TwinTree Insert

04-06 Practical Measurements of T1 and T2

igh-resolution magnetic resonance spectroscopists have measured T1 values since the middle of the last century. Such measurements can be performed in numerous different ways with various degrees of ac­cu­ra­cy — in vitro (ex vivo) and in vivo.

04-06-01 In vitro Determination

In vitro measurements are done on a small samp­les, ap­pro­xi­ma­te­ly 0.1-1.0 ml or slight­ly lar­ger in volume, in an extremely ho­­mo­­ge­­ne­­ous mag­ne­tic field.

A variety of methods has been developed to obtain maximal precision with mi­ni­mal time consumption. Typically, 15 to 30 mag­ne­ti­za­tion measure­ments are per­­for­me­d on the sample for different time delays, TI in inversion-recovery ex­pe­ri­ments or TR in partial saturation experiments. Based on these results, an observed T1 value is calculated, and the error limits are usually better than 5%.

T2 can be calculated with a single multiecho sequence. The more echoes one uses, the more accurate the measurement will be.

Calculations based on fast pulse sequences (so called black box sequences other than IR or SE) lead to rough estimates of T1, T2, T2* (and proton density) values. They might be reproducible when repeated, but the use of relaxation time values acquired with such pulse sequences is not advisable for scientific or clinical com­pa­ri­­sons.

04-06-02 In vivo Determination

Magnet systems with larger bores allowed the examination of whole organisms, ani­mals, and people, and a more physiological determination of relaxation time values than those of excised organs or tissues.

Relaxation time measurements were considered very important during the first years of commercial (medical) MR imaging. All machines were programmed to create true T1 and T2 images (i.e., T1- and T2 mapping), based on SE and IR sequences.

Very early, test objects and the protocols for their use to allow the measurement of T1 and T2 precision and accuracy were introduced in the frame­work of an ex­ten­si­ve Euro­pean project. The findings were sobering, but scientifically predictable [⇒ EEC 1988].

In particular, the accuracy and precision with which the relaxation times T1 and T2 could be measured from the images were found to be rather disappointing. Smal­ler comparison studies followed. It became clear that relaxation time values were not the claimed invaluable addition to diagnostics, and this 'standard' ap­pli­ca­tion was skipped [⇒ Lerski 1988].

A similar multi-center trial was repeated more than 30 years later. Phantom stu­dies — newly introduced in the United States — described the same imprecision and irreproducibility of T1 measurements acquired on different MRI machines, most pro­noun­ced when rapid techniques for the estimation of relaxation constants were used [⇒ Keenan 2021] (cf. Rapid relaxation estimation techniques).

spaceholder redLocalization. One of the major problems of in vivo relaxation time mea­su­re­ments is the lo­ca­li­za­tion of the volume to be observed.

Actual accuracy of in vivo measurements depends on the number of points acquired and the quality of localization. Localization is re­la­ti­ve­ly uncomplicated in little or non-moving organs such as the brain, but de­mand­ing and partly impossible (in par­ti­cu­lar at high/ultrahigh fields) in organs with complex mo­ve­ment patterns such as the heart.

Details of localization tech­ni­ques are given in Chapter 6.

spaceholder redRelaxation Time Values and Proton Density Calculation. The current most de­­pend­­able method used to obtain a T1 image (T1 map), i.e., an image whose picture elements represent pure T1 values, relies on a mathematical manipulation of se­pa­ra­te­ly obtained images with different T1 influence. Measurements are easier and more accurate at low and medium fields, because T1 values are shorter, ECG trig­­ger­­ing less complicated, and artifacts less pronounced at these fields.

Typically, two to four images are used and the signals mathematically processed to cal­cu­late pure T1 values. Bearing in mind that in vivo re­la­xa­tion can be multi­ex­po­nen­tial­, it is somewhat in­ad­equate to perform the analysis by such a limited fit to an ex­ponential curve.

T2 images are calculated from the im­ages of a multi-echo series, e.g. CPMG. In cli­ni­cal settings, usually four or eight echoes are applied.

