Pelvic inflammatory disease (PID) is a general term indicating infection of the female upper genital tract and the surrounding structures. [1, 2] It is a common and serious complication of sexually transmitted disease. Acute episodes need appropriate care and treatment; however, it is the long-term sequelae of chronic pelvic pain, infertility, and ectopic pregnancy due to scarring and adhesions that burden the healthcare system in an adverse manner. 
It encompasses a broad category of diseases, including endometritis, salpingitis, salpingo-oophoritis, tubo-ovarian abscess (TOA), and pelvic peritonitis. The afflicted women may be asymptomatic, present with mild nonspecific symptoms, or may have fulminant disease. Prompt diagnosis and treatment of this condition are critical because the complications of PID can be life and fertility threatening.  The varied clinical presentation and imaging findings may make it difficult to diagnose PID, and sometimes it may remain undetected. 
While clinical and laboratory findings are considered sufficient to initiate treatment of PID, this approach may be incorrect, as seen on laparoscopy by Molander et al.  Laparoscopy has been considered the standard for the diagnoses of PID, but it is costly, invasive, and has a reduced sensitivity in mild disease.  Endometrial biopsy has a sensitivity of 92% and a specificity of 87% compared with laparoscopy, as demonstrated in a study by Paavonen et al. 
Imaging is therefore being increasingly used for the diagnosis of PID, particularly in patients with an uncertain diagnosis, in patients with chronic or complex disease, or in patients who have developed complications. Awareness of the various imaging findings is important to be able to suggest and confirm the clinical diagnosis of PID, facilitate timely and appropriate treatment, avert the chronic complications, and decrease morbidity.
PID is a complex polymicrobial disease that is due to the ascending spread of pathogens from the cervix or vagina, most commonly Chlamydia trachomatis or Neisseria gonorrhoeae (60-75%) , which then spreads into the endometrium, fallopian tubes, ovaries, and adjacent structures.  Of women with inadequately treated chlamydia or gonorrhea, 10-20% may develop PID. [7, 9]
Other pathogens include Mycoplasma hominis, Haemophilus influenzae, Streptococcus pyogenes, Bacteroides species, and Peptostreptococcus species. Less commonly, direct spread from a nearby infection such as appendicitis or diverticulitis may occur. Hematogenous infection is a rare cause of PID except in cases of tuberculous PID. [10, 11]
Douching is a potential risk factor for PID as it can result in a change of the vaginal flora and introduce bacteria from the vagina into the upper reproductive organs. Usage of intrauterine contraceptive device or gynecologic interventions may also predispose a patient to PID. Direct extension of infection from adjacent viscera and uterine instrumentation are more important risk factors in postmenopausal PID. 
Annually, approximately 1 million women develop PID.  An estimated 1 in 8 sexually active adolescent girls develop PID before reaching age 20 years. Because PID may be asymptomatic and is frequently undiagnosed, the incidence rate is likely higher.
PID contributes to approximately 2.5 million office visits and 125,000-150,000 hospitalizations every year. 
The annual incidence of PID in females aged 15-39 years seems to be 10-13 cases per 1000 women, with a peak incidence of about 20 cases per 1000 women in those aged 20-24 years. 
According to 2008 WHO estimates, 499 million new cases of curable sexually transmitted infections (ie, syphilis, gonorrhoea, chlamydia, trichomoniasis) occur annually throughout the world in adults aged 15-49 years.  In developing countries, sexually transmitted infections and their complications rank in the top 5 disease categories for which adults seek health care. In addition, antimicrobial resistance, in particular for gonorrhoea, is becoming increasingly widespread. In sub-Saharan Africa, untreated genital infection may account for up to 85% of infertility among women.
Race- and age-related prevalence
The rate of PID in black women is 2-3 times higher than that in white women. This difference is explained by the marked racial disparity in the rates of chlamydia and gonorrhea. 
PID is most commonly seen in young women and rarely in postmenopausal women. A series reported less than 2% of TOAs in postmenopausal women. 
Mortality and morbidity
Complicated PID resulting in tubo-ovarian or pelvic abscess may contribute to patient mortality.
Lower abdominal pain and abnormal vaginal discharge are common symptoms. Cervical motion and adnexal tenderness are often elicited on physical examination. In severe cases, patients may present with toxemia and signs of infection such as fever, leukocytosis, elevated erythrocyte sedimentation rate or C-reactive protein level, and laboratory documentation of cervical infection. A palpable adnexal mass may be seen in complicated PID with TOA.
