Whilst the DICOM display settings in support us in viewing certain anatomy, this may not be adequate when revealing certain tissue or bone in the scan. Let’s say you have loaded your scan in, and want to start going in and revealing certain tissue or bone within the patient. The ‘Density Display’ sliders can help us in revealing different densities within the scans anatomy.
Hounsfield Units and DICOM Density Settings
The Hounsfield Scale is a quantitive scale constructed by Sir Godfrey Hounsfield. The Hounsfield Scale uses “Hounsfield Units” (HU) which are values used to measure the density of certain anatomy within a subjects body. The use of Hounsfield Units in the density display settings, aids us in revealing specific tissue, muscles, bones or the overall structure of the subject. HU can be used in any dicom or medical imaging file; some of these include computed tomography scan (CT image), magnetic resonance imaging or mr images (MRI), Xray or mammography (mammograms). The Hounsfield Units are found within the ‘Density Display’ sliders and consist of a “Lower/Upper Window” & “Lower/Upper Threshold”.
The use of the Hounsfield Units and the degree to which we use the density display settings depend on how we would like to analyse our subject. The use of the hounsfield display sliders are essential in medical imaging and scan analysis. Some of the most common uses when implementing the hounsfield sliders include identifying cancers (breast cancer, skin cancer, brain cancer) that may be hidden by other layers, identifying tracts or clots (white matter tracts) or detection of specific muscle alteration (hypertrophy, atrophy) all within a patient. Tissues such as air, water and fat have low attenuation resulting in dark areas and low values on the hounsfield scale. On the contrary, dense regions in the dicom image such as bone have high attenuation and high values on the hounsfield scale.
For a basic guide on which anatomy starts to be revealed at different values, refer to the information below.
- Air around the subject is revealed at -1000
- Lung within the subject is revealed at -500
- Fat within the subject can be revealed from -100 to -50
- Soft Tissue within the subject can be revealed from 100 to 300 (other parts of the medical image anatomy may include blood vessels, lesion, cancer, vertebra)
- Bone can be revealed from 700 to 3000
Note: Changing of Hounsfield Units will depend on the subject and the scan. Please only refer to this as a general guide.
How to use the Density Display Sliders
The density display sliders are very simple to use. To begin changing the density values to reveal specific anatomy, we can use the two sliders located under ‘Density Display’ within the ‘Expose’ window. The two sliders named “Lower/Upper Window” and “Lower/Upper Threshold” will aid us in this process.
Lower / Upper Window slider controls the range of Hounsfield values that we would like to specify. This slider will hide anything less dense than the lower part of the slider and anything more dense than the higher value on the slider. For example, let’s say we would like to reveal only the bone within our scan. We would place the lower part of the slider around 700 HU and the upper part around 3000 HU using the basic guide given. This will reveal only the bone in our scan. The result of a scan will differ between patients so the use of these sliders will be a matter of trial and error.
Lower / Upper Threshold slider may seem like it is the same as the above slider, however serves a different purpose. The threshold slider hides or reveals content on the value of black or white. As the HU values are mapped to black and white to allow us to see the difference between densities more clearly, this slider incorporates brightness, contrast, opacity and the HU values- rather than just the HU value alone. Whilst the ‘Window Slider’ may reveal specific densities within the scan, we recommend using the ‘Threshold Slider’ to attain an overall better analysis of the subject.
We urge you to use both the ‘Window Slider’ and ‘Threshold Slider’ simultaneously to achieve the best possible result when analysing a scan. Using both the window slider and the threshold slider will not only increase your segmentation and analysis, but will also provide more clear insight to the patient and anatomy.
Congratulations, now you’ve exposed the anatomy you’re interested in, you can enhance the view with brightness, contrast and opacity settings.