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Purpose To synthesize two low-molecular-weight iron chelates and compare their T1 contrast effects with those of a commercial gadolinium-based contrast agent for their applicability in dynamic contrast material-enhanced (DCE) magnetic resonance (MR) imaging. Materials and Methods The animal experiments were approved by the local ethics committee. Two previously described iron (Fe) chelates of pentetic acid (Fe-DTPA) and of trans-cyclohexane diamine tetraacetic acid (Fe-tCDTA) were synthesized with stability constants several orders of magnitude higher than those of gadolinium-based contrast agents. The T1 contrast effects of the two chelates were compared with those of gadopentetate dimeglumine in blood serum phantoms at 1.5 T, 3 T, and 7 T. For in vivo studies, a human breast cancer cell line (MDA-231) was implanted in five mice per group. The dynamic contrast effects of the chelates were compared by performing DCE MR imaging with intravenous application of Fe-DTPA or Fe-tCDTA on day 1 and DCE MR imaging in the same tumors with gadopentetate dimeglumine on day 2. Quantitative DCE maps were generated with software and were compared by means of a one-tailed Pearson correlation test. Results Relaxivities in serum (0.94 T at room temperature) of Fe-tCDTA (r1 = 2.2 mmol · sec, r2 = 2.5 mmol · sec) and Fe-DTPA (r1 = 0.9 mmol · sec, r2 = 0.9 mmol · sec) were approximately twofold and fivefold lower, respectively, compared with those of gadopentetate dimeglumine (r1 = 4.1 mmol · sec, r2 = 4.8 mmol · sec). Used at moderately higher concentrations, however, iron chelates generated similar contrast effects at T1-weighted MR imaging in vitro in serum, in vivo in blood, and for DCE MR imaging of breast cancer xenografts. The volume transfer constant values for Fe-DTPA and Fe-tCDTA in the same tumors correlated well with those observed for gadopentetate dimeglumine (Fe-tCDTA Pearson R, 0.99; P = .0003; Fe-DTPA Pearson R, 0.97; P = .003). Conclusion Iron-based contrast agents are promising as alternatives for contrast enhancement at T1-weighted MR imaging and have the potential to contribute to the safety of MR imaging. RSNA, 2017 Online supplemental material is available for this article.

The compounds [Rb(Benzo‐18‐crown‐6]2Rb4[Ge9‐Ge9](en)6 (1), [Rb(Benzo‐18‐crown‐6]2Rb4[Ge9‐Ge9](en)4 (2), [K(18‐crown‐6)]2K4[Ge9‐Ge9](en)2 (3), and [K(18‐crown‐6)]3Cs3[Ge9‐Ge9](en)2 (4) have been synthesized by dissolving the phases of the formal composition LiRb3Ge9, NaRb3Ge9, K4Ge9, and K2Cs2Ge9, respectively, in ethylenediamine followed by the addition of the crown ethers under consideration. All four compounds contain a dimeric [Ge9‐Ge9]6— unit. Whereas the [Ge9‐Ge9]6— units in 1, 2 and 3 are connected via the alkali metal atoms to infinite chains of the composition {A6[Ge9‐Ge9]} (A = K or Rb) the connection via the Cs atoms in 4 leads to layers of the composition {K2Cs3[Ge9‐Ge9]}1—. The increased number of dimers, which is now available, establishes the distortion of the clusters: The [Ge9] clusters have the shape of a monocapped quadratic anti prism, which is compressed along one of the diagonal of the open rectangle. In each compound two idealized C2v symmetric [Ge9] clusters are connected via an exo‐bond between two Ge atoms of the open rectangle to form a dimeric [Ge9‐Ge9]6— anion. In all cases the exo‐bond is oriented collinear to the shorter diagonal. Thus the square of the open rectangle of the cluster is compressed along the exo bond. The exo‐bond lenghts are in the range of 2.471Å and 2.521Å.The analysis of the results of an Extended Hückel calculation using the electron localization function (ELF) reveals that a localized two‐centre‐two‐electron bond connects the clusters of the dimeric unit. The results are compared to the ELF of an octahedral [Ge6]2— cluster and of the polymer {[Ge9]}2—.

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We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline ␣Ј-NaV 2 O 5 . We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33 K: A sharp first-order transition at T 1 ϭ(33Ϯ0.1) K slightly below the onset of a second-order transition at T 2 onset Ϸ(34Ϯ0.1) K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T 1 induced by the incipient spin-Peierls order parameter above T 2 ϭT SP .