What determines image storage size?
Image size, generally expressed in megabytes, is determined by spatial resolution and bit depth. Spatial resolution for a two-dimensional (2D) image is defined by a matrix of horizontal and vertical pixels. A single image in a typical CT scan is composed of a matrix of 512 vertical pixels × 512 horizontal pixels, whereas a chest radiograph image might have a matrix size of 2500 vertical pixels × 2000 horizontal pixels. For a given anatomic area of interest, images with a larger matrix size have greater spatial resolution.
Bit depth is defined by the number of shades of gray within the image, where 2 n equals shades of gray and n equals bit depth. An image with a bit depth of 1 has 2 shades of gray (pure black and pure white). A 6-bit image contains 64 shades of gray; 7-bit, 128 shades; 8-bit, 256 shades; and 12-bit, 4096 shades. Most diagnostic-quality digital images in CT, MRI, and computed radiography/digital radiography are displayed in 10 or 12 bits.
The file size of an imaging study also depends on the number of images in that study. A chest radiograph may have 2 images (posteroanterior and lateral), whereas a CT scan of the abdomen may have 50 images. With the advent of multidetector row CT scanners, it is now possible to acquire thinner slices in much less time, often resulting in much larger studies. This capability also allows images to be reconstructed in different planes. All of this contributes to an increased number of images and, overall, larger study sizes for storage. A CT angiogram may contain 500 to 1000 images or more.
How large are these studies?
The below table shows approximate matrix and file sizes for various imaging modalities. These values vary depending on bit depth, number of images acquired, and compression technique.
Approximate Matrix and File Sizes for Various Imaging Studies
|MODALITY||Image Matrix||Images||File Size (in MB)|
|Multidetector row CT||512||512||500||200-1000||250||100-600|
|Digital fluoroscopy (without digital subtraction angiography [DSA])||1024||1024||20||10-50||20||10-50|
|Digital fluoroscopy (with DSA)||1024||1024||150||120-240||450||360-720|