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You can light up 'looking' and follow us, then we will similarly push the world radiology information to you. Abstract Magnetic resonance imaging (MRI) plays an important role in abdominal imaging. The high contrast resolution offered by MRI provides better lesion detection and its capacity to provide multiparametric images facilitates lesion characterization more effectively than computed tomography. However, the relatively long acquisition time of MRI often detrimentally affects the image quality and limits its accessibility. Recent developments have addressed these drawbacks. Specifically, multiphasic acquisition of contrast-enhanced MRI, free-breathing dynamic MRI using compressed sensing technique, simultaneous multi-slice acquisition for diffusion-weighted imaging, and breath-hold three-dimensional magnetic resonance cholangiopancreatography are recent notable advances in this field. 자기 공명 영상 (MRI)은 복부 영상에서 중요한 역할을합니다. MRI가 제공하는 고 대비 해상도는 더 나은 병변 탐지를 제공하며 다중 매개 변수 이미지를 제공하는 기능은 병변을 ct보다 효과적으로 설명하는 데 도움이됩니다. 그러나, 비교적 긴 MRI 획득 시간은 종종 이미지 품질에 악영향을 미쳐서 접근성을 제한합니다. 최근의 개발로 이러한 단점이 해결되었습니다. 구체적으로, 최근에는 다상 강화 자기 공명 영상, 압축 감지 자유 호흡 동적 자기 공명 영상, 확산 가중 영상의 다중 레벨 동시 획득 및 호흡 홀드 3 차원 자기 공명 담관 조영술이 주목할만한 발전이다. 磁共振成像（MRI）在腹部成像中占有重要地位。MRI提供的高对比度分辨率提供了更好的病变检测，其提供多参数图像的能力有助于比ct更有效地描述病变。然而，相对较长的MRI采集时间往往会对图像质量产生不利影响，限制了其可及性。最近的发展已经解决了这些缺点。具体来说，多期增强磁共振成像、压缩传感技术的自由呼吸动态磁共振成像、扩散加权成像的多层面同步采集、屏气三维磁共振胆胰管成像是近年来该领域的显著进展。 The next text will only be in English and Chinese. Have a good time! 接下来的文本将只有英中文，祝您愉快！ INTRODUCTION In recent years, magnetic resonance imaging (MRI) has been widely used for abdominal imaging. The enhanced soft tissue contrast of MRI has improved lesion detection in abdominal organs. In addition, its capability of providing multiparametric images has greatly assisted in the characterization of lesions and monitoring of treatment response. The performance of MRI has led to it being perceived as a problem-solving imaging modality. However, compared with computed tomography (CT) or ultrasonography, MRI requires a long scan time, which often limits its clinical applicability. For abdominal MRI, this lengthy acquisition time often induces motion artifacts, which can significantly hinder image quality. Respiratory-triggering or gating have been used for some sequences, but these techniques do not completely eliminate motion artifacts. Furthermore, they are not feasible to use in dynamic contrast-enhanced MRI, due to scan inefficiency. In addition, the longer acquisition time, often causes MRI to fail in capturing the optimal phase during dynamic phase acquisition. In particular, for liver MRI scans using gadoxetic acid, because the arterial window is relatively short, effective capturing of the arterial phase has been a troublesome issue for radiologists . Efforts have been made to accelerate the scanning speed of MRI, including by making improvements to the hardware related to aspects such as gradient slew rate and amplitude; and also to expand the application of various types of parallel imaging and the adoption of other techniques, such as view-sharing down to keyhole imaging. Recently, compressed sensing has been introduced in abdominal MRI to improve scan speed, in combination with parallel imaging. In this review, we present a brief overview of compressed sensing, focusing on its clinical application in abdominal MRI, in addition to other recently implemented strategies aimed at reducing scan time for diffusion-weighted imaging (DWI) and magnetic resonance cholangiopancreatography (MRCP). 近年来，磁共振成像（MRI）已广泛应用于腹部成像。MRI增强的软组织对比度提高了腹部器官病变的检出率。此外，它提供多参数图像的能力大大有助于病变的特征和治疗反应的监测。磁共振成像的性能使得它被认为是一种解决问题的成像方式。然而，与CT或b超相比，MRI需要较长的扫描时间，这往往限制了其临床应用。对于腹部磁共振成像，长时间的采集往往会产生运动伪影，这会严重影响图像质量。呼吸触发或门控已用于一些序列，但这些技术并不能完全消除运动伪影。此外，由于扫描效率低，它们不适用于动态增强MRI。另外，在动态相位采集过程中，采集时间越长，往往导致MRI无法捕捉到最佳相位。特别是，对于使用钆酸的肝脏核磁共振扫描，由于动脉窗口相对较短，有效地捕捉动脉期一直是放射科医生的一个棘手问题。提高了磁共振成像的扫描速度，包括对梯度转换率和振幅等硬件进行了改进；扩大了各种并行成像的应用范围，并采用了其他技术，如视图共享到锁孔成像。近年来，压缩传感技术被引入腹部MRI，与并行成像相结合，提高了扫描速度。本文综述了压缩传感技术，重点介绍了其在腹部MRI中的临床应用，以及近年来为减少弥散加权成像（DWI）和磁共振胰胆管成像（MRCP）的扫描时间而实施的其他策略。 Parallel Imaging and Compressed Sensing to Accelerate Scan Speed in Abdominal MRI Parallel Imaging The long acquisition time of MRI is closely related with k-space sampling. If it were possible to reconstruct images with partially sampled k-space data, it would improve the temporal resolution of MRI. To reduce the scan time, parallel imaging is often applied. Parallel imaging involves simultaneous data acquisition via the multiple receiver coil elements of phase array coils, a feature available on most clinical scanners; this results in data redundancy. Phased array coils acquire data from multiple elements simultaneously, and variation in sensitivity profiles among those elements can then be exploited in image reconstruction. MRI采集时间长与k空间采样密切相关。如果能够利用部分采样的k空间数据重建图像，则可以提高MRI的时间分辨率。为了减少扫描时间，通常采用并行成像。并行成像涉及通过相位阵列线圈的多个接收器线圈元件同时采集数据，这是大多数临床扫描仪可用的功能；这会导致数据冗余。相控阵线圈同时采集多个单元的数据，利用这些单元之间灵敏度分布的变化进行图像重建。 For conventional parallel imaging, the required phase-encoding steps are undersampled on a regular sublattice. In other words, k-space is undersampled by skipping phase-encoding lines in equidistant steps (Fig. 1). This violates the Nyquist criterion and results in aliasing artifacts in a naïve zero-padded reconstruction, and affects image quality and diagnostic performance. Aliasing artifacts are mitigated by parallel imaging reconstruction, which can be done in image (x, y), k-space (kx, ky), or in hybrid (x, ky) domains (Fig. 2A). The methods involved are classified into two categories, depending on where the images are reconstructed and artifacts are corrected: image domain or k-space domain. The most commonly used techniques in the clinical field are sensitivity encoding (SENSE), which performs post-Fourier transformation in the image domain (4) and generalized autocalibrating partially parallel acquisition (GRAPPA), which works in the k-space domain (3). The former uses coil sensitivity maps to disentangle aliased data, while the latter exploits correlations among neighboring k-space lines and coil elements to calculate weights by which non-acquired k-space data are reconstructed. SENSE has the advantage of numerical efficiency, but is prone to a mismatch of reference data and scan data. Thus, SENSE requires compensatory strategies to overcome the possible mismatch. Conversely, GRAPPA usually uses auto-calibration and deals with the mismatch between measured and reference data, but it has a lower numerical efficiency than SENSE. Parallel imaging is well-established for both two-dimensional (2D) and three-dimensional (3D) Cartesian data acquisitions. It can also be extended to non-Cartesian samplings with the underlying concept being to utilize coil sensitivity data or correlations between different data from different coil elements. Indeed, parallel imaging has been shown to improve abdominal MRI through efficient k-space sampling, thereby improving spatial resolution, and reducing acquisition time and sensitivity to motion. 