EL FUNCIONAMIENTO OBJETIVO

The basic physics of magnetic resonance imaging (MRI) have been covered in the previous chap-ters, and the equipment used in the process is discussed in this and the following three chapters.

An MRI system contains three major compo-nents, each of which consists of several subsys-tems. The major components are the gantry, the operating console, and the computer. In this regard, an MRI system is similar to a computed tomography (CT) imaging system. Here, how-ever, most similarities end. These principal components are shown in Figure 10-1.

Gantry, operating console, and computer are the three principal components of an MRI system.

The gantry contains the main magnet and sev-eral other electromagnetic devices essential to MRI. Unlike CT, there are no moving parts in the MRI gantry. The operating console resem-bles a CT console, and although many of the control designations are similar, they also serve different functions. The MRI computer is pow-erful and fast such as that used in multislice CT.

Each of the three types of MRI systems—

superconducting electromagnet, resistive electromagnet, and permanent magnet—uses similar computers. The operating consoles have similar functional controls that appear the same. However, the gantries are com-pletely different. Each has a distinctive appearance that makes them easily recogniz-able. Because most MRI systems are super-conducting, this discussion focuses on that type.

THE GANTRY

The gantry can be intimidating to a patient, especially after it is placed on the examination couch and moved into the patient aperture. A patient then hears the resounding thump, thump, thump of the gradient coils, suggesting that this is indeed a big and intimidating machine. However, it is not a machine because there are no moving parts. The gantry does have many subsystems and several different electromagnetic coils.

Operating

console Gantry

Computer

Figure 10-1 The principal components of a magnetic resonance imaging system are the gantry, the operating console, and the computer.

Superconducting Magnetic Resonance Imaging System

Figure 10-2 is a typical superconducting MRI magnet. Superconducting MRI magnets are approximately 3 m across by 3 m high, with a length of 2 m. The massive size is due princi-pally to the requirement of maintaining the primary magnetic coils at a super-cooled tem-perature or cryogenic state. This cryogenic state is accomplished with multiple insulating chambers.

Cryogens are liquefied gases that produce supercold temperatures near absolute zero. Liquid nitrogen and liquid helium are used in MRI.

The gantry of a superconducting MRI system can be considered to have three subassem-blies: the patient couch, the primary electro-magnet assembly, and the various secondary electromagnets (Figure 10-3). Often the sec-ondary electromagnets are at room tempera-ture, unlike the primary electromagnet that is immersed in liquid helium.

The patient couch performs the two func-tions of support and position. The couch should be able to accept patients who weigh

up to 130 kg (286 lb) at near floor level and raise the patient with a power assist to the level of the gantry patient aperture. From this position outside the gantry, the couch should be capable of moving to the imaging position under power assist to within ± 1 mm.

Precise positioning is essential during examination. Once at the imaging position, the patient is not moved during the exami-nation.

Figure 10-2 A, This 1.5-T superconducting magnet is actively shielded. B, The active shield reduces the magnetic fringe field distance by one third. (Courtesy Oxford Magnet Technology.)

Primary magnets

Secondary magnets

Figure 10-3 The patient couch, the primary elec-tromagnet assembly, and the secondary electromag-nets are the three superconducting magnet subassemblies.

A B

This precision is obtained with specialty gears and electronic registers. Each revolution of the smallest drive gear corresponds to a 1-mm movement of the couch. Each revolution also activates an electronic counter so that the couch position can be visually displayed. Table 10-1 lists minimum acceptable specifications for the patient couch.

At installation, the service engineer adjusts the two or three positioning laser lights to inter-sect at a point on the axis of the MRI gantry (Figure 10-4). Usually, this position is then set at zero on the couch position indicator.

The primary electromagnet assembly is not visible. It is enclosed in a decorative plastic housing. Even with the housing removed, the primary electromagnet assembly cannot be seen because it is deep within the chambers of the cryostat. The innermost chamber of the cryostat houses an aluminum cylinder onto which the superconducting wire is wound (Figure 10-5).

The cryostat is a large, insulating con-tainer of many concentric chambers.

