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What is MRI?

MRI or Magnetic Resonance Imaging is a process of creating images from the manipulation of hydrogen atoms in magnetic fields.

What makes the hydrogen atom important for MRI?

Hydrogen atoms and other atoms with an odd number or protons and neutrons possess a specific quantum property called spin angular momentum. Spin is where an atom rotates about its own axis and because atoms are charged particles they create small magnetic fields. The atoms are able to occupy multiple different states of spin, but are normally randomly distributed between them. However, in the presence of an external magnetic field the atoms will align their spin states either with or against the external magnetic field depending on whether it is in a low or high energy state. This phenomenon results in giving the object inside the magnetic field a very weak magnetic charge described by what is known as the net magnetization vector (NMV).

Nuclear Magnetic Resonance

A spectroscopy technique called Nuclear Magnetic Resonance described in 1946 uses the above nuclear property to determine the chemical composition of an object. It was found that the atoms could be caused to jump between high and low energy states by the application of a particular radio frequency (RF). The transition between high and low energy states would result in a change in the NMV of the object which can be identified using Faraday's law that states that any change in a magnetic field will cause a voltage (emf) to be induced in a coil of wire perpendicular to that field. The radio frequency required to cause the transition is dependent on an intrinsic property of the atom called the gyromagnetic ratio and the strength of the external magnetic field. This relationship is known as Larmor's equation and it is the underpinning principle behind NMR which uses it to determine the chemical composition of an object depending on what radio frequencies are absorbed.

What is measured by MRI?

Lauterbur, in 1973, extended upon the ideas behind NMR to create the first Magnetic Resonance Images. The NMV caused by placing a body, which is 70% water, in an external magnetic field is dominated by the hydrogen atom. Consequently MRI can use Larmor’s equation to send bursts of specific RF’s that will excite hydrogen atoms into their higher energy states and change the NMV. The image creation is then based on the measuring of the return of the NMV back to its original orientation. This is known as relaxation, it is the extent of how quickly this process is achieved that allows MRI to differentiate between the tissues in the body. There are two types of relaxation constants:

T1 relaxation
T1 relaxation is the time constant for the NMV to return to being aligned with the external magnetic field (longitudinal or Spin-lattice relaxation)
T2 relaxation
T2 relaxation is the time constant for the NMV to leave the plane perpendicular to the external magnetic field (transverse or Spin-Spin relaxation)

The value of these constants are material dependent, specifically they depend on the water content of the material. Low water content materials, such as bone and other hard tissues, have very fast relaxation constants while in contrast soft tissues have a high water content to give long relaxation constants. Additionally since the water content in soft tissues is variable this gives MRI the ability not only to measure the relaxation process but to distinguish between different soft tissues by quantifying the relaxation constant.

Creating an image

A T1 or T2 weighted signal can now be created by measuring the change we created by applying the RF pulse. However to create an image we need to investigate the T1/T2 properties of water in very small blocks of tissue. MRI consequently uses three methods that combined, work together to partition the body into small segments or voxels:

Slice selection

The first step is to divide the body up into slices. By applying a magnetic gradient additional to the external magnetic field we cause the hydrogen atoms to experience a different magnetic field strength dependent on their position along that gradient. As stated above, the Larmor frequency is dependent on the external magnetic field. Therefore when a particular RF pulse is applied, only the Slice of atoms experiencing that magnetic field will be excited, resulting in the first partition of our voxels.

Frequency Encoding

The second step is to then slightly adjust the frequency at which the atoms are spinning or the NMV is precessing about its axis. This is achieved by applying a second magnetic gradient after the RF pulse that is perpendicular to the first. Due to Larmor’s equation this causes the hydrogen atoms individual spins to change again according to their position in the second magnetic gradient. The slight modulation of the Larmor frequency causes the atoms to be binned into specific frequencies and results in the second partition of our voxels.

Phase encoding

As a by-product of applying the Frequency encoding gradient, the phase of the spinning atoms has also been changed. As a result when a phase-encoding gradient is applied orthogonally and in between the slice and frequency encoding gradients, only hydrogen atoms that have an equal but opposite phase will be able to produce an emf that is recorded in the RF coil our final means for partitioning our voxels. But due to this technique of signal crushing, the phase encoding and frequency steps must be repeated multiple times in order to build up a complete image in 2D.

In summary the application of the gradients can be thought of as analogous to reading a book. Where the Slice encoding gradient determines which page or slice we want to look at, the phase encoding chooses the line to be read and the frequency encoding reads the characters in each of the columns.

Fitting it together

Above we have introduced the basic concepts behind MRI but how does it all fit together to create a scanner and the scanner an image.

The External magnet

When placed inside an MRI machine, the body is actually being placed inside a magnetic field causing all the hydrogen atoms inside the body to align with or against the field.

Creating the Magnetic Gradients

Surrounding the body again is an integrated system of electromagnetic coils, these coils are there to provide the magnetic gradients used to partition the body into voxels.

Producing and Receiving the RF pulses

The RF coil used to produce the excitatory RF pulse is also surrounding the body and another coil either surrounding or placed directly on the patient is used to receive the resulting electrical signal caused by the relaxation of the NMV.

The Pulse Sequence

The MR scanner co-ordinates these different actions into what is known as a Pulse Sequence. The Pulse sequence contains the information of how and when to control the different gradients and RF pulses that the scanner uses to create the different images. Variations on these Pulse sequences allows the radiologist to manipulate the field of view, resolution and other image parameters used to create the different MR images.

From the scanner to an image

The scanner obtains the signal from each voxel of the image in what is known as K-Space. K-Space is the complex (contains real and imaginary components) representation of the image. The last step in creating an MRI magnitude image is to take the magnitude of each of these pixels to obtain real values which can be transformed into the conventional grayscale images seen in this atlas.

MRI safety

MRI is a completely safe imaging modality. It does not use any ionizing radiation (x-rays) or any radionucletides to form the images. MRI instead works by the manipulation of magnetic fields which naturally occur around us everyday. The only time MRI is unsafe is if the patient has any metallic implants or if they could have metal shavings in their eye. Some patients may also experience symptoms of claustrophobia while in the scanner. For further information about MRI safety ask your clinician or visit the official website of the Institute for Magnetic Resonance Safety.

Where to learn more about MRI?

Above we have briefly outlined various ideas behind the acquisition of an MRI image. MRI is a growing field and has had numerous advancements that have resulted in improved acquisition technologies that produce sharper images at a reduced imaging time. Additionally MRI has moved beyond providing only conventional images to being able to provide information about velocities and displacements. If you desire to learn more about the mathematical and physical principles behind MRI we can suggest the following links: