Computed Tomography Scan

CT scan ( Figures 12-2 to 12-15 ) is sensitive for many types of spontaneous disease of the kidneys, ureters, and bladder, including renal tumors and urolithiasis. It is less helpful in delineating disease such as transitional cell carcinoma within the ureters or bladder. For suspected renal colic, no IV or oral contrast is needed. A calcified stone within the kidney, ureters, or bladder usually is evident as a high-density (white) lesion on abdominal windows (see Figures 12-2 to 12-6 ). These lesions are easily seen without contrast because few other white structures should be present in the vicinity of the kidneys, ureters, or bladder. Calcified phleboliths in the pelvis may occasionally be confused with intraureteral stones.

Urinary stones: Noncontrast CT on soft-tissue windows.

Noncontrast CT has become the standard imaging modality for diagnosis of urinary tract stones and related complications. The study is useful because it can identify the size and location of stones, as well as complications such as hydronephrosis, hydroureter, or rupture of the renal collecting system as a consequence of obstruction. The size and location of stones have some prognostic power and can aid in planning of procedures to remove stones or relieve obstruction. Perhaps most importantly, noncontrast CT may reveal important alternative diagnoses such as aortic pathology and appendicitis. Noncontrast CT has the advantage of being available for rapid diagnosis, because it requires no preprocedural measurement of renal function and no preparation time for ingestion of oral contrast. In some institutions, the examination is performed with the patient in the prone position to allow bowel and other peritoneal organs to fall away from retroperitoneal urinary tract structures. In other institutions, the patient is scanned in a supine position. Thin slices (3 mm) are often obtained to allow detection of small stones. Because the abdomen does not usually contain calcified structures, calcified stones are readily visible against the background of soft tissues and fat, which are nearly black or dark gray on CT soft-tissue windows. Stones are bright white, as is calcified bone. Occasionally, other calcified structures may be found in the abdomen, including vascular calcifications in the aorta and its branches or pelvic vein calcifications called phleboliths. The latter can be difficult to distinguish from urinary stones. Some urinary stones are not visible on CT because they are not calcified. The classic example is indinavir stones—this relatively insoluble protease inhibitor used to treat human immunodeficiency virus infection can precipitate from solution, forming radiolucent stones. However, the sequela of obstruction, such as hydroureter or hydronephrosis, remains visible. Noncontrast CT can be viewed on soft-tissue or bone window settings to detect stones. Soft-tissue windows are appropriate for evaluating complications such as hydroureter and hydronephrosis, as well as perinephric stranding. In this 37-year-old man with left flank pain, a 4-mm stone is seen in the proximal left ureter (A). The ureter proximal to this is slightly dilated, consistent with hydronephrosis (C, one slice cephalad to A). B, Close-up from A. D, Close-up from C.

Urinary stones: Noncontrast CT on soft-tissue windows.

Same patient as in the previous figure. A, a small stone is present in the left mid ureter. B, a slice cephalad to A, The left kidney is shows mild pelviectasis (enlargement of the renal pelvis) but no frank hydronephrosis.

Urinary stone (small stone at ureterovesical junction): Noncontrast CT on soft-tissue windows.

This patient presented with flank pain radiating to the right lower abdomen. A, A 3-mm stone is present at the right ureterovesical junction. B, Close-up. Stones of this size and location pass spontaneously in nearly all cases.

Urinary stones (staghorn calculus) noncontrast CT.

Staghorn calculi are extensive branching urinary stones that may completely fill the intrarenal collecting system. Although small stones within the kidneys are not generally thought to cause pain, staghorn calculi are usually formed in the presence of chronic colonization or infection with the gram-negative rod Proteus and may cause pain. Occasionally, surgery is performed to remove these large stones. In this patient, a noncontrast computed tomography revealed a right staghorn calculus. Urine cultures grew Proteus. A, Soft-tissue window setting. B, Same slice viewed on a bone window.

Urinary stones (staghorn calculus): Intravenous-contrasted CT soft-tissue windows.

Same patient as in the previous figure. The patient received IV contrast. Now, the right kidney enhances, revealing a hypodense region of intrarenal collecting system that is obstructed by a branch of the staghorn. A, Axial image, B, Coronal image.

Hydronephrosis and hydroureter without urinary stones.

