DOI:10.2214/AJR.05.1795
AJR 2007; 188:W37-W43
© American Roentgen Ray Society
High-Resolution MRI in Evaluation of the Surgical Anatomy of the Esophagus and Posterior Mediastinum
A. M. Riddell1,
D. C. Davies2,
W. H. Allum1,
A. C. Wotherspoon1,
C. Richardson1 and
G. Brown1
1 Department of Diagnostic Radiology, Royal Marsden Hospital Foundation Trust,
Downs Rd., Sutton, Surrey, SM2 5PT, United Kingdom.
2 Department of Anatomy, St. George's Hospital Medical School, London SW17 0RE,
United Kingdom.
Received October 12, 2005;
accepted after revision December 7, 2005.
Address correspondence to A. M. Riddell
(Angela.Riddell{at}rmh.nhs.uk).
WEB
This is a Web exclusive article.
Abstract
OBJECTIVE. The purpose of this study was to use high-resolution MRI
to evaluate the surgical anatomy of the posterior mediastinum, in particular
the esophagus and its relation to the surrounding structures. The aim was to
familiarize radiologists with the appearance of structures considered
important in planning surgical resection of the esophagus.
MATERIALS AND METHODS. The thoraces of two cadavers were imaged with
a 1.5-T magnet using a high-resolution T2-weighted sequence. Axial cadaveric
sections of the posterior mediastinum were cut with a band saw at levels
determined from the MR images, and histologic whole-mount sections of the
esophagus and surrounding tissue were prepared from the cadaveric sections.
The appearance of structures identified on the MR images was compared with the
findings on corresponding gross anatomic and histologic whole-mount
sections.
RESULTS. The MR images depicted the esophagus and structures in
close anatomic relation: the pleural reflections and pericardium. The
technique enabled visualization of structures to our knowledge not previously
described on cross-sectional imaging: the individual layers of the esophageal
wall, the thoracic duct, a connective tissue layer attaching the esophagus to
the anterior wall of the aorta, and a fascial plane passing between layers of
the right and left parietal pleura posterior to the esophagus.
CONCLUSION. High-resolution MRI of the posterior mediastinum
provides detailed anatomic information, delineating structures not visible on
other forms of cross-sectional imaging. It can provide important information
for planning surgical intervention.
Keywords: anatomy • esophageal disease • MRI
Introduction
Advances in MRI technology have improved the achievable signal-to-noise
ratio and enabled development of high-spatial-resolution imaging sequences
(small field of view, thin slices). The technique has proved highly successful
for imaging the pelvis, and high-resolution MRI is recognized as the technique
of choice for local staging of cervical, uterine, and rectal cancer
[1-3].
Such imaging findings are pivotal to decision making by a multidisciplinary
team and help to ensure the appropriate therapeutic course.
In the management of early-stage adenocarcinoma of the esophagus, surgical
resection offers the longest disease-free survival. However, locoregional
recurrence remains a problem, and the presence of tumor close (< 1 mm) to
the circumferential resection margin has been shown to reduce survival
[4]. Identification of specific
resection planes with MRI may provide a preoperative means of predicting
resectability of esophageal tumors and thus aid in selection of patients for
surgery. To our knowledge, however, high-resolution MRI has not been developed
for imaging the thorax. Our aim, using cadaver material, was to adapt the
technique used for imaging pelvic organs to imaging of the posterior
mediastinum and to validate the MRI findings by comparing them with the
findings on corresponding axial and histologic whole-mount sections.
The esophagus descends from the pharynx at the lower border of the cricoid
cartilage to the stomach and, depending on the height of the individual adult,
measures 25-30 cm. Although essentially a midline structure, the esophagus
deviates slightly to the left in the neck and to the right in the thorax,
following the curvature of the vertebral column. It then curves to the left
again as it passes through the hiatus in the diaphragm at the level of T10.
