Imaging Modes and Extraction of Nuclear Magnetic Resonance Parameters

The spin density f (x, y, z) obtained by the various imaging methods described earlier is not a real spin density; it is weighted by T1 or T2 or both. Because T1 or T2 varies between normal and abnormal tissues, the image of spin density weighted by T1 or T2 has been found to be clinically useful. With this in mind, several attempts have been made to extract T1 information, as well as spin density and T2. The typical imaging modes and corresponding terminologies currently in use are described in the following subsections.

It is easy to see the increased dependency of the image on T1 over that obtained in saturation recovery. This technique is often used for measuring T1 values in tissues. Figure 22 shows the pulse sequences for 2-D inversion recovery Fourier imaging.

iii. Spin-echo imaging. Through the application of the 180° pulse following the first 90° pulse at t = Ts, spins are refocused at t = 2Ts by the spin echo (Fig. 20). Although the spins are now refocused and coherent, the amplitude of FID decays exponentially with time constant T2. The decayed spin density f '(x, y, z) can be written f '(x, y, z) = f (x, y, z)exp[-2Ts/T2(x, y, z)]. (62)

As explained earlier, the image is now weighted by T2 as well as by T1. By setting the appropriate Ts values, images

FIGURE 22 RF and gradient pulse sequences of inverson recovery direct Fourier imaging.

weighted mainly by T2 can be obtained, provided that the repetition time is sufficiently large.

b. Parameter imaging methods. The capability of extracting many functional parameters is one of the most important advantages of NMR CT. Flow velocity, T1, T2, and chemical shift are some of the interesting parameters in NMR imaging that are discussed in this section.

i. T1 (Spin-lattice relaxation time) and T2 (Spin-spin relaxation time). The effects of T1 and T2 are closely related to the NMR imaging modes. In T1 imaging, both the saturation recovery and inversion recovery modes can be used. By varying the recovery time and observing the resulting image intensity variation, one can deduce T1 values. Similarly, by changing the echo time, that is, varying 2Ts in Eq. (62) for the spin-echo method, one can obtain several images differently weighted by T2. From the images obtained with different echo times, T2 values of each pixel can be calculated.

ii. Flow imaging. In NMR CT, one can also measure the flow or moving velocity of nuclear spins through observation of the FID signal. In the first attempt at flow velocity measurement two RF coils were used—one for the excitation of spins and the other for reception. In this experiment, surface RF coils were used to excite and receive the signal at known locations. If the maximum signal is received at At seconds after the excitation with the distance Al between two RF coils, the velocity can be estimated by Al/At.

Several flow imaging methods have been developed. Among these, two techniques relevant to general flow measurement will be discussed: one using density information and another using phase information.

The RF and gradient pulse scheme of flow imaging using the selective saturation method uses intensity information. In this scheme, the first 90° RF pulse and the homogeneity-spoiling gradients are used to saturate the spins in the selected slice for flow imaging. The 2-D Fourier imaging sequence for the same slice follows after At seconds to measure the signals originating from spins that flowed in from outside the slice, where spins were not saturated. From the density change observed for several different At's, the flow velocity in the selection gradient direction can be determined as Az/AT, where AT is the minimum At with the maximum spin density and Az is the slice thickness.

In another variation of flow imaging, phase information is used to measure flow velocity. Since the pixel values of an image are usually extracted by taking the real part or absolute values of the image data in complex form, it is possible to use the phase information associated with each pixel data. Let us assume that a time-varying gradient Gx (t) is applied to moving spins after RF excitation. The phase coding resulting from the time-varying gradient can be divided into two terms: the spatially coded term 0s and the velocity-coded term 0v, respectively. The sum appears as

where and

In Eq. (63), Gx(t), x, and v represent the time-dependent x gradient, the x coordinate of spins at t = 0, and the x-directional flow velocity of the moving spins, respectively. In the flow measurement, the flow coding gradient is applied in addition to the conventional RF and gradient pulse sequences, so that the phases on the final image are changed only as a result of flow velocity. Flow velocity can be determined from the calculated phase, which is coded according to the velocity of the spins. The unique advantage of this kind of flow velocity imaging method is the capability of multidirection flow imaging by simply applying the additional flow coding gradient in the desired direction. Figure 23 shows a typical gradient waveform for flow phase coding in the x direction, in which the spatially coded phase term is canceled so that 0s = 0, while the velocity-coded phase term remains 0v = 0. This technique, therefore, allows us to measure velocity by simply measuring the phase, which is now purely dependent on velocity.

iii. Chemical-shift imaging. Another important aspect of NMR CT is its spectroscopic imaging capability. Before NMR CT was proposed, NMR had been

FIGURE 23 Gradient waveform for phase coding of flow velocity measurement. Note that the gradient pulsing shown effectively cancels out the spatial coding. The remaining velocity-coded phase 0v can be written 0v = Y t2 Gv, where v is the flow speed.
FIGURE 24 RF and gradient pulse sequences for 4-D chemical shift imaging using 2-D spatial codings. Note that the total number of coding steps for a slice is N2.

used primarily for chemical spectroscopy, in which the frequency spectrum of a specific kind of nuclei distributed over a few tens of parts per million of its Larmor precession frequency was obtained. The chemical shift was usually measured for homogeneous samples under the condition of uniform field.

In spectroscopic NMR imaging, however, the chemical spectrum for each pixel (chemical spectroscopic imaging) is to be measured neither for the homogeneous samples nor under the uniform field condition but with spatially varying gradient pulses.

A few chemical-shift imaging techniques have been proposed. An original spectroscopic imaging pulse sequence is shown in Fig. 24. The essence of this pulse scheme is the absence of the reading gradient during data acquisition. In this scheme, N 2 steps are required for a 2-D spectroscopic imaging of N x N matrix size image. In Fig. 25, a more generalized imaging sequence using echo-time encoding is shown. In this scheme, the 180° spin-echo RF pulses are applied several times, and corresponding FIDs are observed at each time. The notable difference between this scheme and the former is that here the spatial coding is identical to conventional 2-D imaging (i.e., gradient steps required are only N for N x N matrix size image), while in the former, the number of steps required is N2. Also in the former, N determines the spectroscopic resolution. In the latter scheme, on the other hand, the number of RF time positions determines the spectroscopic resolution and, therefore, by varying the number of RF time positions, one can achieve the desired resolution. Often this step turns out to be much less than N in most in vivo spectroscopic imaging.

FIGURE 25 RF and gradient pulse sequences for the echo-time-encoded chemical-shift imaging. Note here that the total number of coding steps for a slice is N.

c. Other imaging methods. In addition to the imaging methods previously mentioned, there are several other imaging schemes of special form. One of these is gated or synchronized imaging, for an object that moves periodically. An example is the gated cardiac imaging of the human heart. In this case, the RF and the gradient pulse sequences are gated in synchronization with the ECG signals, and data in the different parts of the heart cycle are collected.

In the area of imaging methodology, rotating-frame zeugmatography should be noted. In this method, spatial-phase coding is achieved through the RF field gradient rather than spatial field gradients generated by the x-, y-, and z-directional gradient coils as in conventional NMR imaging. Although this method has some advantages, it is rarely used in imaging because of inherent difficulties, such as those found in the realization of the RF field gradient and associated RF coils.

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