Rapid tissue oxygen tension mapping using 19F inversion-recovery echo-planar imaging of P erfluoro-15 -crown-5-ether

Fluorine-19 inversion-recovery, echo-planar imaging (IR-EPI) wais used in conjunction with a new PFC emulsion, perfluoro-15-crown-5-ether, to map the spatial distribution of oxygen tension in murine liver, spleen and radiation induced fibrosarcoma (RIF-1) tumors. Intravenously administered PFC emulsions were allowed to sequester in the liver, spleen, and tumor 3 to 7 days prior to imaging experiments. Seven, 64 × 64 IR-EPls were acquired with successively increasing inversion times (TI). A nonlinear least-squares regression algorithm was used to fit the seven two-dimensional matrices, on a pixel-by-pixel basis, to solve for the relaxation rate, R₁, of the sequestered PFC. From in vitro calibration curves, the oxygen tension (pO₂,) was calculated from the measured R₁. Oxygen tension maps were then murine liver and spleen were produced (in 2.5 min) to demonstrate the technique and changes in tissue oxygenation as a function of breathing gas (air and carbogen (95% O₂ - 5% CO₂)) are presented. Tissue pO₂ maps from RIF-1 tumors (n = 5) were obtained in less than 10 min and changes in tumor pO₂ were studied when the breathing gals was switched from air to carbogen. The results from tumor pO₂ maps were compared with ¹⁹F MR spectroscopy measurements to check for consistency. Histogram analysis yielded an average liver and spleen pO₂ of 43 torr and 26 torr for RIF-1 tumors when the animals were breathing air. Statistically significant changes in tumor oxygenation as a function of breathing gas were obtained from both pO₂ maps (6 ± 2 torr, P < 0.05) and ¹⁹F MR spectroscopy (13 ± 3 torr, P < 0.01) as evaluated using the Student's paired t test.     

Spectral resolution amelioration by deconvolution (SPREAD) in MR spectroscopic imaging

Purpose

To develop, implement, and evaluate a novel postprocessing method for enhancing the spectral resolution of in vivo MR spectroscopic imaging (MRSI) data.

Materials and Methods

Magnetic field inhomogeneity across the imaging volume was determined by acquiring MRI datasets with two differing echo times. The lineshapes of the MRSI spectra were derived from these field maps by simulating an MRSI scan of a virtual sample whose resonance frequencies varied according to the observed variations in the magnetic field. By deconvolving the lineshapes from the measured MRSI spectra, the linebroadening effects of the field inhomogeneities were reduced significantly.

Results

Both phantom and in vivo proton MRSI spectra exhibited significantly enhanced spectral resolutions and improved spectral lineshapes following application of our method. Quantitative studies on a phantom show that, on average, the full width at half maximum of water peaks was reduced 42%, the full width at tenth maximum was reduced 38%, and the asymmetries of the peaks were reduced 86%.

Conclusion

Our method reduces the linebroadening and lineshape distortions caused by magnetic field inhomogeneities. It substantially improves the spectral resolution and lineshape of MRSI data. J. Magn. Reson. Imaging 2009;29:1395–1405. © 2009 Wiley‐Liss, Inc.     

A multislice single breath-hold scheme for imaging alveolar oxygen tension in humans

Reliable, noninvasive, and high-resolution imaging of alveolar partial pressure of oxygen (p(A)O(2)) is a potentially valuable tool in the early diagnosis of pulmonary diseases. Several techniques have been proposed for regional measurement of p(A)O(2) based on the increased depolarization rate of hyperpolarized (3) He. In this study, we explore one such technique by applying a multislice p(A)O(2) -imaging scheme that uses interleaved-slice ordering to utilize interslice time-delays more efficiently. This approach addresses the low spatial resolution and long breath-hold requirements of earlier techniques, allowing p(A)O(2) measurements to be made over the entire human lung in 10-15 s with a typical resolution of 8.3 × 8.3 × 15.6 mm(3). PO(2) measurements in a glass syringe phantom were in agreement with independent gas analysis within 4.7 ± 4.1% (R = 0.9993). The technique is demonstrated in four human subjects (healthy nonsmoker, healthy former smoker, healthy smoker, and patient with COPD), each imaged six times on 3 different days during a 2-week span. Two independent measurements were performed in each session, consisting of 12 coronal slices. The overall p(A)O(2) mean across all subjects was 95.9 ± 12.2 Torr and correlated well with end-tidal O(2) (R = 0.805, P < 0.0001). The alveolar O(2) uptake rate was consistent with the expected range of 1-2 Torr/s. Repeatable visual features were observed in p(A)O(2) maps over different days, as were characteristic differences among the subjects and gravity-dependent effects.     

