Ultrasound imaging is the primary fetal monitoring modality during pregnancy, nevertheless fetal MRI is increasingly used to image anatomical regions and structures difficult to see with sonography. Given its long record of safety, utility, and cost-effectiveness, ultrasound will remain the modality of first choice in fetal screening. However, MRI is beginning to fill a niche in situations where ultrasound does not provide enough information to diagnose abnormalities before the baby's birth. Magnetic resonance imaging of the fetus provides multiplanar views also in sub-optimal positions, better characterization of anatomic details of e.g. the fetal brain, and information for planning the mode of delivery and airway management at birth.

Indications:
Modern fetal MRI requires no sedatives or muscle relaxants to control fetal movement. Ultrafast MRI techniques (e.g., single shot techniques like Half Fourier Acquisition Single shot Turbo spin Echo HASTE) enable images to be acquired in less than one second to eliminate fetal motion. Such technology has led to increased usage of fetal MRI, which can lead to earlier diagnosis of conditions affecting the baby and has proven useful in planning fetal surgery and designing postnatal treatments. As MR technology continues to improve, more advances in the prenatal diagnosis and treatment of fetal abnormalities are to expect. More advances in in-utero interventions are likely as well. Eventually, fetal MRI may replace even some prenatal tests that require invasive procedures such as amniocentesis.

For Ultrasound Imaging (USI) see Fetal Ultrasound at Medical-Ultrasound-Imaging.com.

(ADC) A diffusion coefficient to differentiate T2 shine through effects or artifacts from real ischemic lesions. In the human brain, water diffusion is a three-dimensional process that is not truly random because the diffusional motion of water is impeded by natural barriers. These barriers are cell membranes, myelin sheaths, white matter fiber tracts, and protein molecules.
The apparent water diffusion coefficients can be calculated by acquiring two or more images with a different gradient duration and amplitude (b-values). The contrast in the ADC map depends on the spatially distributed diffusion coefficient of the acquired tissues and does not contain T1 and T2* values.
The increased sensitivity of diffusion-weighted MRI in detecting acute ischemia is thought to be the result of the water shift intracellularly restricting motion of water protons (cytotoxic edema), whereas the conventional T2 weighted images show signal alteration mostly as a result of vasogenic edema.
The reduced ADC value also could be the result of decreased temperature in the nonperfused tissues, loss of brain pulsations leading to a decrease in apparent proton motion, increased tissue osmolality associated with ischemia, or a combination of these factors. The lower ADC measurements seen with early ischemia, have not been fully established, however, a lower apparent ADC is a sensitive indicator of early ischemic brain at a stage when ischemic tissue remains potentially salvageable.

See also Diffusion Weighted Imaging and Diffusion Tensor Tractography.

(DWI) Magnetic resonance imaging is sensitive to diffusion, because the diffusion of water molecules along a field gradient reduces the MR signal. In areas of lower diffusion the signal loss is less intense and the display from this areas is brighter. The use of a bipolar gradient pulse and suitable pulse sequences permits the acquisition of diffusion weighted images (images in which areas of rapid proton diffusion can be distinguished from areas with slow diffusion).
Based on echo planar imaging, multislice DWI is today a standard for imaging brain infarction. With enhanced gradients, the whole brain can be scanned within seconds. The degree of diffusion weighting correlates with the strength of the diffusion gradients, characterized by the b-value, which is a function of the gradient related parameters: strength, duration, and the period between diffusion gradients.
Certain illnesses show restrictions of diffusion, for example demyelinization and cytotoxic edema. Areas of cerebral infarction have decreased apparent diffusion, which results in increased signal intensity on diffusion weighted MRI scans. DWI has been demonstrated to be more sensitive for the early detection of stroke than standard pulse sequences and is closely related to temperature mapping.
DWIBS is a new diffusion weighted imaging technique for the whole body that produces PET-like images. The DWIBS sequence has been developed with the aim to detect lymph nodes and to differentiate normal and hyperplastic from metastatic lymph nodes. This may be possible caused by alterations in microcirculation and water diffusivity within cancer metastases in lymph nodes.

See also Diffusion Weighted Sequence, Perfusion Imaging, ADC Map, Apparent Diffusion Coefficient, and Diffusion Tensor Imaging.

(FRFSE, FR-FSE) The fast relaxation fast spin echo sequence provides high signal intensity of fluids even with short repetition times, and can be used with parallel imaging techniques for short breath hold imaging or respiratory gating for free-breathing, high isotropic resolution MR imaging. After signal decay at the end of the echo train, a negative 90° pulse align spins with long T2 from the transverse plane to the longitudinal plane, leading to a much faster recovery of tissues with long T2 time to the equilibrium and thus better contrast between tissues with long and short T2.
Fast relaxation FSE has advantages also for volumetric imaging as the TR can be substantially reduced and thus the scan time. The sequence can be post processed with maximum intensity projection, surface or volume rendering algorithms to visualize anatomical details in brain or spine MRI. Cerebro spinal fluid pulsation artifacts, often problematic in the cervical or thoracic spine may be reduced by radial sampling, in particular when combined with acquisitions of the PROPELLER type.

See also Fast spin echo, Driven Equilibrium.

(Hb) Haemoglobin is the major endogenous oxygen-binding molecule, responsible for binding oxygen in the lung and transporting it to the tissues by means of the circulation. Haemoglobin is contained in very high concentration in the red blood cells.
Haemoglobin is an Fe chelate tightly binding one Fe ion in its II oxidation state where it carries the charge 2+ (ferrous iron). If an oxygen molecule is bound to Hb, Hb is called oxyhaemoglobin, if no oxygen molecule is bound it is called deoxyhaemoglobin. When haemoglobin is oxidized (i.e. in a haematoma), Fe2+ is transformed into Fe3+. The resulting haemoglobin is then called metoxyhaemoglobin (Hb Fe3+).
Deoxyhaemoglobin and metoxyhaemoglobin act as paramagnetic contrast agents in MR, while oxyhaemoglobin is diamagnetic. This partly explains the special appearance of an aging haematoma in MR imaging and is also the basic of the blood oxygenation level dependent contrast (BOLD) used in functional magnetic resonance imaging of the brain (fMRI).