Blood pool agent

Blood pool agents (BPAs) are a class of magnetic resonance angiography contrast agents.^[1][2] Blood pool agents (also known as intravascular contrast agents) are differentiated from other contrast agents due to their high molecular weight and higher relaxivities.^[3] Their large size prevents diffusion through the vascular epithelium and leakage into the interstitial space, and because of this they stay in the vascular system for a longer time period. Most contrast agents, leave the vascular system within a few minutes, however blood pool agents remain in the circulation for up to an hour, extending the window available for imaging. Longer image acquisition times allow better signal-to-noise ratio and improved image resolution.

Roles of blood pool agents

Due to their extended time in the circulatory system, blood pool agents can be used for delayed steady-state imaging, and additionally these results can be combined with first pass arterial imaging.^[2][3]

Classes of blood pool agents

Albumin-binding gadolinium complexes

This class of BPAs is based on the noncovalent binding of low molecular weight Gd³⁺-based complexes to human serum albumin. The first commercial agent to be approved in this class is gadofosveset trisodium^[4] (also known as Vasovist^[5] or Ablavar,^[6] and previously known as MS-325^[7]). Many clinical and case studies documenting the use of this BPA have been published,^{[8][9][10][11][12][13][14]} and its efficacy in enhancing blood vessels visibility has been demonstrated.^[15] The manufacturer (Lantheus Medical) discontinued production in 2017 though, due to poor sales.^[16]

Gadocoletic acid (Bracco SpA), also known as B-22956 and B22956/1, is a Gd-DTPA derivative that is currently in development, but has not yet been approved for clinical use.^[17]

Gadobenic acid (MultiHance^[18]) is sometimes categorized as a BPA; however, as it only binds weakly to albumin and because hepatobiliary uptake of this compound occurs, this contrast agent should not be classified as a BPA.

Polymeric gadolinium complexes

Polymeric Gd³⁺ chelates are large in size, which prevents leakage into the interstitial space, and provides long imaging windows. Several polymeric gadolinium-based BPAs are currently in development but have not yet been approved for clinical use: Gadomelitol (Guerbet, France), also known as Vistarem and P792^[19] Gadomer-17 (Schering AG, Berlin, Germany) also known as Gd-DTPA-17, SH L 643 A.^[20]