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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）在腹部成像中佔有重要地位。MRI提供的高對比度解析度提供了更好的病變檢測，其提供多參數圖像的能力有助於比ct更有效地描述病變。然而，相對較長的MRI採集時間往往會對圖像質量產生不利影響，限制了其可及性。最近的發展已經解決了這些缺點。具體來說，多期增強磁共振成像、壓縮傳感技術的自由呼吸動態磁共振成像、擴散加權成像的多層面同步採集、屏氣三維磁共振膽胰管成像是近年來該領域的顯著進展。
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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).
近年來，磁共振成像（MRI）已廣泛應用於腹部成像。MRI增強的軟組織對比度提高了腹部器官病變的檢出率。此外，它提供多參數圖像的能力大大有助於病變的特徵和治療反應的監測。磁共振成像的性能使得它被認為是一種解決問題的成像方式。然而，與CT或b超相比，MRI需要較長的掃描時間，這往往限制了其臨床應用。對於腹部磁共振成像，長時間的採集往往會產生運動偽影，這會嚴重影響圖像質量。呼吸觸發或門控已用於一些序列，但這些技術並不能完全消除運動偽影。此外，由於掃描效率低，它們不適用於動態增強MRI。另外，在動態相位採集過程中，採集時間越長，往往導致MRI無法捕捉到最佳相位。特別是，對於使用釓酸的肝臟核磁共振掃描，由於動脈窗口相對較短，有效地捕捉動脈期一直是放射科醫生的一個棘手問題。提高了磁共振成像的掃描速度，包括對梯度轉換率和振幅等硬體進行了改進；擴大了各種並行成像的應用範圍，並採用了其他技術，如視圖共享到鎖孔成像。近年來，壓縮傳感技術被引入腹部MRI，與並行成像相結合，提高了掃描速度。本文綜述了壓縮傳感技術，重點介紹了其在腹部MRI中的臨床應用，以及近年來為減少彌散加權成像（DWI）和磁共振胰膽管成像（MRCP）的掃描時間而實施的其他策略。
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採集時間長與k空間採樣密切相關。如果能夠利用部分採樣的k空間數據重建圖像，則可以提高MRI的時間解析度。為了減少掃描時間，通常採用並行成像。並行成像涉及通過相位陣列線圈的多個接收器線圈元件同時採集數據，這是大多數臨床掃描儀可用的功能；這會導致數據冗餘。相控陣線圈同時採集多個單元的數據，利用這些單元之間靈敏度分布的變化進行圖像重建。
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.
對於傳統的並行成像，所需的相位編碼步驟在規則的子晶格上欠採樣。換言之，k空間通過以等距步驟跳過相位編碼線而欠採樣（圖1）。這違反了Nyquist準則，導致了原始零填充重建中的混疊偽影，影響了圖像質量和診斷性能。可在圖像（x，y）、k空間（kx，ky）或混合（x，ky）域（圖2A）中進行的並行成像重建可減輕混疊偽影。根據重建圖像和校正偽影的位置，所涉及的方法分為兩類：圖像域或k-空間域。臨床上最常用的技術是敏感度編碼（SENSE），它在圖像域中執行傅立葉後變換，在k-空間域中工作的廣義自動校準部分並行採集（GRAPPA）。前者利用線圈靈敏度映射來分離混疊數據，後者利用相鄰k空間線和線圈元素之間的相關性來計算權值，從而重構未採集的k空間數據。SENSE具有數值效率高的優點，但容易出現參考數據與掃描數據的不匹配。因此，感覺需要補償策略來克服可能的不匹配。相反，GRAPPA通常使用自動校準來處理測量數據和參考數據之間的不匹配，但它的數值效率比SENSE低。對於二維（2D）和三維（3D）笛卡爾數據採集，並行成像已經得到很好的證實。它還可以擴展到非笛卡爾採樣，其基本概念是利用線圈靈敏度數據或來自不同線圈元件的不同數據之間的相關性。事實上，並行成像已經被證明可以通過有效的k空間採樣來改善腹部MRI，從而提高空間解析度，並減少採集時間和對運動的敏感性。
Fig. 1. Different k-space sampling schemes.
圖1。不同的k空間採樣方案。
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.
