is a third-year Radiology Resident and, at the time this article
was an Assistant Professor of Radiology, Division of Abdominal
Imaging and the Associate Program Director of the Residency
Training Program Department of Radiology, University of Michigan
Health System, Ann Arbor, MI. † Dr. Adusumilli died March 3,
Magnetic resonance imaging (MRI) is ideally suited for the
evaluation of uterine disease because of its multiplanar imaging
capabilities, excellent tissue contrast, and lack of ionizing
radiation. This article addresses issues related to benign uterine
disease that are relevant to daily practice. We have expanded the
discussion beyond the description of typical MRI features of
adenomyosis and leiomyomas by presenting diagnostic challenges
related to these disease processes. Rarer entities (such as uterine
arterio-venous malformations and retained products of conception)
are introduced in this article because they have considerable
impact on patient management.
Practical issues related to MRI evaluation of
Classic MRI diagnostic criteria
Adenomyosis is characterized by the presence of ectopic
endometrial glands in the uterine myometrium with subsequent
hyperplasia of the myometrium.
The condition usually affects multiparous women who are older than
30 years of age,
with a prevalence of 8.8% to 31%.
Common presenting symptoms include dysmenorrhea, menorrhagia, and
abnormal uterine enlargement.
This clinical presentation poses a diagnostic dilemma, as symptoms
are similar in women with uterine leiomyomas. Differentiation from
leiomyoma is clinically important, as the treatment of adenomyosis
differs from that of fibroid disease. Adenomyosis is typically
treated with hysterectomy or, occasionally, with a
gonadotropin-releasing hormone (GnRH) analog that can decrease the
width of the junctional zone (JZ).
A leiomyoma, on the other hand, is potentially treated with uterine
artery embolization or uterine-sparing myomectomy.
MRI is ideal for differentiating these 2 entities because of its
excellent soft tissue contrast. MRI has a high sensitivity (78% to
88%) and specificity (67% to 93%) for the diagnosis of adenomyosis
because of its ability to delineate the zonal anatomy of the
Hricak et al
described 3 distinct layers of the uterus that can be identified on
T2-weighted (T2W) imaging. The JZ, which is the innermost layer of
the myometrium, is of low signal intensity and is located between
the endometrium (high signal intensity) and the outer myometrium
(intermediate signal intensity) (Figure 1).
The JZ is normally <8 mm in maximal thickness.
Adenomyosis can be diffuse or focal and is often associated with
enlargement of the uterus, which shows a smooth external contour.
Diffuse adenomyosis is diagnosed by the presence of a diffusely
thickened JZ that is homogeneously low in signal intensity on T2W
and isointense to myometrium on T1-weighted (T1W) imaging.
The low T2W signal intensity is thought to be secondary to smooth
muscle hyperplasia that is incited by the ectopic endometrial
The diagnosis can be made with greater certainty if the JZ exceeds
11 mm in thickness (Figure 2) and is equivocal when the JZ measures
between 8 and 11 mm.
One must be careful in distinguishing between adenomyosis and
normal menstrual changes. These changes are most pronounced early
in the menstrual cycle (days 1 and 2) and may cause significant
thickening of the JZ to >12 mm, thus causing a false-positive
diagnosis of adenomyosis.
Characteristic imaging findings that increase diagnostic confidence
and improve specificity for the diagnosis of adenomyosis include
high signal intensity foci in the JZ on T1W and T2W imaging that
represent ectopic endometrial tissue, endometrial cysts, or small
foci of hemorrhage (Figure 2).
Uterine mass: Adenomyoma versus leiomyoma
Focal adenomyosis classically appears as a low-signal-intensity
mass in the myometrium with ill-defined borders on T2W imaging
and isointensity to the surrounding myometrium on T1W imaging.
These ill-defined masses commonly contain small foci of high T1W
and/or T2W signal intensity and do not exhibit a pseudocapsule.
