An understanding of soft tissue and bony pelvic anatomy on computed tomography (CT) aids in the detection of pelvic diseases and recognition of their patterns of spread. Nearly all major organ systems are represented in the pelvis, including the gastrointestinal (GI), genitourinary, central and peripheral nervous, angiolymphatic, and musculoskeletal systems. The limits of the pelvis are defined by the innominate bones, sacrum, and coccyx. The internal and external pelvic muscular and bony frameworks are relevant, as disease processes arising in these structures sometimes manifest within the pelvic cavity (Figs. 29.1,29.2).
The pelvic peritoneal reflections delineate significant but not insurmountable boundaries between extraperitoneal structures and the peritoneal cavity. The peritoneal reflections over the pelvic organs, vessels, and ligaments define several peritoneal pelvic cavity recesses, the largest and most gravity-dependent of which is the rectovesicle pouch. The rectovesicle pouch, along with the pelvic portion of the greater omentum, is one of the most common sites of tumor implants in peritoneal carcinomatosis.
Pelvic peritoneal reflections often form anatomical landmarks recognizable on CT images where they envelop organs and the associated vascular and lymphatic systems. The peritoneal reflection forming the sigmoid mesocolon encompasses a portion of the proximal rectum and sigmoid colon, along with their associated angiolymphatic structures. The sigmoid mesocolon is more easily seen in the presence of ascites (Fig. 29.3). In addition, the small bowel and its mesentery, as well as their associated pathologic processes, often extend into the pelvis (Fig. 29. 4).
In women, the broad ligament is that part of the peritoneal reflection surrounding the margins of the uterus, fallopian tubes, and proximal round ligaments extending to the pelvic sidewall. The ovary is attached to the posterior broad ligament by the mesovarium. The peripheral broad ligament from the ovary to the pelvic sidewall that contains the ovarian vessels is referred to as the suspensory ligament of the ovary. The uterus divides the rectovesical pouch into the anterior vesicouterine pouch and the posterior rectouterine pouch, also known as the cul-de-sac or pouch of Douglas.
Although the remaining pelvic structures, such as the rectum, anus, distal ureters, bladder, cervix, vagina, seminal vesicles, prostate, and sidewall structures, do not readily protrude into the pelvic peritoneal cavity, they are often associated with fascial planes that frequently confine pathologic processes. These boundaries localize lesions to their site of origin, guiding the differential diagnosis of a pelvic mass.
Ectopic organs, such as pelvic kidneys (Fig. 29.5), are readily identified on CT. Pelvic testicles present as elliptical foci along the course of the testicles’ descent, frequently within the inguinal canal (Fig. 29.6).
Some surgical procedures may result in pseudolesions due to the displacement of normal pelvic structures. In particular, abdominoperineal resection of the rectum can result in a posterior presacral position of the uterus in women or the prostate and seminal vesicles in men (Fig. 29.7). These normal presacral masses must be distinguished from presacral recurrent tumor and postoperative fibrosis. If a suspicious mass is encountered, biopsy is warranted. If there is question that a presacral mass is a displaced normal structure such as the seminal vesicles or uterus, magnetic resonance imaging (MRI) is often helpful.
Midline or lateral surgical transposition of the ovaries in women can also be problematic if the history of this procedure is not elicited, especially if there is a superimposed functional cyst. Lateral surgical ovarian transposition procedures are sometimes performed to preserve ovarian function in young women with cervical carcinoma who will receive radiation therapy. The ovaries are placed in the iliac fossae or high in the lateral abdomen (Fig. 29.8). Midline ovarian transposition may be performed in young women who will receive pelvic nodal radiation for lymphoma.
Other iatrogenic “pseudolesions” are intestinal urinary reservoirs in patients with urinary diversions, tissue expanders to displace bowel from radiation ports prior to radiation therapy (Fig. 29.9), and implanted reservoirs for penile prostheses and artificial sphincters.
Renal and pancreatic transplants are usually placed within the pelvis (Fig. 29.10). Although they are typically imaged with ultrasound, CT is occasionally performed. The pancreatic transplant is often connected to the bladder by a segment of bowel to allow drainage of exocrine fluids. Care must be taken to avoid confusing this with an abnormal collection. Renal and pancreatic peritransplant collections, such as hematomas, seromas, abscesses, urinomas, lymphoceles, and pancreatic pseudocysts, are readily detected with CT.
