 This depends on a property of nuclei called spin.  Gyroscope: Principle: As long as its disc remains spinning rapidly the direction of the spin axis will stay pointing in the same direction independently of any movement of its support.  Direction of a gyroscope can be altered by applying a torque to it. The effect of a torque on a gyroscope is to make the gyroscope precess. This means the top of the axle moves round in a horizontal circle.

 Many biological molecules have hydrogen atoms within them. The hydrogen atom has a single proton, which spins on its axis.  This spin gives the proton a very small magnetic property called its magnetic moment.  When in a magnetic field, the proton experiences a torque, and so it precesses.  The frequency of precession is called the Larmor Frequency.  Larmor frequency  4.25X10 7 X B Where B is the total magnetic flux density.

 When the applied magnetic flux density is altered, resonance occurs.  The magnetic field is altered by using an alternating current of radio frequency, f R, in coils placed in the magnetic field.  When the radio frequency is equal to the Larmor frequency, a relatively large amount of energy is absorbed by the proton.  This causes the proton to flip over to its higher energy state.

 Once a proton has gained this energy, it is in a semi-stable state.  Will not remain in this state, but will relax back.  This is the key to MRI.  Relaxation time depends on the magnetic field at the position of any proton.  During relaxation, the energy previously gained is lost in the form of radio-waves, which can be detected, amplified and interpreted.  Relaxation times for H nuclei in water are long (2 s).  Brain tissues = 200 ms  Tumour has a relaxation time between each of these two.

 Quality of info is very high  No ionising radiation is required.  Radiofrequency is no higher than normal radio, so no concerns about electromagnetic waves.  Magnetic field used is 100 time stronger than Earths’.  2 more problems are cost, and that a scan can take 45 mins. (Patient must also be still, not suitable for young children)

 The scanner contains multiple components!  Main Magnet  Additional Magnets  Radiofrequency Coil  Computer  Display We must also look at the advantages and disadvantages!

 These must produce a magnetic field strength of 1.4 T.  Must be over whole patient.  Produced by coils carrying huge currents in wires kept at temperatures near absolute zero.  Principle of superconductivity: Resistance of a wire is 0 provided temp is low.  Liquid He at 4.2 K.  Value of magnetic field strength must be constant over 90 cm.

 Very accurately calibrated additional magnets are positioned to alter the strength of the magnetic field of the main magnet from place to place.  The field needs to be known at all points in a 3D space.  A scan is done point to point, and for each point, the field strength and the Larmor frequency are known.  Transmitter and Receiver are tuned to the f L emitted by a nucleus at the point.  The scanning of the next point is carried out at a slightly different frequecy.

 If the RF EM waves were continuous, then we wouldn’t be able to measure the relaxation time.  This is why RF waves are emitted in pulses.  After each pulse, a coil picks up the emitted RF waves from the patient.

 Amount of Data is huge.  Programming involves  Isolating slightly different radiofrequencies  Linking them to a point in 3D space.  This can only be done if the magnetic field strength is known accurately at all points within the volume being scanned.  The relaxation time for that point needs to be measured, relative to the type of material at that point, and the whole used to provide a display.

 Normally on a computer screen, and print- outs can be made.  Can give slice, or 3D view.  Can be rotated.  False colours can be attached.

 No ionising radiation involved  High quality image  Good distinction between different types of soft tissue.  Bone provides no barrier, so all images can be clear.  So side effects

 No metallic objects can be scanned, or they heat up.  So those with pacemakers and/or surgical pins can’t receive a scan.  Must not have external radio waves.  Machines are very expensive.  Long time for one scan (3/4 hours)

 This detects the presence of Hydrogen Nuclei.  Since body tissue has a high water content, it can be used to produce an image of the body.  Pulses of high intensity & amplitude magnetic fields are applied which results in emission of EM radiation from the nuclei by the…..

 Hydrogen nuclei spin, giving them a magnetic field.  When an external magnetic field is applied, they rotate around the direction of the field.  This rotation is called precession. (They are not all in phase)  The frequency of precession is called the Larmor Frequency.  Application of a pulse of radio waves at the Larmor frequency causes resonance/flipping – when the nuclei absorb the energy into the precession.  When the pulse is removed, the nuclei lose energy, emitting radio waves. This takes place in a short time called the relaxation time.

 All the protons precess at the same frequency.  The value of f depends on the strength of the magnetic field, B 0.

 The emissions from the hydrogen nuclei are detected by a radio aerial and processed by a computer to give a 3D body image.  MRI scanning is non-invasive and does not cause ionisation.  Very Expensive in terms of running costs.

 The response of different tissue types can be enhanced by varying the time between pulses.  Those with large molecules such as fat are best imaged using rapidly repeated pulses.  This technique is used to image the internal structure of the body.  Allowing more time between pulses enhances the response of watery substances.  This is used for diseased areas.

 7)a) An MRI scanner is a valuable item of diagnostic equipment. It is capable of generating 3D images of the patient.  Describe the operation of an MRI scanner with particular reference to  Larmor Frequency  Resonance of the Protons  Relaxation Times  7)b) State one disadvantage and one advantage of MRI scan.

 Outline the main principles of the use of MR to obtain diagnostic information about internal organs.