Diagnostic problems arise when one must differentiate focal
adenomyosis from a leiomyoma. MRI is quite accurate in diagnosing a
leiomyoma with a sensitivity of 88% to 93% and a specificity of 66%
and in differentiating leiomyoma from focal adenomyosis. Togashi et
correctly diagnosed 92 of 93 cases as either leiomyoma or
adenomyosis in women with an abnormally enlarged uterus. A typical
feature of an adenomyoma is a poorly marginated mass with
ill-defined borders that causes relatively little mass effect
(Figure 3). On the contrary, a leiomyoma has a well-defined border
and exerts a greater degree of myometrial mass effect and
distortion (Figure 4).
Leiomyomas usually compress the adjacent myometrium, creating the
appearance of a pseudocapsule, which is not seen with adenomyosis,
as the ectopic endometrial glands interdigitate with the normal
smooth muscle of the myometrium.
Mark et al
also found that leiomyomas were typically round in shape, whereas
adenomyomas were most often oval with their long axis paralleling
the uterus. Another differentiating characteristic is the presence
of adjacent dilated vessels at the interface with the myometrium,
which is commonly associated with leiomyomas and is not seen with
adenomyosis. Mark et al
found that 42% of leiomyomas had such dilated vessels, whereas
Togashi et al
found no cases of adenomyosis with dilated vessels. An additional
imaging pitfall is the incorrect diagnosis of a focal myometrial
contraction as either an adenomyoma or a leiomyoma. A uterine
contraction can appear as a myometrial mass that has low signal
intensity on T2W imaging and that bulges into the endometrial
cavity. The low signal intensity on T2W imaging is secondary to
decreased water content in the area of contracted myometrium.
The distinction can be easily made, as a myometrial contraction is,
by definition, a transient phenomenon that should resolve on
Practical issues related to MRI evaluation of uterine
Uterine leiomyoma: Classic MRI features
Uterine fibroids are composed of a combination of smooth muscle
cells and fibrous connective tissue and can be found in
approximately 20% to 30% of reproductive age females.
As these masses enlarge, they commonly outgrow their blood supply
and undergo varying degrees of necrosis, which accounts for their
variable signal intensities on MRI. Typically, leiomyomas are low
in signal intensity relative to their surrounding myometrium on T2W
imaging (Figure 4) and are isointense to myometrium on T1W imaging.
These signal characteristics are related to the most common form of
degeneration (60%), which is hyalinization throughout the
Weinreb et al
defined diagnostic criteria for a leiomyoma to include a uterine
mass that is predominantly hypointense compared to myometrium on
T2W imaging and predominantly hypointense on T1W imaging.
Solid pelvic mass: Pedunculated uterine fibroid versus
Leiomyomas can be classified by their location in the uterus as
submucosal, intramural, or subserosal. In some instances, it can be
difficult to differentiate a large exophytic, subserosal leiomyoma
from a solid adnexal mass such as an ovarian neo- plasm.
Differentiation is clinically very important because of differences
in treatment and prognostic implications. MRI in combination with
the patient's clinical findings can be invaluable in making this
distinction and can avoid unnecessary laparoscopy and/or
exploratory surgery. Location is an important distinguishing
characteristic. If the mass can be definitively separated from the
ovaries or is contiguous with the round ligament, than an ovarian
etiology is unlikely.
A well-described MRI feature that is helpful in the evaluation
of large pelvic masses has been referred to as the "bridging
vascular sign," which consists of vessels and/or signal voids that
extend from the uterus to supply a pelvic mass.
The identification of the bridging vascular sign increases the
diagnostic confidence that a large pelvic mass is a uterine
leiomyoma. The bridging vessels can be identified as enhancing
tubular structures on contrast-enhanced T1W imaging or as flow
voids on a T2W fast spin-echo sequence (Figure 5). In a study by
Kim et al,
the bridging vascular sign was present in 20 of 26 exophytic
leiomyomas and was absent in all other adnexal masses, resulting in
a diagnostic accuracy of 80%.
Uterine leiomyoma: Degeneration versus malignant
Another diagnostic dilemma encountered on MRI is the
differentiation of a degenerating fibroid from malignant
transformation of a fibroid into a leiomyosarcoma. Sarcomatous
transformation of a pre-existing leiomyoma is thought to be rare.
