Digital imaging refers to the process of capturing medical images digitally and processing them on a computer. A digital image is made up of a matrix of pixels, each with a numeric value representing brightness or color. Key aspects of digital imaging include image acquisition using devices like CR or DR that convert x-rays to electrical signals, processing the image data through techniques like histogram analysis, and displaying the final image on a monitor for diagnosis. Digital images allow for manipulation through various post-processing techniques to enhance visibility of anatomical structures.

1. Fundamentals of Digital
Imaging
PRESENTED BY POONAM RIJAL
BSC.MIT 2ND YEAR
ROLL NO:-154

2. Contents
• Introduction
• Analog vs. digital image
• Features of digital image
• Basic performance parameters(Pixels, Gray scale ,Bit-depth ,Spatial
Resolution, Color ,File size)
• Digital imaging fundamentals (image acquisition , image processing ,
image display, image storage and transmission )

3. Introduction
• Digital imaging in radiography refers to the process of producing
medical images using digital technology that is based on the
conversion of x-ray photons into an electrical signal, which is then
processed and stored digitally.
• It is made up of discrete picture elements, pixels organized in a
matrix of rows and columns.
• The term was first used in 1970s with the development of CT.
• Digital imaging comprise imaging equipment such as USG, computed
radiography (CR), digital radiography (DR), CT scan, MRI, PET scan.

4. Analog vs. Digital Image
• There are two types of images, analog and digital, used in medical imaging.
• In analog image , information is represented by a physical quantity whose
allowed values are continuous.
• Thus, it can have infinite number of possible values ranging from lower limit
to upper limit.
• it include medical images recorded on a film or displayed on various display
devices like computer monitor
Analog Image

5. Digital image
• Digital images are recorded as many numbers which can be divided
into a matrix or array of small picture elements, or pixels. Each pixel
is represented by a numerical value.
• The advantage of digital images is that they can be processed, in
many ways, by computer systems.

7. Features of digital imaging
• Robustness: Do not deteriorate physically and chemically over time.
• Consistency: Allow true reproduction quality from copy to copy.
• Flexibility: Allow variety of manipulation.
• Featured contents: may be easily linked to textual descriptions and
catalogue records.
• Communicability: Access, transfer and storage of digital images is
possible within existing networks, computers, etc

8. Basic performance parameters
1. Pixels
2. Gray scale
3. Bit-depth
4. Spatial Resolution
5. Color
6. File size

9. Pixel
• A pixel short for “picture element” is the smallest unit of a
digital image.
• A digital image stored in a computer is rectangular in format
and made up of small squares called pixels.

10. Pixel
• Each pixel can be only one color at a time , however pixels blend
together to form new colors.
• The number of colors each pixel can be is determined by the binary
number called bits used to represent it.
• 8-bit colour allows 28 colours to be displayed
• 16-bit colour = 216 = 65,536 colours
• Every pixel has a varied set of tones. These tones are black or white,
or shades of gray or color.
• All the individual pixel with their tonal value combine collectively to
create a digital image.

11. Pixel size
• Directly affects image spatial resolution. The smaller the pixel, the
better the resolution.
• Pixel size in CR range from 50 to 200 microns, in DR 100-200 microns
and FFDM 50-100microns
• Pixel size may change
• When matrix size changes
• The field of view (FOV) changes

14. Image matrix
• It is essentially a grid of picture element called pixel arranged in rows
and columns creating small areas having special numerical value.
• It is a fundamental concept that forms the digital image.
• The typical number of pixels in a matrix ranges from about 512 × 512
to 1024 × 1024 and can be as large as 4800 × 6000.
• the size of the matrix determines the size of the pixels For example, if
you have a 10 × 12 and a 14 × 17 computed radiography (CR) cassette
and both have a 512 × 512pixel matrix, then the 10 × 12 cassette will
have smaller pixels.

15. FOV
• It is the size of the area to be imaged.
• a larged fov means a larger area to be imaged.
• When the field of view (FOV) is reduced, but not changing the matrix
size, the pixels become smaller and the visibility of detail is improved.

17. Pixel Bit depth
• The pixel bit depth is the number of bits that indicates the different
shade of the gray used to define each pixel.
• A one bit (2^1) image can display only 2 shades of gray i.e. black &
white & such an image will display very little or no more information.
• Most digital image use a dynamic range of 8,10 or 12 bit meaning a
gray scale range 2^8 , 2^10 , or 2^12.
• Higher bit depths generally result in better color fidelity and image
quality with better contrast resolution.

20. Grayscale (dynamic range )
• The range of values over which a system can respond is called its
grayscale range or dynamic range.
• These shades of gray that compose an image range from minimum to
maximum intensity.

