Image weighting and contrast
The repetition time TR indicates the time between two excitation pulses, the echo time TE is the time after which the signal transmitted by the tissue is read out (Fig. 15).
Fig. 15: Repetition time TR, echo time TE
By choosing different combinations of these two sequence parameters, different tissue contrasts can be achieved in the MR image.
The contrast of the T1Weighting is determined by the TR. It must be so short that not all spins are relaxed when a new excitation pulse occurs. Since only spins that have already been relaxed (folded back) can be excited again, the level of the FID signal and also that of the spin echo depends on how many spins are available for excitation in the RF pulse (saturation) but being able to fold back in the TR depends on the speed of the longitudinal relaxation, i.e. on the T1 time of the corresponding tissue.
However, the signal echo also becomes smaller after the T2 time due to the dephasing of the spins. In order to keep this influence of the dephasing due to spin-spin interactions low, the PD must be selected as small as possible (Fig. 16). For T1-weighted recordings with spin-echo sequences you choose:
TR short:300-800 ms
TE for short:<30 ms
Fig. 16: Representation of the different magnetic properties of the different tissues as a function of time for T1 weighted recordings
For T2-weighted recordings are selected long TR so that the spins of all tissues (including those of a tissue with a high T1 time) have enough time to fully fold back until the next RF pulse. The FID signal and the signal echo are now largely independent of the T1 time: If a long TE time is selected, the dephasing becomes very important due to the spin-spin interactions and the resulting signal is strongly dependent on the T2 time (Fig. 17). For T2-weighted recordings with spin-echo sequences one chooses:
TR long: > 2 sec
TE long:60-120 ms
Fig. 17: Representation of the different magnetic properties of the different tissues as a function of time for T2 weighted recordings
In the PD / T2-SE sequence, the first spin echo is processed into a separate image, the contrast of which depends primarily on the different number of protons (hydrogen atoms) in the various tissues and therefore as a proton density contrast PD is designated (Fig.18).
TR long: > 2 sec
TE for short: <30 ms
Tissue with a high ProtonsDense, e.g. brain tissue, generate a strong signal due to their large transverse magnetization and appear light on these images, tissue less Protonsdensity dark. (Fig.18,19c)
In tissues with high materialdense (e.g. fat), there is more longitudinal magnetization before each excitation pulse due to the rapid T1 relaxation, since the excited protons release their energy much faster than in tissues with less energy due to the shorter mean distances between the individual molecules materialdense (liquids). This results in a higher transverse magnetization which results in a stronger signal. Therefore, fatty tissue appears light and water appears dark in the T1-weighted images. (Fig.19a)
In the T2-weighted images, water appears due to the large number of protons (high Protonsdensity) and the resulting large transverse magnetization is bright, tissue with a low Protonsdensity, on the other hand, appear dark. (Fig.19b)
Fig. 19 a-c: Axial MRI cross-section a = T1 weighted: light parenchyma, dark liquor
b = T2 weighted: parenchyma dark, liquor light
c = PD weighted: parenchyma light, liquor dark