The accompanying tables of differential diagnoses of pelvic pathologic processes are predominantly organized by organ of origin. However, because the exact site of origin is not always evident, tables relating to a variety of lesions disseminating within the peritoneal cavity and originating from the pelvic cavity walls are also included. Table 29.1 discusses pelvic GI lesions, Table 29.2 lesions of the female and male pelvis, Table 29.3 urinary bladder lesions, and Table 29.4 peritoneal and extraperitoneal lesions.
Segmental wall thickening or polypoid luminal masses. Search for lymph nodes, hepatic metastases, and peritoneal or omental tumor implants.
Often difficult to distinguish bowel contractions and luminal fecal material from focal lesion. Rescan area of concern or barium enema versus small bowel follow-through may be necessary to confirm in the absence of associated secondary signs of tumor (e.g., perienteric fat infiltration).
Segmental or diffuse, irregular or smooth bowel wall thickening, sometimes associated with luminal expansion. May occur with or without adenopathy.
These are typically non–Hodgkin lymphomas. Gastrointestinal (GI) lymphoma may also be encountered in patients positive for human immunodeficiency virus (HIV) and in posttransplant lymphoproliferative disorder.
Polypoid luminal or exophytic intermediate-attenuation mass causing luminal narrowing. The presence of calcifications would favor this diagnosis. Lipomas (see also Fig. 29.25) typically are polypoid and usually have uniform fat attenuation.
Other types of mesenchymal neoplasms are leiomyomas, leiomyosarcomas, and carcinoid tumors. Cystic necrosis is often seen in leiomyosarcomas. Carcinoid tumors may have associated mesenteric fibrotic response, which can lead to intestinal ischemia.
Soft tissue nodules, adjacent to or impinging on the small bowel lumen. Scirrhous lesions, intramural deposits, or polypoid intraluminal masses may be present. The greatest portion of the metastatic deposit is located in the mesentery adjacent to the wall of the small bowel. An omental “cake” may be recognized as an extensive soft tissue mass separating the colon or small bowel from the anterior abdominal wall.
Most common primary tumors are carcinomas of lung, breast, colon, pancreas, kidney, uterus, and skin.
Segmental or diffuse, irregular or smooth bowel wall thickening, with or without perienteric infiltration.
Examples include Crohn disease, which may involve several bowel segments, with or without fistulas; typically spares rectum. May also observe associated mesenteric fibrofatty proliferation. Graft versus host disease can lead to fold and wall thickening and can be associated with pneumatosis intestinalis. Radiation enteritis can be acute and present within a few weeks of therapy, or chronic, manifesting a few months or more after therapy. Acute effects are due to direct cellular toxicity and chronic effects from a radiation-induced vasculitis.
Short-axis section of bowel produces target appearance, and long-axis section of bowel produces ribbon appearance of involved segment with or without associated high-attenuation oral contrast and low-attenuation mesenteric fat.
Often idiopathic in children. Identifiable causes are more common in adults and include polypoid neoplasms and Meckel diverticula.
Cystic or tubular masses in or adjacent to the bowel wall.
Duplication cysts may communicate with the bowel or be associated with rectogenitourinary fistulas, duplications of genital structures, or skeletal abnormalities.
Cystic and/or solid masses with or without ascites and omental or peritoneal surface tumor implants. Debris and septations within cysts are sometimes difficult to recognize on CT.
Isolated cystic masses < 5 cm in women of childbearing age are most often benign functional cysts, but if > 2.5 cm, masses require short-interval follow-up by clinical exam or ultrasound to ensure resolution.
Cystic and/or solid masses. In the absence of obvious metastases, benign neoplasms cannot be reliably distinguished from malignant neoplasms on CT.
Includes adenomas, fibromas, serous or mucinous cystadenomas, and dermoids/teratomas. Meigs syndrome is the association of ovarian fibroma, hydrothorax (usually right-sided), and ascites.
