Pulse sequences for DW

Pulsed gradient spin echo (PGSE)

Most MR methods for measuring molecular diffusion rely on the pulsed gradient spin echo (PGSE) sequence introduced by Stejskal-Tanner. The standard PGSE diffusion MRI sequence is schematically shown in Figure 3.2.3 can be explained as follows:

I. the first gradient pulse with a duration labels the initial phase of the spins of the water molecules according to their positions,

II. after the 1800radio frequency pulse which inverts the spin phases, a second gradient pulse at a time delay labels the final positions of the nuclei through their signal phase

III. the receiver coil receives the signal at the echo time TE and three situations are possible at this point: either the water molecules were not in motion, so that spin labels cancel out each other; the spin moves coherently and all

Solid tissue Necrotic tissue Free fluid Vascular flow b-value L o g o f si g n a l in te n si ty

have the same phase, or water molecules move stochastically during the diffusion time lapse which leads to each spin having a different phase and a signal loss, which is thus a measure of molecular diffusion.

Figure 3.2.3 Pulsed gradient spin echo sequence. Diffusion gradients applied both sides of the 1800RF pulse to sensitize the molecular diffusion.

PGSE sequence based DWI suffers from T2 relaxation effects, so that long diffusion encoding time reduces SNR. However, as a basis of DWIBS techniques the PGSE sequence is used in this thesis.

Stimulated echo encoding (STEAM)

For DWI, the stimulated echo encoding (STEAM) technique is also used, it allows increment of diffusion time without T2 relaxation effects. A stimulated echo is generated by introducing three 900RF pulses, placing one diffusion gradient between the first and second pulse and another one after the third pulse of a stimulated echo sequence to generate images (illustrated in Figure 3.2.4).

Figure 3.2.4 Stimulated echo acquisition mode (STEAM) sequence. The echo signal generated from three 900 RF pulses. Corresponding diffusion time ∆ can be longer without TE penalty.

STEAM imaging allows high b-values and much longer diffusion time without T2 decay effects (magnetization is affected only by T1 in the period between 2nd and 3rd 900pulses). However, the STEAM technique provides less intensity (one-half) signals compared to the spin echo signal (Tanner 1970, Frahm et al. 1985, Merboldt et al.

Echo 180o 90o G TE 90o 90o ₉₀o Echo TE/2 G

Oscillating gradient spin echo (OGSE)

For observing restricted diffusion in the short diffusion time regime, oscillating gradient spin echo encoding (OGSE) is a suitable sequence. This technique uses a train of oscillating gradients for diffusion encoding. In this sequence the b-value expressed as, = , where the effective diffusion time is inversely proportional to the frequency of the oscillating gradient, eff~1/f, illustrated in Figure 3.2.5.

Figure 3.2.5 Oscillating gradient spin echo sequence. Gradients applied both sides of the 1800 RF pulse with frequency f and duration T, corresponding diffusion time ∆~1/ f.

Thus the effective diffusion time depends on oscillation period only, no longer related to the time between the gradient pulses. By using OGSE qualitative and quantitative information from in-vivo brain and spinal cord WM have been studied. However, this approach requires powerful gradients and is generally only suitable for animal MRI scanners (Gross et al. 1969, Stepišnik 1981, Callaghan et al. 1996, Schachter et al. 2000, Parsons et al. 2006, Colvin et al. 2008, Xu et al. 2009, Drobnjak et al. 2010, Gore et al. 2010, Aggarwal et al. 2012).

3.2.3 Optimizing DWI

As DWI depends on signal loss, the main acquisition difficulty is linked to signal to noise ratio (SNR) which is usually quite low. To get higher diffusion weighting, a higher b value is required by increasing either the diffusion time which inevitably leads to a rather long TE, this in turn reduces the SNR; or by shortening the diffusion gradient lobe or by increasing the gradient amplitude, these are challenging for human MRI scanners. On the other hand at low b values (0 to 100 / ), DW signals are sensitive to capillary tissue perfusion. Hence, the choice of the optimum b value must always be considered in order to maintain the sufficient SNR and maximize diffusion effect. Another general problem of DWI is the very high motion sensitivity. As DWI

90o 180o

Echo

P =1/f

eff

involves sensitizing MRI to microscopic motion, acquisitions are sensitive to molecular motion and more generally to macroscopic motion (patient motion) as well. The most important source of such motions are CSF pulsation, pulsatile blood flow, cardiac or respiratory motion, and peristaltic bowel motion. To reduce such motion artifacts, images must be acquired fast. The fastest and most frequently used pulse sequence for DWI in general is the single-shot spin-echo echo-planar-imaging (EPI) sequence. This sequence is relatively insensitive to influences from macroscopic patient motion because of the very fast readout of the complete image data, within about 100ms after a single excitation pulse. However, the maximum attainable spatial resolution can be markedly limited by the T2*-decay during the long period of data acquisition. Also, EPI has only a very small bandwidth per pixel along the phase encoding direction. Consequently, EPI is very susceptible to main field (B0)

inhomogeneities, susceptibility variations, and chemical shift, which all may lead to severe image degradation. To reduce the artifacts, relatively recently introduced parallel imaging techniques, such as sensitivity encoding (SENSE), can be combined with DW EPI, which in turn can increase the bandwidth per pixel in the phase encoding direction and shorten the EPI train, thus significantly decrease the image distortion (Jones et al. 2010, Jones 2011).

DW images are inherently acquired with the same T2 weighted contrast, due to long probing time of the magnetic field gradients. Consequently, an area with a very long T2 relaxation time may have high signal at DWI and be mistaken for restricted diffusion. To separate confounding T2 relaxation and diffusion related changes of the signal, it is therefore helpful to calculate an ADC map using the equation 3.2.7. It should be noted that long transverse relaxation does not necessarily hinder image interpretation, but, in fact, often even increases conspicuity of lesions, since many lesions have both a prolonged T2 and an impeded diffusion (Jones et al. 2010).

An overview of DWI for body imaging is described in the next section.