Fast proton spectroscopic imaging with high signal-to-noise ratio: spectroscopic RARE.

A new fast spectroscopic imaging (SI) method is presented which is based on spatial localization by the fast MRI method of rapid acquisition with relaxation enhancement (RARE) and encoding of the chemical shift information by shifting the position of a refocusing 180 pulse in a series of measurements. This method is termed spectroscopic RARE. In contrast to spectroscopic ultrafast low-angle RARE (U-FLARE), the formation of two echo families (odd and even) is suppressed by using a train of 180 RF pulses with an internal four-step phase cycle. By this means a high signal-to-noise ratio (SNR) per unit measurement time is obtained, because the separation of odd and even echoes, as well as dummy echoes to stabilize the echo amplitudes, is not needed anymore. The method is of particular interest for detecting signals of coupled spins, as effective homonuclear decoupling can be achieved by use of constant evolution time chemical shift encoding. The pulse sequence was implemented on a 4.7 T imaging system, tested on phantoms, and applied to the healthy rat brain in vivo. Spectroscopic RARE is particularly useful if T2* double less-than sign T2, which is typically fulfilled for in vivo proton SI measurements at high magnetic field strength.     

Signal-to-noise ratio and signal-to-noise efficiency in SMASH imaging.

A general theory of signal-to-noise ratio (SNR) in simultaneous acquisition of spatial harmonics (SMASH) imaging is presented, and the predictions of the theory are verified in imaging experiments and in numerical simulations. In a SMASH image, multiple lines of k-space are generated simultaneously through combinations of magnetic resonance signals in a radiofrequency coil array. Here, effects of noise correlations between array elements as well as new correlations introduced by the SMASH reconstruction procedure are assessed. SNR and SNR efficiency in SMASH images are compared with results using traditional array combination strategies. Under optimized conditions, SMASH achieves the same average SNR efficiency as ideal pixel-by-pixel array combinations, while allowing imaging to proceed at otherwise unattainable speeds. The k-space nature of SMASH reconstructions can lead to oscillatory spatial variations in noise standard deviation, which can produce local enhancements of SNR in particular regions.     

Precision, signal-to-noise ratio, and dose optimization of magnitude and phase arterial input functions in dynamic susceptibility contrast MRI

PURPOSE: To determine optimal conditions for precise measurement of arterial input function (AIFs) in dynamic susceptibility contrast (DSC) perfusion MRI. MATERIALS AND METHODS: Magnitude-based (DeltaR(2)*) and phase-based (Deltaphi) AIFs were numerically simulated for several doses and baseline MRI noise levels [SNR(I(0))]. Random noise (1000 realizations) was added to real/imaginary MRI signals (derived from an internal carotid AIF), and AIF signal, noise, and signal-to-noise ratio (SNR) were determined. The optimal dose was defined as the dose that maximizes mean AIF SNR over the first-pass (SNR(mean)), rather than SNR at the AIF peak (SNR(peak)) because, compared to SNR(peak), doses predicted by SNR(mean) reduced the AIF-induced variability in cerebral blood flow (CBF) by 24% to 40%. RESULTS: The AIF SNR is most influenced by choice of AIF signal, then optimal dosing, each with little penalty. Compared to DeltaR(2)*, Deltaphi signal has 4 to 80 times the SNR over all doses and time points, and approximately 10-fold SNR(mean) at respective optimal doses. Optimal doses induce 85% to 90% signal drop for the DeltaR(2)* method, and 70% to 75% for Deltaphi, with two-fold dose errors causing approximately 1.7-fold loss in SNR(mean). Increases in SNR(I(0)) proportionally increase AIF SNR, but at a cost. CONCLUSION: AIF SNR is affected most by signal type, then dosing, and lastly, SNR(I(0)).     

Optimizing the intrinsic signal-to-noise ratio of MRI strip detectors.

