Introduction
Brief overview of O-RADS MRI
Pearls and pitfalls
Acquisition pearls
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Sagittal T2-weighted without fat saturation (slice thickness, ≤ 4 mm)
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Axial T2-weighted without fat saturation (section thickness, ≤3 mm)
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Axial unenhanced T1-weighted in-, opposed-phased, fat and water sequence (section thickness, ≤ 4 mm)
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Axial abdo-pelvic DWI scan (section thickness 4–5 mm, low b-values: 0 or 50 − high b-value: 1000–1200)
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Three-dimensional non-dynamic T1-weighted gadolinium sequence with fat sat (slice thickness 3 mm) without contrast
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Three-dimensional dynamic contrast-enhanced (DCE) MRI T1-weighted sequence (if any enhancing solid tissue within the adnexal lesion): 3D axial (15 cm–15 s–3 mm)
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Maximal slice thickness of 3 mm
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Axial plane, in line with the T2-weighted sequence and DWI scan for ease of cross correlation of the signal characteristics
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Minimum temporal resolution of 15 s per acquisition
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Subtraction allows the suppression of any pre-contrast high T1-weighted signal intensity, whether the DCE is acquired with or without fat saturation.
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The acquisition needs to start prior to contrast injection (10 s prior to contrast injection) and should continue for at least 3 min.
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If DCE MRI is not feasible, an alternative option is to perform a T1WI series at 30–40 s post-contrast injection. It is important to note that this approach will have limitations in assessing adnexal lesions with enhancing solid tissue, which can only be categorized as O-RADS MRI 4 or 5.
Sequence | Aim | Pitfall (solution) |
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Sagittal T2-weighted without fat saturation | Tissue characterization Multiplanar analysis Pelvic anatomy analysis Location of the mass analysis Shape, size and component of the mass analysis | 1. Intravenous contrast injection (if very-dark signal on T2-W imaging, perform DWI scan, to search for both low T2, DWI scan signal to avoid gadolinium injection) 2. Failure to recognize intermediate T2W signal (combine with DWI to evaluate the need for contrast injection analysis) 3. Failure to identify the anatomical origin of the pelvic mass (look for “specific” anatomic sign, landmarks displacement and ipsilateral ovary visualization) 4. Failure to identify small tissular component as papillary projections (use thin slices < 4 mm) |
Axial T2-weighted lombo-pelvic sequence | Same as above Extra-pelvic organ analysis (kidney, liver, vascular structure, lymph node) | 1. Failure to identify extra-pelvic disease (analyze all the slices of the sequence) |
Axial unenhanced T1-weighted in-phase sequence | Tissue characterization | 1. Wrong analysis of the component if hypersignal T1-W (analyze complementary T1W sequence to determine blood, high-protein or fat content) |
Axial unenhanced T1-weighted water sequence | Tissue characterization | 1. Wrong analysis of the fat component (analyze the loss of signal and correlate with T1-weighted fat sequence if doubt) 2. Wrong identification of mucinous component (correlate with morphological sequence to look for loci and high-signal T2W) 3. Wrong identification of pus component (correlate with inflammatory clinical findings, perform gadolinium sequence and DWI scan) 4. Wrong identification of colloid component (correlate with morphological sequence to look for loci and low-signal T2W) 5. Wrong analysis between endometriotic and other hemorrhagic content (correlate with clinical findings, T2W sequence “shading” and T1W “rim”) |
Axial abdominal and -pelvic DWI scan | Tissue characterization Identification of pathological lymph node | 1. Technical issue with the acquisition (perform the sequence with a minimum upper b-value of 1000 s/mm2 , section thickness < 4 mm and check the dark DWI signal of the bladder with high-b-value before interpretation) 2. Analyze the apparent diffusion coefficient to conclude (do not use ADC value to interpret adnexal masses) 3. Analyze the dark T2/DWI partially (analyze the whole tumor to avoid missing high-signal component) 4. Misclassifying the normal ovary with ovarian tumor (correlate to morphological T2W sequence) |
Three-dimensional non-dynamic T1-weighted gadolinium sequence | Tissue characterization (identify tissular component) | 1. Technical issue with the acquisition (ensure perform pre-contrast acquisition to enable subtraction sequences and use section thickness < 3 mm) 2. Failure to identify Rokitansky nodule or thin and regular septations (correlate with morphological T2W and T1W sequence) 3. Failure to identify endosalpingial folds and papillary projections in the context of inflammatory pelvic disease (correlate with clinical findings, morphological T2W sequence and DWI scan) 4. Failure to identify a physiological fimbria end of the tube (correlate with multiplanar T2W and search for stellar morphology) 5. Failure to identify hair, calcifications, debris (use T1W in-, opposed-, fat, and water sequence) 6. Failure to identify normal ovarian parenchyma (use T2W and DWI scan) |
