Download COMPUTED TOMOGRAPHY HISTORICAL PERSPECTIVE

COMPUTED TOMOGRAPHY
HISTORICAL PERSPECTIVE
OUTLINE
• Tomography – definition
• Why CT – limitations of radiography and
tomography
• CT- basic physical principle
• Historical trail
• CT generations
Tomography: From the Greek word “tomos”
section. The process for generating a tomogram, a twodimensional image of a section through a threedimensional object. Tomography achieves this result by
simply moving an x-ray source in one direction as the
x-ray film is moved in the opposite direction during the
exposure to sharpen structures in the focal plane, while
structures away from the focal plane appear blurred.
CONVENTIONAL
RADIOGRAPHY HAS
LIMITATIONS:
• Two dimensional image with infinite depth superimposition of underlying structures (lateral and
oblique views don’t solve it completely).
• Inability to demonstrate slight differences in subject
contrast characteristic of soft tissue.
TOMOGRAPHY –
SOLUTION?
• Conventional tomography attempted to eliminate the
superimposition problem by blurring the structures above
and below the tomographic focal plane.
• Contrast of an image can also be changed by varying
tomographic angle (distance of a tube travel)
• Multidirectional tube movement makes the blurring of
unwanted structures even more effective.
Tomography limitations
• Image blurr present
• Excessive scatter radiation – film fog
RADIOGRAPHY AND
TOMOGRAPHY
• Tissue difference sensitivity
5-10%
CT GOALS:
CT –EVOLUTION OF TERMS
COMPUTERIZED TRANSVERSE AXIAL TOMOGRAPHY
COMPUTER ASISSTED TOMOGRAPHY
COMPUTERIZED AXIAL TOMOGRAPHY
COMPUTED TOMOGRAPHY
FORMATION OF CT IMAGE
DATA AQUSITION
IMAGE RECONSTRUCTION
IMAGE:
DISPLAY, MANIPULATION, STORAGE
COMMUNICATIONS & RECORDING
DATA ACQUISITION
• Collection of x-ray photons transmitted
through the patient by the ct detectors.
DETECTORS
IMAGE RECONSTRUCTION
• Transmission measurements collected by
the ct detectors are sent to the computer for
the processing. computers uses
mathematical algorithm to reconstruct the
image.
IMAGE DISPLAY,
MANIPULATION, STORAGE,
COMMUNICATION.
• After reconstruction image can be displayed on
the monitor.
• Image can be manipulated image can be stored –
on MOD or CD.
• During communication phase image may be
transmitted to a remote location.
CONSTRUCTION OF FIRST CT
•
•
•
•
Radiation source – americum gamma source
Scan—9 days
Computer processing—2.5 hours
Picture production 1 day
HOUNSFIELD’S LATHE BED
SCANNER
1972
FIRST CLINICAL PROTOTYPE CT
BRAIN SCANNER
1. First scans—20 min.
2. Later reduced to 4.5 min.
CLINICALLY USEFUL CT
SCANNER
1974
DR. ROBERT LEDLEY
DEVELOPED THE FIRST WHOLE
BODY CT SCANNER .
SCANNING DEVELOPMENT
• 5 min. –1972
• 1 sec – 1993
DATA ACQUISITION GEOMETRIES
Three primary types of acquisition geometries are
parallel beam geometry, fan beam geometry, and CT
scanning in spiral geometry, which is the most
recently developed geometry. As a result, a simple
categorization of CT equipment has evolved based
on the scanning geometry, scanning motion, and
number of detector
CT SCANNING GENERATIONS
The data acquisition process is based on a translaterotate principle, in which a single, highly collimated xray beam and one or two detectors first translate across
the patient to collect transmission readings. After one
translation, the tube and detector rotate by 1 degree and
translate again to collect readings from a different
direction. This is repeated for 180 degrees around the
patient. This method of scanning is referred to
as rectilinear pencil beam scanning.
Second-generation scanners were based on the translaterotate principle of first-generation scanners with a few
fundamental differences, such as a linear detector array
(about 30 detectors) coupled to the x-ray tube and multiple
pencil beams. The result is a beam geometry that describes a
small fan whose apex originates at the x-ray tube.
Also, the rays are divergent instead of parallel, resulting in a
significant change in the image reconstruction algorithm,
which must be capable of handling projection data from the
fan beam geometry.