May appear cystic or solid, depending on the amount of associated internal debris/hemorrhage. Polycystic ovaries (Stein–Leventhal syndrome) often manifest as bilateral ovarian enlargement, but the cysts are small (usually < 1.5 cm).
Often a consideration in women of childbearing age (see comments on carcinoma). Small cystic adnexal lesions in postmenopausal women on CT are not uncommon. If the cysts lack septations or enhancing components and are of water attenuation, they can represent “retained” functional cysts; if < 3 cm, they sometimes can be followed. Caution must be exercised in the setting of family history of ovarian carcinoma and for the larger cystic lesions. Congenital cysts are rare.
Focal or diffuse mass, often lower attenuation than myometrium following intravenous (IV) contrast. Often originates centrally; if associated with obstruction, nonenhancing hydrometrium/hematometrium may be visible.
Actual neoplastic focus can be inapparent, and frequently the only CT manifestation is an obstructed, fluid-filled uterus. Often impossible to distinguish benign cervical stenosis obstructing the uterus from malignant obstruction.
Gestational trophoblastic disease
All forms have a central, predominantly low-attenuation mass with areas of enhancement after contrast administration. The uterus may be enlarged. Chorio-carcinoma and invasive mole may have myometrial invasion. Choriocarcinoma may metastasize, typically to lungs, vagina, brain, liver, kidney, and bones.
Spectrum of proliferative pregnancy-related trophoblastic tissue includes the hydatidiform mole, invasive mole, and choriocarcinoma. They are typically diagnosed with β-human chorionic gonadotropin (HCG) levels and ultrasound. There may be associated enlarged ovaries due to thecal-luteal cysts.
Enlarged uterus with low- or isoattenuating masses often indistinguishable from other malignancies. The tumor may occupy and enlarge the central uterine cavity or be eccentric and focal with endoexophytic appearance.
Large tumors are often associated with “necrotic” appearance. Malignant change in a leiomyoma is rare, and by radiologic means alone, it is difficult to distinguish a focal leiomyoma from leiomyosarcoma. Rapid changes in size of a known lesion should be considered suspicious.
Diffuse uterine and/or cervical enlargement or lobular contours mimicking fibroids.
Secondary involvement of female genitourinary (GU) tract is not uncommon in advanced non–Hodgkin lymphoma; primary lymphoma is rare.
Low- or isoattenuating masses, often more conspicuous after contrast. May be pedunculated, with or without calcifications. Uncommonly associated with areas of fatty change.
The most common uterine mass affecting 25% of women of reproductive age; often present in patients with other types of uterine or adnexal pathology,
Diffuse uterine enlargement or lobular wall thickening.
Benign endometrial proliferative conditions
Central uterine low attenuation following IV contrast; may be indistinguishable from retained fluid.
Types include endometrial hyperplasia and polyps. In postmenopausal women, hormone replacement therapy may stimulate the endometrium. In premenopausal women, the endometrium is most prominent in the secretory (luteal) phase (usually < 1.5 cm thickness). Gestational sacs and decidual reactions (casts) in ectopic pregnancy should be considered, but they are unlikely possibilities because of routine pre-CT screening of women of childbearing age.
Single or multiple typically round, low-attenuation cervical foci.
Form secondary to stenosis of the cervical glands. Differential diagnosis is with small cervical fibroids.
In premenopausal women, uterine infection is usually associated with pelvic inflammatory disease or prior surgery. In postmenopausal women, there is often an associated malignancy. Postpartum uterine cavity air may be seen normally in the immediate period following uncomplicated cesarean sections and vaginal deliveries and is therefore not absolutely pathognomonic of infection in this setting.
Vagina and vulva
For primary malignant tumors, a locally invasive mass may be evident on CT. In vulvar carcinoma, early metastatic nodal involvement of inguinal and subsequently pelvic nodes occurs. Tumors involving the proximal two thirds of the vagina first metastasize to deep pelvic nodes and tumors of the distal one third to inguinal nodes.
