Case #1This abdominal CT examination was performed as a screening study for metastatic disease, using 150-mL Ioversol 320 mgI/mL injected at 3 mL/sec, along with SmartPrep enhancement-triggered acquisition of 7-mm slices at 1:1 pitch. Prior to administration of the intravenous contrast, a scan through the liver was performed to serve as a baseline. Following contrast administration, there was a 25-second delay prior to initiating a series of low-radiation-dose SmartPrep monitoring scans at a rate of one every four seconds. The threshold was set at 50 HU above the baseline liver density. The SmartPrep display (Fig.1) shows the baseline image in the left upper quadrant; the most recently reconstructed image, in the right upper quadrant; and a time-density enhancement curve in the left lower quadrant. The Region of Interest (ROI) #1 is the aorta; ROI #2 is the portal vein; and ROI #3 is the liver. The liver's enhancement curve is tracked until it crosses the physician-specified threshold - in this case, 50 HU over baseline, considered an excellent degree of enhancement for detecting hepatic lesions. At this point, the diagnostic helical scan is initiated. The graph demonstrates a continually steep rise in liver enhancement crossing the threshold level, which is typical at this volume and dose. The resulting diagnostic images (Fig.2) exhibit excellent liver enhancement.
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Case #2This SmartPrep CT examination of the abdomen and liver was performed using the same parameters, but a smaller volume of contrast - 100-mL Ioversol 320 mgI/mL. With this smaller volume, the liver's SmartPrep enhancement curve #2 rises slowly and never reaches the threshold before fall-off (Fig.3). This is typical of curves displayed when only a limited, 100-mL volume and total contrast dose is employed. This indicates an inadequate amount of contrast for consistently detecting liver lesions. Note in particular that the aortic enhancement curve #1 shows a rather dramatic fall-off, indicating that the threshold was not reachable. In cases such as this, the SmartPrep technique's "default time" serves as a fail-safe, allowing the radiologist or technologist to initiate the diagnostic series manually - valuable whenever it appears that the enhancement of a target structure will not reach the threshold. This avoids any scanning delay which might produce images in the equilibrium phase and significantly compromise exam quality. In this paricular case, the default time was 70 seconds, as tracked by the clock on the SmartPrep display. |
Clinical AdvantagesCompared to using a standard, fixed delay time of, for example, 50-70 seconds, SmartPrep has demonstrated its ability to provide greater consistency from exam to exam, and from patient to patient. We have shown that, when the same patient is studied on multiple occasions, liver enhancement can vary dramatically over time when the same fixed time delay is used - presumably because of the portal venous system's variable blood supply, which delivers 75% of the opacified blood to the liver. SmartPrep's major advantage is its ability to eliminate these individual variations, to let opacification rather than time trigger helical scanning, and to provide significantly improved liver enhancement compared with fixed delays such as 60 or 70 seconds. As a result, we are now able to scan at peak liver enhancement, producing examinations of greater consistency and higher quality. |
Economic AdvantagesThe economic advantages of SmartPrep have become evident. In a recent study*, we found that this technique can reduce contrast from 150 mL to 125 mL per patient, while providing equal liver enhancement. This savings of 25 mL of non-ionic contrast material corresponds to approximately $20 per examination. In our institution, approximately 4,000 liver or abdominal CT examinations with intravenous iodinated contrast are performed annually. If one assumes that 50% of these are done with nonionic contrast, an annual cost savings of $40,000 would result without any compromise in exam quality. * Silverman PM, Roberts SC, Ducic I, Tefft MC, et al. Assessment of an individualized Scan Delay Technology for Helical (Spiral) Liver CT: A Technique To Improve Efficiency in Contrast Usage. RSNA presentation, November 26, 1995. Research sponsored in part by Mallinckrodt Medical and GE Healthcare. |
References
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