Fibroids are the most prevalent source of uterine masses, but share clinical features with other uterine masses such as adenomyosis and ovarian cysts. Ultrasound is usually sufficient to detect uterine fibroids and rule out other differentials, but some masses are difficult to differentiate with ultrasound. In these cases, an MRI may be especially useful in confirming uterine fibroids and planning for appropriate treatment. Establishing an accurate differential has important implications on prognosis and treatment, especially as it pertains to uterine fibroids and other conditions of the uterus.
Uterine fibroids (also called ‘myomas’ or ‘leiomyomas’) are non-cancerous masses that develop in the muscular tissue of the uterus. Fibroids can vary in size from small, undetectable seedlings to massive tumors that cause the uterus to expand to the size of a four- or five-month pregnancy.  Although the majority of women with uterine fibroids are not symptomatic, those who are typically suffering from excessive menstrual bleeding or pelvic pain. Larger fibroids can cause weight gain, an enlarged uterus, urgency, and other abnormalities of the urinary and gastrointestinal systems. 
Fibroids are extremely common, especially as women progress through the latter half of their reproductive years. One study in the United States observed that 60% of African American women had developed fibroids by age 35, with that figure increasing to 80% by age 50. Similarly, 40% of Caucasian women had developed fibroids by age 35, and almost 70% by age 50.  Indeed, uterine fibroids are one of the most common causes of abnormal uterine bleeding in reproductive-aged women,  and the number one reason that women have their uterus removed.  With these figures in mind, it’s tempting to default to a uterine fibroid diagnosis when the symptoms align, but on presentation alone, there are a number of other conditions that share the clinical features of fibroids.
Ruling out possible differentials is a critical step in uterine fibroid care, especially when uterine fibroid embolization (UFE) or other fibroid-specific treatments are being considered that won’t have any effect on other types of uterine masses. Beyond the scope of fibroid treatment, differentiation is clinically important because of differences in treatment and prognostic implications between different conditions of the uterus.
Uterine fibroids share clinical features with other conditions that involve masses in the uterus, cervix, ovaries, fallopian tubes, or the connecting tissue around the uterus.  These include:
While some of these masses are straightforward to differentiate via ultrasonography or other imaging modalities due to their unique anatomical locations or defining features, others require an MRI to reliably differentiate from uterine fibroids. 
If uterine fibroids are suspected, ultrasound and MRI imaging are the two most commonly employed imaging methods. In combination with the patient’s clinical findings, these imaging methods are usually sufficient to confirm a diagnosis, and potentially avoid unnecessary laparoscopy and/or exploratory surgery. 
Ultrasound imaging tends to be the first-line detection method for uterine fibroids due to how cost-effective and accessible it is. When a uterine mass is suspected, both transabdominal and transvaginal scans should be performed. In skilled hands, ultrasonography can detect fibroids as small as 5 mm on transvaginal ultrasound.  In most cases, the diagnosis of fibroids and ruling out differentials with ultrasound is relatively straightforward, but there are cases in which focal adenomyosis, another type of adnexal mass, or uterine leiomyosarcoma can be mistaken for a benign uterine fibroid. When there is doubt about the origin of a pelvic mass on ultrasound, further evaluation with MRI should be performed.
MRI is the most sensitive imaging modality for evaluating uterine fibroids, with a demonstrated sensitivity of 88% to 93% and specificity of 66% to 91%. One critique of ultrasound is it’s lack of reproducibility due to operator-dependence. Although more costly, MRI overcomes this shortcoming of ultrasound and can more reliably detect smaller fibroids. 
In addition to more reliable fibroid detection, MRI is useful in the differentiation of uterine fibroids from focal adenomyosis and adnexal masses. [6,7,8]
Adenomyosis occurs when endometrial tissue breaks through the uterine wall and grows into the myometrium (where fibroids also grow), forming a mass of tissue. Unlike uterine fibroids, adenomyosis is usually amorphous and not well-delineated on ultrasound. However, in the case of what are termed ‘focal adenomyosis’ or ‘adenomyoma,’ the mass is confined to a more circumscribed shape, similar to a uterine fibroid.