This 32-year-old man has a history of ureteral obstruction resulting from past ureteral stones. He now presents with flank pain and hydronephrosis, but no stones were seen on noncontrast CT. A “dual renal scan”—noncontrast CT (this figure), followed by CT with intravenous (IV) contrast (see Figure 12-8 )—was performed. This provides functional information, much like standard IV pyelography. A through C, In this noncontrast CT, the right kidney appears to have marked hydronephrosis whereas the left kidney is normal. High in the pelvis, the right ureter is readily recognized and has hydroureter, whereas the left ureter is barely visible in its normal position hugging the psoas muscle. When trying to locate the ureter using a digital picture archiving and communication system, start at the kidney and follow the ureter slice by slice to the bladder. Attempting to locate a normal ureter in midcourse is often difficult because of the small size of the normal ureter.

Hydronephrosis and hydroureter without urinary stones: Intravenous-contrasted CT.

Same patient as in the previous figure. Now, the patient has received IV contrast. A through C, The right kidney is enhancing, but no contrast has been excreted into the ureter, which remains distended with low-density urine. The left kidney is enhancing, and contrast has been excreted from the kidney into the ureter and is present along its entire course, visible as a bright white dot in transverse cross section or as a linear bright structure in coronal section (see Figure 12-4 ).

Hydronephrosis and hydroureter without urinary stones: Intravenous-contrasted computed tomography.

Same patient as in the previous figures. A coronal image shows the kidneys and ureters. The right kidney has obvious hydronephrosis and hydroureter with no contrast in the ureter. The left kidney is excreting contrast normally, and the ureter is filled with contrast. Note the difference in caliber of the right and left ureters. The distal ureters and bladder are not seen because they lie in a more anterior plane. No stones were seen in the right ureter, and no extrinsic compression was witnessed. A stricture of the ureterovesical junction on the right was suspected, likely because of prior stones.

Emphysematous pyelonephritis noncontrast CT.

A, B, Emphysematous pyelonephritis is a urologic emergency in which infection with gas-forming organisms affects the kidney. This patient has a large stone in the left renal pelvis. Notably, air is filling the renal collecting system. On typical soft-tissue windows, the black appearance of air may be difficult to distinguish from dark retroperitoneal and peritoneal fat. Lung windows make every tissue type except air white, leaving black air in stark relief.

Renal or perinephric abscess: CT with no oral or IV.

A, B, In this renal transplant patient presenting with fever and pain overlying his transplanted kidney, a noncontrast CT was performed because of concerns about potential contrast nephropathy. CT revealed perinephric abscess—although the patient’s transplant had been performed more than 3 years prior. This collection was subsequently drained under CT guidance, and cultures of the fluid grew Staphylococcus aureus. In this case, CT findings suggesting perinephric abscess include regional fat stranding near the inferior pole of the kidney, extending to the left psoas muscle—which may also be involved in the abscess. The normally well-defined margins of the psoas are obscured by this fat stranding. Note that inflammatory fat stranding is visible without administration of any contrast. If IV contrast had been administered, rim-enhancement of the abscess would be expected.

Simple renal cyst: CT with IV contrast on soft-tissue windows.

A through C, Simple renal cysts have a rounded appearance without internal septations. They do not enhance with IV contrast because they are not vascular infectious or inflammatory lesions. This patient has a simple cyst in the right kidney, as well as an enormous cystic liver lesion and ascites. These share the same fluid density and thus have the same grayscale appearance on abdominal soft-tissue windows. On the Hounsfield scale used to measure density on CT, water is assigned a value of 0 Hounsfield units (HU), air is assigned a value of 0 HU, and the densest materials such as bone have values as high as +1000 HU. The density of the fluid in the simple renal cyst is around 30 Hounsfield units, similar to that of water.

Renal masses: Renal cell carcinoma.

Noncontrast CT performed for assessment of urinary stone disease sometimes reveals underlying renal malignancies. When a malignancy is suspected, intravenous contrast should be administered to further characterize the lesion. Flank pain and painless hematuria are both sometimes presenting symptoms of renal cell carcinomas (hypernephromas). A through C, In this patient, an aggressive renal cell carcinoma has nearly replaced the left kidney. A small amount of relatively normal renal architecture can be seen anteriorly, whereas the bulk of the tumor is heterogeneously enhancing, likely because of a degree of necrosis. Incidentally, the patient has a retroaortic left renal vein (the normal course being anterior to the aorta). This vein deserves attention because renal cell carcinomas are known to invade the renal vein, enter the inferior vena cava (IVC), and embolize to the lungs. In this patient, a filling defect is seen in the left renal vein, likely representing tumor invasion. The IVC is just beginning to fill with contrast and has a heterogeneous appearance due to mixing of contrast and normal blood, so it cannot be assessed for tumor invasion. Delayed images could be obtained to identify filling defects in the IVC once the contrast appearance of the IVC has become more uniform.

