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46
M.A. Valente
Rectal sensation and compliance can also be evaluated during anorectal
physiology by intermittent balloon distention (300 cc maximum volume) in the distal rectum and recording the responses. Volumetric measurements include the rectal
sensory threshold (fi rst sensation that is felt as the balloon is fi lled), the fi rst urge to
defecate, and the maximum tolerated volume. Rectal compliance is defi ned as the
ability of rectum to accommodate to different volumes without altering pressures.
Conditions that may cause scarring of the rectum and resultant low compliance
include conditions such as infl ammatory bowel disease or radiotherapy. Alternatively,
a highly compliant rectum may be caused by diabetes mellitus, megarectum, or
other neurological conditions.
A relatively easy method to assess the evacuatory function of the rectum is the
rectal balloon expulsion test. A balloon is placed in the rectum and fi rst fi lled with
50cc of either water or air and the patient is then asked to expel the balloon; if the
patient cannot, the balloon is fi lled to 100cc and then to 150cc. If the patient cannot
expel the balloon in under 60 seconds, this may represent a pelvic fl oor dysfunction.
It should be noted that false negatives are common and this test is supplemental to
the other test and examinations mentioned prior.
2.5.2 Endoanal Ultrasound
Endoanal ultrasonography (EUS) is a highly reliable and reproducible imaging
modality that provides information on the pelvic fl oor structures, anorectal disease
processes (such as abscess and fi stula), and anorectal tumors. In experienced hands,
EUS is quite accurate, with high sensitivity and specifi city for detecting anal
sphincter injuries. Advantages of EUS include the relatively inexpensive cost to
perform and its widespread availability. A disadvantage of EUS is that it is an
operator-dependent test, with varied published results on various fi ndings on the
same disease process.
Circumferential assessment of the anal canal and distal rectum is made possible
by a 360° rotating transducer that is either a 7 or 10 megaHertz (MHz) probe for
two-dimensional (2D) units or a 13 MHz probe for three-dimensional (3D)
(Fig. 2.14 ). In recent years, the use of 3D units has increased, with a similar sensitiv-
ity of detecting both EAS and IAS defects, but it has been shown that with the 3D
units, intraobserver variation is decreased and diagnosis of pathology has been
increased [ 7 ] (Fig. 2.14 ).
Prior to testing, patients receive an enema to clear the anorectum of any stool that
may interfere with images due to artifact. Additionally, EUS should not be performed on patients diagnosed with anal stenosis or fi ssure in ano, as this will
undoubtedly render the test painful and diffi cult to perform. EUS is most commonly
performed with the patient in the left lateral recumbent position. After a gentle
DRE, the well-lubricated ultrasound probe is inserted and slowly advanced and then
withdrawn to view the entire area of the anal canal/rectum (in modern systems, a
crystal moves up and down along the transducer to acquire images while the probe
is held still).
2 Investigations for Anorectal Disease
47
Fig. 2.14 B-K Medical (Herlev, Denmark) three-dimensional anorectal ultrasound equipment
The anal canal is divided into three levels on EUS: upper, middle, and lower
based on anatomic landmarks. The upper anal canal is defi ned by the U-shaped
puborectalis muscle, the middle canal has both EAS and IAS muscles (this is also
where the IAS is at maximum width) (Fig. 2.15 ), and in the lower anal canal, where
only the most distal external sphincter fi bers are visualized (Figs. 2.16 and 2.17 ).
Highly refl ective tissue on EUS reveals a hyperechoic (white) image, while poorly
refl ective tissues are hypoechoic (black). Thus, the smooth muscle-based IAS,
which has a high water content, shows up black on EUS. In post-obstetrical sphincter injuries, the defect is usually located anteriorly and encompasses the EAS and
48
Fig. 2.15 Twodimensional endoanal
ultrasound view of the
U-shaped puborectalis
muscle (PR). IAS internal
anal sphincter
Fig. 2.16 Twodimensional ultrasound
from the mid-anal canal.
