Role of CT & MRI in the Evaluation of Pathologies in the Female Pelvis

Role of CT & MRI in the Evaluation of Pathologies in the Female Pelvis Parveen Gulati and Peeyush Pandit Parveen Gulati Head, Dept. of Radiology & Imaging Pushpanjali Crosslay Hospital Vaishali, Ghaziabad (NCR) Peeyush Pandit Consultant Radiologist Dept. of Radiology & Imaging Pushpanjali Crosslay Hospital Vaishali, Ghaziabad (NCR) Normal Pelvic Anatomy Uterine corpus The normal position of the uterus varies widely. It is typically anteflexed and located centrally on the roof of the urinary bladder. Its size and perfusion vary during the menstrual cycle. The diameter of the uterus in women of reproductive age should not exceed 6 cm. Differentiation of the myometrium and endometrium by CT is not possible. With MR imaging, the corpus uteri is characterized by relatively low signal intensity on T1-weighted MR images. On T2-weighted images, the high-signal-intensity endometrium can be distinguished from the myometrium, which has intermediate signal intensity. On these images, the two layers are separated by a hypointense band, the so-called junctional zone, for which there is no clear histomorphologic correlate. The size of the endometrium varies between 1 and 7 mm through the menstrual cycle. In the postmenopausal uterus, these 3 zones are no longer discernible on T2-weighted images. The postmenopausal endometrium has a maximal thickness of 3-5 mm. Cervix and Vagina On CT scans, the cervix/corpus junction is discernible only at the isthmus of the organ. The cervix appears as a roundish to transverse oval structure with a maximum diameter of 3 cm. The cervix or cervical canal is occasionally seen as a central hypointense zone. On MRI, the cervix is characterized by a low signal intensity on T1 and T2-weighted images due to its high proportion of fibrous tissue. A central highsignal-intensity layer on T2-images corresponds to the epithelial and mucosal coat of the cervical canal. T2-weighted sequences allow a clear identification of the anterior and posterior vaginal layers, which are both of low signal intensity, as well as of the ventrally located urethra and the dorsally situated rectum. Parametrium The parametria are the spaces between the layers of the broad ligament extending form the side of the uterus to the wall of the pelvis. CT only visualizes the round and broad ligaments and occasionally the adnexa and ovaries. With MR imaging, the normal parametrial connective tissue can best be evaluated on T1-weighted axial images, on which it is distinguished from the hyperintense surrounding fatty tissue by its intermediate signal intensity. It shows moderate enhancement after application of Gd-DTPA. Ovaries The ovaries in women of reproductive age have a size of about 3-5 cm x 1.5-3 cm x 0.6-1.5 cm. Due to physiological atrophy; the postmenopausal ovaries are difficult to differentiate from surrounding structures. The advantages of CT Include its high spatial resolution and the absence of motion artifacts, while the radiation exposure is a major drawback, particularly in younger women, CT allows a good differentiation of cystic and solid tumors. The densities of cysts may reach rather high densities, but it is not always possible to differentiate them form solid lesions, whereas fat-containing teratomas (dermoid cysts) are easy to diagnose. The differentiation of benign from malignant ovarian tumors by both CT and MR imaging is based on morphologic criteria. Due to its high special resolution, CT offers good visualization of these morphologic features. On T1-weighted images, the normal ovary is characterized by a low signal intensity and isointensity with muscle or uterine tissue. On T2-weighted images, the ovaries have a signal intensity equivalent to that of fatty tissue and show areas of higher signal intensity corresponding to small cysts. The main advantage of MR imaging in the diagnostic evaluation of ovarian tumors is the high soft-tissue contrast, while the ability to freely orient the imaging plane is less important. Both cystic and solid lesions of the ovaries can be reliable delineated from the uterus, since the latter is distinguished by the high-signal-intensity endometrium and the low-signal-intensity myometrium on T2 weighted images. Especially large masses in the true pelvis may be difficult to differentiate by CT. Congenital Anomalies Congenital Anomalies fall into two major categories: a) Atresia and aplasia of uterus and vagina. b) Anomalies due to incomplete fusion or failure of reabsorption of the mullerian ducts. MRI is the imaging modality of choice for the evaluation of complete or partial vaginal agenesis: One can also demonstrate 19

