AJR 2000; 174:1417-1421
© American Roentgen Ray Society
Dual Phase Hepatic CT
Influence of Scanning Direction on Liver Attenuation
E. O'Riordan1,
C. M. Craven1,
D. Wilson2 and
P. J. Robinson1
1
Department of Clinical Radiology, St. James's University Hospital, Beckett
St., Leeds LS9 7TF, United Kingdom.
2
Department of Medical Physics, St. James's University Hospital, Leeds LS9 7TF,
United Kingdom.
Received August 11, 1999;
accepted after revision November 1, 1999.
Address correspondence to P. J. Robinson.
Abstract
OBJECTIVE. We measured changes in hepatic attenuation during
arterial and portal phase acquisition of hepatic CT in the craniocaudal and
caudocranial directions.
SUBJECTS AND METHODS. In 10 of 20 patients undergoing dual phase
helical CT during staging for colorectal cancer, images in both phases were
obtained in the craniocaudal direction. Ten patients underwent imaging in the
caudocranial direction. Attenuation values in the aorta and in the peripheral
and central liver regions of interest were measured on each slice. Central and
peripheral liver attenuation was also measured in 10 additional patients
undergoing unenhanced CT.
RESULTS. Both peripheral and central regions of interest revealed
progressively increasing attenuation during the arterial phase, irrespective
of scanning direction. During the portal phase, hepatic attenuation was stable
in the craniocaudal direction but decreased in the caudocranial direction
(p < 0.05, Wilcoxon's signed rank sum test). Central hepatic
attenuation was lower than peripheral attenuation in unenhanced livers and in
enhanced livers during both phases of caudocranial acquisition. We determined
no significant difference during the arterial phase of enhancement in the
craniocaudal direction.
CONCLUSION. The direction of acquisition does not influence
sequential liver enhancement during the arterial phase. Craniocaudal
acquisition produces more stable enhancement during the portal phase.
Differences in attenuation between the central and peripheral areas of the
liver are probably unrelated to contrast administration.
Introduction
Administration of contrast medium during hepatic CT improves the detection
of focal lesions [1,
2], and dual phase hepatic CT
increases sensitivity even more
[3,4,5].
The hepatic artery delivers 20-25% and the portal vein delivers 75-80% of
blood flow to the liver
[6,7,8,9],
but because most hepatic tumors have only hepatic arterial blood supply, they
are hypoattenuating relative to the normal liver during portal phase CT.
Imaging during the arterial dominant phase improves the detection of
hypervascular liver lesions [3,
10], which may become
isointense during the portal venous phase
[11]. During the hepatic
arterial dominant phase, radiologists should start acquiring hepatic images
during the brief time between the arrival of IV contrast material in the
hepatic artery branches and the later arrival of contrast material via the
portal venous system.
Early studies of dynamic contrast-enhanced CT revealed differences in
enhancement from the cephalad to the caudad aspect of the liver of up to 45 H
[1,
12,
13]. A more recent study using
helical single phase CT acquisition showed that the attenuation difference
between the upper and lowermost levels decreased to a mean of 2 H
[14], providing a more uniform
examination.
Using CT during arterial portography, Bluemke et al.
[15] found higher hepatic
attenuation values near the caudal margin of the liver than near the cephalic
portion (p < 0.01). Nine of their patients were scanned in a
cephalocaudad direction. The researchers suggest that this contrast gradient
could be related to the scanning direction or to the preferential delivery of
contrast material to the caudal part of the liver. From other nuclear medicine
and angiographic studies
[16,17,18,19,20],
it would appear likely that the true arterial phase lasts only 5-10 sec and
hepatic enhancement increases continuously during the initial phase of CT
acquisition, so scanning direction may be important. In the portal venous
phase, the hepatic veins are the last hepatic structures to be opacified after
a bolus of IV contrast material, so a caudocranial scanning direction may be
optimal for the parenchymal—portal venous phase. We examined the
variations in hepatic enhancement during dual phase CT in the craniocaudal
(top down) and caudocranial (bottom up) directions.
Subjects and Methods
Patients
Twenty patients underwent dual phase helical CT during initial staging for
colorectal carcinoma. To ensure that representative attenuation measurements
could be made on each slice, we excluded patients who had large or extensive
hepatic metastases. In 10 patients, arterial dominant and portal phase images
were obtained in a craniocaudal direction, and in 10 other patients images in
both phases were obtained in a caudocranial direction. Six men and four women
participated in the craniocaudal group (age range, 48-78 years; mean age, 65.2
years). Five men and five women participated in the caudocranial group (age
range, 38-71 years; mean age, 60.9 years).
Image Acquisition
All images were obtained on a Somatom Plus S CT scanner (Siemens, Erlangen,
Germany) (full rotation, 1 sec; beam collimation, 8 mm; table increment, 8
mm/sec; reconstruction interval, 8 mm; exposure factors, 210 mAs and 120 kVp).
