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Everything you need to know about Computed Tomography (CT) & CT Scanning


Small Bowel: CT Angiography and MDCT: Detection, Characterization and Staging of Abdominal Disease

Elliot K. Fishman, M.D.
The Russell H. Morgan Department of Radiology and Radiological Science
Johns Hopkins University School of Medicine
Baltimore, Maryland 21287

This manuscript is based on the Refresher Course presented by Elliot K. Fishman at RSNA 2000.

Introduction

The development of subsecond spiral CT, soon followed by multidetector CT (MDCT) has provided the radiologist with unparalleled capabilities to acquire high quality image data (1-2). Whether it be the ability to acquire thin section images (1-1.25 mm), reconstruct data at narrow interscan intervals (1.0 mm) or acquire this data in the arterial and/or portal phase, the newest scanners provide capabilities that were never thought possible a few short years ago. Yet, the true advance of these newer scanners is not just the ability to get thinner sections faster but the ability to truly move from a ‘slice scanning mode’ mentality to a ‘volume acquisition mode.’ Although at first glance the difference may seem more a nomenclature change than a strategic inflection point this is not the case. With a volume acquisition system we are scanning a volume of interest (i.e. the entire abdomen, the liver, the pancreas, etc.) and display that volume not as slices but as a true volume. Therefore, a CT scan is not an axial image (or 300 axial images) but a three-dimensional display of those axial images that is a form that is easier and faster to understand, but also a form that has unique and optimal display parameters. For example, visualization of the superior mesenteric artery can be done as a series of 100 or more slices or just as a single image of the vessel as if you did a selective catheter angiogram (Fig. 1). One big difference of course is that the CT angiogram is done noninvasively, in less time and at a lower cost. Also the CT angiogram can be viewed in any plane or orientation that would be optimal for that patient an obvious advantage for CT angiography (3-5). With CT angiography we do not have the geometric constraints of a C-arm used in classic angiography.

The techniques used for acquiring data for CT angiography are critical but are not the focus of this presentation. However, a few basic principles can never be overemphasized and include:

  • Positive contrast agents are never used in cases of abdominal CT angiography. We only use water as a marker for stomach and bowel distention.
  • Although timing of the arterial and portal phase can be done with either a timing bolus or use of computer-assisted software we have found that we can preselect scan delays without compromising our studies. Typical scan delays are 25-30 seconds for arterial phase imaging and 55-60 seconds for portal venous phase imaging. In patients with a known history of cardiac disease or in patients with a large heart on the topogram (scout view) we will add an additional 10 seconds to the scan delay.
  • For detailed vascular anatomy slice thickness should be 1-1.25 mm and data reconstructed at 1 mm intervals. All data is sent to the workstation in the original resolution.

Once the study acquisition is completed the CT data must be analyzed on a computer workstation. Although discussion of the various workstations as well as the importance of the rendering algorithms is critical they have previously been addressed (6-7) and will not be discussed at length. Rather, several recurring themes will be addressed for completeness. They include:

  • Creation of the 3D angiograms is best done on a freestanding workstation placed away from the actual point of scanning. This also allows consultation with the referring physicians without interfering with the scanner’s core scanning functions.
  • We prefer a system that allows direct interaction with the CT volume rather than a system that creates select views for the user. We believe in the radiologist taking an active role in image analysis as they can best choose the optimal plane and orientation for demonstrating the extent and presence of pathology. Unsuspected pathology is also more likely to be detected with the radiologist interactively viewing the data volume. This is typically referred to as real-time rendering and is currently available on select workstations.
  • Our preferred 3D rendering techniques for CT angiography is volume rendering. Its specific advantages in vascular imaging include more accurate visualization of vessel detail and stenosis, presence and location of vascular anomalies, and the ability to change the select parameters to view multiple tissue types (i.e. both pancreatic mass and SMA/SMV) is ideal for abdominal CT angiography. However, we also find that maximum intensity projection (MIP) rendering is valuable as an adjunct display especially for smaller vessels within an enhancing organ like the liver and kidney. We never use shaded surface display (5-7).
  • Alternative visualization techniques including stereoscopic display are valuable when looking at complex vascular anatomy (8).