Three-dimensional DCE T1-weighted sequence | Tissue characterization using tissular component and enhancement analysis | 1. Technical issue with the acquisition (perform the sequence with a spatial resolution of 3 mm and temporal resolution of 15 s, place the reference ROI on the outer myometrium avoiding arcuate vessels. Start contrast injection 10 s prior and last for at least 3 min) 2. Technical issue with the curve drawing (analysis must be performed using percentage of enhancement of relative enhancement not absolute) 3. Technical issue with the curve drawing in mixt tumor (perform subtraction sequence to put the ROI) 4. Failure to identify a shoulder and a plateau to differentiate low and intermediate-risk TIC (do not consider the slope of the curve but search for a shoulder and a plateau) 5. Being inconclusive in the context of hysterectomy (analyze the 30–40 s post-contrast sequence to search for an early enhancement of the mass) 6. Drawing enhancement curve in the context of adnexal torsion (correlate with clinical findings and T2W morphological sequences and DWI scan) 7. Systematically drawing enhancement curve in the context of dermoid cyst and struma ovarii (intrinsic component of the tumor can lead to misdiagnosis of a malignant tumor) |
O-RADS MRI score 0
Definition: Lesions that have undergone incomplete or inadequate MRI evaluation are included in this category. |
O-RADS MRI score 1
Definition: Normal ovaries or ovaries with physiologic observations, such as a follicle, corpus luteum cyst, and hemorrhagic cyst measuring ≤ 3 cm in premenopausal women. |
Pitfall 1
The “beak sign” is defined as sharp angles between the adnexa and the lesion, causing the edges of the ovary to deform into a beak shape. The presence of the beak sign suggests that the lesion originates from the ovary [10, 12]. The “bridging vessel” sign and “claw sign” are indicative of the lesion originating from the uterus. The bridging vessel sign refers to vessels extending between the uterus and the lesion, as observed in pedunculated uterine leiomyomas [13]. The “claw sign” corresponds to the uterine tissue draping over the lesion and is also typically observed in uterine leiomyomas [14]. |
Pitfall 2
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Endometrioma: Endometriomas are typically larger and demonstrate homogeneous T1-weighted hyperintensity and variable T2-weighted hypointensity (known as “T2 shading”), with a thin enhancing wall. Endometriomas result from chronic repetitive bleeding over multiple menstrual cycles, which is not observed with a corpus luteum [18‐20].
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Tubo-ovarian abscess: Tubo-ovarian abscesses are typically bilateral and asymmetrical (due to different degree of evolution of pelvic inflammatory disease). They usually demonstrate thick enhancing walls, tubular cystic components, and surrounding fatty infiltration/stranding. Pus induces a very high signal intensity on DWI and a low signal intensity on ADC map. Differentiation of tubo-ovarian abscess from corpus luteum is also based on clinical presentation. In addition to pelvic pain, patients with tubo-ovarian abscesses often have fever and elevated white cell count [21, 22].
Pitfall 3
Definition: An adnexal lesion is defined as an adnexal observation which does not meet criteria for a physiologic finding. This is subdivided further into cystic lesion without solid tissue, cystic lesion with solid tissue, and solid lesion. Cystic lesions can be unilocular or multilocular. Purely solid lesions are defined as having less than 20% cystic component. O-RADS MRI score 2 includes: -Unilocular cyst with no wall enhancement and no enhancing solid tissue If no wall enhancement, may have any type of fluid content. If smooth wall enhancement, may have simple or endometriotic fluid content. -Lesion with lipid content and: No enhancing solid tissue. Only a small amount of enhancing tissue (Rokitansky nodule). -Lesion with homogenously hypointense solid tissue on T2WI and DWI. -Dilated fallopian tube with simple fluid content (hydrosalpinx) and no enhancing solid tissue. May demonstrate a thin, smooth wall/endosalpingeal folds with enhancement. -Para-ovarian cyst with no enhancing solid tissue. May contain any type of fluid content, and may have a thin, smooth wall +/− enhancement. |
Pitfall 1
Pearls 1
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Hemorrhagic fluid: The signal intensity can be heterogeneous and variable depending on the age of blood products. For instance, late subacute hemorrhage appears hyperintense on T1-weighted images (T1WI) and T2-weighted images (T2WI) (Fig. 6).
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Endometriotic fluid: Typically appears homogeneously hyperintense on T1WI, with corresponding hypointensity on T2WI, commonly known as “T2 shading.” Only endometriotic fluid may have higher T1W signal than fatty content. Ancillary findings of endometrioma include multiplicity and the presence of “T2 dark spots,” representing blood clots, which support the diagnosis of endometrioma and aid in distinguishing it from hemorrhagic fluid (Fig. 6).
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Proteinaceous fluid: Fluid composed of mucin, colloid, or purulent material may display variable high T1W signal (Fig. 6).
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Fat- or lipid-containing fluid: Expected to appear hyperintense on both T1W and T2W images, with signal loss on fat-saturated sequences. Notably, the presence of intravoxel fat as detected on chemical shift T1WI out-of-phase sequences, even without detectable macroscopic fat, still represents “lipid-containing fluid” (i.e., dermoid) (Fig. 6).
