Basic steps are involved in getting an MR image
- Placing the patient in the magnet
- Sending Radiofrequency (RF) pulse by coil
- Receiving signals from the patient by coil
- Transformation of signals into an image by complex processing in the computer
Now let us understand these steps at the molecular level. Present MR imaging is based on proton imaging. Proto is a positively charged 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 does proton help in MR imaging?
Protons are positively charged and have a rotatory movement called spin. Any moving charge generates a 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 the human body (outside the magnetic field) move randomly in any direction. When the external magnetic field is applied, i.e. the patient is placed in the magnet, these randomly moving protons align and spin in the direction of the external magnetic field. Some of them align parallel and others anti-parallel to the external magnetic field. When a proton aligns an external magnetic field, it rotates around itself (called spin) and its axis of rotation moves to form a'cone'. This movement of the axis of rotation of a proton is called precession.
The number of the precession of a proton per second is called precession frequency. It is measured in Hertz. Precession frequency is directly proportional to the strength of the external magnetic field. The stronger the external magnetic field, the higher is the precession frequency.
Longitudinal Magnetization
Let us go one step further and understand what happens when protons align under the influence of an external magnetic field. For the orientation in space consider X, Y, and Z axes system. The external magnetic field is directed along the Z-axis is the long axis of the patients as well as the bore of the magnet. Protons align parallel and antiparallel to the external magnetic field, i.e. along the positive and negative side of the Z-axis. The Force of protons on negative and positive sides cancel each other. However, more protons spin on the positive side or parallel to the Z-axis than on the negative side.
So after canceling each other's forces. Forces of these protons and up together to form a magnetic vector along the Z-axis. This is called longitudinal magnetization.
Longitudinal magnetization thus formed along the external magnetic field can not be measured directly. For the measurement, it has to be transverse.
Transverse Magnetization
As discussed in the previous paragraph, longitudinal magnetization is formed along the Z-axis when a patient is placed in the magnet. The next step is to send a radiofrequency (RF) pulse. The processing protons pick up some energy from the radiofrequency pulse. Some of these protons go to higher energy levels and start processing antiparallel (along the negative side of the Z-axis). The imbalance results in tilting of the magnetization into the transverse (X-Y) plane. This is called transverse magnetization. In short, the RF pulse causes tilting of the magnetization into the transverse plane.
The processing frequency of protons should be the same as the 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 "resonance"- the R of MRI.
RF pulse not only causes protons to go to higher energy levels but also makes them process in step, in-phase, or synchronously.
MR Signal
The transverse magnetization vector has a precession frequency. It constantly rotates at Larmor frequency in the transverse plane and includes an electric current while doing so. The receiver RF coil receives this current as an MR signal. The strength of the signal is proportional to the magnitude of the transverse magnetization into the MR image by computer using mathematical methods such as Fourier Transformation.
Localization of the Signal
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. These magnetic fields have different strengths in varying locations hence these fields are called "gradient fields" or simply "gradient". The gradient fields are produced by coils called gradient coils.
The three gradients are:
- Slice selection gradient
- Phase encoding gradient
- Frequency encoding (readout) gradient.
Slice Selection Gradient
Slice selection gradient has gradually increasing magnetic field strength from one end to another. It determines the slice position. Slice thickness is determined by the bandwidth of the RF pulse. Bandwidth is the range of frequencies. Wider the bandwidth thicker the slice.
Pase 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.
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)
- The X-axis (Phase encoding gradient)
In a usual sequence, the 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 on in the end at the time of signal reception.
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 an odd number of protons within them. Hence theoretically 13C, 19F, 23Na, 31P, can be used for MR imaging.
The 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.
Post a Comment