Digital Image Storage

1. Digital Image Storage Thanks to the work of Dr. Perry Sprawls of Emory University and the Sprawls Educational Foundation, this material is available on-line.

2. Digital Image Storage • All digital imaging systems have one or more components (media) in or on which the digital images are stored. • Here we are representing these collectively as the “Storage Media”

3. Digital Image Storage • Later we will explore the technologies that make up storage media/ • Writing and reading refers to the process of transferring image data to and from storage media.

4. Characteristics of Storage Media • Characteristic of all storage media that must be considered include: • Capacity (number of images that can be stored) • Speed (time it take to write/record and read/retrieve images)

5. Characteristics of Storage Media • Reliability and Security (to prevent loss of images)

6. Digital Image Numerical • The numerical size of the image is a major factor with respect to image storage. • As we have discussed earlier, the numerical size is the number of pixels (determined by the matrix size) and number of bits (bit depth of each pixel).

7. Digital Image Numerical • There is a considerable range of matrix sizes used for the various digital modalities in imaging. • Each file may have different requirements determined by the imaging process and image quality requirements.

8. Types of Digital Image Storage • There are many different methods and technologies used to store digital data. • Most all fall into one of three categories, depending upon how they record the data.

9. Types of Digital Image Storage • The three types are: • Electronic • Magnetic • Optical

10. Electronic Memory and Storage • All computers and digital imaging systems have some form of electronic storage that is usually referred to as “memory”. • This form of storage or memory consists of an array of many electronic components (like transistors) that can be set to remember bits.

11. Electronic Memory and Storage • This is a form of Random Access Memory (usually called RAM) because data can be written/recorded and read/retrieved to or from any space within the memory array (random access).

12. Electronic Memory and Storage • This is to distinguish it from a storage device, like tape, where the data is stored and retrieved in sequence.

13. Electronic Memory and Storage • General characteristic of Ram include: • Fast writing/recording and reading • Relative limited capacity in comparison to other storage media. • Requires electrical power to maintain storage data.

14. Electronic Memory and Storage • The RAM is used to store images and other data on the short term. • That is while it is being recorded, reconstructed, processed or displayed.

15. Electronic Memory and Storage • If power is lost, the data is lost.

16. Magnetic Storage Media • Image data and other forms of information (such as analog audio and TV) can be stored on surfaces that can be magnetized. • The recording and reading is done as the surface moves close to a device called a head. • The magnetic surface is usually on a spinning disc or moving tape.

17. Magnetic Storage Media • Here we are looking at a magnetic disc that is an integral part of most digital computer and imaging systems. • It is referred to as a hard disk or drive to distinguish it from the flexible “floppy” disk of old.

18. Magnetic Storage Media • The basic principle is illustrated here. • As an electrical pulse representing each digital bit is applied to the write head, it produces a magnetic pulse.

19. Magnetic Storage Media • This magnetic pulse then magnetizes a small spot on the disk with a specific magnetic polarity ( on a larger scale think of these as North or South poles).

20. Magnetic Storage Media • Let us recall that a bit is recorded by marking each bit position (fill in the blank spot) in one of two possible ways. • Filling in the spot or • Leaving it Blank.

21. Magnetic Storage Media • On the digital magnetic disk each bit is recorded by magnetizing each bit position with one of two possible polarizations or directions. • Here we represent it by black and white ends of little magnets or magnetized area of the disk.

22. Magnetic Storage Media • When the magnetized bits pass under the read head, they produce electrical pulses that pass on to the processor.

23. Magnetic Storage Media • The capacity (megabytes) of a specific disk size is determined by how small the individual bit areas can be made.

24. Magnetic Storage Media • Disk technology is advancing rapidly and had advanced from the range of Megabytes to Gigabytes to Terabytes. • The number of images depends upon the numerical value of the image and the disk capacity.

25. Magnetic Disk Characteristics • The advantages of magnetic disk include the ability to rapidly read and write data. • Data no longer needed can be erased to make space for new data.

26. Magnetic Disk Characteristics • A disadvantage is that they may not have the capacity to store all of the images produced in an imaging facility. • If the data is stored on only one disk, there is the possibility for the data being destroyed in the event of a failure or “crash”.

27. Redundant Array of Inexpensive Disk (RAID) • A RAID is a configuration made up of multiple sick that are used together to make multiple copies of data (redundant copies) to reduce the possibility of loss in the case of one disk failure.

28. Redundant Array of Inexpensive Disk (RAID) • A RAID configuration is used in CT to increase the speed of data processing and management operations

29. Optical Storage Media • There are several types of digital storage media that record the data in optical form using light. • One of the most widely used forms of data storage is the Compact Disk (CD).

30. Optical Storage Media • The bit positions are located along a long spiral path running from the outer edge to near the center of the disk. • The surface of the disk is a material that reflects light.

31. Optical Storage Media • Recording is done by a laser. The laser produces a relatively intense beam of light that is directed to the bit positions as the disk rotates. • The laser beam is turned on and off by a series of pulses representing the bits to be written.

32. Optical Storage Media • If the beam is off as it passes over a bit position, the reflective surface is not affected and the area will remain as a little mirror that will reflect light.

33. Optical Storage Media • If the beam is on as it passes over a bit position, it burns the surface and destroys the ability to reflect. • The bit positions are either reflective or not reflective.

34. Optical Storage Media • When reading the image, a read laser light is projected onto the disk. • It is less intense than the write laser. • The reflected or not reflected light is converted into electrical pulses that is sent to the processor.

35. Optical Media Storage Capacity • Optical media, such as CDs, have a relatively large storage capacity because the optical recording process can form very small bits on the surface of the disk.

36. Optical Media Storage Capacity • Optical media, such as CDs, have a relatively large storage capacity because the optical recording process can form very small bits on the surface of the disk. • The CDs are easily exchangeable.

37. Optical Media Storage Capacity • One problem is that the multiple disks must be filed in some system where specific sets of images can be located and retrieved when needed. • This can be manually or with some type of automatic retrieval system.

38. Optical Media Storage Capacity • The Optical Jukebox is a device for filing optical media disks. It is connected to the image management system. • When images of a specific patient are needed for viewing, it locates the disk, inserts it into the reader and the images are made available for display.

39. Complementary Storage and Memory Devices • Most digital imaging systems use a combination of storage and memory devices. • The function of each depends upon it’s capabilities and it’s limitations.

40. Complementary Storage and Memory Devices • Random Access Memory (RAM) stores images and other data short term while they are being recorded, processed, etc. It has a limited capacity.

41. Complementary Storage and Memory Devices • Magnetic Disk store images and data that are generally available for immediate display and viewing. They might be in a RAID configuration for increased security and safety. • Magnetic disks may not have the capacity for storing and archiving large quantities of images.

42. Complementary Storage and Memory Devices • Optical Media in several different forms can be used to store and archive large quantities of images.