Kidney: Spiral CT Evaluation of the Kidneys: State of the Art

Patricia A. Smith, M.D.
Fray F. Marshall, M.D.
Elliot K. Fishman, M.D.

The Departments of Radiology and Urology,
the Russel H. Morgan Department of Radiology and Radiologic Science
and the James Brady Buchanan Urologic Institute
The Johns Hopkins Medical Institutions
Baltimore, Maryland

Introduction

Computed tomography (CT) was introduced for clinical imaging in the late 1970's. Since that time, CT has developed an undisputed role in the evaluation of renal pathology [1-4]. Common indications for CT examination of the kidneys include evaluation of known or suspected renal masses, inflammatory or infectious conditions, renal trauma, and vascular pathology involving the renal artery or vein.

Two types of CT scanners are currently available, conventional CT scanners and spiral CT scanners. Conventional CT scanners obtain individual axial images by alternating Xray exposure with table incrementation. Individual axial images are obtained during suspended patient respiration. Variations in breathing between axial image acquisition frequently cause imaging artifacts including respiratory motion artifacts, gaps in information and artifacts from overlapping information (i.e., partial volume artifacts). The long acquisition times necessary with conventional scanners do not always allow for imaging during the optimal phase of renal contrast enhancement and long acquisition times prevent multiphasic renal imaging. Many of these problems have been alleviated with the more recent introduction of spiral CT imaging.

The major technical advancement of spiral CT was the addition of a continuously rotating Xray tube that constantly exposes as the patient is moved through the CT gantry. This continuously exposing Xray beam creates a spiral tracing on the patient's surface. Continuous data acquisition occurs for 20-40 seconds allowing a study to be performed during a single patient breath hold. The data is continuously acquired leaving no gaps or overlaps of information as seen with conventional scanners. Volume averaging artifacts commonly present with conventional CT scans are therefore less problematic with spiral imaging. Faster scan times have the added benefits of decreased respiratory motion artifacts and the flexibility to scan during the optimal phases(s) of intravenous contrast enhancement. High speed spiral CT techniques also allow multiphasic contrast imaging where the kidneys can be studied at different phases of contrast enhancement such as the arterial phase or corticomedullary phase [1, 4-7].

The CT data set obtained during a spiral study is a continuous, non overlapping volume of data. With the volumetric CT data set, reconstructed axial images are produced retrospectively and at arbitrary levels. Overlapping images are frequently obtained to decrease partial volume artifacts. Small renal masses can be better characterized using spiral CT imaging by centering the reconstructed images at the level of the lesion. The data set also creates a volume of information that can be used for three-dimensional (3D) imaging. 3D images can be created to evaluate the renal vasculature, to better define renal masses and assist in preoperative planning especially nephron sparing procedures. In summary, the advantages of spiral CT technology includes continuous data acquisition, faster scanning times, less motion and partial volume averaging artifacts, and the ability to create quality 3D CT images from the spiral CT data set.

Renal Spiral CT Imaging

The spiral CT technique used to study the kidneys varies according to the diagnostic question that needs to be answered. For CT imaging of the kidneys, four phases of renal contrast enhancement can be utilized including the precontrast phase, corticomedullary phase, nephrographic phase and excretory phase. Baseline, precontrast images are obtained prior to the administration of intravenous contrast agents. The corticomedullary phase, nephrographic phase and excretory phase images are obtained at 50 seconds, 90 seconds and 3 to 4 minutes respectively, following the administration of intravenous contrast material . Not all phases of contrast enhancement are necessary for routine evaluation of the kidneys. Instead, the spiral evaluation of the kidneys must be tailored according to the diagnostic problem.

