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Technical Innovation |
1
Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline
Ave., Boston, MA 02215.
2
Department of Surgery, Beth Israel Deaconess Medical Center, Boston, MA
02215.
3
Present address: Institute of Liver Transplantation, Lahey Clinic Medical
Center, 41 Mall Rd., Burlington, MA 01805.
Received January 20, 2000;
accepted after revision March 7, 2000.
Presented at the annual meeting of the American Roentgen Ray Society,
Washington, DC, May 2000.
Introduction
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TopIntroductionMaterials And MethodsResultsDiscussionReferences |
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CT was performed with a LightSpeed scanner (General Electric Medical Systems, Milwaukee, WI) with multidetector capability. All patients received milk (1200-1800 mL) as an oral contrast agent [2]. An unenhanced helical scan was obtained to provide scanning levels for the contrast study. At a fixed level along the mid liver, four axial (5-mm collimation) images were obtained at different kilovoltage settings (80, 100, 120, and 140 kVp) to assess fatty infiltration [3]. Multiphase scanning was then performed after IV injection of 180 mL of ioversal 68% (Optiray 320; Mallinckrodt, St. Louis, MO) at a rate of 5 mL/sec. Arterial phase images were obtained at 18 sec (1.25-mm collimation; table speed, 7.5) from a level 2 cm below the dome of the diaphragm to a level 2 cm below the origin of the superior mesenteric artery. Portal venous phase images were obtained at 60 sec (2.5-mm collimation; table speed, 15) through the entire liver. The total acquisition time for each phase was approximately 12 sec.
Postprocessing was performed on a commercially available workstation (Advantage Windows 3.1, General Electric Medical Systems). Three sets of volume rendered reconstructions were performed by a technologist. The first set was for the hepatic arteries and was performed in a coronal oblique plane of the arterial phase images (Fig. 1A). The portal venous phase images were used to generate the other two sets that were for the hepatic and portal venous systems and were obtained in the axial and coronal planes, respectively (Figs. 1B and 1C). Three-dimensional reconstructions of the hepatic vessels were rendered with maximum intensity projections, shaded-surface display, and volume rendering. If needed, additional reconstructions were performed by a radiologist.
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Hand tracing of the liver outline was performed on the axial images of the portal venous phase. Afterward, an automated "paintbrush" method was used to determine the liver volume. Subsequently, a three-dimensional model of the liver volume was isolated, and the liver volume was computed with the same software. The liver and hepatic vein models were then superimposed. Using these models for guidance, we manually hand traced a hepatectomy border, avoiding major vascular structures traversing between the right and left lobes (Fig. 2). If needed, additional planes with different distances from the middle hepatic vein were generated, until an appropriate lobar liver volume was achieved.
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TopIntroductionMaterials And MethodsResultsDiscussionReferences |
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The hepatic veins were best displaced in the axial plane, which showed branching and bifurcation of the middle hepatic vein that may alter the hepatectomy plane (Fig. 1B). Special attention was given to examining for the presence of an accessory inferior right hepatic vein, which must be separately dissected during surgery.
Portal venous anatomy was best displayed in the coronal plane (Fig. 1C). Variants in portal vein anatomy that may affect the selection criteria for potential donors were well displayed, including the separate origin of the posterior right portal vein.
In all cases total and lobar liver volumes were provided, and an avascular virtual hepatectomy plane was identified. The measured right lobe volume was within 93% of the actual graft volume in the 10 hemihepatectomies performed.
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TopIntroductionMaterials And MethodsResultsDiscussionReferences |
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Identification before surgery of fatty infiltration is critical because its presence is associated with a high incidence of nonfunctioning grafts [5]. Dual-energy CT can quantify the degree of fatty infiltration [3]. When scanned with 80 and 140 kVp, fatty infiltration of the liver exhibits greater change in attenuation than do normal livers. A change exceeding 7 H is suggestive of fatty infiltration, and a change greater than 10 H is unique to fatty infiltration. Patients with fatty infiltration of less than 25% have a change of 6 H, whereas patients with 50% fatty infiltration have a change of 11 H. Fatty infiltration greater than 75% is associated with a change of 20 H. Additional intermediate energy levels provided with multidetector scanners (100 and 120 kVp) were included to enhance our sensitivity in detecting fatty infiltration.
To avoid postoperative liver failure due to graft size disparity, the implanted graft should be large enough to permit normal metabolic function. The minimum graft volume required is approximately 40% of the standard recipient's liver mass [5]. Small-for-size grafts are prone to dysfunction because of inadequate functional hepatic mass and because the graft may sustain injury from excessive portal perfusion [6]. The critical volume and the quality of the remnant liver should also be a consideration in patient selection. Liver remnant volume of approximately 35% of the total liver volume is sufficient for the donor to survive if the liver parenchyma is normal [5].
Multidetector CT offers several clinical advantages over helical CT angiography [7]. Multidetector CT provides more accurate and clearer anatomic relationships because it allows the lengthwise display of vessels. We consider this imaging to be of increasing importance, especially given the advances in hepatic resection techniques that rely fundamentally on knowledge of hepatic vascular variants. Technical advantages include a standard protocol that allows optimum visualization of all hepatic vasculature for all subjects.
The technique works best with state-of-the-art technology, including multidetector CT for data acquisition and processing. In addition, costly and sophisticated computer equipment is required. Another disadvantage includes the need for training of technologists and the time required for image processing. The total time required for image processing by the technologist is 15-20 min, and interactive examination by the radiologist requires an additional 10 min.
Multiplanar CT has been used to visualize dilated pancreatic and common bile ducts [8], but normal ducts may not be visualized. Nonionic contrast agents have been used to opacify the nondilated biliary tree [9]. However, these agents are not available in the United States, and their use is associated with an increased iodine load in patients undergoing multiphasic scanning. MR imaging provides a potential alternative imaging technique that could be used to visualize both the hepatic vasculature and the biliary tree. However, further optimization of the MR imaging sequences is required to improve spatial resolution of small hepatic artery branches that are of clinical concern.
In conclusion, multidetector multiphase CT is valuable in delineating the hepatic vascular anatomy. The image-processing techniques used in standard orientations provide a highly graphic depiction of the complex and variable hepatic blood supply. Measurement of total and segmental liver volume is accurate, and creating a virtual hepatectomy is critical in patient selection. This information is important in donor selection, for which CT can provide comprehensive examination before surgery.
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