Utility of MRI Before and After Uterine Fibroid Embolization: Why to Do It and What to Look For
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- Kirby, J.M., Burrows, D., Haider, E. et al. Cardiovasc Intervent Radiol (2011) 34: 705. doi:10.1007/s00270-010-0029-2
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The utility of magnetic resonance imaging (MRI) in the selection, procedure planning, and follow-up of patients undergoing arterial embolization for uterine fibroids is reviewed. Advantages of MRI over ultrasound include multiplanar imaging capability, a larger field of view, increased spatial resolution, improved anatomic detail, and the ability to detect other pelvic disorders. MRI can assess fibroid viability by detecting contrast agent enhancement. Magnetic resonance angiography has a useful role in evaluation of pelvic vasculature. Magnetic resonance parameters such as T1 and T2 relaxation times and diffusion-weighted characteristics have an emerging role in predicting outcome before and after embolization. MRI may be used to evaluate technical success and to image potential complications after embolization.
Uterine-sparing, minimally invasive procedures are increasingly preferred over hysterectomy in the management of symptomatic fibroids, and are typically performed without a prior tissue diagnosis. Here, we review the advantages of magnetic resonance imaging (MRI) in patient selection (including diagnostic features, evaluation of size, location, enhancement) and in demonstration of vascular anatomy before arterial embolization. We also review the role of MRI in predicting postembolization outcome and in imaging of postembolization complications.
Pulse sequences and typical scan parameters used for pelvic MRIa
Axial diffusion EPI
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Slab with 60 slices
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512 × 256
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Advantages of MRI over Ultrasound
Both MRI and transvaginal ultrasound (US) are highly accurate in fibroid detection; however, MRI is better for precise anatomic mapping, particularly in large uteri and in cases of multiple fibroids . US has limited ability to diagnose coexistent disease (such as adnexal masses), may be limited in the obese patient, and is insensitive to many findings such as uterine size, infarcted fibroids, adenomyosis, and relationship to other structures/organs. While color flow and power Doppler techniques add to the performance of US, few comparative studies of MRI and Doppler US for evaluation of fibroids exist.
Spielmann et al. reported additional fibroids on MRI in 63% of patients and erroneously suspected on US in 10% of patients being considered for uterine artery embolization (UAE) . MRI findings resulted in cancellation of UAE in 8% of patients; 14% of patients who were thought to be poor candidates on the basis of US findings underwent embolization . Uterine volume was significantly smaller as measured on MRI compared with US. There was good correlation for the volume of the single largest fibroid, but poor correlation for location.
Omary et al. reported that MRI in patients with symptomatic fibroids referred for potential UAE changed initial diagnosis in 18% (e.g., to adenomyosis, adnexal mass), improved diagnostic certainty by 22%, and changed the treatment plan in 22% of patients . Volkers et al. report good inter- and intraobserver reproducibility for MRI in assessment of uterine volume, number of fibroids, and dominant fibroid signal intensity (SI) .
In our institution, patients with symptomatic fibroids referred for consideration for UAE undergo contrast-enhanced pelvic MRI before being seen in office-based consultation by an interventional radiologist.
Typical MRI Appearance
Fibroids with increased cellular content (predominantly densely packed cellular fascicles of smooth muscle with little intervening collagen) and degenerate fibroids may demonstrate high SI on T2-weighted images . Slightly hyperintense tumors are more likely to be cellular, whereas heterogenous or markedly hyperintense T2 signal suggests degeneration [2, 7]. Gadolinium enhancement may range from homogenous and early (cellular) to minimal and irregular (degenerate) . Completely hyalinized fibroids have low T2 SI. Differentiation of fibroid type may also be possible using diffusion-weighted imaging . After successful embolization, fibroids may undergo progressive liquefaction with increasing T2 signal. Volume reduction is greater in T2 hyperintense fibroids [9–11], however such fibroids are uncommon (7–15%) [11, 12].
On T1-weighted sequences, the uterus has homogeneous intermediate SI, and the three layers of uterine tissue are obscured. Most fibroids are isointense to surrounding myometrium on T1-weighted images (Fig. 1B). T1 hyperintensity (6–25%) suggests fatty or hemorrhagic/red degeneration (as a result of T1-shortening effects of methemoglobin)  and is a negative predictor of success with a lower reduction in vascularity compared to fibroids with low T1 signal [9, 10, 12] (Fig. 1B).
