Contrast usage for liver imaging in the era of MSCT, MDCT: Part II

Applied Radiology — Vol. 32 , Issue 6 , pp. 24 -30

DOI: 10.37549/AR1194

Published: June 1, 2003

Paul M. Silverman, MD, Janio Szklaruk, MD, PhD, Eric Tamm, MD

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[Note: Part I of this article was published in the May 2003 issue of Applied Radiology: Silverman PM, Szklaruk J, Tamm E. Contrast usage for liver imaging in the era of multislice (MSCT), multidetector (MDCT) CT: Part I. Appl Radiol. 2003;32(5):30-38.]

In the early 1990s, dual or split detector systems became available allowing the acquisition of 2 slices during a single gantry rotation. In 1998, the first multislice systems were introduced with 4 data channels providing a quantum leap in CT technology.1-5 These scanners have been referred to by various names, including multidetector, multidetector row, and, most appropriately, multislice CT (MSCT). In the past couple of years, 8-slice, 16-slice, and even 32-slice detector systems have been developed, and manufacturers are now testing the incorporation of flat-panel detectors in scanners in an attempt to extend this technology to near-instantaneous data acquisition with volumetric data acquisition.

Scanners already allow the acquisition of multislice data sets with sub-second scan times.6 These advances allow the acquisition of thinner slices, shorter scanning times, and greater volumetric coverage in the z-axis (Figure 1). Depending on the specific clinical application, relative trade-offs between speed and collimation are tailored to create optimal protocols for multislice CT.2,4 The development of MSCT has also allowed for near-isotropic and, in the past year, isotropic voxels that can provide the ability to image in multiple planes without loss of resolution. Three-dimensional (3D) imaging is not only practical but results in extremely high-quality images devoid of previous stair-step artifacts. Reconstructions from very thinly collimated images provide exquisite depiction of anatomy when coupled with optimized timing of scanning with contrast enhancement usually using bolus-tracking approaches. Three-dimensional images readily performed in the arterial and venous phases are valuable for staging tumors, as well as for the evaluation for vascular anomalies and vascular disease as part of a detailed preoperative assessment. Such imaging is also of great value in planning X-ray therapy.

FIGURE 1.
FIGURE 1. Advantages of multislice CT (MSCT). (A) MSCT can be performed with thinner sections in less time, which provides similar coverage, but in essentially half the time of a conventional, helical scanner. This also offers the value of creating thinner sections with less partial volume artifact. (B) MSCT provides twice the anatomic coverage in the z-axis with thinner sections over the same period of time. This offers extended organ coverage and body coverage allowing optimal imaging during a desired phase of contrast enhancement.

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Contrast dynamics

The liver, because of its unique dual blood supply (20% from the hepatic artery and 80% from the portal venous system), remains just as much or more of a challenge for optimizing protocols in the current era of MSCT. These scanners offer a quantum leap in speed and flexibility when compared with even standard single-slice helical scanners. The previously termed portal venous phase (PVP) for single-slice CT (SSCT) has now been more appropriately named the hepatic venous phase (HVP) on MSCT, as this phase captures the opacification of these veins and maximal liver enhancement (Figures 2 through 8).6,7 The main impact of MSCT scanners has been to provide the ability to examine an organ, such as the liver, in multiple phases of contrast dynamics with the hope of allowing increased detection of lesions as well as improved lesion characterization.7-10 Optimizing protocols for multiphasic imaging includes adding phase(s) to the HVP (ie, dual-phase imaging) and/or inclusion of a very early arterial phase for 3D imaging of the vascular system, (ie, triple-phase imaging). In contrast to helical SSCT, MSCT is able to define three distinct phases of contrast enhancement, rather than just two. With SSCT, the two phases are the hepatic arterial dominant phase (HADP) and the PVP; with MSCT these phases have been termed the hepatic arterial phase (HAP), late arterial phase (LAP) or portal venous inflow phase (PVIP), and a hepatic venous phase (HVP).7 The first two phases were incorporated in the HADP described with SSCT (Figures 9 through 12). The ability to scan rapidly with MSCT allows one to separate these and scan in two phases what could only be done in one phase previously. Hypervascular lesions, either primary or metastatic, are usually best seen in the LAP; however, some lesions are seen only in either the HAP or LAP phases (Figures 13 through 15). Hypervascular lesions have always presented a challenge to the radiologist.10 Failing to image hypervascular lesions during the HAP results in an insensitive examination similar to failing to image hypovascular lesions in the PVP. The HAP is best identified 10 to 20 seconds after the administration of contrast and is characterized by enhancement of the hepatic artery. The LAP is best identified 25 to 30 seconds after injection and shows enhancement of the hepatic artery and some enhancement of the portal venous structures. The HVP is marked by opacification of the hepatic veins at the dome of the liver and enhancement of the portal veins. The speed results in one of the most important challenges in developing optimized protocols for this new, robust technology. Although multiphasic studies could be performed with helical scanners, high-quality, whole-organ imaging with multiple phases awaited the introduction of MSCT.11-15

