Chapter 24: Appendix IDigital Image Processing

What is Digital Imaging?

Digital Imaging is the transforming of energy: (from light photon, sonic, magnetic, x-ray, or gamma radiation sources) to electrical signals that are measured and assigned discrete binary values.

Binary data is processed into image information which may be enhanced, printed, displayed on a monitor, and stored as a computer file.

Digital Modalitites

Every imaging modality may be digital. CT and MRIare only digital.

From an equipment standpoint, the major differencebetween the modalities is the type of energy used,how the energy is changed as is traverses the body, and how the remnant energy is measured as it leaves the body.

Computed Tomography (CT)

X-radiation passes through,and is attenuated.

Is only digital

Cardiovascular Interventional Technology (CVI) digital application started in the 1980s

X-radiation passes through,and is attenuated

Magnetic Resonance Imaging (MRI)

Is only digital

Hydrogen atoms excited by radio frequencies (RF)create magneticvectors that sweepan antenna.

Nuclear Medicine Technology

An isotope is injected, ingested or inhaled. After being metabolized, concentrations of the isotope are collected by the nuclear medicine camera, which was originally viewed on a scope. Nucsare now digital.

Diagnostic Medical Sonographyand Vascular Technology

Sound waves pass into, andare reflected off of interfacesof tissues and organs. Virtuallyall ultrasound is digital today.

Digital Radiography (DR)

X-radiation passes through,and is attenuated

Digital applications were available in the early 1980s, but the difficulties of displaying radiographic quality (in terms of spatial resolution) limited its use. By 2000 high resolution systems becameIncreasingly popular. By 2006 they have become the standard..

Digital Mammography

X-radiation passes through, and is attenuated Like digital radiography, highly dependant on excellent spatial resolution: evenmore so.

Question: How is an analog radiographic image created?

• Begin with photons coming off the anode.

• Outline the process, as each major step.

• Use the appropriate terminology.

Answer: How is an analog radiographic image created?

• Incident beam leaves anode.• Attenuation in body.• Remnant radiation exits as the aerial image.• Photons interact with silver halide crystals.• Latent image is formed.• Latent image is manifest on development.

Question: What does a graphic representation of density building on a film look like, and what is it

called?D log E (or)H & D Curve (or)Hurter & Driffield Curve (or) Characteristic Curve (or)Sensitometric Curve

Producing a digital radiograph is the same as for analog film, up to the point of the photons interacting with the film.

Digital imaging samples the remnant radiation with (some kind of) a detector,

not film.

Analog (343): Information display that is continually changing. Every value, to infinity, exists. For examples: a sweep second hand on a clock, a mercury thermometer, and the gray scale of an x-ray film. (See digital)

Digital (343): Information (or a display of information) that is discrete. Values are absolute, with nothing in between. For examples: the second by second increments of a digital clock, a digital thermometer, and the gray scale of a digital image. (See analog)

Analog is continuous. Digital is discrete.

= 0

= 1

Off (Open)

On (Closed)

Data (343): The smallest unit of information.If a brick building were used for analogy, a single brick would be data. (See information)

Computer circuitry is a series of switches that store data in one of two elementary discrete- states: on, or off.

Digital computers store data as binary digits. Question: How do they do that?

Information: Processed data. If a brick building were used for analogy, a single brick would be data, and the building itself would be information. (See data)

0 and 1 are binary digitsor bits

Digital computers store data as binary digits.Question: How do theydo that?

Bit (Binary digit) (344): In the binary numbering system, two symbols, 0 and 1, are used to represent any value from null to infinity. Like base 10 numbering, binary (base 2), is derived from the base number raised to every exponent of itself: 20, 21, 22, 23 etc. Bit depth refers to the dynamic range (gray scale) a microprocessor can display. A 4 bit processor displays 32 shades of gray, an 8 bit processor 256.

