Pseudocalculi due to excreted gadolinium

Imaging description Gadolinium is a rare-earth metal used as an MRI contrast agent because of its paramagnetic properties. Gadolinium has a high atomic number (64, compared to 53 for iodine) and absorbs x-rays, and so functionally can act as a radiographic contrast agent. Before the recognition of nephrogenic systemic fibrosis as a complication of gadolinium administration in patients with renal failure, gadolinium was advocated as an angiographic contrast agent for such patients [1,2]. Like iodinated contrast, gadolinium is excreted by the kidneys. Concentrated excreted gadolinium is radiodense within the collecting system at CT [3, 4], and this radiodensity can mimic renal calculi when non-enhanced CT is performed within the first few hours after a gadolinium-enhanced MRI study (Figure 47.1) [4, 5]. The phenomenon has not been extensively studied, but limited data suggest the dense appearance of excreted gadolinium at CT in the collecting systems is variable from patient to patient, and cannot be reliably predicted from the time interval since gadolinium administration, patient weight, or simple indices of renal function [4]. Importance Misdiagnosis of excreted gadolinium as renal calculi can result in unnecessary additional investigations, such as abdominal radiography or intravenous pyelography [5]. Typical clinical scenario Pseudocalculi due to excreted gadolinium can be seen in any patient who undergoes non-enhanced abdominal CT after a gadolinium-enhanced MRI.