Helical scanning is the norm in the majority if communities across the US today. And, as for our acquisition protocols, spiral CT has altered traditional imaging, as well as expand the repertoire of procedures performed, even in small general CAT Scan Departments. Abdominal imaging with emphasis on ruling out liver pathology has been profoundly impacted by the myriad of choices of scan delays and which physiological phases to image. Beginning with the basics of hepatic hemodynamics and incorporating several years of spiral scanning research, criteria can be established for selecting appropriate protocols for comprehensive imaging and detection of pathology.

The hepatic vascular supply system consists primarily of the hepatic artery and the portal vein. Oxygenated blood arrives via the hepatic artery, which is approximately 20% of the incoming volume. The remaining 80% gushes in through the portal vein having been routed from the spleen and mesentery fresh with nutrients for the liver, which acts as our personal power plant. Processing nutrients and proteins into fuel, storing vitamins as reserve energy, and extracting waste by products, which exit via the hepatic duct to the common bile duct, the liver receives a liter of blood per minute. Depending on the literature, 75-100% of hepatic neoplasms are primarily supplied with the oxygenated arterial blood, but these are the less commonly occurring growths, so that the majority of tumors we image are visualized in the portal phase. The synchronization of the hepatic blood supply will become the basis of the scan delays selected for the biphasic protocols.

Many studies have documented contrast arrival times from antecubital venous injections. There are also software programs that can graph this arrival time when preexisting medical conditions belie the use of standard timing. These conditions include cardiac insufficiency, diabetes, vascular disease, cirrhosis, hypertension, and age, some of which may impede or accelerate normal blood flow velocity. However, when no prevailing conditions exist, standard contrast arrival time to the hepatic artery is 20-25 seconds from the initiation of the injection. The end of the arterial phase is noted by a decrease in the HU of the aorta and an increase of 20 HU in the liver parenchyma. Approximately 20-30 seconds later the contrast will be reentering the liver via the portal vein, which is now 60-70 seconds from the start of the injection. In approximately 60 seconds the infusion of portal blood plus contrast will fill in the remaining gaps around the edges of the lobules and into the sinusoids of the cells. The end of the portal phase is marked by the onset of equilibrium. This stage of homogeneity can occur as early as 100 seconds, but more commonly it is reached at 120-180 seconds from the start of the injection. Once the liver is at this congruous level of density, the ability to detect early neoplasms diminishes; hence, the need to scan in the 120 second timeframe after contrast is initiated. Of the potential types of liver tumors, 75-100% receive their blood supply from the hepatic artery. These are known as hypervascular neoplasms. The more commonly occurring hypovascular neoplasms, which represent about 25% of the tumor types, are best visualized during the portal phase. This discretionary blood supply can aid in differentiation of neoplastic growth or indicate portal involvement to hypervascular lesions.

Unenhanced - note left lobe cyst	Arterial Phase
Portal Venous Phase	Onset of Equilibrium

The caveat to perfectly timed imaging is the complexity of other pathology present that may alter the appearance of the infusion patterns to tumors. Some of the above illnesses may be accompanied by cirrhosis, which disrupts normal hepatic hemodynamics. Characteristically, cirrhosis is accompanied by portal venous hypertension and hepatic artery hypertrophy, thus giving some hypervascular tumors an appearance of infusing in the portal venous phase. In addition the presence of portal vein thrombosis can delay or restrict the portal blood flow and imaging phase.

Not to be overlooked by a discussion of hemodynamics is unenhanced liver imaging and the hyper or hypoattenuatting appearance of neoplasm versus normal liver parenchyma. One clinical survey indicated that 74% [12] of reporting facilities were no longer doing unenhanced phase imaging and yet a Duke University study on Metastatic Liver Disease reports that 35% [10] of detected lesions were best visualized on the noncontrasted images.

Note the 4 phase appearance of the neoplasm in the central posterior right lobe.

How, then, are these imaging and timing requirements translated into an acquisition protocol? With consideration given to the capabilities of the various scanners, it is very acceptable to scan the unenhanced phase with conventional scanning. The injection is readied and at 25 seconds the hepatic arterial phase (HAP) sequence is started. Using one-second scantime and 30 rotations, the arterial phase is completed at 55 seconds. The patient relaxes and regains a breathhold while the scanning cradle indexes back to the dome of the liver. The portal venous phase (PVP) is started 70 seconds after the start of injection and 30 seconds is completed. At 105 seconds we have accomplished a biphasic scan prior to equilibrium of the liver with 15 seconds to spare - well done! If this is beyond the capabilities of your system, there are several potential solutions. Utilizing an extended pitch of 1.4, it is possible to cover the same anatomy with 20 rotations in 20 seconds instead of 30, thus meeting the tube heat limitations of some scanners. Perhaps you have a scanner that uses a 2 second scantime for helical scanning, and again the extended pitch would permit the acquisition to be completed in the given time parameters. If your scanner needs 20-25 seconds to return to the top of the liver, this time can often be reduced to 10 seconds or better by scanning the PVP from the bottom of the liver up to the dome.

The above scanning delays are based upon a 3ml per second injection rate, which is derived from published scanning protocols from various facilities. Please note that a more rapid injection flow rate will potentially result in shorter time to optimal contrast enhancement and a slower infusion rate will extend the scan delay times. Also, the interval to equilibrium is extended to about 240 seconds with an injection rate of 2ml per second. Contrast timing protocols with test injections of 10 milliliters can provide graphic information for succinctly measuring the optimal delays to be utilized for your patients.

Geared with a comprehensive patient history and the knowledge of the requirements to image the hepatic phases, biphasic or triphasic CT scanning of the liver should be the protocol of choice for detection of hepatic diseases.

References

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