Martin C. Powell

Magnetic resonance imaging (MRI) in obstetrics. I. Maternal anatomy

Summary. We assessed the ability of magnetic resonance, a recently introduced imaging technique, to demonstrate the maternal anatomy in obstetrics. The signal intensity of different maternal tissues using T1 and T2 weighted sequences was examined. The bony pelvis is depicted with sufficient clarity to provide an alternative to conventional X‐ray pelvimetry. The placenta and cervix have a distinctive appearance facilitating the diagnosis of placenta praevia. The unique demonstration of cervical morphology will offer the potential for investigation into the illunderstood conditions of cervical dystocia and cervical incompetence.

Management of a primary abdominal pregnancy

Abstract The first reported case of abdominal pregnancy managed laparoscopically is described.

Diagnostic imaging of gynecologic tumors with the monoclonal antibody 791T/36

One hundred twenty-three patients with suspected primary or recurrent gynecologic cancer were imaged after infusion with radiolabeled antitumor monoclonal antibody (791T/36). Overall sensitivity of tumor detection was 87% and specificity was 75%. Indium 111- and iodine 131-labeled antibody were compared, the former yielding a higher specificity for tumor detection. This antibody shows potential for routine tumor detection, particularly in cases of suspected recurrent cancer, but the development of antimouse antibodies is a drawback to this technique still to be overcome. The specific targeting of cytotoxic agents with this antibody is in progress.

Gestational trophoblastic neoplasia

This chapter provides an overview of the various techniques employed in diagnosis of gestational trophoblastic neoplasia (GTN). It also discusses the role of magnetic resonance imaging (MRI) in diagnosis of GTN. GTN is an infrequent complication of pregnancy. It presents a spectrum of disease from the relatively benign hydatidiform mole to the malignant choriocarcinoma. GTN falls into the category of disease where little is known about the cause, while the diagnosis and management have developed to such a point where morbidity and mortality are rare. The chapter discusses the variable sonographic appearances of hydropic degeneration of the placenta and its confusion with true hydatidiform mole. Angiography has been employed not only as a method for visualizing the position and extent of trophoblastic tumors, but also for differentiating benign from malignant disease and for monitoring treatment response. This is obviously an invasive technique and is not widely accepted in current radiological practice. Accurate pre-treatment metastatic work-up is essential in all patients with a presumptive evidence of persistent GTN, whether invasive mole or choriocarcinoma. Computer tomography (CT) is in widespread use in the evaluation of pulmonary, cerebral, and hepatic metastases. CT is accurate in defining the extent of myometrial and adnexal disease.

Principles of magnetic resonance imaging

This chapter explores that all matter is made up of atoms, which in the classical model consist of a nucleus of protons and neutrons orbited by electrons. The chemical properties of the atom depend on the electrons, but its physical properties are largely dependent on the nucleus which constitutes the majority of the atomic mass. The number of electrons and protons are always equal in order to maintain electrical neutrality. The neutrons and protons possess an intrinsic angular momentum or spin; pairs of protons or pairs of neutrons align in such a way that their spins cancel out. However, where the nucleus has a net spin, they also have a magnetic moment that is a physical vector quantity expressing the strength and direction of the magnetic field. The nucleus behaves as a small bar magnet or magnetic dipole, which in the absence of an externally applied magnetic field is randomly oriented. The chapter explains that the hydrogen atom consists of one proton and one electron, has a net spin, and it is well-suited to form the basis of magnetic resonance imaging owing to its strong nuclear magnetic resonance signal and high abundance in the body.

The maternal anatomy and the placenta

This chapter discusses use of magnetic resonance imaging (MRI) technique in the examination of maternal or fetal anatomy and placenta. It also discusses that an ultrasound scan in early pregnancy serves the purpose of establishing gestational age, diagnosing multiple pregnancies, and in the detection of fetal anomalies. In the later stages of pregnancy, ultrasound is beneficial in the assessment of fetal well-being, the identification of intrauterine growth retardation, and in the evaluation of placental site. The technique does have limitations in the third trimester, particularly in the depiction of the maternal deep pelvis and the structures within. In relation to the assessment of patients with suspected placenta previa, the lower edge of a posterior placenta is obscured by the fetal head, or the cervix distorted by overdistension of the bladder. With MRI, both the cervix and placental have a distinctive appearance, and the technique is not beset by the problems encountered with ultrasound. This has advantages in determining the management of patients with suspected placenta previa, when the exact position of the placenta is uncertain. MRI also provides an excellent display of maternal anatomy which is unaffected by respiratory movement. The bony pelvis is well seen with MRI and has been proposed as an alternative method for performing pelvimetry avoiding the use of ionizing radiation.