Clinical Applications

Although there are numerous clinical applications of CT Angiography in the abdomen which seem to be increasing on nearly a daily basis this presentation will focus on applications in four major areas; the kidney, pancreas, mesenteric vessels/bowel and liver. Although there are many other applications including evaluation of the aorta that could also be discussed they will not be addressed in this presentation.

Renal Applications

There are numerous potential clinical applications for CT angiography of the kidney documented in the literature. The most common applications include evaluation of potential renal transplant donors, detection, classification and staging of renal masses, determining if a patient with renal cancer is eligible for nephron sparing surgery, and evaluation of renal artery stenosis (9-12). Although all of these applications can be done with a single detector spiral CT scanner, the introduction of MDCT has significant advantages especially when looking at the main renal artery and its branches. The increased speed of data acquisition in addition to eliminating artifacts due to patient motion or breathing provides the ability to use thinner slice collimation even when larger volumes are to be imaged. The use of 1-1.25 mm thick sections is essential when evaluating stenosis of vessels as well as identifying small branch vessels.

Evaluation of Potential Renal Donors

The classic scenario for renal transplantation was a cadaveric transplant with occasional use of living related donors. Yet, the waiting list for transplant continued to grow and now for example is over 4 years in the State of Maryland. Against these gloomy statistics a recent development that could go a long way in turning the tide is the introduction of laparoscopic nephrectomy. No longer does a classic nephrectomy with all its potential complications and postoperative recovery times have to be performed. Laparoscopic nephrectomy has been shown to lower the length of hospital stays, and decreases the recovery time and time back to work. Ratner et al. have shown that with laparoscopic nephrectomy the average hospital stay decreases from 5.7 ± 1.7days to 2.7 ± 1.0 days, return to full activity decreases from 4.2 ± 2.4 weeks to 2.3 ± 1.1 weeks and the time before return to work 6.4 ± 3.1 to 3.9 ± 1.6 weeks when compared to a classic renal nephrectomy for a donor (13-14).

Although laparoscopic surgery has tremendous advantages it also provides additional challenges to the transplant surgeon. Vascular mapping of both the arterial and venous system as well as the pelvocalyceal systems and ureters must be accurately defined. Aberrant or anomalous vessels including lumbar and gonadal veins needs to be recognized to avoid potential vascular injury. Renal arterial anatomic variations are common with 25% of individuals having multiple renal arteries (Figs. 2-3). Venous anatomic variations are less common with left side variation up to 28% of patients representing multiple right renal veins; left sided variants include a single retroaortic renal vein in 3%, circumaortic in 17% and lumbar veins joining the left renal vein 75% (15) (Fig. 4). Similarly with the increased age of donors it is not surprising that incidental renal cell carcinomas will be detected. In an era when nearly half of all renal cell carcinomas are detected by serendipity this should come as no surprise (Figs. 5-6). The presence of renal artery stenosis in these older patients must also be searched for and excluded.

As with all clinical applications for CT angiography, the design of the study protocol for a renal transplant donor must be optimized to answer all of the question's of the urologic surgeon. We must also take into account the goal of limiting the radiation dose to the renal donor. Therefore our study protocol requires dual phase acquisitions at 25 and 60 seconds after contrast injection begins, representing an arterial dominant phase and a late cortical-medullary phase. We do not obtain a noncontrast study of the kidney nor do we obtain a set of scans in the excretory phase (16). Rather, we will obtain a delayed topogram at around 4 minutes after the study to obtain a classic IVP style film. Our routine scan protocol uses 120 kVp and 150 mAs with 1.25 mm thick slices (1 mm collimations) and a pitch of 6. Typical scan protocols will take under 20 seconds per acquisition.