It is believed that most leiomyosarcomas actually arise
independently from the myometrial smooth muscle cells. The clinical
significance of this differentiation is important, in light of the
growing use of uterine-sparing leiomyoma treatment such as
myomectomy, GnRH analog, and uterine artery embolization (UAE).
Clinically, malignancy can be suspected if there is a rapidly
enlarging pelvic mass, if a mass does not involute after menopause,
or if a mass does not decrease in size following GnRH therapy.
Unfortunately, the diagnosis of sarcomatous degeneration is most
commonly made as an incidental pathologic diagnosis in 0.5% of
Suspicious imaging findings of malignant degeneration on pelvic MRI
include ascites, lymph node enlargement, and peritoneal seeding.
However, earlier detection is obviously desired.
Initially, investigators thought that rapid growth of a
leiomyoma over time would be an accurate predictor of sarcomatous
degeneration. However, in a retrospective review of 580
leiomyosarcomas, <3% of sarcomas were associated with a rapidly
enlarging uterus. Additionally, in the same study, only1 of 371
women who underwent resection for a rapidly growing leiomyoma was
found to have a leiomyosarcoma.
Therefore, rapid growth is now believed to be an unreliable
indicator of malignant transformation. The same investigators
proposed that a better indicator for predicting malignant
transformation was an irregular margin of the uterine leiomyoma on
Pattani et al
concluded that malignant potential should be considered whenever a
degenerating fibroid has an ill-defined contour. Utilizing an
ill-defined margin as their criteria for diagnosing malignant
potential, Schwartz et al
achieved a 100% preoperative accuracy in diagnosing 4 of 4
leiomyosarcomas; they also did not incorrectly label benign
fibroids as malignant. However, the specificity of this finding has
not been established.
A small study of 12 patients with known malignant tumors
performed by Tanaka et al
found that high signal intensity on T2W imaging in >50% of a
uterine mass was predictive of malignancy. On T2W imaging, high
signal intensity due to normal cellular degeneration does not
typically involve >50% of a fibroid, although overlap can be
seen between both entities (Figure 6). Similarly, the presence of
small areas of nonenhancing high signal intensity on T1W imaging
was also found to be predictive.
When a mass is predominantly hyperintense on T2W imaging, one must
be careful not to dismiss the lesion as a simple leiomyoma, as this
signal alteration can be due to malignancy or sarcomatous
Role of MRI in the evaluation of women undergoing uterine
Uterine artery embolization is a safe and effective option for
the treatment of symptomatic leiomyomas. Common symptoms that often
require treatment include pain, bulk symptoms, and abnormal uterine
bleeding. Uterine artery embolization is an option when uterine
preservation is desired or when the patient is unable to tolerate
an open surgical procedure.
Appropriate patient selection is important when planning UAE to
avoid serious complications. Absolute contraindications include
ongoing pregnancy, active infection, and uterine or adnexal
malignancy. Relative contraindications based on imaging studies are
continually evolving but have traditionally included pedunculated
fibroids (subserosal or submucosal) that are thought to be at
increased risk for detachment from the uterus postembolization.
Detached subserosal fibroids can cause intraperitoneal adhesions
and chronic peritonitis, whereas detached submucosal leiomyomas can
undergo transcervical expulsion with prolonged recovery and
sometimes require additional surgery (Figure 7).
Previously, leiomyomas >10 cm in dimension were considered a
relative contraindication for UAE. For instance, Pelage et al
stated that UAE should not be performed for fibroids >10 cm in
diameter because of predisposition to complications such as
infection and uterine injury. Additionally, large leiomyomas are
more likely to have a collateral blood supply that is separate from
the uterine arteries, making UAE less effective with a smaller
volume reduction postembolization.
However, subsquent studies performed by Katsumori et al
reported no increased risk to patients who underwent UAE for
fibroids on the basis of tumor size. They found no difference in
postprocedural pain, recovery, or leiomyoma size reduction at 4
months and 1 year postprocedure.
