Advanced Imaging and Contrast Concepts
ver the years, several ideas and concepts were developed on how to influence and enhance contrast by either suppressing or highlighting certain tissue structures. These concepts have added to the diagnostic options of MR imaging and are commonly used to solve specific questions or particular research tasks (more or less similar to those in Figure 11-01).
What are the bright spots in the sky on these images: sun or moon? Think about it. Sometimes one cannot determine exactly what is seen on a picture — even when the details are clearly visible. Then, additional information or specific approaches are helpful.
Left: Moonrise in southern Switzerland. Right: Sunset in Manhattan.
We will introduce some of these techniques on the following pages:
Diffusion Imaging, and
11-02 Suppression Techniques
Fat and, in a similar way, water can create contrast problems for a number of clinical issues. It possesses high signal on T1-weighted SE images, which can obscure other tissues or pathologies with high signal adjacent to the fatty tissue. In certain cases, it would be of great advantage to eliminate its signal. This includes lesions in fatty tissues such as the orbit or in examinations of fatty livers, in heart examinations, and in the differentiation of bone and marrow diseases.
We have already described two of the suppression techniques in Chapter 10: fat and fluid suppression with STIR and FLAIR. We will discuss three different approaches below.
11-02-01 Phase-Sensitive Methods
In Chapter 5, we have introduced chemical shift: the molecular difference between fat and water makes them precess at slightly different frequencies. If MR imaging is performed at high fields, chemical shift can lead to two different images of the same anatomical structure, which is known as chemical-shift artifact. Figure 11-02 explains the origin of this artifact.
There is a positive side to this feature: it can be used to eliminate the unwanted fat signal. In gradient-echo sequences chemical-shift effects are not refocused and will depend on the echo time, as the following description exemplifies. Water and fat have a chemical shift of 145 Hz at 1.0 T or of 225 Hz at 1.5 T. At the latter frequency, the off-resonance fat signal rotates through 360° every 4.4 ms.
Thus, at echo times which are even multiples of 4.4 ms, the fat and water signals are in phase, while for echo times which are odd multiples of 2.2 ms, the signals are out of phase (Figure 11-03). ΔB0 effects cause local variations in the exact phase of each component, but their phase difference is preserved.
By choosing an appropriate echo time, we can emphasize or minimize the contribution of the fat signal and by adding two averages which use in-phase and out-of-phase echo times respectively, the fat signal can be removed. This kind of fat suppression sequence is also known as the Dixon method. It is similar to chemical shift imaging or phase contrast [⇒ Dixon 1984, 1985, ⇒ Szumowski].
By applying an RF pulse of the appropriate frequency before the regular imaging pulse sequence, one can eliminate the signal of a specific tissue. Again, this method is field strength-dependent and best used at high fields where water/fat shifts are high.
A presaturation pulse is applied at the precession frequency of fat (or the compound to be saturated); this pulse does not influence the water component of the tissue (Figure 11-04).
Usually a chemical-shift selective pulse sequence (CHESS) or a variation of this sequence is used. With a frequency-selective 90º pulse, the magnetization of fat is rotated into the transverse plane where its dephasing is accelerated by a spoiler (or crusher) gradient. Then the regular pulse sequence follows, but it only excites the water in the sample. Figure 11-05 shows an example of the application of fat suppression.
A different kind of presaturation is used for artifact suppression in flow imaging (see Chapter 17).
Example of fat suppression – tumor in the right orbit. T1-weighted SE images.
(a) Plain image. (b) Contrast enhancement of the tumor after Gd-DTPA. The tumor has become bright. The fat signal has been eliminated; both orbits now are dark and the enhancing parts of the tumor are easily delineated.
11-02-03 Magnetization Transfer
The idea of altering contrast by off-resonance irradiation of the sample was first described by Muller and collaborators in 1983 [⇒ Muller]. Wolff and Balaban coined the term magnetization transfer (magnetization transfer contrast = MTC) for this kind of alteration of image contrast [⇒ Wolff]. Lipton, Sepponen and collaborators improved contrast enhancement of the method [⇒ Lipton].
MTC is a suppression of protein-bound water and related to spin-lock imaging. It is based on the fact that in most biological tissues there is a cross relaxation between the free proton pool Hf representing mobile water protons and the restricted proton pool (Hr) representing the protons associated with macromolecules or immobile water [⇒ Edzes, ⇒ Lipton].
The restricted Hr pool has a much shorter T2 than the mobile Hf pool, and consequently is not directly observed with standard MR techniques. Thus, its influence upon image contrast cannot be exploited with standard pulse sequences. The cross relaxation and/or chemical exchange between these two pools means that saturating the resonance corresponding to one of them also affects the second pool (Figure 11-06).
Saturating the Hr pool leads to a loss of signal from the Hf pool. The cross relaxation is a short range process and, therefore, the direct effect is limited to interfaces between the two pools, although diffusion relays the effect to the bulk of free water. The Hr pool is known to have a very short T2 value; thus, the behavior of the magnetization during the RF pulse is dominated by relaxation.
The majority of sequences developed to date for MTC imaging use a relatively long, low-power, off-resonance saturation pulse to selectively saturate Hr [⇒ Jones 1991, ⇒ Wolff], however, new pulse sequences have been proposed to optimize MTC [⇒ Jones 1992].
To date, the clinical applications of MTC are limited; it can be used in time-of-flight MR angiography to suppress background tissue. In T2-weighted images, MTC may help to detect early demyelination.
A combination of MTC and contrast agent application enhances contrast in cases where one of the techniques alone does not create sufficient enhancement, for instance in multiple sclerosis and other brain lesions, in brain infarction, and in the detection of recent myocardial infarction (Figure 11-07) [⇒ Jones 1993, ⇒ Tanttu].
Figure 11-07: Example of magnetization transfer contrast. Patient with multiple sclerosis. (a): T1-weighted brain images after enhancement with a gadolinium-based contrast agent. (b): Image with additional magnetization transfer contrast. The combination of contrast agent and MTC clearly enhances contrast and shows more lesions, although it remains unclear whether all of these lesions are active.