21. Grayscale
• Dynamic range of the image doesn’t equate to higher spatial
resolution. The greater the dynamic range, the better the contrast
resolution.
• For acceptable contrast resolution on radiographic CT and MR images,
a 12 bit dynamic range is usually required.
• For routine visual display grayscale images contain 8 bits per sampled
pixel which allows 28 or 256 intensities to be recorded.

22. Spatial Resolution
• Ability of imaging system to distinguish between small structures or
details in the patient anatomy .
• Spatial resolution in digital acquisition systems is expressed as pixel
size, pixels/mm or line pairs per mm (lp/mm)
• Resolution is governed by the term called spatial frequency which is
used while sampling a digital image
• An increase in sample frequency helps to increase resolution
• Images composed with a greater no. of pixel have a higher spatial
resolution- hence require larger file sizes.

24. Contrast Resolution
• It refers to the amount of grayscale or color differentiation that exists
in an image.
• For digital image acquisition equipment, this means the number of
shades of gray that a detector can capture.
• In digital imaging due to their wide dynamic range, a wide range of
low-to-high signal intensities can be captured and due to their high
contrast resolution, thousands of shades of gray can be displayed.

25. Color
• the bulk of radiology practice is performed in grayscale, there is a
growing trend towards using virtual color, in color Doppler, CT, and
MRI, particularly during post-processing.
• A color model called RGB (red, green, blue) is commonly used for
generating on-screen color in televisions, computer monitors, and
film recorders.
• In this model, also called as an additive color model, three different
phosphors are displayed on a monitor screen.
• Here the pixel is a red, green, and blue phosphor dot, which when
struck by an electron beam causes it to emit light of the same colour.

26. File size
• File size is the final outcome, indicating the size of information data
representing an image.
• It is a mathematical relationship integrating the total number of
pixels in an image with grayscale or color depth.
• Three additional parameters can influence file size namely,
1. file format
2. amount of data compression and
3. cropping to include the relevant region of interest (ROI).

27. File size
• File size is essentially dependent on pixel dimensions and bit-depth.
• Predictably, it is derived by multiplying the (number of horizontal
pixels) × (number of vertical pixels) × (number of bits in shades of gray
or color, i.e., bit-depth).

28. File size
• File size is represented by two elementary units of measurement,
namely a bit and byte.
• A bit is the smallest measurement indicating '1 or 0' or 'on or off.'
• Eight such bits comprise a byte.
• When file size increases further, units such as kilobyte (1024 bytes),
megabyte (1024 KB), and gigabyte (1024 MB) are used.
• File sizes larger than these are represented by terabytes (1024 GB),
which is relevant, in practice, for picture archival and communications
system (PACS) and large data storage.

30. Image compression
• Image compression is the process of reducing the numerical size of
digital images.
• Transmitting and archiving this amount of data is technically difficult;
therefore, compression techniques are used.
• The level of compression is the factor by which the numerical size is
reduced. It depends on the compression method and the selected
level of compression.
• Lossless
• Lossy

31. Lossless compression
• compression is when there is no loss of image quality, and is
commonly used in many medical applications.
• An image file that is compressed in a lossless mode is one that can be
reconstructed to be exactly the same as the original image.
• Lossless compression reduces the data file to 10% (10:1) to 50% (2:1)
of the original file.
• Lossless compression up to 3:1 generally is considered acceptable
and helpful in digital radiographic image management.

32. Lossy compression
• compression results in some loss of image quality and must be used with
care for diagnostic images .
• compression greater than 10:1 can be considered lossy and it can provide
compression factors of up to 100:1 or greater.
• Such a level of compression supports teleradiology but not computer-aided
detection (CAD) or image archiving.
• Compressed images may cause the CAD system to miss lesions because of
the compression artifact, which actually represents a lack of data.
• Lossy compression is not acceptable for archiving mammography images
for this reason

34. Human vision compared to digital devices
• Initial observations proposed that the human eye could distinguish 30
to 50 shades of gray.
• It is now established that the human eye can discriminate 'between
700 and 900 simultaneous shades of gray for the available luminance
range of current medical displays and in optimal conditions.'
• It is presently felt that it is of 'no use to simultaneously display more
than 10 bits of gray (1,024 gray shades) because this already exceeds
the capabilities of the human visual system.’

