DICOM display

The DICOM display dialog box is used to control the display and calibration of 16-bit data.

Most DICOM files contain calibration values for converting the raw data to Hounsfield Units (HU). The raw values which correspond to 'air' or 'water' are displayed in this dialog box. If the conversion is wrong (or you are using different data, for instance MRI) then these values can be used to perform any linear conversion of the raw data to the display data.

In order to display HU data, it also needs to be 'windowed', i.e. a specific range of the data values are selected for conversion to the 8-bit range which is actually displayed as an image. This is achieved by specifying a window centre (the CT value that will be displayed mid-grey) and a window width (the range of CT values which will be displayed, with the lowest in this range displayed as black, and the highest as white). This windowing can either be done using the rotate tool, or by using the first two sliders in this dialog.

The DICOM filter extent slider applies a Gaussian filter to the raw data during conversion to display data, which can help to smooth the data if it is particularly noisy.

When changing the windowing or filter, the 8-bit data has to be re-created from the raw (usually 16-bit) data, which can take a while if you are currently displaying a reslice.

If the CT data includes a scan of a bone density phantom, Stradview can also calculate the conversion between the stored Hounsfield Units and bone density in mg/cm3. The location of the phantom in the data is determined by using the first four landmarks. The first two landmarks should be placed near the centre of the rods at each edge of the phantom in the first CT image in which the phantom fully appears, except for the Image Analysis Solid HA phantom. For this phantom, the landmarks should be placed on the rod with the highest density, and the rod with the second-to-lowest density: the lowest density rod is not visible since it looks the same as the background material. The next two landmarks should be placed at the same locations, and in the same order, in the last CT image in which the phantom fully appears. Having done this, pressing the 'Calibrate for BMD' button will calculate the calibration parameters.

Stradview currently recognises the Mindways liquid and solid K2HPO4 phantoms, the Siemens Osteo HA, the Image Analysis Solid HA and the QRM Bone Density 3-insert and 6-insert HA phantoms. If you are not using one of these phantoms, or if Stradview does not detect the phantom which you are using, the calibration values will be incorrect. You can verify whether the calibration is reasonable by looking at the various estimated rod values in the dialog box showing calibration results.