Genitourinary Imaging Review Wolin et al. CT Urography Principles, Patterns, and Genitourinary Imaging Review FOCUS ON: Ely A. Wolin 1 David S. Hartman J. Ryan Olson Wolin EA, Hartman DS, Olson JR Keywords: CT urography, nephrogram, pyelogram, rim, striated DOI:10.2214/AJR.12.9691 Received July 31, 2012; accepted after revision November 25, 2012. 1 All authors: Department of Radiology, Penn State College of Medicine, The Milton S. Hershey Medical Center, 500 University Dr, Hershey, PA 17033. Address correspondence to E. A. Wolin (ewolin@hmc.psu.edu). AJR 2013; 200:1210 1214 0361 803X/13/2006 1210 American Roentgen Ray Society Nephrographic and Pyelographic Analysis of CT Urography: Principles, Patterns, and OBJECTIVE. Careful analysis of the nephrogram is an integral component of the evaluation of the kidneys during CT urography. This is a review of the anatomy and physiology required to produce a normal nephrogram, as well as the basic principles affecting the interpretation of the nephrogram. CONCLUSION. The eight abnormal nephrographic patterns will be illustrated and discussed. This review will enable the reader to recognize and understand the abnormal nephrographic patterns encountered in practice. C T urography provides unmatched morphologic detail of the renal parenchyma and collecting system. Equally important, however, is the unique functional and physiologic information provided by careful evaluation of the nephrogram. Important nephrographic parameters to evaluate include the time of appearance of the contrast agent in the kidneys, the time of disappearance, and the density of the nephrogram and the pyelogram. This article will present the basic principles of nephrographic analysis and discuss the pathophysiological mechanisms of eight basic abnormal nephrographic patterns. The differential diagnosis of each pattern will be discussed. The emphasis in this review article is patterns that involve a single entire kidney. However, these principles may be extrapolated to explain abnormalities in both kidneys or in a portion of one or both kidneys. Although this article will emphasize CT urography, these same principles can be used for the other renal imaging modalities that can assess renal function: MR urography, excretory urography, renal angiography, and nuclear renography. Basic Principles There are several basic principles that affect the density and time of appearance of the normal nephrogram and pyelogram that will be discussed in detail in this section: first, the three phases of contrast media excretion; second, symmetry of these three phases; third, implications of asymmetric appearance of these phases; fourth, factors in the density of the nephrogram; fifth, the transit time of contrast media; sixth, the requirements for normal imaging of the kidneys; seventh, causes of an abnormal nephrogram; and finally, collateral circulation to the kidney. The three phases of contrast media excretion include the vascular phase, nephrographic phase, and pyelographic phase. The vascular phase is also referred to as the angiographic or corticomedullary phase. It appears 25 80 seconds after injection of IV contrast material [1]. The nephrographic phase is present approximately 90 seconds after injection of IV contrast material. The pyelographic or excretory phase is present 3 minutes after injection of IV contrast material (Fig. 1). In normal kidneys, the three phases of contrast media excretion are always symmetric in time of appearance, time of disappearance, and density [2]. In cases of asymmetric appearance of any phase, the delayed side is the diseased side. There is no cause for a premature corticomedullary, nephrographic, or pyelographic phase. The density of the nephrogram and pyelogram are dependent on the plasma concentration of iodine, the glomerular filtration rate, and the transit time of contrast agent through the kidney [2]. The normal transit time of contrast media from the renal artery to the renal pelvis may be delayed by diminished systemic blood pressure; obstruction of the renal artery, vein, or collecting system; or abnormal nephron function. There is no mechanism to increase the transit time of contrast agent through the kidney. The den- 1210 AJR:200, June 2013
CT Urography Principles, Patterns, and sity of the pyelogram is also dependent on the degree of hydration. Normal imaging of the kidneys requires an anatomically normal kidney, normal blood flow into the kidney, normal blood flow away from the kidney, normal unobstructed urine flow from the kidney, and normally functioning nephrons. In a similar fashion, an abnormal renal nephrogram will result from alterations in one of these five parameters. Most of the arterial blood flow is to the cortex via the renal artery, the interlobar arteries, the arcuate artery, and the interlobular arteries. The renal medulla is supplied by the vasa recta [3]. With occlusion of renal arteries, collateral vessels may supply the nephrons, which are subcapsular or adjacent to the renal medulla (see the Rim Nephrogram subsection). Renal cortical necrosis results in vascular redistribution, with blood flow away from the cortex and to the medulla (see the Reverse Rim Nephrogram subsection). The main driving force for urine production is filtration pressure, which is related to blood