Noncontrast images are used to obtain baseline density measurements for evaluating renal masses or renal cysts, urolithiasis, nephrolithiasis, renal calcifications or in patients unable to receive intravenous contrast (i.e., contrast allergy, poor renal function, etc.). The corticomedullary phase of enhancement occurs at approximately 50 seconds following intravenous contrast administration. During this phase there is maximal contrast differentiation between the cortical and medullary portions of the kidney. Many studies have shown that small (< 3 cm) intramedullary masses can be missed during this phase due to poor enhancement within the medullary portion of the kidney[1, 8]. However, in many cases, subtle changes in renal contrast enhancement are best seen in this phase and can be useful for the diagnosis of pyelonephritis. This phase is useful for creating 3D images of the renal vasculature as the renal vasculature enhancement is maximized without contrast in the collecting system which can cause potential artifacts or obscure visualization of the renal vessels. The nephrographic phase occurs 90 seconds after intravenous contrast administration is initiated. The renal parenchyma during this phase demonstrates homogenous enhancement and minimal excretion of contrast in the collecting system. Several recent studies have shown this to be the most accurate phase for identifying small renal masses [8]. Imaging of the excretory phase of enhancement occurs at approximately 3 to 4 minutes following intravenous contrast administration. At this time the collecting system is opacified and filling defects within the collecting system such as calculi, uroepithelial tumors, etc., can be detected. Excretory phase imaging is also used to demonstrate the integrity of the collecting system in the setting of trauma.

Several scanning parameters need to be prescribed for spiral CT imaging. Collimator width, which is equivalent to slice thickness, table translation speed and the width of the reconstructed axial images must be chosen. As a general rule, thinner collimator widths combined with low table translation speed and thin reconstruction intervals produce images with the lowest degree of volume averaging artifacts. Scanning parameters must be chosen to cover the region of interest in a single breath hold while attempting to limit the degree of volume averaging. If the kidneys are included with an abdominal spiral CT scan, typical settings include a collimator width of 5-8 mm with a table speed of 8-10 mm/second. Thinner collimator widths (3-5 mm) and a table speed of 3-5 mm/second are used with a dedicated spiral CT study of the kidneys. For contrasted studies, a bolus injection of 100 to 120 cc of ionic contrast at a rate of 3 to 5 cc/second is administered via peripheral vein using power injector. Initiation of the scan must be accurately timed to the bolus contrast infusion to obtain the study during the optimal phase of renal contrast enhancement. Most renal spiral CT studies include a set of noncontrasted images followed by either corticomedullary or nephrographic phase imaging. Excretory phase images are used when clinically indicated (i.e., hematuria, renal obstruction) or when findings on the earlier images require further evaluation. For each examination, care must be taken when choosing individual scan parameters to optimize visualization of the kidneys and defining renal pathology.

Applications

Evaluation of a known or suspected renal mass

With the increased utilization of cross sectional imaging including CT, ultrasound and magnetic resonance imaging, as many as 25 to 33% of renal masses are now discovered serindipitously during evaluation of the abdomen for unrelated processes [1,9]. Spiral CT imaging is frequently called upon to evaluate the known or suspected renal masses. For this evaluation, both noncontrasted and intravenous contrasted images are utilized. CT is well established as the gold standard to determine the presence and size of a renal mass, as well as determining the density and location of a mass. Density measurements are obtained before and after intravenous contrast administration to assess for the presence of enhancement within the mass. If precontrast density measurements are between 0 and 20 Hounsfield units (HU), and later phase density measurements demonstrate less than 12 HU increase in enhancement with the mass, the diagnosis of a cyst can be made. To classify a cyst as a benign simple cyst, additional findings including a well defined, thin, smooth, nonenhancing wall must be documented. Complicated cysts are those low density structures not complying with the criteria stated above. The Bosniak classification uses CT characteristics to determine the probability that a cystic mass represents malignancy [1,4]. Further evaluation including recommendation of follow up or biopsy is determined according to the CT characteristics of the mass.

A renal mass with negative HU, in other words a fat containing lesion, can be classified as a benign angiomyolipoma. These lesions are a mixture of fat, soft tissue and abnormal blood vessels occurring typically as a solitary lesion in middle age females. The presence of multiple angiomyolipomas can be associated with tuberous sclerosis. These lesions can be entirely intrarenal in location or have an exophytic component. Demonstrating the relationship to the kidney is a key finding in differentiating an exophytic angiomyolipoma from a retroperitoneal sarcoma. Other tumors which have rarely been reported to contain fat include renal lipomas, liposarcomas, Wilm's tumors, oncocytomas and renal cell carcinoma. Fortunately this is an exceedingly rare finding in these tumors. Although angiomyolipoma is considered a benign lesion, the risk lies in this lesion's tendency to hemorrhage. Surgical intervention is necessary for symptomatic tumors or as a prophylactic measure in patients with tumors greater than 4 cm. Spiral CT can diagnosis these lesions and be used for preoperative assessment.

