AJR 2000; 174:661-666
© American Roentgen Ray Society
Dynamic MR Imaging of the Pelvic Floor in Asymptomatic Subjects
Vicky Goh1,
Steve Halligan1,
Glenda Kaplan1,
Jeremiah C. Healy2 and
Clive I. Bartram1
1
Intestinal Imaging Centre, Level 4V, St. Mark's Hospital, Northwick Park,
Watford Rd., Harrow, London, HA1 3UJ, United Kingdom.
2
Department of Radiology, Chelsea & Westminster Hospital, Fulham Rd.,
London, SW10 9NH, United Kingdom.
Received May 24, 1999;
accepted after revision August 11, 1999.
Supported by Lister Bestcare, Selby, United Kingdom.
Address correspondence to S. Halligan.
Abstract
OBJECTIVE. Dynamic MR imaging may be used as an alternative to
dynamic cystoproctography for the evaluation of pelvic floor prolapse and
configuration. MR criteria for normality are derived from proctographic
studies because no large MR study of asymptomatic individuals has been
performed. Our study aimed to define the normal range of dynamic pelvic MR
appearances in a large group of asymptomatic individuals.
SUBJECTS AND METHODS. Fifty healthy adult volunteers (25 men and 25
women; age range, 20-66 years; mean age, 34 years) were prospectively
recruited and examined using dynamic MR imaging. All subjects were interviewed
and established as healthy using a validated questionnaire. Axial, coronal,
and sagittal MR imaging was performed at rest and during maximum pelvic strain
using a static 1.0-T unit and a fast-field-echo sequence, providing 10 slices
in 31 sec. Standardized measurements of pelvic configuration were taken, and
rest and strain imaging were compared to determine the range of normal
appearances.
RESULTS. Three women developed a cystocele during maximum pelvic
strain, two of whom also showed grade 1 uterocervical prolapse, which was also
seen in another woman. Three men showed posterior pelvic floor descent in
excess of 3 cm during straining. No rectocele, enterocele, rectal prolapse, or
perineal hernia was seen in any subject.
CONCLUSION. The normal range of pelvic organ descent in asymptomatic
subjects seen on dynamic MR imaging included cystocele, uterocervical
prolapse, and excessive anorectal junction descent. Although we encountered
pelvic prolapse in seven volunteers, it was infrequent and low grade,
suggesting that criteria for abnormality derived from proctography are
generally applicable to MR imaging.
Introduction
Pelvic floor weakness and consequent organ prolapse may result in a variety
of symptoms, including pain, urinary or fecal incontinence, and constipation.
The pelvic floor has traditionally been divided into three
compartments—anterior, middle, and posterior—each attracting its
own specialist interest from urologists, gynecologists, and proctologists,
respectively [1]. Clinical
examination either underestimates or inaccurately diagnoses the site of
prolapse in a significant proportion of patients, and preoperative imaging has
assumed a prominent role because of this
[2]. Furthermore, it is now
apparent that pelvic floor weakness is usually generalized, so that the
various pelvic floor compartments are best imaged simultaneously
[3]. Conventionally, this has
been achieved using evacuation proctography modified by additional
opacification of the bladder and small bowel, an examination termed dynamic
cystoproctography
[4,5,6].
However, cystoproctography is a relatively invasive procedure, involves
ionizing radiation, and images only the lumen of the organs opacified. In an
attempt to overcome these limitations, dynamic pelvic MR imaging has been
introduced and is superseding fluoroscopic methods in some centers
[7,8,9].
An MR diagnosis of abnormality has been based on concordance with established
proctographic findings. However, proctographic studies of healthy subjects
have unexpectedly revealed findings often assumed to be abnormal
[10,
11]. To our knowledge, no
corresponding study has been performed for dynamic pelvic MR imaging. The aim
of this prospective study was to define the normal range of dynamic pelvic MR
appearances in a large group of healthy volunteers.
Subjects and Methods
Subjects
Our local ethics committee approved the study. Fifty adult subjects, 25 men
and 25 women, who were 20-66 years old (mean, 34 years), were recruited
prospectively after giving informed written consent. Seven women were parous
(range, one to three; median parity, two). Twenty-six subjects were volunteers
recruited from hospital personnel; the remaining 24 were recruited from
patients in the MR unit for musculoskeletal examinations. All subjects were
interviewed before examination and completed a questionnaire that detailed
bowel frequency; the need for laxatives; use of digital maneuvers to assist
evacuation; incontinence to urine, gas, or feces; pelvic pain; organ prolapse;
and any need to consult a physician regarding these symptoms. Any abnormal
response excluded the subject from the study. Any patient with a history of
pelvic surgery was also excluded.