This ultrasound image
represents normal, intact
internal anal sphincter
(IAS) (hypoechoic) and
external anal sphincter
(EAS) (hyperechoic)
M.A. Valente
may involve the IAS as well. In cases of postsurgical or posttraumatic injuries of the
anal sphincters, defects can involve either or both muscles and may be unifocal or
multifocal in nature (Fig. 2.18 ). The accuracy of EUS compared to surgical fi ndings
has been reported to be as high as 90–100 % by some authors, and additionally, EUS
has been used after operative sphincter repair to show the overlap of the muscles and
to confi rm a proper repair has been performed.
Anal ultrasonography has also been used to help diagnose and manage anorectal
abscess and fi stulae. In most patients, surgical examination will reveal the abscess
2 Investigations for Anorectal Disease
Fig. 2.17 Threedimensional coronal view
of the upper, middle, and
lower anal canal. EAS
external anal sphincter, IAS
internal anal sphincter
Fig. 2.18 Anteriorly
located defect of both the
EAS and IAS in the
mid-anal canal
49
and/or fi stulous tracts, but some patients may have deep-seated or complex collections or fi stulae which may be diffi cult to fi nd on clinical examination. EUS may be
used in these situations to try and elucidate abscesses and fi stulae. Additionally, the
addition of injecting diluted hydrogen peroxide into an external fi stulous opening
may help identify complex fi stulous tracts, resulting in a hyperechoic image. Several
studies have been performed on the use of EUS for anorectal sepsis and fi stulae with
good to excellent correlation of ultrasound fi ndings to surgical fi ndings [
8 , 9 ]. EUS
provides a useful tool in the work-up and treatment of anorectal abscess and fi stulae
and should be used when the exact location of the sepsis is in question and especially with recurrent, complex fi stulae .
50
M.A. Valente
2.6 Conclusion
Successful diagnosis and subsequent treatment of anorectal diseases must fi rst
begin with a thorough history and well-performed physical examination. The anorectal history is of paramount importance and will guide further testing and examination. The examination portion is one that gives much anxiety and fear to patients.
The examination must be performed with comfort and modesty in mind. A wellperformed anorectal examination will give much information and will lead to an
accurate diagnosis and allow proper treatment to commence. Endoscopic evaluation
allows for a more detailed evaluation of the anorectum and distal colon with the
advantage of timely diagnosis and sometimes treatment in the offi ce setting. Anal
manometry and endoanal ultrasound may be performed in select patients in which
certain anorectal or pelvic fl oor diseases may exist.
References
1. Slezak FA. Indirect anoscopy. Perspect Colon Rectal Surg. 1992;5:235–42.
2. Ashburn J, Church J. Open sesame revisited. Am J Gastroenterol. 2013;108(1):143.
10.1038/ajg.2012.382 .
doi:
3. Farmer KC, Church JM. Open sesame: tips for traversing the anal canal. Dis Colon Rectum.
1992;35(11):1092–3.
4. Nivatvongs S, Fryd DS. How far does the proctosigmoidoscope reach? A prospective study of
1000 patients. N Engl J Med. 1980;303(7):380–2.
5. Lehman GA, Buchner DM, Lappas JC. Anatomical extent of fi beroptic sigmoidoscopy.
Gastroenterology. 1983;84(4):803–8.
6. Hanson JM, Atkin WS, Cunliffe WJ, et al. Rectal retrofl exion: an essential part of lower
gastrointestinal endoscopic examination. Dis Colon Rectum. 2001;44(11):1706–8.
7. Christensen AF, Nyhuus B, Nielsen MB, Christensen H. Three-dimensional anal endosonogra-
phy may improve diagnostic confi dence of detecting damage to the anal sphincter complex. Br
J Radiol. 2005;78(928):308–11.
8. Law PJ, Talbot RW, Bartram CI, et al. Anal endosonography in the evaluation of perianal sepsis
and fi stula in ano. Br J Surg. 1989;76(7):752–5.
