Which parameter describes the number of bits used for each pixel in a CT image?

The parameter that describes the number of bits used for each pixel in a CT image is known as pixel depth. Pixel depth is crucial as it defines the color or grayscale level that each pixel can represent. For instance, a pixel depth of 8 bits means that each pixel can represent 256 different shades, whereas a pixel depth of 16 bits allows for 65,536 possible shades. This greatly influences the image quality and the ability to discern fine details within the CT images.

The other options do not accurately define this parameter. Bit rate refers to the amount of data processed per unit of time and is not directly related to the number of bits per pixel. Matrix size relates to the dimensions of the image in terms of the number of pixels along the width and height, but it does not indicate the bit depth per pixel. Pixel density, on the other hand, refers to the number of pixels per unit area, which is more relevant for understanding image resolution rather than the depth of color or grayscale each pixel contains.