Uterine leiomyoma, also known as uterine fibroid, is the most common uterine neoplasm, affecting almost 70-80% of women.
They are benign tumors of smooth muscle origin, with varying amounts of fibrous connective tissue.
Fibroids usually arise in the myometrium but may occasionally be found in the cervix.
Most women with fibroids are asymptomatic.
However, 20–50% become symptomatic, with symptoms including abnormal uterine bleeding, pelvic pain, bowel or urinary obstruction.
Due to their hormonal sensitivity, responding to both estrogen and progesterone, they usually decrease in size in menopause and may rapidly grow and bleed during pregnancy.
Torsion or infarction can cause an acute abdomen.
MRI is the best imaging modality for characterizing uterine fibroids and identifying their exact anatomical location, though initial identification is usually by ultrasound.
MRI, due to its better soft tissue contrast, larger field-of-view and multiplanar imaging capabilities, assists clinicians in treatment mapping (exact location, number and size) and in differential diagnostic issues.
Gadolinium can be used to determine vascularization pattern when assessing the suitability of a fibroid for uterine artery embolization (UAE).
The FIGO classification allows a uniform classification of fibroids based on their location and subdivides fibroids into submucosal (FIGO types 0–2), intramural and subserosal (FIGO types 3–7), hybrid types and other locations (cervical or parasitic, FIGO type 8).
Accurately classifying uterine fibroids allows clinicians to select the best treatment plan for the patient (hysteroscopy, laparoscopy/laparotomy, or UAE).
Precise classification is also necessary in the post-treatment setting to assess treatment response, change in overall tumor burden and presence of recurrent lesions.
The typical MRI appearance of a leiomyoma is a well-defined mass of low signal intensity compared to the myometrium on T2 images, isointense to the myometrium on T1 images and homogeneous contrast enhancement like adjacent myometrium (Fig 1).
However, up to 65% of leiomyomas manifest with degenerative changes due to inadequate blood supply and therefore will not have this typical appearance on MRI, showing a heterogeneous appearance, with minimal or irregular enhancement.
There are different types of degeneration: The most common is hyaline degeneration (60%) and produces low signal on T2 images with variable enhancement, so they are difficult to distinguish from non-degenerated fibroids on MRI.
Cystic degeneration usually occurs after hyaline degeneration, and it results in high-signal intensity on T2 images and low-signal on T1 images.
Fibroids with myxoid degeneration are filled with a gelatinous material and can be difficult to differentiate from fibroids that have undergone cystic degeneration; however, they typically appear as more complex cystic masses.
Cellular leiomyoma presents high signal on T2 due to its high cell density and shows reduced diffusion in DWI.
Red degeneration is secondary to hemorrhagic infarction due to obstructed draining veins.
This usually occurs in pregnancy or with the use of oral contraceptives.
The fibroid may have a peripheral rim of low signal on T2 images and high signal on T1 images and there will be no enhancement.
Lipoleiomyomas are rare leiomyoma containing mature macroscopic fat.
High T1 signal within the leiomyoma, which suppresses on fat suppressed sequences is characteristic.
Some lipoleiomyomas contain microscopic fat which can only be detected on in-and-out of phase sequences.
The appearance of fat is regarded as characteristic for lipoleiomyoma as liposarcoma arising within the uterus is extremely rare.
Hydropic degeneration is characterized by the accumulation of fluid within the fibroid that leads to the formation of cystic cavities showing high signal on T2 with hypointense linear images inside, which correspond to smooth muscle fibers.
It mimics malignancy due to rapid growth and atypical imaging appearances (Fig 2).
The heterogeneous appearance of some types of degenerated leiomyomas and their rapid growth may be confused with the behavior of a leiomyosarcoma.
Hence, there is an overlap in image findings for benign uterine myoma and malignant uterine sarcoma, indicating that there is a limitation in establishing an accurate diagnosis of malignancy from image findings alone.
Therefore, when interpreting an MRI, clinical data should also be communicated to radiologists, including patient age, clinical symptoms, and examination findings.
If the radiological findings are correlated with the clinical context, it is possible to better determine uterine masses that are potentially malignant.
Leiomyosarcoma is the most common histological variant of uterine sarcomas.
It is a rare malignant neoplasm with a smooth muscle origin and is considered an aggressive tumor associated with poor prognosis, with a 5-year survival rate of 17%–55%, even when it is discovered at an early stage.
Clinical manifestations of uterine sarcomas and leiomyomas are similar, with increased uterine size, abdominal pain and vaginal bleeding.
Imaging techniques are crucial to be able to differentiate between these diagnoses since the treatment of each one is very different.
Leiomyomas usually require conservative treatment or simple intervention such as uterine artery embolization (UAE) or limited surgical resections (myomectomy), whereas leiomyosarcomas require an early extensive surgery to reduce future morbidity and mortality.
On MRI leiomyosarcomas show intermediate or high signal on T2, heterogeneous enhancement with areas of necrosis, calcifications, nodular or irregular borders, restricted diffusion with low ADC values, heterogeneous contrast enhancement and invasion of adjacent tissues.
In these cases, an intravenous contrast medium is useful to identify solid components of leiomyosarcomas.
Based on the current literature DWI is not mandatory for assessing leiomyomas in a clinical routine setting.
However, it might have a potential for characterization of T2 hyperintense leiomyomas or in rapid growth.
Both leiomyosarcomas and benign cellular leiomyomas display restricted diffusion with high b values.
However, several reports found significantly lower ADC values in leiomyosarcomas than in degenerated leiomyomas, showing ADC value of 0.905 x 10-3 mm2/sec or less for leiomyosarcomas (Fig 3).
Typical fibroids can also be confused with other pathologies.
They can mainly be confused with focal adenomyosis or fibrous ovarian tumors.
Focal adenomyosis, also known as adenomyoma, is a morphological subtype of adenomyosis, a focal ectopic endometrial tissue located in the myometrium, sometimes considered a spectrum of endometriosis.
Focal adenomyosis appears on MRI as focal thickening of the junctional zone, isointense to the surrounding myometrium, which makes it possible to be confused with a submucosal leiomyoma.
Other findings may be useful to differentiate these two pathologies.
Adenomyoma shows ill-defined borders and no pseudocapsule, and small foci of high T1 and/or T2 signal representing endometrial glands, and lower mass efect compared to leiomyomas (Fig 4).
A large pedunculated subserosal uterine leiomyoma may be confused with fibrous ovarian tumors.
In a T2 hypointense solid parauterine mass, studies have shown that ovarian fibromas enhance late and less compared to the myometrium, whereas typical subserosal leiomyomas tend to have a similar enhancement pattern to adjacent myometrium.
Another finding that may be useful for correct localization of the lesion is the beak sign, showing sharp angles between ovarian parenchyma and the lesion, indicating an ovarian origin of the mass (Fig 5).
Furthermore, when the ipsilateral ovary is not identified (phantom sign) or when the ovarian vessels reach the lesion, it is considered that the mass most likely has an ovarian origin.
However, when uterine tissue drapes around the solid mass, known as claw sign or there are enlarged and tortuous vessels that extend from the uterus to supply a pelvic mass, known as bridging vessels sign, a uterine origin is suspected.