V08: Contrast media
V08 holds the contrast media: iodinated and non-iodinated X-ray contrast media, and contrast media for magnetic resonance imaging (MRI) and ultrasound [1].
Iohexol (V08AB02) has a second use: its plasma clearance is a sufficiently accurate measure of the glomerular filtration rate (GFR), and so of kidney function [2].
How do contrast media work?
On computed tomography (CT), contrast comes from atoms that attenuate X-rays strongly, and the current water-soluble CT contrast media use iodine [3]. MRI contrast agents are almost all small, water-soluble chelates of gadolinium(III), meaning the metal ion is held in a binding molecule [4].
Imaging with X-ray contrast media relies on their uneven distribution: to some organs more than others, and to abnormal tissue differently from normal tissue. Apart from the biliary agents, iodinated X-ray contrast media bind little to plasma proteins and are not metabolized. For iopamidol (V08AB04), 66–72% of a dose is excreted in the urine. In patients with renal impairment, the metabolic clearance of X-ray contrast media was abnormally high, which suggests compensating routes such as biliary elimination [5].
Efficacy
The effect of a contrast medium is the enhancement it produces on the image. On CT, that enhancement depends on patient factors, on the contrast medium and how it is injected, and on scanner factors including scan timing. One review used contrast medium pharmacokinetics and computer simulation to propose injection protocols and scan timing for common CT examinations [6].
For MRI, the benefit of the contrast agent is improved diagnostic accuracy [4].
Safety
A fall in renal function after contrast medium is now called post-contrast acute kidney injury (PC-AKI), and it has many possible causes. The risk of acute kidney injury caused by intravascular contrast medium has been overstated. The important patient risk factors are chronic kidney disease and dehydration [7].
Renal function also sets how long to wait between two contrast-enhanced examinations. With an estimated GFR (eGFR) above 60 mL/min/1.73 m2, the Contrast Media Safety Committee of the European Society of Urogenital Radiology suggests waiting optimally 12 hours between iodinated contrast examinations, which allows near-complete clearance, and at least 4 hours if a rapid follow-up is needed. The safe waiting time increases as renal function falls [8].
Gadolinium-based agents can cause a disease that thickens and scars the skin and internal organs (nephrogenic systemic fibrosis) in patients with severe renal impairment. Exposure to these agents can also leave gadolinium deposited in bone and brain, even when renal function is normal. Whether this deposition causes disease has been debated [9].