Vaginal and vulvar carcinomas are most frequently squamous cell carcinomas (85%–95%). Clear cell vaginal adenocarcinoma is associated with in utero exposure to diethylstilbestrol (DES). Direct vaginal invasion by cervical or uterine malignancies and metastatic vaginal lesions are additional considerations when encountering vaginal masses. Nonmalignant lesions, including endometriosis, can occur in the vagina and mimic malignancies.
Various malformations, including vaginal absence, atresia, septations, and duplications. Hydro- or hematocolpos (with or without uterine obstruction) may present as a low-attenuation central mass. Cystic peripheral vaginal area lesions may be due to Gartner duct cysts (wolffian duct remnants) or ectopic ureters.
Variable attenuation, frequently unrecognizable on CT in the absence of gross extraprostatic extension of disease. Often superimposed on benign prostatic hypertrophy (BPH) and biopsy changes.
Associated signs of advanced disease include seminal vesicle enlargement due to invasion (vs obstruction), adenopathy, and blastic pelvic bone metastases (vs bone islands).
Benign prostatic hypertrophy (BPH)
Diffuse, usually symmetric homogeneous or heterogeneous gland. Associated secondary signs of outlet obstruction may be present (thick wall bladder, bladder diverticula, and hydronephrosis).
Weight can be estimated from volume assuming a specific gravity for prostate tissue of 1.05 using the formula weight = 0.55 − length − AP − transverse extent.
Normal prostates are usually < 30 g in adults.
Generalized enlargement, sometimes with focal fluid collections.
Focal low-attenuation lesions. If transurethral resection of the prostate (TURP) defect at bladder base, may fill with bladder contrast.
Nonsurgical midline foci are typically congenital, such as utricle and müllerian duct cysts. Eccentric foci often represent acquired processes, such as retention cysts, cystic degeneration in BPH, and abscesses. Ectopic ureters may open into the urethra or genital tract, and the most distal ureter is often dilated.
Usually unilateral, homogeneous or inhomogeneous mass.
Primary benign and malignant seminal vesicle tumors are uncommon; secondary involvement from prostate, bladder, or rectal carcinomas occurs more frequently. May be difficult to distinguish from nonneoplastic cystic seminal vesicles.
Low-attenuation mass, unilateral or bilateral; look for associated urinary tract abnormalities (including cryptorchidism).
Congenital: Often associated with renal and/or collecting system anomalies (e.g., agenesis, duplication, and ectopic ureters).
Acquired: As a result of obstruction, (e.g., due to prostate carcinoma). An association between bilateral cystic seminal vesicle changes and polycystic kidneys has been described.
Infection and hemorrhage
Often indistinguishable from and sometimes superimposed on seminal vesical neoplasms or obstructive processes. On unenhanced CT scan, acute hemorrhage (e.g., from prostate biopsies) may be of high attenuation compared with adjacent tissue.
Polypoid or plaquelike areas of wall thickening. Perivesicle fat involvement manifests as irregular projections from bladder tumor margins.
Urachal remnant tumors (see also Fig. 29.58,p. 831) may be bilobed with intra- and extravesicle involvement of the bladder dome. Most commonly transitional cell carcinoma (~90%). Other types include squamous cell carcinoma (can be associated with schistosomiasis or bladder diverticula) and adenocarcinoma (associated with urachal remnant at the bladder dome).
Focal wall thickening or discontinuity with bladder contrast and/or fluid in an intra- or extraperitoneal location with or without pelvic fractures. Interstitial (mural) extravasation should be distinguished from diverticula. Diverticula are typically associated with other signs of outlet obstruction.
In the setting of blunt trauma, extraperitoneal bladder rupture often associated with pelvic fractures and intraperitoneal bladder rupture typically occurs with a full bladder.
Delayed images, to allow bladder filling, may be needed to document bladder injury.
Focal mass at bladder dome aligned with umbilicus with or without communication with bladder lumen.
Spectrum of benign urachal remnant findings includes cysts, diverticula, and sinuses.
Single or multiple rounded or elliptical foci, typically in a dependent location.
Frequently associated with signs of bladder outlet obstruction, such as diverticula and thick wall bladder.
Malacoplakia may present as a mass in a setting of recurrent infections and may extend outside the bladder wall. Wall calcifications described in alkaline encrustation cystitis, tuberculosis (TB), schistosomiasis, and, rarely, with tumors.
