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


Kidney: Three-Dimensional CT Stereoscopic Visualization of Renal Masses: Impact on Diagnosis and Patient Management

Patricia A. Smith, MD, Bruce A. Urban, MD, Frey F. Marshall, MD, Elliot K. Fishman, MD

 

Abstract

Purpose: To determine whether three-dimensional(3D) reconstruction with stereoscopic display of helical CT data sets with CT angiography are of value in the evaluation of patients with known or suspected renal masses.

Conclusion: Volume rendering techniques applied to helical CT data sets coupled with 3D stereoscopic imaging provides a complete evaluation of patients with known or suspected renal masses. This information can help dictate patient management and provide a single study for comprehensive preoperative evaluation for possible renal sparing surgery versus total nephrectomy.

Introduction

The role of CT in the evaluation of renal lesions is well established (1). CT is widely accepted as the preferred imaging modality for suspected renal tumors, tumor staging and detecting metastatic disease. Benign renal processes which can simulate tumors including cystic diseases, renal infections and benign tumorous conditions (i.e. angiomyolipoma) can also be defined by CT (2).

The advent of helical CT has further improved renal imaging by decreasing volume averaging artifacts, eliminating respiratory misregistration artifacts and allowing imaging during peak intravenous contrast enhancement (3). However with improvements in CT imaging and increased utilization of cross sectional imaging the detection of small or indeterminate renal masses occurs with greater frequency. As many as 33% of renal masses are now detected serindipitously during the evaluation of the abdomen or retroperitoneum for other reasons (4). Smaller, lower grade renal tumors are being detected at an earlier stage. These smaller, lower grade tumors are more amenable to renal sparing procedures with favorable results (4).

Helical CT image acquisition has also lead to improved data sets available for three-dimensional imaging (5). The created data sets are volumetric and lack motion and respiratory misregistration artifacts. These improvements combined with continued advancements in computer hardware and software have lead to the production of higher quality 3D CT images. Volume rendering is the most advanced form of 3D CT imaging available and is currently implemented with real-time interaction, without need for preliminary editing and has the ability to display 3D images in stereo mode (5, 6). With quality 3D CT images, 3D information no longer needs to be extrapolated from two-dimensional axial images alone. In addition, a 3D CT angiogram can be created from the same data set providing important information regarding the renal vasculature with an accuracy equal to standard angiography (7, 8).

To date, the major role of 3D CT imaging for renal pathology has centered on the noninvasive evaluation of renal artery stenosis and other renal vascular pathology (8-10). However, the role of 3D helical CT using volume rendering techniques for the assessment of renal masses has not been addressed. The purpose of this study was to determine if 3D CT imaging using volume rendering and CT angiography are useful in the evaluation of patients with known or suspected renal masses.

Materials and Methods

The CT data sets of 17 consecutive patients (10 men, 7 women; age range, 38 - 78 years) with suspected renal masses were evaluated retrospectively. Renal masses were confirmed by surgery, renal biopsy, CT diagnosis or follow up CT examinations. Helical CT examinations were performed on Somatom Plus or Plus-S scanners (Siemens Medical Systems, Iselin, NJ). For each study, 100-120 mL of nonionic contrast (Omnipaque 350, Nycomed Inc., Princeton, NJ) was injected intravenously at 2-3 mL/second. Images were routinely obtained during the corticomedullary phase of renal enhancement following a delay of 40-50 seconds. The optimal helical CT scanning protocol included 0.75 second scanning, 3-5 mm collimation, pitch of 1-2, and reconstruction interval of 2-3 mm. Ideally patients received p.o. water for GI contrast so that oral contrast did not obscure vessels during real-time 3D CT angiography.

CT data sets were transferred to a free standing work station (Silicon Graphics Onyx with RealityEngine option; Silicon Graphics, Mountain View, CA) running the IPDOC medical imaging software (Advanced Medical Imaging Lab, Johns Hopkins Hospital; Baltimore, MD) and modified VolRen volume render. The volume rendered 3D images were reviewed in standard and stereoscopic modes of display with real-time interaction. Image analysis was performed by the consensus of two radiologists and one urologist. The goal of the evaluation was to determine extent of disease, normal and pathologic anatomy and plan subsequent patient management.

Results

All of the 17 patients were successfully scanned and the data sets were viewed interactively at the workstation in 3D without the need for preliminary editing of the scan data. The presence and location of the renal mass or renal pathology was shown in all cases as well as defining the relationship and interface of the mass to the adjacent normal renal parenchyma. The final diagnoses included renal cell carcinoma (10), transitional cell carcinoma (1), angiomyolipoma (3), complicated renal cyst (2) and pyelonephritis (1).

The renal vessels, including main renal and hilar vessels, were demonstrated in 17/17 cases. Among the 10 patients with renal cell carcinoma, the presence or absence of venous invasion was correctly established in all patients. Four patients with renal cell carcinoma demonstrated venous invasion and within this subset of patients, the extent of venous invasion in all four patients could be demonstrated preoperatively and was confirmed at surgery. One patient with renal cell carcinoma had developed extensive collateral vascularity which was well defined on the 3D images.

In the 10 patients with renal cell carcinoma, transaxial images were able to correctly stage all 10 lesions. The 3D images did not alter the stage of the tumor. However, analysis of the 3D images coupled with CT angiography provided a more complete and accurate preoperative assessment for these cases of renal cell carcinoma and assisted in planning conservative management, renal sparing operation or complete nephrectomy.