9. Deen KI, Williams JG, Hutchinson R, et al. Fistulas in ano: endoanal ultrasonographic assess-
ment assists decision making for surgery. Gut. 1994;35(3):391–4.
CT and MRI of the Pelvis for Anorectal Disease
Myra K. Feldman , Zachary E. Friess , and Joseph C. Veniero
Although direct visualization with or without the aid of sigmoidoscopy and
colonoscopy is the primary screening and diagnostic tool for identifying anorectal
disease, cross-sectional imaging has become indispensable in the workup of many
pathologic entities [ 1 ]. From cancer staging and mass characterization to fi stula
identifi cation and procedural guidance, computed tomography (CT) and magnetic
resonance imaging (MRI) are critical in the detection and treatment of anorectal
pathology and have largely supplanted fl uoroscopic barium studies as the radiologic
contribution in the primary workup of most of these patients [ 1 ].
The main advantage of cross-sectional imaging over direct visualization and
fl uoroscopic barium studies is the ability to see beyond the mucosal surface [ 1 , 2 ].
A fl exible sigmoidoscopy can show you that there is a mass effacing or indenting
the rectal wall, but the useable information ends there. Cross-sectional imaging can
show the anatomic structure that mass originates from and identify other structures
involved in the disease process. It can suggest a diagnosis based on benign or malignant imaging characteristics as well as indicate the severity of disease to help guide
treatment [ 1 , 2 ].
3
3.1 Computed Tomography
CT is an imaging technique that creates images utilizing X-ray beams which are
generated and detected in a 360° scan performed around the patient. The data that is
acquired is put through a computed reconstruction algorithm to create a stack of
sequential axial images [ 3 , 4 ]. In addition, computer-generated, three-dimensional
reconstructions can be rendered in any plane to create the desired image.
M. K. Feldman , MD • Z. E. Friess , DO • J. C. Veniero , MD, PhD (*)
Imaging Institute, Section of Abdominal Imaging, Cleveland Clinic ,
9500 Euclid Ave, Radiology L-10 , Cleveland , OH 44195 , USA
feldmam2@ccf.org; zacharyefriess@gmail.com; venierj@ccf.org
e-mail:
© Springer International Publishing Switzerland 2016
M. Zutshi (ed.), Anorectal Disease, DOI 10.1007/978-3-319-23147-1_3
51
52
M.K. Feldman et al.
Table 3.1 Sample CT
densities of common tissues
Air −1000
Fat −30 to −100
Simple fl uid −10 to 20
Water 0
Blood 30–50
Muscle 45
Cortical bone 200–600
Density (Hounsfi eld units)
CT images are graphical density maps created from the X-ray data. Everything
present on a CT image has a density value which is expressed in Hounsfi eld units
(Table 3.1 ). Water is considered to be a neutral reference density on CT and is
assigned a value of 0 Hounsfi eld units. The other reference is air which is assigned
a value of −1000 Hounsfi eld units. When comparing different regions of an image,
the brighter region is referred to as being hyperdense or having increased attenuation relative to the darker region. Conversely, darker parts of a CT image are considered hypodense or having decreased attenuation relative to a reference [ 3 , 4 ].
A limitation often encountered with CT imaging is that it is diffi cult to distinguish many biologic tissues since they have similar densities. The administration of
different contrast agents introduces other densities that better defi ne and distinguish
biologic tissues. For example, when evaluating the GI tract, one of two different
types of enteric contrast can be used depending on the type of pathology that is
suspected. When evaluating the bowel wall for enhancement, luminal narrowing,
and infl ammatory changes, a neutral density or “negative” enteric contrast is used.
This type of contrast distends the bowel lumen and, because it is less dense than the
adjacent bowel wall, allows evaluation of their mural enhancement patterns [ 5 ].
When evaluating for obstruction, leak, or differentiation of the bowel from adjacent
structures, a high density or “positive” enteric contrast is used. This type of contrast
makes the bowel stand out against less dense abdominal structures and helps to
assess transit through the bowel [ 6 ]. It is important to remember that positive con-
trast can linger in the bowel for several days and follow-up studies may be affected
by residual contrast [ 7 ].