The axial scans are reviewed initially to look for any gross renal or extrarenal pathology and then three-dimensional maps are generated for vascular mapping. For renal imaging we routinely use volume rendering for the 3D display and supplement this with maximum intensity projection (MIP) images. The arterial dominant images are viewed with a real time display and the individual renal arteries defined. Care is taken to view the entire aorta through the bifurcation and iliac vessels to detect any accessory renal arteries. The renal arteries are then followed from their origin off the aorta into the renal hilum and parenchyma. Perihilar branching when present is best defined on the 3D images which are easily missed on axial CT alone. The renal arteries are also studied to rule out renal artery stenosis or aneurysms. On the venous phase images the renal veins are carefully outlined to determine any variations including retroaortic or circumaortic renal arteries. Definition of the left adrenal vein as well as the gonadal vein is also deformed for vascular mapping. All vessels are defined and films documenting their appearance are sent to the transplant surgeon (17-18). CT using single detector CT has been shown to be nearly as accurate as classic angiography and our experience with MDCT is that MDCT and surgery are nearly 1:1 in accuracy.

The dual phase CT angiogram also can define the presence of pathologies that will exclude the patient from being a recent donor. These include polycystic kidney disease, horseshoe kidney with a thick isthmus, as well as an occult renal cell carcinoma.

Renal Carcinoma

CT has become the study of choice for the evaluation of suspected renal tumors. Whether the question be is there a mass present, or to classify a mass into the Bosniak classification, or to stage a renal tumor, CT has become the accepted gold standard. CT angiography builds on our post imaging success in CT and completes the full circle to provide "single stop shopping" for renal pathology. CT angiography with dual phase acquisition provides better detail into the status of the renal artery and vein and allows excellent preoperative planning. The venous phase is especially valuable to determine the presence of renal vein thrombus and well as potential extension of clot into the intrahepatic IVC as well as up into the right atrium. A 3D-volume display is ideal for this application. Real time rendering is especially valuable for determining the presence and extent of clot in vessels like the renal vein, which run obliquely through the study volume. The presence of extensive collateralization is the perirenal and pararenal spaces are not uncommon in hypervascular renal cell carcinomas. The presence and extent of these collaterals is well defined on these dual phase angiographic maps (Fig. 7).

Although CT angiography with 3D reconstruction optimally define the extent of disease there has been no study to date which suggests that it changes the staging of tumor. Rather it helps the surgeon decide on the appropriate course of patient management. This includes the decision as to whether performing a classic nephrectomy or a nephron sparing surgery (partial nephrectomy) in a specific patient (19-20).

Nephron Sparing Surgery

One of the key applications for 3D CT angiography is in the selection of the patients with renal tumors who are candidates for nephron sparing surgery or a partial nephrectomy. With up to half of all renal cell carcinomas now being detected by serendipity, we are seeing tumors that are smaller (< 4 cm) and appearing in younger patients. Unlike the classic patient who presents with symptoms of flank pain, hematuria or metastases, these patients tumor is typically detected because of an ultrasound, CT or MRI for other reasons than suspected renal pathology.

In the past, partial nephrectomy was typically an option for patients where a total nephrectomy would leave the patient dependent on dialysis for survival. These include patients with a mass in a solitary kidney, a patient with bilateral renal masses, and a patient with borderline renal function where nephrectomy would result in renal failure or a patient with a syndrome, which predisposed the patient for renal tumors.

With advances in surgical technique and increased clinical experience urologic surgeons are opting for partial nephrectomy in a new group of patients. The ideal candidate for a partial nephrectomy should have a mass under 4 cm, the mass to be exophytic in location, no evidence of nodes or tumor spread, and the mass be away from the renal pelvis and cortical vessels. 3D angiography is ideal for providing this information. On a single dual phase examination all of the information needed by the urologic surgeon can be provided in a single comprehensive examination. Coll et al. (21) reviewed a total of 97 renal masses in 60 cases and found that 3D volume rendered CT integrated "essential information from angiography, venography, excretory urography and conventional 2D CT into a single imaging modality." At their institution this became the preferred means of data display. Several different series have looked carefully at this subgroup of patients.

