Page 14 - Rob Holtackers

| Chapter 1
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Figure 1.2: Schematic illustration of the contrast agent (CA) concentration in both normal
myocardium (green) and scar tissue (cyan) after a bolus injection. While the CA quickly washes
in and out of the normal myocardium, a delayed wash-in and wash-out is observed for scar tissue.
At approximately ten minutes post-injection, a phase known as ‘late (gadolinium) enhancement’
commences (gray area), where scar tissue contains a significant larger concentration of CA
compared to normal myocardium.

Aside from the different wash-in and wash-out rates, the local distribution is of equal
importance for LGE. The gadolinium-based CAs used for LGE are extracellular and
cannot cross the intact cell membranes of normal myocardium, thereby limiting their
distribution volume to the interstitial space. However, in patients with acute MI, the
cell membranes of the affected myocytes have ruptured, and the CA can now also
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access the ‘intracellular’ space leading to an increase in the distribution volume. Over
time, scar tissue forms, and the affected myocytes are replaced by a collagen matrix.
As a result, the interstitial space is increased and thus also the CA’s distribution volume.
Ultimately, both in patients with acute and chronic MI, the distribution volume of the
CA is increased compared to normal subjects. The combined effect of reduced wash-
in/wash-out rates and the increased distribution volume leads to a delayed
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accumulation of the CA in areas of MI after approximately ten minutes. Because of
the strong T 1-shortening effect of the gadolinium-based CA, these areas of
accumulation lead to a delayed hyperenhancement of the signal, hence the name ‘late
enhancement’ or ‘delayed enhancement’.

Conventional LGE pulse sequence

While the areas of MI experience a strong decrease in T 1 relaxation time due to the
accumulation of CA, most of the CA has already been cleared from the normal
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myocardium, resulting in a much longer T 1 relaxation time. In order to maximize
contrast between the normal myocardium and areas of MI, a heavily T 1-weighted
segmented inversion-recovery (IR) gradient-echo pulse sequence is used (Figure 1.3).
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IR pulse sequences start with a non-selective 180 degrees inversion radiofrequency
(RF) pulse that inverts the magnetization levels of all the different tissue