An increase in SI on T1-weighted images is typically observed immediately after embolization. This is likely due to a combination of reduced blood volume in the fibroid, the T1 effects of the accumulation of iodine-based contrast medium, and later by the presence of methemoglobin and blood breakdown products.
Differential Diagnosis: Is It a Fibroid?
Adenomyosis, endometriosis, adnexal masses, and musculoskeletal and genitourinary pathology may be “incidental” MRI findings in patients being assessed for fibroid embolization, or they may provide an alternative explanation for the patient’s symptoms. Anatomic ovarian localization may facilitate ovarian parenchymal protection by coil placement during UAE since the ovary may not be readily apparent by angiographic parenchymal blush alone. MRI demonstration of continuity of an adnexal mass with adjacent myometrium can help differentiate a pedunculated fibroid from an adnexal mass.
Symptomatic adenomyosis (without coexisting fibroids) may be treated by UAE to decrease junctional zone vascularity, and several small series report clinical improvement after embolization [15–17]. Kim et al. embolized the uterine arteries to near stasis with polyvinyl alcohol (PVA) particles and then completely occluded flow with gelatin sponge pledgets in all cases . This secondary embolization may help to prevent recanalization of the uterine arteries and ensure enough time for ischemia of adenomyosis to occur [17, 18]. Where this approach is used, evidence of more pronounced ischemia may be expected on post-UAE imaging.
Recent advances in MR technology, including higher magnetic field strengths, parallel imaging techniques, and phased-array receiver coils, have driven investigation of the role of diffusion-weighted imaging in differentiating benign fibroids from sarcomas [23–26]. As expected, the mean apparent diffusion coefficient value of sarcomas is lower than that of normal myometrium as a result of histopathological characteristics, such as hypercellularity, enlargement of nuclei, and hyperchromatism. Both Takeuchi et al. and Tamai et al. report an overlap between sarcomas and cellular leiomyomas when diffusion-weighted imaging is used alone [24, 26]. A combination of T2-weighted MRI and 3-T diffusion-weighted imaging has been shown to greatly improve sensitivity and specificity .
Size: Does It Matter?
Spies et al. reported that larger baseline dominant fibroid volume predicted less volume reduction at both 3 and 12 months after UAE . In contrast, Katsumori et al. reported no difference in complications or outcome for fibroids 10–19 cm in size compared to those <10 cm . Firouznia et al. found no relationship between size or number and success or complication rates . Despite this, many practitioners maintain a threshold for embolization of 13–15 cm. Above this, the postembolization volume may still result in bulk symptoms, and the necrosis from a large fibroid may result in a protracted postembolization syndrome.
Location: Does It Matter?
Submucosal location has been shown to have a strong correlation with positive outcome after UAE by some authors [12, 27], although not by others . Submucosal and intramural fibroids that have a large submucosal component may be at increased risk for post-UAE sloughing or expulsion, which can result in significant pain, bleeding, infection, and prolonged vaginal discharge . This may relate to the distribution of particles or to the vascular anatomy of the uterus . Changes in fibroid location after UAE (such as endometrial becoming endocavitary, or subserosal becoming intramural or even submucosal) may occur in 1–5% of cases .
Approximately 3–8% of fibroids arise from the uterine cervix. A fibroid growing down into the cervix from higher up in the uterus is more common than a true cervical fibroid. Cervical fibroids tend to be small (5–10 mm in diameter). Enlargement causes upward displacement of the uterus, and the fibroid may become impacted. The risk of post-UAE sloughing is slightly increased.
Fibroids may also arise from the broad ligament and the round ligament. MRI may be helpful in demonstrating a pedicle from the uterine vasculature. Very rarely, a fibroid may attach itself to another organ and establish a new source of blood, such that its uterine stalk degenerates and the fibroid is no longer attached to the uterus; this is the so-called parasitic fibroid.
Enhancement: Does It Matter?
Gadolinium is essential for assessment of viability and treatment planning. Enhancement of fibroids depends on their vascularity. Areas of degeneration or necrosis appear heterogeneous or hypointense compared with the normal myometrium on T1-weighted images after gadolinium injection. Cellular fibroids enhance homogeneously , while fibroids with hyaline degeneration may show a cobblestone appearance .
The goal of performing UAE is to produce irreversible ischemic injury of fibroids while maintaining endometrial and myometrial perfusion . Fibroids that have already infarcted are therefore unlikely to show volume reduction with UAE. Nikolaidis et al. reported a prevalence of nonviable fibroids of 20% with gadolinium-enhanced MRI; 6% of patients had nonviable dominant fibroids .
