Intensity Calibration for BMD and TMD Computations

Measurements of bone mineral density (BMD) and tissue mineral density (TMD) require calibrated bone data, which is calibrated by means of phantoms with a known density of calcium hydroxyapatite (CaHA). Calibration can be done beforehand (see Intensity Scale Calibration) and is also available on demand in the Bone Analysis Wizard by clicking the Calibrate Intensity Now button in the Global Measurements dialog. You can then calibrate the data in the Intensity Calibration dialog, shown below. The dialog includes preset calibration points for air, water, and CaHA densities.

Intensity Calibration dialog

In the Intensity Calibration dialog, you can:

Select the required calibration points. For example, water and your CaHA phantom.
Probe the selected dataset to extract the mode of the raw intensity values. Mode is the intensity value that corresponds to the center of the bin having the maximum element count. The histogram is built with 1024 bins spread over the range of data of the probed dataset.
Adjust the probed values with the indicators on the histogram.
Toggle between raw intensity values and calibrated intensity values.

How to Calibrate the Intensity Scale for BMD and TMD Computations

In the Global Measurements dialog, select a measurement for bone mineral density (BMD) or tissue mineral density (TMD).
Select the dataset from which BMD or TMD will be computed in the Bone dataset drop-down menu, which is available at the bottom of the Global Measurements dialog.
Click the Calibrate Intensity Now button.

The Intensity Calibration dialog appears.
Choose the two labels that will provide the calibration points as follows:
- Double-click inside the Selection column and then select the lower value calibration point. For example, water.
- Double-click inside the Selection column and then select the upper value calibration point. For example, HA1200.
Do the following for each selection:
- Select the Probe tool, as shown below, for the required calibration point.
- Extract the 'mode' of the raw intensity values of the selection by clicking on your image.
  
  Note You can increase or decrease the size of the spherical brush with the mouse scroll wheel or with your configured actions to Decrease/Increase probe size (in Intensity Calibration).
  
  Note You can also move the markers on the histogram to update the raw intensity value of the selected labels.
- You can also measure mean raw intensity values with the Probe tools available on the Probe panel (see Using the Probe Tools). You can then enter the measured values as the raw intensities.
Click the Calibrate button to calibrate the selected dataset.

The dataset is calibrated and the current unit is updated to Bone Mineral Density (mg.cm^-3).

Note After your image is calibrated, you can toggle between the raw intensity values and the calibrated intensity values by selecting the required scale in the Presentation scale box.
Close the Intensity Calibration dialog.