By correlating the 1D projections obtained at two different
times, it is possible to obtain precise information about the
relative displacement that occurred. Previous results made
with a sphere at a static position give an average error of
0.064 mm between the real displacement and the one
measured experimentally. In the dynamic case (i.e. when
several acquisitions are repeated rapidly), this value is found
to be approximately 1.2 mm [2].
To be able to get an absolute position out of this method,
we need to know the exact position of the device during the
acquisition of the correlation mask. This could theoretically
be done by placement of the ferromagnetic bead in a precise
phantom that would be centered as precisely as possible
inside the MRI bore. The correlation mask would be
acquired, then the phantom removed prior to place the
patient inside the bore. This method has several drawbacks.
The most important drawback is that the center of the MRI
gradients (where the magnetic field is always Bo when
gradients are applied) is known to migrate with time and itsposition is difficult to determine precisely. This would add
an uncertainty to the position of the bead.
A more suitable approach has been developed, allowing a
fast and precise determination of the position of the
ferromagnetic entity at any time without requiring any
special setup or manipulation while not being affected by
the ferromagnetic susceptibility distortion. This autocalibration
method simply involves the convolution of two
projections acquired with opposite readout gradients.
參考答案:由相关的维推算取得两个不同时代 就有可能获得确切消息相对位移发生. 以往成绩取得了一球静止的位置,平均误差为0.064毫米的位移和现实之间的一个实验测量. 在动态情况下(即当数收购是迅速重复) 这个价值被发现有大约1.2毫米[2]. 能够得到一个绝对位置出这个方法 我们需要知道确切位置在仪器购置相关面罩. 这可以从理论上应由放置在一个确切的铁磁珠串幽灵将集中尽可能准确的MRI膛内. 后天将相关掩模,然后拆除前广场歌声病人膛内. 这种方法有几个缺点. 最重要的缺点是正中的梯度磁共振(凡波当梯度磁场总是适用)已知迁移时间和itsposition恰恰是难以确定. 这将增添不明朗的珠子位置. 较适合已发展、 允许快速、精确测定铁磁实体的地位随时无需任何特殊设置或操纵,而不是受铁磁畸变敏感性. 这仅仅涉及autocalibration褶两种方法推算后天相向读取梯度.
