The introduction of dual-detector CT scanners beginning in the 1990s made it possible to acquire images two slices at a time (during one rotation of the gantry). Four-slice systems, referred to mainly as multislice CT (MSCT) systems and also as multidetector CT or multidetector row CT, became available in 1998 and enhanced this new capability still further. Since that time, manufacturers have developed MSCT units capable of acquiring eight, 16, and 32 image slices at once; the incorporation of flat-panel detectors now being tested will bring MSCT into the realm of nearly instant volumetric image acquisition.
Today’s MSCT scanners need less than a second to acquire multislice images. Increased volumetric coverage is made possible because shorter scanning times make it practical to image thinner slices.
For MSCT, the best imaging protocols for each clinical application are created by balancing collimation needs with scanning-speed coverage requirements. The ability of MSCT to generate images composed of isotropic (or nearly isotropic) voxels prevents loss of resolution, even in multiplanar images. This has made it practical to create three-dimensional images that are of extremely high quality and that lack the stair-step artifacts that were formerly common in them. Very thinly collimated images, acquired with scanning that is perfectly timed (using bolus tracking) for the best possible contrast enhancement, can be reconstructed to create amazing views of anatomical structures. Three-dimensional images can be acquired first when the contrast medium reaches the arterial circulation and then when it enhances the venous structures. Having images of both phases of contrast enhancement is, of course, very helpful in evaluating vascular diseases and anomalies, but it is also of great value in the staging of tumors and the planning of radiotherapy.
Of patients who die with malignant disease, 24% to 36%1 have metastases present in the liver. In fact, metastatic lesions of the liver may be the first tumors found for 25% to 40%1 of patients whose primary cancer location is gastrointestinal. Metastatic involvement of the liver is most likely if the cancer’s primary location is the esophagus, stomach, colon, pancreas, biliary tract, or lung.
Although MSCT has made imaging of many body areas both better and easier, liver imaging can be as challenging as ever (and may even have become more difficult, despite stunning MSCT results). Protocols for liver imaging are still complex constructions because the liver has two blood supplies, unlike other organs. The portal venous system delivers 80% of the liver’s blood supply, with the hepatic artery delivering the remaining 20%. For this reason, any contrast medium injected into the bloodstream arrives at the liver not only via two pathways, but at two different times. Imaging must be timed precisely to capture the structures of interest at their maximal enhancement.
Two-phase imaging of the liver is possible using single-slice helical CT: the hepatic arterial dominant phase (HADP) and the portal venous phase (PVP) are captured separately. MSCT is capable of triple-phase hepatic imaging, so data are now acquired at the hepatic arterial phase (HAP), the late arterial phase (LAP) or portal venous inflow phase, and the hepatic venous phase (HVP).2 The first two phases constituted HADP for single-slice helical CT and have now been divided because of the higher speed of MSCT, with PVP simply being renamed for clarity.
Imaging at different phases of contrast dynamics increases the radiologist’s ability to detect lesions and also improves the characterization of the lesions detected. HAP images permit three-dimensional imaging of the liver’s vascular system and are acquired 10 to 20 seconds following the administration of contrast. These images are extremely valuable for surgical and therapeutic planning.
LAP images allow the best visualization of hypervascular tumors, whether these are primary or metastatic (although some of these tumors are visible only during the HAP). Primary hypervascular malignancies encountered in the liver include hepatocellular carcinoma, hemangioendothelioma, and angiosarcoma. Hypervascular metastatic tumors of the liver may have their sources in islet-cell tumors of the pancreas, renal-cell carcinoma, carcinoid, thyroid carcinoma, neuroendocrine tumors, melanoma, choriocarcinoma, sarcoma, or breast cancer. Hypervascular tumors have always been difficult to detect radiologically; if they are not seen during the HAP, the examination can be considered insensitive. The LAP occurs 25 to 30 seconds after contrast administration and is characterized by full enhancement of the hepatic artery and partial enhancement of the portal vein.
Opacification of the hepatic veins at the dome of the liver indicates that the HVP has been reached. This phase is best for the identification of hypovascular tumors, such as the vast majority of metastases.
