DOI:10.2214/AJR.07.2697
AJR 2008; 191:550-554
© American Roentgen Ray Society
Portal Venous System: Evaluation with Unenhanced MR Angiography with a Single-Breath-Hold ECG-Synchronized 3D Half-Fourier Fast Spin-Echo Sequence
Katsuyoshi Ito1,2,
Shinji Koike1,
Ayame Shimizu1,
Masahiro Tanabe1,
Chisaki Jo3,
Mitsue Miyazaki3 and
Naofumi Matsunaga1
1 Department of Radiology, Yamaguchi University School of Medicine, Yamaguchi,
Japan.
2 Present address: Department of Diagnostic Radiology, Kawasaki Medical School,
577 Matsushima, Kurashiki, Okayama 701-0192, Japan.
3 Toshiba Medical Engineering Center, Tochigi, Japan.
Received June 7, 2007;
accepted after revision February 23, 2008.
Address correspondence to K. Ito.
Abstract
OBJECTIVE. Eighteen healthy persons underwent unenhanced MR
angiography with a breath-hold ECG-synchronized 3D half-Fourier fast spin-echo
technique to evaluate the visibility of the portal vein and its branches.
CONCLUSION. Our results indicated that unenhanced MR angiography
with a singlebreath-hold ECG-synchronized 3D half-Fourier fast spin-echo
sequence facilitates precise visualization of the anatomic features of the
portal vein and its branches without the use of contrast agents.
Keywords: MR angiography • portal vein • portography • venography
Introduction
Agadolinium-enhanced breath-hold 3D MR angiographic technique has been used
successfully to study abdominal vessels and has increased the quality of MR
angiography of the portal venous system
[1–7].
We have found, however, that this technique has limitations. Overlapping of
arteries on the portal vein and its branches occurs in almost all patients.
Although arteries can be eliminated by subtraction of images acquired in the
arterial phase from images acquired in the portal phase, image quality is
often degraded by respiratory misregistration
[4,
5]. Another limitation is that
contrast between the portal vein and background tissue becomes relatively poor
because of increased enhancement of the liver, pancreas, and intestines. In
addition, imaging with injection of a contrast agent cannot be repeated when
acquisition timing or slab coverage is inappropriate.
Improvements in gradient technology and software design have led to
development of an unenhanced breath-hold MR angiographic technique consisting
of coronal in-plane 3D half-Fourier fast spin-echo synchronization with ECG
gating at every slice encoding
[8,
9]. Without use of a contrast
agent, imaging with this technique depicts slow blood flow, such as portal
venous flow, as high signal intensity. The technique therefore has the
potential for use in MR angiography of the portal venous system to improve
visualization of the portal vein and its branches, including the
peripancreatic veins, without contrast administration. To the best of our
knowledge, there have been few reports of evaluations of the visibility of the
portal venous system with 3D unenhanced MR angiography. The purposes of this
study were to use a breathhold ECG-synchronized 3D half-Fourier fast spin-echo
technique to evaluate the visibility of the portal vein and its branches in
healthy subjects and to determine the feasibility of this technique for MR
angiography of the portal venous system.
Subjects and Methods
Study Population
The study was approved by the institutional review board, and written
informed consent was obtained from all participants. The study population was
18 healthy persons (13 men, five women; age range, 21–43 years) who did
not have a history of hepatic or pancreatobiliary disease. The subjects fasted
for at least 5 hours before the MRI examination.
MRI Technique
All subjects underwent imaging with a 1.5-T clinical MRI system (Visart/EX,
Toshiba) equipped with a body flex array coil. Before MR angiography, an ECG
preparation acquisition with a 2D half-Fourier fast spin-echo sequence was
performed to determine the appropriate ECG triggering delay time to depict the
abdominal aorta in reduced signal intensity, or black blood
[8]. This technique was used to
avoid overlapping of arteries on the portal vein and its branches. The ECG
preparation acquisition was a single slice in multiple cardiac phases
(100-millisecond intervals starting with zero delay from the R wave) in the
coronal plane. Images were acquired in phases of 0, 100, 200, 300, and 400
milliseconds and so on. ECG preparation acquisition was performed with the
following parameters; TR/TEeff, two R-R intervals/60; echo train
spacing, 5.0 milliseconds; matrix size, 128 x 256; slice thickness, 40
mm; field of view, 38 x 38 cm; total acquisition time, approximately 20
seconds during a single breath-hold.
