Computed tomography

Formation of a CT image

Data acquisition Image reconstructionImage display, manipulation

Storage, communicationAnd recording

Elements of data acquisition

Two elements in the basic scheme:1. Beam geometry:

The size, shape and motion of the beam and its path.

2. Components comprising the scheme:Those physical devices that shape and define the beam, measure its transmission through the patient, and convert this information into digital data for input into the computer.

The x-ray system

The x-ray generator The x-ray tube The x-ray beam The collimator

Data acquisition

The x-ray tube traces a spiral/helical path with a radius equal to the distance from the focal spot to the center of rotation. This results in an entire volume of tissue being scanned during a single breath hold compared with slice-by-slice.

Data acquisition

Image reconstruction

Algorithms:

It is a set of rules or directions for getting a specific output from a specific input. The distinguishing feature of an algorithm is that all vagueness must be eliminated, the rules must describe operations that are so simple and well defined, they can be executed by a machine. Furthermore, an algorithm must always terminate after a finite number of steps.

Image display

Display Monitor

Manipulation Image reformation, 3D processing such as image smoothing, edge enhancement, grey scale manipulation.

Recording Films

Storage Magnetic tapes and magnetic disks, laser storage.

CT scannerHigh voltage generator

System control

Computer

DAC

Storagedevice

Recording device

Displaydevice

ADC

CT gantry

Gantry aperture

Detectors

Contrast media

Physical properties

Side effects

Types

Helical scanning

Requirements for volume data acquisition:

1-Continuously rotating scanner based on slip-ring technology

2-Continuous couch movement

3-Increase in load ability of the x-ray tube, capable of delivering at least 200 mA per revolution continuously throughout the time it takes to scan the volume of tissue.

Helical scanning

4-Increased cooling capacity

5-Spiral/helical weighting algorithm

6-Mass memory buffer to store the vast amount of data collected.

Helical scanning

Helical scanning

Advantages

Limitations

Multislice CT

Multislice or multidetector CT scanners

Are capable of acquiring several tomographic slices in a single rotation of the x-ray tube and detector assembly.

Multislice CT

MSCT scanners can be divided into two categories:

1-Fixed or symmetric matrix type:Have detector elements that are essentially of equal length in the z-direction.

2-Adaptive or asymmetric type:Have detector elements that increase in length with distance along the z-axis from the centre of the array

MSCT scanners offer the following improvements

1-Imaging up to 4,16,32, 64 and more times the volume in a single rotation.

2-Rotation speed is 50-100% greater3-Minimum slice width is halved

4-Improved x-ray utilization.

These advantages when combined, offer many benefits in a wide range of clinical examination.

Image manipulation

Do not produce any additional information. The information content in the processed image is always less than or equal to that in the original image.

Windowing:

The picture contrast is easily changed with two control mechanisms:

The window width and window level.

Detector characteristics:

Efficiency Stability Response time Dynamic range.

Types of data

Measurementdata Processing Raw data

ConvolutionFiltered data

OrConvolved data

Back-projection

CT image

Equipment configuration

Three major systems:1. Imaging systems-----located in the

scanner room2. Computer system----- located in the

computer room3. Image display, recording---- located

in the operator room

Major components of the imaging system

X-ray tube Generator Collimator Filter Detectors Detector electronics

Imaging system

Scanner:

Is usually referred to the gantry and patient couch

The quality of a CT image is determined by:

Resolution1. Spatial resolution2. Contrast resolution

Noise Radiation dose Linearity Cross-field uniformity Image artifacts

Sources of artifacts:

1. Patients

2. Imaging process

3. Equipment

Types

CT artifacts can be classified according to appearance:

1. Streak artifact

2. Shading artifact

3. Rings and bands

Quality control:

It is the continuous testing and maintenance of equipment.

It ensures the optimal performance of the CT scanner through daily, monthly, annual test.

The test include:Spatial resolution, contrast resolution, noise,

slice width, kVp wave form, average CT number of water, standard deviation of CT numbers in water, radiation scatter and leakage.