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Everything you need to know about Computed Tomography (CT) & CT Scanning


Kidney: Spiral CT of the Kidney: Principles, Technique, and Applications

Susan H. Wyatt, M.D.1
Bruce A. Urban, M.D.
Elliot K. Fishman, M.D.

All authors: the Department of Radiology and Radiological Science, The Johns Hopkins Hospital, 600 N. Wolfe St., Baltimore, MD 21205.

 

Introduction

Spiral CT provides many advantages in evaluation of the kidney. Spiral scanning essentially eliminates data misregistration, and allows for visualization of the entire kidney during peak contrast enhancement (1,2). The generated dataset is free from motion artifact, which improves multiplanar reconstructions. These factors are of vital importance for the evaluation and characterization of small renal tumors (3). Increased sensitivity in the detection of subtle asymmetries of the cortical nephrogram is also obtained, which is useful in diagnosing disorders such as renal artery stenosis, renal vein thrombosis, acute pyelonephritis, and renal obstruction. This chapter provides an overview of spiral CT evaluation of renal pathology, illustrating the advantages of spiral scanning in the evaluation of both renal inflammatory diseases and renal masses.

 

Spiral Technique: Kidney

Dynamic CT scanning of the kidneys is accepted as the preferred method for evaluation of renal pathology (4-6). Bolus IV enhancement aids in the detection and characterization of renal masses and parenchymal disease, allows for assessment of tumor vascularity, and helps to differentiate juxtarenal from intrarenal processes. Dynamic scanning is most sensitive during the early cortical nephrogram phase of renal enhancement, when corticomedullary differentiation is maximal (5). However, in obtaining images during this early phase of enhancement, conventional scanners limit evaluation to the kidneys alone. A major advantage of spiral CT as compared to conventional dynamic CT is the ability to image the entire length of the kidneys during this ideal period of enhancement, in conjunction with routine dynamic evaluation of the liver and pancreas (7). With newer software providing for extended spiral capabilities, we can now obtain ideal contrast-enhanced images of the kidneys in routine evaluation of the entire abdomen.

Therefore, we perform spiral scanning of the kidneys using the same protocol as described for the liver (8). A localizing topogram is obtained, and an initial axial section is selected in the upper third of the liver. Subsequently, 120 cc of non-ionic contrast is peripherally administered through a power injector, ideally at a rate of 3 cc/sec. Spiral scanning begins within 10 seconds following the termination of contrast injection. The table incrementation rate is 8 mm/sec, with 8 mm collimation at 210 mAs and 120 kVp. Overlapping reconstruction is usually obtained using 4 mm interscan spacing. Reconstructed multiplanar and three-dimensional analysis is performed in selected cases, especially in evaluation of vascular anatomy or suspected renal artery stenosis.

A few clinical vignettes should alter the routine renal spiral protocol. Initial pre-contrast scanning is performed if renal calculi are clinically suspected. In cases of suspected transitional cell carcinoma, renal calculi, or obstruction, delayed scans are obtained to allow for collecting system opacification. In patients with known pathology presenting for follow-up exam, or if a more directed evaluation of the kidney is desired, evaluation with more narrow collimation can be performed. This results in smaller reconstructed slices, which can improve multiplanar reconstruction and lesion characterization (9,10). To reconstruct thinner slices, a localizing topogram is obtained just superior to the kidneys. Spiral parameters here are 4 mm/sec incrementation, 4 mm collimation, with 2 mm interscan spacing. Other parameters and contrast rates are identical to the routine spiral protocol. Finally, three-dimensional spiral CT angiography may be beneficial in the evaluation of suspected renal artery stenosis (see chapter ______ ), as well as for presurgical planning in cases of renal tumors.

