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Genitourinary Imaging |
1 From the Department of Radiology, New York University Medical Center, Tisch Hospital, 560 First Ave, Suite HW 202, New York, NY 10016. From the 1998 RSNA scientific assembly. Received January 29, 1999; revision requested March 24; revision received April 14; accepted June 21. Address reprint requests to M.M. (e-mail: michael.macari@med.nyu.edu).
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Abstract |
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MATERIALS AND METHODS: In 25 patients, 26 well-demarcated, homogeneous high-attenuating renal masses (mean diameter, 2.5 cm; range, 1–4 cm) detected at initial postcontrast CT were further evaluated with delayed CT (mean, 38 minutes; range, 15–240 minutes) performed with identical parameters. On both the initial postcontrast and delayed CT scans, region-of-interest measurements were obtained in renal masses and in the gallbladder or low-density renal cysts as controls. Correlation with surgical or additional imaging findings was used to determine proof of diagnosis.
RESULTS: Nine of the masses demonstrated no change in attenuation between initial postcontrast and delayed CT, indicating that they represented avascular lesions consistent with high-density cysts. These cases were confirmed with prior or follow-up imaging studies that demonstrated stability. Seventeen masses (nine surgically proved neoplasms and eight neoplasms that demonstrated interval growth at follow-up or previous CT) demonstrated decreased attenuation at delayed CT compared with initial postcontrast CT, which indicates vascularity.
CONCLUSION: Delayed CT of incidentally discovered well-demarcated homogeneous high-attenuating (>30-HU) renal masses detected at postcontrast CT enables differentiation of high-density cysts from renal neoplasms by demonstrating deenhancement as a proof of vascularity and, hence, neoplasm.
Index terms: Computed tomography (CT), tissue characterization, 81.12114, 81.12115 • Kidney, CT, 81.12114, 81.12115 • Kidney neoplasms, CT, 81.1211 • Kidney neoplasms, diagnosis, 81.31, 81.32
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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However, delayed CT can be used as another way of determining the vascularity of the lesion by demonstrating "deenhancement." If the lesion is a vascular neoplasm, it will deenhance and have a lower Hounsfield unit reading on the delayed scan, but if it is a high-density cyst, no change in the Hounsfield unit measurement will occur since the lesion is avascular. Establishment of deenhancement can be used as proof of vascularity just as enhancement of a lesion denotes vascularity indicative of neoplasm. The purpose of this study was to demonstrate this principle.
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MATERIALS AND METHODS |
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In nine of the study patients, the indication for abdominal CT was nonrenal in origin and included trauma in six patients and abdominal pain in three. In these nine patients, a pathologic renal condition was not suspected clinically, and the renal mass was incidentally detected during prospective monitoring of the CT study.
Delayed CT was performed in the remaining 16 patients with 17 known renal masses who had previously undergone CT or US examinations in which the diagnosis of the mass was unclear. In this group of patients, precontrast CT scans of the kidneys were obtained in addition to the initial postcontrast and delayed CT scans that were obtained as part of the study.
CT Technique
CT examinations were performed with scanners (HiSpeed Advantage; GE Medical Systems, Milwaukee, Wis) that undergo weekly calibration.
In the nine patients with incidentally discovered, well-circumscribed dense renal masses, contrast material–enhanced CT was generally performed with 7-mm-thick sections, table speed of 7–10 mm/sec, pitch of 1.0–1.4, 120 kV, and 210–260 mA. Initial postcontrast CT was performed 60 seconds after intravenous administration of a 150-mL bolus of iothalamate meglumine (Conray 60; Malinckrodt Medical, St Louis, Mo) with a power injector through an arm vein at 1.5–2.5 mL/sec. In these nine patients, the CT study was prospectively monitored, and the renal mass was identified. Delayed CT was performed 15–240 minutes (mean, 87 minutes) after the initial postcontrast CT study, with use of the same CT scanner and scanning parameters as were used in the initial postcontrast CT study.
