10-05 Signal Inversion: TI – the Inversion Time

In the early 1980s, the research group at Hammersmith Hospital in London suc­cee­ded in imaging the brain with such an excellent spatial and contrast re­so­lu­tion that they could compare their images with slices seen before only in ne­crop­sy. They used inversion-recovery (IR) pulse sequences [⇒ Bydder].

In IR sequences, the initial magnetization is turned around by a 180° pulse. Thus, the recovery starts with a negative value, reaches 0 after 0.69 times the re­spec­ti­ve T1, then becomes positive and returns to its equilibrium after ap­pro­xi­ma­te­ly five times T1 (Figure 10-08 top). At some stage during this period, a 90° pulse is transmitted, which changes the partly recovered longitudinal mag­ne­ti­za­tion into observable transverse magnetization. Commonly this is followed by another 180° pulse which creates a spin echo.

A detailed description of this pulse sequence can be found in Chapter 4.

In reality, the initial negative magnetization is lost due to the calculation of the magnitude or modulus image (solid lines in Figure 10-08 top) from the real and imaginary signals, which is the standard procedure on most systems. Therefore, negative signals are recorded as positive signals of the same strength, and the sig­nal is positive on either side of the null point. Image contrast in IR sequences reflects this behavior (Figure 10-08 bottom).

Figure 10-08:
Graph top: Signal-intensity (SI) behavior of an inversion-recovery sequence with a repetition time TR = 2000 ms, B0 = 1.5 T. Note that until 0.69 × T1 has been reached, the signal is negative (dot­ted lines). The solid lines depict the IR signal intensity behavior in reality (magnitude image): the (positive) signal decreases to zero, then increases again until it reaches equilibrium.

Graph bottom: Relative contrast between gray and white matter and CSF in the same inversion-recovery experiment. There is poor contrast with short inversion times. It then increases and rea­ches a peak at approximately 400 ms. Immediately afterwards, there is a sharp drop in gray/ white-matter contrast; it disappears completely, and then turns negative, reaching another peak. With long inversion times, it disappears again. CSF/white-matter contrast behaves similarly. Note that peaks of optimal contrast can be very close to zero contrast. When looking for pathology, this means that lesions may easily be overlooked if wrong inversion times are chosen.

Images bottom: One of the main problems with the IR sequence is that its contrast behavior can change dramatically with only minimal changes of the inversion time (see also Figure 10-09 below).

Figure 10-09: Animation: In all images, TR = 4000 ms (different from the graph in Figure 10-08). TI from 100 ms to 2000 ms, increasing in steps of 100 ms. Simulation software: MR Image Expert®

To retain the signal information, a reference image is needed so that the phase in each pixel can be compared. An interlaced IR/PS sequence is one way of achie­ving this.

Images with long inversion times (TI) have hardly any contrast between neigh­bo­ring tissues, except the one created by differences in proton density, where­as images with short TI show high contrast.

Like partial saturation sequences, IR emphasizes the longitudinal relaxation time T1. The intensity of an averaged signal can be calculated with the following equation:

SI = K × ρ × M₀ (1 - 2exp {-TI / T1} + exp {-TR / T1})

where SI is signal intensity, K is a constant comprising bulk flow, diffusion, per­fu­sion and other parameters, ρ is proton density, M0 is magnetization at time 0, TI is the inversion time, TR is the repetition time, and T1 is the longitudinal re­la­xa­tion time.

The signal intensity for a given T1 is strongly dependent on inversion and re­pe­ti­tion times.

The IR sequences implemented in clinical machines add a second 180° pulse after the 90° pulse to rephase T2 influences. Thus, clinical IR images are also af­fec­ted by T2; however, T2 is not a primary source of contrast in IR imaging and will be neglected in this context.

If a shorter TR is chosen than the T1 of a particular component of the sample, it can happen that — at a certain TI, which is shorter than T1 — the relative sig­nal intensity of this tissue component will be higher than that of a neighboring com­po­nent with a shorter T1. This complicated and ambiguous contrast be­ha­vi­or is best explained in Figures 10-08 and 10-09. The graphs and images show that the gaps between no contrast and high contrast are very small. If there are no un­pre­dic­table pathological changes present in the sample, contrast can be fore­told. Problems arise when unknown lesions have to be found.

To avoid false interpretations of MR images, several pictures at different TI are desirable. This, however, prolongs imaging and examination times. It is pos­si­ble to produce several images with different TI values in a single interval using a spe­cial IR sequence, but the signal-to-noise ratio of this sequence is reduced be­cause flip angles of <90° are applied for each excitation [⇒ Graumann; ⇒ Young].

Usually, IR images are acquired as multislice pictures. This means that se­ve­ral parallel slices, rather than one single slice, are imaged at a time. This is an elegant way to save time and shorten an examination, and greatly increases the ef­fi­ci­en­cy of IR examinations, which are very time-consuming.

The pulse sequence used for multislice IR is the BIR (balanced IR) sequence, which is also known as MDEFT (modified driven-equilibrium Fourier trans­for­ma­tion). These sequences provide superior T1-weigh­ted contrast to T1- weigh­ted SE sequences.