Page 16 - Rob Holtackers
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| Chapter 1

            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
            underestimating the apparent scar volume.
            Such  misinterpretations  can  be  mitigated  using  a  phase-sensitive  inversion-recovery
            (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
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