the presence or absence of functional endometrium and cervix on MRI. This information is important for patient counseling regarding reproductive capability. Uterine agenesis or hypoplasia: T2 weighted sagittal images are best for determining the presence or absence of a uterus. There is no identifiable uterine tissue on MR images in uterine agenesis. In uterine hypoplasia, the endometrial cavity is small with a reduced intercornual diameter (less than 2 cm). Unicornuate uterus: The MRI appearance of unicornuate uterus is the elongated banana shape. MRI is more accurate than endovaginal Ultrasound (EVUS) for diagnosing the unicornuate uterus. Didelphys uterus (nonfusion of mullerian ducts): The MRI appearance is of two separate nominal sized uteri and cervices with a septum extending into upper vagina. The two uterine horns are usually widely separated and the endometrial and myometrial width are preserved. Rarely, separate hemiuteri with completely separate vagina, with two vaginal orifices can be seen. Bicornuate uterus (partial failure of mullerian duct fusion): The division may extend to the external os (uterus bicornuate bicollis) or to the internal os (uterus bicornuate unicollis). On MRI scans, the inter cornual distance is increased and there is an outward fundal concavity compared with the normal convexity. In arcuate uterus, the fundal surface is flattened or slightly concave, with a heart-shaped endometrial cavity. Hematosalpix or hematometra can develop in any uterine anomaly. MRI will demonstrate a dilated uterine horn distended with material showing signal intensity consistent with hemorrhage. Septate uterus (Failure of resorption of the final fibrous septum between the mullerian duct components): Septate uteri cause more frequent reproductive problems than bicornuate uteri, and are repaired by different surgical procedures. The septum is composed mainly of fibrous tissue, showing low signal intensity on T2 weighted images. If septum reaches to internal os, it is complete and if it terminates above the internal os, it is partial. The fundal contour on MRI is usually normal with an outward fundal convexity, differentiating it from bicornuate uterus. Accurate differentiation between the septate and bicornuate uterus is extremely important for patient management. The septate uterus is associated with higher rate of reproductive failure but can be repaired by hysteroscopic metroplasty. The bicornuate uterus which is a less often cause of fetal loss is repaired by a trans abdominal approach because of risk of perforating the myometrium. Uterine anomalies have been reported to be present in up to 50% of cases exposed in utero to DES (synthetic estrogen). Associated anomalies include hypoplasia, T-shaped uterus, contractions and marginal irregularities of endometrial cavity and result in an increased incidence of early reproductive loss. MRI may also be useful in evaluating patients with ambiguous genitalia. These patients are at a higher risk for gonadal tumors and may require radiologic evaluation. Uterine Tumors Adenomyosis: This is the presence of heterotopic endometrium within the myometrium. There are two forms of adenomyosis, a diffuse and a focal type. Diffuse type: On MRI images, the low signal intensity junctional zone is increased in thickness to varying degrees. Use of a junctional zone thickness of 12mm or more results in a high MRI accuracy in detection of adenomyosis. In some patients, high signal intensity foci can be detected within the thickened junctional zone on both T1W and T2W images, thought to represent hemorrhage. Focal type: This is seen as a mass of low signal intensity that appears similar to a leiomyoma and is sometimes called an adenomyoma. Differentiation between leiomyomas and focal adenomyosis is of primary clinical importance. An adenomyoma is oval in contour where as the leiomyoma is rounded. The margin of the adenomyoma at its interface with the myometrium is