AJR 2000; 175:1459-1461
© American Roentgen Ray Society
Respiratory Variation of the Diameter of the Pancreatic Duct on Sonography
Ronald H. Wachsberg1
1
Department of Radiology, University Hospital, UMDNJ-New Jersey Medical School,
150 Bergen St., Rm. C-320, Newark, NJ 07103-2406.
Received February 24, 2000;
accepted after revision April 14, 2000.
Address correspondence to R. H. Wachsberg.
Abstract
OBJECTIVE. The objective of this study was to assess the prevalence
of a 1-mm or greater increase in the diameter of the pancreatic duct during
deep inspiration in patients without pancreatic disease.
MATERIALS AND METHODS. A retrospective review was performed of
normal findings on pancreatic sonograms of 25 consecutive lean patients
without pancreatic disease who were capable of taking deep breaths. The
anteroposterior diameter of the pancreatic duct in the body of the gland was
measured at end-expiration and end-inspiration. A significant change was
defined as a 1-mm or greater difference between the end-inspiratory and
end-expiratory diameters for at least two of three consecutive breaths.
RESULTS. Seven patients (28%) had a significant increase in the
diameter of the pancreatic duct at end-inspiration. These included four
patients (16%) in whom the diameter of the duct was less than or equal to 2.5
mm (i.e., normal) at end-inspiration and three patients (12%) in whom the
diameter of the duct was greater than 2.5 mm at end-inspiration.
CONCLUSION. The diameter of the pancreatic duct can increase during
deep inspiration in some adults without pancreatic disease. This finding
should be borne in mind as a potential pitfall during pancreatic
sonography.
Introduction
The pancreatic duct is important to examine in pancreatic imaging because
dilatation of the duct can be the most salient finding or even the sole
abnormal finding detected in some patients with pancreatic disease
[1]. On sonography, the most
frequently imaged and reproducibly measured segment of the duct is in the body
of the gland, in which a luminal diameter of 2.5 mm is the upper limit of
normal [1]. A 1-mm or greater
increase in diameter of the duct that persists for longer than 15 min after
secretion administration also constitutes evidence of ductal obstruction and
is a finding of particular importance in patients with dysfunction of Oddi's
sphincter and a normal diameter of the duct
[2,
3].
Because of anecdotal observations that the diameter of the pancreatic duct
sometimes increases during inspiration, a study was performed to evaluate the
prevalence of the phenomenon in patients without evidence of pancreatic
disease.
Materials and Methods
A retrospective review was performed of measurements of the diameter of the
pancreatic duct obtained in 25 consecutive lean patients who were referred for
normal abdominal sonographic findings and who had met the following criteria:
the pancreas was easily visualized without the need to apply pressure via the
transducer, the patient was subjectively judged as able to take deep breaths,
and a normal serum amylase level was documented within 2 weeks of the
sonographic examination.
Each patient was examined after having fasted for at least 4 hr. The body
of the pancreas was identified via an epigastric window with the scan plane
parallel to the long axis of the gland. The patient was instructed to
completely exhale and to suspend respiration, and a long axis image was
obtained of the pancreatic duct in the body of the gland. The patient was then
instructed to take a deep breath and to suspend respiration, and another long
axis image of the pancreatic duct was obtained. This procedure was repeated
twice for a total of three breaths. On each image, the greatest
anteroposterior diameter of the lumen of the pancreatic duct (i.e.,
inner-to-inner diameter) in the body of the gland was measured with electronic
calipers.
End-expiratory and end-inspiratory measurements of the diameter of the
pancreatic duct were compared for each of the three breaths. A difference of 1
mm or greater between end-inspiratory and end-expiratory measurements was
defined as significant. If a significant change was elicited by all three
breaths, the end-inspiratory and end-expiratory measurements were averaged. If
a significant change was noted after two of the three breaths, it was assumed
that the patient had not inhaled deeply for the breath that did not provoke a
significant change, and data from that breath were discarded, whereas
measurements from the two breaths that had provoked a significant change were
averaged. If a significant change was elicited by only one of the three
breaths, this was considered to be a nonreproducible chance event.