Similarly, the secondary magnetic coils are not visible. They are also covered by a decorative protective housing. The relative position of these magnetic coils in the gantry is shown in Figure 10-6.

There are three secondary magnetic coils, and they are independent. Nearest to the patient is the radiofrequency (RF) coil. The RF coil is not an electromagnet in the normal sense. It does produce a magnetic field, an

TABLE10-1 Minimum Specifications for an MRI Patient-Positioning Couch

Descriptor Performance Standard Patient capacity 130 kg

Lift speed 1 cm/s

Translation speed 10 cm/s

Position accuracy ±1 mm

MRI, Magnetic resonance imaging.

Localizing

Figure 10-4 Two or three positioning laser lights are adjusted to intersect on the axis of the primary magnetic field of a magnetic resonance imaging system.

Primary coils in a cryostat

Patient aperture

Figure 10-5 Relative position of the primary mag-netic coils in the magnet.

Secondary electromagnet coils

Patient aperture

Figure 10-6 Relative position of the secondary magnetic coils in the magnet.

incidental byproduct to its use as an RF antenna. The RF coil is a separate, removable assembly and therefore is not cryogenic but kept at room temperature (Figure 10-7).

The secondary magnetic coils adjacent to the patient aperture are the gradient coils. These coils are large electrical conductors that produce the transient gradient magnetic fields. They are switched on and off rapidly. This current switch-ing results in the conductors’ heatswitch-ing and expanding, and these cause the thumping sound.

These coils are also usually at room temperature.

Between the gradient coils and the primary electromagnet assembly for earlier systems, shim coils were positioned to make the B₀field more homogeneous (uniform field intensity). Usually the shim coils were at room temperature, but in advanced MRI systems, they were in the cryostat.

However, shim coils have largely been aban-doned because of expense and the bore space they occupied. Now, small pieces of ferromag-netic materials are used along with gradient magnetic field offsets to obtain excellent B₀ homogeneity.

Resistive Electromagnet Imaging System The patient couch of a resistive electromagnet imaging system appears the same as that of a superconducting MRI system and will have similar performance characteristics. Unlike the superconducting MRI system, the primary magnet can usually be visualized.

Figure 10-8 shows the typical configuration for the primary magnet of the resistive electro-magnet imaging system. In this illustration there are three white rings, two large and one small. There is a fourth small ring on the back-side of this electromagnet.

These rings each contain a coil of wire con-ducting a large electrical current, approximately 30 to 50 amperes (A). The two large coils pro-duce the B₀ magnetic field, whereas the two smaller coils on each end help to extend the length of the field and make it uniform.

The secondary electromagnets of a resistive magnet imaging system are often visible. The shim coils and gradient coils are usually

con-tained within the primary magnet. This sub-assembly defines the patient aperture. The RF probe is a separate operator-interchangeable assembly.

Resistive electromagnet MRI systems have made something of a comeback. Many 0.2-T and 0.5-T magnets are now resistive with an iron core. These magnets look like large C-arms with a vertical B₀field.

Permanent Magnet Imaging System

The subassemblies of a permanent magnet imaging system are not visible because of a decorative housing. There is no hint of how

Shim coils

Gradient coils

RF coil

Figure 10-7 Relative position of the shim coils, gradient coils, and radiofrequency (RF) coil.

Figure 10-8 A four-coil resistive electromagnet for magnetic resonance imaging. (Courtesy Bruker Instruments.)

the subassemblies appear except for the RF probe, which is identifiable and operator inter-changeable.

The cutaway view of the permanent mag-net MRI gantry in Figure 10-9 shows that the primary magnetic field is produced by two assemblies of bricklike magnets. These pri-mary magnets are attached to a massive iron yoke.

The iron yoke plays the same role with the MRI gantry as it does for a transformer and is similar in design. The iron yoke provides a return path for the lines of the primary mag-netic field. The result of the yoke’s presence is to increase the B₀ magnetic field intensity within the patient aperture.

There are no shim coils in a permanent magnet MRI system. Shimming the primary magnetic field is accomplished by mechani-cally adjusting the two finely machined pole pieces.