Year : 2004 | Volume
: 14 | Issue : 3 | Page : 285--290
Dual-phase helical CT of kidney : Comparison of corticomedullary and nephrographic phases in detection and characterization of renal masses
CM Shetty, BN Lakhar, BV Devi, BS Lakshmi
Department of Radio Diagnosis and Imaging, Kasturba Medical College and Hospital, Manipal-576104, India
C M Shetty
Department of Radio Diagnosis and Imaging, Kasturba Medical College and Hospital, Manipal-576104
OBJECTIVES : To evaluate the role of dual phase helical CT in the detection and characterization of renal masses. MATERIALS AND METHODS : A prospective study of twenty two patients was undertaken with CT to characterize indeterminate renal masses. Inall patients, five mm thick contiguous sections were obtained from the kidneys before and after injection of intravenous contrast material in corticomedullary and nephrographic phases. RESULTS : Renal neoplasms and normal renal cortex showed significantlygreater enhancement in the nephrographic phase compared with that in corticomedullary phase (p= 0.01). Renal veins are best visualised in corticomedullary phase and inferior vena cava is better visualized in nephrographic phase. CONCLUSION : Conspicuity of renal neoplasms is better in nephrographic phase compared to corticomedullary phase. Renal veins are best visualised in corticomedullary phase and inferior vena cava is better visualized in nephrographic phase. If dedicated renal CT is requested for a suspected renal mass, the helical scan series to be obtained are unenhanced, corticomedullary and nephrographic phase images.
|How to cite this article:|
Shetty C M, Lakhar B N, Devi B V, Lakshmi B S. Dual-phase helical CT of kidney : Comparison of corticomedullary and nephrographic phases in detection and characterization of renal masses.Indian J Radiol Imaging 2004;14:285-290
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Shetty C M, Lakhar B N, Devi B V, Lakshmi B S. Dual-phase helical CT of kidney : Comparison of corticomedullary and nephrographic phases in detection and characterization of renal masses. Indian J Radiol Imaging [serial online] 2004 [cited 2019 Dec 9 ];14:285-290
Available from: http://www.ijri.org/text.asp?2004/14/3/285/28604
The introduction of helical computed tomography (CT) has created many important advances in the detection, characterization and treatment of disease throughout the body. Helical CT is widely accepted as the state-of-the art technology for evaluation of abdomen . It is widely accepted as the preferred imaging technique for suspected renal tumor, tumor staging and detecting metastases, because of its low cost, high accuracy and ready accessibility.
Helical CT has many advantages over conventional CT for the evaluation of renal disease. Because of its high speed of image acquisition, helical CT can image kidneys during each of the three phases of contrast enhancement. The elimination of respiratory misregistration ensures that the entire lesion is imaged and that the chance of identifying small enhancing lesions is maximized. The acquisition of volumetric data during a single breath hold allows a comparison of identical levels on scans obtained before and after administration of contrast material. Partial volume averaging is minimized because a section through the center of a lesion is assured with helical CT when overlapping sections are reconstructed. Another advantage is that raw data can be reconstructed at any level. This improves the accuracy of Region-of-interest measurements and the ability to characterize a lesion. Thus, helical CT is the best technique for characterization of renal masses.
The purpose of our study was to compare thin-section corticomedullary and nephrographic phase images of the kidneys to determine whether one of these phases of parenchymal enhancement is superior in the characterization of a previously detected indeterminate renal mass.
MATERIALS AND METHODS
Subjects: Twenty three patients were referred for dual phase helical CT. The study population composed of fifteen men and eight women. Patients were referred for characterization of renal masses that did not meet the criteria of simple cysts at ultrasound.