Renal cell carcinoma is the most common malignant neoplasm involving the kidneys. The typical CT appearance is of a soft tissue attenuation mass which displays inhomogenous contrast enhancement following intravenous contrast administration. A mass with attenuation values which increase greater than 12 HU following contrast administration is suspicious for renal cell carcinoma. Additional CT appearances of renal cell carcinoma include a diffusely infiltrative mass, a complicated cystic mass or multiple masses . Spiral CT is also useful to evaluate for the presence or absence of renal venous invasion and the development of collateral vasculature . Screening for signs of metastatic disease is also a primary role for spiral CT imaging. Common sites for spread of renal cell carcinoma include extension to the renal vein and inferior vena cava, local lymph nodes, the adrenal glands, bone and lung. Following removal of a renal tumor by complete or partial nephrectomy, CT is used to evaluate for local recurrence or metastatic spread of disease.

Metastatic involvement of the kidneys is not uncommon, but rarely is clinically evident. Common primary tumors which metastasize to the kidneys include lung, breast, lymphoma and melanoma. The CT appearance is usually multiple, bilateral, homogeneously enhancing masses in a patient with other CT findings of metastatic disease. The patients will frequently have metastatic involvement of adjacent organs including retroperitoneal lymph nodes, liver, adrenal glands, etc.

Renal involvement with lymphoma is frequently clinically silent. The CT appearance of renal lymphoma can have a variety of patterns including a solid mass, discrete nodules, perinephric involvement or diffuse renal involvement which may cause renal enlargement. Renal function is rarely impaired with lymphomatous involvement of the kidneys unless the kidney is extensively replaced by tumor or retroperitoneal lymph nodes obstruct the urinary tract.

The improved detection and characterization of a known or suspected renal mass using spiral CT is secondary to decreased respiratory motion artifacts, fewer volume averaging artifacts, and finer control of imaging following intravenous contrast administration.

Renal cystic diseases

Simple renal cysts are the most common renal mass. Approximately 50% of the general population, over the age of 50, will have at least one renal cyst. The CT criteria of a simple cyst include a well defined, low density cystic structure (0-20 HU), which is thin walled and does not enhance following contrast administration. Spiral CT assists the diagnosis of simple cysts as partial volume averaging artifacts are less common allowing more accurate density measurements. Accurate density measurements are crucial for the evaluation of renal cysts. The use of overlapping, arbitrarily set reconstructed images allows the creation of identical levels for comparison of density measurements on pre and post contrast images. The Bosniak classification system for renal cysts is employed to determine the likelihood of a cystic lesion representing malignancy. This four scale classification system separates cystic lesions into categories of benign simple cyst, probably benign, indeterminate or probably malignant [1,4,8]. Each category also has recommendations for further work up of a cystic lesion.

The most commonly inherited cystic diseases include adult type polycystic kidney disease (APCKD), tuberous sclerosis and von Hippel-Lindau Syndrome. All of these diseases have renal cysts as a primary component, but renal malignancy is also a concern in these patient groups . Spiral CT can be used to assist in screening and diagnosis of these diseases as well as assessing for complications.

APCKD frequently demonstrated renal enlargement and multiple, variably sized, non communicating renal cysts . Cysts can be demonstrated within other solid organs including the liver, pancreas and spleen. The degree of renal impairment with this disorder can be variable with the majority of patients presenting in the third to fifth decades with hypertension and renal failure. These patients are at increased risk for the development of renal cell carcinoma especially following long term dialysis therapy. The diagnosis of renal tumors can be elusive in this patient population as their kidneys are frequently enlarged, demonstrate calcifications and many times have atypical appearing cysts. Spiral CT is useful in diagnosing renal tumors in these patients but can also be used to evaluate complications associated with APCKD including hemorrhagic cysts, renal infections and renal lithiasis.

Renal involvement of tuberous sclerosis includes renal cysts, multiple renal angiomyolipomas and an increased incidence of renal cell carcinoma.

Angiomyolipomas have a typical appearance on CT due to the fat content of these tumors. Because renal cell carcinoma can occasionally mimic an angiomyolipoma by ultrasound, spiral CT is used as a tool for screening and diagnosis of malignancy in these patients.

