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 Tomography: From the Greek word “tomos” section. The process for generating a tomogram, a two-dimensional image of a section through a three-dimensional 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

DISPLAY, MANIPULATION, STORAGE COMMUNICATIONS & RECORDING 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

FIRST CLINICAL PROTOTYPE CT BRAIN SCANNER 1972 FIRST CLINICAL PROTOTYPE CT BRAIN SCANNER First scans—20 min. Later reduced to 4.5 min.

CLINICALLY USEFUL CT SCANNER

DR. ROBERT LEDLEY DEVELOPED THE FIRST WHOLE BODY 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 translate-rotate principle, in which a single, highly collimated x-ray 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 translate-rotate 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 first-generation 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 x-ray 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 SPIRAL-HELICAL GEOMETRY BASED ON SLIP RING TECHNOLOGY Slip rings

DUAL SLICE CT HELICAL SCANNER 1992 DUAL SLICE CT HELICAL SCANNER

MULTISLICE CT SCANNERS 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.