Another type of contrast that can be used to differentiate tissues on CT scans is
intravenous contrast. Although many brands are available, they all contain iodine, a
relatively dense atom with properties that result in increased absorption of X-rays.
IV contrast is typically administered through a peripheral vein and increases the
density wherever it accumulates. Initially it is located in the intravascular space,
allowing studies such as arteriograms to be acquired if the scan is performed at the
appropriate time after contrast is given. As contrast passes through capillary beds,
some of it leaks out of the vessels into most of the interstitial tissues of the body,
increasing their density to different degrees, helping to differentiate them on the
scan. All living tissue enhances, a fact that helps in the characterization of nonliving
3 CT and MRI of the Pelvis for Anorectal Disease
53
tissue such as cysts, abscesses, and necrosis. Intravenous contrast is eventually
eliminated through the kidneys.
CT is commonly used in imaging anorectal disease because of its widespread
availability and relatively lower cost when compared to MRI. However, CT uses
ionizing radiation like any other X-ray technique; a fact that needs to be considered
when deciding among the different imaging modalities. CT is often used as a screening exam for a multitude of abdominal and pelvic pathologies, both in the hospital
and in the outpatient setting. As a result of this, anorectal disease is often fi rst identifi ed on a CT scan as an incidental fi nding [ 8 ].
Scanning methods have been developed to screen the colon for polyps and cancer. Colonographic screening can identify previously unknown rectal masses. CT
colonography is a low-dose screening technique which provides anatomic detail of
the colon and rectum without the inherent invasiveness of a standard colonoscopy.
Although no colonoscope is used in the procedure, adequate preparation and colonic
distention is crucial to achieve a diagnostic result [ 9 ]. A full discussion of this tech-
nique is beyond the scope of this chapter.
3.2 Magnetic Resonance Imaging
MRI is an imaging technique that generates images based on the behavior of the
hydrogen atoms in water molecules. The majority of the human body is composed
of water molecules. When placed in the strong magnetic fi eld of the MR machine,
the hydrogen nuclei tend to align either with the fi eld, in a lower-energy state, or
against the fi eld, in a higher-energy state. At equilibrium, there are more nuclei in
the lower-energy state. When radio-frequency (RF) energy is applied at specifi c frequencies, that energy is absorbed by the low-energy nuclei, raising them into the
higher-energy state. As they relax back to the lower-energy state, they release RF
energy which is recorded by the system as the MRI signal. This signal is detected
and reconstructed into images through a process called spatial localization. The
result is an image that shows the amount of signal returning from each region of the
tissue represented on the image with brightness that is proportional to the amount of
signal returning [ 10 ].
MRI images can be acquired in multiple planes. A typical MRI examination consists of a combination of multiple views of the area of interest, each obtained to
emphasize different components of the tissue and/or obtained in different orientations. These views are referred to as imaging sequences. They can be tailored to
emphasize or eliminate fl uid or fat. In addition, both intraluminal and intravascular
contrast can be utilized to further enhance tissue differences in similar ways to those
previously described for CT [ 11 , 12 ]. It is important to use proper terminology when
describing MRI fi ndings. In general, things that appear brighter or more white are
said to have “increased signal intensity” or be “ hyperintense,” and things that appear
darker or more black are said to have “decreased signal intensity” or be “hypointense.” Two common MRI sequences are referred to as “T1” and “T2” weighted.
Only a few things appear hyperintense on T1-weighted sequences including fat,
54
M.K. Feldman et al.
Table 3.2 General intensity
on MR images
Air
a
Fat
Simple fl uid
Hemorrhagic fl uid
Muscle
Cortical bone
a
Fat appears hypointense on all fat-saturated sequences
T1 appearance
Black Black
Hyperintense Hyperintense
Hypointense Hyperintense
Hyperintense Hypointense
Hypointense Hypointense
Black Black
T2 appearance
some hemorrhage, melanin, protein, and gadolinium-based contrast agents. Simple
fl uid is typically hypointense on T1-weighted sequences. T2-weighed images are
“fl uid sensitive,” and many water-containing substances typically appear hyperintense including fl uid, fat, edema, and tumor (Table 3.2 ). Air has no signal and is
black on both sequences. If desired, the signal from fat can be suppressed or eliminated from T2-weighted sequences in order to differentiate between fl uid and fat.
