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
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
. 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:
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:
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.
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.
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. 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). 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. 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.
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.
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).
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".
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. 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.
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.
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. 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).
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.
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
. 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. 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. 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.
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). 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:
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. 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.
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.
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.
This information may be helpful in biopsy planning and in defining candidates
who are transplant eligible.
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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