Convert multiple Dicom images(.dcm files) into grayscale images

I have a listbox, pushbutton and axes to display dicom images. After I have load the images into listbox, I want to convert all the images to grayscale images. After I had created dicom2gray function, it shows black screen images. Can someone help me check the code? Thanks

function varargout = fyp2016(varargin)
% FYP2016 MATLAB code for fyp2016.fig
%      FYP2016, by itself, creates a new FYP2016 or raises the existing
%      singleton*.
%
%      H = FYP2016 returns the handle to a new FYP2016 or the handle to
%      the existing singleton*.
%
%      FYP2016('CALLBACK',hObject,eventData,handles,...) calls the local
%      function named CALLBACK in FYP2016.M with the given input arguments.
%
%      FYP2016('Property','Value',...) creates a new FYP2016 or raises the
%      existing singleton*.  Starting from the left, property value pairs are
%      applied to the GUI before fyp2016_OpeningFcn gets called.  An
%      unrecognized property name or invalid value makes property application
%      stop.  All inputs are passed to fyp2016_OpeningFcn via varargin.
%
%      *See GUI Options on GUIDE's Tools menu.  Choose "GUI allows only one
%      instance to run (singleton)".
%
% See also: GUIDE, GUIDATA, GUIHANDLES

% Edit the above text to modify the response to help fyp2016

% Last Modified by GUIDE v2.5 10-Apr-2016 14:29:03

% Begin initialization code - DO NOT EDIT
gui_Singleton = 1;
gui_State = struct('gui_Name',       mfilename, ...
                   'gui_Singleton',  gui_Singleton, ...
                   'gui_OpeningFcn', @fyp2016_OpeningFcn, ...
                   'gui_OutputFcn',  @fyp2016_OutputFcn, ...
                   'gui_LayoutFcn',  [] , ...
                   'gui_Callback',   []);
if nargin && ischar(varargin{1})
    gui_State.gui_Callback = str2func(varargin{1});
end

if nargout
    [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});
else
    gui_mainfcn(gui_State, varargin{:});
end
% End initialization code - DO NOT EDIT

% --- Executes just before fyp2016 is made visible.
function fyp2016_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to fyp2016 (see VARARGIN)

% Choose default command line output for fyp2016
% Choose default command line output for DICOMFiles
handles.center = 1024; handles.width = 80;
handles.output = hObject;
handles.cdir = pwd;
set(handles.DicomListBox,'enable','off');

set(handles.BrightnessSlider, 'Enable', 'off');
set(handles.ContrastSlider, 'Enable', 'off');

set(handles.editBrightness,'String', sprintf('%10s:%4.0f%%', 'Brightness', 100*get(handles.BrightnessSlider,'Value')));
set(handles.editContrast,'String', sprintf('%10s:%4.0f%%', 'Contrast', 100*get(handles.ContrastSlider,'Value')));

% Update handles structure
guidata(hObject, handles);

% UIWAIT makes fyp2016 wait for user response (see UIRESUME)
% uiwait(handles.figure1);

% --- Outputs from this function are returned to the command line.
function varargout = fyp2016_OutputFcn(hObject, eventdata, handles) 

% varargout  cell array for returning output args (see VARARGOUT);
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Get default command line output from handles structure
varargout{1} = handles.output;

% --- Executes on button press in InsertPushButton.
function InsertPushButton_Callback(hObject, eventdata, handles)
% hObject    handle to InsertPushButton (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
handles.output = hObject;

fn = uigetdir(handles.cdir,'Select directory');

if fn ~= 0   
    handles.cdir = fn;    
    img = dir(fullfile(handles.cdir,'*.dcm'));
    for x = 1 : length(img)
        handles.I{x} = dicomread(fullfile(handles.cdir,img(x).name));
    end   

      if length(img) ~= 0, set(handles.DicomListBox,'enable','on');
      else 
          set(handles.DicomListBox,'enable','off');
      end

      set(handles.NofFiles,'string',handles.cdir);
      set(handles.DicomListBox,'string',{img.name});  
      set(handles.BrightnessSlider, 'Enable', 'on');
      set(handles.ContrastSlider, 'Enable', 'on');
  end
  guidata(hObject, handles);

% --- Executes on selection change in DicomListBox.
function DicomListBox_Callback(hObject, eventdata, handles)
% hObject    handle to listbox1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% Hints: contents = cellstr(get(hObject,'String')) returns listbox1 contents as cell array
%        contents{get(hObject,'Value')} returns selected item from listbox1
handles.output = hObject;
index = get(handles.DicomListBox,'value');
MD=max(max(handles.I{index}));
ND=min(min(handles.I{index}));
axes(handles.OriginalDicom);
imshow(handles.I{index},[ND MD]);
axes(handles.HistOriginal); 
imhist(handles.I{index});
%Display Images
im = handles.I{index};
handles.Im2 = dicom2gray(im, handles.center, handles.width);
axes(handles.OutputDicom);
imshow(handles.Im2);
axes(handles.HistOutput); 
imhist(handles.Im2);
guidata(hObject, handles);

% --- Executes during object creation, after setting all properties.
function DicomListBox_CreateFcn(hObject, eventdata, handles)
% hObject    handle to DicomListBox (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: listbox controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end
return

*******And my dicom2gray function is *************

function im_gray = dicom2gray(im, center, width)

% -- convert 16-bit DICOM image into unsigned integer 8-bit based on
% window width & window center and pixel value / Hounsfield Unit(HU) -- %
% -- pixel < 0, new pixel = 0;      pixel >= 81, new pixel = 255
% -- 0 < pixel <= 80, new pixel = ceil((old pixel-min)*255/max)

min = center - width/2;
max = center + width/2; % max = min + width; 

[row, column] = size(im);
im = double(im);
im_gray = double(ones(row, column));
for a = 1:column
    for b = 1:row
        if im(b,a)<min
            im_gray(b,a) = 0;
        elseif im(b,a)>=min && im(b,a)<=max
            im_gray(b,a) = (im(b,a)-min)*255/width;
        elseif im(b,a)>max
            im_gray(b,a) = 255;
        end
    end
end
im_gray = uint8(im_gray);