1. History of CT

2. Why CT??? Deals with the issue of superimposition of structures
Provides excellent low-contrast resolution because of beam geometry and sensitive detectors
Spiral/Helical volume data acquisition leading to major imaging innovations (MPR, 3D, etc.)

3. Basic Principles of CT
X-ray beam is passed at a cross section through a patients body. This eliminates superimposition
The beam is finely collimated this reduces scatter and gives better contrast resolution
The collimated beam pass thru the body , the body tissue absorbs the beam.
The beam exits the body and strikes the detectors. The detectors are quantitative and distinguish differences in tissue contrast
Detector converts photons to a analog signal
The ADC converts it to digital signal
Digital data is sent to CPU for reconstruction

4. History of CT Johann Radon - Theory was actually developed in 1917
Showed that an image can be reconstructed of a 2 or 3 dimensional object from a large number of projections stemming from different directions
Referred to Astronomy et al

5. History of CT
Godfrey Hounsfield 1967 – Researched x-ray beam being passed through object in all directions while obtaining measurements of transmission that information about internal structures can be obtained and presented in 3d representation

6. History of CT
Through the 1960’s, mathematicians continued to investigate possibility of image reconstruction in medicine
1963 – Alan Cormack (r.) was able to apply techniques in nuclear medicine for such which was then seen as the solutions to the mathematical problems developed at the advent of the CT scanner

7. History of CT Original “CT” Scanner Did not utilize x-ray
Gamma Source coupled with a single crystal detector
9 days to scan object
Extremely low radiation output
Computer utilized processed 28,000 measurements in 2.5 hours
When decision was made to replace radiation source with x-ray tube cut time down to 1 day…

8. History of CT
Early experiments of brain tissue in conjunction with a radiologist showed the ability to differentiate between tumor tissue and gray and white brain matter
Also able to differentiate details like ventricles and pineal gland
1971 – First clinical prototype CT Brain Scanner was installed in England under direction of Dr. James Ambrose
Processing time = 20 minutes
Minicomputer introduction = 4.5 minutes

9. History of CT 1972 – First patient scanned
Suspected brain lesion which turned out to be a large cyst
1979 – Hounsfield and Cormack receive Nobel Prize in Medicine for their contributions to the development of CT

10. History of CT 1974 – Dr. Robert Ledley
First whole body CT scanner
1975 – Dynamic Spatial Reconstructor (DSR)
Image dynamics of organ system with high spatial resolution – Advanced speed of scanner
1983 – Electron Beam CT Scanner introduced
First cardiac imager

11. History of CT
1989 – First practical spiral CT scanner introduced at RSNA
Dr. Willi Kalender
Single Slice spiral/helical
Allowed volumetric scanning which allowed scanning larger volumes in less time
1998 – Multislice CT introduced
4 or more slices per revolution
2005 – Dual Source CT
Developed by Siemens
Advanced cardiac imaging by utilizing 2 x-ray tubes with 2 detector arrays
Between the beats of the heart…
AND ON, AND ON, AND ON…

12. Generation 1

13. CT Generations First-Generation Systems:
Original scan geometry used by Hounsfield
Set of parallel rays that generate a projection profile
Translate-Rotate
Single collimated beam with one or two detectors translate across the patient collecting readings
After translation tube and detectors rotate 1 degree and begin the process again
Repeated for 180 degrees – AKA rectilinear pencil beam scanning
4.5 – 5.5 minutes to produce scan

14. Generation 2

15. CT Generations Second Generation System
Still based on the original translate/rotate principle
Introduced a detector array (approx. 30 detectors)
Multiple pencil beams which resembled a small fan
Ray now assumes divergence
Resulted in different reconstruction computations
Rectilinear multiple pencil beam scanning
After a translation, rotation is by larger increments over 180 degrees
Shorter scan times depending on number of detectors
20 seconds to 3.5 minutes

16. Generation 3

17. CT Generations Third Generation System
Based on fan beam geometry rotating continuously 360 degrees
Curved detector array
30 – 40 degree arc
Continuously rotating fan beam scanning
As tube and detectors rotate, projection profiles are obtained for every fixed point
Much faster
Allowed for single breath hold scans

