Chapter 5v5.docx

1、Chapter 5v5Receive CoilsAs stated earlier, RF coils serve two functions in MRI. They must provide an RF magnetic field B1 during the transmit mode when the nuclear spins are excited and/or manipulated. They must also pick up the small signal originating from the precessing nuclei during the receive

2、mode following the excitation. In early scanners, these two functions were performed by the same coil. As a producer of B1 field, the coils transmit function could be characterized by its transducer factor B1xy(r)/I, which specifies the strength of the x-y component of the B1 field produced at posit

3、ion r by a unit of coil current I. The coils ability to pick up a signal from a unit of precessing magnetization was governed by this same factor. The reciprocity theorem states that the voltage induced in coil by a unit of magnetization located at a position r is proportional to B1xy(r)/I. During t

4、he transmit mode in most imaging applications, the spins are manipulated to reveal properties of the tissue including pathological conditions. Therefore, it is desirable to apply the same B1 to all of the nuclei. The factor B1xy(r)/I specifies the degree of homogeneity of the transmit B1. Initially,

5、 it was thought that all the nuclei should be observed with uniform sensitivity to avoid interpreting any inhomogeneity in the signal sensitivity as due to pathology. Therefore, using the same coil for receiving with a uniform B1xy(r)/I seemed reasonable. However, the introduction of highly sensitiv

6、e surface coils as receive coils demonstrated that radiologists were willing to accept very non-uniform sensitivity to gain greatly increased signal sensitivity. Since then, the majority of new coils have been receive-only surface coils or coil arrays targeting specific regions of the body. The T/R

7、SwitchOne must note the extreme difference in the magnitude of the RF pulses transmitted by the RF power amplifier and the resulting MRI signal processed by the receive channel. Whether the input of the receiver is connected to the transmit coil directly or is located in close proximity to it, the r

8、eceiver must withstand a large RF pulse without damage and amplify a small signal afterward. The receiver must be protected during the transmit pulses. The transmitter must be disconnected from the transmit coil during the receive mode to prevent it from generating noise in the receiver. These two f

9、unctions are performed by what is called the transmit/receive or T/R switch. A typical circuit is Fig. 5.1The two pin diodes are turned on during the RF power pulse by applying a voltage by way of the inductor (called a choke when used in the manner) located at point B. A pin diode exhibits a very s

10、mall resistance when a relatively small direct current flows through it. It appears to be a small capacitor when no current flows through it. When the pin diode #1 is turned on, it connects the power amplifiers output to the coil. The quarter wavelength coaxial line appears to be a high impedance at

11、 its end near the coil during the transmit pulse because it is terminated by a short circuit created by pin diode #2. The pin diode #2 has a low resistance because it is turned on by the same current that passes through pin diode #1. It prevents a large voltage from being applied to the preamplifier

12、 during the transmit pulse. During the receive mode, both pin diodes appear to be high impedances. The transmit power amplifier is disconnected from the coil and the small signal from the coil can reach the preamp unimpeded Receive Coil PerformanceThe receive coil provides the interface between pati

13、ent and computer in the scanner. In performing this task, the receive coil determines the intrinsic quality of the data that is used to form the image. The receive coil picks up the signal generated by the precessing nuclei and the noise that must accompany the signal. Therefore, the primary goal of

14、 the coil designer is to maximize the signal-to-noise ratio (SNR) of the resulting image. Our treatment will cover the two factors, signal and noise, separately. We will consider only surface coils as opposed to the volume coils considered in the previous chapters. The term surface coil originally r

15、eferred to a single coil loop that was placed on the outer surface of the sample to obtain a signal from a local, superficial region. More recently, arrays of surface coil elements are used to image large superficial regions or the whole volume of the sample. Coil SensitivityWe will begin by conside

16、ring the signal sensitivity of a single surface coil element. The reciprocity theorem states that the coil pickups up the signal from a region located at the position r proportional to the x-y component of the field B1xy(r) that would have been produced by a unit current in the coil. Hence, our disc