MR Imaging
MR imaging was performed with a 1.0-T static unit (Gyroscan NT 1.0;
Philips, Hammersmith, United Kingdom). No preparation of the subject was
required. The subject lay supine on a waterproof pad placed on the MR table.
Maximum pelvic straining was practiced with the patient before examination;
patients were encouraged to bear down as if emptying their bowels. Images were
obtained in sagittal, axial, and coronal orientations using the body coil,
first with the patient at rest and then again during maximum pelvic strain. A
T1-weighted fast-field-echo sequence was used with the following parameters:
flip angle, 55°; TR/TE, 79/240 msec; field of view, 34 cm; slice
thickness, 8 mm; interslice gap, 2 mm; matrix size, 256 x 256; and four
excitations. This sequence gave 10 slices in 31 sec. The examinations were
downloaded onto a dedicated workstation (Easyvision; Philips) for radiologist
review.
Image Analysis
All examinations were analyzed by two radiologists in consensus, who
recorded standard measurements of pelvic floor anatomy and also noted the
presence or absence of any structural abnormality. The pubococcygeal line,
defined as the line that joined the inferior border of the pubic symphysis to
the last coccygeal joint [7,
11], was drawn onto the
midline sagittal resting image. The position of the bladder base, cervix, and
anorectal junction, defined as the junction of the rectal ampulla and anal
canal, were measured at 90° to the pubococcygeal line. The anorectal
angle, defined as the angle between the longitudinal axis of the anal canal
and the posterior rectal wall, and the levator plate angle, defined as the
angle between the levator plate and the pubococcygeal line, were measured
[7]. These values were then
compared with measurements taken from the corresponding sagittal straining
image, and the change on straining was calculated. A cystocele was diagnosed
if the bladder base descended below the inferior border of the symphysis pubis
[1,
5]. Uterocervical prolapse was
defined as cervical descent below the pubococcygeal line (grade 1), to the
introitus (grade 2), or to the exterior (grade 3). Anorectal junction descent
was defined as excessive if more than 2.5 cm below the pubococcygeal line at
rest or if more than 3 cm on maximum strain
[10,11,12].
The axial images were used to calculate the pelvic floor hiatal area and
perimeter at rest and during maximum strain
[7,
8], which were measured at the
level of the most inferior point of the symphysis pubis. All images at rest
and during maximum strain were also evaluated for the presence or absence of
any other structural abnormality as follows: enterocele was defined as small
bowel within the rectovaginal septum that reached or crossed the junction of
the upper one third and distal two thirds of the vagina
[13]. Rectocele was defined as
an anterior rectal wall bulge exceeding 2 cm
[10,
11]. Rectal prolapse and any
perineal hernia through the levator plate were also noted if present.
Statistical Analysis
Descriptive statistics were performed on all data at rest and during
maximum strain. The Student's t test was used to compare
parametrically distributed continuous data, and statistical significance was
assigned to a p value of less than 0.05. Calculations were performed
using Arcus Quickstat Biomedical 1.0 (Research Solutions, Cambridge, United
Kingdom).
Results
The examination was well tolerated by all volunteers. All MR images both at
rest and during maximum strain were considered technically adequate.
Mean values with standard deviations (SDs) for the bladder base, cervix,
and anorectal junction at rest, during maximum strain, and the descent on
straining are shown in Table 1.
The bladder base lay above the pubococcygeal line in all subjects at rest but
descended below this line during maximum strain in three women, two of whom
were parous (one with two and one with three vaginal deliveries), resulting in
a diagnosis of cystocele (Table
1, Figs.
1A,1B
and
2A,2B).
Although the mean bladder base was significantly higher in men than in women
at rest and during maximum strain, no significant difference was seen between
men and women with respect to the distance descended. No statistical
difference was noted between any bladder base value when parous and
nulliparous women were compared.
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TABLE 1 MR Imaging Measurements of Anterior, Middle, and Posterior Pelvic Floor
Descent in Asymptomatic Men and Women
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Fig. 1A. —Position of bladder base relative to pubococcygeal line. Ladder
plots show position of bladder base relative to pubococcygeal line at rest and
during maximum strain for 25 asymptomatic men (A) and 25 asymptomatic
women (B). Note that bladder base descends below this line in three
women, indicating cystocele.  = multiparous, • = nulliparous.
