TRANSVERSE RELAXATION OF SOLVENT PROTONS INDUCED BY MAGNETIZED SPHERES - APPLICATION TO FERRITIN, ERYTHROCYTES, AND MAGNETITE
TRANSVERSE RELAXATION OF SOLVENT PROTONS INDUCED BY MAGNETIZED SPHERES - APPLICATION TO FERRITIN, ERYTHROCYTES, AND MAGNETITE
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DOI：
10.1002/mrm.1910050404
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发表时间：
1987-10-01
影响因子：
3.3
通讯作者：
KOENIG, SH
中科院分区：
文献类型：
--
作者：
GILLIS, P;KOENIG, SH
由于在典型的成像场中，组织水的质子的1/T2通常远大于1/T1，因此小型单离子造影剂，如Gd(DTPA)，对1/T2的增量贡献相当，因此与1/T1.sbd相比，对1/T2的分数贡献较小。与优先增强1/T2的造影剂相比，不太理想。原则上，这种专门的药剂只在较高的场域中有效，因为1/T1的场依赖性（色散）在高场域中接近零，而1/T2接近恒定值。余量1/T2称为“长期”贡献，它是由溶解溶剂或组织水分子的质子所感应到的时间波动引起的。对于足够大的顺磁性或铁磁性颗粒的溶液或悬浮液(.gtoreq。250 .ANG。直径)，顺磁对弛豫率的贡献满足1/T2 .mchgt。1/T1在典型成像场。我们详细研究了长期松弛理论，特别是当它适用于与磁共振成像相关的系统时，然后分析了溶液、悬浮液或含有铁蛋白、红细胞、琼脂结合磁性颗粒和低密度复合聚合物包覆磁铁矿的肝脏的数据。在大多数情况下，我们可以根据质子（水分子）在磁化粒子的外层环境中扩散的弛豫理论，定量地解释弛豫数据。这些粒子产生的偶极场具有很强的空间依赖性，其明显的时间波动（由扩散的质子看到）产生的自旋跃迁在低场下对1/T1和/T2都有贡献；对于较大的粒子，由于色散，长期项在感兴趣的场中占主导地位。根据理论与小顺磁配合物、大磁铁矿颗粒和含有低密度聚合物包覆磁团块的肝脏溶液的数据的一致，认为该理论是足够可靠的，因此，例如，对于1/T2出乎意料地大的铁蛋白，其大弛豫的来源必须存在于铁蛋白核的非理想化学中。对于血液，似乎是细胞内梯度的扩散决定了1/T2。
Since 1/T2 of protons of tissue water is generally much greater than 1/T1 at typical imaging fields, small single-ion contrast agents.sbd.such as Gd(DTPA), which make comparable incremental contributions and therefore smaller fractional contributions to 1/T2 compared to 1/T1.sbd.are not as desirable for contrast-enhancement as agents that could enhance 1/T2 preferentially. In principle, such specialized agents will only be effective at higher fields because the field dependence (dispersion) of 1/T1 is such that it approaches zero at high fields whereas 1/T2 approaches a constant value. The residual 1/T2 is called the "secular" contribution and arises from fluctuations in time.sbd.as sensed by the proton of difusing solvent or tissue water molecules.sbd.of the component of the magnetic field parallel to the static applied field. For solutions or suspensions of sufficiently large paramagnetic or ferromagnetic particles (.gtoreq. 250 .ANG. diameter), the paramagnetic contributions to the relaxation rates satisfy 1/T2 .mchgt. 1/T1 at typical imaging fields. We examine the theory of secular relaxation in some detail, particularly as it applies to systems relevant to magnetic resonsance imaging, and then analyze the data for solutions, suspensions, or tissue containing ferritin, erythrocytes, agar-bound magnetic particles, and liver with low-density composite polymer-coated magnetite. In most cases we can explain the relaxation data, often quantitatively, in terms of the therory of relaxation of protons (water molecules) diffusing in the outer sphere environments of magnetized particles. The dipolar field produced by these particles has a strong spatial dependence, and its apparent fluctuations in time as seen by the diffusing protons produce spin transitions that contribute to both 1/T1 and /T2 comparably at low fields; for the larger particles, because of dispersion, the secular term dominates at fields of interest. On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low-density polymer-coated magnetic agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin.sbd.for which 1/T2 is unexpectedly large.sbd.the source of its large relaxivity must reside in nonideal chemistry of the ferritin core. For blood, it appears that diffusion through intracellular gradients determines 1/T2.