- Placing the patient in the magnet
- Sending radiofrequency (RF) pulse by coil
- Receiving signals from the patient again by coil
- Signals are sent to computers for complex processing to get image.
Now let us understand these steps at molecular level. Present MR imaging is based on proton imaging. Proton is a positively charged particle in the nucleus of every atom. Since hydrogen ion (H+) has only one particle i.e. proton, it is equivalent to a proton.
How does this proton help in MR imaging?
Protons are positively charged and have rotatory movement called spin. Any charge, which moves, generates current. Every current has a small magnetic field around it. So every spinning proton has a small magnetic field around it.
Without any influence of external magnetic field protons in the patient's body move randomly in any direction. When external magnetic field is applied, i.e. patient is placed in the magnet, these randomly moving protons align and spin in the direction of external magnetic field. Some of them align parallel and some anti-parallel to the external magnetic field. When protons align, not only they rotate around themselves (called spin) but also their axis of rotation moves such that it forms a ‘cone’. This movement of axis of rotation of proton is called as precession (Fig. 1.1).5
Fig. 1.1: Spin versus precession. Spin is rotation of protons around its own axis while precession is rotation of the axis itself under the influence of external magnetic field such that it forms a ‘cone’
The number of precessions of proton per second is precession frequency in Hertz. Precession frequency is directly proportional to strength of external magnetic field. Stronger the external magnetic field, higher is precession frequency. This relationship is expressed by Larmor's equation–
Wo = yBo
Where wo = Precession frequency in Hz
Bo = Strength of external magnetic field in Tesla
Y = Gyromagnetic ratio, which is specific to particular nucleus
Precession frequency of hydrogen proton for 1 Tesla is 42 MHz and for 1.5 Tesla it is 64 MHz.
MAGNETIZATION
Now let us go one step further and understand what happens when protons align under influence of external magnetic field.6
For the orientation in space consider X,Y, and Z axes system. External magnetic field is directed along Z-axis. Conventionally, Z-axis is the long axis of the patient as well as bore of the magnet. Protons align parallel and anti-parallel to external magnetic field i.e. along positive and negative sides of Z-axis. Forces of protons on negative and positive side cancel each other. However, there are always more protons spinning on the positive side or parallel to Z-axis than negative side. So, after canceling each other few protons remain on positive side, which are not cancelled. Forces of these protons add up together to form a magnetic vector along Z-axis. This is longitudinal magnetization (Fig. 1.2).
Longitudinal magnetization along external magnetic field cannot be measured directly. For measurement it has to be transverse.7
Fig. 1.3: Transverse magnetization. Magnetization vector is flipped in transverse plane by 90 degree RFp
Transverse Magnetization
As we discussed when patient is placed in the magnet, longitudinal magnetization vector forms along Z-axis and in the long axis of the patient. At this stage radiofrequency pulse is sent. Precessing protons pick up some energy from radiofrequency pulse. Some of these protons go to higher energy level and start precessing anti-parallel. This results in reduction in the magnitude of longitudinal magnetization. Forces of protons now add up to form a new magnetic vector in transverse (X-Y) plane. This is called as transverse magnetization (Fig. 1.3). In short, RF pulse causes longitudinal magnetization to reduce and establishes a new transverse magnetization.
For exchange of energy to occur between protons and RF pulse, precession frequency of protons should be same as RF pulse frequency. When RF pulse and protons have same frequency protons can pick up some energy from 8RF pulse. This phenomenon is called as “resonance”—the R of MRI.
RF pulse not only causes protons to go to higher level but also makes them precess in step or in phase.
MR Signal
Transverse magnetization vector formed has a precession frequency. When it moves it produces electric current. The coils receive this current as MR signal (Fig. 1.4). Strength of the signal depends upon magnitude of the transverse magnetization. MR signals are Fourior Transformed into MR image by computers.
Fig. 1.4: MR signal. When RFp is switched off TM vector goes on reducing in its magnitude and LM goes on increasing. The resultant NMV formed by addition of these two (LM and TM vectors) gradually moves from transverse X-Y plane into vertical Z-axis. During this movement it produces current in receiver coil. This current received by the coil is MR signal
Revision
- Patient is placed in the magnetAll randomly moving protons in patent's body align and precess along external magnetic field. Longitudinal magnetization is formed long Z-axis.
- RF pulse is sentPrecessing protons pick up energy from RF pulse to go to higher energy level and precess in phase. This results in reduction in longitudinal magnetization and formation of transverse megnetization in X-Y plane.
- MR signalTransverse magnetization vector precess and generates current. When RF pulse is switched off, this current produces signal in the coil.
- Image formationSignal is transformed into image by complex mathematical process—Fourior Transformation by computers.
Localization of Signal
To localize from where in the body signals are coming three more magnetic fields are superimposed on main magnetic field along X, Y, and Z axes. These magnetic fields have different strength in varying location hence these fields are called “gradient fields” or simply “gradients”.
The three gradients are:
- Slice selection gradient
- Phase encoding gradient
- Frequency encoding gradient
Slice Selection
Slice selection gradient has gradually increasing magnetic field strength from one end to another (Fig. 1.5). It determines slice position. Slice thickness is determined by bandwidth of RF pulse. Bandwidth is range of frequencies. Wider the bandwidth thicker is the slice.
To determine the point in a slice from where certain signal is coming, two more gradients—phase encoding and frequency encoding, are applied perpendicular to each other and perpendicular to slice selection gradient (Fig. 1.6).
Typically, for transverse or axial sections following are axes and gradients applied, even though X and Y axes can be varied.
- Z-axis—Slice selection gradient
- Y-axis—Frequency encoding gradient
- X-axis—Phase encoding gradient.
In a usual sequence, slice selection gradient is sent at the time of RF pulse. Phase encoding gradient is turned on for a short-time after slice selection gradient. Frequency encoding or readout gradient is sent in the end at the time of signal reception.11
Information from all three axes is sent to computers to get the particular point in that slice from which the signal is coming.
Why Proton Only?
Other nuclei can be used for MR imaging. Requirement is that they should have spin and should have odd number of protons in the nucleus. Hence, theoretically 13C, 19F, 23Na, 31P can be used for MR imaging.