Magnetic Resonance Imaging Physics PDF

MAGNETIC RESONANCE Imaging PHYSICS Dr / Bothaina Kandil  INTERACTION OF NUCLEI WITH A STATIC MAGNETIC FIELD  In addition to aligning with a magnetic field, a magnetic moment also processes about the field. Precession is easily demonstrated in rotating objects (another reason wh...

MAGNETIC RESONANCE Imaging PHYSICS Dr / Bothaina Kandil  INTERACTION OF NUCLEI WITH A STATIC MAGNETIC FIELD  In addition to aligning with a magnetic field, a magnetic moment also processes about the field. Precession is easily demonstrated in rotating objects (another reason why we infer the property of “spin” for protons). A spinning top, for example, will “wobble” about a vertical axis defined by the earth’s gravitational field. This wobbling motion is precession.  Precession is a type of motion that is distinct from rotation. Rotation is the spinning of an object about its axis (an imaginary line through the center of mass of the object). The rapid spin of a top that causes its surface to blur is rotation. Precession is a “second-order” motion. It is the “rotation of a rotating object.”  In the macroscopic world, rotating objects have the property of angular momentum that causes them to behave as gyroscopes and tops. Nuclei react to forces in the microscopic world just as objects with angular momentum respond to forces in the macroscopic world. Protons and other subatomic particles are assumed to rotate about their axes and are described as having “spin.”  Precession results from the interaction of forces with a rotating object. where the force is gravity, and for a proton, where the force is a magnetic field.  The frequency of precession is known as the Larmor frequency. The frequency f of precession of a proton in units of megahertz (106 cycles or rotations per second) depends upon its gyromagnetic ratio γ (in megahertz per tesla) and the strength B(in tesla, T) of the static magnetic field. This relationship is described by the Larmor equation. Magnetic Resonance Properties of Selected Nuclei  INTERACTION OF NUCLEI WITH A RADIO FREQUENCY WAVE: NUTATION  There is a “third-order” property of an object that is rotating (first-order property) and precessing (second- order property). This property, called nutation, is the result of forces that rotate with the precession of the object.  When force is applied to an object with angular momentum, the object tends to move at right angles to the force. If we try to speed up the precession of a gyroscope by using a finger to push it in a circle, we will not affect the precessional speed.  Instead, the angle of precession will change. That is, if we push in the direction of precession, the gyroscope precesses at a greater angle until it finally lies flat on the table. This result is best explained by a diagram that is drawn as if we were precessing along with the object. Such a “rotating-frame-of-reference”.  This change in angle, called nutation, is a third-order circular motion (after rotation and precession).  INDUCTION OF A MAGNETIC RESONANCE SIGNAL IN A COIL  A changing magnetic field can induce a current in a loop of conducting wire. This principle is known as Faraday’s law, or the law of electromagnetic induction.  A proton has a magnetic moment and therefore acts as a small magnet.  Protons that precess so that their magnetic fields intersect the plane of a nearby coil will induce an electrical current in the coil. This current is the MR “signal” induced in the receiver coil. Radio Frequency Pulse Before the use of MRI a pre scan is made to determine at which frequency the protons are spinning ( Larmor Frequency). This frequency is also known as centre frequency. System uses this frequency in the next steps during MRI process. Once the centre frequency is determined the system starts acquisition. Now RF (Radio Frequency ) pulse is sent into the patient‟s body and observed.  For example, in the quantum mechanical interpretation of a system of spins (such as protons) in a magnetic field, there are two possible “energy states,” either parallel or antiparallel to the magnetic field. The protons having magnetic moments aligned with the magnetic field have slightly less energy (are in a lower energy state) than do protons with magnetic moments opposing the magnetic field. A photon with an energy equal to the energy difference between the two states can promote, or “flip,” protons from the lower to the higher energy state.  RELAXATION PROCESSES: T1 AND T2  We have seen that when an RF pulse is applied to a sample, the bulk magnetization may be nutated into the xy plane and induce an MR signal in a receiver coil positioned perpendicular to the xy plane. When the radio wave is switched off, the signal decays away. This decay is the result of the return of protons to the state that existed before the radio wave was applied. This return is termed relaxation of the protons. There are two basic relaxation processes at work in the sample. Both processes account for the observed decay of the MR signal. longitudinal or spin-lattice relaxation  One relaxation process involves a return of the protons to their original alignment with the static magnetic field. This process, called longitudinal or spin-lattice relaxation, is characterized by a time constant T1. The term spin-lattice refers to the interaction of the protons (spins) with their surroundings (the “lattice” or network of other spins). This interaction causes a net release of energy to the surroundings as the protons return to the lower energy state of alignment. Transverse or Spin–spin Relaxation  The other relaxation process is a loss of synchrony of precession among the protons. Before a radio wave is applied, the precessional orientation of the protons is random. The application of a radio wave brings the protons into synchronous precession, or “in phase.” When the radio wave is switched off, the protons begin to interact with their neighbors and give up energy in random collisions.  In so doing, they revert to a state of random phase. As the protons revert to random orientation, the bulk signal decreases because the magnetic moments tend to cancel each other. This process is called transverse or spin–spin relaxation and is characterized by a time constant T2  In a patient undergoing MRI, both longitudinal and transverse relaxation processes occur at the same time. The transverse (T2) relaxation time is always shorter than the longitudinal (T1) relaxation time. That is, magnetic moments dephase faster than they move into alignment with the static magnetic field.  For typical biological materials, T1 may be on the order of several hundred milliseconds while T2 is a few tens of milliseconds  Relaxation (decay) of the MR signal is characterized by exponential expressions analogous to those used to describe radioactive decay, absorption of photons, and growth of cells in culture. For longitudinal relaxation, reveals that the MR signal S decreases exponentially in time from the signal S0 that was present immediately following an RF pulse. The value of S0 is influenced by factors such as the number of protons in the sample, length of time that the radio wave was applied to the sample, sensitivity of the receiver coil, and overall sensitivity of the electronics. William R. Hendee, E. Russell Ritenour. MEDICAL IMAGING PHYSICS.2002. 4th Edition Two student will be selected for assignment. Explain what is meant by a longitudinal wave, and describe how an ultrasound wave is propagated through a medium.