Pretest Grupo Experimental 2bm

The frequency, power and duration of RF pulses can be variously manipulated to produce specific effects on protons. The combination of a 90° pulse followed by a 180° pulse is termed a spin echo sequence (Figure 1.6), and this is the most commonly used sequence. Following a 90° pulse, protons are tipped into the xy plane and they precess in phase. This generates a detectable transverse magnetic field. However, signal immediately begins to decline due to a combination of Tj decay and dephasing due to external magnetic field inhomogeneity, the so- called Tj* or free induction decay (FID). After a delay, termed the echo time (TE), a 180° pulse is applied. This rotates the protons’ magnetic moments through 180°, reversing the direction of

precession. Thus there is a return of all the transverse magnetisation that was lost due to T2*

effects, and an “echo” is generated. Therefore, the amplitude of the resulting echo is dependent primarily on Tj relaxation. Generation of an image requires several repetitions of the spin echo sequence, with the time between successive 90° pulses termed the repetition time (TR).

180 180 RF p u lse TE TR N M R S ig n a l E ch o _FID E ch o FID

Manipulation of both the TR and TE allows images to be generated with contrast dependant on varying degrees of T,, Tj or proton density (PD) characteristics. Weighting refers to the type of contrast that dominates an image. With a short TR, those protons with longer Tj will not have

returned to align along before the next excitation. The combination of a short TR with a short

TE produces an image that is T, weighted, those tissues with a long Tj appearing dark and those with a shorter Tj appearing bright. The combination of a long TR with a short TE generates a PD weighted image, where contrast is less dependent on either T, or Tj. With both a long TR and TE, those protons with a short Tj dephase to a greater degree than those with a longer Tj, yielding a Tj weighted image, where tissues with short T^ appear dark relative to those of a longer T^.

1.2.1.2 Spatial localisation

Localisation of the MR signal is required for image generation. This is achieved through the modification of the external magnetic field by the application of magnetic field gradients. The application of a slice selection gradient along the z axis allows selective excitation of a particular xy slice according, to the frequency of the RF pulse used. This allows localisation of signal along the z axis.

A similar approach is used to allow localisation along the x axis. Thus, a frequency encoding gradient is applied perpendicular to the slice select gradient, allowing spatial localisation along the X axis. To encode spatial information along the y axis requires a phase encoding gradient. This gradient is applied only briefly to alter the phase, but not the frequency, of the derived signal. The stronger the phase encoding gradient, the greater the difference in phase angle along the y direction. The phase of different components of the signal therefore identifies their origin.

1.2.1.3 Techniques fo r 7^ weighted imaging

With conventional spin echo (CSE) imaging, a two-feature approach is adopted, using the early

and late echos to generate proton density (PD) and T2 weighted images, respectively. One line

of k-space is acquired per TR, which is usually within the range of 2000 to 3000 msec. K-space is where the raw data of spatially encoded MR signals is collected during application of magnetic field gradients. In CSE imaging, one line of k-space is encoded per TR interval, and the pulse sequence is repeated typically 128 or 256 times (phase encodings) per image. Typically, TEs of 25 to 50 msec for PD weighting and 80 to 120 msec for Tj weighting are used. The exact parameters will depend to an extent on scanner field strength, since while Tg is virtually field strength independent, T, is positively correlated with field strength. Therefore, to obtain similar CSF suppression across different Tesla imagers (a Tj effect on PD weighted images), the TR is

lower for low-field strength machines (Filippi et al, 1997a). MS lesions appear hyper-intense

relative to background white matter on both PD and T2 weighted images (Figure 1.7) by virtue

of their higher water content.

More recently, fast spin echo (FSE) imaging has become increasingly used. The FSE sequence

is based on the rapid acquisition with relaxation enhancement (RARE) sequence (Hennig et al,

1986; Hennig et al, 1988). With FSE imaging, multiple phase encodings are performed in each

TR and multiple echoes per TR are acquired. Therefore, instead of a single line of k-space per TR interval, fi*om 2 to 16 or more lines are encoded per TR, resulting in a considerable reduction in acquisition time in proportion to the number of echoes collected.

Although the FSE sequence produces images that are broadly similar to a corresponding CSE image, they are not identical.

,/m.

Figure 1.7 a Figure 1.7 b

l i # #

Figure 1.7 c

Figure 1.7 Axial images of a patient with clinically definite MS; PD (Figure 1.7 a) and Tj weighted (Figure 1.7 b) images from a dual echo CSE sequence, and corresponding fast FLAIR image (Figure 1.7 c). Several periventricular and discrete white matter lesions are demonstrated with all three sequences. Note the CSF suppression with the fast FLAIR sequence.

The FSE sequence may produce increased signal at tissue interfaces (for example, in periventricular regions), making detection of MS lesions in this region more difficult (Bastienello

et al, 1997; Thorpe et al, 1994). It may also be less sensitive in detecting small lesions due to

point spread function effects (Constable & Gore 1992). This latter effect describes the blurring that can occur in the phase encoding direction due to the collection of different lines of k-space at different times, obscuring detection of small lesions.

While long TR long TE sequences offer a higher level of contrast between lesions and white matter (determined by Tj mechanisms), an undesirable consequence is reduced lesion-to-CSF contrast, making identification of periventricular and subcortical lesions more difficult (Figure 1.7). The high CSF signal can be suppressed by application of a 180° inversion pulse, with an appropriately long inversion time (TI) before each excitation pulse, thereby allowing longer TEs

while at the same time suppressing CSF signal (De Coene et al, 1992; Thorpe et al, 1994). This

is known as the fluid attenuated inversion recovery (FLAIR) sequence, and increases the number

of MR visible lesions (White et al, 1992; De Coene et al, 1993). However, the long Tls and TRs

required demand longer acquisition times with a standard FLAIR sequence. More recently, the inversion pulse has been combined with an FSE pulse sequence to produce the fast FLAIR sequence (Figure 1.7). This sequence can acquire 36 slices of 5 mm slice thickness in just over

5 minutes (Rydberg et al, 1994).