The superior and inferior boundaries of the posterior mediastinum are the
level of the T4 vertebral body and the diaphragm, respectively. The posterior
pericardium forms the anterior, the pleura the lateral, and the vertebral
column the posterior border of the posterior mediastinum. Within the posterior
mediastinum, the esophagus descends to the right of the descending thoracic
aorta. Its anterior relations (from superior to inferior) are the trachea,
right pulmonary artery, left main bronchus, pericardium (separating it from
the left atrium), and diaphragm. In the posterior aspect, the esophagus is
separated from the vertebral column by the azygos vein, thoracic duct, five
right upper intercostal arteries, and aorta. An elongated recess of pleura
intervenes between the esophagus and the azygos vein on the right side. On the
left side, the esophagus is related to the descending thoracic aorta and
pleura.
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Fig. 1A —Cadaver of 86-year-old woman. Images show layers of
esophageal wall: mucosa (thin arrow), submucosa (arrowhead),
and muscularis propria (thick arrow). Nearby structures are azygos
vein (A), vertebral body (V), and descending thoracic aorta (TA).
High-resolution T2-weighted axial MR image.
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Fig. 1B —Cadaver of 86-year-old woman. Images show layers of
esophageal wall: mucosa (thin arrow), submucosa (arrowhead),
and muscularis propria (thick arrow). Nearby structures are azygos
vein (A), vertebral body (V), and descending thoracic aorta (TA). Photograph
shows anatomic section.
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Fig. 1C —Cadaver of 86-year-old woman. Images show layers of
esophageal wall: mucosa (thin arrow), submucosa (arrowhead),
and muscularis propria (thick arrow). Nearby structures are azygos
vein (A), vertebral body (V), and descending thoracic aorta (TA). Photograph
shows whole-mount histologic section. (H and E)
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Surgical resection of tumors of the middle and lower portions of the
esophagus is generally approached with a two-stage procedure involving midline
laparotomy for mobilization of the stomach followed by nodal dissection in the
territories of the left gastric and common hepatic arteries. The patient is
then turned to the left lateral decubitus position, and right thoracotomy is
performed to access the esophagus, the level of the thoracotomy being
determined by the position of the tumor. Extensive periesophageal nodal
dissection is performed and typically requires ligation, resection, or both,
of the azygos vein for adequate exposure. The thoracic duct is resected
together with the surrounding lymphatic tissue, primarily to prevent
postoperative complications of chylothorax because it is rarely the site of
metastatic disease. Lymph nodes are dissected to the level of the carina. The
esophagus is transected, and the stomach elevated into the thoracic cavity for
restoration of gastrointestinal continuity
[5,
6].
Understanding the appearance and MR signal characteristics of the esophagus
and relations to nearby structures, such as the pleura, pericardium, azygos
vein, and aorta, is essential for evaluation of potential surgical resection
planes. In the treatment of patients with esophageal cancer, knowledge of the
extent of tumor spread in relation to resection planes should facilitate
preoperative surgical planning and identification of patients likely to
benefit from neoadjuvant therapy.
Materials and Methods
The institutional ethics committee approved the protocol for imaging and
correlation with MRI and macroscopic and microscopic anatomic and
histopathologic studies of two human cadavers (women 72 and 86 years old).
High-resolution MRI was performed with a 1.5-T magnet (Philips Intera,
software version 9.5.2) with an external surface coil (Philips Sense cardiac
coil) placed over the thorax. Axial T2-weighted fast spin-echo images of the
two cadaveric thoraces were obtained with the following parameters: TR/TE,
5,300/100; field of view, 22.5 cm; slice thickness, 2 mm; interslice gap, 0.3
mm; matrix size, 312 x 512; echo-train length, 16; number of
acquisitions, 10. Images were acquired from the superior aspect of the aortic
arch to the gastric cardia.