全採樣k-空間（A）、加速度因子為2（B）的均勻欠採樣或徑向採樣中具有跳躍輻條的非笛卡爾採樣（C）。實線和虛線分別指採集的和跳過的k空間數據。Kx=頻率編碼方向，Ky=相位編碼方向。
Fig. 2. Graphical representation of principles of parallel imaging and compressed sensing.
圖2。並行成像和壓縮傳感原理的圖形表示。
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.
在並行成像（A）中，均勻子採樣產生典型的混疊偽影。並行成像重建可以實現無混疊圖像。在廣義自校正部分並行採集（GRAPPA）中，並行成像重建是在圖像空間進行的，而去混疊是在FFT之前進行的。在壓縮感知（B）中，可變密度的偽隨機子採樣在FFT後產生非相干噪聲，如混疊偽影。稀疏性（在這種情況下，小波）變換允許設置稀疏性約束。圖像經IWT後進入圖像域。IFFT返回k空間允許數據一致性檢查。經過多次迭代，最終圖像在數據一致性和稀疏性約束之間達到最佳平衡。FFT=傅立葉變換，IFFT=傅立葉逆變換，IWT=逆變換，PI=並行成像，SENSE=靈敏度編碼，WT=小波變換。
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，加速度係數，由完全採樣的k空間數據與欠採樣的k空間數據的比率指定，受到幾個因素的限制。高加速因子可導致信噪比降低，由線圈設置引起的幾何因子量化的附加噪聲放大，以及剩餘的混疊偽影。因此，在臨床實踐中，當加速度因子大於4時，要獲得穩定且可接受的圖像質量往往是一個挑戰。
Compressed Sensing
壓縮傳感
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.
為了進一步加快MR採集速度，研究了壓縮傳感的概念；這依賴於自然圖像是可壓縮的前提。基於我們使用圖像存檔和通信系統的長期經驗，我們可以安全地假設醫學圖像也是可壓縮的，不會丟失基本信息或降低圖像質量。這也意味著在核磁共振掃描中存在冗餘。如果大多數採集到的數據可以在沒有感知損失的情況下被丟棄，那麼首先就不需要獲取不必要的數據，而利用MRI數據的冗餘意味著應該有較少的樣本足以用相關信息重建圖像，從而減少掃描時間。相反，採集後醫學圖像的壓縮不會改變採集時間。然而，壓縮傳感技術在磁共振成像採集中的應用一直比較緩慢，因為目前還不清楚信號的哪些部分含有重要信息，哪些部分是冗餘的。
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.
壓縮感知需要三個條件：稀疏、非相干和非線性重建。稀疏性是指在適當的變換域中，只有少量係數攜帶圖像中的相關信息的情況。為了獲得更高的壓縮效果，需要具有強稀疏性，即只包含少量的非消失係數。如前所述，MRI在其變換域中本質上是稀疏的。即使一幅圖像可能是稀疏的，但在固定的採集方案下，要從任何一幅圖像中找到不消失的分量仍然是一個挑戰。壓縮感知的基礎是選擇與變換域中任何稀疏表示有足夠重疊的採樣。非相干和隨機採樣方案實現了這一要求，因為相關聯的混疊偽影類似於噪聲，因此在變換域中的閾值允許識別相關係數。然而，在實踐中，由於硬體或生理上的限制，例如轉換率、渦流和神經刺激，純隨機抽樣不太可能是可行的。因此，通常使用偽隨機抽樣模式。圖2B說明了使用壓縮傳感的圖像採集過程。
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.
與並行成像一樣，壓縮傳感可以與各種序列相結合，包括笛卡爾和非笛卡爾捕獲方案。特別是，非笛卡爾採樣方案在壓縮感知方面有一些優勢。在非笛卡爾序列中，徑向或螺旋採樣方案的典型混疊偽影比笛卡爾並行成像中使用的常規欠採樣偽影的相干度低，因此更自然地符合先決條件。此外，中心k空間固有的密集採樣，即使在捕獲時間減少的情況下，它也有助於提高信噪比。
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
壓縮感知的最後一個前提是非線性重建，這是確定上述稀疏表示的必要條件。圖像重建策略通常通過優化以下成本函數在數據一致性和稀疏性之間提供平衡：
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).
