| 摘要 |
A less expensive apparatus and associated method for Magnetic Resonance Imaging (MRI) of body tissues are disclosed. A physician uses an ultrasonic detector in combination with a weak magnetic field and pulsed, low frequency R.F. generator. The patient lies partially within the field of a magnet and receives low frequency radio pulses. As is shown below, ultrasonic waves are produced within the patient's body by the ordinary occurrence of nuclear magnetic resonance. For this reason, ultrasonic acoustic detection replaces the usual radio detection of the magnetic resonance induced spin echoes. The physician manipulates a hand-held ultrasonic detector with real time image feedback in much the same way that an ultrasonic transducer is used to provide the usual ultrasonic images. In the present case, however, no ultrasonic waves are emitted by the transducer; instead, the waves emitted by the "slice" selected by the radio frequency resonance with tissue protons are the source of the ultrasonic image produced. The MRI advantages of image production emphasizing longitudinal, T1, spin relaxation or transverse, T2, relaxation are retained. The usual problem that MRI has in low-noise measurement and location of spin echoes is obviated by the intrinsic ability to locate possessed by short wavelength ultrasonic signals. The need for very high magnetic fields in MRI to improve signal-to-noise ratio is removed in the present system. This removes a large capital expense. Another capital expense required by MRI is its complex magnetic field pulsing equipment. Such equipment is necessary to locate echoes using long wavelength R.F. detection. For a given frequency, of magnetic moment precession, the length of the acoustic waves, which are emitted at twice the precession frequency, are 100,000 times shorter than the length of radio waves, which are emitted at the precession frequency. Thus acoustic detection easily locates and measures the sources of magnetic spin echoes with a smaller burden of pulsed and steady magnetic fields. |