Focal inflammatory bladder masses, such as malacoplakia, occur as a response to bladder infection or secondary to bladder involvement from the sigmoid colon (e.g., diverticulitis) or adnexal (e.g., TOA) inflammatory processes adjacent to the bladder.
Cystic focus most commonly at the ureterovesicle junction with or without distal ureteral dilation.
Intra- or extramural ureteroceles can mimic diverticula. Ectopic ureteroceles extend into the bladder neck and/or urethra.
Bladder outlet obstruction
Most commonly associated with diffuse wall thickening, bladder diverticula, and enlarged prostate gland in men.
Typically uniform, diffuse, smooth or irregular wall thickening with or without perivesical fat infiltrative changes.
Includes infectious causes as well as mechanical or chemical irritation (e.g., Foley catheter or cyclophosphamide) and radiation changes. Emphysematous cystitis most commonly occurs in diabetics secondary to Escherichia coli. In hemorrhagic cystitis, high-attenuation bladder contents due to blood may be seen.
Carcinoma and lymphoma
Focal or diffuse process.
Primary bladder lymphoma is uncommon. Uniform, circumferential transitional cell carcinoma is uncommon, and there is often variable wall thickening when there is widespread mural tumor (see also Fig. 29.49p. 828).
Table 29.4 Pelvic peritoneal and extraperitoneal lesions
Low-attenuation pelvic cavity material with mass effect with or without septations or calcifications. May be of higher attenuation than water.
This is a special form of peritoneal dissemination of disease with gelatinous material distributed in the peritoneal cavity. Etiologies for the mucinous material include cystadenomas and cystadenocarcinomas of the appendix or ovary.
Solid or cystic masses or plaques involving peritoneal margins, mesentery, or omentum with or without ascites.
Malignant peritoneal mesothelioma is an extremely rare cancer, accounting for < 30% of all mesothelioma cases. Because pleural mesothelioma is more common and often spreads to the peritoneal cavity, it is important to determine if pleural mesothelioma is the primary cancer.
Venous thrombi typically are of lower attenuation than enhanced blood; however, acute thrombus can also be iso- or hyperattenuating. A varix is a focal dilation of a vein.
A varix is often idiopathic or may result from more cephalad vascular obstructions, such as IVC thrombosis and left common iliac artery compression of vein. Uncommonly, a varix is due to arteriovenous fistula where venous distention results from shunting of arterial pressures.
Most commonly musculoskeletal or GI origin fluid and/or air collections. Lymphoceles, urinomas, and hematomas may become secondarily infected.
Most commonly result of trauma or GI fistulas, such as Crohn disease.
Mixed attenuation masses with or without associated pelvic fractures. Rectus sheath hematomas may track into retropubic space or pelvic sidewall tissue planes.
May follow minor trauma in anticoagulated patients. May see typical “molar tooth” configuration in retropubic space of Retzius. Hemophiliacs may present with pelvic musculoskeletal pseudotumors secondary to hemorrhage.
Fluid or higher attenuation collection (with or without iodinated contrast) adjacent to ureters or bladder. Delayed scans following IV contrast administration may be helpful.
Etiologies include blunt and penetrating trauma and from surgical procedures, such as abdominoperineal resections, urinary diversions, and prostatectomies.
Soft tissue attenuation mass (es). Most commonly affects area of lower lumbar spine/lumbosacral junction. May cause vascular or urinary tract obstructions. Most commonly idiopathic, but may occur as a desmoplastic response to tumors.
Bilateral, symmetric, fat-attenuation masses displacing and compressing central and lateral pelvic structures.
May cause vascular or urinary tract obstructions.
Typically arise from pelvic musculoskeletal framework or supporting tissues.
Uncommon; most pelvic sidewall masses are of lymph node or vascular origin. Anterior pelvic wall tumors include desmoids (especially in the rectus sheath; see Fig. 29.1,p. 814). These show intense contrast enhancement. They may enlarge during pregnancy. Desmoids are sometimes seen in Gardner syndrome. Posterior midline lesions include chordomas, sacrococcygeal teratomas, and metastases or local recurrence of rectal carcinomas.