MRI - Magnetic Resonance Imaging (NMR)

Magnetic Resonance Imaging (MRI)

MRI is a non-invasive imaging technique. Its primary field of application includes the display of morphology, that is, tissue structures in a series of slice images through the body. Just like CT scan it also have the same based cross sectional imaging modality, but the physical principles of MRI are completely unlike from those of CT scan or and to the Conventional Radiography. The main advantage of MR imaging is that, no emission of ionizing radiation to produce a MRI images. This images is produced through the interactions of magnetic fields and radio waves with soft tissues.

The original name called after MRI is Nuclear Magnetic Resonance (NMR) imaging, with the word nuclear signifying that the nonradioactive atomic nuclear energy and nuclear weapons. But, neither of them is related with MRI.

WHAT IS MRI?

Magnetic resonance imaging (MRI) is a spectroscopic imaging technique used in medical settings to produce images of the inside of the human body.
It is based on the principles of nuclear magnetic resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules.

MRI Machine

In 1977 the first MRI exam was performed on a human being. It took 5 hours to produce one image.
The MR imaging is accomplished through the absorption and emission of energy of the radio frequency (RF) range of the electromagnetic spectrum.

Advantages of MR imaging

There are 3 main advantages of MR imaging these includes:

Excellent soft tissue contrast with high resolution
Display of several images and oblique cuts
No ionizing radiation

Modern MR systems allow for fast images of the body from head to toe. For example, examination of the entire spine can be completed in as little as two step.

As pacemaker of the MR industry, Siemens has incorporated a unique coil concept known as Integrated Panoramic Array or IPA. Integrated Panoramic Array (IPA) is a Siemens term referring to the connectability of various RF coils and now more commonly known as Total Imaging Matrix (TIM).

Components of MRI and Magnets

A magnet which produces a very powerful uniform magnetic field.
Gradient Magnets which are much lower in strength.
Equipment to transmit radio frequency (RF).
A very powerful computer system, which translates the signals transmitted by the coils.
The most important component of the MRI scanner is the magnet: The magnets currently used in scanners today are in the .5-tesla to 2.0-tesla range (5,000 to 20,000-gauss).
Higher values are used for research.
Earth magnetic field: 0.5-gauss

Type of Magnets

Resistive magnets
Permanent magnets
Super conducting magnets (the most commonly used type in MRI scanners).

Aside from these magnets the MRI machine also contains three gradient magnets. These magnets have a much lower magnetic field and are used to create a variable field.

Signal, Noise and Contrast

One of the objectives of the MR examination is to obtain the diagnostic image information necessary in the shortest possible measurement time. The image quality has to be of diagnostic usefulness. How can we affect image quality? What constitutes a good image? The most important criteria for image quality are: A strong signal, low noise, good contrast as well as sufficient resolution.

How image form from a MRI signal?

The MR image consist of a multitude of image pixels. Each pixel has a specific grey value. The pixels in the image represent the individual volume elements voxels in the slice.

After the RF pulse stimulates a slice in the patient’s body, each voxel in this slice emits an MR signal.

Among other things, the SIGNAL STRENGHT depends on the quantity of signal generating proton spins in the respective voxel (proton density).

What is Spin in MRI?

The atoms that compose the human body have a property known as spin (a fundamental property of all atoms in nature like mass or charge).
Spin can be thought of as a small magnetic field and can be given a + or – sign and a mathematical value of multiples of ½.
Components of an atom such as protons, electrons and neutrons all have spin.
Protons and neutron spins are known as nuclear spins.
An unpaired component has a spin of ½ and two particles with opposite spins cancel one another.
In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field.
Human body is mainly composed of fat and water, which makes the human body composed of about 63% hydrogen.

Why Are Protons Important?

Because it is positively charged
Spin about a central axis
A moving (spinning) charge creates a magnetic field.
The straight arrow (vector) indicates the direction of the magnetic field.

Energy Absorption of Machine

The MR machine applies radio frequency (RF) pulse that is specific to hydrogen.
The RF pulses are applied through a coil that is specific to the part of the body being scanned.
RESONANCE
The gradient magnets are rapidly turned on and off which alters the main magnetic field.
The pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance.
Frequency (Hz) of energy absorption depends on strength of external magnetic field.
The resonance frequency, w0, is referred to as the Larmor frequency.
When the RF pulse is turned off the hydrogen protons slowly return to their natural alignment within the magnetic field and release their excess stored energy. This is known as relaxation.

What happens to the released energy?

Released as heat
OR
Exchanged and absorbed by other protons
OR
Released as Radio Waves.

Measuring the MR Imaging Signal

The moving proton vector induces a signal in the RF antenna
The signal is picked up by a coil and sent to the computer system.
The received signal is sinusoidal in nature
The computer receives mathematical data, which is converted through the use of a Fourier transform into an image.