generations of CT, explains each generations of CT, muti detector computer tomography, slip ring technology, main terminologies such as FOV , pitch, voxel and matrix, pixel size equation. EBCT, Basic configuration of CT, Data acquisition systems DAS, multi-slice CT

1. GENERATIONS OF CT
ANANYA GIRISH BABU
LECTURE RADIOLOGY

2. GENERATIONS
1. First-generation scanners were based on the parallel beam geometry and translate-rotate scanning
motion.
2. Second-generation scanners were based on the fan beam geometry and translate-rotate motion.
3. Third-generation scanners were based on fan beam geometry and complete rotation of the tube and detectors.
4. Fourth-generation scanners were based on fan beam geometry and complete rotation of the x-ray tube around a
stationary ring of detectors.
5.Fifth-generation scanners were developed primarily for high-speed CT scanning. These scanners are based
on special configurations intended to facilitate very fast scanning
6.Sixth-generation scanners have multiple x-ray tubes and detectors. These scanners are intended specifically to
image moving structures, such as the heart. One such recent scanner is the dual source CT
(DSCT) scanner
7. Seventh-generation scanners use flat-panel digital area detectors similar to the ones used in digital radiography

3. TERMINOLOGY
• Translation is the linear movement of X-ray tube and detector. Rotation is the rotary movement of X-ray tube and
detector
• Human body is imagined as a matrix and is divided into number of columns and rows. Each matrix element is
named as picture element (pixel) in a 2-dimensional (2D) concept.
• Volume element (voxel) represents a volume of tissue in the patient and it is a three dimensional (3D) concept.
Each pixel on the monitor display represents a voxel in the patient.
• The field of view (FOV) is the diameter of the area being seen by the X-ray at the isocenter. The relation between
the matrix size, pixel size and FOV is given below.

4. FIRST-GENERATION CT SCANNERS
• Parallel beam geometry was first used by Hounsfield (1973).
• The first EMI brain scanner and other earlier scanners were based on this concept. Parallel beam
geometry is defined by a set of parallel rays that generates a projection profile
• 1st generation CT scanner is a rotate or translate pencil beam system.
• Single detector is used
• First-generation CT scanners took at least 4.5 to 5.5 minutes to produce a complete scan of the
patient
• Detector is made up of sodium iodide
• Head is enclosed in a water bath
• At the end of translation the tube and detector were rotated 1° and translation repeat , it repeat till 180
projection for a complete circulation
• Limitation- high scanning time , poor spacial resolution , shorter rotation angle 1°
• Advantage – pencil beam geometry reduced scatter radiation

6. SECOND –GENERATION CT SCANNER
• Narrow beam ,rotate translate
• Multi-slice system
• 30 detectors are used made up of Nal
• Scanning time reduced The larger rotational increments and increased number of detectors result in
shorter scan times that range from 20 seconds to 3.5 minutes. In general, the time decrease is
inversely proportional to the number of detectors. The more detectors, the shorter is the total scan
time
• Bowtie filters are used here
• Each detector viewed the x ray tube in a diff angle ,single translation produced 3 projection .system
could rotate 3 degree instead of 1 degree hence needed 60 translation instead of 180 [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. ]
• Advantage- shorter scanner time ,speed increase due to more number of detector
• Disadvantage- scattered radiation

8. THIRD GENERATION CT SCANNER
• Rotate-rotate system
• Detector arranged in arch , x ray tube and detector length is constant
• Both xenon and scintillation detector are used
• Modern spiral scan is modification of this type
• Third-generation CT scanners were based on a fan beam geometry that rotates
continuously around the patient for 360 degrees
• 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
• 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.

10. FOURTH GENERATION CT SCANNER
• Rotating tube and stationary fixed detector
• Designs to eliminate ring artifact
• Only the source rotate with the fixed detector .Aline as ring and the ring is completely surrounded the pt
• Multidetector more than 2000
• Duration of scanning is few sec
• Limitation is less efficiency ,use of detector since only ¼th of detector are used at any point of time
• Essentially, fourth-generation CT scanners feature two types of beam geometries:
1. A rotating fan beam [within a stationary ring of detectors ]
2. A nutating fan beam [which the apex of the fan (x-ray tube) is located outside a nutating ring of
detectors.]

12. ROTATING FAN BEAM NUTATING FAN BEAM
1. The x-ray tube is positioned within a stationary,
circular detector array
2. The beam geometry describes a wide fan.
3. The apex of the fan now originates at each detector..
4. As the tube moves from point to point within the
circle, single rays strike a detector. These rays are
produced sequentially during the point’s circular travel.
5. Scan times are very short and vary from scanner to
scanner, depending on the manufacturer.
6. The x-ray tube traces a circular path.
• 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.

