Vučković Ivan, Nayfeh Tarek, Mishra Prasanna K, Periyanan Sigapriya, Sussman Caroline R, Kline Timothy L, Macura Slobodan
Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, MN 55905, USA.
Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA.
Magn Reson Imaging. 2020 Apr;67:7-17. doi: 10.1016/j.mri.2019.11.019. Epub 2019 Dec 7.
In MRI of formalin-fixed tissue one of the problems is the dependence of tissue relaxation properties on formalin composition and composition of embedding medium (EM) used for scanning. In this study, we investigated molecular mechanisms by which the EM composition affects T2 relaxation directly and T1 relaxation indirectly.
To identify principal components of formaldehyde based EM and the mechanism by which they affect relaxation properties of fixed tissue.
We recorded high resolution H NMR spectra of common formalin fixatives at temperatures in the range of 5 °C to 45 °C. We also measured T1 and T2 relaxation times of various organs of formalin fixed (FF) zebrafish at 7 T at 21 °C and 31 °C in several EM with and without fixative or gadolinium contrast agents.
We showed that the major source of T2 variability is chemical exchange between protons from EM hydroxyls and water, mediated by the presence of phosphate ions. The exchange rate increases with temperature, formaldehyde concentration in EM and phosphate concentration in EM. Depending on which side of the coalescence the system resides, the temperature increase can lead to either shortening or prolongation of T2, or to no noticeable change at all when very close to the coalescence. Chemical exchange can be minimized by washing out from EM the fixative, the phosphate or both.
The dependence of T2 in fixed tissue on the fixative origin and composition described in prior literature could be attributed to the phosphate buffer accelerated chemical exchange among the fixative hydroxyls and the tissue water. More consistent results in the relaxation measurements could be obtained by stricter control of the fixative composition or by scanning fixed tissue in PBS without fixative.
在福尔马林固定组织的磁共振成像（MRI）中，问题之一是组织弛豫特性取决于福尔马林的成分以及用于扫描的包埋介质（EM）的成分。在本研究中，我们探究了EM成分直接影响T2弛豫以及间接影响T1弛豫的分子机制。
确定基于甲醛的EM的主要成分及其影响固定组织弛豫特性的机制。
我们记录了常见福尔马林固定剂在5℃至45℃范围内的高分辨率氢核磁共振（H NMR）谱。我们还在7T场强下，于21℃和31℃时，在添加或不添加固定剂或钆造影剂的几种EM中，测量了福尔马林固定（FF）斑马鱼各个器官的T1和T2弛豫时间。
我们表明，T2变化的主要来源是EM羟基中的质子与水之间的化学交换，这一过程由磷酸根离子介导。交换速率随温度、EM中的甲醛浓度和EM中的磷酸盐浓度增加而增加。根据系统处于合并的哪一侧，温度升高可能导致T2缩短或延长，或者当非常接近合并时根本没有明显变化。通过从EM中洗去固定剂、磷酸盐或两者，可以使化学交换最小化。
先前文献中描述的固定组织中T2对固定剂来源和成分的依赖性，可能归因于磷酸盐缓冲液加速了固定剂羟基与组织水之间的化学交换。通过更严格地控制固定剂成分或在不含固定剂的磷酸盐缓冲液（PBS）中扫描固定组织，可以在弛豫测量中获得更一致的结果。