Two preservation techniques were used. One cadaver was perfused with a
mixture of formalin, phenol, and polyethylene glycol, and the other was
soft-fixed with a mixture of phenol, glycerol, alcohol, and water. The thorax
of each cadaver was imaged in its entirety and then sectioned in the axial
plane with a band saw at the following levels considered on the MR images to
be of surgical and radiologic interest: carina, insertion of the pulmonary
veins into the left atrium, and lower esophagus approximately 3 cm above the
gastroesophageal junction. The axial cadaver sections were immersed in 5%
hydrochloric acid for 10 weeks to allow bone demineralization before
preparation of whole-mount histologic sections of the posterior mediastinum
and staining with H and E and elastin-van Gieson stain. The histologic
sections of the posterior mediastinum were prepared to provide detailed
information about the morphologic features of the esophagus and their
relations to other structures, in particular the fascial planes surrounding
the esophagus at specific levels within the posterior mediastinum. The
appearance of the anatomic structures identified within the gross cadaveric
and whole-mount histologic sections was compared with their appearance on the
corresponding MR images.
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Fig. 3A —Cadaver of 72-year-old woman. Images show right pleural space
(arrow) extending to esophageal wall. RL = right lung, V = vertebral
body, TA = descending thoracic aorta. Axial MR image.
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Fig. 3B —Cadaver of 72-year-old woman. Images show right pleural space
(arrow) extending to esophageal wall. RL = right lung, V = vertebral
body, TA = descending thoracic aorta. Photograph shows anatomic section.
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Fig. 4A —Cadaver of 72-year-old woman. Line drawing shows site of
folds of parietal serous pericardium forming oblique sinus posterior to left
atrium and transverse sinus posterior to ascending aorta.
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Fig. 4B —Cadaver of 72-year-old woman. MR image shows pericardium
(arrows) as low signal intensity, oblique sinus (double
asterisk) posterior to left atrium, and transverse sinus
(asterisk) posterior to ascending aorta.
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Fig. 4C —Cadaver of 72-year-old woman. Photograph of anatomic section
shows pericardium (arrows) as shimmering fibrous tissue layer,
oblique sinus (double asterisk) posterior to left atrium, and
transverse sinus (asterisk) posterior to ascending aorta.
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Results
The MR signal characteristics were similar for the two cadaver preservation
techniques.
Esophageal Wall
Both cadavers had hiatal hernia, which distorted normal anatomic features
at the level of the diaphragm. The esophageal lumen of both cadavers contained
fluid, which distended the esophagus sufficiently to allow measurement of wall
thickness. The wall was 3 mm thick on both MRI and histologic sections,
confirming the findings of an earlier study in which a lower-resolution MRI
technique was used [7]. The
layers of the esophageal wall were clearly visible on high-resolution MRI.
Normal mucosa produced a fine intermediate signal layer, which was often
corrugated. This layer was surrounded by high-signal-intensity submucosa and
the outer low-signal-intensity muscularis propria. Figures
1A,
1B, and
1C shows the layers of the
esophageal wall revealed on MRI with the corresponding gross anatomic and
histologic whole-mount sections for comparison.
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Fig. 5A —Cadaver of 86-year-old woman. Close anatomic structures are
left atrium (LA), vertebral body (V), and descending thoracic aorta (TA). MR
image shows thoracic duct (arrow) as small structure of low signal
intensity posterolateral to aorta. Azygos vein (arrowhead) is to
right of duct.
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Fig. 5B —Cadaver of 86-year-old woman. Close anatomic structures are
left atrium (LA), vertebral body (V), and descending thoracic aorta (TA).
Photograph of histologic whole-mount section shows thoracic duct
(arrow) as fine endothelial-lined vessel. Azygos vein
(arrowhead) is to right of duct. (H and E)
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Fig. 6A —Cadaver of 86-year-old woman. Images show fascial attachment
to aorta. Nearby anatomic structures are left atrium (LA), azygos vein (A),
vertebral body (V), and descending thoracic aorta (TA). MR image shows fascial
plane (arrow) as linear band of low signal intensity passing to
aortic adventitia and extending posterior to esophagus toward right parietal
pleura (arrowheads).