If a significant change in the diameter of the pancreatic duct was observed
between end-expiration and end-inspiration, the transducer was rotated 90°
to perform short axis sonography of the duct. The transducer was moved back
and forth parallel to the long axis of the pancreas to confirm that true short
axis images were being obtained, and a short axis sonogram was obtained where
the anteroposterior diameter of the duct was greatest. On this image, the
anteroposterior and cephalocaudad diameters of the pancreatic duct were
measured and compared. The short axis configuration of the pancreatic duct was
judged as circular if the difference between the anteroposterior and
cephalocaudad diameters was less than 1 mm and as oval if the difference
between the anteroposterior and cephalocaudad diameters was 1 mm or
greater.
Sonography was performed with 128 XP-10 (Acuson, Mountain View, CA), 700MR
(General Electric Medical Systems, Milwaukee, WI) or HDI (Advanced Technology
Laboratories, Bothell, WA) scanners. Pancreatic sonograms were obtained with
broadband convex array transducers at selected center frequencies between 3.5
and 5.0 MHz. All sonograms and measurements were obtained by a single
experienced abdominal sonographer. Over the 2-month period during which the
patients included in the study were examined, the abdominal sonography
protocol called for images of the pancreatic duct to be obtained at both
end-inspiration and end-expiration. Our study was approved by the
institutional review board.
Results
Our patients were 11 men and 14 women between 21 and 58 years. In these
patients, we found the mean diameter of the pancreatic duct at end-expiration
to be 1.2 mm (range, 0.7-2.0 mm; standard deviation, 0.4 mm). A significant
increase in the diameter of the duct after inspiration was observed in seven
patients (28%), among whom the mean increase was 1.3 mm (range, 1.0-2.2 mm;
standard deviation, 0.4 mm). These seven included four patients (16%) in whom
the diameter of the duct was less than or equal to 2.5 mm (i.e., normal) at
end-inspiration and three patients (12%) in whom the diameter of the duct was
greater than 2.5 mm at end-inspiration (Figs.
1A,1B
and
2A,2B).
The increase in diameter of the duct was immediately apparent at
end-inspiration in all patients in whom the phenomenon was observed. Short
axis sonography confirmed that in all patients in whom a significant increase
in the diameter of the duct was noted at end-inspiration, the cross-sectional
configuration of the duct was circular throughout the respiratory cycle.
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Fig. 1A. —36-year-old man with epigastric pain and normal serum amylase
level. Transducer center frequency was 4 MHz. Transverse sonogram obtained at
end-expiration shows pancreatic duct in body of gland. E = 0.8 mm.
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Fig. 1B. —36-year-old man with epigastric pain and normal serum amylase
level. Transducer center frequency was 4 MHz. Transverse sonogram obtained at
end-inspiration at same level as A shows that diameter of pancreatic
duct increases by 1.1 mm (from 0.8 to 1.9 mm) at end-inspiration. Although
diameter at end-inspiration is normal, 1.1-mm change might be misinterpreted
as evidence of ductal obstruction if secretin had been administered and
postsecretin measurement was obtained at end-inspiration. F = 1.9 mm.
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Fig. 2A. —45-year-old woman with dyspepsia that was subsequently
attributed to gastritis. Transducer center frequency was 4 MHz. Transverse
sonogram obtained at end-expiration shows pancreatic duct in body of gland. B
= 1.6 mm.
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Fig. 2B. —45-year-old woman with dyspepsia that was subsequently
attributed to gastritis. Transducer center frequency was 4 MHz. Transverse
sonogram obtained at end-inspiration at same level as A shows that
diameter of pancreatic duct increases by 2.2 mm (from 1.6 to 3.8 mm) so that
it exceeds the 2.5-mm upper limit of normal at end-inspiration. A = 3.8
mm.