 

The Normal Kidney

Following bolus intravenous injection, the renal parenchyma demonstrates four distinct phases of enhancement: the cortical angiogram, the glomerulogram, the cortical nephrogram, and the tubular nephrogram (11). The first two phases occur within a few seconds and are not routinely imaged. Routine spiral CT scanning occurs during the peak of the third phase of renal enhancement, the cortical nephrogram phase. At this time (approximately 20 to 70 seconds after injection), contrast fills the cortical capillaries, peritubular spaces, and proximal convoluted tubules, resulting in exquisite corticomedullary differentiation (5, 11,12). The density of the cortical nephrogram and the degree of corticomedullary differentiation obtained on the spiral scan depends on the patient's renal function and cardiac output, as well as on the rate and volume of contrast administration (11). In our experience, bolus injection at 2 to 3 cc/sec usually results in cortical enhancement ranging from 100 to 200 HU.

Unlike conventional dynamic scanning, spiral CT is routinely able to image the entire length of the kidneys during this ideal period of enhancement. This can be extremely helpful in the detection of subtle parenchymal disease or small masses. This can also help to discriminate normal cortical variants such as a prominent column of Bertin or dromedary hump from an abnormal renal mass. Spiral CT provides increased sensitivity in the detection of subtle asymmetry of the cortical nephrogram, which reflects abnormal hemodynamics in the setting of either abnormal perfusion or delayed tubular transit (12).

The renal arteries are confidently displayed on spiral CT, and this has application in the evaluation of renal artery stenosis or thrombosis (13,14). Most spiral scans also provide sufficient opacification of the renal veins, although opacification may be delayed in patients with poor renal function. The inferior vena cava is normally non-opacified on spiral scan. In patients with renal cell carcinoma and suspected vascular invasion, delayed scans may to be obtained to better visualize the renal veins and inferior vena cava.

Following spiral data acquisition, conventional delayed images may be obtained during the pyelogram phase of renal enhancement, which routinely begins three to four minutes following injection. At this time, contrast normally fills the renal collecting system and ureter. We routinely obtain delayed scans in evaluation for suspected tumor or calculi in the collecting system, or in cases of obstruction.

Renal Neoplastic Disease

Renal Cell Carcinoma

CT is widely accepted as the preferred modality for detection and staging of renal cell carcinoma (15-22). The typical carcinoma presents as a bulky, heterogeneous mass that demonstrates inhomogeneous enhancement, yet remains relatively hypodense compared to the normally enhancing renal parenchyma. Cystic renal cell cancers are differentiated from complicated benign cysts by their thick irregular wall and tumor nodularity. Local spread of renal cell carcinoma typically occurs into the renal vein or perinephric spaces.

Spiral CT offers many advantages in the evaluation of renal cell carcinoma. The uniform peak vascular opacification that is routinely achieved with spiral scanning improves both detection and characterization of the typically hypervascular renal cell carcinoma. Prognostic significance may also follow, in that tumor size and vascularity may correlate with a renal cell carcinoma's ability to metastasize (23). Optimal vascular opacification also enables the detection of extension of a renal cell carcinoma into the renal vein and inferior vena cava. Venous extension of renal cell carcinoma appears to be more common with right sided tumors, with approximately 10% of renal cell cancers invading the inferior vena cava (24, 25). Tumor thrombus presents as a luminal filling defect , and vascular collaterals can be occasionally be demonstrated. Delayed scans may improve visualization of the inferior vena cava, which can normally be nonopacified on an early spiral scan.

A particular problem with conventional CT in the precise delineation of the degree of tumor extension into the inferior vena cava. This is particularly true when thrombus approaches the right atrium. Currently, MR imaging is the most useful modality for tumor thrombus evaluation (26). However, multiplanar reconstruction and 3D imaging obtained from the motion-free spiral dataset may prove as effective in the depiction of tumor thrombus extension. Three dimensional models of the kidneys and their vascular supply can also be helpful in surgical planning for patients eligible for partial nephrectomy.

A major advantage of spiral CT over conventional scanning is the increased frequency of detection of small renal cell carcinomas. This advantage results in part from data continuity provided by a single-breath-hold acquisition, which eliminates breathing misregistration and partial volume averaging. This allows for increased visualization of smaller lesions that may otherwise have been missed. Peak contrast enhancement also helps to differentiate small tumors from brightly enhancing normal parenchyma.

