Magnet Types PDF

Magnet Types Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Magnets. ✓ Magnetic field. ✓ Classification of magnets. ✓ Magnets used in MRI. ✓ Magnetic states of matter.  Magnets An object that is surrounded by a magnetic field and t...

Magnet Types Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Magnets. ✓ Magnetic field. ✓ Classification of magnets. ✓ Magnets used in MRI. ✓ Magnetic states of matter.  Magnets An object that is surrounded by a magnetic field and that has the property, either natural or induced, of attracting iron or steel. To obtain a magnetic resonance (MR) signal from tissues, a large static magnetic field is required. The primary purpose of the static magnetic field (known as “Bo” field) is to magnetize the tissue.  Magnetic field A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles. A vector quantity consisting of both a north and south pole; it exerts an induction force on ferromagnetic and paramagnetic substances. Bipolar or Dipolar Magnets Always has a north or south pole  Classification of magnets Magnets are classified according to the origin of the magnetic property. Natural Occurring magnets Permanent magnets Electromagnets  Magnets used in MRI Magnets used in MRI types of magnet used in MRI Resistive Superconductive Permanent  MR magnets Permanent magnet – Consists of a material which has been magnetized such that it won’t loose its magnetic field – Commonly made of an alloy ALNICO and Rare earth materials Resistive magnet – Consists of huge copper or aluminum coils – They produce a lot of heat so requires water cooling Superconducting magnet – Special alloys of Niobium-titanium in a copper matrix when super cooled to a temperature of 4K (-2690C) becomes super conductor – Niobium-Tin alloy , Magnesium diboride is emerging new alloy – The coolant or the cryogen used is liquid helium  Resistive Magnets Resistive magnets : Are simple, although large, electromagnets. Earliest types of magnets used in MRI They consist of coils of wire. A magnetic field is produced by passing an electric current through the coils. The electrical resistance of the wire produces heat and limits the maximum magnetic field strength of resistive magnets. The heat produced is conducted away from the magnet by cooling system. Whenever electrical current is applied to a wire, a magnetic field is induced around the wire. To produce a static field, (NOT ALTERNATING), direct current is required  Resistive Magnets Characteristics Resistive magnet – Field strength up to 0.5 T – Magnetic field inhomogeneity - 10-50 ppm – Power consumption - 50 to 100kW – Weight of magnet- 4 tons – Field can be switched off immediately(when not in use) – Flux lines run horizontally – Modest fringe fields ~ 2m (0.5mT) Superconductive (cryogenic) Magnets Are also electromagnet. Most are solenoid in design However, their wire loops are cooled to very low temperatures with liquid helium and liquid nitrogen (cryogens) to reduce the electrical resistance.  Superconducting Magnet A superconducting magnet is build up by a number of vacuum vessels which acts as temperature shields The length of superconducting wire in the magnet is typically several miles The coil of wire is kept at a temperature of 4.2K by immersing it in liquid helium The coil and liquid helium is kept in a large dewar The typical volume of liquid helium in an MR magnet is about 1700 ltrs The dewar is surrounded by liquid nitrogen at 77.4K (-1690C) which acts as a thermal buffer between room temperature (293K) and liquid helium Superconducting Magnet Superconducting Magnet Superconducting Magnet Characteristics Superconducting magnet – High field strength 0.37 to 4 T (up to 14Tfor research) – Magnetic field inhomogeneity - 0.1-5 ppm – Expensive to buy and run & difficult to maintain – Flux lines run horizontally – Large fringe fields ~10m (0.5mT) – Weight of magnet- 10 tons – Power consumption ~ 20kW  Superconducting Magnet Advantage Disadvantage Advantage Disadvantage High High field strength field strength High capital cost High capital cost High High field homogenecity field homogenecity High cryogen cost High cryogen cost Low Low power power consumption consumption Acoustic noise Acoustic noise High SNR High SNR Motion artifact Motion artifact Fast scanning Fast scanning Technical complexity Technical complexity  Magnet Quench Loss of magnet super conductivity with sudden boil- off of liquid helium Patient and staff must be immediately evacuated from the scanner room if quench occurs Liquid helium in large quantities can completely displace oxygen and cause unconsciousness within 10 secs when inhaled Quench can be planned as well as accidental All supercon magnets have a quench button which can turn the field off within few seconds  Permanent Magnets Consist of blocks or slabs of naturally occurring ferrous material It has a constant field that does not require additional electricity or cooling to low temperatures Permanents magnets have the advantage that their magnetic field does not extend as far away from the magnet (fringe field) as do the other magnetic field of other types  What is Fringe Field? The portion of the magnetic field extending away from the confines of the magnet that cannot be used for imaging but can affect nearby equipment or personnel.  Permanent Magnets Open bore permanent magnet (0.2-0.7T)  Permanent Magnets Advantage Disadvantage Low power consumption Limited field strength. Low operating cost. Very heavy. Small fringe field. No quench possibility. No cryogen. 20 MR Field strengths Low field → below 0.3T Mid-field → 0.3T to 1.0T High field → 1.0T to 3.0T Very high field → 3.0T to 7.0T Ultra high field → above 7.0T Advantages of low field scanners Open design Lower fringe field Reduction of certain MR artifacts – Chemical shift, susceptibility & flow/motion artifact Lower energy deposition on tissues (SAR) – SAR is proportional to square of Magnetic field Lower initial purchase price Lower operational cost Disadvantages of low field scanners Lower signal to noise – Signal to noise is approximately proportional to field strength Lower homogeneity – limits the ability to perform certain pulse sequences (such as chemical shift fat suppression, echo-planar imaging, and MR spectroscopy) Impaired detection of calcification and hemorrhage – Poor in detection of calcification, iron accumulation, or hemorrhage Reduced detection of gadolinium enhancement – Enhancement is less apparent in low field scanners