這裡，x̂是重建圖像，y表示獲得的k-空間數據，A是將圖像x映射到k-空間數據的系統算符。系統算子A包括關於k-空間軌跡、線圈靈敏度和傅立葉變換的信息。此外，Ψ是應用於圖像的稀疏變換，λ是正則化因子。第一項確保了數據的一致性，並且與非正則意義重建中的項相同。第二項通過選擇l1範數在變換域中增強稀疏性。正則化因子（λ）平衡了數據一致性和數據稀疏性（圖3）。
Fig. 3. Effect of regularization parameters.
圖3。正則化參數的影響。
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.
使用無正則化參數（A）、建議的正則化參數（B）和比建議的正則化參數（C）高10倍的正則化參數重建同一數據集。根據正則化參數，圖像顯示不同的成像紋理和信噪比。
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.
與並行成像不同，壓縮感知使用稀疏性來減少所需的相位編碼步驟，而這些步驟與線圈設置無關。然而，如上面的方程所示，它可以自然地與並行成像結合起來。因此，壓縮傳感和並行成像相結合比單獨並行成像更容易加速MRI速度（25）。此外，這種組合降低了丟失小係數（進而降低對比度對象）和時間或空間模糊的風險。
Clinical Applications
Current Issues Related to Obtaining Dynamic 3D T1-Weighted Imaging
動態3D T1加權成像的研究現狀
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，延長的採集時間是腹部成像的一個限制因素，這通常會導致屏氣檢查中的運動偽影。動態T1加權成像是病灶檢測和定性的最重要的序列之一，它的應用尤其麻煩。對於使用肝細胞特異性造影劑的肝臟磁共振成像，特別是瞬時運動經常發生，並在關鍵時間點產生運動偽影。此外，與細胞外造影劑相比，較短的動脈時間窗對獲得最佳動脈相位成像提出了挑戰。眾所周知，通過縮短捕獲時間），可以降低由於屏氣能力有限或瞬時運動而產生的運動偽影的發生率。為此，多動脈期圖像廣泛應用於肝臟MRI。儘管並行成像提高了時間解析度，但多動脈相位往往以犧牲空間解析度為代價獲得較高的時間解析度，這可能會對圖像質量和潛在的診斷性能產生負面影響。為了進一步加速核磁共振成像，同時平衡這兩種類型的解析度，已經提出了其他策略，包括視圖共享到鎖孔技術。這些策略已應用於動態增強序列，以獲得肝臟MRI的最佳動脈期。然而，由於運動，所有相位都可能退化，並且由於採樣數據在各個相位之間共享，可能發生時間模糊。因此，我們一直在嘗試在沒有明顯運動偽影的情況下獲得最佳動脈相位。
3D T1-Weighted Images with High Temporal Resolution
高時間解析度三維T1加權圖像
Static Imaging
靜態成像
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.
利用空間域的稀疏性，壓縮傳感與並行成像採集可以獲得更高的靜態成像加速度。使用壓縮傳感和並行成像的組合允許加速度大於4倍，並且在不顯著影響圖像質量的情況下，採集時間可以減少到小於10秒（圖4）。屏氣序列的高時間解析度有助於限制屏氣能力的患者。
Fig. 4. Hepatobiliary phase of gadoxetic acid-enhanced MRI in 69-year-old man.
圖4。69歲男性肝膽期gadoxetic增強MRI表現。
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、 第一幅圖像是單獨使用平行成像（SENSE）獲得的，加速度因子為2.8。B、 下一幅圖像採用壓縮感知和加速度係數為7.17的感知。儘管兩幅圖像顯示出可比較的圖像質量和空間解析度（重建體素大小為0.99×0.99×3mm），但圖像採集時間分別為（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.
對比度增強動態序列是壓縮感知的一個很好的應用，因為在時域上的稀疏性可以得到進一步的利用。嘗試用壓縮感知和笛卡爾或非笛卡爾捕獲實現高時間解析度的T1加權動態圖像。
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).
