CT Scan of the Kidneys and Ureters
Most of the renal abnormalities are best seen on CT after intravenous contrast media administration. Plain CT scan is generally reserved to demonstrate calcification and calculi that may be obscured by contrast agent or it is used as a baseline of attenuation measurement when contrast enhancement is calculated as a feature of renal mass characterization. MDCT is the current modality of choice for renal evaluation.
Evaluation of Renal Mass
When the examination is performed to evaluate a renal mass, scans are typically taken before the contrast bolus and at one or more phases after intravenous (IV) contrast administration. The conticomedullary phase typically occurs approximately 30 to 70 seconds after the contrast bolus; the nephrogram phase is seen 80 to 120 seconds after the contrast bolus; the excretory phase begins about 3 minutes after injection and can last 15 minutes or longer. 2-dimentional and 3-dimentional reformations may be useful in defining certain types of renal abnormalities, such as renal cell carcinoma and ureteropelvic junction UPJ obstruction.
What is CT Urography?
The CT urography (CTU) is a relatively new imaging examination designed to provide a comprehensive evaluation of the upper and lower urinary tract. Many different CT urography protocols are currently being used. CT urography is defined as a diagnostic examination optimized for imaging the kidneys, ureters and bladder. The examination involved the use of MDCT with thin slice imaging, intravenous administration of contrast media, and imaging in the excretory phase.
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| CT Urography showing Calculi |
CT Urography Protocols and Phases
The CT urography examination protocols is often tailored to the clinical question. Protocols may include only the excretion phase, or may contain as many as 4 four phases, the plain CT study, corticomedullary, nephrographic and excretory. Excretory phase imaging for CT urography study methods can range from 3 to 16 minutes after injection. Longer delays are beneficial for opacification of the distal ureters.
Contrast administration is accomplished using 1 of 2 different approaches (a single bolus injection or a spit bolus injection)
- A single bolus injection administers 100 to 150 mL of Low Osmolality Contrast Media injected at rate of 2 to 3 mL/s, scans are typically obtained in the nephrographic phase to assess the renal parenchyma and in the excretory phase to assess the urinary tract mucosa.
- Protocols for split bolus injections vary somewhat, but all divide the contrast media dose into 2 bolus injections with a delay of 2 to 5 minutes between injections
- The patient is scanned once after the second injection, reducing the radiation dose, at this point, the contrast material injected first is providing excretory phase opacification and the contrast material injected 2nd is providing renal parenchymal enhancement. The goal of the split bolus is to image a combined nephrographic excretory phase.
Other Technique of CT Urography
Other techniques that may be used during CT urography to optimize the visualization of the urinary tract include the use of abdominal compression bands, IV saline hydration it is approximately 250mL of 0.9% of normal saline, and low-dose furosemide (Lasix) injection.
Multiphase CT urography imaging is associated with a relatively high radiation dose. The benefits of the examination must be carefully weighed against the risks on a patient by patient basis. The split bolus technique has been gaining in popularity because by combining 2 phases, the radiation dosed is reduced.
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