Four basic steps are involved in acquiring a magnetic resonance (MR) image:
- Placing the patient in the magnet
- Sending radiofrequency (RF) pulse by a coil
- Receiving signals from the patient by a coil
- Transformation of signals into the image by complex processing in the computers.
Now, let us understand these steps at the molecular level. Present magnetic resonance imaging (MRI) 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. Most of the signal on clinical MR images comes from water molecules that are mostly composed of hydrogen.
How do protons help in MRI?
Protons are positively charged and have rotatory movement called spin. Any moving charge generates current. Every current has a small magnetic field around it. So every spinning proton has a small magnetic field around it, also called magnetic dipole moment.
Normally, the protons in human body (outside the magnetic field) move randomly in any direction. When external magnetic field is applied, i.e., patient is placed in the magnet, these randomly moving protons align (i.e., their magnetic moment align) in the direction of external magnetic field. Some of them align parallel and others antiparallel to the external magnetic field. When a proton aligns along external magnetic field, not only it rotates around itself (called spin) but also its axis of rotation moves forming a “cone”. This movement of the axis of rotation of a proton is called as precession (Fig. 1).
The number of precessions of a proton per second is termed precession frequency. It is measured in Hertz (Hz). Precession frequency is directly proportional to strength of external magnetic field. Stronger the external magnetic field, higher is the precession frequency. This relationship is expressed by Larmor's equation:
f0 = γ B0
Where, f0 = precession frequency in Hz
B0 = strength of external magnetic field in Tesla
Fig. 1: Spin versus precession: Spin is rotation of a proton 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”.
Precession frequency of the hydrogen proton at 1, 1.5, and 3 Tesla is roughly 42, 64, and 128 MHz respectively.
LONGITUDINAL MAGNETIZATION
Let us go one step further and understand what happens when protons align under the influence of external magnetic field. For the orientation in space consider X, Y, and Z coordinate system. External magnetic field is directed along the Z-axis. Conventionally, the Z-axis is the long axis of the patient as well as bore of the magnet. Protons align parallel and antiparallel to external magnetic field, i.e., along positive and negative sides of the Z-axis respectively. Protons that are diagonally opposite on negative and positive sides cancel each other's forces. However, there are always more protons spinning on the positive side of Z-axis as it takes less energy to be on positive side of Z-axis. So, after canceling each other's forces, there are a few protons on positive side that retain their forces. Forces of these protons add up together to form net magnetization represented by a vector along the Z-axis. This is called as longitudinal magnetization (Figs. 2A to C).
Figs. 2A to C: Longitudinal magnetization: (A) More protons precess along positive side of Z-axis; (B) Protons that are diagonally opposite to each other cancel out each other's forces. A few protons with uncanceled forces remain along positive side; (C) Forces of these proton add up to form longitudinal magnetization, represented as a vector along positive side of Z-axis.
5Longitudinal magnetization thus formed along the external magnetic field can not be measured directly. It can be measured when it is tipped away from Z-axis and precesses at Larmor frequency.
TRANSVERSE MAGNETIZATION
As discussed in the previous paragraph when patient is placed in the magnet, longitudinal magnetization is formed along the Z-axis. The next step is to send RF pulses. The precessing protons pick up some energy from the RF pulse. Some of these protons go to higher energy level and start precessing antiparallel (along negative side of the Z-axis). The imbalance results in tilting of the magnetization into the transverse (X-Y) plane. This is called as transverse magnetization (Figs. 3A to C). In short, RF pulse tilts the magnetization into the transverse plane.
The precession frequency of protons should be same as RF pulse frequency for the exchange of energy to occur between protons and RF pulse. When RF pulse and protons have the same frequency protons can pick up some energy from the RF pulse. This phenomenon is called as “resonance”—the R of MRI.
Radiofrequency pulse not only causes protons to go to higher energy level but also makes them precess in phase or synchronously.
MAGNETIC RESONANCE SIGNAL
Transverse magnetization vector has a precession frequency. It constantly precesses at Larmor frequency in the transverse plane and induces electric current while doing so. The receiver RF coil receives this current as the MR signal (Fig. 4). The strength of the signal is proportional to the magnitude of the transverse magnetization. MR signals are transformed into MR image by computers using mathematical methods such as Fourier transformation.
Figs. 3A to C: Transverse magnetization: (A) Longitudinal magnetization (LM) is along positive side of Z-axis; (B) 90-degree radiofrequency (RF) pulse creates imbalance of proton forces resulting in decrease in LM and increase in magnetization in transverse plane; (C) Magnetization vector is flipped in transverse plane, called as transverse magnetization.
Fig. 4: MR signal. Magnetization tilted in the transverse plane precesses at Larmor frequency and induces current in the receiver coil while do so. This current in the receiver coil is the MR signal. The TM vector starts reducing in its magnitude immediately after its formation because of dephasing of protons. The LM starts gradually increasing in its magnitude. The net magnetization vector (NMV) formed by addition of these two (LM and TM) vectors gradually moves from transverse X-Y plane toward vertical Z-axis. As long as the NMV is away from the Z-axis, there is some component of the magnetization in the transverse plane inducing current in the receiver coil.(LM: longitudinal magnetization; MR: magnetic resonance; TM: transverse magnetization)
Revision
Basic four steps of MR imaging include (Fig. 5):
- Patient is placed in the magnet: All randomly moving protons in patent's body align and precess along the external magnetic field. Longitudinal magnetization is formed along the Z-axis.
- Radiofrequency pulses sent: Precessing protons pick up energy from RF pulse to go to higher energy level and precess in phase with each other. This results in reduction in longitudinal magnetization and formation of transverse magnetization in X-Y plane.
- Precession of transverse magnetization vector: The transverse magnetization vector precession at Larmor frequency induces current in receiver RF coil. This is MR signal.
- Image formation: MR signal received by the coil is transformed into image by complex mathematical process such as Fourier transformation by computers.
Three more magnetic fields are superimposed on the main magnetic field along X, Y, and Z axes to localize from where in the body signals are coming. The strength of these magnetic fields varies from one end to other hence these fields are called “gradient fields” or simply “gradients”. The gradient fields are produced by coils called as gradient coils.
The three gradients are:
- Slice selection gradient
- Phase encoding gradient
- Frequency encoding (read out) gradient
Slice Selection Gradient
Slice selection gradient has gradually increasing magnetic field strength from one end to another (Fig. 6). It determines the slice position. Slice thickness is determined by the bandwidth of RF pulse. Bandwidth is the range of frequencies. Wider the bandwidth thicker is the slice.
Phase Encoding and Frequency Encoding Gradients
These gradients are used to localize the point in a slice from where the signal is coming. They are applied perpendicular to each other and perpendicular to the slice selection gradient (Fig. 7).
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 turned on 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 turned on in the end at the time of signal reception.
Fig. 6: Slice selection gradient. Magnetic field strength varies from one end to other. The magnetic field variation is in the range of few Gauss.(RF: radiofrequency)
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 substances can also be utilized for MR imaging. The requirements are that their nuclei should have spin and should have odd number of protons within them. Hence theoretically 13C, 19F, 23Na, and 31P can be used for MR imaging.
Hydrogen atom has only one proton. Hence H+ ion is equivalent to a proton. Hydrogen ions are present in abundance in body water. H+ gives the best and most intense signal among all nuclei.