MRI is an excellent modality for the evaluation of potential UAE
patients. It is superior to ultrasound based on its larger field of
view, better spatial resolution, and ability to detect concomitant
MRI is accurate in localizing a leiomyoma as subserosal,
intramural, or submucosal and in the identification of a thin
stalk. Pedunculated, subserosal fibroids have previously been
defined as having a stalk that is <50% the diameter of the
This is considered a contraindication to UAE. However, Katsumori et
studied 196 women, 12 of whom had pedunculated subserosal fibroids
with a mean stalk diameter of 3.1 cm (2.0 to 5.5 cm). Patients
whose fibroid had a stalk <2 cm were excluded from undergoing
UAE. The final study group had no complications, such as detachment
of the leiomyoma from the uterus or other permanent adverse
Patients were followed at both 4 months and 1 year
postembolization, and the leiomyoma stalk diameter was found to be
stable, whereas the leiomyoma had decreased in size. Therefore, it
was concluded that UAE was safe for pedunculated, subserosal
leiomyomas with stalk diameters >2 cm.
Another imaging feature that is important to assess on pre-UAE
MRI is the presence of enhancement within a leiomyoma. The degree
of enhancement correlates with the vascularity of the mass and
suggests a better response after embolization (Figure 8).
Nikolaidis et al
studied 94 women and found that 20% of women had nonenhancing
leiomyomas on their preprocedural MRI that were considered
nonviable for UAE. However, in two thirds of the women, the
nonviable leiomyoma was not the dominant fibroid. In only 6% of the
women was the nonviable, avascular leiomyoma also the dominant
fibroid, in which case UAE would not be expected to reduce blood
flow or result in a favorable response.
Some studies have shown that cellular leiomyomas, with high
signal intensity on T2W imaging, exhibit a greater percentage of
volume reduction postembolization. In contrast, degenerating
leiomyomas with high signal intensity on T1W imaging are associated
with a smaller volume reduction postembolization.
Location can also be predictive of successful postembolization
volume reduction. Smaller leiomyomas in a submucosal location are
more likely to have the greatest degree of volume reduction.
Studies have shown that a submucosal location is strongly
predictive of outcome with a 30% to 40% greater volume reduction
following embolization compared with either intramural or
Role of MRI in diagnosis of postpartum and postabortion
Uterine arteriovenous malformation
A uterine arteriovenous malformation (AVM), which is classified
as either a congenital or an acquired lesion, is composed of a
tangle of variously sized arteries and veins without histologic
evidence of an intervening capillary bed.
Uterine AVMs are most commonly seen in women who are 20- to
40-years-old and can result in symptoms of menorrhagia or
Typically, an AVM is acquired and secondary to trauma related to
spontaneous or therapeutic abortion, endometrial carcinoma, or
gestational trophoblastic disease.
There is also a greater potential for the formation of abnormal
arteriovenous connections following hysterectomy and cesarean
Uterine AVMs have nonspecific and often confusing imaging
features on gray-scale sonography, including multiple tubular
anechoic spaces within the myometrium.
Color Doppler is valuable and can show marked hypervascularity of
the mass (Figure 9) as well as reversal of flow.
Spectral Doppler reveals low-resistance high-velocity arterial flow
(resistive index of 0.25 to 0.55), and low pulsatility (pulsatility
index 0.40 to 0.59). Sonography may detect a vascular abnormality
in the myometrium and offer a potential diagnosis of AVM. However,
arteriovenous shunting, in itself, is a nonspecific finding that
can occur in other conditions such as pregnancies, missed
abortions, trophoblastic disease, and ectopic pregnancies. MRI is
able to confirm the diagnosis, better define the extent of the
and offer a more accurate characterization in the setting of
equivocal sonographic findings.
Typical MRI findings of an AVM include a focal uterine mass that
consists of a group of distinct, serpiginous flow voids on T2W
imaging with an ill-defined border that disrupts the JZ and is
associated with prominent parametrial vessels (Figure 9).
Dynamic contrast-enhanced T1W gradient-echo imaging will also
reveal opacification of the vessels within the uterus and adjacent
parametrium as well as depict an early draining vein. An AVM has a
propensity to erode into the endometrial cavity if it reaches a
large enough size (Figure 9).
The diagnosis of uterine AVM on MRI has important therapeutic
implications because it 1) may prevent dilatation and curettage
that could otherwise result in catastrophic bleeding and 2) may
lead to catheter angiography that can both diagnose and treat the
lesion with embolization, thereby avoiding more invasive treatment
such as hysterectomy.