Application of a superparamagnetic iron oxide (Resovis®) for MR imaging of human cerebral blood volume

The authors describe the feasibility of dynamic MRI using a novel superparamagnetic iron oxide contrast agent. Resovist® was injected as a bolus at doses of 4, 8, and 16 μmol Fe/kg bodyweight in three consented patients participating in a Phase 2 clinical multicenter trial for hepatic MRI. Dynamic images of the brain were obtained with a conventional FLASH sequence. Results were analyzed by evaluation of dynamic images, cerebral blood volume maps, and normalized signal intensity time curves. Resovist® enabled rapid injections and a dose-dependent strong reduction in gray and white matter signal intensity. The small injection volume and good tolerability may enable Resovist® to serve as a perfusion agent. Dedicated clinical trials are warranted to assess the potentials of Resovist® for perfusion MRI and fMRI.     

Faster dynamic imaging of speech with field inhomogeneity corrected spiral fast low angle shot (FLASH) at 3 T

Purpose

To evaluate the impact of magnetic field inhomogeneity correction on achievable imaging speeds for magnetic resonance imaging (MRI) of articulating oropharyngeal structures during speech and to determine if sufficient acquisition speed is available for visualizing speech structures with real‐time MRI.

Materials and Methods

We designed a spiral fast low angle shot (FLASH) sequence that combines several acquisition techniques with an advanced image reconstruction approach that includes magnetic field inhomogeneity correction. A simulation study was performed to examine the interaction between imaging speed, image quality, number of spiral shots, and field inhomogeneity correction. Six volunteer subjects were scanned to demonstrate adequate visualization of articulating structures during simple speech samples.

Results

The simulation study confirmed that magnetic field inhomogeneity correction improves the available tradeoff between image quality and speed. Our optimized sequence co‐acquires magnetic field maps for image correction and achieves a dynamic imaging rate of 21.4 frames per second, significantly faster than previous studies. Improved visualization of anatomical structures, such as the soft palate, was also seen from the field‐corrected reconstructions in data acquired on volunteer subjects producing simple speech samples.

Conclusion

Adequate temporal resolution of articulating oropharyngeal structures during speech can be obtained by combining outer volume suppression, multishot spiral imaging, and magnetic field corrected image reconstruction. Correcting for the large, dynamic magnetic field variation in the oropharyngeal cavity improves image quality and allows for higher temporal resolution. J. Magn. Reson. Imaging 2010;32:1228–1237. © 2010 Wiley‐Liss, Inc.     

Multislice first-pass myocardial perfusion imaging: Comparison of saturation recovery (SR)-TrueFISP-two-dimensional (2D) and SR-TurboFLASH-2D pulse sequences

PURPOSE: To compare signal-to-noise ratio (SNR), contrast-to-noise (CNR) ratio, and diagnostic accuracy of a newly developed saturation recovery (SR)-TrueFISP-two-dimensional (2D) sequence with an SR-TurboFLASH-2D sequence. MATERIALS AND METHODS: In seven healthy subjects and nine patients with coronary artery disease (CAD), contrast-enhanced perfusion imaging (with Gd-DTPA) was performed with SR-TrueFISP and SR-TurboFLASH sequences. Hypoperfused areas were assessed qualitatively (scale = 0-4). Furthermore, SNR and CNR were calculated and semiquantitative perfusion parameters were determined from signal intensity (SI) time curves. Standard of reference for patient studies was single-photon emission computer tomography (SPECT) and angiography. RESULTS: The perception of perfusion deficits was superior in TrueFISP images (2.6 +/- 1.0) than in TurboFLASH (1.4 +/- 0.6) (P < 0.001). Phantom measurements yielded increased SNR (143 +/- 34%) and CNR (158 +/- 64%) values for TrueFISP. In patient/volunteer studies SNR was 61% to 100% higher and signal enhancement was 110% to 115% higher with TrueFISP than with TurboFLASH. Qualitative and semiquantitative assessment of perfusion defects yielded higher sensitivities for detection of perfusion defects with TrueFISP (68% to 78%) than with TurboFLASH (44% to 59%). CONCLUSION: SR-TrueFISP-2D perfusion imaging provides superior SNR and CNR than TurboFLASH imaging. Moreover, the dynamic range of SIs was found to be higher with TrueFISP, resulting in an increased sensitivity for detection of perfusion defects.     

Quantitative mapping of total choline in healthy human breast using proton echo planar spectroscopic imaging (PEPSI) at 3 Tesla

Purpose:

To quantitatively measure tCho levels in healthy breasts using Proton‐Echo‐Planar‐Spectroscopic‐Imaging (PEPSI).

