Three-phase CT protocols are the state of the art for imaging the kidneys and provide all the information necessary to plan for NSS (Fig. 1) (9-11). Scans should be performed on a multidetector helical scanner, which allows for the efficient use of intravenous contrast and facilitates the creation of thin-slice datasets for smooth 2-D and 3-D reformations. Technical parameters, such as kVp and mAs, should be kept consistent across the scan phases.
The first scan phase is a noncontrast CT of the abdomen, including the adrenal glands and both kidneys. The noncontrast CT is essential not only because it helps to plan for the contrast-enhanced portion of the study but also because it provides baseline attenuation values for any detected renal masses and also because it allows for the identification of any calcifications in the urinary tract or in renal lesions.
The second scan phase is a vascular phase CT scan (34). The timing for this phase can be determined either by scanning after a test bolus of 20 mL of contrast material has been injected, usually at a rate of 3 or 4 mL/sec or by using an automated bolus-tracking technique set to trigger from a threshold value, set from enhancement in the upper abdominal aorta. Additional time is needed to assure enhancement of the renal veins. An additional five seconds is usually sufficient to allow renal venous enhancement. In otherwise healthy patients, most vascular-phase CT scan delays are between 25 and 35 seconds after the initiation of the contrast material injection.
The third scan phase is obtained during the parenchymal phase of enhancement, obtained after a 120- to 150-second delay from the initiation of the bolus contrast injection (the longer delay times are used for older patients or patients with cardiac dysfunction). The parenchymal phase images are the most sensitive and specific for lesion detection and characterization, although the vascular phase images can also be useful when characterizing masses (1,3-5,28,29,34).
For each scan phase, thin sections, typically obtained at 3 mm, are reconstructed without image overlap for diagnostic interpretation and filming. Softcopy reading is recommended using either a picture archiving and communication system (PACS) workstation or the scanner console. In addition to the 3 mm slices for diagnostic interpretation, a separate reconstruction set of 1 mm thick slices with 20% overlap (reconstruction interval of 0.8 mm) is also created and used for multiplanar reformations and 3-D real-time volume-rendering reconstructions. Multiplanar reformatted (MPR) images are created through the abnormal kidney for interpretation and sent to the referring urologist. True sagittal and coronal oblique images oriented parallel to the long axis of the kidney are helpful for localization of the tumor within the kidney. Thin-section (3-5 mm thickness) coronal oblique thin-slab maximum intensity projection (MIP) images through the aorta and kidneys are helpful for delineating the renal vasculature. These thin-slab MIP images improve
visualization of the renal vasculature and facilitate measurements of the distance to the first renal arterial branches and distances between renal arterial ostia in those patients with multiple renal arteries. MPR and MIP reformations are performed at the scanner console by the technologists using the thin-section (1 mm) dataset and then sent for image review along with the diagnostic axial images (Figs. 2 and 3).
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