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1.
The conversion of hyperpolarized 13C pyruvate to metabolic products in the Krebs cycle provides valuable information about the metabolic status and the viability of the myocardium. Therefore, imaging methods must be able to spectrally discriminate different 13C metabolites. However, the requirement for spectral selectivity conflicts with the demands for rapid image acquisition and high spatial resolution in cardiac imaging. In this work, the feasibility of a balanced steady state free precession sequence with low flip angles was investigated in the pig heart after injection of hyperpolarized 13C1‐pyruvate. Using cardiac gating, it was possible to acquire 13C‐bicarbonate images within a single heartbeat (acquisition time 150 ms) without destroying the substrate signal from the hyperpolarized pyruvate. Therefore, the technique may be useful in dynamic studies of cardiac metabolism. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.  相似文献   

2.
Dynamic nuclear polarization can create hyperpolarized compounds with MR signal‐to‐noise ratio enhancements on the order of 10,000‐fold. Both exogenous and normally occurring endogenous compounds can be polarized, and their initial concentration and downstream metabolic products can be assessed using MR spectroscopy. Given the transient nature of the hyperpolarized signal enhancement, fast imaging techniques are a critical requirement for real‐time metabolic imaging. We report on the development of an ultrafast, multislice, spiral chemical shift imaging sequence, with subsecond acquisition time, achieved on a clinical MR scanner. The technique was used for dynamic metabolic imaging in rats, with measurement of time‐resolved spatial distributions of hyperpolarized 13C1‐pyruvate and metabolic products 13C1‐lactate and 13C1‐alanine, with a temporal resolution of as fast as 1 s. Metabolic imaging revealed different signal time courses in liver from kidney. These results demonstrate the feasibility of real‐time, hyperpolarized metabolic imaging and highlight its potential in assessing organ‐specific kinetic parameters. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.  相似文献   

3.
Fast chemical shift imaging (CSI) techniques are advantageous in metabolic imaging of hyperpolarized compounds due to the limited duration of the signal amplification. At the same time, reducing the acquisition time in hyperpolarized imaging does not necessarily lead to the conventional penalty in signal‐to‐noise ratio that occurs in imaging at thermal equilibrium polarization levels. Here a high‐performance gradient insert was used in combination with undersampled spiral CSI to increase either the imaging speed or the spatial resolution of hyperpolarized 13C metabolic imaging on a clinical 3T MR scanner. Both a single‐shot sequence with a total acquisition time of 125 ms and a three‐shot sequence with a nominal in‐plane resolution of 1.5 mm were implemented. The k‐space trajectories were measured and then used during image reconstruction. The technique was applied to metabolic imaging of the rat brain in vivo after the injection of hyperpolarized [1‐13C]‐pyruvate. Dynamic imaging afforded the measurement of region‐of‐interest‐specific time courses of pyruvate and its metabolic products, while imaging at high spatial resolution was used to better characterize the spatial distribution of the metabolite signals. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.  相似文献   

4.
Hyperpolarized [1‐13C]‐pyruvate is an exciting new agent for the in vivo study of cellular metabolism and a potential cancer biomarker. The nature of the hyperpolarized signal poses unique challenges because of its short duration and the loss of magnetization with every excitation. In this study, we applied a novel and efficient time‐resolved MR spectroscopic imaging (MRSI) method to investigate in a prostate cancer model the localized temporal dynamics of the uptake of [1‐13C]‐pyruvate and its conversion to metabolic products, specifically [1‐13C]‐lactate. This hyperpolarized 13C method used multiband excitation pulses for efficient use of the magnetization. This study demonstrated that regions of tumor were differentially characterized from normal tissue by the lactate dynamics, where tumors showed later lactate detection and longer lactate duration that was statistically significant (P < 0.001). Compared to late‐pathologic‐stage tumors, early‐ to intermediate‐stage tumors demonstrated significantly (P < 0.01) lower lactate total signal‐to‐noise ratio (SNR), with similar temporal dynamic parameters. Hyperpolarized pyruvate dynamics provided uptake, perfusion, and vascularization information on tumors and normal tissue. Large, heterogeneous tumors demonstrated spatially variable uptake of pyruvate and metabolic conversion that was consistent with cellularity and necrosis identified by histology. The results of this study demonstrated the potential of this new hyperpolarized MR dynamic method for improved cancer detection and characterization. Magn Reson Med 63:582–591, 2010. © 2010 Wiley‐Liss, Inc.  相似文献   