对于传统的并行成像，所需的相位编码步骤在规则的子晶格上欠采样。换言之，k空间通过以等距步骤跳过相位编码线而欠采样（图1）。这违反了Nyquist准则，导致了原始零填充重建中的混叠伪影，影响了图像质量和诊断性能。可在图像（x，y）、k空间（kx，ky）或混合（x，ky）域（图2A）中进行的并行成像重建可减轻混叠伪影。根据重建图像和校正伪影的位置，所涉及的方法分为两类：图像域或k-空间域。临床上最常用的技术是敏感度编码（SENSE），它在图像域中执行傅立叶后变换，在k-空间域中工作的广义自动校准部分并行采集（GRAPPA）。前者利用线圈灵敏度映射来分离混叠数据，后者利用相邻k空间线和线圈元素之间的相关性来计算权值，从而重构未采集的k空间数据。SENSE具有数值效率高的优点，但容易出现参考数据与扫描数据的不匹配。因此，感觉需要补偿策略来克服可能的不匹配。相反，GRAPPA通常使用自动校准来处理测量数据和参考数据之间的不匹配，但它的数值效率比SENSE低。对于二维（2D）和三维（3D）笛卡尔数据采集，并行成像已经得到很好的证实。它还可以扩展到非笛卡尔采样，其基本概念是利用线圈灵敏度数据或来自不同线圈元件的不同数据之间的相关性。事实上，并行成像已经被证明可以通过有效的k空间采样来改善腹部MRI，从而提高空间分辨率，并减少采集时间和对运动的敏感性。 Fig. 1. Different k-space sampling schemes. 图1。不同的k空间采样方案。 Cartesian acquisition with fully sampled k-space (A), uniformly undersampled with acceleration factor of 2 (B), or non-Cartesian acquisition with skipping spokes in radial acquisition (C). Solid and dashed lines refer to acquired and skipped k-space data, respectively. Kx = frequency encoding direction, Ky = phase encoding direction. 全采样k-空间（A）、加速度因子为2（B）的均匀欠采样或径向采样中具有跳跃辐条的非笛卡尔采样（C）。实线和虚线分别指采集的和跳过的k空间数据。Kx=频率编码方向，Ky=相位编码方向。 Fig. 2. Graphical representation of principles of parallel imaging and compressed sensing. 图2。并行成像和压缩传感原理的图形表示。 In parallel imaging (A), uniform subsampling gives typical aliasing artifacts. Parallel imaging reconstruction makes it possible to achieve alias-free image. For SENSE, parallel imaging reconstruction is done in image space, whereas de-aliasing is already done before FFT in generalized autocalibrating partially parallel acquisitions (GRAPPA). In compressed sensing (B), variable-density, pseudo-random subsampling produces incoherent noise-like aliasing artifact after FFT. Sparsity (in this case, wavelet) transform allows setting of sparsity constraints. Image is obtained after IWT into image domain. IFFT back to k-space allows data consistency checking. After several iterations, final image is delivered with optimal balance between data consistency and sparsity constraints. FFT = Fourier transform, IFFT = inverse Fourier transform, IWT = inverse transform, PI = parallel imaging, SENSE = sensitivity encoding, WT = wavelet transform. 在并行成像（A）中，均匀子采样产生典型的混叠伪影。并行成像重建可以实现无混叠图像。在广义自校正部分并行采集（GRAPPA）中，并行成像重建是在图像空间进行的，而去混叠是在FFT之前进行的。在压缩感知（B）中，可变密度的伪随机子采样在FFT后产生非相干噪声，如混叠伪影。稀疏性（在这种情况下，小波）变换允许设置稀疏性约束。图像经IWT后进入图像域。IFFT返回k空间允许数据一致性检查。经过多次迭代，最终图像在数据一致性和稀疏性约束之间达到最佳平衡。FFT=傅里叶变换，IFFT=傅里叶逆变换，IWT=逆变换，PI=并行成像，SENSE=灵敏度编码，WT=小波变换。 However, the acceleration factor, specified by the ratio of fully sampled k-space data to undersampled k-space data, is limited by several factors. A high acceleration factor may cause a decreased signal-to-noise ratio, additional noise amplification quantified by the geometry factor due to the coil setup, and remaining aliasing artifacts. Consequently, it is often challenging to achieve stable and acceptable image quality for acceleration factors larger than 4 in clinical practice. 然而，加速度系数，由完全采样的k空间数据与欠采样的k空间数据的比率指定，受到几个因素的限制。高加速因子可导致信噪比降低，由线圈设置引起的几何因子量化的附加噪声放大，以及剩余的混叠伪影。因此，在临床实践中，当加速度因子大于4时，要获得稳定且可接受的图像质量往往是一个挑战。 Compressed Sensing 压缩传感 For further acceleration of MR acquisition speed, the concept of compressed sensing has been investigated (12); this relies on the premise that a natural image is compressible. Based on our long-standing experience with using picture archiving and communication systems, we can safely assume that medical images are also compressible, without loss of essential information or degradation of image quality. This also implies redundancy in MRI scans. If most acquired data can be discarded without perceptual loss, there is no need to acquire unnecessary data in the first place, and exploiting the redundancy of MRI data would mean that fewer samples should be sufficient for reconstructing images with relevant information, thereby reducing the scan time. In contrast, compression of medical images after acquisition does not change the acquisition time. However, the application of compressed sensing for MRI acquisition has been slow, because it is not clear which parts of the signals contain essential information and which parts are redundant. 为了进一步加快MR采集速度，研究了压缩传感的概念；这依赖于自然图像是可压缩的前提。基于我们使用图像存档和通信系统的长期经验，我们可以安全地假设医学图像也是可压缩的，不会丢失基本信息或降低图像质量。这也意味着在核磁共振扫描中存在冗余。如果大多数采集到的数据可以在没有感知损失的情况下被丢弃，那么首先就不需要获取不必要的数据，而利用MRI数据的冗余意味着应该有较少的样本足以用相关信息重建图像，从而减少扫描时间。相反，采集后医学图像的压缩不会改变采集时间。然而，压缩传感技术在磁共振成像采集中的应用一直比较缓慢，因为目前还不清楚信号的哪些部分含有重要信息，哪些部分是冗余的。 Compressed sensing requires three conditions: sparsity, incoherence, and non-linear reconstruction. Sparsity refers to a condition in which only a small number of coefficients carries the relevant information in images in a suitable transform domain. Strong sparsity, i.e., involving only a few non-vanishing coefficients, is desired to achieve higher compression. As described earlier, MRI is inherently sparse in its transform domain. Even though an image may be sparse, the challenge remains to find the non-vanishing components from any image with a fixed acquisition scheme. The basis of compressed sensing is choosing samplings that have sufficient overlap with any sparse representation in the transform domain. An incoherent and random sampling scheme achieves this requirement, because the associated aliasing artifacts are noise-like, and thus thresholding in the transform domain allows identification of relevant coefficients. In practice, however, pure random sampling is not likely to be feasible, because of hardware or physiological constraints, such as slew rate, eddy currents, and nerve stimulation. Consequently, pseudo-random sampling patterns are often used. Figure 2B illustrates the process of image acquisition using compressed sensing. 压缩感知需要三个条件：稀疏、非相干和非线性重建。稀疏性是指在适当的变换域中，只有少量系数携带图像中的相关信息的情况。为了获得更高的压缩效果，需要具有强稀疏性，即只包含少量的非消失系数。如前所述，MRI在其变换域中本质上是稀疏的。即使一幅图像可能是稀疏的，但在固定的采集方案下，要从任何一幅图像中找到不消失的分量仍然是一个挑战。压缩感知的基础是选择与变换域中任何稀疏表示有足够重叠的采样。非相干和随机采样方案实现了这一要求，因为相关联的混叠伪影类似于噪声，因此在变换域中的阈值允许识别相关系数。然而，在实践中，由于硬件或生理上的限制，例如转换率、涡流和神经刺激，纯随机抽样不太可能是可行的。因此，通常使用伪随机抽样模式。图2B说明了使用压缩传感的图像采集过程。 