LA RETICENCIA

If room temperature shim coils are used, another cylinder is positioned inside the cylin-der on which the shim coils are wound. The gradient coils are positioned on this second cylinder. If room temperature shims are not used, the gradient coils are just inside the pri-mary magnet housing.

There are three sets of gradient coils, one each for the X, Y, and Z directions. Figure 12-2 shows the configuration of such coils.

Gradient coils are not coils of wire in the normal sense. Instead, they are broad, thick copper-conducting bands (Figure 12-3).

These bands are referred to as conductors and typically measure 10 mm wide and 4 mm thick.

The large size is necessary to reduce resistance and carry the intense electric current (up to 30 A) required to generate the gradient magnetic fields (up to 40 mT/m). Gradient magnetic fields are measured in millitesla per meter, and when fast imaging is required, bigger is better.

These coils must be able to switch on and off rapidly. Switching times of less than 500 µs are necessary for many MRI applications.

Advanced gradient coils, such as those used for echo planar imaging, can produce gradient fields of up to 40 mT/m, with switching times of as little as 50 µs.

The rate of rise to maximum gradient amplitude is the slew rate. The time required to switch gradient coils on or off is rise time.

A typical slew rate curve is shown in Figure 12-4. This representation shows that the maxi-mum gradient field, 40 mT/m, is obtained in a rise time of 250 µs. Therefore the slew rate is 160 T/m/s.

Because the gradient coils are not brought to design amplitude instantaneously, they are generally represented as a trapezoid, as in Figure 12-5.

For routine SE imaging, slew rate is impor-tant because it limits the minimum

time-to-echo (TE) that is available. For fast imaging, slew rate is exceptionally important because it is the limiting specification for short repetition time (TR), short TE, and total imaging time.

The switching of the gradient coils produces the “thump, thump, thump” in the imaging Figure 12-3 Gradient coils consist of large, band-type conductors.

Z Coils

X Coils

Y Coils

Figure 12-2 The positioning of the three sets of gradient coils.

aperture that is heard by the patient. When a gradient coil is switched on, the electric current causes the conductor to expand as a result of resistive heating. When the gradient is switched off, the conductor cools and contracts. The alternate expansion and contraction simulates an audio speaker cone, and a thumping sound is the result.

Because the gradient coils are generating magnetic fields, they also tend to torque or twist. If this happens without constraints, the coils change shape, and the resulting magnetic field is altered.

MRI systems incorporating high slew rate gra-dient coils induce eddy currents more efficiently.

The rapidly changing gradient magnetic field can induce eddy currents in anything nearby (patient, coils, probes), but most important, in the con-ductive structures of the interior of the annulus of the cryostat, and can create image artifacts and loss of spatial and contrast resolution.

The principal method for controlling eddy currents is the use of shielded gradients. The gradient magnetic fields are effectively shielded from producing eddy currents at the surface of the cryostat.

time (µs) Gradient

amplitude (mTm)

0 10 20 30 40

100

rise time

= 250µs

rise time = 500µs Slew rate = 30T/m/s Slew rate =

160T/m/s

15mT/m 40mT/m

200 300 400 500

Figure 12-4 The slew rate is the time required to energize or switch off a gradient coil.

Unidirectional gradient pulse

Bipolar gradient pulse

Figure 12-5 The representation of gradient pulses in a pulse sequence diagram.

For the prevention of physical distortion, the gradient coils are imbedded in a strong epoxy resin casing. This casing prevents the coils from moving when they are energized and dissipates the heat generated by the gradients. The casing is also designed to muffle the sound of the gra-dient switching for patient comfort.

Because gradient coils are designed to have low resistance, heating does not really become a problem until echo planar imaging (EPI) speeds are approached. Sometimes water-cooling is used.

Z Gradient Coils (G_Z, G_SS)

The Z gradient coils are usually a pair of circu-lar coils, each of which is wound on the cylin-der at opposite ends of the imaging volume (Figure 12-6). If a direct current with opposite polarity is passed through the two coils, a small change in the magnetic field along the Z-axis of the gantry is produced. The currents used for SE imaging are approximately 20 A, producing a linear change in main magnetic field strength of 10 to 40 mT/m.

This change in magnet field strength allows for selection of a slice along the axis of the gantry. When transverse slices are required,

the Z gradient is the slice selection gradient.

The stronger the Z gradient electric current is, the stronger will be the Z gradient magnetiza-tion, and this will result in thinner slices.

The Z gradient magnetic field is symbol-ized by B_Zor B_SS, for slice selection.

The B_Zcoils are much more efficient than the B_X and B_Y coils because all segments of the coils contribute to the gradient magnetic field.

There is a larger gradient per ampere-turn.

X Gradient Coils (G_X, G_R)

As shown in Chapter 2, the magnetic field lines of a circular coil of wire are along the axis of that coil. As a result, the X and Y gradient coils are more difficult to fabricate and position on the cylinder because they cannot be made as circles. Figure 12-7 shows the way the X gradi-ent magnetic field is induced by a pair of coils—

actually four saddle-shaped coils in sets of two—positioned on either side of the cylinder.

By convention, these coils are positioned so that the gradient magnetic field is across the patient laterally. The axis is therefore the hor-izontal axis across the patient from side to side.

Z Coils

Figure 12-6 Z gradient coils change the gradient magnetic field along the Z-axis. This field, B_SS, is often used to select a transverse section of the patient for imaging.

X Coils

Figure 12-7 X gradient coils produce a gradient magnetic field across the patient. For transverse imaging, this field is frequency encoded and called the read gradient magnetic field (B_R).

These coils behave in precisely the same way as the B_Zcoils. Direct current of opposite polarity is applied to produce a gradient mag-netic field. The X gradient current and induced gradient magnetic field are similar in magni-tude to the Z gradients.

The X gradient magnetic fields provide spa-tial localization along the X-axis, side-to-side across the patient, and can also be used for slice selection (sagittal images), phase encod-ing, or frequency encoding.

For transverse images, the X gradient magnetic field is usually frequency encoded and symbolized as B_Xor B_R, for the read gradient.

Y Gradient Coils (G_Y, G_Φ)

A gradient magnetic field along the Y-axis of the patient is produced by a set of coils that look and operate exactly like the X gradient coils (Figure 12-8). By convention, the Y-axis is the vertical axis through the patient in the anterior-posterior direction.

The Y gradient is normally, though not nec-essarily, used for phase encoding the MR

sig-nal during transverse imaging and is symbol-ized as B_Yor B_Φ. It can also serve to perform slice selection (coronal images) and to fre-quency encode. Together, the Y and X gradient allows precise determination of where the con-tribution to the MR signal from each voxel originated within the transverse imaging section.

For transverse images, the Y gradient magnetic field is usually phase-encoded and symbolized as B_Y or B_Φ.

The previous discussion is an instructional con-venience that correctly applies to images of the chest and abdomen. Transverse images of the head are usually acquired with the phase-encoded gradient running from left to right.

With this technique, the number of phase-encoding steps can be reduced to produce a rec-tangular field of view that better conforms to the elliptical shape of the head.

Combined Gradients

Magnetic fields add vectorially. When ener-gized simultaneously, the currents in the three pairs of gradient coils do not produce three separate magnetic fields but rather a single composite magnetic field.

All three gradients are energized simultane-ously to obtain an oblique image (Figure 12-9).

If the current through each pair of gradient coils is precisely controlled, the plane for imag-ing can be precisely specified.

The gradient coils are under coordinated electronic control during the MRI examination by the pulse programmer to achieve the desired gradient magnetic fields. Oblique sections can also be imaged with three-dimensional Fourier transformation (3DFT) techniques.