5.
Hyperpolarized 13C MR spectroscopic imaging can detect not only the uptake of the pre‐polarized molecule but also its metabolic products in vivo, thus providing a powerful new method to study cellular metabolism. Imaging the dynamic perfusion and conversion of these metabolites provides additional tissue information but requires methods for efficient hyperpolarization usage and rapid acquisitions. In this work, we have developed a time‐resolved 3D MR spectroscopic imaging method for acquiring hyperpolarized 13C data by combining compressed sensing methods for acceleration and multiband excitation pulses to efficiently use the magnetization. This method achieved a 2 sec temporal resolution with full volumetric coverage of a mouse, and metabolites were observed for up to 60 sec following injection of hyperpolarized [1‐13C]‐pyruvate. The compressed sensing acquisition used random phase encode gradient blips to create a novel random undersampling pattern tailored to dynamic MR spectroscopic imaging with sampling incoherency in four (time, frequency, and two spatial) dimensions. The reconstruction was also tailored to dynamic MR spectroscopic imaging by applying a temporal wavelet sparsifying transform to exploit the inherent temporal sparsity. Customized multiband excitation pulses were designed with a lower flip angle for the [1‐13C]‐pyruvate substrate given its higher concentration than its metabolic products ([1‐13C]‐lactate and [1‐13C]‐alanine), thus using less hyperpolarization per excitation. This approach has enabled the monitoring of perfusion and uptake of the pyruvate, and the conversion dynamics to lactate and alanine throughout a volume with high spatial and temporal resolution. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.  相似文献   

6.
This study describes a technique for fast imaging of x‐nuclei metabolites. Due to increased sensitivity and larger chemical shift dispersion at high magnetic fields, images of multiple metabolites can be obtained simultaneously by selective excitation of their resonances with a multifrequency selective radiofrequency pulse at any desired flip angle. This aim is achieved by combining a three‐dimensional gradient echo imaging sequence with a Shinnar‐LeRoux optimized excitation pulse. A proper choice of bandwidth, imaging matrix size, and field of view allows using the chemical shift dispersion of the different resonances to completely separate their images within one large field of view. The method of fast metabolic imaging is illustrated with 13C measurements of a phantom containing a solution of 13C labeled glucose, lactate, and sodium octanoate and by dynamic measurements of the 31P metabolites phosphocreatine and β‐adenosine triphosphate in human femoral muscle in vivo, both at 7T. With dynamic selective 31P imaging of the larger part of the upper leg, phosphocreatine signal intensity changes of specific muscles can be studied simultaneously by analyzing the sum of phosphocreatine signals within arbitrarily shaped regions of interest following the muscles' contours. This concept of dynamic metabolic imaging can be applied to other organs and further expanded to other MR‐detectable nuclei and metabolites. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.  相似文献   

7.
Phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo imaging was recently introduced, producing high‐resolution isotropic cerebrospinal fluid attenuated brain images without long inversion recovery preparation. Despite the advantages, the weighted‐averaging‐based technique suffers from noise amplification resulting from different levels of cerebrospinal fluid signal modulations over the two acquisitions. The purpose of this work is to develop a signal‐to‐noise ratio‐optimized version of the phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo. Variable refocusing flip angles in the first acquisition are calculated using a three‐step prescribed signal evolution while those in the second acquisition are calculated using a two‐step pseudo‐steady state signal transition with a high flip‐angle pseudo‐steady state at a later portion of the echo train, balancing the levels of cerebrospinal fluid signals in both the acquisitions. Low spatial frequency signals are sampled during the high flip‐angle pseudo‐steady state to further suppress noise. Numerical simulations of the Bloch equations were performed to evaluate signal evolutions of brain tissues along the echo train and optimize imaging parameters. In vivo studies demonstrate that compared with conventional phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo, the proposed optimization yields 74% increase in apparent signal‐to‐noise ratio for gray matter and 32% decrease in imaging time. The proposed method can be a potential alternative to conventional fluid‐attenuated imaging. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.  相似文献   

