Hounsfield Units


Hounsfield Units (HU) are used in CT images it is a measure of radio-density, calibrated to distilled water and free air.

HUs can be calculated from the pixel data with a Dicom Image vir two methods.

The first (and harder) method involves using the Slope and intercept value from the Dicom image and applying it to a target pixel.

HU = m * P + b

where: m is the Dicom attribute (0028,1053) "Rescale slope" b is the Dicom attribute (0028,1052) "Rescale intercept" P is the value of that particular pixel in the pixels array.

 Public Function CalculatePixelHU(ByVal x As Integer, ByVal yAs Integer) As Short 
 
  'get the slope and intercept
  Dim m As Short = CType(Viewer.CurrentImage.Attributes.Item(&H28, &H1053).Value, Int32)
  Dim b As Short = CType(Viewer.CurrentImage.Attributes.Item(&H28, &H1052).Value, Int32)

  'Move pixels into an array and select the pixel under the mouse
  pixels = Viewer.CurrentImage.Pixels 'where pixels is a Private Object
  Dim xo = Viewer.ImageXPosition(x, y) 'where x & y are the mouse coordinate on the viewer
  Dim yo = Viewer.ImageYPosition(x, y)
   
  'calculate HU
  Return pixels(xo, yo, 1) * m + b
End Function


Alternately the better solution is to allow DicomObjects to handle the calculation for you. The ROI mean function will return a HU value without having to add in the slope and intercept.

Here we are creating a rectangular label, 1 pixel big

Public Function CreateRoiLabel()                       ' this is run once
  If RoiLabel Is Nothing Then                          ' where RoiLabel is a DicomLabel (global)
    RoiLabel = New DicomObjects.DicomLabel             ' create a label 
    RoiLabel.LabelType = DicomObjects.doLabelType.doLabelRectangle
    RoiLabel.Width = 1                                 ' one pixel big
    RoiLabel.Height = 1
    RoiLabel.ImageTied = True                          ' must be image tied for ROI mean to work
    RoiLabel.Visible = False    
    RoiLabel.Left = 1
    RoiLabel.Top = 1
    currentImage.Labels.Add(RoiLabel)                  ' add the label to the current image
   End If
End Function

We then place the label at the mouse location and then get DicomObjects to calculate the Hounsfield Units

Public Function CalculatePixelHU(ByVal x As Integer, ByVal y As Integer) As Short
  RoiLabel.Left = Viewer.ImageXPosition(x, y)         'convert viewer coordinate to image coordinate 
  RoiLabel.Top = Viewer.ImageYPosition(x, y)          'and move RoiLabel to mouse 
  Return (currentImage.Labels(currentImage.Labels.IndexOf(RoiLabel)).ROIMean())
End Function