When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [-(E1 - E2)/kT] where k is the Boltzmann constant and T is the absolute temperature.
For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon.

(MR mammography) Magnetic resonance imaging of the breast is particularly useful in evaluation of newly diagnosed breast cancer, in women whose breast tissue is mammographically very dense and for screening in women with a high lifetime risk of breast cancer because of their family history or genetic disposition.
Breast MRI can be performed on all standard whole body magnets at a field strength of 0.5 T - 1.5 Tesla. Powerful gradient strengths over 15 mT/m will help to improve the balance between spatial resolution, scanning speed, and volume coverage. The use of a dedicated bilateral breast coil is obligatory.
Malignant lesions release angiogenic factors that increase local vessel density and vessel permeability. Breast cancer is detectable due to the strong enhancement in dynamic breast imaging that peaks early (about 1-2 min.) after contrast medium injection. If breast cancer is suspected, a breast biopsy may be necessary to secure the diagnosis.

See also Magnetic Resonance Imaging MRI, Biopsy and MR Guided Interventions.

Requirements in breast MRI procedures:
For Ultrasound Imaging (USI) see Breast Ultrasound at Medical-Ultrasound-Imaging.com.

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'

In the last years, cardiac MRI techniques have progressively improved. No other noninvasive imaging modality provides the same degree of contrast and temporal resolution for the assessment of cardiovascular anatomy and pathology. Contraindications MRI are the same as for other magnetic resonance techniques.
The primary advantage of MRI is extremely high contrast resolution between different tissue types, including blood. Moreover, MRI is a true 3 dimensional imaging modality and images can be obtained in any oblique plane along the true cardiac axes while preserving high temporal and spatial resolution with precise demonstration of cardiac anatomy without the administration of contrast media.
Due to these properties, MRI can precisely characterize cardiac function and quantify cavity volumes, ejection fraction, and left ventricular mass. In addition, cardiac MRI has the ability to quantify flow (see flow quantification), including bulk flow in vessels, pressure gradients across stenosis, regurgitant fractions and shunt fractions. Valve morphology and area can be determined and the severity of stenosis quantified. In certain disease states, such as myocardial infarction, the contrast resolution of MRI is further improved by the addition of extrinsic contrast agents (see myocardial late enhancement).
A dedicated cardiac coil, and a field strength higher than 1 Tesla is recommended to have sufficient signal. Cardiac MRI acquires ECG gating. Cardiac gating (ECGs) obtained within the MRI scanner, can be degraded by the superimposed electrical potential of flowing blood in the magnetic field. Therefore, excellent contact between the skin and ECG leads is necessary. For male patients, the skin at the lead sites can be shaved. A good cooperation of the patient is necessary because breath holding at the end of expiration is practiced during the most sequences.

See also Displacement Encoding with Stimulated Echoes.
For Ultrasound Imaging (USI) see Cardiac Ultrasound at Medical-Ultrasound-Imaging.com.

See also the related poll results: 'In 2010 your scanner will probably work with a field strength of' and 'MRI will have replaced 50% of x-ray exams by'

GE Healthcare is the result of the merger between GE Medical and Amersham Health in Nov. 2004, after GE acquired Amersham Health for 9.5 billion in Oct. 2003. Jeffrey R. Immelt, Chairman of the Board and Chief Executive of General Electric, said, 'Amersham's diagnostic pharmaceutical and life sciences business will add new, high growth platforms to GE Medical's diagnostic imaging, services and healthcare information technology businesses'. GE Healthcare, a UK company, is a unit of General Electric (NYSE: GE). GE Healthcare is a global leader in medical imaging, diagnostic imaging contrast agents, interventional procedures, healthcare services, and information technology. For more than 100 years, health care providers have relied on GE Medical Systems, now GE Healthcare, for high quality medical technology and productivity solutions. GE Healthcare, headquartered now at formerly seat of Amersham Health in Great Britain, operates facilities around the world. Global Operations include organizations on the Americas, Europe, and Asia, including India, Japan, Korea China, Thailand and Vietnam.

MRI Scanners:

0.2T to 1.0T:
to 1.5T:
to 3.0T:

MRI Contrast Agents:

Hepatobiliary chelates used in MRI are paramagnetic contrast agents consisting of a metal ion bound to an organic ligand. Paramagnetic metal ions such as gadolinium improve the MRI signal, but the toxicity of these uncomplexed metal ions makes the use of a chelate to bind the metal ion essential. Due to the hepatocyte uptake of this chelate complex, the different contrast between normal parenchyma and liver lesions improves the detection and characterization of specific diseases. In addition, the hepatobiliary excretion allows the assessment of the hepatobiliary system.
Chelates for hepatobiliary imaging: MultiHance® (Gadobenate Dimeglumine), Teslascan® (Mangafodipir Trisodium), Gd-HIDA, Cr-HIDA, and Fe-EHPG IronIII or other derivatives.

See also Hepatobiliary Contrast Agents, Liver Imaging.