8.
A radiofrequency (RF) excitation scheme is presented in which flip angle is encoded in the phase of the resulting excitation. This excitation is implemented with nonselective hard pulses, and is used to give flip angle maps over three‐dimensional volumes. This phase‐sensitive B1 mapping excitation can be combined with various acquisition methods such as gradient recalled echo (GRE) and echo‐planar (EP) readouts. Imaging time depends primarily on the readout method, and is roughly equivalent to the imaging time of conventional double‐angle techniques for three‐dimensional acquisition. The phase‐sensitive method allows imaging over a much wider range of flip angles than double‐angle methods. Phantom and in vivo results are presented comparing the phase‐sensitive method with the conventional double‐angle method, demonstrating the ability of the phase‐sensitive method to measure a wider range of flip angles than double‐angle methods. Magn Reson Med 60:889–894, 2008. © 2008 Wiley‐Liss, Inc.  相似文献   

9.
Measurements of the conversion of hyperpolarized [1‐13C]pyruvate into lactate, in the reaction catalyzed by lactate dehydrogenase, have shown promise as a metabolic marker for the presence of disease and response to treatment. However, it is unclear whether this represents net flux of label from pyruvate to lactate or exchange of isotope between metabolites that are close to chemical equilibrium. Using saturation and inversion transfer experiments, we show that there is significant exchange of label between lactate and pyruvate in a murine lymphoma in vivo. The rate constants estimated from the magnetization transfer experiments, at specific points during the time course of label exchange, were similar to those obtained by fitting the changes in peak intensities during the entire exchange time course to a kinetic model for two‐site exchange. These magnetization transfer experiments may therefore provide an alternative and more rapid way of estimating flux between pyruvate and lactate to serial measurements of pyruvate and lactate 13C peak intensities following injection of hyperpolarized [1‐13C]pyruvate. Magn Reson Med 63:872–880, 2010. © 2010 Wiley‐Liss, Inc.  相似文献   

10.
In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look‐Locker (LL) imaging. It is shown that frequency‐selective FS prep pulses can invoke MTC through the exchange between free and motion‐restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T1 measurement bias (up to 20%) when using inversion recovery (IR) LL echo‐planar imaging (EPI) sequences. Furthermore, it is shown that water‐selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T1 measurement without fat artifacts in MR images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.  相似文献   

11.
Spin‐echo‐based pulse sequences are desirable for the application of high‐resolution imaging of trabecular bone but tend to involve high‐power deposition. Increased availability of ultrahigh field scanners has opened new possibilities for imaging with increased signal‐to‐noise ratio (SNR) efficiency, but many pulse sequences that are standard at 1.5 and 3 T exceed specific absorption rate limits at 7 T. A modified, reduced specific absorption rate, three‐dimensional, fast spin‐echo pulse sequence optimized specifically for in vivo trabecular bone imaging at 7 T is introduced. The sequence involves a slab‐selective excitation pulse, low‐power nonselective refocusing pulses, and phase cycling to cancel undesired out‐of‐slab signal. In vivo images of the distal tibia were acquired using the technique at 1.5, 3, and 7 T field strengths, and SNR was found to increase at least linearly using receive coils of identical geometry. Signal dependence on the choice of refocusing flip angles in the echo train was analyzed experimentally and theoretically by combining the signal from hundreds of coherence pathways, and it is shown that a significant specific absorption rate reduction can be achieved with negligible SNR loss. Magn Reson Med 63:719–727, 2010. © 2010 Wiley‐Liss, Inc.  相似文献   

12.
Pyruvate is included in the energy production of the heart muscle and is metabolized into lactate, alanine, and CO(2) in equilibrium with HCO(3) (-). The aim of this study was to evaluate the feasibility of using (13)C hyperpolarization enhanced MRI to monitor pyruvate metabolism in the heart during an ischemic episode. The left circumflex artery of pigs (4 months, male, 29-34 kg) was occluded for 15 or 45 min followed by 2 hr of reperfusion. Pigs were examined by (13)C chemical shift imaging following intravenous injection of 1-(13)C pyruvate. (13)C chemical shift MR imaging was used in order to visualize the local concentrations of the metabolites. After a 15-min occlusion (no infarct) the bicarbonate signal level in the affected area was reduced (25-44%) compared with the normal myocardium. Alanine signal level was normal. After a 45-min occlusion (infarction) the bicarbonate signal was almost absent (0.2-11%) and the alanine signal was reduced (27-51%). Due to image-folding artifacts the data obtained for lactate were inconclusive. These studies demonstrate that cardiac metabolic imaging with hyperpolarized 1-(13)C-pyruvate is feasible. The changes in concentrations of the metabolites within a minute after injection can be detected and metabolic maps constructed.  相似文献   

13.

