MRI的基本知识.ppt

Magnetic resonance imaging（MRI） 3.1 Basic Principles of MRI MRI reveals chemical compositions of objects as well as their molecular structure arrangement , but not density. Electromagnetic wave is used in MRI , not ionizing radiation wave (X-ray ,γ-ray) or mechanical wave(ultrasound) EXCITATION : causing the magnetic moments flip away from equilibrium position (z- axis) Relaxation: Return to equilibrium T2 relaxation Decay of the transverse component of magnetization T2 relaxation constant: the elapsed time between the peak transverse signal and 37% of the peak level (1/e) T1 relaxation Increase in the longitudinal magnetization T1 relaxation constant: the time needed to recover 63% of the longitudinal magnetization T1 is significantly longer than T2 3.2 Basic Equipments of MRI Classification of MR according to the type of main magnet: permanent, resistive, superconductive Generation, detection coding of MR signal Main magnet radiofrequency (RF) transmitter and receiver (coil) gradient coil Data processing, image reconstruction, displaying and storage analog converter Computer with monitor Storage devices (disk…) printer 3.3 Factors Affecting MR Signals Flow void effect 3.4 MR Signals of Some  Normal Pathological Tissues Body fluidincluding cerebrospinal fluid, urine, edema fluid and fluid with low protein content in cavity and cyst, etc T1WI low signal intensity T2WI high signal intensity Fatincluding subcutaneous fat, bone marrow, well differentiated fatty tumors, etc T1WI high signal intensity T2WI high signal intensity Tumormajority: hypointense on T1WI, hyperintense on T2WI cause: more water contentminority: hyperintense on T1WI, hypointense on T2WI cause: special component, eg, melanin in melanoma Magnetic resonance angiography (MRA) Methods of producing angiograms using MR techniques 1. Time of flight (TOF) MRA 2. Phase contrast (PC) MRA 3. Contrast enhanced (CE) MRA no contrast TOF-MRA CE-MRA Diffusion weig