Foundations of Medical Imaging and Signal Recording - Biomedical Signal Analysis: Contemporary Methods and Applications - page 21

FFT

frequency

v

time

Figure 1.11
Frequency-domain transformation of the measured temporal course. The amplitude
in the spectrum is exhibited at the Larmor frequency.

(a) (b) (c)

Figure 1.12
Brain MRI showing (a) T 1 ,(b)T 2 , and (c) hydrogen density-weighted images.
(Image courtesy Dr. A. Wismuller, Dept. of Radiology, University of Munich.)

“Weighted” means that the differences in intensity observed between
different tissues are mainly caused by the differences in T 1 ,T 2 ,and P D ,
respectively, of the tissues. The basic way to create contrast based on
the above parameters is show in table 1.3.
The pixel intensity I ( x, y ) of an MR image obtained using a spin-echo
sequence is given by

P D ( x, y ) 1 exp exp

T R
T 1

T E
T 2

I ( x, y ) ∝ − − − (1.15)

T2−weighting

T1−weighting

Varying the values of T R and T E will control the sensitivity of the signal
to the T 1 /T 2 relaxation process and will produce different weighted