GENERATION OF CT SCANNERS

GENERATION OF CT SCANNERS

A variety of CT geometries have been developed to acquire the

X-ray transmission data for image reconstruction. These geometries

are commonly called generations. The main objective of different

generation is (i) scanning time reduction and (ii) simplification of

mechanical motion.

FIRST GENERATION

The first generation CT scanner is a rotate/translate, pencil beam system.

It had two X-ray detectors and used parallel ray geometry with NaI detector . It is translated linearly to acquire 160 rays across

a 24 cm FOV and rotated between translations to acquire 180 projections

at 1° interval. It took about 4.5 minutes per scan with 1.5 minutes

to reconstruct a slice with a linear measurements of 28,800 rays (160

× 180).

There is a large change in signal due to increased X-ray flux outside

of head and hence patient’s head is pressed into a flexible membrane

surrounded by a water bath. The NaI detector signal decayed slowly,

affecting measurements. The advantage of the system is the efficient

scatter reduction, best of all scanner generations. The disadvantage

includes (i) water bath acted as a bolus and (ii) afterglow of NaI.

SECOND GENERATION

The second generation CT scanner is also rotate/translate system

(11.11B), with narrow beam geometry (10°). Linear array of 30 detectors

were used to acquire more data, to improve image quality (600 rays

× 540 views = 3,24000). These scanners provided larger rotational

increments and faster scans. The shortest scan time was 18 seconds

per slice. Narrow fan beam allows more scattered radiation to be detected.

THIRD GENERATION

The third generation scanner is a rotate/rotate system with wide

beam geometry (Fig.11.12A). The number of detectors has increased

substantially (> 800 detectors) and the angle of fan beam is increased

to cover entire patient. It eliminated the need for translational motion.

The X-ray tube and detector array are mechanically joined androtate together.

Newer systems have scan times of the order of

< 0.5 second.

The 3rd generation scanners lead to a situation in which each detector

is responsible for the data corresponding to a ring in the image. Any

drift in the signal levels of the detectors over time affects the mt values

that are back projected to produce the CT image, causing ring artifacts.

FOURTH GENERATION

The fourth generation scanners are designed to overcome the problem

of ring artifacts. It has a stationary ring of about 4,800 detectors, and

the X-ray tube has to move inside this detector. Since it is rotated

continuously, very fast scan time is possible. Geometrical misalignment

between detector ring radius and X-ray beam origin may be possible.

It has inter scan delay times, since the X-ray tube had to return to

its starting position (home).

              Third generation fan beam geometry has the X-ray tube as the

apex of the fan. In the 4th generation, the individual detector is the

apex . Though the X-ray tube forms the fan beam, data

are processed for fan beam reconstruction with each detector as the

vertex of a fan. The rays acquired by each detector are fanned out

to different positions of the X-ray source. In the 3rd generation, It

and Io are measured at the center and at the edge of the detector

array. Hence, the gain of the reference detector and the individual

detector may not be equal. In the 4th generation, each detector has

its own reference detector, and hence the gain of the reference and

individual detector is equal.

FIFTH GENERATION

The fifth generation scanner is a stationary/stationary system, developed

specifically for cardiac tomography imaging. No conventional X-ray tube

is used, instead large arc of tungsten (210°) encircles patient and lies

directly opposite to the detector ring. It uses an electron gun that deflects

and focuses a fast moving electron beam along tungsten target ring

in the gantry. Since the detector is also in the form of ring, it permits

simultaneous acquisition of multiple image sections.

The images are obtained in 50 ms times and can produce fast frame

rate CT movies of the beating heart with minimum motion artifacts.

The advantage is the speed of data acquisition. The whole heart can

be acquired in 0.2 s. These scanners are useful in cardiac imaging,

pediatric and trauma patients. It can also be used as conventional

CT by averaging multiple images with repeat scans.

SIXTH GENERATION

Third/fourth generation + slip ring technology + helical motion = Sixth

generation (1990). Slip ring is a circular contact with sliding brushes

and allows the gantry to rotate continuously. It eliminates inertial limitations

at the end of each slice and has greater rotational velocities, with shorter

scan time. Helical CT scanners acquire data while the table is moving.

The total scan time required to image the patient can be much shorter,

excluding time required to translate the patient table.

It allows the use of less contrast agent and increases patient throughput.

In some instances, the entire scan can be done within a single breathhold

of the patient. Raw data from helical scans can be interpolated to approximate acquisition of planar reconstruction data. The speed of

the couch motion is very important; hence the term pitch is defined.

SEVENTH GENERATION

Seventh generation uses multidetector array (MDA), the collimator spacing

is wider and more X-rays are used in producing image data. Opening

up the collimator in a single array scanner increases the slice thickness,

but reduces spatial resolution in the slice thickness dimension. Hence,

slice thickness is controlled by detector size, not by the collimator.

A 4 contiguous 5 mm detector array gives 20 mm collimator spacing.

The number of X-rays detected is 4 times higher than that of single

array of 5 mm. Further, 10 mm,15 mm, 20 mm slices may be obtained

from the same acquisition. Though it offers flexibility of CT acquisition

protocol, the number of parameters have increased. It has better efficiency

for patient imaging and detector pitch needs to be defined.