Vascular Anatomy: Practical Relevance for Optimal Embolization
The optimal strategy for imaging blood supply to uterine fibroids before, during, and after UAE is unclear.
3D contrast-enhanced MRA has been shown to accurately map the pelvic vasculature before UAE . Gadolinium may not be essential. Mori et al. demonstrated that unenhanced 3D water-excitation sensitivity-encoding time-of-flight MRA was superior to gadolinium-enhanced MRA (correlated with digital subtraction angiography) in uterine arterial visualization .
The use of pre-embolization MRA to identify the optimal tube angle for angiographic selection of the uterine artery has recently been shown to reduce radiation dose, fluoroscopy time, and volume of contrast medium used .
Women over 45 years of age have been shown to have a higher prevalence of uterine–ovarian arterial anastomoses (up to 43%) compared with women under 45 years of age (<5%) [42, 43] and are at increased risk for ovarian dysfunction after UAE through undetected nontarget embolization of the ovaries or a direct effect on the uterus . Thresholds for transient amenorrhea of 10% and permanent amenorrhea of 3% for <45 years, 15% for >45 years, have been suggested . Gomez-Jorge et al. reported transient menopausal symptoms in 65% of patients with tubo-ovarian arteries identified by catheter angiography prior to UAE .
Visualization of ovarian arteries at MR angiography (MRA) equal in size or exceeding the diameter of a 4F pigtail catheter and extending visibly into the pelvis is suggestive of a significant contribution to the uterine circulation . Kroencke et al. reported a 100% sensitivity and 77% specificity for MRA in depicting ovarian artery supply . Restricting aortography or selective uterine angiography to those cases in which MRA suggests a high probability of fibroid supply may save time, contrast, and radiation.
What Happens after Embolization?
The pathophysiology of fibroid infarction remains incompletely understood. Burbank has postulated a mechanism similar to that for separation of the postpartum placenta whereby blood flow is slowed in myometrial vessels, resulting in transient uterine ischemia . Fibroids (like the placenta) seem to be more susceptible to this period of ischemia, whereas uterine viability is maintained through various small anastomotic vessels, including ovarian and cervical arteries, the round ligament artery (via the inferior epigastric artery), and the external iliac artery. Later, fibrinolysis lyses clot in the uterine arteries and veins, and uterine perfusion is restored .
DeSouza and Williams have shown by MRI that uterine perfusion is decreased immediately after UAE, whereas perfusion to fibroids is completely suppressed . On follow-up imaging at 1 and 4 months, myometrial perfusion returns to normal, whereas perfusion to tumors remains suppressed . Therefore, the uterus reestablishes perfusion, whereas tumors preferentially undergo necrosis. Aziz et al. have shown that 150–250-μm-diameter PVA particles used for UAE 1 day prior to hysterectomy lodge in arteries with a diameter of 1–2 mm but not in the arterioles that supply the layers of the myometrium .
Shimada et al. used dynamic triple-phase MRI to demonstrate a close relationship between the quantity of vessels, the degree of hyalinization, and fibroid vascularity . Kosaka et al. used perfusion-weighted MRI with a double-echo T2*-weighted spoiled gradient-recalled acquisition sequence to demonstrate that relative blood volume correlated with histologic vascular area in uterine fibroids . Jha et al. reported that hypervascular fibroids showed a 35% greater reduction in vascularity than hypovascular fibroids . Similar findings were reported by Harman et al. , but other groups have failed to show a relationship between perfusion or degree of gadolinium enhancement and fibroid volume reduction [10, 11]. Likewise, Goodwin et al., using milligrams of PVA per unit of uterine volume as a measure of vascularity, found no difference in outcome for hypervascular fibroids ; and the value of MRI for assessment of vascularity remains work in progress.
Visible uterine arteries after embolization are indicative of failure . The association of incomplete infarction with poor clinical response has been reported by several groups [53–55]. Katsumori et al. reported that complete fibroid infarction on contrast-enhanced MRI 1 week after embolization was associated with a higher rate of symptom control and a lower rate of additional gynecologic intervention at 5 years compared with incomplete infarction .
There is limited literature on the use of echoplanar diffusion-weighted imaging in the assessment of response of uterine fibroids and myometrium to embolization. Liapi et al. reported a pattern of restricted diffusion in fibroids before embolization, thought to be due to clusters of uniform smooth muscle cells and intervening collagen . Embolized fibroids had low apparent diffusion coefficient values compared to untreated fibroids consistent with increased dehydration at a cellular level. Apparent diffusion coefficient values of the myometrium did not significantly change after embolization, suggesting maintained myometrial viability . The study was limited by small numbers and a large interval from UAE to MRI. Clark et al. suggest that the use of high b-value apparent diffusion coefficient may assist in identifying patients whose fibroids are most likely to show a significant decrease in volume after embolization, but they note that prospective evaluation within a large patient cohort is required .
