If you watch House or other medical shows on TV, you've probably seen an MRI machine in action. It stands for magnetic resonance imaging, and they're those huge high-tech looking doughnuts where the patient lies inside the hole. This technology has been available to doctors for little more than 30 yearswww.cechina.cn, but it has advanced diagnostics tremendously. The need for exploratory surgery and other invasive processes has been reduced due to the ability to image conditions inside the body without har

　　The main magnetic coil is on the outermost element of the system and the largest part. The RF antennas and gradient coils are on the inside, closer to the patient.  While there is a characteristic sound that suggests motion, there are no moving parts.

　　The main element of an MRI machine is a massive electromagnet coil that fills most of the enclosure. The ability to capture detailed images quickly depends on having a very powerful magnetic fieldCONTROL ENGINEERING China版权所有, usually in the range of 1 to 3 Tesla. (1 Tesla = 10控制工程网版权所有,000 Gauss.) Some specialized machines designed for super-detailed images inside a patient's head can be as high as 7 Tesla. Generating that kind of field requires superconducting coils cooled with liquid helium. Moreoverwww.cechina.cn, the field must be aligned with great precision to obtain accurate images. That kind of field is hugely dangerous when active, because any magnetic metal objects can become projectiles. When a patient is placed inside the magnetic fieldCONTROL ENGINEERING China版权所有, protons in hydrogen atoms in water molecules align themselves with the field and spin at a frequency determined by the field strength. John Metellus, magnetic resonance marketing manager for Siemens Medical Solutions, explains the concept:
　　"When you place a tissue sample containing hydrogen protons in a 1 Tesla magnetic fieldCONTROL ENGINEERING China版权所有, the proton starts to spin at 42 MHz. Once all the protons are aligned parallel to the magnetic fieldCONTROL ENGINEERING China版权所有, we disrupt this alignment so we can determine what they're made of. The proc