In the 16 patients with known renal masses, precontrast CT of the kidneys was performed with helical CT, 5-mm-thick sections, table speed of 7–10 mm/sec, pitch of 1.4–2.0, 120 kV, and 210–260 mA. In some cases, transverse cluster scanning was also performed. This was followed by initial postcontrast CT of the kidneys with use of the same parameters. In these 16 patients, postcontrast CT was performed after intravenous administration of 150 mL of iothalamate meglumine with a power injector through an arm vein at 2 mL/sec. Initial postcontrast CT of the kidneys was started 80 seconds after initiation of the intravenous bolus injection. In all 16 patients, initial postcontrast CT of the kidneys was performed in the nephrographic phase of renal enhancement. Delayed CT was performed at 15 minutes or longer with use of the same parameters as were used in the precontrast and initial postcontrast CT studies.
In the nine patients with well-circumscribed renal masses that were incidentally discovered, the time of delayed CT was not standardized and was dependent on the CT schedule and patient factors. However, in all patients the delayed CT study was performed at least 15 minutes or longer after the initial postcontrast CT study. In the patients with a known renal lesion, a 15-minute delayed CT study was performed. Fifteen-minute delayed CT was chosen because demonstration of washout of contrast material from renal neoplasms in this time period seemed adequate and because it enabled the study to be performed while the patient stayed on the CT table without disruption of a busy CT schedule.
Evaluation of Renal Lesions
Attenuation coefficients were obtained with use of region-of-interest cursors placed in all portions of the renal lesion. The region-of-interest cursors were carefully placed (M.M. or M.A.B.) to encompass as much of the renal lesion as possible while avoiding adjacent structures. Region-of-interest measurements were obtained for all lesions on initial postcontrast and delayed CT studies and, when available, on the precontrast study. Region-of-interest measurements were performed by either author. In all cases, the studies were reviewed at a later time in consensus to ensure that region-of-interest measurements were satisfactory. Several region-of-interest measurements were obtained for each lesion, and the mean attenuation value was used. The mean attenuation value and SD were recorded for each measurement obtained. In all cases, the individual attenuation measurements were very close to the mean.
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RESULTS |
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The renal mass in one patient, with tuberous sclerosis and known stable nonlipomatous hamartoma, demonstrated a 62-HU decrease in attenuation at delayed CT. The remaining five lesions were in patients who were not considered surgical candidates, and they were followed up. These five renal masses were also evaluated with precontrast CT, which demonstrated enhancement when compared with the initial postcontrast CT findings and were clearly indicative of neoplasm. Minimal growth was seen in some of these lesions during relatively short follow-up (2).
Of the nine renal masses that did not demonstrate deenhancement at delayed CT, two were incidentally identified at CT studies performed for nonrenal indications and seven were indeterminate lesions present at a previous imaging study. The mean diameter of these renal masses was 2.3 cm (range, 1–4 cm). Table 2 gives the individual attenuation values for these masses. The patients with the two renal masses that were incidentally identified did not undergo precontrast CT, but the patients with the remaining seven lesions underwent pre- and postcontrast CT. For initial postcontrast CT, the mean attenuation value for these masses was 56.6 HU (range, 35–81 HU). This measurement was obtained during the early nephrographic phase of contrast enhancement. At delayed CT, the mean attenuation value was 56.0 HU (range, 35–82 HU). In these nine masses, the mean difference in attenuation values between the initial postcontrast and delayed CT scans was 2.5 HU (range, 0–6 HU). In one patient, a low-density renal cyst (attenuation, <20 HU) was present. In this patient, delayed region-of-interest measurements in the low-density cyst demonstrated no change in attenuation.
The patients with seven of the nine renal lesions without deenhancement also underwent precontrast CT, and the renal lesion did not enhance. These lesions fulfilled the CT criteria for high-density cysts and were therefore not surgically proved (1). Further proof of many of the cases was afforded by the fact that these lesions showed stability in size when compared with prior or follow-up imaging studies (mean, 10 months; range, 4–48 months).