Smith et al. (19) found similar results and noted that "the 3D helical CT uniquely assists the urologist by providing preoperative information in a flexible display that aid in determining whether nephron sparing surgery is possible and planning the surgical procedure. With an ever-increasing number of patients presenting with incidentally detected tumors the use of 3D CT angiography for this application will continue to grow"(Figs 5-6).

Renal Artery Stenosis

Another renal application for CT angiography is the evaluation of renal artery stenosis. Although there has been much debate as to what the ideal imaging modality is for renal artery stenosis there is little doubt that CT scanning can play a major role especially with multidetector CT. Previous articles by Brink et al. (22) have shown that thin section CT with a maximum thickness of 2 mm is necessary for accurately determining renal artery stenosis. With single detector scanners the ability to use thin sections and cover a large enough volume was often a challenge. With multidetector CT with its rapid acquisition, thin collimation and long data acquisition possibilities study design becomes ideal. In this application we use 1-1.25 mm slice thickness although we are currently looking at using .5 mm slice thickness which provides isotropic data. CT can clearly define the presence of single or multiple renal arteries and accurately define the presence of stenosis. Articles by Kuszyk (23) and Ebert (24) have shown that 3D CT is accurate for determining degree of stenosis as long as correct rendering parameters are selected.

For three-dimensional imaging of the renal arteries we routinely use volumetric rendering technique and supplement this with MIP. The MIP technique can be problematic in the presence of calcification and can overestimate the degree of stenosis present. With volume rendering we can clearly define vessel lumen from calcification and so this problem does not exist (Fig. 8). Please note that one can err in setting the parameters of volume rendering and either over- or underestimate the degree of stenosis present. However, with careful attention to detail and experience this becomes less of an issue. On the other hand, with MIP due to the inherent limitations of the technique, which is a projection technique, the presence of calcification on a vessel wall can easily stimulate total or extensive stenosis when none may be present. Therefore, care must be taken when using rendering techniques in this application.

Pancreatic Imaging

The protocols for CT angiography of the pancreas include dual phase imaging with data obtained 25 and 60 seconds after the injection of 120 cc of Omnipaque -350 (Nycomed Amersham, Princeton, NJ) at a rate of 3 cc/second. Scan parameters for MDCT are 1.25 mm scan width reconstructed at 1 mm intervals and 3 mm scan width at 1mm intervals for single detector CT. After data acquisition the images are sent over the hospital-imaging network to the workstation and select 3D images are generated from both phases of acquisition.

The classic description for a pancreatic tumor was a mass within the pancreas commonly ranging in size from 3-6 cm. Detection of the mass was typically based on size parameters with less attention paid to the differential enhancement of normal and abnormal pancreatic tissue. In part this was due to the slow injection rates as well as longer scan times on early generation CT scans. Additional parameters commonly used include the presence of pancreatic or common bile duct dilatation, as well as changes in the pancreatic contour. Most pancreatic masses are best seen on the portal venous phase images, as they are hypovascular. Other tumors including islet cell tumors and metastases are hypervascular and are best seen on arterial phase images. Graf et al. found that tumor conspicuity of pancreatic adenocarcinoma was better in the portal phase where the tumor to pancreas contrast difference was 54+/-31 H than in the arterial phase when it was 31 +/- 29H (25). Regardless of the tumor type, the use of thin section CT with close interscan spacing allows smaller tumor detection when changes in the gland enhancement patterns are detected. Although axial imaging and review of these images alone has been the standard mode of CT review, it will not remain the gold standard, as newer technologies became available. Bonaldi et al. (26) found that simply reviewing the images on a workstation with a cine display provided better definition of tumors as well as vascular anatomy and ductal anatomy. The use of 3D imaging can be very helpful in select cases by defining the mass or suspected mass in multiple viewing planes. In many cases this may help distinguish a true pancreatic mass from adjacent duodenal or small bowel tumor or peripancreatic adenopathy.