An MRI detector is formed from a conducting strip separated by a dielectric substrate from a ground plane, and tuned to a quarter-wavelength. By distributing discrete tuning elements along the strip, the geometric design may be adjusted to optimize the signal-to-noise ratio (SNR) for a given application. Here a numerical electromagnetic (EM) method of moments (MoM) is applied to determine the length, width, substrate thickness, dielectric constant, and number of tuning elements that yield the best intrinsic SNR (ISNR) of the strip detector at 1.5 Tesla. The central question of how strip performance compares with that of a conventional optimized loop coil is also addressed. The numerical method is validated against the known ISNR performance of loop coils, and its ability to predict the tuning capacitances and performance of seven experimental strip detectors of varying length, width, substrate thickness, and dielectric constant. We find that strip detectors with low-dielectric constant, moderately thin-substrate, and length about 1.3 (+/-0.2) times the depth of interest perform best. The ISNR of strips is comparable to that of loops (i.e., higher close to the detector but lower at depth). The SNR improves with two inherently-decoupled strips, whose sensitivity profile is well-suited to parallel MRI. The findings are summarized as design "rules of thumb."     

MRI扫描参数与信噪比关系的实验研究

目的　对MRI信噪比影响因素进行实验研究 ,验证信噪比与部分扫描参数的关系。方法　利用Magphan体模在 1 5T高场和 0 15T低场MR设备上进行实验 ,利用体模均匀层图像测量信噪比。改变的扫描参数有激励次数 (NEX)、矩阵和层厚。使用的序列有自旋回波 (SE)、快速自旋回波(FSE)和梯度回波 (GRE)序列。结果　 (1)NEX :得到NEX分别为 2、3、4的信噪比与NEX为 1的信噪比的比值。高场设备A结果为 1 13、0 99、0 99(10mm层厚 ) ;高场设备B的结果为 1 44、1 68、2 0 2(6mm层厚 )和 1 42、1 75、1 93 (3mm层厚 ) ;低场设备C结果为 1 5 1、2 0 3 (NEX分别为 2和 4)。 (2 )矩阵 (保持扫描野不变 ) :得到设备A矩阵为 192× 2 5 6、5 12× 5 12的信噪比与 2 5 6× 2 5 6的信噪比的比值。SE结果为 10 5 %和 45 % ;FSE结果为 96%和 5 4% ;GRE(T1)结果为 111%和 71% ;GRE(T2 )结果为 110 %和 63 %。 (3 )层厚 :得到设备A层厚为 2、6mm的信噪比与 10mm信噪比的比值。SE结果为 41%、10 5 % ;FSE结果为 43 %、69% ;GRE(T1)结果为 43 %、80 % ;GRE(T2 )结果为 3 3 %、75 %。结论　 (1)对于高场设备 ,如无特殊要求NEX应≤ 2 ,低场设备增加NEX对提高信噪比有效。 (2 )矩阵对信噪比的影响比理论值小 ,矩阵的选择应考虑信噪比     

眼眶磁共振扫描blade技术和常规技术对信噪比的影响

目的通过常规技术和Blade技术检查眼眶疾病患者图像信噪比,探讨Blade技术对伪影发生、影像质量的改善情况。方法选取2012-07至2013-06行眼眶磁共振扫描的患者,并且行常规眼眶Tse T2WI序列扫描时显现不同程度的不自主运动伪影,最终纳入136例。采用德国西门子公司Trio Tim3．0T超导磁共振成像系统,以在相同位置对所有患者先后进行常规磁共振扫描和Blade技术扫描,并以常规磁共振扫描图像的图像质量分级,将136例分为轻度、中度和重度伪影组,获取不同组两种扫描图像的信噪比,从而评价Blade技术对图像质量的改善情况。结果轻度伪影组眼球区（t=4．159,P〈0．05）和骨膜外区（t=-4．306,P〈0．05）Blade技术信噪比明显高于常规技术;中度伪影组各眼眶分区内Blade技术信噪比与常规技术比较无统计学差异;重度伪影组眼球区（t=-4．917,P〈0．05）、视神经鞘区（Z=-4．687,P〈0．05）和骨膜外区（Z=-3．431,P〈0．05）Blade技术信噪比明显高于常规技术。结论Blade技术可提高眼眶扫描中图像的信噪比,且在同一伪影程度组中不同眼眶分区的信噪比差异有区别。     

