DOI: 10.1148/radiol.2241010361
(Radiology 2002;224:199-201.)
© RSNA, 2002
The Shading Sign1
Christine M. Glastonbury, MBBS, FRANZCR
1 From the Department of Radiology, Veterans Affairs Medical Center, 4150 Clement St, San Francisco, CA 94121. Received January 25, 2001; revision requested March 6; revision received March 29; accepted April 3. Address correspondence to the author (e-mail: Christine.Glastonbury@radiology.ucsf.edu).
Index terms: Endometriosis, 85.3192 • Ovary, cysts, 852.3192 • Ovary, MR, 852.121411, 852.121415 • Signs in Imaging
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APPEARANCE
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The shading sign is the magnetic resonance (MR) imaging finding of T2 shortening in an adnexal cyst that is hyperintense on T1-weighted images. The hypointensity was initially described as either focal or diffuse; however, the most common manifestation is complete loss of signal intensity or dependent layering with a hypointense fluid level (Figs 1, 2) (1–3).
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Figure 1a. Transverse MR images in a 33-year-old woman with a history of endometriosis and menorrhagia. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 (repetition time msec/echo time [effective] msec) image demonstrates a diffusely hyperintense right adnexal mass (arrow). (b) Corresponding T1-weighted fast spin-echo (496/14; echo train length, four) image obtained with chemical-selective fat saturation shows no loss in signal intensity of the mass (arrow), confirming that T1 shortening is not due to the presence of fat. (c) T2-weighted fast spin-echo (6,443/96; echo train length, 16) image obtained at the same level shows diffuse hypointensity (ie, shading) of the mass (large arrow). Small follicles are also present, confirming the ovarian nature of the mass (small arrows). This is a right ovarian endometrioma.
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Figure 1b. Transverse MR images in a 33-year-old woman with a history of endometriosis and menorrhagia. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 (repetition time msec/echo time [effective] msec) image demonstrates a diffusely hyperintense right adnexal mass (arrow). (b) Corresponding T1-weighted fast spin-echo (496/14; echo train length, four) image obtained with chemical-selective fat saturation shows no loss in signal intensity of the mass (arrow), confirming that T1 shortening is not due to the presence of fat. (c) T2-weighted fast spin-echo (6,443/96; echo train length, 16) image obtained at the same level shows diffuse hypointensity (ie, shading) of the mass (large arrow). Small follicles are also present, confirming the ovarian nature of the mass (small arrows). This is a right ovarian endometrioma.
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Figure 1c. Transverse MR images in a 33-year-old woman with a history of endometriosis and menorrhagia. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 (repetition time msec/echo time [effective] msec) image demonstrates a diffusely hyperintense right adnexal mass (arrow). (b) Corresponding T1-weighted fast spin-echo (496/14; echo train length, four) image obtained with chemical-selective fat saturation shows no loss in signal intensity of the mass (arrow), confirming that T1 shortening is not due to the presence of fat. (c) T2-weighted fast spin-echo (6,443/96; echo train length, 16) image obtained at the same level shows diffuse hypointensity (ie, shading) of the mass (large arrow). Small follicles are also present, confirming the ovarian nature of the mass (small arrows). This is a right ovarian endometrioma.
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Figure 2a. Transverse MR images in a 40-year-old infertile woman with a complex left ovarian mass that had been followed by means of serial ultrasonographic (US) examination over 3 months. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 image demonstrates diffuse hyperintensity of the left adnexal mass (arrow), which retains its signal intensity on (b) the T1-weighted fast spin-echo 496/14; echo train length, four image (arrow) obtained with chemical-selective fat saturation. (c) Corresponding T2-weighted fast spin-echo 6,443/96; echo train length, 16 image shows a fluid-fluid level (arrow) with marked shading (hypointensity) of the dependent portion. Pathologic examination after cystectomy revealed an ovarian endometrioma. f = uterine fibroid tissue.
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Figure 2b. Transverse MR images in a 40-year-old infertile woman with a complex left ovarian mass that had been followed by means of serial ultrasonographic (US) examination over 3 months. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 image demonstrates diffuse hyperintensity of the left adnexal mass (arrow), which retains its signal intensity on (b) the T1-weighted fast spin-echo 496/14; echo train length, four image (arrow) obtained with chemical-selective fat saturation. (c) Corresponding T2-weighted fast spin-echo 6,443/96; echo train length, 16 image shows a fluid-fluid level (arrow) with marked shading (hypointensity) of the dependent portion. Pathologic examination after cystectomy revealed an ovarian endometrioma. f = uterine fibroid tissue.