As with parallel imaging, compressed sensing can be combined with various sequences, including Cartesian and non-Cartesian acquisition schemes. In particular, non-Cartesian sampling schemes have several advantages for compressed sensing. The typical aliasing artifacts from radial or spiral sampling schemes in non-Cartesian sequences are less coherent than those of the regular undersampling used for Cartesian parallel imaging, and therefore comply more naturally with the prerequisites. In addition, central k-space is inherently densely sampled and it contributes to a better signal-to-noise ratio, even when acquisition time is reduced. 与并行成像一样，压缩传感可以与各种序列相结合，包括笛卡尔和非笛卡尔捕获方案。特别是，非笛卡尔采样方案在压缩感知方面有一些优势。在非笛卡尔序列中，径向或螺旋采样方案的典型混叠伪影比笛卡尔并行成像中使用的常规欠采样伪影的相干度低，因此更自然地符合先决条件。此外，中心k空间固有的密集采样，即使在捕获时间减少的情况下，它也有助于提高信噪比。 The final prerequisite for compressed sensing is non-linear reconstruction, which is necessary to determine the sparse representation discussed above. The strategy of image reconstruction provides a balance between data consistency and sparsity, typically through optimizing the following cost function: χ^=argminχy−Aχ22+λΨχ1 压缩感知的最后一个前提是非线性重建，这是确定上述稀疏表示的必要条件。图像重建策略通常通过优化以下成本函数在数据一致性和稀疏性之间提供平衡： χ^=argminχy−Aχ22+λΨχ1 Here, χ̂ is the reconstructed image, y represents the acquired k-space data, and A is the system operator that maps the image χ to the k-space data. The system operator A includes information about the k-space trajectory, the coil sensitivities, and Fourier transformation. Furthermore, Ψ is a sparsifying transformation applied to the image and λ is the regularization factor. The first term ensures data consistency and is identical to the term in a non-regularized SENSE reconstruction. The second term enforces sparsity in the transform domain though the chosen l1 norm. The regularization factor (λ) balances the data consistency and data sparsity (Fig. 3). 这里，x̂是重建图像，y表示获得的k-空间数据，A是将图像x映射到k-空间数据的系统算符。系统算子A包括关于k-空间轨迹、线圈灵敏度和傅里叶变换的信息。此外，Ψ是应用于图像的稀疏变换，λ是正则化因子。第一项确保了数据的一致性，并且与非正则意义重建中的项相同。第二项通过选择l1范数在变换域中增强稀疏性。正则化因子（λ）平衡了数据一致性和数据稀疏性（图3）。 Fig. 3. Effect of regularization parameters. 图3。正则化参数的影响。 Same dataset was reconstructed using no regularization parameter (A), suggested regularization parameter (B), and 10-fold higher regularization parameter than that suggested (C). Images show different imaging textures and signal-to-noise ratios, according to regularization parameters. 使用无正则化参数（A）、建议的正则化参数（B）和比建议的正则化参数（C）高10倍的正则化参数重建同一数据集。根据正则化参数，图像显示不同的成像纹理和信噪比。 Unlike parallel imaging, compressed sensing uses sparsity to reduce the number of required phase-encoding steps, which are independent of the coil setup. However, as shown by the equation above, it can be naturally combined with parallel imaging. Thus, a combination of compressed sensing and parallel imaging tends to accelerate the MRI speed more than does parallel imaging alone (25). In addition, this combination reduces the risk of losing small coefficients (and in turn low-contrast objects) and of temporal or spatial blurring. 与并行成像不同，压缩感知使用稀疏性来减少所需的相位编码步骤，而这些步骤与线圈设置无关。然而，如上面的方程所示，它可以自然地与并行成像结合起来。因此，压缩传感和并行成像相结合比单独并行成像更容易加速MRI速度（25）。此外，这种组合降低了丢失小系数（进而降低对比度对象）和时间或空间模糊的风险。 Clinical Applications Current Issues Related to Obtaining Dynamic 3D T1-Weighted Imaging 动态3D T1加权成像的研究现状 As mentioned above, the extended acquisition time is a limiting factor for abdominal imaging, and this often causes motion artifacts in breath-hold examinations. It is particularly troublesome for dynamic T1-weighted imaging, which is the most important sequence for lesion detection and characterization. For liver MRI using a hepatocyte-specific contrast agent, in particular, transient motion often occurs and results in motion artifacts at critical time points. Furthermore, compared with extracellular contrast media, a shorter arterial time window challenges the acquisition of optimal arterial phase imaging. The incidence of motion artifacts due to limited breath-hold capacity or transient motion is known to be reduced by shortening the acquisition time. For this purpose, multiple arterial phase images are widely performed for liver MRI. Although parallel imaging has improved the temporal resolution, multi-arterial phase often achieves a high temporal resolution at the expense of spatial resolution; and this may negatively affect image quality, and potentially diagnostic performance. For further acceleration of MRI while balancing both types of resolution, other strategies have been suggested including view-sharing down to keyhole techniques . Such strategies have been applied to dynamic contrast-enhanced sequences to acquire an optimal arterial phase for liver MRI. However, it is possible that all phases may be degraded due to motion and that temporal blurring may occur because sampled data are shared throughout the phases. Hence, there has been an ongoing attempt to achieve optimal arterial phase timing without significant motion artifacts. 如上所述，延长的采集时间是腹部成像的一个限制因素，这通常会导致屏气检查中的运动伪影。动态T1加权成像是病灶检测和定性的最重要的序列之一，它的应用尤其麻烦。对于使用肝细胞特异性造影剂的肝脏磁共振成像，特别是瞬时运动经常发生，并在关键时间点产生运动伪影。此外，与细胞外造影剂相比，较短的动脉时间窗对获得最佳动脉相位成像提出了挑战。众所周知，通过缩短捕获时间），可以降低由于屏气能力有限或瞬时运动而产生的运动伪影的发生率。为此，多动脉期图像广泛应用于肝脏MRI。尽管并行成像提高了时间分辨率，但多动脉相位往往以牺牲空间分辨率为代价获得较高的时间分辨率，这可能会对图像质量和潜在的诊断性能产生负面影响。为了进一步加速核磁共振成像，同时平衡这两种类型的分辨率，已经提出了其他策略，包括视图共享到锁孔技术。这些策略已应用于动态增强序列，以获得肝脏MRI的最佳动脉期。然而，由于运动，所有相位都可能退化，并且由于采样数据在各个相位之间共享，可能发生时间模糊。因此，我们一直在尝试在没有明显运动伪影的情况下获得最佳动脉相位。 3D T1-Weighted Images with High Temporal Resolution 高时间分辨率三维T1加权图像 Static Imaging 静态成像 Compressed sensing with parallel imaging acquisition can achieve higher acceleration for static imaging by using sparsity in the spatial domain. Using a combination of compressed sensing and parallel imaging allows acceleration by a factor greater than four, and the acquisition time can be reduced to less than 10 seconds, without significant compromise of image quality (Fig. 4). High temporal resolution of breath-hold sequences can be helpful for patients with limited breath-holding capacity. 利用空间域的稀疏性，压缩传感与并行成像采集可以获得更高的静态成像加速度。使用压缩传感和并行成像的组合允许加速度大于4倍，并且在不显著影响图像质量的情况下，采集时间可以减少到小于10秒（图4）。屏气序列的高时间分辨率有助于限制屏气能力的患者。 Fig. 4. Hepatobiliary phase of gadoxetic acid-enhanced MRI in 69-year-old man. 图4。69岁男性肝胆期gadoxetic增强MRI表现。 A. First image was obtained with parallel imaging alone (SENSE) with acceleration factor of 2.8. B. Next image was acquired using compressed sensing and SENSE with acceleration factor of 7.17. Although both images show comparable image quality and spatial resolution (reconstruction voxel size 0.99 × 0.99 × 3 mm), image acquisition time was 15 seconds in (A) and 6 seconds in (B). A、 第一幅图像是单独使用平行成像（SENSE）获得的，加速度因子为2.8。B、 下一幅图像采用压缩感知和加速度系数为7.17的感知。