When trying to decide if a sequence is T1 or T2 weighted, look for structures that
usually contain fl uid such as the bladder or spinal canal; if they are hyperintense, the
sequence is likely T2 weighted. Intravenous contrast, which is gadolinium based,
can be added to T1-weighted sequences to image vessels or look for abnormal
enhancement. Like CT, tissues with leakier capillary beds, such as those in areas of
infl ammation and tumor, will tend to accumulate more contrast and enhance more
avidly than normal tissue [ 13 ].
Standard rectal protocol MRI images are oriented in three planes to the rectum
(axial, sagittal, and coronal). In the setting of cancer, the most important images for
staging are high-resolution T2-weighted images that are oriented perpendicular to
the long axis of the rectum. Air within the rectal lumen appears black on
MRI. Certain lesions, especially small or polypoid masses, can be diffi cult to identify in the collapsed rectum. In order to accentuate the rectal wall or rectal lesions
extending into the lumen, an aqueous gel can be used as a luminal contrast agent to
fi ll the cavity. This appears very hyperintense on T2-weighted images and is
hypointense on T1-weighted images [ 13 ]. In other cases, especially those with ses-
sile lesions, overdistention of the rectum by the gel can cause underestimation of
tumor size and involvement. Advanced techniques such as diffusion-weighted
imaging (DWI) may be used for identifi cation of subtle lesions and pelvic lymphadenopathy [ 13 ].
Some of the advantages of MR imaging for evaluating anorectal disease include
superior resolution, increased anatomic detail, and imaging without ionizing radiation. Often, MRI can identify disease characteristics that suggest a specifi c pathology or pathologic subtype. Although there are many advantages to MRI,
disadvantages include higher cost, longer exam times, and technical limitations that
can result in imaging artifacts. MR is not considered safe in patients with certain
implanted mechanical devices. Fortunately, developing technical innovations and
improvements in methodologies continue to reduce these disadvantages.
3 CT and MRI of the Pelvis for Anorectal Disease
3.3 Imaging Anatomy
The anal canal is the channel extending from the perineum, at the anal verge, cranially to the anorectal junction, where the rectal ampulla narrows at the puborectalis
sling. The dentate line, an important morphologic landmark, can’t be seen on MRI
but is located in the upper anus and is the level of transition between the rectal
mucosa of the upper anus and the squamous epithelium of the lower anus [ 14 ]. The
internal anal sphincter (IAS) which is formed by the circular muscle layer of the
rectal wall appears hypointense on T2-weighted MRI. The muscular external anal
sphincter (EAS), the inferiormost extension of the levator ani, also appears hypointense on T2-weighted MRI (Fig. 3.1a ).
55
Fig. 3.1 Normal rectal anatomy on T2-weighted MR images. Coronal ( a ) and axial ( b ) images
through the lower rectum, axial image through the mid-rectum ( c ) and sagittal near-midline image
( d ) including the upper rectum. The sphincter complex is seen in ( a ) including the levator ani
( white arrow ) which is contiguous with the external anal sphincter ( white star ) that surrounds the
internal anal sphincter ( white circle ). The layers of the rectal wall are seen in coronal ( a ) and axial
( b ) including the outer T2 hyperintense mesorectal fat ( black star ), T2 hypointense muscularis
propria ( dashed black arrow ), T2 intermediate to hyperintense submucosa ( dashed white arrow ),
and thin T2 hypointense mucosa ( black arrow ). The mesorectal fascia ( black arrow heads ) is
shown in the mid-rectum ( c ). The thin hypointense band representing the peritoneal refl ection
( white arrowheads ) is seen on the sagittal image ( d )