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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_1056_Библиотеки_им_академика_М_И_Перельмана.pdf
11 Magnetic Resonance and Traditional Radiology in the Diagnosis of... 169
patients usually suffer from perineal pain, fever, and leukocytosis (Khati et al. 2015;
Guniganti et al. 2017). The advantages of CT include its high availability, relatively
low cost, rapid execution, great patient tolerance, and excellent spatial resolution. It
can precisely reveal even small amounts of free air, presence of calcifications, and
internal bleeding (Guniganti et al. 2017).
The most appropriate CT protocol varies, depending upon patient presentation
and differential diagnosis (Guniganti et al. 2017). Rectal contrast (gastrografinor
barium) is not always required, and it is usually administered through a welllubricated small rubber catheter gently inserted up the anal canal into the rectum
prior to scanning (Khati et al. 2015). This catheter, left in place during scanning, may
help to differentiate the anal canal from any potential fistula and abscess.
Reformatted sagittal and coronal images are also obtained if needed. The creation
of multiplanar reconstructions in CT provides a better recognition and characterization of anorectal pathology, and they are extremely useful in order to evaluate the
supra-levator extension of disease (Guniganti et al. 2017).
Administration of intravenous contrast is much preferred to non-contrast examination, as it helps to detect smal l fluid collections and presence of inflammation and
to better delineate the anatomic relationship among perianal structures (Guniganti
et al. 2017) (Fig. 1).
Using CT technique, a fistulous tract appears as a well-defined tubular, soft tissue,
or fluid/air-filled structure that arises from the anal sphincter (Fig. 2). A thick,
enhanced wall along with surrounding inflammatory changes usual ly refers to an
infected fistula (Khati et al. 2015; Guniganti et al. 2017). An abscess is identified as a
fluid collection surrounded by a thick wall that may contain some air, and sometimes
it is associated with a fistulous tract (Khati et al. 2015; Guniganti et al. 2017).
Surrounding inflammatory changes among perianal structures are usually present
(Khati et al. 2015).
Fig. 1 (a) Axial and (b) coronal CT images show the anatomy of the perianal region. AC anal
canal, IS internal sphincter, ES external sphincter, IAF ischiatic-anal fossa, IRF ischiorectal fossa, SS
supra-levator space, R rectum, LAM levator ani muscle
170 L. M. Minordi et al.
Fig. 2 Contrast-enhanced
axial CT image shows a
complex fistula with an
internal opening in the
anterior midline and bilateral
inter-sphincteric extensions
(white arrows) into the
anterolateral direction, with a
horseshoe configuration
CT fistulography is performed after the injection of contrast medium into the
fistula (Liang et al. 2014; Soker et al. 2016).
In the study performed by Liang et al. in 2014 (Liang et al. 2014), the patients
were placed in a prone position. Before performing the scan, an enema tip was
inserted into the rectum to the near maximum patient tolerance, and it was used to
insufflate air in order to discover the internal fistula opening. Adequate intestinal
distension is crucial for a correct visualization of the internal opening. CT scan views
allowed a vigilant monitoring of the insufflation, gradually performed, in order to
avoid the risk of perforation. An infusion tube with the needle removed was cut in at
the site of the external opening under rigorous disinfection measures. The tip of the
infusion tube was dipped in xylocaine gel for a local anesthetic effect and lubrication. The site of the external opening was cleaned well with alcohol and a povidoneiodine solution. The infusion tube was cleaned thoroughly using a compatible
enzymatic detergent. A prepared solution (1 mL gastr ografin mixed in 10 mL of
sterile normal saline) was gradually injected into the fistula. Different volumes of the
contrast mixture were utilized depending on the branching, width, and length of the
perianal fistula. Reflow of contrast from the external opening indicated that the
appropriate amount of contrast had been injected. At that point the external opening
was closed with sterile gauze, and any contrast refluxed on the skin’s surface was
cleaned off. Then, the contrast was injected through all openings to completely fill
the perianal fistula. The images were reconstructed on a workstation, including
techniques such as maximum intensity projection, volume rendering (VR), and
multiple planar reconstructions. The authors concluded that fistulography improved
the diagnostic effectiveness in the assessment of the fistulous course.
