Yonago Acta medica 1996;39:119–125

Comparison of Visualization of the Intrahepatic Portal Venous System
by Two-Dimensional Time-of-Flight MR Angiography among Different
Imaging Planes
Yuji Suto, Ken Yamamoto, Hiroyuki Fujiwara, Shuji Sugihara, Kotaro
Yoshida and Yoshio Ohta
Department of Radiology, Faculty of Medicine, Tottori University, Yonago 683, JapanThe differences in visualization of the intrahepatic portal system among different
imaging planes obtained by two-dimensional (2D)-time-of-flight (TOF) magnetic
resonance angiography (MRA) was examined comparatively. Ten healthy volunteers
were subjected to 2D-TOF-MRA of the entire liver from three directions — sagittal,transverse and coronal — during breath-holding using 2D-fast low angle shot sequences.
Using the obtained consecutive image data, MRA was reconstructed by the maximum
intensity projection algorithm. MRA was observed in a cine mode display, andvisualization of each portal branch was classified into 3 groups: A, B and C (excellent,fair and poor, respectively). The visualization rate from the portal vein trunk to thesegmental branches in the right lobe excluding the right anterior superior branch was
highest on the sagittal section. The visualization rate of each branch in the left lobe was
low as a whole on images taken from the 3 directions; a particularly low rate was foundon the transverse images so all of these cases were classified into Group C. In conclusion,for visualizing from the portal vein trunk to the branches in the right hepatic lobe as awhole, the sagittal section was good, but visualization of branches in the left lobe waspoor in all sections. Therefore, visualization of the entire intrahepatic portal system by
the ordinary 2D-TOF technique appears to have limits.
Key words: liver; MR angiography; MR imaging; portal vein

system has never been examined by research,
the optimum section plane is not yet known. In
this study, the differences in visualization of theintrahepatic portal system among imaging
sections by the 2D-TOF technique was examined comparatively.

For MR imaging of the portal venous system,
the two-dimensional (2D)-time-of-flight (TOF)
technique has currently been used (Gullberg etal., 1987; Edelman et al., 1989a, 1989b). Although this technique is used in the evaluationof the extrahepatic portal system such as theesophageal varix (Finn et al., 1991), it has notbeen used for evaluation of the intrahepatic
portal system because of its low spacial resolution. Ordinarily, a magnetic resonance angiogram (MRA) is reconstructed from the image
data of the transverse section (Gullberg et al.,
1987; Edelman et al., 1989a, 1989b). However,since the difference in visualization among theimaging sections of the intrahepatic portal

Subjects and Methods
The subjects were 10 healthy adult volunteers
aged 30-40 years (mean: 36 years). Prior to thestart of each study, informed consent was
obtained from each subject.

Abbreviations: 2D, two-dimensional; FA, flip angle; FLASH, fast low angle shot; FOV, field of view; Gd-DTPA,
gadopentetate dimeglumine; GMN, gradient motion nulling; MIP, maximum intensity projection algorithm; MRA,
magnetic resonance angiography; TE, echo time; TOF, time-of-flight; TR, repetition time.

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A
TR

B

RB

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0

TRA COR SAG

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TRA COR SAG

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C

RPB

RAB

TRA COR SAG

0

TRA COR SAG

Fig. 1. Visualization rate of the portal vein trunk (TR), right branch (RB), right anterior branch (RAB) andright posterior branch (RPB). Group A (excellent): the entire length of blood vessel from the branch iscontinuously identifiable. Group B (fair): the blood vessel is identifiable but not continuously. Group C(poor): the blood vessel cannot be identified. COR, coronal plane; SAG, sagittal plane: TRA, transaxial
plane.

MRA
A Magnetom H15 (Siemens, Erlangen, Germany)
was used and a body coil was used as an imaging coil. For imaging sequences, a 2D-fast low
angle shot (FLASH) (Haase et al., 1986) which
had a gradient motion nulling (GMN) function(Haacke and Lenz, 1987; Laub and Kaiser,
1988) for the slice selection gradient and readout gradient was used. Multiple imagings of theentire liver were performed on the sagittal,transverse and coronal sections during breathholding. The MRA was reconstructed fromobtained image data by the maximum-intensity
projection (MIP) algorithm (Laub, 1990). A
series of projection images were generated withviewing angles of -45˚ to +45˚ from the coronalplane at fixed 5˚ intervals. The parameters ofimaging were as follows: repetition time (TR),20 ms; echo time (TE), 8 ms; flip angle (FA),40˚; field of view (FOV), 350 × 350 mm; matrix,
128 × 256; slice thickness, 4 mm; overlap, 2 mm;
signal averaging, 1 (19 s, 5 multi-slice images).