尽管两幅图像显示出可比较的图像质量和空间分辨率（重建体素大小为0.99×0.99×3mm），但图像采集时间分别为（A）中的15秒和（B）中的6秒。 Dynamic Imaging Contrast-enhanced dynamic sequence is a good application of compressed sensing, because sparsity in the temporal domain can additionally be exploited. Achieving T1-weighted dynamic images with high temporal resolution has been attempted using compressed sensing and either Cartesian or non-Cartesian acquisition. 对比度增强动态序列是压缩感知的一个很好的应用，因为在时域上的稀疏性可以得到进一步的利用。尝试用压缩感知和笛卡尔或非笛卡尔捕获实现高时间分辨率的T1加权动态图像。 Cartesian sampling schemes should follow a pseudorandom, underdamping pattern to utilize compressed sensing . Multiple arterial phase acquisitions can be obtained in a single breath-hold. This is similar to previous studies in which dual or triple arterial phase acquisitions were obtained in a single breath-hold. However, this combination of compressed sensing and parallel imaging allows both higher temporal and spatial resolution than does multi-arterial phase acquisition using parallel imaging only (Fig. 5) ). In addition, it minimizes the concerns about temporal blurring with appropriate reconstruction algorithms and the contamination by motion artifacts as compared to view-sharing techniques (Figs. 6, 7). If high temporal resolution is critical, view-sharing techniques can be combined with compressed sensing and parallel imaging for further acceleration of scan speed . As parallel imaging, a combination of compressed sensing and parallel imaging can be applied to both spectrally adiabatic inversion recovery and the Dixon technique for fat suppression (Figs. 5, 7). 笛卡尔采样方案应遵循伪随机、低阻尼模式，以利用压缩传感）。一次屏气可获得多个动脉期采集。这类似于以往的研究，在一次屏气中获得双动脉或三动脉相位。然而，与仅使用并行成像的多动脉相位采集相比，压缩传感和并行成像的组合允许更高的时间和空间分辨率（图5）。此外，与视图共享技术相比，它通过适当的重建算法和运动伪影污染最小化了对时间模糊的关注（图。6，7）。如果高时间分辨率是关键，则可将视图共享技术与压缩感知和并行成像相结合，以进一步加快扫描速度。作为并行成像，压缩传感和并行成像的结合可应用于光谱绝热反演恢复和Dixon脂肪抑制技术（图。5，7页）。 Fig. 5. Multi-arterial phase of gadoxetic acid-enhanced MRI in 63-year-old man obtained using compressed sensing and parallel imaging. 图5。应用压缩传感和平行成像技术对63岁男性患者进行了多动脉期的钆酸增强磁共振成像。 First (A), second (B), and third (C) arterial phases clearly captured different timings of contrast-enhancement of liver, with sufficient spatial resolution (reconstruction voxel size of 0.98 × 1.41 × 3 mm), without noticeable temporal blurring in single breath-hold. Temporal resolution of each phase was 5.3 seconds. 第一（A）、第二（B）和第三（C）动脉期清楚地捕捉到肝脏造影增强的不同时间，具有足够的空间分辨率（重建体素大小为0.98×1.41×3mm），在单次屏气中没有明显的时间模糊。每个相位的时间分辨率为5.3秒。 Fig. 6. Multi-arterial phase of gadoxetic acid-enhanced MRI in 66-year-old man obtained using view-sharing technique. 图6。应用视图共享技术获得66岁男性患者的多动脉期gadoxetic增强MRI。 All three arterial phases (A–C) show persistent motion artifacts, which decrease image quality. 所有三个动脉期（A-C）都显示持续的运动伪影，从而降低图像质量。 Fig. 7. Multi-arterial phase of gadoxetic acid-enhanced MRI in 88-year-old woman obtained using compressed sensing and parallel imaging. 图7。应用压缩传感和平行成像技术获得88岁妇女的多动脉期伽氧西酸增强MRI。 Even though patient failed to hold her breath during scan, first (A) and second (B) phases were saved because motion artifact was limited to last phase (C). 即使病人在扫描过程中没有屏住呼吸，第一（A）和第二（B）阶段还是被保存了下来，因为运动伪影被限制在最后阶段（C）。 Non-Cartesian acquisition schemes, such as radial or spiral sampling, can be performed for dynamic imaging in combination with compressed sensing and parallel imaging. Stack-of-stars sampling, which uses radial acquisition (Fig. 1C), is the most common acquisition scheme for body imaging. It uses in-plane radial acquisition and through-plane Cartesian acquisition. For the radial trajectory, a golden-angle ordering scheme is beneficial. With a suitable increment between angles of subsequent spokes, e.g. 112.2°, it can be guaranteed that k-space is almost uniformly sampled in almost arbitrary time intervals. In golden-angle radial sparse parallel (GRASP) imaging, a following spoke fills the largest gap between prior spokes, which results in uniform k-space coverage at any time during a scan. Using the technique, high temporal resolution and high spatial resolution can be obtained for dynamic phases. In other words, images can be “retrospectively” reconstructed using subsets of data owing to the uniform coverage. Using GRASP, images with variable temporal resolution can be reconstructed from a single examination (Fig. 8), and the highest temporal resolution has been reported as less than 3 seconds per volume for liver MRI (38). Although images with the highest temporal resolution (less than 3 seconds) are not desired in routine clinical practice, due to substantial artifacts, the achievable temporal resolution of GRASP is encouraging. In addition, the radial acquisition itself is motion-resistant as compared with Cartesian sampling, and data are often acquired under free-breathing conditions. The unique features of GRASP are further discussed in the following subsection. 非笛卡尔采集方案，如径向或螺旋采样，可与压缩传感和并行成像相结合用于动态成像。利用径向采集（图1C）的星堆采样是体成像中最常用的采集方案。它使用平面内径向采集和平面内笛卡尔采集。对于径向轨迹，黄金角排序方案是有益的。如果后续辐条的角度之间有适当的增量（例如112.2°），则可以保证k空间在几乎任意的时间间隔内几乎均匀地采样。在黄金角径向稀疏平行（GRAP）成像中，后续辐条填补了先前辐条之间的最大间隙，从而在扫描期间的任何时间产生均匀的k空间覆盖。利用该技术可以获得动态相位的高时间分辨率和高空间分辨率。换言之，由于均匀覆盖，可以使用数据子集“回顾性”重建图像。使用GRASH，可以从单个检查39）重建具有可变时间分辨率的图像（图8），对于肝脏MRI，最高时间分辨率报告为每体积小于3秒。尽管在常规临床实践中不需要具有最高时间分辨率（小于3秒）的图像，但由于存在大量伪影，抓取的时间分辨率是令人鼓舞的。此外，与笛卡尔采样相比，径向采集本身具有运动阻力，而且数据通常是在自由呼吸条件下采集的。抓取的独特特征将在下面的小节中进一步讨论。 Fig. 8. Flexible temporal resolution of GRASP imaging of liver MRI in 61-year-old man. Images with temporal resolution of 13.3 seconds (A) and 3.3 seconds (B) were retrospectively reconstructed from single free-breathing examination. GRASP = golden-angle radial sparse parallel 图8. 61岁男性肝脏MRI的GRASP成像的灵活时间分辨率。 从一次自由呼吸检查中回顾性地重建了时间分辨率为13.3秒（A）和3.3秒（B）的图像。GRASP =黄金角径向稀疏平行 Free-Breathing T1-Weighted Images with Continuous Data Acquisition 具有连续数据采集功能的自由呼吸的T1加权图像 Although imaging with high temporal resolution can provide sufficient image quality in most patients, it does not eliminate the demand for breath-holding, nor address existing motion artifacts. Free-breathing images provide a potential solution to this issue. Free-breathing T1-weighted imaging can also be achieved using either Cartesian or non-Cartesian acquisitions. 尽管具有高时间分辨率的成像可以为大多数患者提供足够的图像质量，但它并不能消除屏气的需求，也不能解决现有的运动伪影。自由呼吸的图像为该问题提供了潜在的解决方案。也可以使用笛卡尔或非笛卡尔采集来获得自由呼吸的T1加权成像。 