Reduction of spectral ghost artifacts in high-resolution echo-planar spectroscopic imaging of water and fat resonances.

Echo-planar spectroscopic imaging (EPSI) can be used for fast spectroscopic imaging of water and fat resonances at high resolution to improve structural and functional imaging. Because of the use of oscillating gradients during the free induction decay (FID), spectra obtained with EPSI are often degraded by Nyquist ghost artifacts arising from the inconsistency between the odd and even echoes. The presence of the spectral ghost lines causes errors in the evaluation of the true spectral lines, and this degrades images derived from high-resolution EPSI data. A technique is described for reducing the spectral ghost artifacts in EPSI of water and fat resonances, using echo shift and zero-order phase corrections. These corrections are applied during the data postprocessing. This technique is demonstrated with EPSI data acquired from human brains and breasts at 1.5 Tesla and from a water phantom at 4.7 Tesla. Experimental results indicate that the present approach significantly reduces the intensities of spectral ghosts. This technique is most useful in conjunction with high-resolution EPSI of water and fat resonances, but is less applicable to EPSI of metabolites due to the complexity of the spectra.     

Trial of signal-to-noise ratio adjustment by repetition time in magnetic resonance imaging

The purpose of this study was to find an equation between signal-to-noise ratio (SNR) and repetition time (TR) in order to adjust SNR by changing TR in magnetic resonance imaging (MRI) examinations. Using a phantom for SNR measurement, according to NEMA MS 1-1988, measurement of SNR was performed by spin-echo pulse sequences for various TR values. An equation of SNR and scanning parameters including TR were obtained from these results. In order to determine the range of TR where the images showed contrast suitable for diagnosis, the contrast-to-noise ratio (CNR) was measured for various TR values. CNR measurement was performed by scanning a brain phantom, and CNR was defined as the contrast of white matter and gray matter divided by noise. Scanning of a resolution phantom was carried out with various scanning parameters, and the usefulness of the equation obtained was determined by whether or not pins in the phantom were visible. The reliability of the equation was confirmed from this verification. Results showed that TR can be used for the adjustment of SNR using the equation obtained in this study.     

Measurement of visceral fat/subcutaneous fat ratio by 0.3 tesla MRI

PURPOSE: Visceral fat-type obesity is known to be closely related to hyperlipidemia and diabetes. The visceral fat area/subcutaneous fat area ratio is used for the diagnosis of visceral fat-type obesity. In this study, we measured the visceral and subcutaneous fat areas in the fat images obtained using 0.3 Tesla open-type MRI, and investigated their usefulness. MATERIALS AND METHODS: A short TR was set to shorten the acquisition time, and in-phase and out-of-phase images were acquired during holding of breath. The visceral and subcutaneous fat areas were automatically measured from the fat image using a workstation. The measurements were compared with the visceral and subcutaneous fat areas measured by CT as the gold standard. RESULTS: No major differences were observed in the fat areas measured by MRI and CT. This method was capable of imaging during holding of breath, and clearly imaged visceral and subcutaneous fat. CONCLUSION: CT is not free from the concern of radiation exposure, whereas MRI is free from radiation. For measurement of the visceral and subcutaneous fat areas, 0.3 Tesla MRI was useful.     

Improved fat water separation with water selective inversion pulse for inversion recovery imaging in cardiac MRI

Purpose:

To develop an improved chemical shift‐based water‐fat separation sequence using a water‐selective inversion pulse for inversion recovery 3D contrast‐enhanced cardiac magnetic resonance imaging (MRI).