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Fig. 6B —Cadaver of 86-year-old woman. Images show fascial attachment
to aorta. Nearby anatomic structures are left atrium (LA), azygos vein (A),
vertebral body (V), and descending thoracic aorta (TA). Photograph of
histologic whole-mount section confirms findings in A. Fascial plane
passes to aortic adventitia (arrow) and posterior to esophagus toward
right parietal pleura (arrowheads). (Elastin-Van Gieson)
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Fig. 7A —Cadaver of 86-year-old woman. RL = right lung, A = azygos
vein, V = vertebral body, TA = descending thoracic aorta. Axial MR image shows
fine line (arrow) of high signal intensity interposed between
esophagus and left main bronchus at level of carina.
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Fig. 7B —Cadaver of 86-year-old woman. RL = right lung, A = azygos
vein, V = vertebral body, TA = descending thoracic aorta. Photograph of
histologic whole-mount section confirms presence of narrow layer of connective
tissue (arrow) between esophagus and left main bronchus at level of
carina. (Elastin-van Gieson)
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Parietal and Visceral Pleura
The pleura is a serous membrane that invests each lung in the form of a
closed invaginated sac. The parietal pleura lines the thoracic cavity,
including the superior surface of the diaphragm and pericardium. The visceral
pleura reflects from the medial wall of the pleural cavity onto the surface of
the lungs. In living persons there is a potential space between the two
layers. In both cadavers, however, fluid was present within this pleural
space, separating the parietal and visceral layers (Figs.
2A and
2B). On MRI, the parietal
pleura appeared as a linear band of low signal intensity, and the visceral
pleura was not clearly discernible separate from the surface of the underlying
lung (Figs. 2A and
2B). Below the level of the
pulmonary veins, the condensed right parietal pleural layer extended to
contact the right side of the esophageal wall. Examination of the anatomic
sections confirmed the presence of direct continuity between the right
parietal pleura and the esophageal wall (Figs.
3A and
3B).
Endothoracic Fascia
In the cadaver sections, the endothoracic fascia, a layer of loose areolar
tissue, was observed to attach the parietal pleura to the deep surface of the
chest wall. In the lateral aspect, this fascia lines the surface of the
innermost intercostal muscles and the intervening ribs. In the anterior aspect
it blends with the periosteum of the sternum, and in the posterior aspect it
becomes continuous with the prevertebral fascia. The endothoracic fascia was
not distinguishable from the parietal fascia on MRI of the cadaver
sections.
Pericardium
The fibrous pericardium encases the heart and blends with the adventitia of
the roots of the great vessels. In the inferior aspect it also blends with the
central tendon of the diaphragm. Deep to the fibrous pericardium is a closed
sac, the serous pericardium, which is invaginated by the heart. The serous
pericardium comprises visceral and parietal layers that enclose a narrow
pericardial cavity. The visceral pericardium covers the surface of the heart
and is continuous with the thin parietal layer that lines the inner surface of
the fibrous pericardium. The parietal pleura lines the outer surface of the
fibrous pericardium, but these two layers could not be differentiated either
in the anatomic sections or on the corresponding MR images. The parietal
pleura is absent in the posterior aspect because the pleural reflection
courses around the hilar structures; therefore the anterior wall of the
esophagus abuts the pericardium directly.
On MR images the pericardium appears as a distinct structure of low signal
intensity surrounding the heart. However, the individual layers of the
pericardium are beyond the resolution of MRI (Figs.
4A,
4B, and
4C). On the posterior surface
of the heart, reflections of serous pericardium around the pulmonary veins
form a recess, the oblique sinus. The transverse sinus is formed by reflection
of serous pericardium between the aorta and pulmonary trunk anteriorly and the
pulmonary veins posteriorly. Both these recesses can be identified on MR
images and corresponding anatomic and histologic whole-mount sections (Figs.
4A,
4B, and
4C).