pressure. Hypotension or any factor affecting filtration pressure (e.g., obstruction of the renal artery, vein, or collecting system) will affect the transit time of contrast media through the kidney. Once contrast material is within the renal parenchyma, the vast majority is filtered. As a result, most of the contrast agent that gets into the nephrons will eventually arrive within the collecting system. Eight Abnormal Nephrographic and Pyelographic Patterns and In this section, we will review eight abnormal nephrographic and pyelographic patterns: unilateral absent nephrogram, unilateral delayed pyelogram or nephrogram, unilateral hyperdense nephrogram, bilateral persistent nephrogram or bilateral delayed pyelogram, rim nephrogram, reverse rim nephrogram, striated nephrogram, and spotted nephrogram. Unilateral Absent Nephrogram The term absent nephrogram refers to a lack of any functioning kidney in the renal fossa. This, therefore, includes an absent kidney as well as an abnormal kidney, which is present but does not enhance (Fig. 2). Mechanisms for this pattern include congenital or surgical absence of the kidney, renal ectopia, complete arterial occlusion (embolic or thrombotic), complete renal vein thrombosis, complete longstanding collecting system obstruction with parenchymal atrophy, or absent nephrons (congenital or acquired). Before making a diagnosis of absent kidney, it is important to look carefully for an ectopic kidney (especially a pelvic kidney or a crossed fused ectopic kidney). Unilateral Delayed Pyelogram or Nephrogram With the unilateral delayed pattern, the diseased kidney is delayed in either the angiographic or nephrographic phase while the normal kidney is in the nephrographic or pyelographic phase, respectively (Fig. 3). Mechanisms for this pattern include increased transit time of contrast agent in the diseased kidney or unilateral abnormal nephron function. Increased transit time may result from slow blood in, slow blood out, or slow urine out. Unilateral poor nephron function is usually a result of acute pyelonephritis. Unilateral Hyperdense Nephrogram With the unilateral hyperdense pattern, the nephrogram of the diseased kidney becomes denser than that of the contralateral normal kidney and has delayed excretion (pyelographic phase) (Fig. 4). Contrast agent in the delayed or diseased kidney has delayed transit. Because of the continued tubular absorption of the sodium and water, the concentration of the iodine within the nephrons is increased. With the exception of pyelonephritis, the entities in this pattern are identical to the differential diagnoses of the unilateral delayed pyelogram or nephrogram pattern. Acute pyelonephritis does not have a hyperdense nephrogram unless there is concomitant obstruction. Bilateral Persistent Nephrogram or Bilateral Delayed Pyelogram With the bilateral persistent or delayed pattern, both kidneys remain in the angiographic or nephrographic phase longer than 3 minutes after contrast agent injection, which is the usual time of the pyelographic phase (Fig. 5). The bilateral pyelograms are also delayed. Contrast agent remains in the cortex or cortex and collecting tubules. The mechanism of this pattern is slow progression of contrast material through both kidneys. If this pattern is present days after contrast agent administration, it is usually because of functional tubular obstruction (e.g., acute tubular necrosis). When there is an absence of both pyelograms on a 3-, 5-, or 10-minute image, systemic hypotension is the probable cause. Rim Nephrogram With the rim pattern, the diseased kidney only has enhancement peripherally within the subcapsular zone and centrally in the cortex immediately adjacent to the medulla [4, 5] (Fig. 6). The pathophysiology of this pattern is the result of occlusion of the normal vascular blood supply. After occlusion of the main renal artery or vein, the capsular, peripelvic, and periureteric vessels enlarge. These collateral vessels supply the nephrons in the subcapsular and juxtamedullary regions [6]. The remaining renal parenchyma is not perfused and does not enhance. The rim nephrogram is not seen immediately after global infarction. Rather, it usually takes several days or weeks to become apparent. Eventually, all nonperfused parenchyma will undergo atrophy, with a resultant small smooth kidney. Reverse Rim Nephrogram With the reverse rim pattern, the only enhancement is seen centrally within the renal medulla [4] (Fig. 7). There is no cortical enhancement. This pattern is almost always identified in the setting of acute cortical necrosis, a rare form of acute renal failure. Cortical necrosis may result from any cause of prolonged systemic hypotension, especially obstetrical complications, transfusion reactions, and transplant rejections [4]. The exact pathophysiology of vascular redistribution is a poorly understood process. It may be due to vasospasm of the cortical vessels due to circulating toxins. With prolonged systemic hypotension, there may be intrarenal vascular shunting redirecting blood from the cortex to the medulla. The necrotic cortex will appear as a hypodense rim surrounding the perfused medullary system [7, 8]. With time, the necrotic cortex atrophies and calcifies resulting in cortical nephrocalcinosis. Striated Nephrogram With the striated pattern, the normally homogenous nephrogram is replaced by alternating bands of enhanced and unenhanced renal parenchyma (Fig. 8). The bands are arranged radially, similar to the orientation of the collecting ducts. The striations extend through the entire renal parenchyma from the capsule to the papilla. There are several theories of the pathophysiology of the various densities. The lowattenuating bands may represent collecting tubules that do not contain contrast material or edema between nephrons. The bands that enhance may be of normal density or hyperdense relative to normal parenchyma. The hyperdense bands result from tubular obstruction (e.g., intratubular obstruction from pus in pyelonephritis) with resultant hyperconcen- AJR:200, June 2013 1211