Materials and Methods:

The two‐dimensional mapping of tCho at 3 Tesla across an entire breast slice using PEPSI and a hybrid spectral quantification method based on LCModel fitting and integration of tCho using the fitted spectrum were developed. This method was validated in 19 healthy females and compared with single voxel spectroscopy (SVS) and with PRESS prelocalized conventional Magnetic Resonance Spectroscopic Imaging (MRSI) using identical voxel size (8 cc) and similar scan times (～7 min).

Results:

A tCho peak with a signal to noise ratio larger than 2 was detected in 10 subjects using both PEPSI and SVS. The average tCho concentration in these subjects was 0.45 ± 0.2 mmol/kg using PEPSI and 0.48 ± 0.3 mmol/kg using SVS. Comparable results were obtained in two subjects using conventional MRSI. High lipid content in the spectra of nine tCho negative subjects was associated with spectral line broadening of more than 26 Hz, which made tCho detection impossible. Conventional MRSI with PRESS prelocalization in glandular tissue in two of these subjects yielded tCho concentrations comparable to PEPSI.

Conclusion:

The detection sensitivity of PEPSI is comparable to SVS and conventional PRESS‐MRSI. PEPSI can be potentially used in the evaluation of tCho in breast cancer. A tCho threshold concentration value of ～0.7 mmol/kg might be used to differentiate between cancerous and healthy (or benign) breast tissues based on this work and previous studies. J. Magn. Reson. Imaging 2012;36:1113–1123. © 2012 Wiley Periodicals, Inc.     

Comparison of a 28-channel receive array coil and quadrature volume coil for morphologic imaging and T2 mapping of knee cartilage at 7T

Purpose:

To compare a new birdcage‐transmit, 28‐channel receive array (28‐Ch) coil and a quadrature volume coil for 7T morphologic MRI and T2 mapping of knee cartilage.

Materials and Methods:

The right knees of 10 healthy subjects were imaged on a 7T whole body magnetic resonance (MR) scanner using both coils. 3D fast low‐angle shot (3D‐FLASH) and multiecho spin‐echo (MESE) sequences were implemented. Cartilage signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), thickness, and T2 values were assessed.

Results:

SNR/CNR was 17%–400% greater for the 28‐Ch compared to the quadrature coil (P ≤ 0.005). Bland–Altman plots show mean differences between measurements of tibial/femoral cartilage thickness and T2 values obtained with each coil to be small (?0.002 ± 0.009 cm / 0.003 ± 0.011 cm) and large (?6.8 ± 6.7 msec/?8.2 ± 9.7 msec), respectively. For the 28‐Ch coil, when parallel imaging with acceleration factors (AF) 2, 3, and 4 was performed SNR retained was: 62%–69%, 51%–55%, and 39%–45%.

Conclusion:

A 28‐Ch knee coil provides increased SNR/CNR for 7T cartilage morphologic imaging and T2 mapping. Coils should be switched with caution during clinical studies because T2 values may differ. The greater SNR of the 28‐Ch coil could be used to perform parallel imaging with AF2 and obtain similar SNR as the quadrature coil. J. Magn. Reson. Imaging 2012;441‐448. © 2011 Wiley Periodicals, Inc.

     

In vivo T(1rho) mapping in cartilage using 3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS)

For T(1rho) quantification, a three-dimensional (3D) acquisition is desired to obtain high-resolution images. Current 3D methods that use steady-state spoiled gradient-echo (SPGR) imaging suffer from high SAR, low signal-to-noise ratio (SNR), and the need for retrospective correction of contaminating T(1) effects. In this study, a novel 3D acquisition scheme-magnetization-prepared angle-modulated partitioned-k-space SPGR snapshots (3D MAPSS)-was developed and used to obtain in vivo T(1rho) maps. Transient signal evolving towards the steady-state were acquired in an interleaved segmented elliptical centric phase encoding order immediately after a T(1rho) magnetization preparation sequence. RF cycling was applied to eliminate the adverse impact of longitudinal relaxation on quantitative accuracy. A variable flip angle train was designed to provide a flat signal response to eliminate the filtering effect in k-space caused by transient signal evolution. Experiments in phantoms agreed well with results from simulation. The T(1rho) values were 42.4 +/- 5.2 ms in overall cartilage of healthy volunteers. The average coefficient-of-variation (CV) of mean T(1rho) values (N = 4) for overall cartilage was 1.6%, with regional CV ranging from 1.7% to 8.7%. The fitting errors using MAPSS were significantly lower (P < 0.05) than those using sequences without RF cycling and variable flip angles.     