Materials and Methods:

In inversion recovery sequences the fat signal is substantially reduced due to the application of a nonselective inversion pulse. Therefore, for simultaneous visualization of water, fat, and myocardial enhancement in inversion recovery‐based sequences such as late gadolinium enhancement imaging, two separate scans are used. To overcome this, the nonselective inversion pulse is replaced with a water‐selective inversion pulse. Imaging was performed in phantoms, nine healthy subjects, and nine patients with suspected arrhythmogenic right ventricular cardiomyopathy plus one patient for tumor/mass imaging. In patients, images with conventional turbo‐spin echo (TSE) with and without fat saturation were acquired prior to contrast injection for fat assessment. Subjective image scores (1 = poor, 4 = excellent) were used for image assessment.

Results:

Phantom experiments showed a fat signal‐to‐noise ratio (SNR) increase between 1.7 to 5.9 times for inversion times of 150 and 300 msec, respectively. The water‐selective inversion pulse retains the fat signal in contrast‐enhanced cardiac MR, allowing improved visualization of fat in the water‐fat separated images of healthy subjects with a score of 3.7 ± 0.6. Patient images acquired with the proposed sequence were scored higher when compared with a TSE sequence (3.5 ± 0.7 vs. 2.2 ± 0.5, P < 0.05).

Conclusion:

The water‐selective inversion pulse retains the fat signal in inversion recovery‐based contrast‐enhanced cardiac MR, allowing simultaneous visualization of water and fat. J. Magn. Reson. Imaging 2013;37:484–490. © 2012 Wiley Periodicals, Inc.     

Rapid fat/water assessment in knee bone marrow with inner-volume RARE spectroscopic imaging.

An inner-volume modification of a spectroscopic imaging technique based on RARE (rapid acquisition with relaxation enhancement) sequences is shown to provide, in less than 1 minute, spectra suitable for fat/water peak-area integration from selected imaging columns containing more than 1,000 5 x 5 x 1-mm voxels. A limitation is the unavoidable T2 weighting of the spectral peaks, which influences estimations of true fat/water composition from the spectra. Studies obtained in the knees of healthy volunteers demonstrate the ability of the technique to enable characterization of the two major proton peaks in bone marrow. Data collected at two or more effective TEs are suitable for extrapolation of fat/water estimates to zero TE.     

Incremental flip angle snapshot FLASH MRI of hepatic lesions: improvement of signal-to-noise and contrast.

Incremental flip angle (IFA) snapshot fast low angle shot (FLASH) is a new modification of inversion recovery snapshot FLASH MR imaging. The method changes the flip angle incrementally from low to high during data acquisition and was applied in the evaluation of 16 focal hepatic lesions in 10 patients. Sequence comparisons were performed with a fixed flip angle inversion recovery snapshot FLASH sequence (standard), a T1- and T2-weighted spin-echo (SE) sequence, and a T1-weighted breath-hold FLASH sequence. Whereas snapshot FLASH images in both pulse sequences were free from physiological motion artifacts, SE and FLASH images showed respiratory artifacts in some patients. Quantitative analysis of IFA snapshot FLASH images at low hepatic and low lesion signal revealed both superior lesion-liver signal-difference-to-noise ratio (SD/N) and superior contrast compared with standard snapshot FLASH without additional artifacts. Unless motion artifacts were evident, SE and FLASH images showed a higher anatomic resolution but lower SD/N and lower contrast than IFA snapshot images. Because of its superior SD/N and contrast, IFA snapshot FLASH will likely widen the application of fast MR imaging techniques.     