Patients with von Hippel-Lindau Syndrome can have multiple renal and pancreatic cysts as well as renal adenomas and renal cell carcinomas. These patients are at particular risk of renal cell carcinoma occurring at an earlier age than the general population . At presentation, these renal tumors are frequently bilateral and multiple. Those with von Hippel-Lindau are screened for the development of renal malignancy on a regular basis.

Acquired renal cystic disease occurs in patients following long term dialysis. Whether patients undergo peritoneal dialysis or hemodialysis, 90% of patients will develop acquired renal cystic disease following 5-10 years of dialysis treatment [10]. The typical CT appearance of this disorder is the presence of multiple renal cysts that can be found in small or enlarged kidneys. Renal cyst hemorrhage occurs in this group of patients as a result of underlying hypertension, heparin administration during hemodialysis, or anticoagulation therapy in patients with dialysis access grafts. It is generally accepted that this renal cell carcinoma occurs at a higher incidence in this patient population. Studies show a 3 to 6 fold increased incidence of renal malignancy with acquired renal cystic disease [10]. CT can be used to assist in the diagnosis of renal malignancy in this patient group as well as in the evaluation of renal complications.

Renal inflammatory diseases

Acute renal infections occur frequently and are commonly diagnosed and treated clinically without sequelae. Ascending urinary infection and hematogenous spread of infection are the commonest routes for development of pyelonephritis. Spiral CT evaluation is used for the evaluation of recurrent or complicated renal infections. The CT appearance of pyelonephritis can be variable including homogenous renal enlargement, segmental or wedged-shaped perfusion defects, focal striations within renal parenchyma, or asymmetry of the nephrogram . More aggressive infections such as emphysematous pyelonephritis can be diagnosed by the demonstration of gas within the renal parenchyma. Renal cysts can become infected. The CT changes of an infected cyst include enhancement of the wall, irregularity of the wall, and inflammatory changes in the adjacent tissues. CT imaging can also be used to detect and assist interventions for complications associated with renal infections such as renal abscess and perinephric spread of infection.

The spiral CT appearance of chronic pyelonephritis includes cortical scarring with distortion of the underlying calyx. Occasionally the unaffected adjacent renal parenchyma can mimic a renal mass. Spiral CT evaluation is utilized to demonstrate normal renal parenchymal enhancement with these regions. Spiral CT can also demonstrate complications associated with chronic pyelonephritis including renal calculi and renal abscesses.

Renal trauma

Evaluation of the acutely traumatized patient commonly involves CT scanning of the abdomen especially in patients suffering blunt or penetrating abdominal trauma. Patients who present with hypotension, multisystem injury, direct major flank injury or gross hematuria are particularly suspected of having a renal injury. According to a study at the University of Maryland, 8-10% of patients suffering blunt trauma to the abdomen with incur significant renal injury [11]. CT is the most sensitive imaging modality available for the detection of renal injury. The speed of spiral CT imaging is particularly advantageous in the evaluation of the acutely traumatized patient.

Renal evaluation following trauma is typically included in the study of the entire abdomen. Intravenous contrast (120 cc) is administered at 3 cc/second via peripheral injection with scanning initiated following a 40 second delay following contrast administration. The abdomen is the commonly surveyed from the dome of the diaphragm through the pubic symphysis using 5-8 mm collimation and 8-10 mm/second table speed. Depending on the spiral CT scanning system available, the entire abdomen and pelvis may or may not be studied in a single spiral acquisition. To complete the study through the pelvis, a second spiral scan can be obtained or alternatively, axial, non-spiral images can be used.

Spiral CT findings in the acutely traumatized patient include renal parenchymal injuries include contusions, lacerations, renal fractures, and subcapsular hematomas. Spiral CT is also used to evaluate for renal pedicle injury including injury to the renal artery or renal vein . Diagnosis of these conditions needs to be quick with prompt surgical intervention. If intervention is delayed, renal function is rarely restored following renal pedicle injury.

With suspected collecting system injury, delayed axial images obtained during the excretory phase of renal contrast enhancement are obtained. With injury to the collecting system, contrast extravasation from the collecting system will be identified on the delayed images. Injuries to the collecting system include lacerations, ureteropelvic junction avulsion, or penetrating injury. Delayed images through the urinary bladder are obtained if indicated on a clinical basis, such as patients sustaining bony pelvic fractures or when findings on the earlier phase images suggest possible bladder disruption.