低场磁共振CISS序列内耳水成像相关参数的控制

目的:探讨低场磁共振CISS序列内耳水成像相关参数的选择,以提高其临床应用水平。方法:对10例正常成人的10对内耳分别使用常规CISS序列、参数调整后的CISS序列进行配对扫描,之后对图片质量进行评分形成配对数据资料。结果:参数调整后的CISS序列的内耳水成像图像质量明显优于常规CISS序列。结论:在低场磁共振上对CISS序列相关参数的选择优化后,能明显提高内耳水成像图像质量,从而提高其临床应用水平,适合基层医院使用。     

Optimization of parameter values for complex pulse sequences by simulated annealing: Application to 3D MP-RAGE imaging of the brain

A number of pulse sequence techniques, including magnetization-prepared gradient echo (MP-GRE), segmented GRE, and hybrid RARE, employ a relatively large number of variable pulse sequence parameters and acquire the image data during a transient signal evolution. These sequences have recently been proposed and/or used for clinical applications in the brain, spine, liver, and coronary arteries. Thus, the need for a method of deriving optimal pulse sequence parameter values for this class of sequences now exists. Due to the complexity of these sequences, conventional optimization approaches, such as applying differential calculus to signal difference equations, are inadequate. We have developed a general framework for adapting the simulated annealing algorithm to pulse sequence parameter value optimization, and applied this framework to the specific case of optimizing the white matter-gray matter signal difference for a T₁-weighted variable flip angle 3D MP-RAGE sequence. Using our algorithm, the values of 35 sequence parameters, including the magnetization-preparation RF pulse flip angle and delay time, 32 flip angles in the variable flip angle gradient-echo acquisition sequence, and the magnetization recovery time, were derived. Optimized 3D MP-RAGE achieved up to a 130% increase in white matter-gray matter signal difference compared with optimized 3D RF-spoiled FLASH with the same total acquisition time. The simulated annealing approach was effective at deriving optimal parameter values for a specific 3D MP-RAGE imaging objective, and may be useful for other imaging objectives and sequences in this general class.     

Proton echo-planar spectroscopic imaging of J-coupled resonances in human brain at 3 and 4 Tesla.

In this multicenter study, 2D spatial mapping of J-coupled resonances at 3T and 4T was performed using short-TE (15 ms) proton echo-planar spectroscopic imaging (PEPSI). Water-suppressed (WS) data were acquired in 8.5 min with 1-cm(3) spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. Constrained spectral fitting in reference to a non-WS (NWS) scan was performed with LCModel using correction for relaxation attenuation and partial-volume effects. The concentrations of total choline (tCho), creatine + phosphocreatine (Cr+PCr), glutamate (Glu), glutamate + glutamine (Glu+Gln), myo-inositol (Ins), NAA, NAA+NAAG, and two macromolecular resonances at 0.9 and 2.0 ppm were mapped with mean Cramer-Rao lower bounds (CRLBs) between 6% and 18% and approximately 150-cm(3) sensitive volumes. Aspartate, GABA, glutamine (Gln), glutathione (GSH), phosphoethanolamine (PE), and macromolecules (MMs) at 1.2 ppm were also mapped, although with larger mean CRLBs between 30% and 44%. The CRLBs at 4T were 19% lower on average as compared to 3T, consistent with a higher signal-to-noise ratio (SNR) and increased spectral resolution. Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter (GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies.     

磁共振成像定位在低位直肠癌术前放疗大体肿瘤靶区勾画中的应用

目的 探讨磁共振成像（MRI）定位在直肠癌术前放疗中的应用价值。方法 选择经肠镜下病理活检和全身分期检查明确诊断为局部晚期低位直肠癌的患者40例,男性22例,女性18例,年龄31~80岁,中位年龄58岁。先后在相同的体位和固定装置下行CT和MRI定位扫描。由两名直肠癌放疗专业的医生分别在两种定位图像上进行大体肿瘤靶区（GTV）勾画。通过放疗计划系统（TPS）计算出CT和MRI定位图像上勾画的直肠原发肿瘤GTV（GTV_CT、GTV_MRI）的长度、体积及GTV下界距肛缘的距离,并比较这两组数据的差异。结果 肛门指诊提示肿瘤下界距肛缘均≤ 5 cm。GTV_CT的平均长度为（5.21±1.65）cm,长于GTV_MRI的平均长度（4.46±1.51）cm（t=5.059,P<0.05）。GTV_CT的平均体积为（55.71±31.57）cm³,大于GTV_MRI的平均体积（44.02 ±25.11）cm³（t=6.977,P<0.05）。通过肛门指诊判断的肿瘤下界距肛缘的距离为（3.72±0.93）cm,长于GTV_CT下界距肛缘的距离（t=-5.503,P<0.05）,但与GTV_MRI有较高的一致性（P>0.05）。应用CT-MRI融合定位制定调强放疗计划,40例患者中无3、4级放疗不良反应发生。术后pCR率为32.5%。结论 MRI定位勾画出的GTV范围更小,对于肿瘤的下界的确定也更为准确。MRI定位的应用可能会通过提高靶区勾画的精确性,从而提高放射治疗的疗效,减少不良反应的发生率。     