Renal masses: Smaller renal cell carcinoma identified on CT with IV contrast.

A through D, A renal cell carcinoma is seen arising from the right kidney. Typical features of this malignancy include a septated or heterogeneous appearance with the administration of intravenous contrast. In comparison, benign simple renal cysts are homogeneous and have a single cyst cavity. Small renal cell carcinomas can be missed on CT without IV contrast, so IV contrast should be administered when malignancy is suspected, such as in adults with painless hematuria.

Renal infarcts: Contrast-enhanced CT on soft-tissue windows.

A, B, CT with intravenous (IV) contrast gives important information on renal perfusion. This patient presented with flank pain and hematuria and was noted to be in new-onset atrial fibrillation. Renal infarction was suspected, and both oral and IV contrast were given because of a broad differential, including possible mesenteric ischemia. The renal findings could have been delineated using IV contrast alone. The left kidney enhances uniformly. The right kidney has a peripheral wedge-shaped area of hypodensity and nonenhancement, consistent with renal infarction. This appearance is similar to what might be seen with a traumatic renal contusion, but here no history of trauma exists. Severe focal pyelonephritis has been reported to have a similar appearance, but here the history of atrial fibrillation makes infarction the leading diagnosis. In the coronal image ( B ), the window level has been adjusted slightly, accounting for the brighter appearance of bone and solid organs.

Noncontrast CT has become a test of choice because it can be performed immediately and provides several key pieces of information:

• •
  

  Acute abdominal pathology outside of the urinary tract, such as ruptured AAA, appendicitis, bowel obstruction, and free air. Studies suggest that between 4% and 10% of CT scans performed for evaluation of suspected renal colic demonstrate alternative or additional diagnoses.

  
  
• •
  

  Presence of stones within the entire urinary tract, from kidneys to urethra.

  
  
• •
  

  Size and location of urinary stones, information that is useful in predicting chance of spontaneous stone passage or need for urologic intervention.

  
  
• •
  

  Presence of hydronephrosis or hydroureter resulting from an obstructing stone (see Figures 12-7 to 12-9 ).

  
  
• •
  

  Additional stone burden within the kidney, which may predict future symptomatic urolithiasis. Stones currently in the kidney are rarely thought to cause symptoms, except in the case of staghorn calculi (see Figures 12-5 and 12-6 ).

Compared with IV urography, CT is also advantageous because it does not require contrast administration, which can cause nephrotoxicity in patients with already obstructed urinary systems. In addition, IVU does not routinely reveal nonurinary pathology. Studies comparing emergency department length of stay show an advantage to CT over IVU, the traditional standard.

Is Intravenous or Oral Contrast Needed for Detection of Stones? Does Contrast Interfere With the Diagnosis of Urinary Stones?

For detection of intrarenal and intraureteral stones, no contrast of any form is needed. Administration of oral and IV contrast can interfere with diagnosis of urologic stone disease; for this reason, noncontrast CT is often performed immediately before an IV-contrasted study. This also allows comparison of the noncontrast and contrasted CT for identification of lesions that enhance with IV contrast. The addition of contrast makes identification of stones more difficult by providing an array of white (contrast-filled) structures among which a white urolith must be sought. The kidneys enhance intensely after administration of IV contrast, so stones within the kidneys may be difficult or impossible to discern. Soon after IV contrast is administered, contrast is filtered and excreted by the kidneys and begins to fill the ureters, again potentially disguising stones within the ureters (see Figures 12-8 and 12-9 ). In cases of suspected high-grade obstruction, additional delayed images of the kidneys and ureters can be performed following IV contrast administration to assess for normal filling of the ureters, or pathologic nonfilling. This information is similar to that obtained from a standard intravenous pyelography (IVP) using conventional radiography. Oral contrast agents are not needed to detect urologic stone disease. Some CT protocols place the patient in a prone position, rather than the supine position commonly used for abdominal CT. This position leaves bowel and other intraabdominal organs in a dependent position, whereas ureters and kidneys are held tightly in their retroperitoneal position. Calcifications in the ureters may be more easily discriminated from nonurinary calcifications by this technique.

When Should Noncontrast CT Be Performed? When Should Noncontrast CT Be Followed by Contrasted CT?

