In the mid 1980s, development began on MR imaging contrast agents with special properties aimed at liver imaging. These efforts evolved during a time when abdominal MR imaging had not been accepted and when the strengths and limitations of MR imaging for imaging the liver had not been fully tested. Nevertheless, liver-directed contrast agents seemed attractive for two reasons: (1) the prevalence of visceral malignancies and the desire to treat aggressively patients with metastatic disease made liver imaging an economically viable and clinically important public health concern, and (2) the many functions of the liver and its specialized tissue components made possible the rational design of liver-directed contrast media.

Gadolinium (Gd) chelate contrast agents introduced initially to image the neuraxis were quickly applied to abdominal imaging as well. At that time, fast imaging techniques, such as gradient recalled echoes, were unavailable on commercial scanners. Like the nephrographic iodinated contrast agents used in CT scans, these Gd chelate contrast agents rapidly leave the vascular space and after about 3 minutes reach equilibrium throughout the extracellular fluid compartment. Experience with CT scan indicated that hepatic images acquired over several minutes, during the equilibrium phase of contrast distribution, provided little benefit over unenhanced images. Therefore, liver contrast media research sought contrast agents with prolonged retention, targeted specifically at liver tissue and simultaneously excluded from liver neoplasms.

Most imaging of the liver is performed to evaluate focal disease. MR imaging contrast media development likewise has centered on enhanced capability to evaluate the liver for focal lesions. Detection of focal liver abnormalities and their characterization are distinct but complementary components of liver imaging. The initial thrust of contrast media development was for improved detection of focal lesions, such as liver metastases. The success of this program led to detection of tiny lesions previously unseen. Because up to 20% of adults have hemangiomas and other small benign focal liver lesions,<sup>23 Karhunen P. Benign hepatic tumors and tumor-like conditions in men J Clin Pathol 1986 ;  39 : 183-187  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup> it became imperative to study ways to characterize lesions using the newer contrast materials. Therefore, evaluation of contrast material efficacy requires consideration of both lesion detection and lesion characterization.