Three-Dimensional (3D) spiral CT imaging

Spiral CT imaging has caused dramatic improvements in 3D display of CT data sets. Spiral CT technology allows rapid data acquisition of volumetric data sets obtained at optimal contrast enhancement with minimal artifact from patient respiration or volume averaging. These qualities produce data sets which are optimal for 3D reconstructed images and have broadened the applicability of 3D CT imaging [12-16].

Two main steps are involved in creating a 3D spiral CT image. First is acquisition of the CT data set followed by the application of the 3D rendering algorithm. Data acquisition for a 3D spiral CT study requires attention to the protocol as the protocol chosen will influence the quality of the 3D image produced. As a general rule, slice thickness, table speed and reconstruction intervals are kept to a minimum to produce high quality 3D displays of CT data sets. The acquisition of CT data must also be accurately timed to bolus administration of intravenous contrast agents to assure accurate opacification of the target vessels and organs.

The second step of creating 3D spiral CT data sets is the application of the specific rendering technique. Several rendering techniques are currently available for creating 3D CT images but a complete discussion of 3D rendering techniques is beyond the scope of this paper. Volume rendering techniques are used at our institution. Unlike other rendering techniques available, this technique uses the entire CT data set to create a 3D display. The resultant 3D images display depth, surface relationships and relative CT attenuation values. Real-time interactive manipulation of the data set is used to best display the anatomy of interest. Advancements in display technology allow review of 3D spiral CT data sets using 3D stereoscopic display. The advantage of the 3D stereoscopic display includes the ability to separate overlying structures and to visualize complex relationships between the normal anatomy and adjacent pathologic changes.

3D spiral CT of the renal vasculature

Spiral CT angiography offers a fast and minimally invasive evaluation of the renal vasculature. With optimization of data acquisition and proper rendering techniques, accurate 3D spiral CT images of the renal vasculature can be produced. Renal artery stenosis is a rare cause of hypertension and attempts have been made to accurately diagnose renal artery stenosis using techniques less invasive than standard catheter angiography. Many investigators have compared the use of spiral CT and 3D techniques to the gold standard, conventional angiography, for the evaluation of renal artery stenosis. Early results have been promising for the use of maximum intensity projection 3D images, another algorhithm commonly used to created 3D CT displays, which have been shown to have 92% specificity and a 82% sensitivity for the diagnosis of renal artery stenosis [12, 17-21]. Other signs of hemodynamically significant renal artery stenosis include subtle asymmetry of the nephrogram following intravenous contrast administration, decreased size of the affected kidney, and calcification within the renal artery.

3D spiral CT is also used to preoperatively evaluate potential living related renal donors. Sensitivity and specificity for 3D spiral CT angiography to define renal vascular anatomy so far has had favorable results compared to catheter angiography. Newer surgical procedures for living related renal donor nephrectomy include laproscopic nephrectomy. The preoperative evaluation for this procedure needs good definition of the number and position of the renal arteries, the presence of renal venous anomalies and the presence of additional vessels draining into the renal veins . Investigation is currently underway to determine whether 3D spiral CT with volume rendering techniques can replace standard angiography for the preoperative evaluation of this patient population.

3D spiral CT display for preoperative planning

The indications for nephron sparing surgery are broadening. Traditionally this procedure was performed in patients with renal insufficiency, solitary kidney, bilateral renal tumors or patients with contralateral renal abnormalities. Renal masses are being identified with greater frequency with increasing use of cross sectional imaging. Incidental renal tumors are frequently lower grade tumors and improved long term outcomes are being found with the use of nephron sparing surgery [9]. 3D display of spiral CT data sets can be used to optimally visualize the renal mass and determine resectability of the mass. Using the innumerable imaging angles available with 3D spiral CT, the mass and it's relation to renal parenchyma and major renal vessels can be shown . In addition, spiral CT angiography can be produced from the same spiral CT data set to evaluate for renal venous invasion by the tumor. 3D spiral CT has proven useful for the preoperative assessment for renal masses, but also for determining if nephron sparing procedures can be performed.

Conclusion

CT is well established as the gold standard for imaging the kidneys. Spiral CT with it's numerous advantages have improved the evaluation of renal pathology. Future developments in spiral CT technology and 3D CT imaging will assure this imaging modality's role for the evaluation of the kidneys.

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