Analysis of MRA
The blood vessels evaluated for this analysis
were the portal vein trunk, right branch, left

branch, right anterior branch, right posterior
branch and each segmental branch (P2–P8).
The MRA was observed in a cine mode display
with the axis of head to tail rotation. Evaluation
was based on the consensus of 3 trained radiologists. The visualization of each portal segment
was classified into the following 3 groups:
Group A (excellent), the entire length of bloodvessel from the branch is continuously identifiable; Group B (fair), the blood vessel is identifiable but not continuously; and Group C(poor), the blood vessel cannot be identified.

Results
The percentage of each group is shown in Figs.
1–3, and the visualization rate is schematically
depicted in Fig. 4.
In the vessels from the portal vein trunk to
the right secondary branches, 40 to 70% of thesubjects were classified as Group A on theimage of the sagittal section. The percentage ofGroup A from the portal vein trunk to the rightsecondary branches was highest on the image ofthe sagittal section (Fig. 1). In the right segmental branches (P5–P7) excluding the rightanterior superior branch (P8), 30 to 40% of the120

2D-TOF MR angiography

A
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TRA COR SAG

Fig. 2. Visualization rate of the portal branches in the right segment (P5—P8). Group A (excellent): theentire length of blood vessel from the branch is continuously identifiable. Group B (fair): the blood vessel is
identifiable but not continuously. Group C (poor): the blood vessel cannot be identified. COR, coronal plane;
SAG, sagittal plane: TRA, transaxial plane.

subjects were classified as Group A on theimage of the sagittal section. The percentage
from Group A of these branches was also highest on the image of the sagittal section. In P8,
only 30% of the subjects were classified asGroup A + Group B on the image of the sagittal
section. However, 60% of the subjects wereclassified as Group A + Group B on the imageof the transverse section (Fig. 2). The visualization rate of the left branch was low on images of
all 3 directions. The rate was particularly low
on images of the transverse section and every
case was classified into Group C. The rate of
the left regional branches (P2–P4) was also low
as a whole. A particularly low rate was observed on images of the transverse and sagittal secA

LB

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Fig. 3. Visualization rate of the portal left branch (LB) and branches in the left segment (P2–P4). Group A(excellent): the entire length of blood vessel from the branch is continuously identifiable. Group B (fair): theblood vessel is identifiable but not continuously. Group C (poor): the blood vessel cannot be identified.
COR, coronal plane; SAG, sagittal plane; TRA, transaxial plane.

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A + B > 50%–

50% > A + B –> 30%

P8

A + B < 30%
P7

P4

RAB
RB

P2 + P3

LB

P5
RPB
TR
P6

Fig. 4A. Map indicating the degree of visualization rate of eachportal branch (coronal).
A + B > 50%–

50% > A + B > 30%–

P8
A + B < 30%
P7

P4

RAB
RB

P2 + P3

LB

P5
RPB
TR
P6

Fig. 4B. Map indicating the degree of visualization rate of eachportal branch (transaxial).

A + B > 50%–

50% > A + B –
> 30%

P8

A + B < 30%
P7

P4

RAB
RB

P2 + P3

LB

P5
RPB
TR
P6

Fig. 4C. Map indicating the degree of visualization rate of eachportal branch (sagittal).

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tions and every case was classified
into Group C (Fig. 3).
In summary, the sagittal section plane was best for visualizing
from the portal vein trunk to theregional branches in the right lobe
other than P8. However, the visualization rate of each branch in theleft lobe was low as a whole on
images of all 3 directions (Figs. 4
and 5).

Discussion
MRA of the abdominal region is
performed mainly by the 2D-TOF
technique (Gullberg et al., 1987;
Edelman et al., 1989a, 1989b; Finn
et al., 1991). This technique employs multiple repetitions of the
2D flow compensated gradientecho method by which imaging is
performed in relatively short TR
during breath-holding. Inflow
enhancement produces a brighter
signal for blood relative to therepeatedly excited stationary tissues which have a low signal(Edelman, 1993). When the flowcompensation technique is applied,the phase shift that would normally
result in subsequent dephasing andsignal loss are minimized (Haacke
and Lenz, 1987; Laub and Kaiser,
1988; Suto et al., 1994a). Signalsof blood vessels on 2D-TOF depend on the flow rate and geometrical distribution of blood vessels(Marchal et al., 1990). Therefore,
signal intensity is affected by the
direction of blood vessels to theplane of the imaging section.
When a blood vessel is vertical to
the section plane of imaging, theinflow enhancement is the largest.
When a blood vessel is running
diagonally to the slice or present in
the slice plane, the flowing spin