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DISCUSSION |
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Simple renal cysts are readily diagnosed because of their well-known and established characteristics (1,3,7). However, a subset of renal cysts are those that have undergone interval hemorrhage or contain high concentrations of protein and are called "high-density," "hyperdense," or "high-attenuation" cysts because their attenuation is greater than that of water, usually 30–90 HU (1,8). These lesions are more difficult to diagnose definitively because the contents of the cyst render US findings less characteristic and the thickness of the wall of the cyst cannot be truly assessed at CT. These lesions can easily be confused with well-marginated renal neoplasms. The most important criteria to distinguish the two is to determine the vascularity of the lesion, that is, whether or not the lesion enhances with intravenous contrast material. High-density cysts are homogeneous and well-marginated but most important do not enhance with contrast material. Renal neoplasms can also be homogeneous and well-marginated but they are vascular and enhance with contrast medium (1).
If these high-density cysts or well-marginated neoplasms are incidentally discovered at postcontrast CT, they pose a problem in diagnosis because, as noted, they display similar features. Whether or not a lesion enhances with contrast material cannot be known if a precontrast CT scan is not available, and under these circumstances the patient must undergo a US study (which is often not definitive especially if attenuation of the lesion is greater than 40 HU because it contains blood or debris) or a repeat CT study with pre- and postcontrast scanning (1,4). A US examination might be helpful in hyperattenuating lesions that measure approximately 40 HU or less, but findings are usually not definitive in lesions with attenuation above 40 HU because of the presence of echogenic material within hemorrhagic cysts. An alternative strategy, as suggested in this study, would be to obtain delayed scans while the patient is still in the radiology department to determine the vascularity of the lesion by noting whether or not it deenhances. Renal neoplasms, which are vascular, demonstrate deenhancement over time as the contrast medium drains out of the lesion (Figs 1–3), whereas a high-density cyst will maintain the same attenuation (over time) because it is not vascular and its attenuation is due to internal contents (blood products, debris, and protein) (Fig 4). Although the longer the wait to demonstrate deenhancement the more obvious it becomes, we found that a 15-minute delay was usually sufficient for contrast material drainage to occur in most renal neoplasms. Fifteen minutes was chosen since it is a reasonable time to keep a patient on the CT table to make a definitive diagnosis. If there is any question as to deenhancement, the patient could be removed from the table and imaged again after 30 minutes or more.
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It must be emphasized that diagnostic use of deenhancement of a renal lesion assumes that the initial postcontrast CT study was performed with an adequate blood level of contrast material. If the blood level is not adequate, a vascular lesion may not reach a level of enhancement sufficiently high to demonstrate deenhancement at delayed imaging that is obvious enough to allow differentiation from an avascular high-density cyst (Fig 3). Although this is usually not a problem in most neoplasms, which are vascular enough to enhance noticeably, it can create a problem in hypovascular lesions and is a potential pitfall in the use of deenhancement as a diagnostic tool as we found in our study.
The extent of enhancement of a renal neoplasm on a CT scan is dependent on two major factors: the nature of the tissue in the tumor and the level of contrast medium in the tissues of the tumor at the moment the scan is obtained. Some tumors are very vascular and will enhance substantially. Other tumors are hypovascular and will enhance much less. Most renal tumors fall between these extremes but contain vascular tissue sufficient for enhancement to be readily observed.
The level of contrast medium in the tissues of a renal neoplasm is dependent on a number of factors including the integrity of the renal vessels, the amount of contrast medium administered, the speed of the injection, and the timing of scan acquisition with respect to administration of the contrast material.