The Centers for Disease Control and Prevention (CDC) has established criteria for the diagnosis of PID. 
Ultrasound- or CT-guided aspiration/drainage may be performed for tubo-ovarian or pelvic abscess, with the addition of antibiotic coverage. Preservation of the ovaries is an advantage of image-guided drainage over surgery. 
Ultrasonography should be the first diagnostic imaging examination to be performed in cases of suspected PID in which there are ambiguous or unexplained clinical findings or an inability to perform an adequate clinical examination. Ultrasonography is also indicated to evaluate for complications of PID, which may impact operative versus nonoperative management or the decision to hospitalize a patient. This modality is readily available, noninvasive, and can be performed at the patient's bedside. [4, 21, 22, 23, 24]
Most often, ultrasonography is preferred over CT scanning as the triaging tool in a female child or adolescent with right lower quadrant or pelvic pain, particularly because of concerns about radiation exposure. Transvaginal sonography allows detailed visualization of the uterus and adnexa, including the ovaries and thickened fallopian tubes. Transabdominal sonography is complementary to the endovaginal examination because it provides a more global view of the pelvic contents. Whether transabdominal sonography (bladder filling required) or transvaginal sonography (bladder filling not required) is performed first and whether the complementary examination is needed for a final diagnosis is a matter of individual clinical imaging practice. [25, 22, 26, 27]
MRI serves as an excellent imaging modality in cases in which the ultrasonographic findings are equivocal. In a study by Tukeva et al, the authors compared findings from MRI with sonograms and found that MRI was more accurate than ultrasonography in the diagnosis of PID. 
Occasionally, CT scanning may be used as the initial diagnostic study for the investigation of nonspecific pelvic pain in a female, and PID may be found incidentally. CT scanning is very sensitive for the detection of pelvic pathology; however, it may not be as specific as sonography when an adnexal pathology must be differentiated into a tubal or ovarian one. If the diagnosis of PID is still in question, confirmation with ultrasonography is suggested.
See the PID and tubo-ovarian images below.
In early pelvic inflammatory disease (PID), CT findings are obscuration of the normal pelvic floor fascial planes, thickening of the uterosacral ligaments, cervicitis, oophoritis, salpingitis, and accumulation of simple fluid in the endometrial canal (see the image below), fallopian tubes, and pelvis.
In fact, mild inflammatory changes are seen better with CT scanning than with sonography. Pelvic fat stranding and enhancement of the endocervical canal, endometrium, and peritoneum are well appreciated on CT scans (see the images below). 
Later, there may be reactive inflammation in the adjacent organs manifesting as small or large bowel ileus or obstruction, hydronephrosis or hydroureter, and right upper quadrant inflammation (Fitz-Hugh-Curtis syndrome) (see the images below). 
In a retrospective study of CT images of 32 women with clinically proven acute PID, diagnostic morphological findings in women with clinically proven PID included hepatic capsular enhancement on late arterial phase and tubal thickening. 
As the disease progresses, the fallopian tubes become thicker with enhancement and fill with complex fluid forming a pyosalpinx. Later, frank tubo-ovarian and pelvic abscesses form with thick-walled, complex fluid collections that may contain internal septa (see the images below), a fluid-debris level, or, less commonly, gas. [32, 33, 34] The presence of gas is a definite sign of infection.
The inflammation can spread to involve the peritoneum, which shows enhancement on a postcontrast study. There can also be contiguous inflammation of the adjacent small bowel loops, appendix, and colon, causing thickening and enhancement with reactive ileus. There can also be secondary involvement of the urinary bladder and ureters with cystitis and hydroureteronephrosis.  A peritoneal inclusion cyst may also be seen as a loculated fluid collection surrounding the uterus and ovaries. See the image below.
It is a syndrome associated with acute PID characterized by perihepatitis and associated right-sided abdominal pain. [36, 37] The imaging findings include thickening and abnormal enhancement of the anterior capsule of the liver and perihepatic ascites. [38, 39, 40] There can be thickening of the gallbladder wall and pericholecystic inflammation mimicking cholecystitis. A reversible perfusion defect may be seen in the posterior segment of the right lobe of liver, due to either portal vein compression/occlusion or partial hepatic venous flow obstruction.
Other conditions that can mimic pelvic abscess/tubo-ovarian abscess (TOA) include necrotic pelvic neoplasm, hematoma, hemorrhagic physiologic cyst, and endometrioma. Other tubular structures in the pelvis that may resemble a dilated fallopian tube include hydroureter, an inflamed appendix, and gonadal vein thrombophlebitis (see the image below).