Thoracic Duct
The thoracic duct extends from the level of the T12 vertebral body to the
root of the neck and conveys lymph from the lower limbs and left upper limb to
the venous system. The duct passes upward from behind the right diaphragmatic
crus to the right of the aorta and posterior to the esophagus, inclining to
the left to eventually drain into the venous system at the confluence of the
left internal jugular and subclavian veins
[8]. The MR images confirmed
the presence of a fine continuous tubular structure passing between the azygos
vein and the aorta. In one cadaver the duct returned high signal intensity,
and in the other, low signal intensity. In both cadavers the structure
corresponded histologically to a small endothelium-lined vessel (Figs.
5A and
5B).
Fascial Attachment to Aorta
The lower part of the esophagus is attached to the anterior wall of the
aorta by a band of connective tissue. On high-resolution T2-weighted MR images
of the cadaver sections, the attachment appeared as a fine band of low signal
intensity extending from the left lateral wall of the esophagus to the aortic
adventitia, which continued laterally to the left parietal pleura. On the
right, this low-signal-intensity band passed posterior to the esophagus to
fuse with the right parietal pleura. The fascial band extended distally from
the level of the pulmonary veins over a length of 4 cm. The transverse course
of this fascial plane on MRI and corresponding histologic whole-mount section
is shown in Figures 6A and
6B.
Relation of Esophagus to Principal Airways
MRI of the cadavers showed the esophagus posterior to the membranous
trachea and to the left of the midline with a minimal layer of
high-signal-intensity periesophageal fat interposed between the structures. At
the level of the carina the esophagus was posterior to the left main bronchus,
again with minimal tissue separating the structures on MR images. The
histologic whole-mount section at this level showed a thin fascial plane that
divided the esophagus from the posterior wall of the left main bronchus, but
this finding was not clearly resolved on MRI (Figs.
7A and
7B). In contrast, the distinct
layer of periesophageal fat separating the esophagus from the right main
bronchus was readily visible on MR images, and its presence was confirmed on
the corresponding histologic section.
Discussion
The MR images obtained in the current study with a high-resolution
T2-weighted fast spinecho technique showed in exquisite detail the esophagus
and posterior mediastinal structures. The individual components of the
esophageal wall have been described previously and shown on MR images obtained
with endoluminal coils both in vivo and in cadavers
[9,
10]. The in vivo technique,
however, is limited by motion artifacts, inability to traverse strictures
within the esophagus, and a limited field of view, which prevent assessment of
the surrounding anatomic structures. In previous studies, images were obtained
with an external coil technique and conventional spin-echo T1-weighted
sequences, primarily because these sequences are faster than conventional
spin-echo T2-weighted sequences and therefore are not as susceptible to motion
artifact [11,
12]. With T1-weighted imaging,
however, the individual wall layers cannot be differentiated. Therefore any
disruption of the layers of the gastrointestinal tract, by tumor for example,
is likely to be much better delineated with the fast spin-echo T2-weighted
technique described in this current study.
Although the study was conducted with only two cadavers and therefore was
limited in scope, our findings provide important new information. The relation
of the pleura to the esophagus and the fascial attachment of the esophagus to
the aorta have not previously been delineated on MRI. Unlike the rectum, the
esophagus does not appear to have a complete envelope of fascia (comparable to
the mesorectum) surrounding it to serve as a specific circumferential
resection plane. In the current study, examination of the histologic
whole-mount sections confirmed the presence of a distinct fascial layer,
identifiable on the corresponding MRI images, that passes posterior to the
esophagus within the posterior mediastinum and condenses bilaterally with the
parietal pleura. This layer may provide the lateral and posterior margins for
surgical resection of the lower esophagus up to the level of the pulmonary
veins. In the anterior aspect, periesophageal fat extends to the thin
connective tissue layer posterior to the left main bronchus and inferior in
relation to the fibrous pericardium. The MR images highlighted the intimate
relation between the esophagus and the posterior wall of the left main
bronchus. It is therefore not surprising that direct invasion of the left main
bronchus by tumor is seen as a complication of advanced cancer of the middle
third of the esophagus.
