Bach-Gansmo T
Department of Diagnostic Radiology, Uppsala University, Sweden.
Acta Radiol Suppl. 1993;387:1-30.
Contrast agents based on superparamagnetic particles have been in clinical development for more than 5 years, and the complexity of their effects is still not elucidated. The relaxivities are frequently used to give an idea of their efficacy, but these parameters can only be used if they are concentration independent. For large superparamagnetic systems, the evolution of the transverse magnetization is biexponential, after an initial loss of magnetization. Both these characteristics of large superparamagnetic systems should lead to prudence in using the relaxivities as indicators of contrast medium efficacy. Susceptibility induced artefacts have been associated with the use of superparamagnetic contrast agents since the first imaging evaluation took place. The range of concentrations where good contrast effect was achieved without inducing artefacts, as well as blurring and metal artefacts were evaluated. The influence of motion on the induction of artefacts was studied, and compared to the artefacts induced by a paramagnetic agent subject to motion. With a suitable concentration of a negative contrast agent, a signal void could be achieved in the region prone to motion, and no artefacts were induced. If the concentration was too high, a displacement of the region close to the contrast agent was observed. The artefacts occurred in a volume surrounding the contrast agent, i.e., also outside the imaging plane. In comparison a positive, paramagnetic contrast agent induced heavy artefacts in the phase encoding direction, appearing as both high intensity regions and black holes, in a mosaic pattern. Clinical trials of the oral contrast agent OMP for abdominal MR imaging showed this agent to be safe and efficacious. OMP increased the diagnostic efficacy of abdominal MR imaging in 2 of 3 cases examined, with a significant decrease in motion artefacts. Susceptibility contrast agents may also be of use in the evaluation of small lesions in the liver. Particulate material injected i.v. will be targeted to the liver and spleen by way of the mononuclear phagocyte system (MPS). Small particles, without specific receptor affinities were targeted to the hepatocytes and the MPS. The distribution correlated with a high efficiency as a contrast agent, whereas no correlation to in vitro relaxation rates and relaxivities could be found. Superparamagnetic particles have important possibilities as contrast agents. The identification of in vitro properties of these agents may help the comparison of various agents before in vivo imaging.
基于超顺磁性颗粒的造影剂已进入临床研发阶段超过5年，但其效应的复杂性仍未阐明。弛豫率常被用来反映其功效，但只有在与浓度无关时，这些参数才能使用。对于大型超顺磁性系统，横向磁化强度的演变在初始磁化强度损失后呈双指数形式。大型超顺磁性系统的这两个特性都应促使人们谨慎使用弛豫率作为造影剂功效的指标。自首次进行成像评估以来，敏感性诱导伪影就一直与超顺磁性造影剂的使用相关。评估了在不诱导伪影的情况下实现良好对比效果的浓度范围，以及模糊和金属伪影。研究了运动对伪影诱导的影响，并与顺磁性造影剂在运动时诱导的伪影进行了比较。使用合适浓度的阴性造影剂，可以在易发生运动的区域实现信号缺失，且不会诱导伪影。如果浓度过高，则会观察到靠近造影剂的区域发生位移。伪影出现在造影剂周围的体积中，即在成像平面之外。相比之下，阳性顺磁性造影剂在相位编码方向上会诱导严重伪影，表现为高强度区域和黑洞，呈马赛克图案。口服造影剂OMP用于腹部磁共振成像的临床试验表明，该造影剂安全有效。在3例接受检查的病例中，OMP提高了2例腹部磁共振成像的诊断效能，运动伪影显著减少。敏感性造影剂也可能用于肝脏小病变的评估。静脉注射的颗粒物质将通过单核吞噬细胞系统（MPS）靶向肝脏和脾脏。没有特定受体亲和力的小颗粒靶向肝细胞和MPS。这种分布与作为造影剂的高效率相关，而与体外弛豫率和弛豫性没有相关性。超顺磁性颗粒作为造影剂具有重要的应用潜力。确定这些造影剂的体外特性可能有助于在体内成像之前比较各种造影剂。
