Teaching Case

Young adult woman with unremarkable medical history, presenting to the emergency department complaining of right flank pain, nausea and mild fever a few days earlier. Currently apyretic, without previous therapies.
Emergency laboratory tests revealed leukocytosis (17.000 cells/mmc), mild anaemia (11.6 g/dl haemoglobin), normal C-reactive protein and urinalysis.

Emergency noncontrast CT for acute renal colic (Fig. 1a) showed normal-sized, symmetric kidneys without calculi, hydronephrosis and perinephric fat stranding. The right kidney had a 3.5-cm centrally hypoattenuating mass, which showed thin peripheral enhancement on immediate contrast-enhanced study (Fig. 1b-e), without any other signs of acute pyelonephritis. Sonographically, the renal lesion showed heterogeneous hypoanechoic appearance.
Upon hospitalisation, further investigation using unenhanced and post-gadolinium MRI (Fig. 3) confirmed normally functioning kidneys and right-sided lesion with internal moderately high T2-signal and frank restricted diffusion, thin T2-hypointense and strongly enhancing periphery, consistent with an abscess.
After initial improvement on intensive antibiotics, early follow-up noncontrast MRI (Fig. 4) showed initial decrease in size of the renal abscess with unchanged signal and partial regression of diffusion restriction. At discharge, leukocyte count had normalised. Repeated noncontrast MRI at 1 month (Fig. 5) showed markedly decreased size, hypointense signal consistent with regression of necrotic-purulent content.

Renal abscesses (RA) may develop from either haematogenous septic dissemination or fusion of tiny suppurative foci in acute pyelonephritis, and may be detected sonographically as hypo-anechoic cavities or complex structures without internal colour Doppler signals. However, the vast majority of RA are currently encountered during CT studies requested to investigate suspected pyelonephritis, flank pain or unexplained fever. The well-known CT appearance of a RA is a variable-sized, round or geographic collection with central near-water hypoattenuation (corresponding to pus, liquefaction and debris), demarcated by a more or less thick (typically several millimetres) peripheral enhancing “rim”, often surrounded by hypoenhancing (infected but non-necrotic) renal parenchyma [1-3].
Particularly in adolescents and young individuals, MRI is increasingly used to investigate abdominal disorders without use of ionising radiation, including suspected pyelonephritis in women of childbearing age. Diffusion-weighted imaging (DWI) is now implemented in most MRI protocols as it provides information regarding cellular density, cytotoxic oedema and abscess formation, and increases conspicuity of both neoplastic and inflammatory lesions. As in this patient, RA appear as fluid-like heavily T2-hyperintense collections with lower-signal periphery which enhances after gadolinium contrast. High b-value DWI further confirms a diagnosis of RA by showing strong hyperintensity in the liquefied centre. The DWI-derived apparent diffusion coefficient (ADC) quantifies the degree of molecular water motion. Restricted diffusion is more pronounced in RA’s fluid components compared to renal carcinomas, where diffusion is free in cystic/necrotic portions, thus resulting in high sensitivity and specificity for differential diagnosis [4-7].
Particularly in those patients without frank clinical and laboratory signs of renal infection, such as when lower urinary tract symptoms are absent and urinary cultures test negative after empiric antibiotics, the use of MRI is helpful to correctly diagnose RA which requires tailored therapy including prolonged antibiotics and percutaneous drainage of larger collections [8, 9].
If other cross-sectional signs of acute pyelonephritis (including perinephric fat inflammation and thickening of the retroperitoneal fasciae) are absent, the other key differential diagnosis is a complex cystic renal lesion, which requires careful MRI assessment of loculation, number and thickness of septa, mural thickness, nodularity and enhancement. Finally, the high diagnostic agreement between the restricted diffusion and the nonenhancing portion of RA enables reliable lesion follow-up without use of paramagnetic contrast, particularly in patients with contraindications such as impaired renal function, pregnancy and lactation. MRI may be repeated every 3 weeks and regression of treated RA may take up to 8-10 weeks [4-7].