Free-breathing Cartesian acquisitions have been reported for dynamic imaging of liver MRI. Variable density undersampling scheme is combined with parallel imaging and compressed sensing using a prototypical sequence (compressed-sensing volumetric interpolated breath-hold examination [CS-VIBE], Siemens Healthineers, Erlangen, Germany), with an 11-second temporal resolution . Because the Cartesian acquisition is more sensitive to motion, motion correction is mandatory for achieving acceptable image quality. However, current respiratory triggering or gating techniques are not suitable for dynamic phase acquisition, due to scan inefficiency. For this sequence, the navigator signal is aligned with a preparation pulse and no temporal penalty is observed. In combination with motion correction, free-breathing Cartesian undersampling provided fewer motion artifacts and better overall image quality than breath-hold Cartesian dynamic images in patients with limited breath-holding capacity or those at high risk of transient motion (Fig. 9) . 据报道，自由呼吸的笛卡尔采集可用于肝脏MRI的动态成像。可变密度欠采样方案与并行成像和压缩传感结合使用原型序列（压缩传感体积插值屏气检查[CS-VIBE]，西门子医疗公司，德国埃尔兰根），具有11秒的时间分辨率。因为笛卡尔采集对运动更敏感，所以为了获得可接受的图像质量，必须进行运动校正。但是，由于扫描效率低下，当前的呼吸触发或门控技术不适用于动态相位采集。对于此序列，导航信号与准备脉冲对齐，并且未观察到时间损失。与运动校正相结合，在屏气能力有限或有短暂运动风险的患者中，自由呼吸的笛卡尔欠采样比屏气笛卡尔动态图像提供更少的运动伪像和更好的整体图像质量（图9）。 Fig. 9. T1-weighted images of gadoxetic acid-enhanced liver MRI in 56-year-old woman with limited breath-holding capacity. 图9.屏气能力有限的56岁女性用牛磺酸增强肝脏MRI的T1加权图像。 Motion artifacts are significantly less in free-breathing, motion-resolved reconstructed images (extra-dimension-VIBE, A) than in subsequent breath-hold 3D GRE transitional phase images (breath-hold VIBE, B). GRE = gradient-echo, VIBE = volumetric interpolated breath-hold examination, 3D = three-dimensional 在自由呼吸，运动分解的重建图像（超维VIBE，A）中，运动伪影明显少于随后的屏气3D GRE过渡阶段图像（屏气VIBE，B）。GRE =梯度回波，VIBE =容积插值屏气检查，3D =三维 Non-Cartesian acquisitions are often less sensitive to motion, which is an advantage for free-breathing imaging. GRASP, describe above, belongs to this category. However, substantial motion also creates motion artifacts in radial acquisitions, and motion correction would be helpful for improving image quality as compared with non-gated images. 非笛卡尔采集通常对运动不太敏感，这对于自由呼吸成像是一个优势。如上所述，GRASP属于此类。但是，大量运动还会在径向采集中产生运动伪影，与非门控图像相比，运动校正将有助于改善图像质量。 For both free-breathing Cartesian and non-Cartesian sampling schemes, motion correction can be performed retrospectively. For Cartesian sampling, an implemented navigator signal can be used. For GRASP, central k-space is sampled continuously and a self-gated signal (Fig. 10) can be extracted without additional navigator signals or respiratory bellows. The simplest method of motion correction is “hard gating” in which an acceptance window is defined and the obtained motion signal is used to determine whether the data would be used or discarded for image reconstruction. “Soft gating” is another option for reducing motion-related image blurring; it incorporates motion state weighting to penalize the motion state inconsistency. Motion-resolved reconstruction is a more advanced option in which motion state data is implemented during the image reconstruction process. Because the reconstruction includes extra dimensions of the motion state, it is referred to as either eXtra-Dimension (XD)-GRASP or XD-VIBE (Fig. 11). Compared with hardgating, XD-VIBE showed better image quality due to further reduction of motion artifacts. 对于自由呼吸的笛卡尔采样方案和非笛卡尔采样方案，都可以追溯地进行运动校正（41）。对于笛卡尔采样，可以使用已实现的导航器信号。对于GRASP，中央k空间是连续采样的，并且可以提取自选通信号（图10），而无需其他导航器信号或呼吸波纹管。最简单的运动校正方法是“硬门控”，其中定义一个接受窗口，并使用获得的运动信号来确定是将数据用于图像重建还是丢弃。“软门控”是用于减少与运动有关的图像模糊的另一种选择；它结合了运动状态权重以惩罚运动状态不一致。解决运动的重建是一种更高级的选项，其中在图像重建过程中实现运动状态数据。因为重建包括运动状态的额外维度，所以将其称为超维度（XD）-GRASP或XD-VIBE（图11）。与强化相比，由于进一步减少了运动伪像，XD-VIBE显示出更好的图像质量。 Fig. 10. Self-gated signals extracted from k-space in GRASP imaging sequence. Regular breathing pattern (A) and irregular breathing pattern (B) are seen. 图10.从GRASP成像序列的k空间提取的自门控信号。 可以看到规律的呼吸模式（A）和规律的呼吸模式（B）。
Fig. 11. Fast fat-saturated T1-weighted imaging acquires imaging data in form of echo trains following fat-suppression pulse. For free-breathing acquisitions, additional GRE with same excitation but with selectable readout direction can be inserted (top). Consequently, head-feet projections for each coil element can be obtained along with imaging data (middle). This can either be used for gated reconstruction that only utilizes specified fraction of data with smallest variation (bottom left) or for extracting gating signal to assign each echo train to motion state, followed by motion-resolved reconstruction (bottom right). FS = fat-suppressed, SI = signal intensity, TR = repetition time. 图11.快速脂肪饱和T1加权成像在抑制脂肪脉冲之后以回波序列的形式获取成像数据。 对于自由呼吸采集，可以插入其他具有相同激励但具有可选读出方向的GRE（顶部）。因此，可以获得每个线圈元件的头部-脚部投影以及成像数据（中间）。这既可以用于仅利用变化最小的指定部分数据的门控重建（左下），也可以用于提取门控信号以将每个回波序列分配给运动状态，然后进行运动解析重建（右下）。FS =抑制脂肪，SI =信号强度，TR =重复时间。 Both free-breathing Cartesian and non-Cartesian sequences are clinically important in several ways. First, they reduce the need for patients to hold their breaths by providing motion robustness. This is presumably beneficial for patients with limited breath-holding capacity, non-cooperative patients, including children, and patients at risk of transient motion after administration of contrast media. Even for patients with sufficient breath-holding capacity, it may enhance comfort by eliminating repeated breath-holding. Second, they provide a better workflow for radiology technicians by allowing continuous data acquisition (Fig. 12). Repeated instructions for optimal respiration would be omitted and arterial phase acquisition using MR fluoroscopy may be unnecessary. Furthermore, they potentially reduce the need for re-examination of patients with motion during the acquisition. Currently, patients are referred for re-examination if motion artifacts cannot be corrected. Using the aforementioned sequences, retrospective motion correction is possible, and it provides clinically acceptable images in patients with motion during the scan. Third, GRASP provides a flexible temporal resolution from a single examination (Fig. 8). It makes it possible to recover a missing “arterial phase” without readministration of contrast media. Furthermore, the high temporal resolution images can capture critical hemodynamic information related to abdominal organs as well as tumors. Thus, current issues relating to dynamic imaging can be mitigated in coming years. 自由呼吸的笛卡尔序列和非笛卡尔序列在临床上都具有多种重要意义。首先，它们通过提供运动鲁棒性来减少患者屏住呼吸的需要。据推测，这对于屏气能力有限的患者，包括儿童在内的非合作患者以及在服用造影剂后有短暂运动风险的患者都是有益的。即使对于具有足够屏气能力的患者，也可以通过消除重复屏气来提高舒适度。其次，它们允许连续的数据采集，从而为放射技术人员提供了更好的工作流程（图12）。将省略关于最佳呼吸的重复说明，并且可能不需要使用MR透视检查获取动脉相。此外，它们潜在地减少了在采集过程中对运动患者进行重新检查的需求。当前，如果无法纠正运动伪影，则将患者转诊接受复查。使用上述序列，可以进行追溯运动校正，并且可以在扫描过程中为运动患者提供临床可接受的图像。第三，GRASP通过一次检查即可提供灵活的时间分辨率（图8）。这样就可以在不重新使用造影剂的情况下恢复丢失的“动脉期”。此外，高时间分辨率图像可以捕获与腹部器官以及肿瘤有关的重要血液动力学信息。因此，与动态成像有关的当前问题可以在未来几年中得到缓解。