In the study performed by Soker et al. in 2016 (Soker et al. 2016), fistulography
CT exams were evaluated in comparison with MRI and surgical reports. In CT the
11 Magnetic Resonance and Traditional Radiology in the Diagnosis of... 171
location of the external orifice was found on the prone position of the patient, and the
region was cleaned with alcohol and povidone-iodine solution. Then, a mixture of
1 mL of nonionic iodinated contrast material and 10 mL of saline was injected
without a needle into the orifice, following an application of xylocaine gel to
anesthetize the local area. Intravenous iodinated contrast media was injected in all
patients. Axial and three-dimensional (3D) recons tructed images of all patients were
evaluated on a separate workstation. CT fistulography allowed an exact evaluation of
the fistula classification in 30 (73.1%) of the 41 patients; on the other hand, MRI
correctly defined fistula classification in 38 (92.7%) of these patients. CT
fistulography revealed 29 secondary extensions in 16 patients, whereas MRI
revealed 28 secondary extensions in 15 patients. There were no major discordances
between surgical findings and these two modalities. Comparing the ability to localize
internal openings, CT fistulography was able to detect the locations in 28 patients
(68.2%), whereas MRI was more accurate in this aspect, allowing a correct evaluation in 35 patients (85.3%).
CT fistulography is a technique with many disadvantages: it is an invasive, not
well-tolerated, and time-consuming procedure. It carries risks associated with an
inappropriate injection, and it may cause tissue injuries and false passages and fail to
demonstrate details of the fistulous tract. Other complications associated with this
procedure include infection, sepsis, and aggravation of pain (Soker et al. 2016).
Other relevant drawbacks of CT compared to MRI are its lower soft tissue contrast
resolution and exposure to ionizing radiation (Guniganti et al. 2017). The superior
soft tissue contrast resolution of MRI makes it a superior imaging technique in most
clinical scenarios, allowing an accurate diagnosis, depicting perianal fistulae and
their relationship to the anal sphincter, and revealing fistulas between pelvic organs,
especially in patients with cryptogenic fistulas or abscesses (Guniganti et al. 2017).
2.3 Magnetic Resonance Imaging
Over the past two decades , MR imaging has gained a prominent role in the
management of patients with anal fistulas. This is due to the fact that MRI can
classify fistulas preoperatively with very high precision. Many studies have demonstrated the remarkable diagnostic value of MRI, showing high accuracy of up to 93%
(Beets-Tan et al. 2001; Halligan 2020). The favora ble results of these studies confirm
that MRI is presently the method of choice for the assessment of perianal fistulas and
associated complications due to its high soft tissue contrast and the multiplanar
capability (Vanbeckevoort et al. 2014; Baskan et al. 2014; Balcı et al. 2019; Vo et al.
2019).
2.3.1 Anatomy MRI
On phased-array 1-3-T MRI, the mean length of the anal canal is 4.4 cm measured
from the anorectal junction to the most caudal extent of the subcutaneous external
anal sphincter (Erden et al. 2017) (Fig. 3).
172 L. M. Minordi et al.
Fig. 3 (a) Axial T2-weighted image, (b) axial contrast-enhanced fat-suppressed T1-weighted
image, (c) coronal T2-weighted image, and (d) coronal contrast-enhanced fat-suppressed
T1-weighted image show anatomy of the perianal region. AC anal canal, IS internal sphincter, ES
external sphincter, IAF ischiatic-anal fossa, IRF ischiorectal fossa, SS supra-levator space, R rectum;
LAM levator ani muscle
The dentate line is situated at approximately half of the length of the canal, which
is usually at the middle point between the superior border of the puborectalis muscle
and the most caudal tip of the subcutaneous external sphincter. These structures
demarcate the surgical anal canal, different from the anatomic anal canal, which is
shorter and defined as the canal caudal to the anal valves. The dentate line is
generally not directly identifiable at MRI. However, its placement can be estimated
especially in coronal reconstructions, where the cranio-caudal extent of the
puborectalis muscle and external sphincter can be more easily appreciated (Erden
et al. 2017; Halligan 2020).
The wall of the anal canal is formed by roughly cylindrical-shaped layers: the
mucosa, submucosa, and muscularis. The inner muscular layer forms the internal anal
sphincter and the outer layer the external anal sphincter complex (Erden et al. 2017).