Purpose

To evaluate the effect of flip angle on volume flow rate measurements obtained with nontriggered phase‐contrast magnetic resonance imaging (MRI) in vivo.

Materials and Methods

We prospectively measured volume flow rates of the bilateral internal carotid artery and the basilar artery with cine and nontriggered phase‐contrast MRI. For nontriggered phase‐contrast imaging, flip angles of 4, 15, 60, and 90° were used for 40 volunteers and of 8, 15, and 30° for 54 volunteers. Lumen boundaries were semiautomatically determined by pulsatility‐based segmentation using cine phase‐contrast MRI. Identical lumen boundaries were used for nontriggered phase‐contrast imaging.

Results

The ratio of volume flow rate obtained with nontriggered phase‐contrast imaging to that obtained with cine phase‐contrast imaging significantly increases with an increase in the flip angle. The mean ratios lie within a relatively narrow range of ±15% with a wide range of flip angles of 8–90°. As the flip angle increases, ghost artifacts become prominent and signal‐to‐noise and contrast‐to‐noise ratios increase.

Conclusion

Flip angles between 8 and 60° are most appropriate for nontriggered phase‐contrast MR measurements in the internal carotid and the basilar artery. J. Magn. Reson. Imaging 2009;29:1218–1223. © 2009 Wiley‐Liss, Inc.  相似文献   

14.
Chemical exchange saturation transfer (CEST) provides a new imaging contrast mechanism sensitive to labile proton exchange. Pulsed‐CEST imaging is better suited to the hardware constraints on clinical imaging systems when compared with traditional continuous wave‐CEST imaging methods. However, designing optimum pulsed‐CEST imaging sequences entails complicated and time‐consuming numerical integrations. In this work, a simplified and computationally efficient technique is provided to optimize the pulsed‐CEST imaging sequence. An analysis was performed of the optimal average irradiation power and the optimal irradiation flip angle as a function of the acquisition parameters and sample properties in both a two‐pool model and a three‐pool model of endogenous amine exchange. Key simulated and experimental results based on a creatine/agar tissue phantom show that ( 1 ) the average irradiation power is a more meaningful sequence metric than is the average irradiation field amplitude, ( 2 ) the optimal average powers for continuous wave and pulsed‐CEST imaging are approximately equal to each other for a relevant range of solute frequency offsets, exchange rates, and concentrations, ( 3 ) an irradiation flip angle of 180° is optimal or near optimal, independent of the other acquisition parameters and the sample properties, and ( 4 ) higher duty cycles yield higher CEST contrast. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.  相似文献   

15.
Spatially resolved images of hyperpolarized 13C substrates and their downstream products provide insight into real‐time metabolic processes occurring in vivo. Recently, hyperpolarized 13C pyruvate has been used to characterize in vivo cardiac metabolism in the rat and pig, but accurate and reproducible measurements remain challenging due to the limited period available for imaging as well as physiological motion. In this article, time‐resolved cardiac‐ and respiratory‐gated images of [1‐13C] pyruvate, [1‐13C] lactate, and 13C bicarbonate in the heart are acquired without the need for a breathhold. The robustness of these free‐breathing measurements is demonstrated using the time‐resolved data to produce a normalized metric of pyruvate dehydrogenase and lactate dehydrogenase activity in the heart. The values obtained are reproducible in a controlled metabolic state. In a 60‐min ischemia/reperfusion model, significant differences in hyperpolarized bicarbonate and lactate, normalized using the left ventricular pyruvate signal, were detected between scans performed at baseline and 45 min after reperfusion. The sequence is anticipated to improve quantitative measurements of cardiac metabolism, leading to feasible validation studies using fewer subjects, and potentially improved diagnosis, serial monitoring, and treatment of cardiac disease in patients. Magn Reson Med 69:1063–1071, 2013. © 2012 Wiley Periodicals, Inc.  相似文献   