For instance, if only a small amount of contrast material is used in the study, enhancement of the tumor will obviously be less than if a larger amount of contrast material were used. Likewise, if a delay in scanning occurs, so that a few minutes elapse between the injection of contrast material and the beginning of scanning, the degree of enhancement will be reduced compared to what it would have been if scanning had occurred soon after contrast material injection (Fig 3). Also, if contrast material is given slowly, the blood level of contrast material will not increase as much as if a rapid bolus of the same amount of contrast material were given, and therefore enhancement will not be as great. Likewise, if scanning is started too soon after contrast material administration, a peak level of contrast enhancement might not be achieved (12).
All of these factors that affect enhancement are also involved in deenhancement of the tumor and must be kept in mind when interpreting deenhancement. For example, a hypovascular lesion that does not enhance very much will similarly not deenhance very much. This is shown in Table 1 where it can be seen that the lowest amount of deenhancement (15 HU) occurred in hypovascular lesions that enhanced the least. Also, a lesion that enhances only slightly because scanning was delayed after the contrast material was administered will not deenhance very much on delayed scans because it already began to deenhance when the initial postcontrast CT study was performed. These are the type of potential errors that can occur with the use of deenhancement as in our study. The use of enhancement or deenhancement criteria is dependent on well-performed studies in which a full bolus of intravenous contrast material is administered rapidly and scanning is performed within 60–90 seconds. Studies that do not fulfill these criteria can still be used if they show obvious deenhancement, but they must be evaluated with caution.
It must also be remembered that pseudo- enhancement secondary to technical factors might play a role. Beam-hardening artifacts and scatter from adjacent high-attenuating structures can create changes in Hounsfield unit readings that can affect the enhancement and deenhancement measurements (13). In general, these technical errors are not a problem.
There is no absolute number of Hounsfield units that can be used to signify enhancement or deenhancement. What is needed is an understanding of the possible pitfalls involved in these measurements so that the necessary adjustments can be made. Whatever the value of tissue enhancement or deenhancement used, it must be stressed that enhancement or deenhancement must be unequivocal to be used in the diagnosis (1). If there is uncertainty concerning deenhancement, then repeat scanning before and after injection of contrast material is indicated.
A final potential pitfall to the use of deenhancement to diagnose neoplasm would be in the case of a well-delineated vascular malformation, aneurysm, or fistula. In such a case, the mass would deenhance with time, and unless the lesion can be suspected as being an abnormality of a vessel, neoplasm could be wrongly suspected. Usually there are clues that the lesion is a vascular abnormality, however, since the lesion has the same attenuation as that of other vessels on the scan (aorta or vena cava), and the renal vein or artery is often unusually large in these cases. In addition, neoplasms other than renal cell carcinoma would be expected to deenhance on delayed CT scans. Lesions such as metastases and hamartomas without fat have demonstrated deenhancement in our series. Likewise, renal lymphoma would also be expected to demonstrate deenhancement over time.
A final limitation of our study is that histologic proof was not obtained in all cases. The nine renal masses that demonstrated no change in attenuation and were diagnosed as high-density cysts on the basis of established criteria (1) remained stable over time. Of the 17 renal masses demonstrating deenhancement, nine were resected surgically and were confirmed renal neoplasms. In one patient with carcinoma of the lung, two homogeneous renal masses were identified. These masses demonstrated interval growth when compared with prior CT findings. This patient also had widely disseminated metastatic cancer, and these masses were most consistent with metastatic disease. One patient with tuberous sclerosis had a known stable nonlipomatous hamartoma, and the remaining five lesions that demonstrated characteristic findings of renal neoplasm at CT were not resected surgically because of their small size and because the patients were not considered surgical candidates (2). Minimal interval growth was seen in some of these cases during a short follow-up (Fig 2).
In summary, delayed contrast-enhanced CT can be used to differentiate high-density cyst from renal neoplasm when a high-attenuating renal mass is incidentally detected. It must be stressed that technical factors of scanning and contrast material injection need to be maximized to use deenhancement as a diagnostic tool. Finally, deenhancement needs to be demonstrated unequivocally to reliably diagnose neoplasm.
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Footnotes |
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