Binary numbering

1248163264128256512

20

21

22

23

24

= 140 0 0 0 0 0 0 1 1 1 0= 421 0 1 0 1 0

1 1 1 1 1 1 1

= 127

0123410 10 10 10 10

Binary numbering system (344): In the base 2 numbering system all values are represented by 0 or 1. Zero = off when representing an open switch. One = on.

Binary numbering

1248163264128256512=841

1280 1 0 1 0 0

Binary numbering

1248163264128256512=4001

5121 0 0 1 0 0

384

0 0

Byte (344): A group of 8 bits, used in computer programming for organization of data. An 8 bit processor (28) stores 256 bitsof data. When the data is an image, than image can have up to 256 shades of gray. (See appendix I)

On + Off = Off

Off + Off = Off

On + On = On

AND Gate

Machine language: (344) is a programming language that sets the configuration of switches in a computer chip, which determines how data is processed. Though cryptic to humans (10010111), machine language is the most efficient programming code, for it does not require interpretationfor the computer uses it. (See bits, bytes, appendix I)

Three logic gates:AND, OR, NOT,form a half adder.Any combinationof 0 and 1 can beadded in a halfadder

On + Off = On

Off + Off = Off

OR Gate

On + On = On

On = Off

Off = On

NOT Gate

0+0

0 0

Operation of a Half Adder

= 0

A digital computer processes and stores data by configuring circuit pathways: opening and closing semiconductor switches of integrated circuits (IC). When a switch is closed a current will flow through that branch of the circuit. When it is open current will not flow.

1+0

0 1

Operation of a Half Adder

= 1

0+1

0 1

Operation of a Half Adder

= 1

1+1

1 0

Operation of a Half Adder

= 2

Program (343): Operating instructions in the form of operating systems or application programs. (See software)

Software (343): Software is the program (machine language, BASIC, Visual BASIC, Fortran, C, C+, C++, Magic) that directs a computer’s function by configuring the switches in the semiconductor material of a computer chip. (See program, hardware)

Central Processing Unit (CPU)

ALU

CUPrimaryMemory(RAM &ROM)

Arithmetic LogicUnit (contains logic gates in registers)

Control Unit (directs the flow of input and output)

Central processing unit (CPU) (344): The major components of a digital computer: the arithmetic logic unit (ALU), the control unit (CU), and primary memory (Random access memory or RAM), comprise the CPU. Input and output devices, as well as secondary memory are peripherals (CD, DVD, floppy drives, flash drives) that communicate with the CPU.

Hardware (344): The computer chips or integrated circuits (IC), the mother board they reside on, the DVD, hard drive, ports, plugs, box, and anything else coffee can be spilled on.

Random access memory (RAM) (345): Semiconductor switches in microchips have addresses: locations which may be accessed directly (randomly): a quick, electronic process. When a program (such as word processing) is loaded from a secondary storage device (such as a hard drive) it is copied into a RAM chip. As changes are made they are made in RAM. The program must be saved back to the hard drive to be retained. RAM is also called volatile memory, which means that when power is turned off data in RAM is lost.

Read only memory (ROM) (345): Memory that can only be read (used) and not written to (changed).

35013502

41074108

Welcome to RAM

Population: 376,243,101,765

RAM: addressable memoryin an integrated circuit (IC)

Input

ALU

CUPrimaryMemory(RAM)

ADC

When input is analog, (as sampled by a conventional video camera), an Analog to Digital Converter (ADC) digitizes the signal before being sent to the control unit.

When input is digital (keyboard, CT, MRI, DR etc.)the digital signal is sent directly to the contol unit

Processing& SavingALU

CUPrimaryMemory(RAM)

ADC1

2

3

4

5

Secondary Memory

1. Raw data (digital signal) enters the CU from a digital imaging device or ADC

2. The signal is identified and sent to the ALU for processing

3. Processing complete, the CU routes the image data to RAM

4. From Primarymemory the imageis most often sent to a monitor, and, in the case ofimaging equipment, it is SAVEd in secondary memory.

Secondary memory (floppy, hard, optical disk or tape) is most often measured in megabytes, or gigabytes.

