Question

Explain Magnetic Resonance Imaging.What is the fundamental physics concepts behind Magnetic Resonance Imaging? How does it work?

Answer 1

What Is MRI ?

Magnetic resonance imaging (MRI), also known as nuclear magnetic resonance imaging, is a scanning technique for creating detailed images of the human body.

The scan uses a strong magnetic field and radio waves to generate images of parts of the body that can't be seen as well with X-rays, CT scans or ultrasound. For example, it can help doctors to see inside joints, cartilage, ligaments, muscles and tendons, which makes it helpful for detecting various sports injuries.

MRI is also used to examine internal body structures and diagnose a variety of disorders, such as strokes, tumors, aneurysms, spinal cord injuries, multiple sclerosis and eye or inner ear problems . It is also widely used in research to measure brain structure and function, among other things.

How it Works

The human body is mostly water. Water molecules (H2O) contain hydrogen nuclei (protons), which become aligned in a magnetic field. An MRI scanner applies a very strong magnetic field (about 0.2 to 3 teslas, or roughly a thousand times the strength of a typical fridge magnet), which aligns the proton "spins."

The scanner also produces a radio frequency current that creates a varying magnetic field. The protons absorb the energy from the magnetic field and flip their spins. When the field is turned off, the protons gradually return to their normal spin, a process called precession. The return process produces a radio signal that can be measured by receivers in the scanner and made into an image.

Protons in different body tissues return to their normal spins at different rates, so the scanner can distinguish among various types of tissue. The scanner settings can be adjusted to produce contrasts between different body tissues. Additional magnetic fields are used to produce 3-dimensional images that may be viewed from different angles. There are many forms of MRI, but diffusion MRI and functional MRI (fMRI) are two of the most common.

Physics Behind MRI

The physics of magnetic resonance imaging(MRI) involves the interaction of biological tissue with electromagnetic fields. MRI is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body. The human body is largely composed of water molecules, each containing two hydrogen nuclei, or protons. When inside the magnetic field (B0) of the scanner, the magnetic moments of these protons align with the direction of the field. They may align in two configurations parallel (in the direction of B0), or anti-parallel opposing B0. While each individual proton can only have one of two alignments the collection of protons appear to behave as though they can have any alignment. More protons align parallel to B0 as this is a lower energy state.

A radio frequency pulse is then applied, which can excite protons from parallel to anti-parallel alignment, only these protons are relevant to the rest of the discussion. In response to the force bringing them back to their equilibrium orientation, the protons undergo a rotating motion (precession), much like a spin wheel under the effect of gravity. The protons will return to the low energy state and do so by emitting photons corresponding to the energy difference between the two possible alignments. This appears as a magnetic flux, which yields a changing voltage in receiver coils to give the signal. The frequency at which a proton or group of protons in a voxel resonates depends on the strength of the local magnetic field around the proton or group of protons, a stronger field corresponds to a larger energy difference and higher frequency photons. By applying additional magnetic fields (gradients) that vary linearly over space, specific slices to be imaged can be selected, and an image is obtained by taking the 2-D Fourier transformof the spatial frequencies of the signal (a.k.a., k-space). Due to the magnetic Lorentz forcefrom B0 on the current flowing in the gradient coils, the gradient coils will try to move. The knocking sounds heard during an MRI scan are the result of the gradient coils trying to move against the constraint of the concrete or epoxy in which they are secured. These sounds can be very unnerving to the patient, particularly given the tight space in which the patient lies. This behaviour of MRI scanners can be described in terms of a fully coupled acousto-magneto-mechanical system.Solutions to such systems can provide useful insight for design engineers.