def splitter_base_comparison(comparison_scan, original_df, roi):
    """Splits the original csv into two dataframes - base scan and comparison scan for ROI.
    
    Arguments:
    comparison_scan = comparison week scan in pandas dataFrame
    original_df = full original csv in pandas dataFrame
    roi = string of region of interest, ie. "Bladder"
    
    """
    # get indices for region of interest
    df_idx = comparison_scan.index[comparison_scan['roi'] == roi].tolist()
    
    # get base week df
    df_base = original_df[original_df['roi'].str.match(roi)]
    df_base = df_base[df_base['R.x'].eq(0)] # get base week data
    
    return df_base, comparison_scan.loc[df_idx]