What is the normal appearance of the kidneys on CT and MRI?
The kidneys have an attenuation of 30 to 50 Hounsfield units (HU) on unenhanced CT. On MRI, the renal cortices have low T1-weighted signal intensity relative to the medullary pyramids and low T2-weighted signal intensity relative to the medullary pyramids. The normal kidney is “bean shaped” and has a smooth contour. Persistent fetal lobation may sometimes be seen and can give the kidney a lobulated contour.
Unenhanced CT images (but not MR images) are useful to detect renal calcifications that may be related to urinary stone disease, parenchymal calcifications due to varied causes such as hyperparathyroidism, or dystrophic calcifications in renal neoplasms. However, both CT and MR unenhanced images are useful to (1) detect presence of macroscopic fat within a renal mass to establish a specific diagnosis of renal angiomyolipoma (AML), a benign lesion, as well as to (2) serve as a reference of comparison for contrast-enhanced images to determine whether there are enhancing components within a renal lesion. In the vast majority of renal lesions, the presence of a soft tissue enhancing component is suspicious for a malignant lesion such as RCC.
The kidneys receive ≈25% of the cardiac output and therefore enhance avidly and rapidly after contrast administration. Following the intravenous administration of contrast material, sequential phases of enhancement of the kidney are seen as follows:
- • The arterial phase of enhancement occurs 10 to 30 seconds after contrast administration, where the renal arteries predominantly enhance. This phase is useful for delineation of renal arterial anatomy and pathology, as in preoperative planning for renal donor evaluation or for a complex tumor resection.
- • The corticomedullary (or cortical) phase of enhancement occurs 25 to 70 seconds after contrast administration. Here, the renal cortices are enhanced but the medullary pyramids have not yet enhanced. In this phase, the intrarenal vasculature and hypervascular renal lesions are seen to best effect. However, some small renal lesions, particularly those located in the medullary pyramids, may be difficult to visualize during this phase of enhancement.
- • The nephrographic phase of enhancement occurs 60 to 180 seconds after contrast administration, where the renal cortices and medullary pyramids are homogeneously enhanced ( Figure 35-1, C ). In this phase, hypovascular renal lesions and the renal veins are seen to best effect; most renal lesions are seen as areas with less enhancement in the enhanced normal renal parenchyma. Hypervascular renal lesions may also have washout of enhancement relative to the enhanced normal renal parenchyma, and thus are easily identified.
- • The delayed (or excretory or equilibrium) phase of enhancement occurs starting >120 to 180 seconds after contrast administration, where renal parenchyma decreases in enhancement, and the renal collecting systems increasingly opacify with excreted intravenous contrast material ( Figure 35-1, D ). This phase is most useful to evaluate the collecting systems for urothelial tumors as in a CT urogram and to evaluate the integrity of the collecting systems in patients with severe renal trauma (blunt or penetrating).