FIGURE 2.
FIGURE 2. Time density curves for multislice CT (MSCT). Optimal scanning of the entire liver can now be completed during the hepatic venous phase (HVP), the period of peak liver enhancement, which offers the maximal difference between liver and tumor enhancement and improved conspicuity of lesions compared with standard helical scanning. The scan time for HVP is much shorter and does not encroach on the early equilibrium phase. This allows for complete organ imaging during the optimal period for detecting hypovascular lesions. The term “HVP” is substituted for “PVP” (portal venous phase) when using helical CT.
FIGURE 3.
FIGURE 3. Sigmoid colon carcinoma scanned during peak hepatic venous phase with multislice CT. (A) Metastatic colon cancer lesion is seen readily in the posterior segment of the right lobe of the liver. (B) Primary sigmoid cancer can be identified at the lateral part of the sigmoid colon.
FIGURE 4.
FIGURE 4. Contrast enhancement curves comparing single-slice CT (SSCT) with multislice CT (MSCT). Using MSCT, scanning during the hepatic venous phase (HVP) (which is synonymous with the SSCT portal venous phase [PVP]) is much shorter than the PVP using SSCT, capturing peak enhancement.
FIGURE 5.
FIGURE 5. Tiny metastatic colon carcinoma lesions are now detectable with multislice CT. (A) On noncontrast CT, a tiny lesion located posteriorly in right lobe (arrow) is not well seen. (B) The lesion is best seen (arrow) during the hepatic venous phase (HVP). (C) The lesion is still visible during the late HVP stage (arrow).
FIGURE 6.
FIGURE 6. Contrast enhancement curves for dual-phase imaging of hypervascular liver metastases. Multislice CT allows excellent dual-phase imaging with a late arterial phase (LAP) and hepatic venous phase (HVP).
FIGURE 7.
FIGURE 7. Dual-phase multislice CT scans of angiosarcoma metastatic to the liver. (A) Numerous hypervascular lesions to the right lobe and an incidental cyst are visible in the late arterial phase. (B) The lesions are poorly seen shortly later during the peak hepatic venous phase.
FIGURE 8.
FIGURE 8. Stage IV hepatocellular carcinoma. (A) During the hepatic arterial phase, there is irregular enhancement (arrow) in the region of the main portal vein and the right portal vein, which reflects collateralization. (B) A scan from the portal venous phase demonstrates the lack of enhancement of the portal vein (arrow), which is indicative of portal vein thrombosis.
FIGURE 9.
FIGURE 9. Contrast dynamics of triphasic imaging of hypervascular liver metastases. Three distinct phases can be identified with multislice CT, the hepatic arterial phase (HAP), the late arterial phase (LAP), and the hepatic venous phase (HVP).
FIGURE 10.
FIGURE 10. Hepatic contrast dynamics for triphasic multislice CT. (A) In the hepatic arterial phase, the hepatic artery is seen opacified in the porta hepatis. (B) In the late arterial phase, the portal vein begins to receive some contrast, which accounts for an alternative name for this phase, portal venous inflow phase. (C) The hepatic venous phase is the routine phase for best detecting hypovascular lesions.
FIGURE 11.
FIGURE 11. Multiphasic imaging of metastatic thyroid carcinoma. (A) Lesion representing a metastasis in the dome of the liver (arrow) is poorly seen in the hepatic arterial phase. (B) Optimal enhancement of the metastasis (arrow) is shown during the late arterial phase. (C) Washout of the lesion (arrow) is seen in the hepatic venous phase.
FIGURE 12.
FIGURE 12. Triphasic examination of a metastatic carcinoid. (A) During the hepatic arterial phase, multiple lesions in the right lobe secondary to metastatic carcinoid appear hypovascular. (B) During the late arterial phase, lesions develop a peripheral rim of enhancement and are seen more clearly. (C) During the hepatic venous phase, the peripheral rim fades, making lesions slightly less conspicuous.
FIGURE 13.
FIGURE 13. Multiphasic imaging of islet cell carcinoma. (A) Numerous lesions with either peripheral or complete enhancement (arrows) are visible during the hepatic arterial phase. Note the relatively low density liver. (B) During the late arterial phase, the lesions are seen optimally (arrows). (C) Lesions become much less conspicuous during the hepatic venous phase (arrow).
FIGURE 14.
FIGURE 14. Multiphasic imaging of neuroendocrine metastases. (A) During the hepatic arterial phase, densely enhancing lesions are seen against the background of dark liver not yet enhanced. (B) The lesions are not seen as well during the late arterial phase. (C) In the hepatic venous phase, the lesions lose their enhancement and are poorly seen.
FIGURE 15.
FIGURE 15. (A) Hypervascular metastasis from carcinoid is seen during the hepatic arterial phase. The densely enhancing small (1 cm) metastasis in the right lobe (arrow) is seen against the dark, unenhanced liver. (B) The lesion no longer seen during the late arterial phase. (C) Lesion is also undetectable during the hepatic venous phase.