One hundred and fifty milliliters of contrast medium (300 mg I/ml) (Omnipaque;
Nycomed, Oslo, Norway) was administered at 5 ml/sec (power injection) (Medrad;
Wolverson, Willenhall, United Kingdom) with a delay of 20-25 sec for the
arterial phase and 65-70 sec for the portal venous phase
(Table 1). Patients were
instructed to hold their breath during acquisition. Before spiral acquisition,
a topogram was obtained to define the upper and lower margins of the
liver.
Image Analysis
Circular regions of interest (area, 3.26 cm2) (Fig.
1A,1B)
were selected on each reconstructed slice in the arterial dominant and portal
venous phases. Then the regions were placed peripherally (in the 9-o'clock
position) and centrally avoiding the main hepatic veins and the inferior vena
cava. Additionally, a region of interest (area, 1.44 cm2) was
placed in the aorta on each slice in both phases. The change in attenuation of
sequential slices during acquisition was measured for each patient, and mean
values for all patients were obtained. The significance of changes in hepatic
attenuation and aortic attenuation during both phases of craniocaudal and
caudocranial acquisition were assessed using the Wilcoxon's signed rank sum
test.
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Fig. 1A. —61-year-old man with colorectal cancer. A and B,
Arterial phase (A) and portal venous phase (B) hepatic CT scans
show placement of peripheral hepatic (1), central hepatic
(2), and aortic (3) regions of interest for attenuation
measurement. Metastasis is present in segment 7.
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Fig. 1B. —61-year-old man with colorectal cancer. A and B,
Arterial phase (A) and portal venous phase (B) hepatic CT scans
show placement of peripheral hepatic (1), central hepatic
(2), and aortic (3) regions of interest for attenuation
measurement. Metastasis is present in segment 7.
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To compare central attenuation with peripheral attenuation, the mean value
of all slices in each patient was obtained for the 9-o'clock and central
positions. These values were then compared using the Student's t test
for paired data. Having found a significant difference between the mean
attenuation in the peripheral and central positions during the arterial and
portal venous phases for the caudocranial group and during the portal venous
phase in the craniocaudal group, we measured the attenuation of similar
peripheral and central regions of interest in the livers of 10 patients
undergoing unenhanced CT for lymphoma staging. Again, the mean value for each
patient's liver was obtained for the peripheral and central positions. Mean
peripheral and central values for all 10 patients were obtained, and the
Student's t test for paired data was used to analyze statistical
significance. The number of slices analyzed varied according to the size and
shape of each patient's liver. We included 6-14 slices (mean, 9.7 slices) in
the craniocaudal group, and 8-16 slices (mean, 12 slices) in the caudocranial
group. Because measurements were not obtained on slices that did not include a
section of liver large enough for placement of a 3.26-cm2 region of
interest, there was a delay between the start of acquisition and the timing of
the first slice on which measurements were made (slice 1). This delay averaged
4.7 sec (range, 3-6 sec) for craniocaudal and 9 sec (range, 7-12 sec) for
caudocranial acquisition. We used a longer delay for caudocranial acquisition
because of the greater difficulty in defining the lower extent of the
liver.
Results
Craniocaudal Acquisition
Both peripheral and central regions of the hepatic parenchyma showed a
significant increase in attenuation during the arterial phase in the
craniocaudal direction (p < 0.05)
(Fig. 2). The mean increase in
hepatic attenuation during the arterial phase was 16 H. We measured no
significant change in hepatic attenuation during the portal venous phase in
either central or peripheral regions of the liver
(Fig. 3). Aortic attenuation
during the arterial phase showed a significant decrease (p <
0.05), but we measured no significant change in aortic attenuation during the
portal venous phase.
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Fig. 2. —Graph shows mean attenuation of central ([UNK]) and peripheral
([UNK]) hepatic parenchyma during arterial phase acquisition in craniocaudal
direction. Error bars represent standard deviation.
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Fig. 3. —Graph shows mean attenuation of central ([UNK]) and peripheral
([UNK]) hepatic parenchyma during portal venous phase acquisition in
craniocaudal direction. Error bars represent standard deviation.
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Caudocranial Acquisition
When data was acquired in the caudocranial direction, peripheral and
central regions of the hepatic parenchyma showed a significant increase in
attenuation during the arterial phase (mean, 22 H)
(Fig. 4). We measured a
significant decrease (mean, -7 H) in hepatic attenuation during the portal
venous phase (Fig. 5). We
measured no significant change in aortic attenuation during the arterial phase
or the portal venous phase.
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Fig. 4. —Graph shows mean attenuation of central ([UNK]) and peripheral
([UNK]) hepatic parenchyma during arterial phase acquisition in caudocranial
direction. Error bars represent standard deviation.
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Fig. 5. —Graph shows mean attenuation of central ([UNK]) and peripheral
([UNK]) hepatic parenchyma during portal venous phase acquisition in
caudocranial direction. Error bars represent standard deviation.