Imaging technique: All CT examinations were performed on a helical CT scanner (General Electrical medical systems prospeed sx scanner). The scan parameters remained constant throughout the study. For acquisition of all images, a tube voltage of 120 kvp, a tube current of between 200-300 mA, collimation of five mm and a pitch of 1:1 are used. In all patients, initial images were obtained before administration of contrast material. Patients then received 100 ml of Iopromide (Ultravist 300 mg%) with a 20- gauge plastic venous cannula into the antecubital vein through calibrated power injectors at a rate of three ml / second. The first scan series (corticomedullary phase images) were obtained after a delay of 25 seconds after initiation of injection of contrast material. The second scan series(nephrographic phase images) were obtained after a delay of 120 seconds after initiation of contrast material injection.
Image analysis: All CT diagnoses were obtained by a consensus of two radiologists. The size and location of the mass in question were obtained from the nephrographic images. The mass was then characterized by evaluating its features and by its contrast enhancement. The enhancement of all lesions was determined by the region-of-interest technique. All solid lesions with attenuation similar to other soft tissue abdominal structures and if interrogation of region of interest revealed tissue enhancement of 10 HU or greater were classified as malignant. For characterization of cystic renal lesions, Bosniak criteria were used.
In our study, there were fifteen men and eight women. In one patient left kidney upper pole showed hypoechoic mass on Ultrasonography, which turned out to be dromedary hump and was excluded from the study. For the final analysis twenty two patients were included (fourteen men and eight women) who ranged in age from 22 to 82 years (mean age- 53 years, maximum age-82 years and minimum age-22 years). Sixteen patients presented with hematuria, eleven patients with loin pain, four patients with Weight loss, one with fever and one patient was asymptomatic.
There are twenty three lesions in twenty two patients. The size of the lesion ranged from 2.4-14 cm with mean size of 9.7 cm. Of these, twenty lesions were neoplastic lesions and three lesions were cystic lesions.
The attenuation values of neoplasms in corticomedullary and nephrographic phases were measured and listed in [Table 1].
On unenhanced CT scans, neoplasms demonstrated a mean attenuation of 23 ± 7 HU [Table 2]. Renal neoplasms showed average enhancement of 26 ± 14HU during corticomedullary phase and 45 ±19HU during nephrographic phase. One tumor, which showed greater degree of enhancement during corticomedullary phase, showed less enhancement during nephrographic phase.
The renal cortex demonstrated a mean attenuation of 32 ± 3HU on unenhanced CT images [Table 3]. Cortical mean enhancement was 105 ± 29 HU during corticomedullary phase and 131 ± 40 HU during nephrographic phase.
Three radiologically benign cysts were diagnosed. Cysts demonstrated a mean attenuation of 12 ± 2 HU. The mean cyst enhancement was 1 ± 2 HU during corticomedullary phase and 2 ± 3 HU during nephrographic phase.
The visibility of renal veins and inferior vena cava was assessed in corticomedullary and nephrographic phases. Renal veins were better visualized in corticomedullary phase and inferior vena cava was better visualized in nephrographic phase.
Summary of enhancement data, listed in table 4, showed that renal neoplasms and renal cortex showed greater enhancement in the nephrographic phase compared with that in the corticomedullary phase. The statistical significance of both renal neoplasms and normal renal cortex enhancement in the nephrographic and corticomedullary phases were calculated using student t test. Renal neoplasms and renal cortex showed significantly greater enhancement in the nephrographic phase compared with that in the corticomedullary phase (p =.001 and p =.001 respectively).
The widespread use of US and CT for survey examinations of abdomen has resulted in an increased rate of detection of both neoplastic and nonneoplastic renal masses. Accurate differentiation of a renal neoplasm and a simple cyst or minimally complicated cyst has become an increasingly important CT application. In cases in which sonography can not give a definitive diagnosis of a simple cyst, a dedicated renal CT is indicated. Assuming that the lesion does not represent normal renal parenchyma (pseudotumor) or inflammatory tissue, the presence of unequivocal lesion enhancement is indicative of neovascularity and neoplasm.
Previous studies ,,, have shown that nephrographic phase images are superior to corticomedullary phase images in the detection of renal masses. The purpose of the present study is to compare corticomedullary and nephrographic phases in the characterization of a renal mass and to evaluate the characteristics of renal parenchymal enhancement during these two phases.