2D-TOF MR angiography

receives multiple radiofrequency pulses andtends to be saturated (Lewin, 1992; Edelman,
1993). Consequently, signal intensity inside
the blood vessel decreases and visualization of
the blood vessel becomes incomplete.
In order to depict blood vessels clearly, it isfundamental to set the imaging section plane sothat more components of vertical blood flow
and less components of parallel blood flow to

the slice section are included. The present
study shows that the visualization rate from theportal vein trunk to the right segmental branches
other than P8 was highest on the image of thesagittal section. The reason for this appears tobe that blood vessels running vertically to theslice section were more in the saggital section
than in other sections. Since P8 runs diagonallyforward and upward in many cases, the visual-Fig. 5. Case 1. A: Magnetic resonance angiography (MRA) reconstructed from the coronal
section image. B: MRA reconstructed from thetransverse section image. C: MRA reconstructedfrom the sagittal section image. On MRAs reconstructed from images of the transverse (A) andcoronal (B) sections, discontinuation (arrows) of
the portal branch in the right hepatic lobe is observed, while on the MRA from the sagittal section image (C), a better depiction of the entire
branche system in the right hepatic lobe is found.
Visualization of the branches in the left hepatic
lobe is poor on imaging in all of the section
planes.

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ization rate seems to be higher on images of thetransverse section, as demonstrated in thepresent study. Although the sagittal section isbetter relatively, even in the portal trunk region
where the diameter is large, the rate of the sumof Groups A and B did not reach 100%, indicating a low visualization rate of blood vessels. As
for the cause of low visualization of the entire
intrahepatic portal venous system by the 2DTOF technique, factors other than the imaging
section plane should be considered. The biggest problem in the application of the 2D-TOF
technique to the abdominal region is the decrease in spacial resolution due to a stair-shapechange of the blood vessel margin or discontinuation of blood vessels on an MRA reconstructed by the MIP method caused by phase
discrepancy among images attributable tofrequent breath-holdings (Bosmans et al., 1992;
Suto et al., 1994a). Therefore, it is necessary to
sufficiently explain to the subjects that breathholding should be done consistently, with thesame amount and speed of air intake for eachbreath interval. However, even the problem ofbreathing inconsistency cannot be completely
at fault since there is another explanation forphase discrepancy: the voxel size in the 2D
method is larger than that in the three-dimensional
one (Bosmans et al., 1992; Lewin, 1992; Suto etal., 1994a), and a decrease in signal intensity
due to phase dispersion caused by flow tends tooccur, and eventually, cases of false positive inthe diagnosis of intrahepatic portal obstruction
or stenosis will increase (Suto et al., 1994a).
In the present study, the visualization rate
was low despite the fact that the diameter of theportal left branch was relatively thick. As forthe reason, saturation due to increased bloodflow component parallel to the slice section
plane can be considered. Phase dispersion also
tends to occur when the left branch runs in a
curve. For identification of peripheral bloodvessels, identification of the root blood vessels
is a prerequisite. The low visualization rate ofbranches in the left region in the present study
can be attributed to the low visualization rate of
the portal left branch in the root. Also, draining
blood vessels which are saturated upstream ofthe portal left branch and a component with low

blood flow signal intensity are other factors
which reduce the visualization rate of bloodvessels.
We think that the selection of an optimum
imaging section plane is most successful when
the researcher can find a section plane on which
continuous depiction from the upstream thickblood vessels to the periphery is possible. For
that purpose, the sagittal section plane is goodfor visualizing from the portal trunk to the regional branches in the right lobe. However, even inhealthy volunteers in this study, the visualization rate from the left branch to its peripheral
branches was low. Hence, in patients withdecreased hepatic circulation such as cirrhosisof the liver, the visualization rate of the portal
veins by the 2D-TOF technique will be lower.
As for measures against this shortcoming,
we are now performing three-dimensional
imaging by bolus intravenous injection ofgadopentetate dimeglumine (Gd-DTPA)(Weinmann et al., 1984; Niendorf et al., 1991)
during a single breath-hold instead of using the
2D-TOF technique, and reconstructing an MRA
from the image data. This technique improved
visualization of the intrahepatic portal vein withina short time. Blood saturation is suppressed by
the T1 shortening effect of the contrast agent. A
decrease in the spacial resolution due to phasedispersion in respiration which occurred during
the 2D-TOF technique can be avoided becauseof the three-dimensional data acquisition. In
addition, the reduced number of breath-holds
significantly increased patient comfort andtolerance of the MR examination. (Suto et al.,1994b, 1996).
In conclusion, the sagittal section plane isexcellent for securing good visualization fromthe portal vein trunk to branches in the righthepatic lobe, but visualization of the left lobe
branches is poor in the imaging of all the section planes. Therefore, the conventional 2DTOF technique appears to have its limits forvisualization of the entire intrahepatic portal
venous system.

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2D-TOF MR angiography

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(Received August 19, Accepted September 9, 1996)

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