11 Magnetic Resonance and Traditional Radiology in the Diagnosis of... 173
The internal sphincter can be recognized as the thickened extension of the circular
smooth muscle layer of the rectum, and it ends proximal to the lower part of the
external anal sphincter. It is homogenous and moderately hyperintense on
T2-weighted images and enhances significantly after administration of contrast
agent. On axial MR images, the internal anal sphincter is seen as an inner circular
layer of the anal canal. It appears as a longitudinal band on the coronal plane. The
mean thickness of the internal anal sphincter is about 3.5 mm, being approximately
3.2 0.7 mm in women and 2.9 0.7 mm in men. The thickness of the internal anal
sphincter increases with age. This is most likely due to the infiltration of connective
tissue rather than genuine muscular hypertrophy (Erden et al. 2017).
The external sphincter complex is formed by skeletal muscle. On axial
T2-weighted images, it can be recognized as an outermost circular layer of the
anal canal. It has low signal intensity with “striated” appearance. After administration of intravenous contrast agent, the external anal sphincter appears less enhanced
than the internal anal sphincter. It is thicker than the internal sphincter, with a mean
value of about 4 mm. According to the traditional description, the external anal
sphincter has three separa te muscular fiber bundles: deep, superficial, and subcutaneous. All these three components can be distinguished only on high-resolution
T2-weighted images. The deep part of the external anal sphincter surrounds the
upper third of the anal canal and joins proximally with the sling-like puborectalis
muscle. The superficial part is the largest of the three layers, it strengthens the bulk of
the internal anal sphincter on all sides and surrounds the anal canal above the
subcutaneous external anal sphincter; it continues within the ano-coccygeal ligament, which adheres posteriorly to the coccyx. The subcutaneous part is an annular
muscle, which circles the anal canal at the infero-lateral region of the internal anal
sphincter, and is situated immediately under the perianal skin; the subcutaneous part
of the external anal sphincter extends under the internal anal sphincter and curves
medially with a typical J shape. On MRI, the mean thickness of the subcutaneous
external anal sphincter on each side is about 5 mm in normal subjects (Erden et al.
2017).
In the space that separates the external and the internal anal sphincter, there is a
fibro-fatty muscular layer called conjoint longitudinal muscle (CLM). It continues
superiorly with the longitudinal muscle layer of the rectum. On the axial T1- and T2weighted MR sections, the CLM shows a thin hypointense circular configuration. It
enhances after injection of intravenous contrast agent. It is a very thin layer with a
mean thickness of 1.63 0.44 at histo-topographic study, and it varies between 0.9
and 1.6 mm on phased-array MRI (Erden et al. 2017).
The inter-sphincteric space is a potential space between the conjoint longitudinal
muscle and the external anal sphincter. It shows as a thin hyperintense area on the
axial T1- and T2- weighted images. Most perianal fistulas originate from this space
(Erden et al. 2017).
Puborectalis muscle is the part of the levator ani group of muscles, which emerges
from the body of the pubic bone and forms a sling around the anorectal junction.
This sling causes angulation of the rectum, partially contributing to continence. It
typically has a shape of a “U,” which can be easily recognized on the axial plane. Its
174 L. M. Minordi et al.
mean thickness is 4.28 0.13 mm on phased-array MRI. The absence of
puborectalis muscle weakens the anterior aspect of the canal (Erden et al. 2017).
The anorectal angle determines a fundamental parameter of the anorectal configuration. It is defined as the angle formed by the lines passing from the posterior wall
of the rectum and through the center of the anal canal. The normal range of this angle
can be quantified in MRI and measures, in asymptomatic subje cts at rest, between
93 and 108 (Erden et al. 2017).
Ano-coccygeal ligament (also called ano-coccygeal raphe) is a complex
musculotendinous structure extending between the coccyx and anal canal. It is a
crucial structure for decision-making regarding rectal and upper anal canal mobilization. Its mean diameter on the axial MR images is 7.28 0.17 mm (Erden et al.
2017).
2.3.2 MRI Technique (Coils, Volume, and Sequences) and Findings
Endoluminal anal coil and body phased-array coils can be used.
