Computers in Radiology

1

What is PACS?

PACS stands for Picture Archiving and Communication System. On the most basic level, PACS integrates image acquisition modalities, workstation displays, the image archiving system, and the underlying network.

2

How are PACS images stored?

The PACS archive traditionally has been composed of short-term and long-term storage. Short-term storage is usually composed of redundant arrays of inexpensive (or independent) discs (RAID) arrays that provide quick access to image data. After a certain amount of time (3 to 30 days, depending on the size of the short-term archive), images from the short-term archive are moved to the long-term archive, which is usually composed of magnetic tape or magneto-optical media. Images cannot be viewed directly from the long-term archive. Instead, images need to be “fetched” from the long-term archive and copied back to the short-term archive before being viewed on a workstation. This compromise was made because of the high cost of RAID storage. The cost of RAID storage has decreased enough more recently so that several PACS archives are now being designed as “always online” systems. These new systems are composed only of RAID arrays, which essentially place all images in the short-term archive and eliminate the need for fetching.

3

What is image compression?

Image compression is the process of reducing image file size using various mathematical algorithms. Compression is usually expressed as a ratio (e.g., 10 : 1). A 10-megabyte (MB) file that is compressed at a ratio of 10 : 1 would have a final size of 1 MB. Generally, as compression ratios increase, file sizes decrease. However, a price is inevitably paid in decreased image fidelity.

4

What is the difference between “lossy” and “lossless” compression?

Encoding an image is a process that converts a raw image (e.g., the original radiograph) into a more compact coded file. Decoding converts the coded file to a decoded image. If the raw image and the decoded image are the same, the compression method is considered lossless. If there is a difference between the raw image and the decoded image, the method of encoding and decoding is considered lossy. Lossless compression can usually achieve ratios of 2 : 1 or 3 : 1. Lossy compression can achieve much higher compression ratios; however, overcompression may destroy fine detail, making the image unacceptable for diagnostic purposes.

5

What is RIS?

RIS stands for Radiology Information System. RIS is the core system responsible for managing workflow within a radiology department. Tasks include patient scheduling and tracking, examination performance tracking, examination interpretation, billing, and handling of radiology reports including result distribution. RIS must work optimally with the PACS to properly enable information sharing. Over the past several years, with the evolution of radiology to filmless environments, RIS capabilities have increased tremendously in addition to merging of RIS/PACS interdependent functionality.

6

What is HIS?

HIS stands for Hospital Information System. HIS manages patient demographics; insurance and billing; and often other clinical information systems, including laboratory results, physician orders, and electronic medical records.

7

What is DICOM?

DICOM stands for Digital Imaging and Communications in Medicine. It is a standard that establishes rules that allow medical images and associated information to be exchanged between imaging equipment from different vendors, computers, and hospitals. A computed tomography (CT) scanner produced by vendor A and a magnetic resonance imaging (MRI) scanner produced by vendor B can send images to a PACS from vendor C using DICOM as a common language. In addition to storing image information, other DICOM standard services include query/retrieve, print management, scheduling of acquisition and notification of completion, and security profiles.

8

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.

9

How large are these studies?

Table 5-1 shows approximate matrix and file sizes for various imaging modalities. These values vary depending on bit depth, number of images acquired, and compression technique.