18. Generation 4

19. CT Generations Fourth Generation System Two types of beam geometries
Rotating fan beam within stationary ring of detectors
Tube is within stationary circular array which line 360 degrees of gantry
Rotating fan beam outside nutating detector ring
Tube rotates outside detector ring which tilts so that beam strikes detectors on the far side which allowed detectors nearest the tube to be outside the array
No Longer manufactured…

20. Generation 5

21. CT Generations Fifth Generation System
Acquired scan data in milliseconds
Electron beam CT Scanner (EBCT) and Dynamic Spatial Reconstructor (DSR…and obsolete…)
No Moving Parts…beam of electrons that scans stationary tungsten rings
No X-Ray Tube – Electron Gun in which electrons are emitted in a beam which is accelerated, focused and deflected at precise angles
When beam collides with ring, x-ray is produced, collimated into a fan beam through the patient
Extremely fast reconstructions
Cardiac Scanner….Siemens Evolution

22. Generation 6

23. CT Generations Sixth Generation Dual Source CT Scanner
2 x-ray tubes with 2 sets of detectors offset 90 degrees
Cardiac CT
Better temporal resolutions
Reduced CTA artifact
Seventh Generation
Flat Panel CT – Still in prototype stages
Similar to digital radiography systems in that utilize TFT array as a detector
Excellent spatial resolution but lack contrast resolution
Angiography??

24. CT Physics
Lecture 2: Review of Basic Computing and introduction to Digital image processing

25. Computers and Digital Imaging

26. The Computer
By definition – high speed electronic machine utilized which accepts information in data format through an input device and processes this information with arithmetic and logic operations from a program stored in memory
Results can be displayed, stored, recorded or transmitted
Introduced to radiology in 1955 in order to calculate radiation dose distribution in cancer patients
Imaging Applications – Digital Imaging
Non-imaging Applications – PACS , RIS

27. The Computer Analog Computers Digital Computers
handle data composed of continuously varied electrical currents
Analog watch – displays time with hands
Digital Computers
Handle data composed of definite quantities of current
Digital watch – displays numerical readout
All medical imaging achieved now with digital

28. The Computer Hardware Software Physical components
Set of instructions upon which the computer operates
Computer Languages
Fortran – Formula Translations / Engineering
Basic – All purpose contains symbols and codes
Cobol – Buisness oriented
Pascal – High Level Math

29. The Computer Computer Architecture
General structure of a computer and includes all elements of hardware and software – chips, circuitry, and systems software
Terminology
Serial / Sequential Processing
Data and Instructions is processed in the order in which items are stored – one instruction at a time
Distributed Processing
Information processed by several computers on a network – highly structured, free exchange
Multitasking
more than one task at a time
Multiprocessing
two or more separate processors working differing sets of instructions
Parallel processing
task distributed over multiple available processors carrying out at the same time
Pipelining
fetching and decoding instructions in which at any time several programs instructions are in varying stages

30. Components
Central Processor Unit – heart of computer, directs information.. Capable of performing multiple tasks (parallel processing)
Consists of the control unit – tells computer how to carry out software instructions
Arithmetic/Logic Unit (ALU) – performs arithmetic or logic calculations.
These are connected to the BUS -
Bus – conductors which connects various components (provides path for the flow of electrical signals)
2 basic types of internal memory
RAM – Random Access Memory
Temporary storage.
ROM – Read Only Memory
Contain data and programs to make computer work.
Basic Operating Instructions

31. Components Array Processor –
Primary data processing component. Has its own CPU and uses CPU to perform simultaneous mathematical operations in a parallel fashion at high speeds
Is responsible for receiving the scan data from the host computer, performing all of the major processing of the CT image, and returning the reconstructed image to the storage memory of the host computer.
Hard Disk Drive – is a rewritable, nonremovable storage system that must be capable of storing a lot of data and transferring data fast.
Operating System is the primary software of the CT computer. It controls the usage of computer hardware resources, such as available memory, CPU time, and disk space.
Common OS – Windows, MS-DOS, OS/2, and UNIX.

32. Digital Fundamentals Operates on a binary number system
Base 2 = 0, 1
Yes, No system representing when current is present
Individual binary digit = bit
Bit, like an atom, the smallest unit of storage
A bit stores just a 0 or 1
"In the computer it's all 0's and 1's" ... bits
How much exactly can one byte hold?

