Image Viewer > Advanced Viewing > Digital Subtraction

This is a premium feature and may not be available in all installations.

Digital subtraction enhances the visibility of a contrast agent in a series of images by masking each image by one or more pre-contrast images. Currently, only greyscale CT, MR, RF and XA images are supported.

Digital Subtraction Angiography is commonly used to enhance the visibility of blood vessels in the image.

Activating Subtraction

To enable digital subtraction mode, select a mask from the Digital Subtraction submenu on the toolbar or context menu. The following initial masks are available:

•	Use First Image as the Mask. This will create and apply a new mask using only the first image in the display set.

•	Use Current Image as the Mask. This will create and apply a new mask using only the currently selected image in the display set.

•	DICOM Masks. This will apply a mask that was defined by the modality and stored in the image's DICOM header.

To disable digital subtraction, simply reopen the submenu and select Reset.

To start Digital Subtraction, select from the list of available masks.

Modifying the Mask

Once in digital subtraction mode, the mask can be further refined by adding or removing additional images to the mask. The Workstation will then, for each pixel location, subtract the average of the pixel across all images in the mask from the pixel in the current image.

Noise and other artifiacts may be averaged out by adding and removing images to the mask.

Measuring Pixel Differences

You can use the Measurement Tools on subtracted images to quantify the difference between the viewed image and the mask.

The pixel values of the subtracted images represent the pixel value of the image, less the value of the corresponding pixel in the mask image (or average of the values of the corresponding mask pixels, if more than one mask image is used). Any modality LUTs present in the original images (whether the image being viewed or being masked) are applied before computing the difference. Positive values thus indicate that the pixel value is greater than the mask value, and negative values indicate that the pixel value is less than the mask value.

Measurements on the subtracted image represent the difference between the original pixel values and the pixel values of the mask.

Technical Notes

•	The subtracted image is always signed, and the bit depth is always one greater than that of the original images up to a maximum of 16 bits.

•	Pixel differences are limited to the range of signed 16-bit values. A subtracted pixel value of 32767 indicates that the difference is greater than or equal to 32767, while a subtracted pixel value of -32768 indicates that the difference is less than or equal to -32768.