Since MRI field strengths and phased-array coils have advanced their diagnostic
performance, dedicated endoluminal anal coils are now infrequently utilized,
although they provide the best spatial resolution. Other factors that led to this
tendency are their restricted field of view (which may not allow to visualize all the
fistulous tracts and abscesses outside the sphincter complex), low tolerance by
patients, and frequent motion artifacts (Dwarkasing et al. 2005; George et al.
2011; Vanbeckevoort et al. 2014).
On the other hand, MRI exam performed with body phased-array coils does not
need any particular patient preparation and is better tolerated. Advantages of the
body phased-array coils include a larger field of view (avoiding fistula extensions
from being overlooked) and a good spatial resolution (even though not as much as
endoluminal coils) (George et al. 2011; Vanbeckevoort et al. 2014; Baskan et al.
2014).
The imaged volume should include all the frequent sites of the fistula extensions:
the levators, the whole presacral space, and the perineum (Baskan et al. 2014).
Another important advantage of MR imaging in fistula evaluation is the capacity
to study the anal sphincter complex in any surgically relevant plane. For this reason,
it is crucial that imaging planes are correctly oriented with respect to the anal canal.
The anal canal is leaned forward from the vertical by approximately 45
in the
sagittal plane, making it necessary to obtain axial and coronal images aligned
orthogonal and parallel to the anal canal, respectively. Normally oriented axial and
coronal images would not allow precise analysis of the source and the fistulous track
(de Miguel Criado et al. 2012; Baskan et al. 2014). At first the precise inclination of
the anal canal is evaluated in a T2-weighted sequence in the sagittal plane in order to
acquire these correctly orientated planes (de Miguel Criado et al. 2012; Baskan et al.
2014). This sequence provides the correct orientation of the anal canal, so that truly
axial (Fig. 4a) and coronal (Fig. 4b) images along the long axis of the anal canal can
be obtained. Axial images frequently allow an accurate evaluation of the primary
track (e.g., ischioanal or inter-sphincteric), and they are also ideal to show the radial
site of the internal opening. Coronal images are excellent for displaying the levator
11 Magnetic Resonance and Traditional Radiology in the Diagnosis of... 175
Fig. 4 Sagittal T2-weighted image shows the orientation of (a) oblique axial and (b) oblique
coronal planes (white lines), according to, respectively, anal canal short and long axes
plates, which are used as a landmark to separate supra-levator from infra-levator
disease. Coronal images allow evaluating the height of the internal opening with the
caveat that the anal canal must be imaged along its entire cranio-caudal extent
(Halligan 2020).
In our institution the protocol for the evaluation of perianal fistulas consists of the
following sequences: oblique axial T1-weighted FSE, oblique axial T2-weighted
FSE, and oblique axial and oblique coronal fat-suppressed T1-weighted FSE with
gadolinium-based contrast material, oriented perpendicular or parallel (in the case of
the latter) to the long axis of the anal canal.
T2-weighted sequences are crucial in assessing the perianal disease, due to their
exceptional soft tissue contrast, allowing appropriate distinction of the anatomic
boundaries between the internal and external sphincters (Figs. 5 and 6). All pathological processes (fistulas, including every secondary fistulous track, and fluid
collections) can be clearly identified (Baskan et al. 2014). Active fistulas have a
characteristic aspect, with a central high-signal-intensity tract surrounded by a lowsignal-intensity wall (Dwarkasing et al. 2005). The inner high-signal-intensity
region of fistulas represents the true lumen with granulation tissue, and the external
part of fistulas with lower signal intensity indicates fibrotic tissue (Dwarkasing et al.
2005). Therefore, active fistulous tracks and extensions have high signal intensity on
T2-weighted images, and they can be easily distinguished from the sphincters and
muscles, which have low signal intensity (de Miguel Criado et al. 2012; Balcı et al.