Vascular Mapping

The area of greatest challenge in pancreatic imaging has been the ability of CT to accurately determine the presence of vascular invasion. The accuracy of CT to define arterial or venous invasion has been the subject of numerous articles often focusing on its accuracy when compared to either catheter based angiography or to surgical findings. Several articles even in the pre-spiral CT era has clearly shown the equivalence of carefully performed CT and catheter angiography for vascular encasement (27-28).

Yet, with spiral and then multidetector CT our capabilities have gone far beyond looking at vessels in the axial plane. As shown in figure x there are significant limitations when looking at vessels in the axial plane. Partial averaging of data, questions as to whether a tumor is adjacent to rather than encasing a vessel as well as the lack of an ideal display are problems with axial CT alone. The use of image reconstruction especially with a 3D vascular map has obvious advantages especially to the referring surgeon who is more comfortable with a volumetric display.

The accurate display of arterial anatomy (SMA, celiac axis) is the major focus of arterial phase 3D mapping in the evaluation of the pancreas. The goal of preoperative vascular mapping in patients who are potential candidates for a Whipple procedure is to clearly define the angiographic map with accuracy equal to or exceeding a classic catheter angiogram. With 3D CT angiography this goal can be obtained. Anatomic variation such as a common celiac and SMA trunk or the presence and location of a right hepatic artery arising off the SMA are all easily detailed on the 3D display. The viewing of this data with a stereoscopic display may add further information in cases of complicated vascular anatomy.

In cases where there is potential vascular invasion the 3D display will define the course of the vessel and its relationship to the pancreatic mass. The ability to determine whether a mass actually encases or just abuts a vessel will be clearly shown in most cases. The ability to view images in any plane or perspective is an important tool supplementing the information from the axial CT and in replacing angiography. Although no large series has yet to be published, our personal experience has been suggestive of a near one to one correlation with surgical findings (29)(Figs. 9-12).

With axial images alone, numerous articles tried to develop strategies for defining resectability. Lu et al. (30) graded vessel invasion on a 0-4 scale based on circumferential contiguity of tumor to vessel. That is, a grade 0 was no contiguity of tumor to vessel, grade 1 was tumor contiguous with less than one quarter circumference, grade 2 between one quarter and one half circumference, grade 3 between one half and three quarters circumference and grade 4 greater than three quarters circumference or any vessel constriction. Involvement of more than one half the circumference or grade 3 was highly specific for unresectable tumor. Yet, even half of the grade 2 cases were proven to be unresectable and 12.5% of grade 3 were resectable. We believe these studies highlight some of the disadvantages of viewing vessels with an axial plane only. This is especially true when it comes to vessel narrowing or constriction. The use of MDCT increases the number of slices through the pancreas over even the best single detector spiral protocol by a factor of 2 or 3. This increased data sampling at 1-1.25 mm slice thickness and 1 mm interscan intervals coupled with the higher resolution of MDCT should prove better at defining the key arterial vessels even if only the axial images are reviewed. However, the ability to view the vessels in multiple orientations provides a more complete and accurate display where even subtle vessel invasion can be detected. The extent of vessel encasement is optimally defined with this visualization.

Numerous articles including one by Vedantham et al. (31) have shown that helical CT performed in the portal venous phase at 40-70 seconds after injection of contrast is ideal for defining peripancreatic venous anatomy for determining tumor invasion. However as noted previously the use of axial CT alone is not ideal for pancreatic imaging and that a more volumetric display is required. With the development of single detector and now multidetector CT we believe that a volume display is the most accurate technique for evaluating venous invasion. Even with the use of single detector CT Graf et al. (32) were able to create accurate CT venograms of the mesenteric veins that were able to equal angiography for defining variations in vascular anatomy. However careful analysis of the images would suggest that their detail was not adequate to define early vessel invasion as in pancreatic cancer. Novick et al. (33) did show that using advanced 3D techniques like volume rendering those vascular maps could not only define the venous anatomy but also accurately predict vascular invasion. Raptopoulos et al. (34) had similar results with CT Angiography from single detector spiral CT data sets. They found that by adding the CT angiogram to the axial images alone the negative predictive value of a resectable tumor was 96% compared to 70% for axial images alone. The studies were especially valuable in determining unresectability.

