10-03 The Basic Processes

In the conventional pulse sequences image contrast is calculable, but there are in­crea­sing­ly other sequences where it is not predictable. On the following pages, we will first look at the basic pulse sequences and their contrast-influencing com­po­nents TR, TE, TI, FA, and others step-by-step. Many other pulse sequences can be derived from the basic sequences; their contrast behavior follows the fun­da­men­tals described here.

10-03-01 TR – the Repetition Time

In the early days of MR imaging one plain radiofrequency pulse sequence was used to create images: the partial-saturation (PS) sequence. It consists of 90° pulses transmitted in a train. The sequence is discussed in detail in Chapter 4.

Its signal intensity (SI) can be determined by the following equation:

SI = K × ρ × (1 - exp {-TR / T1})

where K is a constant comprising bulk flow, diffusion, perfusion, and other parameters, ρ is proton density, TR the repetition time between the 90° pulses, and T1 the spin-lattice re­la­xa­tion time.

The nuclei are exposed to repeated RF pulses which cause a free induction de­cay of a specific initial amplitude. If the repetition time between two subsequent pulses is less than 5×T1, magnetization has not completely recovered and the sig­nal intensity will be lower than the initial amplitude. The 90° pulse saturates the spin system for a certain time. If another RF pulse is transmitted during this pe­ri­od, a lower signal will be received. Therefore, equilibrium signals and thus ima­ge contrast differ, depending on the length of TR.

Partial-saturation images are only slightly influenced by T2, the spin-spin re­la­xa­tion, but heavily by T1, the spin-lattice relaxation, until the interpulse delay equals approximately 2×T1. At 2×T1, 90% of the magnetization has recovered. Afterwards, proton density is responsible for contrast (Figure 10-02). In general, bright regions on a PS image resemble object areas of short T1 and/or high pro­ton density, while dark regions depict areas of long T1 and low proton density.

Hardly anybody uses the original PS sequences today because of their limited diagnostic value. However, partial saturation becomes a gradient-echo se­quen­ce when a field gradient is added. Most sequences dubbed saturation recovery to­day are in reality gradient-echo sequences.

Figure 10-02:
Signal-intensity (SI) behavior of a partial saturation pulse sequence showing the dependence of SI of white matter (WM), gray matter (GM), and cerebrospinal fluid (CSF) at B₀ = 0.5 T. If the re­pe­ti­tion time chosen is long enough, signal intensities differ by the factor of proton density only (WM: 72%, GM: 82%, CSF: 100%). The images are of a healthy adult brain.

In this animation we fly along the time line of the graph above and watch how the contrast changes. Simulation software: MR Image Expert®

10-03-02 TE – the Echo Time

The principle elements of a spin-echo (SE) sequence are a 90° pulse followed by a 180° pulse after a time interval τ which builds up a spin echo after TE, the echo time (details were explained in Chapter 4). Signal decay depends on the re­la­xa­tion times and ρ of the respective tissue. The curves reach zero signal intensity faster or slower, owing to the composition of the tissue.

When researchers in the early 1980s started applying the spin-echo (SE) se­quen­ce for medical imaging, they found better contrast but also more com­pli­ca­ted contrast behavior. Some already known brain lesions were not seen because there was a lack of contrast. In particular, SE images with a TE shorter than 60 ms sometimes did not reveal multiple sclerosis plaques, astrocytomas, me­nin­gi­o­mas, infarctions, or other lesions. The reason for this behavior can be explained by the signal decay curves of spin-echo sequences.

The signal intensity of different compounds decreases with longer TE. The most interesting features of these curves, however, are the points of intersection. At these points, the respective brain tissues are isointense and there is no con­trast between them: they are indistinguishable. Many pathologies possess signal in­ten­si­ties similar to normal brain tissue on images and therefore are invisible on images with short TE (T1- and intermediately weighted images) (Figure 10- 03).

Figure 10-03:
Spin-echo sequence: decay curves of gray matter (GM), white matter (WM, and cerebrospinal fluid (CSF) at high field strength (B₀ = 1.5 T). Relative signal intensity, SI, versus echo time, TE, at a given repetition time of TR = 2000 ms. The images are of a healthy adult brain.