13. FIFTH-GENERATION SCANNERS
• Fifth-generation scanners are classified as high-speed CT scanners because they can acquire scan data in milliseconds.
• Two such scanners are the electron beam CT scanner (EBCT) and the dynamic spatial reconstructor (DSR) scanner

14. EBCT [Electron Beam CT]
• 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
• The principles and operation of the EBCT scanner were first described by Boyd et al. (1979) .At that time, the machine was
referred to by such names as the cardiovascular computed tomography scanner and the cine CT scanner. Today, the
machine is known as the EBCT scanner (McCollough, 1995)
• The overall goal of the EBCT scanner is to produce high-resolution images of moving organs (e.g., the heart) that are free
of artifacts caused by motion.
• These scanners are design enables it to acquire CT data 10 times faster than conventional CT scanners.
• The EBCT scanner is based on electron-beam technology and no x-ray tube is used.
• There is no mechanical motion of the components.
• The acquisition geometry of the EBCT scanner is fundamentally different compared with those of conventional systems

15. BASIC CONFIGURATION OF EBCT
• At one end of the scanner is an electron gun that generates a 130-kilovolt (kV) electron beam. This beam is accelerated,
focused, and deflected at a prescribed angle by electromagnetic coils to strike one of the four adjacent tungsten target rings
• The electron beam is steered along the rings, which can be used individually or in any sequence.
• When the electron beam collides with the tungsten target, x rays are produced. Collimators shape the x rays into a fan
beam that passes through the patient, who is positioned in a 47-cm scan field, to strike a curved, stationary array of
detectors positioned opposite the target rings
• Each solid-state detector consists of a luminescent
• crystal and cadmium tungstate (which converts x rays to light) coupled optically with silicon photodiodes (which convert
light into current) connected to a preamplifier. The output from the detectors is sent to the data acquisition system (DAS)
• The computer for the EBCT scanner is capable of very fast reconstruction speeds, and image reconstruction is based on the
filtered back-projection algorithm used in conventional CT systems
• Data acquisition systems (DAS) are the heart of CT scanners. These systems perform detector signal analog-to-digital
conversion and data preprocessing for the scanner’s reconstruction system. An ideal DAS would accurately and
consistently digitize the input signal from the detectors with no noise.

16. MULTISLICE CT SCANNERS: CT SCANNING IN SPIRAL-HELICAL
GEOMETRY
• Scanning in spiral-helical geometry is the most recent development in CT data acquisition. The need for faster scan times
and improvements in 3D and multiplanar reconstruction have encouraged the development of continuous rotation
scanners, or volume scanners, in which the data are collected in volumes rather than individual slices.
• As the tube rotates, the patient is transported through the gantry aperture for a single breath-hold. Because this results in a
volume of the patient being scanned, the term volume CT is also used.
• The overall goal of the MSCT scanner is to improve the volume coverage speed performance of both single-
slice and dual-slice CT scanners.

17. DUAL SLICE CT SCANNER
• These scanners used two detectors based on the translate/rotate method of data collection over 180 degrees. The next major
step to MSCT scanning appeared in 1993, with the introduction of the first dual-slice volume CT scanner
• The dual-slice whole-body fan-beam CT scanner offers improved volume coverage speed performance compared with the
single-slice volume CT scanner, reducing the scan time by 50% while maintaining image quality for the same scanned
volume.

18. 6TH GENERATION CT
• 1990,Significant advancement in technology
• Allowed 3D image acquisition within a single breath hold
• Third / forth generation + slip ring technology + helical motion = sixth generation
Spiral/Helical CT
• Design: x-ray tube rotates as patient is moved smoothly into x-ray scan field
• Simultaneous source rotation, table translation and data acquisition Produces one continuous volume set of data for entire
region
• Data for multiple slices from patient acquired at 1sec/slice
ADVANTAGES OF SPIRAL CT ARE …..
• Speed: patient movement continuous…………shorter exam time ; entire abdomen or chest: 30 sec (1BH)
• Improved detections: differences in BHs in standard CT, small lesions fall out of plane for each continuous slice
• Improved contrast: image a region in a short period, contrast can be timed
• Improved reconstruction & manipulation: volume of data collected, transverse data can be reconstructed in any plane- strip
away skin, muscles, etc

19. SPLIP RING TECHNOLOGY
• Three technological developments: by spiral CT
1. Slip-ring gantry designs
2. Very high power x-ray tubes
3. Interpolation algorithms to handle projection data
• Spiral-helical CT is made possible through the use of slip-ring technology, which allows for
continuous gantry rotation. Slip rings are “electromechanical devices consisting of circular
electrical conductive rings and brushes that transmit electrical energy across a rotating
interface”
• In addition, slip rings not only provide the electrical power to operate the x-ray tube but also
transfer the signals from the detectors for input into the image reconstruction computer

20. 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
• The x-ray tube and detectors are coupled and positioned in the CT gantry. The
detector consists of a cesium iodide (CsI) scintillator coupled to an amorphous,
silicon thin-film transistor array.
• These flat-panel detectors produce excellent spatial resolution but lack good
contrast resolution; therefore, they are also used in angiography to image blood
vessels, for example, where the image sharpness is of primary importance

21. THANK YOU