Different classes of small- or nano-sized calcium-sensitive probes for magnetic resonance imaging (MRI) have been proposed in the last two decades. These compounds have been developed mainly for functional MRI purposes and tested in vivo in different animal models. Most of them are paramagnetic systems that change their relaxivity in the presence of the divalent ion calcium, resulting in increased T1 or T2 contrast. In this work, we report the investigation of their relaxometric behavior at low magnetic fields, specifically the comparison of the monomeric Ca-sensitive probe and the corresponding dendrimer conjugates of generations 0, 1 and 2 (G0, G1 and G2, respectively). As a result, a relaxivity hump between 10 and 100 MHz of the Larmor frequency progressively appeared with an increase in the size of the investigated contrast agent, indicative of a restricted rotational motion of the complexes as long as the size of the molecule increases. The same trend with a more pronounced effect was detectable in the presence of calcium. The relaxivity enhancement for the Ca2+ adducts, primarily caused by an increase of the hydration state of Gd3+, went from ca. 130% for the monomeric probe to ca. 310% for the G2 dendrimer conjugate at 0.5 T and 25 °C. T1 weighted magnetic resonance images acquired at 1 T displayed the strong ability of these systems to change their contrast according to the presence of calcium at this field, thus laying the basis for promising future in vivo applications.

中文翻译：

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树枝状钙敏感的MRI探头：第一个低场弛豫研究。

在过去的二十年中，已经提出了不同种类的用于磁共振成像（MRI）的小型或纳米级钙敏感探针。这些化合物的开发主要是为了功能性MRI目的，并在不同的动物模型中进行了体内测试。它们中的大多数是顺磁性系统，在存在二价离子钙的情况下会改变其弛豫性，从而导致T1或T2对比度增加。在这项工作中，我们报告了对它们在低磁场下的弛豫行为的研究，特别是对单体Ca敏感探针与第0、1和2代（分别为G0，G1和G2）的相应树枝状大分子共轭物的比较。结果，随着所研究造影剂尺寸的增加，在拉莫尔频率的10至100 MHz之间逐渐出现了弛豫峰，只要分子的大小增加，表明复合物的旋转运动受限制。在钙的存在下，可以检测到具有更明显效果的相同趋势。Ca2 +加合物的弛豫度增强主要由Gd3 +的水合态增加引起。单体探针的约130％。在0.5 T和25°C下，G2树状大分子共轭物的含量为310％。在1 T时采集的T1加权磁共振图像显示出这些系统具有强大的能力，可根据该场中钙的存在来改变其对比度，从而为有希望的未来体内应用奠定了基础。Ca2 +加合物的弛豫度增强主要由Gd3 +的水合态增加引起。单体探针的约130％。在0.5 T和25°C下，G2树状大分子共轭物的含量为310％。在1 T时采集的T1加权磁共振图像显示出这些系统具有强大的能力，可根据该场中钙的存在来改变其对比度，从而为有希望的未来体内应用奠定了基础。Ca2 +加合物的弛豫度增强主要由Gd3 +的水合态增加引起。单体探针的约130％。在0.5 T和25°C下，G2树状大分子共轭物的含量为310％。在1 T时采集的T1加权磁共振图像显示出这些系统具有强大的能力，可根据该场中钙的存在来改变其对比度，从而为有希望的未来体内应用奠定了基础。