Page 14 - Rob Holtackers
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| Chapter 1

            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
            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
            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
            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).
            IR  pulse  sequences  start  with  a  non-selective  180  degrees  inversion  radiofrequency
            (RF) pulse that inverts the magnetization levels of all the different tissue
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