Fig. 12. Shift of acquisition scheme in contrast-enhanced abdominal MRI. Current protocol of dynamic sequence (A) includes several pauses and instances of breath-holding. In dynamic sequences using compressed sensing VIBE or GRASP, continuous data acquisition is possible (B) without breath-holding, because images are retrospectively reconstructed including motion correction. CM = contrast medi 图12.腹部对比增强MRI中采集方案的变化。 当前的动态序列协议（A）包括多个暂停和屏气实例。在使用压缩感应VIBE或GRASP的动态序列中，无需屏住呼吸就可以进行连续数据采集（B），因为可以回顾性地重建图像，包括运动校正。CM =造影剂。 3D T1- and T2-Weighted Images with High Spatial Resolution 具有高空间分辨率的3D T1和T2加权图像 Compared with CT or ultrasound, MRI has a lower spatial resolution. High spatial resolution images on MRI have been attempted using various techniques, and they have shown better lesion conspicuity. However, there are inevitable drawbacks of aliasing artifacts and lowering the signal-to-noise ratio by increasing the acceleration factor of parallel imaging. By combining compressed sensing and parallel imaging, we may obtain images with high spatial resolution in an acceptable time frame. The aforementioned combination would reduce unfolding artifacts, which occur in images obtained with parallel imaging. In addition, the signal-to-noise penalty can be also decreased by combining parallel imaging and compressed sensing. By using compressed sensing, parallel imaging, and contrast media, MRI can achieve both high contrast resolution and high spatial resolution, which would be useful for detecting small lesions and anatomic structures (Fig. 13). Because compressed sensing can be combined with other sequences in addition to T1-weighted sequences, T2-weighted imaging can also be acquired with better spatial resolution in an acceptable time frame by using compressed sensing (compressed SENSE, Philips Healthcare, Best, the Netherlands) (Fig. 14). 与CT或超声相比，MRI的空间分辨率较低。尝试使用各种技术在MRI上获得高空间分辨率的图像，它们显示出了更好的病变显着性。然而，通过增加并行成像的加速因子，混叠伪影和降低信噪比存在不可避免的缺点。通过组合压缩传感和并行成像，我们可以在可接受的时间范围内获得具有高空间分辨率的图像。前述组合将减少在通过平行成像获得的图像中出现的展开伪像。另外，还可以通过组合并行成像和压缩传感来降低信噪比。通过使用压缩感测，并行成像和造影剂，MRI可以实现高对比度分辨率和高空间分辨率，这对于检测小病变和解剖结构将很有用（图13）。由于压缩感知可以与T1加权序列之外的其他序列组合，因此通过使用压缩感知（压缩SENSE，Philips Healthcare，Best，荷兰），还可以在可接受的时间范围内以更好的空间分辨率获取T2加权成像。（图14）。
Fig. 13. Hepatobiliary phase of gadoxetic acid-enhanced liver MRI in 51-year-old man. Image obtained with compressed sensing and parallel imaging (A) shows less image noise and better overall image quality than that obtained with parallel imaging only (B). Treated hepatocellular carcinoma (arrowheads) is more visible in image obtained by using both compressed sensing and parallel imaging, than in that obtained with parallel imaging alone. Acquisition time is 15 seconds for both images and spatial resolution is same (reconstruction voxel size: 0.98 × 0.98 × 1.5 mm). 图13.一名51岁男性在服用葡萄糖酸增强肝MRI的肝胆期。 与仅通过并行成像获得的图像（B）相比，通过压缩感测和并行成像获得的图像（A）显示更少的图像噪声和更好的总体图像质量。与单独使用平行成像相比，在通过压缩传感和平行成像获得的图像中，治疗的肝细胞癌（箭头）更为明显。图像的采集时间为15秒，空间分辨率相同（重建体素大小：0.98×0.98×1.5 mm）。
Fig. 14. T2-weighted image of 49-year-old man with hemangiomas. 2D T2-weighted images using compressed sensing and parallel imaging with 4-mm slice thickness and 4-mm gap (A, B) provide better conspicuity of small hemangiomas (arrowheads) than that obtained in 2D T2-weighted images with 8-mm slice thickness and 8-mm gap (C, D). TR/TE were 3240/80 ms (A, B) and 2050/83.6 ms (C, D), respectively. TE = echo time, 2D = two-dimensional. 图14. 49岁血管瘤患者的T2加权图像。 使用压缩感应和4毫米切片厚度和4毫米间隙（A，B）的并行成像的2D T2加权图像比在8毫米切片的2D T2加权图像中获得的小血管瘤（箭头）更明显 厚度和8毫米间隙（C，D）。TR / TE分别为3240/80 ms（A，B）和2050 / 83.6 ms（C，D）。TE =回声时间，2D =二维。 3D MRCP 3D MRCP is one of the key sequences for evaluating the bile duct or pancreatic diseases. It is non-invasive, as compared with endoscopic retrograde cholangiopancreatography, and provides near isotropic volumetric data, as compared with 2D MRCP. Indeed, there have been several reports that 3D MRCP showed better duct visibility or diagnostic performance than 2D MRCP (47, 48, 49). However, the acquisition time is long because respiratory triggering is routinely used to obtain volumetric data of a large field of view. Compressed sensing is an effective strategy for acquiring 3D MRCP because 3D MRCP is sparse in the image domain. In other words, 3D MRCP uses the high contrast of fluid-containing structures, such as the bile duct or pancreatic duct, and any other background signal is suppressed. Thus, it is easily anticipated that image reconstruction can be achieved from a small number of data samples, and therefore, that scan time can be reduced. 3D MRCP是评估胆管或胰腺疾病的关键序列之一。与内窥镜逆行胰胆管造影术相比，它是非侵入性的，与2D MRCP相比，它具有近乎各向同性的体积数据。确实，有几篇报道表明3D MRCP比2D MRCP表现出更好的导管可见性或诊断性能。但是，获取时间很长，因为常规使用呼吸触发来获取大视野的体积数据。压缩感测是获取3D MRCP的有效策略，因为3D MRCP在图像域中比较稀疏。换句话说，3D MRCP使用诸如胆管或胰管之类的含流体结构的高对比度，并且抑制了任何其他背景信号。因此，容易预期可以从少量的数据样本中实现图像重构，因此可以减少扫描时间。 To reduce data sampling, variable-density, random undersampling has been used; this includes a variable-density Poisson disk pattern or a variable-density Gaussian incoherent sampling model . In addition, either GRAPPA or SENSE can be applied to accelerate the scan speed, as well as to aid in the preservation of data consistency and reduction of the reconstruction time. As expected, the combination of compressed sensing and parallel imaging shortened the scan time of 3D MRCP in previous studies. In those studies, applying compressed sensing and parallel imaging reduced acquisition time by 50%. Specifically, respiratory-triggered 3D MRCP can be achieved within 2 or 3 minutes. In addition, overall image quality was not significantly different from that of 3D MRCP using parallel imaging only (Fig. 15). This was an encouraging result in terms of improving MRI workflow and reducing the burden for radiologists, radiology technicians, and patients. 为了减少数据采样，使用了可变密度的随机欠采样。这包括可变密度的Poisson圆盘模式或可变密度的高斯非相干采样模型。此外，可以使用GRAPPA或SENSE来加快扫描速度，并有助于保持数据一致性并减少重建时间。不出所料，在以前的研究中，压缩传感和并行成像的结合缩短了3D MRCP的扫描时间。在那些研究中，应用压缩感测和并行成像将采集时间减少了50％。具体而言，可以在2或3分钟内实现呼吸触发的3D MRCP。此外，总体图像质量与仅使用并行成像的3D MRCP相比没有显着差异（图15）。就改善MRI工作流程和减轻放射科医生，放射技师和患者的负担而言，这是令人鼓舞的结果。