This is a clinical decision, and it should be driven by the differential diagnosis. Several factors warrant consideration. First, is a life-threatening diagnosis, such as ruptured AAA or aortic dissection, under serious consideration? If so, immediate CT should be considered, perhaps without any contrast agents, depending on the stability of the patient (bedside ultrasound and surgery consultation may be appropriate in an unstable patient). AAA can be diagnosed without any contrast agents, whereas aortic dissection requires IV contrast for diagnosis. Second, is the patient old enough that the radiation exposure from multiple CT scans is not an important consideration? In patients older than 50 years, the radiation exposure from CT is unlikely to cause a clinically important cancer, and rapid diagnosis of an immediate life threat such as AAA easily trumps radiation risk. In younger patients, especially those in whom an imminent life threat is not suspected, it may be more reasonable to perform a single CT scan with IV and oral contrast to maximize the possibility of detecting nongenitourinary pathology. This strategy may be more beneficial to the patient in the long run than scanning without contrast and then scanning with contrast if no pathology is detected. At the same time, studies suggest that CT without contrast has good sensitivity for many conditions traditionally examined with contrast, including appendicitis. These studies are discussed in more detail in Chapter 9 , Imaging of Nontraumatic Abdominal Conditions. Depending on the specific differential diagnosis being considered, noncontrast CT may be the only imaging needed—for example, contrasted CT may not be required if the appendix is well visualized on noncontrast CT. An additional factor to be considered is the patient’s renal function and allergies—IV-contrasted CT should be avoided in patients with renal insufficiency or dye allergies, and noncontrast CT may be adequate to evaluate fully the differential diagnosis under consideration. Noncontrast CT generally is not performed after contrasted CT because orally administered agents may remain present for a day or more and IV contrast agents are visible for minutes to hours (in the case of urinary obstruction) within the renal collecting system, ureters, and bladder as they are excreted.

Besides the Presence of Stones, What Genitourinary Abnormalities Can Noncontrast CT Identify? What Genitourinary Abnormalities Can Be Seen With the Addition of IV Contrast?

Noncontrast CT can reveal the presence of ureteral obstruction. Hydronephrosis is recognized by the presence of a dilated renal collecting system. The unobstructed contralateral side serves as a useful comparison (see Figures 12-7 to 12-9 ). Variation in size of the proximal ureter can occur because of the presence of a normal variant extrarenal pelvis, which can simulate significant proximal hydroureter. Hydroureter (see Figures 12-7 to 12-9 ) proximal to an obstructing stone can be detected on noncontrast CT. The upper limit of normal diameter for an unobstructed ureter is 3 mm. Stranding of perirenal fat (see Figures 12-8 and 12-11 ) can be seen without any contrast agents and can occur in the setting of obstruction or infection. Stranding is caused by lymphatic capillary leak, resulting in infiltration of fluid into the perirenal fat. This increases the density of the perirenal fat relative to normal fat. Increased density on CT results in a whiter appearance, compared with the usual dark gray appearance of fat. This finding may also occur in pyelonephritis, so the urinalysis should be examined for infection. Importantly, stranding alone does not indicate infection, unless other clinical indicators of infection (such as a positive urinalysis) are present. Emphysematous pyelonephritis (see Figure 12-10 ) can be observed on noncontrast CT because air appears black and does not require contrast for visualization. Perinephric abscess (see Figure 12-11 ) can be seen as a low-density (dark gray) fluid collection adjacent to the kidney, often with stranding. Addition of IV contrast enhances an abscess. Urine collections (urinomas) surrounding a kidney because of a leaking renal pelvis or ureter have a similar appearance to abscess on noncontrast CT but may lack stranding and do not enhance with IV contrast. Isolated simple renal cysts (see Figure 12-12 ) are visible on noncontrast CT. These structures have a low Hounsfield unit density near zero because they contain fluid similar in density to water. The surrounding renal parenchyma is slightly denser and thus slightly brighter without contrast. When IV contrast is administered, renal cysts become more conspicuous because the surrounding normal kidney enhances dramatically but cysts do not. Solitary simple cysts are not usually diagnostically important because they are not typically a cause of acute pain. Polycystic kidneys may be the cause of acute or chronic pain and are readily seen on noncontrast CT. Unilateral, horseshoe, or pelvic kidneys are recognized on noncontrast CT, though these are not associated with acute abdominal or flank pain. Solid renal tumors such as renal cell carcinoma (see Figures 12-13 and 12-14 ) can be difficult to identify on noncontrast CT when small. They may be exophytic and can invade adjacent structures (see Figure 7-56 ). Addition of IV contrast helps in detection of small lesions by allowing enhancement. Renal infarction is not easily identified on noncontrast CT but is readily apparent with IV contrast, as the infarcted region fails to enhance while normal renal parenchyma vividly enhances (see Figure 12-15 ). Retroperitoneal hemorrhage ( Figure 12-16 ), whether spontaneous or resulting from trauma, can be seen on noncontrast CT. Addition of IV contrast is important in these cases as it can reveal active bleeding.