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Figure 2c. Transverse MR images in a 40-year-old infertile woman with a complex left ovarian mass that had been followed by means of serial ultrasonographic (US) examination over 3 months. Images were obtained with a 256 x 256 matrix, a 6-mm section thickness, and a 1-mm gap. (a) T1-weighted spin-echo 764/10 image demonstrates diffuse hyperintensity of the left adnexal mass (arrow), which retains its signal intensity on (b) the T1-weighted fast spin-echo 496/14; echo train length, four image (arrow) obtained with chemical-selective fat saturation. (c) Corresponding T2-weighted fast spin-echo 6,443/96; echo train length, 16 image shows a fluid-fluid level (arrow) with marked shading (hypointensity) of the dependent portion. Pathologic examination after cystectomy revealed an ovarian endometrioma. f = uterine fibroid tissue.
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EXPLANATION
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The precise mechanism of the shading sign is complex. Endometriotic cysts are highly viscous and have a high concentration of protein and iron from recurrent hemorrhage. All of these components can shorten T2 and may contribute to signal intensity loss, described as shading (3–5). In addition, intra- and extracellular methemoglobin markedly shorten the T1 of fluids (2,6). This results in hyperintensity on T1-weighted images and hypointensity (shading) on T2-weighted images.
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DISCUSSION
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The shading sign is a distinguishing feature of endometriotic cysts (endometriomas) at MR imaging. Such cysts are the principal imaging finding of endometriosis, a common condition in women of reproductive age. It is important to recognize endometriosis, since it may cause pelvic pain and reduce fertility (7).
Ectopic foci of endometrium seed the pelvic cavity and respond to estrogen and progesterone stimulation with cyclical bleeding. Endometriotic cysts are produced as these implants enlarge and are most commonly seen on the ovaries (8). Concentration of the cyst contents forms thick, chocolate-colored, aged blood, which has led to the gross description of "chocolate cysts." Recurrent internal hemorrhage and repeated rupture also produce multiple cysts and pelvic adhesions. These morphologic characteristics are particular to endometriomas and are key to the differentiation of adnexal masses at MR imaging (1,3).
The classic description of an endometrioma at US is that of a cystic mass with diffuse low-level echoes. The findings at computed tomography (CT) are more varied, and endometriomas may appear solid (9). The MR imaging criteria for diagnosis are either multiple cysts with hyperintensity on T1-weighted images or one or more cysts with hyperintensity on T1-weighted images and hypointensity (shading) on T2-weighted images. With these criteria, MR imaging has a diagnostic accuracy of 91%–96%, a sensitivity of 90%–92%, and a specificity of 91%–98% (1,7,10,11). MR imaging has been found to be more specific than either US or CT (3).
Any adnexal mass that is bright on T1-weighted images must be imaged with a chemical-selective fat-saturated sequence to exclude the possibility of a fat-containing teratomatous lesion (5,7,12). The most difficult differential diagnosis to make when the mass remains bright on fat-saturated T1-weighted images is that of a hemorrhagic adnexal cyst (most commonly, a corpus luteum cyst). While these cysts may be bright on T1-weighted images, they are usually solitary and thin walled and are brighter on T2-weighted images than are endometriomas (5,9). Marked loss of signal intensity on T2-weighted images is not usually seen with hemorrhagic cysts, since they do not repeatedly bleed. Without recurrent hemorrhage and concentration of contents, viscosity of the cyst remains lower, and shading is unlikely to be present (5,7). Endometriomas that are not bright on T1-weighted images may be difficult to distinguish from other adnexal masses (3).
MR imaging can demonstrate some solid endometrial implants but will not demonstrate adhesions and small focal implants (3,13). There is subsequently incomplete correlation of MR images with the true severity of disease, and laparoscopy remains the reference standard for diagnosis. MR imaging as a noninvasive modality is still useful when it can accurately characterize an adnexal mass and alter surgical decision making.
Patient symptoms and desire for fertility direct the treatment of endometriosis to medical, surgical, or "expectant" pathways (3). Medical treatment currently consists of hormones such as danazol or gonadotropin-releasing hormone analogues that suppress cyclical hemorrhage (3). Results of clinical studies have suggested that MR imaging may be useful in assessing therapeutic response to medical management, thereby limiting the necessity for repeat laparoscopy (14). The demonstration of a further decrease in signal intensity on T2-weighted images in response to gonadotropin-releasing hormone analogues may correlate with a good response to medical therapy (15). Results of another study suggested that larger endometriotic cysts that initially showed the shading sign were less likely to respond to medical treatment (8). It was proposed that more pronounced shading on T2-weighted images resulted from the presence of more concentrated blood products in older endometriomas. These cysts may have obliterated their glandular lining by means of internal pressure, and, thus, would not respond to hormone manipulation (8).
In summary, endometriosis is a common condition in women of childbearing age that may result in pelvic pain and infertility. The shading sign is an MR imaging feature of endometriosis that increases diagnostic specificity. Recognition of this sign may yield an earlier accurate diagnosis and aid in minimization of pelvic pain and optimization of fertility.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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