Pitfall 2
Pitfall 3
Definition of O-RADS MRI score 3: -Unilocular cyst with proteinaceous, hemorrhagic, or mucinous fluid content and No enhancing solid tissue Smooth wall enhancement -Multilocular cyst with any type fluid content No enhancing solid tissue No lipid content May have smooth enhancing septae and wall -Lesion with solid tissue (excluding dark T2/dark DWI) with low-risk time intensity curve (TIC) on DCE MRI. -Dilated fallopian tube with no enhancing soft tissue nodule and: Non-simple fluid and thin walls/folds Simple fluid with thick, smooth walls/folds |
• Papillary projection: An enhancing solid component emerging from the inner or outer wall or septation, displaying a branching architecture. • Mural nodule: An enhancing solid component measuring greater than 3 mm, originating from the wall or septation, with a nodular appearance. • Irregular septation: An enhancing linear strand extending from one internal surface of the cyst to the contralateral side, demonstrating an uneven margin. • Irregular wall: An enhancing cyst wall displaying an uneven margin. • Larger solid portion: An enhancing component of an adnexal lesion that does not fit into the categories of papillary projection, mural nodule, or irregular septation/wall. |
Pitfalls 1: What is not considered solid tissue?
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Fat, clot, debris. Use of dedicated axial unenhanced T1-weighted in-, opposed-phased, fat and water sequence and subtraction images are particularly helpful in this setting.
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Normal ovarian parenchyma
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Rokitansky nodule (Figs. 8 and 10). Rokitansky nodule is a small solid component that may show enhancement, but it is not categorized as solid tissue. Thomassin-Naggara et al. reported that twenty mature cystic teratomas were misclassified, with 19/20 classified as O-RADS 4 or 5. These errors occurred because readers examined TIC and recorded intermediate or high-risk TIC in a Rokitansky nodule which should not considered soft tissue [7]. Rokitansky nodule can demonstrate enhancement due to the presence of smooth muscular cells and fibrous, neuroglial, or thyroid tissue [28].×
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Endosalpingeal folds (Fig. 11). Another common cause of false positives is the misinterpretation of endosalpingeal folds as papillary projections, especially in the context of pelvic inflammatory disease (PID). PID is typically associated with inflammatory processes, leading to thickening, and marked enhancement of the fallopian tube wall and endosalpingeal folds. A study by Thomassin-Naggara et al. reported that 11 out of 12 PID cases were incorrectly scored as O-RADS MRI category 4 or 5 because the TIC analysis was performed by placing a region of interest (ROI) on endosalpingeal folds or thickened wall [7]. Radiologists should exercise caution and consider all imaging findings, such as premenopausal status, tubular shape, inflammatory changes in adjacent structures, and the presence of pus within the lesion. Pus can be recognized as high signal intensity on high-b-value DWI with a corresponding low signal intensity on the ADC map.×
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Fimbriated end of the tube may look like irregular solid tissue with strong enhancement and may be recognized thanks to its stellar morphology (Fig. 12)×
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Smooth wall or septa (Fig. 13) as opposed to irregular wall or septa.×
Pitfalls 2: Failure to recognize a low-risk TIC curve
O-RADS MRI score 4 and 5
Definition of O-RADS MRI score 4: Adnexal lesions with an intermediate risk for malignancy as defined by a PPV of approximately 50%. O-RADS MRI score 4 lesions include: -Lesion with solid enhancing tissue (except for dark T2/dark DWI) with: Intermediate-risk TIC on DCE MRI. (If DCE MRI is not feasible, any lesion with enhancement ≤ myometrium at 30–40 s on non-DCE MRI. This analysis is less accurate than TIC analysis with a loss of specificity as no difference is feasible in the presence or absence of a plateau). -Lesion with lipid content and large volume enhancing solid tissue. |
Definition of O-RADS MRI score 5: Lesions in this category are considered high-risk with a PPV for malignancy of about 90%. O-RADS MRI score 5 lesions include: -Lesion with solid tissue (except for dark T2/dark DWI) with: High-risk TIC on DCE MRI. (If DCE MRI is not feasible, any lesion with enhancement > myometrium at 30–40 s on non- DCE MRI. This analysis has the same accuracy as TIC analysis). -Peritoneal, mesenteric, or omental nodularity or irregular thickening with or without ascites. |
Pearls
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When dealing with small amounts of solid tissue, be cautious about motion artifacts that may affect the region of interest (ROI) in each phase. Note, a small amount of solid tissue is defined as solid tissue measuring up to 3 mm.
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In cases of weak enhancement, consider evaluating the presence of a potential plateau before comparing it to the outer myometrium.
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For mixed solid tissue with cystic content demonstrating high signal intensity on T1WI, it is beneficial to examine subtraction images.