Current Clinical Concerns in CT: Results : Protocols
CT of the Kidney
1. I am in a large practice with multiple locations and multiple protocols for different scanners. In some locations, we do our enhanced abd/pelvic scans (for misc. indication such as abd. pain) in one shot and often get the kidneys in the corticomedullary phase, known to have a lower sensitivity for renal lesions. In other locations, we routinely do delayed scans through the kidneys. There is an effort to standardize our practice and do all abd/pelvic CTs in one shot without delays. I am worried we are doing a disservice to our pts and referrers by degrading the quality of the interpretation of the kidneys. Do you have any thoughts on this or any advice?
| Answer: With new MDCT scanners, it is not uncommon to get images in the CM phase. Although, one can miss small lesions, they are usually benign cysts. A key to reviewing CM phase images is a wide window/level such as 550/50. We also will get selected delays in cases with suspected renal pathology, or if something is notes on the CM phase. Please note that in cases of suspected renal disease, we also do 4 minute delays. |
2. In patients with renal mass (as opposed to hematuria) what is the value of delayed images in the diagnostic/staging CT examinations? Isn't a 3 phase scan (unenhanced, corticomedullary and nephrographic) sufficient?
| Answer: The value of late phase imaging in this case is less than in hematuria, but may help define the presence of calyceal invasion. If you have an obvious tumor, then staging is done with arterial and venous phase as you note. |
3. In your article "Current Concepts in the Diagnosis and Management of Renal Cell Carcinoma: Role of MDCT and 3D CT" (in Radiographics 2001; 21: S237-254) you state "the nephrographic phase is the most valuable for detecting renal masses and characterizing indeterminate lesions…This phase is best imaged after a scanning delay of at least 80 seconds and lasts up to 180 seconds after the start of injection. The renal parenchyma enhances homogeneously, allowing the best opportunity for discrimination between the normal renal medulla and masses." However, in you Siemens Sensation 16 protocols for kidney (mass and hematuria) you image in the arterial and excretory phase. Shouldn't the nephrographic phase be added or replace the arterial phase?
| Answer: We like the arterial phase because we are able to get vascular mapping and look at vascularity of tumors. For staging renal tumors we also routinely get nephrographic phase images at 60 seconds. Excretory phase is necessary at 4 to 5 minutes to look at the renal pelvis and calyces, as well as finding small hypovascular tumors, or in cases of inflammatory disease. |
4. If urology needs to r/o renal mass, suspected as being a tumor:
a. should we do a noncontrast sequence. Your protocol says not, but urology has specifically asked me to revisit this.
b. do you see it as important to see increased vascularity in the arterial phase as well? Should we add a 3rd sequence at 25-30 seconds?
| Answer: A non-contrast CT is indeed needed to determine the presence of enhancement and r/o a high density renal cyst. They seem to be more common lately. Arterial phase is best for neovascularity, and venous phase best for renal vein. Late phase is best to r/o TCC, as you know. I will repost the kidney protocols and make it clearer. Please note that if a prior CT or US shows a definite tumor, then in those cases, a non-contrast study is not needed. |
References
RH Cohan, LS Sherman, M Korobkin, JC Bass and IR Francis
Renal masses: assessment of corticomedullary-phase and nephrographic- phase CT scans Radiology 1995; 196: 445-451.
- Summary: Helical CT was used compare corticomedullary phase (CMP- 40 seconds after initiating contrast infusion) and nephrographic phase (NP- following CMP) for detection and characterization of renal masses in 33 patients. While 111 lesions in the medulla were identified on NP images, only 25 lesions were identified on CMP images. Addition of NP images decreased false negatives and false positives.
Birnbaum BA, Jacobs JE, Langlotz CP, Ramchandani P. Assessment of a bolus-tracking technique in helical renal CT to optimize nephrographic phase imaging. Radiology 1999; 211:87-94.
- Summary: Bolus tracking was used during CT of the kidneys in 75 patients, and images were reviewed to determine the onset of the nephrographic phase by identifying homogeneous enhancement of the kidney parenchyma. Contrast (150 ml of 60%) was infused at 2 cc/second for patients with 2 kidneys and 3 cc/second in those with a previous nephrectomy (100 cc). The results revealed that the onset of the nephrographic phase ranged from 60 to 136 seconds, with a mean of 89 seconds. Parameters which impact the onset of nephrographic phase include patient age, volume of contrast material and injection rate.
Catalano C, Frailoi F, Laghi A et al. High-resolution MDCT in the preoperative evaluation of patients with renal cell carcinoma. AJR 2003; 180; 1271-1277.
- Summary: Forty patients with suspected renal cell carcinoma (RCC) were evaluated with MDCT, all of whom had RCC confirmed. The protocol was as follows:
Four slice MDCT was conducted with 4 x 2.5 mm unenhanced sequence (5 mm slice thickness and reconstruction interval). The contrast enhanced acquisitions included 4 x 1 mm collimation, 1.25 mm slice thickness with 1 mm reconstruction interval. Patients were administered 140 ml of 350 mgI/ml contrast material at 4 mL/second. Arterial phase imaging was initiated at 22 seconds and parenchymal/venous phase at 50-60 seconds, with excretory phase imaging at 5 minutes.