Multidetector-row helical computed tomography (MDCT) has the potential to improve the diagnos- tic assessment of the abdomen. CT colonography is an emerging method to screen for colonic polyps [1] and CT remains one of the main diag- nostic methods for imaging the hepatobiliary sys- tem and the pancreas. The high speed of MDCT can be used to cover larger anatomical volumes, to increase the spatial resolution along the z-axis, and to decrease the acquisition time. With a short ac- quisition time, the different phases of enhance- ment in the liver and pancreas can be easily sepa- rated, and with thin collimation, high-quality mul- tiplanar and three-dimensional images can be ob- tained. This may be particularly important to as- sess vascular invasion in pancreatic tumors [2].
Optimization of the injection of contrast me- dia in MDCT requires an understanding of con- trast media dynamics. The magnitude of enhance- ment of an organ is determined by the net iodine flux into the organ. During the early phase after injection, enhancement depends on both intrinsic factors, i.e., circulation time and central volume, and extrinsic factors, i.e., contrast medium vol- ume, contrast medium concentration, and injec- tion rate. Increasing the injection rate improves the enhancement of the arteries and organs with arterial perfusion, such as the pancreas, by in- creasing the iodine flux into these organs. In con- trast, increasing the injection rate has little effect on the enhancement of the liver, which is mainly perfused through the portal vein. Indeed, the dis- persion of the bolus in an additional capillary bed before reaching the liver through the portal vein has a buffering effect opposing that of the in- creased injection rate. Therefore, a rapid injection rate (5 ml/s) does not increase the magnitude of hepatic enhancement compared with an interme- diate rate (2–3 ml/s) [3]. As the liver enhancement depends only on the contrast agent volume and concentration, higher-concentration contrast
agents should be used to improve the enhance- ment of the liver, without increasing the volume of the contrast agent [4, 5]. In contrast, the enhance- ment of vessels, pancreas, and hypervascular liver tumors can be improved by increasing the injec- tion rate or the concentration of the contrast agent [6].
In addition, parenchymal and vascular en- hancement can be improved by using a double-sy- ringe injector with saline flush following contrast material bolus [7]. This improvement is explained by the fact that the saline pushes the contrast ma- terial that otherwise would be retained in the brachial vein and the superior vena cava.
With MDCT, automated coordination of con- trast material arrival and initiation of scanning is useful. For arterial-phase imaging, CT examina- tions are triggered after aortic enhancement of 150 HU. Triggering can also be performed during the hepatic portal venous phase at 55-HU hepatic enhancement [8].
In conclusion, optimization of the scanning technique and contrast material injection is im- portant for MDCT of the abdomen. The protocols will continue to evolve with the development of new CT scanners with increased detector rows.
References
1. Pickhardt PJ, Choi JR, Hwang I et al (2003) Comput- ed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med 349:2191-2200
2. Fenchel S, Boll DT, Fleiter TR et al (2003) Multislice helical CT of the pancreas and spleen. Eur J Radiol 4:S59-S7
3. Bae KT, Heiken JP, Brink JA (1998) Aortic and he- patic peak enhancement at CT: effect of contrast medium injection rate. Pharmacokinetic analysis and experimental porcine model. Radiology 206:455-464
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Introduction
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24 MDCT: Scanning and Contrast Protocols
4. Brink JA (2003) Use of high concentration contrast media (HCCM): principles and rationale. Body CT.
Eur J Radiol 45:S53-S58
5. Furuta A, Ito K, Fujita T et al (2004) Hepatic en- hancement in multiphasic contrast-enhanced MD- CT: comparison of high- and low-iodine-concentra- tion contrast medium in same patients with chronic liver disease. AJR Am J Roentgenol 183:157-162 6. Awai K, Takada K, Onishi H et al (2002) Aortic and
hepatic enhancement and tumor-to-liver contrast:
analysis of the effect of different concentrations of
contrast material at multi-detector row helical CT.
Radiology 224:757-763
7. Schoellnast H, Tillich M, Deutschmann HA et al (2004) Improvement of parenchymal and vascular enhancement using saline flush and power injection for multiple-detector-row abdominal CT. Eur Radi- ol 14:659-664
8. Saini S (2004) Multi-detector row CT: principles and practice for abdominal applications. Radiology 223:323-327
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