often irregular or ill defined, where as leiomyomas are sharply defined. Distortion of endometrium may occur if adenomyosis is extensive. The numerous dilated veins which often accompany a leiomyoma are not associated with adenomyosis. The signal intensity of adenomyosis is low on T2W images and on T1W images they blend with the surrounding myometrium. MRI is helpful in differentiating adenomyosis from leiomyomas. Leiomyomas: They are the most common uterine tumor. These may be solitary or multiple, and are found in submucosal, intramural, or subserosal sites within the uterine corpus (90%) or cervix (5%). A small number are found in the broad ligament or even detached from the uterus. Leiomyomas often calcify (mostly in post menopausal women), and large tumors may develop hyaline, myxomatous, cystic or hemorrhagic degeneration. Torsion, infection and sarcomatous degeneration are infrequent complications. In patients desirous of preserving fertility, myomectomy is increasingly performed as an alternative to hysterectomy. Ultrasound remains the initial imaging modality of choice MRI is the most accurate imaging technique for the detection and localization of leiomyomas. Tumors as small as 0.5 cm can be accurately seen and the precise location can be determined. On T2 weighted images, leiomyomas appear as sharply marginated homogeneous areas of decreased signal intensity. On T1weighted images, non degenerative leiomyomas display a well circumscribed, rounded appearance with medium signal intensity which is often indistinguishable from adjacent myometrium. Occasionally, calcification causes leiomyomas to appear low in signal intensity on both T1W and T2W scans. Degenerative leiomyomas demonstrate variable signal intensities on MRI imaging. The appearance of leiomyomas after administration of gadolinium is variable. Non degenerative leiomyomas may or may not enhance after contrast, depending on the vascularization, and degenerative lesions demonstrate variable enhancement. Endometrial polyps: These may be sessile or pedunculated, and are almost always benign. Malignant change is only rarely identified within them. On T2 W images endometrial polyp are suspected when the endometrial clarity is distended by an intermediate signal mass. On T1W images, endometral polyps demonstrate medium signal intensity similar to that of normal endometrium. Endometrial polyps enhance after gadolinium contrast, facilitating their detection. Endometrial carcinoma: Adenocarcinomas account for 90% to 95% of all endometrial carcinomas. Endometrial carcinoma initially invades the myometrium and the the endo cervix. After trans-serosal spread, direct invasion of the parametrium, bladder, or bowel occurs. Lymphangitic spread to pelvic and para aortic lymph nodes are frequent. Hematogeneous and intraperitoneal spread is rare, with the lungs and peritoneal surfaces of upper 20

abdomen being the most frequent sites of distant metastasis. Metastasis also may occur to liver, bone and brain. The uterus (if not removed surgically) and vaginal apex is the most common sites of local recurrence, which occurs most often within the first 3 years. Endometrial carcinoma is staged according to the FIGO staging system, which is now a surgical staging system. FIGO Staging I. Carinoma confined to uterine corpus A Tumour limited to endometrium B Tumour invasion through <50% myometrial thickness C Tumour invasion through >50% myometrial thickness II. Cervical Invasion A Endocervical glandular involvement only B Cervical stromal invasion III. Invasion of true pelvis A Invasion of serosa, adenexa, positive peritoneal cytology B Vaginal metastases. C Metastasis to Pelvic or para aortic lymph or inguinal nodes IV. Extension beyond true pelvis A Invasion of bladder or bowel mucosa. B Distant metastases. The prognosis is related to the nuclear grade, depth of myometrial invasion, stage of tumour and presence of lymph node metastasis. In the presence of deep (>50%) myometrial or cervical stromal invasion, preoperative intracavity radiation therapy or more extensive para-aortic lymph node sampling could be planned. CT has limitations in differentiating between stages I and II and also in consistently determining the depth of myometrial invasion. On non-enhanced MR images, the signal intensity of small endometrial cancers is