Teaching Case

A 78-year-old woman with a history of recurrent low-grade papillary transitional cell carcinoma lesions in the bladder was referred to the radiology department for CT to rule out upper urinary tract lesions. Multiphasic abdominal CT was performed.

Figure 1: axial contrast-enhanced CT-image in the arterial phase (A) shows a symmetric nephrogram with normal corticomedullary differentiation. Venous phase image (B) shows some areas with faint hypodense aspect of the medulla (for example in the upper pole of the right kidney). Excretory phase images (C) show a symmetrical contrast excretion. Although there were no visible calcifications in the arterial and venous phase, the absence of calcifications (D) was confirmed on older non-enhanced CT examinations available in PACS.
Figure 2: CT urography images, in different window setting, in axial plane (A) and coronal MIP reformations (B) show a typical ‘paintbrush’ appearance of the renal medulla in both kidneys, due to dilated collecting ducts. Also note the duplicated collecting system of the left kidney.

Background: Benign tubular ectasia is a benign developmental condition where dilatation of the collecting tubules in the medullary pyramids of the kidney occurs. The term ‘benign’ is used to differentiate this entity from medullary sponge kidney. It is mostly seen as incidental finding on imaging studies. The pathogenesis is unknown. [1]

Imaging Perspective: In general, CT-images in the arterial and venous phase show no abnormalities at the renal medulla. Imaging modality of choice to depict tubular ectasia is excretory phase CT urography. CT urography images show parallel streaks of contrast in the medullary pyramids, corresponding to dilatation of the renal collecting ducts. On imaging this creates the typical ‘paintbrush’ appearance. Benign tubular ectasia can involve a different number of calyces, ranging from a few to all. [2, 3] CT urography in our case was performed to rule out malignancy, since fully opacified urinary tract is crucial for detection of upper urinary tract lesions. In general, subtle abnormalities of the upper urinary tract (e.g. papillary necrosis, urothelial cancer,…) will be better portrayed in the excretory phase whereas arterial and venous phase images can be normal. [4, 5]
In case of renal tubular ectasia associated with medulla sponge kidney, nonenhanced CT shows small linear calculi in the renal pyramids. These calculi are formed when there is sufficient stasis of urine in the tubules. When extended, these calcifications can be seen on conventional radiography. CT, however, is more sensitive in detecting a smaller and/or limited number of calcifications. It can also more exactly define the location of these calcifications. In medullary sponge kidney, the tubular ectasia is more prominent and dilatation may be cylindrical or saccular. In contrast to patients with benign tubular ectasia, patients with medullary sponge kidney may show symptoms such as microhaematuria, episodes of renal colic, renal insufficiency and infection. [2, 5]

Take-Home Message: A typical ‘paintbrush’ appearance of the renal medulla on CT-urography should alert the radiologist to the diagnosis of renal tubular ectasia, in particular in the absence of medullary calcifications. Recognition of this benign entity is important in order to avoid unnecessary follow-up or other interventions.