Angiography reveals abnormal stasis of contrast within a tangle of
vessels that are supplied by enlarged feeding arteries and have
early venous drainage during the arterial phase of image
Management of a patient with a symptomatic AVM also includes
control of vaginal bleeding with uterine packing, maintenance of
hemodynamic stability with blood transfusions, and administration
of medications (such as 15-methyl prostaglandin F2 alpha and
parenteral estrogen and progestin).
Stable patients without heavy bleeding can be treated
conservatively and followed expectantly.
Retained products of conception (placental polyps)
Another vascular-appearing uterine abnormality that is
well-suited to MRI evaluation is retained products of conception
(RPOC)-the retention of placental tissue within the uterine cavity
after delivery or abortion. The placental fragment may eventually
form a placental polyp that can be retained long after the initial
pregnancy. Retained products of conception have been reported to
complicate approximately 1% of all pregnancies
and commonly present with vaginal bleeding either late in the
postpartum period or even months to years after the pregnancy. The
diagnosis may be evident when considering the patient's clinical
presentation and laboratory data (β-HCG).
Patients with postpartum hemorrhage are initially evaluated with
ultrasound, which reveals echogenic material within the endometrial
However, occasionally, RPOC may appear as an intramural mass
(consisting of serpiginous vessels) that shows vascularity on color
Doppler and thus mimics a uterine AVM (Figure 10). It is important
to make the distinction between RPOC and AVM, as RPOC are routinely
treated with dilation and curettage that could result in hemorrhage
in the presence of a uterine AVM.
In cases of confusing sonographic findings, MRI is the ideal next
step for identifying imaging features that would suggest an AVM and
also for delineating the exact location of the lesion.
MRI findings of a classic RPOC include a soft tissue mass within
the endometrial cavity that shows heterogeneous signal intensity on
T1W and T2W imaging and avid enhancement on gadolinium-enhanced T1W
images (Figure 10). The mass usually disrupts the JZ and causes
some degree of myometrial thinning. While the lesion can show avid
enhancement (Figure 11), it is less likely to be associated with an
early draining vein and an increased number and size of parametrial
However, there has been a case report of an RPOC showing
serpiginous signal voids on T1W imaging and irregular enhancement
in the uterine wall, thereby masquerading as a uterine AVM.
No enhancing soft tissue mass was identified in the endometrial
cavity to confidently diagnose retained placental tissue. Although
rare, this case illustrates the overlap of uterine AVM and the
atypical form of aged RPOC that presents as an intramural mass with
prominent arteriovenous fistulas secondary to necrosis of chorionic
villi in the placenta.
Gestational trophoblastic disease (GTD) or placental site
trophoblastic site disease (PSTD) can also have very similar MRI
features to that of RPOC. Again, this distinction is extremely
important because of differences in treatment, with trophoblastic
disease requiring chemotherapy. Prior studies have suggested that
RPOC can be distinguished from GTD based on the lack of enhancement
of RPOC and brisk enhancement of GTD.
However, a small series of RPOC described by Noonan et al
found this sign to be unreliable, as RPOC was shown to enhance in
every case. Therefore, the differentiation may not always be made
solely based on imaging findings and requires correlation with
serial β-HCG levels. While the β-HCG level remains elevated to 1000
mIU/mL or more in the setting of GTD, it generally decreases to
zero or near zero with RPOC and should not be elevated with uterine
MRI is an invaluable problem-solving tool when evaluating women
with benign uterine conditions. The use of T2W and
gadolinium-enhanced T1W imaging is particularly important in
accurately localizing and characterizing uterine masses. An
awareness of important imaging principles related to specific
uterine diseases, as discussed in this article, should help the
practicing radiologist solve these radiologic dilemmas.
Dr. Livermore and the editors of this journal would like to
thank Hero K. Hussain, MD (Assistant Professor of Radiology,
Director of Clinical MR Service, and Chief, Body MRI, Department of
Radiology, University of Michigan Health Service, Ann Arbor, MI)
for her assistance in reviewing this article following Dr.