Because of its high speed of image acquisition, helical CT can image kidneys during each of the three phases of contrast enhancement. During the first phase, the corticomedullary phase, much of the injected contrast material resides within the vascular system, including the renal cortical capillaries. Contrast material is also present in the peri tubular spaces and in the proximal convoluted tubules and in the columns of Bertin. The corticomedullary phase usually occurs between 25 and 80 seconds after initiation of contrast injection. During the corticomedullary phase, the renal cortex enhances briskly, the medulla enhances only minimally. Differences in enhancement between cortex and medulla are often pronounced.
During the nephrographic phase, which usually begins 85-120 seconds after initiation of contrast injection, contrast material filters through the glomeruli and enters the loop of Henle and the collecting ducts. During the nephrographic phase, renal medullary enhancement is similar to renal cortex enhancement.
The excretory phase begins when the contrast material is first excreted into the calyces. This phase usually occurs approximately 3-5 minutes after contrast injection begins. Nephrograms remain homogeneous during this phase; however, the attenuation is lower during the excretory phase as compared to that encountered during the nephrographic phase and gradually decreases over time.
When a patient is referred specifically for evaluation of the kidneys, initial unenhanced images should be obtained through the kidneys. These images are obtained to identify any renal calcifications and for baseline attenuation of masses before administration of contrast material. These images can be acquired in either axial or helical mode. In helical mode the scan parameters frequently used are voltage of 120 kvp, a tube current of 220-320 mA, table speed of 5.0-7.5 mm/sec and collimation of 5 mm. The scan parameters remained constant throughout all the scanned series.
IV contrast material is then administered via a mechanical injector into an angiocatheter, which is preferably inserted into a large vein into the antecubital fossa. The total volume of contrast material injected ranging between 90 ml and 150 ml. There are various contrast injection techniques. Monophasic injection rates of 1-4 ml/sec or biphasic injection rates of 2-3 ml/ sec for the first 50-75 ml of contrast material and of 1.0-1.5 ml/sec for the remaining volume of contrast material, have been used.
Corticomedullary phase images are acquired helically beginning 25-70 seconds after initiation of contrast material injection. We used a standard delay of 25 seconds. To ensure that enhancement of both renal cortex and medullae is homogeneous in all patients, nephrographic images should not be acquired until at least 100 seconds after initiation of contrast injection . Excretory phase images are usually obtained 3-5 minutes after injection of contrast material has begun.
If dedicated renal CT is requested, initial unenhanced scans are obtained throughout the kidneys followed by corticomedullary phase images and nephrographic or excretory phase images using the previously described protocols.
Our study results demonstrate that renal neoplasms show more enhancement in nephrographic phase(p =.001) compared to that in corticomedullary phase [Figure 2],[Figure 3].Normal renal cortex also showed more enhancement in nephrographic phase (p =.001) compared to that in corticomedullary phase. No statistically significant differences (p >.05) in enhancement were noted for the radiologically benign cysts when the two phases are compared. Tables 5 and 6 show that our results correlated with the study conducted by Birnbaum et al .
We demonstrated that renal veins were best visualized in corticomedullary phase and inferior vena cava was better visualized in nephrographic phase [Figure 4]. This correlated with studies published in literature. In a study conducted by Dahlman et al , two thrombi in renal vein were best visualized in corticomedullary phase and one thrombus in inferior vena cava was best visualised in nephrographic phase images. They concluded that renal veins were best visualized in corticomedullary phase and inferior vena cava is better visualized in nephrographic phase.
Conspicuity of renal neoplasms is better in nephrographic phase compared to corticomedullary phase. Renal veins are best visualised in corticomedullary phase and inferior vena cava is better visualized in nephrographic phase.
To conclude, if dedicated renal CT requested for a suspected renal mass, the helical scan series to be obtained are unenhanced, corticomedullary and nephrographic phase images.
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