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Higher concentration contrast in multislice CT

Detection of liver lesions is dependent on scanning during the phase that optimally distinguishes normal from abnormal tissue as discussed. Optimized imaging requires using adequate amounts of contrast, ie, grams of iodine.16 The grams of iodine have a direct impact on the difference in hepatic attenuation relative to lesion detection that defines the relative conspicuity of lesions (normal hepatic attenuation – liver lesion attenuation = lesion conspicuity).

Most recently, with the rapid proliferation of MSCT technology, the concept of using higher concentrations of contrast material has begun to be explored.16-18 The impetus for this has been that the standard contrast concentrations of 300 to 320 mg I/mL have required volumes on the order of 150 mL to deliver adequate grams of iodine to image the liver effectively. This is in contrast to examining other areas of the body, such as the chest, where the dose and volume of iodinated contrast can be significantly reduced (ie, 150 to 100 mL [helical CT] to 60 to 75 mL [MSCT]). Imaging of liver lesions requires more precise protocols. Studies of the liver with less than optimal contrast enhancement result in compromised lesion detectability. Fortunately, to date, prices of contrast material are not directly tied to grams of iodine within the product, but are most closely linked with the volume of contrast. Thus, if we can use lower volumes and higher concentrations of contrast, it has the additional benefit of becoming highly cost effective. With SSCT and helical scanning, protocols for body CT required volumes of contrast in the range of 150 mL with 300 mg I/mL and 320 mg I/mL to be able to have optimal enhancement of the liver and also provide adequate enhancement of abdominal and pelvic structures. With MSCT, this can be accomplished without requiring such large volumes since scans can be completed so rapidly. Thus, it becomes the challenge for radiologists to adopt new protocols to take advantage of this continually evolving technology.

Higher concentrations of contrast, 350, 370, and even 400 mg I/mL, have been developed and are being used clinically. If a target range of 37 to 48 grams of iodine is considered to image the liver, then this can be achieved by a number of different permutations of volume and concentration of contrast (Table 1). Higher concentrations of contrast also allow contrast delivery of the same grams of iodine per second to the target organ at lower rates. For example, the administration of 150 mL of 300 mg I/mL at 5 mL/sec delivers an iodine dose of 1.5 g/sec whereas the administration of 100 mL of 370 mg I/mL at only 4 mL/sec delivers essentially the same iodine dose of 1.48 g/sec. The ability to decrease the total volume of contrast will result in overall substantial cost savings in a busy clinical CT service.

Table 1. Volumes of contrast that can be used for different contrast concentrations*

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Conclusion

The introduction of MSCT has created a new challenge for radiologists. These very fast scanners provide a great deal of flexibility for body imaging, especially in the liver. It also provides for very high-quality 3D vascular imaging, which can aid in surgical and therapeutic planning. It is only by understanding the flexibility of this new technology and new developments made by contrast companies in providing a variety of concentrations of contrast material that we can take full advantage and harness its potential for the benefit of our patients.  AR

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Citation

Silverman PM, Szklaruk J, Tamm E. Contrast usage for liver imaging in the era of MSCT, MDCT: Part II. Applied Radiology. 2003;32(6):24-30. doi:10.37549/AR1194.