35. Human vision compared to digital devices
• Images in radiology produced by x-ray detectors, CT scans, etc.,
contain 12 to 16 bits/pixel, corresponding to 4,096 to 65,536 shades
of gray, while images visualized by medical displays in general, have
much lower number of gray shades, often in the range of 8 bits or 256
shades of gray per pixel

36. Digital imaging
The fundamentals of digital imaging in radiography include :-
1. Image acquisition
2. Image processing
3. Image display
4. Image storage
5. Image transmission

37. Image acquisition
• In Digital imaging three components capture element , coupling
element and collection element are essential for image acquisition.
1. Capture element:- x-rays are captured by PSP in CR , CsI ,Gd-Os or a-
se in DR.
2. Coupling element :- transfers x-ray generated signal to the collection
element . The coupling element may be lens or fiber optic
assembly, a contact layer or a-se.
3. Collection element :- the collection element may be a photodiode, a
CCD or TFT.

41. Formation of the digital image (digitization)
• The formation of digital image involves several steps, beginning with
analog processing to converting it into a digital format .
• Each pixel consists of individual electronic detector absorb x-ray
generate small voltage proportional to amount of x-ray absorbed by
each individual pixels within the detector.
• The voltage fluctuation at each pixel is known as ‘analog signal ‘
• To make it compatible with digital system , the signal undergoes ADC (
Analog to Digital).

42. Analog to digital conversion
• The analog data obtained from detector or transducers must be
converted to digital forms if they are to be processed by computer,
transferred over a network, or shared on a digital storage device- such
device called ADCs- the conversion called digitization
• Digitization has 2 steps:
i) Sampling
ii) Quantization

44. Image processing
• A principal advantage of digital radiographic imaging over screen-film
radiographic imaging is the ability to manipulate the image before
display—preprocessing— and after display—post processing.
• Preimage processing and post image processing alter image
appearance, usually for the purpose of improving image contrast.
• The x-ray photons converted into electrical signal undergo processing
and manipulation with the help of computer system.
• In radiology,processing of a digital image involves several steps to
enhance image quality for accurate diagnosis

45. Pre-processing
• Preprocessing of a digital image is largely automatic that takes place
in the computer where the algorithms determine the image
histogram.
• Pre-processing is designed to produce artifact-free digital images.

46. Histogram Analysis
• Is an image processing technique used to identify the edges of the
image and assess the raw data prior to image display
• Histogram is a graphical representation of pixel intensity distribution
for a digital image
• The computer analyzes the histogram using processing algorithms
and compares with a pre-established histogram specific to the
anatomic part being imaged

47. Histogram Analysis
• The computer software has stored histogram models, each having a
shape characteristics of the selected region and projection
• These histogram models have VOI, which determine the range of
histogram data set that should be included in the displayed image
• Histogram analysis is employed to maintain consistent image
brightness despite over and underexposure of the IR-procedure
known as rescaling.

49. Pre-processing
• preprocessing provides electronic calibration to reduce pixel-to-pixel,
row-to-row, and column-to-column response differences. The
processes of pixel interpolation, lag correction, and noise correction
are automatically applied with most systems.
• Automatic calibration images designed to make the response of the
image receptor uniform are called Offset images and gain images.
• These preprocessing calibration techniques are identified as
flatfielding.

50. Pre-processing
• signal interpolation- corrects the defective individual pixel The
response of pixels surrounding the defective pixel is averaged, and
that value is assigned to the defective pixel.
• image lag is seen which can be troublesome when one is switching
from high-dose to low-dose techniques can be solved application of
an offset voltage before the next image is acquired.
• line noise along the buses that drives the pixel can cause linear
artifacts to appear on the final image. The solution is to apply a
voltage correction from a row or a column of pixels in a dark,
unirradiated area of the image receptor.

51. Post processing
• Post processing requires intervention by the radiologic technologist
and the radiologist.
• Post processing refers to anything that can be done to a digital
radiographic image after it is acquired by the imaging system.
• Postprocessing of the digital radiographic image is performed to
optimize the appearance of the image for the purpose of better
detecting pathology

52. Addition and subtraction of a digital image
• Addition or subtraction of images require the images to be of same
format and the resulting image is a sum image or difference image in
that same format.
• In image subtraction , each pixel in one image is subtracted from the
corresponding in a second image yields the corresponding pixel value
in the different image.
• Addition is similar to subtraction but with pixel by pixel addition
instead of subtraction.
• Image subtraction is used in DSA to remove the effects of stationary
anatomic structure not of interest from the images of contrast
enhanced blood vessels

53. Spatial filtering
• Medical images commonly have a grainy appearance-quantum
mottle, caused by the statistical nature of the acquisition process
• The visibility of quantum mottle can be reduced by a spatial filtering
operation called smoothing but blurs the image
• In most spatial filtering algorithms, each pixel value in the smoothed
image is obtained by a weighted averaging of the corresponding pixel
in the unprocessed image with its neighbors
• Also enhances the edges of structure in an image

54. Other Post processing techniques
• Annotation is the process of adding text to an image helps in patient
identification informs clinician about anatomy and diagnosis.
• Digital imaging allows magnification of a region of an image to
render the smallest detail visible.