Embryologically the esophagus is a foregut derivative that originates as a
short tube on the posterior aspect of the septum transversum (primitive
central tendon of the diaphragm)
[13]. The primitive gut is
enveloped by a mesentery that has a dorsal and a ventral aspect. The foregut
ventral mesentery involutes, and the caudal part develops into specialized
structures such as the lienorenal and gastrosplenic ligaments. The dorsal
mesentery is formed by a double layer of mesothelium, which in the abdomen
develops into the parietal and visceral layers of the peritoneum. The
embryologic origin of the fascial layer that we identified passing posterior
to the esophagus in the posterior mediastinum is uncertain, but it may
represent the residuum of the embryonic dorsal mesentery of the foregut within
the thorax. From a surgical perspective, this fascial layer may form a
potential posterior resection plane once access to the posterior mediastinal
structures has been achieved. Knowledge gained from the current study
increases understanding of the anatomic relations of the posterior mediastinum
and aids surgical planning, particularly in the context of esophageal
carcinoma, for which high-resolution MRI is likely to provide valuable
information that enables prediction of resectability.
Acknowledgments
We thank Geraint Williams and the pathology staff at Cardiff University
Hospital for their painstaking work preparing the histologic whole-mount
sections for this study.
References
- Choi SH, Kim SH, Choi HJ, Park BK, Lee HJ. Preoperative magnetic
resonance imaging staging of uterine cervical carcinoma: results of
prospective study. J Comput Assist Tomogr2004; 28:620
-627[CrossRef][Medline]
- Manfredi R, Gui B, Maresca G, Fanfani F, Bonomo L. Endometrial
cancer: magnetic resonance imaging. Abdom Imaging2005; 30:626
-636[CrossRef][Medline]
- Brown G, Davies S, Williams GT, et al. Effectiveness of
preoperative staging in rectal cancer: digital rectal examination, endoluminal
ultrasound or magnetic resonance imaging? Br J Cancer2004; 91:23
-29[CrossRef][Medline]
- Dexter SP, Sue-Ling H, McMahon MJ, Quirke P, Mapstone N, Martin IG.
Circumferential resection margin involvement: an independent predictor of
survival following surgery for oesophageal cancer. Gut2001; 48:667
-670[Abstract/Free Full Text]
- Lewis I. The surgical treatment of carcinoma of the oesophagus with
special reference to a new operation for growths of the middle third.
Br J Surg 1946;34:18
-31[CrossRef]
- Griffin M, Raimes S. Upper gastrointestinal surgery: a
companion to specialist surgical practice, 2nd ed. Philadelphia,
PA: Saunders, 2001: 138-139
- Quint L, Glazer G, Orringer M. Esophageal imaging by MR and CT:
study of normal anatomy and neoplasms. Radiology1985; 156:727
-731[Abstract/Free Full Text]
- McMinn RMH. Last's anatomy: regional and applied,
9th ed. London, UK: Churchill Livingstone, 1994:279
- Dave UR, Williams AD, Wilson JA, et al. Esophageal cancer staging
with endoscopic MR imaging: pilot study. Radiology2004; 230:281
-286[Abstract/Free Full Text]
- Yamada I, Murata Y, Izumi Y, et al. Staging of esophageal carcinoma
in vitro with 4.7-T MR imaging. Radiology1997; 204:521
-526[Abstract/Free Full Text]
- Nakashima A, Nakashima K, Seto H, Kakishita M. Normal appearance of
the esophagus in sagittal section: measurement of the anteroposterior diameter
with ECG gated MR imaging. Radiat Med1996; 14:77
-80[Medline]
- Nakashima A, Nakashima K, Seto H, et al. Thoracic esophageal
carcinoma: evaluation in the sagittal section with magnetic resonance imaging.
Abdom Imaging1997; 22:20
-23[CrossRef][Medline]
- Moore KL. Before we are born, 3rd ed.
Philadelphia, PA: Saunders, 1989:166
-167
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Abstract
Introduction