笛卡爾採樣方案應遵循偽隨機、低阻尼模式，以利用壓縮傳感）。一次屏氣可獲得多個動脈期採集。這類似於以往的研究，在一次屏氣中獲得雙動脈或三動脈相位。然而，與僅使用並行成像的多動脈相位採集相比，壓縮傳感和並行成像的組合允許更高的時間和空間解析度（圖5）。此外，與視圖共享技術相比，它通過適當的重建算法和運動偽影汙染最小化了對時間模糊的關注（圖。6，7）。如果高時間解析度是關鍵，則可將視圖共享技術與壓縮感知和並行成像相結合，以進一步加快掃描速度。作為並行成像，壓縮傳感和並行成像的結合可應用於光譜絕熱反演恢復和Dixon脂肪抑制技術（圖。5，7頁）。
Fig. 5. Multi-arterial phase of gadoxetic acid-enhanced MRI in 63-year-old man obtained using compressed sensing and parallel imaging.
圖5。應用壓縮傳感和平行成像技術對63歲男性患者進行了多動脈期的釓酸增強磁共振成像。
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）動脈期清楚地捕捉到肝臟造影增強的不同時間，具有足夠的空間解析度（重建體素大小為0.98×1.41×3mm），在單次屏氣中沒有明顯的時間模糊。每個相位的時間解析度為5.3秒。
Fig. 6. Multi-arterial phase of gadoxetic acid-enhanced MRI in 66-year-old man obtained using view-sharing technique.
圖6。應用視圖共享技術獲得66歲男性患者的多動脈期gadoxetic增強MRI。
All three arterial phases (A–C) show persistent motion artifacts, which decrease image quality.
所有三個動脈期（A-C）都顯示持續的運動偽影，從而降低圖像質量。
Fig. 7. Multi-arterial phase of gadoxetic acid-enhanced MRI in 88-year-old woman obtained using compressed sensing and parallel imaging.
圖7。應用壓縮傳感和平行成像技術獲得88歲婦女的多動脈期伽氧西酸增強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).
即使病人在掃描過程中沒有屏住呼吸，第一（A）和第二（B）階段還是被保存了下來，因為運動偽影被限制在最後階段（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.
非笛卡爾採集方案，如徑向或螺旋採樣，可與壓縮傳感和並行成像相結合用於動態成像。利用徑向採集（圖1C）的星堆採樣是體成像中最常用的採集方案。它使用平面內徑向採集和平面內笛卡爾採集。對於徑向軌跡，黃金角排序方案是有益的。如果後續輻條的角度之間有適當的增量（例如112.2°），則可以保證k空間在幾乎任意的時間間隔內幾乎均勻地採樣。在黃金角徑向稀疏平行（GRAP）成像中，後續輻條填補了先前輻條之間的最大間隙，從而在掃描期間的任何時間產生均勻的k空間覆蓋。利用該技術可以獲得動態相位的高時間解析度和高空間解析度。換言之，由於均勻覆蓋，可以使用數據子集「回顧性」重建圖像。使用GRASH，可以從單個檢查39）重建具有可變時間解析度的圖像（圖8），對於肝臟MRI，最高時間解析度報告為每體積小於3秒。儘管在常規臨床實踐中不需要具有最高時間解析度（小於3秒）的圖像，但由於存在大量偽影，抓取的時間解析度是令人鼓舞的。此外，與笛卡爾採樣相比，徑向採集本身具有運動阻力，而且數據通常是在自由呼吸條件下採集的。抓取的獨特特徵將在下面的小節中進一步討論。
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
圖8. 61歲男性肝臟MRI的GRASP成像的靈活時間解析度。
從一次自由呼吸檢查中回顧性地重建了時間解析度為13.3秒（A）和3.3秒（B）的圖像。GRASP =黃金角徑向稀疏平行
Free-Breathing T1-Weighted Images with Continuous Data Acquisition
具有連續數據採集功能的自由呼吸的T1加權圖像
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.
儘管具有高時間解析度的成像可以為大多數患者提供足夠的圖像質量，但它並不能消除屏氣的需求，也不能解決現有的運動偽影。自由呼吸的圖像為該問題提供了潛在的解決方案。也可以使用笛卡爾或非笛卡爾採集來獲得自由呼吸的T1加權成像。
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) .
