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Chapter 10

10-01
Introduction

10-02
Main Contrast Factors in MR Imaging

10-03
The Basic Processes

Repetition Time (TR)
Echo Time (TE)
10-04
Multiecho Sequences

Rapid Spin Echo
10-05
Signal Inversion:
TI – the Inversion Time

10-06
Fat and Water Suppression

10-07
Gradient Echo Sequences

FA – the Flip Angle
10-08
Static Field Strength and Contrast


10-08 Static Field Strength and Contrast

Table 10-01 lists a large number of factors influencing contrast. Among the main contrast parameters are the relaxation times T1 and T2. If they change, contrast also changes.

While within today’s imaging range, T2 is only minimally influenced by field alterations, T1 is strongly field-dependent. Therefore, at given pulse sequence pa­ra­me­ters, contrast changes with field strength. The way these changes occur is nearly impossible to predict because it is extremely difficult to extrapolate the T1 values of a particular tissue acquired at one field strength to another: T1 is de­ter­mi­ned by several factors which vary at different fields.

The best method to examine these features of relaxation is relaxometry, the study of the behavior of longitudinal and transversal nuclear relaxations and of their dependence on internal and external parameters. Among them are mo­le­cu­lar and supramolecular structures, temperature, viscosity, pH, magnetic field strength, and paramagnetic and ferromagnetic agents. Field-cycling relaxometry deals with the relaxation behavior and its changes with the strength of the mag­ne­tic field [⇒ Rinck 1988]. It requires special, purpose-built machines, field cyc­ling relaxometers.

Figure 10-15a depicts the T1 relaxation times of white and gray matter of an adult versus field strength. They both increase with field, but with a different ratio to each other. When comparing the contrast behavior of different tissues within the imaging range of MR fields, one finds that pure T1-contrast increases from low and ultra-low fields and reaches a peak in medium fields between 0.6 and 0.9 Tesla; then it worsens again in high fields (Figure 10-15b). At low and medium fields, the relevance of this finding for clinical imaging is relatively un­im­por­tant because pure T1 images are not used.

Usually, clinical images (i.e., T1-, T2- or intermediately weighted) possess suf­fi­ci­ent contrast even at high fields if parameters and pulse sequences are cho­sen properly [⇒ Chen; ⇒ Fischer; ⇒ Hoult]. However, the loss of gray/white matter contrast between 1.5 and 3.0 Tesla machines is distinctly noticeable.


However, for heavily T1-weighted images of any pulse sequence, this feature becomes important because it means that diseases cannot be de­tec­ted at certain fields with T1- weigh­ted sequences.

Multiple sclerosis plaques and other brain lesions have a similar sig­nal intensity as their sur­roun­ding tissue at medium and high fields when, e.g., imaged with a heavily T1-weighted conventional SE sequence (Figure 15c). Contrast exists only at ultra-low fields and diagnostic pulse se­quen­ce pa­ra­me­ters have to be adjusted ac­cor­ding­ly.


Figure 10-15:
(a) T1 relaxation time values of gray (GM) and white matter (WM) versus field strength.
(b) Approximation of pure-T1 contrast (C) between gray and white matter: Contrast is relatively poor at low and high fields.
(c) The signal intensity behavior of multiple sclerosis plaques (field strength in log scale).


On the other hand, it is important to keep in mind that T1 grows with field strength. This increase is the reason why images taken with the same pulse pa­ra­me­ters, but at different fields, change their contrast appearance. Thus, they can­not be directly compared with each other, as the image comparison of Figure 10-16 shows.

Figure 10-16:
The image series on the left side was taken at 0.5 Tesla, the one to the right at 1.5 Tesla at a re­pe­ti­tion time TR = 2000 ms. Compare how contrast changes and differs between the two field strengths at the same imaging parameters.

Simulation software: MR Image Expert®

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