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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_1125_Библиотеки_им_академика_М_И_Перельмана.pdf
56
M.K. Feldman et al.
The rectum extends from the anorectal ring approximately 15 cm cephalad to
about the level of the sacral promontory and is divided into upper, middle, and
lower thirds. The fi ve layers of the rectum, the mucosa, submucosa, muscularis
propria, mesorectal fat, and mesorectal fascia, can be readily distinguished on a
good- quality scan. On T2-weighted MR images, the rectal mucosa is represented by
a thin hypointense line adjacent to the rectal lumen. The underlying submucosa is
higher in signal intensity. The muscularis propria appears as a thin hypointense line
surrounding the submucosa. The mesorectal fat, as most fat, appears hyperintense
on both T1- and T2-weighted imaging. Finally, the mesorectal fascia is a thin
hypointense line surrounding the mesorectal fat [ 13 ] (Fig. 3.1 ). Although all fi ve
layers are present through most of the rectum, the anatomy varies at both ends. The
mesorectal fat and fascia are not present around the uppermost portion of the rectum, above the level of the peritoneal refl ection, or around the lowest portion of the
rectum as it transitions into the anus.
3.4 Anorectal Neoplasms
Imaging evaluation of primary anal and rectal carcinoma is currently performed
with high-fi eld pelvic MRI using dedicated rectal protocols that provide highresolution, small fi eld-of-view images. In everyday practice, this technique is supplanting previously standard endorectal ultrasound in the initial staging of disease.
3.4.1 Rectal Adenocarcinoma
As stated previously, the high inherent tissue contrast of MRI in combination with
the high resolution possible with newer techniques allows visualization of the stratifi ed rectal wall (Fig. 3.1 ). This level of detail makes it possible to stage nonlocally
advanced rectal carcinoma (T1–T3) based on the depth of invasion of the tumor into
the rectal wall. In many clinical cases, the distinction between T1 and T2 disease
cannot be reliably made by MRI. However, the more clinically relevant distinction
between those lower-grade tumors and T3 disease can usually be made. MRI is also
excellent for the identifi cation and evaluation of the local invasion found in T4
disease.
Originating in the mucosal layer, rectal adenocarcinomas are seen as intermediatelow signal lesions on T2-weighted images. Typically, they are hyperintense relative
to muscle and hypointense relative to fat. The depth of invasion, best visualized on
T2-weighted images obtained perpendicular to the rectal wall, corresponds to the
tumor stage on MRI evaluation. T1-stage tumors may extend into the submucosal
layer of the bowel wall, whereas T2-stage tumors extend into but not beyond the
muscularis propria. Tumors that extend beyond this point, into the mesorectal fat,
are designated T3 stage and are sometimes subtyped depending on their depth of
invasion into the fat. When tumors invade into adjacent pelvic organs or extend into
the peritoneal fat, they are T4 stage. Some modifi cation of this system is required
for tumors that extend very low or very high in the rectum [ 15 ] (Fig. 3.2 ).
3 CT and MRI of the Pelvis for Anorectal Disease
57
Fig. 3.2 Rectal cancer T-staging. Axial T2-weighted images through the rectum ( a ) showing a T1
tumor ( black arrow ) without invasion into the rectal wall. ( b ) A T2 tumor ( thick black arrow )
disrupting ( white arrow ) the rectal mucosa ( white arrow head ). ( c ) A T3 tumor with nodular exten-
sion into the mesorectal fat ( white dashed arrows ). ( d ) A T4 tumor ( white stars ) extending through
the mesorectal fascia into the prostate ( short white arrows )
3.4.2 Circumferential Resection Margin (CRM)
Among the more important measurements provided by the radiologist, the radiographic CRM is the shortest radial distance from the tumor or a tumor deposit in the
mesorectal fat to the mesorectal fascia. A CRM of 1 mm or less is considered involving the mesorectal fascia, increasing the likelihood that the mass will not be completely resected with a typical mesorectal excision [ 16 – 18 ].
58
M.K. Feldman et al.
3.4.3 Low Rectal Cancer
Because of the anatomic changes present in the bowel wall as the rectum transitions
into the anus, the classifi cation system described previously does not necessarily
apply here. At this level, the mesorectal fat thins and ends at the puborectalis sling.
Tumors that extend below this level, to the internal anal sphincter, do not fi t this
staging system. Taylor et al. propose a staging system for these tumors that is based
on the degree of extension through the muscle layers [ 15 ]. In this system, stage 1
tumors involve the rectal wall but do not extend into the sphincter complex. Stage 2
tumors extend into but not beyond the internal anal sphincter. Stage 3 tumors extend
into the intersphincteric space and stage 4 tumors invade the external anal sphincter
and beyond (Fig. 3.3 ).
