M R I Physics Course. Jerry Allison Ph.D., Chris Wright B.S., Tom Lavin B.S., Nathan Yanasak Ph.D. Department of Radiology Medical College of Georgia

Transcription

1 M R I Physics Course Jerry Allison Ph.D., Chris Wright B.S., Tom Lavin B.S., Nathan Yanasak Ph.D. Department of Radiology Medical College of Georgia

2 M R I Physics Course Spin Echo Imaging Hahn Spin Echo Spin - Warp Imaging Carr Purcell (CP) Pulse Sequence Carr Purcell Meiboom Gill (CPMG) Pulse Sequence Spin Echo Imaging Multiplanar Imaging Multislice Imaging Oblique imaging Spin Echo Variations

3 Spin Echo Imaging The Spin Echo imaging technique has the advantage that it is not as sensitive to inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue. 3

4 Hahn Spin Echo The concept of Spin Echo production was developed by Hahn. Spin echoes are sometimes referred to as Hahn spin echoes. The Hahn Spin Echo technique consists of a 90 o flip (see Lecture 3), followed by an interpulse delay time, τ,, a 180 o flip, followed by a second interpulse delay (τ).( A spin echo occurs at echo time TE (2τ) following the initial 90 o flip as shown in the figure. 4

5 Hahn Spin Echo (continued) The Hahn Spin Echo was developed in 1950 for use 5 in nmr long before the advent of MRI.

6 Hahn Spin Echo (continued) The Hahn Spin Echo was used for the measurement of T 2 values. It is possible to measure T 2 since spin echo techniques compensate for local inhomogeneities of the static magnetic field. In order to measure T 2, the sequence had to be repeated with several different values of the interpulse delay (τ).( It was necessary to wait approximately 5 x T 1 (~4sec) for relaxation between sequences. 6

7 Hahn Spin Echo (continued) Due to the length of some interpulse delays, diffusion of nuclei (Brownian motion) contributed to relaxation and corrupted the measured T 2 values. Diffusion errors depend on gradient strength, diffusion coefficient (D) and diffusion time. In a spin echo technique, τ determines the diffusion time. 7

8 Spin-Warp Imaging Spin-warp imaging is a basic spin echo technique which reduces corruption of T 2 data due to magnet inhomogeneity and tissue susceptibility. In 2DFT techniques the variations in phase encoding associated with each TR can be accomplished by varying the magnitude of the gradient or the duration of the gradient. Spin-warp imaging varies the magnitude of phase encoding gradients (not the duration). Relaxation caused by flow, diffusion or proton exchange are not compensated by the spin echo technique, as mentioned before. 8

9 Here is a pulse- sequence diagram. This shows a timeline for: 1) RF pulses; 2) gradient amplitudes for Gx, Gy, Gz; 3) the readout (i.e., A/D), and 4) the signal of the excited nuclei. 9

10 Spin-Warp Imaging (continued) ****************** 10

11 Carr-Purcell (CP) Pulse Sequence A modified spin echo technique is the Carr Purcell spin echo technique. The Carr-Purcell technique (CP pulse sequence) uses a 90 o RF pulse followed by a train of evenly spaced 180 o RF pulses. A series of echoes which have alternating signs are produced. The first echo is negative, the second echo is positive, the third echo is negative, etc. 11

12 Carr-Purcell (CP) Pulse Sequence An envelope connecting the echo amplitudes decays exponentially with a rate constant accurately reflecting the T 2 of the sample (as * opposed to T 2 ). Since τ is relatively short in the CP technique, diffusion errors in T 2 measurement are much smaller. One can use the exponential decay envelope to calculate T 2 values. 12

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14 Carr-Purcell-Meiboom-Gill (CPMG) Pulse Sequence The Carr-Purcell-Meiboom-Gill ( CPMG sequence) The Carr-Purcell-Meiboom-Gill (CPMG technique is a modification of the Carr-Purcell technique. The CPMG technique applies the 180 o RF pulses along the Y axis of the rotating frame (rather than the X axis as in the CP technique). This modification makes the accuracy of the 180 o RF pulse much less critical. Each echo signal is positive in the CPMG technique. Variants of the CPMG technique have been widely used in MRI. 14

