Imaging technique—Cystic masses are evaluated with the same protocol used to image solid renal masses, including unenhanced and multiphasic (corticomedullary, nephrographic, and excretory) contrast-enhanced acquisitions. Narrow detector thickness (< 1 mm) is essential in these studies, to optimally visualize thin septations (Fig. 1A, 1B) and small nodules (Figs. 2A, 2B, 2C, 2D and 3A, 3B). These lesions must be carefully evaluated across phases, with attention to several specific findings, including thickness, contour, and enhancement of septations and wall and the presence of solid nodules, the latter of which is the paramount finding indicative of potential malignancy [4]. In addition, interval increase in size or nodularity of septations is suspicious for malignancy. Of note, the amount, location, and configuration of calcification are not associated with malignancy; calcification is seen in both benign and malignant masses [5].

Imaging review—In addition to the requirement for multiple contrast-enhanced acquisitions, evaluation requires 2D and 3D multiplanar reconstruction techniques to carefully inspect cystic masses.

Imaging pearl—A small nodular component may not be readily apparent on axial sections. Specifically, interactive data set inspection with multiplanar reconstruction tailored to the exact orientation of the mass can confirm the presence of a nodule that is subtle on axial sections, to increase diagnostic confidence (Fig. 2A, 2B, 2C, 2D).

Imaging pitfall—Proper timing of the contrast-enhanced acquisition is essential for detection of small soft-tissue nodules, because the nodules may be revealed only after contrast infusion. If the corticomedullary phase image is acquired too early, a subtle enhancing nodule may be missed (Fig. 3A, 3B).

Imaging technique—Data acquisition for transitional cell carcinoma detection also requires a high-resolution technique; we acquire data with 0.6-mm detectors. The excretory phase is the crux of this protocol, because the opacified collecting system outlines mural pathologic abnormalities (Fig. 4A, 4B). It is important that the delay be extended to around 7 minutes to optimize opacification of the collecting system. Corticomedullary phase and nephrographic acquisitions are also performed to identify involvement of the renal vasculature or tumor infiltration of the kidney in larger lesions (Fig. 5A, 5B). For each acquisition, we perform thin section (0.75 × 0.5 mm) and thicker section (3 mm) reconstructions.

Imaging review—Subtle tumor nodules are often difficult to visualize on axial corticomedullary phase and nephrographic phase acquisitions (Fig. 4A, 4B) but can be identified by reviewing the delayed acquisition in multiple orientations. This acquisition is optimally displayed with coronal-oblique volume rendering and multiplanar reconstruction using a wide window, to clearly delineate any masses or nodules within the renal collecting system without beam-hardening artifact from dense contrast [6]. Patterns of transitional cell carcinoma include small nodules, larger masses, and wall thickening [7, 8]. Caoili et al. [7] performed MDCT urography with various postprocessing tools, including maximum intensity projection, average intensity projection, and volume rendering. Using a 4-MDCT scanner with 4 × 1.25 mm configuration for the excretory phase, nine transitional cell carcinomas were not identified prospectively, and three were undetected retrospectively [7]. Although current-generation MDCT scanners are capable of higher resolution, some small lesions are below the resolution of CT. The entire collecting system needs to be carefully inspected for synchronous lesions, and the coronal oblique display technique facilitates evaluation for multifocality throughout the collecting system.

Imaging pearl—When an intraluminal mass is identified in the collecting system, the most likely diagnosis is transitional cell carcinoma. However, other types of neoplasms can also arise in this location. Of note, one tumor has specific imaging and clinical findings. The mixed epithelial and stromal tumor is a usually benign lesion that occurs most commonly in perimenopausal women [9, 10]. It is believed to be hormonally influenced. Pathologically, these lesions are composed of spindle cell stroma and glandular structures, accounting for the CT appearance, which is a cystic and septated mass that often is larger than transitional cell carcinoma [9, 10] (Fig. 6A, 6B).

Imaging pitfalls—Underdistention or inadequate opacification of the ureter may prevent identification of a small transitional cell carcinoma [7].

If excretory images are obtained later than 8 minutes, contrast in the collecting system may be very dense and can cause beam-hardening artifacts.

Imaging technique—Many renal artery aneurysms are identified incidentally. However, if a patient is referred for known or suspected aneurysm, a biphasic acquisition is performed, including an early arterial phase with submillimeter detector configuration (0.6 mm) to define the aneurysm, reconstructed with narrow overlapping sections (e.g., 0.75-mm sections by 0.5-mm intervals). A late venous phase acquisition follows, at 80–90 seconds.

Imaging review—Data set interrogation for evaluation of the arterial phase volume requires multiplanar display with both 2D multiplanar reconstructions and 3D rendering (volume rendering or maximum intensity projections or both) (Figs. 7A, 7B and 8A, 8B, 8C). Aneurysm size and location should be described, with attention to the relationship of the aneurysm to nearby arterial branches (Fig. 7A, 7B). If one aneurysm is identified, carefully inspect both kidneys for others. In one study using arteriography, 32% of patients had multiple aneurysms and 19% had bilateral aneurysms [11]. The late venous phase acquisition is evaluated to assess renal perfusion.