often similar to that of normal endometrium. Indirect signs of presence of endometrial cancer include increased thickness or lobulation of the endometrial cavity, or the presence of heterogeneous but lower signal intensity mass on T2W images. After I/V contrast administration endometrial carcinoma demonstrates variable enhancement, which may be less or greater than that of myometrium. MRI is accurate is staging histologically documented endometrial carcinomas. In general, endometrial carcinomas enhance later than normal myometrium, hence on early phase dynamic scanning endometrial carcinomas will appear hypointense relative to endometrium. Early tumor enhancement may indicate more aggressive pathology. MRI is more accurate than CT or USG for detecting myometrial invasion. MRI is also sensitive in detecting cervical invasion. The most reliable MRI criterion for myometrial invasion is disruption of the functional zone. MRI can also be used to evaluate extrauterine spread (Stage III and IV) tumor Carcinoma of the Cervix: The commonest malignancy of female genital tract in India. Nearly 90% of all cervical carcinomas are squamous cell carcinomas. The prognosis is determined primarily by the tumor stage, histolgic grade, size (transverse diameter), location within the cervix (exocervix versus endocervix), depth of stromal invasion, adjacent tissue extension and presence of lymph node metastasis. Accurate staging is important not only for prognosis but also for the choice of optimal therapy. Cervical carcinoma spreads primarily via local extension or lymphangitic spread. Tumor may locally invade the vagina, lower uterine segment and parametrium to finally reach the pelvic sidewall. The ureters, bladder and rectum may be invaded as the disease advances. Lymphangitic spread occurs first to the internal and external iliac lymph nodes and later to the common iliac and paraaortic lymph nodes. If the tumor extends to lower vagina, metastasis may occur to inguinal nodes. Hematogenous spread occurs only in advanced disease, most commonly involves the chest. FIGO Staging for Carcinoma of Cervix 0 Carcinoma in situ I Carcinoma strictly confined to cervix I.a.1 Microscopic invasion I.a.2 Measurable invasion-depth of invasion from base of epithelium <5mm; horizontal spread <7mm. I.b.1 Lesion of greater dimension than stage I.a.2 but 4 cms. I.b.2 lesions >4cm II Carcinoma extends beyond cervix, but not to pelvic side wall or lower third of vagina. II.a No obvious parametrial involvement II.b Obvious parametrial involvement III Carcinoma extends to lower third of vagina or to pelvic side wall III.a Lower third of vagina involved, no extension to pelvic side wall. III.b Extension to pelvic sidewall, all cases with hydronephrosis or nonfunctioning kidney IV Carcinoma extends beyond true pelvis or involves bladder or rectal nucosa. Patients with carcinoma in situ (Stage 0) or micro invasive disease (Stage I.a.1 and I.a.2) are usually treated with simple hysterectomy. Patients with invasive carcinoma (Stage I.b) or Stage II.a disease are treated with radical hysterectomy and pelvic lymph node dissection. A patient with a large tumor or a barrel shaped cervix may be a candidate for adjacent radiation therapy. Patients with more advanced disease typically are treated with radiation therapy CT imaging has several drawbacks. Identifying parametrial extension by CT is difficult and CT is inaccurate in differentiating stage I.b from II.b disease, and in advanced disease, CT under stages pelvic sidewall involvement. Neither carcinoma in situ nor stage I.a tumor will be routinely identified on MRI. On T2W images, cervical carcinoma is seen as an abnormal area of high signal intensity, distinct from the normal lower signal intensity cervical stroma. On T1W images, the cervical mass is isointense with normal cervix, uterus and vagina. Macroinvasive cervical carcinoma, stage I.b appears on T2W images as intermediate signal mass that may expand the endocervical canal, the outer low signal intensity cervical stroma is preserved, the peri cervical tissue is normal. A tumor is considered stage II.a, when it extends into the upper two thirds of vagina. On T2W images, there is loss of the low signal intensity from the normal vaginal wall. Parametrial involvement (Stage II.b) is diagnosed when in 21

