MRI Instrumentation

� Dr. Abdullah Jamea

The The

physical physical

basics ofbasics of

-Main parts for MRI scanner:

- Magnet.

- Gradient system.

- RF transmitter receiver system

- Computer and accessories.

Physical Basis of MRI

-Magnetic field can be produced naturally using ferromagnetic materials or through the passage of electric current in a good conductor.

-Two properties of magnetic fields are of central importance in MRI:

- Intensity or strength.

- Uniformity or homogeneity.

Physical Basis of MRI

- There are three types of magnets that used in MRI:

1- Permanent magnets:

-it is constructed of ferromagnetic materials.

-These ferromagnetic materials (e.g., iron, cobalt, nickel) in

certain crystalline forms, contain magnetic domains that can be

aligned by an external magnetic field. When the external field is

removed, the material remain aligned and a permanent magnet

results

- Dose not require the supply of an external current because it

is constructed of ferromagnetic materials. It can produce a

maximum magnetic field of 0.2T.

Physical Basis of MRI

Permanent magnet

Physical Basis of MRI

2- Resistive magnets:

-It is an electromagnet in which the magnetic field is generated

by the passage of electric current through a wire that is a good

conductor. It require a continuous high current supply.

Maximum field generated is 0.3T.

-The power to energize such a resistive magnet is dissipated

as heat in the coil. This is removed by passing water over the

ends of the coil.

Configuration of resistive magnet

3- Superconducting magnets:

This type of magnets use a superconducting materials such as

Nb/Ti. This alloy becomes superconducting at ~10ok (-283oc)

leading to decrease in electric resistance.

- There is no power loss in a superconductor and, after it has

been energized, the current continues as long as the

superconducting wire is maintained in a low temperature

state.

- This type of magnets are very powerful and can produce a

very homogenous high magnetic field strengths. It is the most

common magnet type used in MRI.

Physical Basis of MRI

Superconducting magnet

Magnetic field shield:

There are tow type of magnetic field shielding:

-Passive shield.

-Active shield.

Physical Basis of MRI

Magnetic field homogeneity:

-To generate a reasonably homogenous field in the bore of

acceptable length (~2 meters), the coil segments are wound

with a smaller coils to compensate for the divergence of the field at the ends of the magnet.

-To achieve the high homogeneity required for MRI, the

magnet field must be adjusted at the site of diagnostic operation by process called shimming.

Physical Basis of MRI

-The gradient system consists of three sets of coils, which can be switched on and off frequently to allow slice selection, phase encoding and frequency encoding.

- This coils fit within the room of magnet bore.

Physical Basis of MRI

- Gradient coils are powered by gradient amplifiers. Fault in the gradient coils or gradient amplifiers can result in geometric distortion in the MR image.

-The steepness of the magnetic field gradient (change in magnetic field per unit distance) affects the slice thickness sampled and the minimum field of view (FOV) that can be employed.

- Other parameters that characterized the quality of the gradient subsystem are the gradient linearity and their rise and fall times.

Physical Basis of MRI

- Primary functions of the RF system are to generate and collect MR signal.

- The induced voltage in the receiver is in order of

one tenth of microvolt (0.1µµµµV), whereas the transmitter output is in order of hundreds of volts.

Physical Basis of MRI

- The RF coil must generate a field that is perpendicular to the orientation of the main static B0 magnetic field. These considerations severely limit the geometry of the transmitter and receiver coils.

- Most of RF coils can be used as transmitter/ receiver coils.

Physical Basis of MRI

- There are several types of coil currently used in MR imaging. The more common ones are:

1- The volume coils: are used to image relatively large areas and yield uniform SNR over the entire imaging volume. However, because of their large size they generally produce images with lower SNR than other types of coil.

Physical Basis of MRI

Neurovascular

Head

Breast

Pediatric

2- Surface coils: are used to improve the SNR when imaging structures near the surface of the patient (such as the lumbar spine). Generally, the nearer the

coil is situated to the structure under examination, the greater the SNR.

3- The phased array coils: took the advantages of using small surface coils (increased SNR and resolution) to form a combination that covers a large FOV or to improve uniformity across a whole volume.

Physical Basis of MRI

Surface coil volume coil

Phased array coil

B1

B1

B1