Following the HVP, the equilibrium phase occurs. Liver tissue and tumors begin to be equally enhanced by contrast media at this time; lesions become very difficult to detect because contrast is no longer useful in distinguishing normal tissues from various types of lesions. If images mistakenly acquired during this phase are believed to represent the earlier phases of contrast dynamics, lesions may be missed. In addition, known lesions that are being tracked during treatment may appear smaller, possibly leading to the false assumption that their regression or cure has occurred.3,4
Unfortunately, many imaging practices have employed a protocol that requires a waiting period of 60 to 70 seconds between the beginning of contrast injection and the initiation of CT scanning. This does not take into account the great variations in body mass among patients, nor does it allow for the broad differences seen in cardiac output and circulation. These factors may change the arrival time of contrast at the structures of interest not only from patient to patient, but from scan to scan of the same patient, especially if his or her clinical status has changed between examinations. The delay that provides ideal contrast enhancement for one patient is, therefore, unlikely to be suitable for the next (see Figure 1, page 97).
Because MSCT scanning is much faster than its predecessors, the need for perfect timing becomes still more crucial, and the consequences of poor timing become still worse. A positive development in this area has been the advent of computer-automated scanning technology (CAST).5 Bolus tracking via multiple scans at low radiation levels allows CAST users to place cursors over the anatomical areas of interest on their monitors. Enhancement of the marked structures is graphed automatically. When optimal contrast enhancement has been reached, users are alerted that diagnostic scanning will begin (although they can override this triggering manually). For imaging of the liver, beginning diagnostic scanning when the level of contrast enhancement has reached 50 Hounsfield units above the baseline reading promotes capture of images at the optimal level of enhancement. CT manufacturers have introduced CAST capabilities for use with their own scanners, and work in this area indicates that bolus tracking is highly effective in pinpointing ideal scan timing. Although CAST is typically used for timing of the HVP, which begins 65 to 75 seconds after contrast injection, it can also be used to pinpoint the earlier contrast phases for the detection of hypervascular lesions.6,7
Acquiring images during the phase that provides the highest degree of lesion differentiation is clearly vital to examination quality, but the best possible hepatic imaging also requires the use of an adequate amount of iodine (see sidebar).
MSCT already benefits patients by making CT studies faster and more thorough, and imaging facilities are able to achieve increased patient throughput using MSCT. By developing new protocols for MSCT imaging of the liver, radiologists are rising to the challenge created by this continually evolving technology. Used optimally, MSCT can make whole-organ imaging a reality and has the ability to replace more complex procedures (such as CT angiography and CT arterial portography) for hepatic imaging.
|Figure.1: Liver enhancement above baseline (Hounsfield units).|
Paul M. Silverman, MD, is professor of radiology and Gerald D. Dodds Distinguished Chair in Diagnostic Imaging, director of the academic development program at the University of Texas MD Anderson Cancer Center, Houston. This article has been excerpted from MSCT Implications: Liver Imaging, which he presented at Oncologic Imaging for the Practicing Radiologist on Sunday, Sept 14, in San Antonio, Tx.
- Baker ME. Hepatic metastases: basic principles and implications for radiologists. Radiology.1995;197(2):329-37.
- Foley WD, Mallisee TA, Hohenwalter MD, Wilson CR, Quiroz FA, Taylor AJ. Multiphase hepatic CT with a multirow detector CT scanner. AJR Am J Roentgenol. 2000 Sep;175(3):679-85.
- Silverman PM, Szklaruk J, Tamm EP. Contrast usage for liver imaging in the era of multislice (MSCT), multidetector (MDCT): Part I, Appl Radiol. 2003;32(5):30-38.
- Silverman PM, Szklaruk J, Tamm EP. Contrast usage for liver imaging in the era of multislice (MSCT), multidetector (MDCT): Part II, ApplRadiol. 2003; 32(6): 26-34.
- Silverman PM, O’Malley, J, Cooper, C, Zeman, RK, Tefft, MC. Detection of hepatic metastases by helical (spiral) CT: Optimization of timing after contrast administration for assessing the conspicuity of metastatic disease. AJR Am J Roentgenol. 1995;164: 619-623.
- Silverman PM, Brown, B, Wray, H, Fox, SH, Cooper, C, Roberts, S., Zeman, RK. Optimal contrast enhancement of the liver using helical (spiral) CT: Value of SmartPrep. AJR Am J Roentgenol. 1995;164:1169-1171.
- Silverman PM, Roberts, S, Teffet, MC, Brown, B, Fox, SH, Cooper, C., Zeman, RK. Helical (spiral) CT of the liver: Value of an automated computer technique, SmartPrep for obtaining images with optimal contrast enhancement. AJR Am J Roentgenol.1995;165:73-78.