We have recently shown that the use of MDCT coupled with three-dimensional imaging provides an even better way to image vessel patency. By acquiring data sets of narrow collimation with short acquisitions we are able to obtain a true volume data sets for evaluation of their arterial and venous system. Using the 3D display we can define vessel patency as well as determine early vessel encasement or invasion. The use of these display tools decreases the potential for false positive studies as well as indeterminate studies. Areas where the 3D display is especially helpful are at the confluence of the portal and superior mesenteric vein as well as the more distal portions of the portal vein. As surgeons become more aggressive in putting vascular grafts when only limited invasion is present the use of these 3D angiographic maps will become even more valuable (Figs. 13-14).

Mesenteric Vasculature and Bowel

When analyzing the detail of the display of the mesenteric artery and veins routinely demonstrated in patients with suspected pancreatic disease it becomes clear that there are other applications in the abdomen are ideal for this imaging technology. One such application is in the evaluation of the patient with suspected ischemic bowel. Although CT has been long used as a diagnostic study for the evaluation of ischemic bowel the classic CT findings are more often seen in patients with more advanced disease. CT signs of early ischemic may only be several dilated loops of bowel and be a nonspecific finding based on analysis of the CT study. The more classic CT signs including pneumatosis of the bowel wall, edema and/or thickening of the bowel wall, inflammed, pericolonic fat as well as the presence of a clot in the superior mesenteric artery or vein are seen in later stages of ischemia. Ideally, we would like to be able to detect ischemic bowel at an earlier state where intervention can have a greater impact on decreasing morbidity and mortality.

The introduction of MDCT with 3D rendering provides unique capabilities for this application (Fig. 15). The presence of SMA or SMV stenosis or occlusion, narrowing of proximal or distal mesenteric branch vessels or occlusion, as well as patterns of collateralization can be clearly defined (Fig. 16). The use of both volume rendering technique and MIP based 3D reconstruction can define the branching of the mesenteric vessels in similar or better detail when compared to classic angiography without the need for catheter placement. An additional advantage of CT angiography over conventional angiography is the ability to evaluate bowel enhancement. In cases of ischemia there are often changes in bowel enhancement, which may be demonstrated as focal decreased enhancement of the small bowel. These changes can be seen on the CT images (35). The presence of pneumatosis can be seen but ideally we would like to detect the presence of ischemic changes before it progresses to infarction.

Our experience with MDCT for CT Angiography shows the need for dual phase acquisitions. The arterial phase acquisition is best for the arterial mapping especially of smaller more distal branch vessels while the venous map is best for defining SMV patency as well as for patency of the portal vein, SMV and its tributaries. The arterial phase acquisition is best for the arterial mapping especially of smaller more distal branch vessels while the venous map is best for defining SMV patency as well as for patency of the portal vein, SMV and its tributaries. The later phase images are also best for detecting changes in bowel enhancement in ischemia.

Not surprisingly the ability to visualize small vessels typically requires narrow scan widths (1-1.25 mm) and close interscan spacing (1 mm). The data must be acquired in a successful single breathhold so it is not surprising that MDCT is essential. Although single detector CT can be used it has significant limitations for this application.

Another application that we believe may be developed with MDCT and CT angiography for bowel is a more functional examination of the bowel in Crohns disease (Fig. 17). Although CT has long been used for determining extent of involvement especially for extraluminal disease, the question of disease activity has always been a challenge.

If bowel is thickened it is simply a sign of disease but not determinate of activity. Can we potentially obtain additional information from CT with CT angiography regarding disease activity? Preliminary work suggests that this is indeed the case. We have found two important signs of active disease when doing CT angiography with dual phase imaging. The first is that the distant arterial branches to bowel are dilated and often serpidinous in appearance. The second is that the areas of active disease are increasingly enhanced on the early phase images, which are probably a result of hyperemia and increased blood flow. Prior reports of ultrasound and MRI have suggested increased blood flow in active disease. Larger studies with surgical and pathologic correlation will be needed to document the frequency of these important findings.