55. Other Post processing techniques
• At times, multiple digital images must be flipped horizontally or
vertically. This process, called image flip, is used to bring images into
standard viewing order.
• image inversion, which results in a black appearance of bone and a
white appearance of soft tissue .

56. Other Post processing techniques
• Greater use is being made of quantitative imaging, that is, use of the
numeric value of pixels to help in diagnosis. This requires identifying a
region of interest (ROI) and computing the mean pixel value for that
ROI.
• Edge enhancement is effective for fractures and small, high-contrast
tissues.
• Highlighting can be effective in identifying diffuse, nonfocal disease.
• Pan, scroll, and zoom allows for careful visualization of precise
regions of an image.

57. Windowing and leveling
• The ability to window is a valuable feature of all digital images.
• Windowing and leveling is a common method for contrast
enhancement.
• It permits the viewer to use the entire range of display intensities to
display just a portion of the total range of pixel values.
• Most display workstations have level and window controls.
• The level control determines the midpoint of the pixel values to be
displayed and
• the window width control determines the range of pixel values about
the level to be displayed.

59. Image Display
• Display interfaces send the image information in digital form to the
display monitors; these monitors contain DACs to convert the digital
pixel values to analog signal which is displayed on a monitor .
• Image display permit technologists to visually assess the adequacy of
acquired images, for physician to interpret, or to guide radiologists
performing interventional procedures.
• cathode ray tubes (CRTs) were used for the electronic display of
images which are being replaced by flat-panel displays.
• Several types of flat panel displays are gas plasma, light emitting
diode, organic light emitting diode, and field emissive displays

60. Image Display
• Most flat panel displays used today have LCDs because of their
superior performance and longevity
• LCDs provide higher small spot contrast ratio and a larger dynamic
range and are less vulnerable towards ambient light than CRTs
displays
• A limitation is that contrast resolution drastically decreases when
visualized from an angulated position

61. Display of image contrast and contrast
enhancement
• A display system must be able to display a range of pixel values from
the minimal to the maximal value in the image
• For displaying the image, a range of light intensities is available, from
the darkest to the brightest ,for which the display monitor is
calibrated .

62. Display of image contrast and contrast
enhancement
• Lookup tables (LUTs) are commonly used by medical image processing
and display computers to affect the display of image contrast.
• A LUT is simply a table containing a value for each possible pixel value
in an image.
• In practice, each pixel value in the transformed image is determined
by selecting the value in the LUT corresponding to the pixel value in
the unmodified image
• a digital radiography system may use a LUT to modify the acquired
pixel values, which are proportional to the detected x-ray signals, to
cause the display of contrast to resemble that of a film .

64. Image storage and transmission
• Mass storage devices permit the non-volatile storage of information
• Mass storage devices have mechanical components that include
magnetic disk drives, magnetic tape drives, optical (laser) disk units-
CD, DVDs
• Electronic storage modality- flash memory
• A technology called RAID can provide a large amount of on-line
storage. RAID permits several or many small and inexpensive hard
magnetic disk drives to be linked together to function as a single very
large drive

65. PACS
• Is a system for the storage, transfer and display of radiological images
• Consists of a digital archive to store medical images, display
workstations to permit physicians to view the images and a computer
network to transfer images and related information between the
imaging device and the archive and between the archive and the
display workstations

66. PACS
• DICOM is the standard format for storing, transmitting and sharing
medical image and related information in digital format that ensures
interoperability and compatibility between different medical imaging
devices and software systems

67. PACS
• For efficiency of workflow and avoidance of errors, the PACS should
exchange information system with other information system such as
HIS, RIS, EMR system
• Many PACS incorporate teleradiography. Teleradiography is the
transmission of such images for viewing at a site or sited remote from
where they are acquired

68. SUMMARY
• The current practice of radiology across the world, involves extensive
handling of digital images, often in black and white and varying shades of
gray.
• Digital imaging was introduced to reduce the problems of conventional film
screen system which has better storage capacity More efficient in time &
personnel Wide range acquisition high flexibility and consistency .
• At its very core, digital imaging is governed by a few important
performance parameters like pixels, grayscale, bit-depth, resolution, color,
and fi le size.
• The fundamentals of digital imaging are image acquisition, image
processing, image display, image storage and transmission.

69. References
• The Essential Physics Of Medical Imaging by Bushberg.
• Radiologic Science For Technologists by Bushong.
• Digital imaging in radiology practice: An introduction to few
fundamental concepts, article in The Indian Journal of radiology and
imaging, November 2007
• Previous presentations

70. Thank you!!