Retroperitoneal hemorrhage.

CT with IV contrast gives important information on retroperitoneal hemorrhage. Blood that has escaped before the CT scan appears dark gray and can be seen on noncontrast CT. Addition of IV contrast allows recognition of active bleeding and demonstrates normal and abnormal renal perfusion. Oral contrast is not needed. This 50-year-old man on warfarin and enoxaparin for a Saint Jude aortic valve presented with flank pain after undergoing a CT-guided percutaneous ablation of a renal mass. The CT shows no enhancement in the upper pole of the right kidney, as expected following the ablation. Fluid density (blood) is seen in the retroperitoneum surrounding the right kidney. No blush of contrast is seen within this collection, indicating that no active bleeding is present. A, Axial view. B, Coronal view.

IV contrast results in intense enhancement of the normal kidney because of the enormous blood flow to this organ. An abnormal kidney may fail to enhance compared with the normal kidney. Examples include aortic dissection (see Figure 7-83 ) or renal artery dissection. In addition, hypoperfused renal segments may not enhance normally. Examples include renal infarcts in the context of atrial fibrillation (see Figure 12-15 ). Hypoperfusion of renal segments also is seen in some cases of pyelonephritis, although no clinical significance is known and contrasted CT is not recommended for this diagnosis. As mentioned earlier, renal abscesses with rim enhancement may also be demonstrated on CT with IV contrast, though noncontrast CT or ultrasound may show a fluid collection around the kidney (see Figure 12-11 ). Perirenal fluid collections may also indicate urinomas because of a leaking urinary tract—these do not demonstrate rim enhancement with the addition of IV contrast and can be seen on noncontrast CT. Renal masses such as renal cell carcinoma (hypernephroma; see Figures 12-13 and 12-14 ) are also detected as enhancing masses on contrasted CT (these lesions may be detected, though not as perfectly delineated, on noncontrast CT). Exophytic bladder lesions such as transitional cell carcinomas may become visible as IV contrast excreted by the kidneys fills the bladder. The tumor mass may be visible as a void or filling defect in the contrast-filled bladder. Clot may have a similar effect and appearance. Dense contrast settles in the dependent portion of the bladder. As a consequence, if the bladder is not completely filled with contrast, lesions in the nondependent portion of the bladder may not be visible.

How Sensitive Is Noncontrast CT for Renal Stones?

CT is highly sensitive for most calcified ureteral stones. Its sensitivity and specificity approach 100%, with better positive and negative likelihood ratios than IVU. In some cases, no stone is seen because the patient passed the stone just before CT scan. Stones formed from indinavir, a poorly soluble protease inhibitor used in the treatment of human immunodeficiency virus infection, are not visible on CT because they are isodense with urine. However, findings of obstruction from these stones, such as hydroureter and hydronephrosis, are still readily recognized on CT. These stones can be seen by modalities such as ultrasound, and on IVU they create an obstruction picture identical to that seen with other stones.

What Is the Prognostic Value of CT for Renal Stones? What Size of Stone Will Pass? How Quickly can Stones Develop? What If the Patient Had a Scan Without Stones 1 Week Ago?

Stone size as measured on CT scan is predictive of the rate of spontaneous passage or need for surgical or procedural intervention. Only 2.2% of patients with stones smaller than 6 mm require a procedure, whereas 80% of patients with stones of at least 6 mm require a procedure. The rapidity with which stones can develop is not well studied, but in general, the absence of stones on recent CT imaging likely indicates that a large obstructing stone is unlikely.