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Discussion
Dilatation of the pancreatic duct indicates obstruction of the normal flow
of pancreatic secretions or, rarely, tumoral hypersecretion of mucin
[4]. In some patients with
obstruction of the pancreatic duct, dilatation of the duct is the most
conspicuous abnormal finding detected on imaging. Because the detection of a
dilatated pancreatic duct typically leads to additional imaging (e.g., CT, MR
cholangiography, and endoscopic retrograde cholangiography) that involves
substantial cost and risk, it is important to avoid a mistaken diagnosis of
dilatation of the pancreatic duct. Indeed, several pitfalls that can lead to
erroneous assessment of the duct have been reported
[1]. A pitfall that has
apparently not been previously described is brought to light in this report.
Atri and Finnegan [1] observed
that the diameter of the normal pancreatic duct may change during the course
of sonography, but they had not noted a correlation between such fluctuation
and respiration (Atri M, personal communication).
In this study, the diameter of the pancreatic duct exceeded the 2.5-mm
upper limit of normal at end-inspiration in 12% of patients who lacked
clinical, laboratory, or imaging evidence of pancreatic disease and in whom
the end-expiratory diameter of the duct was normal. Had secretin been
administered to increase the sensitivity of sonography for obstruction of the
pancreatic duct, an additional 16% of patients (i.e., the 4 patients in whom
the diameter of the duct increased by 1 mm or more but was always 
2.5 mm)
might have potentially been misdiagnosed as having an obstructed pancreatic
duct if presecretin measurements were obtained at end-expiration and
postsecretin measurements were obtained at end-inspiration. Such a
modification in technique might occur if different examiners performed the
presecretin and postsecretin sonographic examination or if the distribution of
bowel gas shifted during the interval between the two examinations so that a
deep breath was necessary to allow the pancreas to be visualized on the
postsecretin sonographic examination.
A mechanism whereby an increase in the diameter of the pancreatic duct
might occur during inspiration is suggested. It is known that the pancreas is
quite mobile during respiration
[5,
6]. Bryan et al.
[5] postulated and Kivisaari et
al. [6] later confirmed that
the excursion of the pancreatic tail during inspiration exceeds that of the
remainder of the gland in healthy subjects, presumably because the tail
extends into the lienorenal ligament and the spleen is displaced considerably
by the descending diaphragm. If movement of the pancreatic tail during
inspiration is not only caudad but also toward the midline because of
displacement by the descending spleen, the result might be shortening of the
long axis dimension of the pancreas. This result would shorten the length of
the pancreatic duct because the duct is entirely surrounded by pancreatic
parenchyma and spans the length of the gland. To accommodate the finite volume
of fluid in the lumen of the duct, a decrease in the length of the duct would
necessitate a concomitant increase in its diameter if Oddi's sphincter
remained closed. During expiration, the duct would reassume its resting
length, and elastic recoil would restore its diameter to the baseline value.
This mechanism is similar to the one that has been proposed to explain the
increase in diameter of the bile duct that occurs during inspiration in some
healthy individuals [7].
One could alternatively consider the possibility that the normally circular
cross section of the pancreatic duct might become oval (i.e., that the
anteroposterior diameter increases and the superoinferior diameter decreases)
during inspiration in some individuals. If true, such a phenomenon would cause
an apparent increase in the diameter of the duct on transverse sonography
because it is the anteroposterior dimension of the duct that is measured.
However, short axis sonography confirmed that the cross section of the duct
remained circular throughout the respiratory cycle in all patients in whom a
significant change in diameter was observed during inspiration.
A limitation of this study is that a single non-blinded investigator
performed all sonographic examinations and measurements. However, most
sonographic examinations were observed by other experienced individuals who
concurred that the diameter of the pancreatic duct did indeed increase.
Because only healthy patients between 21 and 58 years old were studied, the
conclusions may not apply to children or to older patients, in whom the duct
is normally wider and possibly more compliant than in younger subjects
[8]. Because only patients who
could take a deep breath were studied, the prevalence of the phenomenon
described is presumably lower in an unselected population than the 28%
prevalence observed in the study population.
To summarize, the diameter of the pancreatic duct can increase
substantially during inspiration in some healthy adults. This observation
should be borne in mind as a potential pitfall during pancreatic
sonography.
References
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Abstract
Introduction