Diagnosis of the small renal cell carcinoma also depends, in part, on the detection of "significant" contrast enhancement. Currently, only relatively large increases in attenuation value (>20 HU) indicate true enhancement on conventional CT, in part because smaller increases of attenuation are artificially produced by partial volume averaging (27). By eliminating this potential source of error, spiral scanning may allow for a decrease in the currently accepted "threshold" level of significant enhancement, thereby increasing our specificity in the characterization of the small renal mass.

Many of these small (< 3 cm.) renal cell carcinomas are discovered incidentally. Because of disagreement about their clinical course, detection of these lesions poses a problem in clinical management (18). In the past, these were largely felt to represent adenomas with a low potential for metastasis, although presently, most believe these lesions should all be considered carcinomas (16, 20). Because there is no effective cure for advanced renal cell cancer, a potential increase in early detection of small renal cell carcinomas with spiral scanning could provide a valuable role in increasing overall patient survival.

Lymphoma

Renal lymphoma invariably results from hematogenous dissemination. Lymphoma can demonstrate multiple appearances on CT (6). Most commonly, the kidneys show diffuse enlargement, with other variations including bilateral focal masses, a single dominant mass, or direct invasion by adjacent adenopathy. Lymphomatous masses typically are homogeneous, and demonstrate minimal contrast enhancement (10-20 HU). The appearance of focal lymphoma is indistinguishable from renal cell carcinoma. Small or subtle lesions are detected on spiral CT with increased frequency, accuracy, and confidence.

Angiomyolipoma

Angiomyolipomas are focal hamartomas classically seen in middle-aged females or patients with tuberous sclerosis (28). Spiral scanning can help in the detection of subtle foci of fat within a renal mass. This is considered almost pathognomonic for angiomyolipoma, although recent reports have described subtle fat deposits in renal cell carcinoma (29, 30). Improved lesion characterization provided by the virtual elimination of partial volume averaging can help to differentiate angiomyolipomas from small cysts. Angiomyolipomas can grow quite large and appear almost entirely eccentric to the renal parenchyma. Spiral scanning can aid in demonstration of the invariable attachment to the underlying kidney, thus allowing for diagnostic differentiation from retroperitoneal liposarcoma. Finally, accurate evaluation of larger lesions (greater than three or four centimeters) can be helpful for potential presurgical planning.

Transitional Cell Carcinoma

Transitional cell carcinoma accounts for approximately 5% of all renal tumors. Patients frequently present with hematuria. In conjuction with intravenous urography, CT has an important role in the evaluation and staging of transitional cell carcinoma. Several distinct patterns of involvement are described on conventional CT (31). Most often, tumors present as sessile intraluminal filling defects. Contrast can pool around the tumor mass on delayed views. Other presentations include diffuse ureteral or pelvic-wall thickening, or renal parenchymal invasion that can mimic the appearance of renal cell carcinoma.

When performing a spiral CT scan of the kidneys, careful attention must be directed to the renal pelvis. Routine spiral scanning does not opacify the renal collecting system. Therefore, in patients with any subtle suggestion of intraluminal filling defect, or in studies performed for evaluation of hematuria or known transitional cell carcinoma, delayed scans are necessary for complete evaluation of the renal pelvis and ureter. While delayed scanning markedly improves detection and conspicuity of transitional cell carcinoma, we have found that most tumors can be suggested even before obtaining the delayed scan.

Other Tumors

A variety of other neoplasms can commonly involve the kidney. Wilm tumor is the most common pediatric renal tumor, and usually presents as a large mass which can invade the renal vein and inferior vena cava. Collateral vessels may be demonstrated. Tumor extension is accurately demonstrated using multiplanar reconstruction generated by the spiral dataset. Oncocytomas usually demonstrate features identical to renal cell carcinoma, although demonstration of a central scar may suggest the diagnosis. Most, if not all, patients still require nephrectomy to exclude renal cell carcinoma. Metastases may present as single or multiple masses. Some have advocated partial nephrectomy as treatment for small metastases, which are detected with increased frequency on spiral scanning (20).