3.4.4 High Rectal Cancer
Similarly, tumors that extend very high in the rectum require different evaluations
when staging. If a high rectal tumor is above the level of the peritoneal refl ection,
any extension beyond the muscularis propria involves the peritoneal space, similar
to typical colon carcinomas. It has been suggested that upper rectal cancers can be
treated like colon cancer with limited benefi t of neoadjuvant radiation [ 19 ]. It has
also been suggested that these high intraperitoneal tumors are less aggressive,
stressing the importance of localizing them before treatment [ 20 ]. On MR imaging,
the peritoneal refl ection can sometimes be seen directly. When it is not seen, its
location can be inferred at the superior margin of the mesorectal fat. Since there are
implications on surgical planning for resection, localization of the tumor in relation
to the peritoneal refl ection is important in the radiologic evaluation of these masses.
Relationship to the sacral promontory can be easily determined and is important to
report since it can be a helpful landmark during surgery as well.
Fig. 3.3 Coronal ( a ) and axial ( b ) T2-weighted MR images showing a stage 4 low rectal tumor
( white star ) with extension into the IAS ( white arrow heads ), intersphincteric space ( white arrows ),
and the EAS ( black arrow heads )
3 CT and MRI of the Pelvis for Anorectal Disease
59
3.4.5 Lymph Nodes
MRI excels at providing the anatomic detail needed to identify lymph nodes in the
perirectal/mesorectal tissue and more distant locations in the pelvis and retroperitoneum. The diffi culty is in the distinction between normal and abnormal nodes.
Abdominal and pelvic lymph nodes are usually considered abnormal on crosssectional imaging studies if their short axis measures 1 cm or more since statistically, these nodes are signifi cantly more likely to be abnormal. However, in rectal
cancer, the size distribution of normal and abnormal nodes shifts; there is a much
higher percentage of abnormal nodes harboring metastatic disease with short axes
less than 1 cm [ 21 ].
In order to increase the likelihood of correctly identifying abnormal lymph
nodes, the evaluation of these nodes takes into account not only size but morphologic characteristics as well. Imaging features that make a node suspicious for neoplastic involvement include irregular borders and heterogeneous signal intensity
[ 22 – 24 ] (Fig. 3.4 ). Nodal staging in rectal cancer is based on the number of abnor-
mal nodes present. N1 disease is characterized by the presence of 1–3 abnormal
nodes; four or more abnormal nodes indicate N2 disease [ 15 ].
3.4.6 Vascular Invasion
Tumor growth that extends beyond the muscularis propria into small vessels associated with the rectum in the region of the tumor is associated with poor prognosis,
independent of other factors [ 25 , 26 ]. High-resolution MR imaging can identify these
small vessels and can evaluate them for the absence of fl ow and changes in morphology that suggest vascular invasion. It is also possible on post-contrast scans to distinguish tumoral enhancement seen in invasion from normal vascular opacifi cation.
3.4.7 Mucinous Tumors
Although staged similarly, mucinous tumors of the rectum typically have a markedly different appearance on T2-weighted imaging because of their mucin content.
Mucin, having high water content, is usually hyperintense and these mucinous
tumors usually appear as multiseptated cystic masses (Fig. 3.5 ). Recent data sug-
gests that patients with these tumors may not benefi t from neoadjuvant therapy [ 27 ].
The ability of MRI to characterize these lesions allows this to be taken into consideration at the time of treatment planning.
3.4.8 Surgical Planning
In addition to traditional small fi eld-of-view T2-weighted imaging performed to
evaluate the stage of the tumor, at our institution, we utilize additional large-fi eld
T2-weighted imaging and post-contrast T1-weighted imaging in multiple planes.
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M.K. Feldman et al.
Fig. 3.4 Axial T2-weighted MR image through the rectum ( a ) showing a small smoothly margin-
ated, oval mesorectal lymph node ( white arrow ) which is likely benign, inferior to an enlarged,
irregularly marginated rounded lymph node ( black arrow ) which is likely malignant. A more cau-
dal axial image ( b ) shows a similar-appearing malignant lymph node ( black arrow head ) and an
opposite tumor nodule ( white star ). The corresponding slice from the apparent diffusion coeffi -
cient map ( c ) derived from the diffusion-weighted images (not shown) shows low signal within
both lesions ( dashed white arrows ) indicating restricted water diffusion, compatible with tumor
These images help identify tumor involvement with adjacent structures. This can
aid the surgeon in planning complex resections near the spine and the pelvic sidewall and in and around the sciatic foramen (Fig. 3.6 ).
