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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
               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
               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
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