Page 16 - Rob Holtackers

| Chapter 1
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Figure 1.4: Example of a ‘Look-Locker’ scan that provides a series of low-resolution images with
increasing inversion time (TI). After injection of the contrast agent, the scar tissue and blood pool
have a shorter T1 relaxation time compared to the normal myocardium, leading to a faster
recovery of the longitudinal magnetization (Mz). Therefore, the scar tissue (cyan line) reaches the
zero-magnetization level first, shown as a black appearance on the magnitude image (cyan
arrowhead in image 1). Soon after, the blood pool follows (orange line and dot in image 2), and,
finally, the normal myocardium (green line and dot in image 4). In this example, the TI that
corresponds to image 4 should be set for conventional myocardium-nulled (bright-blood) LGE.

Image reconstruction
When all required MR signals have been acquired, the reconstruction of the image is
started. Conventional reconstruction mechanisms render so-called magnitude or
modulus images. In these images, the magnitude of the MR signal is determining the
local signal intensity, regardless of the sign of the signal. This implies that both positive
and negative magnetization levels, with equal magnitude, lead to identical image
intensities. In case of LGE, where a specific TI has to be determined to null the normal
myocardium, this could lead to misinterpretation of the tissue type. In particular, when
the TI is chosen too short, normal myocardium will yield a negative signal and appear
bright just like the positive signals of MI, potentially obscuring these areas and
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underestimating the apparent scar volume.
Such misinterpretations can be mitigated using a phase-sensitive inversion-recovery
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(PSIR) sequence. Compared to a conventional IR sequence, the PSIR sequence applies
a 180 degrees inversion RF pulse only once every two heartbeats (Figure 1.5). During
every second heartbeat, a reference signal is acquired using a low flip angle excitation
(usually only 5 degrees). This reference signal is used to determine the phase, and thus
the polarity, of the signals acquired every first heartbeat. As PSIR is able to