 

Renal Cystic Disease

The Simple Cyst

Renal cysts are round and usually cortically-based. On CT, cysts demonstrate sharp smooth margins with the normal renal parenchyma. Attenuation values are near that of water (-10 to 20 HU), with no increase after the administration of contrast. Because of peak contrast enhancement, spiral CT is ideally suited to help define the walls and clearly define the smooth transition with normal, uniformly enhancing renal cortex.

Conventional CT can result in errors in the diagnosis of the simple cyst (32). Errors can result from partial volume averaging of small (< 1 cm) cysts or those that are small and completely intrarenal, both of which may falsely elevate the attenuation number. Another common error is the impression of a thick cyst wall that occurs when the parenchymal beak is imaged in cross-section (33). By virtually eliminating partial volume averaging, spiral scanning can increase our specificity in the diagnosis of the simple renal cyst, thereby avoiding the erroneous diagnosis of an "indeterminate renal mass." Similarly, "indeterminate masses" found on ultrasound or conventional CT can be confidently characterized on spiral CT (3).

Renal cysts are associated with syndromes such as tuberous sclerosis or von Hippel-Lindau disease. In tuberous sclerosis, the cysts are often small, rarely exceeding three centimeters in diameter (34). Multiple angiomyolipomas are characteristic of tuberous sclerosis. Patients with von Hippel-Lindau disease can also demonstrate multiple renal cysts; however, these cysts may be a precursor to malignancy. Tumors in this setting often present as masses arising from the cyst wall, and are frequently bilateral. Careful evaluation is necessary in these patients, as tumors are often small (< 2 cm) and very difficult to distinguish from other cysts (35).

The Complicated Cyst

Complicated cysts can be seen in the setting of superimposed infection or hemorrhage. The benign, complicated cyst can be difficult to differentiate from a cystic and septated renal cell carcinoma. In general, if the septae are thin (1 mm or less) and smooth, a benign diagnosis is made with confidence. The septations should not demonstrate enhancement. Minimally complicated cysts may demonstrate calcification in the septations or cyst wall(36). Infected cysts can demonstrate markedly thickened walls that occasionally calcify (32).

If septations measure greater than 1 mm in thickness or are associated with solid or enhancing elements at their attachment to the cyst wall, malignancy must be considered (36). Because of data continuity provided from a single breath hold acquisition, spiral CT can visualize the margins of cystic lesions with great precision. This can be helpful in the detection of subtle nodularity or enhancement in the wall of a cystic neoplasm (3).

Hyperdense renal cysts can be confused with solid masses on CT. Most hyperdense cysts result from hemorrhage or proteinaceous debris, with hemorrhage complicating approximately 6% of simple cysts (32). Two-thirds of patients with polycystic kidney disease have hyperdense cysts (18, 37, 38).

Parapelvic Cysts

Parapelvic cysts are likely lymphatic in origin. They can be single or multiple, and commonly present in the perihilar region. CT characteristics follow those of the simple cyst. Parapelvic cysts can mimic hydronephrosis or the renal pelvis on dynamic spiral CT evaluation. Delayed scans, which demonstrate characteristic compression of the renal pelvis, are necessary to establish the proper diagnosis.

Polycystic Kidney Disease

Polycystic kidney disease usually manifests clinically during the third or fourth decade. Patients can present with flank pain, hematuria, hypertension, infection, or a palpable abdominal mass. Some patients are asymptomatic. Spiral CT demonstrates enlarged kidneys with multiple, bilateral cysts of varying size. Cysts can also be demonstrated in the liver, spleen and pancreas. The hyperdense cyst (60-90 HU) is a common complication, seen in up to two-thirds of patients (37, 38). It usually results from prior post-traumatic hemorrhage, followed by clot retraction and protein concentration (39, 40). Cyst wall calcification then results in approximately 25% of patients, usually those with more advanced disease. This finding alone is not worrisome for neoplasm unless an associated soft tissue mass also is present (41).