信噪比在MR图像质量控制中的作用

优质的MR图像能准确地显示介剖结构和病理情况，为临床提供足够的诊断信息。优化信噪比(signal noise ratio，SNR)是获得满意MR图像的基础。而如何提高图像信噪比就成了MR图像质量控制的主要内容我们通过调试影响信噪比的五个主要扫描参数，来观察研究它们各自对信噪比的影响程度，以及对MR图像质量产生的效果。     

Pelvic imaging with phased-array coils: quantitative assessment of signal-to-noise ratio improvement.

The signal-to-noise ratios (S/Ns) of two different pelvic magnetic resonance (MR) imaging phased arrays were compared with that of the body coil. Each array consisted of two coils placed anteriorly and two posteriorly, oriented transversely in one array and longitudinally in the other. S/N measurements were obtained in an adjustable water-filled phantom that stimulated the shape and radio-frequency loading effects of various-size patients. Depending on the simulated anterior-posterior thickness of the patient, the S/N produced by the longitudinal array ranged from 2.3 to 3.1 times higher than that of the body coil. The S/N of the transverse array was 3.1 to 3.4 times higher. The increased coil sensitivity permits imaging with shorter acquisition times, smaller fields of view, finer resolution, and/or thinner sections. Two examples in patients demonstrate the enhanced imaging capability of the phased arrays.     

Dixon techniques for water and fat imaging

In 1984, Dixon published a first paper on a simple spectroscopic imaging technique for water and fat separation. The technique acquires two separate images with a modified spin echo pulse sequence. One is a conventional spin echo image with water and fat signals in-phase and the other is acquired with the readout gradient slightly shifted so that the water and fat signals are 180 degrees out-of-phase. Dixon showed that from these two images, a water-only image and a fat-only image can be generated. The water-only image by the Dixon's technique can serve the purpose of fat suppression, an important and widely used imaging option for clinical MRI. Additionally, the availability of both the water-only and fat-only images allows direct image-based water and fat quantitation. These applications, as well as the potential that the technique can be made highly insensitive to magnetic field inhomogeneity, have generated substantial research interests and efforts from many investigators. As a result, significant improvement to the original technique has been made in the last 2 decades. The following article reviews the underlying physical principles and describes some major technical aspects in the development of these Dixon techniques.     

Parallel imaging compressed sensing for accelerated imaging and improved signal-to-noise ratio in MRI-based postimplant dosimetry of prostate brachytherapy

PurposeTo investigate the feasibility of using parallel imaging compressed sensing (PICS) to reduce scan time and improve signal-to-noise ratio (SNR) in MRI-based postimplant dosimetry of prostate brachytherapy.Methods and MaterialsTen patients underwent low-dose-rate prostate brachytherapy with radioactive seeds stranded with positive magnetic resonance-signal seed markers and were scanned on a Siemens 1.5T Aera. MRI comprised a fully balanced steady-state free precession sequence with two 18-channel external pelvic array coils with and without a rigid two-channel endorectal coil. The fully sampled data sets were retrospectively subsampled with increasing acceleration factors and reconstructed with parallel imaging and compressed sensing algorithms. The images were assessed in a blinded reader study by board-certified care providers. Rating scores were compared for statistically significant differences between reconstruction types.ResultsImages reconstructed from subsampling up to an acceleration factor of 4 with PICS demonstrated consistently sufficient quality for dosimetry with no apparent loss of SNR, anatomy depiction, or seed/marker conspicuity when compared to the fully sampled images. Images obtained with acceleration factors of 5 or 6 revealed reduced spatial resolution and seed marker contrast. Nevertheless, the reader study revealed that images obtained with an acceleration factor of up to 5 and reconstructed with PICS were adequate-to-good for postimplant dosimetry.ConclusionsCombined parallel imaging and compressed sensing can substantially reduce scan time in fully balanced steady-state free precession imaging of the prostate while maintaining adequate-to-good image quality for postimplant dosimetry. The saved scan time can be used for multiple signal averages and improved SNR, potentially obviating the need for an endorectal coil in MRI-based postimplant dosimetry.