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Central Versus Peripheral Attenuation
The mean attenuation in the central hepatic area was significantly lower
than that in the peripheral area during three of four acquisitions: the
arterial (p < 0.005) and portal venous phases (p <
0.005) with caudocranial acquisition and the portal venous phase with
craniocaudal acquisition (p < 0.001). There was no significant
difference in central versus peripheral hepatic attenuation during the
arterial phase in the craniocaudal direction
(Table 2).
In unenhanced livers, the means and standard deviations of the hepatic
attenuation measurements in the central and peripheral regions of interest
were 57 ± 6.3 H and 61 ± 6.9 H, respectively (p <
0.001).
Discussion
The optimization of dual phase hepatic CT requires the separation of the
arterial dominant phase from the portal venous phase, but the arterial phase
is difficult to define. Previous data from nuclear medicine and angiography
studies show that the time between the start of the arterial phase and the
start of the portal venous phase may be as short as 5-10 sec. During
angiography, portal venous opacification may begin within 5-6 sec of contrast
material injection in the celiac or splenic artery, and within 8-10 sec in the
superior mesenteric artery
[16,
17]. Studies of hepatic blood
flow using peripheral vein bolus injections of radionuclides also show that
the arterial component arrives about 7-8 sec before the portal venous
component
[18,19,20].
These data suggest that if the duration of acquisition during CT is 20 sec,
then the arterial dominant phase includes some portal inflow, explaining the
appearance of contrast material in the portal veins during the arterial
phase.
Investigations into the effects of various contrast medium concentrations,
volumes, and rates of injection report that increasing the rate of injection
from 2 ml/sec to 5-6 ml/sec results in earlier peak hepatic enhancement
[21] and increases the
magnitude of arterial enhancement
[22], possibly improving the
temporal separation of the arterial dominant and portal phases of enhancement
[22]. Peak hepatic enhancement
has been shown to occur 25-32 sec after contrast material injection,
irrespective of the injection rate (between 2 ml/sec and 5 ml/sec)
[23,
24]. Some researchers argue
that bolus tracking provides more reproducible timing of the arterial phase
[25] than the use of a fixed
interval after injection, as in our study. However, in all our patients, the
measured attenuation in the aorta remained stable during the arterial phase,
suggesting that acquisition did not start too early.
We used 150 ml of IV contrast material at 5 ml/sec for 30 sec and acquired
images during the portal venous phase starting at 65-70 sec, attempting to
encompass the peak of hepatic enhancement. Our acquisition of images during
the arterial dominant phase started at 20 sec and lasted an average of 20 sec.
Our timing was within the window suggested by Kopka et al.
[25], who proposed the onset
of the arterial phase at 15-30 sec, lasting 8-16 sec, and ending at 27-42 sec.
Frederick et al. [26] suggest
that the arterial phase is complete in 44 sec and that the detection of
hypervascular lesions could be compromised if acquisition lasts longer than 44
sec. In our study, arterial phase acquisition in the craniocaudal direction
was complete at a mean time of 40 sec. Arterial acquisition in the
caudocranial direction lasted an additional 4 sec (mean) to arrive at the
position of slice 1, resulting in an arterial phase of 44 sec in some
patients. This may explain the slight fall in hepatic attenuation during the
portal phase with caudocranial acquisition. The number of slices measured in
the craniocaudal group (range, 6-12 slices; mean, 9.6 slices) was less than
that measured in the caudocranial group (range, 8-16 slices; mean, 12 slices),
so the duration of acquisition of slices in which measurements were made was
greater in the caudocranial patients than in the craniocaudal patients.
However, this difference is small compared with the duration of the
continually rising part of the time-enhancement curve. The gradual increase in
attenuation during the arterial phase was the same in both groups, so it is
unlikely that a small difference in acquisition time influenced the
results.
On average, central hepatic attenuation was 5 H lower than peripheral
attenuation on unenhanced acquisitions. This difference was maintained during
both phases of contrast enhancement, suggesting no difference in perfusion
between the central and peripheral zones of the liver. This may be caused by a
mild beam hardening effect.
Multiarray detector CT scanners may provide equivalent image quality at 2-3
times the volume coverage speed of single slice helical CT
[27]; with axial acquisition
speeds of 8 cm/sec, the entire hepatic volume can be scanned in 3-4 sec. The
increased speed of multiarray CT allows more uniform acquisition during the
same phase of contrast enhancement and should eliminate the change in hepatic
attenuation during scan acquisition, which is seen with single slice helical
CT.
In conclusion, the direction of acquisition does not influence sequential
increase in hepatic enhancement during the arterial phase of dual phase
helical CT. Craniocaudal acquisition produces a more stable enhancement during
the portal venous phase. The difference in attenuation between peripheral and
central areas of the liver is unrelated to contrast enhancement.
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Abstract
Introduction