Output

ALU

CUPrimaryMemory(RAM)

ADCDAC

Secondary Memory

Joe’s colon

If the image file had been saved to a secondary memory device and purged from RAM, it must be loaded back to RAM before being sent to output.

When the output deviceis digital the file is sentdirectly to it

When the outputdevice is analog,it is sent to the DAC

Output is routed from RAM to the CU, then to the output device.

One bit of computer memory (on or off) is all it takes to light up a pixel (on), or not (off).

One Pixel

Pixel (Picture element) (346): Commonly thought of as the dots on a monitor or TV screen, but pixels also refer to the elements of certain digital detectors. The term conveys the concept of reducing nformation into a matrix of discreet elements (See matrix)

Matrix (346): An array of pixels arranged in rows and columns. The pixels of a monitor or TV screen form a matrix. A large matrix (more pixels) = better spatial resolution (See pixels)

A conventional (not HDTV orhigh resolution monitor) isa matrix of 525 x 525 pixels.

Only two bits of data (2 bit processor) is needed to control each pixel when the dynamic range is 21: on or off.

MRI midsagittal head scandisplayed in 2 bits.

250

The number 47 defines theshade of gray for the pixelin column 250, row 210.

Printout of the datain the matrix of aCT image

Column 250

Row 210

Values of digits stored in bytes of computer memory directly correspond to the illumination of pixels.

In this case, the pixel in column 250, row 210.

Bandwidth (Bandpass) (350): The range of frequencies in a signal. Conventional commercial TV has a 525 x 525 matrix (1-2 lp/mm), which is very poor spatial resolution. Increasing the matrix size (more pixels) improves spatial resolution but also requires the pixels to be scanned faster, which requires the electron beam to modulate (change from pixel to pixel) more rapidly. The frequency of modulation is measured in Hz, and is referred to as the bandwidth. Commercial TV has a bandwidth of 4 MHz. A 1000 line (actually 1050 pixels vertically) high definition monitor requires 20 MHz, and resolves 5-7 lp/mm.

Voxel (Volume element) (346): Although each pixel on a monitor displays a two dimensional representation of data, the data did not come from a finite plane. For example, a sectional image of a CT scan is sampled by a fan beam which has a thickness from 1mm to a cm. The volume of the tissue within the sampled tissue contributes to the brightness of the pixels. This is the voxel.

Sampling the Voxel in Cross Sectional Images

Cross sectional images have depth, which is selected priorto a scan. When a two dimensional section is viewed, thedensity of each pixel actually represents all the tissues inthe volume of the section. This density represents the volume element, or the voxel.

Three contiguous CTsections with parts ofa frontal lobe mass ineach one

The CT numbers from these samples would represent an average of the mass and healthy tissue.

The CT number from this sample would represent the true density of the mass.

Using the region of interest (ROI) cursor to sample CT numbers from the voxels of pixels.

This ROI has beensized to measure adensity in the right kidney.

The mean density of 15.9 indicates a fluidfilled cyst.

Filtering (Convolution) (350-35): With image data stored as binary numbers, mathematical algorithms (repeated applications of a formula, applied to raw data) are used to enhance the appearance of the image. Digital filters do not add to the data. They accentuate features by suppressing frequencies that may obscure detail, like blowing dust from a written page enhances readability without altering the print. This is done by suppressing spatial frequencies (which enhances others). An example of anatomy with low spatial frequencies is the liver, for it is a homogeneous shade of gray. High spatial frequencies occur in trabecular bone, which demonstrates heterogeneous shades of gray. Band-pass filtering: Demonstration (display) of a selected range of frequencies. High-pass filtering: Suppression of all but high frequency signals. Also called sharpening or edge enhancement. Low-pass filtering: Suppression of all but low frequency signals. Smoothing filter: A low-pass that averages adjacent pixels

Plain film x-ray of the abdomendemonstratinglow spatial frequencies(few changes in density)of water density organs