Hepatic Imaging: Clinical Applications

CT Angiography is becoming an important primary or secondary imaging modality in the evaluation of hepatic disease. As a primary imaging modality, CT is used as a replacement for conventional angiography in such applications as pre-operative planning for hepatic resection, preoperative evaluation and planning in potential liver transplant recipients as well as living related liver donors (both adult-child and adult-to-adult transplantation) as well as for the evaluation of portal vein patency in a range of potential cases including pre-TIPS placement (36-42). As a secondary imaging modality, CT can supplement axial information in patients with cirrhosis, upper gastrointestinal tract bleeding due to varices or primary extrahepatic neoplasms.

Regardless of its use in the liver, the study design and scanning protocol will remain as the most critical steps in a successful CT Angiographic study of the liver. In most applications, two phases of data acquisition is needed: an arterial phase (25-second delay) and a portal venous phase (60-second delay). In select cases, a third phase (either non-contrast or a 90-second delay post contrast) may be needed. In other cases such as evaluation of portal vein patency only a single phase of acquisition will be needed. Although CT of the liver for suspected, metastases or primary tumor is obtained with 5 mm slice thickness, cases with CT Angiography require the use of 1-1.25 mm slice thickness. The scan data is then reconstructed at 1 mm intervals, which usually results in an average of 200-230 slices per patient per acquisition. Although single detector scanners can be used the protocols with 1-1.25 mm slice width are typically only possible with a MDCT scanner. When a single detector scanner is used the slice collimation used is typically 3 mm with data reconstruction at 2 mm intervals.

One of the most common applications for CT angiography in the liver today is in the evaluation of potential liver transplant candidates (40)(Fig. 18). In the patient who has liver disease that may require a transplant, the study provides a comprehensive examination that answers a number of specific questions including:

  • what is the status of the liver parenchyma?
  • is there a hepatoma present?
  • is their extrahepatic disease present?
  • are there varices present and if so to what extent and where?
  • is the portal vein patent?
  • what is the origin and branching pattern of the hepatic arterial system?

Smith et al. (40) reviewed dual phase spiral CT scans with 3D volume rendering in 50 consecutive patients and found that the study provided a comprehensive preoperative liver transplant evaluation, supplying both the information necessary for patient selection as well as for surgical planning. In the series of 50 patients, ten patients (20%) had anomalous origin of the hepatic artery and six patients (12%) had cavernous transformation of the portal vein. Five patients had hepatic masses of which one was a hepatoma and four non-neoplastic tumors. Ngheim et al. (45) similarly reviewed a series of 80 patients preliver transplant using double helical CT and 3D CT angiography (DHCT/3D-CTA). The authors found that "DHCT/3D-CTA provides noninvasive means to identify findings that have significant impact on surgical planning for hepatic transplantation including celiac axis stenosis, diameter of inflow arterial vessel <= 3 mm, complete replacement of hepatic arterial supply, portal vein thrombosis, and splenic artery aneurysm." The use of MDCT has made the opportunity for a successful study more likely and the volume data sets provide superior anatomic detail.

The use of dual phase acquisition with 3-D reconstruction provides unique tools for data display, orientation and interaction. This is helpful in better defining vessels like the hepatic artery or vein depending on the clinical situation. Not surprisingly portal vein patency occlusion requires use of portal or later phase imaging as well.

