THE MOIRE EFFECT IN RADIOGRAPHY by xyh18958

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									VOL.        115,     No.        3



                            THE                    MOIRE                        EFFECT                             IN                 RADIOGRAPHY*
                                                                           By      FRANK              A.        RIEBEL,                   M.D.
                                                                                           COLUMBUS,                OHIO


I F TWO                         Bucky          grids        quality
                                                             of identical                                         2     grids              of         different             peniodicity                     are       used.
        are         precisely                           they
                                              superimposed        will                                           These                 may be:
function       as one. If, however,            they differ sig-                                                          I.          Grids     which       differ   in lines            per inch.
nificantly,      as, for example,          in lines per inch,                                                           2.           Identical       grids      which     are           in contact,
they       will produce        patterns          called      moire.                                               but          one          is notated                 in relation  to the other.
The       term      is derived         from        moire        (pro-                                                3. Grids                     which              are kept parallel,     but one
nounced        mw#{224}r),a textile      which       has a wavy                                                   is separated                     from              the other.
appearance;           and indeed,         the x-ray          moines                                                     4.           When             one        grid      is tilted               in     relation            to
frequently        give a similar         pattern.                                                                 the     other.
    Actually       these   patterns         appear        in many                                                       5. Grids                      which             differ           in      focal       distance.
products                   of       human              workmanship.                   Thus,           if              Enquin2                     has          used   radiographic                         moines         cx-
light   passes     through         2 or       more      layers    of                                              penimentally                                as attractive                      ant      forms.          He
tulle   (a fine net-like            textile),       alternating                                                   has    also suggested          the possibility      of produc-
bands     of light    and dark,         called      fringes,    are                                               ing   magnified        images     suitable     for evaluating
seen. The wavy          pattern       can be produced             as                                              poorly       manufactured           or damaged         grids.
well by printing        2 overlapping            sets of paral-                                                      The radiologist            is likely     to encounter       the
id lines of different         ratios5      and also appears                                                       moire      effect    in the event         that film in a grid
during               electron               microscopy                   when            gold         is          cassette                   is        exposed             in       a         spot-film              device
overlaid      with palladium.3        Multichannel                                            col-                through       a built-in    grid. If the lines are pan-
limators        used   with    a scintillation                                           camera                   allel to each other,         a gross pattern   of paral-
can      produce      comparable          effects.’                                       All of                  lel bands        will be produced       as in Figure     i.
these      are examples      of interference                                            by light                  The     distance        (D)  between     the bands    fol-
or other       rays.                                                                                              lows the law D=              d2/d’-d2    when  d’ and    d2



       In       radiography,                      moines          are producedwhen




FIG.          I.   A moire              pattern          of coarse         transverse             lines           FIG.          2. Image         produced     by exposing    film through                                      2
       resulted            when         a   cassette          with      built-in        grid was                        stationary                      which
                                                                                                                                                      grids      were slightly    rotated                                     in
       used        in a spot_film             device        which      also     had     a grid,                         relation            to each other.
   *     From       the    Department             of Radiology,       College      of Medicine,           The    Ohio        State      University,            Columbus,         Ohio.


                                                                                                           641
642                                                                                     Frank            A. Riebel                                                                                                JULY     ,   1972




                                                                                                            grees.               Note                that           the        original                 pattern                   in
                                                                                                            Figure                 2    has      become                    magnified                   and          fainter,
                                                                                                            but         at night                angles              to it there                 are         now           more
                                                                                                            noticeable          wavy                             lines         which                 were             barely
                                                                                                            visible     before.
                                                                                                                 Figure     4 was                           produced                  by       using               3 grids,
                                                                                                            set       at        various                  angles.
                                                                                                                   In        Figure                  5    an       IIXI4               inch            cross-hatch
                                                                                                            grid was used.    This  has 75                                             lines            the
                                                                                                                                                                                                       to                 inch,
                                                                                                            an 8 : I ratio, and is simply                                              an       assembly                    of        2

                                                                                                            unfocused                        grids             of half         the       usual              thickness,
                                                                                                            built             at        right             angles.             Although                        the         focal
                                                                                                            distance                    was          6 feet,               there       is a definite                           hot
                                                                                                            spot           at          the       center.              There              is also               a coarse
                                                                                                            plaid               pattern                   of       moire             which                  thins              out
                                                                                                            penipherall.        The                              moire              is of low                     contrast
                                                                                                            and     is obscured                                 when               a patient                      is inter-
                                                                                                            posed.
FIG.     3.       By   increasing          the     angle       of rotation           a different
                                                                                                                                                               SUMMARY
                                pattern           is produced.
                                                                                                                   J\IoirC              is a term                  which             indicates                    a wave-
are       the          distances                 between              the       respective                  like,          clouds’              or       frosted             appearance                       of textile
grid       lines.                                                                                           fabrics                on        metallic               surfaces.               It can                 also        oc-
       Other      mathematical                               formulae3               apply          to      cur         in          the        x-ray             spectrum                   when              2      Buckv
the true moire-like        pattern     seen in Figure        2,                                             grids    are superimposed.                                             The           2     most              likely
where    an 8:1     8o lines       to the inch      parallel                                                circumstances       under                                      which           the         appearance
grid was used,        with     a grid similar,       except
for having   a 32 inch focus,          superimposed          at
an       angle
       In Figure
                 of rotation
                      3 one
                                                   of 7 degrees.
                                                 grid        was      rotated             i        de-
                                                                                                             I




FIG.          .   Three        grids       at       varied         angles       of      rotation                             FIG.        .      A faint            moire       was       produced                  by
                                          were       used.                                                                                   exposing            a cross-hatch                 grid.
VOL.      115,   No.      3                                  The       Moire        Effect      in Radiography                                                                     643


might            be      observed                by        the      radiognapher                        collimators            for sci ntilla tion       camera.          Radiology,
                                                                                                        1971,       98,    329-334.
would           be coincident                       to      the      exposure              of
                                                                                                2.   ENQUIN,          B.       X-rays      and         moire       images.         Rev.
films        through    cross-hatch                        grids,      on on       films
                                                                                                         ‘Irgent.     de     Radiologia,    1970,33,           154-161.
exposed           through               a grid        cassette         placed  in a             3. HAINE         and C05LE’n’.          The     Electron       Microscope.
spot-film              device           which         already         has a built-                     Clowes       & Son, London,           1961,     pp. 270.
in grid.                                                                                        4. HIEDENREICH,            R. D. Fundamentals                 of Transmis-
                                                                                                       sion. Electron         Microscopy.          John     Wiley      & Sons,
2565     Leeds         Road                                                                            Inc., New York,            1964, 294-296.
Columbus,             Ohio      4322!                                                           . OSLER,          G., and       NISHIJIMA,         G. Moire-patterns.
                                                                                                       Scientflc      American,        1963, 208, pp. 54.
                                   REFERENCES
                                                                                                6. SIEGEL,         B. M. Modern             Developments             of Elec-
I.     BONTE,          l. J.,     GRAHAM,             K. I)., and DOWDY,                   J.          tron      Microscopy.         Academic          Press,     Inc., New
         Image          aberrations             produced     by multichannel                           York,       pp. 162-171.

								
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