Fig. 15. Respiratory-triggered 3D MRCP in 67-year-old man. Conventional 3D MRCP (A) and compressed sensing 3D MRCP (B) show comparable image quality, with acquisition times of 5 minutes 35 seconds and 2 minutes 4 seconds, respectively. TR/TE was 4172/702 ms for conventional 3D MRCP (A) and 3861/725 ms for compressed sensing 3D MRCP (B). MRCP = magnetic resonance cholangiopancreatography. 图15.一名67岁男子的呼吸触发3D MRCP。 常规3D MRCP（A）和压缩感测3D MRCP（B）表现出可比的图像质量，采集时间分别为5分钟35秒和2分钟4秒。传统3D MRCP（A）的TR / TE为4172/702毫秒，压缩感测3D MRCP（B）的TR / TE为3861/725毫秒。MRCP =磁共振胰胆管造影。 Regardless of these benefits, respiratory-triggered of 3D MRCP still has several issues. A scan time of 2 or 3 minutes is still lengthy as compared with thick-slab 2D MRCP. Moreover, image quality and acquisition time depend heavily on patients' respiratory pattern. In other words, the scan time is often unpredictable and image quality is often unsatisfactory when patients have an irregular breathing rhythm. In those patients, respiratory-triggered 3D MRCP, even using both compressed sensing and parallel imaging, is unable to solve the problem. 不管这些好处如何，呼吸触发的3D MRCP仍然存在几个问题。与厚板2D MRCP相比，2或3分钟的扫描时间仍然很长。此外，图像质量和采集时间在很大程度上取决于患者的呼吸模式。换句话说，当患者的呼吸节律不规则时，扫描时间通常是不可预测的，并且图像质量通常不令人满意。在这些患者中，即使使用压缩感测和并行成像，呼吸触发的3D MRCP也无法解决问题。 Continuous efforts have been made to reduce scan time further. Now 3D MRCP can be acquired with a single breath-hold by exploiting the capability of a combination of compressed sensing and parallel imaging. In these studies, only 4.5–5% of k-space data was sampled, resulting in a more than 10 times faster acquisition speed than that of conventional respiratory-triggered 3D MRCP sequences. The image quality was not significantly inferior to that of conventional respiratory-triggered 3D MRCPand was even better in terms of blurring and motion artifacts. Compared with respiratory-triggered 3D MRCP, breathhold 3D MRCP also reduced the incidence of undiagnostic scans or severe artifacts. This is a remarkable advance in abdominal MRI and compressed sensing implementation in clinical practice. Even though breath-hold 3D MRCP has been attempted since its initial presentation, using techniques and sequences other than compressed sensing, those techniques were not implemented in clinical practice due to the inconsistent and unsatisfactory image quality as compared with conventional respiratory-triggered 3D MRCP. However, compared with prior methods, the new breath-hold 3D MRCP using compressed sensing seems to provide consistent image quality and is useful in patients failing to breathe regularly (Fig. 16). 一直在努力减少扫描时间。现在，通过利用压缩感测和并行成像相结合的功能，可以一次屏住呼吸就可以获取3D MRCP。在这些研究中，仅采样了4.5-5％的k空间数据，其采集速度比传统的呼吸触发3D MRCP序列快10倍以上。图像质量并不明显低于传统的呼吸触发3D MRCP，并且在模糊和运动伪像方面甚至更好。与呼吸触发的3D MRCP相比，屏气3D MRCP还减少了无法诊断的扫描或严重伪影的发生率。在临床实践中，这是腹部MRI和压缩感测实现的显着进步。即使自首次展示以来就一直尝试屏住呼吸的3D MRCP，但使用的不是压缩感测技术，但由于与常规呼吸相比图像质量不一致且不令人满意，这些技术并未在临床实践中实施触发的3D MRCP。但是，与以前的方法相比，使用压缩感应的新型屏气3D MRCP似乎可以提供一致的图像质量，并且对于无法正常呼吸的患者很有用（图16）。
Fig. 16. 3D MRCP using compressed sensing in 68-year-old woman. Conventional respiratory-triggered 3D MRCP using parallel imaging (A) shows substantial motion artifacts due to irregular breathing patterns, whereas breath-hold 3D MRCP using compressed sensing and parallel imaging (B) shows acceptable image quality. TR/TE was 4421/699 ms for conventional respiratory-triggered 3D MRCP (A) and 1700/674 ms for breath-hold 3D MRCP (B). 图16.在68岁女性中使用压缩感测的3D MRCP。 传统的使用并行成像的呼吸触发3D MRCP（A）由于呼吸模式不规则而显示出大量的运动伪影，而使用压缩感测和并行成像的屏气3D MRCP（B）显示出可接受的图像质量。传统的呼吸触发3D MRCP（A）的TR / TE为4421/699 ms，屏气3D MRCP（B）的TR / TE为1700/674 ms。 A recent study has suggested a modified protocol for breath-hold 3D MRCP with high resolution and a small field of view, using a large acceleration factor, oversampling, and saturation band (56). As expected, high-resolution breath-hold 3D MRCP was better than the original breath-hold 3D MRCP in terms of image quality and peripheral bile duct and pancreatic duct visualization, resulting in a better depiction of pancreatic duct abnormality. 最近的一项研究提出了一种改进的协议，该协议使用较大的加速度因子，过采样和饱和带，从而具有高分辨率和小视野的屏气式3D MRCP。不出所料，高分辨率屏气3D MRCP在图像质量以及外周胆管和胰管可视化方面优于原始屏气3D MRCP，从而更好地描绘了胰管异常。 Other Methods for Rapid Abdominal MRI Simultaneous Multi-Slice DWI Although compressed sensing is a promising technique for accelerating the acquisition speed of MRI, its benefit seems to be limited in DWI. This is because the single-shot echo-planar imaging (EPI) scheme that is the most commonly used in DWI is highly effective, and there is insufficient room to achieve sparsity in 2D static images as compared with 3D or dynamic images. Simultaneous multi-slice DWI (SMS-DWI) would be an option for reducing the scan time for DWI. SMS imaging excites several slices simultaneously using a multiband pulse (57, 58) that is carefully designed with consideration of constraints from increased specific absorption rates (59, 60). Slices are separated using information about coil sensitivities from phased array coils, gradients, or radiofrequency encoding. Because phased array coils typically have limited encoding power in transverse abdominal protocols in the slice direction, slice shifting is applied to separate slices and to improve the geometric factor (16, 61). A combination of parallel imaging and SMS can create a synergistic effect for the following reasons. First, parallel imaging improves in-plane resolution and reduces echo train lengths. It can improve image quality by reducing blurring and distortion. SMS works in the slice direction, which significantly reduce scan time. Furthermore, SMS does not suffer from the inherent signal-to-noise ratio loss of undersampling acquisition. 尽管压缩感测是加速MRI采集速度的有前途的技术，但其优势似乎在DWI中受到限制。这是因为在DWI中最常用的单脉冲回波平面成像（EPI）方案非常有效，并且与3D或动态图像相比，没有足够的空间实现2D静态图像的稀疏性。同时多切片DWI（SMS-DWI）将是减少DWI扫描时间的一种选择。SMS成像使用多波段脉冲同时激发几个切片，该多波段脉冲经过精心设计，并考虑到比吸收率增加的限制。使用有关相控阵线圈的线圈灵敏度，梯度或射频编码的信息来分离切片。由于相控阵线圈在横向腹部协议中通常在切片方向上具有有限的编码能力，因此将切片移位应用于单独的切片并改善几何因子。并行成像和SMS的组合可以产生协同作用，原因如下。首先，平行成像可提高平面分辨率并减少回波列的长度。它可以通过减少模糊和失真来提高图像质量。SMS在切片方向上工作，从而大大减少了扫描时间。此外，SMS不会遭受欠采样采集固有的信噪比损失。 It has been reported that SMS-DWI can provide acceptable image quality with a shorter acquisition time than conventional DWI (Fig. 17). For abdominal protocols, SMS acceleration is now limited to a factor of 2 to ensure acceptable image quality, because higher slice accelerations result in noise enhancement, related to increased g-factors, as well as more aliasing artifacts. This needs to be addressed in future. In addition, apparent diffusion coefficients should be compared between SMS and conventional DWI for quantitative measurements. 据报道，SMS-DWI可以以比传统DWI短的采集时间提供可接受的图像质量（图17）。对于腹部方案，SMS加速度现在被限制为2倍，以确保可接受的图像质量，因为更高的切片加速度会导致噪声增强，这与增加的g因子以及更多的混叠伪影有关。将来需要解决此问题。另外，应该在SMS和常规DWI之间比较视在扩散系数，以进行定量测量。