One may also encounter an edematous ovary secondary to adjacent visceral inflammation, such as in a case of sigmoid diverticulitis (see the image below).
Magnetic Resonance Imaging
Pelvic inflammatory disease (PID) findings on MRIs are similar to those found on CT scans. Pelvic inflammation is seen as ill-defined hyperintense areas on fat-suppressed T2-weighted images and enhancement on gadolinium-enhanced fat-suppressed T1-weighted images (see the image below).
Because of superior tissue contrast, MRI can distinguish hematosalpinx from pyosalpinx. MRI may also be used to differentiate a tubo-ovarian abscess (TOA) from a possible ovarian neoplasm in cases of suspected PID. In a study comparing ultrasonography and MRI in the diagnosis of laparoscopically confirmed PID, MRI was found to be more sensitive and specific than ultrasonography.  Given the high costs and less availability, MRI is better used as a problem-solving tool.
Ultrasonography is the most frequently requested imaging examination for suspected pelvic inflammatory disease (PID). Transabdominal sonography may be useful in depiction of the extent of disease. Transvaginal sonography helps detect subtle abnormalities and delineation of tubal or ovarian involvement. 
According to a study of 100 women of reproductive age, ultrasound-guided drainage of tubo-ovarian abscess (TOA) in combination with antibiotics seems to preserve fertility in approximately half of the patients. Twenty of 38 (52.6%) of women who intended to have a child achieved pregnancy naturally and became mothers. In addition, 7 (50%) of 14 women who were not on birth control on a regular basis became pregnant. No ectopic pregnancies were registered. 
The findings may be nonspecific and subtle in early or mild PID, and correlation with clinical and laboratory findings is helpful.
The uterus may be enlarged with indistinct borders and demonstrate fluid in the endometrium with thickening. Increased echogenicity of the pelvic fat and free or complex fluid may also be seen. 
The ovaries may be enlarged with increased number of follicles as a result of inflammation. 
Normal fallopian tubes are difficult to visualize on sonography. Salpingitis may be diagnosed on ultrasound as a hyperechoic structure with a hypoechoic rim of edema.  With disease progression, adhesion may cause tubal blockage, with resultant pus accumulation and a pyosalpinx. The dilated tubes with thickened endosalpingeal folds resemble a cogwheel in cross-section. Hyperemia of the walls and folds of the fallopian tube is seen on color Doppler. 
The ovaries may become inflamed. When the ovary is adhered to the fallopian tube but visualized as a discrete structure, it is termed a tubo-ovarian complex. A breakdown of the ovarian and tubal architecture in a complex results in a thick-walled, ill-defined multiloculated cystic/solid lesion called a tubo-ovarian abscess (TOA). Untreated TOA can occasionally rupture, resulting in peritonitis and intraperitoneal abscesses (see the image below). The infection may also spread from one side to the other healthy adnexa.
A hydrosalpinx may develop from fluid accumulation in an occluded tube. It may be seen as a thin-walled fallopian tube containing anechoic fluid, incomplete septa, and beads-on-a-string sign, which are remnants of the endosalpingeal folds. A peritoneal inclusion cyst may also be seen as a loculated fluid collection surrounding the uterus and ovaries.  This is a sequela of adhesions inhibiting absorption of fluid from a ruptured ovarian cyst.
Hydrosalpinx and pyosalpinx can usually be readily distinguished from pelvic veins and bowel by visualizing the color flow within the patent blood vessels and peristalsis within the bowel.
Imaging findings in TOAs may be nonspecific and must be distinguished from endometriomas, ectopic pregnancies, hemorrhagic cysts, ovarian tumors, and abscesses from adjacent organs.
Degree of confidence
Transvaginal sonography may be limited by the patient's inability to tolerate the transvaginal examination (although this is not usual). In such cases, only transabdominal findings may be available. Occasionally, the higher frequency and the lower position of the transvaginal transducer limits penetration of the sound beam, and transvaginal imaging of an unusually high adnexa may not be possible. Sometimes, a patient's large body habitus or abdominal wall scarring limits penetration of the sound beam, adversely affecting transabdominal sonography. 
The findings may be nonspecific in early or mild PID, and correlation with clinical and laboratory findings is helpful to increase the accuracy. Equipment quality and sonographer experience affect the sensitivity and specificity of sonography.