How are US CT and MRI of the female pelvis performed?
US is performed using a transducer probe that sends and receives the ultrasound waves. Typically, images of the pelvis are initially obtained using a transabdominal approach (i.e., the transducer is placed over the abdomen and a conductive gel is applied) with a fluid-filled bladder. This is routinely followed by a transvaginal approach (i.e., a condom-covered sterilized transducer is placed in the vagina and covered in sterile conductive gel), preferably with an empty bladder. Transvaginal US typically demonstrates better delineation of the endometrium and adnexal regions, but these techniques are complementary. Images of the pelvic organs are obtained in the longitudinal and transverse planes. 2D and 3D imaging can be obtained, but 3D US requires specialized 3D transducers. Sonohysterography may be performed which combines hysterography with US. This is a slightly more invasive procedure because saline is infused into the endometrial canal. It is reserved for select patients with an abnormal endometrium. Transperineal and transrectal US are reserved for patients who are not candidates for transvaginal US and for assessment of pelvic organ prolapse. Although US is operator dependent and requires physical examination of the patient, it allows real-time imaging and is free of ionizing radiation. It takes at least 15 to 30 minutes to complete an examination, depending on its complexity.
MRI utilizes strong magnetic fields to produce images, is similarly free of ionizing radiation, and is operator independent. For optimal image acquisition, a phased array coil is placed around the patient to optimize signal in the area of interest and to speed up image acquisition time. Patients must be screened prior to entering the MRI for any metallic devices. It can take up to 30 to 40 minutes to complete the examination, but with improvements in the technology, scan time continues to improve. 2D images may be acquired in any plane, and 3D imaging can also be performed. Multiple imaging sequences are obtained with different contrast weightings such as T1-weighted and T2-weighted images. Endovaginal and endorectal coils are occasionally used for urethral and perineal imaging. Gadolinium-based intravenous contrast material may be used if indicated. An antiperistalsis agent may sometimes be administered to decrease motion artifact from bowel peristalsis. Oral contrast material is sometimes administered for dedicated bowel imaging.
CT is a technique that utilizes ionizing radiation. It is fast and nonoperator dependent. Scanning is performed in a single breath hold. Images are acquired in the transverse plane but can be reconstructed in coronal and sagittal planes, which are particularly helpful for evaluation of female pelvic anatomy and pathology. Iodinated intravenous contrast material and oral contrast material are often used for CT imaging.