After ECG preparation acquisition, the appropriate delay time was selected
and applied to MR angiography of the portal venous system with a breath-hold
ECG-synchronized 3D half-Fourier fast spin-echo sequence to trigger every
slice encoding. The imaging parameters for this sequence were as follows:
TR/TEeff, 2 or 3 R-R intervals (1,500)/80 or longer; echo train
spacing, 5 milliseconds; matrix size, 256 x 256; field of view, 35
x 35 cm; slice thickness, 3.5 mm; 16 slice sections with
single-breath-hold technique. A fatsaturation pulse was applied to suppress
signal intensity from fatty tissue. Interpolation in the slice direction was
performed to improve the apparent resolution, and the 3D data were processed
with maximum-intensity-projection processing. Images were obtained in two
oblique coronal planes. The first plane included the splenic vein and the
bifurcation of the portal vein (left anterior oblique view) to show the
branches of the superior mesenteric vein and the splenic vein. The second
plane included the left portal vein and the right portal vein (right anterior
oblique view) to show the intrahepatic portal branches.
Image Analysis
The MR images were interpreted on a clinical workstation (Image VINS,
version 1.06, Yokogawa Electric) by two radiologists experienced in abdominal
MRI. When there was a discrepancy in the readings of the two radiologists,
they reached consensus by reviewing the images together. All evaluations were
categorized and documented on standardized data sheets. The source images and
3D reconstructed images with maximum intensity projection were reviewed. Image
analyses for visibility of the portal venous system were performed for the
following vessels: main portal vein, right portal vein, right anterior portal
branches, right anterior superior portal branches (portal vein branch P8),
right anterior inferior portal branches (P5), right posterior portal branches,
right posterior superior portal branches (P7), right posterior inferior portal
branches (P6), left portal vein, left anterior portal branches (P3), left
posterior portal branches (P2), left medial portal branches (P4), splenic
vein, left gastric vein, superior mesenteric vein, inferior mesenteric vein,
posterior superior pancreaticoduodenal vein, gastrocolic trunk, right
gastroepiploic vein, right colic vein, anterior superior pancreaticoduodenal
vein, right gastric vein, and first jejunal vein.
The visibility of each vessel was rated and recorded on a 4-point scale (3,
excellent; 2, good; 1, poor; 0, not visible). The ratings were defined as
follows: 3, excellent, if the vessel was clearly seen as a
high-signal-intensity structure; 2, good, if the vessel was seen with moderate
visibility between excellent and poor; 1, poor, if the vessel was visible but
the image of the vessel was blurred; 0, if the vessel was not seen.
Satisfactory visualization was assessed by averaging the visibility scores of
each vessel [10]. Venous
anatomy as described in the literature
[11–14]
was used as the reference for evaluation of images obtained with MR
angiography.
Results
The frequency of MRI visualization of the portal vein and its tributaries
is shown in Table 1.
Approximately one half of the vessels were identified in all subjects on
maximum-intensity-projection (Figs.
1A and
1B) or source (Figs.
1C,
1D,
1E, and
1F) images or both.
Table 2 lists the averaged
score of visibility of each vessel. Satisfactory visualization (averaged
scores of 2 or higher) was achieved for 16 of the vessels. The relatively low
averaged visibility scores of the right posterior portal branches (2.4), right
posterior superior portal branches (2.2), and right posterior inferior portal
branches (2.1) were associated with overlapping of the gallbladder and
incomplete coverage of the right posterior segment. Although they were visible
on the source images, the inferior mesenteric vein and left gastric vein were
sometimes obscured by the high signal intensity of overlapping stomach or
small intestine.
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Fig. 1A —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Left anterior oblique
maximum-intensity-projection MR portogram shows superior mesenteric vein and
splenic vein joining to form portal vein.
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Fig. 1B —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Right anterior oblique
maximum-intensity-projection MR portogram shows portal vein dividing into left
and right portal veins. Intrahepatic portal branches are evident.
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Fig. 1C —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Right (C and D) and left
(E and F) anterior oblique source MR images show each branch
more clearly than do A and B.
|
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Fig. 1D —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Right (C and D) and left
(E and F) anterior oblique source MR images show each branch
more clearly than do A and B.
|
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View larger version (151K):
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Fig. 1E —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Right (C and D) and left
(E and F) anterior oblique source MR images show each branch
more clearly than do A and B.
|
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View larger version (153K):
[in this window]
[in a new window]
[as a PowerPoint slide]
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Fig. 1F —36-year-old man with normal portal venous system. Unenhanced
MR angiograms obtained with single-breath-hold 3D half-Fourier fast spin-echo
sequence show excellent detail of anatomic features. GB = gallbladder, CBD =
common bile duct, MPV = main portal vein, SV = splenic vein, GT = gastrocolic
trunk, LPV = left portal vein, RPV = right portal vein, LGV = left gastric
vein, IMV = inferior mesenteric vein. Right (C and D) and left
(E and F) anterior oblique source MR images show each branch
more clearly than do A and B.