addition to loss of the low signal intensity cervical stroma, there is irregularity of the lateral cervical margin, parametrial mass, or stranding within the parametrial fat. Encasement of uterine vessels is helpful in diagnosing parametrial invasion. A tumor is classified as Stage III.a when it extends to the lower third of vagina, and as Stage III.b when it extends to the pelvic sidewall. The criterion for stage IV.a disease is tumor involvement of the bladder or bowel wall, which appears on T2W images as low of the normal low signal intensity or the wall of the organ. The use of contrast enhanced imaging is helpful in evaluation of advanced disease, especially when invasion of the bladder or rectum results in fistula formation. Presence of lymph node metastasis is an important prognostic factor, as it adversely affects prognosis. Lymph nodes larger than 1.5cm are considered abnormal. Lymph nodes between 1-1.5 cm in size, multiple lymph nodes are considered suspicious. MRI is both an accurate and a cost effective means of staging invasive cervical carcinoma, it is superior to either CT or clinical staging. MRI is also more accurate than CT or clinical staging in identifying stromal invasion, depth of invasion, assessing tumor size and parametrial status. MRI and CT are roughly equivalent in identifying lymph node metastasis. Fat suppressed imaging on MRI however improves the detection of parametrial spread. Ovary Ovarian Cysts: Simple serous cysts include follicular cysts, corpus luteal cysts, theca lutein cysts, and paratubal cysts. On MRI examination such cysts are homogenous with signal intensity isointense to urine on all pulse sequences - low signal intensity on T1W images and very high signal intensity on T2W images. Cysts filled with proteinaceous fluid may be high signal intensity on both T1W and T2W images. The cyst wall is thin and smooth. Hemorrhagic cysts are most often corpus luteal cysts. On MRI hemorrhagic cysts are commonly intermediate to high signal intensity on T1W images and lower signal intensity on heavily T2W images, although cysts appearing high signal intensity on both T1W and T2W images may also be seen. The cyst wall should be thin and smooth and may demonstrate intense enhancement after gadolinium. Layering (hematocrit effect) or debris may also be seen. Contrast administration may differentiate adherent clot from mural nodule because clot will not enhance after administration of gadolinium. Endometriosis: It is the presence of ectopic functioning endometrial cavity. It occurs predominantly in the reproductive age women (mostly between 25 to 30 yrs) and affecting as many as 40% of infertile women. The most common sites of involvement in descending order of frequency include ovaries, uterine ligaments, cul-de-sac and pelvic peritoneum covering the uterus, fallopian tubes, rectosigmoid and bladder. However, endometriosis in distant sites, including vagina, lymph nodes, lung, skeletal tissue and bone has been reported. On CT the appearance of endometrioma is varied, and sometimes lesions may appear solid. USG and CT are neither sensitive nor specific in diagnosis or staging of endometriosis. USG or CT cannot accurately identify small implants or adhesions. MRI has been reported to have a sensitivity of 90 to 92%, a specificity of 91 to 98% and an accuracy of 91 to 96% for diagnosing endometriomas. On MRI, endometriomas appear most commonly as multiple lesions with signal behavior consistent with hemorrhage of varying age. The most specific MR criteria include multiplicity, angular margins or distorted shape, and high signal intensity on T1W images with shading or low signal intensity on T2W images. Small implants may appear as areas of signal void, or solid contrast-enhancing lesions of intermediate signal on T1W images with punctate foci of high signal intensity. Other less specific features include adhesions (obliteration of fat planes between adjacent organs, low signal bands connecting or surrounding pelvic organs, and angulation of bowel loops) and a thick low signal intensity rim, which may enhance after contrast administration. Endometriomas may be high signal intensity on both T1W and T2W images. Endometriomas are bilateral in one third to one half of patients. T1W fat saturated images increase the sensitivity of MRI for diagnosis of