MR Techniques
How Are Images Obtained?
MRI uses magnetic fields and radio frequency (RF) waves to create images. It is most often tuned to create images by manipulating protons (hydrogen atoms, high in concentration in water and fat within the body). Differences in proton density and the molecular structure surrounding them leads to protons giving off different RF signal characteristics which results in tissue contrast in the MR image.
Image courtesy of Aaron Field MD PhD
How it all works in a nutshellThe basic components of an MRI scanner include the main magnet, gradients, and coils. The patient lies on the table along the direction of the magnetic field. All of the protons within the patient's body align with this main magnetic field. The protons are then excited by an RF (Radiofrequency) pulse which elevates the hydrogen atoms to an excited state. These hydrogen atoms then relax to their normal resting state. However the time that it takes for each hydrogen atom to relax depends on their local tissue environment. Hydrogen atoms in water relax at different rates than hydrogen in soft tissue. It is this difference in relaxation times that enables us to generate an MR image. For additional information read on....
Image courtesy of Aaron Field MD PhD
Main magnet (B0)
Magnetic fields are referred to as “B” in physics and engineering. The main magnetic field is called B0. B0 is aligned with the MRI tube, or bore, that the patient lies in while being scanned. This magnetic field is strong. In clinical scanners these days it can range from 0.7 to 3.0T. The way the magnetic field is created is “cool.” There are several loops of wire wrapped around the bore (the center of the scanner). The wires are bathed in liquid helium causing them to be very cold and superconducting. A service engineer then applies a large current to the superconducting wire loops. This current stays in the loops without needing a recharge. The current creates a magnetic field (remember the right hand rule? Probably not. Thank goodness for Wikipedia!). For this reason the magnet is ALWAYS ON. There is no switch. The magnet is turned off by releasing the helium and quenching the magnet. This occurs very rarely, only in the context of a safety emergency or malfunction. Quenching the magnet leads to down-time and expense.
Gradients
Gradients are electronics which create magnetic fields that are small relative to B0. These can be quickly turned on and off and reoriented by the scanner. The small magnetic fields applied by the gradients cause very small variations in the magnetic field each proton experiences (ie variation from1.5T) which causes small changes in the response a proton has to an RF pulse. These small changes allow the scanner to know where a proton is in the patient to reconstruct 2d and 3d images. For more details read on...
Coils
Coils are essentially RF antennas which have both transmit and receive capabilities. A clinical scanner has a large coil which surrounds the magnet bore. However, the further away from the patient the coil is the less signal to noise you get. For this reason there are several coils designed to specifically fit around the head, close to the spine, around the body, or around an extremity to maximize signal to noise contribution from the coil. The purpose of RF transmission and receiving is described below.
Above is a 1.5T wide bore MR scanner. This special scanner is connected with an operating room and can be used for intraoperative imaging during neurosurgical procedures. A head coil is placed around the patient's head in this example. Before imaging, the patient table would be moved so that the head and coil are in the center of the magnet bore.
B0 is oriented directly down the center of the bore. The wires wrapped around the bore that make the main field and the gradients are covered by the housing.
This particular head coil can transmit and receive RF pulses. The closer the coil is to the anatomy of interest the better the image. There are several specialty coils designed for specific anatomy to wrap as close to the anatomy as possible.
Image Acquisition
Spinning hydrogen protons act as a magnetic dipole (link). When in a magnetic field these will realign with the magnetic field. Think of what happens when you bring two magnets close together (but not stuck together). They will try to align parallel. This is what happens to many of the hydrogen protons in the body when the body enters the magnet bore and thus the strong B0 field.
An RF transmission is applied at the resonant frequency of a lone proton (hydrogen). This frequency is proportional to the magnet strength and is equal to 64 MHz for a 1.5T scanner or 128 MHz for a 3.0T scanner. A resonant RF pulse will cause the protons that are aligned with the B0 field to become unaligned with B0. When the RF pulse is stopped the protons quickly re-align with B0. When the proton realigns with B0 it gives off energy in the form of a RF signal. Milliseconds after the transmitted RF pulse the receive coil/antenna listens for this RF signal radiating from the body. This signal is used to create the image.
Additional Resources and References
Basic Explanation of MRI physics: click HERE
Advanced physics tutorials:
Click HERE for a great advanced MRI tutorial in pdf format (start on page 10).
Click HERE for a good website with more advanced explanation of MRI physics and sequences.