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Fig. 1B. —Position of bladder base relative to pubococcygeal line. Ladder
plots show position of bladder base relative to pubococcygeal line at rest and
during maximum strain for 25 asymptomatic men (A) and 25 asymptomatic
women (B). Note that bladder base descends below this line in three
women, indicating cystocele. = multiparous, • = nulliparous.
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Fig. 2B. —40-year-old asymptomatic multiparous woman. Sagittal T1-weighted
fast-field-echo MR image of pelvis at strain shows cystocele diagnosed because
bladder base descent is below pubococcygeal line (white arrow).
Cervix has also descended below pubococcygeal line (black arrow).
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The cervix descended below the pubococcygeal line in two (one nulliparous
and one with three vaginal deliveries) of the three women with cystocele, and
in a third (who did not have a cystocele), resulting in a diagnosis of grade 1
uterocervical prolapse (Figs. 3
and
4A,4B).
Grade 2 or 3 uterocervical prolapse was not seen in any woman. No significant
difference was seen when nulliparous and parous women were compared for any
uterocervical measurement.
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Fig. 3. —Ladder plot shows position of uterocervical junction relative to
pubococcygeal line at rest and during maximum strain for 25 asymptomatic
women. Note that uterocervical junction descends below this line in three
women, indicating prolapse. = multiparous, • = nulliparous.
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Fig. 4A. —50-year-old asymptomatic nulliparous woman. Sagittal T1-weighted
fast-field-echo MR image of pelvis at rest shows pubococcygeal line (black
line) and cervix (arrowhead). No apparent abnormality is
seen.
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Fig. 4B. —50-year-old asymptomatic nulliparous woman. Sagittal T1-weighted
fast-field-echo MR image of pelvis at strain shows grade 1 uterocervical
prolapse (white arrow) indicated by cervical descent below
pubococcygeal line (black line). Note also cystocele (black
arrow).
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The mean anorectal junction position lay on the pubococcygeal line in both
men and women (Table 1) and was
not more than 2.0 cm below this level in any subject at rest. However,
anorectal junction descent was more than 3.0 cm in three men during maximum
strain (Fig.
5A,5B).
No overall statistical difference was noted between men and women
(Table 1) or between parous and
nulliparous women.
Mean values at rest and during maximum strain for the anorectal angle, the
levator plate angle, the pelvic floor hiatal area, and the hiatal perimeter in
men and women are shown in Table
2. The anorectal angle during maximum pelvic strain was
significantly more acute in men than in women, but no significant difference
was seen either between sexes or between women of different parity for any
other measurement. No rectocele, enterocele, rectal prolapse, or perineal
hernia was seen in any subject when all images were examined for further
structural abnormality. An unsuspected ovarian dermoid cyst was seen in one
woman, and this diagnosis was subsequently confirmed surgically.
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TABLE 2 MR Imaging Measurements of Anorectal and Levator Plate Angles and Pelvic
Floor Hiatus Area and Perimeter in Asymptomatic Men and Women
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Discussion
The effects of pelvic floor weakness may not be localized to one organ or
compartment, and failure to identify all sites of prolapse may lead to
incomplete surgical repair and subsequent recurrence
[11]. Clinical examination
tends to underestimate the degree of prolapse or may miss it altogether; in
one study of 300 women, enteroceles were revealed on dynamic cystoproctography
in 111, of which 93 (84%) were missed clinically
[4]. Because of this, many
investigators take the opportunity to modify the standard proctographic
examination by either administering an oral barium suspension approximately
1-2 hr before the procedure [2]
or, alternatively, using a vaginal marker so that enteroceles can be diagnosed
by rectovaginal separation. The best choice is probably a barium paste because
a tampon may inhibit prolapse by splinting the vagina
[14]. Dynamic
cystoproctography is essentially evacuation proctography preceded by a
cystogram [5]; it extends the
examination into the anterior pelvic floor so that any cystocele can be
diagnosed.
Although evacuation proctography is rapid and easy to perform, the
modifications necessary to image other organs may be time-consuming, invasive,
and complex, and the musculature of the pelvic floor itself is not visualized.
