History of CT

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.)

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

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

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

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

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…

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

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

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

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…

Generation 1

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

Generation 2

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

Generation 3

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

Generation 4

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…

Generation 5

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

Generation 6

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??

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

Computers and Digital Imaging

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

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

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

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

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

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.

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?

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

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

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.

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

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

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

Digital Imaging Characteristics Matrix Pixels Voxels Bit Depth

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

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

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

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

Bit Depth

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.

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

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

PACS

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

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