Flank pain with hematuria is a common clinical dilemma in a patient with polycystic kidney disease. This may result from cyst hemorrhage, calculi (found in 20-36% of patients), or infection (41). Spiral CT is ideally suited for differentiating these complications. We have found the early dynamic images obtained during spiral CT to be more sensitive than conventional delayed scanning in the detection of secondary pyelonephritis. Very rarely, neoplasm can complicate polycystic kidney disease (42, 43).

 

Acquired Cystic Disease of Uremia

While the exact etiology is unclear, cysts can complicate renal failure, typically three or more years after the onset of hemodialysis. Uremic cysts can regress after renal transplantation (44). The kidneys in acquired cystic disease of uremia are typically small, and the cysts are commonly intrarenal in location. There is a significant risk of renal adenomas and carcinomas in these patients. Spiral CT can help differentiate simple uremic cysts from complicating tumor.

 

Renal Inflammatory Disease

Acute Pyelonephritis

Acute pyelonephritis is usually a clinical diagnosis, although increasingly, patients are referred for CT evaluation. Ascending retrograde infection typically spreads through the collecting ducts into the renal parenchyma (45). Increased interstitial pressure results in obstruction of the tubules and intense focal vasoconstriction of the blood vessels. Involvement may be focal or diffuse, and the infected kidney is generally enlarged. Enhanced CT scans demonstrate focal areas of striated or wedge-shaped perfusion abnormality, resulting in a characteristic "patchy" nephrogram (46).

Poorly enhanced areas of focal pyelonephritis can mimic a renal mass on conventional CT. Spiral scanning is probably more specific in differentiating infection from tumor. Spiral CT is also helpful in detecting subtle cases of acute pyelonephritis. Abnormalities only visualized during early dynamic contrast enhancement such as loss of the normal, sharp corticomedullary differentiation and delayed appearance of the cortical nephrogram, are clues to the diagnosis. Delayed views of the infected kidney may demonstrate a dense nephrogram.

In the absence of direct visualization of thrombus or vascular collaterals, acute renal vein thrombosis can sometimes mimic conventional CT findings of acute pyelonephritis (47). Both entities can demonstrate renal enlargement and poor renal function. In acute pyelonephritis, low attenuation wedge-shaped defects extend to the renal cortex, with ill-defined or poorly enhancing parenchymal lesions occasionally seen (48, 49). In renal vein thrombosis, focal low attenuation areas are better defined, representing the renal medulla surrounded by high attenuation cortex (47). Spiral scanning during peak contrast enhancement can help to differentiate these patterns.

Chronic Pyelonephritis

Changes resulting from prior renal infection constitute chronic pyelonephritis. Spiral CT demonstrates focal parenchymal scars with underlying calyceal distortion, typically most pronounced in the polar regions of the kidney. The kidneys are often small, and renal calculi are frequently seen.

In addition, chronic scarring can produce an irregular, lobulated cortical surface that may mimic a renal mass on sonography or conventional CT. Spiral CT nicely demonstrates the normally enhancing cortex, and thus improves detection of focal areas of cortical scarring or thinning. Thus, spiral CT helps to distinguish focal scarring from a renal mass. Delayed scans are often helpful to demonstrate the underlying blunted calyces, and to confirm the location of renal calculi.

Xanthogranulomatous pyelonephritis is an uncommon form of chronic inflammatory disease, felt to result from obstruction in the presence of infection. CT findings here include a staghorn calculus, an absent or markedly diminished nephrogram, and dilated calyces (48).

Renal Abcess

Abscess formation can complicate pyelonephritis or an infected cyst. Lesions are low in attenuation, and classically demonstrate an enhancing, thick and irregular wall, especially in cases of chronic abscess formation (49). Spiral CT improves lesion characterization due to bolus contrast enhancement. Furthermore, the virtual elimination of partial volume averaging provided by spiral scanning can aid in the detection of smaller lesions and in the demonstration of gas bubbles which are virtually pathognomonic for renal abscess formation. A perinephric abscess can result from extension through the renal capsule.