16.
Two approaches to high‐resolution SENSE‐encoded magnetic resonance spectroscopic imaging (MRSI) of the human brain at 7 Tesla (T) with whole‐slice coverage are described. Both sequences use high‐bandwidth radiofrequency pulses to reduce chemical shift displacement artifacts, SENSE‐encoding to reduce scan time, and dual‐band water and lipid suppression optimized for 7 T. Simultaneous B0 and transmit B1 mapping was also used for both sequences to optimize field homogeneity using high‐order shimming and determine optimum radiofrequency transmit level, respectively. One sequence (“Hahn‐MRSI”) used reduced flip angle (90°) refocusing pulses for lower radiofrequency power deposition, while the other sequence used adiabatic fast passage refocusing pulses for improved sensitivity and reduced signal dependence on the transmit‐B1 level. In four normal subjects, adiabatic fast passage‐MRSI showed a signal‐to‐noise ratio improvement of 3.2 ± 0.5 compared to Hahn‐MRSI at the same spatial resolution, pulse repetition time, echo time, and SENSE‐acceleration factor. An interleaved two‐slice Hahn‐MRSI sequence is also demonstrated to be experimentally feasible. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.  相似文献   

17.
Conventional T2‐weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid‐attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. The purpose of this work is to develop a phase‐sensitive, dual‐acquisition, single‐slab, three‐dimensional, turbo/fast spin echo imaging, simultaneously achieving both conventional T2‐weighted and fluid‐attenuated inversion recovery–like high‐resolution whole‐brain images in a single pulse sequence, without an apparent increase of imaging time. Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip‐down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF‐suppressed image is then reconstructed by weighted averaging the in‐ and opposed‐phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T2‐weighted imaging and fluid‐attenuated inversion recovery. Magn Reson Med 63:1422–1430, 2010. © 2010 Wiley‐Liss, Inc.  相似文献   

18.
Contrast‐enhanced three‐dimensional T1‐weighted imaging based on magnetization‐prepared rapid‐gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T1‐weighted, black‐blood version of single‐slab three‐dimensional turbo/fast spin echo whole‐brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. For blood suppression, variable refocusing flip angles with flow‐sensitizing gradients are employed. To avoid a signal loss resulting from the flow‐sensitizing scheme, the first refocusing flip angle is forced to 180°. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T1‐weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases. Magn Reson Med 63:553–561, 2010. © 2010 Wiley‐Liss, Inc.  相似文献   

19.
Radiofrequency (RF)‐spoiled gradient‐echo imaging provides a signal intensity close to pure T1 contrast by using spoiler gradients and RF phase cycling to eliminate net transverse magnetization. Generally, spins require many RF excitations to reach a steady‐state magnetization level; therefore, when unsaturated flowing spins enter the imaging slab, they can cause undesirable signal enhancement and generate image artifacts. These artifacts can be reduced by partially saturating an outer slab upstream to drive the longitudinal magnetization close to the steady state, while the partially saturated spins generate no signal until they enter the imaging slab. In this work, magnetization evolution of flowing spins in RF‐spoiled gradient‐echo sequences with and without partial saturation was simulated using the Bloch equations. Next, the simulations were validated by phantom and in vivo experiments. For phantom experiments, a pulsatile flow phantom was used to test partial saturation for a range of flip angles and relaxation times. For in vivo experiments, the technique was used to image the carotid arteries, abdominal aorta, and femoral arteries of normal volunteers. All experiments demonstrated that partial saturation can provide consistent T1 contrast across the slab while reducing inflow artifacts. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.  相似文献   

20.
The selective multiple quantum coherence technique is combined with a read gradient to accelerate the measurement of a specific scalar‐coupled metabolite. The sensitivities of the localization using pure phase encoding and localization with the read gradient are compared in experiments at high magnetic field strength (17.6 T). Multiple spin‐echoes of the selective multiple quantum coherence edited metabolite are acquired using frequency‐selective refocusing of the specified molecule group. The frequency‐selective refocusing does not affect the J‐modulation of a coupled spin system, and the echo time is not limited to a multiple of 1/J to acquire pure in‐phase or antiphase signal. The multiple echoes can be used to accelerate the metabolite imaging experiment or to measure the apparent transverse relaxation T2. A simple phase‐shifting scheme is presented, which enables the suppression of editing artifacts resulting from the multiple spin‐echoes of the water resonance. The experiments are carried out on phantoms, in which lactate and polyunsaturated fatty acids are edited, and in vivo on tumors, in which lactate content and T2 are imaged. The method is of particular interest when a fast and sensitive selective multiple quantum coherence editing is necessary, e.g., for spatial three dimensional experiments. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.  相似文献   

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