據報導，自由呼吸的笛卡爾採集可用於肝臟MRI的動態成像。可變密度欠採樣方案與並行成像和壓縮傳感結合使用原型序列（壓縮傳感體積插值屏氣檢查[CS-VIBE]，西門子醫療公司，德國埃爾蘭根），具有11秒的時間解析度。因為笛卡爾採集對運動更敏感，所以為了獲得可接受的圖像質量，必須進行運動校正。但是，由於掃描效率低下，當前的呼吸觸發或門控技術不適用於動態相位採集。對於此序列，導航信號與準備脈衝對齊，並且未觀察到時間損失。與運動校正相結合，在屏氣能力有限或有短暫運動風險的患者中，自由呼吸的笛卡爾欠採樣比屏氣笛卡爾動態圖像提供更少的運動偽像和更好的整體圖像質量（圖9）。
Fig. 9. T1-weighted images of gadoxetic acid-enhanced liver MRI in 56-year-old woman with limited breath-holding capacity.
圖9.屏氣能力有限的56歲女性用牛磺酸增強肝臟MRI的T1加權圖像。
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，運動分解的重建圖像（超維VIBE，A）中，運動偽影明顯少於隨後的屏氣3D GRE過渡階段圖像（屏氣VIBE，B）。GRE =梯度回波，VIBE =容積插值屏氣檢查，3D =三維
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.
非笛卡爾採集通常對運動不太敏感，這對於自由呼吸成像是一個優勢。如上所述，GRASP屬於此類。但是，大量運動還會在徑向採集中產生運動偽影，與非門控圖像相比，運動校正將有助於改善圖像質量。
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.
對於自由呼吸的笛卡爾採樣方案和非笛卡爾採樣方案，都可以追溯地進行運動校正（41）。對於笛卡爾採樣，可以使用已實現的導航器信號。對於GRASP，中央k空間是連續採樣的，並且可以提取自選通信號（圖10），而無需其他導航器信號或呼吸波紋管。最簡單的運動校正方法是「硬門控」，其中定義一個接受窗口，並使用獲得的運動信號來確定是將數據用於圖像重建還是丟棄。「軟門控」是用於減少與運動有關的圖像模糊的另一種選擇；它結合了運動狀態權重以懲罰運動狀態不一致。解決運動的重建是一種更高級的選項，其中在圖像重建過程中實現運動狀態數據。因為重建包括運動狀態的額外維度，所以將其稱為超維度（XD）-GRASP或XD-VIBE（圖11）。與強化相比，由於進一步減少了運動偽像，XD-VIBE顯示出更好的圖像質量。
Fig. 10. Self-gated signals extracted from k-space in GRASP imaging sequence.
Regular breathing pattern (A) and irregular breathing pattern (B) are seen.
圖10.從GRASP成像序列的k空間提取的自門控信號。
可以看到規律的呼吸模式（A）和規律的呼吸模式（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.
圖11.快速脂肪飽和T1加權成像在抑制脂肪脈衝之後以回波序列的形式獲取成像數據。
對於自由呼吸採集，可以插入其他具有相同激勵但具有可選讀出方向的GRE（頂部）。因此，可以獲得每個線圈元件的頭部-腳部投影以及成像數據（中間）。這既可以用於僅利用變化最小的指定部分數據的門控重建（左下），也可以用於提取門控信號以將每個回波序列分配給運動狀態，然後進行運動解析重建（右下）。FS =抑制脂肪，SI =信號強度，TR =重複時間。
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.
自由呼吸的笛卡爾序列和非笛卡爾序列在臨床上都具有多種重要意義。首先，它們通過提供運動魯棒性來減少患者屏住呼吸的需要。據推測，這對於屏氣能力有限的患者，包括兒童在內的非合作患者以及在服用造影劑後有短暫運動風險的患者都是有益的。即使對於具有足夠屏氣能力的患者，也可以通過消除重複屏氣來提高舒適度。其次，它們允許連續的數據採集，從而為放射技術人員提供了更好的工作流程（圖12）。將省略關於最佳呼吸的重複說明，並且可能不需要使用MR透視檢查獲取動脈相。此外，它們潛在地減少了在採集過程中對運動患者進行重新檢查的需求。當前，如果無法糾正運動偽影，則將患者轉診接受複查。使用上述序列，可以進行追溯運動校正，並且可以在掃描過程中為運動患者提供臨床可接受的圖像。第三，GRASP通過一次檢查即可提供靈活的時間解析度（圖8）。這樣就可以在不重新使用造影劑的情況下恢復丟失的「動脈期」。此外，高時間解析度圖像可以捕獲與腹部器官以及腫瘤有關的重要血液動力學信息。因此，與動態成像有關的當前問題可以在未來幾年中得到緩解。
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
圖12.腹部對比增強MRI中採集方案的變化。
當前的動態序列協議（A）包括多個暫停和屏氣實例。在使用壓縮感應VIBE或GRASP的動態序列中，無需屏住呼吸就可以進行連續數據採集（B），因為可以回顧性地重建圖像，包括運動校正。CM =造影劑。
3D T1- and T2-Weighted Images with High Spatial Resolution
具有高空間解析度的3D T1和T2加權圖像
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).