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Discussion
The results of our study indicated that the portal vein and its branches
can be well delineated with unenhanced MR angiography performed with a
single-breath-hold coronal in-plane ECG-synchronized 3D half-Fourier fast
spin-echo sequence. Although contrast-enhanced breath-hold MR portography has
been found effective for imaging the portal venous system
[1–7],
the technique has the following shortcomings. First, contrast-enhanced MR
portography suffers from overlay of arteries that reduces image quality. Even
though subtraction techniques are used, artifacts caused by respiratory
misregistration degrade the quality of subtracted images. Second, enhancement
of background tissues, such as pancreas, kidney, and intestine disturbs
visualization of the portal vein and its tributaries. Third, bolus-timing
difficulties can be encountered in patients with irregular cardiac rhythms or
poor cardiac output.
Our unenhanced MR angiographic technique has advantages over conventional
techniques, including contrast-enhanced MR angiography. It does not require
injection of contrast material into the portal circulation, therefore the data
acquired are expected to depict more accurately the portal vein and its
tributaries under physiologic conditions. In addition, because MR angiography
can be performed during a single breath-hold without a contrast agent, images
can be repeatedly obtained in various planes according to the area of
interest. Another advantage is that arteries overlapping the portal vein and
its tributaries can be avoided by use of ECG preparation acquisition. In
addition, extrahepatic biliary tracts, which commonly have a signal intensity
higher than that of the portal veins, can be depicted with our technique,
facilitating visualization of the relations between the portal venous system
and the bile ducts. Bile ducts can have high signal intensity, similar to that
of the portal veins. Bile ducts, however, usually are differentiated from
portal vessels on 3D images (stereo view) on the basis of the course of the
ducts parallel to the course of the portal vessels. One issue with this
technique is that gastrointestinal fluid with a long T2 value showing high
signal intensity can interfere with visualization of the portal venous
branches. Therefore, fasting before examinations is indispensable.
Knowledge of portal venous anatomy is crucial before liver transplantation,
hepatic tumor resection, and shunt construction for gastrointestinal varices
[15–18].
Our results showed that our unenhanced 3D MR angiographic technique may have
the potential to depict various collateral pathways in patients with cirrhosis
and portal hypertension, contributing to decisions about therapeutic options.
Results of previous studies
[19–23]
have indicated that analysis of the peripancreatic veins is valuable for
precise CT staging of pancreatic carcinoma. The results of our study indicate
that visualization of peripancreatic veins, such as the superior mesenteric
vein, splenic vein, gastrocolic trunk, inferior mesenteric vein, and right
gastroepiploic vein, is sufficient for evaluation of these vessels in MR
staging of pancreatic carcinoma. Therefore, unenhanced MR angiography with the
breath-hold ECG-synchronized 3D half-Fourier fast spin-echo technique may have
potential for early detection of vascular invasion by tumors. Further
prospective study is necessary to determine whether this technique can improve
the accuracy of MRI in the staging of pancreatic carcinoma.
Our study was limited in that conventional angiography was not performed as
an anatomic reference standard. Because there is no clinical indication for
conventional angiography of persons acting as controls, that procedure was not
justified. In addition, precise identification of small branches of the portal
venous system can be difficult with conventional angiography. Another
limitation was that only healthy persons were subjects and no patients were
included. The quality of this technique may be degraded in patients with
ascites or biliary ductal dilatation or in evaluation of small collateral
vessels.
A final limitation of our study was that we did not perform a double-blind
comparison between our technique and contrast-enhanced MR portography. The aim
of this study, however, was to evaluate the feasibility of unenhanced MR
angiography with a single-breath-hold 3D half-Fourier fast spinecho sequence
to depict the portal venous system in the normal state. Imaging of a group of
healthy subjects was expected to answer our research question. In future
studies, it is necessary to evaluate the optimized unenhanced MR angiographic
sequence in patients with portal hypertension and liver disease in comparison
with contrast-enhanced MR portography. The limitations do not reduce the
validity of the basic conclusions of this study.
An unenhanced MR angiographic technique with a single-breath-hold
ECG-synchronized 3D half-Fourier fast spin-echo sequence can facilitate
precise visualization of the anatomic features of the portal vein and its
branches under physiologic conditions.
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Abstract
Introduction