Ultrasound

Ultrasound is a valuable tool for the assessment of obstruction of the ureters. Ultrasound has the advantage of being noninvasive, with no radiation exposure and no need for IV contrast. It is relatively inexpensive compared with CT and can be repeated over time without harm to the patient. Ultrasound is portable, and it can be used to assess for important alternative causes of symptoms, from aortic aneurysm to appendicitis to complications of pregnancy. It is the standard test for suspected renal colic in the pregnant patient because it does not expose the fetus to ionizing radiation. Renal ultrasound assesses for hydronephrosis and hydroureter ( Figures 12-17 and 12-18 ). It is limited to some extent by body habitus and operator experience, and it does not always allow visualization of the cause of the obstruction, whether that is an intraureteral stone or extrinsic compression of the ureter by a mass or retroperitoneal fibrosis. On ultrasound, calcified renal stones are hyperechoic (bright white) (see Figure 12-17 ), reflecting the ultrasound beam and preventing its transmission deep to the stone. As a consequence, they also create an acoustic shadow deep to the stone. Ultrasound can be used to detect other renal disease including atrophic kidneys, renal cysts including polycystic kidney disease, and renal masses. With the addition of Doppler ultrasound, flow in the renal arteries and renal artery stenosis can be diagnosed.

Ultrasound of renal stone.

Ultrasound can be used to assess for renal stones and complications such as hydronephrosis. Stones can be difficult to detect, whereas hydronephrosis is usually readily observed. Because stones are dense, they reflect sound and prevent its through transmission. As a result, stones are echogenic (bright) on ultrasound, and cast an acoustic shadow (black). A, Short-axis view of kidney. B, Close-up.

Hydronephrosis: Ultrasound.

Ultrasound can detect hydronephrosis readily, whereas renal stones are not as easily observed. The normal kidney has a hypoechoic (dark gray) outer cortex, whereas the renal pyramids of the inner medulla are hyperechoic (bright). The renal calyces of a normal kidney are barely visible because urine drains from them readily in the absence of obstruction. As obstruction develops, the calyces fill with urine, which is hypoechoic (black). Normal kidney (A) and mild (B), moderate (C), and severe (D) grades of hydronephrosis. Kidneys are shown in the long-axis view.

Studies comparing ultrasound and CT for the diagnosis of ureterolithiasis show comparable sensitivity and specificity (91% and 95%, respectively, for CT and 93% and 95%, respectively, for ultrasound). Some studies have shown lower sensitivity of ultrasound (61%) compared with CT (96%). Overall, ultrasound is a reasonable alternative to CT when the differential diagnosis is limited to ureteral stones and radiation reduction is a priority, as in pregnancy or young patients with multiple episodes of renal colic.

Plain Film X-ray

Plain film x-ray plays a limited role in evaluation of nontraumatic urologic emergencies. In the setting of suspected renal colic, a single plain film of the abdomen was commonly performed before the era of CT. This x-ray is often called a KUB (for kidneys, ureters, bladder)—ironically, because none of these structures is usually visible, though they may be included in the field of view. X-ray is likely insensitive for detection of renal stones: 18.6% in one study compared with a gold standard of unenhanced CT. The specificity is high, around 95%. This method may have some limited value when other imaging techniques are unavailable. It can demonstrate likely ureteroliths, although it may fail to detect noncalcified stones. In addition, extraurinary calcifications such as phleboliths may be misidentified as urinary stones ( Figures 12-19 and 12-20 A ). Although the size of the stones may be measured, plain film does not allow assessment for obstruction. In addition, sensitivity for a broader differential diagnosis, including aortic aneurysm, appendicitis, and bowel obstruction, is quite limited. Perhaps the greatest benefit of a single plain film in this setting is that the radiation exposure to the patient is low—though the information gleaned from the study is so limited that no imaging is nearly as useful. Sometimes a single x-ray to evaluate for radiodense stones is combined with ultrasound to evaluate for hydronephrosis and hydroureter. When a stone has been visualized on a prior x-ray, x-ray can be used to monitor its progression. X-ray is also used to assess the position of medical devices such as ureteral stents.

X-ray of renal stones.

X-rays of the abdomen have been used historically to diagnose and monitor urinary stones. Unfortunately, x-rays are neither sensitive nor specific. Some renal stones are relatively radiolucent and not visible on x-ray. In addition, other calcifications in the abdomen and pelvis may be indistinguishable from ureteral stones. X-ray does not show complications of urinary stones, such as ureteral obstruction and hydronephrosis. In this patient, the right ureter has been stented to relieve obstruction. Calcifications are visible in the pelvis, but these are likely phleboliths (venous calcifications) rather than ureteral stones. Notice how they lie lateral to the position of the stent in the ureter. Some also have a radiolucent (dark) center, which is a common but not pathognomonic feature of phleboliths. A, X-ray, often called a KUB (for kidneys, ureters, and bladder) because these structures lie within the field of view. Ironically, none of these structures is well seen with plain x-ray. B, Close-up.