33. Digital Fundamentals Individual binary digit = bit
4 binary bits (0.5 byte) = nibble
8 binary bits (1 byte) = byte = one addressable location in memory
16 binary bits (2 bytes) = word
32 binary bits (4 bytes) = double word
1 thousand bytes = 1 KB
1 million bytes = 1 MB
1 billion bytes = 1 GB
1 trillion bytes = 1 TB

34. Analog Image… Analog images – Digital Image
Images that we, as humans, look at. 
Example photographs and all of our medical images recorded on film or displayed on various display devices, like computer monitors. 
What we see in an analog image is various levels of brightness (or film density) and colors.  It is generally continuous and not broken into many small individual pieces.
Digital Image
Numerical representations of images as 1 and 0…
Requires computer

35. Digital Image… Digital Image
Numerical representations of images as 1 and 0…
Requires computer
A digital image is a matrix of many small elements, or pixels.
Each pixel is represented by a numerical value.  In general, the pixel value is related to the brightness or color that we will see when the digital image is converted into an analog image for display and viewing.
Generally, at the time of viewing, the actual relationship between a pixel numerical value and it's displayed brightness is determined by the adjustments of the window control as discussed in other modules.

36. Analog to Digital Conversion
Converting an analog signal into “a sequence of numbers having finite precision”
3 part process
Sampling = conversion of continuous signal into discrete signal from sampling stream at certain increments
Quantization = conversion of discrete signal into a value
Coding = assignment of a bit sequence to the discrete output

37. Digital Imaging Systems
Generic Digital Imaging System
Components:
Data Acquisition
Attenuation Data
Image Processing
Input digital image to output digital image utilizing binary
Image Display, Storage and Communication

38. Digital Image Processing
CT based on a reconstruction process where a digital image is changed into a visible physical image
CT acquires images in the spatial location domain
Location of each number in an image identified by x-y coordinate

39. Digital Imaging Characteristics
Matrix
Pixels
Voxels
Bit Depth

40. Matrix
2 dimensional array of numbers consisting of columns and rows defining small square regions of “picture elements”
Diagnostic images generally are rectangular in shape
When imaging a patient, the operator usually selects the matrix size or aka FOV…
Standard CT Matrix is 512 * 512

41. Pixels Smallest picture element
Generally square in shape and measured in the X-Y dimension
Contains a discrete value representing a brightness level
Calculated using:
Pixel size = FOV / Matrix
DETERMINES RESOLUTION: Larger the matrix size, smaller the pixel, better the spatial resolution

42. Voxels Represents the volume of tissue being imaged
Measured in the Z dimension
3 dimensional volume of tissue

43. Bit Depth Determines shades of gray that a pixel can take on
Number of bits per pixel
Uses the base 2 system
CT utilizes a bit depth of 12
-1024 to 3071

44. Bit Depth

45. Image Digitization Three distinct steps:
Scanning: Division of the picture into small regions (pixels) onto a grid (matrix)
Sampling: Measures brightness of each pixel in the image by utilizing a photomultiplier tube to detect transmitted light
Quantization: Brightness value is assigned a numerical value depending on strength which could be positive or negative called a gray level. Total number of gray levels is called gray scale.

46. Spatial Resolution
The ability to see the difference in small objects that are next to each other
Determined by the pixel size in the monitor matrix
Parameters that can affect spatial resolution:
Filters in high frequency regions
SFOV
Matrix size
Detector width and spacing
Number of projections
Focal spot size

47. PACS (P)icture (A)rchiving and (C)ommunication (S)ystems Components –
Computer System which is used to capture, store, distribute and then display medical images
Components –
Network Switches
PACS Controller with database image server
Short and Long term archives
RIS/PACS broker
Web server
Various displays
Integrated with both RIS and HIS

48. PACS

49. Communication Protocol Standards
HL-7
Health Level 7: standard application protocol for use in most HIS and RIS
DICOM
Digital Imaging and Communications in Medicine: developed by the ACR and NEMA (National Electrical Manufacturers Association) – Standard for handling, storing printing and transmitting information in medicine
File format definitions and network communications protocols
Enables integration of scanners, servers, workstations, printers and network hardware from multiple manufactures into a PACS system

50. References Image courtesy of Sprawls.com
Stewart Bushong “Radiologic Science for Technologists”
Bushberg et al., “The Essential Physics of Medical Imaging”
Wikipedia