【APT 白皮书】Philips 3D APT (6)

2021

09/27

CTMR技术园蒋强盛

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非饱和的 S0 图使用一大的频率偏置射频脉冲进行采集，这样可以对 APTw% 进行归一化处理，同时以百分比进行显示与测量（%）。

How does mDIXON XD TSE shorten the acquisition of APT-weighted images?

mDIXON XD TSE 是如何缩短 APT 加权成像采集时间的？

APTw MRI examinations in 3D may be very time consuming, because multiple imaging volumes need to be acquired for different RF saturation frequency offsets. Keeping the acquisition time within a clinically acceptable duration of 3-5 minutes while providing suitable robustness, signal-to-noise ratio and spatial resolution further guide the choice of APT imaging parameters. The Philips implementation leverages two major principles to allow an acceptable scan time. First it does not acquire an extensive Z-spectrum, but only nine image volumes at critical frequencies for the Z-spectrum. Second, a B0 map acquisition for the asymmetry calculation can be derived from three acquisitions at +3.5ppm using the mDIXON algorithm.

APT 加权磁共振成像检查如果使用 3D 扫描模式可能会花费很长时间，因为需要在不同的射频饱和频率偏置处采集多个成像容积。保持整体采集时间在临床可接受的 3~5 分钟时间范围内，同时提供适当的稳健性、信噪比和空间分辨率，然后进一步指导其他 APT 成像参数的选择。飞利浦平台使用两个主要原理来使得扫描时间在一可接受的范围内：一、不去广泛大量采集整个 Z-谱，而是仅在 Z-谱一些关键频率处采集 9 个容积图像；二、为了非对称计算，B0 图的采集通过使用 mDIXON 算法在 +3.5ppm 处采集三次容积数据进行导出。

Figure 4 contains a schematic representation of the Philips implementation; in total nine image volumes are acquired at seven different frequency offsets, using a 3D TSE read-out that allows volumetric coverage as well as geometrically undistorted images. We found that nine volumes are the minimum required to precisely assess APTw%, including magnetic field (B0) field homogeneity correction via a Lagrange interpolation among the different saturation frequency offsets ∆ω on both sides of the Z-spectrum.The non-saturated S0 image is acquired with a large offset of the RF pulses to allow normalization of APTw%, which is measured in percent (%).

图 4 包含了飞利浦 APTw 的执行方案的示意图；总共 9 个容积图像是在 7 个不同的频率偏置处使用 3D TSE 序列进行采集，这样能够实现容积覆盖以及得到的图像没有几何变形。我们发现，9 次容积采集是精准评估 APTw% 所需要的最小数值，包括了 Z-谱两边不同饱和频率偏置通过拉格朗日插值对主磁场 B0 场的均匀性校正。非饱和的 S0 图使用一大的频率偏置射频脉冲进行采集，这样可以对 APTw% 进行归一化处理，同时以百分比进行显示与测量（%）。

Figure 4: Z-spectrum acquisition as implemented on the Philips platform for APTw MRI. Nine Z-spectral images are acquired (blue dots); eight of those are used to assess the Z-spectrum itself (1-8). Image 9 is the S0 image, acquired at a large ∆ω which is used for normalization of APTw%. At 3.5 ppm, three of the Z-spectral images (1,2,3) are acquired using a different echo time shift. This allows calculating a magnetic field (B0) map, via the mDIXON algorithm, known from water-fat turbo spin echo (TSE) imaging. The B0 map is used to correct the Z-spectrum origin on a voxel-by-voxel basis.

图 4 ：飞利浦平台 APTw 磁共振成像的 Z-谱采集方案。采集 9 个 Z-谱图像（蓝色点）；其中 8 个被用来评估 Z-谱（1-8）。第 9 个是 S0 图，在一大的频率偏置 ∆ω 处采集，为的是将 APTw% 进行归一化处理。在 3.5ppm 处，使用不同的回波时间采集 3 个 Z-谱图像（1,2,3），为的是使用 mDIXON 算法（即水-脂快速自旋回波（TSE）成像）来计算主磁场的 B0 图，而 B0 图是用来校正每个体素 Z-谱的原点的。

The three acquisitions at +3.5 ppm are performed with slightly different echo shifts on the order of 0.5 milliseconds.This allows calculation of a B0 field map directly from the APTw image acquisition based on mDIXON algorithms. This B0 field map is used for the B0 correction to allow calculation of APTw% on a voxel-by-voxel basis. With this approach, no separate B0 field map or separate Z-spectrum acquisition (WASSR = WAter Saturation Shift Referencing) is needed because the B0 field information was obtained simultaneously. As B0 fields may change due to physiological or drift effects during or after the APT image acquisition, the integrated B0 mapping approach using mDIXON approach provides enhanced robustness to these B0 field alterations.

在 +3.5ppm 处所采集的三个容积图像拥有不同的回波时间，回波位移大约为 0.5ms。这样根据 mDIXON 算法可以直接在 APTw 采集过程中得到 B0 图；在计算每个体素的 APTw% 时使用 B0 图进行 B0 校正。通过这一方法，不需要再单独采集 B0 图或单独的 Z-谱采集（水饱和偏移参考），因为 B0 场信息在 ATPw 扫描时就一同采集了。因为 B0 场可能由于生理效应产生变化，或在 APT 成像采集时与采集后的磁场漂移效应导致 B0 场发生变化，那么使用 mDIXON 方法将 B0 图的采集集成内嵌在了 APTw 的扫描中，进一步地提升了鲁棒性，对 B0 场的变化不敏感。