3.4.9 Posttreatment
Patients who have undergone neoadjuvant therapy, typically those with T3 tumors
and/or those with N1 or N2 disease, are usually restaged with pelvic MRI. The
extent of disease is reassessed using the same criteria used in the primary evaluation. Downstaging after treatment can result in modifi cation of the patient’s treatment plan [ 28 ].
3 CT and MRI of the Pelvis for Anorectal Disease
Fig. 3.5 High-resolution axial ( a ) and sagittal ( b ) T2-weighted images through the lower rectum
showing a large multiloculated high-signal mucinous neoplasm extending into the anal canal.
Multiple thin septations within the lesion are hypointense
61
Fig. 3.6 Post-contrast T1-weighted axial MRI images showing ( a ) direct tumor extension of
tumor into the mesorectal fat and infl ammation extending to the pelvic sidewall. Recurrent adenocarcinoma ( b ) involving the pelvic sidewall to the hypointense cortical bone ( short arrow ) without
extension into the greater sciatic notch ( long arrow )
3.4.10 Anal Carcinoma
As opposed to rectal carcinoma staging, where depth of invasion determines the
stage, anal cancer is primarily staged based on its size, with the length of the long
axis of the tumor determining the T1–T3 stage. Less than 2 cm, 2–5 cm, and greater
than 5 cm correspond to stages 1, 2, and 3, respectively [ 29 ]. The stage is unaffected
by involvement of local structures such as the sphincter complex, rectum, or anal
verge. Involvement of distal structures, however, such as the bladder and vagina,
does upstage the mass to stage 4 (Fig. 3.7 ).
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M.K. Feldman et al.
Fig. 3.7 Anal carcinoma . Coronal ( a ) and axial ( b ) T2-weighted images through the anus showing
an intermediate intensity T4N0 anal carcinoma involving the IAS ( white arrow heads ) on the right
and extending into the expanded intersphincteric space ( white star ) and through the posterior,
inferior aspect of the EAS ( black arrowheads ) into the gluteal fat and fi nally through the skin (not
shown)
3.4.11 Lymph Node Staging
Also contrary to rectal carcinoma staging, nodal staging of anal cancer is not based
on the number of involved nodes but rather their distribution. So, involvement of
perirectal nodes alone is considered N1; unilateral involvement of the inguinal or
internal iliac nodes is N2 and involvement of both groups or bilateral involvement
is considered N3 [ 30 , 31 ].
3.4.12 Posttreatment Imaging
Posttreatment imaging can be quite challenging. After treatment fi ndings can range
from interval progression of disease to a complete radiologic response. Newer techniques, such as diffusion-weighted imaging, that assign image contrast based on the
freedom of water movement within tissues can be used to help identify residual
disease [ 14 ].
3.4.13 Distant Metastatic Disease
Evaluation of or screening for distal metastatic anorectal disease , when appropriate,
is best performed with whole-body CT or PET imaging. Although PET/CT is a
common modality employed in this evaluation, recently emerging PET/MRI examinations performed in high-fi eld magnets can combine local staging studies and
evaluation of/for distant metastases [ 32 ].
3 CT and MRI of the Pelvis for Anorectal Disease
63
3.5 Other Rectal Neoplasms
3.5.1 Mesenchymal Lesions
Mesenchymal lesions of the rectum include lipomas, leiomyomas, leiomyosarcomas, and gastrointestinal stromal tumors (GIST). Rectal lipomas have a characteristic appearance on MR that makes their diagnosis straightforward. Lipomas are
smoothly contoured ovoid or broad-based lesions which follow fat signal on all
sequences. This means they are hyperintense on T1 and T2 images and hypointense
on fat-suppressed sequences. These lesions will also show a particular artifact pattern on a specifi c type of T1-weighted imaging sequence referred to as out-of-phase
images [ 2 , 33 ]. There is overlap in the imaging features of the other mesenchymal
tumors and diagnosis is usually made by histology. GISTs are the most common
mesenchymal tumor to involve the rectum. These lesions are typically smoothly
contoured with T1 hyperintense signal, variable precontrast T2 signal, and heterogeneous post-contrast enhancement [ 2 , 33 , 34 ] (Fig. 3.8 ).
3.5.2 Neuroendocrine Tumors
Neuroendocrine tumors and neuroendocrine carcinomas, also referred to as
“carcinoid tumors,” occur more frequently in the rectum than the colon. T he usual
imaging appearance of a rectal neuroendocrine tumor is a mural-based polypoid
mass which is mildly T2 hyperintense and isointense to the rectal wall on T1. These
lesions are very vascular and show avid post-contrast enhancement on both MRI
and CT [ 35 ]. Poorly differentiated neuroendocrine carcinomas of the rectum show
imaging features similar to rectal adenocarcinoma.
