Shahid Younas Lecture 5

INTRODUCTION TO CT PHYSICSIntroduction CT Physics

Computed Tomography

A single transmission measurement through the patient made by a

single detector at a given moment in time is called a

A. Projection

B. ray

C. view

D. beam

E. voxel

Computed Tomography

A series of rays that pass through the patient at the same

orientation is called a

A. Projection

B. ray

C. view

D. beam

E. pixel

Computed Tomography

A beam geometry, in which all of the rays in a projection are

parallel to each other, is named as

A. Parallel

B. divergent

C. orthogonal

D. tangential

E. anti-parallel

Computed Tomography

In fan beam geometry, the rays at a given projection angle

A. diverge

B. converge

C. split

D. tilt

E. spread

Computed Tomography

If It is the intensity produced by x-ray source and Io is the intensity

detected by the CT detector, then, relation between It and Io is,

Computed Tomography

Which beam geometry was used in first generation CT

SCANNER?

A. Pencil

B. Fan

C. Narrow Fan

D. Wide Fan

E. Helical

Computed Tomography

Which one describes the advantage of first generation CT scanner?

A. efficient scattered rejection

B. proper usage of x-ray beam

C. huge change in x ray flux

D. higher quantum noise

E. patient friendly

Computed Tomography

Which beam geometry was used in second generation CT scanner?

A. Pencil

B. Fan

C. Narrow Fan

D. Wide Fan

E. Helical

Computed Tomography

Which group describes the third generation CT?

A. Rotate / Translate Pencil Beam

B. Rotate / Translate Narrow Fan Beam

C. Rotate / Rotate Wide Fan Beam

D. Rotate / Stationary

E. Helical

Computed Tomography

Ring Artifact is a result of ?

A. Pencil Beam

B. Narrow Fan Beam

C. Gain of detectors remains equal

D. Gain of detectors does not cancel out

E. Heel effect

Fifth Generation CT : Stationary / Stationary

5th Generation CT

Electron Beam CT Scanner

Fifth Generation CT : Stationary / Stationary

Cine CT

Electron Cone Beam Tomography (ECBT)

does not use a conventional x-ray tube

large arc of tungsten encircles the patient and lies directly

opposite to the detector ring.

Fifth Generation CT : Stationary / Stationary

Ref:

Fifth Generation CT : Stationary / Stationary

X-rays are produced from the focal track as a high-energy

electron beam strikes the tungsten.

No moving parts to this scanner gantry

Primarily to cardiologists.

They are capable of 50-msec scan times and can produce fast-

frame-rate CT movies of the beating heart.

Sixth Generation CT : Helical

6th Generation CT

Helical

Sixth Generation CT : Helical

Helical CT (inaccurately

called spiral CT) acquire

data while the table is

moving.

“Volume Scanning”

Sixth Generation CT : Helical

Sixth Generation CT : Helical

By avoiding the time required to translate the patient table, the

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

(~ 30 seconds for the entire abdomen).

Consequently, helical scanning allows the use of less contrast

agent and increases patient throughput. In

Sixth Generation CT : Helical

The table movement in the z direction during the acquisition

will naturally generate inconsistent sets of data, causing every

image reconstructed directly from a volume data set to be

degraded by artifacts.

However, special reconstruction principles generates a planar set

of data for each table position – produce artifact-free images

Second Generation CT : Rotate Translate Narrow Fan Beam

Do you know which special

reconstruction technique is used

in Spiral CT?

interpolation techniques

Sixth Generation CT : Helical

Software applications enable the clinical use of spiral CT

even for regions which are subject to involuntary

movements.

The speed of the table motion relative to the rotation of

the CT gantry is a very important consideration and the

pitch is the parameter that describes this relationship.

Sixth Generation CT : Helical

Pitch = table feed per rotation/collimation

The larger the table feed, the faster (i.e. with fewer

rotations) a body region can be scanned.

However, if the table feed is too large, image quality will

be impaired.

Sixth Generation CT : Helical

Advantages of spiral CT in clinical use

1. Complete coverage of organs in a single respiratory position.

2. Short scan times (resulting in fewer motion artifacts and a

lower contrast medium requirement)

3. Additional diagnostic information due to improved resolution

(thinner slices) and 3D visualization in routine operation

Slip Ring Technology

1990s, the design of 3rd and

4th generation.

A slip ring is a circular

contact with sliding brushes

that allows the gantry to

rotate continually, untied by

wires.

Sixth Generation CT : Helical

Sixth Generation CT : Helical

Sixth Generation CT : Helical

Seventh Generation CT : Multiple Detector Array

7th Generation

Seventh Generation CT : Multiple Detector Array

1. An approach to make better use of the x-rays.

2. Collimator spacing is wider and therefore more of the x-rays

are used in producing image data.

3. Slice thickness is determined by the detector size.

4. four contiguous 5-mm detector arrays and 20-mm c. spacing

Thanks