Magnetic Resonance Imaging PDF

Magnetic Resonance Imaging Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Cooling of Magnets. ✓ LCS. ✓ Shape of MRI Machine. ✓ Comparison Between MRI Machine. Cooling of Magnets MRI (magnetic resonant imaging) machines work by generating a very large magnetic...

Magnetic Resonance Imaging Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Cooling of Magnets. ✓ LCS. ✓ Shape of MRI Machine. ✓ Comparison Between MRI Machine. Cooling of Magnets MRI (magnetic resonant imaging) machines work by generating a very large magnetic field using a super conducting magnet and many coils of wires through which a current is passed. Maintaining a large magnetic field needs a lot of energy, and this is accomplished using superconductivity, which involves trying to reduce the resistance in the wires to almost zero. This is done by bathing the wires in a continuous supply of liquid helium at -269.1C. A typical MRI scanner uses 1,700 liters of liquid helium, which needs to be topped up periodically. Recently small special purpose refrigerators have been proposed for recondensation of evaporated helium, which together with a cryocooler for the radiation shields give a complete closed refrigeration system. 3 Cooling of Magnets 4  Notes In this figure the cryostat has an outer vacuum case (OVC) made of metal, one thermal shield (usually at a temperature of 40–50 K) and the helium vessel, housing the magnet assembly. Top left shows a typical cryocooler in its vertical orientation, ready to fit into the cryocooler sleeve, as indicated. The liquid helium fill level to keep the magnet superconducting at 4 K is also shown. For a complete fill, typically 1500– 2000 l is used. Depending on the temperature gradient that may develop inside the magnet (from bottom to top) and on the superconducting coil design, which defines coil stability, lower fill volumes may be tolerable. The minimum allowable volume may also differ between the ramping process and the subsequent persistent operation of the ramped magnet. Any advanced/alternative cryogenic concept for MRI applications needs to address all the following operating modes: Energy saving pre-cooling of the magnet down to the operating temperature (usually done with liquid nitrogen or a recoverable liquid helium facility). 5  Notes Magnet ramp up to full field, preferably with captured boil-off helium gas during ramp. Normal operating condition (NOC) with extra heat loads (due to gradient heating) that reduce the cryogenic margin, and ensure no helium loss (zero boil-off/recovery). Ramp down. Shipping ‘ride-through’ (from factory to MRI site), optimizing losses to minimize the cost. Cooldown to operating temperature or refill at the customer site, with high-efficiency transfer. Safe ramp up at the customer site. Cryocooler technology is constantly progressing. Currently, the dual-stage cryocooler cools the thermal shield thermally linked to its first stage. The second stage is connected to the recondenser which re-liquefies escaping helium gas from the helium vessel. 6 Cooling of Magnets LASER COOLING SYSTEM(LCS) LCS is one of the recent technologies used to cool magnet in MRI. The temperature of a laser system can determine its lifetime, performance and safety. In laser cooling, atomic and molecular samples are cooled down to nearly absolute zero through the interaction with one or more laser fields. The basic principle of laser cooling is Doppler effect. The Doppler effect, or Doppler shift, is the change in wavelength and frequency caused by the movement of an observer relative to the source. 7  LCS In Doppler effect the frequency of light is tuned slightly below an electronic transition in the atom. Because the light is detuned to lower frequency, the atom will absorb more photons if they move towards the light source. If light is applied from two opposite directions, the atom will scatter more photons. If this process continuous, the speed of the atom reduces and hence the kinetic energy also reduces. Which reduces the temperature of the atom, and hence cooling of the atom is achieved. As per Doppler cooling, if a stationary atom sees the laser neither red shifted nor blue shifted, it does not absorb the photon. An atom moving away from the laser sees that the laser is red shifted, then also it does not absorb photon. If an atom is moving towards the laser and sees that it is blue-shifted, the it absorbs the photon and thus the speed of the atom will get reduced. 8 LCS 9 Notes In this proposed system four temperature sensors are fixed on the four sides of the superconducting magnet. It can predict the temperature level at the superconducting magnet, and transmit it to the controller. So the controller has to be designed for making the cooling effective. And we have to place our model in controller so that it can provide the corresponding wavelength of laser for the predicted temperature. 10 Shape of MRI Machine CLOSED MRI OPEN MRI 11 Comparison CLOSED MRI OPEN MRI High field typically 1.5T – 3T. Low field typically 0.2T – 0.4T High image quality Low image quality Fast imaging Slow imaging Advanced application Limited application Increased patient anxiety. Less patient anxiety. Claustrophobic patients Claustrophobic patients handling. problems. Lower acoustic noise levels. High acoustic noise levels. 12