small endometriomas (<1 cm). In larger lesions, presence of a malignant neoplasm arising within the cyst should be excluded, as this complication has been documented in 0.3 to 0.8% of patients with ovarian endometriosis. Complications are rare, but include massive ascites, hemorrhage of spontaneous rupture of an endometrioma. MRI may also have a role in monitoring treatment response in patients with endometriosis. There should be no loss of signal intensity of endometriomas on fat-saturation images. Blood clots or intracystic debris within the endometrioma should not enhance after gadolinium administration where as the presence of enhancing nodules or papillary excrescences raises the possibility of underlying malignancy. Benign Ovarian Neoplasms Benign cystic teratomas of ovary (Dermoid cysts): Most commonly diagnosed in women of reproductive age. On CT, the presence of fat (-130 to -90 ho) in an ovarian mass is pathognomonic of a dermoid cyst. On MRI, examination the key to diagnosing a dermoid cyst is the identification of fat within an adenxal mass. On SE sequences, lipid within the lesion will be isointense to subcutaneous or pelvic fat on all pulse sequences. The presence of chemical shift artifact at interface between fatty and non fatty tissue is a specific, but not sensitive MRI finding for teratoma. A T1 weighted fatsaturation sequence must be obtained to distinguish teratomas from endometriomas. MRI findings of fat fluid levels, gravity dependent layering of floating debris or hairball, palm tree like protrusions, mural nodules and areas of signal void assist in the diagnosis of cystic teratoma. Serous Cystadenomas: Cystadenomas are the most common benign ovarian neoplasms. They are bilateral in up to 20% of patients. Serous cystadenomas are usually cystic, unilocular, thin-walled lesions with a smooth outer surface. The signal intensity usually follows that of simple fluid but may vary with cyst contents. The presence of mucin or hemorrhage within the lesion may cause hyperintensity on T1W as wall as T2W image. Layering, fluid-fluid of levels and internal septations may be noted. Occasionally, regions of low signal intensity on all pulse sequences (psammomatous calcification) may be identified. Mucinous cystademomas: These are typically multilocular benign cystic neoplasms occurring as frequently as serous cystadenomas. Unlike serous cystadenomas, they are less 22

commonly bilateral and tend to be much larger. Pseudomyxoma peritonei is a complication that may result if contents of a mucinous cyst are spilled into peritoneal cavity. MRI appearance may be similar to that of serous cystadenomas. There is a tendency toward greater complexity, with layering, fluid-fluid levels, multiple septations and multilocularity more often noted. The mucinous material within cysts may appears high in signal intensity on T1W images Fibromas: These are solid benign ovarian neoplasms, usually seen in peri and post menopausal women. They are most commonly 5-10 cm in diameter. Bilaterality is seen in up to 10% of cases. On MRI, fibromas are usually well defined solid tumors, with intermediate signal intensity on T1W images and low signal intensity on T2W images. Irregular foci of hyperintensity may be seen on T2W images (areas of hyalinization and myxomatous change). The constellation of findings seen in Meig s syndrome (ovarian fibroma, hydrothorax and ascites) occur in 1% of patients with ovarian fibromas. Ovarian Carcinoma It is predominantly a disease of peri and postmenopausal women. Ovarian carcinoma usually spreads by direct shedding into the peritoneal cavity, local extension or lymphatic dissemination. Hematogeneous spread is rare. The most common pathway for spread is through peritoneal space. Malignant cells are carried by peritoneal fluid from the pelvis and cul-de-sac inferiorly to the diaphragm superiorly. Peritoneal fluid extends mainly along the right paracolic gutter to the liver surface and right hemidiaphragm. Malignant cells may pass through diaphragmatic lymphatics to implant on the pleura producing a malignant pleural effusion. The omentum is involved in nearly 100% of patients dying of ovarian carcinoma. The ovarian lymphatics drain primarily to para-aortic nodes and secondarily to pelvic and inguinal lymph nodes. Hematogenous spread is less common and