Computed tomography

Formation of a CT image Data acquisitionImage reconstruction Image display, manipulation Storage, communication And 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 scanner High voltage generator System control Computer DAC Storage device Recording device Display device 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

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 % greater 3-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 Measurement data Processing Raw data Convolution Filtered data Or Convolved data Back-projection CT image

Equipment configuration Three major systems: 1. Imaging systems-----located in the scanner room 2. Computer system----- located in the computer room 3. 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: Resolution 1. Spatial resolution 2. 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.