Discussion

CT is well established as the gold standard for evaluating renal masses. CT provides information on the size, number, location, tissue attenuation, presence of calcification or fat and signs of local or metastatic spread (3). Helical CT has improved the evaluation of renal masses by allowing for study during peak contrast enhancement as well as minimizing or eliminating patient motion artifacts and volume averaging artifacts (3). Subsequently, a greater number of smaller and earlier stage renal masses are now being discovered (4, 11). This finding has changed the surgeons management of renal masses including an increase in the number of renal sparing procedures (4, 12)(Figure 1).

Three-dimensional imaging is improving with advancements in scan data acquisition and computer technology. Higher quality images are now produced using complicated rendering algorithms with availability of real-time interaction and stereo mode display. Images can be displayed in an infinite number of imaging planes to optimize visualization of the renal lesions and the associated vascular components. The stereo display is especially useful in demonstrating overlapping structures such as vascular anatomy In addition, a true 3D appearance is available to visualize the renal mass with demonstration of the relationship to adjacent parenchyma and local structures. The information obtained with 3D helical scanning and 3D CT angiography provides the necessary information for patient management with a single study.

In this era of health care reform, minimally invasive and outpatient procedures are being emphasized. Improved 3D CT images have lead to increasing use of 3D CT angiography as a rapid and minimally invasive study of the vasculature. This procedure can frequently be performed at a lower cost compared to standard diagnostic angiography and cavography in many cases can replace the more invasive study.

Results in our 17 patients demonstrate good visualization of renal masses using 3D CT imaging. The location, number, size and anatomic relationships of the renal mass(es) were all shown. The renal vasculature was shown in all cases which is necessary in the preoperative assessment of renal masses. It is especially valuable in complex cases such as the patient with a renal crossed fused ectopia on the left and an exophytic renal cell carcinoma at the upper pole of this anomalous kidney. This patient was evaluated with 3D spiral CT and CT angiography for possible renal sparing surgery. The multiple renal arteries and anomalous venous system was shown preoperatively as well as the location of her primary tumor. This exam provided adequate information to be used as a single study in the preoperative assessment. Thus, standard renal angiography, a more invasive, time consuming and costly examination was not necessary (figure 3).

When evaluating renal tumors, the presence of venous invasion must be assessed and accurate preoperative assessment requires determination of the extent of venous invasion. Four patients in this study with the diagnosis of renal cell carcinoma demonstrated venous invasion. In all of these cases the extent of venous invasion could be shown (figure 4). Venography was not necessary in the preoperative evaluation of any of these four patients.

Of the benign tumors found in this study, including 3 patients with angiomyolipoma, 3D CT imaging were used to better define the extent of involvement and to determine patient management. For example, the specific diagnosis of renal angiomyolipoma could be defined on transaxial images based on the presence of fat within the lesion. One patient, in this study, presenting with hematuria had a 4 cm angiomyolipoma discovered by helical CT exam. This angiomyolipoma involved the lower pole of the left kidney and extension to the renal hilum could not be determined with certainty with the transaxial images alone. Volumetric 3D display clearly showed renal hilar extension and involvement of the main hilar vessels (figure 5). In this case, it was determined that renal sparing surgery could not be performed due to location and vascular involvement. Conservative follow up was recommended.

Although the results of this initial series are potentially limited by size, it leads to several questions in the sequencing of radiologic studies of the kidneys. First, is helical CT with 3D stereoscopic display and 3D CT angiography satisfactory as a single study for evaluation of renal lesions? Will MRI, standard angiography or ultrasound be relegated to answering specific questions raised or unanswered by the helical CT? Does venography need to be performed preoperatively? Lastly, can arterial and venous extension be accurately and reliably defined with 3D helical CT? Our study focused on the potential of this single CT study, but larger multiinstitutional series will need to be performed to answer these important questions and before this practice becomes accepted. We personally believe 3D helical CT with CT angiography will become the standard.

Another potential limitation of this paper is it's subjective nature, relying on the impression of the radiologist and urologist. The purpose of this paper was to determine if there was a role for helical 3-D imaging using volume rendering techniques for the evaluation of renal masses. Larger studies will be necessary to determine changes in surgical technique or alterations in patient management as a result of this technique.

Conclusion

Proper helical CT protocols and volume rendering techniques produces high quality 3D CT images. The flexibility of volume rendering to display images in the optimal plane as well as in stereo display makes 3D CT imaging as well as 3D CT angiography useful for evaluation of renal masses. This technique has many advantages compared to the standard radiologic evaluation of renal masses. A renal mass can be more accurately delineated and planes between the normal renal parenchyma and adjacent organs can be shown using 3D rotation of the kidney. Venous invasion is a frequent occurrence associated with renal cell carcinoma and can be difficult to assess using standard transaxial CT images. MRI or venography are frequently obtained for this reason. Our study shows that 3D helical CT with CT angiography can accurately demonstrate the presence or absence of venous invasion in patients with renal cell carcinoma. MRI or venography therefore may become unnecessary in the preoperative evaluation of renal cell carcinoma. In conclusion, three-dimensional helical CT with CT angiography, as a single study, can potentially supplant other radiologic studies in the preoperative evaluation of renal masses.

References

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