| 摘要 |
<p>A main magnetic field (B0) is applied through an imaging region. High power RF excitation or manipulation electrical pulses are applied to transmit coils. The transmit coil is inherently tuned to a frequency that is different from the resonance frequency of dipoles in the imaging region that are to be resonated. The transmit coil includes a reactive element (Ct) which is switched in and out of electrical communication with other inductive and capacitive portions (L), (C0) of the transmit coil to change its tuning between the resonance and off-resonance frequencies. A rectifying diode (V2) and a resistor are connected in parallel with the PIN diode (V1) to rectify RF signals and generate a D.C. voltage across the resistor which biases the PIN diode to its conductive state. Preferably, before an RF excitation or manipulation electrical pulse is sent to the transmit coils, a tuning synthesizer (60) generates an off-resonance radio frequency pulse which is applied to the transmit coil, causing the rectifying diode and resistor to bias the PIN diode conductive, hence retune the transmit coil to the resonance frequency by the time the resonance frequency excitation or manipulation electrical signal is applied. After the radio frequency excitation or manipulation pulse is terminated, the PIN diode (V1) switches non-conductive detuning through transmit coil. When the transmit coil is detuned, it does not interfere with a receive only coil (50) as it receives resonance signals from the resonating dipoles in the imaging region. <IMAGE></p> |