3.5.3 Lymphoma
The cecum and rectum are the most common areas in the large bowel affected by
primary lymphoma. Primary rectal lymphoma can present as a focal mass or infi ltrative process. One distinguishing feature of lymphoma is that, unlike adenocarcinoma,
it typically does not cause luminal obstruction. Lymphoma tends to show intermediate
signal intensity on T1-weighted images and high signal intensity on T2-weighted
sequences. Although imaging features may suggest the diagnosis, biopsy is required
to distinguish these lesions from adenocarcinoma and to guide management [ 36 ].
3.5.4 Metastatic Disease
The anorectum can be involved in metastatic disease by direct extension of adjacent
neoplasms such as ovarian or prostate carcinoma as well as from hematogenous
spread. Hematogenous metastatic disease will have variable imaging appearance
64
M.K. Feldman et al.
Fig. 3.8 High-resolution coronal T2-weighted image ( a ) through the rectum showing a lobulated
intermediate-/high-signal GIST ( white arrow ) extending into the rectal lumen and mesorectal fat
( black star ). Axial post-contrast T1-weighted image ( b ) through the mass ( white arrow ) shows
avid enhancement
that may mirror the primary tumor. For example, malignant melanoma usually
produces a mass with characteristic T1 hyperintense signal due to melanin. Other
hematogenous metastasis may show T2 hyperintense signal and heterogeneous
enhancement. Large metastatic lesions can cause luminal obstruction [ 2 ].
3.5.5 Other Lesions
Rectal hemangioma is a rare, benign vascular neoplasm of the rectum. These lesions
cause rectal wall thickening and prominence of the adjacent vasa recta. On CT,
calcifi ed phleboliths in the wall of the rectum are a key imaging feature that suggests the diagnosis (Fig. 3.9 ).
Endometriosis occurs when functioning endometrial tissue is present outside of
the uterine cavity. Endometriosis deposits typically contain blood products and
appear as T1 hyperintense lesions. They may also have internal T2 hypointense
signal in a gradient pattern classically described as T2 “shading” related to layering
blood products of differing ages. Atypical endometriomas that are hypointense on
T1-weighted images can be confused with other adnexal masses [ 37 , 38 ].
3.5.6 Retrorectal Cystic Lesions
The differential for a predominately cystic mass in the retrorectal space includes
developmental cysts, teratoma, meningocele, lymphangioma, neurogenic tumor,
and infection (Fig. 3.10 ). Retrorectal cystic lesions are often surgically excised due
to the malignant potential of some of the lesions in this group. MRI adds value to
the care of these patients in presurgical planning both directly and indirectly:
directly, by showing the exact location of the lesion and its relationship to adjacent
3 CT and MRI of the Pelvis for Anorectal Disease
Fig. 3.9 Axial
unenhanced CT of the
pelvis shows diffuse
rectal wall thickening
( white star ). Multiple
calcifi cations are present in
the rectal wall compatible
with venous phleboliths
( white arrow )
characteristic of a rectal
hemangioma
anatomic structures, and indirectly, by narrowing the differential and potentially
changing the surgical plan.
Of the developmental lesions, retrorectal hamartomas , also called tailgut cysts ,
tend to be multiloculated with internal septations. Duplication cysts will have a
muscular wall and may also have identifi able mucosa and submucosa. Another
developmental cyst, the epidermoid cyst, is T1 hypointense, T2 hyperintense, and
shows restricted diffusion on diffusion-weighted MR sequences [ 2 , 39 , 40 ].
Teratomas are usually fi rst identifi ed in children but may be suggested when a
presacral T2 hyperintense, T1 hypointense cystic, and solid mass is present, especially if it contains fat or calcifi cation [
39 ].
A meningocele is a presacral meningeal cyst that develops secondary to a defect
in the anterior sacrum. It has fl uid intensity, is very hyperintense on T2-weighted
images, and does not have enhancing components. A specifi c diagnosis of a meningocele is made when the stalk communicating with the thecal sac is identifi ed [ 39 ].
65
3.6 Inflammatory and Infectious Diseases
MR imaging studies tailored for anorectal evaluation have become an important
tool in diagnosis, surgical planning, and management of patients with complex anorectal infl ammatory and infectious disease. The resolution and anatomic detail
afforded by MR make it possible to delineate disease extent and sphincter involvement, information crucial for surgical planning and treatment monitoring.
3.6.1 Anorectal Abscess
Anal abscess can occur in healthy individuals or patients with underlying Crohn’s
disease. The widely accepted “cryptoglandular theory” suggests an anal abscess