與CT或超聲相比，MRI的空間解析度較低。嘗試使用各種技術在MRI上獲得高空間解析度的圖像，它們顯示出了更好的病變顯著性。然而，通過增加並行成像的加速因子，混疊偽影和降低信噪比存在不可避免的缺點。通過組合壓縮傳感和並行成像，我們可以在可接受的時間範圍內獲得具有高空間解析度的圖像。前述組合將減少在通過平行成像獲得的圖像中出現的展開偽像。另外，還可以通過組合併行成像和壓縮傳感來降低信噪比。通過使用壓縮感測，並行成像和造影劑，MRI可以實現高對比度解析度和高空間解析度，這對於檢測小病變和解剖結構將很有用（圖13）。由於壓縮感知可以與T1加權序列之外的其他序列組合，因此通過使用壓縮感知（壓縮SENSE，Philips Healthcare，Best，荷蘭），還可以在可接受的時間範圍內以更好的空間解析度獲取T2加權成像。（圖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).
圖13.一名51歲男性在服用葡萄糖酸增強肝MRI的肝膽期。
與僅通過並行成像獲得的圖像（B）相比，通過壓縮感測和並行成像獲得的圖像（A）顯示更少的圖像噪聲和更好的總體圖像質量。與單獨使用平行成像相比，在通過壓縮傳感和平行成像獲得的圖像中，治療的肝細胞癌（箭頭）更為明顯。圖像的採集時間為15秒，空間解析度相同（重建體素大小：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.
圖14. 49歲血管瘤患者的T2加權圖像。
使用壓縮感應和4毫米切片厚度和4毫米間隙（A，B）的並行成像的2D T2加權圖像比在8毫米切片的2D T2加權圖像中獲得的小血管瘤（箭頭）更明顯 厚度和8毫米間隙（C，D）。TR / TE分別為3240/80 ms（A，B）和2050 / 83.6 ms（C，D）。TE =回聲時間，2D =二維。
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是評估膽管或胰腺疾病的關鍵序列之一。與內窺鏡逆行胰膽管造影術相比，它是非侵入性的，與2D MRCP相比，它具有近乎各向同性的體積數據。確實，有幾篇報導表明3D MRCP比2D MRCP表現出更好的導管可見性或診斷性能。但是，獲取時間很長，因為常規使用呼吸觸發來獲取大視野的體積數據。壓縮感測是獲取3D MRCP的有效策略，因為3D MRCP在圖像域中比較稀疏。換句話說，3D MRCP使用諸如膽管或胰管之類的含流體結構的高對比度，並且抑制了任何其他背景信號。因此，容易預期可以從少量的數據樣本中實現圖像重構，因此可以減少掃描時間。
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.
為了減少數據採樣，使用了可變密度的隨機欠採樣。這包括可變密度的Poisson圓盤模式或可變密度的高斯非相干採樣模型。此外，可以使用GRAPPA或SENSE來加快掃描速度，並有助於保持數據一致性並減少重建時間。不出所料，在以前的研究中，壓縮傳感和並行成像的結合縮短了3D MRCP的掃描時間。在那些研究中，應用壓縮感測和並行成像將採集時間減少了50％。具體而言，可以在2或3分鐘內實現呼吸觸發的3D MRCP。此外，總體圖像質量與僅使用並行成像的3D MRCP相比沒有顯著差異（圖15）。就改善MRI工作流程和減輕放射科醫生，放射技師和患者的負擔而言，這是令人鼓舞的結果。
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.
圖15.一名67歲男子的呼吸觸發3D MRCP。
常規3D MRCP（A）和壓縮感測3D MRCP（B）表現出可比的圖像質量，採集時間分別為5分鐘35秒和2分鐘4秒。傳統3D MRCP（A）的TR / TE為4172/702毫秒，壓縮感測3D MRCP（B）的TR / TE為3861/725毫秒。MRCP =磁共振胰膽管造影。
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.