Intravenous urography, also called pyelography.

In intravenous pyelography (IVP), contrast is injected intravenously and then excreted by the kidneys, providing a cast image of the renal calyces, pelvis, ureters, and bladder. This provides structural information because hydronephrosis, hydroureter, and points of ureteral stricture or obstruction are revealed. This also provides functional information because a nonfunctioning kidney does not excrete contrast. Together, delayed appearance of contrast in a renal collecting system and slow excretion from the collecting system is called a delayed nephrogram and is a sign of partial obstruction. An IVP consists of a preinjection plain x-ray of the abdomen and pelvis, followed by a series of x-rays of the same region conducted at intervals of 5, 15, and 30 minutes, as well as a final image after voiding of urine or contrast from the bladder. Additional delayed images may be obtained. Sometimes exophytic lesions in the bladder (neoplasms) are seen as filling defects. A through E, In this IVP series, the patient has a normal left kidney, with a collecting system that briskly fills with contrast and then empties. Structurally, this collecting system also appears normal, with no dilated calyces and a thin normal ureter. In contrast, the right renal calyces are slow to fill (because of high pressure in the collecting system from distal obstruction). The right kidney not only fills more slowly than the left but also is slower to empty, remaining markedly visible even on postvoid images. The right renal calyces and pelvis are extremely dilated consistent with hydronephrosis, and the proximal right ureter is dilated, consistent with hydroureter. There is relatively little contrast in the distal right ureter at the level of L3-L4, consistent with an obstruction proximal to this level. The patient had a ureteral stone at this level previously and likely has a stricture at this location. Phleboliths are also visible in the pelvis and may be mistaken for urinary stones.

Intravenous Urography

IVU (also called IVP) is a series of plain film images taken over time (see Figure 12-20 ). In addition to providing information about the structure of the urinary system, including the presence of stones or obstruction, this is a “functional study,” because it provides a view of renal excretion of injected contrast. First, a plain film image of the abdomen and pelvis is obtained, before the administration of contrast. This image can be examined for radiopaque stones and medical devices such as ureteral stents, as described earlier. Occasionally, an alternative diagnosis such as appendicitis or AAA may be recognized, although this modality should never be relied on to exclude these diagnoses because it is insensitive. Next, around 50 mL of iodinated contrast is injected intravenously, and the plain film (KUB) is repeated after a delay of 5 minutes. Additional images are repeated at approximately 15-minute intervals until both kidneys have been observed to fill and then empty of contrast, allowing excretion of the contrast material to be observed over time. Depending on the speed of excretion, the examination usually takes less than 1 hour, though several hours may be required in cases of severe obstruction. A normal IVU demonstrates rapid and symmetrical enhancement of both kidneys, followed by complete filling of the ureters and bladder. Several abnormalities may be observed. First, congenital anomalies may be observed, including duplications of the collecting system, and unilateral, horseshoe, or abnormally positioned (e.g., pelvic) kidneys. In the setting of ureteral obstruction from any cause, the kidney on the affected side may show delayed enhancement and delayed emptying, called a “delayed nephrogram” (see Figure 12-20 E ). This is because of diminished renal filtration and excretion of contrast in the setting of obstruction. In addition, the affected ureter may fail to fill with contrast beyond the point of a complete obstruction, or the contrast may appear less intense beyond the point of a partial obstruction. IVU provides anatomic and functional information about the location and degree of obstruction, which may be important for planning urologic intervention.

Is Intravenous Urography Preferable to CT for Evaluation of Renal Colic or Ureteral Obstruction?

IVU and CT provide similar but not identical information. CT is superior in delineating a range of emergency alternative diagnoses, explaining its rapid incorporation into the imaging algorithm for suspected renal colic in the United States. CT also has the advantage of not requiring the administration of any contrast material, with the obvious benefit of not posing a threat of contrast nephropathy or dye allergy. In addition, CT can be performed immediately and diagnostic results are available within minutes, in comparison to IVU, which takes around 1 hour in most cases and longer in cases of severe obstruction. A number of studies have compared the time, expense, and radiation exposure of CT and IVU. CT generally results in slightly shorter emergency department length of stay, similar costs (depending on whether institutional or patient costs are compared), and somewhat higher radiation exposure, though new protocols have reduced radiation exposures.