 

Vascular Disease

Renal Artery Stenosis

Renal artery stenosis is implicated in approximately 5% of patients with hypertension. With currently available technology, neither color duplex ultrasound nor magnetic resonance angiography has proven adequate in screening patients for significant renal artery stenosis (50-53). Three-dimensional imaging using the spiral CT dataset may provide a reliable, non-invasive screening method for direct visualization of renal artery stenosis. Benefits from spiral-generated multiplanar reconstructions and three-dimensional angiography have already been established elsewhere in the abdomen and neck (10, 13, 14, 54-56).

Indirect, corroborative evidence of renal artery stenosis is deduced by noting a small and smooth, non-obstructed kidney. Spiral CT may demonstrate an asymmetric delay in the appearance of the cortical nephrogram. The cortical nephrogram is then prolonged on delayed images, resulting from decreased glomerular filtration (12). Frequently, other clues to the diagnosis of renal vascular disease are present on the CT scan, such as severe abdominal aortic atherosclerosis or aneurysm formation. The role of spiral CT angiography is discussed in detail in chapter _______.

Renal Infarction

Renal infarcts are commonly the result of embolic phenomenon, usually from a diseased heart. The kidney in acute global infarction is rarely enlarged. A characteristic rim of enhancement, representing collateral capsular perfusion, is seen surrounding a hypodense, non-functioning infarcted kidney (57). The rim usually measures no more than 4 mm in depth (12). Occasionally, vermiform and globular medullary enhancement is demonstrated; while this finding is of unknown origin, it probably reflects collateral blood flow (58).

Renal infarcts can also be focal, presenting as wedge-shaped defects that extend to the capsular surface. Over time, atrophy occurs in the vascular distribution of the infarction. Segmental infarctions of the anterior or posterior renal arteries demonstrate a characteristic appearance on CT, with the larger ventral branch supplying the anterior lateral portion of the kidney and the smaller dorsal branch supplying the posterior medial aspect.

Renal Vein Thrombosis

The spiral CT diagnosis of renal vein thrombosis is based on direct visualization of an intraluminal filling defect within an enlarged vein. In patients with poor renal function or decreased renal perfusion, care must be taken not to misdiagnose normal delayed filling of the renal veins for thrombus. Delayed scans are often helpful here in questionable cases. Indirect signs of renal vein thrombosis include demonstration of collateral vessels, renal enlargement, thickening of Gerota'a fascia, and retroperitoneal hemorrhage (12).

Spiral CT is sensitive in detection of the asymmetric, abnormally delayed cortical nephrogram seen in patients with renal vein thrombosis. This delay results from diminished glomerular filtration and slowed tubular transit due to compression of the tubules by interstitial edema. Perhaps most importantly, spiral scanning should prove more sensitive than conventional CT in the demonstration of underlying etiologies of renal vein thrombosis, the most significant of which is renal cell carcinoma.

Small Vessel Disease

Many diseases can involve the intrarenal vasculature (49). These include the collagen vascular diseases, radiation nephritis, and hypertension. The spiral CT appearance is non-specific in these entities. with parenchymal scarring frequently seen. The involved kidneys may be small and poorly functioning, with marked cortical thinning. Areas of hemorrhage may result from rupture of microaneurysms in patients with polyarteritis nodosa, Wegener's granulomatosis, lupus, or intravenous drug abuse. Radiation nephritis may be acute or chronic, with renal failure ensuing from tubular and glomerular degeneration, as well as vascular fibrinoid necrosis.

Vascular Anomalies

One or more accessory renal arteries are present in up to 40 % of normal patients. These usually arise distal to the major artery. Accessory renal branches are best appreciated with spiral angiography. Common venous anomalies include the circumaortic and retroaortic renal vein (59, 60). The retroaortic left renal vein is seen in approximately 2% of patients(60). Demonstration of these vascular anomalies is of vital importance for pre-surgical planning.

Other Vascular Disease

Peak contrast enhancement provided by spiral scanning allows for improved detection of congenital and acquired vascular disease. Renal artery aneursyms can be seen in patients with arteriosclerosis or as a result of septic emboli (61). Congenital arteriovenous malformations are rare. Acquired AVM's most commonly result from prior renal biopsy. Dynamic scanning can reveal enlarged feeding vessels and collateral circulation.