不管這些好處如何，呼吸觸發的3D MRCP仍然存在幾個問題。與厚板2D MRCP相比，2或3分鐘的掃描時間仍然很長。此外，圖像質量和採集時間在很大程度上取決於患者的呼吸模式。換句話說，當患者的呼吸節律不規則時，掃描時間通常是不可預測的，並且圖像質量通常不令人滿意。在這些患者中，即使使用壓縮感測和並行成像，呼吸觸發的3D MRCP也無法解決問題。
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).
一直在努力減少掃描時間。現在，通過利用壓縮感測和並行成像相結合的功能，可以一次屏住呼吸就可以獲取3D MRCP。在這些研究中，僅採樣了4.5-5％的k空間數據，其採集速度比傳統的呼吸觸發3D MRCP序列快10倍以上。圖像質量並不明顯低於傳統的呼吸觸發3D MRCP，並且在模糊和運動偽像方面甚至更好。與呼吸觸發的3D MRCP相比，屏氣3D MRCP還減少了無法診斷的掃描或嚴重偽影的發生率。在臨床實踐中，這是腹部MRI和壓縮感測實現的顯著進步。即使自首次展示以來就一直嘗試屏住呼吸的3D MRCP，但使用的不是壓縮感測技術，但由於與常規呼吸相比圖像質量不一致且不令人滿意，這些技術並未在臨床實踐中實施觸發的3D MRCP。但是，與以前的方法相比，使用壓縮感應的新型屏氣3D MRCP似乎可以提供一致的圖像質量，並且對於無法正常呼吸的患者很有用（圖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).
圖16.在68歲女性中使用壓縮感測的3D MRCP。
傳統的使用並行成像的呼吸觸發3D MRCP（A）由於呼吸模式不規則而顯示出大量的運動偽影，而使用壓縮感測和並行成像的屏氣3D MRCP（B）顯示出可接受的圖像質量。傳統的呼吸觸發3D MRCP（A）的TR / TE為4421/699 ms，屏氣3D MRCP（B）的TR / TE為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.
最近的一項研究提出了一種改進的協議，該協議使用較大的加速度因子，過採樣和飽和帶，從而具有高解析度和小視野的屏氣式3D MRCP。不出所料，高解析度屏氣3D MRCP在圖像質量以及外周膽管和胰管可視化方面優於原始屏氣3D MRCP，從而更好地描繪了胰管異常。
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.
儘管壓縮感測是加速MRI採集速度的有前途的技術，但其優勢似乎在DWI中受到限制。這是因為在DWI中最常用的單脈衝回波平面成像（EPI）方案非常有效，並且與3D或動態圖像相比，沒有足夠的空間實現2D靜態圖像的稀疏性。同時多切片DWI（SMS-DWI）將是減少DWI掃描時間的一種選擇。SMS成像使用多波段脈衝同時激發幾個切片，該多波段脈衝經過精心設計，並考慮到比吸收率增加的限制。使用有關相控陣線圈的線圈靈敏度，梯度或射頻編碼的信息來分離切片。由於相控陣線圈在橫向腹部協議中通常在切片方向上具有有限的編碼能力，因此將切片移位應用於單獨的切片並改善幾何因子。並行成像和SMS的組合可以產生協同作用，原因如下。首先，平行成像可提高平面解析度並減少回波列的長度。它可以通過減少模糊和失真來提高圖像質量。SMS在切片方向上工作，從而大大減少了掃描時間。此外，SMS不會遭受欠採樣採集固有的信噪比損失。
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.
據報導，SMS-DWI可以以比傳統DWI短的採集時間提供可接受的圖像質量（圖17）。對於腹部方案，SMS加速度現在被限制為2倍，以確保可接受的圖像質量，因為更高的切片加速度會導致噪聲增強，這與增加的g因子以及更多的混疊偽影有關。將來需要解決此問題。另外，應該在SMS和常規DWI之間比較視在擴散係數，以進行定量測量。
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
圖17.呼吸觸發的DWI在64歲男性中使用800 s / mm2的b值。
傳統DWI（A）和SMS-DWI（B）的圖像顯示出可比的圖像質量，但是SMS-DWI的掃描時間明顯短於傳統DWI。傳統DWI（A）的TR / TE為2100/60 ms，SMS-DWI（B）的TR / TE為2200/62 ms。視場（400×320 mm2）和矩陣（150×120）相同。DWI =擴散加權成像，SMS =同時多層
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).