Benign disease of the ovary

This chapter discusses the benign diseases of ovary. The normal ovary is the most difficult of the female pelvic organs to visualize with magnetic resonance imaging (MRI). This is in part due to its inherent signal characteristics, but also relates to anatomical position and size. The position of the ovary is subject to a wide range of variations, particularly in multiparous women. Ovary is displaced by pregnancy and does not return to its previous non-pregnant site. It is mobile and can change its position to a varying degree, according to the state of the surrounding organs, such as the small intestines or bladder. The healthy ovary with MRI is usually seen in an oblique transection, but can be identified in most patients on a transverse axial plane of view. The anatomical landmarks, such as, the vessels surrounding the ovary, identified as dark tubular structures at their periphery, are easily seen. The ovary gives rise to a wider variety of tumors than any other organ in the body, which may be completely asymptomatic. Enlargements outside the pelvis may subsequently occur. They may be solid, cystic or a combination of both, and benign, malignant or borderline in character. Although the ability to differentiate between benign and malignant tumors of the ovary may be of less importance from a surgical standpoint, it is desirable to make a specific histological diagnosis of an ovarian mass prior to surgery.

The normal uterus and vagina

The uterus not only has its lining membrane renewed every 4 weeks for 35 years or more, but has to enlarge out of all proportion to accommodate a growing fetus, and return to something approaching its normal size shortly after parturition. It therefore undergoes constant structural alterations throughout the entire female life and possesses an extreme degree of normal variation in anatomical and functional states. This chapter highlights the potential for magnetic resonance in imaging of the male and female non-pregnant pelvis. It also discusses that unsurpassed soft tissue contrast provided by magnetic resonance imaging (MRI) allows visualization of internal structures of the uterus. The imaging of both the physiological and pathological changes which the uterus undergoes has been done by ultrasound and X-rays. X-rays consist of computerized tomography (CT), arteriography and hysterosalpingography.. The uterus on a magnetic resonance image is easily distinguished from the other surrounding pelvic organs, particularly when viewed on the sagittal plane. The chapter also discusses that with MRI, the dimensions of the uterine cavity are significantly greater in a multiparous than in a nulliparous non-pill-taking woman. MRI can also reflect minor pathological changes within the uterine cavity or myometrium which may not be apparent during clinical examination.

11 – The fetus

Publisher Summary This chapter discusses that fetal movement that occurs during the course of an imaging sequence can lead to significant degradation of the quality of the final image. Magnetic resonance gating techniques have been successfully employed to contend with respiratory and cardiac movement; however, fetal movement is of random nature and such methods are not helpful. Fetal movement is significantly reduced during the latter part of pregnancy, and is restricted mainly to the limbs. At this stage, detailed observations of the fetal anatomy are possible, without the deleterious effect of movement artifact. By utilizing the tissue differentiation qualities of magnetic resonance imaging (MRI) in conjunction with good anatomical resolution, detailed images of the fetal brain, cardiovascular system, lungs, abdomen, and musculoskeletal system are obtained. The chapter also explains that a central low intensity area in the center of the thorax representing the fetal heart is seen with MRI from early in the second trimester. By the final trimester the cardiac image can be sufficiently clear to identify the individual chambers. This interpretation can be difficult because the image plane obtained through the heart is fortuitous and dependent on the fetal lie.

3 – Safety considerations

Publisher Summary The medical community has been sensitized as a result of experience with ionizing radiation, which has a known genotoxic and oncogenic effect. The potential adverse bioeffects of ultrasound, which is now routinely used in early pregnancy, have come under scrutiny. Therefore, any new technology, such as magnetic resonance imaging (MRI) with potential widespread use in reproductive medicine, elicits appropriate caution and enquiry. This chapter examines the bioeffects of the individual modalities within nuclear magnetic resonance (NMR) for likely endpoints, and also combinations in extreme conditions of their applications. The potential adverse effects of static magnetic field include flashes, induced currents that affect muscle and nerve conduction, and bone healing. The chapter also provides an overview of the bioeffects of NMR, from research using cell cultures, on human populations. The photon energy changes that are associated with nuclear excitation and relaxation are lesser than those associated with ionization by many orders of magnitude. There is low probability of long-term ill effects of MRI because it is based on a weak radiofrequency phenomenon.