2019). Fat suppression sequences help the differentiation from the surrounding
pelvic fat, which also appears hyperintense (Balcı et al. 2019). Abscesses have
also high signal intensity on T2-weighted images due to the presence of purulent
176 L. M. Minordi et al.
Fig. 5 (a) Sagittal T2-weighted image, (b) axial T2-weighted image, (c) axial fat-suppressed
T2-weighted image, (d) coronal T2-weighted image, (e) coronal fat-suppressed T2-weighted
11 Magnetic Resonance and Traditional Radiology in the Diagnosis of... 177
Fig. 6 A 53-year-old man. (a) Axial T2-weighted image, (b, c axial contrast-enhanced
fat-suppressed T1-weighted images show a trans-sphincteric fistula with an internal opening at
11 o’clock position and directed anteriorly towards scrotum (white arrows)
secretions in the central cavity (de Miguel Criado et al. 2012). Having both high
signal intensity on T2-weighted image, it may be difficult to assess fistulous tracts
with a concomitant acute abscess, and some underlying fistula tracks may be
obscured (Baskan et al. 2014). The low-signal-intensity areas are expected to
increase over time as a fistula becomes more chronic, indicating the forming of an
increasing quantity of fibrotic tissue (Dwarkasing et al. 2005).
ä
Fig. 5 (continued) image, (f, g) axial and coronal contrast-enhanced fat-suppressed T1-weighted
images show a right posterolateral inter-sphincteric fistula, confined within external sphincter
(white arrows)
178 L. M. Minordi et al.
Unenhanced T1-weighted images allow a detailed anatomic overview of the
sphincter complex, levator plate, and ischiorectal fossa. However, both pathological
areas (fistulous tracks, inflammation, and abscesses) and normal structures (such as
the sphincters and levator ani muscles) have low-intermediate-signal intensity and
may not be differentiated. Unenhanced T1-weighted images can clearly reveal the
presence of hemorrhage in the early postoperative phase (de Miguel Criado et al.
2012; Baskan et al. 2014; Balcı et al. 2019); hemorrhage produces high signal
intensity on T1-weighted images and therefore can be distinguished from the
lower signal of the residual tracks (de Miguel Criado et al. 2012). Hemorrhagic
material can also be differentiated from active granulation tissue having hemorrhage
a high signal intensity in T1-weighted images, contrasting the low signal intensity
due to the presence of fluid or pus in the granulation tissue (de Miguel Criado et al.
2012).
The routine MRI protocol for the anorectal disease usually comprehends a
gadolinium contrast-enhanced study (Figs. 5 and 6). This is due to its advantage of
rapid dynamic acquisition of data during enhancement of the inflammatory tracts and
associated abscess (Baskan et al. 2014; Balcı et al. 2019). Normal anorectal structures usually do not enhance significantly on gadolinium-enhanced fat-suppressed
T1-weighted images. Internal anal sphincter and blood vessels (including hemorrhoidal vessels) are an exception; there fore, they should not be mistaken for fistulous
tracks or fluid collections. At contrast-enhanced fat-suppressed T1-weighted imaging, a fistula can distinctly be identified as well as its extensions and its relationship
to the anal canal, especially to the external sphincter. It is relatively simple to assess
the extension of a fistula, whether it is contained within the external sphincter or has
extended beyond it. The fluid inside the track is hypointense, while the fistulous
tracks and active granulation tissue produce intense enhancement. A plausible
reason for high signal intensity within the fistulous track on contrast-enhanced
fat-suppressed T1-weighted images is the presence of hemorrhagic material from
recent surgical intervention; however, this finding is not related to contrast enhancement. Abscesses are seen as a central area of low signal intensity due to purulent
material that is surrounded by intense ring enhancement. On the other hand, chronic
fistulas and fibrotic tissue do not enhance with gadolinium contrast material
(de Miguel Criado et al. 2012). T1-weighted contrast-enhanced fat-suppressed
MRI sequences are used to differentiate inflamed and normal perineal tissues as
well as fluid and scarring/granulation tissue, which is an important aspect in the
imaging characterization of an abscess (Baskan et al. 2014).
Fibrotic fistula tracks and scars appear typically as linear structures producing low
signal intensity on T1- and T2-weighted images with no significant enhancement
after administration of contrast material (de Miguel Criado et al. 2012).
Diffusion-weighted imaging (DWI) provides functional information regarding
the motion of water molecules, tissue cellularity, and the integrity of the cellular
membranes. Additionally to its many oncologic applications, DWI is demonstrating
promising results in the evaluation of inflammation. In fact, DWI was proven to be
able to assess the infl
ammatory activity in patients with Crohn’s disease (Oto et al.
2009; Oussalah et al. 2010).