子成像技术在大鼠放射性皮肤损伤修复评估中的应用

目的 采用双光子激发荧光（TPEF）成像技术,无创、活体评估X射线引起大鼠皮肤放射性损伤发展和修复过程。方法 24只SD大鼠采用随机数表法分为4组,健康对照组、25 Gy组、35 Gy组和45 Gy组,每组6只。照射后不同时间评估皮肤损伤程度,通过TPEF成像技术在体检测表皮细胞尼克酰胺腺嘌呤二核苷酸（磷酸）[NAD（P）H]和真皮胶原纤维荧光信号的病理改变。结果 第10天,各辐射组大鼠出现红斑和脱皮;第15~20天,随着辐射剂量地增加,辐射组出现递进性的渗出、水肿和溃疡;第25天,25 Gy组开始修复,其他组仍有渗出和溃疡。第10天,25、35和45 Gy组表皮细胞NAD（P）H荧光信号减少,乳头层和网状层荧光信号值减少,与健康对照组比较,差异有统计学意义（t=24.145、28.303、26.989、6.654、7.510、7.997,P<0.05）;第30天,25 Gy组表皮细胞NAD（P）H和真皮胶原纤维开始修复,颗粒层、棘层和基底层细胞出现荧光信号,35、45 Gy组未出现NAD（P）H荧光信号;25 Gy组乳头层和网状层荧光信号均逐渐增高,但仍低于健康对照组（t=115.133、17.431,P<0.05）,而45 Gy组未出现荧光信号。结论 TPEF技术可以无创、活体检测X射线照射后表皮细胞和真皮胶原纤维信号损伤和修复的病理变化。     

Accelerated fluorine‐19 MRI cell tracking using compressed sensing

Cell tracking using perfluorocarbon labels and fluorine‐19 (¹⁹F) MRI is a noninvasive approach to visualize and quantify cell populations in vivo. In this study, we investigated three‐dimensional compressed sensing methods to accelerate ¹⁹F MRI data acquisition for cell tracking and evaluate the impact of acceleration on ¹⁹F signal quantification. We show that a greater than 8‐fold reduction in imaging time was feasible without pronounced image degradation and with minimal impact on the image signal‐to‐noise ratio and ¹⁹F quantification accuracy. In ¹⁹F phantom studies, we show that apparent feature topology is maintained with compressed sensing reconstruction, and false positive signals do not appear in areas devoid of fluorine. We apply the three‐dimensional compressed sensing ¹⁹F MRI methods to quantify the macrophage burden in a localized wounding‐inflammation mouse model in vivo; at 8‐fold image acceleration, the ¹⁹F signal distribution was accurately reproduced, with no loss in signal‐to‐noise ratio. Our results demonstrate that three‐dimensional compressed sensing methods have potential for advancing in vivo ¹⁹F cell tracking for a wide range of preclinical and translational applications. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Clinically constrained optimization of flexTPI acquisition parameters for the tissue sodium concentration bioscale

The rapid transverse relaxation of the sodium magnetic resonance signal during spatial encoding causes a loss of image resolution, an effect known as T₂‐blurring. Conventional wisdom suggests that spatial resolution is maximized by keeping the readout duration as short as possible to minimize T₂‐blurring. Flexible twisted projection imaging performed with an ultrashort echo time, relative to T₂, and a long repetition time, relative to T₁, has been shown to be effective for quantitative sodium magnetic resonance imaging. A minimized readout duration requires a very large number of projections and, consequentially, results in an impractically long total acquisition time to meet these conditions. When the total acquisition time is limited to a clinically practical duration (e.g., 10 min), the optimal parameters for maximal spatial resolution of a flexible twisted projection imaging acquisition do not correspond to the shortest possible readout. Simulation and experimental results for resolution optimized acquisition parameters of quantitative sodium flexible twisted projection imaging of parenchyma and cerebrospinal fluid are presented for the human brain at 9.4 and 3.0T. The effect of signal loss during data collection on sodium quantification bias and image signal‐to‐noise ratio are discussed. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

DC‐gated high resolution three‐dimensional lung imaging during free‐breathing

Purpose:

To use the acquisition of the k‐space center signal (DC signal) implemented into a Cartesian three‐dimensional (3D) FLASH sequence for retrospective respiratory self‐gating and, thus, for the examination of the whole human lung in high spatial resolution during free breathing.