梯度和自旋迴波（GRASE）是GRASE序列的組合。它使用快速自旋迴波或渦輪自旋迴波採集方案和EPI讀數，這些讀數跟隨快速自旋迴波序列的重新聚焦脈衝。渦輪自旋迴波序列的turbo因子和梯度回波序列的EPI因子均可加快採集速度。因此，與可比的turbo自旋迴波序列相比，GRASE提供了更短的採集時間。另外，通過使用較少的射頻脈衝，在GRASE中比吸收率較低。與EPI相比，GRASE具有多個優點，包括由於多個重新聚焦脈衝而具有更好的信噪比和更少的磁化偽影。即使GRASE是在1990年代初期開發的，它仍被選擇性地用於神經成像，直到被用於體積3D MRCP。這是因為與turbo自旋迴波序列相比，其固有的局限性以及缺乏並行成像。儘管與EPI相比，GRASE不易受磁化偽影的影響，但與Turbo自旋迴波序列相比，GRASE更容易受到磁場不均勻性的影響。在舊掃描儀的情況下，要實現覆蓋大視場的3D MRCP的足夠的場均勻性將是一個挑戰，導致使用GRASE的3D MRCP的圖像質量不一致。幸運的是，最近的掃描儀可提供更好的場均勻性，並且可以使用平行成像。最近的論文報導，結合使用GRASE和並行成像，可以在一次屏氣中實現3D MRCP，並且與傳統的呼吸觸發3D MRCP相比，它可以減少運動偽影並提供更好的整體圖像質量（圖18）。儘管壓縮感測和並行成像技術提供了出色的結果，但對於無法使用壓縮感測的掃描儀，使用GRASE的3D MRCP將是替代選擇（圖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).
圖18.屏氣能力有限的61歲女性的3D MRCP。
使用梯度和自旋迴波的屏氣3D MRCP（A）與使用壓縮感應（B）的屏氣3D MRCP相比具有可比的圖像質量，並且比通過呼吸觸發的3D MRCP（C）更好的圖像質量。
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.
由於對屏氣的嚴格限制，快速MRI對於腹部成像至關重要。當與並行成像結合使用時，壓縮傳感可以改善腹部MRI的時間解析度和空間解析度。此外，它結合了其他技術和3D MRCP屏氣量數據採集功能，可在動態圖像自由呼吸中連續採集數據。因此，壓縮感測技術可能會在未來幾年改變臨床實踐。對於動態圖像，壓縮感測和並行成像的組合將通過同時使用運動校正技術來最小化屏氣持續時間或消除屏住呼吸的需要。另外，自由呼吸技術可以潛在地回顧性地校正MR圖像中的運動偽像。如前所述，通過回顧性運動校正進行的自由呼吸，連續數據採集，不僅可以減少放射科醫師和放射技師的工作量，而且可以減少重新檢查的必要性並提高整體圖像質量，從而可以提高患者的舒適度。
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.
在可用的掃描儀中，傳統的經呼吸觸發的3D MRCP已被壓縮感測的經呼吸觸發的3D MRCP取代，從而將掃描時間減少了約50％。這將產生顯著的臨床影響，尤其是在屏住呼吸的3D MRCP是首次執行時；掃描時間將從5–6分鐘減少到不到20秒。此外，壓縮感測呼吸觸發的3D MRCP僅在屏氣量有限的患者中執行。它將大大改善當前的工作流程和圖像質量。此外，SMS-DWI可以減少DWI的掃描時間。
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.
在臨床實踐中，壓縮感測的主要局限性是計算量大，導致重建時間長。儘管可以通過使用圖形處理單元來改善這一點，但是仍然需要高級的重構，包括額外的尺寸重構。我們希望這個問題也將在未來幾年內通過技術發展（包括用於圖像重建的人工智慧）解決。
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.
總之，在臨床檢查中已經實現了使用壓縮傳感，並行成像和SMS採集的快速MRI。它們有助於增強腹部MRI的時間和空間解析度，優化工作流程並改善患者體驗。