17、ussion of coil sensitivity focuses on computing the B1xy(r) created by the coil even through as a receive-only coil, we will not use it to generate a B1 field. In principle, one can get a pretty good estimate of B1xy(r) by using Biot-Savants law. This will require a numerically calculation on a comp

18、uter. Instead, we will consider some very simple coil geometries to demonstrate their relevant features of B1xy(r). We will assume that the sample volume consists of the semi-infinite space having positive x-values. Our coil elements will be placed on the x = 0 plane. In other words, we would observ

19、e the performance of our surface coil by placing it on a large flat water filled phantom. Coils Made from Long WiresWe start by considering a very simple, but impractical, coil geometry, namely a single long wire located on the z-axis with current in the positive z-direction: Fig. 5.2The field at r

20、has a strength inversely proportional to r and is directed tangential to the circle with radius r. Currents directed in the z-direction produce fields only in the x-y direction. This current distribution is highly efficient for MRI because there is no wasted field directed along the z-direction. We

21、define the coils field-of-view (FOV) as that region where the signal is greater than a certain cutoff value equivalent to field strength B1xy(r) greater than Bc. For the case of the long wire, we will consider that it has an adequate signal sensitivity in the region falling within the circle of radi

22、us r1. Therefore, we would say that this coils FOV is 2r1 wide and r1 deep. Since all real coils must have a return path for their current to form a closed loop, we consider two long wires running in the z-direction. One wire with a positive current is located at y = a and the other one with a negat

23、ive current is located at y = -a. Fig. 5.3The B1xy(r) field is the vector sum of the two individual fields. In the region between the wires, the fields add constructively. For locations on the x = 0 plane outside of the coil, the fields add destructively. For positions along the x-axis, the field st

24、rength decreases more rapidly than the 1/r dependence of the individual wires. The red lines in the figure indicate contours of constant field strength. The numerical estimate of the coils FOV will be left as an exercise for the reader. But qualitatively, one can estimate that the lateral FOV is sig

25、nificantly wider than the width of the coil d. Now consider a single long wire running along the y-axis. In this case, the field generated has components directed in the x- and/or z-directions. Fig. 5.4Again, the field strength decreases as 1/r. However, the useful field component for picking up the

26、 MRI signal only occurs above and below the wire. On the x-axis, the field has only a z-component and so the signal sensitivity is zero. The red lines represent the contours of constant signal sensitivity. Finally, we consider two wires oriented in the y-direction. Fig. 5.5The fields add constructiv

27、ely in the region between the wires. The regions where there is only a z-component are now displaced away from centerline. If a image were acquired in the y = 0 plane with this configuration, it would look something like the figure below: Fig. 5.6The two dark bands of poor signal strength limit the

28、useful FOV in the z-direction to be only the coil height d. The red line delineates the region of high signal. One other current distribution that is sometimes used can be modeled with three long wires with current in the z-direction and located at y= d, y =0, and y = -d. The center wire has one uni

29、t of current in the positive direction and the two other wires have one half unit of current in the negative direction. The field distribution is Fig. 5.7 The notable feature of this current distribution is that the field at locations along the x-axis is directed in the y-direction. That is, in the

30、central region of this coil, the field is parallel to the surface of the phantom. This is in contrast with the coil in Fig. 5.2, where the field is primarily perpendicular to the surface of the phantom. In our discussion of the reciprocity theorem in Chapter 2, Eqn 10 indicated that the phase of a s

31、ignal induced in a coil is determined by the direction of its B1 field. Therefore, the signals picked up by coils in Figures 5.6 and 5. 2 will have a phase difference of 900. Likewise, if the two coils were placed on the phantom together, they could be driven as a quadrature transmit coil to produce

32、 a circularly polarized B1 field in their central region.Practical coils must be made in finite sizes and form closed loops. The trace around a given coil must include portions that are parallel to the z-axis, perpendicular to the z-axis, or some combination of both orientations. The purpose of discussing the simpler examples given above was to emphasis that different parts of the coil make different contributions to its signal sensitivity. For example, a circular coil with diameter d and placed on the surface of a phantom (or patient) will have an elliptical shaped FOV. Fig. 5.8The FOV wou