In this animation we fly along the time line of the graph above and watch how the contrast changes. Simulation software: MR Image Expert®

The spin-echo signal contains information about proton density as well as spin-lattice and spin-spin relaxation. Signal intensity of a spin-echo sequence can be calculated by the following equation:

SI = K × ρ × (1 - exp { - [TR - TE] / T1} × exp {-TE / T2 })

Again, SI stands for signal intensity, K represents the influence of flow, perfusion and diffusion, ρ is proton density, TR repetition time, TE echo time, and T1 and T2 are the relaxation times.

This equation reveals that T2-weighting of an SE image increases as the echo time advances. T1-weighting of the signal intensity depends on both TR and TE. In a single echo SE sequence, T1-weighting usually is created by both short TR and short TE. In general, the early echoes (= short TE) of an SE sequence are ρ- and T1-weighted. The later echoes are increasingly intermediately and T2- weigh­ted [⇒ Harms].

Figure 10-04 explains with two examples the interdependence of the echo time and the repetition time upon signal intensity and signal contrast. When a short TR is chosen, the initial signal intensity will be low and SE images with short echo times will be heavily T1-weighted. With longer repetition times, sig­nal in­ten­si­ties will be higher. The relation of signal intensities to each other chan­ges de­pen­ding on TR, and thus contrast is also strongly influenced by TR.

Figure 10-04:
Top: Short repetition times (TR = 250 ms) emphasize T1-weighting.
Bottom: Long repetition times (TR = 1500 ms) emphasize T2-weighting. In brain imaging, the crossover points of no contrast move to shorter TE values when TR is increased.
Animation: Left sequence: TR = 250 ms; right sequence TR = 1500 ms.

Simulation software: MR Image Expert^®

Contrast might be predictable with normal anatomy; however, when sear­ching for lesions, prediction may become impossible. Figure 10-05 gives an ex­amp­le of how a pathological lesion can stay hidden or be highlighted.

Sometimes the signal-decay curves of different tissues cross each other, lea­ding to a complete extinction of contrast between them. The best contrast is seen when the relative ratio between them is highest. If the parameters (i.e., T1, T2, ρ, TR and TE) are known, the decay curves can be precalculated within a certain error range. However, if one does not know T1, T2 and ρ and use the wrong pulse se­quen­ce parameters, one can overlook a lesion.

Figure 10-05: The necessity of image acquisitions with different pulse sequence parameters: Brain, transverse view. SE sequence: TR = 1500 ms; TE from 15 ms to 225 ms. B0 = 0.5 T. On the more T2-weighted images, there is a clear­ly visible lesion: an old brain infarction. If you would just perform a T1-weighted study only very suspicious radiologists will describe possible pa­tho­lo­gi­cal changes: there is a slight mass effect.

Simulation software: MR Image Expert^®

As we have seen in Chapter 4, one must clearly distinguish between T1- and T2-images ('pure T1- and T2-images') on the one hand, and T1-, T2- and in­ter­me­di­a­te­ly (proton-density weighted) images on the other. Whereas in the for­mer ima­ges calculated relaxation times are depicted, the latter images show sig­nal intensities with, e.g., in the case of a T1-weighted image, a high influence of T1, but at the same time also with T2 and proton-density contributions.

Table 10-02 gives an approximation to image weighting of SE sequences. Short TE and TR em­pha­si­ze T1 influence, long echo and repetition times em­pha­si­ze T2 influence. The combination of long repetition times and short echo times eli­mi­na­te part of the T1 and T2 effects, thus emphasizing ρ.

Table 10-03 depicts the signal intensity behavior in 'weighted' images.

Table 10-02:
Image weighting. Proton density (ρ-) images should preferably be called intermediately weighted images, because the highest signal intensity in these images might not present the highest water content. TR and TE depend on field strength. At high fields, TR and TE for both T1- and T2- weigh­ting are shorter than at medium or low fields.

Table 10-03:
Signal-intensity behavior in weighted images. Only weighted spin-echo images are used in routine clinical examinations. Note that the respective signal intensities on weighted images vary ac­cor­ding to the TE and TR chosen and with the strength of the magnetic field. The easiest way to dis­tin­guish T1- and T2-weighted images is by looking at water-like liquids: on T1-weighted ima­ges, they are dark; on T2-weighted ima­ges, they are bright.