An ankle with Paget’sdisease demonstratinghigh spatial frequencies(many changes in density) of the diseased trabecular bone

Repeated applicationsof a high passfilter (also knownas sharpening or edge enhancement)demonstratesthe effect of suppressing lowspatial frequencies

No filter High

Ultra High Ridiculously High

No filter

Low pass or smoothingfilter

Edge sharpening filterhas an algorithm similarto a high pass filter

Windowing (Window level and width) (349): Digital images may have a dynamic range of thousands of shades of gray (depending on the bit depth of the processor) but the human eye can only distinguish about 32. To utilize this much information a feature called windowing allows selective display of the dynamic range. (See appendix I) Window width: If 256 shades of gray were acquired during imaging, but only half of those were chosen to be displayed on the monitor, the window width would be 128. Widow width can be described as controlling density (or brightness). Window level (or center): The displayed dynamic range could be at the high or low end of the scale, or anywhere in between. The window level (or center) is the number the display is centered on. For example, if the window width were 105, and the window center was 70, the display of grays would extend from 18 to 122. (52 shades of gray below 70, and 52 above, which = 105 shades of gray. Window level can be described as controlling contrast.

Windowing

The human eye candistinguish 32 shadesof gray.

25

It’s like contrast and brightness, but it’s not.

32

Windowing

25

But a digitized image may contain thousands of shadesof gray, known as the dynamic range

210

102432

Bone = + 1000

CT Numbers (Hounsfield Units)

Based on Water = 0

Air = - 1000 CT numbers (HU), express attenuation values relative to water

The range of displayed pixel values

Window Width

For example: A window width of 5

A narrow window is the digitalequivalent of a short scale ofcontrast on a radiograph

The range of displayed pixel values

Window Width

For example: A window width of 500

A wide window is the digitalequivalent of a long scale ofcontrast on a radiograph

The middle of the range of any given window width

Window Center (or level)

At a center of 2

For example: A windowwidth of 5

0

7

-7

12345

*012

Window Center (or level)

At a center of - 2

A window width of 5

0

7

-7

*-2-10

This CT section through the abdomenwas windowed with a width of 300 shadesof gray

The median number(the level or center)is 2 HU below water,-2

A soft tissue window set at a width of 110, at a levelof 43

The same CT section of the head set at a width of 2010,at a level of 800

Note the fracture through the frontal sinus

Width 2290Level 907

Bone windows demonstratingtrauma to the left orbit, maxilla, and sinuses.

Two windows of the same section through the thorax

Width 1269Level 202

Width 1269Level 2

Digital subtractionangiography(DSA) images usenarrow windows toenhance the contrastdifference betweenthe vessel and thesurrounding tissue

Signal to noise ratio (SNR) (347): Along with a signal (such as the video signal) there are stray electrical currents that degrade image information. The higher the ratio the better the signal. For example, a vidicon camera has a SNR of 200:1 due the heat produced by an electron tube. A digital signal must be no less than 1000:1 to be of acceptable quality.

Question: How can a simple on/off switch be used to store

complex information that contains many shades of gray?

Answer: Many switches are used in combination.

Question: If one bit of data is enough to turn a pixel off or on, what can a byte of data do for a

single pixel?

Answer: A byte of image data stores values for 256 shades of gray.

How many KB of computer memory is required for a monitor with a 512 x 512 matrix displaying a gray scale of 2?

512 x 512 = 262,144 bits

262,144 / 8 bits per byte = 32,768 bytes

32,768 bytes / 1024 bytes in a kilobyte = 32KB

Answer = 32KB (compared to the 3.2 KB file for all the text of chapter 24)

How many bytes of computer memory is required for a for a monitor with a 512 x 512 matrix, displaying 256 shades of gray (2 )?

512 x 512 = 262,144 bits

262,144(8bits)/8 bits per bytes = 262,144 bytes

262,144 bytes/1024 bytes in a kilobyte =262KB

Answer = 262KB Conclusion: Images are memory hogs.

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