参考资料-2.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,Magnetic resonance imaging(MRI)is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body.,MRI is based on the principles of nuclear magnetic resonance(NMR),a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules.,The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging(NMRI)because of the negative connotations associated with the word nuclear in the late 1970s.,MRI started out as a tomographic imaging technique,that is it produced an image of the NMR signal in a thin slice through the human body.MRI has advanced beyond a tomographic imaging technique to a volume imaging technique.,Felix Bloch and Edward Purcell,both of whom were awarded the Nobel Prize in 1952,discovered the magnetic resonance phenomenon independently in 1946.In the period between 1950 and 1970,NMR was developed and used for chemical and physical molecular analysis.,In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumors differed,thus motivating scientists to consider magnetic resonance for the detection of disease.,In 1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding,and the Fourier Transform.This technique is the basis of current MRI techniques.A few years later,in 1977,Raymond Damadian demonstrated MRI called field-focusing nuclear magnetic resonance.In this same year,Peter Mansfield developed the echo-planar imaging(EPI)technique.,Lets begin with a pictorial introduction to some basic MRI.Magnetic resonance started out as a tomographic imaging modality for producing NMR images of a slice through the human body.Each slice had a thickness(Thk).,This form of imaging is in some respects equivalent to cutting off the anatomy above the slice and below the slice.The slice is said to be composed of several volume elements or voxels.,The volume of a voxel is approximately 3 mm,3,.The magnetic resonance image is composed of several picture elements called pixels.The intensity of a pixel is proportional to the NMR signal intensity of the contents of the corresponding volume element or voxel of the object being imaged.,Microscopic Property Responsible for MRI,The human body is primarily fat and water.Fat and water have many hydrogen atoms which make the human body approximately 63%hydrogen atoms.Hydrogen nuclei have an NMR signal.For these reasons magnetic resonance imaging primarily images the NMR signal from the hydrogen nuclei.Each voxel of an image of the human body contains one or more tissues.,For example here is a voxel with one tissue inside.Zooming in on the voxel reveals cells.Within each cell there are water molecules.Here are some of the water molecules.,Each water molecule has one oxygen and two hydrogen atoms.If we zoom into one of the hydrogens past the electron cloud we see a nucleus comprised of a single proton.,The proton possesses a property called spin which can be thought of as a small magnetic field,and will cause the nucleus to produce an NMR signal.,Not all nuclei possess the property called spin.,