03-06 Transmitter and Receiver

As mentioned earlier, the initial high power excitation of the nuclei is ac­comp­li­shed with a shortlasting RF pulse at a frequency close to or at the Larmor fre­quen­cy. Both the radio waves and pulses are generated in the transmitter section of the magnetic resonance equipment.

The desired frequency is produced by a frequency syn­the­si­zer. The syn­the­si­zer output is then modulated with an 'envelope' to provide the required pulse shape for the RF excitation. The receiver is basically a highly sensitive low-noise detector of signals in the high- and very-high-frequency (HF and VHF) range. The magnetic resonance signals are typically a few microvolts in amplitude. In the receiver, the signal is improved by a factor of 500 to 1,000.

After this stage, the signal is converted from an HF resonance signal (MHz) to an audio frequency signal (kHz).

03-06-01 "Regular" Transmitter and Receiver Coils

The object to be studied, whether it is a 1 mm³ sample for chemical analysis or a whole human body, is placed inside an antenna or coil. It should fill the coil as much as possible — the coil should have a good filling factor (> 70%).

The oscillating magnetic field B1 of the RF coil has to be perpendicular to the main magnetic field B0 generated by the magnet if the spins are to be excited. The most common configuration is to have the main field orientated along the bore of the magnet, so the coil must produce a field perpendicular to the bore of the magnet (Figure 03-09).

Figure 03-09: The direction of the main magnetic field (B0) de­pends on the orientation of the coils of the magnet. The field can either be vertical as in (a) or horizontal as in (b). In superconducting and some resistive systems, the field is usu­ally horizontal and thus parallel to the patient in the magnet.

Coils consist of one or more windings of low-resistance wire, usually copper. The geometry of the single or multiple windings is crucial for the proper ex­ci­ta­tion and subsequent signal detection.

Separate coils can be used for transmitting and receiving, but in most cases a single coil is used for both excitation and detection (transceiver coil). Since the excitation pulse is many orders of magnitude stronger than the responding mag­ne­tic re­so­nan­ce signal emitted by the human body the receiver will be damaged if it is subjected to all or part of the RF pulse. To overcome this problem, a device known as a transmit/receive switch is used which can very rapidly switch the rou­ting of signals.

Coils come in different categories and for different magnet types. Some of the most popular coil geometries are the solenoid, saddle-shape (Helmholtz), bird­cage, and slotted resonator. This group of coils is identified as volume coils. Since a solenoid coil generates an oscillating magnetic field parallel to the coil's axis it has to be aligned across the bore of the magnet. This limits the use of this kind of coil to magnetic fields that are orientated perpendicular to the patient couch.

Figure 03-10 illustrates the form of some commonly used coils in MR ima­ging. Typical examples are head and body coils, or coils for studying the knees or the neck.

Figure 03-10:
Three different types of coils: (a) solenoidal coil; (b) Helmholtz or saddle-shape coil; (c) birdcage coil. Their oscillating magnetic field (B1) must be perpendicular to the main magnetic field (B0). Their RF field is uniform within the volume.

03-06-02 Surface Coils

All coils except surface coils are designed to produce a very homogeneous RF field such that all of the sample experiences the same degree of excitation. Sur­face coils have a much higher sensitivity than homogeneous volume coils. They are used to detect magnetic resonance signals from a small region close to the coil when it is placed, for instance, on a patient's spine, orbit, or temporo-man­di­bu­lar joint.

The advantage of using surface coils is that in the small volume of tissue close to the coil, one can obtain a better signal-to-noise ratio than that obtained by a standard volume RF coil. However, surface coils receive a high signal only from an approximately hemispherical area below the coil with a depth of half its dia­me­ter (Figure 03-11). Thus, both the RF fields and the detection sensitivity of a surface coil are highly inhomogeneous, leading to a position-dependent ex­ci­ta­tion.

The variation of RF intensity with depth causes the flip angle to vary with depth when the surface coil is used as a transmitter. The problems of varying pulse angle with depth in surface coils can be overcome either by using special RF pulses or by transmitting the RF pulse on the standard body coil and only detecting the signal with the surface coil.

Figure 03-11: Surface coils. (Top) Diagram of a simple surface coil. Since the intensity of the RF field varies with depth, the pulse angle will also vary with depth unless special ('adiabatic') pul­ses are used. Similarly, the detection sen­si­ti­vi­ty will also decrease with increasing depth. (Center) T1- and T2-weighted images of surface coil acquisitions of the lumbar spine. Spinal cord and spine are well de­pic­ted, but there is hardly any signal from the anterior parts of the pelvis. (Bottom) Wrap-around or half saddle- shape surface coil.

An increasingly common modification to the standard coil design is the qua­dra­ture coil (or circularly polarized coil) which uses at least two RF fields or­tho­go­nal­ly to each other and improves both the efficiency of the coil and the SNR (signal-to-noise ratio) of the resulting signal by √2.

A phased-array is a group of antennas in which the relative phases of the re­spec­tive signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In MR imaging, phased-array coils (or synergy coils) consist of a number of small surface coils [⇒ Roemer]. The signals received by these coils can be collected simultaneously and the data can be combined to con­struct a single image of the body region covered (Figure 03-12).

Figure 03-12: Top. Phased-array coils, sketch. Four sur­face coils each left and right have been pla­ced in an array with overlapping sen­si­ti­vi­ty volumes. Bottom. Phased-array transceiver coil for breast imaging.

Compared to a large surface coil with poor SNR, phased-array coils have the ad­van­ta­ge of better signal-to-noise. However, each single subunit requires its own receiver channel, which makes image reconstruction difficult and coils ex­pen­si­ve. Still, they had a major impact upon MR imaging and opened the way for parallel acquisition techniques combining the signals of several coils to re­con­struct an image.