may occur as a late manifestation. Distant organs at risk of metastasis in decreasing order of frequency are liver, lung, pleura, kidney, bone, adrenal gland, bladder and spleen. FIGO Staging System I. Limited to ovaries A. One ovary/no tumor on external surface B. Both ovaries/no tumor on external surface C. Malignant ascites/positive peritoneal washings/tumor on surface of ovaries II. Pelvic extension. A. Uterus or fallopian tube involved B. Extension to other pelvic tissues C. Malignant ascites/positive peritoneal washings III. Introperitoneal metastases outside of pelvis, including small bowel and omentum, positive retroperitoneal or inguinal lymphnodes A. Microscopic seeding. B. Implants < 2 cm. C. Implants > 2 cm and/or positive lymphnodes IV. Distant metastases, including positive pleural fluid, parenchymal liver metastases. Aggressive debulking surgery, followed by combination chemotherapy is the gold standard of treatment. The prognosis is dependent on the stage of tumor at presentation, volume of tumor remaining after debulking surgery, histological grade, and the age of patient. MRI Characterization of Carcinoma Ovary The role of imaging lies in lesion detection, characterization, staging and follow up. In the detection of adenexal lesions, sensitivity of MRI is comparable to USG and CT. The ability of MRI to characterize adenexal tumors as benign or malignant with high specificity has also been shown. In the characterization of adenexal masses, MRI is more accurate than either USG or CT. MRI is particularly useful for differentiating the uterine versus adenexal origin ovarian masses. On MRI, ovarian carcinoma usually appears as a large (>4cm), heterogenous, solid and cystic lesion which is not uncommonly bilateral. Well differentiated tumors of low malignant potential appear primarily cystic, with intracystic vegetations present, while undifferentiated tumours have large amounts of solid tissue, necrosis and hemorrhage. Solid components of ovarian carcinoma usually appear as low to intermediate signal intensity on T1W images and high signal intensity on T2W images, however signal may vary depending on whether hemorrhage or necrosis is present. The cystic component of ovarian carcinoma may also have a variable appearance on MRI, depending on cyst contents. Cystic ovarian carcinomas containing proteinaceous or hemorrhagic material may manifest high signal intensity on both T1W and images. Cystic carcinomas may demonstrate thick walls or septa (<3mm), and contain vegetations or regions of soft tissue nodularity. The findings are best demonstrated on either T2W or Gadolinium enhanced T1W images. Use of Gadolinium expedites lesion characterization by facilitating assessment of intratumoral architecture. Gadolinium is useful in determining wall thickness, presence and thickness of septations, identification of vegetations or soft tissue nodules, and depiction of tumor necrosis. Gd enhanced MRI is more accurate than noncontrast MRI and endovaginal sonography in differentiating benign from malignant lesions. MRI is as accurate as CT in ovarian cancer staging. Prediction of tumor resectability is excellent for both MRI and CT. Criteria for tumor non resectability include: a) Metastases >2 cm at root of mesentry, gastrosplenic ligament, omentum of the lesser sac, porta hepatis, intersegmental tissue of the liver, diaphragn or dome of liver b) Enlarged (short axis >1cm) lymph nodes at or superior to the oveliac axis c) Presence of presacral extraperitoneal disease MRI may be superior to CT in assessment of pelvic involvement by ovarian carcinoma. The presence of ureteral obstruction with hydronephrosis as well as disease extension to the pelvic side wall or retroperitoneum may be seen on MRI. Outside the pelvis, MRI maybe useful in detecting ascites, peritoneal implants, omental cake and mesenteric disease. Implants demonstrate low signal intensity on TW images and enhance post gadolinium. When involved by metastasis, omentum is enlarged and may be seen as intermediate signal intensity crescent-shaped mass surrounded by ascites. It enhances diffusely after Gadolinium administration MRI detection rate for pelvic and retroperitoneal lymphadenopathy is very good and equals that of CT. MRI is slightly better than CT in assessing colonic and uterine invasion as well. 23