Furthermore, examination involves irradiation, and many patients will be women
in their childbearing years, especially if the examination is being performed
to investigate constipation. In an attempt to overcome these limitations, MR
imaging has been applied to pelvic floor dynamics with promising results.
Initial reports were necessarily compromised by slow acquisition times
[15,
16], but advancing MR
technology has enabled multislice imaging during a single straining effort. A
study of constipated and anally incontinent women using this technique
revealed unsuspected visceral prolapse at multiple sites
[7]. However, these studies
have recruited limited numbers of control subjects. Previous proctographic
studies of asymptomatic control subjects have unexpectedly shown that many of
these individuals show findings previously considered abnormal
[10,
11]. MR criteria for
abnormality have been based on proctographic examinations but it is likely
that values will differ between the two techniques, primarily because the
seated position achieved during proctography is available only to the few
investigators with access to open-architecture magnets
[17,
18]. Supporting this, a study
of 10 women who underwent both proctography and dynamic MR imaging found good
correlation but poor agreement between corresponding measurements
[19]. Because of this, the
range of normal values for dynamic MR imaging needs to be defined. We have
attempted to do this in the largest dynamic MR study to date, to our
knowledge.
All subjects examined in this study satisfied vigorous and validated
criteria for health. Despite this, we encountered some findings con-sidered
abnormal. We showed significant differences between men and women with respect
to the position of the bladder base, as would be expected, but a cystocele
developed in three women during maximum pelvic strain, two of whom also
developed grade 1 uterocervical prolapse; the latter was also seen in one
additional woman. However, no large cystocele was seen, nor was any higher
grade of uterocervical prolapse. Similarly, three men showed significant
pelvic floor descent (
3 cm) when judged by proctographic criteria, but
this also was low grade. These findings are broadly in agreement with those
encountered in proctographic studies of asymptomatic individuals in which most
subjects were "normal," but there is some crossover with findings
considered abnormal [10,
11]. Supporting this, no
rectocele, enterocele, rectal prolapse, or perineal hernia was seen in any
subject when all images were reviewed. The range of normal values for the
anorectal and levator plate angle has also been defined. Although the
anorectal angle during maximum pelvic strain was significantly more acute in
men, this result is likely to have occurred because of multiple statistical
testing. These measurements were included for completeness but are rarely of
major clinical significance and do not influence management, as would
diagnosis of a prolapse site. The pelvic floor hiatus is a relatively new
measurement that is already clinically relevant and is increased in women with
pelvic organ prolapse [20]. We
were unable to find any significant difference between the sexes for this
measurement, nor were we able to show any significant difference between
parous and nulliparous women for any parameter, possibly because of the
relatively few parous women recruited. The vigorous questionnaire excluded
many older and parous women from the study, predominantly because of minor
anal incontinence, a symptom strongly associated with previous vaginal
delivery [21].
Dynamic pelvic MR imaging is evolving, and an optimal technique remains to
be defined. A major disadvantage of the technique may relate to the inability
to ensure an adequate straining effort. In an attempt to minimize this effect,
straining was practiced with the subject before the examination, and the
volunteer was placed on an absorbent pad to minimize the fear of leakage.
Despite this, some subjects raised their pelvic floor during straining,
evidenced by ano-rectal junction ascent, which suggests pelvic floor
contraction rather than relaxation. To eliminate this effect, some
investigators fill the rectum and bladder and encourage evacuation in the
magnet despite the supine position
[9]. This approach will provide
additional information relating to the rate and completeness of rectal
evacuation, important parameters when constipation is being investigated
[22]. A seated position is
optimal but only achievable in an open magnet
[17,
18].
If prolapse is shown, it is tempting to attribute symptoms to the prolapse,
but the prolapse may merely be secondary to an underlying functional disorder.
For example, constipated patients who strain excessively may develop
cystocele, enterocele, rectocele, rectal prolapse, and pelvic floor descent
[23], but surgery to correct
these will not treat the underlying disorder
[24]. The results of any
dynamic pelvic examination, be it cystoproctography or MR imaging, need to be
considered in the light of the patient's history and other tests so that
therapy is appropriate.
In summary, we examined 50 healthy volunteers using dynamic pelvic MR
imaging to define the range of normal appearances encountered during this
relatively new test. Although we encountered some pelvic floor prolapse, it
was infrequent and of low grade, suggesting the criteria for abnormality
derived from proctographic examinations are generally applicable to MR
imaging.
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Abstract
Introduction