In second-generation scanners, the fan beam translates
across the patient to collect a set of transmission
readings. After one translation, the tube and detector
array rotate by larger increments (compared with firstgeneration scanners) and translate again. This process is
repeated for 180 degrees and is referred to as rectilinear
multiple pencil beam scanning. The x-ray tube traces a
semicircular path during scanning.
Third-generation CT scanners were based on a fan beam
geometry that rotates continuously around the patient for 360
degrees. The x-ray tube is coupled to a curved detector array
that subtends an arc of 30 to 40 degrees or greater from the
apex of the fan. As the x-ray tube and detectors rotate,
projection profiles are collected and a view is obtained for
every fixed point of the tube and detector. This motion is
referred to as continuously rotating fan beam scanning. The
path traced by the tube describes a circle rather than the
semicircle characteristic of first— and second-generation CT
scanners.
Third-generation CT scanners collect data faster than
the previous units (generally within a few seconds).
This scan time increases patient throughput and limits
the production of artifacts caused by respiratory motion.
Fourth-Generation Scanners
Essentially, fourth-generation CT scanners feature two types
of beam geometries: a rotating fan beam within a stationary
ring of detectors and a nutating fan beam in which the apex
of the fan (x-ray tube) is located outside a nutating ring of
detectors.
Rotating Fan Beam Within a Circular Detector Array
The main data acquisition features of a fourth— generation
CT scanner are as follows:
1.The x-ray tube is positioned within a stationary, circular
detector array.
2.The beam geometry describes a wide fan.
Rotating Fan Beam Outside a Nutating Detector Ring
In this scheme, the x-ray tube rotates outside the detector
ring. As it rotates, the detector ring tilts so that the fan beam
strikes an array of detectors located at the far side of the xray tube while the detectors closest to the x-ray tube move
out of the path of the x-ray beam.
The term nutating describes the tilting action of the detector
ring during data collection. Scanners with this type of
scanning motion eliminate the poor geometry of other
schemes, in which the tube rotates inside its detector ring,
near the object.
However, nutate-rotate systems are not currently
manufactured.
HIGH SPPED CT
V GENERATION
( CARDIVASCULAR CT)
Fifth-generation scanners are classified as high-speed CT
scanners because they can acquire scan data in
milliseconds. In the EBCT scanner, the data acquisition
geometry is a fan beam of x rays produced by a beam of
electrons that scans several stationary tungsten target rings.
The fan beam passes through the patient and the x-ray
transmission readings are collected for image
reconstruction
EBCT ( SIEMENS)
The design configuration of the EBCT scanner is different
from that of conventional CT systems in the following
respects:
1.The EBCT scanner is based on electron beam technology
and no x-ray tube is used.
2.There is no mechanical motion of the components.
3.The acquisition geometry of the EBCT scanner is
fundamentally different compared with those of
conventional systems.
1990
SPIRAL CT ( HELICAL) –SLIP
RING TECHNOLOGY
CT SCANNING IN SPIRALHELICAL GEOMETRY BASED ON
SLIP RING TECHNOLOGY
Slip rings
1992
DUAL SLICE CT HELICAL
SCANNER
1998
MULTISLICE CT SCANNERS
Spiral/Helical Geometry Scanners
These systems have evolved through the years from two to
eight slices per revolution of the x-ray tube and detectors
(360-degree rotation) to 16, 32, 40, 64, and 320 slices per
360-degree rotation. As of 2007, a prototype scanner
featuring 256 slices per 360-degree rotation is being
developed by Toshiba Medical Systems (Japan) for
imaging moving structures such as the heart and lungs. One
striking feature of this scanner compared with other
multislice scanners is that it covers the entire heart in a
single rotation.
An interesting point with respect to scanners capable of
imaging 16 or greater slices per 360-degree rotation is that
the beam becomes a cone. These systems are therefore
based on cone-beam geometries (as opposed to fan-beam
geometries) because the detectors are two-dimensional
detectors
Sixth-Generation Scanners:
The Dual Source CT Scanner
The overall goal of the MSCT scanners mentioned previously
is to improve the volume coverage speed while providing
improved spatial and temporal resolution compared with the
older four slices per 360-degree rotation scanner. This scanner
consists of two x-ray tubes and two sets of detectors that are
offset by 90 degrees. The DSCT scanner is designed for
cardiac CT imaging because it provides the temporal
resolution needed to image moving structures such as the
heart.
Seventh-Generation Scanners:
Flat-Panel CT Scanners
Flat-panel digital detectors similar to the ones used in
digital radiography are now being considered for use in
CT; however, these scanners are still in the prototype
development and are not available for use in clinical
imaging.