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 via 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.

However, it can be done in a much easier and better by using the ROIMean function of DicomLabel as demonstrated in the code below.


public DicomLabel CreateRoiLabel()      // this is run once
    // create a label
    return new DicomLabel()
        LabelType =  LabelType.Rectangle,
        Width = 1,   // one pixel big
        Height = 1,
        ScaleMode = ScaleMode.Image, // must be image tied for ROI mean to work
        Visible = false,
        Left = 1,
        Top = 1,                

public double CalculatePixelHU(int x, int y)
    PointF p= Viewer.ImagePosition(new Point(x, y));	//convert viewer coordinate to image coordinate
    DicomLabel l = CreateRoiLabel();
	  return l.ROIMean(Viewer.CurrentImage, true);


    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	

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