Fig. 17. Respiratory-triggered DWI using b-value of 800 s/mm2 in 64-year-old man. Images of conventional DWI (A) and SMS-DWI (B) show comparable image quality, but scan time was significantly shorter in SMS-DWI than in conventional DWI. TR/TE were 2100/60 ms for conventional DWI (A) and 2200/62 ms for SMS-DWI (B). Field of view (400 × 320 mm2) and matrix (150 × 120) were identical. DWI = diffusion-weighted imaging, SMS = simultaneous multi-slice 图17.呼吸触发的DWI在64岁男性中使用800 s / mm2的b值。 传统DWI（A）和SMS-DWI（B）的图像显示出可比的图像质量，但是SMS-DWI的扫描时间明显短于传统DWI。传统DWI（A）的TR / TE为2100/60 ms，SMS-DWI（B）的TR / TE为2200/62 ms。视场（400×320 mm2）和矩阵（150×120）相同。DWI =扩散加权成像，SMS =同时多层 Gradient and Spin-Echo for 3D MRCP Gradient and spin-echo (GRASE) is a combination of GRASE sequences. It uses a fast spin-echo or turbo spin-echo acquisition scheme and EPI readouts that follow the refocusing pulse of a fast spin-echo sequence. The acquisition speed can be accelerated by both the turbo factor of the turbo spin-echo sequence and the EPI factor of the gradient-echo sequence. Consequently, GRASE provides a lower acquisition time than a comparable turbo spin-echo sequence. In addition, the specific absorption rate is lower in GRASE, by using fewer radiofrequency pulses. Compared with EPI, GRASE has several advantages, including a better signal-to-noise ratio and fewer susceptibility artifacts, owing to multiple refocusing pulses. Even though GRASE was developed in the early 1990s, it has been selectively used for neuroimaging, until being applied for volumetric 3D MRCP . This is because of its inherent limitation compared with turbo spin-echo sequences and a lack of parallel imaging. Although GRASE is less prone to susceptibility artifacts than EPI, it is more susceptible to field inhomogeneity than a turbo spin-echo sequence. In the case of old scanners, it would be challenging to achieve sufficient field homogeneity for 3D MRCP that covers a large field of view, resulting in inconsistent image quality in 3D MRCP using GRASE. Fortunately, recent scanners offer better field homogeneity and parallel imaging is available. Recent papers have reported that 3D MRCP can be achieved in a single breath-hold using a combination of GRASE and parallel imaging and that it provided reduced motion artifacts and better overall image quality than conventional respiratory-triggered 3D MRCP (Fig. 18). Although compressed sensing and parallel imaging techniques provide excellent results, 3D MRCP using GRASE would be an alternative option for scanners where compressed sensing is not available (Fig. 18). 梯度和自旋回波（GRASE）是GRASE序列的组合。它使用快速自旋回波或涡轮自旋回波采集方案和EPI读数，这些读数跟随快速自旋回波序列的重新聚焦脉冲。涡轮自旋回波序列的turbo因子和梯度回波序列的EPI因子均可加快采集速度。因此，与可比的turbo自旋回波序列相比，GRASE提供了更短的采集时间。另外，通过使用较少的射频脉冲，在GRASE中比吸收率较低。与EPI相比，GRASE具有多个优点，包括由于多个重新聚焦脉冲而具有更好的信噪比和更少的磁化伪影。即使GRASE是在1990年代初期开发的，它仍被选择性地用于神经成像，直到被用于体积3D MRCP。这是因为与turbo自旋回波序列相比，其固有的局限性以及缺乏并行成像。尽管与EPI相比，GRASE不易受磁化伪影的影响，但与Turbo自旋回波序列相比，GRASE更容易受到磁场不均匀性的影响。在旧扫描仪的情况下，要实现覆盖大视场的3D MRCP的足够的场均匀性将是一个挑战，导致使用GRASE的3D MRCP的图像质量不一致。幸运的是，最近的扫描仪可提供更好的场均匀性，并且可以使用平行成像。最近的论文报道，结合使用GRASE和并行成像，可以在一次屏气中实现3D MRCP，并且与传统的呼吸触发3D MRCP相比，它可以减少运动伪影并提供更好的整体图像质量（图18）。尽管压缩感测和并行成像技术提供了出色的结果，但对于无法使用压缩感测的扫描仪，使用GRASE的3D MRCP将是替代选择（图18）。
Fig. 18. 3D MRCP in 61-year-old woman with limited breath-holding capability. Breath-hold 3D MRCP using gradient and spin-echo (A) shows comparable image quality to that of breath-hold 3D MRCP using compressed sensing (B), and better image quality than that of respiratory-triggered 3D MRCP (C). 图18.屏气能力有限的61岁女性的3D MRCP。 使用梯度和自旋回波的屏气3D MRCP（A）与使用压缩感应（B）的屏气3D MRCP相比具有可比的图像质量，并且比通过呼吸触发的3D MRCP（C）更好的图像质量。 CONCLUSION Rapid MRI is crucial for abdominal imaging due to the critical limitation with regard to breath-holding. Compressed sensing has improved the temporal resolution and spatial resolution of abdominal MRI, when used in combination with parallel imaging. In addition, it allows continuous data acquisition in free-breathing for dynamic images in combination of other techniques and breath-hold volumetric data acquisition for 3D MRCP. Therefore, the compressed sensing technique is likely to change clinical practice in the coming years. Regarding dynamic images, a combination of compressed sensing and parallel imaging would minimize breath-holding duration or eliminate the need for holding the breath by simultaneously using a motion correction technique. In addition, free-breathing techniques can potentially correct motion artifacts retrospectively in MR images. As mentioned earlier, free-breathing, continuous data acquisition with retrospective motion correction would improve patients' comfort, in addition to reducing the workload of radiologists and radiology technicians, by reducing the necessity of re-examination, and enhancing overall image quality. 由于对屏气的严格限制，快速MRI对于腹部成像至关重要。当与并行成像结合使用时，压缩传感可以改善腹部MRI的时间分辨率和空间分辨率。此外，它结合了其他技术和3D MRCP屏气量数据采集功能，可在动态图像自由呼吸中连续采集数据。因此，压缩感测技术可能会在未来几年改变临床实践。对于动态图像，压缩感测和并行成像的组合将通过同时使用运动校正技术来最小化屏气持续时间或消除屏住呼吸的需要。另外，自由呼吸技术可以潜在地回顾性地校正MR图像中的运动伪像。如前所述，通过回顾性运动校正进行的自由呼吸，连续数据采集，不仅可以减少放射科医师和放射技师的工作量，而且可以减少重新检查的必要性并提高整体图像质量，从而可以提高患者的舒适度。 In available scanners, conventional respiratory-triggered 3D MRCP has already been replaced with compressed sensing respiratory-triggered 3D MRCP that reduces scan time by approximately 50%. This will have a marked clinical impact, especially when breath-hold 3D MRCP is the first sequence performed; the scan time will then be reduced from 5–6 minutes to less than 20 seconds. Furthermore, compressed sensing respiratory-triggered 3D MRCP will be performed only in patients with limited breath-holding capacity. It will substantially improve the current workflow and image quality. Furthermore, SMS-DWI can reduce the scan time for DWI. 在可用的扫描仪中，传统的经呼吸触发的3D MRCP已被压缩感测的经呼吸触发的3D MRCP取代，从而将扫描时间减少了约50％。这将产生显着的临床影响，尤其是在屏住呼吸的3D MRCP是首次执行时；扫描时间将从5–6分钟减少到不到20秒。此外，压缩感测呼吸触发的3D MRCP仅在屏气量有限的患者中执行。它将大大改善当前的工作流程和图像质量。此外，SMS-DWI可以减少DWI的扫描时间。 The major limitation of compressed sensing in clinical practice would be the computational burden, leading a long reconstruction time. Although this can be ameliorated by using a graphics processing unit, advanced reconstruction, including extra dimension reconstruction, is still demanding. We expect that this issue will also be resolved by technical developments, including artificial intelligence for image reconstruction, in the coming years. 在临床实践中，压缩感测的主要局限性是计算量大，导致重建时间长。尽管可以通过使用图形处理单元来改善这一点，但是仍然需要高级的重构，包括额外的尺寸重构。我们希望这个问题也将在未来几年内通过技术发展（包括用于图像重建的人工智能）解决。 In summary, rapid MRI using compressed sensing, parallel imaging, and SMS acquisition have been implemented in clinical examinations. They have contributed to enhancing both the temporal and spatial resolution of abdominal MRI, optimizing the workflow, and improving patients' experience. 总之，在临床检查中已经实现了使用压缩传感，并行成像和SMS采集的快速MRI。它们有助于增强腹部MRI的时间和空间分辨率，优化工作流程并改善患者体验。 |
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