With the severe shortage of available livers for transplantation, there has much interest in alternative solutions including living related donors. In the past, parent to child liver donation was successfully attempted and has been a viable alternative for children with hepatic failure. In these cases the adult typically donated the lateral segment of the left lobe. CT Angiography was done of the adult liver to define the vascular map and to obtain liver volumes. A scan of the child was also done to obtain the volume of the liver to be removed in order to make sure the donor liver would fit into the patient recipient. More recently the procedure has been attempted in adults with adult-to-adult-living donor liver evaluation. In these cases the portion of the ‘donor’s liver’ used is the right lobe and CT angiography appears to be an idea way to non-invasively evaluate these patients. Kamel et al. (43-44) reviewed 40 consecutive potential donors with multidetector CT and found that based on the CT findings 15 patients were excluded as donors (Fig. 19). MDCT in this series "provided comprehensive parenchymal, and volumetric preoperative evaluation of potential donors undergoing living adult right lobe liver transplantation." 3D mapping of arterial and venous anatomy with CT angiography are rapidly becoming the state of the art for this clinical application.

Once the liver has been transplanted CT Angiography can also be used for evaluation of potential transplant complications in either the transplant donor or recipient. Katyal et al. (39) was successful in detecting common and potentially lethal vascular complications including hepatic artery stenosis, hepatic artery thrombosis, and portal vein stenosis.

Liver Resection Planning for Hepatic Tumors

The use of subsecond single detector spiral CT, soon followed by MDCT with its ability to acquire well timed datasets of multiple scan acquisition sequences has increased our ability to detect and classify hepatic tumors. Whether the lesions be hypovascular or hypervascular, the use of thin collimation and multiphasic data acquisition has helped us to optimize both detection and calcification of liver disease (Figs. 20-22).

Once the presence and extent of disease is defined decisions as to resectability need to be made. The use of 3D CT angiography is a natural progression of the work done over the past decade in surgical planning for hepatic resection. Three-dimensional maps from arterial and portal venous datasets allow us to construct highly detailed an accurate vascular maps that are used as a guide for surgical planning. These vascular maps provide better detail for vascular invasion of the portal or hepatic veins by displaying the course of the vessel in optimal planes. Uchida et al. (42) compared maximum intensity projection technique with volume rendering technique and found that both techniques could provide valuable information. However, "the VRT was needed to sufficiently depict the relationship of the hepatic and intrahepatic portal veins to the segmental anatomic structure and to the tumor." VRT was therefore felt to be critical in surgical planning and to provide detailed anatomic displays of the normal ‘liver’ tumor and vascular map. This is similar to our experience where the volume rendered images are the primary display mode and the MIP is used to supplement this display. However, with today’s workstations with real-time rendering, it is literally a split second to go from one rendering technique to the other so there is little need for controversy and both techniques have select advantages.

Cirrhosis

The ability to evaluate the liver in multiple phases of enhancement is ideal for the evaluation of hepatic parenchymal disease. Although numerous articles have clearly show the role of SDCT or MDCT for looking at the extent of cirrhosis and its complication we have found that MDCT with 3D Angiography to have certain select advantages. The arterial phase as noted previously is ideal for determining the presence of hepatoma as well as to define variations in hepatic arterial anatomy. The portal venous phase is best for defining the patency of the portal vein as well as the SMV and in the patterns of collateral flow. Whether the collaterals are dilated coronary veins, gastroepiploic veins or splenorenal venous shunting 3D angiographic mapping is ideal for defining extent and location of these collaterals (Figs. 23-24). This information may be helpful in biopsy planning and in defining candidates who are transplant eligible.

Conclusion

The advances in single detection spiral CT, and most recent multidetector CT, has provided the radiologist with unique imaging capabilities that provide the opportunity to revolutionize how we image and evaluate patients across a wide range of clinical applications. The ability to provide a non-invasive exam while may cost 25-33% of the cost of a more invasive study with equal or greater ease of use is a very exciting development and provides unique opportunities across the enterprise.

The development of faster workstations, coupled with better and more useable user interfaces and investigative tools promises to help drive the entire CT arena. The introduction over the next 12-24 of newer multidetector scanners with 8-24 detectors and acquisition speeds in the 100-200 millisecond range coupled with routine slice collimation of .5 mm resulting in isotropic datasets with continue to drive the film. Changes in our workflow will be needed if we are to take full advantage of this new technologist. We look forward to these exciting challenges and opportunities to help meet our goals of improving patient diagnosis, care and management.

 

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