Wolin et al. tration. Alternatively, the dense bands may be secondary to contrast agent extravasation from collecting tubules and small vessels or variable perfusion due to vasospasm [5]. Spotted Nephrogram With the spotted pattern, the normally homogenous nephrogram is replaced by segmental and subsegmental areas of nonenhancement [4] (Fig. 9). The zones of nonenhancement are more amorphous than the linear bands seen in the striated nephrogram (Fig. 8). The pathophysiologic basis for the zones of nonenhancement is segmental (lobar) and subsegmental infarctions as a result of small-vessel occlusion. Conclusion CT urography provides invaluable information about the morphologic and physiologic information when used to evaluate the kidneys. The basic principles and patterns involving the density and timing of contrast enhancement of the renal parenchyma can help in the diagnosis of different disease process. References 1. Yuh BI, Cohan RH. Different phases of renal enhancement: role in detecting and characterizing renal masses during helical CT. AJR 1999; 173:747 755 2. Davidson AJ, Hartman DS, Choyke PL, Wagner BJ. Nephrographic analysis. In: Davidson AJ, Hartman DS, Choyke PL, Wagner BJ, eds. Davidson s radiology of the kidney and genitourinary tract, 3rd ed. Philadelphia, PA: Saunders, 1999:667 683 3. Davidson AJ, Hartman DS, Choyke PL, Wagner BJ. Radiologic anatomy and anomalies of the kidney and ureter. In: Davidson AJ, Hartman DS, Choyke PL, Wagner BJ, eds. Davidson s radi- Fig. 1 Images show different phases of CT urogram as contrast agent enters and leaves left kidney over time. ology of the kidney and genitourinary tract, 3rd ed. Philadelphia, PA: Saunders, 1999:41 72 4. Dyer RB, Chen MY, Zagoria RJ. Classic signs in uroradiology. RadioGraphics 2004; 24:S247 S280 5. Saunders HS, Dyer RB, Shifrin RY, Scharting ES, Bechtold RE, Zagoria RJ. The CT nephrogram: implications for evaluation of urinary tract disease. RadioGraphics 1995; 15:1069 1085 6. Amilineni V, Lackner DF, Morse WS, Srinivas N. Contrast-enhanced CT for acute flank pain caused by acute renal artery occlusion. AJR 2000; 174:105 106 7. Kawashima A, Sandler CM, Ernst RD, Tamm EP, Goldman SM, Fishman EK. CT evaluation of renovascular disease. RadioGraphics 2000; 20:1321 1340 8. Jordan J, Low R, Jeffrey RB. CT findings in acute renal cortical necrosis. J Comput Assist Tomogr 1990; 14:155 156 1212 AJR:200, June 2013
CT Urography Principles, Patterns, and Fig. 2 32-year-old man with absent left kidney (absent nephrogram). Right kidney is normal (pyelographic phase). Colon fills left renal fossa. There was no ectopic kidney. Fig. 4 29-year-old man with distal ureteral obstruction. Unilateral hyperdense pattern is seen on nephrogram. CT was obtained 12 hours after administration of IV contrast agent. Unilateral hyperdense pattern is due to slow urine out secondary to distal ureteral obstruction. Fig. 3 Patient with obstruction and hydronephrosis. Unilateral delayed pattern is seen on nephrogram, which shows that left kidney is in corticomedullary phase (delayed) while right kidney is in pyelographic phase. Fig. 5 82-year-old man with acute tubular necrosis. Bilateral persistent or delayed pattern is seen on nephrogram. Contrast agent was given 3 days ago, and both kidneys are in corticomedullary phase with absent pyelograms. Note absence of contrast bolus in aorta. These findings are consistent with acute tubular necrosis. Fig. 6 21-year-old man with complete occlusion of main renal artery resulting from intimal flap dissection 10 days before. Rim pattern is seen on nephrogram. Only enhancement is peripherally in subcapsular zone and centrally in cortex adjacent to medulla. Fig. 7 33-year-old man with acute cortical necrosis. Reverse rim sign is seen. With certain disease states, such as prolonged systemic hypotension resulting from hemorrhage, there may be intrarenal vascular shunting from cortex to medulla, as in this case. If not reversed, affected nephrons undergo necrosis and cortical nephrocalcinosis. AJR:200, June 2013 1213
Wolin et al. Fig. 8 25-year-old woman with bilateral pyelonephritis. Striated pattern is seen on nephrogram. There are alternating bands of enhanced and nonenhanced renal parenchyma. Fig. 9 13-year-old boy with polyarteritis nodosa. Spotted pattern is seen on nephrogram (left). Contrast-enhanced CT shows that normally homogenous nephrogram is replaced by segmental and subsegmental areas of nonenhancement. Aortogram (right) shows multiple small aneurysms involving both kidneys. 1214 AJR:200, June 2013