15 Carr-Purcell-Meiboom-Gill (CPMG) Pulse Sequence Another note of interest: the signal envelope following the 90 o RF pulse reflects T * 2, while the signal envelope connecting the magnitude of succeeding echoes reflects T 2. 15

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17 Spin Echo Imaging First, let s go through the spin echo imaging sequence, see a demonstration of this, then finish up by discussing some timing issues of image acquisition: 17

18 1. Using a Z gradient for slice selection, the macroscopic magnetization is nutated into the transverse plane using a 90 o flip. Nutation is about the X axis. The Z gradient is reversed briefly to rephase the spins within the selected slice. 18

19 2. The Y phase encode gradient is applied with the first phase encode value. 19

20 3. After the interpulse delay time, τ, a slice selective 180 o RF pulse is applied to flip the transverse plane about the X axis (Spin-Warp, CP) or the Y axis (CPMG). The effect of the 180 o RF pulse is to retard the spins that were ahead in phase and to advance the spins that had retarded phase. 20

21 3. (continued): At time τ after the 180 o RF pulse (2τ after the 90 o RF pulse), the slow spins having advanced phase and the fast spins having retarded phase briefly reestablish phase coherence and a spin echo occurs. After phase coherence occurs the fast spins once again advance in phase and the slow spins fall behind in phase. 21

22 3. (continued): A second 180 o RF pulse at time 3τ 3 after the 90 o RF pulse can cause a second echo to occur at time 4τ 4 after the 90 o RF pulse. The second echo is smaller than the first echo, primarily due to T 2 relaxation. 22

23 4. During each echo period, the frequency encoding gradient is applied, during the collection of the signal induced in the RF coil. Notice that the Analog to Digital converter (A/D) is enabled while the frequency encode gradient is active. 23

24 Spin Echo Imaging (continued) 5. Pulse sequences are repeated. Consider a 256 x 256 acquisition matrix. 256 pulse sequences are executed with a different value of the phase encoding gradient to fill k-space (raw data). 24

25 Spin Echo Imaging (continued) It s movie time again before proceeding, let s see the spin-echo sequence in action to visualize how it works. 25

26 Spin Phase Plot Discussion We can overlay all excited spins onto one orbit, to show phase differences easily between all of them. The overlay is plotted on the right, from the perspective of the lab frame. Excited spins precessing in slice Overlay image, lab frame 26

27 Spin Phase Plot Discussion Phase differences between the spins are easier to see if we plot spin position while rotating the slice. Compare a plot on the left of the excited slice in rotation, to the simple overlay plot on the right. Excited spins in slice, showing rotation of the slice. Overlay image, rotating frame 27

28 Spin Phase Plot Discussion Because the total MRI signal is a sum of the signals from all spins, we see a maximum echo amplitude when all of the phases are nearly the same. Same phase, large echo Similar phases, noticeable echo Disparate phases, no echo 28

29 Spin Echo Movies Basic Carr-Purcell sequence (90 o x,, 180 o x) Spin in red leads in phase, and always progresses clockwise. Spin in blue lags in phase, and always progresses counter- clockwise. 29

30 Spin Echo Movies (continued) Basic Carr-Purcell-Meiboom-Gill sequence (90 o x,, 180 o ) y Spin in red leads in phase, and always progresses clockwise. Spin in blue lags in phase, and always progresses counter- clockwise. 30

31 Spin Echo Movies (continued) Basic Carr-Purcell-Meiboom-Gill sequence (90 o x,, 180 o y), including T 2 decay. Spin in red leads in phase, and Spin in blue lags in phase, but notice the jitter in phase (T 2 ). Spin-echo sequence cannot correct for this, and echo is 31 smaller.

32 Spin Echo Imaging (continued) The time interval between each execution of the pulse sequence is termed the Repetition Time (TR). In the previous examples, each movie showed <1 TR worth of the sequence. 32

33 Spin Echo Imaging (continued) 6. The value of the repetition time (TR) and the echo time (TE) can be varied to control contrast in spin echo imaging. For example: TR = 2000 msec, TE = 20 msec Proton Density Weighting TR = 2000 msec TE = 80 msec T 2 Weighting TR = 600 msec TE = 20 msec T 1 Weighting Note that the echo time is typically short compared to the repetition time. We will return to this point in our discussion of multislice imaging. 33