Lasix Nuclear Medicine Scan: An Alternative Test for Ureteral Obstruction

A patient with recurrent renal calculi may develop an appearance of chronic obstruction with a dilated renal pelvis and ureter by ultrasound, CT scan, or IVP. A Lasix nuclear medicine scan ( Figures 12-21 and 12-22 ) can discriminate obstruction from a flaccidly dilated but currently unobstructed collecting system. Though rarely used in the emergency department, this test may be requested by a urologic consultant. In this test, a radiopharmaceutical, technetium-99m mercaptoacetyltriglycine, is administered intravenously. Dynamic blood flow phase images of the kidneys are obtained during bolus injection of the radiotracer. Sequential 2-minute acquisitions posteriorly over the abdomen are obtained through 30 to 60 minutes, allowing generation of graphs of radioactivity over time for the renal cortex and entire kidney. Lasix (50 mg IV) is administered after around 20 minutes. In a normal scan, rapid uniform excretion occurs after administration of Lasix. This is consistent with a flaccid but unobstructed collecting system. Lack of excretion of the tracer following Lasix administration suggests obstruction.

Lasix renal scan.

Same patient as in the CT scan in Figures 12-7 to 12-9 . A Lasix renal scan is a nuclear medicine study that provides quantitative information about renal perfusion and excretion. Though rarely used in the emergency department, this study may occasionally be requested by consulting urologists when uncertainty exists about the functional effect of a chronically dilated urinary system. Note that the right–left convention for this study is reversed from that used for CT. The frames are numbered sequentially and occur at 2-minute intervals. In this scan, 10.2 mCi of a radiopharmaceutical, technetium-99m mercaptoacetyltriglycine, are administered intravenously. Dynamic blood flow phase images of the kidneys are obtained during bolus injection of the radiotracer. Sequential 2-minute acquisitions posteriorly over the abdomen are obtained through 42 minutes. In addition to anatomic images, radioactivity can be graphed over time for the renal cortex and whole kidney, providing quantitative measurements of renal parenchymal perfusion. Lasix (50 mg intravenously) is administered at 22 minutes. In this patient, the dynamic blood flow images above demonstrate relatively prompt and symmetrical perfusion to the bilateral kidneys. On the initial renogram images, the right kidney appears slightly larger than the left kidney, and there is central photopenia in the right kidney that suggests hydronephrosis. On subsequent renogram images over 42 minutes, the left kidney demonstrates relatively prompt uptake, excretion, and clearance of radiotracer with no evidence of obstruction. The right kidney demonstrates mildly delayed uptake and excretion of radiotracer into a prominent collecting system, from which there is only minimal clearance of tracer activity until the administration of Lasix. After Lasix administration, there is somewhat better, though incomplete, clearance of tracer activity over the duration of the study, with residual tracer activity seen in the right collecting system and the right ureter to level of the urinary bladder on the postvoid image. These findings suggest partial obstruction of the right ureter.

Lasix renal scan.

Same patient as in the previous figure. Quantitative information about renal perfusion can be obtained from this study. In this patient, both kidneys were found to be well perfused despite poor excretion from right kidney, consistent with obstruction. (See Color Plate 3 for color image of this figure.)

Is Any Imaging Needed for Suspected Renal Colic?

When a broad differential diagnosis is being considered, nongenitourinary diagnoses may drive the imaging strategy. Important diagnoses such as suspected AAA rupture or appendicitis may require CT or other imaging for diagnosis. In effect, though CT may confirm urolithiasis, the most important function of CT may be to exclude other diagnoses. When renal colic is the only serious diagnosis under consideration, the emergency physician should carefully consider whether any imaging is required. CT carries a moderately high radiation exposure, and patients with renal colic often have recurrent symptoms, resulting in multiple CT exposures over time. When complications such as infection or renal insufficiency are not present, deferring CT imaging may be reasonable. In first-time episodes of suspected renal colic, clinical judgment is incorrect in as many as 20% of cases, even when suspicion of stone is 90% or greater. Imaging of first episodes is likely warranted. Alternative or additional diagnoses are reported in approximately 10% of patients undergoing CT for suspected renal colic, although not all of these diagnoses require specific emergency treatment. In patients with recurrent symptoms, another reasonable strategy may be ultrasound to evaluate for hydronephrosis or hydroureter. Ultrasound may show a stone, though with lower sensitivity than CT. In patients older than 50 years, the risk of repeat CT radiation exposures may be relatively unimportant, and the risk of alternative causes of symptoms rises. CT may be a prudent strategy in this age group.