MRI Physics – Resonance – Deconstructing MRI Physics Test Review.
In our previous article we discussed atoms, their subatomic particles, as well as the three distinct spins occuring within the atom. We also covered the terms Magnetic Moment, Precession, Net Magnetization Vector, as well as defined the Larmor Equation and broke it down into easily digestible steps and terms. In this article we will discuss the concept of Resonance, and its relation and importance in MRI.
In physics, resonance is a process in which a vibrating object or external force causes another system to vibrate with greater amplitude at specific frequencies. While this definition may sound somewhat dense, as is the case with many things in MRI, it can be broken down into simpler terms. This can be thought of as follows: if a force is vibrating at a certain frequency and it comes into contact with a seperate force vibrating at a similar frequence, that second force will become excited and gain energy.
Such excitation is on of the fundamental aspects of MRI.
In MRI the “external force” causing all of the excitement discussed above is a radiofrequency (RF) pulse and the “other system” gaining energy, becoming excited, is the precessing hydrogen atoms nucleus. If we recall from the previous article, hydrogen is precessing in accordance to the Larmor Equation. For every 1T of external magnetic field strength, hydrogen precesses at 42.57MHz or millions of times per second. If we send in a vibrating object, in this case a radiofrequency pulse, at a similar frequency to this precession, resonance should, and does occur. If, however, radiofrequency is sent in at a frequency different to the precessional frequency of the hydrogen atom, resonance and excitation will not occur. It is also important to note that 42.57MHz/T is the precessional frequency of hydrogen, but other elements have their own individual precessional frequencies, and as such when RF is sent in at the precessional frequency of hydrogen, these atoms do not achieve resonance, or excitation. Conversely, RF sent in at the precessional frequency of these other atoms will have no effect on hydrogen.
You may also recall from the previous article that the Net Magnetization Vector is a byproduct of an excess of low energy (parallel, spin up) hydrogen atoms. In using the analogy of a lazy river at a waterpark, we discussed how there will be more low energy, lazy patrons who are willing to be pulled by the current of the river than there are patrons attempting to swim against the current. This excess of low energy patrons, or in the case of MRI, precessing hydrogen atoms, are what compose the net magnetization vector. This NMV is, prior to excitation, is pointing north, or upward Y axis of an XY axis graph.
The introduction of RF at the resonant frequency of hydrogen causes excitation, and some of the hydrogen atoms that were previously spin up, north oriented, low energy prior to excitation, gain energy and transition into a spin down, high energy state. The introduction of RF at hydrogens resonant frequency and the excitation caused can be thought of as analogous to introducing energy drinks to the patrons in the lazy river. While many of the patrons will continue to lazily be pulled by the flow of the river, a few will find motivation from the new found energy and decide to swim against the current. This shift in hydrogen nuclei (patrons) will cause a shift in the Net Magnetization Vector, tilting it away from north on the Y axis of our XY axis graph. The tilt of this vector away from north on the Y axis can vary in its degrees of flip and is known as the Flip Angle. Flip Angle’s vary by pulse sequence which will be discussed in a future article, but for a point of reference common flip angles for Gradient Echo Sequences typically range between 1 and 89 degrees. Common values for Spin Echo Sequences are 90 and 180 degrees.
The XY graph described above, when pertaining to MRI, has unique names for the X and Y axis. The Y axis, which the Net Magnetization Vector is typically oriented toward in the presence of an external magnetic field, prior to excitation by an RF pulse, is known as the longitudinal plane. The X axis in MRI is referred to as the Transverse plane. Upon receiving excitation from an RF pulse the Net Magnetization Vector flips itself toward the transverse plane, and away from the longitudinal plane.
It is also important to note that the Radio Frequency pulse causing excitation and resonance must come in perpendicular to the angle of the Net Magnetization Vector. Again reinforcing our lazy river analogy, an energy drink thrown from behind cannot be caught by the lazy river patron. One being thrown from the front could potentially come in too fast and hit the patron in the head. An energy drink gently tossed from the side however would be easily caught and cause excitation, and resonance.
One final item of importance regarding excitation and resonance in MRI is the concept of Phase. When the net magnetization vector becomes excited, its protons begin to precess in-phase. This can be thought of as follows: prior to excitation from their energy drinks all of our patrons in the lazy river were pointed either directly with, or directly against the current of the lazy river. These patrons were all spinning, or precessing, at the same rate in their inner tubes, but were spinning independently from one another. While patrons were spinning at the same speed, one could be facing to the right while another could be facing to the left. Patrons who are spinning at the same rate, but have different orientation are out-of-phase from one another. In the presence of an RF pulse, which produces excitation or resonance, however, these patrons or independent magnetic moments would all align themselves with one another and spin at the same frequency, and have the same orientation. This means that all patrons would be spinning at the same rate in the lazy river, and would all be facing the same directing at the same time. This level of choreography from our patrons is short lived, however, and as soon as the RF pulse or energy drinks are removed our patrons begin to lose phase, and transition back into their natural out-of-phase state.