34 Spin Echo Imaging (continued) 7. The image acquisition time can be calculated as follows: T S = N Y x TR x NEX where N Y = number of phase encodings (512, 256, 192, 128, n) TR = repetition time NEX = number of excitations (1, 2, n). Siemens terminology for NEX is Number of Acquisitions (No. Acq.). 34

35 Spin Echo Imaging (continued) 7. (continued): MRI images are sometimes acquired with a NEX greater than 1. For example, the number of excitations (NEX) might be set to 4 for a particular study. The result is that each line of k-space is sampled 4 times in order to improve signal-to-noise in the image. Image acquisition time is increased by 4. Signal in this case is improved by the square root of 4, (i.e., a factor of 2). In essence, each image is acquired 4 times and averaged together as 1 image. 35

36 Multiplanar Imaging Spin echo imaging techniques (as well as other MRI techniques) can be used to acquire axial, sagittal, coronal, or oblique images. The spin echo technique described above used the Z gradient for slice selection, the Y gradient for phase encoding and the X gradient for frequency encoding. This described the acquisition of an axial image. 36

37 Multiplanar Imaging (continued) Axial, sagittal, and coronal images can be acquired as follows: Notice that for each plane, the choice of axis for phase and frequency encoding can vary. 37

38 Multiplanar Imaging (continued) The MRI system usually chooses to apply the phase encoding axis along the thinner body dimension. For example, when acquiring an axial image of the thorax the phase encoding gradient is applied along the Y axis (anterior to posterior) since the AP dimension of the thorax is smaller than the left to right dimension. This selection helps to prevent wrap around in the phase encoding direction and may enable use of a rectangular field of view for faster scanning. The MRI system operator can choose to swap the direction of phase encoding and frequency encoding if necessary. 38

39 Multiplanar Imaging (continued) Aliasing example Phase- encode direction is A-P (longer axis of head), creating aliasing. Phase- encode direction is L-R (shorter axis of head), eliminating aliasing. Images from MRI Tutor website (Copyright( , All Rights Reserved) 39

40 Multiplanar Imaging (continued) The MRI system operator may also choose to swap the direction of phase encoding and frequency encoding to minimize flow artifacts in particular organs. Flow artifact Phase encode in the A-P direction. Phase encode in the 40 L-R direction.

41 Oblique Imaging Imaging of oblique planes can be accomplished by applying more than one gradient during the slice selective 90 o and 180 o RF pulses. If a Y gradient is applied during slice selection, an axial slice is defined. If Z and X gradients of equal magnitude are applied during slice selection, an axial oblique slice is defined. The axial oblique slice would be at an angle of 45 o to both the axial and sagittal planes. 41

42 Multislice Imaging As shown earlier, echo time TE is typically short compared to the repetition time TR. The long TR is necessary to allow the excited slice to relax sufficiently between the first phase encoding sequence and subsequent phase encoding sequences. This time can be used efficiently by performing the first phase encoding on other slices while waiting to perform the second phase encoding sequence on the first slice. 42

43 Multislice Imaging (continued) For a T 1 weighted pulse sequence having a TR of 600 msec and TE of 20 msec, it is possible to perform the first phase encoding on approximately 12 slices before performing the second phase encoding on the first slice. For a 256 x 256 matrix, this means that data can be acquired from 12 different anatomic slices in the same time ( 2:41 minutes) required for a single slice. 43

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45 Spin Echo Variations 1. MEMP (Multi Echo Multi Planar) techniques on a GE Signa allow the production of up to 4 evenly spaced (e.g. TE = 20, 40, 60, 80) echo images during image acquisition. 2. VEMP (Variable Echo Multi Planar) techniques on a GE Signa allow the production of 2 echo images during image acquisition. The echo times are variable and are not required to be evenly spaced (e.g. TE = 30,80) 45

46 Spin Echo Variations (continued) 3. Siemens supports single echo, dual echo and triple echo spin echo sequences. Echo times can be set by the operator (and need not be multiples). There is also a multiple echo technique that allows for production of up to 16 echoes for measurement of T 2 values. 46

47 Spin Echo Variations (continued) 4. k-space variations It is possible to reduce scan time by filling only part of the k-space or raw data matrix. The missing data is then synthesized using the symmetric properties of the matrix. If half of k-space is filled (NEX = 0.5) the scan takes less time to acquire but has a lower signal-to-noise ratio. We will discuss how this is possible in a later lecture. 47