Potential complications after UAE include sequelae of nontarget embolization (including premature ovarian failure), transcervical fibroid expulsion, uterine (myometrial) infarction, endometritis, and fibroid infection.
Walker and Pelage reported permanent amenorrhea after UAE in 7% women, 2% under the age of 45 . Infective complications requiring emergency hysterectomy occurred in 1% patients followed for a mean of 16.7 months . Spies et al. reported a periprocedural complication rate of 8.5% and a serious (Society of Cardiovascular and Interventional Radiology class D) complication rate of 1.25% . The complication-related hysterectomy rate in the Ontario uterine fibroid embolization trial was 1.5% within 3 months of embolization .
The EMMY trial , a multicenter, randomized comparison of UAE and hysterectomy, reported a relatively high technical failure rate of 5.3%, mainly as a result of difficult anatomy, and a 17.3% procedural failure rate, which included patients with aberrant anatomy, extensive collateral vessels to the cervix or vagina, and no flow in uterine arteries after catheterization . The overall complication rates were 28.4% during the patients’ hospital stay and 60.5% after 6 weeks . Patients underwent preprocedure MRI, but no details were provided regarding the use of gadolinium contrast or MRA, and routine postprocedure imaging was not performed.
Transcervical expulsion may occur in up to 15% of women, typically 6 weeks to 3 months after uterine fibroid embolization . Submucosal and intramural fibroids with a submucosal component are at increased risk [20, 37]. It has been postulated that the endometrium and inner myometrium have fewer available collateral blood vessels and are therefore more susceptible to infarction and subsequent sepsis . The typical MR appearances are of an infarcted fibroid, with increased T1-weighted SI, decreased T2 SI, and no enhancement, accompanied by distension of the endometrial canal and migration toward the cervix or vagina . Contrast-enhanced MRI may demonstrate any viable attachment to the uterine wall .
Ischemia leading to uterine necrosis is rare [61, 62], with proposed causes including poor collateral circulation, superinfection, large solitary fibroids, and small particle size (that occlude distal arterial branches and impede collateral vessel formation) .
Gabriel et al. reported the necrotic uterus as having homogeneously low SI on T1-weighted images, areas of higher SI on T2-weighted images possibly related to hemorrhage, a widened endometrial stripe, and nonenhancement of the entire uterus with the exception of a few capsular vessels . In a second case report, Torigian et al. describe intermediate to high SI on T1-weighted images and high SI on T2-weighted images, with no enhancement after gadolinium .
Inflammation of the endometrium (endometritis), can occur days or weeks after UAE and may have an infectious or noninfectious cause . Most patients with endometritis respond well to antibiotics. On MRI, endometritis may manifest as uterine enlargement with T1 hyperintense intracavitary hematoma. Associated gas appears as a signal void on all sequences . Contrast enhancement may increase the conspicuity of intracavitary fluid collections.
Fibroid infection is defined as bacterial infection of one or more fibroids as a result of colonization by either blood-borne or ascending vaginal pathogens, the latter more common in the setting of arrested transcervical passage . Various patterns of gas may be seen in the uterine vessels and fibroids after UAE; however, the presence of gas alone does not necessarily imply infection .
A detailed analysis of imaging costs in uterine fibroid embolization is beyond the scope of this review. The 24-month cumulative cost of UAE in the EMMY trial was lower than that for hysterectomy, with the UAE analysis providing for pre-embolization MRI, compared to US for the surgical arm . The HOPEFUL study recently concluded that UAE is a less expensive option to the health service compared with hysterectomy, even when the costs of repeat procedures and associated complications are factored in. The authors included similar imaging requirements for both surgical and embolization arms .
We advocate baseline MRI for better characterization of fibroid size, position, and vascularity, and for more accurate detection of adenomyosis, all of which may affect a decision to offer embolization. MRA may provide useful information for the interventional radiologist. MR parameters such as T1 and T2 relaxation times and diffusion-weighted characteristics have an emerging role in predicting outcome after embolization. We also advocate follow-up MRI to ensure adequate devascularization of the fibroids and to assess their shrinkage; and to evaluate for complications.
Conflict of interest
The authors declare that they have no conflict of interest.