Physiological origin of low‐frequency drift in blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI)

We investigated the biophysical mechanism of low‐frequency drift in blood‐oxygen‐level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) (0.00–0.01 Hz), by exploring its spatial distribution, dependence on imaging parameters, and relationship with task‐induced brain activation. Cardiac and respiratory signals were concurrently recorded during MRI scanning and subsequently removed from MRI data. It was found that the spatial distribution of low‐frequency drifts in human brain followed a tissue‐specific pattern, with greater drift magnitude in the gray matter than in white matter. In gray matter, the dependence of drift magnitudes on TE was similar to that of task‐induced BOLD signal changes, i.e., the absolute drift magnitude reached the maximum when TE approached T whereas relative drift magnitude increased linearly with TE. By systematically varying the flip angle, it was found that drift magnitudes possessed a positive dependence on image intensity. In phantom experiments, the observed drift was not only much smaller than that of human brain, but also showed different dependence on TE and flip angle. In fMRI studies with visual stimulation, a strong positive correlation between drift effects at baseline and task‐induced BOLD signal changes was observed both across subjects and across activated pixels within individual participants. We further demonstrated that intrinsic, physiological drift effects are a major component of the spontaneous fluctuations of BOLD fMRI signal within the frequency range of 0.0–0.1 Hz. Our study supports brain physiology, as opposed to scanner instabilities or cardiac/respiratory pulsations, as the main source of low‐frequency drifts in BOLD fMRI. Magn Reson Med 61, 2009. © 2009 Wiley‐Liss, Inc.     

High-resolution DTI of a localized volume using 3D single-shot diffusion-weighted STimulated echo-planar imaging (3D ss-DWSTEPI).

Diffusion tensor MRI (DTI) using conventional single-shot (SS) 2D diffusion-weighted (DW)-EPI is subject to severe susceptibility artifacts. Multishot DW imaging (DWI) techniques can reduce these distortions, but they generally suffer from artifacts caused by motion-induced phase errors. Parallel imaging can also reduce the distortions if the sensitivity profiles of the receiver coils allow a sufficiently high reduction factor for the desired field of view (FOV). A novel 3D DTI technique, termed 3D single-shot STimulated EPI (3D ss-STEPI), was developed to acquire high-resolution DW images of a localized region. The new technique completes k-space acquisition of a limited 3D volume after a single diffusion preparation. Because the DW magnetization is stored in the longitudinal direction until readout, it undergoes T(1) rather than T(2) decay. Inner volume imaging (IVI) is used to limit the imaging volume. This reduces the time required for EPI readout of each complete k(x)-k(y) plane, and hence reduces T(2)(*) decay during the readout and T(1) decay between the readout of each k(z). 3D ss-STEPI images appear to be free of severe susceptibility and motion artifacts. 3D ss-STEPI allows high-resolution DTI of limited volumes of interest, such as localized brain regions, cervical spinal cord, optic nerve, and other extracranial organs.     

低场磁共振FLAIR序列对脑脊液内钆剂浓度检出的实验研究

 目的 探讨低场磁共振液体衰减反转恢复序列T2WI对人工脑脊液(cerebral spinal fluid,CSF)内钆浓度的识别水平,为临床应用提供依据.方法 采用浓度0～8 mmol/L的Gd-DTPA人工CSF溶液模拟CSF内不同的强化水平,新鲜熟蛋清模拟正常脑实质.使用0.35T机型,对样本分别行SE序列T1WI和快速FLAIR序列T2WI扫描,测算出各被检样本与参照样本间的对比噪声比(contrast-to-noise ratios,CNR),比较两种成像序列所能检出的最低钆剂浓度.结果 FLAIR序列T2WI所能识别的最低钆剂浓度为0.0078 mmol/L,比SE序列T1WI约低10倍;当样本内钆剂浓度超过0. 8 mmol/L时,FLAIR序列T2WI的检出能力不及SE序列T1WI.结论 低场磁共振FLAIR序列T2WI对人工CSF内低浓度钆剂的检出能力明显优于其SE序列T1WI.