Page 17 - Rob Holtackers

Introduction | 7

Figure 1.5: Phase-sensitive inversion-recovery (PSIR) versus standard inversion-recovery (IR):
whereas a 180 degrees inversion radiofrequency pulse is applied in IR every heartbeat, this same
pulse is applied only once every two heartbeats in PSIR. In the second heartbeat, a low flip angle
(5 vs. 25 degrees) reference acquisition is performed at exactly the same trigger delay time as the
actual image acquisition (of the first heartbeat). This reference acquisition helps to distinguish
between positive and negative longitudinal magnetization (Mz) levels and enables the
construction of the additional corrected real (or PSIR) image. This resulting PSIR image uses a
different grayscale compared to the conventional magnitude grayscale used for standard IR.

distinguish positive from negative magnetization levels, negative magnetization levels
will be represented differently in the additional corrected real (or PSIR) image that is
reconstructed: negative magnetization levels appear black, nulled magnetization levels
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appear dark-gray, and positive magnetization levels appear bright. This is in contrast
to a conventional magnitude image reconstruction where nulled magnetization levels
appear black, and both negative and positive magnetization levels appear bright. When
PSIR is used for conventional myocardium-nulled LGE, the clinical observer may adjust
window levels to further darken normal myocardial tissue, mimicking a magnitude
image representation.

In clinical practice, PSIR is recommended to make LGE image quality less sensitive to
the chosen TI, leading to a reduction in image artifacts and potential
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misinterpretations. Additionally, in contrast to standard IR, PSIR is a so called ‘two-
beat’ sequence with only a single inversion RF pulse every two heartbeats. Therefore,
PSIR is more robust to heart rate variations and cardiac arrhythmias since it relies to a
lesser extent on a constant time delay between successive inversion RF pulses,
averaging irregular heartbeats over two heart beats. Inherently, however, scan duration
doubles when using PSIR instead of standard IR.

Limitations of conventional bright-blood LGE
Although conventional bright-blood (PSIR) LGE can adequately distinguish hyper-
enhanced scar regions from normal myocardium, LGE has its limitations. Since
ischemia of the cardiac muscle causes a wavefront-phenomenon of necrosis that affects
subendocardial fibers of the myocardium first, a large proportion of MIs are