Page 15 - Rob Holtackers

Introduction | 5

Figure 1.3: Standard inversion-recovery sequence used for conventional (bright-blood) late
gadolinium enhancement (LGE). A non-selective 180 degrees inversion radiofrequency pulse
inverts all magnetization levels. A time delay follows, called the inversion time or TI, in which
all longitudinal magnetization (Mz) levels recover with a rate determined by the tissue-dependent
T1 relaxation time. Following the TI, acquisition of the signal takes place. Tissues with shorter T1
relaxation times recover faster and therefore yield more signal in the resulting MR image. Note
that for conventional LGE the TI is set to null the magnetization level of the normal myocardium
magnetization level (green line at 0), leading to a black appearance of the normal myocardium
on a magnitude image. The magnitude grayscale bar is shown on the right, indicating only minor
differences between the scar (cyan) and blood pool (orange) magnetization levels.

types. A time delay follows, called the ‘inversion time’ (TI). During this delay, all
magnetization levels will recover back to their equilibrium with a rate determined by
the tissue-specific T 1 relaxation time. Finally, after the TI, a spoiled gradient-echo
readout of approximately 100-150 ms is performed to acquire the MR signal.

A ‘Look-Locker’ scan, also known as TI scout, that provides a series of low-resolution
images with increasing TI is usually performed beforehand to determine the desired TI
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(Figure 1.4). Traditionally, the TI is set so that the longitudinal magnetization level of
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normal myocardium is exactly zero when the MR signal is acquired. Nulling the
normal myocardium will then lead to a black appearance on magnitude images, while
the blood pool and areas of MI appear bright due to their shortened T 1 relaxation time.
As the blood pool is appearing bright using the conventional LGE sequence (with
myocardium nulling), it is referred to as ‘bright-blood’ LGE.