Because similar anatomy is affected, adenomyosis presents with virtually all of the same clinical features as uterine fibroids: heavy bleeding, pain, pelvic pressure, and enlarged uterus. These similarities in presentation, along with similar anatomical features make it difficult to reliably distinguish focal adenomyosis from uterine fibroids with ultrasound. The distinction between the two on MRI, on the other hand, tends to be easy for a radiologist to distinguish.  On MRI, adenomyosis appears as an ill-defined, homogenous low-signal intensity area embedded with sparse high-intensity spots, as opposed to fibroids which appear as circumscribed masses with a spectrum of signal intensity.
Adnexa is an anatomical term that describes the ovaries, fallopian tubes, and the ligaments that hold the uterus in place. An adnexal mass describes any growth that occurs on these structures, including ovarian cysts, benign ovarian tumors, ovarian cancer, ectopic pregnancy, and uterine fibroids.  MRI imaging allows the detection and characterization of pedunculated uterine fibroids and differentiation from other types of solid adnexal masses. An MRI scan allows a radiologist to reliably determine if a mass has continuity with the myometrium, which establishes the uterine fibroid diagnosis.
Uterine leiomyosarcoma is a rare cancer of the uterus that is especially difficult to diagnose, even with MRI imaging. Leiomyosarcoma may arise in previously existing fibroids or independently from the smooth muscle cells of the myometrium, and with the state of current medical technology, it can only be reliably detected via the surgical removal of an affected uterine mass.
While ultrasound has a well-defined role in the diagnosis of uterine fibroids, there are situations in which an MRI is a smart move to definitively establish a diagnosis and move forward with fibroid treatment.
Dr. Michael Lalezarian is a practicing interventional radiologist with the Fibroid Specialists of University Vascular in Los Angeles, CA. In addition to patient care, Dr. Lalezarian teaches and supervises medical students, residents, and fellows as a full-time teaching Professor in the Department of Radiology at UCLA. He is regarded as an expert in uterine fibroid embolization.
 Duhan, N., & Sirohiwal, D. (2010). Uterine myomas revisited. European Journal of Obstetrics Gynecology and Reproductive Biology, 152(2), 119–125.
 Day Baird, D., Dunson, D. B., Hill, M. C., Cousins, D., & Schectman, J. M. (2003). High cumulative incidence of uterine leiomyoma in black and white women: Ultrasound evidence. American Journal of Obstetrics & Gynecology, 188(1), 100–107.
 Munro, M. G., Critchley, H. O. D., & Fraser, I. S. (2011). The FIGO classification of causes of abnormal uterine bleeding in the reproductive years. Fertility and Sterility, 95(7), 2204–2208.e3.
 Bonafede, M. M., Pohlman, S. K., Miller, J. D., Thiel, E., Troeger, K. A., & Miller, C. E. (2018). Women with Newly Diagnosed Uterine Fibroids: Treatment Patterns and Cost Comparison for Select Treatment Options. Population Health Management, 21(S1), S-13-S-20.
 Bukulmez, O., & Doody, K. J. (2006). Clinical features of myomas. Obstetrics and Gynecology Clinics of North America, 33(1), 69–84.
 Khan, A. T., Shehmar, M., Gupta, J. K., & Gupta, J. (2014). Uterine fibroids: current perspectives. International Journal of Women’s Health, 6, 95–114.
 Perez-jaffe, L. A., & Tureck, R. W. (1999). Uterine Leiomyomas: Histopathologic Features, MR Imaging Findings, Differential Diagnosis, and Treatment. RadioGraphics, 1179–1197.
 Biggs, W. S., & Marks, S. T. (2016). Diagnosis and management of adnexal masses. American Family Physician, 93(8), 676–681.