Imaging pitfalls—In the setting of vasculitis, small intrarenal aneurysms will not be apparent during the nephrographic phase because they are obscured by parenchymal enhancement (Fig. 9A, 9B, 9C). Other findings indicative of vasculitis include subcapsular and perirenal hematoma, linear bands of decreased density, multiple infarctions of varying age, and nonspecific focal regions of decreased attenuation [12].

Small focal vascular lesions can be missed on CT. Sountoulides et al. [13] describe a patient with flank pain and hematuria who underwent imaging with contrast-enhanced CT. The arterial phase acquisition revealed a soft-tissue mass within the renal pelvis compatible with clot, versus transitional cell carcinoma; no vascular malformation was seen on CT. After a ureteroscopy was performed, with negative results, arteriography revealed an arteriovenous malformation. The CT technique was not reported, but the case reveals the importance of considering an arteriovenous malformation or vascular abnormality when hemorrhage is identified within the collecting system but no lesions are visualized at CT.

Imaging technique—For patients with suspected pyelonephritis (Figs. 10A, 10B, 10C and 11A, 11B), nephrographic and excretory phase contrast-enhanced acquisitions are performed. Compared to a 15-second acquisition, a 45-second acquisition was more sensitive for demonstrating altered perfusion [14]. It is important to recognize that pyelonephritis is the most common alternative genitourinary diagnosis in patients imaged with unenhanced CT for suspected renal calculus [15]. Referring physicians need to be educated that, if clinical findings suggest pyelonephritis, rather than performing an unenhanced CT that will potentially show no renal abnormality, it would be optimal to perform a single contrast-enhanced acquisition.

In the setting of suspected ischemia or infarct, an arterial phase acquisition is required to evaluate the arterial supply to the kidney, to exclude thrombosis, embolism, stenosis, dissection, or aneurysm (Figs. 9A, 9B, 9C and 12A, 12B, 12C, 12D). A late venous acquisition (80–90 seconds) is performed to assess for parenchymal perfusion abnormality. However, in some cases, altered perfusion may be apparent only on the arterial phase images (Fig. 12A, 12B, 12C, 12D).

Imaging review—On the nephrographic and excretory contrast-enhanced acquisitions, striated perfusion defects in the renal parenchyma reflect the presence of pyelonephritis (Figs. 10A, 10B, 10C and 11A, 11B). Wedge-shaped perfusion defects are also seen with ischemia (Figs. 9A, 9B, 9C and 12A, 12B, 12C, 12D). Both types of perfusion abnormality are generally well-visualized if the correct acquisition is inspected, regardless of display plane.

Imaging pearls—If the corticomedullary phase sequence is performed, diminished corticomedullary differentiation is a good indicator of pyelonephritis (Figs. 10A, 10B, 10C and 11A, 11B).

Perfusion defects due to either ischemic changes or pyelonephritis are very well seen on the excretory phase acquisition (Figs. 9A, 9B, 9C, 10A, 10B, 10C, 11A, 11B, 12A, 12B, 12C, 12D). Given the choice of one acquisition for pyelonephritis, we would select the excretory phase; the classic excretory pyelography findings of striated nephrogram originally described by Berliner and Bosniak [16] were based on excretory phase images.

The regions of altered perfusion on nephrographic or excretory contrast-enhanced acquisitions would actually appear as regions of hyperattenuation if the kidney was imaged 3–6 hours after the initial CT, owing to a decreased rate of contrast transit through the tubules [17].

Imaging technique—Trauma is one application where multiphasic CT is essential. The range of potential injuries mandates arterial and delayed phase acquisitions [18]. We acquire the arterial phase with a fixed delay between 30 and 60 seconds, because most trauma patients are young adults. The use of submillimeter detectors (0.6 mm) and narrow overlapping reconstructions (0.75 × 0.5 mm) for the early phase are essential to adequately visualize subtle renal artery injury, such as a small pseudoaneurysm (Fig. 13A, 13B, 13C). The delayed acquisition is performed 4–5 minutes after initiating contrast infusion. For both volumes, a second reconstruction is generated using a thicker section (3 mm).

Imaging review—In addition to axial review, the high-resolution volume should be interrogated interactively with 2D multiplanar reconstructions and 3D postprocessing tools (i.e., volume rendering and maximum intensity projection), to thoroughly evaluate the vasculature. Coronal display often improves visualization of vascular pathologic abnormalities, such as aneurysm (Fig. 7A, 7B) and pseudoaneurysm (Fig. 13A, 13B, 13C). On the early acquisition, the presence of active extravasation indicates that the patient may require emergency surgery or other interventional procedure [19]. The thicker reconstruction section volumes are reviewed for evidence of renal or other intraabdominal injury, and the delayed acquisition is timed to elucidate collecting system or bladder injury.