 

Renal Calculi

Conventional CT has been shown to be sensitive in the detection of renal calculi (62, 63). Theoretically, smaller calculi should be detected with increased frequency and sensitivity on spiral CT. Lack of breathing misregistration ensures continuous coverage of the entire kidney which improves detection of small stones. Density measurements of small renal calculi are more accurate, as partial volume averaging is virtually eliminated.

In performing a spiral CT for suspected stone disease, precontrast scans should be obtained. However, even on enhanced studies alone, one is usually able to distinguish renal calculi from potentially obscuring contrast. This is in part due to spiral CT's ability to image the kidney during the cortical nephrogram phase of enhancement, before contrast enters the medulla and collecting system. Delayed scans may then be obtained to help localize calculi within a calyx or the renal pelvis.

Obstruction

Early scanning during the cortical nephrogram phase of enhancement can be useful in the detection of subtle obstruction (12). Asymmetry in the appearance of the cortical nephrogram on spiral CT, manifested by early delay and late persistence, signifies the presence of either abnormal perfusion or tubular transit(12, 64). The finding of a dilated collecting system is then a specific finding for obstruction.

However, acute cases of obstruction may demonstrate little to no calyceal dilitation, making spiral CT findings similar to those of renal artery stenosis, renal vein thrombosis, or pyelonephritis. On later images, the nephrogram can appear mottled or striated, and hyperdense (64, 65). Delayed images of the collecting system and ureter may help define the cause of obstruction at any point from renal pelvis to the bladder. Suspicious areas can then be re-scanned with narrow collimation spiral CT to help characterize the obstructing lesion.

Renal Trauma

CT is the imaging modality of choice in the evaluation of renal trauma. (66-68). Technical advances in spiral scanning are making "survey" coverage of the abdomen possible during a single breath-hold acquisition. This will allow for ideal peak contrast opacification of the entire abdomen. In the kidney, this may be useful in detecting subtle lacerations, contusions, segmental or cortical infarcts, and subcapsular hematomas. Traumatic occlusion or avulsion of the renal artery produces complete absence of renal contrast enhancement shortly after injury. With time, collateral capsular flow is seen, identical to that seen in non-traumatic renal infarction.

Congenital Anomalies

Early fusion of the lower poles of the kidneys can result in the horseshoe kidney. This is found in 1 in 600 people and represents a very common renal fusion anomaly (69). The lower poles fuse anterior to the aorta and inferior vena cava via an isthmus of functioning renal tissue or fibrous tissue. The majority of patients with horseshoe kidney demonstrate multiple, bilateral renal arteries. Other fusion anomalies include crossed fused ectopia and the pancake kidney. Spiral scanning can help define the relationship of the abnormal kidney to the major vessels. Other common congenital anomalies include renal agenesis, renal duplication, and malrotation.

Renal Transplants

Spiral CT has a potential role in the evaluation of the renal transplant patient. The most important application here may lie in the evaluation of vascular complications, which are a significant cause of graft dysfunction and occur in up to 10% of patients. Common types of vascular complications include arterial and venous stenosis or occlusion, arteriovenous fistulas, and pseudoaneurysms (70). Peak vascular enhancement increases detection of these potential complications. Other common complications include peri-transplant fluid collections and obstruction.

Summary

Spiral CT represents an exciting advance in evaluation of renal disease, offering several technical advantages in evaluation of the kidney. Scans are obtained during peak contrast enhancement with minimal respiratory artifact. As a result, smaller cancers are detected with increased frequency, which will likely impact upon overall patient survival. Subtle asymmetries of the cortical nephrogram can be readily detected, increasing sensitivity and oftentimes specificity in the diagnosis of renal obstruction, renal artery stenosis, or pyelonephritis. Artifactual findings from partial volume averaging of benign lesions with normal parenchyma are minimized. Finally, improved lesion characterization should decrease the incidence of the diagnostically frustrating indeterminate renal mass.

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