Diffusion, flow, and multiexpo­nential decays are hardly ever taken into account in the fits and noise as well as motion artifacts add to inaccuracies.

spaceholder redMatrix size and slice thickness as well as partial volume effects are limiting fac­tors in relaxation-time measurements in vivo. Par­tial volume effects and other fac­tors influ­ence the measurements. Variations within the same lesion related to vas­cu­la­ri­ty, necrosis, and cell behavior (macro­scopic compartmentalization) contribute to the overlapping of relaxation times values.

All methods re­ly­ing on slices through the ex­amined object will have as additional error source par­tial volume effects from the edges of the slices; the only method which avoids this slice problem is the true 3D vol­ume imaging method.

Standard deviation in fitting, artifacts, and variations in the selec­tion of vol­ume ele­ments by the opera­tors are all possible sources of error (Figure 04-21).

Figure 04-21:
Relaxation time measurements in vivo can be per­formed pixel by pixel and by re­gions of interest of different size.
Left: Small regions of interest cover­ing edema (green), tumor (pink), necrosis (red), etc.
Right: Large region covering the entire tumor.

Furthermore, similar lesions may have a more than single exponential relaxation rate, e.g., brain tumors and mul­ti­ple sclero­sis plaques. This is not unexpected, con­si­d­er­ing the he­te­ro­ge­neous nature of tumors — and tissues in general.

Reproducibility of such measurements is also limited.

spaceholder redThe multilayered complexity of factors and features influencing and creating relax­ation time and proton distribution changes is not completely understood yet [⇒ Springer 2014]. A simplified view offered by Koenig suggests that water molecules can wander rather ex­tensively by thermally-induced diffusion throughout the intra- and extracullular regions of tissue, and that the exploration is rather thorough in a time of the order of T1 (or even T2).

Another con­cept is the highly structured water, re­strained for a significant time in a geometry defined by various ionic and molecular constituents of the cytoplasm [⇒ Koenig 1985, 1988].

However, some features of the T1-dis­persion do not fit easily into these concepts, for instance cross relaxation phenomena that lead to quadrupolar dips in the T1-dis­persion plot. They are dependent on field strength and temperature (Figure 04-22) [⇒ Rinck 1988]. More about the dependence of relax­ation times on static field strength and its influ­ence upon contrast can be found in Chapter 10.

Figure 04-22:
The dispersion of T1 in tissues (ms) ver­sus field strength (log Tesla) is not as monotonic and smooth as shown e.g. in Figure 04-04 and Figure 10-16.
This high resolution nu­clear magnetic relaxation dis­persion (NMRD) curve of a multiple sclerosis tissue sam­ple reveals two dips (quadrupolar dips) at 0.0505 and 0.0660 T (2.1 and 2.8 MHz) where the oth­er­wise steady increase of T1 is interrupted.

04-06-03 T1 and T2 Images and Weighted Images

Relaxation time and proton density values can be used to create synthetic or si­mu­lat­ed ima­ges for training and teaching purposes.

In clinical routine, people often talk about T1, T2 or proton-density images. The cor­rect terms are T1-weighted (or dependent), T2-weighted (or dependent), and pro­ton den­si­ty (ρ)-weighted (or better, intermediately weighted) images, because these images have only a cer­tain T1, T2, or proton-density dependence. They are not calculated pure relaxation time or proton density images. Chapter 10 will ex­plain this in detail.

Figures 04-23c and d are T1-weighted and T2-weighted images, to be compared with the pure T1 and T2 images of Figure 04-23a and 04-23b.

Figure 04-23:
Top row: Images of a patient with a polyp in the left paranasal sinus.
Bottom row: images of a cervical spine.
(a) Calculated (pure) T1, and (b) calculated (pure) T2 image. Figures (c) and (d) show T1- and T2-weighted images. Pure T1 and T2 ima­ges are of very limited diagnostic value. Multiparameter weighted images are far more valuable for clinical diagnosis and commonly used in patient studies.
Simulation software: MR Image Expert®