Materials and Methods:

Volunteer as well as patient measurements were performed under free breathing conditions. The DC signal is acquired after the actual image data acquisition within each excitation of a 3D FLASH sequence. The DC signal is then used to track respiratory motion for retrospective respiratory gating.

Results:

It is shown that the acquisition of the DC signal after the imaging module can be used in a 3D FLASH sequence to extract respiratory motion information for retrospective respiratory self‐gating and allows for shorter echo times (TE) and therefore increased lung parenchyma SNR.

Conclusion:

The acquisition of the DC signal after image signal acquisition allows successful retrospective gating, enabling the reconstruction of high resolution images of the whole human lung under free breathing conditions. J. Magn. Reson. Imaging 2013;37:727–732. © 2012 Wiley Periodicals, Inc.     

Fast,simple gradient delay estimation for spiral MRI

Timing delays between data acquisition and gradient transmission result in image degradation. This is especially true in spiral MRI, where delays can alter data in a nonuniform manner, generating significant artifact in the reconstructed data. The many methods that exist to mitigate these delays or measure the k‐space coordinates require long measurement times, complicated analysis, specialized phantoms or hardware, or significant changes to the sequence of interest. A fast and simple method is proposed to measure delays on each gradient channel. It requires only minimal modification to an existing spiral sequence and can be used to measure independent delays on three gradient channels and any scan subject within six sequence repetition times. The effectiveness and accuracy of this method are analyzed. Magn Reson Med 63:1683–1690, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

High efficiency multishot interleaved spiral‐in/out: Acquisition for high‐resolution BOLD fMRI

Growing demand for high spatial resolution blood oxygenation level dependent (BOLD) functional magnetic resonance imaging faces a challenge of the spatial resolution versus coverage or temporal resolution tradeoff, which can be addressed by methods that afford increased acquisition efficiency. Spiral acquisition trajectories have been shown to be superior to currently prevalent echo‐planar imaging in terms of acquisition efficiency, and high spatial resolution can be achieved by employing multiple‐shot spiral acquisition. The interleaved spiral in/out trajectory is preferred over spiral‐in due to increased BOLD signal contrast‐to‐noise ratio (CNR) and higher acquisition efficiency than that of spiral‐out or noninterleaved spiral in/out trajectories (Law & Glover. Magn Reson Med 2009; 62:829–834.), but to date applicability of the multishot interleaved spiral in/out for high spatial resolution imaging has not been studied. Herein we propose multishot interleaved spiral in/out acquisition and investigate its applicability for high spatial resolution BOLD functional magnetic resonance imaging. Images reconstructed from interleaved spiral‐in and ‐out trajectories possess artifacts caused by differences in T2* decay, off‐resonance, and k‐space errors associated with the two trajectories. We analyze the associated errors and demonstrate that application of conjugate phase reconstruction and spectral filtering can substantially mitigate these image artifacts. After applying these processing steps, the multishot interleaved spiral in/out pulse sequence yields high BOLD CNR images at in‐plane resolution below 1 × 1 mm while preserving acceptable temporal resolution (4 s) and brain coverage (15 slices of 2 mm thickness). Moreover, this method yields sufficient BOLD CNR at 1.5 mm isotropic resolution for detection of activation in hippocampus associated with cognitive tasks (Stern memory task). The multishot interleaved spiral in/out acquisition is a promising technique for high spatial resolution BOLD functional magnetic resonance imaging applications. Magn Reson Med 70:420–428, 2013. © 2012 Wiley Periodicals, Inc.     

Real time blood flow imaging by spiral scan phase velocity mapping

The work describes the development of a novel sequence that uses rapid spiral k-space sampling, combined with phase velocity mapping, for real time flow velocity imaging. The performance of the technique is assessed on phantoms for both through-plane and in-plane flows. The flow measurements compared well with those measured using a bucket and stopwatch. One advantage of the technique is that flow related signal loss is minimal due to the early acquisition of the center of k-space data. Flow artifacts were observed for in-plane flow and these were understood with the aid of computer simulations. In vivo studies involved cine velocity mapping in normal volunteers; aortic blood flow waveforms acquired by spiral scanning in two cardiac cycles compared well with data from a conventional gradient-echo sequence. Potential applications of the method are demonstrated by studying the response of aortic flow to physical exercise and the real time monitoring of aortic flow during a valsalver maneuver.